Exciton Conformational Dynamics of Poly(3-hexylthiophene) (P3HT) in Solution from Time-Resolved Resonant-Raman Spectroscopy.

We have used time-resolved resonant-Raman spectroscopy to investigate the picosecond conformational relaxation of regioregular poly(3-hexylthiophene) (RR-P3HT) in chlorobenzene after 510 nm photoexcitation. Vibrational signatures from modes along and peripheral to the exciton's backbone have been identified according to the time dependence of excited-state Raman features and from comparisons to Raman spectra of other polymer states. Measured spectral dynamics reflect initial changes in the resonance enhancement of backbone modes on a time scale of 9 ± 1 ps. In contrast, contributions from peripheral modes exhibit time-dependent decay determined only by exciton intersystem-crossing kinetics. Spectral dynamics are interpreted in terms of evolution in bond lengths along the exciton's backbone resulting from increased conjugation allowed by torsional reordering. Possible origins of peripheral features are discussed, including distorted inter-ring modes at exciton termini. Findings provide a glimpse of the underlying molecular dynamics responsible for the red shift in the exciton's near-IR transient absorption occurring on the same time scale.