Nandiraju V. S. Rao

Organization of the polarization splay modulated smectic liquid crystal phase by topographic confinement

Recently, the topographic patterning of surfaces by lithography and nanoimprinting has emerged as a new and powerful tool for producing single structural domains of liquid crystals and other soft materials. Here the use of surface topography is extended to the organization of liquid crystals of bent-core molecules, soft materials that, on the one hand, exhibit a rich, exciting, and intensely studied array of novel phases, but that, on the other hand, have proved very difficult to align. Among the most notorious in this regard are the polarization splay modulated (B7) phases, in which the symmetry-required preference for ferroelectric polarization to be locally bouquet-like or “splayed” is expressed. Filling space with splay of a single sign requires defects and in the B7 splay is accommodated in the form of periodic splay stripes spaced by defects and coupled to smectic layer undulations. Upon cooling from the isotropic phase this structure grows via a first order transition in the form of an exotic array of twisted filaments and focal conic defects that are influenced very little by classic alignment methods. By contrast, growth under conditions of confinement in rectangular topographic channels is found to produce completely new growth morphology, generating highly ordered periodic layering patterns. The resulting macroscopic order will be of great use in further exploration of the physical properties of bent-core phases and offers a route for application of difficult-to-align soft materials as are encountered in organic electronic and optical applications.

Four-ring achiral unsymmetrical bent core molecules forming strongly fluorescent smectic liquid crystals with spontaneous polar and chiral ordered B7 and B1 phases

Achiral molecules based on a novel four-ring core with an ester linkage at the molecular bend are shown to exhibit smectic liquid crystal phases with spontaneously chiral and polar layers (SmCP phases), including their polarization splay modulated and layer undulated (PMLU) variants, B7 and B1. Additionally, these compounds exhibit strong photoluminescence, the first family of SmCP phases to do so.

Ferroelectric-like switching in the nematic phase of four-ring bent-core liquid crystals

The recent discovery of a ferroelectric response to a switching electric field in nematic phases exhibited by bent-core polar molecules, particularly 1,2,4-oxdiazole derivatives, could lead to new avenues for the development of electro-optic devices. For the first time, we report ferroelectric-like switching under the influence of a triangular wave electric field in the nematic phase exhibited by unsymmetrical achiral four-ring bent-core compounds. These bent-core molecules, exhibiting a large nematic phase range (>70 °C), consist of two unequal lengths in two wings and possess a polar moiety at one end and an alkyloxy chain at the other end. An anomalous variation in spontaneous polarization as a function of temperature in the nematic phase is distinctly observed, which is similar to the results reported only in the low temperature region of the nematic phase. Electro-optical, current-response and dielectric studies of aligned samples corroborate earlier reports of the proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases.

Viscoelasticity, dielectric anisotropy, and birefringence in the nematic phase of three four-ring bent-core liquid crystals with an L-shaped molecular frame

Molecular shape is an important factor in determining the material properties of thermotropic liquid crystals (LCs). We synthesized and investigated several LC compounds formed by asymmetrically bent molecules with a rigid four-ring core in the shape of the letter ‘L’. We measured the temperature dependencies of dielectric permittivities, birefringence, splay K1 and bend K3 elastic constants, splay viscosity ηsplay and flow viscosities η|| and η⊥. The bend–splay anisotropy δK31 = K3 − K1 is negative, similar to the case of nematic LCs formed by symmetrically bent molecules of V-shape. The dielectric anisotropy Δe and birefringence are positive in the entire nematic range. The splay viscosity ηsplay and the flow viscosities η|| and η⊥ are smaller than the viscosities measured for the symmetric V-shaped bent-core materials at similar temperatures. The ratio Γ = ηsplay/η||,⊥ is in the range 5–4 that is typical for rod-like LCs. The reported L-shaped bent-core nematic LCs combine the useful features of bent-core LCs (such as a negative δK31, suitable for formulation of broad-range blue phases) with the relatively low viscosities, a property typical for rod-like LCs and beneficial for electro-optic switching applications.

Homeotropic alignment and director structures in thin films of triphenylamine-based discotic liquid crystals controlled by supporting nanostructured substrates and surface confinement.

