# On the Vapor Pressures, Phase Transitions, and Solid‐State Fluorescence of 2‐(2‐Hydroxyphenyl)benzoxazole and 2‐(2‐Hydroxyphenyl)benzothiazole

## TL;DR

This study explores how replacing oxygen with sulfur in certain chemical compounds affects their thermal and optical properties, which could help in designing better materials for light-emitting devices and sensors.

## Contribution

The paper provides new insights into how substituting sulfur for oxygen in benzazole derivatives influences their sublimation energetics, phase transitions, and solid-state fluorescence.

## Key findings

- Sulfur substitution increases intermolecular interactions, affecting sublimation and vaporization enthalpies.
- Both compounds show strong solid-state fluorescence with large Stokes shifts due to ESIPT.
- Molecular packing in the solid state leads to complex emission spectra.

## Abstract

Benzazole derivatives exhibit distinctive photophysical behavior due to excited‐state intramolecular proton transfer (ESIPT), making them promising candidates for optoelectronic applications such as organic light‐emitting diodes (OLEDs) and fluorescent sensors. Understanding their sublimation energetics, phase behavior, and emissive properties is essential for both fundamental studies and materials design. This article reports an investigation on two benzazole derivatives—2‐(2‐hydroxyphenyl)benzothiazole and 2‐(2‐hydroxyphenyl)benzoxazole (HBO)—through studies of thermal analysis, vapor pressure measurements, and fluorescence spectroscopy to establish structure–property relationships. Thermal stability and phase transitions are characterized using simultaneous thermogravimetry‐differential scanning calorimetry (TG‐DSC) and heat‐flux DSC. Vapor pressures are determined using both Knudsen effusion mass loss and mass spectrometry. The derived standard molar enthalpies of sublimation, vaporization, and fusion highlight the presence of heteroatom (S versus O) on intermolecular interactions. Solid‐state fluorescence measurements reveal strong emission in both compounds, with a large Stokes shift—consistent with ESIPT—and complex spectra attributed to solid‐state molecular packing. This comprehensive experimental strategy delivers benchmark thermodynamic and photophysical data, offering new insights into the interplay between molecular structure, thermal behavior, and fluorescence of benzazole derivatives. Such understanding is relevant for the development of advanced optoelectronic materials.

Benzazoles 2‐(2‐hydroxyphenyl)benzothiazole and 2‐(2‐hydroxyphenyl)benzoxazole exhibit tunable optoelectronic properties via excited‐state intramolecular proton transfer. Combining thermogravimetry (TG‐DSC), vapor pressure measurements, and fluorescence spectroscopy, how sulfur (S) versus oxygen (O) substitution dictates their stability, sublimation energetics, and solid‐state emission is revealed—guiding design for organic light‐emitting diodes and sensors.© 2026 WILEY‐VCH GmbH