Femtosecond investigations of photophysics of ultrafast intersystem crossing in terthiophene by wavelength dependent excitation

Ultrafast intersystem crossing (ISC) and triplet generation in terthiophene (3T) was investigated by femtosecond time-resolved spectroscopy in dependence of excitation wavelengths. Triplet yield was monitored by examining transient spectra of the triplet absorption band. We find that triplets are generated by different processes: (i) ISC starting from the lowest vibrational S1 state and (ii) ultrafast ISC from an intermediate state. We discuss the probable formation mechanism and the nature of the intermediate state. The results suggest that a planarisation process of excited twisted conformers, which proceeds on a 2 ps time scale, is responsible for the highly effective triplet generation.