Property-Driven Design of Thermally Robust Organophosphorus Ionic Liquids for High-Temperature Applications

We have developed a class of organophosphorus ionic materials featuring tetraarylphosphonium cations with extended π-conjugated systems via a facile and modular approach. These mesothermal ionic liquids demonstrate exceptional thermal stability, maintaining their structural integrity when heated at 300 °C for 96 h under aerobic conditions without decomposition. Their negligible volatility and strategic exclusion of aliphatic C­(sp3)–H bonds from our molecular architecture yields materials with outstanding resistance to thermo-oxidative degradation. Our rigorous investigation using comprehensive single-crystal X-ray diffraction and thermodynamic studies validates the design principles while providing detailed insights into the structure–property relationships governing their thermal stability, melting behavior, and photophysical properties. Our studies reveal a systematic correlation between the nature of the cations and the resulting phase transitions. Additionally, detailed photophysical characterization demonstrates that select derivatives exhibit strong fluorescence with quantum yields up to 42%, suggesting potential applications in optoelectronic devices. These thermally robust organic-ion materials with tunable properties have potential applications ranging from thermally demanding environments (thermoresponsive materials, advanced nuclear reactor coolants, and thermal energy storage) to optoelectronic devices that capitalize on their unique photoluminescent characters.