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.

Observation of linear solid-solid phase transformation in silicon nanoparticles

“…High pressure experiments were carried out on the nanoparticles using piston-cylinder style diamond anvil cell 27 . The particles were dispersed in a PMMA matrix.…”

Analysis of Pressure-induced Variations in the Crystalline Structures of Polyimides Having Flexible Linkages by Wide-Angle X-ray Diffraction

“…Applying a high pressure to induce external perturbation onto the aggregation structures of polymer chains in the solid state is more effective than varying temperature. We have measured WAXD patterns [18][19][20][21][22], infrared absorption (IR) [23,24], ultraviolet/visible (UV/Vis) absorption, fluorescence spectra [25][26][27][28][29][30][31][32][33], and Brillouin scattering spectra [34] of various PIs under high pressure in order to examine the relationships between the molecular aggregation structures and the physical and optical properties in the solid state. Recently, we discovered that the optical properties of PI films are well correlated with their molecular aggregation structures based on the UV/Vis absorption and fluorescence spectra at elevated pressures up to 8.0 GPa [33].…”

Absorption and Emission Sensitivity of 2‐(2′‐Hydroxyphenyl)benzoxazole to Solvents and Impurities

“…Conformational isomers , solvated, and/or deprotonated HBO (Scheme ) may also exist depending on solvent and other conditions . Considering these scenarios, the molecular photophysics of ESIPT molecules has long been baffling due to their capricious spectral sensitivity in both ground and excited states to solvation and polar/hydrogen bond‐capable impurities , and other environmental parameters such as pH , pressure and viscosity . Therefore, the photophysical models of the ESIPT compounds have been constantly updated as new experimental facts and techniques continue to emerge over the past few decades.…”