Excited states of C-70 and the intersystem crossing process studied by picosecond time-resolved spectroscopy in the visible and near-IR region

The photophysical properties of C-70 excited states were investigated by some time-resolved spectroscopic techniques. The absorption bands of singlet excited state (C-1(70)*) and triplet excited state (C-3(70)*) in the visible region were observed by time-resolved absorption spectroscopy using a streak camera. The intersystem crossing rate constant k(isc) from C-1(70)* to C-3(70)* was determined to be 1.25 x 10(9) s(-1) by the analysis of the absorption-time profiles considering the overlap of the decay of C-1(70)* and the growth of C-3(70)*, where the curve fitting was carried out by using the decay rate constant of C-1(70)* (1.61 x 10(9) s(-1)) determined by the picosecond time-resolved emission spectroscopy. The picosecond time-resolved near-IR spectra of C-3(70)* were obtained by the pump-probe technique, using a probe beam based on the broad-band optical parametric generation (OPG) of a beta-barium berate (BBO) crystal pumped by 532-nm laser pulse. The lowest transition band of the T-T absorption of C-3(70)* was found in the near-IR region (960 nm) by the new technique.