Photophysical properties of C₆₀H₁₈ and C₆₀H₃₆: a laser flash photolysis and pulse radiolysis study

Abstract

Spectroscopic and photophysical properties of two hydrogenated fullerenes, namely, C₆₀H₁₈ and C₆₀H₃₆, have been studied in benzene solution using laser flash photolysis and pulse radiolysis techniques. Samples of C₆₀H₁₈ and C₆₀H₃₆ used were prepared by the reduction of C₆₀ with Zn/concentrated HCl in benzene or toluene at normal (for C₆₀H₃₆) or at high (for C₆₀H₁₈) temperature and pressure. It is reported that C₆₀H₁₈ and C₆₀H₃₆ prepared by these methods have tetrahedral (T) symmetry containing four highly delocalized benzenoid rings, and a crown structure with C_3v symmetry, respectively. Owing to hydrogenation, the fluorescence band of C₆₀ shifts toward the blue and both the fluorescence quantum yields and the singlet-state (S₁) lifetimes of the more hydrogenated species increase compared with those of the parent fullerene molecule. Triplet quantum yields reduce considerably (e.g., φ _T for C₆₀H₁₈ and C₆₀H₃₆ is 0.15 and 0.1, respectively) compared with unity for C₆₀. Also the singlet-singlet and triplet-triplet absorption spectra show considerable differences from those of C₆₀. Owing to successive hydrogenation of C₆₀, the intensity of the near-IR band (at ca. 880 nm) in the singlet-singlet absorption spectra decreases and that in the visible (at ca. 500 nm) becomes more prominent. In the triplet-triplet absorption spectrum, the major band, which is seen at 740 nm for C₆₀, gradually shifts to the blue in the more hydrogenated species. The spectroscopic properties of the hydrogenated fullerenes used by us have been seen to be different in many aspects compared with those reported by Bensasson et al. (Chem. Phys. 1997, 215, 111), possibly owing to the different symmetries of these derivatives prepared by different hydrogenation methods.