Ferroelectricity of Hydrogen-Bonded Azobenzene Derivatives

Hydrogen-bonded azobenzene derivatives (1a, 2, and 3a) bearing a −CONHC10H21 chain were designed, and their ferroelectric responses were analyzed. 1a and 2 had one hydrogen-bonded alkylamide chain, whereas 3a had two alkylamide chains at 4,4′-positions. To improve the flexibility of the molecular assembly of 2, one decyl (−C10H21) chain was further substituted into 2. Solid–solid phase transitions (S1–S2) were induced in 1a and 2, whereas lamellar-type liquid crystalline M1 and M2 phases were observed in 3a. Single-crystal X-ray structural analyses of short-chain derivatives 1b and 3b bearing −CONHC4H9 were performed to clarify the hydrogen-bonding and packing structures of 1a, 2, and 3a. All compounds formed a layered structure comprising azobenzene π-core and alkyl chains, which were connected by one-dimensional N–H···O═ hydrogen-bonding units to form the M1 or M2 phase of 3a. The magnitude of thermally activated motional freedom decreased in the following order: 1a (S2 phase) > 2 (S2 phase) > 3a (M1 phase) > 3a (M2 phase). Among them, ferroelectric polarization–electric field hysteresis loops were observed for the S2 phase of 2 as well as M1 and M2 phases of 3a, where the dynamic inversion of the polar hydrogen-bonded chains was activated by applying an outer electric field. Additionally, 3a formed an organogel, which was applied to adjust the surface morphologies of the trans- and cis-isomers of 3a on the mica surface.