Relation between Morphology and Chiroptical Properties in Chiral Conducting Polymer Films: A Case Study in Chiral PEDOT

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.