PHOTOINDUCED PROTON TRANSFERS IN METHYL SALICYLATE AND METHYL 2-HYDROXY-3-NAPHTHOATE

The effects of solvent and temperature on the dual fluorescence emission of methyl salicylate (MSA) have been reinvestigated, and new insight regarding the photoinduced proton-transfer reactions is reported. The steady-state spectral data obtained in this work are found to be consistent with the spectral assignments proposed by previous investigators. Specifically, the dual emission bands in alcohols are shown to occur from excited states derived from two ground-state rotamers, a and b. The emission band from excited a (the normal band) is in mirror-image relationship with the absorption band and the Stokes shift of this band is approximately 5000 cm-1. The long wavelength emission band, which has a Stokes shift of approximately 10 700 cm-1, was postulated to be an emission from an excited zwitterion resulted from an intramolecular proton transfer in excited b. Both emission bands exhibit monoexponential decays. The fluorescence lifetimes for the normal and the long wavelength band are 1.2 and 0.29 ns, respectively. The monoexponential decays indicate that the two emitting states are not in thermodynamic equilibrium. This model is supported by time-resolved emission spectra. New evidence for the occurrence of tautomerization associated with the intramolecular proton-transfer process in ''cited MSA is provided by a structural effect study. We have extended our measurements to methyl 2-hydroxy-3-naphthoate (MNA). In analogy to hydroxyazo compounds 1-(phenylazo)-2-hydroxybenzene and 1-(phenylazo)-2-naphthol, where the hydroxy/azo --> keto/hydrazone tautomerization in the latter compound is favorable due to the smaller loss in resonance energy, the excited enol tautomer of NMA, if formed, should be better stabilized. Experimentally, MNA is found to exhibit dual fluorescence emission bands. The Stokes shift of the long wavelength emission ranges from 6300 to 9900 cm-1 depending on the solvent and the temperature and is smaller than that of MSA by 800 cm-1. It is argued, based on the smaller Stokes shift and the thermochromic and solvatochromic shifts of the long-wavelength emission, that keto --> enol tautomerization occurs in excited NMA. The similarity in spectral properties between MSA and MNA suggests that a similar tautomerization process also occurs in excited MSA. Temperature and D-isotope effects on the fluorescence decay of the long wavelength emission band enable us to conclude that regeneration of the ground-state keto tautomer of MSA is the major radiationless decay for the excited enol tautomer. The conclusions drawn from the spectroscopic data in this work are in total agreement with those advanced by Herek et al., who recently studied the H-transfer reactions of excited MSA by femtosecond depletion techniques under collisionless conditions.