Relaxation processes in the excited state of C-60-o-quinodimethane adducts

Intramolecular electron transfer and exciplex formation in a C-60 adduct possessing an N,N-dimethylaniline (DMA) moiety, C-60-DMA, in cyclohexane and benzonitrile have been investigated in comparison with the intermolecular reactions between C-60-o-quinodimethane (OQD) and DMA. The fluorescence of C-60-DMA in cyclohexane was little quenched by the DMA moiety and was almost the same as that of C-60-OQD, in contrast with the decrease and red shift in the fluorescence of C-60-OQD in the presence of excess DMA in cyclohexane, mainly resulting from exciplex formation. The difficulties in the intramolecular exciplex formation of C-60-DMA in cyclohexane are considered to result from its unfavourable molecular structure, which also affects charge-transfer complex formation. On the other hand, in benzonitrile the fluorescence of C-60-DMA was significantly quenched and red shifted. The quenching is due to the intramolecular electron transfer from the DMA moiety to the S-1 state of the C-60 moiety, where the rate constant was estimated as 1.5 x 10(10) s(-1). The red shift is brought about by the charge-transfer character in the S-1 state, resulting from the through-bond interaction between the two chromophores. The transient absorption spectra of C-60-DMA in cyclohexane exhibited the T-n<--T-1 bands, in good agreement with those of C-60-OQD without a dimethylamino group. In benzonitrile, however, the T-n<--T-1 bands were considerably weakened by the decrease in the triplet population, and the lifetime (7 mu s) of T-1 was reduced relative to that in cyclohexane (28 mu s). The decrease in the lifetime in benzonitrile can be ascribed to the weak charge-transfer interaction between the T-1 of the C-60 moiety and the S-0 of the DMA moiety.