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 C₇₀ excited states were investigated by some time-resolved spectroscopic techniques. The absorption bands of singlet excited state (¹C₇₀*) and triplet excited state (³C₇₀*) in the visible region were observed by time-resolved absorption spectroscopy using a streak camera. The intersystem crossing rate constant k_isc from ¹C₇₀* to ³C₇₀* was determined to be 1.25 × 10⁹ s^-1 by the analysis of the absorption-time profiles considering the overlap of the decay of ¹C₇₀* and the growth of ³C₇₀*, where the curve fitting was carried out by using the decay rate constant of ¹C₇₀* (1.61 × 10⁹ s^-1) determined by the picosecond time-resolved emission spectroscopy. The picosecond time-resolved near-IR spectra of ³C₇₀* 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 ³C₇₀* was found in the near-IR region (960 nm) by the new technique.