Tailoring Chiral Discotic Liquid Crystals: Mesophase Engineering through Alternative Approaches and Chain Lengths.

Hydrogen (H)-bonding is crucial in constructing superstructures in chemical (such as chiral discotic liquid crystals (DLCs)) as well as in biological systems due to its specific and directional nature. In this context, we achieved the successful synthesis of two branches of heptazine-based H-bonded complexes using distinct strategies. Hpz*-Es-Cn , we incorporated chiral alkyl tails (Hpz-chiral) onto the central C3 symmetric heptazine core, connected to achiral benzoic acid derivatives (Es-Cn acid) through H-bonding. In Hpz-Es-Cn -acid*, we used an achiral heptazine derivative (Hpz-Es-Cn ) linked to a chiral acid via H-bonding. On the other hand, based on the DSC results, we observed that Hpz*-Es-Cn complexes exhibited three distinct phases, whereas Hpz-Es-Cn -acid* complexes displayed only a single mesophase. In polarized optical microscopy (POM) observations, all the complexes displayed birefringence at room temperature, with the color of the POM images changing as the temperature varied. X-ray diffraction (XRD) studies at lower temperatures confirmed that Hpz*-Es-C8 exhibited the columnar rectangular (Colr ) phase, while Hpz*-Es-C10/12 exhibited the columnar oblique (Colob ) phase. However, all the H-bonded complexes exhibited the columnar hexagonal (Colh ) phase at higher temperatures. The chiroptical spectra recorded by Circular dichroism (CD) highlight the specific observations in the columnar phase of two complexes, Hpz*-Es-C10 and Hpz*-Es-C12 . This behavior has potential applications in various fields, including sensors, displays, and responsive materials.

Biogenic Silver Nanoparticles/Mg-Al Layered Double Hydroxides with Peroxidase-like Activity for Mercury Detection and Antibacterial Activity.

Over the past decade, the attention of researchers has been drawn to materials with enzyme-like properties to substitute natural enzymes. The ability of nanomaterials to mimic enzymes makes them excellent enzyme mimics; nevertheless, there is a wide berth for improving their activity and providing a platform to heighten their potential. Herein, we report a green and facile route for Tectona grandis leaves extract-assisted synthesis of silver nanoparticles (Ag NPs) decorated on Mg-Al layered double hydroxides (Mg-Al-OH@TGLE-AgNPs) as a nanocatalyst. The Mg-Al-OH@TGLE-AgNPs nanocatalyst was well characterized, and the average crystallite size of the Ag NPs was found to be 7.92 nm. The peroxidase-like activity in the oxidation of o-phenylenediamine in the presence of H2O2 was found to be an intrinsic property of the Mg-Al-OH@TGLE-AgNPs nanocatalyst. In addition, the use of the Mg-Al-OH@TGLE-AgNPs nanocatalyst was extended towards the quantification of Hg2+ ions which showed a wide linearity in the concentration range of 80-400 µM with a limit of detection of 0.2 nM. Additionally, the synergistic medicinal property of Ag NPs and the phytochemicals present in the Tectona grandis leaves extract demonstrated notable antibacterial activity for the Mg-Al-OH@TGLE-AgNPs nanocatalyst against Gram-negative Escherichia coli and Gram-positive Bacillus cereus.

Dimer-parity-dependent odd-even effects in photoinduced transitions to cholesteric and twist grain boundary smectic-C

We describe investigations on the influence of the flexible spacer parity and length of the guest photoactive liquid-crystalline dimers in guest-host mixtures exhibiting photoinduced transitions involving isotropic (I), cholesteric (N^{*}), and twist grain boundary smectic-C^{*} (TGBC^{*}) phases. Despite a small concentration (3 wt. %) of the guest molecules, the transition temperatures and their photodriven shift (δT) show a strong odd-even parity (of the dimer) dependent effect, with the even-parity systems having a larger value than their odd-parity counterparts; δT is larger for the N^{*}-TGBC^{*} transition than for the I-N^{*} one. The photocalorimetric measurements corroborate these features in addition to showing that, in comparison with the absence-of-ultraviolet (UV) case, the transition enthalpy (ΔH) of the I-N^{*} transition in the UV-on case is diminished by 33 and 12% for the mixtures with even- and odd-parity dimers, respectively. The duration for relaxation from the isothermal photodriven transition also exhibits general features of an odd-even influence. Molecular dynamics simulations demonstrate the presence of significant conformational heterogeneity and associated shift in the conformational space on photostimulation of the guest molecules. The change in the effective shape and nematic order parameter is more pronounced in the even-parity system.

