Controlling electronic states and photochemical reactivities of C₆₀ by chemical functionalization: Ortho-substituent effect of 1,6-N-(phenyl)aza-[60]fulleroids and 1,2-(N-phenyl)aziridino-[60]fullerenes

The reaction rate of the photochemical rearrangement 1,6-N-(2-substituted)aza-[60]fulleroids (1) → 1,2-(N-substituted)aziridino-[60]fullerenes (2) differed ca. 3.1-fold as the ortho-substituent of the N-phenyl group changed from 2-methylphenyl (b), 2-ethylphenyl (c), to 2-iso-propylphenyl (d). The retardation of the rearrangement is due to the contribution of charge-separated triplet state in the excited states in addition to the normal triplet state, which was confirmed by transient absorption measurements. A semi-empirical calculations on the ground state conformation of 1 and 2 showed that 1 with phenyl (a) and 2,6-dimethylphenyl (e) N-substituents and 2a,e have C_{2 v} symmetry but the rotation of the phenyl group along the N-Ph bond occurs with the increase of steric hindrance between the phenyl group and C₆₀ moiety (1b-d and 2b-d). These results indicate that the rate of the rearrangement, i.e., the nature of the excited states, is probably controlled by the conformation of the molecules at their excited states rather than that of their ground states. The generation of charge-separated triplet state in the excited state can be explained by the occurrence of phenyl group rotation along the N-Ph bond.