Room Temperature Nematic Liquid Research Articles

Synergistically controlled in situ growth of silver nanoparticles leading to organization of anisotropic liquid crystalline medium

ABSTRACT An in-depth macroscopic and microscopic investigation is carried out regarding understanding phase ordering and organisation of nematic phase of a room temperature nematic liquid crystal (NLC) MBBA (N-(4-methoxybenzylidene)-4-butylaniline) in the presence of silver nanoparticles (AgNPs) which are synthesised in situ in a single step using MBBA as a template as well as a reducing and stabilising unit. A synergistic effect between anisotropic LC structure and morphologies of AgNPs is observed. The LC-AgNP composite exhibits better thermal stability and phase ordering compared to its neat LC counterpart as indicated by combined study of optical polarization microscopy (OPM) and differential scanning calorimetry (DSC). While OPM study exhibits emergence of smectic mesophase in LC-AgNP composites, DSC manifests significant enhancement in the isotropic temperature and associated enthalpy as well as a change in the kinetic behaviour in LC-AgNP composites compared to neat LC. Also, an alternation in the electronic properties of LC-AgNP composites compared to neat LC is recognised. Such ordering at the macroscopic scale is well supported by our microscopic observations through spectroscopic studies such as Fourier transform infrared (FTIR) and X-ray diffraction (XRD) which provides valuable insights regarding understanding the organisation process more closely.

Tailoring the physical parameters of nematogen using non-mesogenic ferromagnetic unit: dielectric and photoluminescence study

Liquid crystals are self-organized anisotropic fluids which exhibit ordered states ranging from nematics to smectics/hexatics. Hybridized liquid crystals with nanoparticles broaden the research area in finding the novel functional devices. Reported herein, the study of highly stable room temperature nematic liquid crystal E7 and carbon-coated cobalt (Co) nanoparticles. Dispersion of 0.50 wt.% Co into E7 has been introduced to tailor the physical parameters of host E7. Dielectric studies have been performed in planar as well as homeotropic cells in the frequency range of 20 Hz–10MHz while photoluminescence spectra have been recorded at excitation wavelength of 325 nm in planar cells. Dielectric studies reveal the change in phase transition temperature and molecular dynamics associated with magnetic nanoparticles doped liquid crystals. Quenching in photoluminescence intensity has been observed with the inclusion of Co nanoparticles due to the formation of non-radiative channels in the composite system. The intercalation of Co nanoparticles into E7 studies manifests the strong correlation between liquid crystals and nanoparticles. Allocation of magnetic attributes onto liquid crystal molecules finds potential applications in advanced magnetic materials, information devices and optical devices.

Tuning liquid crystal properties with ZnO nanodiscs: a study on order parameter and conductivity.

Anisotropic nanostructures dispersed in nematic liquid crystal, 4-(trans-4'-pentylcyclohexyl)-benzonitrile (PCH5), tend to improve the properties of host nematic liquid crystal owing to its structure and strong dipolar interactions. The experimental investigations of such composite systems are intriguing as well as challenging. Here, we have presented a systematic experimental study of effect of incorporation of two-dimensional anisotropic nanostructure, i.e., nanodisc on properties of room temperature nematic liquid crystal. Zinc oxide nanodiscs (ZD) of thickness ∼ 50 nm and diameter ∼ 200 nm are synthesized using hydrothermal method and characterized. Dispersion of zinc oxide nanodiscs in PCH5 is observed to increase the dielectric anisotropy and elastic constants which indicate enhancement in the order parameter of the composite system as compared to host nematic liquid crystal. Bulk ac conductivity is also observed to be decreasing by ∼60% by dispersion of zinc oxide nanodiscs in host liquid crystal. The increase in the ratio of bend to splay elastic constant makes the ZD composites of PCH5 potential candidates for higher multiplexing devices.

