Molecular Engineering of Tetra(hetero)arylphosphonium Ionic Liquids: Thermally Robust, Photoluminescent Materials with Tunable Melting Thermodynamics

Abstract

Ionic liquids (ILs) are deployed for high-temperature technologies, where sustained thermo-oxidative stability must be combined with additional functions in a single organic material. We report a second-generation library of 34 tetra(hetero)arylphosphonium cations paired with the [NTf2]− anion, spanning seven π-conjugated scaffolds in mono-, di-, and tricationic forms with a matched substituent series, characterized by thermal, thermodynamic, photophysical, and crystallographic methods. Thermogravimetric analysis shows that all compounds lacking aliphatic C(sp3)−H bonds show decomposition onsets above 400 °C in air, and isothermal aging confirms that they withstand 300 °C for 96 h with less than 8% mass loss. Most ILs form glasses rather than crystalline solids, consistent with the increased asymmetry and conformational flexibility of the extended scaffolds. Normalizing ΔHfus and ΔSfus to [Ph4P][NTf2] reveals tunable melting thermodynamics: cation charge raises ΔHfus and produces enthalpy-driven melters, whereas conformational rigidity lowers ΔSfus and produces entropy-driven melters. Of the 34 ILs, 33 emit with quantum yields (QYs) up to 0.87 and emission maxima ranging from 374 nm to 522 nm. Extending the π-system leaves the emission unchanged, and QY tracks cation conformation rather than conjugation length; peripheral substitution tunes both emission wavelength and quantum yield: methoxy substitution blue-shifts emission by 15 nm on average, and 2-pyridyl substitution by up to 59 nm; 2-pyridyl substitution also raises QY up to twofold. Single-crystal X-ray diffraction provides structural context for both correlations: conformationally locked cations melt entropy-driven whether or not they π-stack, the propeller-shaped triarylamine cations cohere through anion contacts, multiplying with charge, and the measured twist angles are consistent with the quantum-yield differences among the triarylamine cations. These ILs are photoluminescent in solution and thermo-oxidatively stable beyond the range of conventional organic fluorophores. This combination of properties in a single platform motivates evaluation for high-temperature applications, e.g., phosphor thermometry, solid-state lighting, and luminescent solar concentrators.