FEMTOSECOND DOUBLE PROTON-TRANSFER DYNAMICS IN [2,2'-BIPYRIDYL]-3,3'-DIOL IN SOL-GEL GLASSES

Intramolecular excited state double proton-transfer dynamics has been studied for [2,2'-bipyridyl]-3,3'-diol (BP(OH)(2)) in sol-gel glass. B y means of the femtosecond fluorescence up-conversion technique, the s pectral dependence of the fluorescence transients obtained for BP(OH)( 2) in a few sol-gel glasses has been followed. From the temporal behav ior of the reconstructed spectra, two concurrent double proton-transfe r pathways are concluded to occur in the sol-gel hosts: the first is a concerted double proton-transfer process (within 100 fs after the exc itation pulse) and the second is a two-step process involving the reac tion from the excited dienol-to-monoketo tautomer (<100 fs), followed by the monoketo-to-diketo step (with a time constant of a few ps). The proton-transfer dynamics has been studied in the sol-gels TMSPM and T MOS, both as a function of the temperature and as a function of the wa velength of the excitation pump pulses. In the sol-gel TMSPM, the prot on-transfer dynamics in photoexcited BP(OH)(2) is faster than the dyna mics of the solute in TMOS and also faster than that in liquid solutio n (as known from our previous work). In the sol-gel TMOS, which contai ns residual amounts of methanol and water, the proton-transfer dynamic s is found to be influenced by the salvation of the BP(OH)(2) solute t o the nearby layers of methanal and water. In the two-step process, th e ratio of the yields of the monoketo and diketo tautomers is signific antly reduced when excitation is to lower excited-state energies. It i s discussed that this effect is typical of an energy barrier in the di enol-to-monoketo reaction pathway, the barrier height being more prono unced in the TMSPM glass than in the case of the TMOS glass (similar t o 300 cm(-1)). At higher excitation energies, the results for the rati o of the yields of the monoketo and diketo tautomers are suggestive of additional dark processes due to coupling to the rigid sol-gel networ k.