Abstract
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.
Highlights
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
In ionic liquids (ILs) containing the imidazolium ring, dispersion and π−π interactions compete with hydrogen bonding (Hbonding), which in part determines the structuring in the IL.[5]
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 5-doxyl-stearic acid (5DSA) in these systems because it only solubilizes into the hydrophobic region formed by polymer aggregates
Summary
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 possess the capability of coordination through electrostatic forces and hydrogen bonding (H-bonding) either with solvents, helping in solubilization, or with other dissolved molecules.[1−3] The ionic groups in the polymers introduce ion−dipole and ion− ion forces.[4]. 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 ( known as π+) has to be taken into account. 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+)
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