The mesophases of octa-alkanoyloxy-9,10-anthraquinone

Abstract

Nine members of the octa-n-alkanoyloxy-9,10-anthraquinone series, ranging from octanoyloxy (n = 8) to hexadecanoyloxy (n = 16) (where n is the number of carbon atoms per chain), were prepared and their mesomorphic properties studied by differential scanning calorimetry and optical microscopy. All the compounds studied are mesomorphic with the higher homologues exhibiting two (for n ≧ 12) or even three (for n ≧ 14) columnar mesophases. For n = 8 to 11, the phase sequence is C-D_B-I, where D_B is an optically biaxial mesophase with a two-dimensional pgg symmetry. For n = 12, 13, the phase sequence is C-M₃-D_B-I, where M₃ is a highly ordered phase whose structure has not been identified. Finally for n = 14 to 16, the phase sequence is C-M₃-D_B-D_A-I, where D_A is an hexagonal phase. In all cases, the phases change from high to low order with increasing temperature and all transitions are first order. Compared with the corresponding hexa-substituted naphthoquinones the present series generates larger mesomorphic ranges and the higher homologues exhibit a uniaxial phase which is lacking in the hexa-substituted series. The increase in volume occupied by the aliphatic chains in the octa-substituted series is evidently sufficient to reduce the interaction between the aromatic cores of the different columns to permit the stability of a uniaxial phase.