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 (∼4000 cm^-1) involving high-frequency skeletal symmetric stretching modes (∼900-1600 cm^-1) in addition to large displacements of low-frequency torsional motions (∼300-340 cm^-1). Transient absorption spectroscopy reveals the emergence of distorted singlet (S₁) 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 μs. Potential energy surfaces calculated along the dominant displaced out-of-plane torsional mode reveal shallow energy barriers for entering the triplet manifold from S₁. We propose that dTV is a good model system for understanding vibrational contributions to intersystem crossing events in related polymer systems.