Conformational diversity of symmetric dimer mesogens, α,ω-bis(4,4′-cyanobiphenyl)octane, -nonane, α,ω-bis(4-cyanobiphenyl-4′-yloxycarbonyl)propane, and -hexane in crystal structures

Abstract

The crystal structures of the title compounds show a variety of molecular conformations: a twisted chain conformation to one side and biphenyl moieties with highly different twisted angles (8.0(3) and 31.0(2)°) in the octane ( I-8), two largely different conformers in the nonane ( I-9) with and without twofold rotation symmetry in the space group of P3 121 in the trigonal system, a twisted conformation in the propane ( II-3), and the all-trans conformation with the inversion center in the hexane ( II-6). Bent molecules of odd members and straight molecules of even ones form wavy and straight stackings, respectively, in imbricated structures. The twisted conformers of I-8, I-9, and II-3 are compared with the hypothetical all-trans conformers using semi-empirical MO calculation. In I-8, another crystalline phase appears below 10 °C, the structure of which is energetically very similar to the structure determined at room temperature in this study.