Cationic Imidazolium Polythiophenes: Effects of Imidazolium-Methylation on Solution Concentration-Driven Aggregation and Surface Free Energy of Films Processed from Solvents with Different Polarity
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Cationic Imidazolium Polythiophenes: Effects of Imidazolium-Methylation on Solution Concentration-Driven Aggregation and Surface Free Energy of Films Processed from Solvents with Different Polarity
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  • Sergio E. Domínguez*
    Sergio E. Domínguez
    Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    *E-mail: suesdo@utu.fi (S.E.D.).
    More by Sergio E. Domínguez
    Orcidhttp://orcid.org/0000-0002-5702-9889
  • Antti Vuolle
    Antti Vuolle
    Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    More by Antti Vuolle
  • Michela Cangiotti
    Michela Cangiotti
    Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy
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  • Alberto Fattori
    Alberto Fattori
    Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy
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  • Timo Ääritalo
    Timo Ääritalo
    Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
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  • Pia Damlin
    Pia Damlin
    Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
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  • M. Francesca Ottaviani
    M. Francesca Ottaviani
    Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy
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    Orcidhttp://orcid.org/0000-0002-4681-4718
  • Carita Kvarnström*
    Carita Kvarnström
    Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    *E-mail: carkva@utu.fi (C.K.).
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    Orcidhttp://orcid.org/0000-0002-8734-2294
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Langmuir

Cite this: Langmuir 2020, 36, 9, 2278–2290
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https://doi.org/10.1021/acs.langmuir.9b03095
Published February 6, 2020

Copyright © 2020 American Chemical Society. This publication is licensed under CC-BY.

Abstract

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Cationic imidazolium-functionalized polythiophenes with single- or double-methylation of the imidazolium ring were used to study the impact of imidazolium-methylation on (i) the solution concentration-driven aggregation in the presence of paramagnetic probes with different ionic and hydrophobic constituents and (ii) their surface free energy (SFE) as spin-coated films deposited on plasma-activated glass. Electron paramagnetic resonance spectroscopy shows that the differences in film structuration between the polymers with different methylations originate from the early stages of aggregation. In the solid state, higher degree of imidazolium-methylation generates smaller values of total SFE, γS, (by around 2 mN/m), which could be relevant in optoelectronic applications. Methylation also causes a decrease in the polar contribution of γS (γSp), suggesting that methylation decreases the polar nature of the imidazolium ring, probably due to the blocking of its H-bonding capabilities. The values of γS obtained in the present work are similar to the values obtained for doped films of neutral conjugated polymers, such as polyaniline, poly(3-hexylthiophene), and polypyrrole. However, imidazolium-polythiophenes generate films with a larger predominance of the dispersive component of γS (γSd), probably due to the motion restriction in the ionic functionalities in a conjugated polyelectrolyte, in comparison to regular dopants. The presence of 1,4-dioxane increases γSp, especially, in the polymer with larger imidazolium-methylation (and therefore unable to interact through H-bonding), probably by a decrease of the imidazolium–glass interactions. Singly-methylated imidazolium polythiophenes have been applied as electrode selective (“buffer”) interlayers in conventional and inverted organic solar cells, improving their performance. However, clear structure–function guidelines are still needed for designing high-performance polythiophene-based interlayer materials. Therefore, the information reported in this work could be useful for such applications.

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Introduction

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Conjugated polyelectrolytes (CPEs) possess physical–chemical properties related to both “π” systems, like acting as chromophores and fluorophores, and properties of polyelectrolytes, such as solubility in high dielectric media (e.g., water and other polar solvents). They also possess the capability of coordination through electrostatic forces and hydrogen bonding (H-bonding) either with solvents, therefore helping in solubilization, or with other dissolved molecules. (1−3) The ionic groups in the polymers introduce ion–dipole and ion–ion forces. (4)
Furthermore, when CPEs contain functional groups with cationic π-rings (such as the heteroatomic imidazolium or pyridinium rings), the coaction between the noncovalent cation and π forces (also known as π+) has to be taken into account. In recent years, π+–π and π+–π+ interactions have been recognized as a distinctive contributing factor in structuring in the context of host–guest chemistry and fundamental ab initio studies of π–π interactions. (5) In ionic liquids (ILs) containing the imidazolium ring, dispersion and π–π interactions also compete with hydrogen bonding (H-bonding), which in part determines the structuring in the IL. (5) When protonated, imidazoliums can establish H-bonds through their N–H group as it happens in the “doubly ionic” low-energy H-bonds present between histidine and aspartate during enzymatic catalysis. (6) According to qualitative molecular orbital computational analyses, methylation of the nitrogen atoms in imidazolium rings (known as aprotic imidazolium rings) does not cancel the H-bonding capabilities of the ring, which considers the cationic C–H group (C–H+) to possess H-bonding donor capabilities. (7) This has also been considered in molecular dynamics simulations in order to explain the cooperative–competitive interplay between H-bonding and π-type interactions in an IL consisting of aprotic-imidazolium and oxalatoborate. (8)
Notice that because of the different definitions of H-bonding, in numerous studies, the classification of any interaction considered may be equivocal, as pointed in the review by Grabowski. (9) For example, numerous C+–H···Y interactions have been classified as H-bonds; however, they could not be classified as such in the Pauling definition because carbon is not an electronegative atom. (9) In his review, Steiner pointed that despite the role of C–H groups as H-bond donors being underexplored, it could be predicted to occur when very acidic C–H group donors or very basic acceptor groups are involved. (6,10)
If present in CPEs, all of these forces are expected to impact their (i) solubility, (ii) conformation in solution, (iii) aggregation between polymer chains (intermolecular aggregation) and between different segments of the same chain (intramolecular aggregation), and (iv) interaction with other molecules either in solution (e.g. complex formation and assembly) or in solid state.
In the solid state, the solubility of CPEs is important in the fabrication of optoelectronic devices. For example, the use of water-soluble polythiophenes with ionic ammonium pendant groups allow orthogonal processing on top of the photoactive layer of organic solar cells (OSCs). This grants the formation of a capacitive double layer, enabling improved charge extraction and, thus, device efficiency. (11)
The power conversion efficiency of OSCs can be improved significantly by using electrode selective (“buffer”) interlayers made of cationic or anionic CPEs, regardless of the ion functionality. (12) Such phenomena improve the efficiency of organic photovoltaics, (11−13) and therefore CPEs containing different ionic moieties are frequently used as electrode selective “buffer” layer materials.
Kelvin probe force microscopy (KPFM) or ultraviolet photoelectron spectroscopy (UPS) has been used to gain insight into the structure-function dependence and effect on the working mechanism of these buffer layers. (13) From these studies, different mechanisms have been proposed, such as (i) preferred orientation of the ionic moieties, (ii) energy level alignment at the organic/metal interface or active layer doping, (iii) formation of an image charge, causing alterations in the work functions, (13) or (iv) capability to show spontaneous permanent dipoles, poling-induced dipole alignment, and interfacial energy barrier control. (14,15)
Besides KPFM and UPS measurements, another approach is to characterize the photovoltaic properties of OSCs after including buffer layers made of CPEs, among other materials (e.g., LiF and Cs2CO3 of fullerene derivatives), affording remarkable improvements in conversion efficiencies. (16)
In the particular case of cationic polythiophenes used as buffer layers in a conventional OSC architecture (i.e., the bottom-metallic electrode act as the cathode, extracting electrons), Seo et al. (17) reported one of the first studies on improvement of OSCs by adding a cationic trimethylammonium polythiophene next to the metallic cathode. Later, Kesters et al. (16) compared the effect of applying cationic polythiophenes with either trimethylammonium or imidazolium side chains as buffer layers. The results showed that the presence of a cation−π system is desirable because the imidazolium functionality generates better device performance. In a subsequent study, the same group compared two polythiophenes containing either imidazolium or pyridinium side chains. (11,13) From their studies, it was concluded that a larger cation−π system is preferred, whereas the polymer having a pyridinium functionality performs better than that with an imidazolium group. With regard to the use of cationic polythiophenes in inverted architectures (i.e., with the metallic electrode acting as the anode by extracting holes), Zilberberg et al. (18,19) applied an ultrathin cathode buffer layer made of the same imidazolium polythiophene used by Kesters et al. (13,16) It was found that the buffer layer reduced the work function of the indium tin oxide (ITO) electrode (which under inverted architecture extracts electrons). In another work, Rider et al. (20,21) reported stable inverted OSCs fabricated using a cathodic buffer layer consisting of a mixture of a cationic pyridinium polythiophene and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate.
Despite the mentioned studies, a clear mechanistic model to explain the working mechanism of buffer layers is not available yet. Therefore, clear structure-function guidelines are still needed for designing high-performance polythiophene-based interlayer materials. (11)
Contact angle (CA) goniometry is a useful route to gain insight into the properties of films of CPEs because it allows estimating the surface free energy (SFE or, simply, surface energy) of polymeric films. For example, in OSCs, increments in the total SFE (γS) of around 4 mN/m have been observed in poly(3-hexylthiophene) (P3HT) films because of a decrease in polymer regioregularity. (22) This was interpreted as a difference in the packing of the alkyl chains in P3HT, following previous studies on pentacene films, which showed (by means of CA goniometry) that a decreased film order increases γS (in less than 1 mN/m). (23) This result allowed explaining the high miscibility observed between P3HT and [6,6]-phenyl-C61-butyric acid methyl ester PC60BM (24) and later PC70BM. (25) SFE also has an impact on the morphology, miscibility, and segregation between adjacent layers or between layers and electrodes in OSCs, in the end affecting the efficiency of the devices. For example, a difference of around 10 mN/m in γS between layers (29.1 and 41.1 mN/m) promotes poor miscibility, producing a slightly larger phase-separated film morphology. (22,25,26) However, when this difference decreases to around 2.5 mN/m (29.1 and 31.6 mN/m), penetration and diffusion of the fullerene into the polymer region are promoted. (11,22,25,26)
SFE analyses have been utilized to study the following: (i) the impact of CPE buffer layers on the short-circuit current and fill factor of OSCs, (27) (ii) the impact of surface treatments of buffers on the adhesion and power conversion efficiency of OSCs, (28) and (iii) the adhesive properties of the constituent layers in OSCs, which impact the mechanical stability of the device. (29)
The energy level, electrical conductivity, and SFE of films made of CPEs can be modified by means of molecular structure, for example, by changing the polymer backbone and the lengths of alkyl side chains. (15)
It is also possible to dope the films; for example, the archetypical P3HT generates films with a low surface energy. However, doped P3HT generates high surface energies mainly due to its conductivity, namely, the presence of radical cations and anions. The use of dopants with strong hydrophobic groups (e.g., tolyl groups), hydroxyl groups, and carboxyl groups also promotes intermolecular hydrogen bonds, modifying the wetting properties of the polymer. Polypyrrole (PPy) possesses Lewis acid–base contributions, predominately Lewis acidity. The most energetic part of the molecule is the acidic sites, possibly due to N–H bonds on the pyrrole acting as electron-pair acceptors (i.e., act as H-bonding donors) and/or the cationic nature of the backbone. (30)
Besides modifying the molecular structure of the polymer and/or doping the polymeric films, a judicious selection of the polarity of the solvent mixtures allows modulation of the nanomorphology of self-assembled aggregates (e.g., vesicles, rods etc.) as well as the optical properties of conjugated polymers and CPEs. (11) Cosolvents (also known as “additives” in the field of OSCs) provide an extra level of control over the main parameters that dominate the OSC formation during solution processing: (1) in solution, the thermodynamic parameters, such as the solubility of donor and acceptor materials in the solvent(s), their ability to undergo crystallization/aggregation, and the mutual interactions between the solvents and the donor and acceptor solutes and (2) the kinetic parameters, such as the vapor pressure of the solvents and the deposition conditions that collectively define the drying kinetics of the mixture. (31) CPEs are particularly tunable by means of solvents because these molecules allow the use of high dielectric media (e.g., water and hydroxylic solvents), offering a wider window of conditions and maximizing the possibility to study interaction forces. For the particular case of cationic imidazolium polythiophenes, Urbánek et al. have found that an imidazolium polythiophene shows solvatochromic concentration-driven aggregation, with methanol decreasing the extent of aggregation. (32) Our previous studies agree with this reference because we observed that a cationic isothiouronium polythiophene (which possess H-bonding donor capabilities) shows larger Stokes shifts when dissolved in water than when dissolved in mixtures with decreased polarity/H-bonding capacity, both in disaggregated (33) and aggregated (34) states.
This work presents a study on the effect of methylation of an imidazolium functionality in cationic polythiophenes. The focus is on their (i) concentration-driven aggregation in water and (ii) SFE of films processed from solvents with different polarities. The study of aggregation in aqueous media is studied by electron paramagnetic resonance (EPR) spectroscopy with the aid of paramagnetic probes of different polarities. The computer-aided analysis of the EPR spectra of polymers that are able to self-aggregate in aqueous solutions has demonstrated to be a useful tool to obtain information on the aggregation behavior and the interactions occurring in solution, (35,36) as shown in our previous study on the concentration-driven aggregation of cationic polythiophenes in water. (34) Therefore, EPR spectroscopy is an ideal complementary technique to further study the concentration-driven aggregation of imidazolium polythiophenes reported previously. (32)
The SFE was studied by CA goniometry measurements on spin-coated films of the polymers on plasma-activated microscope glass coverslips. The effect of the polarity and H-bonding capacity of the processing solvent was studied by using either water or a water–1,4-dioxane (DI) 50:50 (v/v) mixture (W–DI).
With regard to the imidazolium polythiophenes used here, the one with less extent of methylation is analogous to that used previously in studies in solution (32) or applied as buffer layers, (13,16,18) whereas that with methylation in the C+–H group (see Figure 1b) has not been analyzed yet in such type of studies, to the best of our knowledge.

Figure 1

Figure 1. EPR spectra of the paramagnetic probe 5DSA in solutions of (a) PIMa and (b) PIMb at 25 °C and 0.5 mM.

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With regard to the solvents selected, DI is a nonpolar aprotic solvent with a boiling point and density similar to water, which also is miscible with water in all proportions. DI is also capable of disrupting the H-bonding structure in water by accepting two H-bonds, without donating any, because of its relatively bulky structure consisting of ether groups. (37) Besides this, the 50:50 v/v mixture of water and DI (W–DI), has a dielectric constant ≈50% smaller than that of water and a viscosity double that of water (see Table 1). Density functional theory studies have shown that complexation of molecules can be modulated by changing the amount of DI in water. (37) Molecular dynamics simulations of the interactions between the oligomers of an anionic phenylene–fluorene copolymer in water or the W–DI mixture showed that DI forms a “coating,” displacing water from the immediate environment of the molecule, whereas the ionic parts are preferentially solvated by water. This coating reduces interchain and side-chain interactions and leads therefore to aggregation. (38) This coating effect is in agreement with the experimental study by Luong et al., (39) who reported that heteromolecular water–DI H-bond dominates only at low concentrations of water, whereas at water mole fractions above 0.1, it generates a bulklike, intermolecular, three-dimensional H-bonded water network dynamics. Experimental studies of quenching in solution have used the W–DI system because it provides a wide range of variation of solvent dielectric constant and viscosity, allowing to analyze their effects. (40)
Table 1. Values of Physical–Chemical Parameters Relevant for the Studies, from All Solvents (at 20 or 29 °C)a
solventdensity (g/cm3)dynamic viscosity (mPa s)dielectric constantrefractive index
water0.99 (49)b0.754 (50)80.38 (51)b1.33 (50)
W–DI1.03 (52)b1.4 (50)36.89 (51)b1.40 (50)
solventH-bonding capacitycδD dispersiondδP polardδH hydrogen bondingd
waterstrong15.516.042.3
DImoderate17.51.89.0
a

Also shown are the H-bonding capacities of each pure solvent (according to the Hildebrand scale) and the values of the H-bonding interactive force (δH) of the Hansen solubility parameters of each pure solvent. Next to each value is provided the reference number.

b

At 29 °C.

c

Ref (53).

d

Ref (54).

Besides the computational and empirical studies in solution, with regard to studies focused on films, to the best of our knowledge, DI has been used as a cosolvent at very low concentrations (1–2%) when studying OSCs made of hydrophobic molecules. (41,42) It has not, however, been used in studies on thermodynamics in solution or drying kinetics of films using either hydrophobic or hydrophilic molecules. (31)
With regard to the glass substrates used, the polymeric films produced in this work can be considered as model surfaces similar to buffer layers in contact with oxide electrodes because both ITO (43,44) and plasma-activated glass (45) possess surface −OH groups. As mentioned before, in the context of OSCs, cationic imidazolium polythiophenes next to ITO substrates has been reported in devices with inverted architecture, in which the ITO electrode acts as the cathode, extracting holes. (18,20)
However, regardless of the surface properties of the substrate, the present work allows to study the effect of modifying the conjugated nature of the cationic functionality, as observed in the works of Kesters et al. cited before, having films of cationic polythiophenes next to aluminum substrates as a part of OSCs. (13,16)
Furthermore, the films obtained in this work allow comparisons with previously reported films made of neutral (nonionic) conjugated polymers, such as P3HT, polyaniline (PANI), and PPy, doped with different dopants (and also dedoped). These polymers showed similar ratios between the polar and dispersive contributions of the SFE, regardless of whether they were deposited onto glass or metallic (e.g., gold) substrates. (30)

Materials and Methods

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Unless otherwise stated, all reagents and solvents used are of analytical reagent grade, commercially available, and used as supplied (Sigma-Aldrich). Deionized water was used for the preparation of the stock solutions.
Scheme 1a shows the skeletal structure of the cationic imidazolium polythiophenes used in this work: poly-3-(1-methylimidazolium)hexyloxy-4-methylthiophene (PIMa) and poly-3-(1,2-dimethylimidazolium)hexyloxy-4-methylthiophene (PIMb), whose self-assembling capacity has been previously described. (46)

Scheme 1

Scheme 1. Skeletal Structures of (a) the Cationic Imidazolium Polythiophenes PIMa and PIMb and (b) the Paramagnetic Probes Used in the EPR Aqueous Study, TOH, CAT8, CAT16, and 5DSA
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These polymers are assumed to have mainly head-to-tail regioregularity because the syntheses were performed using 3-alkoxy-4-methylthiophene monomers in an oxidative polymerization (using FeCl3), conditions that are known to generate mainly 2,5 linkages. (47) The polymers are also assumed to have a similar degree of polymerization (20–30 repeating units) and dispersity (D̵ = Mw/Mn = 1–3). For further details on these assumptions, please see ref (33).
Scheme 1b shows the structures of the paramagnetic spin probes used in this work. 4-hydroxy-2,2,6,6-tetramethyl-piperidine-1-oxyl (TOH) and 5-doxyl-stearic acid (5DSA) are commercially available, and 4-octyl dimethyl ammonium, 2,2,6,6 tetramethyl-piperidine-1-oxyl bromide (CAT8) and 4-cetyl dimethylammonium, 2,2,6,6 tetramethyl-piperidine-1-oxyl bromide (CAT16) are a gift from Dr. Xuegong Lei, Columbia University, NY, USA. These probes were selected after accurate screening and found to be the most suitable to get information about the formation of aggregates and about the interacting ability of PIMa and PIMb in water because they were previously demonstrated to be informative on the structure and aggregation of polymers and surfactants. (35,36) These spin probes have also been used in our previous EPR study on the concentration-driven aggregation of cationic polythiophenes. (34)

EPR Spectroscopy

EPR spectra were recorded by an EMX-Bruker spectrometer operating at X band (9.5 GHz) and interfaced with a PC (software from Bruker for handling and recording the EPR spectra). The temperature was controlled by a Bruker ST3000 variable temperature assembly cooled with liquid nitrogen. The reproducibility was verified by repeating each experiment at least three times.
The concentration of 0.05 mM was selected for all probes because it showed to be nonperturbative of the systems on the basis of the invariability of the spectral line shape by further decreasing this concentration.
The computation of the spectra was accomplished by means of the well-established procedure of Budil et al. (48) The EPR spectral line shape is determined by the molecular reorientational dynamics of the spin probe and its constraints over correlation times ranging from 10–11 to 10–6 s. According to the Kubo–Tomita theory, it is possible to simulate EPR spectra on the basis of peculiar dynamic models. (48) Anisotropies of the reorientational motion of anisotropic molecules, for example,, nitroxide molecules, mainly surfactants, were accounted for by introducing simple potentials. A modification of the Levenberg–Marquardt minimization algorithm was used for the analysis of the EPR spectra. The dynamic parameters describing the slow motion are obtained from the least-squares fitting of model calculations based on the stochastic Liouville equation of the experimental spectra. The correlation time obtained provides a measure of microviscosity at the nitroxide site.
The main parameters extracted from computation were the following: (i) the Aii components of the hyperfine coupling tensor A for the coupling between the electron spin and the nitrogen nuclear spin. These components measure the environmental polarity. Unless otherwise specified, for simplicity, the Axx and Ayy components were assumed to be constant (6 G), whereas only Azz was changed. The accuracy of this parameter is ±0.01 G; (ii) the correlation time for the diffusional rotation motion of the probe (τ), which measures the microviscosity around the probe, in turn monitoring the interactions occurring among the molecules at the probe site. The accuracy in this parameter is ±1 ps. (35)
The total intensity of well-reproducible EPR spectra was evaluated by the double integral of the spectra in arbitrary units (A.U.). Quantitative EPR measurements of spin concentration cannot be performed in the absence of an internal reference, but, in the present case, we trusted the intensity values only in a comparative way for a series of samples for an indirect measure of the spin-probe solubility.

Solvent Systems

Table 1 shows some relevant physical–chemical properties of water and W–DI.

Spin-Coating Preparation of Glass Blanks and Polymeric Films

Spin-coated films of PIMa and PIMb were deposited either from water or W–DI on air-plasma-cleaned microscope borosilicate glass coverslips (VWR International). Air-plasma decreases the number of siloxane groups while increasing the surface concentration of H-bonding donor OH groups (55) and thus increasing the value of the “silanol number.” (56) Besides the polymers, the plasma-activated glass slips were spin-coated only with water or DI in order to obtain the “glass–water” and “glass–DI” blanks, respectively.
The polymeric films were produced by adding 3 μL of 0.2 mg/mL solutions of PIMa or PIMb (for concentrations ≈ 0.8 mM, monomer based) dissolved either in water or W–DI on an already 500 rpm spinning substrate (i.e., dynamic dispense). Previously, a PIMa concentration of 0.25 mg/mL in water was used in this group to produce self-assembled multilayers of CPEs. (46) Also, in the previously mentioned studies of Kesters et al. using cationic polythiophenes, concentrations ≤ 0.25 mg/mL (in methanol) showed to be optimal to observe differences in OSC efficiencies as a function of the cationic functionality in the polythiophene. (13)
Despite these references, we analyzed the effect of increasing the surface concentration by using multiple depositions (with a drying of 60 s between each) using different cationic polythiophenes. Larger surface concentrations did not increase the difference between polymers (results not shown).
All solutions were obtained from the same aqueous stock solution (2.1 mM). The polymers are expected to interact with the plasma glass through electrostatic interactions between the cationic imidazolium units and the partial negatively charged surface −OH groups.
In order to maximize reproducibility (i.e., decrease the experimental error), all films were produced from the same batch solutions and by the same operator. In order to minimize the biased data due to the learning curve of the process, the production of films and the CA measurements were randomized as much as possible by avoiding to systematically produce or measure films exposed to the same treatment (i.e., same polymer or processing solvent) or similar measurements (e.g., same probe liquids).

Expected Interactions with Plasma-Glass

Scheme 2 shows the expected interactions between the plasma-activated glass substrates and the polymers.

Scheme 2

Scheme 2. Expected Interactions between the Imidazolium CPEs Used in This Work and the Plasma-Activated Glass, Modified from Refs (57−59); Notice That the Glass Substrate Here Is Presented as Partially Activated, i.e., with a Partial Surface Concentration of Si–OH Having Also Si–O–Si Groups
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CA Goniometry and SFE Estimations

CA measurements allow estimating the SFE of films made of conducting polymers in a relatively simple way (when compared with other techniques such as inverse gas chromatography), however, providing high sensitivity. (30) The CA between a liquid and a surface of interest can be related to the surface tension or energy via Young’s equation together with different models (details ahead). (30) If CA values with two or more test (or “probe”) liquids, with known and convenient surface tension components, are available, then it is possible to estimate the total free energy and also its Liftshitz–van der Waals (dispersive) and Lewis acid–base (polar) components. (30)
The estimations of the total SFE (γS), together with their polar (γSp) and dispersive (γSd) contributions, of the substrate blanks and polymer films were obtained using two models: the Owens, Wendt, Rabel, and Kaelble (OWRK) model and the Wu model (also known as the harmonic mean method). (60) Both methods have been described elsewhere, (61) and it is known that they require less measuring data than other models for estimation of γS, γSp, and γSd while avoiding generating negative values as other methods (e.g., the acid–base method). (61) Wu’s model has already been used to study films made of conjugated polymeric molecules. (30,62−64)
The SFE estimations by Wu’s method were obtained considering the four probe liquids shown in Table 2 (glycerol, ethylene glycol, formamide, and diiodomethane), whereas the OWRK estimations were obtained considering two probe liquids (glycerol and diiodomethane). (61)
Table 2. Total Surface Tension (γL) of the Probe Liquids Used in This Work Together with Their Constituting Dispersive (γLd) and Polar (γLp) Contributions
 glycerolethylene glycolformamidediiodomethane
γL (mN/m)63.447.758.250.8
γLd (mN/m)3726.439.548.5
γLp (mN/m)26.421.318.72.3
The calculations to estimate the SFE were performed with the aid of the software KSV Surface Free Energy Analysis (version 3.0), copyright KSV Instruments, Ltd. (1997–2005), using the averages of at least triplicate CA measurements from different experimental units.
In this work, the CAs between the blank surface or polymeric films and different probe liquids (glycerol, ethylene glycol, formamide, and diiodomethane) were measured using the sessile drop method, with 3 μL drops of each probe liquid. The CA value was taken from the stabilized reading. The surface tension values of the respective liquids (γL) and their constituting polar and dispersive forces (γLp and γLd, respectively) are shown in Table 2.
Notice that the surface tension of liquids and SFE of solids are commonly reported in the literature either with units of force/unit length (mN/m) or energy/unit area (mJ/m2), with both scales being numerically equivalent.

