Phase Structures and Self-assembled Helical Suprastructures via Hydrogen Bonding in a Series of Achiral 4-Biphenyl Carboxylic Acid Compounds

Abstract

A series of achiral 4-biphenyl carboxylic acid compounds (BPCA-Cn-PmOH) connected with alkoxyl chains having various carbon numbers (n ) 6-10) and terminated by phenyl groups with meta-positioned hydroxyl groups was synthesized. Different phase structures including nematic, smectic A (SmA), smectic C (SmC), and highly ordered smectic liquid-crystalline phases along with crystalline phases were identified based on wide-angle X-ray diffraction and electron diffraction experiments. It was found via infrared spectroscopy that the hydrogen (H)-bonds were formed between the carboxylic acids to construct headto-head dimers as the building blocks for these ordered structural formations. H-bonds formed via the meta-positioned hydroxyl groups also played an important role in forming ordered layers in these structures. The morphology of this series of BPCA-Cn-PmOH as observed under polarized light microscopy showed an oily streak (cylinder) texture with Myelin-figure in the SmA phase. When temperature cools to enter the SmC phase, these streaks (cylinders) started to twist into helical suprastructures, which were not only by the birefringence changes but also by the three-dimensional helical geometry observed in other microscopic techniques. The dynamic conformational changes of the aromatic and aliphatic parts in this series of BPCA-Cn-PmOH at different temperatures correspond well with the thermal transitions via solid-state carbon-13 nuclear magnetic resonance experiments. Computer simulation indicated that the head-to-head dimers possess a twisted rather than a bent conformation. It was deduced that the twisted conformation of the dimers and the terminal meta-substituted phenyl groups at both ends of the dimers are critically important in forming the helical suprastructures.

Structures and phase transformations of odd-numbered asymmetric main-chain liquid crystalline polyesters

“…For the high-temperature transition, the heat of transition and the onset transition temperature at the high-temperature side do not exhibit cooling rate dependence. This indicates that the high-temperature transition takes place close to thermodynamic equilibrium, and it may be associated with the transitions between an isotropic melt (I) and a low ordered LC phase [28][29][30][31][32][33][34][35]. In contrast, the heat of transition (6.5 kJ/mol) at the lower transition temperature and its onset transition temperature are cooling rate dependent, which is often observed during polymer crystallization processes.…”

“…In the odd-numbered BPE-Cn, there is neither the helical supra-molecular texture (which exists in BPCA-Cn-PmOH, its monomer) [28,29], nor the ring striation with three different colors (which are observed in the corresponding evennumbered BPE-Cn) [30]. This is because that the oddnumbered BPE-Cn have neither a large zigzag conformation in their polymer chains nor a continuous twisting of the chemical repeat unit from one to another along the same twisting direction.…”

“…We recently reported that a three-dimensional (3D) helical supramolecular structure formed in a synclinically tilted smectic C (SmC S ) phase of the achiral a,u-carboxylic acidhydroxyl terminated series of compounds with different methylene spacer units (BPCA-Cn-PmOH, nZ6-10) [28,29]. When this series of A-B type asymmetric BPCA-Cn-PmOH compounds was polymerized, the corresponding polyesters (BPE-Cn) having a bend due to the meta-linkage in its chemical structure were obtained.…”

Novel Tetrahedratic Smectic C and Nematic Mesophases in Unsymmetrically 1,1′‐Bis‐substituted Ferrocenomesogens

“…[11] Beside the above mentioned banana liquid crystals spontaneously generated chirality from achiral molecules has been observed earlier in some nematics formed from hydrogen-bonded carboxylic acids [12] and bow-shaped molecules, [13] and also in double layered anticlinic SmCA mesophases. [14] In our opinion, the origin of macroscopic chiral domains and helical superstructures in 1 is adequately explainable considering pseudotetrahedratic associates. One can see that there are several kinds of mirror relationships involving the central ferrocene fragment can be established in 1 ( Figure 2).…”

Self-assembly and supramolecular liquid crystals based on organic cation encapsulated polyoxometalate hybrid reverse micelles and pyridine derivatives

“…3. For SEP-Si, the absorption band at 1683 cm À1 , which is assigned to C@O stretching mode, and the weak double absorption bands at 2551 and 2661 cm À1 (normally called satellite bands), which represent the formation of hydrogen-bonding, indicate that the terminal carboxylic acids exist in the intermolecular cyclic dimer state definitely [28][29][30][31]. The detailed assignments of other absorption bands are summarized in SI.…”