Intermolecular photoinduced proton transfer in nile blue and oxazine 720

The S1 excited state lifetimes of laser dyes nile blue (NB) and oxazine 720 (OX) have been determined in acidic aqueous solutions as functions of pH. The decay constant show a linear dependence on [H+], with an identical slope for both molecules. The reason for the faster decay in acidic solutions results from the formation of dications by reaction of excited state monocations with hydrogen ions. These results suggest that in the excited state reaction the additional proton locates on the ring nitrogen atom rather than on the terminal amino groups.