We explore the effects of nanoscale morphology of supporting solid substrates on alignment, defects, and director structures exhibited by thin films of triphenylamine-based discotic liquid crystals. Fluorescence confocal polarizing microscopy and intrinsic polarized fluorescence properties of studied molecules are used to visualize three-dimensional director fields in the liquid crystal films. We demonstrate that, by controlling surface anchoring on supporting or confining solid substrates such as those of carbon nanotube electrodes on glass plates, both uniform homeotropic and in-plane (edge-on) alignment and nonuniform structures with developable domains can be achieved for the same discotic liquid crystal material.

Synthesis and Mesomorphic Properties of N(4-n-alkyloxy salicylidene)4′-n-alkylanilines and their Copper Complexes I

A series of metal (Cu) complexes with bidentate Schiff-bases, i.e., N(4-n-alkyloxy salicylidene)4′-n-alkylanilines obtained by the 1:1 condensation of 4-n-alkyloxy salicylaldehyde and 4-n-alkylanilines, i.e., 4-n-hexylaniline, 4-n-heptylaniline, and 4-n-octylaniline, have been prepared and characterized. The synthesis of the ligands N(4-n-butyloxysalicylidene)-4′-n-alkylanilines as well as the complexes and characterization by thermal microscopy for mesomorphism are presented. The ligands are found to exhibit interesting smetic phases (mostly tilted phases) and polymorphism, while the complexes showed orthogonal smectic A, smectic B, and smectic E phases. The mesomorphic range as well as the polymorphism decreases with increase in chain length. However, the coordination with copper leads to thermally stable metallomesogens possessing larger mesomorphic range but with a reduced number of mesomorphic phases.

Influence of the chloro substituent on the mesomorphism of unsymmetrical achiral four-ring bent-core compounds: 2D polarization modulated banana phases

The synthesis and characterization of stable achiral unsymmetrical four-ring banana-shaped molecules consisting of a laterally chloro substituted 3,4′-disubstituted biphenyl unit with an ester linkage between the phenyl rings as the central unit in 4-(N-4′-n-tetradecyloxysalicylidene)aminophenyl [2-chloro-5-(N-4′-n-tetradecyloxysalicylidene) aminobenzoate] and 4-(N-4′-n-alkyloxysalicylidene)aminophenyl [4-chloro-5-(N-4′-n-alkyloxysalicylidene) aminobenzoate] are presented. These compounds are thermally and hydrolytically stable due to intramolecular hydrogen bonding and exhibit polarization splay modulated and layer undulated (PMLU) B7/B1Rev/Tilted (2D-polarization modulated layer undulated smectic phase, B1RevTilt phase) phase variants. The phase transitions have been confirmed by differential scanning calorimetry and the phases are characterized by polarized optical microscopy. Two representative examples have been characterized by X-ray diffraction studies. DFT calculations of the bending angle, dipole moments, molecular polarizabilities and voltage holding ratio are also presented.

Unusual temperature dependence of smectic layer structure associated with the nematic–smectic C phase transition in a hockey-stick-shaped four-ring compound

In a newly designed four-ring asymmetrical bent-core compound, we observed smectic-C-type diffuse layer reflection over the entire nematic temperature range. At the nematic–smectic C phase transition, a sharp layer reflection emerges in addition to the diffuse reflection with different layer tilt angles.

A novel smectic liquid crystalline phase exhibited by W-shaped molecules

A novel smectic phase has been discovered in the W-shaped molecular system of N,N′-bis[2-hydroxy-5-(4′-n-tetradecylphenylazo)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.

Novel chiral filament in an achiral W-shaped liquid crystalline compound

Anomalous optical birefringence and optical rotation have been observed in a liquid crystal phase comprising of W-shaped liquid crystal molecules with hydrogen bonds. The texture of this phase exhibits characteristic chiral filaments, which form strongly segregated chiral domains similar to the B4 phase of conventional bent-core mesogens, despite of having no chiral carbon. Based on X-ray microbeam diffraction, polarizing and confocal optical microscope observations, the molecular arrangement in the filament is discussed, and the novel structure different from that in B4 is proposed; the smectic layers form concentric cylinder, on which the c-director of the W-shaped molecules form screw-like and/or polar arrangement, inducing herringbone anisotropic structure and chirality in each filament, as observed in the experiments.