Chiral Incommensurate Helical Phase in a Smectic of Achiral Bent-Core Mesogens.

An achiral, bent-core mesogen forms several tilted smectic liquid crystal phases, including a nonpolar, achiral de Vries smectic A which transitions to a chiral, ferroelectric state in applied electric fields above a threshold. At lower temperature, a chiral, ferrielectric phase with a periodic, supermolecular modulation of the tilt azimuth, indicated by a Bragg peak in carbon-edge resonant soft x-ray scattering, is observed. The absence of a corresponding resonant umklapp peak identifies the superlayer structure as a twist-bend-like helix that is only weakly modulated by the smectic layering.

Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of achiral molecular dimers.

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.

Arrays of Molecular Rotors with Triptycene Stoppers: Surface Inclusion in Hexagonal Tris(o-phenylenedioxy)cyclotriphosphazene.

A new generation of rod-shaped dipolar molecular rotors designed for controlled insertion into channel arrays in the surface of hexagonal tris(o-phenylenedioxy)cyclotriphosphazene (TPP) has been designed and synthesized. Triptycene is used as a stopper intended to prevent complete insertion, forcing the formation of a surface inclusion. Two widely separated (13)C NMR markers are present in the shaft for monitoring the degree of insertion. The structure of the two-dimensional rotor arrays contained in these surface inclusions was examined by solid-state NMR and X-ray powder diffraction. The NMR markers and the triptycene stopper functioned as designed, but half of the guest molecules were not inserted as deeply into the TPP channels as the other half. As a result, the dipolar rotators were distributed equally in two planes parallel to the crystal surface instead of being located in a single plane as would be required for ferroelectricity. Dielectric spectroscopy revealed rotational barriers of ∼4 kcal/mol but no ferroelectric behavior.

Chiral isotropic sponge phase of hexatic smectic layers of achiral molecules.

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.

P2/O3 Biphasic Cathode Material through Magnesium Substitution for Sodium-Ion Batteries.

P2-type Fe-Mn-based oxides offer excellent discharge specific capacity and are as affordable as typical layered oxide cathode materials for sodium-ion batteries (SIBs). After Cu modification, though they can improve the cycling performance and air stability, the discharge specific capacity will be reduced. Considering the complementary nature of biphasic phases in electrochemistry, hybridizing P2/O3 hybrid phases can enhance both the storage performance of the battery and specific capacity. Herein, a hybrid phase composite with high capacity and good cycle performance is deliberately designed and successfully prepared by controlling the amount of Mg doping in the layered oxide. It has been found that the introduction of Mg can activate anion redox in the oxide layer, resulting in a significant increase in the specific discharge capacity of the material. Meanwhile, the dual-phase structure can produce an interlocking effect, thus effectively alleviating structure strain. The degradation of cycling performance caused by structural damage during the high-voltage charging and discharging process is clearly mitigated. The results show that the specific discharge capacity of Na0.67Cu0.2Mg0.1Fe0.2Mn0.5O2 is as high as 212.0 mAh g-1 at 0.1C rate and 186.2 mAh g-1 at 0.2C rate. After 80 cycles, the capacity can still maintain 88.1%. Moreover, the capacity and cycle performance as well as the stability can still remain stable even in the high-voltage window. Therefore, this work offers an insightful exploration for the development of composite cathode materials for SIBs.

Inclusion compound based approach to arrays of artificial dipolar molecular rotors: bulk inclusions.

We examine the insertion of two dipolar molecular rotors as guests into a host, tris(o-phenylenedioxy)cyclotriphosphazine (TPP, 1), using differential scanning calorimetry, solid-state NMR, powder X-ray diffraction, and dielectric spectroscopy. The rotors are 1-(4'-n-pentylbiphenyl-4-yl)-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane and 1,12-bis(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane. Both enter the bulk even though their nominal diameter exceeds the nominal channel diameter and although a closely related rotor, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, is known to produce a surface inclusion compound. Rotational barriers of 5.4-9.3 kcal/mol were found for the dichlorophenyl rotator contained within the TPP channel. Clearly, van der Waals diameters in themselves do not suffice to predict TPP channel entry. It is suggested that the efficacy of the p-carborane stopper is reduced by the presence of the two relatively bulky adjacent benzene rings, which help to stretch the channel, and by the axial direction of its axis, which prevents the attached rotator from contributing to the stopping action.

Topological ferroelectric bistability in a polarization-modulated orthogonal smectic liquid crystal.

