Influence of Side Chain Composition and Polarity of the Environment on the Electrochemical Doping Mechanism in Poly(3-hexylthiolhene) and Dioxythiophene Derivatives

We address the nature of electrochemically induced polarons in conjugated polymers and their evolution as a function of electrochemical potential. We investigate the photo-physical mechanism driving their dynamics and coupling to the local environment by means of transient absorption and Raman spectroscopies synergistically performed in situ throughout the electrochemical doping process. We employ two prototypical system to benchmark the experimental observations -- one is an oligoether-functionalized 3,4-propylenedioxythiophene (ProDOT) copolymer and the other is a poly(3-hexylthiolhene) derivative, where the alkyl sidechains have been terminated by a hydroxyl group. The changes embedded in both linear and transient absorption features show that while in the case of the ProDOT copolymer the neutral transition and the polaron have distinct ground states, in the case of the P3HT derivatives the ground state is shared between these two species. In the case of the ProDOT copolymer we also identify a precursor electronic state with charge-transfer (CT) character that precedes polaron formation and bulk electronic conductivity and it is instead absent in the case of the P3HT derivative. These marked differences can be rationalized in the context of a very different polar environment pertaining to these two classes of polymers, where the P3HT derivatives show a lower polarity with respect to the ProDOT family. This work provides insight into the energetic landscape of a heterogeneous polymer-electrolyte system and demonstrates how such coupling depends on environmental parameters, such as polymer structure, electrolyte composition, and environmental polarity.