Scientific developments of liquid crystal-based optical memory: a review

Highly luminescent, fast switching electro-optical device based on core-shell bimetallic nanoparticles/ ferroelectric liquid crystal composites

Owing to the passive nature of liquid crystal (LC) materials, achieving luminous displays using pure LC materials is challenging. In addition, it is difficult to achieve a fast switching time using pristine ferroelectric LC devices without compromising their cell thickness. Herein, we have developed a fast switching and highly luminescent electro-optical device by dispersing a minute concentration of bimetallic nanoparticles (Au@Ag NPs) having a spherical gold core and a silver shell within a ferroelectric liquid crystal (FLC) host matrix, ZLI3654. Au@Ag core-shell NPs having synergic attributes of both counterparts were successfully synthesized by a facile seed-mediated route. The Au core helps to tune the shape of the Ag shell and provides enhanced electron density as well as improved stability against oxidation. Introducing nanoparticles induces little structural modifications to the host FLC, resulting in an improvement in the mesogenic alignment. Interestingly, ∼29-fold enhancement in the photoluminescence (PL) intensity is observed on dispersing 0.25 wt% of Au@Ag NPs into the FLC host matrix. The enhanced electromagnetic field in the FLC-nanocomposite is attributed to the Localized Surface Plasmon Resonance of Au@Ag NPs, which strengthens the photon absorption rates by the FLC molecules, culminating in the massive enrichment of the PL intensity. In addition, the improved localized electric field inside the FLC device led to a noticeable enhancement in the spontaneous polarization, dielectric permittivity, and, most interestingly, ∼53% fastening in the switching time at an optimum concentration (0.25 wt%) of Au@Ag NPs. The improved electro-optical parameters of the Au@Ag NPs/FLC composite have been compared with the performance of both pristine Au NPs/FLC and Ag NPs/FLC composites, respectively, for the comprehensiveness of the study. The present study paves a systematic way to develop FLC-based advanced electro-optical devices with faster switching and higher luminescence properties.

Thermotropic liquid crystals in drug delivery: A versatile carrier for controlled release

Responsive and adaptive soft-matter systems represent an advanced category of materials with potential applications in drug delivery. Among these, liquid crystals (LCs) emerge as multifunctional anisotropic scaffolds capable of reacting to temperature, light, electric or magnetic fields. Specifically, the ordering and physical characteristics of thermotropic LCs are primarily contingent on temperature as an external stimulus. This comprehensive review aims to bridge a notable gap in the biomedical application of thermotropic mesogens by exclusively focusing on drug delivery. Anticipated to inspire diverse ideas, the review intends to facilitate the elegant exploitation of controllable and temperature-induced characteristics of LCs to enhance drug permeation. Here, we delineate recent advancements in thermally-driven LCs with a substantial emphasis on LC monomer mixtures, elastomers, polymers, microcapsules and membranes. Moreover, special emphasis is placed on the biocompatibility and toxicity of LCs as the foremost prerequisite for their application in healthcare. Given the promising prospect of thermotropic LC formulations in a clinical context, a special section is devoted to skin drug delivery. The review covers content from multiple disciplines, primarily targeting researchers interested in innovative strategies in drug delivery. It also appeals to those enthusiastic about firsthand exploration of the feasible biomedical applications of thermotropic LCs. To the best of our knowledge, this marks the first review addressing thermotropic LCs as tunable soft-matter systems for drug delivery.

Frequency-dependent nonlinear optical response and refractive index investigation of lactone-derived thermochromic compounds