Results and Discussion

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EPR

EPR spectroscopy was successfully applied to investigate the aggregation mechanism of the differently methylated polymers described above. Characterization of the interaction between EPR probes and the polymer system is given by the interpretation of the experimental data by the use of the computer aided analysis described in the experimental section. The completely hydrophobic probe (5DSA) generated different results with respect to probes containing hydrophilic and hydrophobic groups (TOH, CAT8, and CAT16). Therefore, the results obtained with 5DSA will be discussed separately.
Figure 1 shows the spectra recorded for 5DSA with (a) PIMa and (b) PIMb at 0.5 mM.
The spectra shown in the figure were recorded under the same experimental conditions as used for the other spin probes, for a matter of comparison. These spectra are quite noisy, demonstrating the low intensity due to the low solubility of 5DSA in these systems because it only solubilizes into the hydrophobic region formed by polymer aggregates. However, we clearly see that the noise is lower for PIMb than for PIMa, indicating higher solubilization of the hydrophobic probe in PIMb aggregates because of methylation of the imidazolium ring, which increases hydrophobicity.
The computations of the spectra (red lines; the main parameters Azz and τ are listed in the figure) interestingly indicate that the radical group (at position 5 of the stearic chain) is located in a region at low polarity and quite high microviscosity, as expected for the hydrophobic core of a lipid aggregate. A significantly higher microviscosity (more than the double) was calculated for PIMb with respect to PIMa, indicating that methylation of the imidazolium function increases the microviscosity in the environment of the hydrophobic probe inserted into the polymer aggregates.

Effect of Imidazolium Methylation on the EPR Intensity

With regard to the results from the probes TOH, CAT8, and CAT16, Figure 2 shows the intensity variation (measured as a double integral of the spectra) as a function of the polymer concentration (in the 0–0.5 mM polymer concentration range) for TOH (a), CAT8 (b), and CAT16 (c).

Figure 2

Figure 2. EPR spectral intensity (measured as a double integral of the spectra in A.U.) as a function of the concentration of PIMa (green upward triangles) and PIMb (blue downward triangles) in the presence of 0.1 mM TOH (a), CAT8 (b), and CAT16 (c).

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Figure 2 shows that for all paramagnetic probes, a maximum intensity is observed at about 0.25 mM of polymer concentration. Assuming that the intensity measures the probe solubility, this result indicates increased probe solubility around this concentration. All probes contain a hydrophilic and hydrophobic portion and solubilize better when they insert in aggregates where the hydrophobic part is protected from the hydrophilic one. Therefore, we may consider the intensity increase as a proof of the formation of polymer aggregates, where the hydrophobic parts of the polymers condense surrounded by the hydrophilic parts. A previous study from our group using steady-state fluorescence on solutions of cationic isothiouronium polythiophenes also showed aggregation of the polymers around a polymer concentration of 0.2 mM. (34)
The equivalent solubilization of TOH in the aggregates of PIMa and PIMb shown in Figure 2a is reasonable because TOH is the most hydrophilic probe and therefore interacts with the cationic imidazolium groups regardless of their degree of methylation.
The fact that both polymers interact to a similar extent with TOH, despite the difference in the H-bonding capabilities between them, can be explained because water (and other hydroxylic solvents) is known to compete for intermolecular H-bonding; this is why they are known as “competitive solvents” in the contexts of molecular recognition (65,66) or polymer solvation. (67) Therefore, in this case, water would compete with TOH for the H-bonding, nulling the structural difference between PIMa and PIMb with regard to their H-bonding capabilities.
On the other hand, Figure 2b shows that for polymer concentrations associated with aggregation of cationic polythiophenes, (33,34) CAT8 generates larger intensities in the presence of the polymer with less extent of methylation in the imidazolium ring (PIMa). This is because this probe solubilizes at the hydrophilic/hydrophobic interface of the aggregates, and the methyl groups partially impede solubilization.
Interestingly, Figure 2c shows that, for a probe with a larger hydrophobic nature (CAT16), larger intensities are obtained in the presence of the polymer with larger extent of methylation in the imidazolium ring (PIMb). In this case, the methyl groups favor the solubilization in the disordered aggregates of the CAT16 probe whose hydrophobic portion has good affinity for hydrophobic methyl groups.
Figure 2b,c shows that, when interacting with CAT8 and CAT16, the highest polymer concentrations cause the curves of PIMa and PIMb to diverge. This behavior is opposite to that observed during the concentration-driven aggregation of isothiouronium polythiophenes with spacers of different lengths, under identical experimental conditions to the present work. (34)
In the previous study, the intensity increase to the maximum and, then, the decrease at the highest concentration have been ascribed to the formation of aggregates at the maximum, which became progressively less organized with the further increase in the concentration. For the isothiouronium polythiophenes, the longer spacer provokes the formation of better organized aggregates at the maximum, whereas the highest concentration of polymers equivalently leads to disorganization of the aggregates for the two polymers, despite the spacer length. In the present case, the methyl group is not perturbing the aggregate formation at the maximum, but only a high concentration of the polymer lets the methyl group differently affect the disorganizing process. This is because the methyl group is located at the charged head and starts being perturbative only when the concentration of polymers is high and the charged head groups start repulsing each other.

Effect of Imidazolium-Methylation on Microviscosity

Figure 3 shows the microviscosity (interaction) parameter (τ) as a function of the concentration of PIMa and PIMb (also in the 0–0.5 mM polymer concentration range) for TOH (a), CAT8 (b), and CAT16 (c).

Figure 3

Figure 3. Microviscosity (interaction) parameter (τ) as a function of the concentration of PIMa (blue upward triangles) and PIMb (green downward triangles) in the presence of 0.1 mM TOH (a), CAT8 (b), and CAT16 (c).

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In Figure 3, it is observed that the differences between PIMa and PIMb at high concentrations are small (e.g., in Figure 3a,c); however, these differences are above the experimental error and in agreement with the other results. Figure 3a,c shows that the more hydrophilic and more hydrophobic probes, respectively, indicate a larger viscosity in the aggregates of the polymer with less extent of methylation in the imidazolium ring. Interestingly, Figure 3b shows that the probe with a middle extent of hydrophilic and hydrophobic components (compared with TOH and CAT16) indicates the same viscosity, regardless of the polymer. As suggested on the basis of the intensity data, the positively charged CAT group of CAT8 is hosted at the hydrophilic/hydrophobic interface, and it is repulsed by the positively charged polymer head. Therefore, the interactions do not feel the presence of the methyl groups. Conversely, both the neutral probe (TOH) and the largely hydrophobic probe CAT16 feel the presence of the methyl groups in the aggregates, which perturb the hydrophilic interactions at the highest PIMb concentrations, thus decreasing the microviscosity.
To better understand the intensity and microviscosity variations and the consequent information on the system structures, it is interesting to compare the behavior of two more hydrophobic probes, 5DSA and CAT16, with respect to the two polymers. Both probes show higher solubilization in PIMb aggregates because of methylation and increased hydrophobicity. However, the microviscosity for the methylated-PIMb sample, compared to PIMa, increases for 5DSA, whereas it decreases for CAT16. The radical group of 5DSA is at position 5 of the carbon chain, and hence it is embedded into the hydrophobic portion of the aggregates in proximity to the interface. Therefore, PIMb aggregates are more packed in their hydrophobic region than PIMa aggregates because of the presence of the methyl group in PIMb, which is thus located in the lipidic region where the doxyl group of 5DSA is situated, close to the interface, and increases the PIMb aggregate packing. Conversely, the radical CAT group of CAT16 is positively charged and stays outside the lipidic region. The long C16 chain forces this probe to solubilize in the PIMb aggregates (while CAT8 can escape!). However, by itself, the CAT group of CAT16 is also forced to approach the positively charged imidazolium group. Therefore, charge repulsion provokes the weakening of hydrophilic interactions and the consequent decrease in microviscosity.

CA Goniometry and SFE

Table 3 shows the average CA values of each of the four probe liquids on three blank surfaces: (i) plasma-activated glass (plasma glass), (ii) plasma glass spin-coated with water (glass–water), and (iii) plasma glass spin-coated with DI (glass–DI). Table 4 shows the OWRK and Wu estimations of the total SFE (γS) in the three blank surfaces and in the films of PIMa and PIMb on plasma glass. The values of the polar (γSp) and dispersive (γSd) contributions are also shown.
Table 3. CA Values of the Four Probe Liquids onto (i) Plasma-Activated Glass, (ii) Plasma-Glass Spin-Coated with Water (Glass–Water), and (iii) Plasma-Glass Spin-Coated with DI (Glass–DI)
 blank surface
probe liquidplasma glass CA (deg) ± SDaglass–water CA (deg) ± SDaglass–DI CA (deg) ± SDa
glycerol37.55 ± 7.3539.39 ± 6.644.44 ± 5.39
ethylene glycol21.87 ± 5.1424 ± 5.8230.05 ± 2.76
formamide7.23 ± 1.9414.4 ± 1.5716.76 ± 1.87
diiodomethane38.46 ± 3.8643.22 ± 3.7143.35 ± 2.66
a

SD values based on at least triplicate (see Table S1 in the Supporting Information).

Table 4. SFE and Its Components Estimated from Films Processed from Water and the W–DI Mixture According to OWRK Model (Estimated Using Data from Glycerol and Diiodomethane) and Wu’s Model (Estimated Using Data from the Four Probe Liquids)
  SFE
 surfaceOWRK γS (mN/m)OWRK γSp (mN/m)OWRK γSd (mN/m)Wu γS (mN/m)Wu γSp (mN/m)Wu γSd (mN/m)
blankplasma-glass54.1313.7640.3854.8913.6441.25
 glass–water52.3914.4337.9553.1614.139.06
 glass–DI49.8611.9837.8951.7812.4439.34
PIMsPIMa from water58.7913.9544.8457.7213.2844.45
 PIMa from W–DI58.915.4643.4457.5214.243.32
 PIMb from water56.3711.9844.3955.8411.4344.4
 PIMb from W–DI54.2510.4342.8155.4312.3343.10
The data presented in Table 4 shows that OWRK and Wu models do not generate the same values of γS, γSp, and γSd. In the case of PIMa, Wu’s model estimates smaller values than OWRK, regardless of the processing solvent. In the case of PIMb, when processed from water, the OWRK model estimates larger SFE values, whereas when processed from W–DI, the Wu’s model estimates a larger value of SFE.
Regardless, both models generate the same trends with regard to the effect of the molecular structure and the processing solvent. Therefore, for the sake of simplicity and also to allow comparing with previous reports (all references cited ahead used Wu’s model), Figure 4 shows only the SFE values estimated with Wu’s model for blank surfaces and polymeric films.

Figure 4

Figure 4. Wu’s model estimations of (a) γS and its (b) polar component (γSp) and (c) dispersive component (γSd) of films made of PIMa and PIMb, processed from water (solid color bars) or W–DI (dash-patterned bars). Dashed horizontal lines indicate the SFE, SFEp, and SFEd values of the blank surfaces of (1) plasma glass and (2) glass–water.

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CA Values of Blanks

The average CA of formamide on the glass–water blank (14.4°, Table 2) is 40% smaller than that reported by Rymuszka et al. of formamide on glass previously exposed to water during ultrasonic cleaning and drying (≈25°). (68) However, the average CA from diiodomethane on glass–water blank (43°) is similar to that of diiodomethane reported in the same reference (≈45°). Concerning the SFE values of the blanks, Table 4 shows that both models estimate similar total SFE values of the glass–water blank (≈53 mN/m). This value is 15% smaller than that estimated for nonheated glass with controlled porosity reported by Jańczuk et al. (≈70 mN/m). (69)
With regard to the CA values, the difference between the cited reference and our data could be due to (i) a possible difference in the type of glass, for example, soda lime glass instead of borosilicate, which are known to have different smoothness, see ref (70), and/or (ii) the difference in the drying conditions after exposure to water. These factors, alone or combined, would generate a different hydration in each glass. Because of the hydrophilic interactions of water, such a difference in hydration is expected to be clearer when using a polar probe liquid and less clear when reducing the polarity of the probe liquid. This is in agreement with the fact that the CA and SFE results obtained in the present work are similar to those of diiodomethane in the work of Rymuszka et al. (68) and SFE estimations of Jańczuk et al., (69) respectively.
Given the experimental design of the present work, the glass–water blanks are useful regardless of previous reports; however, the references cited show that our results lie within the range of previously reported values.

SFE of Polymeric Films and Blanks and Previously Reported Values

Figure 4a shows that, from both the processing solvents, the films of PIMa have larger values of γS (by at least 2 mN/m) than those of any of the blanks. The films of PIMb also generate larger values of γS than the blanks, albeit in a smaller range. With regard to the components of γS in the polymeric films, Figure 4b shows that γSp of the glass–water blank is larger than those of the polymeric films (with the exception of the PIMa films processed from W–DI).
This indicates that, with the exception of the PIMa films processed from W–DI, the surface concentration and/or energy of imidazolium cationic units in the films are smaller than those of the −OH groups present in the glass–water blank. Thus, the PIMa films processed from W–DI would have a similar surface concentration of ionic groups (imidazolium and/or −OH).
Figure 4b also shows that the polymer with larger imidazolium ring methylation (PIMb) has smaller γSp than PIMa. This indicates that the alkylation impacts the polarity of the film. On the other hand, Figure 4c shows that, for both polymers, the γSd component is always larger than that in any of the blanks, regardless of the processing solvent, which gives evidence of the presence of the hydrophobic components in the polymers (i.e., thiophene rings and alkoxy spacer) on the glass substrate.

Effect of Imidazolium-Methylation on the SFE

Figure 4a shows that regardless of the processing solvent, the PIMa films have a larger γS (≈57 mN/m) than the PIMb films (≈55–56 mN/m). This decreased imidazolium-methylation increases γS. In this regard, a previous study on the effect of the regioregularity of P3HT on its surface energy as films showed that decreased packing of alkyl chains (due to smaller regioregularity) increases γS ≈ 4 mN/m. (22) Such a conclusion was made after studies on pentacene films, which showed (by means of CA goniometry) that a decreased film order increases γS (in less than 1 mN/m). (23) Therefore, our results suggest that the smaller extent of imidazolium-methylation in PIMb decreases film ordering, thereby decreasing γS.
The magnitude of the change in γS because of methylation could be useful when tuning the morphology, miscibility, and segregation between adjacent layers or between layers and electrodes in applications such as OSCs. In such devices, a difference of around 10 mN/m in γS between two layers (having 29.1 and 41.1 mN/m) promote poor miscibility, generating a larger phase-separated film morphology. However, when this difference decreases to around 2.5 mN/m (29.1 and 31.6 mN/m), penetration and diffusion of the fullerene into the polymer region are promoted. (11,22,25,26)
With regard to similarities with previously reported films of neutral conjugated polymers, PIMa films show γS values similar to those of HCl-doped films of PANI (57.9 mN/m), whereas the PIMb films show values similar to those reported from doped P3HT films on glass (54 mN/m) and FeCl3-doped PPy films (55.4 mN/m). Notice that, as in the review by Higgins and Wallace, (30) these similarities have only a qualitative nature because the references cited were obtained using different methods and substrates. However, though qualitatively, these previous reports of neutral conjugated polymers allow analyzing the effects of regular- and “self”-doping present in neutral conjugated polymers and CPEs, respectively.
The γSp and γSd contributions provide further information. For the case of γSp, Figure 4b shows that regardless of the processing solvent, PIMa has larger values of γSp (by around 1.5 mN/m) than PIMb. As mentioned before, differences of around 2 mN/m are relevant when it comes to the SFE of films of conjugated polymers. (22,25,26)
In the case of γSd, when processed from W–DI, PIMa has a larger value of γSd (by around 1 mN/m) than PIMb.
In the case of films processed from water, the difference in γSp between polymers is negligible. In this case, PIMa and PIMb have γSd values of 44.45 and 44.4 mN/m, respectively.
In resume, from both processing solvents, imidazolium-methylation causes a clear decrease only in the case of γSp. This suggests that the imidazolium-methylation has an impact mainly on the polar component of the SFE. This could be related to a decrease of the π-enhanced cationic nature of the imidazolium functionality caused by methylation.
It is beneficial to use the ratio between the dispersive and polar contributions (γSd/γSp), because the relative contribution of the γSp and γSd components provides information about the structural differences of the films produced using CPEs and those produced from neutral CPs, doped or dedoped.
PIMa films generate γSd/γSp ratios of ≈3.3 and ≈3 when processed from water or W–DI, respectively. These values are at least 33% larger than those reported in films of HCl-doped PANI on glass (γSd/γSp ≈ 2). (30)
On the other hand, PIMb films generate values of ≈3.9 and ≈3.3 when processed from water and W–DI, respectively. These values are at least 60% larger than those reported in doped films of P3HT on glass or PPy on polyethylene terephthalate, which generate γSd/γSp ratios of 1.42 and 2, respectively. (30,63)
The PIMa and PIMb films have similar γS values of previously reported doped-P3HT or PPy films, respectively, but the similarity does not hold concerning the value of the ratios γSd/γSp: the PIMa-PIMb films have larger γSd/γSp than these references. In fact, the γSd/γSp ratio of the PIMa–PIMb films (ranging in values of 3–4) are similar to those of dedoped films of P3HT and PPY, which have ratios of 4.14 and 2.63, respectively. (30,63)
These results indicate that the cationic functionalities in CPEs do not contribute to the polar nature of the films as much as regular dopants do. This could be related to the fact of the freedom of mobility that regular dopants have, in comparison with the restricted nature of the cationic functionalities attached to a CPE.

Effect of the Processing Solvent on the SFE

The components of the SFE (Table 4 and Figure 4b) show that the presence of DI increases the value of γSp: for PIMa and PIMb films, these increases are 7 and 13%, respectively. On the other hand, Figure 4c shows that DI decreases γSd in a similar extent for both polymers (for PIMa and PIMb films, these decreases are of 3 and 5%, respectively).
To further analyze the causes behind the increase in γSp due to the presence of DI, it is useful to use the CA data from the most nonpolar probe liquid, following the contribution by Tsai et al. (64) These authors studied a film of PPy, electropolymerized in the presence of dodecylbenzenesulfonate (DBS), on top of Si coated with Au/Cr. Then, the PPy in the film was electrochemically reduced or oxidized in an aqueous solution using sodium nitrate as the electrolyte while measuring in situ the CA values of dichloromethane, the least probe liquid tested.
Electrochemical reduction caused larger dichloromethane CA values, indicating a larger surface concentration of ionic sulphonate groups (from DBS).Contrarily, the oxidized state of the film caused smaller CA values, indicating a larger surface concentration of the dodecyl chain in DBS.
Thus, Table S1 shows that the CA values of diiodomethane on PIMa films processed from water is 28.5 ± 2.45°. This value is ≈3° smaller than that on films processed from W–DI (31.85 ± 6.2°). In the case of PIMb films, the effect of W–DI is larger: the CA values of diiodomethane on films processed from water (29.59 ± 0.48°) is also ≈3° smaller than those on films processed from W–DI (32.94 ± 2.6°).
These results suggest that, in the same extent for both polymers, the presence of DI in the processing solvent generates films with larger surface concentration of ionic imidazolium groups, which generate larger CA values with the most nonpolar probe liquid diiodomethane. Considering these results, a possible mechanistic explanation would be that DI decreases the number of contacts between the imidazolium group and glass, causing therefore a larger number of unattached imidazole rings, which could then contribute to the polarity of the films.
On the other hand, the larger sensitivity of the γSp component of PIMb to DI (in comparison with γSp of PIMa) shows that the methylation in the C+–H group of PIMa has an effect on the adhesion, regardless of the fact that the H-bonds associated with PIMa are considered to have low energies (e.g., less than 17 kJ/mol). (5)
As mentioned before, there is a lack of understanding on the effect of cosolvents on the solution-thermodynamics and drying-kinetics of conjugated molecules, and that for the case of DI, there are no reports available. (31) However, from computational and empirical studies in solution-phase, it is known that DI disrupts the H-bonding structure of water, (37) causing a “coating” effect of groups of DI molecules surrounding the hydrophobic parts of the solutes, (38) probably due to the presence of “clusters” of water and DI being formed at binary 50/50 mixtures. (38,39)
Therefore, the detailed mechanism behind the different effect DI has on PIMa and PIMb during the processing (i.e., deposition of films) involves thermodynamics of solvation in solution and drying kinetics and requires further studies.

Conclusions

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Our results show that methylation of the imidazolium functionality modifies the concentration-driven aggregation, which ends the impact on the surface properties of spin-coated films.
With respect to the films, larger extent of imidazolium-methylation generates smaller values of total SFE, γS, (by around 2 mN/m). This could indicate a larger degree of film ordering in comparison to the polymer with decreased methylation. In optoelectronic devices, such a change in γS would be capable of changing the morphology, miscibility, and segregation between adjacent layers or between layers and electrodes.
Imidazolium-methylation causes a decrease in both the components of γS (γSp and γSd), regardless of the processing solvent. However, the decrease is much larger in γSp, which decreases around 9–14%, whereas γSd decreases only 0.1–2%. This indicates that methylation decreases the polar nature of the imidazolium ring, which could be related to the blocking of the H-bonding donor capabilities in the imidazolium ring, regardless of the fact that the H-bonds associated with PIMa are considered to have low energies (e.g., less than 17 kJ/mol). (5)
The values of γS obtained in the present work are similar to those of doped films of neutral conjugated polymers: the polymer with the smaller extent of imidazolium-methylation (PIMa) shows values of γS similar to doped films of PANI, whereas PIMb shows values of γS similar to doped films of P3HT or PPy. However, the PIMa–PIMb films have a larger predominance of γSd (larger values of the ratio γSd/γSp) than the films of neutral conjugated polymers. This indicates that the cationic functionalities in PIMa–PIMb (and CPEs in general) contribute in a smaller extent to the surface energy, in comparison with regular dopants in films made of neutral conjugated polymers. This could be explained by the restriction in mobility in the ionic functionalities in a CPE (i.e., attachment to a polymer backbone).
With regard to the effect of DI, its presence slightly increases γSp, especially in the polymer with larger imidazolium-methylation (PIMb). On the other hand, DI causes negligible changes in γSd for both polymers. The CA values of diiodomethane suggest that DI decreases the number of contacts between the imidazolium group and glass, causing therefore a larger number of unattached imidazole rings, which could then contribute to the polarity of the films. Therefore, DI seems to decrease the imidazolium–glass interactions, particularly, for the polymer with larger extent of methylation (and therefore capable of interacting only through electrostatic interactions, see Scheme 2), whereas the polymer with H-bonding donor capabilities is less affected.
EPR results show that the differences in film structuration between the polymers with different methylations originate in the early stages of aggregation. Four different spin probes provide different points of view about the polymer structure with respect to the differently polar and charged regions. The hydrophobic probe (5DSA) better solubilizes in the methylated-PIMb aggregates, indicating higher packing of the hydrophobic region with respect to PIMa aggregates.
In the case of probes with hydrophobic and hydrophilic components (CAT8 and CAT16), in aggregates, their solubilization varies as a function of the extent of imidazolium-methylation. In aggregates, the probe possessing an octyl chain (CAT8) stays at the interface. Therefore, the methyl group of PIMb imidazolium repulses its charged group, whereas better solubility is obtained in the presence of PIMa. Conversely, the probe possessing a hexyldecyl chain (CAT16) is better solubilized by PIMb aggregates. EPR results also show that PIMb aggregates, because of the presence of the methyl group, are more packed in their hydrophobic region than PIMa aggregates. On the contrary, surface packing decreases for PIMb with respect to PIMa, and hence, methyl groups repulse the positively charged groups on the surface.
Finally, the small probe without a hydrophobic chain (TOH) is equivalently solubilized by both polymers, regardless of the extent of imidazolium-methylation, probably due to water competition for H-bonding.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.9b03095.

  • CA values of each of the probe liquids (glycerol, ethylene glycol, formamide, and diiodomethane) on plasma-activated glass, plasma-glass spin-coated with water (glass–water), and plasma-glass spin-coated with DI (glass–DI) and plots of the estimated values of the SFE and its dispersive and polar contributions, as estimated using the OWRK and Wu’s models, from films of PIMa or PIMb processed from water or W–DI (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Antti Vuolle - Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    • Michela Cangiotti - Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy
    • Alberto Fattori - Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy
    • Timo Ääritalo - Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    • Pia Damlin - Department of Chemistry, Turku University Centre for Materials and Surfaces (MATSURF), University of Turku, 20014 Turku, Finland
    • M. Francesca Ottaviani - Department of Pure and Applied Sciences (DiSPeA), University of Urbino, Via Ca’ Le Suore 2/4, 61029 Urbino, Italy;  Orcidhttp://orcid.org/0000-0002-4681-4718
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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SED deeply acknowledges the Mexican National Council for Science and Technology (CONACyT) for the international doctoral scholarship no. 310828, the Turku University Foundation (Turun Yliopistosäätiö), the Real estate Foundation (Kiinteistösäätiö), the partial support from the Finnish National Doctoral Programme in Nanoscience (NGS-NANO), and the Doctoral Programme in Physical and Chemical Sciences (PCS) from the University of Turku. M.F.O., M.C., and A.F. thank DiSPeA, University of Urbino, for funding and Dr. Xuegong Lei from Columbia University, NY, USA, for the paramagnetic probes CAT8 and CAT16.

