Emissive Room-Temperature Ionic Liquids Enabled by Dialkylphospholium-Based Tetracyclic Cores with Bulky Counter...

Metal-free ionic liquids that exhibit intense fluorescence in the visible region of over 500 nm in the fluid state remain scarce. Although small π-conjugated frameworks combining electron-donating and electron-accepting groups represent a promising structural motif, such modifications often lead to fluorescence quenching in polar environments, including neat ionic liquids. Herein, we report fluorescent ionic liquids based on a phospholium containing a tetracyclic core. Incorporation of both an amino group and the phosphonium center in the same benzene ring prevents excessive spatial separation of HOMO and LUMO, thereby suppressing the increase in intramolecular charge-transfer character in the excited state. This push-pull architecture enables the ionic liquids to display bright yellow fluorescence with a large Stokes shift under neat conditions. Furthermore, bridging with a dialkylphosphonium unit effectively suppresses aggregation-induced quenching while lowering the glass transition temperature. The introduction of bulky counter anions, such as a tetrakis-(3,5-bis-(trifluoromethyl)-phenyl)-borate, further decreases the glass transition temperature by hindering π-core proximity and significantly increases the fluorescent quantum yield up to 0.64. A neat film of one derivative, doped with an additional red-emissive dye, exhibited red fluorescence through a Förster resonance energy transfer.