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 H2n+1CnO (4 less than or equal to n(even) less than or equal to 14) in the 1, 4, 5, and 8 positions were synthesized. All of t he molecules, except A(4), yield the smectic A mesophase below the isotropi c phase, and with decreasing temperature, ordered smectic phases are observ ed. 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 degreesC shows a crystalline smectic H ' arrangement and revea ls that the molecule is nonplanar. Consequently, the symmetry of the rigid core decreases from D-2h to C-2v, with a dipole moment along the binary axi s. 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 sublay ers. 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 intr oducing a strong dipole moment (about 5 D) perpendicular to the director of a mesogen.