The design of a novel fluorescent PET sensor for proton and water: A phenylaminonaphtho[1,2-d]oxazol-2-yl-type fluorophore containing proton donor and acceptor groups

Novel phenylaminonaphtho[1,2-d]oxazol-2-yl-type fluorophores having a diethylamino group as proton binding site and a carboxyl group as proton donating site, for sensing protons and water, have been designed and developed, and their photophysical properties were investigated in solution. In 1,4-dioxane, fluorophores that did not contain a proton binding site exhibited an intense fluorescence band whereas fluorophores that contained a proton binding site displayed only a weak fluorescence band. However, this behavior was reversed when the two types of fluorophore were dissolved in aq. acetic acid solution. The fluorophore that contained both a proton binding site and proton donating site showed weak fluorescence in organic solvents, but very intense fluorescence accompanied an increase in the water content of such solvents. Semi-empirical molecular orbital calculations (AM1 and CNDO/S) and spectral analyses revealed that such fluorophores are capable of sensing protons or water by photo-induced electron transfer.