We report a bent-core liquid crystal (LC) compound exhibiting two fluid smectic phases in which two-dimensional, polar, orthorhombic layers order into three-dimensional ferroelectric states. The lower-temperature phase has a uniform polarization field which responds in an analog fashion to applied electric field. The higher-temperature phase is a new smectic state with periodic undulation of the polarization, structurally modulated layers, and a bistable response to applied electric field which originates in the periodically splay-modulated bulk of the LC rather than by surface stabilization at the cell boundaries.

Inclusion compound based approach to arrays of artificial dipolar molecular rotors. A surface inclusion.

We describe an approach to regular triangular arrays of dipolar molecular rotors based on insertion of dipolar rotator carrying shafts as guests into channels of a host, tris(o-phenylenedioxy)cyclotriphosphazene (TPP). The rotor guests can either enter the bulk of the host or stay at or near the surface, if a suitable stopper is installed at the end of the shaft. Differential scanning calorimetry, solid-state NMR, and powder X-ray diffraction were used to examine the insertion of a dipolar rotor synthesized for the purpose, 1-n-hexadecyl-12-(2,3-dichlorophenyl)-p-dicarba-closo-dodecaborane, and it was found that it forms a surface inclusion compound. Rotational barriers from 1.2 to 9 kcal/mol were found by dielectric spectroscopy and were attributed to rotors inserted into the surface to different degrees, some rubbing the surface as they turn.

Boosting performance of Co-free Li-rich cathode material through regulating the anionic activity by means of the strong TaO bonding.

Benefiting from the extra contribution of O redox, Co-free Li-rich layered oxides (LRNMO) can satisfy the requirement of high specific capacities. However, during the high-voltage charging process, lattice oxygen being oxidized to O- or O2 leads to a gradual transition of the structure from layered to spinel phase, capacity and voltage decline, hindering the practical application of LRNMO in the lithium-ion battery. Here, a surface modification strategy of Li1.2Ni0.32Mn0.48O2-δ doped with Ta5+ ions is proposed, in which the Ta5+ ions occupy the lithium sites of the lattice structure on the surface layer of LRNMO and form a Ta2O5 coating layer. The modified electrode exhibits excellent rate performance and cycling stability, with 94.9% and 85.5% capacity retention rate and voltage retention rate, respectively, after 200 cycles at 1C. Moreover, the initial coulomb efficiency and ionic conductivity of the modified electrode are also apparently enhanced. Simultaneously, the decreased Li/Ni mixing degree of the modified electrode reflects the improvement of the structural stability. Therefore, the modification strategy through strong metal-oxygen bonding to integrate the surface structure to regulate the oxygen activity provides a new direction for the design of high energy density Co-free Li-rich cathode materials.

New SmAPF Mesogens Designed for Analog Electrooptics Applications.

We have previously reported the first realization of an orthogonal ferroelectric bent-core SmAPF phase by directed design in mesogens with a single tricarbosilane-terminated alkoxy tail. Given the potentially useful electrooptic properties of this phase, including analog phase-only electrooptic index modulation with optical latching, we have been exploring its "structure space", searching for novel SmAPF mesogens. Here, we report two classes of these-the first designed to optimize the dynamic range of the index modulation in parallel-aligned cells by lowering the bend angle of the rigid core, and the second expanding the structure space of the phase by replacing the tricarbosilane-terminated alkyl tail with a polyfluorinated polyethylene glycol oligomer.

Selection and Validation of Reference Genes for Gene Expression Analysis in Vigna angularis Using Quantitative Real-Time RT-PCR.

Adzuki bean (Vigna angularis) is one of the most important legume crops in Asian countries like China, Japan and Korea due to its nutritious protein and starch contents. In spite of its economic importance, gene expression analysis system for gene function verification of adzuki bean is still absent. Therefore, reference genes for gene expression analysis based on the quantitative real time PCR (qRT-PCR) were screened in current study. A total of nine general housekeeping genes, including ACT, Fbox, ZMPP, GAPDH, EF, PP2A, UBC, UBN and PTB were evaluated for their expression stability by qRT-PCR in four adzuki bean cultivars, three different tissues, four abiotic stress and one biotic stress. The best group of candidates as reference genes were as follows: PTB and ACT for different cultivars; EF and UBN for different tissues; ACT and ZMPP for biotic stress and waterlogging stress; Fbox and UBC for salinity-alkalinity stress; Fbox and PTB for drought stress. Our results will provide a more accurate and reliable normalization of qRT-PCR data in adzuki bean.

Airflow-aligned helical nanofilament (B4) phase in topographic confinement.

