EFFECT OF INTERMOLECULAR HYDROGEN-BONDING AND PROTON-TRANSFER ON FLUORESCENCE OF SALICYLIC-ACID

Effects of intermolecular interactions, in particular the influence of intermolecular hydrogen bonds formed by salicylic acid (SA) as a prot on donor with proton acceptors of different strength, on fluorescence spectra of SA in non-aqueous solutions have been investigated. Infrare d spectra of studied systems have been analyzed in order to elucidate the ground state structure of the complexes formed. It has been found that at the room temperature in dilute solutions in non-polar or sligh tly polar aprotic solvents, where the SA molecule is not involved in i ntermolecular hydrogen bonding, the position of the main (blue) fluore scence component is determined by the excited state intramolecular pro ton transfer (ESIPT) in the lowest singlet excited state S-1. With inc reasing proton acceptor ability of the environment, when formation of weak or middle strength intermolecular H-bonds is possible, the emissi on band shifts gradually to lower frequency, the quantum yield falls a nd poorly resolved doublet structure becomes more pronounced, especial ly in the solvents containing heavy bromine atoms. As a possible reaso n for these effects, coupling between the S-1 and closely lying triple t term is considered. With the strongest proton accepters like aliphat ic amines, intermolecular proton transfer with ionic pair formation in the ground state and double (intra- and intermolecular) proton transf er in the excited state take place, resulting in a blue shift of the e mission band. Similar emission is typical for the SA anion in aqueous solutions. The pK(a) value of SA in S-1 state has been found to be 3.1 . Such a small value can be explained taking into account the ESIPT re action following the excitation. The SA complex with pyridine exhibits emission spectrum containing both molecular-like and anion-like bands with relative intensities strongly dependent on the temperature and s olvent properties. The most probable origin of this dual emission is t he molecular-ionic tautomerism caused by the existence of two potentia l minima and reversible intermolecular proton transfer in the excited state. (C) 1997 Published by Elsevier Science B.V.