Ultrafast dynamics of photogenerated styrylpyridinyl radical and its dimerradical cation with styrylpyridinium cation studied by femtosecond laser flash photolysis

Ultrafast photoinduced electron transfer reaction and dimer radical cation formation have been studied at room temperature in less polar solvents by f emtosecond laser flash photolysis of ion-pair charge-transfer complexes bet ween a styrylpyridinium cation and a tetraphenylborate anion. In addition t o the visible absorption due to the styrylpyridinyl radicals, the charge re sonance absorption band (CR band) due to the electronic interaction between the photogenerated styrylpyridinyl radical and the styrylpyridinium cation was observed in the near infrared region. Transient absorptions of both ra dicals and the CR band showed very rapid rise in less than 1 ps, which was limited by the time resolution of the flash photolysis system. No interacti on of styrylpyridinium cations was observed before photoexcitation. These r esults strongly suggested that the electronic interaction between the styry lpyridinium ion and its reduced form occurred without a diffusion process i mmediately after photoinduced electron transfer. From the decay dynamics, t he dimer radical cation was found to show a faster back electron transfer t han the radical itself.