Insights into the Structural and Vibrational Properties of Polaron in Doped Poly(3-alkyl-thiophene), P3HT

DFT-based investigation is here reported for a wide series of oligo-thiophenes, considered as effective molecular models to study the polaron vibrational dynamics of doped poly(3-alkyl-thiophene), P3HT. DFT calculations capture the nuclear relaxations occurring upon charging the polymer, namely, a structural reorganization from an aromatic- to a quinoid-like form. Polaron self-confinement, described at different DFT levels, extends over circa four thiophene units, in accordance with recent experimental findings. A complete rationalization of the doping-induced IR active vibrations (IRAVs) of P3HT is achieved through the analysis of both the IR local electro-optical parameters and the eigenvectors of the vibrational normal modes. The vibrational analysis distinctively unveils the IRAVs activation mechanism and intensity pattern of doped P3HT. IRAV normal modes exhibit collective character, resembling the effective conjugation coordinate (ECC) normal mode that dominates the Raman spectrum of pristine (undoped) P3HT. DFT calculations on model oligomers bearing lateral alkyl substituents provide a one-to-one correspondence with the experimental IRAV bands, thus allowing a full assignment of the IR spectrum of doped P3HT.