The discotic mesophases of octaalkyloxy-and octaalkanoyloxyorthocyclophanes

Abstract

The mesomorphic properties of 3,4,10,11,17,18,24,25-octaalkyloxy and octaalkanoyloxycyclotetraveratrylene (orthocyclophane) have been investigated by differential scanning calorimetry (DSC), polarizing optical microscopy, NMR and X-ray diffraction. Twelve homologues of the octaalkyloxy series, with equal side chains containing n = 5 to 16 carbons per side chain, were studied and found to be mesomorphic with at least one columnar mesophase. All members of the series exhibit a biaxial columnar mesophase denoted by D₁. The homologues, with n = 10 to 16, also exhibit a uniaxial columnar mesophase, between the D₁ and the liquid phase, which is denoted by D₂. Miscibility studies show that the D₁ phases of all the compounds, and the D₂ mesophases of the high members of the series are isomorphic. The D₁ and D₂ phases are completely miscible with M₁ and M₂ of the hexaalkanoyloxytriphenylene series respectively. They may, therefore, be classified as discotic B and discotic A respectively. Both phases remain stable upon addition of substantial amounts of non-mesogenic aromatic compounds. The transition between D₂ and D₁ is weakly first order (not observed by DSC), but the interconversion is spread over a wide temperature range (∼20°C for the n = 13 homologue). The analysis of the X-ray results shows that D₂ is hexagonal and that D₂ is rectangular, with one and two molecules per unit cell respectively. The lattice parameters for all measured compounds in the two mesophases are reported.

Four homologues of the octaalkanoyloxy derivatives of cyclotetraveratrylene with side chains containing 12, 14, 15 and 16 carbons were also studied by X-ray diffraction. These compounds exhibit a single discotic columnar mesophase with essentially rectangular symmetry.