Competition between Intramolecular Electron-Transfer and Energy-Transfer Processes in Photoexcited Azulene-C₆₀ Dyad

Photoinduced intramolecular processes in a dyad of azulene and C ₆₀ (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 C ₆₀(T₁). In Az-C₆₀, it was revealed that the S₂ state of the Az moiety (Az(S₂)-C₆₀(S ₀)) 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(S ₁), from which the Si state of the Az moiety (Az(S ₁)-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)-C ₆₀(S₀) → Az(S₀)-C₆₀(S _n), Az(S₁)-C₆₀(S₀) → Az(S ₁)-C₆₀(S₀), where n ≥ 2} demonstrate that the multiple energy transfer is achieved in a simple dyad molecule. On the other hand, Naph-C₆₀ dyad did not show charge separation upon excitation of the C₆₀ moiety, but deactivated via intersystem crossing, generating almost quantitatively the C₆₀(T₁) moiety. These findings indicate the favorable donor ability of azulene compared to that of naphthalene, even though both azulene and naphthalene have the same 10-π-electron system.