Absorption Spectra of C60-Excited States in Various Solvents: Their Dependence on the Ionization Potential of Solvent Molecules

Abstract

Using a picosecond laser photolysis system, the absorption spectra of excited states of C₆₀ in twelve solvents were measured in the 590—1190 nm wavelength range. The absorption spectra observed immediately after a 532 nm picosecond laser excitation and those in the late-time region depended upon the solvents. In particular, the former dependence was much larger than the latter one. Their spectral shapes did not correlate with either dielectric constant of the solvents or their refractive index, but with their ionization potential (I_p). The early-gated absorption spectra of C₆₀ in solvents with a small I_p were extraordinary broad compared with those in solvents with a large I_p. In addition, the absorption changed to those observed in the late-time region both with a decay time constant of 1.2±0.05 ns and with keeping the isosbestic points. This time constant was the same as the lifetime of the C₆₀-singlet excited state (¹C₆₀^*) reported previously, and the presence of the isosbestic points was also the same as that observed during spectral evolution from S_n←S₁ absorption of C₆₀ to its T_n←T₁ one. Combining the results of the resonance Raman spectra by Gallagher et al. with the present ones suggested that the solvent-dependent early-gated absorption spectra of the C₆₀-excited state are ascribed to ¹C₆₀^* with a distortion induced by specific interactions with solvents molecules through the donation of their π-electron density without forming obviously excited-state complexes. The transient absorption spectra observed immediately after the direct excitation of ground-state charge-transfer complexes formed between C₆₀ and solvent molecules with a small I_p were not due to ionic species, but to ¹C₆₀^*, suggesting that the photogenerated ionic species undergo a very rapid charge-recombination reaction to result in ¹C₆₀^* and the ground-state solvent molecule. The absorption spectra in the late-time region, which showed a slight dependence on I_p of the solvent molecules, as aforementioned, were assigned to the T_n←T₁ absorption of C₆₀; the dependence was considered to be also ascribed to the same reason as that for ¹C₆₀^*.