Transition from lamellar to nanostructure mesophases in azobenzene-based hockey-stick polycatenars

Abstract

Photoresponsive nanostructured materials are of great importance for photonic applications. Herein we report the design, synthesis, and liquid crystal (LC) self-assembly of two new series of photoresponsive hockey-stick LCs (HSLCs). The new HSLCs are azobenzene-based polycatenars derived from the 4-cyanoresorcinol bent-core unit. The long arm is a fork-like triple alkoxylated wing, while the short arm is terminated with a single variable alkoxy chain. The molecular self-assembly of these HSLCs is characterized using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electro optical measurements. In both series of LC materials differing in the chain volume at the crowded end, wide temperature ranges of lamellar Smectic A phases (up to 170 K) are observed beside achiral isotropic liquid phases (Iso₁) for the shorter derivatives. On chain elongation helical self-assembly of the π-conjugated rods in networks occurs resulting in the formation of nanostructured bicontinuous cubic network phases with Iad space group (Cub_bi/Iad), in most cases stable even around room temperatures on cooling. At the transition from SmA to Cub_bi phases, an additional unknown three-dimensional (3D) phase is formed. A possible explanation for the disappearance of the triple network Cub_bi phases with I23 symmetry (Cub_bi^[*^]/I23) in these HSLCs is represented based on the phase behaviour of a new material having the central 4-cyanoresorcinol unit shifted to a terminal position of the rod-like core. Finally, the reversible trans–cis photoisomerization for some selected examples was investigated in chloroform solution.