Excited state proton transfer: A new feature in the fluorescence of methyl 5-chlorosalicylate and methyl 5-methoxysalicylate

Abstract

The fluorescence of methyl 5-methoxysalicylate and, to a lesser extent, methyl 5-chlorosalicylate departs appreciably from the usual emission of salicylic acid derivatives. The emission spectra, quantum yields and lifetimes of these compounds are compared here with those of the 4-methoxy, 3-chloro and 4-chloro derivatives which show dual emission and excitation spectra similar to those of the parent compound methyl salicylate. Moreover, the photophysics of the 3- and 4-derivatives can be understood in terms of a conformational equilibrium in the ground state and an excited state proton transfer reaction. In contrast, a new emission process with the same excitation spectrum as the proton transfer band is needed to explain the fluorescence of the 5-derivatives. It is speculated that the new flourescence originates in those excited species which fail to undergo the proton transfer reaction despite having the appropriate intramolecular hydrogen bond in the ground state; this explanation has already been considered by Weller in the excited state equilibrium hypothesis.