Nonlinear optical (NLO) switchable materials play a crucial role in the fields of electronics and optoelectronics. The selection of an appropriate switching approach is vital in designing such materials to enhance their NLO response. Among various approaches, thermos-switching materials have shown a 4-fold increase in NLO response compared to other photo-switching materials. In this study, we computationally investigated the geometric, electronic, and nonlinear optical properties of reversible lactone-based thermochromic compounds using the ωB97XD/6-311+G (d,p) level of theory. Molecular orbital studies are employed to analyze the electronic properties of the close and open isomers of these compounds, while time-dependent density functional theory (TD-DFT) analysis is utilized to evaluate their molecular absorption. Our findings reveal that the π-electronic conjugation-induced delocalization significantly influences the ON-OFF switchable nonlinear optical response of the lactone-based thermochromic compounds. Notably, among all compounds, the open isomer of lactone 2 exhibits the highest hyperpolarizability value (6596.69 au). Furthermore, we extended our analysis to investigate the frequency-dependent second and third-order hyperpolarizabilities. The most pronounced frequency-dependent NLO response is observed at 532 nm. Additionally, we calculated the refractive index of these thermochromic compounds to further assess their nonlinear optical response. The open isomer of lactone 1 demonstrates the highest refractive index value (3.99 × 10-14 cm2/W). Overall, our study highlights the excellent potential of reversible thermochromic compounds as NLO molecular thermos-switches for future applications.

Enhancement in electrical conductivity of liquid crystals by graphene metal oxide composites

Enhancing the electrical conductivity of liquid crystal (LC) circumvents challenges for application in advanced electronic components. Toward this, using additives made of different nanostructures that could result in functional LCs is suggested. In this paper, various concentrations of graphene (Gr)/metal-oxide (Fe₃O₄) nanocomposite (GMN) (0.0001-1 w%) were added to E7 nematic LC. We found that the role of anisotropic Gr flakes, their edges as well as surface-decorated-metal-oxide-additives have significant impact on electrical properties of E7. A range of appropriate additives of such a nanocomposite enhances the electrical conductivity of LCs. This effect can be traced through the decrease in the formation of GMN aggregates in the E7 and increase in the electrostatic field at the edges of the Gr sheets. Moreover, the presence of metal-oxide nanoclusters due to the presence of oxygen vacancies and defects facilitates the construction of conductive network for improving the charge transfer pathways and contributes to a stronger interaction of the Gr surface with charged species. These factors can provide Gr layers as dipole moments and lead to signal propagation in the dielectric medium. Our finding conveys a pathway toward significant enhancement of electrical conductivity in the LC family which can be useful for functional applications.

Dynamic memory tuned by frequency in a homologous thermotropic liquid crystal

With scaling of dynamic RAM and NAND memory technologies reaching a limit, there is a need for dynamic memory with high density. In this work, an investigation on existence of dynamic memory feature in a frequency tuned homologous series of thermotropic liquid crystals has been carried out. A homologues series of five thermotropic liquid crystalline compounds comprising of 4-butyl benzoic acid and various alkyloxy benzoic acids are prepared, and all these mesogens exhibit only nematic phase. Liquid crystal dynamic memory storage setup consists of a conducting transparent glass cell with two indium tin oxide coated transparent glass plates acting as electrical electrodes in which the selected thermotropic liquid crystal is filled by capillary action. The temperature dependent dielectric relaxation studies enable to elucidate the relaxation frequency of each of these mesogens in nematic phase. The liquid crystal at different temperatures is excited with the relaxation frequency at various chosen fields, and the dielectric hysteresis is recorded. The magnitude of the hysteresis loop is directly proportional to the memory storage capacity. The main objective of this work is observation of dynamic memory storage in liquid crystals exhibiting nematic phase, at different frequencies in a thermotropic liquid crystal as the ingredient in a conducting polyamide buffed glass cell excited by an external electrical dc stimulus. The variation of the hysteresis loop with varying field; temperature and frequency are also studied and reported in this work.

A protet-based, protonic charge transfer model of energy coupling in oxidative and photosynthetic phosphorylation

Textbooks of biochemistry will explain that the otherwise endergonic reactions of ATP synthesis can be driven by the exergonic reactions of respiratory electron transport, and that these two half-reactions are catalyzed by protein complexes embedded in the same, closed membrane. These views are correct. The textbooks also state that, according to the chemiosmotic coupling hypothesis, a (or the) kinetically and thermodynamically competent intermediate linking the two half-reactions is the electrochemical difference of protons that is in equilibrium with that between the two bulk phases that the coupling membrane serves to separate. This gradient consists of a membrane potential term Δψ and a pH gradient term ΔpH, and is known colloquially as the protonmotive force or pmf. Artificial imposition of a pmf can drive phosphorylation, but only if the pmf exceeds some 150-170mV; to achieve in vivo rates the imposed pmf must reach 200mV. The key question then is 'does the pmf generated by electron transport exceed 200mV, or even 170mV?' The possibly surprising answer, from a great many kinds of experiment and sources of evidence, including direct measurements with microelectrodes, indicates it that it does not. Observable pH changes driven by electron transport are real, and they control various processes; however, compensating ion movements restrict the Δψ component to low values. A protet-based model, that I outline here, can account for all the necessary observations, including all of those inconsistent with chemiosmotic coupling, and provides for a variety of testable hypotheses by which it might be refined.