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    In this paper we have explored the structural and energetic landscape of potential π+-π+ stacked motifs, hydrogen-bonding(HB) arrangements and anion-π+ interactions for gas-phase ion pair (IP) conformers and IP-dimers of 1,3-dimethylimidazolium chloride, [C1C1i.m.]Cl. We classify cation-cation ring stacking as an electron deficient π+-π+ interaction, and a competitive anion on-top IP motif as an anion-donor π+-acceptor interaction. 21 stable IP-dimers have been obtained within an energy range of 0-126 kJ mol-1. The structures have been found to exhibit a complex interplay of structural features. We have found that low energy IP-dimers are not necessarily formed from the lowest energy IP conformers. The sampled range of IP-dimers exhibits new structural forms that cannot be recovered by examg. the ion-pairs alone, moreover the IP-dimers are recovering addnl. key features of the local liq. structure. Including dispersion is shown to impact both the relative energy ordering and the geometry of the IPs and IP-dimers, however the impact is found to be subtle and dependent on the underlying functional.
  6. 6
    Hunt, P. A.; Ashworth, C. R.; Matthews, R. P. Hydrogen Bonding in Ionic Liquids. Chem. Soc. Rev. 2015, 44, 1257– 1288,  DOI: 10.1039/C4CS00278D
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    Hydrogen bonding in ionic liquids
    Hunt, Patricia A.; Ashworth, Claire R.; Matthews, Richard P.
    Chemical Society Reviews (2015), 44 (5), 1257-1288CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    A review. Ionic liqs. (IL) and hydrogen bonding (H-bonding) are two diverse fields for which there is a developing recognition of significant overlap. Doubly ionic H-bonds occur when a H-bond forms between a cation and anion, and are a key feature of ILs. Doubly ionic H-bonds represent a wide area of H-bonding which has yet to be fully recognized, characterized or explored. H-bonds in ILs (both protic and aprotic) are bifurcated and chelating, and unlike many mol. liqs. a significant variety of distinct H-bonds are formed between different types and nos. of donor and acceptor sites within a given IL. Traditional more neutral H-bonds can also be formed in functionalized ILs, adding a further level of complexity. Ab initio computed parameters; assocn. energies, partial charges, d. descriptors as encompassed by the QTAIM methodol. (ρBCP), qual. MO theory and NBO anal. (E(2)n→σ*) provide established and robust mechanisms for understanding and interpreting traditional neutral and ionic H-bonds. In this review the applicability and extension of these parameters to describe and quantify the doubly ionic H-bond has been explored. Estg. the H-bonding energy is difficult because at a fundamental level the H-bond and ionic interaction are coupled. The NBO and QTAIM methodologies, unlike the total energy, are local descriptors and therefore can be used to directly compare neutral, ionic and doubly ionic H-bonds. The charged nature of the ions influences the ionic characteristics of the H-bond and vice versa, in addn. the close assocn. of the ions leads to enhanced orbital overlap and covalent contributions. The charge on the ions raises the energy of the Ylp and lowers the energy of the X-H σ* NBOs resulting in greater charge transfer, strengthening the H-bond. Using this range of parameters and comparing doubly ionic H-bonds to more traditional neutral and ionic H-bonds it is clear that doubly ionic H-bonds cover the full range of weak through to very strong H-bonds.
  7. 7
    Matthews, R. P.; Welton, T.; Hunt, P. A. Hydrogen Bonding and π–π Interactions in Imidazolium-Chloride Ionic Liquid Clusters. Phys. Chem. Chem. Phys. 2015, 17, 14437– 14453,  DOI: 10.1039/C5CP00459D
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    7
    Hydrogen bonding and π-π interactions in imidazolium-chloride ionic liquid clusters
    Matthews, Richard P.; Welton, Tom; Hunt, Patricia A.
    Physical Chemistry Chemical Physics (2015), 17 (22), 14437-14453CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A systematic electronic structure anal. of hydrogen bonding (H-bonding), anion-π+ and π+-π+ interactions present in [C1C1i.m.]Cl ion-pairs (IPs) and selected [C1C1i.m.]2Cl2 IP-dimers has been carried out. Interactions have been characterized using a combination of QTAIM, NCIPLOT, NBO and qual. MO theory. IP-dimers form non-directional charge quadrupolar arrangements due to Coulombic interactions. These are found to assoc. either as clusters or as loosely assocd. IP-IP structures. Large conformational changes are found to occur for very little cost in energy, indicating that charge screening is essentially independent of the cation ring orientation. H-bond formation is accompanied by charge transfer and polarization of the entire [C1C1i.m.]+ ring. Charge transfer does not follow the same trend for the CHelpG, QTAIM and NBO methods. Weak "stacked" π+-π+ interactions are stabilized in the presence of anions, which locate between and at the periphery of the rings, novel strongly bent H-bonds are also present. Primary (ring; C-H···Cl-) H-bonds and anion-π+ (C2···Cl-) interactions are found to decay more rapidly with distance than secondary (aliph.; CM-H···Cl-) H-bonds. This leads to an increase in the relative importance of secondary H-bond interactions in the IP-dimers. Moreover, rotation of the Me groups within the "stacked" π+-π+ IP-dimers facilitates the formation of (stronger) linear secondary H-bonds. Thus, compared to isolated IPs, secondary H-bonds may play an increased role within the condensed phase. Overall we find that structural fluidity is facilitated by fluctuating hydrogen bond, π+-π+ and anion-π+ interactions.
  8. 8
    Wang, Y.-L.; Laaksonen, A.; Fayer, M. D. Hydrogen Bonding versus π–π Stacking Interactions in Imidazolium–Oxalatoborate Ionic Liquid. J. Phys. Chem. B 2017, 121, 7173– 7179,  DOI: 10.1021/acs.jpcb.7b05564
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    8
    Hydrogen Bonding versus π-π Stacking Interactions in Imidazolium-Oxalatoborate Ionic Liquid
    Wang, Yong-Lei; Laaksonen, Aatto; Fayer, Michael D.
    Journal of Physical Chemistry B (2017), 121 (29), 7173-7179CODEN: JPCBFK; ISSN:1520-5207. (American Chemical Society)
    Intermol. features like hydrogen bonding and π-type interactions play pivotal roles in stabilizing mol. structures in ionic liqs. with planar rings and hydrogen-bond donors and acceptors. However, the delicate interplay among these interactions is complicated and depends on specific ion types. In this work, ab initio mol. dynamics simulations were performed to reveal competitive and cooperative characteristics among hydrogen bonding and π-type interactions in a typical imidazolium-oxalatoborate ionic liq. Imidazolium rings take preferential on-top parallel orientations, leading to their particular π-π stacking distributions at short distances. Intermol. interactions between imidazolium and oxalato rings are manifested by short-range on-top parallel orientations and in-plane hydrogen bonding interactions, promoting their parallel displaced offset stacking arrangements. However, on an intermediate distance scale, attractive Coulombic interactions between imidazolium and oxalato rings dominate and contribute to their perpendicular orientations. Spatial coordination patterns between intermol. oxalato rings are balanced by repulsive electrostatic interactions and steric hindrance effects, leading to their tilted orientations in local environments.
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    Grabowski, S. J. What Is the Covalency of Hydrogen Bonding?. Chem. Rev. 2011, 111, 2597– 2625,  DOI: 10.1021/cr800346f
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    9
    What Is the Covalency of Hydrogen Bonding?
    Grabowski, Slawomir Janusz
    Chemical Reviews (Washington, DC, United States) (2011), 111 (4), 2597-2625CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The following topics are discussed: geometrical parameters of hydrogen bonding, H-bond energy, QTAIM theory in anal. of H bonding and interrelations of QTAIM and the interaction energy components, covalency of different types of H bonds.
  10. 10
    Steiner, T. The Hydrogen Bond in the Solid State. Angew. Chem., Int. Ed. 2002, 41, 48– 76,  DOI: 10.1002/1521-3773(20020104)41:1<48::AID-ANIE48>3.0.CO;2-U
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    10
    Reviews: The hydrogen bond in the solid state
    Steiner, Thomas
    Angewandte Chemie, International Edition (2002), 41 (1), 48-76CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH)
    A review. The hydrogen bond is the most important of all directional intermol. interactions. It is operative in detg. mol. conformation, mol. aggregation, and the function of a vast no. of chem. systems ranging from inorg. to biol. Research into hydrogen bonds experienced a stagnant period in the 1980s but re-opened around 1990, and has been in rapid development since then. In terms of modern concepts, the hydrogen bond is understood as a very broad phenomenon, and it is accepted that there are open borders to other effects. There are dozens of different types of X-H···A hydrogen bonds that occur commonly in the condensed phases, and in addn. there are innumerable less common ones. Dissocn. energies span more than two orders of magnitude (about 02.-40 kcal mol-1). Within this range, the nature of the interaction is not const., but its electrostatic, covalent, and dispersion contributions vary in their relative wts. The hydrogen bond has broad transition regions that merge continuously with the covalent bond, the van der Waals interaction, the ionic interaction, and also the cation-π interaction. All hydrogen bonds can be considered as incipient proton transfer reactions, and for strong hydrogen bonds, this reaction can be in a very advanced state. In this review, a coherent survey is given on all these matters.
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    Houston, J. E.; Richeter, S.; Clément, S.; Evans, R. C. Molecular Design of Interfacial Layers Based on Conjugated Polythiophenes for Polymer and Hybrid Solar Cells. Polym. Int. 2017, 66, 1333– 1348,  DOI: 10.1002/pi.5397
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    11
    Molecular design of interfacial layers based on conjugated polythiophenes for polymer and hybrid solar cells
    Houston, Judith E.; Richeter, Sebastien; Clement, Sebastien; Evans, Rachel C.
    Polymer International (2017), 66 (10), 1333-1348CODEN: PLYIEI; ISSN:0959-8103. (John Wiley & Sons Ltd.)
    A review. In the past two decades, bulk heterojunction org. photovoltaic (OPV) devices have emerged as attractive candidates for solar energy conversion due to their lightwt. design and potential for low-cost, high-throughput, soln.-phase processability. Interfacial engineering is a proven efficient approach to achieve OPV devices with high power conversion efficiencies. This mini-review provides an overview of the key structural considerations necessary when undertaking the mol. design of conjugated polyelectrolytes, for application as interfacial layers (ILs). The different roles of ILs are outlined, together with the advantages and disadvantages of competing classes of IL materials. Particular emphasis is placed on the design and synthesis of water-sol. polythiophene-based IL materials and the effect of their structural characteristics on their performance as a promising class of IL material. Finally, the challenges and opportunities for polythiophenes as IL materials for OPV devices and other soln.-processed solar cell technologies (e.g. perovskite solar cells) are discussed. © 2017 Society of Chem. Industry.
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    Lim, K.-G.; Park, S. M.; Woo, H. Y.; Lee, T.-W. Elucidating the Role of Conjugated Polyelectrolyte Interlayers for High-Efficiency Organic Photovoltaics. ChemSusChem 2015, 8, 3062– 3068,  DOI: 10.1002/cssc.201500631
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    Elucidating the Role of Conjugated Polyelectrolyte Interlayers for High-Efficiency Organic Photovoltaics
    Lim, Kyung-Geun; Park, Sung Min; Woo, Han Young; Lee, Tae-Woo
    ChemSusChem (2015), 8 (18), 3062-3068CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Despite the promising function of conjugated polyelectrolytes (CPEs) as an interfacial layer in org. photovoltaics (OPVs), the underlying mechanism of dipole orientation and the elec. characteristics of CPE interlayers remain unclear. Currently, the ionic functionality of CPEs (i.e., whether they are cationic or anionic) is believed to det. the interfacial dipole alignment and the resulting electron or hole extn. properties at the interface between an org. photoactive layer and a metal electrode. In this research, we find that in contrast to this common belief, the photovoltaic efficiency can be improved significantly by both cationic and anionic CPE layers regardless of the ion functionality of the CPE. This improvement occurs because the interfacial dipoles of cationic and anionic CPEs are realigned in the identical direction despite the different ionic functionality. The net dipole is detd. not by the intrinsic mol. dipole of the CPE but by the ionic redistribution in the CPE layer and the resulting interfacial dipole at the intimate contact with adjacent layers. We also demonstrated that the energy level alignment and performance parameters of OPVs can be controlled systematically by the elec. poled CPE layers with the oriented interfacial dipoles; the distribution of pos. and neg. ions in the CPE layer was adjusted by applying an appropriate external elec. field, and the energy alignment was reversible by changing the elec. field direction. The anionic and cationic CPEs (PSBFP-Na and PAHFP-Br) based on the same π-conjugated backbone of fluorene-phenylene were each used as the electron extn. layer on a photoactive layer. Both anionic and cationic CPE interlayers improved the energy level alignment at the interface between the photoactive layer and the electrode and the resulting performance parameters, which thereby increased the power conversion efficiency to 8.3 %.
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    Kesters, J.; Govaerts, S.; Pirotte, G.; Drijkoningen, J.; Chevrier, M.; Van Den Brande, N.; Liu, X.; Fahlman, M.; Van Mele, B.; Lutsen, L. High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 6309– 6314,  DOI: 10.1021/acsami.6b00242
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    High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells
    Kesters, Jurgen; Govaerts, Sanne; Pirotte, Geert; Drijkoningen, Jeroen; Chevrier, Michele; Van den Brande, Niko; Liu, Xianjie; Fahlman, Mats; Van Mele, Bruno; Lutsen, Laurence; Vanderzande, Dirk; Manca, Jean; Clement, Sebastien; Von Hauff, Elizabeth; Maes, Wouter
    ACS Applied Materials & Interfaces (2016), 8 (10), 6309-6314CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Conjugated polyelectrolyte (CPE) interfacial layers present a powerful way to boost the I-V characteristics of org. photovoltaics. Nevertheless, clear guidelines with respect to the structure of high-performance interlayers are still lacking. In this work, impedance spectroscopy is applied to probe the dielec. permittivity of a series of polythiophene-based CPEs. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extn. and device efficiency. A counteracting effect is the diminishing affinity with the underlying photoactive layer. To balance these two effects, we found copolymer structures contg. nonionic side chains to be beneficial.
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    Lee, S.; Nguyen, T. L.; Lee, S. Y.; Jang, C. H.; Lee, B. R.; Jung, E. D.; Park, S. Y.; Yoon, Y. J.; Kim, J. Y.; Woo, H. Y. Conjugated Polyelectrolytes Bearing Various Ion Densities: Spontaneous Dipole Generation, Poling-Induced Dipole Alignment, and Interfacial Energy Barrier Control for Optoelectronic Device Applications. Adv. Mater. 2018, 30, 1706034,  DOI: 10.1002/adma.201706034
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    Lee, W.; Seo, J. H.; Woo, H. Y. Conjugated Polyelectrolytes: A New Class of Semiconducting Material for Organic Electronic Devices. Polymer 2013, 54, 5104– 5121,  DOI: 10.1016/j.polymer.2013.07.015
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    Conjugated polyelectrolytes: A new class of semiconducting material for organic electronic devices
    Lee, Wonho; Seo, Jung Hwa; Woo, Han Young
    Polymer (2013), 54 (19), 5104-5121CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)
    A review. This feature article presents a short review of the recent developments in the synthesis of conjugated polyelectrolytes (CPEs) along with their applications in org. optoelectronic devices with particular focus on the mol. structures of CPEs with ionic functionality, synthetic approaches, and their utilization as an interfacial layer. The orthogonal soly. of the CPEs allows the simple prepn. of multilayer org. devices by soln. casting on top of a nonpolar org. photoactive layer without disturbing the interfaces, showing their effectiveness in tuning the electronic structures at the interfaces for improving the charge carrier transport and resulting device properties. These achievements highlight the dynamic nature of CPEs and their applicability to a wide range of optoelectronic devices.
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    Kesters, J.; Ghoos, T.; Penxten, H.; Drijkoningen, J.; Vangerven, T.; Lyons, D. M.; Verreet, B.; Aernouts, T.; Lutsen, L.; Vanderzande, D. Imidazolium-Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance. Adv. Energy Mater. 2013, 3, 1180– 1185,  DOI: 10.1002/aenm.201300049
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    16
    Imidazolium-Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance
    Kesters, Jurgen; Ghoos, Toon; Penxten, Huguette; Drijkoningen, Jeroen; Vangerven, Tim; Lyons, Dani M.; Verreet, Bregt; Aernouts, Tom; Lutsen, Laurence; Vanderzande, Dirk; Manca, Jean; Maes, Wouter
    Advanced Energy Materials (2013), 3 (9), 1180-1185CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivs. have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I-V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium-substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short-circuit current. The mol. wt. of the ionic polythiophenes has been identified as a crucial parameter influencing performance.
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    Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer. J. Am. Chem. Soc. 2011, 133, 8416– 8419,  DOI: 10.1021/ja2037673
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    17
    Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer
    Seo, Jung Hwa; Gutacker, Andrea; Sun, Yanming; Wu, Hongbin; Huang, Fei; Cao, Yong; Scherf, Ullrich; Heeger, Alan J.; Bazan, Guillermo C.
    Journal of the American Chemical Society (2011), 133 (22), 8416-8419CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The power conversion efficiencies of bulk heterojunction (BHJ) solar cells can be increased from 5 to 6.5% by incorporating an ultrathin conjugated polyelectrolyte (CPE) layer between the active layer and the metal cathode. Poly[N-9''-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-Ph C71 butyric acid Me ester (PC71BM) were chosen for the photoactive layer. CPEs with cationic polythiophenes, in both homopolymer and block copolymer configurations, were used to improve the electronic characteristics. The improvement in device performance and the simplicity of fabrication by soln. processing suggest a promising and practical pathway for improving polymer solar cells with high efficiencies.
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    Zilberberg, K.; Behrendt, A.; Kraft, M.; Scherf, U.; Riedl, T. Ultrathin Interlayers of a Conjugated Polyelectrolyte for Low Work-Function Cathodes in Efficient Inverted Organic Solar Cells. Org. Electron. 2013, 14, 951– 957,  DOI: 10.1016/j.orgel.2013.01.018
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    18
    Ultrathin interlayers of a conjugated polyelectrolyte for low work-function cathodes in efficient inverted organic solar cells
    Zilberberg, Kirill; Behrendt, Andreas; Kraft, Mario; Scherf, Ullrich; Riedl, Thomas
    Organic Electronics (2013), 14 (3), 951-957CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    Inverted polymer:fullerene solar cells are reported using the conjugated polyelectrolyte poly(3-[6-(N-methylimidazolium)hexyl]thiophene) bromide P3ImHT as ultra-thin cathode interlayer to reduce the work-function (WF) of ITO for improved electron selectivity. Similar to ITO/TiOx cathodes, UV exposure is found necessary in the case of ITO/CPE to achieve a high fill factor on the order of 60%. UV illumination is shown to lead to a redn. of the ITO WF which in combination with WF lowering due to the dipole of the CPE leads to a suitable electron selective cathode. As a result, inverted solar cells with an efficiency of 4.8% are achieved. This is the highest efficiency of an inverted OSC using a conjugated polyelectrolyte as interface modifier directly on top of ITO.
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    Wang, K.; Liu, C.; Meng, T.; Yi, C.; Gong, X. Inverted Organic Photovoltaic Cells. Chem. Soc. Rev. 2016, 45, 2937– 2975,  DOI: 10.1039/C5CS00831J
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    Inverted organic photovoltaic cells
    Wang, Kai; Liu, Chang; Meng, Tianyu; Yi, Chao; Gong, Xiong
    Chemical Society Reviews (2016), 45 (10), 2937-2975CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    The advance in lifestyle, modern industrialization and future technol. revolution are always at high expense of energy consumption. Unfortunately, there exist serious issues such as limited storage, high cost and toxic contamination in conventional fossil fuel energy sources. Instead, solar energy represents a renewable, economic and green alternative in the future energy market. Among the photovoltaic technologies, org. photovoltaics (OPVs) demonstrate a cheap, flexible, clean and easy-processing way to convert solar energy into electricity. However, OPVs with a conventional device structure are still far away from industrialization mainly because of their short lifetime and the energy-intensive deposition of top metal electrode. To address the stability and cost issue simultaneously, an inverted device structure has been introduced into OPVs, bridging lab. research with practical application. In this review, recent progress in device structures, working mechanisms, functions and advances of each component layer as well their correlations with the efficiency and stability of inverted OPVs are reviewed and illustrated.
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    Rider, D. A.; Worfolk, B. J.; Harris, K. D.; Lalany, A.; Shahbazi, K.; Fleischauer, M. D.; Brett, M. J.; Buriak, J. M. Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface. Adv. Funct. Mater. 2010, 20, 2404– 2415,  DOI: 10.1002/adfm.201000304
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    Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface
    Rider, David A.; Worfolk, Brian J.; Harris, Kenneth D.; Lalany, Abeed; Shahbazi, Kevin; Fleischauer, Michael D.; Brett, Michael J.; Buriak, Jillian M.
    Advanced Functional Materials (2010), 20 (15), 2404-2415CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A cationic and water-sol. polythiophene [poly[3-(6-pyridiniumylhexyl)thiophene bromide] (P3PHT+Br-)] is synthesized and used in combination with anionic poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS)- to produce hybrid coatings on indium tin oxide (ITO). Two coating strategies are established: (i) electrostatic layer-by-layer assembly with colloidal suspensions of (PEDOT:PSS)-, and (ii) modification of an electrochem. prepd. (PEDOT:PSS)- film on ITO. The coatings are found to modify the work function of ITO such that it could act as a cathode in inverted 2,5-diyl-poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid Me ester (PCBM) polymer photovoltaic cells. The interfacial modifier created from the layer-by-layer assembly route is used to produce efficient inverted org. photovoltaic devices (power conversion efficiency ∼2%) with significant long-term stability in excess of 500 h.
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    Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285– 310,  DOI: 10.1039/C0EE00273A
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    The role of buffer layers in polymer solar cells
    Po, Riccardo; Carbonera, Chiara; Bernardi, Andrea; Camaioni, Nadia
    Energy & Environmental Science (2011), 4 (2), 285-310CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    This review rationalizes the information spread in the literature concerning the use and role of buffer layers in polymer solar cells. Usual device structures include buffer layers, both at the anode and at the cathode interface, mainly to favor charge collection and extn., but also to improve the overall performance of the device. Buffer layers are actually essential for achieving highly efficient polymer solar cells and can no more be considered as optional, thus the need and importance of understanding their properties and role. The aim of this review is to give the reader an overview of this topic and to provide a practical and useful tool for the daily activities of researchers in the field of polymer photovoltaics.
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    Kim, M.; Lee, J.; Jo, S. B.; Sin, D. H.; Ko, H.; Lee, H.; Lee, S. G.; Cho, K. Critical Factors Governing Vertical Phase Separation in Polymer–PCBM Blend Films for Organic Solar Cells. J. Mater. Chem. A 2016, 4, 15522– 15535,  DOI: 10.1039/C6TA06508B
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    Critical factors governing vertical phase separation in polymer-PCBM blend films for organic solar cells
    Kim, Min; Lee, Jaewon; Jo, Sae Byeok; Sin, Dong Hun; Ko, Hyomin; Lee, Hansol; Lee, Seung Goo; Cho, Kilwon
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (40), 15522-15535CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    In org. bulk-heterojunction solar cells, the vertical distribution of the compn. of the active layers as well as the lateral morphol. is one of the crit. issues that can significantly affect charge transport and recombination characteristics. Here we studied the crit. parameters that can affect the formation of vertically stratified bulk heterojunction org. solar cells based on various polymers with different side chains, and investigated the effect of the miscibility of the polymer-fullerene blend and the crystallinity of the polymer on vertical morphol. The major factor that affected the vertical phase sepn. was the interaction parameter χ between the polymer and phenyl-C61-butyric acid Me ester (PCBM). Polymer-PCBM blends with high values of χ tended to trigger surface-directed vertical phase sepn. during rapid solvent evapn. However, strong aggregation of polymers with low soly. counteracted this surface-directed vertical stratification. Moreover, solvent additives strongly affected the vertical phase sepn. processes, and caused the compn. of the active layer to fluctuate dramatically. We also found the photovoltaic characteristics, including charge recombination time, to be strongly affected by the vertical distribution of the compn. The modulation of the compn. in the vertical direction should therefore be optimized to increase the efficiency of charge collection and hence achieve high-efficiency org. solar cells.
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    Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K. Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics. J. Am. Chem. Soc. 2008, 130, 10556– 10564,  DOI: 10.1021/ja800142t
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    Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics
    Lee, Hwa Sung; Kim, Do Hwan; Cho, Jeong Ho; Hwang, Minkyu; Jang, Yunseok; Cho, Kilwon
    Journal of the American Chemical Society (2008), 130 (32), 10556-10564CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of org. thin-film transistors (OTFTs), we deposited pentacene mols. on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temps. (30, 60, and 90°C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temp. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temp.-dependent alkyl chain mobility of the ODTS monolayers.
  24. 24
    Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater. 2011, 1, 82– 89,  DOI: 10.1002/aenm.201000023
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    Interdiffusion of PCBM and P3HT reveals miscibility in a photovoltaically active blend
    Treat, Neil D.; Brady, Michael A.; Smith, Gordon; Toney, Michael F.; Kramer, Edward J.; Hawker, Craig J.; Chabinyc, Michael L.
    Advanced Energy Materials (2011), 1 (1), 82-89CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
    Developing a better understanding of the evolution of morphol. in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies >10%. In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphol. during processes such as thermal annealing are not well understood. The authors use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. PCBM aggregates and/or mol. species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the advantages of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these 2 components.
  25. 25
    Singh, R.; Suranagi, S. R.; Lee, J.; Lee, H.; Kim, M.; Cho, K. Unraveling the Efficiency-Limiting Morphological Issues of the Perylene Diimide-Based Non-Fullerene Organic Solar Cells. Sci. Rep. 2018, 8, 2849,  DOI: 10.1038/s41598-018-21162-x
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    Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells
    Singh Ranbir; Suranagi Sanjaykumar R; Lee Jaewon; Lee Hansol; Kim Min; Cho Kilwon
    Scientific reports (2018), 8 (1), 2849 ISSN:.
    Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
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    Kouijzer, S.; Michels, J. J.; van den Berg, M.; Gevaerts, V. S.; Turbiez, M.; Wienk, M. M.; Janssen, R. A. J. Predicting Morphologies of Solution Processed Polymer:Fullerene Blends. J. Am. Chem. Soc. 2013, 135, 12057– 12067,  DOI: 10.1021/ja405493j
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    Predicting Morphologies of Solution Processed Polymer:Fullerene Blends
    Kouijzer, Sandra; Michels, Jasper J.; van den Berg, Mauricio; Gevaerts, Veronique S.; Turbiez, Mathieu; Wienk, Martijn M.; Janssen, Rene A. J.
    Journal of the American Chemical Society (2013), 135 (32), 12057-12067CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The performance of soln. processed polymer:fullerene thin film photovoltaic cells is largely detd. by the nanoscopic and mesoscopic morphol. of these blends that is formed during the drying of the layer. Although blend morphologies have been studied in detail using a variety of microscopic, spectroscopic, and scattering techniques and a large degree of control has been obtained, the current understanding of the processes involved is limited. Hence, predicting the optimized processing conditions and the corresponding device performance remains a challenge. We present an exptl. and modeling study on blends of a small band gap diketopyrrolopyrrole-quinquethiophene alternating copolymer (PDPP5T) and [6,6]-phenyl-C71-butyric acid Me ester ([70]PCBM) cast from chloroform soln. The model uses the homogeneous Flory-Huggins free energy of the multi-component blend and accounts for interfacial interactions between (locally) sepd. phases, based on phys. properties of the polymer, fullerene, and solvent. We show that the spinodal liq.-liq. demixing that occurs during drying is responsible for the obsd. morphologies. The model predicts an increasing feature size and decreasing fullerene concn. in the polymer matrix with increasing drying time in accordance with exptl. observations and device performance. The results represent a first step toward a predictive model for morphol. formation.
  27. 27
    Manders, J. R.; Tsang, S.-W.; Hartel, M. J.; Lai, T.-H.; Chen, S.; Amb, C. M.; Reynolds, J. R.; So, F. Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells. Adv. Funct. Mater. 2013, 23, 2993– 3001,  DOI: 10.1002/adfm.201202269
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    Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells
    Manders, Jesse R.; Tsang, Sai-Wing; Hartel, Michael J.; Lai, Tzung-Han; Chen, Song; Amb, Chad M.; Reynolds, John R.; So, Franky
    Advanced Functional Materials (2013), 23 (23), 2993-3001CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The detailed characterization of soln.-derived nickel (II) oxide (NiO) hole-transporting layer (HTL) films and their application in high efficiency org. photovoltaic (OPV) cells is reported. The NiO precursor soln. is examd. in situ to det. the chem. species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decomp. to form non-stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor soln. The defect-induced Ni3+ ion, which is present in non-stoichiometric NiO and signifies the p-type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using XPS. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly-dithienogermole-thienopyrrolodione (pDTG-TPD) and [6,6]-phenyl-C71-butyric acid Me ester (PC71BM) are fabricated using these soln.-processed NiO films. The resulting devices show an av. power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphol. in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.
  28. 28
    Lee, I.; Noh, J.; Lee, J.-Y.; Kim, T.-S. Cooptimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers. ACS Appl. Mater. Interfaces 2017, 9, 37395– 37401,  DOI: 10.1021/acsami.7b10398
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    Co-optimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers
    Lee, Inhwa; Noh, Jonghyeon; Lee, Jung-Yong; Kim, Taek-Soo
    ACS Applied Materials & Interfaces (2017), 9 (42), 37395-37401CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    Here, the authors demonstrate the Co-optimization of the interfacial fracture energy and power conversion efficiency (PCE) of poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT)-based org. solar cells (OSCs) by surface treatments of the buffer layer. The studied surface treatments of the buffer layer simultaneously changed the crack path and interfacial fracture energy of OSCs under mech. stress and the work function of the buffer layer. To study the effects of surface treatments, the work of adhesion values were calcd. and matched with the exptl. results based on the Owens-Wendt model. Subsequently, the authors fabricated OSCs on surface-treated buffer layers. In particular, ZnO layers treated with poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) simultaneously satisfied good mech. reliability and PCE of OSCs by achieving high work of adhesion and optimized work function.
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    Dupont, S. R.; Voroshazi, E.; Heremans, P.; Dauskardt, R. H. Adhesion Properties of Inverted Polymer Solarcells: Processing and Film Structure Parameters. Org. Electron. 2013, 14, 1262– 1270,  DOI: 10.1016/j.orgel.2013.02.022
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    Adhesion properties of inverted polymer solar cells: Processing and film structure parameters
    Dupont, Stephanie R.; Voroshazi, Eszter; Heremans, Paul; Dauskardt, Reinhold H.
    Organic Electronics (2013), 14 (5), 1262-1270CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
    We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO3), as the hole transport layer. The PEDOT:PSS OPVs were prepd. by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we obsd. adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m2, but only 0.5 J/m2 for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10-40 nm. A marginal increase in adhesion energy was measured with increased O2 plasma power. Compared to soln. processed PEDOT:PSS, we found that thermally evapd. MoO3 adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO3/P3HT:PCBM interface during the thermal evapn. process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable org. photovoltaic devices.
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    Higgins, M. J.; Wallace, G. G. Surface and Biomolecular Forces of Conducting Polymers. Polym. Rev. 2013, 53, 506– 526,  DOI: 10.1080/15583724.2013.813856
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    Surface and Biomolecular Forces of Conducting Polymers
    Higgins, Michael J.; Wallace, Gordon G.
    Polymer Reviews (Philadelphia, PA, United States) (2013), 53 (3), 506-526CODEN: PRPPCY; ISSN:1558-3716. (Taylor & Francis, Inc.)
    In this review, the authors provide insight into the surface forces of conducting polymers, a class of "intelligent" materials that offer unique strategies for controlling biomol. interactions in wide-ranging biomedical applications. Crit. to the success of these applications is that the polymer interface is exposed to biol. fluids whose interactions are controlled through the polymer surface chem. and electrochem. switching of the surface properties. There is, however, little known about the intermol. and surface forces that govern these interactions. Therefore, the purpose of this review is to more closely examine the forces that mediate interactions with biol. entities, including forces such as van der Waals, electrostatic, hydrophobic, and hydrogen bonding. The authors introduce relevant surface properties such as surface energy and surface potential, and demonstrate how they manifest as forces. In particular, the authors highlight the emerging use of At. Force Microscopy for directly measuring these forces at the single mol. level; a unique capability that is enabling deconvolution of complex biomol. interactions with conducting polymers. Finally, the authors provide an overview of biomol. interactions, namely model proteins and DNA, and conclude by discussing a growing area of interest; the spatio-temporal and reversible control of biomol. forces via elec. stimulation.
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    McDowell, C.; Abdelsamie, M.; Toney, M. F.; Bazan, G. C. Solvent Additives: Key Morphology-Directing Agents for Solution-Processed Organic Solar Cells. Adv. Mater. 2018, 30, 1707114,  DOI: 10.1002/adma.201707114
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    Urbánek, P.; di Martino, A.; Gladyš, S.; Kuřitka, I.; Minařík, A.; Pavlova, E.; Bondarev, D. Polythiophene-Based Conjugated Polyelectrolyte: Optical Properties and Association Behavior in Solution. Synth. Met. 2015, 202, 16– 24,  DOI: 10.1016/j.synthmet.2015.01.015
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    Polythiophene-based conjugated polyelectrolyte: Optical properties and association behavior in solution
    Urbanek, Pavel; di Martino, Antonio; Gladys, Sviatoslav; Kuritka, Ivo; Minarik, Antonin; Pavlova, Ewa; Bondarev, Dmitrij
    Synthetic Metals (2015), 202 (), 16-24CODEN: SYMEDZ; ISSN:0379-6779. (Elsevier B.V.)
    Conjugated polyelectrolyte with polythiophene main chain and imidazolium side groups, poly{3-[6-(1-methylimidazolium-3-yl) hexyl]thiophene-2,5-diyl bromide}, is investigated. Polymer architecture with hydrophobic main chain and hydrophilic side groups is responsible for the inconvenient behavior of the polymer in soln. Absorption and emission characteristics and supramol. assembling are investigated by spectroscopic, microscopic and light-scattering methods. We report significant solvatochromism which is observable by a naked eye. Water solns. and mixed solvent systems exhibit most interesting features: supramol. assembling was investigated by DLS, AFM and TEM techniques and common spectroscopic methods such as UV-vis and fluorescence spectroscopy. Moreover, slow relaxation processes take part in water as it is evidenced by time evolution of absorption spectra. It is a result of vanishing of inter- and intramol. interactions. These interactions are responsible for extraordinary soln. behavior of the ionic polythiophene.
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    Domínguez, S. E.; Meriläinen, M.; Ääritalo, T.; Damlin, P.; Kvarnström, C. Effect of Alkoxy-Spacer Length and Solvent on Diluted Solutions of Cationic Isothiouronium Polythiophenes. RSC Adv. 2017, 7, 7648– 7657,  DOI: 10.1039/C6RA21451G
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    Effect of alkoxy-spacer length and solvent on diluted solutions of cationic isothiouronium polythiophenes
