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 S-1 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. (C) 2000 Elsevier Science B.V. All rights reserved.