Vapour Pressures, Phase Transitions, and Solid-State Fluorescence of Three Benzazoles

Benzazole derivatives are a class of organic compounds with significant potential in optoelectronic applications due to their excited-state intramolecular proton transfer (ESIPT) capability, which enables unique photophysical properties[1,2]. Understanding their thermodynamic stability[3], phase behaviour, and emissive properties is crucial for their application in organic light-emitting diodes, fluorescent sensors, and other optoelectronic devices[4]. In this study, three structurally related benzazoles - featuring oxazole, thiazole, and imidazole, were investigated. Their general molecular structure is depicted in figure 1. The thermal stability and phase behaviour of these compounds were examined using simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC), as well as standalone DSC. Vapour pressures were measured by Knudsen effusion mass-loss (KEML) technique, providing data to calculate the enthalpy of sublimation of each one of compounds. The thermodynamic properties studied (enthalpies of sublimation, vaporisation, and fusion) were correlated with molecular structure, highlighting the influence of heteroatom substitution on inter/intramolecular interactions. Furthermore, solid-state fluorescence spectroscopy demonstrated that the three benzazoles exhibit strong emission, with the benzimidazole derivative showing particularly intense fluorescence. This study reports benchmark thermodynamic data as a potential contribution to the development of optoelectronic materials. The combination of TG, DSC, Knudsen effusion, and fluorescence spectroscopy studies provides a multitechnique framework to evaluate the stability of benzazoles, correlating phase transitions with structure and inter/intramolecular interactions.