Excited state intramolecular proton transfer and dual emission of the cyclic homo- and heterodimers of 2-hydroxy and 2,6-dihydroxy benzoic acids

The fluorescence and excitation spectra of homo- and heterodimers formed by salicylic and 2,6-dihydroxy benzoic acids (SA and DHBA respectively) with various carboxylic acids were obtained in the crystalline state, liquid and frozen glassy solutions within the temperature range 300-90 K. In all the cases, dual emission has been found with the low frequency band attributed to the excited state with intramolecular proton transfer (ESIPT) and the high frequency one to a state without ESIPT. The relative intensities of these bands depend strongly on the temperature, excitation wavelength and the pK(a) value of the second acid molecule involved in a cyclic dimer. The dual emission of molecules like methyl salicylate is conventionally explained in terms of the ground state tautomeric equilibrium with the OH ... OH intramolecularly hydrogen bonded isomer incapable of ESIPT. Since in the case of 2,6-dihydroxy benzoic acid the only stable isomer with two hydrogen bonds can exist, this explanation seems to be at least insufficient. Other explanations of the dual emission origin are discussed. An attempt is made to explain the available experimental data using the hypothesis involving double well potential surface of the cyclic dimer in the excited state. In the framework of this hypothesis, the barrier between two wells emerges from the reorganization of the whole hydrogen bonded system of a dimer.