Liquid-crystalline [2]rotaxanes and their phase transition behavior

Supramolecular liquid crystals (LCs), which integrate non-covalent molecular interactions with long-range structural order, represent a promising platform for the development of advanced functional materials. Rotaxanes and pseudorotaxanes, characteristic molecular architectures in which axle-shaped molecules are threaded through ring components, are notable for their dynamic behavior and molecular recognition capabilities. However, liquid-crystalline materials based on rotaxane architectures remain largely unexplored, likely due to the difficulty of incorporating liquid-crystalline features into rotaxane architectures, as suggested by the limited number of reported molecular design strategies. In this study, we propose a new molecular design concept for liquid-crystalline [2]rotaxanes: integrating an axle molecule as mesogen core with a ring molecule as flexible tail via a threaded structure. We successfully synthesized a representative [2]rotaxane based on this concept. Although neither the axle nor the ring component exhibited liquid crystallinity on its own, the resulting [2]rotaxane exhibited thermotropic liquid crystallinity. Furthermore, we extended this concept to a pseudorotaxane system capable of reversible assembly and disassembly, and successfully synthesized a liquid-crystalline [2]pseudorotaxane. These findings demonstrate a novel design strategy for constructing supramolecular liquid crystals based on rotaxane architectures.