EXCITED-STATE INTRAMOLECULAR PROTON-TRANSFER AND DUAL EMISSION OF THECYCLIC HOMODIMERS AND HETERODIMERS OF 2-HYDROXY AND 2,6-DIHYDROXY BENZOIC-ACIDS

The fluorescence and excitation spectra of homo- and heterodimers form ed by salicylic and 2,6-dihydroxy benzoic acids (SA and DHBA respectiv ely) with various carboxylic acids were obtained in the crystalline st ate, liquid and frozen glassy solutions within the temperature range 3 00-90 K. In all the cases, dual emission has been found with the low f requency band attributed to the excited state with intramolecular prot on transfer (ESIPT) and the high frequency one to a state without ESIP T. The relative intensities of these bands depend strongly on the temp erature, excitation wavelength and the pK(a) value of the second acid molecule involved in a cyclic dimer. The dual emission of molecules li ke methyl salicylate is conventionally explained in terms of the groun d state tautomeric equilibrium with the OH ... OH intramolecularly hyd rogen bonded isomer incapable of ESIPT. Since in the case of 2,6-dihyd roxy benzoic acid the only stable isomer with two hydrogen bonds can e xist, this explanation seems to be at least insufficient. Other explan ations of the dual emission origin are discussed. An attempt is made t o explain the available experimental data using the hypothesis involvi ng double well potential surface of the cyclic dimer in the excited st ate. In the framework of this hypothesis, the barrier between two well s emerges from the reorganization of the whole hydrogen bonded system of a dimer.