Columnar Liquid Crystals in Cylindrical Nanoconfinement

Abstract

Axial orientation of discotic columnar liquid crystals in nanopores of inorganic templates, with the columns parallel to the axis of the nanochannels, is considered desirable for applications such as production of molecular wires. Here, we evaluate experimentally the role of the rigidity of the LC columns in achieving such orientation in nanopores where the planar anchoring (i.e., columns parallel to wall surface) is enforced. We studied the columnar phase of several discotic compounds with increasing column rigidity in the following order: dendronized carbazole, hexakis(hexyloxy)triphenylene (HAT6), a 1:1 HAT6-trinitrofluorenone (TNF) complex, and a helicene derivative. Using 2-D X-ray diffraction, AFM, grazing incidence diffraction, and polarized microscopy, we observed that the orientation of the columns changes from circular concentric to axial with increasing column rigidity. Additionally, when the rigidity is borderline, increasing pore diameter can change the configuration from axial back to circular. We derive expressions for distortion free energy that suggest that the orientation is determined by the competition between, on the one hand, the distortion energy of the 2-d lattice and the mismatch of its crystallographic facets with the curved pore wall in the axial orientation and, on the other hand, the bend energy of the columns in the circular configuration. Furthermore, the highly detailed AFM images of the core of the disclinations of strength +1 and +1/2 in the center of the pore reveal that the columns spiral down to the very center of the disclination and that there is no amorphous or misaligned region at the core, as suggested previously.

Inhomogeneous relaxation dynamics and phase behaviour of a liquid crystal confined in a nanoporous solid

“…Both the collective orientational (isotropic-to-nematic) and the translational (smectic-toliquid or smectic-to-nematic) transitions have turned out to be significantly affected by finite size and interfacial (solid-liquid or liquid-liquid) interactions introduced by confining walls [1][2][3][4][5][6] or the geometrical constraints in nanoporous media [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24].…”

High-resolution dielectric study reveals pore-size-dependent orientational order of a discotic liquid crystal confined in tubular nanopores

“…This competition between surface anchoring and tendency to form rigid, hexagonal columnar phases has turned out to be decisive for the formation of either axial or radial or circular concentric arrangements of the columns in cylindrical nano confinement [27][28][29][30][31][32][48][49][50]. For edge-on anchoring Zhang et.…”

“…DLCs can also easily be embedded into nanoporous media by melt infiltration [25][26][27][28][29][30][31]. One of the promising ideas in this respect is the synthesis of nanowires by this technique as pioneered by Steinhart et al [32].…”

“…Possible strategies to overcome this problem are modifications of host-guest interactions, i.e. chemical modification of the pore wall along with a proper choice of the guest DLC material and the pore diameter [30], or controlling the packing structure by applying external fields, e.g. by magnetic fields [33].…”

“…However, similarly as the dynamical and structural behavior of rod-like liquid crystalline systems [34][35][36][37][38][39][40][41][42][43][44][45][46][47], the properties of DLCs has turned out to be particularly susceptible to nano confinement [27][28][29][30][31][32][48][49][50]. Thus, new, simple experimental methods that are able to monitor the type of molecular ordering inside nanoporous media are strongly demanded.…”