Responsive molecular liquid materials

Abstract

This review focuses on solvent‐free molecular liquid materials that exhibit changes in response to stimuli. Along with the recent development of soft functional materials, research on functional liquids possessing free deformability and ease of molding is becoming a new trend. A typical method for designing functional molecular liquids (FMLs) introduces flexible alkyl side‐chains around a π‐conjugation unit at the center of the molecule. As a result, the aggregation between π‐conjugated units is inhibited, and an entropy‐rich room temperature liquid is obtained. FMLs designed in this way can exhibit unique and exciting stimulus responses due to the high density of functional units and fluidity. Such notable features include electrochromism, phase transitions, and dynamic changes in the nanostructure. In addition, some stimulus responses are reversible and thanks to the fluidity, return to their initial state under relatively mild conditions, such as room temperature or slight heating. To elicit such effective stimulus responses, selecting an optimal π‐core, and modifying it with the appropriate alkyl side‐chains (length, branching, and substitution position) is essential. A deep understanding of the alkyl–π molecular design will enable the creation of more attractive stimuli‐responsive FMLs, which are of high value in advanced fields such as healthcare, security, sensors, soft electronics, and robotics.

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