Design of ionic liquid crystals enabled by [2]rotaxane structure formation†

We report a new synthetic concept for converting isotropic ionic molecules into thermotropic ionic liquid crystals by forming [2]rotaxane structures. Our results demonstrate the synthesis of liquid-crystalline (LC) rotaxane from an ionic axle molecule as a mesogen core, and a molecular ring as flexible tails, neither of which possess LC properties. The [2]rotaxane obtained exhibited an interdigitated smectic A phase at around 140 °C. A simple mixture of the axle and the ring, which cannot form a rotaxane structure, did not show an LC phase. A [2]rotaxane compound having a ring with shorter flexible tails did not show an LC phase, either. These comparisons revealed that the integration of the mesogen core and flexible tails of a sufficient length in one molecule via the rotaxane structure enables the emergence of LC nature. Our results prove that the rotaxane structure serves as a connection to spatially introduce flexible tails into the mesogen core, pioneering a new approach to LC molecular design.