Yongqiang Shen

A new class of polyphilic mesogens composed of a tolanphenyl carboxylate core, a chiral alkoxy tail, and a tricarbosilane terminated alkoxy tail was synthesized, and shown to self-organize into nanophase segregated, smectic liquid crystalline (LC) phases. It was found that the number of carbons in the alkyl spacer between the tricarbosilane and the core controlled the observed phase with perfect fidelity: odd-carbon spacers gave antiferroelectric phases, and even-carbon spacers gave ferroelectric phases. Even more interestingly, homologues with odd carbon number show the rare and useful chiral orthoconic antiferroelectric SmC_A* phase, where the optic axis tilt alternates from layer to layer between +45° and −45°, making the molecular directors in adjacent layers orthogonal. The even-carbon homologues exhibit the SmC* phase with close to 45° tilt. The findings demonstrate that the supramolecular self-organization of polyphilic mesogens may be controlled by the length of hydrocarbon spacers.

A rod-shaped molecular rotor consisting of a p-terphenyl shaft attached to p-carborane whose antipodal position carries a dipolar 2,3-dichlorophenyl rotator forms an inclusion compound with hexagonal tris-o-phenylenedioxycyclotriphosphazene (TPP). Results of solid-state NMR spectroscopy, X-ray powder diffraction, dielectric loss spectroscopy, and density functional theory calculations lead us to propose that the whole molecule inserts into the TPP channels, with the rotator located in the outermost surface layer. Although the placement and alignment of the dipoles at the surface appear favorable, the sample does not exhibit collective behavior even at 7 K, presumably due to the relatively large barrier to rotation (∼8.6 kcal/mol). In incompletely annealed samples of the inclusion compound, some of the rotators protrude outside the surface and have a rotational barrier of ∼3.4 kcal/mol. In the inclusion compound of an analog in which the rotator is replaced with a methyl group, some of the methyl substituents are located inside the surface layer of TPP and others protrude above it.

Smectic layers of tilted, bent‐core liquid crystals have a tendency to exhibit spontaneous saddle‐splay curvature, a mechanical response that relieves the internal strain of the layers. When this tendency is strong enough, the smectic layers form complex, equilibrium, non‐planar structures such as the helical nanofilaments in the B4 phase and the disordered focal conics in the chiral dark conglomerate (DC) phase. The DC phase is usually observed on cooling directly from the isotropic phase, with the disordered focal conics analogous to the disordered sponge phase found in lyotropic systems. We report a DC phase observed below a B2 phase that is stable down to room temperature. In mixtures with the calamitic liquid crystal 8CB, the low‐temperature DC phase forms a more ordered, bicontinuous structure, resembling the cubic phase observed in the lyotropic systems, which is attributed to the enhanced intralayer ordering of the bent‐core molecules in the mixtures.

A novel hexa‐peri‐hexabenzocoronene/oligothiophene hybrid is shown to self‐assemble into a hexagonal columnar liquid crystalline phase, and respond to applied electric fields resulting in uniform homeotropic or parallel alignment depending upon the electrode structure. Furthermore, the columnar orientation can be maintained even after removal of the electric field unless the material is heated above the clearing temperature.

Freeze-fracture transmission electron microscopy study of the nanoscale structure of the so-called “twist–bend” nematic phase of the cyanobiphenyl (CB) dimer molecule CB(CH₂)₇CB reveals stripe-textured fracture planes that indicate fluid layers periodically arrayed in the bulk with a spacing of d ∼ 8.3 nm. Fluidity and a rigorously maintained spacing result in long-range-ordered 3D focal conic domains. Absence of a lamellar X-ray reflection at wavevector q ∼ 2π/d or its harmonics in synchrotron-based scattering experiments indicates that this periodic structure is achieved with no detectable associated modulation of the electron density, and thus has nematic rather than smectic molecular ordering. A search for periodic ordering with d ∼ in CB(CH₂)₇CB using atomistic molecular dynamic computer simulation yields an equilibrium heliconical ground state, exhibiting nematic twist and bend, of the sort first proposed by Meyer, and envisioned in systems of bent molecules by Dozov and Memmer. We measure the director cone angle to be θ_TB ∼ 25° and the full pitch of the director helix to be p_TB ∼ 8.3 nm, a very small value indicating the strong coupling of molecular bend to director bend. Significance The appearance of new nematic liquid crystal (LC) equilibrium symmetry (ground state) is a rare and typically important event. The first and second nematics were the helical phase and blue phase of chiral molecules, both found in 1886 in cholesteryl benzoate by Reinitzer, discoveries that marked the birth of LC science. The third nematic, the achiral uniaxial phase, also found in the 19th century, ultimately formed the basis of LC display technology and the portable computing revolution of the 20th century. Despite this achievement, the 20th can claim only the fourth nematic, the lyotropic biaxial phases found by Saupe. Now, early in the 21st, the heliconical structure of the fifth nematic is observed, an exotic chiral helix from achiral molecules.

Hexagonal tris(o‐phenylenedioxy)cyclotriphosphazene (TPP) is used as ahost for organizing dipolar molecular rotor guests into regular trigonal arrays. Inclusion of molecular rotors with transversely dipolar rotators into TPP channels is followed by solid‐state nuclear magnetic resonance, diifferential scanning calorimetry, X‐ray diffraction, and dielectric spectroscopy. The more polar of the two rotors does not form an inclusion. The second rotor forms two different inclusions differing in crystallite size and the rotational barriers.