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

The effects of solvent and temperature on the dual fluorescence emissi on of methyl salicylate (MSA) have been reinvestigated, and new insigh t regarding the photoinduced proton-transfer reactions is reported. Th e steady-state spectral data obtained in this work are found to be con sistent with the spectral assignments proposed by previous investigato rs. Specifically, the dual emission bands in alcohols are shown to occ ur from excited states derived from two ground-state rotamers, a and b . The emission band from excited a (the normal band) is in mirror-imag e relationship with the absorption band and the Stokes shift of this b and is approximately 5000 cm-1. The long wavelength emission band, whi ch has a Stokes shift of approximately 10 700 cm-1, was postulated to be an emission from an excited zwitterion resulted from an intramolecu lar proton transfer in excited b. Both emission bands exhibit monoexpo nential decays. The fluorescence lifetimes for the normal and the long wavelength band are 1.2 and 0.29 ns, respectively. The monoexponentia l decays indicate that the two emitting states are not in thermodynami c equilibrium. This model is supported by time-resolved emission spect ra. 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 me thyl 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. Experim entally, 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 sh ift and the thermochromic and solvatochromic shifts of the long-wavele ngth emission, that keto --> enol tautomerization occurs in excited NM A. The similarity in spectral properties between MSA and MNA suggests that a similar tautomerization process also occurs in excited MSA. Tem perature and D-isotope effects on the fluorescence decay of the long w avelength emission band enable us to conclude that regeneration of the ground-state keto tautomer of MSA is the major radiationless decay fo r the excited enol tautomer. The conclusions drawn from the spectrosco pic data in this work are in total agreement with those advanced by He rek et al., who recently studied the H-transfer reactions of excited M SA by femtosecond depletion techniques under collisionless conditions.