Photoinduced proton transfer in 3-methyl-6-hydroxy-m-phthalic acid

The ground and excited state photophysics of 3-methyl-6-hydroxy-m-phthalic acid (HmPA) has been studied in homogeneous solvents by steady state, nanos econd transient emission spectroscopy and semiemperical AM1 calculations. H mPA shows Variations in steady-state spectra depending on the solvent chara cteristics. A large Stokes-shifted emission in the nonpolar solvent (simila r to 9500 cm(-1), lambda(a) = 325 nm and lambda(f) = 470 nm) indicates that on UV excitation the keto form of HmPA undergoes intramolecular proton tra nsfer (ESIPT) leading to tautomerization in the singlet excited state. This particular emission (type-I emission) attributed to the enol tautomer of H mPA, is characterized by a very low emission quantum yield (phi(f) similar to 0.08). But in strong hydrogen bonding solvents like ethanol and water an other relatively stronger (type-LI emission, phi(f) similar to 0.3-0.5 depe nding on the solvent) but small Stokes-shifted fluorescence(lambda(a) = 360 nm and lambda(f) = 410-430 nm) is observed. The fluorescence maximum for t his particular type of emission depends on the static polarity of the solve nt (lambda(f) varies from 410 nm in ethanol to 430 nm in water). In the gro und state, Very rapid deprotonation from the carboxylic acid groups occurs in the hydrogen bonding solvent medium resulting in dicarboxylate anion of HmPA. The ground state closed conformer (keto form) of this dicarboxylate a nion of HmPA undergoes thermodynamically favourable intramolecular proton t ransfer in the ground state (GSIPT) leading to the formation of proton tran sferred form (enol tautomer). All the experimental findings were explained with the theoretical results obtained from CNDO/S-CI calculations. (C) 1999 Elsevier Science B.V. All rights reserved.