    Dominguez, S. E.; Merilainen, M.; Aaritalo, T.; Damlin, P.; Kvarnstrom, C.
    RSC Advances (2017), 7 (13), 7648-7657CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    This contribution presents a study on two cationic isothiouronium polythiophenes (poly-3-(N,N-diethyl-S-iso-thiouronium)alkoxy-4-Me thiophene), with alkoxy chains of different length in the 3-position of the thiophene ring, results of steady-state absorption and fluorescence spectroscopies in four solvents of different polarities: protic water, protic-protic water-isopropanol, protic-aprotic water-tetrahydrofuran and protic-non polar water-1,4-dioxane (all mixts. 0.5 vol./vol.) are presented. Molar absorption coeff. (ε), quantum yield (Φf), spectral maxima (λmax, λem) and Stokes shift (Δν) are presented, and the effect of solvents on these parameters is discussed. Empirical optical and electrochem. energy levels were estd. using drop cast films on glass and on glassy carbon electrode, resp. D. functional theory was used to gain supporting structural, energy and electronic information of different oligomers, simulating solvents of different polarity.
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    Domínguez, S. E.; Cangiotti, M.; Fattori, A.; Ääritalo, T.; Damlin, P.; Ottaviani, M. F.; Kvarnström, C. Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes. Langmuir 2018, 34, 7364– 7378,  DOI: 10.1021/acs.langmuir.8b00808
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    Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes
    Dominguez, S. E.; Cangiotti, M.; Fattori, A.; Aaritalo, T.; Damlin, P.; Ottaviani, M. F.; Kvarnstrom, C.
    Langmuir (2018), 34 (25), 7364-7378CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aq. solns. was studied by ESR spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption-fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concn. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic-hydrophilic surface area in aq. soln. aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concn., whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.
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    Ottaviani, M. F.; Cangiotti, M.; Fiorani, L.; Barnard, A.; Jones, S. P.; Smith, D. K. Probing Dendron Structure and Nanoscale Self-Assembly Using Computer-Aided Analysis of EPR Spectra. New J. Chem. 2012, 36, 469– 476,  DOI: 10.1039/C1NJ20685K
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    Probing dendron structure and nanoscale self-assembly using computer-aided analysis of EPR spectra
    Ottaviani, M. Francesca; Cangiotti, Michela; Fiorani, Luigi; Barnard, Anna; Jones, Simon P.; Smith, David K.
    New Journal of Chemistry (2012), 36 (2), 469-476CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
    The self-assembly of dendritic mols. is an effective way of generating functionalized nanoscale architectures. This study reports a computer-aided anal. of the EPR spectra of 5 doxyl-stearic acid (5DSA) and Cu(II) probes interacting with self-assembling dendrons in water. The dendrons studied have previously been reported as potential gene delivery vehicles, and possess amine surface groups, different dendritic architectures based on ether-amide or ester branching, and hydrophobic groups at the focal point which can encourage self-assembly in aq. soln. The parameters extd. from computation provide information about both the structure and dynamics in soln. and the interacting ability of the dendrons to be used in gene therapy. The hydrophobic 5DSA probe is able to effectively probe the hydrophobic core of self-assembled dendron nanostructures. It reports on the polarity of its local environment and is most affected by dendrons with two cholesterol units at the focal point, partly affected by dendrons with cholesterol groups at their focal point, but is unaffected by dendrons with a simple Ph group at the focal point. This reflects the different modes of self-assembly obsd. for these dendrons. When Cu(II) is used as an EPR probe of the branched environment, at pH7, much of the Cu(II) was external' to the dendritic structure, presumably due to protonation of the peripheral amine groups. On gradually increasing the Cu(II) loading, and using computer-aided anal., it was possible to quantify the levels of 'internal' (dendron-bound) and 'external' Cu(II) and this was strongly dependent on the structure of the dendritic branching and the ability of the dendron to self-assemble, with self-assembling ester dendrons being best able to bind the Cu(II). It was also possible to propose the nature of the copper binding sites assocd. with the internal' signal as either Cu-NO3 and Cu-N2O2 distorted square-planar coordination sites. The self-assembling ester based dendrons which also contain 1,2,3-triazole units, had higher levels of 'internal' Cu(II) and showed the latter form of coordination, while the other dendrons, with lower levels of Cu(II) uptake showed the former. In summary, this paper demonstrates that two complementary EPR probes can be used to provide information about different regions of a self-assembled dendritic architecture.
  36. 36
    Santeusanio, S.; Attanasi, O. A.; Majer, R.; Cangiotti, M.; Fattori, A.; Ottaviani, M. F. Effect of Hydrogenated Cardanol on the Structure of Model Membranes Studied by EPR and NMR. Langmuir 2013, 29, 11118,  DOI: 10.1021/la402008n
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    Effect of Hydrogenated Cardanol on the Structure of Model Membranes Studied by EPR and NMR
    Santeusanio, Stefania; Attanasi, Orazio Antonio; Majer, Roberta; Cangiotti, Michela; Fattori, Alberto; Ottaviani, Maria Francesca
    Langmuir (2013), 29 (35), 11118-11126CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Hydrogenated cardanol (HC) is known to act as an antiobesity, promising antioxidant, and eco-friendly brominating agent. In this respect, it is important to find the way to transport and protect HC into the body; a micellar structure works as the simplest membrane model and may be considered a suitable biocarrier for HC. Therefore, it is useful to analyze the impact of HC in the micellar structure and properties. This study reports a computer aided ESR (EPR) and 1H NMR investigation of structural variations of cetyltrimetylammonium bromide (CTAB) micelles upon insertion of HC at different concns. and pH variations. Surfactant spin probes inserted in the micelles allowed us to get information on the structure and dynamics of the micelles and the interactions between HC and CTAB. The formation of highly packed HC-CTAB mixed micelles were favored by the occurrence of both hydrophobic (chain-chain) and hydrophilic (between the polar and charged lipid heads) interactions. These interactions were enhanced by neutralization of the acidic HC heads. Different HC localizations into the micelles and micellar structures were identified by changing HC/CTAB relative concns. and pH. The increase in HC concn. generated mixed micelles characterized by an increased surfactant packing. These results suggested a rod-like shape of the mixed micelles. The increase in pH promoted the insertion of deprotonated HC into less packed micelles, favored by the electrostatic head-head interactions between CTAB and deprotonated-HC surfactants.
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    Kris, M. R. Effect of 1,4-Dioxane on the Complexation of Lanthanides with α-Hydroxy-Isobutyrate. Doctor of Philosophy, Department of Chemistry, Washington State University, 2010, No. December.
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    Burrows, H. D.; Fonseca, S. M.; Silva, C. L.; Pais, A. A. C. C.; Tapia, M. J.; Pradhan, S.; Scherf, U. Aggregation of the Hairy Rod Conjugated Polyelectrolyte Poly{1,4-Phenylene-[9,9-Bis(4-Phenoxybutylsulfonate)]Fluorene-2,7-Diyl} in Aqueous Solution: An Experimental and Molecular Modelling Study. Phys. Chem. Chem. Phys. 2008, 10, 4420– 4428,  DOI: 10.1039/b800773j
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    Aggregation of the hairy rod conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} in aqueous solution: an experimental and molecular modelling study
    Burrows, Hugh D.; Fonseca, Sofia M.; Silva, Claudia L.; Pais, Alberto A. C. C.; Tapia, Maria J.; Pradhan, Swapna; Scherf, Ullrich
    Physical Chemistry Chemical Physics (2008), 10 (30), 4420-4428CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The aggregation of the fluorescent hairy rod, anionic conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) has been studied in aq. solns. by mol. dynamics simulations, fluorescence and light scattering. Formation of clusters leads to considerable increases in light scattering, decreases in the fluorescence quantum yields and red shifts in emission maxima. Mol. dynamics simulations considering two isolated tetramers in aq. soln. show that they rapidly form aggregates, and support exptl. evidence for the assocn. of polymer chains involving both electrostatic and hydrophobic interactions. They also provide indications for proximity of arom. rings, which is likely to be the main factor responsible for the obsd. fluorescence behavior. However, there are no indications of extensive π-stacking. The org. co-solvents methanol, acetonitrile and dioxane break up these aggregates. From studies of the dependence of the aggregation behavior on dielec. const. or the empirical solvent parameters ENT and BKT for binary mixts. with water, it can be seen that this is not simply an effect of changing solvent polarity, but is due to preferential solvation of the polymer chains. This is supported by mol. dynamic simulations on two tetramers in water-dioxane mixts. (70 : 30%). It is suggested that similar factors are involved in both the assocn. behavior and aggregate disruption with other hairy rod conjugated polyelectrolytes in water.
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    Luong, T. Q.; Verma, P. K.; Mitra, R. K.; Havenith, M. Onset of Hydrogen Bonded Collective Network of Water in 1,4-Dioxane. J. Phys. Chem. A 2011, 115, 14462– 14469,  DOI: 10.1021/jp204927r
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    Onset of Hydrogen Bonded Collective Network of Water in 1,4-Dioxane
    Luong, Trung Quan; Verma, Pramod Kumar; Mitra, Rajib Kumar; Havenith, Martina
    Journal of Physical Chemistry A (2011), 115 (50), 14462-14469CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    We have studied the evolution of water hydrogen bonded collective network dynamics in mixts. of 1,4-dioxane (Dx) as the mole fraction of water (Xw) increases from 0.005 to 0.54. The inter- and intramol. vibrations of water have been obsd. using terahertz time domain spectroscopy (THz-TDS) in the frequency range 0.4-1.4 THz (13-47 cm-1) and Fourier transform IR (FTIR) spectroscopy in the far-IR (30-650 cm-1) and mid-IR (3000-3700 cm-1) regions. These results have been correlated with the reactivity of water in these mixts. as detd. by kinetic studies of the solvolysis reaction of benzoyl chloride (BzCl). Our studies show an onset of intermol. hydrogen bonded water network dynamics beyond Xw ≥ 0.1. At the same concn., we observe a rapid increase of the rate const. of solvolysis of BzCl in water-Dx mixts. Our results establish a correlation between the onset of collective hydrogen bonded network with the solvation dynamics and the activity of clustered water.
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    Bhavya, P.; Melavanki, R.; Kusanur, R.; Sharma, K.; Muttannavar, V. T.; Naik, L. R. Effect of Viscosity and Dielectric Constant Variation on Fractional Fluorescence Quenching Analysis of Coumarin Dye in Binary Solvent Mixtures. Luminescence 2018, 33, 933– 940,  DOI: 10.1002/bio.3492
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    Effect of viscosity and dielectric constant variation on fractional fluorescence quenching analysis of coumarin dye in binary solvent mixtures
    Bhavya, P.; Melavanki, Raveendra; Kusanur, Raviraj; Sharma, Kalpana; Muttannavar, V. T.; Naik, L. R.
    Luminescence (2018), 33 (5), 933-940CODEN: LUMIFC; ISSN:1522-7243. (John Wiley & Sons Ltd.)
    Photo phys. properties of fluorescent org. compds. give an immense improved knowledge on characteristics of excited state that is beneficial to devise innovate mols. and understand their performance in particular applications. Coumarin derivs. have been extensively investigated in this regard. This article narrates steady state fluorescence quenching measurements of a coumarin deriv. namely 3-hydroxy-3-[2-oxo-2-(3-oxo-3H-benzo[f]chromen-2-yl)-ethyl]-1,3-dihydro-indol-2-one (3HBCD) in a binary mixt. of acetonitrile and 1,4-dioxane. Aniline is used as quencher. Fluorescence intensity is large in acetonitrile and decreases as the percentage of 1,4-dioxane in the solvent mixt. increases. With modest quencher concn. a deviation towards the x axis is noticed in the Stern-Volmer (S-V) plots. This downward curvature is interpreted as due to the presence of 3HBCD in different conformers in the lowest energy level. Ground state intramol. hydrogen bonding formation is obsd. due to the conformational changes in the solute. Figured estns. of various quenching parameters recommend that, while dynamic quenching prompts linearity in S-V plot at lower quencher concn., increasing quenching efficiency with increasing medium viscosity suggests that reaction is not entirely controlled by material diffusion. Stern-Volmer const. increases with decreasing medium dielec. const.
  41. 41
    Nakagawa, T.; Hatano, J.; Matsuo, Y. Influence of Additives in Bulk Heterojunction Solar Cells Using Magnesium Tetraethynylporphyrin with Triisopropylsilyl and Anthryl Substituents. J. Porphyrins Phthalocyanines 2014, 18, 735– 740,  DOI: 10.1142/S1088424614500655
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    Influence of additives in bulk heterojunction solar cells using magnesium tetraethynylporphyrin with triisopropylsilyl and anthryl substituents
    Nakagawa, Takafumi; Hatano, Junichi; Matsuo, Yutaka
    Journal of Porphyrins and Phthalocyanines (2014), 18 (8-9), 735-740CODEN: JPPHFZ; ISSN:1088-4246. (World Scientific Publishing Co. Pte. Ltd.)
    We designed and synthesized anthryl-disubstituted magnesium tetraethynylporphyrin([{5,15-bis(anthracen-9'-yl)ethynyl}-10,20-bis{(triisopropylsilyl)ethynyl}porphyrinato] magnesium(II)), and applied it as an electron donor to soln.-processed bulk heterojunction small mol. org. solar cells. The compd. was characterized by single crystal X-ray crystallog. as well as UV-vis light absorption spectrum showing the absorption max. and onset at 700 and 740 nm, resp. Org. solar cells using this compd. and [6,6]-phenyl-C61-butyric acid Me ester (PC61BM) as electron donor and acceptor, resp., showed power conversion efficiency of 1.31% at the donor and acceptor ratio of 1:3. The use of pyridine as a coordinating additive increased power conversion efficiency to 1.61%, which was the best among tested additives, THF, pyradine, dioxane, and 1,8-diiodooctane.
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    Chen, S.; Liu, Z.; Ge, Z. Synthesis, Characterization and Photovoltaic Properties of Three New 3,4-Dithienyl-Substituted Polythiophene Derivatives. Polym. J. 2016, 48, 101– 110,  DOI: 10.1038/pj.2015.85
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    Synthesis, characterization and photovoltaic properties of three new 3,4-dithienyl-substituted polythiophene derivatives
    Chen, Shaojie; Liu, Zhiyang; Ge, Ziyi
    Polymer Journal (Tokyo, Japan) (2016), 48 (1), 101-110CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
    Three 3,4-dithienyl-substituted polythiophene derivs. have been synthesized and characterized. Polymer P2 exhibits the widest light absorption spectrum of the polymers in its polymer chain, which has an alternative donor-acceptor structure. Copolymers blended with 6,6-phenyl-C71-butyric acid Me ester(PC71BM) as active layers are used to fabricate polymer solar cells (PSCs), and a variety of post-treatments are employed to optimize the PSCs performance. A max. power conversion efficiency (PCE) of 1.22% is achieved for the conventional configuration device based on P2, and the inverted configuration device of P2 is also prepd. for comparison. The results show that the inverted P2 device exhibits a better PCE (1.47%) than that of the conventional device due to the TiO2 nanoparticles and the close energy alignment between the work function of the MoO3 and the HOMO energy levels, which facilitate a light absorption increase and improved charge transport.
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    Khan, M. Z. H. Effect of ITO Surface Properties on SAM Modification: A Review toward Biosensor Application. Cogent Eng. 2016, 3, 1170097,  DOI: 10.1080/23311916.2016.1170097
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    Khan, M. Z. H.; Nakanishi, T.; Kuroiwa, S.; Hoshi, Y.; Osaka, T. Effect of Surface Roughness and Surface Modification of Indium Tin Oxide Electrode on Its Potential Response to Tryptophan. Electrochim. Acta 2011, 56, 8657– 8661,  DOI: 10.1016/j.electacta.2011.07.068
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    Effect of surface roughness and surface modification of indium tin oxide electrode on its potential response to tryptophan
    Khan, Md. Zaved Hossain; Nakanishi, Takuya; Kuroiwa, Shigeki; Hoshi, Yoichi; Osaka, Tetsuya
    Electrochimica Acta (2011), 56 (24), 8657-8661CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The effect of surface modification of indium tin oxide (ITO) electrode on its potential response to tryptophan was studied for ITO substrates with different surface roughness. A small difference in surface roughness, between ∼1 and ∼2 nm of R a evaluated by at. force microscopy, affects the rest potential of ITO electrode in the electrolyte. A slight difference in In:Sn ratio at the near surface of the ITO substrates, measured by angle-resolved x-ray photoelectron spectrometry and Auger electron spectroscopy is remarkable, and considered to relate with surface roughness. Successive modification of the ITO surface with aminopropylsilane and disuccinimidyl suberate, of which essentiality to the potential response to indole compds. the authors previously reported, improved the stability of the rest potential and enabled the electrodes to respond to tryptophan in case of specimens with R a values ranging between ∼2 and ∼3 nm but not for those with R a of ∼1 nm. Probably there are optimum values of effective work function of ITO for specific potential response to tryptophan, which can be obtained by the successive modification of ITO surface.
  45. 45
    Xu, J.; Wang, C.; Tian, Y.; Wu, B.; Wang, S.; Zhang, H. Glass-on-LiNbO3 Heterostructure Formed via a Two-Step Plasma Activated Low-Temperature Direct Bonding Method. Appl. Surf. Sci. 2018, 459, 621– 629,  DOI: 10.1016/j.apsusc.2018.08.031
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    Glass-on-LiNbO3 heterostructure formed via a two-step plasma activated low-temperature direct bonding method
    Xu, Jikai; Wang, Chenxi; Tian, Yanhong; Wu, Bin; Wang, Shang; Zhang, He
    Applied Surface Science (2018), 459 (), 621-629CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Lithium niobate (LiNbO3) crystal has been widely used in integrating optical and acoustic devices. However, direct bonding of LiNbO3 to Si-based materials is still a challenge due to the large coeff. of thermal expansion mismatches. In this paper, strong bonding strength of the LiNbO3/glass pair was obtained via the optimized O2 plasma activation followed by N2 plasma treatment. After the pre-bonding was accomplished at room temp., the bonded pair was annealed at 150°C for 12 h. A defect-free and tight bonding interface was confirmed by the SEM and transmission electron microscopy. Based on at. force microscopy, water contact angle and XPS characterizations, both LiNbO3 and glass surfaces became smoother and more hydrophilic due to the densities of -OH and N-related groups increased. Addnl., combined with the bonding strength obtained via different plasma activated processes, we demonstrated that the mechanism of the enhanced bonding strength could be attributed to N-related covalent bonds formed across the bonding interface. Furthermore, the excellent optical transmittance of LiNbO3/glass bonded pairs indicated this facile bonding process can meet the demand of high-performance glass-on-LiNbO3 devices.
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    Viinikanoja, A.; Areva, S.; Kocharova, N.; Ääritalo, T.; Vuorinen, M.; Savunen, A.; Kankare, J.; Lukkari, J. Structure of Self-Assembled Multilayers Prepared from Water-Soluble Polythiophenes. Langmuir 2006, 22, 6078– 6086,  DOI: 10.1021/la060519u
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    Structure of self-assembled multilayers prepared from water-soluble polythiophenes
    Viinikanoja, Antti; Areva, Sami; Kocharova, Natalia; Aeaeritalo, Timo; Vuorinen, Maarit; Savunen, Arto; Kankare, Jouko; Lukkari, Jukka
    Langmuir (2006), 22 (14), 6078-6086CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    We have studied the structure and morphol. of self-assembled polyelectrolyte multilayers prepd. using poly(styrene sulfonate) (PSS) and four different cationic poly(alkoxy-thiophene) derivs. bearing methylimidazolium-terminated ionic side chain at the 3-position of the thiophene ring: poly(1-methyl-3-[3-[3-thienyl-oxy]-propyl]-1H-imidazolium), poly(1-methyl-3-[6-[3-thienyl-oxy]-hexyl]-1H-imidazolium), poly(1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium ), and poly(1-methyl-3-[6-[(4-methyl-3-thienyl)oxy]-hexyl]-1H-imidazolium ). All the multilayers exhibited regular growth. The thickness of the multilayers was measured with ellipsometry, their layer-by-layer growth was followed by polarization modulation IR reflection-absorption spectroscopy and ellipsometry, and the morphol. of the films was studied by at. force microscopy. The length of the methylimidazolium-terminated side chain (Cn, n = 2, 3, 6) and the substituent (H or Me) at the 4-position of the thiophene ring were varied. All multilayers were inhomogeneous in the sub-micrometer scale and contained aggregates of two kinds. The large ones with a low and const. surface no. d. were attributed to PSS, whereas the small aggregates were polythiophene-based. The surface d. of these org. semiconducting nanoparticles greatly depended on the structure of polythiophene, being favored by polymer regio-regularity and the length of the side chain. The side chains remained disordered in all the multilayers, but with polythiophenes having hexyl chains both the imidazolium and thiophene rings tended to orient themselves more perpendicular to the surface than in films contg. shorter chains (C2 or C3). The relative water content of the multilayers (at 7.1% relative humidity) did not depend on the film thickness. As the no. of bilayers increased the methylimidazolium-sulfonate ion pairs gradually weakened and became more individually hydrated.
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    Malika Jeffries-El, R. D. M. 9. Regioregular Polythiophenes. In Handbook of Conducting Polymers, 3rd ed.; Terje, A., Skotheim, J. R. R., Eds., Conjugated Polymers Theory, Synthesis, Properties, and Characterization; CRC Press Taylor & Francis Group, 2007.
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    Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. Nonlinear-Least-Squares Analysis of Slow-Motion EPR Spectra in One and Two Dimensions Using a Modified Levenberg-Marquardt Algorithm. J. Magn. Reson., Ser. A 1996, 120, 155– 189,  DOI: 10.1006/jmra.1996.0113
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    Nonlinear-least-squares analysis of slow-motion EPR spectra in one and two dimensions using a modified Levenberg-Marquardt algorithm
    Budil, David E.; Lee, Sanghyuk; Saxena, Sunil; Freed, Jack H.
    Journal of Magnetic Resonance, Series A (1996), 120 (2), 155-189CODEN: JMRAE2; ISSN:1064-1858. (Academic)
    The application of the model trust region modification of the Levenberg-Marquardt minimization algorithm to the anal. of 1-dimensional continuous-wave EPR and multidimensional Fourier-transform (FT) EPR spectra esp. in the slow-motion regime is described. The dynamic parameters describing the slow motion were obtained from least-squares fitting of model calcns. based on the stochastic Liouville equation (SLE) to exptl. spectra. The trust-region approach is inherently more efficient than the std. Levenberg-Marquardt algorithm, and the efficiency of the procedure may be further increased by a sepn.-of-variables method in which a subset of fitting parameters is independently minimized at each iteration, thus reducing the no. of parameters to be fitted by nonlinear least squares. A particularly useful application of this method occurs in the fitting of multicomponent spectra, for which it is possible to obtain the relative population of each component by the sepn.-of-variables method. These advantages, combined with recent improvements in the computational methods used to solve the SLE, led to an order-of-magnitude redn. in computing time, and have made it possible to carry out interactive, real-time fitting on a lab. workstation with a graphical interface. Examples of fits to exptl. data will be given, including multicomponent continuous-wave EPR spectra as well as two- and three-dimensional FT EPR spectra. Emphasis is placed on the analytic information available from the partial derivs. used in the algorithm, and how it may be used to est. the condition and uniqueness of the fit, as well as to est. confidence limits for the parameters in certain cases.
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    Belandria, V.; Mohammadi, A. H.; Richon, D. Volumetric Properties of the (Tetrahydrofuran + Water) and (Tetra-n-Butyl Ammonium Bromide + Water) Systems: Experimental Measurements and Correlations. J. Chem. Thermodyn. 2009, 41, 1382– 1386,  DOI: 10.1016/j.jct.2009.06.014
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    Volumetric properties of the (tetrahydrofuran + water) and (tetra-n-butyl ammonium bromide + water) systems: Experimental measurements and correlations
    Belandria, Veronica; Mohammadi, Amir H.; Richon, Dominique
    Journal of Chemical Thermodynamics (2009), 41 (12), 1382-1386CODEN: JCTDAF; ISSN:0021-9614. (Elsevier Ltd.)
    We report exptl. d. data for the binary mixts. of (water + THF) and (water + tetra-Bu ammonium bromide) at atm. pressure and various temps. The densities were measured using an Anton Paar digital vibrating-tube densimeter. For the (THF + water) system, excess molar volumes were calcd. using the exptl. densities and correlated using the Redlich-Kister equation. The Redlich-Kister equation parameters were adjusted on exptl. results. The partial molar volumes and partial excess molar volumes at infinite diln. were also calcd. for each component. A simple d. equation was finally applied to correlate the measured d. of the (tetra-Bu ammonium bromide + water) system.
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    Nayak, J. N.; Aralaguppi, M. I.; Kumar Naidu, B. V.; Aminabhavi, T. M. Thermodynamic Properties of Water + Tetrahydrofuran and Water + 1,4-Dioxane Mixtures at (303.15, 313.15, and 323.15) K. J. Chem. Eng. Data 2004, 49, 468– 474,  DOI: 10.1021/je030196t
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    Thermodynamic Properties of Water + Tetrahydrofuran and Water + 1,4-Dioxane Mixtures at (303.15, 313.15, and 323.15) K
    Nayak, Jyoti N.; Aralaguppi, Mrityunjaya I.; Naidu, B. Vijaya Kumar; Aminabhavi, Tejraj M.
    Journal of Chemical and Engineering Data (2004), 49 (3), 468-474CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
    Properties like d. and viscosity at (303.15, 313.15, and 323.15) K and refractive index at 303.15 K have been measured for binary mixts. of water + THF and water + 1,4-dioxane. The excess molar volume (VE) has been calcd. from the d. results. Viscosity and refractive index data have been used to compute resp. deviations in viscosity (Δη) and molar refraction (ΔR) of the mixts. Excess molar volume and deviations in viscosity and molar refraction data have been fitted to the Redlich-Kister equation to calc. binary coeffs. and std. deviations. Furthermore, literature published vapor-liq. equil. data of the mixts. have been used to construct x-y compn. curves and compared with the pervaporation curves. Results of this study indicate the importance of the pervaporation technique for the sepn. of mixts. over that of simple distn. at the azeotropic compn. Permeate samples were analyzed by refractive index as well as gas chromatog. measurements to get the exact content of water.
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    Critchfield, F. E.; Gibson, J. A., Jr.; Hall, J. L. Dielectric Constant for the Dioxane-Water System from 20 to 35°. J. Am. Chem. Soc. 1953, 75, 1991– 1992,  DOI: 10.1021/ja01104a506
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    Dielectric constant for the dioxane-water system from 20° to 35°
    Critchfield, Frank E.; Gibson, John A., Jr.; Hall, James L.
    Journal of the American Chemical Society (1953), 75 (), 1991-2CODEN: JACSAT; ISSN:0002-7863.
    The dielec. consts. for the dioxane-water system ranging from 0 to 100% by wt. dioxane at 10% intervals were detd. at 20, 25, 30, and 35°. The results differ from those of Akerl.ovrddot.of and Short by as much as 12% (C.A. 30, 6616.8).
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    Omota, L.-M.; Iulian, O.; Omota, F.; Ciocirlan, O. Densities and Derived Properties of Water, 1,4-Dioxane and Dimethyl Sulfoxide Binary and Ternary Systems at Temperatures from 293.15 K to 313.15 K. Rev. Roum. Chim. 2009, 54, 63– 73
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    Densities and derived properties of water, 1,4-dioxane and dimethyl sulfoxide binary and ternary systems at temperatures from 293.15 K to 313.15 K
    Omota, Ligia-Maria; Iulian, Olga; Omota, Florin; Ciocirlan, Oana
    Revue Roumaine de Chimie (2009), 54 (1), 63-73CODEN: RRCHAX; ISSN:0035-3930. (Editura Academiei Romane)
    Excess molar volumes (VE) and excess partial molar volumes (‾VEi) of the binary systems: water + 1,4-dioxane, 1,4-dioxane + DMSO, water + DMSO and of the corresponding ternary system water + 1,4-dioxane + DMSO have been evaluated from exptl. d. at five temps. ranging from 293.15 K to 313.15 K. For all binary mixts., the exptl. data were correlated with Redlich-Kister's equation. For ternary system, the excess molar volume was estd. by correlative and predictive (sym. and asym.) methods and the results were compared. The calcd. functions have been used to explain the intermol. interactions between the mixing components.
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    Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed.; Barton, A. F. M., Ed.; CRC Press, 1991.
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    Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, 2nd ed.; Hansen, C. M., Ed.; CRC Press, 2007.
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    Dalstein, L.; Potapova, E.; Tyrode, E. The Elusive Silica/Water Interface: Isolated Silanols under Water as Revealed by Vibrational Sum Frequency Spectroscopy. Phys. Chem. Chem. Phys. 2017, 19, 10343– 10349,  DOI: 10.1039/C7CP01507K
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    Silica/water interface: isolated silanols under water as revealed by vibrational sum frequency spectroscopy
    Dalstein, Laetitia; Potapova, Elizaveta; Tyrode, Eric
    Physical Chemistry Chemical Physics (2017), 19 (16), 10343-10349CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    It has been long recognized that the surface chem. of silica, and in particular the type and relative amt. of surface bound silanol groups, plays a crit. role in many of the properties assocd. with the material, where a typical example is the discrepant adsorption behavior obsd. depending on the pretreatment history of the surface. However, in spite of its importance, the direct probing of sp. surface silanol groups under water has been hampered by instrumental limitations. Here we make use of vibrational sum frequency spectroscopy (VSFS) to first, identify under water the OH stretch of isolated surface silanols, and second, explore its acid/base behavior and dependence on the surface pretreatment method. The properties of other types of silanol groups (i.e. hydrogen bonded/geminal) are also inferred from the data. The ability to directly probe these functional groups under water represents a crucial step to further improving our understanding of this widely used mineral oxide.
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    Concentration of hydroxyl groups on the surface of amorphous silicas
    Zhuravlev, L. T.
    Langmuir (1987), 3 (3), 316-18CODEN: LANGD5; ISSN:0743-7463.
    The concn. of hydroxyl groups in amorphous silicas, viz., silica gels, aerosilogels, and porous glasses, which differ in prodn. conditions, surface area, and pore size distribution. was studied. The surface d. of OH groups (the silanol no.) is a physicochem. const. for a fully hydroxylated surface and the d. as a function of temp. of vacuum treatment does not depend in a significant way on the type of silica.
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    Pouryousefy, E.; Xie, Q.; Saeedi, A. Effect of Multi-Component Ions Exchange on Low Salinity EOR: Coupled Geochemical Simulation Study. Petroleum 2016, 2, 215– 224,  DOI: 10.1016/j.petlm.2016.05.004
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    Matisons, J. G. Silanes and Siloxanes as Coupling Agents to Glass: A Perspective. In Silicone Surface Science; Owen, M. J., Dvornic, P. R., Eds.; Advances in Silicon Science; Springer Netherlands: Dordrecht, 2012; pp 281– 298.
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    Wu, J.; Liu, F.; Yang, H.; Xu, S.; Xie, Q.; Zhang, M.; Chen, T.; Hu, G.; Wang, J. Effect of Specific Functional Groups on Oil Adhesion from Mica Substrate: Implications for Low Salinity Effect. J. Ind. Eng. Chem. 2017, 56, 342– 349,  DOI: 10.1016/j.jiec.2017.07.030
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    Effect of specific functional groups on oil adhesion from mica substrate: Implications for low salinity effect
    Wu, Jiazhong; Liu, Fanghui; Yang, Hui; Xu, Shijing; Xie, Quan; Zhang, Minghui; Chen, Ting; Hu, Guangxin; Wang, Jinben
    Journal of Industrial and Engineering Chemistry (Amsterdam, Netherlands) (2017), 56 (), 342-349CODEN: JIECFI; ISSN:1226-086X. (Elsevier B.V.)
    Low salinity effect has been in the center of attention as a cost-effective and environmentally friendly technique. Wettability alteration of the oil/brine/mica system appears to be the identified mechanism(s) to trigger the low salinity effect. While the effect of water chem. and minerol. on the wettability of the system has been extensively investigated, few studies have investigated the effect of specific function groups from crude oil on the system wettability, limiting the understanding of how specific functional group contributes to the wettability. We thus exptl. measured the adhesion forces between mica surfaces and functional groups (e.g., C6H5-, CH3-, COOH-, and NH2-) in the presence of different aq. ionic solns. using chem. force microscopy (CFM). Moreover, to understand the contribution of the structural force, the traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was extended (denoted as EDLVO) to fit the force profiles using a Gauss model. Our results showed that the adhesion force between mica and functional groups in a decreasing order was -NH2 > -COOH > -CH3 > -C6H5. We also found that while DLVO forces strongly affected the tip-surface contact due to the interactions among oil/brine/mica interfaces, the structural forces also played an important role in a distance of 1-20 nm due to the presence of H-bonds between COOH-terminated or NH2-terminated tip and mica surface. We therefore conclude that the structural force largely contributes to the adhesion force due to the hydrophilicity or polarity of functional groups, and nucleophilic property (such as Ph group). Our results suggest that the polarity of the crude oil needs to be considered to screen a candidate reservoir for low salinity water flooding projects.
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    Schuster, J. M.; Schvezov, C. E.; Rosenberger, M. R. Analysis of the Results of Surface Free Energy Measurement of Ti6Al4V by Different Methods. Procedia Mater. Sci. 2015, 8, 732– 741,  DOI: 10.1016/j.mspro.2015.04.130
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    Analysis of the Results of Surface Free Energy Measurement of Ti6Al4V by Different Methods
    Schuster, Jonathan M.; Schvezov, Carlos E.; Rosenberger, Mario R.
    Procedia Materials Science (2015), 8 (), 732-741CODEN: PMSREM; ISSN:2211-8128. (Elsevier B.V.)
    Surface Free Energy (SFE) of solids should be calcd. using theor. models. The contact angle (CA) measurement on a surface is considered the most practical way to obtain the SFE. The results of five methods based in the models are compared: the method of Zisman (ZI), the geometric mean (GM), the harmonic mean (HM), the Lifshitz-van der Waals/ Acid-Base (LW/AB) and the equation of state (ES). The SFE calcd. with GM, HM and LW/AB methods change with the amt. and type of liq. used, however, when water, glycerol and DMSO are used together the SFE and its dispersive and polar components are similar in value for the three methods. In the case of the ES model the values of SFE change with the liq. used; finally using the ZI method the SFE values are 20% lower than the values of SFE obtained with the other methods.
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    Thomsen, F. KRUSS Technical Note TN315e. Custom-Made Models: From Contact Angle to Surface Free Energy , December 2008.
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    Shigematsu, Mikiji; Morita, Mitsuhiro; Sakata, Isao
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    The phase sepn. states of hemicellulose (I) and lignin (II) in their blends were investigated by means of contact angle measurements. The hysteresis behavior in dynamic contact angles suggested that a phase sepn. of cluster-state type (not of random dispersion type) took place at I- and II-rich mixing ratios, and it may be caused by the low cohesive force between I and II mols. Further, the total surface tension, and the balance between dispersion and polar forces in the total surface tension were evaluated from the equation of Wu. In the region of a I-rich mixing ratio, the polar and dispersion forces of the surface tension of the solid water decreased and raised, resp., and in contrast to this, in the II-rich region, the former were raised and the latter were decreased. The polymer-polymer interaction parameter of Flory between I and II depends on the mixing ratio as reported previously; this may be ascribed to a difference of the balancing of polar-dispersion forces.
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    Liu, M. J.; Tzou, K.; Gregory, R. V. Influence of the Doping Conditions on the Surface Energies of Conducting Polymers. Synth. Met. 1994, 63, 67– 71,  DOI: 10.1016/0379-6779(94)90251-8
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    Synthetic Metals (1994), 63 (1), 67-71CODEN: SYMEDZ; ISSN:0379-6779.
    The surface energies and polarities of the conducting polymers, polyaniline, polypyrrole and poly(3-hexylthiophene), dedoped and doped, were investigated by the harmonic-mean equation based on contact angle measurements. The values of the surface energies and polarities of dedoped conducting polymers are similar to those of conventional polymers. However, the surface energies of the doped conducting polymers vary greatly depending on the choice of the dopants and doping level. The change in surface energy of conducting polymers may result from the contribution of the polar components, depending on the types and concn. of the dopants, since the dispersion components of the measured surface energies of the examd. conducting polymers do not change significantly from doping to dedoping.
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    Tsai, Y.-T.; Choi, C.-H.; Gao, N.; Yang, E.-H. Tunable Wetting Mechanism of Polypyrrole Surfaces and Low-Voltage Droplet Manipulation via Redox. Langmuir 2011, 27, 4249– 4256,  DOI: 10.1021/la104403w
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    Tunable Wetting Mechanism of Polypyrrole Surfaces and Low-Voltage Droplet Manipulation via Redox
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  • Abstract

