Ground-state proton transfer tautomer of Al(III)-salicylate complexes in ethanol solution

The tautomerization of salicylate anion in the presence of Ai(III) in ethanol was studied by UV-visible absorption spectroscopy and fluorescence spectroscopy, anisotropy, and lifetime measurements from 100 to 298 K. Complexation with AI(III) causes an equilibrium shift from the normal form of the salicylate anion toward the tautomer form, demonstrating that the presence of a highly charged cation, Al(III), stabilizes the tautomer form of salicylate. Spectra and fluorescence lifetimes of salicylate and other salicyl derivatives in the presence of AI(III) reveal three types of Al(III)-saIicylate complexes. In type I complexes, salicylate binds to Al(III) through the carboxylate group, preserving the intramolecular hydrogen bend between the carbonyl oxygen and the phenol group, as indicated by the largely Stokes-shifted fluorescence emission following an excited state proton transfer process. In type II complexes, salicylate binds to Al(III) through the carboxylate group, but the phenol proton is oriented away from the carbonyl oxygen so that the complex shows short wavelength fluorescence emission characteristic of substituted phenolic compounds. In type III complexes, Al(III) stabilizes and binds to the tautomer form of salicylate through the phenolate oxygen, in which salicylate exists in its proton transferred tautomer form. Absorption spectra recorded at temperatures between 100 K anf 298 K indicate that the type III tautomer complex is energetically favored at low temperature, although type I is the dominant species at room temperature. All three types of complexes are interconvertible above the ethanol glass transition temperature. However, below the glass transition temperature interconversion ceases, indicating large amplitude atomic motion is involved in the conversion.