Search for microscopic and macroscopic biaxiality in the cybotactic nematic phase of new oxadiazole bent-core mesogens

Abstract

The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales. Locally, at the scale of a few molecules, x-ray-diffraction data demonstrate biaxial packing. However, above the mesoscopic scale, the global orientational order in all three compounds is uniaxial, as evidenced by uniform homeotropic alignment of the nematic phase which is optically tested over the entire temperature range and by the observations of topological defects induced by individual and aggregated colloidal spheres in the nematic bulk.

Control of the Folding Dynamics of Self‐Reconfiguring Magnetic Microbots Using Liquid Crystallinity

“…As our microcubes have edge lengths ( L ) of 10 μm, which is larger than K/W = 0.1 to 1.0 μm (i.e., W r I 2 K r I ), the microcubes reorient the neighboring LCs from n 0 . The microcubes thus generate distinct sets of topological defects to satisfy the topological conservation law (theorems of Poincaré and Gauss) that demand a certain number of singularities around microinclusions with a nonzero Euler characteristic (e.g., 1 for a square, 2 for a cube) …”

“…From the optical characterization discussed above and by assuming that the microcubes exhibit strong surface anchoring (i.e., near‐surface n is strictly tangential or homeotropic), we mapped the director fields and associated sets of topological defects around the microcubes (Figure d,h). To satisfy the conservation law, the defects observed at the corners of nontreated microcubes (circles in Figure c) should correspond to line defects (disclinations) along edges perpendicular to n 0 (// z ; axes defined in Figure a) with topological strengths of m = +1/4 (red dots in Figure d) and m = −1/2 (blue dots in Figure d). In contrast, the DMOAP‐treated microcubes exhibited a disclination loop of m = −1/2 (Saturn‐ring defect; ellipsoid in Figure g and blue lines and dots in Figure h), and thus must possess surface disclinations of m = +1/4 at all edges (red dots and lines in Figure h).…”

“…In the presence of (// x ), the nontreated microbots exhibited six point defects of m = −1/2 at the center of their surfaces (circles in Figure a(i) and open blue dots in Figure a(iii)). To satisfy the conservation law, we predicted that surface disclinations of m = +1/4 form along the edges of microbot surfaces in the y–z plane (red dots and lines in Figure a(iii)), although it is difficult to experimentally locate surface disclinations . When was removed, however, the nontreated microbots folded and no longer possessed the point defects, leading to the formation of disclinations of m = +1/4 and −1/2 (red and blue dots in Figure b(iii), respectively) along the edges perpendicular to n 0 .…”

Comparative 2H NMR and X-Ray Diffraction Investigation of a Bent-Core Liquid Crystal Showing a Nematic Phase

“…Indeed, the accepted model of the biaxial 'cybotactic nematic' phase assumes the existence of a peculiar short-range ordering in the form of nanometer-sized clusters having a layered supramolecular structure (usually tilted, i.e., smectic C-like) with intrinsic biaxial orientational order [15]. The main evidence of this 'cybotactic' local arrangement is given by the characteristic four-spot small-angle (SA) X-ray XRD feature [15][16][17][18], which is sensibly different from the two-spot pattern exhibited by conventional calamitic nematics.The occurrence of nano-sized aligned domains or local clusters in the nematic phase of bent-core LCs was also supported by independent studies based on Dynamic Light Scattering (DLS) [20] and on a combination of NMR experiments performed on similar five-rings banana-shaped molecules [21][22][23]. In particular, 2 H NMR static spectra recorded in the isotropic and in the nematic phase of these BLC nematogens were characterized by an unusual line-broadening [24], related to both dynamic and static reasons, which were further investigated by means of 2 H NMR relaxation times [25], 1 H NMR relaxometry [26] and 1 H NMR diffusometry [27].…”

“…Indeed, the accepted model of the biaxial 'cybotactic nematic' phase assumes the existence of a peculiar short-range ordering in the form of nanometer-sized clusters having a layered supramolecular structure (usually tilted, i.e., smectic C-like) with intrinsic biaxial orientational order [15]. The main evidence of this 'cybotactic' local arrangement is given by the characteristic four-spot small-angle (SA) X-ray XRD feature [15][16][17][18], which is sensibly different from the two-spot pattern exhibited by conventional calamitic nematics.…”