Multi‐Stimuli Responsive Ionic Liquid Crystals: The Fluorenoviologens

Materials capable of tunable optical absorption and fluorescence properties in response to multiple external stimuli, while providing a readable signal, have garnered significant scientific interest. Such materials hold promise for applications in wearable electronics, anticounterfeiting technologies, self-powered light sources and displays, human–machine interfaces, and intelligent sensing systems. A highly effective approach to achieving multi-stimuli optical responsiveness is to integrate various functionalities into a single structure, such as reversible electrochemistry, ion and electronic charge transport, photoluminescence, and supramolecular organization (e.g., mesomorphism). Here, we introduce a new class of thermotropic smectic ionic liquid crystals, composed of the bistriflimide salts of π-conjugated fluorenoviologen dications. The dications feature a central fluorene core functionalized in position 2,7 with two pyridine moieties, whose nitrogen atoms are alkylated with promesogenic alkyl chains of varying lengths. In their bulk liquid crystalline phases, these materials exhibit ON/OFF electrofluorochromism (under UV photoexcitation), with voltage-triggered fluorescence quenching and a shift from yellow to dark electrochromism. Additionally, they display thermofluorochromism, showing a striking fluorescence color change from green to blue on going from the crystalline solid phase at room temperature to the liquid crystalline phases at high temperatures.