Abstract
The electronic properties of conducting polymers are influenced by their micro- and macrostructural orders, which can be tailored by substituent modification. However, while the effect of substituents on conducting polymers is extensively investigated, chiral substituents are far less studied. Furthermore, many chiral conducting polymers have regioirregular structures, which result in polymer films with inferior properties. In this work, we apply electronic circular dichroism (ECD) spectroscopy to study the morphological changes to the chiral polymers under different polymerization conditions. For this purpose, we investigated 3,4-ethylenedioxythiophene (EDOT) derivatives having two stereogenic centers on each monomer and bearing methyl or phenyl side groups (dimethyl-EDOT and diphenyl-EDOT, respectively). Polymerizing the enantiomerically pure monomers produces regioregular and stereoregular dimethyl-PEDOT and diphenyl-PEDOT, respectively. The effect of the electrolyte and solvent on polymer film morphology was studied using scanning electron microscopy (SEM) and ECD, showing a correlation between the polymer’s morphology and the chiroptical properties of its films. We found that, for diphenyl-PEDOT, the combination of perchlorate anion electrolyte and acetonitrile solvent resulted in a unique morphology characterized by significant intermolecular interactions. These interactions were clearly observable in the ECD spectra in the form of exciton couplings, whose presence was supported by TD-DFT calculations. A small enantiomeric excess was sufficient to induce very intense ECD signals, demonstrating chiral amplification in electropolymerized films.
Highlights
Chiral conducting polymers have numerous applications, including chiral sensors and catalysts, chemical separation materials, and spintronic devices.[1−5] Among conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most extensively studied because of the stability of its doped and undoped states, the ease with which it is structurally modified, and its processability.[6−8] The disubstitution of the monomer at both β-positions reduces its tendency to engage in cross-linking during the polymerization process.[7,9] Properties such as solubility, band gap, and redox potentials can be tailored via substitution with additional side groups.[10]
We investigated the applicability of electronic circular dichroism (ECD) as an analytical tool for the characterization of film morphology
We found a direct correlation between the ECD spectra, representing arrangements at the supramolecular level, and the film morphology observed in scanning electron microscopy (SEM) images
Summary
Chiral conducting polymers have numerous applications, including chiral sensors and catalysts, chemical separation materials, and spintronic devices.[1−5] Among conducting polymers, poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most extensively studied because of the stability of its doped and undoped states, the ease with which it is structurally modified, and its processability.[6−8] The disubstitution of the monomer at both β-positions reduces its tendency to engage in cross-linking during the polymerization process.[7,9] Properties such as solubility, band gap, and redox potentials can be tailored via substitution with additional side groups.[10]. One explanation for this observation is chiral amplification, or the “majority rule”, which was observed by Meijer’s group for chemically polymerized polythiophenes.[24] To study this phenomenon further, we electropolymerized solutions containing both S,S-1 and R,R-1 with different enantiomer excesses. This finding indicates that chiral amplification, which was previously observed for chemically polymerized films, can occur during electropolymerization
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