Tunable optical, electro-optical and dielectric properties of eco-friendly graphene quantum dots-nematic liquid crystal composites

ABSTRACT The changes in physical properties obtained by dispersing hydrophilic carboxylated graphene quantum dots (GQDs, <10 nm), which are non-toxic, biocompatible, and eco-friendly, into a room-temperature nematic liquid crystal, 5CB (4-pentyl-4ˈ- biphenylcarbonitrile) are described. Nematic–isotropic transition temperatures, polarising microscopy, electro-optic, photoluminescence, and dielectric investigations are reported for two composites having concentrations of GQDs of 0.25 and 1.0 wt.% in 5CB and compared to pristine 5CB. At the lower concentration (0.25%) of GQDs there is an enhancement in the order parameter by 5.49%, the birefringence by 1.28%, and the activation energy for dielectric relaxation by 4.4% compared to pure 5CB. However, at the higher concentration (1%), aggregation of GQDs occurs. This is apparent in optical textures and resulted in the deterioration of optical and electrical properties compared to 5CB. The photoluminescence spectrum of both concentrations of GQDs are centred at the same wavelength; however, the 1% GQD-5CB mixture exhibits significant quenching and may be suitable for some optical devices. Overall, our results support that, in addition to promoting sustainable, heavy-metal-free, and environmentally safe technologies, a low concentration of GQDs in LCs produces composites that have superior electrical and optical properties; thus, making them suitable for a variety of photonics and display applications.

Room temperature nematic liquid crystals: Synthesis and characterisation of pentabromophenol and cyanobiphenyl derived rod-shaped molecules

ABSTRACT Calamitic molecules consisting of pentabromophenol tethered to cyanobiphenyl via a flexible alkyl chain were synthesised and characterised for their mesomorphic properties. New chemical structures differing in the length of the spacer (6a–f) were confirmed by FT-IR, 1H-NMR and 13C-NMR spectroscopies, and by elemental analysis. Compounds having six, eight, nine and ten methylene units in the spacer show room-temperature nematic liquid crystalline phases and the other derivatives are soft solids. The mesomorphic behaviour and thermal stability of all compounds have been investigated by polarised optical microscopy, differential scanning calorimetry and thermal gravimetric analysis, respectively.

Enhancement of the physical parameters due to the dispersion of functionalised gold nanoparticles in a room temperature nematic liquid crystal

ABSTRACT A room temperature nematic liquid crystal (N-3017) mixed with surface-stabilised gold nanoparticles was examined for its thermodynamic, dielectric, and electro-optical properties. The isotropic to nematic transition temperature decreases in both the nanocomposites as compared to that of the pure nematic material, according to thermodynamic findings. In the frequency range of 1 Hz to 40 MHz, dielectric data of both the dispersed nanocomposites in homeotropic and planar aligned samples are collected. The dispersion of capped GNPs enhanced the dielectric anisotropy of the dispersed nanocomposites, according to the dielectric results. As the quantity of capped GNPs in the host matrix increases, the threshold voltage for LC reorientation decreases due to the decreased elastic constant and increased dielectric anisotropy. The dielectric study also signifies a prominent relaxation mode due to the flip-flop motion of the nematic molecules about their short axes. To analyse this mode, experimental data have been fitted with the modified Cole–Cole equation. This removes the parasitic effects in the low- and high-frequency domains.

Impact of strontium titanate nanoparticles on the dielectric, electro-optical and electrical response of a nematic liquid crystal

The impact of ferroelectric nanoparticles (FNPs), namely strontium titanate (SrTiO3) on the dielectric, electro-optical and electrical response of a room temperature nematic liquid crystal (NLC), 5CB (4-pentyl-4′-cyanobiphenyl) is investigated through dielectric, electro-optical and electrical techniques. The dielectric studies clearly show that low frequency dielectric relaxation shifts towards higher frequency as concentration of FNPs increases but decreases to the value even lower than pure sample at 2 wt% concentration. Afterwards, the short axis molecular relaxation observed in homeotropic cell obtained from homogeneous aligned cell by applying bias voltage higher than threshold shifts towards higher frequency by ∼ 28 % in composites but remains independent of concentration. Interestingly, after the removal of bias voltage, dielectric spectra does not return to its original state even after ∼ 22 min in the composites unlike pure sample and could be considered as apparent dielectric memory (i.e. memory effect). However, the voltage dependent optical textures confirm the short term memory in the composites. The electrical response recorded as a function of frequency in the composite is also significantly different than pure sample which further confirms the impact of FNPs on NLC. We believe that the observed effects of FNPs on dielectric, electro-optical and electrical behavior could be due to the plausible favorable interaction among FNPs and NLC molecules. Such investigations would certainly be pivotal in tuning the dielectric and electro-optical properties of NLCs using FNPs and helpful in designing the next-generation tunable electro-optical and soft memory based devices.