Nematic twist-bend phase of a bent liquid crystal dimer: field-induced deformations of the helical structure and macroscopic polarization.

The twist-bend nematic (Ntb) phase is a recent addition to the family of nematic (N) phases of liquid crystals (LCs). A net polar order in the Ntbphase under an external electric field is interesting and it was predicted in several recent theoretical studies. We investigated the field-induced polarization behaviour, dielectric, and electro-optic properties of a bent LC dimer CB7CB in the N and Ntbphases. A threshold-dependent polarization current response was obtained in both the phases under triangular and square-wave input electric fields, existing till frequencies as high as 150 Hz. The polarization switching times were found in ∼1 ms region, especially in the N phase. In the Ntbphase, electric field-induced deformation of the helical structure was observed, like ferroelectric LCs. Dielectric measurements revealed the presence of cybotactic clusters via collective relaxations. The dielectric anisotropy (Δϵ) is negative at the frequencies of polarization measurements. The net polarization resulted from field-induced reorientation of cybotactic clusters and additionally from the field-induced deformation of helical structures in the Ntbphase. We explored the possibility of ionic contributions to the net polarization by synthesizing TiO2nanoparticles (NPs) dispersed CB7CB LC nanocomposite. Incorporation of the NPs resulted in reduction of the collective order, increase in the ionic impurity content and conductivity, but an extinction of the field-induced polarization response. Our results demonstrate that the net polarization has competing contributions from both ferroelectric-like and ionic origin (up to ∼10 Hz) in the LC phases, but it becomes dominantly ferroelectric-like at higher frequencies.

Green and low-cost synthesis of zinc oxide nanoparticles and their application in transistor-based carbon monoxide sensing.

There has been steady progress in developing reliable and cost-effective strategies for the clean production of zinc oxide (ZnO) nanoparticles (NPs) owing to their unique structural and wide functional characteristics. While the green synthesis of such NPs from plant extracts has emerged as a sustainable and eco-friendly protocol, it is greatly restricted owing to the scarcity of potential natural precursors necessitating comprehensive investigations in this direction. Herein, we report a facile, low-cost green synthesis and characterization of ZnO NPs along with the demonstration of their usage as an active media in organic field-effect transistor (OFET) devices for sensing carbon monoxide (CO) gas. The ZnO NPs obtained from Nelumbo nucifera (lotus) leaf extract-mediated solution combustion synthesis at a much lower initiation temperature, the first of its kind, were characterized by various techniques such as UV-vis spectroscopy, XRD, EDX analysis, TEM and FESEM. The data derived from these experiments clearly evidence the formation of very pure and crystalline ZnO NPs possessing nearly spherical-shape with a size of 3-4 nm. The p-type organic field-effect transistor (OFET) device, fabricated using poly(3-hexylthiophene-2,5-diyl) (P3HT) and ZnO NPs, showed a field-effect mobility of 10-2 cm2 V-1 sec-1 with a slightly enhanced response of detecting CO gas at room temperature (RT). The phenomenon was further confirmed by the variation in electrical parameters of the OFET such as field-effect mobility (µ), on-current (I on), and off-current (I off). The selectivity and sensitivity of the fabricated device in CO gas detection was found to be more prominent than the other reducing gases (hydrogen sulphide, H2S and ammonia, NH3) and methanol vapours tested.

Effect of gelation on the Frank elastic constants in a liquid crystalline mixture exhibiting a twist bend nematic phase.

