Highly pi-conjugated poly(3-hexylthiophene) (P3HT) shows an effective formation of polaron, which is critical in organic photovoltaic performance. In our theoretical and experimental studies, this polaron generation can be stimulated by a highly oscillating vibrational motion of C=C. We exploited excited-state femtosecond stimulated Raman spectroscopy (FSRS) and found the two prominent Raman peaks at ~1450 cm-1(C=C stretching) and 1350 cm-1 (C-C stretching) which are ~20 cm-1 red-shifted from ground-state Raman peak locations. This red-shift dynamics is a pure excited-state reaction which occurs within 5 picoseconds after the excitation. Using density functional theory, we have correlated the redshifts to asymmetric or half-symmetric C=C and C-C stretches, not the entire symmetric stretching modes vibrating along the whole chain. Additionally, we also have found the elongated C=C motion can effectively increase the electron density localization and shortened C=C does vice versa. Thus, this motion can effectively initiate the formation of intramolecular polarons, supporting that the nuclear wave packet motions and electron densities are strongly correlated in P3HT.
In addition, we have studied vibrational Raman modes and depolarization ratios of annealed and non-annealed P3HT films to account for the structural dependence on intermolecular interactions.