Photoinduced proton transfer in 3-hydroxy-2-naphthoic acid

Spectral and photophysical properties of 3-hydroxy-2-naphthoic acid (3HNA) have been investigated experimentally and theoretically. In addition to its normal fluorescence, 3HNA exhibits a large Stokes-shifted emission that depends on its concentration, the nature of the solvent, pH, temperature and excitation wavelength. 3HNA seems to form different emitting species in different media. The large Stokes shift is attributed to species undergoing excited state intramolecular proton transfer (ESIPT). Ab initio calculations using configuration interaction (single excitation) reveal a single minimum in the potential energy profile corresponding to the primary form in the ground state. While semi-empirical calculations with CI (AM1/PECI = 8) predict a double well potential, single point density functional theoretic calculations (B3LYP/6-31G**) confirm the absence of a barrier in the ground state for proton transfer. Ln the first excited singlet state, however, there are two minima corresponding to the primary and tautomeric forms at both ab initio CIS and AM1/PECI = 8 levels, thus accounting for the dual emission in 3HNA. The theoretical methods also account for the observed pH dependence of the spectral characteristics qualitatively correctly. (C) 2001 Elsevier Science B.V. All rights reserved.