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 de pends on its concentration, the nature of the solvent, pH, temperature and excitation wavelength. 3HNA seems to form different emitting species in dif ferent media. The large Stokes shift is attributed to species undergoing ex cited 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 g round state. While semi-empirical calculations with CI (AM1/PECI = 8) predi ct a double well potential, single point density functional theoretic calcu lations (B3LYP/6-31G**) confirm the absence of a barrier in the ground stat e for proton transfer. Ln the first excited singlet state, however, there a re 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 i n 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.