POSS ionic liquid crystals

Here we report the use of unique organic−inorganic hybrid materials composed of octa-substituted polyhedral oligomeric silsesquioxane (POSS) cores as ionic liquid (IL) crystals. These materials have a wide temperature range in which they exist in liquid crystal (LC) phase because of the stabilizing effect of the POSS core. We synthesized ion pairs composed of alkyl chain-substituted imidazolium and carboxylates of various lengths that were or were not connected to the POSS core; then the thermal properties of these materials were investigated. It was found that both ion salts and the octadecyl-substituted imidazolium ion pairs with or without connection to POSS could form LCs. Interestingly, the LC phase of the POSS-tethered ion salts was maintained until decomposition. In contrast, the octadecyl-substituted imidazolium ion salts that were not tethered to the POSS core showed a clearance point during heating. The highly symmetric structure of POSS contributes not only to the suppression of the molecular motion of the ion salts, but also results in the formation of regular structures, leading to thermally stable, thermotropic IL crystals. Ionic liquid crystals (ILCs) with intriguing thermal properties can now reach practical working temperatures using siloxane cubes. ILCs are promising as optical materials and platforms for chemical templating, but their thermal stability remains problematic. Now, Kazuo Tanaka, Yoshiki Chujo and co-workers from Kyoto University in Japan present a new concept for overcoming this problem that involves using polyhedral oligomeric silsesquioxane (POSS), a silicon–oxygen compound with a rigid cubic nanostructure. On tethering ion pairs to POSS, the researchers observed a simultaneous reduction in melting temperature and enhacement in thermal stability — evidence that POSS isolates the ion pairs during melting and reduces molecular motions that destabilize the liquid crystal phase. Introduction of POSS produced an ILC stable from 24 °C to 163 °C, a seven times larger working temperature range than that of non-POSS stabilized ILCs. The thermotropic ionic liquid crystal with the wide temperature range of the liquid crystal phase is reported using polyhedral oligomeric silsesquioxane. We found the liquid crystal phase of the POSS-tethered ion salts can be maintained until the pyrolysis occurs.