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    Figure 1

    Figure 1. EPR spectra of the paramagnetic probe 5DSA in solutions of (a) PIMa and (b) PIMb at 25 °C and 0.5 mM.

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    Scheme 1

    Scheme 1. Skeletal Structures of (a) the Cationic Imidazolium Polythiophenes PIMa and PIMb and (b) the Paramagnetic Probes Used in the EPR Aqueous Study, TOH, CAT8, CAT16, and 5DSA
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    Scheme 2

    Scheme 2. Expected Interactions between the Imidazolium CPEs Used in This Work and the Plasma-Activated Glass, Modified from Refs (57−59); Notice That the Glass Substrate Here Is Presented as Partially Activated, i.e., with a Partial Surface Concentration of Si–OH Having Also Si–O–Si Groups
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    Figure 2

    Figure 2. EPR spectral intensity (measured as a double integral of the spectra in A.U.) as a function of the concentration of PIMa (green upward triangles) and PIMb (blue downward triangles) in the presence of 0.1 mM TOH (a), CAT8 (b), and CAT16 (c).

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    Figure 3

    Figure 3. Microviscosity (interaction) parameter (τ) as a function of the concentration of PIMa (blue upward triangles) and PIMb (green downward triangles) in the presence of 0.1 mM TOH (a), CAT8 (b), and CAT16 (c).

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    Figure 4

    Figure 4. Wu’s model estimations of (a) γS and its (b) polar component (γSp) and (c) dispersive component (γSd) of films made of PIMa and PIMb, processed from water (solid color bars) or W–DI (dash-patterned bars). Dashed horizontal lines indicate the SFE, SFEp, and SFEd values of the blank surfaces of (1) plasma glass and (2) glass–water.

