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

Abstract

Ultrafast photoinduced electron transfer reaction and dimer radical cation formation have been studied at room temperature in less polar solvents by femtosecond laser flash photolysis of ion-pair charge-transfer complexes between a styrylpyridinium cation and a tetraphenylborate anion. In addition to the visible absorption due to the styrylpyridinyl radicals, the charge resonance 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 radicals 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 interaction of styrylpyridinium cations was observed before photoexcitation. These results strongly suggested that the electronic interaction between the styrylpyridinium ion and its reduced form occurred without a diffusion process immediately after photoinduced electron transfer. From the decay dynamics, the dimer radical cation was found to show a faster back electron transfer than the radical itself.