Experimental and theoretical aspects of proton transfer in 3-methyl-6-hydroxy-m-phthalic acid

Abstract

The ground- and excited-state proton transfer reactions of 3-methyl-6-hydroxy-m-phthalic acid (HmPA) have been studied in different protic and aprotic solvents at room temperature and 77 K both in the presence and absence of a base. In the ground state, intramolecularly hydrogen-bonded closed conformer has been found to be the only component in nonpolar and weakly polar aprotic solvents, whereas anion is detected in highly polar aprotic solvents. The excited-state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission in nonpolar and weakly polar solvents due to the formation of enol tautomer of HmPA. Solute-solvent interaction appears to play a major role in determining the nature of the absorbing and fluorescing species. At 77 K, HmPA shows phosphorescence both in the presence and absence of a base. The observed fluorescence decay rates are relatively slow in polar solvents compared to those in nonpolar and weakly polar solvents. Semiempirical calculations with limited configuration interaction demonstrate that ESIPT is favored only in the S₁ electronic state both kinetically and thermodynamically, while the ground singlet and the excited triplet are predicted to have considerably higher barriers on the respective proton transfer paths.