Mesomorphism and Polar Distortion in 1,4,5,8-Tetrasubstituted Anthraquinones and Anthracenes

The derivatives of anthraquinone, AQ n, and anthracene, A n, substituted with four alkoxy chains H 2 n +1 C n O (4 ≤ n even ≤ 14) in the 1, 4, 5, and 8 positions were synthesized. All of the molecules, except A 4, yield the smectic A mesophase below the isotropic phase, and with decreasing temperature, ordered smectic phases are observed. For the A 4 molecule, the high-temperature phase is monoclinic with a molecular arrangement of the smectic H type. Single-crystal diffraction of AQ 4 at −50 °C shows a crystalline smectic H‘ arrangement and reveals that the molecule is nonplanar. Consequently, the symmetry of the rigid core decreases from D 2 h to C 2 v, with a dipole moment along the binary axis. The structures of A 4 and AQ 4 observed in the ordered smectic phases (S H, S H ‘ ) afford a picture of well-defined sheets with segregated sublayers. Despite its bowed shape, AQ 4 forms a herringbone arrangement in the aryl sublayer, but with four molecules in the unit cell that are associated in pairs with antiparallel dipole moments. The molecular distortion in 1,4,5,8-tetrasubstituted anthraquinones is proposed as a novel method for introducing a strong dipole moment (about 5 D) perpendicular to the director of a mesogen.