We report studies on the Frank elastic constant behaviour of a liquid crystal gel system exhibiting the twist bend nematic (Ntb) phase. Physical gelation is observed to ease the splay and stabilize the twist deformations in the nematic phase preceding the Ntb. More importantly, the ultra-low bend elastic constant (K33) of the system is enhanced by an order of magnitude on gelation. The magnitude of K33 remains high even in the vicinity of the Ntb phase, which otherwise is susceptible to bend deformations. This phenomenon is explained from the point of view of polar interactions in the Ntb system. XRD and dynamic rheology along with the elastic constant data validate this argument. Another salient feature of the system is that gel fibers grown in the direction orthogonal to the helical axis vanish in the Ntb phase as observed from polarizing optical microscopy. A possible reason for this is discussed on the basis of ordering developed in the surrounding medium. This feature gives the possibility of using the Ntb phase as a tool to imprint directional microstructures with a gel network.

Influence of chirality on the thermal and electric properties of the columnar mesophase exhibited by homomeric dipeptides.

We present the first investigation of the influence of chirality on the thermal and electric properties in a biologically important homomeric dipeptide that exhibits a hexagonal columnar liquid crystal mesophase. The peptide employed has two chiral centres, and thus the two possible enantiopures are the (R,R) and (S,S) forms having opposite chirality. The measurements reported the span of the binary phase space between these two enantiopures. Any point in the binary diagram is identified by the enantiomeric excess Xee (the excess content of the R,R enantiopure over its S,S counterpart). We observe that the magnitude of Xee plays a pivotal role in governing the properties as evidenced by X-ray diffraction (XRD), electric polarization (Ps), dielectric relaxation spectroscopy (DRS) measurements, and the isotropic-columnar transition temperature. For example, XRD shows that while other features pointing to a hexagonal columnar phase remain the same, additional short-range ordering, indicating correlated discs within the column, is present for the enantiopures (Xee = ±1) but not for the racemate (Xee = 0). Similarly, an electric-field driven switching whose profile suggests the phase structure to be antiferroelectric is seen over the entire binary space, but the magnitude is dependent on Xee; interestingly the polarization direction is axial, i.e., along the column axis. DRS studies display two dielectric modes over a limited temperature range and one mode (mode 2) connected with the antiferroelectric nature of the columnar structure covering the entire mesophase. The relaxation frequency and the thermal behaviour of mode 2 are strongly influenced by Xee. The most attractive effect of chirality is its influence on the polar order, a measure of which is the magnitude of the axial polarization. This result can be taken to be a direct evidence of the manifestation of molecular recognition and the delicate interplay between chiral perturbations and the magnitude of the polar order, a feature attractive from the viewpoint of devices based on, e.g., remnant polarization-a currently hot topic. To add further dimension to the work, the DRS measurements are also extended to elevated pressures.

Effect of Pressure on Dielectric and Frank Elastic Constants of a Material Exhibiting the Twist Bend Nematic Phase.

We report the first investigation on the effect of applied pressure on the now well-known dimer α,ω bis(4,4'-cyanobiphenyl)heptane (CB7CB) that exhibits two types of nematic: the regular uniaxial nematic (N) and the recently discovered twist-bend nematic (NTB) phase. At atmospheric pressure, the thermal behavior of ε⊥, the permittivity normal to the director in the N phase decreases on entering the NTB wherein the value represents permittivity orthogonal to the helical axis. Application of pressure initially decreases the magnitude of the change in ε⊥ and with further increase in pressure exhibits an increase in the value. Such a change in the feature of ε⊥ is similar to that obtained at room pressure when the monomeric heptyloxy cyanobiphenyl (7OCB) is doped to CB7CB at a high concentration of 50%. The dielectric anisotropy exhibits a trend reversal with temperature, the extent of which is affected at high pressures. Another salient feature of the study is the effect that pressure has on the Frank bend elastic constant K33. Over the pressure range studied K33 enhances by a large factor of 5. In contrast, the splay elastic constant exhibits a much smaller change of only 70%. The pressure-temperature phase boundary has a much smaller slope for the N-NTB transformation than for the isotropic-N transition. We propose that all these features can be understood in terms of the relative population of the more energetic horseshoe and lower energy extended conformer adopted by the CB7CB molecule. The extended conformer is favored at lower temperatures or at higher pressures. This argument is validated by X-ray diffraction experiments at atmospheric pressure on the binary mixture of CB7CB and 7OCB, mentioned above.

