Absorption spectra of C-60-excited states in various solvents: Their dependence on the ionization potential of solvent molecules

Using a picosecond laser photolysis system, the absorption spectra of excit ed states of C-60 in twelve solvents were measured in the 590-1190 nm wavel ength range. The absorption spectra observed immediately after a 532 nm pic osecond laser excitation and those in the late-time region depended upon th e solvents. In particular, the former dependence was much larger than the l atter one. Their spectral shapes did not correlate with either dielectric c onstant of the solvents or their refractive index, but with their ionizatio n potential (I-p). The early-gated absorption spectra of C-60 in solvents w ith a small I-P were extraordinary broad compared with those in solvents wi th a large I-P. In addition, the absorption changed to those observed in th e late-time region both with a decay time constant of 1.2+/-0.05 ns and wit h keeping the isosbestic points. This time constant was the same as the lif etime of the C-60-singlet excited state (C-1(60)*) reported previously, and the presence of the isosbestic points was also the same as that observed d uring spectral evolution from S-n<--S-1 absorption of Csa to its T-n<--T-1 one. Combining the results of the resonance Raman spectra by Gallagher et a l. with the present ones suggested that the solvent-dependent early-gated a bsorption spectra of the C-60-excited state are ascribed to C-1(60)* with a distortion induced by specific interactions with solvents molecules throug h the donation of their pi-electron density without forming obviously excit ed-state complexes. The transient absorption spectra observed immediately a fter the direct excitation of ground-state charge-transfer complexes formed between C-60 and solvent molecules with a small I-P were not due to ionic species, but to C-1(60)*, suggesting that the photogenerated ionic species undergo a very rapid charge-recombination reaction to result in C-1(60)* an d the ground-state solvent molecule. The absorption spectra in the late-tim e region, which showed a slight dependence on I-P of the solvent molecules, as aforementioned, were assigned to the T-n<--T-1 absorption of C-60; the dependence was considered to be also ascribed to the same reason as that fo r C-1(60)*.