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. By means of the femtosecond fluorescence up-conversion technique, the spectral dependence of the fluorescence transients obtained for BP(OH)(2) in a few sol-gel glasses has been followed. From the temporal behavior of the reconstructed spectra, two concurrent double proton-transfer pathways are concluded to occur in the sol-gel hosts: the first is a concerted double proton-transfer process (within 100 fs after the excitation pulse) and the second is a two-step process involving the reaction 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 TMOS, both as a function of the temperature and as a function of the wavelength of the excitation pump pulses. In the sol-gel TMSPM, the proton-transfer dynamics in photoexcited BP(OH)(2) is faster than the dynamics of the solute in TMOS and also faster than that in liquid solution (as known from our previous work). In the sol-gel TMOS, which contains residual amounts of methanol and water, the proton-transfer dynamics is found to be influenced by the salvation of the BP(OH)(2) solute to the nearby layers of methanal and water. In the two-step process, the ratio of the yields of the monoketo and diketo tautomers is significantly reduced when excitation is to lower excited-state energies. It is discussed that this effect is typical of an energy barrier in the dienol-to-monoketo reaction pathway, the barrier height being more pronounced in the TMSPM glass than in the case of the TMOS glass (similar to 300 cm(-1)). At higher excitation energies, the results for the ratio of the yields of the monoketo and diketo tautomers are suggestive of additional dark processes due to coupling to the rigid sol-gel network.