Quantum chemical studies on excited state intermolecular proton transfer of oxazine dyes

The excited state lifetimes of oxazine dyes nile blue and oxazine 720 show a significant solvent dependence. As an aid to the interpretation of the de cay in acidic environments the ground electronic states and the first singl et excited states of oxazine monocations and dications have been studied by methods of quantum chemistry. Ab initio calculations carried out on the lo wer members on this series show a significant increase of the electron dens ities of the ring nitrogen on excitation. The calculated electron energies also suggest that the second proton attaches to the ring nitrogen in the ex cited states, in contrast to the ground states where tin the case of oxazin e 720) the second proton attaches to the terminal nitrogen. A kinetic scheme is proposed for the decay process in which in acidic envir onments the main deactivation path is the reaction of excited state monocat ions with protons, followed by internal conversion and subsequent deprotona tion of the ground state dications. (C) 2000 Elsevier Science B.V. All righ ts reserved.