Nematic liquid crystal/dimethylsulphoxide mixture based tuning condenser

Here, we demonstrate experimentally the existence of temporally variable dielectric behavior of a nematic liquid crystal (NLC), the most widely used liquid crystal (LC) phase among all LCs materials, by mixing dimethylsulphoxide (DMSO) into former. The intermolecular interactions and nanosegregation of the molecular LC structures have influenced their self-assembly by mixing DMSO into it. We mainly examined the dielectric parameters such as dielectric permittivity, dielectric loss factor, and absorption/energy dissipation and observed that the NLC/DMSO mixture shows a nearly dielectric-conductor (D-C) transition as time passes. The presence of DMSO in this analysis was confirmed by Fourier transform infrared spectroscopy while time-dependent dielectric studies were carried out using dielectric spectroscopic techniques. The promising idea of showing D-C transition is truly proven that may open the possibilities for real-time variable and supercapacitors.

The ultra-photostable and electrically modulated Stimulated Emission in perylene-based dye doped liquid crystal

One of the most important drawback of organic dyes is their low photo-stability which reduces possibility of their commercial utilization. In this article we employ the strategy of dye re-crystallization from oversaturated matrix in order to enhance material's durability. One of the main advantages of perylene derivative is ability to form emissive j-aggregates, good miscibility and incorporation into liquid crystalline matrix. Investigation of perylene-based dye and LC matrix brought as the result very efficient light amplification modulation by applied external electric field. In our article we show that Stimulated Emission (STE) is possible to achieve from perylene-derivative based system, at typical fluence thresholds for laser dyes: 3.9 mJ/cm2. Moreover, presented system proves ultra-high photostability, showing lack of STE reduction even after 12 000 excitation laser pulses. Furthermore, we proved the possibility of light emission intensity control using external electric field.

Synthesis and thermotropic properties of new green electrochromic ionic liquid crystals

New liquid crystal viologens, which exhibit green electrochromic behavior in the LC phase, rarely observed for small molecules, have been developed.

Interfacial behavior of confined mesogens at smectic-C*-water boundary

In this paper, we have investigated the behavior of mesogens at smectic-C*-water interface confined in a liquid crystal (LC) cell with interfacial geometry. Polarized optical microscopy was used to probe the appearance of various smectic-C* domain patterns at water interface owing to the reorientation of mesogens. The undulated stripe domains observed at the air interface of smectic-C* meniscus vanished as the water entered into the smectic layers and focal conical domain patterns appeared at smectic-C*-water boundary. A spatially variable electro-optical switching of LC molecules was also observed outside the electrode area of the interfacial cell. The electrode region at the interface, as well as on the water side, was damaged upon application of an electric field of magnitude more than 150 kV/m. The change in dielectric parameters of mesogens was extensively studied at interface after evaporating the water. These studies give fundamental insights into smectic-C*-water interface and also will be helpful in fabricating better LC devices for electro-optical and sensing applications.

Interferometric measurement of temporal behavior of linear birefringence with extended range

A heterodyne interferometer is developed to measure the static and temporal behaviors of birefringence of a liquid crystal variable retarder. The interferometer is designed based on the analysis of the polarization state of a coherent wave. Since the optical components of the interferometer are fixed without any adjustment, the phase retardation and the azimuthal angle of a liquid crystal variable retarder is measured independently in real time, where the environmental perturbations and common mode noises can be reduced. From the analysis and experimental demonstrations, the phase retardation can be determined in the [0, 4π] range. Meanwhile, the orientational variation of the azimuthal angle of the optic axis is found.