We investigated a controlled helical nanofilament (HNF: B4) phase under topographic confinement with airflow that can induce a shear force and temperature gradient on the sample. The resulting orientation and ordering of the B4 phase in this combinational effort was directly investigated using microscopy. The structural freedom of the complex B7 phase, which is a higher temperature phase than the B4 phase, can result in relatively complex microscopic arrangements of HNFs compared with the B4 phase generated from the simple layer structure of the B2 phase. This interesting chiral/polar nanofilament behaviour offers new opportunities for further exploration of the exotic physical properties of the B4 phase.

Time-Resolved Fluorescence Anisotropy of Bicyclo[1.1.1]pentane/Tolane-Based Molecular Rods Included in Tris(o-phenylenedioxy)cyclotriphosphazene (TPP).

We examine the fluorescence anisotropy of rod-shaped guests held inside the channels of tris(o-phenylenedioxy)cyclotriphosphazene (TPP) host nanocrystals, characterized by powder X-ray diffraction and solid state NMR spectroscopy. We address two issues: (i) are light polarization measurements on an aqueous colloidal solution of TPP nanocrystals meaningful, or is depolarization by scattering excessive? (ii) Can measurements of the rotational mobility of the included guests be performed at low enough loading levels to suppress depolarization by intercrystallite energy transfer? We find that meaningful measurements are possible and demonstrate that the long axis of molecular rods included in TPP channels performs negligible vibrational motion.

An electric-field-responsive discotic liquid-crystalline hexa-peri-hexabenzocoronene/oligothiophene hybrid.

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.

Cybotactic behavior in the de Vries smectic-A* liquid-crystal structure formed by a silicon-containing molecule.

We have identified a metastable liquid-crystal (LC) structure in the de Vries smectic-A* phase (de Vries Sm-A*) formed by silicon-containing molecules under certain boundary conditions. The phase transition with the metastable structure was observed in a LC droplet placed on a planar aligned substrate and LCs confined in the groove of a silicon microchannel. During the rapid cooling step, a batonnet structure was generated as an intermediate and metastable state prior to the transition that yielded the thermodynamically stable toric focal conic domains. This distinctive behavior was characterized using depolarized reflection light microscopy and grazing incidence x-ray diffraction techniques. We concluded that the silicon groups in the molecules that formed the de Vries phase induced the formation of layered clusters called cybotactic structures. This observation is relevant to an exploration of the physical properties of cybotactic de Vries phases and gives a hint as to their optoelectronic applications.

Inclusion compound based approach to forming arrays of artificial dipolar molecular rotors: a search for optimal rotor structures.

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.

Generalized Langevin-Debye model of the field dependence of tilt, birefringence, and polarization current near the de Vries smectic-A* to smectic-C* liquid crystal phase transition.

In chiral smectic-A (Sm-A) liquid crystals, an applied electric field induces a tilt of the optic axis from the layer normal. When these materials are of the de Vries type, the electroclinic tilt susceptibility is unusually large, with the field-induced director reorientation accompanied by a substantial increase in optical birefringence with essentially no change in the smectic layer spacing. In order to account for the observed electro-optic behavior, we assume that the molecular orientation distribution in the Sm-A has two degrees of freedom: azimuthal orientation and tilt of the molecular long axis from the layer normal, with the tilt confined to a narrow range of angles. We present a generalized Langevin-Debye model of the response of this orientational distribution to applied field that gives a field-induced optic axis tilt, birefringence, and polarization dependence that agrees well with experimental measurements and reproduces the double-peaked polarization current response characteristic of a first-order Sm-A(*)-Sm-C(*) transition. Additionally, we find that the measured field-induced polarization and the Langevin-Debye model predictions can be quantitatively described as pre-transitional behavior near the tricritical point of a recently published generalized 3D XY model of interacting hard rods confined to reorient on a cone in the presence of an applied field.

Transitions between paraelectric and ferroelectric phases of bent-core smectic liquid crystals in the bulk and in thin freely suspended films.

We report on the contrasting phase behavior of a bent-core liquid crystal with a large opening angle between the mesogenic units in the bulk and in freely suspended films. Second-harmonic generation experiments and direct observation of director inversion walls in films in an applied electric field reveal that the nonpolar smectic C phase observed in bulk samples becomes a ferroelectric "banana" phase in films, showing that a mesogen with a small steric moment can give a phase with polar order in freely suspended films even when the corresponding bulk phase is paraelectric.

Effective conductivity due to continuous polarization reorientation in fluid ferroelectrics.

A smectic-A (SmA) liquid crystal phase of fluid layers with in-plane polarization P is shown to exhibit effective conductivity in the semiconducting range during electric-field-induced polarization reorientation, but becomes insulating once the polarization is aligned with the field. Such fluid ferroelectrics sandwiched between highly insulating layers enable electro-optic devices with long-term dc electrostatic control of polarization and optic axis orientation.