Excited state prototropic activities in 2-hydroxy 1-naphthaldehyde

Excited state proton transfer in 2-hydroxy 1-naphthaldehyde (HNL) has been reported on the basis of steady state absorption and emission, time-resolved emission, and semiempirical quantum chemical calculations. The existence of open and closed ring conformers in the ground state gives rise to different emissions upon excitation in hydroxylic and hydrocarbon solvents, respectively. Formation of the ground state HNL ion in open conformer was observed with addition of base. Excitation of the closed conformer leads to intramolecular proton transfer across the preexisting intramolecular hydrogen bond to give excited zwitterion in hydrocarbon solvents. On excitation of the open ring conformer in hydroxylic solvents, emission from the neutral excited state could be observed for the first time apart from the emission of ionic intermolecular proton transfer species. An increase or decrease in pH results in enhancement or a decrease of emission from the ionic conformer with parallel dwindling or intensification of emission of the neutral species. Phosphorescence in both the solvents arises from the open conformer.