Competition between Intramolecular Electron-Transfer and Energy-Transfer Processes in Photoexcited Azulene−C60Dyad

Abstract

Photoinduced intramolecular processes in a dyad of azulene and C60 (Az−C60) were compared with those of a dyad of naphthalene and C60 (Naph−C60) on the basis of laser flash photolysis experiments. Upon photoexcitation of C60 in the presence of azulene, intermolecular electron transfer proceeded from azulene to the triplet state of C60 (C60(T1)), although the rate constant was small (107 M-1 s-1), because of the small free-energy change for electron transfer via C60(T1). In Az−C60, it was revealed that the S2 state of the Az moiety (Az(S2)−C60(S0)) donates the excited energy to the C60 moiety, effectively generating Az(S0)−C60(S1). In polar solvents, a charge-separated state (Az•+−C60•-) was generated from Az(S0)−C60(S1), from which the S1 state of the Az moiety (Az(S1)−C60(S0)) was also generated by competitive energy transfer. The lifetimes of the charge-separated states were on the order of nanoseconds. Successive energy-transfer processes {Az(S2)−C60(S0) → Az(S0)−C60(Sn), Az(S0)−C60(S1) → Az(S1) −C60(S...