Evidence for Triclinic Symmetry in Smectic Liquid Crystals of Bent-Shape Molecules (original) (raw)
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Unusual Sequences of Tilted Smectic Phases in Liquid Crystals of Bent-Shape Molecules
Physical Review Letters, 2002
The first theoretical description of the experimentally often-observed isotropic-polar smectic phase transitions of symmetric and asymmetric bent-shape molecules is presented. The isotropic-tilted polar smectic phase transition requires a coupling among three types of ordering processes: polar packing, director tilt, and layer ordering. For explaining the unusual triclinic-monoclinic phase transition with the lower symmetry phase at higher temperature, observed in fluoro-containing compounds recently, a mechanism is proposed in which the layer ordering triggers the simultaneous onset of the tilt and polar order.
Occurrence of Five Different Orthogonal Smectic Phases in a Bent-Core (BC) Liquid Crystal
Molecular Crystals and Liquid Crystals, 2015
Five orthogonal smectic phases (Sm-A, Sm-A b , Sm-AP R , Sm-AP AR , and Sm-AP A) were observed in a 4-cyanoresorcinol bisbenzoate with two terephthalate-based wings achiral bent-core (BC) compound. The phase behavior was investigated by polarising optical microscopy, second harmonic electro-optic response (EO 2) and dielectric spectroscopy. The field dependent biaxiality of polar and non-polar Sm-A phases was studied. It is obviously of enormous scientific significance and practical interest to find five different orthogonal smectic phases in a low molecular weight bent-core (BC) compound at relatively low temperatures.
A novel smectic liquid crystalline phase exhibited by W-shaped molecules
Journal of Materials Chemistry, 2003
A novel smectic phase has been discovered in the W-shaped molecular system of N,N'-bis[2-hydroxy-5-(4'-ntetradecylphenylazo)benzylidene]-4-nitro-1,3-phenylenediamine. On slowly cooling this material from the isotropic phase, fine filaments of a chiral nature appear. When they cover the whole surface, schlieren-like textures with two-brush defects are observed. In the texture, fine stripes formed by the filaments exist, which represent an orientational map. Each filament in a specific region has its own chiral character and forms chiral domains, as identified by uncrossed polarizers. According to X-ray analysis, a layer structure with a periodicity of the molecular length is clearly seen. Several diffuse but distinct wide-angle diffraction peaks are observed in the smectic phase and become sharp in the lower-temperature crystalline phases. Based on these experimental results, we can conclude the existence of a new phase with a chiral nature.
Two distinct modulated layer structures of an asymmetric bent-shape smectic liquid crystal
Liquid Crystals, 2012
We present chemical synthesis, polarising optical microscopy, electric current, and small-angle X-ray scattering measurements on a strongly anisotropic bent-shape liquid crystal material. We find that it has two layer-undulated ferroelectric phases, M1 and M2. In the higher temperature M2 phase the undulation amplitude and period increases on cooling, similar to other published materials. It can be identified with a tilted modulated phase. In the M1 phase the molecular plane is not tilted, and -in sharp contrast with all prior observations -the modulation amplitude and period decrease on cooling. These observations can be explained with a 'leaning' director structure, where the leaning angle is decreasing on cooling.
Chirality and polarity transfers between bent-core smectic liquid-crystal substances
2006
Liquid-crystal phases with both solid and liquid characteristics are unique, since they exhibit chirality on both molecular and mesoscopic levels under special circumstances. We have studied binary composites formed by mixing of a chiral dimer ͑B-Ch͒, in which a bent core is covalently linked to a promesogenic cholesterol unit through a flexible spacer, and an achiral bent-core mesogen having two vicinal fluorine atoms ͑B-2F͒. The pure materials show smectic phases featuring C 2 symmetry. The dimer B-Ch displays conventional chiral smectic C ͑SmC * ͒ phase with a chirality-induced polarization P c ϳ 30 nC/ cm 2 , whereas B-2F possesses synclinic and anticlinic antiferroelectric "banana" smectic ͑SmCP͒ phase with a relatively high-polarization ͑P b ͒ value of about 900 nC/ cm 2. Mixing these two materials we were able to measure P c , and we have verified that the molecular chirality contributes to the polarization of banana phases only in synclinic configurations.
2011
The mesomorphic properties of an achiral bent-core liquid crystal derived from 4-cyanoresorcinol are studied by polarizing optical microscopy, x-ray diffraction, and second harmonic electro-optic response. It shows a novel sequence of four nontilted or orthogonal smectic phases on cooling: SmA-SmAP R-SmAP X-SmAP A. Here SmAP X is the new orthogonal polar uniaxial smectic phase. The electric-field-induced transformations in the SmAP X phase give rise to two biaxial states separated by a uniaxial one. The second harmonic electro-optic response in this phase is interpreted in terms of the polar interaction with the electric field. A comparison of the experimental results with the next-nearest-neighbor model for the structure of the SmAP X phase shows it to be an SmAP phase.
Physical Review Research, 2020
The physical properties and the phase transitions of a smectic liquid crystal material built from achiral bent-core molecules are investigated by electro-optics, polarizing microscopy, birefringence, and spontaneous polarization. This material is shown to exhibit a number of orthogonal and tilted smectic phases. The observation of an anomalous phase transition on cooling from a tilted to orthogonal antiferroelectric smectic phase at lower temperatures is made, and a proof of the evidence is presented. The importance of the biaxial anchoring term in the free energy is modeled and discussed.