Journal of Luminescence 81 (1999) 61—70 Photoinduced proton transfer in 3-methyl-6-hydroxy-m-phthalic acid Ranjan Das, Sivaprasad Mitra, Dipanwita Guha, Samaresh Mukherjee*  Department of Chemistry, J.K. College, Purulia, W. Bengal, India  Department of Chemistry, School of Science, Kwamsei Gakuin University, Nishinomiya 662, Japan  Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India Received 25 February 1998; received in revised form 13 July 1998; accepted 27 July 1998 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 calcu- lations. HmPA shows variations in steady-state spectra depending on the solvent characteristics. A large Stokes-shifted emission in the nonpolar solvent (&9500 cm,)  "325 nm and   "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 (  &0.08). But in strong hydrogen bonding solvents like ethanol and water another relatively stronger (type-II emission,   &0.3—0.5 depending on the solvent) but small Stokes-shifted fluorescence (  "360 nm and   "410—430 nm) is observed. The fluorescence maximum for this particular type of emission depends on the static polarity of the solvent (  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 dicar- boxylate 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.  1999 Elsevier Science B.V. All rights reserved. Keywords: Proton transfer; 3-methyl-6-hydroxy-m-phthalic acid; Solvent effect 1. Introduction Molecules, undergoing excited state intra- molecular proton transfer (ESIPT) reaction, are important in connection with promotion of laser action [1], conversion of solar energy into electrical * Corresponding author. Tel.: #91 33 473 5374; Fax: #91 33 473 2805; e-mail: pcsm@mahendra.iacs.res.in. energy [2], processes related to the absorption of ultraviolet light by chemical and biochemical sys- tems [3,4] and solvation dynamics [5]. Due to its wide spread applications in science and technology, ESIPT reaction has attracted a number of experi- mental and theoretical scientists [6]. Although proton transfer reaction of salicylic acid (SA) was known at least four decades ago [7], studies on this system are not very common. This is due to the fact that o-hydroxy carboxylic 0022-2313/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 8 ) 0 0 0 5 0 - 7