Polaronic states in inherently chiral polythiophenes revealed by circular dichroism spectroelectrochemistry and DFT calculations

Conducting polymers with a helical twist have attracted the attention of researchers, since they combine two apparently conflicting characteristics, namely extended conjugation, which favors conduction, and winding conformation, which confers the system the possibility of preferred interaction with biomolecules or of second order susceptibility, due to non centrosymmetric condition. We have obtained films based on inherently chiral 2,2’-Bis(2,2’-bithiophene-5-yl)-3,3’-bithianaphthene (T4-BT2) and recorded their Circular Dichroism (CD) and UV absorption spectra, during a full voltammetric cycle. Besides the bisignate CD features in the UV range recorded at zero potential, a few features extending into the visible and NIR range are observed at progressively higher potential values. Time Dependent Density Functional Theory (TD-DFT) calculations on the monomer, dimer, trimer and tetramer of T4-BT2, both in the neutral and charged state, allow to monitor the conformational properties and to describe the electronic transitions involved in the spectroscopic signals. The CD and UV spectra result interpreted by these calculations on a quantitative basis, beyond a simple molecular exciton interpretation. The description of the charged vs. neutral species emerging from the DFT calculations, in terms of structures and electronic states, may provide hints on the conduction mechanism and sensing of analytes by devices based on T4-BT2.