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.