Four-ring Bent-core Liquid Crystals Research Articles

Blue Phase III from Bent-Core Liquid Crystals: Unveiling Thermal Stability & Unique Traits - A Comprehensive Review.

This review article mainly delves into the comprehensive development, thermal stabilization, characteristics, and applications of Blue Phase III (BPIII) derived from non-calamitic, mainly T-shaped and bent-core liquid crystals (BCLC). The discussion begins with discovering and characterizing various liquid crystal (LC) phases of BCLCs, emphasizing the significance of the nematic (N) phase in three and four-ring BCLCs. Following this, the focus shifts to the stabilization, properties, and potential applications of BPIII, particularly those derived from non-conventionalliquid crystals. The review highlights the exceptional electro-optical (E-O) properties of BPIII, including high Kerr constants and distinct phase transitions. Studies reveal the impact of chirality on thermal behavior, microscopic observations, and the influence of molecular structures on mesophase formation. Investigations into asymmetrical chiral liquid crystal diads and hydrogen-bonded complexes underscore the importance of molecular design in expanding BPIII ranges. Furthermore, achiral unsymmetrical BCLC designs reveal significant insights into the interplay between molecular structure, phase transitions, and E-O behavior.Additionally, the structural transformation and E-O properties of highly polar BCLCs are examined to stabilize BPIII at room temperature, achieving notable Kerr constants and low voltage requirements. These collective studies provide a thorough understanding of BPIII and its promising applications in materials science and technology.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly. There is no analysis of the photo-physical properties, in particular, the excited state dynamical behaviour, of these molecules. Because of the availability of two proton transfer sites in these systems, there is no clear explanation of the involvement of single or double proton transfer reactions in the origin of the large Stokes shifted green fluorescence band. Therefore, we employ the femtosecond transient absorption spectroscopy technique to recognize the formation of transient species in the excited state and its associated dynamics in the femto-picosecond time domain. In order to validate the experimental photo-physical properties, the time-dependent density functional theory (TDDFT) calculations have been performed. Our results indicate that the four-ring bent-core system is an excellent example of systems exhibiting two proton transfer reactions in a sequential process. Further, these two proton transfer sites are not electronically coupled to each other; therefore, monoketo and diketo tautomers exhibit very close absorption and emission positions. The large Stokes shifted green emission in these systems is mainly contributed by the monoketo tautomer (MK-C*). The linking ester functional group of both sites plays a significant role in controlling the rate of proton transfer reactions. A good correlation is observed between theoretical and experimental results.

Cybotactic nematic phase of achiral unsymmetrical bent-core liquid crystals – Quelling of polar ordering and the influence of terminal substituent moiety

Nematic phase of bent-core liquid crystals (LCs) exhibiting cybotactic clusters (NCyb) have gained significant importance owing to its promising ability to demonstrate macroscopic biaxiality and the ferronematic phase. In this context, three achiral unsymmetrical four-ring bent-core LC compounds, bearing a long alkyloxy chain and differing only in the terminal substituent moiety (methyl, chloro, nitro), are designed and synthesized followed by their optical, dielectric, electro-optic and structural investigations. The presence of NCyb in the methyl and chloro substituted compounds was confirmed via dielectric spectroscopy and X-ray diffraction observations. The absence of ferroelectric behaviour in any of these compounds, even in the cybotactic nematic phase and in presence of polar substituent moieties (chloro and nitro), is attributed to the increased alkyloxy chain length and antiparallel molecular arrangement. The density functional theory (DFT) optimized molecular structure along with the experimental observations further substantiates these findings. The study establishes that cybotactic clusters and polar end moiety, although being a prerequisite for ferroelectric-like nature, do not necessarily result in a ferronematic phase.

Impact of terminal polar substitution on elastic, electro-optic and dielectric properties of four-ring bent-core nematic liquid crystals.

Here we report the influence of terminal –F, –Cl and –NO2 substitution on the elastic, dielectric and polar switching behavior of four-ring bent-core liquid crystals (LCs). Elastic constants of nematic liquid crystals are the key parameters in determining the threshold voltage and sensitivity to electro-optical response in a device. The elastic properties of bent-core liquid crystal systems show atypical temperature dependence and there is no hard-core theory to explain the behavior. However based on molecular simulation and atomistic calculations it is found in earlier studies that the bend angle dominates the behavior of elastic constants and the terminal or lateral substitutions have very little effect. Here we have studied three bent-core compounds which are differentiated only by their terminal polar substitution. The bend angle is identical (∼146°) for all the three compounds yet they show dramatically different elastic properties. In the fluoro-substituted compound K11 > K33, while for the other two compounds K33 > K11. Thus it is evident that the terminal polar substitution plays vital role in determining the elastic properties of bent-core systems. Correlating the mesophase ranges with the respective dipole moments of the samples it is observed that the fluoro-substituted compound (11-2M-F) with lowest dipole moment favours only nematic phase with smallest mesophase range (46.1 °C), compound 11-2M-Cl with moderate dipole moment favours short range nematic, broad range smectic with moderate mesophase range (53.1 °C), whereas the compound 11-2M-NO2 possesses the widest mesophase range (99.8 °C) with a very narrow nematic and a broad smectic phase amongst the three studied compounds.

