Light-switchable LCST windows in an ionic liquid solution of azobenzene copolymers

Abstract

Photo-induced tuning of lower critical solution temperature (LCST) behavior is an attractive route for reversible phase control, yet the achievable cloud-point shift is often modest in conventional solvent systems. Here, we demonstrate a composition-amplified, light-tunable LCST in ionic liquid polymer solutions using random copolymers of 4-phenylazophenyl methacrylate and phenethyl methacrylate (P(AzoMA-r-PhEtMA)) dissolved in 1-ethyl-3-methylimidazolium trifluoromethylsulfonylimide ([C2mim][TFSI]), a medium in which the azobenzene-containing monomer is readily compatible. Turbidity measurements reveal opposite cloud-point shifts under UV (cis-type) and visible-light (trans-type) conditions, enabling a bistable temperature window that widens monotonically with azobenzene composition. At 37 °C, the solution is reversibly switched between one-phase and two-phase states by light alone and maintains stable cycling for more than ~40,500 s. These results provide a practical molecular design guideline for targeting ambient-to-physiological light-controlled phase separation in nonvolatile ionic liquid media. Random copolymers of 4-phenylazophenyl methacrylate and phenethyl methacrylate in a typical ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethylsulfonylimide, [C2mim][TFSI] exhibit reversible, light-controlled LCST-type phase transitions. Photoisomerization of the azobenzene units shifts the cloud-point temperature, creating a bistable temperature window between 25 and 37 °C, where UV light induces dissolution and visible light restores phase separation. This simple molecular design enables non-volatile, light-switchable thermoresponsive polymer systems.