Excited-state acidity of the bifunctional molecules 3-(2′-hydroxyphenyl)-5-methyl-Δ2-1,2,4-oxadiazoline and 3-(2′-hydroxyphenyl)-5-methyl-1,2,4-oxadiazole

Abstract

3-(2′-Hydroxyphenyl)-5-methyl-Δ²-1, 2,4-oxadiazoline has been synthesized and its spectral properties have been determined, along with those of the previously reported molecule 3-(2′-hydroxyphenyl)-5-methyl-1,2,4-oxadiazole. In spite of the obvious similarities of structure, these two bifunctional compounds have very different fluorescence spectra. The oxadiazoline shows two peaks in both polar and non-polar solvents, attributed to an open structure (short wavelength) and a closed structure (long wavelength), whereas the oxadiazole has only one peak in polar solvents, attributed to the open form. Quantum-mechanical arguments are given to rationalize these results, which are compared with those for the previously reported molecule 2-(2′-hydroxyphenyl)-benzoxazole.