Detection of human serum albumin by room temperature nematic liquid crystals based on polarising microscopy, molecular docking and Raman spectroscopy

ABSTRACT Liquid crystal-based biosensors serve an essential role to detect biomolecules and chemical events as an effective, and simple detection tool. A biosensor that works on the principle of orientational reordering of liquid crystal molecules in presence of protein is reported. We employ a liquid crystal 4´-pentyl-4-biphenylcarbonitrile to detect the protein human serum albumin using multiple techniques. The composite of liquid crystal and protein forms nematic droplets. The transitions in the radial to bipolar configuration of the droplets are investigated via polarising optical microscopy. The detection limit is determined to be 100 µM. The interaction between protein and liquid crystal is performed through molecular docking . The binding sites, active residues, and binding energies are also calculated from the analysis. The binding energy required to bind 5CB with HSA at this site is −8.10 Kcal/mol. Raman spectroscopy is performed to determine the vibrational changes in the active residues, conformations of the disulphide bonds of the protein, which developed during the interaction with the liquid crystal. Raman spectroscopic studies confirm the interactions between the active residues of HSA and 5CB. The Raman parameters are studied at different HSA concentrations. This report demonstrates the applications of liquid crystals as transducing element in biosensing field.

Remarkable Increase of Fluorescence Quantum Efficiency by Cyano Substitution on an ESIPT Molecule 2-(2-Hydroxyphenyl)benzothiazole: A Highly Photoluminescent Liquid Crystal Dopant

Fluorescent molecules with excited-state intramolecular proton transfer (ESIPT) character allow the efficient solid-state luminescence with large Stokes shift that is important for various applications, such as organic electronics, photonics, and bio-imaging fields. However, the lower fluorescence quantum yields (ΦFL) in the solution or viscous media, due to their structural relaxations in the excited state to reach the S0/S1 conical intersection, shackle further applications of ESIPT-active luminophores. Here we report that the introduction of a cyano group (-CN) into the phenyl group of 2-(2-hydroxyphenyl)benzothiazole (HBT), a representative ESIPT compound, remarkably increase its fluorescence quantum yield (ΦFL) from 0.01 (without -CN) to 0.49 (with -CN) in CH2Cl2, without disturbing its high ΦFL (=0.52) in the solid state. The large increase of the solution-state ΦFL of the cyano-substituted HBT (CN-HBT) is remarkable, comparing with our previously reported ΦFL values of 0.05 (with 4-pentylphenyl), 0.07 (with 1-hexynyl), and 0.15 (with 4-pentylphenylethynyl). Of interest, the newly-synthesized compound, CN-HBT, is miscible in a conventional room-temperature nematic liquid crystal (LC), 4-pentyl-4′-cyano biphenyl (5CB), up to 1 wt% (~1 mol%), and exhibits a large ΦFL of 0.57 in the viscous LC medium. A similar ΦFL value of ΦFL = 0.53 was also recorded in another room-temperature LC, trans-4-(4-pentylcyclohexyl)benzonitrile (PCH5), with a doping ratio of 0.5 wt% (~0.5 mol%). These 5CB/CN-HBT and PCH5/CN-HBT mixtures serve as light-emitting room-temperature LCs, and show anisotropic fluorescence with the dichroic ratio of 3.1 upon polarized excitation, as well as electric field response of luminescence intensity changes.

Sequentially amplified circularly polarized ultraviolet luminescence for enantioselective photopolymerization

Chiral optical materials based on circularly polarized luminescence (CPL) have emerged rapidly due to their feasible applications in diverse fields of research. However, limited to the small luminescence dissymmetry factor (glum), real application examples have rarely been reported. Here, we present a complex system, which show intense circularly polarized ultraviolet luminescence (CPUVL) with large glum value, enabling a chiral UV light triggered enantioselective polymerization. By integrating sensitized triplet-triplet annihilation upconversion and CPL, both visible-to-UV upconversion emission and upconverted circularly polarized ultraviolet luminescence (UC-CPUVL) were obtained in the systems, built of chiral annihilator R(S)-4,12-biphenyl[2,2]paracyclophane (R-/S-TP), and a thermally activated delayed fluorescence (TADF) sensitizer. After dispersing this upconversion system into room-temperature nematic liquid crystal, induced chiral nematic liquid crystal could significantly amplify the glum value (0.19) of UC-CPUVL. Further, the UC-CPUVL emission has been used to trigger the enantioselective photopolymerization of diacetylene. This work paves the way for the further development of functional application of CPL active materials.