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      Lim, Kyung-Geun; Park, Sung Min; Woo, Han Young; Lee, Tae-Woo
      ChemSusChem (2015), 8 (18), 3062-3068CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Despite the promising function of conjugated polyelectrolytes (CPEs) as an interfacial layer in org. photovoltaics (OPVs), the underlying mechanism of dipole orientation and the elec. characteristics of CPE interlayers remain unclear. Currently, the ionic functionality of CPEs (i.e., whether they are cationic or anionic) is believed to det. the interfacial dipole alignment and the resulting electron or hole extn. properties at the interface between an org. photoactive layer and a metal electrode. In this research, we find that in contrast to this common belief, the photovoltaic efficiency can be improved significantly by both cationic and anionic CPE layers regardless of the ion functionality of the CPE. This improvement occurs because the interfacial dipoles of cationic and anionic CPEs are realigned in the identical direction despite the different ionic functionality. The net dipole is detd. not by the intrinsic mol. dipole of the CPE but by the ionic redistribution in the CPE layer and the resulting interfacial dipole at the intimate contact with adjacent layers. We also demonstrated that the energy level alignment and performance parameters of OPVs can be controlled systematically by the elec. poled CPE layers with the oriented interfacial dipoles; the distribution of pos. and neg. ions in the CPE layer was adjusted by applying an appropriate external elec. field, and the energy alignment was reversible by changing the elec. field direction. The anionic and cationic CPEs (PSBFP-Na and PAHFP-Br) based on the same π-conjugated backbone of fluorene-phenylene were each used as the electron extn. layer on a photoactive layer. Both anionic and cationic CPE interlayers improved the energy level alignment at the interface between the photoactive layer and the electrode and the resulting performance parameters, which thereby increased the power conversion efficiency to 8.3 %.
    13. 13
      Kesters, J.; Govaerts, S.; Pirotte, G.; Drijkoningen, J.; Chevrier, M.; Van Den Brande, N.; Liu, X.; Fahlman, M.; Van Mele, B.; Lutsen, L. High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 6309– 6314,  DOI: 10.1021/acsami.6b00242
      13
      High-Permittivity Conjugated Polyelectrolyte Interlayers for High-Performance Bulk Heterojunction Organic Solar Cells
      Kesters, Jurgen; Govaerts, Sanne; Pirotte, Geert; Drijkoningen, Jeroen; Chevrier, Michele; Van den Brande, Niko; Liu, Xianjie; Fahlman, Mats; Van Mele, Bruno; Lutsen, Laurence; Vanderzande, Dirk; Manca, Jean; Clement, Sebastien; Von Hauff, Elizabeth; Maes, Wouter
      ACS Applied Materials & Interfaces (2016), 8 (10), 6309-6314CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Conjugated polyelectrolyte (CPE) interfacial layers present a powerful way to boost the I-V characteristics of org. photovoltaics. Nevertheless, clear guidelines with respect to the structure of high-performance interlayers are still lacking. In this work, impedance spectroscopy is applied to probe the dielec. permittivity of a series of polythiophene-based CPEs. The presence of ionic pendant groups grants the formation of a capacitive double layer, boosting the charge extn. and device efficiency. A counteracting effect is the diminishing affinity with the underlying photoactive layer. To balance these two effects, we found copolymer structures contg. nonionic side chains to be beneficial.
    14. 14
      Lee, S.; Nguyen, T. L.; Lee, S. Y.; Jang, C. H.; Lee, B. R.; Jung, E. D.; Park, S. Y.; Yoon, Y. J.; Kim, J. Y.; Woo, H. Y. Conjugated Polyelectrolytes Bearing Various Ion Densities: Spontaneous Dipole Generation, Poling-Induced Dipole Alignment, and Interfacial Energy Barrier Control for Optoelectronic Device Applications. Adv. Mater. 2018, 30, 1706034,  DOI: 10.1002/adma.201706034
      There is no corresponding record for this reference.
    15. 15
      Lee, W.; Seo, J. H.; Woo, H. Y. Conjugated Polyelectrolytes: A New Class of Semiconducting Material for Organic Electronic Devices. Polymer 2013, 54, 5104– 5121,  DOI: 10.1016/j.polymer.2013.07.015
      15
      Conjugated polyelectrolytes: A new class of semiconducting material for organic electronic devices
      Lee, Wonho; Seo, Jung Hwa; Woo, Han Young
      Polymer (2013), 54 (19), 5104-5121CODEN: POLMAG; ISSN:0032-3861. (Elsevier Ltd.)
      A review. This feature article presents a short review of the recent developments in the synthesis of conjugated polyelectrolytes (CPEs) along with their applications in org. optoelectronic devices with particular focus on the mol. structures of CPEs with ionic functionality, synthetic approaches, and their utilization as an interfacial layer. The orthogonal soly. of the CPEs allows the simple prepn. of multilayer org. devices by soln. casting on top of a nonpolar org. photoactive layer without disturbing the interfaces, showing their effectiveness in tuning the electronic structures at the interfaces for improving the charge carrier transport and resulting device properties. These achievements highlight the dynamic nature of CPEs and their applicability to a wide range of optoelectronic devices.
    16. 16
      Kesters, J.; Ghoos, T.; Penxten, H.; Drijkoningen, J.; Vangerven, T.; Lyons, D. M.; Verreet, B.; Aernouts, T.; Lutsen, L.; Vanderzande, D. Imidazolium-Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance. Adv. Energy Mater. 2013, 3, 1180– 1185,  DOI: 10.1002/aenm.201300049
      16
      Imidazolium-Substituted Polythiophenes as Efficient Electron Transport Materials Improving Photovoltaic Performance
      Kesters, Jurgen; Ghoos, Toon; Penxten, Huguette; Drijkoningen, Jeroen; Vangerven, Tim; Lyons, Dani M.; Verreet, Bregt; Aernouts, Tom; Lutsen, Laurence; Vanderzande, Dirk; Manca, Jean; Maes, Wouter
      Advanced Energy Materials (2013), 3 (9), 1180-1185CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      In the field of polymer solar cells, improving photovoltaic performance has been the main driver over the past decade. To achieve high power conversion efficiencies, a plethora of new photoactive donor polymers and fullerene derivs. have been developed and blended together in bulk heterojunction active layers. Simultaneously, further optimization of the device architecture is also of major importance. In this respect, we report on the use of specific types of electron transport layers to boost the inherent I-V properties of polymer solar cell devices, resulting in a considerable gain in overall photovoltaic output. Imidazolium-substituted polythiophenes are introduced as appealing electron transport materials, outperforming the currently available analogous conjugated polyelectrolytes, mainly by an increase in short-circuit current. The mol. wt. of the ionic polythiophenes has been identified as a crucial parameter influencing performance.
    17. 17
      Seo, J. H.; Gutacker, A.; Sun, Y.; Wu, H.; Huang, F.; Cao, Y.; Scherf, U.; Heeger, A. J.; Bazan, G. C. Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer. J. Am. Chem. Soc. 2011, 133, 8416– 8419,  DOI: 10.1021/ja2037673
      17
      Improved High-Efficiency Organic Solar Cells via Incorporation of a Conjugated Polyelectrolyte Interlayer
      Seo, Jung Hwa; Gutacker, Andrea; Sun, Yanming; Wu, Hongbin; Huang, Fei; Cao, Yong; Scherf, Ullrich; Heeger, Alan J.; Bazan, Guillermo C.
      Journal of the American Chemical Society (2011), 133 (22), 8416-8419CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The power conversion efficiencies of bulk heterojunction (BHJ) solar cells can be increased from 5 to 6.5% by incorporating an ultrathin conjugated polyelectrolyte (CPE) layer between the active layer and the metal cathode. Poly[N-9''-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-Ph C71 butyric acid Me ester (PC71BM) were chosen for the photoactive layer. CPEs with cationic polythiophenes, in both homopolymer and block copolymer configurations, were used to improve the electronic characteristics. The improvement in device performance and the simplicity of fabrication by soln. processing suggest a promising and practical pathway for improving polymer solar cells with high efficiencies.
    18. 18
      Zilberberg, K.; Behrendt, A.; Kraft, M.; Scherf, U.; Riedl, T. Ultrathin Interlayers of a Conjugated Polyelectrolyte for Low Work-Function Cathodes in Efficient Inverted Organic Solar Cells. Org. Electron. 2013, 14, 951– 957,  DOI: 10.1016/j.orgel.2013.01.018
      18
      Ultrathin interlayers of a conjugated polyelectrolyte for low work-function cathodes in efficient inverted organic solar cells
      Zilberberg, Kirill; Behrendt, Andreas; Kraft, Mario; Scherf, Ullrich; Riedl, Thomas
      Organic Electronics (2013), 14 (3), 951-957CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      Inverted polymer:fullerene solar cells are reported using the conjugated polyelectrolyte poly(3-[6-(N-methylimidazolium)hexyl]thiophene) bromide P3ImHT as ultra-thin cathode interlayer to reduce the work-function (WF) of ITO for improved electron selectivity. Similar to ITO/TiOx cathodes, UV exposure is found necessary in the case of ITO/CPE to achieve a high fill factor on the order of 60%. UV illumination is shown to lead to a redn. of the ITO WF which in combination with WF lowering due to the dipole of the CPE leads to a suitable electron selective cathode. As a result, inverted solar cells with an efficiency of 4.8% are achieved. This is the highest efficiency of an inverted OSC using a conjugated polyelectrolyte as interface modifier directly on top of ITO.
    19. 19
      Wang, K.; Liu, C.; Meng, T.; Yi, C.; Gong, X. Inverted Organic Photovoltaic Cells. Chem. Soc. Rev. 2016, 45, 2937– 2975,  DOI: 10.1039/C5CS00831J
      19
      Inverted organic photovoltaic cells
      Wang, Kai; Liu, Chang; Meng, Tianyu; Yi, Chao; Gong, Xiong
      Chemical Society Reviews (2016), 45 (10), 2937-2975CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      The advance in lifestyle, modern industrialization and future technol. revolution are always at high expense of energy consumption. Unfortunately, there exist serious issues such as limited storage, high cost and toxic contamination in conventional fossil fuel energy sources. Instead, solar energy represents a renewable, economic and green alternative in the future energy market. Among the photovoltaic technologies, org. photovoltaics (OPVs) demonstrate a cheap, flexible, clean and easy-processing way to convert solar energy into electricity. However, OPVs with a conventional device structure are still far away from industrialization mainly because of their short lifetime and the energy-intensive deposition of top metal electrode. To address the stability and cost issue simultaneously, an inverted device structure has been introduced into OPVs, bridging lab. research with practical application. In this review, recent progress in device structures, working mechanisms, functions and advances of each component layer as well their correlations with the efficiency and stability of inverted OPVs are reviewed and illustrated.
    20. 20
      Rider, D. A.; Worfolk, B. J.; Harris, K. D.; Lalany, A.; Shahbazi, K.; Fleischauer, M. D.; Brett, M. J.; Buriak, J. M. Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface. Adv. Funct. Mater. 2010, 20, 2404– 2415,  DOI: 10.1002/adfm.201000304
      20
      Stable Inverted Polymer/Fullerene Solar Cells Using a Cationic Polythiophene Modified PEDOT:PSS Cathodic Interface
      Rider, David A.; Worfolk, Brian J.; Harris, Kenneth D.; Lalany, Abeed; Shahbazi, Kevin; Fleischauer, Michael D.; Brett, Michael J.; Buriak, Jillian M.
      Advanced Functional Materials (2010), 20 (15), 2404-2415CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A cationic and water-sol. polythiophene [poly[3-(6-pyridiniumylhexyl)thiophene bromide] (P3PHT+Br-)] is synthesized and used in combination with anionic poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (PEDOT:PSS)- to produce hybrid coatings on indium tin oxide (ITO). Two coating strategies are established: (i) electrostatic layer-by-layer assembly with colloidal suspensions of (PEDOT:PSS)-, and (ii) modification of an electrochem. prepd. (PEDOT:PSS)- film on ITO. The coatings are found to modify the work function of ITO such that it could act as a cathode in inverted 2,5-diyl-poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid Me ester (PCBM) polymer photovoltaic cells. The interfacial modifier created from the layer-by-layer assembly route is used to produce efficient inverted org. photovoltaic devices (power conversion efficiency ∼2%) with significant long-term stability in excess of 500 h.
    21. 21
      Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285– 310,  DOI: 10.1039/C0EE00273A
      21
      The role of buffer layers in polymer solar cells
      Po, Riccardo; Carbonera, Chiara; Bernardi, Andrea; Camaioni, Nadia
      Energy & Environmental Science (2011), 4 (2), 285-310CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      This review rationalizes the information spread in the literature concerning the use and role of buffer layers in polymer solar cells. Usual device structures include buffer layers, both at the anode and at the cathode interface, mainly to favor charge collection and extn., but also to improve the overall performance of the device. Buffer layers are actually essential for achieving highly efficient polymer solar cells and can no more be considered as optional, thus the need and importance of understanding their properties and role. The aim of this review is to give the reader an overview of this topic and to provide a practical and useful tool for the daily activities of researchers in the field of polymer photovoltaics.
    22. 22
      Kim, M.; Lee, J.; Jo, S. B.; Sin, D. H.; Ko, H.; Lee, H.; Lee, S. G.; Cho, K. Critical Factors Governing Vertical Phase Separation in Polymer–PCBM Blend Films for Organic Solar Cells. J. Mater. Chem. A 2016, 4, 15522– 15535,  DOI: 10.1039/C6TA06508B
      22
      Critical factors governing vertical phase separation in polymer-PCBM blend films for organic solar cells
      Kim, Min; Lee, Jaewon; Jo, Sae Byeok; Sin, Dong Hun; Ko, Hyomin; Lee, Hansol; Lee, Seung Goo; Cho, Kilwon
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (40), 15522-15535CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      In org. bulk-heterojunction solar cells, the vertical distribution of the compn. of the active layers as well as the lateral morphol. is one of the crit. issues that can significantly affect charge transport and recombination characteristics. Here we studied the crit. parameters that can affect the formation of vertically stratified bulk heterojunction org. solar cells based on various polymers with different side chains, and investigated the effect of the miscibility of the polymer-fullerene blend and the crystallinity of the polymer on vertical morphol. The major factor that affected the vertical phase sepn. was the interaction parameter χ between the polymer and phenyl-C61-butyric acid Me ester (PCBM). Polymer-PCBM blends with high values of χ tended to trigger surface-directed vertical phase sepn. during rapid solvent evapn. However, strong aggregation of polymers with low soly. counteracted this surface-directed vertical stratification. Moreover, solvent additives strongly affected the vertical phase sepn. processes, and caused the compn. of the active layer to fluctuate dramatically. We also found the photovoltaic characteristics, including charge recombination time, to be strongly affected by the vertical distribution of the compn. The modulation of the compn. in the vertical direction should therefore be optimized to increase the efficiency of charge collection and hence achieve high-efficiency org. solar cells.
    23. 23
      Lee, H. S.; Kim, D. H.; Cho, J. H.; Hwang, M.; Jang, Y.; Cho, K. Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics. J. Am. Chem. Soc. 2008, 130, 10556– 10564,  DOI: 10.1021/ja800142t
      23
      Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics
      Lee, Hwa Sung; Kim, Do Hwan; Cho, Jeong Ho; Hwang, Minkyu; Jang, Yunseok; Cho, Kilwon
      Journal of the American Chemical Society (2008), 130 (32), 10556-10564CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of org. thin-film transistors (OTFTs), we deposited pentacene mols. on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temps. (30, 60, and 90°C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temp. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temp.-dependent alkyl chain mobility of the ODTS monolayers.
    24. 24
      Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater. 2011, 1, 82– 89,  DOI: 10.1002/aenm.201000023
      24
      Interdiffusion of PCBM and P3HT reveals miscibility in a photovoltaically active blend
      Treat, Neil D.; Brady, Michael A.; Smith, Gordon; Toney, Michael F.; Kramer, Edward J.; Hawker, Craig J.; Chabinyc, Michael L.
      Advanced Energy Materials (2011), 1 (1), 82-89CODEN: ADEMBC; ISSN:1614-6840. (Wiley-Blackwell)
      Developing a better understanding of the evolution of morphol. in plastic solar cells is the key to designing new materials and structures that achieve photoconversion efficiencies >10%. In the most extensively characterized system, the poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl-C61-butyric-acid-methyl-ester (PCBM) bulk heterojunction, the origins and evolution of the blend morphol. during processes such as thermal annealing are not well understood. The authors use a model system, a bilayer of P3HT and PCBM, to develop a more complete understanding of the miscibility and diffusion of PCBM within P3HT during thermal annealing. PCBM aggregates and/or mol. species are miscible and mobile in disordered P3HT, without disrupting the ordered lamellar stacking of P3HT chains. The fast diffusion of PCBM into the amorphous regions of P3HT suggests the advantages of mixing in this system, opposing the belief that phase-pure domains form in BHJs due to immiscibility of these 2 components.
    25. 25
      Singh, R.; Suranagi, S. R.; Lee, J.; Lee, H.; Kim, M.; Cho, K. Unraveling the Efficiency-Limiting Morphological Issues of the Perylene Diimide-Based Non-Fullerene Organic Solar Cells. Sci. Rep. 2018, 8, 2849,  DOI: 10.1038/s41598-018-21162-x
      25
      Unraveling the efficiency-limiting morphological issues of the perylene diimide-based non-fullerene organic solar cells
      Singh Ranbir; Suranagi Sanjaykumar R; Lee Jaewon; Lee Hansol; Kim Min; Cho Kilwon
      Scientific reports (2018), 8 (1), 2849 ISSN:.
      Herein we report a comparative morphological analysis of the perylene diimide (PDI)- and fullerene-based organic solar cells (OSCs) to identify the factors responsible for low performance of PDI-based devices. A PDI derivative, bis-PDI, and a fullerene derivative, PC70BM, are mixed with an efficient polymer donor, PffBT4T-2OD. The large disparity in power conversion efficiencies (PCEs) of OSCs composed of PffBT4T-2OD:bis-PDI (PCE = 5.18%) and PffBT4T-2OD:PC70BM (PCE = 10.19%) observed are attributed to differences in the nanostructural motif of bulk heterojunction (BHJ) morphologies of these blend systems. The X-ray scattering and surface energy characterizations revealed that the structurally dissimilar bis-PDI and PC70BM molecules determine the variation in blend film morphologies, and in particular, the molecular packing features of the donor PffBT4T-2OD polymer. In addition, high-resolution transmission electron microscopy (HRTEM) images explore the BHJ morphologies and presence of longer polymer fibrils in PffBT4T-2OD:bis-PDI system, justifying the unbalanced charge transport and high hole mobility. The low performance of PffBT4T-2OD:bis-PDI devices was further investigated by studying charge carrier recombination dynamics by using light-intensity-dependent and transient photovoltage (TPV) experiments. Furthermore, the temperature-dependent experiments showed the photovoltaic properties, including charge recombination losses, are strongly affected by energetic disorder present in bis-PDI-based system.
    26. 26
      Kouijzer, S.; Michels, J. J.; van den Berg, M.; Gevaerts, V. S.; Turbiez, M.; Wienk, M. M.; Janssen, R. A. J. Predicting Morphologies of Solution Processed Polymer:Fullerene Blends. J. Am. Chem. Soc. 2013, 135, 12057– 12067,  DOI: 10.1021/ja405493j
      26
      Predicting Morphologies of Solution Processed Polymer:Fullerene Blends
      Kouijzer, Sandra; Michels, Jasper J.; van den Berg, Mauricio; Gevaerts, Veronique S.; Turbiez, Mathieu; Wienk, Martijn M.; Janssen, Rene A. J.
      Journal of the American Chemical Society (2013), 135 (32), 12057-12067CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The performance of soln. processed polymer:fullerene thin film photovoltaic cells is largely detd. by the nanoscopic and mesoscopic morphol. of these blends that is formed during the drying of the layer. Although blend morphologies have been studied in detail using a variety of microscopic, spectroscopic, and scattering techniques and a large degree of control has been obtained, the current understanding of the processes involved is limited. Hence, predicting the optimized processing conditions and the corresponding device performance remains a challenge. We present an exptl. and modeling study on blends of a small band gap diketopyrrolopyrrole-quinquethiophene alternating copolymer (PDPP5T) and [6,6]-phenyl-C71-butyric acid Me ester ([70]PCBM) cast from chloroform soln. The model uses the homogeneous Flory-Huggins free energy of the multi-component blend and accounts for interfacial interactions between (locally) sepd. phases, based on phys. properties of the polymer, fullerene, and solvent. We show that the spinodal liq.-liq. demixing that occurs during drying is responsible for the obsd. morphologies. The model predicts an increasing feature size and decreasing fullerene concn. in the polymer matrix with increasing drying time in accordance with exptl. observations and device performance. The results represent a first step toward a predictive model for morphol. formation.
    27. 27
      Manders, J. R.; Tsang, S.-W.; Hartel, M. J.; Lai, T.-H.; Chen, S.; Amb, C. M.; Reynolds, J. R.; So, F. Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells. Adv. Funct. Mater. 2013, 23, 2993– 3001,  DOI: 10.1002/adfm.201202269
      27
      Solution-Processed Nickel Oxide Hole Transport Layers in High Efficiency Polymer Photovoltaic Cells
      Manders, Jesse R.; Tsang, Sai-Wing; Hartel, Michael J.; Lai, Tzung-Han; Chen, Song; Amb, Chad M.; Reynolds, John R.; So, Franky
      Advanced Functional Materials (2013), 23 (23), 2993-3001CODEN: AFMDC6; ISSN:1616-301X. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The detailed characterization of soln.-derived nickel (II) oxide (NiO) hole-transporting layer (HTL) films and their application in high efficiency org. photovoltaic (OPV) cells is reported. The NiO precursor soln. is examd. in situ to det. the chem. species present. Coordination complexes of monoethanolamine (MEA) with Ni in ethanol thermally decomp. to form non-stoichiometric NiO. Specifically, the [Ni(MEA)2(OAc)]+ ion is found to be the most prevalent species in the precursor soln. The defect-induced Ni3+ ion, which is present in non-stoichiometric NiO and signifies the p-type conduction of NiO, as well as the dipolar nickel oxyhydroxide (NiOOH) species are confirmed using XPS. Bulk heterojunction (BHJ) solar cells with a polymer/fullerene photoactive layer blend composed of poly-dithienogermole-thienopyrrolodione (pDTG-TPD) and [6,6]-phenyl-C71-butyric acid Me ester (PC71BM) are fabricated using these soln.-processed NiO films. The resulting devices show an av. power conversion efficiency (PCE) of 7.8%, which is a 15% improvement over devices utilizing a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HTL. The enhancement is due to the optical resonance in the solar cell and the hydrophobicity of NiO, which promotes a more homogeneous donor/acceptor morphol. in the active layer at the NiO/BHJ interface. Finally, devices incorporating NiO as a HTL are more stable in air than devices using PEDOT:PSS.
    28. 28
      Lee, I.; Noh, J.; Lee, J.-Y.; Kim, T.-S. Cooptimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers. ACS Appl. Mater. Interfaces 2017, 9, 37395– 37401,  DOI: 10.1021/acsami.7b10398
      28
      Co-optimization of Adhesion and Power Conversion Efficiency of Organic Solar Cells by Controlling Surface Energy of Buffer Layers
      Lee, Inhwa; Noh, Jonghyeon; Lee, Jung-Yong; Kim, Taek-Soo
      ACS Applied Materials & Interfaces (2017), 9 (42), 37395-37401CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      Here, the authors demonstrate the Co-optimization of the interfacial fracture energy and power conversion efficiency (PCE) of poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT)-based org. solar cells (OSCs) by surface treatments of the buffer layer. The studied surface treatments of the buffer layer simultaneously changed the crack path and interfacial fracture energy of OSCs under mech. stress and the work function of the buffer layer. To study the effects of surface treatments, the work of adhesion values were calcd. and matched with the exptl. results based on the Owens-Wendt model. Subsequently, the authors fabricated OSCs on surface-treated buffer layers. In particular, ZnO layers treated with poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl-fluorene)] (PFN) simultaneously satisfied good mech. reliability and PCE of OSCs by achieving high work of adhesion and optimized work function.
    29. 29
      Dupont, S. R.; Voroshazi, E.; Heremans, P.; Dauskardt, R. H. Adhesion Properties of Inverted Polymer Solarcells: Processing and Film Structure Parameters. Org. Electron. 2013, 14, 1262– 1270,  DOI: 10.1016/j.orgel.2013.02.022
      29
      Adhesion properties of inverted polymer solar cells: Processing and film structure parameters
      Dupont, Stephanie R.; Voroshazi, Eszter; Heremans, Paul; Dauskardt, Reinhold H.
      Organic Electronics (2013), 14 (5), 1262-1270CODEN: OERLAU; ISSN:1566-1199. (Elsevier B.V.)
      We report on the adhesion of weak interfaces in inverted P3HT:PCBM-based polymer solar cells (OPV) with either a conductive polymer, PEDOT:PSS, or a metal oxide, molybdenum trioxide (MoO3), as the hole transport layer. The PEDOT:PSS OPVs were prepd. by spin or spray coating on glass substrates, or slot-die coating on flexible PET substrates. In all cases, we obsd. adhesive failure at the interface between the P3HT:PCBM with PEDOT:PSS layer. The adhesion energy measured for the solar cells made on glass substrates was about 1.8 J/m2, but only 0.5 J/m2 for the roll-to-roll processed flexible solar cells. The adhesion energy was insensitive to the PEDOT:PSS layer thickness in the range of 10-40 nm. A marginal increase in adhesion energy was measured with increased O2 plasma power. Compared to soln. processed PEDOT:PSS, we found that thermally evapd. MoO3 adheres less to the P3HT:PCBM layer, which we attributed to the reduced mixing at the MoO3/P3HT:PCBM interface during the thermal evapn. process. Insights into the mechanisms of delamination and the effect of different material properties and processing parameters yield general guidelines for the design of more reliable org. photovoltaic devices.
