Relaxation processes in the excited state of C60–o-quinodimethane adducts

Abstract

Intramolecular electron transfer and exciplex formation in a C₆₀ adduct possessing an N,N-dimethylaniline (DMA) moiety, C₆₀–DMA, in cyclohexane and benzonitrile have been investigated in comparison with the intermolecular reactions between C₆₀–o-quinodimethane (OQD) and DMA. The fluorescence of C₆₀–DMA in cyclohexane was little quenched by the DMA moiety and was almost the same as that of C₆₀–OQD, in contrast with the decrease and red shift in the fluorescence of C₆₀–OQD in the presence of excess DMA in cyclohexane, mainly resulting from exciplex formation. The difficulties in the intramolecular exciplex formation of C₆₀–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 C₆₀–DMA was significantly quenched and red shifted. The quenching is due to the intramolecular electron transfer from the DMA moiety to the S₁ state of the C₆₀ moiety, where the rate constant was estimated as 1.5 × 10¹⁰ s^–1. The red shift is brought about by the charge-transfer character in the S₁ state, resulting from the through-bond interaction between the two chromophores. The transient absorption spectra of C₆₀–DMA in cyclohexane exhibited the T_nâ†� T₁ bands, in good agreement with those of C₆₀–OQD without a dimethylamino group. In benzonitrile, however, the T_nâ†� T₁ bands were considerably weakened by the decrease in the triplet population, and the lifetime (7 µs) of T₁ was reduced relative to that in cyclohexane (28 µs). The decrease in the lifetime in benzonitrile can be ascribed to the weak charge-transfer interaction between the T₁ of the C₆₀ moiety and the S₀ of the DMA moiety.