Thermally Reversible On‐Off Switching of Aggregation‐Induced Emission via LCST Phase Transition of Ionic Liquids in Water

Abstract A new phosphonium‐based ionic liquid is designed and synthesized using the 1,1,2,2‐tetraphenylethene‐derived anion ([P₄₄₄₄]₄[TSPE]). Aqueous solutions of [P₄₄₄₄]₄[TSPE] are obtained as transparent liquids (monophasic state), which suspended with increasing temperature. This behavior is found to exhibit a lower critical solution temperature (LCST) phase transition behavior. The LCST phase transition temperature can be controlled from 36 to 55 °C by changing the weight ratio between [P₄₄₄₄]₄[TSPE] and water. When observing the LCST phase transition behavior of aqueous solutions of [P₄₄₄₄]₄[TSPE] under excitation light with a wavelength of 365 nm, negligible photoluminescence in the monophasic state is observed at room temperature. Conversely, strong photoluminescence is observed in the suspended state when the temperature is above the LCST phase transition temperature. This strong photoluminescence is attributed to aggregation‐induced emission (AIE) that is induced by the restriction of the intramolecular rotations of the 4‐sulfophenyl groups against the ethylene core in the [TSPE] anion. This behavior is thermally reversible. This is the first example of on‐off switching of AIE achieved using the LCST behavior of ionic liquids.