    30. 30
      Higgins, M. J.; Wallace, G. G. Surface and Biomolecular Forces of Conducting Polymers. Polym. Rev. 2013, 53, 506– 526,  DOI: 10.1080/15583724.2013.813856
      30
      Surface and Biomolecular Forces of Conducting Polymers
      Higgins, Michael J.; Wallace, Gordon G.
      Polymer Reviews (Philadelphia, PA, United States) (2013), 53 (3), 506-526CODEN: PRPPCY; ISSN:1558-3716. (Taylor & Francis, Inc.)
      In this review, the authors provide insight into the surface forces of conducting polymers, a class of "intelligent" materials that offer unique strategies for controlling biomol. interactions in wide-ranging biomedical applications. Crit. to the success of these applications is that the polymer interface is exposed to biol. fluids whose interactions are controlled through the polymer surface chem. and electrochem. switching of the surface properties. There is, however, little known about the intermol. and surface forces that govern these interactions. Therefore, the purpose of this review is to more closely examine the forces that mediate interactions with biol. entities, including forces such as van der Waals, electrostatic, hydrophobic, and hydrogen bonding. The authors introduce relevant surface properties such as surface energy and surface potential, and demonstrate how they manifest as forces. In particular, the authors highlight the emerging use of At. Force Microscopy for directly measuring these forces at the single mol. level; a unique capability that is enabling deconvolution of complex biomol. interactions with conducting polymers. Finally, the authors provide an overview of biomol. interactions, namely model proteins and DNA, and conclude by discussing a growing area of interest; the spatio-temporal and reversible control of biomol. forces via elec. stimulation.
    31. 31
      McDowell, C.; Abdelsamie, M.; Toney, M. F.; Bazan, G. C. Solvent Additives: Key Morphology-Directing Agents for Solution-Processed Organic Solar Cells. Adv. Mater. 2018, 30, 1707114,  DOI: 10.1002/adma.201707114
      There is no corresponding record for this reference.
    32. 32
      Urbánek, P.; di Martino, A.; Gladyš, S.; Kuřitka, I.; Minařík, A.; Pavlova, E.; Bondarev, D. Polythiophene-Based Conjugated Polyelectrolyte: Optical Properties and Association Behavior in Solution. Synth. Met. 2015, 202, 16– 24,  DOI: 10.1016/j.synthmet.2015.01.015
      32
      Polythiophene-based conjugated polyelectrolyte: Optical properties and association behavior in solution
      Urbanek, Pavel; di Martino, Antonio; Gladys, Sviatoslav; Kuritka, Ivo; Minarik, Antonin; Pavlova, Ewa; Bondarev, Dmitrij
      Synthetic Metals (2015), 202 (), 16-24CODEN: SYMEDZ; ISSN:0379-6779. (Elsevier B.V.)
      Conjugated polyelectrolyte with polythiophene main chain and imidazolium side groups, poly{3-[6-(1-methylimidazolium-3-yl) hexyl]thiophene-2,5-diyl bromide}, is investigated. Polymer architecture with hydrophobic main chain and hydrophilic side groups is responsible for the inconvenient behavior of the polymer in soln. Absorption and emission characteristics and supramol. assembling are investigated by spectroscopic, microscopic and light-scattering methods. We report significant solvatochromism which is observable by a naked eye. Water solns. and mixed solvent systems exhibit most interesting features: supramol. assembling was investigated by DLS, AFM and TEM techniques and common spectroscopic methods such as UV-vis and fluorescence spectroscopy. Moreover, slow relaxation processes take part in water as it is evidenced by time evolution of absorption spectra. It is a result of vanishing of inter- and intramol. interactions. These interactions are responsible for extraordinary soln. behavior of the ionic polythiophene.
    33. 33
      Domínguez, S. E.; Meriläinen, M.; Ääritalo, T.; Damlin, P.; Kvarnström, C. Effect of Alkoxy-Spacer Length and Solvent on Diluted Solutions of Cationic Isothiouronium Polythiophenes. RSC Adv. 2017, 7, 7648– 7657,  DOI: 10.1039/C6RA21451G
      33
      Effect of alkoxy-spacer length and solvent on diluted solutions of cationic isothiouronium polythiophenes
      Dominguez, S. E.; Merilainen, M.; Aaritalo, T.; Damlin, P.; Kvarnstrom, C.
      RSC Advances (2017), 7 (13), 7648-7657CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      This contribution presents a study on two cationic isothiouronium polythiophenes (poly-3-(N,N-diethyl-S-iso-thiouronium)alkoxy-4-Me thiophene), with alkoxy chains of different length in the 3-position of the thiophene ring, results of steady-state absorption and fluorescence spectroscopies in four solvents of different polarities: protic water, protic-protic water-isopropanol, protic-aprotic water-tetrahydrofuran and protic-non polar water-1,4-dioxane (all mixts. 0.5 vol./vol.) are presented. Molar absorption coeff. (ε), quantum yield (Φf), spectral maxima (λmax, λem) and Stokes shift (Δν) are presented, and the effect of solvents on these parameters is discussed. Empirical optical and electrochem. energy levels were estd. using drop cast films on glass and on glassy carbon electrode, resp. D. functional theory was used to gain supporting structural, energy and electronic information of different oligomers, simulating solvents of different polarity.
    34. 34
      Domínguez, S. E.; Cangiotti, M.; Fattori, A.; Ääritalo, T.; Damlin, P.; Ottaviani, M. F.; Kvarnström, C. Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes. Langmuir 2018, 34, 7364– 7378,  DOI: 10.1021/acs.langmuir.8b00808
      34
      Effect of Spacer Length and Solvent on the Concentration-Driven Aggregation of Cationic Hydrogen-Bonding Donor Polythiophenes
      Dominguez, S. E.; Cangiotti, M.; Fattori, A.; Aaritalo, T.; Damlin, P.; Ottaviani, M. F.; Kvarnstrom, C.
      Langmuir (2018), 34 (25), 7364-7378CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Aggregation of cationic isothiouronium polythiophenes with alkoxy-spacers of different lengths at the 3-position of the thiophene ring was studied in solvents of different polarities. Hydrogen-bonding capacity was assessed by steady-state absorption and fluorescence spectroscopy, whereas the aggregation in aq. solns. was studied by ESR spectroscopy, using paramagnetic probes of different polarities. The two polymers displayed similar features in respect to conformation, effect of cosolvents on aggregation, unstructured absorption-fluorescence spectra, Stokes shifts when aggregated, solvatochromic effect, and self-quenching concn. However, these polymers also showed different specific interactions with water, Stokes shifts in water, effect of the solvent on the extent of dominant state of the S1 level, and also different inner cavities and hydrophobic-hydrophilic surface area in aq. soln. aggregates. Water maximized the difference between the polymers concerning the effect of specific increases in concn., whereas the presence of 1,4-dioxane generated almost identical effects on both polymers.
    35. 35
      Ottaviani, M. F.; Cangiotti, M.; Fiorani, L.; Barnard, A.; Jones, S. P.; Smith, D. K. Probing Dendron Structure and Nanoscale Self-Assembly Using Computer-Aided Analysis of EPR Spectra. New J. Chem. 2012, 36, 469– 476,  DOI: 10.1039/C1NJ20685K
      35
      Probing dendron structure and nanoscale self-assembly using computer-aided analysis of EPR spectra
      Ottaviani, M. Francesca; Cangiotti, Michela; Fiorani, Luigi; Barnard, Anna; Jones, Simon P.; Smith, David K.
      New Journal of Chemistry (2012), 36 (2), 469-476CODEN: NJCHE5; ISSN:1144-0546. (Royal Society of Chemistry)
      The self-assembly of dendritic mols. is an effective way of generating functionalized nanoscale architectures. This study reports a computer-aided anal. of the EPR spectra of 5 doxyl-stearic acid (5DSA) and Cu(II) probes interacting with self-assembling dendrons in water. The dendrons studied have previously been reported as potential gene delivery vehicles, and possess amine surface groups, different dendritic architectures based on ether-amide or ester branching, and hydrophobic groups at the focal point which can encourage self-assembly in aq. soln. The parameters extd. from computation provide information about both the structure and dynamics in soln. and the interacting ability of the dendrons to be used in gene therapy. The hydrophobic 5DSA probe is able to effectively probe the hydrophobic core of self-assembled dendron nanostructures. It reports on the polarity of its local environment and is most affected by dendrons with two cholesterol units at the focal point, partly affected by dendrons with cholesterol groups at their focal point, but is unaffected by dendrons with a simple Ph group at the focal point. This reflects the different modes of self-assembly obsd. for these dendrons. When Cu(II) is used as an EPR probe of the branched environment, at pH7, much of the Cu(II) was external' to the dendritic structure, presumably due to protonation of the peripheral amine groups. On gradually increasing the Cu(II) loading, and using computer-aided anal., it was possible to quantify the levels of 'internal' (dendron-bound) and 'external' Cu(II) and this was strongly dependent on the structure of the dendritic branching and the ability of the dendron to self-assemble, with self-assembling ester dendrons being best able to bind the Cu(II). It was also possible to propose the nature of the copper binding sites assocd. with the internal' signal as either Cu-NO3 and Cu-N2O2 distorted square-planar coordination sites. The self-assembling ester based dendrons which also contain 1,2,3-triazole units, had higher levels of 'internal' Cu(II) and showed the latter form of coordination, while the other dendrons, with lower levels of Cu(II) uptake showed the former. In summary, this paper demonstrates that two complementary EPR probes can be used to provide information about different regions of a self-assembled dendritic architecture.
    36. 36
      Santeusanio, S.; Attanasi, O. A.; Majer, R.; Cangiotti, M.; Fattori, A.; Ottaviani, M. F. Effect of Hydrogenated Cardanol on the Structure of Model Membranes Studied by EPR and NMR. Langmuir 2013, 29, 11118,  DOI: 10.1021/la402008n
      36
      Effect of Hydrogenated Cardanol on the Structure of Model Membranes Studied by EPR and NMR
      Santeusanio, Stefania; Attanasi, Orazio Antonio; Majer, Roberta; Cangiotti, Michela; Fattori, Alberto; Ottaviani, Maria Francesca
      Langmuir (2013), 29 (35), 11118-11126CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Hydrogenated cardanol (HC) is known to act as an antiobesity, promising antioxidant, and eco-friendly brominating agent. In this respect, it is important to find the way to transport and protect HC into the body; a micellar structure works as the simplest membrane model and may be considered a suitable biocarrier for HC. Therefore, it is useful to analyze the impact of HC in the micellar structure and properties. This study reports a computer aided ESR (EPR) and 1H NMR investigation of structural variations of cetyltrimetylammonium bromide (CTAB) micelles upon insertion of HC at different concns. and pH variations. Surfactant spin probes inserted in the micelles allowed us to get information on the structure and dynamics of the micelles and the interactions between HC and CTAB. The formation of highly packed HC-CTAB mixed micelles were favored by the occurrence of both hydrophobic (chain-chain) and hydrophilic (between the polar and charged lipid heads) interactions. These interactions were enhanced by neutralization of the acidic HC heads. Different HC localizations into the micelles and micellar structures were identified by changing HC/CTAB relative concns. and pH. The increase in HC concn. generated mixed micelles characterized by an increased surfactant packing. These results suggested a rod-like shape of the mixed micelles. The increase in pH promoted the insertion of deprotonated HC into less packed micelles, favored by the electrostatic head-head interactions between CTAB and deprotonated-HC surfactants.
    37. 37
      Kris, M. R. Effect of 1,4-Dioxane on the Complexation of Lanthanides with α-Hydroxy-Isobutyrate. Doctor of Philosophy, Department of Chemistry, Washington State University, 2010, No. December.
      There is no corresponding record for this reference.
    38. 38
      Burrows, H. D.; Fonseca, S. M.; Silva, C. L.; Pais, A. A. C. C.; Tapia, M. J.; Pradhan, S.; Scherf, U. Aggregation of the Hairy Rod Conjugated Polyelectrolyte Poly{1,4-Phenylene-[9,9-Bis(4-Phenoxybutylsulfonate)]Fluorene-2,7-Diyl} in Aqueous Solution: An Experimental and Molecular Modelling Study. Phys. Chem. Chem. Phys. 2008, 10, 4420– 4428,  DOI: 10.1039/b800773j
      38
      Aggregation of the hairy rod conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} in aqueous solution: an experimental and molecular modelling study
      Burrows, Hugh D.; Fonseca, Sofia M.; Silva, Claudia L.; Pais, Alberto A. C. C.; Tapia, Maria J.; Pradhan, Swapna; Scherf, Ullrich
      Physical Chemistry Chemical Physics (2008), 10 (30), 4420-4428CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The aggregation of the fluorescent hairy rod, anionic conjugated polyelectrolyte poly{1,4-phenylene-[9,9-bis(4-phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) has been studied in aq. solns. by mol. dynamics simulations, fluorescence and light scattering. Formation of clusters leads to considerable increases in light scattering, decreases in the fluorescence quantum yields and red shifts in emission maxima. Mol. dynamics simulations considering two isolated tetramers in aq. soln. show that they rapidly form aggregates, and support exptl. evidence for the assocn. of polymer chains involving both electrostatic and hydrophobic interactions. They also provide indications for proximity of arom. rings, which is likely to be the main factor responsible for the obsd. fluorescence behavior. However, there are no indications of extensive π-stacking. The org. co-solvents methanol, acetonitrile and dioxane break up these aggregates. From studies of the dependence of the aggregation behavior on dielec. const. or the empirical solvent parameters ENT and BKT for binary mixts. with water, it can be seen that this is not simply an effect of changing solvent polarity, but is due to preferential solvation of the polymer chains. This is supported by mol. dynamic simulations on two tetramers in water-dioxane mixts. (70 : 30%). It is suggested that similar factors are involved in both the assocn. behavior and aggregate disruption with other hairy rod conjugated polyelectrolytes in water.
    39. 39
      Luong, T. Q.; Verma, P. K.; Mitra, R. K.; Havenith, M. Onset of Hydrogen Bonded Collective Network of Water in 1,4-Dioxane. J. Phys. Chem. A 2011, 115, 14462– 14469,  DOI: 10.1021/jp204927r
      39
      Onset of Hydrogen Bonded Collective Network of Water in 1,4-Dioxane
      Luong, Trung Quan; Verma, Pramod Kumar; Mitra, Rajib Kumar; Havenith, Martina
      Journal of Physical Chemistry A (2011), 115 (50), 14462-14469CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      We have studied the evolution of water hydrogen bonded collective network dynamics in mixts. of 1,4-dioxane (Dx) as the mole fraction of water (Xw) increases from 0.005 to 0.54. The inter- and intramol. vibrations of water have been obsd. using terahertz time domain spectroscopy (THz-TDS) in the frequency range 0.4-1.4 THz (13-47 cm-1) and Fourier transform IR (FTIR) spectroscopy in the far-IR (30-650 cm-1) and mid-IR (3000-3700 cm-1) regions. These results have been correlated with the reactivity of water in these mixts. as detd. by kinetic studies of the solvolysis reaction of benzoyl chloride (BzCl). Our studies show an onset of intermol. hydrogen bonded water network dynamics beyond Xw ≥ 0.1. At the same concn., we observe a rapid increase of the rate const. of solvolysis of BzCl in water-Dx mixts. Our results establish a correlation between the onset of collective hydrogen bonded network with the solvation dynamics and the activity of clustered water.
    40. 40
      Bhavya, P.; Melavanki, R.; Kusanur, R.; Sharma, K.; Muttannavar, V. T.; Naik, L. R. Effect of Viscosity and Dielectric Constant Variation on Fractional Fluorescence Quenching Analysis of Coumarin Dye in Binary Solvent Mixtures. Luminescence 2018, 33, 933– 940,  DOI: 10.1002/bio.3492
      40
      Effect of viscosity and dielectric constant variation on fractional fluorescence quenching analysis of coumarin dye in binary solvent mixtures
      Bhavya, P.; Melavanki, Raveendra; Kusanur, Raviraj; Sharma, Kalpana; Muttannavar, V. T.; Naik, L. R.
      Luminescence (2018), 33 (5), 933-940CODEN: LUMIFC; ISSN:1522-7243. (John Wiley & Sons Ltd.)
      Photo phys. properties of fluorescent org. compds. give an immense improved knowledge on characteristics of excited state that is beneficial to devise innovate mols. and understand their performance in particular applications. Coumarin derivs. have been extensively investigated in this regard. This article narrates steady state fluorescence quenching measurements of a coumarin deriv. namely 3-hydroxy-3-[2-oxo-2-(3-oxo-3H-benzo[f]chromen-2-yl)-ethyl]-1,3-dihydro-indol-2-one (3HBCD) in a binary mixt. of acetonitrile and 1,4-dioxane. Aniline is used as quencher. Fluorescence intensity is large in acetonitrile and decreases as the percentage of 1,4-dioxane in the solvent mixt. increases. With modest quencher concn. a deviation towards the x axis is noticed in the Stern-Volmer (S-V) plots. This downward curvature is interpreted as due to the presence of 3HBCD in different conformers in the lowest energy level. Ground state intramol. hydrogen bonding formation is obsd. due to the conformational changes in the solute. Figured estns. of various quenching parameters recommend that, while dynamic quenching prompts linearity in S-V plot at lower quencher concn., increasing quenching efficiency with increasing medium viscosity suggests that reaction is not entirely controlled by material diffusion. Stern-Volmer const. increases with decreasing medium dielec. const.
    41. 41
      Nakagawa, T.; Hatano, J.; Matsuo, Y. Influence of Additives in Bulk Heterojunction Solar Cells Using Magnesium Tetraethynylporphyrin with Triisopropylsilyl and Anthryl Substituents. J. Porphyrins Phthalocyanines 2014, 18, 735– 740,  DOI: 10.1142/S1088424614500655
      41
      Influence of additives in bulk heterojunction solar cells using magnesium tetraethynylporphyrin with triisopropylsilyl and anthryl substituents
      Nakagawa, Takafumi; Hatano, Junichi; Matsuo, Yutaka
      Journal of Porphyrins and Phthalocyanines (2014), 18 (8-9), 735-740CODEN: JPPHFZ; ISSN:1088-4246. (World Scientific Publishing Co. Pte. Ltd.)
      We designed and synthesized anthryl-disubstituted magnesium tetraethynylporphyrin([{5,15-bis(anthracen-9'-yl)ethynyl}-10,20-bis{(triisopropylsilyl)ethynyl}porphyrinato] magnesium(II)), and applied it as an electron donor to soln.-processed bulk heterojunction small mol. org. solar cells. The compd. was characterized by single crystal X-ray crystallog. as well as UV-vis light absorption spectrum showing the absorption max. and onset at 700 and 740 nm, resp. Org. solar cells using this compd. and [6,6]-phenyl-C61-butyric acid Me ester (PC61BM) as electron donor and acceptor, resp., showed power conversion efficiency of 1.31% at the donor and acceptor ratio of 1:3. The use of pyridine as a coordinating additive increased power conversion efficiency to 1.61%, which was the best among tested additives, THF, pyradine, dioxane, and 1,8-diiodooctane.
    42. 42
      Chen, S.; Liu, Z.; Ge, Z. Synthesis, Characterization and Photovoltaic Properties of Three New 3,4-Dithienyl-Substituted Polythiophene Derivatives. Polym. J. 2016, 48, 101– 110,  DOI: 10.1038/pj.2015.85
      42
      Synthesis, characterization and photovoltaic properties of three new 3,4-dithienyl-substituted polythiophene derivatives
      Chen, Shaojie; Liu, Zhiyang; Ge, Ziyi
      Polymer Journal (Tokyo, Japan) (2016), 48 (1), 101-110CODEN: POLJB8; ISSN:0032-3896. (NPG Nature Asia-Pacific)
      Three 3,4-dithienyl-substituted polythiophene derivs. have been synthesized and characterized. Polymer P2 exhibits the widest light absorption spectrum of the polymers in its polymer chain, which has an alternative donor-acceptor structure. Copolymers blended with 6,6-phenyl-C71-butyric acid Me ester(PC71BM) as active layers are used to fabricate polymer solar cells (PSCs), and a variety of post-treatments are employed to optimize the PSCs performance. A max. power conversion efficiency (PCE) of 1.22% is achieved for the conventional configuration device based on P2, and the inverted configuration device of P2 is also prepd. for comparison. The results show that the inverted P2 device exhibits a better PCE (1.47%) than that of the conventional device due to the TiO2 nanoparticles and the close energy alignment between the work function of the MoO3 and the HOMO energy levels, which facilitate a light absorption increase and improved charge transport.
    43. 43
      Khan, M. Z. H. Effect of ITO Surface Properties on SAM Modification: A Review toward Biosensor Application. Cogent Eng. 2016, 3, 1170097,  DOI: 10.1080/23311916.2016.1170097
      There is no corresponding record for this reference.
    44. 44
      Khan, M. Z. H.; Nakanishi, T.; Kuroiwa, S.; Hoshi, Y.; Osaka, T. Effect of Surface Roughness and Surface Modification of Indium Tin Oxide Electrode on Its Potential Response to Tryptophan. Electrochim. Acta 2011, 56, 8657– 8661,  DOI: 10.1016/j.electacta.2011.07.068
      44
      Effect of surface roughness and surface modification of indium tin oxide electrode on its potential response to tryptophan
      Khan, Md. Zaved Hossain; Nakanishi, Takuya; Kuroiwa, Shigeki; Hoshi, Yoichi; Osaka, Tetsuya
      Electrochimica Acta (2011), 56 (24), 8657-8661CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The effect of surface modification of indium tin oxide (ITO) electrode on its potential response to tryptophan was studied for ITO substrates with different surface roughness. A small difference in surface roughness, between ∼1 and ∼2 nm of R a evaluated by at. force microscopy, affects the rest potential of ITO electrode in the electrolyte. A slight difference in In:Sn ratio at the near surface of the ITO substrates, measured by angle-resolved x-ray photoelectron spectrometry and Auger electron spectroscopy is remarkable, and considered to relate with surface roughness. Successive modification of the ITO surface with aminopropylsilane and disuccinimidyl suberate, of which essentiality to the potential response to indole compds. the authors previously reported, improved the stability of the rest potential and enabled the electrodes to respond to tryptophan in case of specimens with R a values ranging between ∼2 and ∼3 nm but not for those with R a of ∼1 nm. Probably there are optimum values of effective work function of ITO for specific potential response to tryptophan, which can be obtained by the successive modification of ITO surface.
    45. 45
      Xu, J.; Wang, C.; Tian, Y.; Wu, B.; Wang, S.; Zhang, H. Glass-on-LiNbO3 Heterostructure Formed via a Two-Step Plasma Activated Low-Temperature Direct Bonding Method. Appl. Surf. Sci. 2018, 459, 621– 629,  DOI: 10.1016/j.apsusc.2018.08.031
      45
      Glass-on-LiNbO3 heterostructure formed via a two-step plasma activated low-temperature direct bonding method
      Xu, Jikai; Wang, Chenxi; Tian, Yanhong; Wu, Bin; Wang, Shang; Zhang, He
      Applied Surface Science (2018), 459 (), 621-629CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Lithium niobate (LiNbO3) crystal has been widely used in integrating optical and acoustic devices. However, direct bonding of LiNbO3 to Si-based materials is still a challenge due to the large coeff. of thermal expansion mismatches. In this paper, strong bonding strength of the LiNbO3/glass pair was obtained via the optimized O2 plasma activation followed by N2 plasma treatment. After the pre-bonding was accomplished at room temp., the bonded pair was annealed at 150°C for 12 h. A defect-free and tight bonding interface was confirmed by the SEM and transmission electron microscopy. Based on at. force microscopy, water contact angle and XPS characterizations, both LiNbO3 and glass surfaces became smoother and more hydrophilic due to the densities of -OH and N-related groups increased. Addnl., combined with the bonding strength obtained via different plasma activated processes, we demonstrated that the mechanism of the enhanced bonding strength could be attributed to N-related covalent bonds formed across the bonding interface. Furthermore, the excellent optical transmittance of LiNbO3/glass bonded pairs indicated this facile bonding process can meet the demand of high-performance glass-on-LiNbO3 devices.
    46. 46
      Viinikanoja, A.; Areva, S.; Kocharova, N.; Ääritalo, T.; Vuorinen, M.; Savunen, A.; Kankare, J.; Lukkari, J. Structure of Self-Assembled Multilayers Prepared from Water-Soluble Polythiophenes. Langmuir 2006, 22, 6078– 6086,  DOI: 10.1021/la060519u
      46
      Structure of self-assembled multilayers prepared from water-soluble polythiophenes
      Viinikanoja, Antti; Areva, Sami; Kocharova, Natalia; Aeaeritalo, Timo; Vuorinen, Maarit; Savunen, Arto; Kankare, Jouko; Lukkari, Jukka
      Langmuir (2006), 22 (14), 6078-6086CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      We have studied the structure and morphol. of self-assembled polyelectrolyte multilayers prepd. using poly(styrene sulfonate) (PSS) and four different cationic poly(alkoxy-thiophene) derivs. bearing methylimidazolium-terminated ionic side chain at the 3-position of the thiophene ring: poly(1-methyl-3-[3-[3-thienyl-oxy]-propyl]-1H-imidazolium), poly(1-methyl-3-[6-[3-thienyl-oxy]-hexyl]-1H-imidazolium), poly(1-methyl-3-[2-[(4-methyl-3-thienyl)oxy]-ethyl]-1H-imidazolium ), and poly(1-methyl-3-[6-[(4-methyl-3-thienyl)oxy]-hexyl]-1H-imidazolium ). All the multilayers exhibited regular growth. The thickness of the multilayers was measured with ellipsometry, their layer-by-layer growth was followed by polarization modulation IR reflection-absorption spectroscopy and ellipsometry, and the morphol. of the films was studied by at. force microscopy. The length of the methylimidazolium-terminated side chain (Cn, n = 2, 3, 6) and the substituent (H or Me) at the 4-position of the thiophene ring were varied. All multilayers were inhomogeneous in the sub-micrometer scale and contained aggregates of two kinds. The large ones with a low and const. surface no. d. were attributed to PSS, whereas the small aggregates were polythiophene-based. The surface d. of these org. semiconducting nanoparticles greatly depended on the structure of polythiophene, being favored by polymer regio-regularity and the length of the side chain. The side chains remained disordered in all the multilayers, but with polythiophenes having hexyl chains both the imidazolium and thiophene rings tended to orient themselves more perpendicular to the surface than in films contg. shorter chains (C2 or C3). The relative water content of the multilayers (at 7.1% relative humidity) did not depend on the film thickness. As the no. of bilayers increased the methylimidazolium-sulfonate ion pairs gradually weakened and became more individually hydrated.
