Proton transfer reaction in 4-hydroxy-3-formyl benzoic acid at room temperature and 77 K D. Banerjee, A. Mandal, S. Mukherjee * DepartmentofPhysicalChemistry,IndianAssociationfortheCultivationofScience,2A&BRajaS.C.MullickRoad, Jadavpur,Calcutta700032,India Received 11 February 2003; in final form 31 March 2003 Abstract Proton transfer processes of 4-hydroxy-3-formyl benzoic acid (HFBA) have been investigated by means of ab- sorption, emission and nanosecond transient spectroscopy in some nonpolar and polar aprotic solvents at room temperature and 77 K. The excited state intramolecular proton transfer (ESIPT) is evidenced by a large Stokes shifted emission (12000cm 1 ) in nonpolar solvents which is likely to originate from the enol tautomer of HFBA. On the otherhand,intermolecularinteractionoccursinprotonacceptingpolaraproticsolventsbothinthegroundandexcited state. At 77 K, HFBA shows phosphorescence in DMSO. It is proposed that occurrence of phosphorescence is due to the rupture of intramolecular bond and rotation of the formyl group. From the nanosecond measurements and quantum yield of fluorescence, we have estimated decay rates in polar solvents. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction It is well established now that in the case of salicylic acid (SA), the most stable form in the ground state is the normal primary form and that in the excited state is the tautomeric form [1]. It has also been shown that both the primary and tautomeric forms are intramolecularly hydrogen bonded form. Bisht et al. [2] attributed the large Stokes shifted emission spectra to the signature of tautomerization. In molecules where both inter- and intramolecular proton transfer (PT) are pos- sible, it can represent an important nonradiative channel either to products or to the ground state. The rate could be high enough to make the PT competitive with intersystem crossing and lead to photochemical products. In our earlier work, we have investigated the structures and PT processes of the ground and excited states of 4-methyl-2,6-diformylphenol (MFOH), 4-methyl-2,6-diacetylphenol (MAOH), 4-methyl-2,6-diamidophenol (MDOH), 4-methyl- 2,6-dicarbomethoxyphenol (CMOH) and 3-meth- yl-6-hydroxy-m-phthalic acid (HmPA) in different nonpolar and polar solvents [3–11]. Some of the results obtained from there are as follows: (1) the stablemolecularstructureinthegroundstateisan intramolecularly hydrogen bonded closed con- former; (2) the excited state intramolecular proton Chemical Physics Letters 373 (2003) 464–470 www.elsevier.com/locate/cplett * Corresponding author. Fax: +91-33-2473-2805. E-mailaddress: pcsm@mahendra.iacs.res.in (S.Mukherjee). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00630-4