Excited-state intermolecular proton transfer of lumazine

Time-resolved and steady-state emission spectroscopies, as well as absorption spectroscopy, were employed to study the protolytic reactivity of lumazine in both the ground and the excited states. We found that the luminescence of lumazine consists of two emission bands with their respective band maxima at 375 and 465 nm. We attributed these emission bands to the protonated (375 nm) and the deprotonated (465 nm) forms. The protonated form fluoresces with a short decay time of 35 ps and fits the rise-time of the deprotonated form's fluorescence. A study of the temperature dependence of the time-resolved emission of the protonated band shows that in the wide temperature range of 80-300 K the decay time is only slightly affected by the temperature. On the basis of the experimental results, we propose that an excited-state proton transfer process takes place in lumazine. The weak temperature dependence is explained by an efficient nonradiative decay rather than by a weak temperature dependence of the proton transfer rate.