Ferroelectricity of alkylamide-substituted triptycene derivatives

Abstract

1,8,13-Tris(dodecyloxy)triptycene forms uniform two-dimensional (2D) thin films by self-assembly. We synthesized new 1,8,13- and 1,8-substituted triptycene derivatives in which the alkoxy chains were replaced with alkylamide chains capable of hydrogen-bonding and investigated their phase transition behaviors, molecular assembly structures, and dielectric properties. In the three-chain compounds, the formation of a tightly packed 2D structure dominated by one-dimensional (1D) intermolecular N–H⋯O hydrogen bonds was observed. In the two-chain compounds, the molecular arrangement and hydrogen bonding patterns were different from those of the three-chain molecules due to the decrease in the occupied volume of the alkyl chains and molecular symmetry, and the disorder in the molecular arrangement increased in the hydrophobic moiety. Both compounds exhibit solid-to-solid and solid-to-liquid phase transitions without forming a liquid-crystalline phase. In the temperature- and frequency-dependent dielectric constants, thermally activated dynamics of the polar amide groups with increasing temperature was observed at low frequencies, accompanied by a hysteresis in the polarization–electric field (P–E) curve above 400 K, which is a characteristic of ferroelectrics.