Influence of virtual surfaces on Frank elastic constants in a polymer-stabilized bent-core nematic liquid crystal.

Effect of a polymer network on the threshold voltage of the Fréedericksz transition, Frank elastic constants, switching speed, and the rotational viscosity are investigated in a polymer-stabilized bent-core nematic liquid crystal with different polymer concentrations. These polymer networks form virtual surfaces with a finite anchoring energy. The studies bring out several differences in comparison to similar studies with a calamitic liquid crystal as the nematic host. For example, on varying the polymer content the threshold voltage decreases initially, but exhibits a drastic increase above a critical concentration. A similar feature-reaching a minimum before rising-is seen for the bend elastic constant, which gets enhanced by an order of magnitude for a polymer content of 2.5 wt %. In contrast, the splay elastic constant has a monotonic variation although the overall enhancement is comparable to that of the bend elastic constant. The behavior changing at a critical concentration is also seen for the switching time and the associated rotational viscosity. The presence of the polymer also induces a shape change in the thermal dependence of the bend elastic constant. We explain the features observed here on the basis of images obtained from the optical and atomic force microscopy.

Binary System Exhibiting the Nematic to Twist-Bend Nematic Transition: Behavior of Permittivity and Elastic Constants.

We describe measurements of the permittivity and Frank elastic constant in the nematic phase of a binary system displaying a transition between the nematic (N) and the recently discovered twist-bend nematic (NTB) phase. Among the salient features observed are (i) the existence of the NTB phase even when the system is loaded with a high concentration (∼64 mol %) of a rodlike component; (ii) a clear signature in permittivity of the N-NTB transition; and (iii) a lower value of the bend elastic constant compared to the splay over a large phase space, with the difference between the two becoming a maximum for an intermediate mixture. These studies further support the surprising idea that the elastic features associated with bent molecules can be further augmented by suitable rodlike additives.

s-Triazine-Based Functional Discotic Liquid Crystals: Synthesis, Mesomorphism and Photoluminescence.

A new series of C3 -symmetric, π-conjugated molecules was designed, synthesized and characterized. The materials were derived from electron-accepting s-triazine, appended covalently to electron-donating styrylbenzene arms, and were readily prepared in excellent yield with high purity by means of three-fold condensation of triphosphonate with n-alkoxybenzaldehydes under Horner-Wadsworth-Emmons reaction conditions. Examination of the phase transitional properties by several complementary techniques evidenced self-assembly into a hexagonal columnar phase, occurring over wide and reasonable thermal ranges. The photophysical properties were studied both in solution and in the fluid/frozen columnar states by UV/Vis absorption and photoluminescence spectroscopy. The emission spectra obtained as a function of the temperature rule out the breaking-up of larger columns and a non-radiative, thermally activated process. A study carried out on thin films of the glassy columnar state, which accounts for conserved fluorescence, defect-free orientation, and freezing ionic species, with the help of atomic force microscopy (AFM) images, suggested a homogeneous granular morphology comprising fibrillar structures. Dissimilarities in the surface morphology and birefringence of thin films of the solid and frozen columnar states were clearly shown by Raman spectroscopy. An electrochemical investigation revealed a LUMO energy of -4.0 eV. Thus, the discotic motifs presented herein meet certain criteria of organic materials, which are essential for developing electronic devices.

Influence of polarization-tilt coupling on the ferroelectric properties of smectic gels.