    47. 47
      Malika Jeffries-El, R. D. M. 9. Regioregular Polythiophenes. In Handbook of Conducting Polymers, 3rd ed.; Terje, A., Skotheim, J. R. R., Eds., Conjugated Polymers Theory, Synthesis, Properties, and Characterization; CRC Press Taylor & Francis Group, 2007.
      There is no corresponding record for this reference.
    48. 48
      Budil, D. E.; Lee, S.; Saxena, S.; Freed, J. H. Nonlinear-Least-Squares Analysis of Slow-Motion EPR Spectra in One and Two Dimensions Using a Modified Levenberg-Marquardt Algorithm. J. Magn. Reson., Ser. A 1996, 120, 155– 189,  DOI: 10.1006/jmra.1996.0113
      48
      Nonlinear-least-squares analysis of slow-motion EPR spectra in one and two dimensions using a modified Levenberg-Marquardt algorithm
      Budil, David E.; Lee, Sanghyuk; Saxena, Sunil; Freed, Jack H.
      Journal of Magnetic Resonance, Series A (1996), 120 (2), 155-189CODEN: JMRAE2; ISSN:1064-1858. (Academic)
      The application of the model trust region modification of the Levenberg-Marquardt minimization algorithm to the anal. of 1-dimensional continuous-wave EPR and multidimensional Fourier-transform (FT) EPR spectra esp. in the slow-motion regime is described. The dynamic parameters describing the slow motion were obtained from least-squares fitting of model calcns. based on the stochastic Liouville equation (SLE) to exptl. spectra. The trust-region approach is inherently more efficient than the std. Levenberg-Marquardt algorithm, and the efficiency of the procedure may be further increased by a sepn.-of-variables method in which a subset of fitting parameters is independently minimized at each iteration, thus reducing the no. of parameters to be fitted by nonlinear least squares. A particularly useful application of this method occurs in the fitting of multicomponent spectra, for which it is possible to obtain the relative population of each component by the sepn.-of-variables method. These advantages, combined with recent improvements in the computational methods used to solve the SLE, led to an order-of-magnitude redn. in computing time, and have made it possible to carry out interactive, real-time fitting on a lab. workstation with a graphical interface. Examples of fits to exptl. data will be given, including multicomponent continuous-wave EPR spectra as well as two- and three-dimensional FT EPR spectra. Emphasis is placed on the analytic information available from the partial derivs. used in the algorithm, and how it may be used to est. the condition and uniqueness of the fit, as well as to est. confidence limits for the parameters in certain cases.
    49. 49
      Belandria, V.; Mohammadi, A. H.; Richon, D. Volumetric Properties of the (Tetrahydrofuran + Water) and (Tetra-n-Butyl Ammonium Bromide + Water) Systems: Experimental Measurements and Correlations. J. Chem. Thermodyn. 2009, 41, 1382– 1386,  DOI: 10.1016/j.jct.2009.06.014
      49
      Volumetric properties of the (tetrahydrofuran + water) and (tetra-n-butyl ammonium bromide + water) systems: Experimental measurements and correlations
      Belandria, Veronica; Mohammadi, Amir H.; Richon, Dominique
      Journal of Chemical Thermodynamics (2009), 41 (12), 1382-1386CODEN: JCTDAF; ISSN:0021-9614. (Elsevier Ltd.)
      We report exptl. d. data for the binary mixts. of (water + THF) and (water + tetra-Bu ammonium bromide) at atm. pressure and various temps. The densities were measured using an Anton Paar digital vibrating-tube densimeter. For the (THF + water) system, excess molar volumes were calcd. using the exptl. densities and correlated using the Redlich-Kister equation. The Redlich-Kister equation parameters were adjusted on exptl. results. The partial molar volumes and partial excess molar volumes at infinite diln. were also calcd. for each component. A simple d. equation was finally applied to correlate the measured d. of the (tetra-Bu ammonium bromide + water) system.
    50. 50
      Nayak, J. N.; Aralaguppi, M. I.; Kumar Naidu, B. V.; Aminabhavi, T. M. Thermodynamic Properties of Water + Tetrahydrofuran and Water + 1,4-Dioxane Mixtures at (303.15, 313.15, and 323.15) K. J. Chem. Eng. Data 2004, 49, 468– 474,  DOI: 10.1021/je030196t
      50
      Thermodynamic Properties of Water + Tetrahydrofuran and Water + 1,4-Dioxane Mixtures at (303.15, 313.15, and 323.15) K
      Nayak, Jyoti N.; Aralaguppi, Mrityunjaya I.; Naidu, B. Vijaya Kumar; Aminabhavi, Tejraj M.
      Journal of Chemical and Engineering Data (2004), 49 (3), 468-474CODEN: JCEAAX; ISSN:0021-9568. (American Chemical Society)
      Properties like d. and viscosity at (303.15, 313.15, and 323.15) K and refractive index at 303.15 K have been measured for binary mixts. of water + THF and water + 1,4-dioxane. The excess molar volume (VE) has been calcd. from the d. results. Viscosity and refractive index data have been used to compute resp. deviations in viscosity (Δη) and molar refraction (ΔR) of the mixts. Excess molar volume and deviations in viscosity and molar refraction data have been fitted to the Redlich-Kister equation to calc. binary coeffs. and std. deviations. Furthermore, literature published vapor-liq. equil. data of the mixts. have been used to construct x-y compn. curves and compared with the pervaporation curves. Results of this study indicate the importance of the pervaporation technique for the sepn. of mixts. over that of simple distn. at the azeotropic compn. Permeate samples were analyzed by refractive index as well as gas chromatog. measurements to get the exact content of water.
    51. 51
      Critchfield, F. E.; Gibson, J. A., Jr.; Hall, J. L. Dielectric Constant for the Dioxane-Water System from 20 to 35°. J. Am. Chem. Soc. 1953, 75, 1991– 1992,  DOI: 10.1021/ja01104a506
      51
      Dielectric constant for the dioxane-water system from 20° to 35°
      Critchfield, Frank E.; Gibson, John A., Jr.; Hall, James L.
      Journal of the American Chemical Society (1953), 75 (), 1991-2CODEN: JACSAT; ISSN:0002-7863.
      The dielec. consts. for the dioxane-water system ranging from 0 to 100% by wt. dioxane at 10% intervals were detd. at 20, 25, 30, and 35°. The results differ from those of Akerl.ovrddot.of and Short by as much as 12% (C.A. 30, 6616.8).
    52. 52
      Omota, L.-M.; Iulian, O.; Omota, F.; Ciocirlan, O. Densities and Derived Properties of Water, 1,4-Dioxane and Dimethyl Sulfoxide Binary and Ternary Systems at Temperatures from 293.15 K to 313.15 K. Rev. Roum. Chim. 2009, 54, 63– 73
      52
      Densities and derived properties of water, 1,4-dioxane and dimethyl sulfoxide binary and ternary systems at temperatures from 293.15 K to 313.15 K
      Omota, Ligia-Maria; Iulian, Olga; Omota, Florin; Ciocirlan, Oana
      Revue Roumaine de Chimie (2009), 54 (1), 63-73CODEN: RRCHAX; ISSN:0035-3930. (Editura Academiei Romane)
      Excess molar volumes (VE) and excess partial molar volumes (‾VEi) of the binary systems: water + 1,4-dioxane, 1,4-dioxane + DMSO, water + DMSO and of the corresponding ternary system water + 1,4-dioxane + DMSO have been evaluated from exptl. d. at five temps. ranging from 293.15 K to 313.15 K. For all binary mixts., the exptl. data were correlated with Redlich-Kister's equation. For ternary system, the excess molar volume was estd. by correlative and predictive (sym. and asym.) methods and the results were compared. The calcd. functions have been used to explain the intermol. interactions between the mixing components.
    53. 53
      Barton, A. F. M. CRC Handbook of Solubility Parameters and Other Cohesion Parameters, 2nd ed.; Barton, A. F. M., Ed.; CRC Press, 1991.
      There is no corresponding record for this reference.
    54. 54
      Hansen, C. M. Hansen Solubility Parameters: A User’s Handbook, 2nd ed.; Hansen, C. M., Ed.; CRC Press, 2007.
      There is no corresponding record for this reference.
    55. 55
      Dalstein, L.; Potapova, E.; Tyrode, E. The Elusive Silica/Water Interface: Isolated Silanols under Water as Revealed by Vibrational Sum Frequency Spectroscopy. Phys. Chem. Chem. Phys. 2017, 19, 10343– 10349,  DOI: 10.1039/C7CP01507K
      55
      Silica/water interface: isolated silanols under water as revealed by vibrational sum frequency spectroscopy
      Dalstein, Laetitia; Potapova, Elizaveta; Tyrode, Eric
      Physical Chemistry Chemical Physics (2017), 19 (16), 10343-10349CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      It has been long recognized that the surface chem. of silica, and in particular the type and relative amt. of surface bound silanol groups, plays a crit. role in many of the properties assocd. with the material, where a typical example is the discrepant adsorption behavior obsd. depending on the pretreatment history of the surface. However, in spite of its importance, the direct probing of sp. surface silanol groups under water has been hampered by instrumental limitations. Here we make use of vibrational sum frequency spectroscopy (VSFS) to first, identify under water the OH stretch of isolated surface silanols, and second, explore its acid/base behavior and dependence on the surface pretreatment method. The properties of other types of silanol groups (i.e. hydrogen bonded/geminal) are also inferred from the data. The ability to directly probe these functional groups under water represents a crucial step to further improving our understanding of this widely used mineral oxide.
    56. 56
      Zhuravlev, L. T. Concentration of Hydroxyl Groups on the Surface of Amorphous Silicas. Langmuir 1987, 3, 316– 318,  DOI: 10.1021/la00075a004
      56
      Concentration of hydroxyl groups on the surface of amorphous silicas
      Zhuravlev, L. T.
      Langmuir (1987), 3 (3), 316-18CODEN: LANGD5; ISSN:0743-7463.
      The concn. of hydroxyl groups in amorphous silicas, viz., silica gels, aerosilogels, and porous glasses, which differ in prodn. conditions, surface area, and pore size distribution. was studied. The surface d. of OH groups (the silanol no.) is a physicochem. const. for a fully hydroxylated surface and the d. as a function of temp. of vacuum treatment does not depend in a significant way on the type of silica.
    57. 57
      Pouryousefy, E.; Xie, Q.; Saeedi, A. Effect of Multi-Component Ions Exchange on Low Salinity EOR: Coupled Geochemical Simulation Study. Petroleum 2016, 2, 215– 224,  DOI: 10.1016/j.petlm.2016.05.004
      There is no corresponding record for this reference.
    58. 58
      Matisons, J. G. Silanes and Siloxanes as Coupling Agents to Glass: A Perspective. In Silicone Surface Science; Owen, M. J., Dvornic, P. R., Eds.; Advances in Silicon Science; Springer Netherlands: Dordrecht, 2012; pp 281– 298.
      There is no corresponding record for this reference.
    59. 59
      Wu, J.; Liu, F.; Yang, H.; Xu, S.; Xie, Q.; Zhang, M.; Chen, T.; Hu, G.; Wang, J. Effect of Specific Functional Groups on Oil Adhesion from Mica Substrate: Implications for Low Salinity Effect. J. Ind. Eng. Chem. 2017, 56, 342– 349,  DOI: 10.1016/j.jiec.2017.07.030
      59
      Effect of specific functional groups on oil adhesion from mica substrate: Implications for low salinity effect
      Wu, Jiazhong; Liu, Fanghui; Yang, Hui; Xu, Shijing; Xie, Quan; Zhang, Minghui; Chen, Ting; Hu, Guangxin; Wang, Jinben
      Journal of Industrial and Engineering Chemistry (Amsterdam, Netherlands) (2017), 56 (), 342-349CODEN: JIECFI; ISSN:1226-086X. (Elsevier B.V.)
      Low salinity effect has been in the center of attention as a cost-effective and environmentally friendly technique. Wettability alteration of the oil/brine/mica system appears to be the identified mechanism(s) to trigger the low salinity effect. While the effect of water chem. and minerol. on the wettability of the system has been extensively investigated, few studies have investigated the effect of specific function groups from crude oil on the system wettability, limiting the understanding of how specific functional group contributes to the wettability. We thus exptl. measured the adhesion forces between mica surfaces and functional groups (e.g., C6H5-, CH3-, COOH-, and NH2-) in the presence of different aq. ionic solns. using chem. force microscopy (CFM). Moreover, to understand the contribution of the structural force, the traditional Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was extended (denoted as EDLVO) to fit the force profiles using a Gauss model. Our results showed that the adhesion force between mica and functional groups in a decreasing order was -NH2 > -COOH > -CH3 > -C6H5. We also found that while DLVO forces strongly affected the tip-surface contact due to the interactions among oil/brine/mica interfaces, the structural forces also played an important role in a distance of 1-20 nm due to the presence of H-bonds between COOH-terminated or NH2-terminated tip and mica surface. We therefore conclude that the structural force largely contributes to the adhesion force due to the hydrophilicity or polarity of functional groups, and nucleophilic property (such as Ph group). Our results suggest that the polarity of the crude oil needs to be considered to screen a candidate reservoir for low salinity water flooding projects.
    60. 60
      Schuster, J. M.; Schvezov, C. E.; Rosenberger, M. R. Analysis of the Results of Surface Free Energy Measurement of Ti6Al4V by Different Methods. Procedia Mater. Sci. 2015, 8, 732– 741,  DOI: 10.1016/j.mspro.2015.04.130
      60
      Analysis of the Results of Surface Free Energy Measurement of Ti6Al4V by Different Methods
      Schuster, Jonathan M.; Schvezov, Carlos E.; Rosenberger, Mario R.
      Procedia Materials Science (2015), 8 (), 732-741CODEN: PMSREM; ISSN:2211-8128. (Elsevier B.V.)
      Surface Free Energy (SFE) of solids should be calcd. using theor. models. The contact angle (CA) measurement on a surface is considered the most practical way to obtain the SFE. The results of five methods based in the models are compared: the method of Zisman (ZI), the geometric mean (GM), the harmonic mean (HM), the Lifshitz-van der Waals/ Acid-Base (LW/AB) and the equation of state (ES). The SFE calcd. with GM, HM and LW/AB methods change with the amt. and type of liq. used, however, when water, glycerol and DMSO are used together the SFE and its dispersive and polar components are similar in value for the three methods. In the case of the ES model the values of SFE change with the liq. used; finally using the ZI method the SFE values are 20% lower than the values of SFE obtained with the other methods.
    61. 61
      Thomsen, F. KRUSS Technical Note TN315e. Custom-Made Models: From Contact Angle to Surface Free Energy , December 2008.
      There is no corresponding record for this reference.
    62. 62
      Shigematsu, M.; Morita, M.; Sakata, I.; Sugihara, G. Phase Separation State in Hemicellulose and Lignin Blends Analyzed by Contact Angle Measurements. Macromol. Chem. Phys. 1996, 197, 177– 183,  DOI: 10.1002/macp.1996.021970114
      62
      Phase separation state in hemicellulose and lignin blends analyzed by contact angle measurements
      Shigematsu, Mikiji; Morita, Mitsuhiro; Sakata, Isao
      Macromolecular Chemistry and Physics (1996), 197 (1), 177-83CODEN: MCHPES; ISSN:1022-1352. (Huethig & Wepf)
      The phase sepn. states of hemicellulose (I) and lignin (II) in their blends were investigated by means of contact angle measurements. The hysteresis behavior in dynamic contact angles suggested that a phase sepn. of cluster-state type (not of random dispersion type) took place at I- and II-rich mixing ratios, and it may be caused by the low cohesive force between I and II mols. Further, the total surface tension, and the balance between dispersion and polar forces in the total surface tension were evaluated from the equation of Wu. In the region of a I-rich mixing ratio, the polar and dispersion forces of the surface tension of the solid water decreased and raised, resp., and in contrast to this, in the II-rich region, the former were raised and the latter were decreased. The polymer-polymer interaction parameter of Flory between I and II depends on the mixing ratio as reported previously; this may be ascribed to a difference of the balancing of polar-dispersion forces.
    63. 63
      Liu, M. J.; Tzou, K.; Gregory, R. V. Influence of the Doping Conditions on the Surface Energies of Conducting Polymers. Synth. Met. 1994, 63, 67– 71,  DOI: 10.1016/0379-6779(94)90251-8
      63
      Influence of the doping conditions on the surface energies of conducting polymers
      Liu, M. J.; Tzou, K.; Gregory, R. V.
      Synthetic Metals (1994), 63 (1), 67-71CODEN: SYMEDZ; ISSN:0379-6779.
      The surface energies and polarities of the conducting polymers, polyaniline, polypyrrole and poly(3-hexylthiophene), dedoped and doped, were investigated by the harmonic-mean equation based on contact angle measurements. The values of the surface energies and polarities of dedoped conducting polymers are similar to those of conventional polymers. However, the surface energies of the doped conducting polymers vary greatly depending on the choice of the dopants and doping level. The change in surface energy of conducting polymers may result from the contribution of the polar components, depending on the types and concn. of the dopants, since the dispersion components of the measured surface energies of the examd. conducting polymers do not change significantly from doping to dedoping.
    64. 64
      Tsai, Y.-T.; Choi, C.-H.; Gao, N.; Yang, E.-H. Tunable Wetting Mechanism of Polypyrrole Surfaces and Low-Voltage Droplet Manipulation via Redox. Langmuir 2011, 27, 4249– 4256,  DOI: 10.1021/la104403w
      64
      Tunable Wetting Mechanism of Polypyrrole Surfaces and Low-Voltage Droplet Manipulation via Redox
      Tsai, Yao-Tsan; Choi, Chang-Hwan; Gao, Ning; Yang, Eui-Hyeok
      Langmuir (2011), 27 (7), 4249-4256CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      This paper presents the exptl. results and analyses on a controlled manipulation of liq. droplets upon local redn. and oxidn. (redox) of a smart polymer - dodecylbenzenesulfonate doped polypyrrole (PPy(DBS)). The electrochem. tunable wetting property of PPy(DBS) permitted liq. droplet manipulation at very low voltages (-0.9 to 0.6 V). A dichloromethane (DCM) droplet was flattened upon PPy(DBS) redn. It was found that the surface tension gradient across the droplet contact line induced Marangoni stress, which caused this deformation. Further observation of PPy(DBS)'s color change upon the redox process confirmed that the surface tension gradient was the driving force for the droplet shape change.
    65. 65
      Li, A.-F.; Wang, J.-H.; Wang, F.; Jiang, Y.-B. Anion Complexation and Sensing Using Modified Urea and Thiourea-Based Receptors. Chem. Soc. Rev. 2010, 39, 3729,  DOI: 10.1039/b926160p
      65
      Anion complexation and sensing using modified urea and thiourea-based receptors
      Li, Ai-Fang; Wang, Jin-He; Wang, Fang; Jiang, Yun-Bao
      Chemical Society Reviews (2010), 39 (10), 3729-3745CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This crit. review highlights recent advances in the structurally modified (thio)urea-based receptors for anion complexation and sensing. Modifications of the (thio)urea structure are aimed at a better anion binding in terms of higher binding const., anion selectivity and feasibility. Major (thio)urea receptors are reviewed as N-alkyl, N-aryl and N-amido/N-amino (thio)ureas. Hints for designing (thio)urea-based receptors for anions are discussed (102 refs.).
    66. 66
      Fan, E.; Van Arman, S. A.; Kincaid, S.; Hamilton, A. D. Molecular Recognition: Hydrogen-Bonding Receptors That Function in Highly Competitive Solvents. J. Am. Chem. Soc. 1993, 115, 369– 370,  DOI: 10.1021/ja00054a066
      66
      Molecular recognition: hydrogen-bonding receptors that function in highly competitive solvents
      Fan, Erkang; Van Arman, Scott A.; Kincaid, Scott; Hamilton, Andrew D.
      Journal of the American Chemical Society (1993), 115 (1), 369-70CODEN: JACSAT; ISSN:0002-7863.
      Simple synthetic receptors have been developed that function via directed hydrogen bonding interactions in highly competitive solvents. For example, a mol. contg. two urea sites sepd. by a p-xylylene spacer binds to glutarate derivs. in DMSO via four hydrogen bonds and with an assocn. const. of 6.4 ± 0.4 × 102 M-1. Strong binding of this type in polar solvents may be due to a no. of factors including favorable secondary hydrogen bonding interactions between the carboxylate and urea, the use of charged H-bond acceptors, an inefficient solvation of the closely spaced H-bond donor sites in the urea, and an entropically favorable release of solvent and/or counterion mols. on complex formation. An enhancement of these factors can be achieved in a receptor contg. two alkylguanidinium groups in place of the ureas. This binds very strongly to glutarate even in aq. DMSO. The assocn. const. was >5 × 104 M-1 in neat DMSO, 8.5 ± 1.5 × 103 M-1 in 12% aq. DMSO and 4.8 ± 2.5 × 102 M-1 in 25% aq. DMSO.
    67. 67
      Dormidontova, E. E. Role of Competitive PEO–Water and Water–Water Hydrogen Bonding in Aqueous Solution PEO Behavior. Macromolecules 2002, 35, 987– 1001,  DOI: 10.1021/ma010804e
      67
      Role of Competitive PEO-Water and Water-Water Hydrogen Bonding in Aqueous Solution PEO Behavior
      Dormidontova, Elena E.
      Macromolecules (2002), 35 (3), 987-1001CODEN: MAMOBX; ISSN:0024-9297. (American Chemical Society)
      The behavior of aq. solns. of poly(ethylene oxide) (PEO) is studied theor. by applying a mean field-like approach which includes the effect of the competition of PEO and water as proton acceptors in hydrogen bond formation. Accounting for this effect is of crucial importance for a correct description of all soln. properties. We calc. the temp. and concn. dependence of the av. fraction of hydrogen bonds between PEO and water and find very good agreement between our predictions and exptl. or MD simulation data. We also make predictions concerning the temp. behavior of the second virial coeff. A2 and the effective interaction parameter χeff and compare it with exptl. data. We found that the decrease of A2 with temp. is caused by the delicate balance of the opposing effects of water-PEO and water-water hydrogen bonding. The phase diagram for PEO of different mol. wts. in water is calcd. using exptl. reported data for the energy and entropy of assocn. We achieved very good quant. agreement with most of the exptl. data reported, in particular reproducing the closed loop regions of phase coexistence. We also compare our findings with results of other theor. models.
    68. 68
      Rymuszka, D.; Terpiłowski, K.; Hołysz, L. Influence of Volume Drop on Surface Free Energy of Glass. Ann. UMCS, Chem. 2014, 68, 121– 132,  DOI: 10.2478/umcschem-2013-0010
      There is no corresponding record for this reference.
    69. 69
      Jańczuk, B.; Choma, I.; Dawidowicz, A. L.; Kliszcz, A.; Białopiotrowicz, T. Correlation between the Surface Free Energy of Modified and Non-Modified Glasses with Controlled Porosity and Their Sorption Properties. Chromatographia 1990, 30, 382– 387,  DOI: 10.1007/BF02328502
      69
      Correlation between the surface free energy of modified and nonmodified glasses with controlled porosity and their sorption properties
      Janczuk, B.; Choma, I.; Dawidowicz, A. L.; Kliszcz, A.; Bialopiotrowicz, T.
      Chromatographia (1990), 30 (7-8), 382-7CODEN: CHRGB7; ISSN:0009-5893.
      Contact angles for water and diiodomethane drops were measured on the surface of thermally and chem. (by Carbowax 20M bonding) modified porous glasses and on the surface of naphthalene, di-Ph and anthracene. Using the obtained results to a modified Young equation, dispersion and nondispersion components of the surface free energy of these glasses and org. substances were calcd. The work of adhesion (WA) for benzene, naphthalene, di-Ph, anthracene, nitrobenzene was estd. and correlated with the capacity factors (k') of these substances. On the basis of exptl. and calcd. data it can be stated that the thermal treatment of porous glasses increases their hydrophobicity but the Carbowax layer causes hydrophilicity. There is a linear relationship between k' and WA which can be helpful for predicting retention data of chromatographed substances on the base of surface free energy which can be calcd. from contact angle measurements.
    70. 70
      Yan, R.; Wang, Y.; Duncan, T. V.; Shieh, Y. C. Effect of Polymer and Glass Physicochemical Properties on MS2 Recovery from Food Contact Surfaces. Food Microbiol. 2020, 87, 103354,  DOI: 10.1016/j.fm.2019.103354
      70
      Effect of polymer and glass physicochemical properties on MS2 recovery from food contact surfaces
      Yan R; Wang Y; Duncan T V; Shieh Y C
      Food microbiology (2020), 87 (), 103354 ISSN:.
      Viruses are transmissible via their interaction with contact surfaces of food containers or tools. This study evaluated the recoveries of MS2 coliphage, a virus surrogate, from polypropylene (PP), polyvinyl chloride (PVC), polyethylene (PE), and glass (borosilicate and soda lime), as influenced by the surface chemistry and topography. MS2 (5-6 logs) in PBS with 1% TSB was inoculated onto each of 9 different surfaces, 24-h cold-incubated, and recovery was quantified by infectivity. The order of MS2 recovery efficiency from smooth surfaces was PP > PE ≥ soda lime glass, which classified into 3 ANOVA groups, p = 0.05. The MS2 recovery ratios of smooth vs. rough surfaces were 1.4-1.5. Atomic force microscopy revealed 21-nm diam pinholes (<28-nm of MS2 size) in the borosilicate glass. The lowest and highest MS2 recoveries among the 9 surfaces were demonstrated by the hole-bearing borosilicate glass (34 ± 8%) and smooth PP (69 ± 14%) respectively. Generally greater MS2 recovery was obtained from smooth PP and PE surfaces compared to glass, but topographic alterations (pinholes or increased roughness) decreased recovery possibly by trapping the viruses.

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    • CA values of each of the probe liquids (glycerol, ethylene glycol, formamide, and diiodomethane) on plasma-activated glass, plasma-glass spin-coated with water (glass–water), and plasma-glass spin-coated with DI (glass–DI) and plots of the estimated values of the SFE and its dispersive and polar contributions, as estimated using the OWRK and Wu’s models, from films of PIMa or PIMb processed from water or W–DI (PDF)


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