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

Photoinduced intramolecular processes in a dyad of azulene and C 60 (Az−C 60 ) were compared with those of a dyad of naphthalene and C 60 (Naph−C 60 ) on the basis of laser flash photolysis experiments. Upon photoexcitation of C 60 in the presence of azulene, intermolecular electron transfer proceeded from azulene to the triplet state of C 60 (C 60 (T 1 )), although the rate constant was small (10 7 M -1 s -1 ), because of the small free-energy change for electron transfer via C 60 (T 1 ). In Az−C 60, it was revealed that the S 2 state of the Az moiety (Az(S 2 )−C 60 (S 0 )) donates the excited energy to the C 60 moiety, effectively generating Az(S 0 )−C 60 (S 1 ). In polar solvents, a charge-separated state (Az •+ −C 60 •- ) was generated from Az(S 0 )−C 60 (S 1 ), from which the S 1 state of the Az moiety (Az(S 1 )−C 60 (S 0 )) 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(S 2 )−C 60 (S 0 ) → Az(S 0 )−C 60 (S n ), Az(S 0 )−C 60 (S 1 ) → Az(S 1 ) −C 60 (S 0 ), where n ≥ 2} demonstrate that the multiple energy transfer is achieved in a simple dyad molecule. On the other hand, Naph−C 60 dyad did not show charge separation upon excitation of the C 60 moiety, but deactivated via intersystem crossing, generating almost quantitatively the C 60 (T 1 ) 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.