Phase structures and self-assembled helical supra-molecular structures via hydrogen bonding in achiral 4-biphenyl carboxylic acids and their asymmetric bent polyesters

A series of achiral 4-biphenyl carboxylic acids (BPCA-Cn-P mOH, n = 6--10) and C6 model compounds were studied. Different phase structures including N, SmA, SmC and highly ordered smectic LC phases along with crystalline phases were identified based on WAXD and SAED experiments. It was found via FT-IR that hydrogen (H)-bonds were formed between the carboxylic acids to construct head-to-head dimers as the building blocks for these ordered structural formations. In particular, a series of achiral BPCA-Cn-PmOHs and C6 model compounds exhibited helical supra-molecular structures in the SmC phase. The self-assembled twisted dimers are essential to form a stable helix, but the -OH or -OCH3 groups at the meta- or para-position of phenyl ring are not critical. Computer simulation indicates that the dimers possess a twist rather than a bent conformation. Therefore, we can conclude that the helical supra-molecular structures in achiral carboxylic acid compounds result from axial chiral conformers of individual head-to-head dimers under the compression caused by molecular tilting toward the long axis of the cylinders in a network during the transition from the SmA → SmC phases.; A series of new asymmetric bent-core main-chain LC polyesters (BPE-Cn, n = 6--10) were investigated to understand the effect of the bent-core in determining chiral characteristics. BPE-Cn exhibits a rich set of phase behaviors as observed via DSC measurements. Based on the WAXD experiments, even-numbered polyesters possess three phases: SmCA, SmHA and crystalline phases. On the other hand, odd-numbered polyesters showed two phases: SmA and crystalline phases. A detailed chain-packing model of the crystalline phase has been proposed based on the experimental diffraction patterns and qualitatively proven using simulated structural diffractions. Of particular interest is that, in all of the ordered phases of the even-numbered BPE-Cn, the chains maintain the large zigzag conformation. On the other hand, mesogenic groups of the odd-numbered BPE-Cn are more or less parallel to the smectic layer normal. Although the molecular conformation involves a bent similar to the bent-core LCs in the case of even-numbered BPE-Cn, no helical supra-molecular structure was observed due to the covalent bonding connections in this series of BPE-Cn polymers.