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

Abstract

The ground and excited state photophysics of 3-methyl-6-hydroxy-m-phthalic acid (HmPA) has been studied in homogeneous solvents by steady state, nanosecond transient emission spectroscopy and semiemperical AM1 calculations. HmPA shows variations in steady-state spectra depending on the solvent characteristics. A large Stokes-shifted emission in the nonpolar solvent (∼9500 cm −1,~ λ a=325 nm and λ f=470 nm) indicates that on UV excitation the keto form of HmPA undergoes intramolecular proton transfer (ESIPT) leading to tautomerization in the singlet excited state. This particular emission (type-I emission) attributed to the enol tautomer of HmPA, is characterized by a very low emission quantum yield ( φ f∼0.08). But in strong hydrogen bonding solvents like ethanol and water another relatively stronger (type-II emission, φ f∼0.3–0.5 depending on the solvent) but small Stokes-shifted fluorescence ( λ a=360 nm and λ f=410–430 nm) is observed. The fluorescence maximum for this particular type of emission depends on the static polarity of the solvent ( λ f varies from 410 nm in ethanol to 430 nm in water). In the ground 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 anion of HmPA undergoes thermodynamically favourable intramolecular proton transfer in the ground state (GSIPT) leading to the formation of proton transferred form (enol tautomer). All the experimental findings were explained with the theoretical results obtained from CNDO/S-CI calculations.