Four series of new 1,2,4-oxadiazole derived bent-core liquid crystals incorporating one or two cyclohexane rings are synthesized and investigated by optical polarizing microscopy, differential scanning calorimetry (DSC), X-ray diffraction (XRD), electro-optical, and dielectric investigations. All the compounds exhibit wide ranges of nematic phases composed of tilted smectic (SmC-type) cybotactic clusters with strongly tilted aromatic cores (40-57 ° ) and show a distinct peak in the current curves observed under a triangular wave fi eld. Dielectric spectroscopy of aligned samples corroborates the previously proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases. Hence, it is shown that this is a general feature of the nematic phases of structurally different 3,5-diphenyl-1,2,4-oxadiazole derivatives. In these uniaxial nematic phases there is appreciable local biaxiality and polar order in the cybotactic clusters. As a second point it is shown that electric fi eld induced fan-like textures, as often observed for the nematic phases of bent-core liquid crystals, do not indicate the formation of a smectic phase, rather they represent special electro-convection patterns due to hydrodynamic instabilities.

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Several series of bent-core mesogens derived from 3,5-diphenyl-1,2,4-oxadiazole with or without lateral groups and with different length terminal chains at both ends, and polycatenar molecules with three to six alkoxy chains are synthesized and their mesomorphic behaviour is investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction (XRD), dielectric, electro-optical and second-harmonic generation (SHG) experiments. Most compounds exhibit broad regions of skewed cybotactic nematic (NcybC ) and tilted smectic (SmC) phases with a strong tilt of the aromatic cores (up to 63°), but non-tilted SmA and NcybA phases are also observed for a compound that has only one terminal chain. The XRD patterns of the nematic phases of most of the compounds investigated indicate a 2D periodicity with short correlation length in the magnetically aligned samples. This is of importance for the general interpretation of the small-angle XRD splitting patterns typically observed for aligned samples of bent-core nematic phases. In most nematic phases one current peak is observed in the half period of an applied electric field, though no coherent signal is found in the SHG experiments. Based on additional electro-optical and dielectric results, the nematic phases are considered to be cybotactic nematic phases with local polar order, and show a dielectric reorientation of the polar domains. Only chiral nematic phases (NcybC *), but not blue phases, are obtained for compounds with one or two chiral (3S)-3,7-dimethyloctyloxy tail(s).

Thermal and extensive rheological characterization of a nematic liquid crystal gelated with a novel monodisperse dipeptide, also a liquid crystal, has been carried out. For certain concentrations, the calorimetric scans display a two-peak profile across the chiral nematic-isotropic (N*-I) transition, a feature reminiscent of the random-dilution to random-field crossover observed in liquid crystal gels formed with aerosil particles. All samples show shear thinning behavior without a Newtonian plateau region at lower shear rates. Small deformation oscillatory data at lower frequencies exhibit a frequency dependence of the storage (G') and loss (G'') moduli that can be described by a weak power-law, characteristic of soft glassy rheological systems. At higher frequencies, while lower concentration composites have a strong frequency dependence with a trend for possible crossover from viscoelastic solid to viscoelastic liquid behavior, the higher-concentration gels show frequency-independent rheograms of entirely elastic nature G' > G''. The plateau modulus of G' is described by a power-law with an exponent again common to soft materials, such as foams, slurries, etc. Other features which are a hallmark of such materials observed in the present study are: (i) above a critical strain, a strain softening of the moduli with a peak in the loss modulus, (ii) power-law variation of the storage modulus in the nonlinear viscoelastic regime, and (iii) absence of Cox-Merz superposition for the complex viscosity. An attractive feature of these gels is the fast recovery upon removal of large strain and qualitatively different temporal behavior of the recovery between the low and high concentration composites, with the latter indicating the presence of two characteristic time scales.

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Herein, we demonstrate that with the widespread theme of residue patterning and stereochemical restraints of self-complimenting proteinogenic amino acids, a new and rich class of homomeric dipeptides exhibiting two-dimensional fluid aggregates with hierarchical ordering can be obtained. In particular, a simple way of achieving a class of functional dipeptides, wherein the first and the second residues chosen are L-/D-alanines and L-/D-leucines, has been accomplished. The supramolecules synthesized can be regarded as intermediates between polycatenars and taper-shaped amphiphiles because they possess two lipophilic segments interlinked by a peptide unit (spacer). Two pairs of enantiomers and their respective diastereomers derived from these amino acids are evidenced to self-organize into a helical columnar phase through hydrogen bonding by means of FTIR, UV/Vis, and chiroptical circular dichroism (CD) spectral analyses as well as by optical, calorimetric, electrical switching, and X-ray studies. The CD and X-ray studies have revealed that the form chirality (handedness) and the magnitude of out-of-plane fluctuations of the lattice planes of the fluid supramolecular columnar structures are solely directed by the stereochemistry encoded in the spacer. Of special significance, the less frequently found oblique helical columnar phase formed by a pair of enantiomers derived from L-/D-alanines, unlike those derived from other amino acids, exhibit ferroelectric behavior; the measured spontaneous polarization is as high as 440 nC cm(-2). Besides, all these supramolecules form stable organogels in ethanol and the CD and SEM studies on a representative gel suggest the presence of helical structures.

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The dielectric spectra of the twist bend nematic phase (N_{TB}) of an achiral asymmetric bent-core liquid crystalline compound are studied for determining the various relaxation modes. Dielectric measurements are also carried out under the bias field E up to 8 V/µm. Two molecular and two collective relaxation processes are observed. The orientational order parameters with respect to the local and the main directors determined using molecular modes are used to find the heliconical angle. The results also show that the order parameter with reference to the main director reverses its trend from increasing to decreasing at temperatures of a few degrees above the N_{TB} to N transition. The collective relaxation modes are assigned to (a) distortions of the local director by the electric field at a frequency of ∼100kHz while the periodic helical structure remains unaltered (mode attributed to flexoelectricity); (b) changes in the periodic structure arising from a coupling of the dielectric anisotropy with the electric field at the lowest frequency in the range of 100 Hz-10 kHz. Frequency of the higher frequency collective mode (∼100kHz) depends primarily on the heliconical angle and has anomalous softeninglike behavior at the N-N_{TB} transition. The lowest frequency mode is studied under the bias field E; the modulus of the wave vector gradually vanishes on increasing E (except for an initial behavior, E^{2}<0.1V^{2}/μm^{2}, which is just the opposite). The transition from the twist bend to splay bend structure is observed by a sudden drop in the frequency of this mode, followed by a linear decrease in frequency by increasing E. The results agree with the predictions made from the currently proposed models for a periodically distorted N_{TB} phase.