Femtosecond investigation of photophysics of intersystem crossing in terthiophene

Photophysics of ultrafast intersystem crossing and triplet yield in 2,2':5' ,2 "-terthiophene (3T) was investigated in dependence of excitation wavelen gth by means of femtosecond time-resolved absorption/emission spectroscopy. Triplet generation yield was monitored by using the product of maximum and width of the photoinduced triplet absorption band near 460 nm. We found th at triplets were generated by two different processes: (i) the well-known i ntersystem crossing from the lowest vibrational S-1 state and (ii) ultrafas t intersystem crossing from an intermediate state generated by photoexcitat ion. We discuss the probable formation mechanism and the lifetime of an int ermediate state. The results suggest that this intermediate state is a nonp lanar conformer within the S-1 manifold; it seems to be responsible for the high effective triplet generation.