Femtosecond time-resolved measurements of terthiophene: fast singlet–triplet intersystem crossing

Femtosecond time-resolved studies using pump–probe spectroscopy were performed on 2,2′:5′,2″-terthiophene in solution. Optical density changes ΔD were observed in the bleaching region at different excitation wavelengths. The decay curves could be described using a bi-exponential fitting procedure. A fast decay time of 2 ps and a slower one varying with excitation wavelengths (400 nm: 126 ps; 385 nm: 74 ps; 374 nm: 51 ps) were determined. To explain this time behavior we suggest a model with two channels for intersystem crossing starting from different vibronic states in the singlet system into suitable states of the triplet manifold. Experimental data, as well as the applied model, indicate that we observed a fast two-channel intersystem crossing process.