We have studied composites of a ferroelectric liquid crystal mixture with a simple organic gelating agent, employing structural, thermal, electrical and mechanical probes, investigating the influence of the coupling between the polarization and the tilt angle on the ferroelectric properties of smectic gels. The calorimetric data, presenting clear signatures of the gelation occurring in the smectic A (SmA) phase or the isotropic phase, depending on the concentration of the gelator, help in constructing a rich diagram in the temperature-gelator concentration phase space. The atomic force microscopy imaging brings out the interesting feature of the transfer of chirality from the ferroelectric liquid crystal (FLC) to the gel strands, as exemplified by the creation of nanorope structures which have attracted much attention in recent times. The influence of gelation on the magnitude of the tilt angle appears to be dependent on the probe employed: there is no change in the values obtained by X-ray diffraction, which looks at the projection of the entire molecular length onto the layer normal. In contrast, the value from the electro-optic method, wherein the molecular-core is responsible for the results, diminishes with gelator concentration. The latter feature is copied by the magnitude of the polarization also. Dielectric spectroscopy shows that gelation weakly influences the soft mode in the SmA phase. However, the Goldstone mode behaviour is strongly dependent on the gelator concentration, with the appearance of two modes in the smectic C* (SmC*) phase of higher gel concentrations. With information obtained upon application of DC bias, the origin of the two relaxations is discussed. These data are analyzed in terms of the predictions of the Landau model proposed for the ordinary (non-gel) SmA-SmC* transition showing that the gel network enhances the linear polarization-tilt coupling over the biquadratic one. Upon gelation the system becomes mechanically strong with a large increase in the elastic moduli.

Effect of pressure on the dielectric behavior of a bent-core liquid crystal.

We report the effect of applied pressure on the dielectric properties of the B2 phase of a bent-core liquid crystal. This study on bent-core banana-shaped molecules shows that while the dielectric anisotropy hardly varies with pressure, the relaxation parameters associated with the rotation around the long axes of the molecules are significantly influenced. These studies also bring out the fact that there are additional phases between the B2 phase and the true crystalline solid. Indeed, the existence of another variant of the B2 phase (labeled B2'), is revealed only in dielectric studies but not seen in x-ray and calorimetric measurements. Employing the dependence of the relaxation frequency along isobaric as well as isothermal paths, different activation parameters are determined and their behavior is compared with those of rodlike systems. The influence of dc bias on the dielectric behavior obtained at atmospheric pressure is also presented, which exhibits features similar to chiral antiferroelectric smectics, and further shows an additional relaxation over a small window of dc voltages.

Reentrant nematic phenomenon in a new class of low molar mass, single-component liquid crystals.

We report the observation of reentrant nematic behavior in a relatively new class of compounds, namely, dimerlike mesogens that are made by covalently linking nematogenic cyanobiphenyl and nonmesogenic N-(n-alkyl)salicylaldimine segments through a central flexible spacer of varying length and parity. The existence of reentrant phenomenon in this class of mesogens, evidenced indubitably by means of several complementary studies, appears to be crucially dependent on the length of the terminal alkyl tail and parity of the spacer.

High-pressure dielectric investigations of nanocolloidal Aerosil-nematic liquid crystal composites.

We report the effect of applied pressure on the dielectric properties of composites of a nematic liquid crystal with hydrophilic Aerosil particles. The study, which is the first of its kind on the nanocolloidal Aerosil systems, also has the novelty that a weakly polar nematogen is used as the host material. In conjunction with differential scanning calorimetric data, these measurements show that not only T(N-Iso), the nematic-isotropic transition at room pressure, but several other parameters exhibit a nonmonotonic variation with increasing Aerosil concentration. The dependence of the relaxation frequency of the mode connected with the reorientations of the molecules around their short axis is probed along isobaric as well as isothermal paths, and the determined activation parameters are discussed in terms of the chemical structure of the host molecule. The temperature dependence of the relaxation frequency at atmospheric pressure, exhibiting non-Arrhenius behavior, has been analyzed using the Vogel-Fulcher-Tamann expression, yielding information regarding the influence of the Aerosil concentration on the fragility strength as well as the glass transition temperature. The calorimetric data is also discussed in terms of the concept of a crossover from the random-dilution to the random-field limits.