Effect of iron oxide (γ-Fe2O3) nanoparticles on the morphological, electro-optical and dielectric properties of a nematic liquid crystalline material

The effect of Iron Oxide Nanoparticles (γ-Fe2O3 NPs) on a room temperature nematic liquid crystalline material namely 4-(4′-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) has been studied. The ferronematic composites have been characterized using differential scanning calorimetry, polarizing optical microscopy, dielectric spectroscopy and electro-optical measurements. It is found that the ferronematic with the low concentration of γ-Fe2O3 NPs enhanced the orientational ordering of nematic matrix due to the strong orientational coupling between magnetic dipole moments associated to nanoparticles and nematic liquid crystal director, which is the essential feature of these systems. It resulted the improvement of various parameters of liquid crystals important for the application. In the ferronematics, the nematic to isotropic transition temperature increased as compared to that of the pure material. Dielectric spectroscopy has shown an increase of dielectric anisotropy in the composite systems. Electro-optical results reveal a decrement in both the splay elastic constant and the electric threshold voltage, in line with a noteworthy improvement of the steepness of transmission- voltage curves.

Enhancement of electro-optical and dielectric parameters of a room temperature nematic liquid crystalline material by dispersing multi-walled carbon nanotubes

ABSTRACT We have prepared the composites of a room temperature nematic liquid crystal mixture by dispersing small fractions of Multi-Walled Carbon Nanotubes into its matrix. The electro-optical and dielectric properties of the prepared samples were investigated. The effect of Multi-Walled Carbon Nanotubes on various display parameters of host nematic liquid crystal mixture was observed. The variations in some of the display parameters like dielectric anisotropy, threshold voltage, splay elastic constant and activation energy have been observed for composite systems with respect to pure nematic liquid crystal mixture. Dispersion of Multi-Walled Carbon Nanotubes enhanced the nematic order of liquid crystal host, resulting in the improvement of dielectric and electro-optical parameters of studied composite systems.

Textural and electro-optical study of a room temperature nematic liquid crystal 4̍-pentyl-4-biphenylcarbonitrile doped with metal oxide nanowires in planar and in-plane switching cell configurations

ABSTRACT This work shows the doping effect of metal oxide nanowires (alumina nanowires) on the textural and electro-optical characteristics of nematic liquid crystal (5CB) in planar and in-plane switching (IPS) cell configurations. Results indicate the increase in nematic to isotropic phase transition temperature after doping in both planar and IPS cells. A decrease in threshold voltage in addition to increase in contrast ratio, birefringence, and band gap energy was observed after doping with alumina nanowires in both types of cells. Overall analysis shows that IPS cell configuration has more improved threshold voltage and contrast ratio in addition to decrease in birefringence and transmission intensity, as compared to planar cell configuration. No effect of cell configuration was observed on energy band gap and found same value for both planar and IPS cell configurations, as energy band gap is a material specific property.

Extended conjugation of ESIPT-type dopants in nematic liquid crystalline phase for enhancing fluorescence efficiency and anisotropy.

Organic compounds capable of excited-state intramolecular proton transfer (ESIPT) show fluorescence with a large Stokes shift and serve as solid-state emitters, luminescent dopants, and fluorescence-based sensing materials. Fluorescence of ESIPT molecules is usually increased in the solid state, but is weak in solvents due to the accelerated non-radiative decays by rotational motions of a part of the molecular core in these environments. Here we report, using a representative ESIPT motif 2-(2-hydroxyphenyl)benzothiazole (HBT), the extended-conjugation strategy of keeping sufficient fluorescence efficiency both in the solid state and in organic media. The introduction of an alkyl-terminated phenylene-ethynylene group into the HBT molecule dramatically enhances the fluorescence quantum yield from 0.01 to 0.20 in toluene and from 0.07 to 0.32 in a representative room-temperature nematic liquid crystal, 4-pentyl-4'-cyano biphenyl (5CB). The newly-synthesized CnP-C[triple bond, length as m-dash]C-HBT (n = 5 or 8) serves as a fluorescent dopant in 5CB and exhibits anisotropic fluorescence with the order parameter of 0.48, where the luminescence is controlled by the applied electric-field. The enhanced emission efficiency is rationalized by the larger height of energy barrier for the ESIPT process due to the introduction of phenylene-ethynylene groups.

