Ion pair formation ia photoinduced proton transfer in excited hydroxynaphthalimide-N-methylimidazole hydrogen bonded complex: effect of temperature and viscosity on dual fluorescence

Abstract

The kinetics of photoinduced processes in the hydrogen bonded complex between N-methyl-3-hydroxynaphthalimide and 1-methylimidazole was studied in a wide temperature range in ethyl acetate and glycerol triacetate. The proton transfer within the excited complex was found to be very fast because of its negligible activation energy. The fairly intense dual fluorescence was assigned to hydrogen bonded and solvent separated ion pairs. Kinetic parameters for the various deactivation pathways of these excited species were derived from the combined analysis of the steady-state and the time-resolved fluorescence results. The Arrhenius pre-exponential factor of the transition from the solvent separated into the hydrogen bonded ion pair proved to be more than two orders of magnitude larger in glycerol triacetate compared with that in ethyl acetate, whereas the other processes showed less viscosity dependence. The radiationless energy dissipation rate of the hydrogen bonded ion pair was insensitive to the experimental conditions. However, thermal enhanced internal conversion was observed for the solvent separated ion pair.