New SmCG Phases in a Hydrogen-Bonded Bent-Core Liquid Crystal Featuring a Branched Siloxane Terminal Group

In this study, we synthesized three analogous bent-core molecules, a hydrogen-bonded complex and a covalent-bonded compound with branched siloxane units (H–SiO and C–SiO, respectively) and a hydrogen-bonded complex with an alkyl unit (H–Alk), and investigated the effects of the hydrogen bonding and branched siloxane terminal units on their mesomorphic properties. The covalent-bonded compound C–SiO and the hydrogen-bonded complex H–Alk exhibited typical SmCP phases; in contrast, the hydrogen-bonded complex H–SiO exhibited a series of general tilt smectic (SmCG) phases with highly ordered layer structures (i.e., SmC̃G₂P_F–USmCG₂P_A–SmCG₂P_F–SmCGP_F upon cooling). During the SmCG-type phase transition process, a 2D-modulated ribbon structure transferred into highly ordered layers via undulated layers, as the hydrogen-bonding strength increased with reduced temperatures. As the SmCG domains were aligned under dc electric fields, a gradual decrease in the leaning angle from ca. 60° to 50° (while the tilt angle kept at ca. 31°) could be determined by in situ wide-angle X-ray scattering (WAXS). Combined with Fourier transform infrared and Raman spectroscopic data, our results suggest that the change in the leaning angle was governed by the competition of the hydrogen bonds and microsegregation of siloxane units within the bilayer structure of the hydrogen-bonded complex H–SiO. In addition, the ferroelectric–(antiferroelectric)–ferroelectric transitions proven by the switching current responses in the SmCG-type phases of H–SiO reveal that the polar switching occurred through collective rotations around the long axis of H–SiO. Therefore, novel SmCG phases with a series of highly ordered 2D-structures were induced by the effects of the hydrogen bonding and branched terminal siloxane unit in the bent-core hydrogen-bonded LC complex H–SiO.