Origin of self-assembled helical supramolecular structures in achiral C6 biphenyl carboxylic acid compounds

To understand the formation mechanism of helical supramolecular structures in the smectic C (SmC) liquid crystalline (LC) phase of BPCA-Cn-PtnOH (it is the number of methylene units, n = 6-10) (see: Jeong, K.-U.; Jin, S.; Ge, J. J.; Knapp, B. S.; Graham, M. J.; Ruan, J.; Guo, M.; Xiong, H.; Harris, F. W.; Cheng, S. Z. D. Client. Mater. 2005, 17, 2852), a series of newly designed achiral C6 4-biphenyl carboxylic acid compounds was synthesized, of which all of them have six methylene units (n = 6). To recognize the importance of hydrogen (H)-bonding between hydroxyl groups and the small kink at the end of the phenyl groups, the hydroxyl group at the meta-position of phenyl groups in BPCA-C6-PmOH was first moved to the para-position to become BPCA-C6-PpOH, and then changed to a methoxy group to become BPCA-C6-PmOH3. BPCA-C6-P, having no functional groups on the phenyl groups, was also synthesized. Finally, one more compound was synthesized to disrupt the formation of the H-bonded head-to-head dimers by replacing the H group on the carboxylic acid with an ethyl group to become BPCE-C6-PmOH. Phase structures of this series of compounds were characterized by wide-angle X-ray diffraction (WAXD), selected area electron diffraction, and differential scanning calorimetry. The phase identifications were also supported by the observation of morphological changes via polarized light microscopy. The formation of H-bonds between carboxylic acids as well as between hydroxyl groups was also studied using infrared spectroscopy (FT-IR). The two-dimensional WAXD alone, with the FT-IR experiments allowed us to understand the existence and stabilization of the synclinically tilted SmC (SmCs) phase in the cases of BPCA-C6-PmOH and BPCA-C6-PpOH. In both cases, dimers can form H-bonds between the hydroxyl groups at the end of the dimers in the SmCs phases. In the cases of BPCA-C6-PmOH3 and BPCA-C6-P, they packed into an anticlinically tilted SmC (SMCA) phase. Except for BPCE-C6-PmOH, this series of compounds formed helical supramolecular structures in the SmC phases similar to those of BPCA-Cn-PmOH, regardless of whether their dimers were packed into synclinically or anticlinically tilted SmC phases. Therefore, the construction of the dimers was essential to form stable helical structures, but the -OH or -OCH3 groups at the meta- or para-position of the phenyl groups were not critical. Computer simulation results indicated that the head-to-head dimers possessed a twist rather than a bent conformation. These could be the chemical origin of the helical supramolecular structures. Furthermore, the physical origin of the helical structure could be the dimer tilting toward the long axis of LC cylinders in the SmA networks during the transition from the SmA to SmC phases. This dimer tilting led to an elongation of the LC cylinders along their long axis and a decrease in the diameter of the cylinders. Since the cylinder lengths are fixed in the SmA networks, a mechanical compression was generated on the cylinders to force them to be twisted when the compounds entered the SmC phase.