Conformational diversity of symmetric dimer mesogens, a,v-bis(4,40-cyanobiphenyl)octane, -nonane, a,v-bis(4-cyanobiphenyl-40- yloxycarbonyl)propane, and -hexane in crystal structures

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)8) 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 P3121 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 8C, the structure of which is energetically very similar to the structure determined at room temperature in this study. q 2004 Elsevier B.V. All rights reserved