Pressure-Induced Isomerization of 2-(2‘-Hydroxyphenyl)benzoxazole in Solid Media

Abstract

2-(2‘-Hydroxyphenyl)benzoxazole (HBO) has three isomers: a ketone (Kt), an enol (1) which is intramolecularly hydrogen bonded, and a second enol (2) which is not. Enol 1 does not emit but transforms in the excited state to the ketone Kt which can emit fluorescence or return by various paths to the ground state of enol 1. Enol 2 fluoresces but at atmospheric pressure is present, in general, in very low concentration except in hydrogen-bonding media. The fluorescence of HBO was measured as a function of pressure to 60 kbar in five media: poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA), poly(butyl methacrylate) (PBMA), poly(2-hydroxyethyl methacrylate), (PHEMA), and poly(vinyl chloride) (PVCl). In all cases enol 2 is stabilized relative to enol 1 by pressure. With reasonable assumptions it was possible to extract ΔV̄, the difference in partial molar volume of enol 1 − enol 2, as a function of pressure. For the methacrylates ΔV̄ decreases strongly with increasing pressure. The indications are that enol 2, which hydrogen bonds to the methacrylate, has initially a smaller ΔV̄ but also the configuration has a smaller compressibility. In PVCl ΔV̄ is relatively small but independent of pressure.