Permittivity, conductivity, elasticity, and viscosity measurements in the nematic phase of a bent-core liquid crystal.

We report on measurements of dielectric permittivity epsilon, electrical conductivity sigma, elastic moduli k(ii), and rotational viscosity gamma for a bent-core nematic liquid crystal. The static permittivity anisotropy epsilon(a) = epsilon(parallel)-epsilon(perpendicular) is negative; at a given temperature in the interval 107-123 degrees C, epsilon(parallel) shows two relaxations falling in the frequency bands 20-200 kHz and 0.9-2 MHz; epsilon(perpendicular) also shows a relaxation between 0.9 and 5 MHz. The conductivity anisotropy sigma(a) = sigma(parallel)-sigma(perpendicular) is negative at low frequencies; it changes sign twice at frequencies f(1) and f(2) that increase with temperature, in the ranges 6.5-10 and 95-600 kHz, respectively. Surprisingly, the splay modulus k(11) is considerably greater than the bend modulus k(33) in the entire nematic range. Viscous relaxation is more complex than in calamitic systems involving at least a two-step process. The gamma values are an order of magnitude greater compared to calamitics.

Soft glass rheology in liquid crystalline gels formed by a monodisperse dipeptide.

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.

Patterned electroconvective states in a bent-core nematic liquid crystal.

We report the results of investigations on the anisotropic electrohydrodynamic states arising in a highly conducting, planarly aligned, bent-core nematic liquid crystal driven by ac fields of frequency f in the range from 10 Hz to 1 MHz. Pattern morphologywise, two f regimes are distinguished. The low-f regime, wherein the primary bifurcation is to a state of periodic longitudinal stripes (LS), extends to an unprecedentedly large f, in the range 150-550 kHz, depending on the temperature T. This is followed by the high-f regime wherein periodic normal stripes (NS) constitute the primary instability. Both instabilities involve predominant director modulations and streamlines in the layer plane. The transitional frequency between the two regimes is linear in temperature. The curve V(c)(f) shows a nonlinear increase for the LS state and decrease for the NS state. V(c)(T) is an ever increasing curve close to the nematic-isotropic point for both states. The wavenumber of LS varies directly as V, and that of NS shows nearly the same behavior. The pattern period versus f is increasing for LS but decreasing for NS. Both instability states exhibit complex, light-polarization-dependent lens action. Well above the threshold, disclination loops of regular geometry appear along the stripes. They drift in a coordinated manner along the flow lines. At very high voltages, the instability turns strongly time dependent. The current models of anisotropic convection based on static electrical parameters fail to account for the observed instabilities.

Converse flexoelectric effect in bent-core nematic liquid crystals.

We report on the converse flexoelectric effect in two bent-core nematic liquid crystals with opposite dielectric anisotropies. The results are based on electro-optic investigations of inplane field-driven distortions in homeotropic samples (the Helfrich method). They are interpreted by an extension of the Helfrich theory that takes into account the higher order distortions. The bend flexocoefficient for both the compounds is of the usual order of magnitude as in calamitics, unlike in a previously investigated bent-core nematic for which giant values of the bend flexocoefficient are reported. In order to resolve this discrepancy, we propose a molecular model with nonpolar clusters showing quadrupolar flexoelectricity. The study also includes measurements on surface polarization instabilities in the dielectrically positive material; the splay flexocoefficient thereby deduced is also of the conventional order.

Fast responding robust nematic liquid crystalline gels formed by a monodisperse dipeptide: electro-optic and rheological studies.

Realization of mechanically robust electrically fast responding liquid crystal devices with low operating voltage is one of the current research interests. Here we report a gel system comprising a commercially available nematic liquid crystal material and a new monodisperse dipeptide liquid crystalline organogelator that results in these properties. The gels exhibit nearly 2 orders of magnitude faster switching response than the pure nematic liquid crystal while having 3 orders of magnitude higher zero shear rate viscosity, and with the attractive feature that the switching threshold voltage is hardly altered. Electro-optic and rheological studies of this system are described here.