Large Excited-State Conformational Displacements Expedite Triplet Formation in a Small Conjugated Oligomer

Intersystem crossing in conjugated organic molecules is most conveniently viewed from pure electronic perspectives; yet, vibrational displacements may often drive these transitions. We investigate an alkyl-substituted thienylene-vinylene dimer (dTV) displaying efficient triplet formation. Steady-state electronic and Raman spectra display large Stokes shifts (similar to 4000 cm(-1)) involving high-frequency skeletal symmetric stretching modes (similar to 900-1600 cm(-1)) in addition to large displacements of low-frequency torsional motions (similar to 300-340 cm(-1)). Transient absorption spectroscopy reveals the emergence of distorted singlet (S-1) and triplet signatures following initial vibrational relaxation dynamics that dominate spectral dynamics on time scales > 100 ps, with the latter persisting on time scales up to ca. 7 mu s. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S-1. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.