Molecular design of viologens to exhibit low-order liquid-crystalline phases

Abstract

Viologens that are useful as stimulus-responsive compounds and exhibit a low-order liquid-crystalline (LC) phase at relatively low temperatures (approximately 20 °C) can be developed into materials that combine redox properties with LC properties. We synthesised a viologen (12NNP12) having an asymmetric core, while maintaining its stimulus-responsive properties, with the aim of forming a low-order LC phase at ambient temperature. The synthesised 12NNP12 exhibited low-order LC phases: smectic-A (SmA) and smectic-C (SmC). The SmC phase appeared over a wide temperature range (approximately 170 °C) above −4.2 °C, and the layer shrinkage determined by X-ray diffraction measurements was small, up to 1.3% over a temperature range of 115 °C. Furthermore, we synthesised a viologen derivative (12NPN12) having a symmetric core and a compound (12NPP12) having an asymmetric core and a single ionic bonding site, and investigated the effect of the asymmetry of the core on the LC properties. When the rod core was asymmetrised by changing the position and number of ionic bonds, the development of low-order LC phases was observed. It was suggested that the intermolecular interactions were weakened by varying the position of the ionic bonding sites to asymmetrise the core. Even for viologens with an asymmetric core (nNNP12), an LC phase was not formed if the length (n) of the alkyl chain was too short. However, it was found that the LC properties were affected more by the asymmetry of the rigid ionic part than that of the alkyl chain.