Excited States of C₇₀ 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 ( 1 C 70 *) and triplet excited state ( 3 C 70 *) in the visible region were observed by time-resolved absorption spectroscopy using a streak camera. The intersystem crossing rate constant k isc from 1 C 70 * to 3 C 70 * was determined to be 1.25 × 10 9 s -1 by the analysis of the absorption−time profiles considering the overlap of the decay of 1 C 70 * and the growth of 3 C 70 *, where the curve fitting was carried out by using the decay rate constant of 1 C 70 * (1.61 × 10 9 s -1 ) determined by the picosecond time-resolved emission spectroscopy. The picosecond time-resolved near-IR spectra of 3 C 70 * were obtained by the pump−probe technique, using a probe beam based on the broad-band optical parametric generation (OPG) of a β-barium borate (BBO) crystal pumped by 532-nm laser pulse. The lowest transition band of the T−T absorption of 3 C 70 * was found in the near-IR region (960 nm) by the new technique.