Dielectric and electro-optic properties of 6CHBT nematic liquid crystals and silver nanoparticles composites

ABSTRACTWe have prepared the composite of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and silver nanoparticles (AgNPs). Thermodynamic, electro-optic and dielectric properties of the prepared composite have been investigated. From these investigations, parameters such as transition temperature, dielectric anisotropy, dielectric permittivity and splay elastic constant have been determined for the pristine as well as composite systems. Threshold switching voltage of the pure and composite samples has also been determined from transmission voltage curve. The effects of dispersion of AgNPs on dielectric and electro-optic parameters of LC-NPs composite have been discussed in terms of existing standard models.

Temperature dependent Raman spectroscopy of a nematic liquid crystal compound 6CHBT

In this study, we have chosen a room temperature nematic liquid crystal (NLC) compound namely, 6CHBT [(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene] having clearing temperature at 42.8 °C. This particular liquid crystal (LC) exhibits a broad nematic phase from 13 °C to 42.8 °C. Among all the available spectroscopy techniques, Raman spectroscopy is one of the most convenient techniques to understand the various phase transitions of a LC compound. In order to understand the phase change and the change of the molecular orientations along with their change in the chemical bond, the temperature dependent Raman study has been conducted for the LC compound 6CHBT. The transition temperatures of various phases obtained from the linewidth peak positions of Raman spectra agrees well with the literature data. From the Raman spectra, the change in chemical bonds is observed which is due to the increase in the molecular orientation of the alkyl chain in the LC compound. The bond assignment of the experimental value agrees well with the Raman assignment obtained from literature.

Dielectric and electro-optic behaviour of nematic-SWCNT nanocomposites under applied bias field

ABSTRACTWe have prepared the composites of a room temperature Nematic Liquid Crystals and Single-Walled Carbon Nanotubes. Effect of dispersion of nanotubes on various dielectric and electro-optic parameters on host nematic liquid crystals were investigated. The changes in dielectric and electro-optic parameters (viz: relative permittivity, dielectric anisotropy, switching threshold voltage and splay elastic constant) were observed for composite systems. The composites filled in the cells have been probed under applied bias electric field and it enhanced the nematic ordering of the liquid crystal molecules in the composites which results overall improvement of dielectric and electro-optic parameters of the prepared composites.

Photoluminescence and electrical conductivity measurement of liquid crystal doped with ZnO nanoparticles

ABSTRACTWe report the effect of dispersion of zinc oxide (ZnO) nanoparticles (NPs) on the conductivity, birefringence and fluorescence properties of commercially available room temperature nematic liquid crystal (LC) with the variation of dopant concentration. Significant changes have been observed in transition enthalpy, DC conductivity, photoluminescence and birefringence values of the LC material by the addition of ZnO NPs. While the inclusion of NPs enhances the electrical conductivity of the composite system, it results in a reduction in the birefringence value, which can be attributed to a decrease in the order parameter of the system due to the perturbed geometry of the LC. This also results in the increase in threshold voltage value, which has been speculated as due to the piezoelectric nature of the ZnO NPs. The analysis of the fluorescence spectrum indicates that ZnO NPs enhance the intensity in the LC phase along with a small blue shift.

Effects of manganese (II) titanium oxide nano particles on the physical properties of a room temperature nematic liquid crystal 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene

In the present work, Manganese (II) Titanium Oxide nanoparticles (MnTiO3-NPs) were dispersed in a room temperature nematic liquid crystal 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzene (6CHBT) at the concentrations of 0.5 wt% and 1 wt% and their thermodynamic, optical and dielectric properties were investigated. Effects of dispersion of MnTiO3-NPs on various electro-optical and display parameters of host liquid crystalline material have been studied. Physical parameters, such as threshold voltage, dielectric anisotropy and splay elastic constant have enhanced for composite systems. Due to the dispersion of MnTiO3-NPs, nematic to isotropic transition temperature has increased for the composite systems.

Dielectric and electro-optical properties of a nematic liquid crystalline material with gold nanoparticles

ABSTRACTWe have prepared the composites of a room temperature nematic liquid crystalline material namely 4-(trans-4′-n-hexylcyclohexyl) isothiocyanatobenzoate (6CHBT) and gold nanoparticles (GNPs). Thermodynamic, electro-optical and dielectric properties have been investigated. Effect of dispersion of GNPs on various electro-optical and display parameters of host liquid crystalline material have been studied. Physical parameters such as threshold voltage, dielectric anisotropy and splay elastic constant have altered for composite systems. Due to the dispersion of GNPs, nematic to isotropic transition temperature is significantly increased. Relaxation frequency corresponding to flip-flop motion of the 6CHBT molecules about their short axes has increased due to the presence of GNPs.