Hydrogen bonded arylamide-linked cholesteryl dimesogenic liquid crystals: a study of the length and side chain effects

Dimesogenic compounds 1a-c, 2a-i, and 3, that are composed of a hydrogen bonding-induced straight arylamide spacer and two appended cholesterol groups, have been designed and synthesized. The backbones of the rigid spacers of 1a-c, 2a-i, and 3 contain one, three, and five benzene units, which bear two, six, and ten alkoxyl (methoxyl, n-octoxyl, or n-dodecoxyl) groups, respectively. The thermal and optical properties of the compounds are investigated by using the differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder X-ray diffraction (PXRD) analysis. It is revealed that 1a-c exhibit one or two liquid crystalline (LC) phases, 2a-i exhibit no, one or two LC phases, while 3 exhibits one LC phase in a wide temperature range. Generally, the more and longer alkoxyl chains facilitate the formation of the LC phases at low temperature. Notably, compound 2g, which bears two methoxyl and four dodecoxyl groups, displays a blue-red color change during both the heating and cooling cycle. The result illustrates that dimesogens with large rigid spacers can exhibit different LC phases when long aliphatic chains are appended to balance the strong stacking of the rigid backbones.