Temperature-dependent infrared study of phase transitions in bent-core liquid crystalline compound 12O(OH)2MeO10

Temperature-dependent Fourier-transform infrared (FTIR) investigation on a novel four-ring bent-core liquid crystal ((4′-n-decyloxyphenylazo)-phenyl-4-yl-3-[N-(4′-n-dodecyloxy-2-hydroxybenzylidene) amino]-2-methyl benzoate) has been done to study the dynamics of the molecules in mesophases at the molecular level. The spectroscopic analysis revealed that specific modes gave a clear evidence of Cr⇄SmC and SmC⇄NcybC phase transitions and successfully depicts the mechanism at these transitions. One of the important objectives to find the spectroscopic signature of the coexistence of SmC and cybotactic nematic NcybC phases below NcybC→SmC phase transition up to 3°C could not be achieved. FTIR spectroscopy confirmed the phase reversibility. In order to get the theoretical support density functional theory (DFT) has been adopted for conformational analysis that predicted the most stable conformer I. Conformer I might be expected closer to the real system and utilized for vibrational assignment along with potential energy distribution (PED). It has been demonstrated that the combination of FTIR spectroscopy and DFT paved a way to understand a candid picture of structural modification and dynamics of mesophases at the macromolecular level during the phase transitions.

Study of Cr → SmA phase transition and hydrogen bonding in four-ring bent-core liquid crystal

A newly designed asymmetrical four-ring bent-core compound (4′-n-decyloxyphenylazo)-phenyl-4-yl-3-[N-(4′-n-octadecyloxy-2-hydroxybenzylidene) amino]-2-methyl benzoate exhibiting liquid crystalline behavior was synthesized and characterized. The thermal and textural morphology were studied using differential scanning calorimetry and polarizing optical microscopy, respectively. The study of hydrogen bonding and dynamics of the phase transition has been performed at the molecular level using temperature dependent Fourier transform infrared (FTIR) spectroscopy. The spectral analysis of OH, CH2/CH3, CO, and CN stretching vibrational bands revealed clear signatures of Cr→SmA phase transition at 125°C. Density functional theory has been adopted for the geometry optimization and conformational study of the monomer using the B3LYP/6-31G(d) method. The conformational analysis has been performed to predict the most stable conformer along with the possible conformers using one-dimensional potential energy scan employing the same level of theory. The combination of experimental findings and theoretical analysis helped to understand the mechanism of phase transitions at the molecular level.

Existence of polar switching in the nematic and orthogonal smectic phases in novel four-ring bent-core compounds

Bent-core liquid crystals have set the first example of forming polar superstructures from achiral molecules. Polar switching studies in smectic phases have revealed several exciting sub-phases which have never been observed in rod-like liquid crystals. In this study, mesomorphic and polar switching properties of three bent-core compounds belonging to a homologous series have been investigated using polarizing optical microscopy (POM), differential scanning calorimetry, XRD studies, electro-optics, and dielectric spectroscopy. These achiral, unsymmetrical four-ring bent-core liquid crystals with a polar fluoro substituent at one end and n-alkoxy chain at the other terminal end possess azo, ester, and imine linkages between the four phenyl rings and different lateral substituents. The compounds 16-F and 18-F exhibit orthogonal smectic phase with antiferroelectric polar order, and additionally, the compound 16-F exhibits a short range nematic phase with a polar order. The compound 7-F exhibits broad enantiotropic nematic mesomorphism without appearance of any smectic ordering at low temperature. The smectic and nematic phases were identified by their optical textures observed by POM. Distinct polarization current peaks under triangular wave voltage are observed for all the compounds in the entire mesophase range. Relaxation phenomena corresponding to polar associations of the molecules are observed in dielectric spectroscopy. The cybotactic nature of the nematic phase is established by the XRD and electro-optic studies of 16-F. Although it is rather difficult to form mesophase when the number of aromatic rings is reduced from five, we have successfully demonstrated the bent-core compounds with four-ring which exhibit orthogonal smectic phases as well as nematic mesomorphism with unusual cybotactic signature.

A combined experimental and theoretical approach to study SmC → NcybC phase transition in a four-ring bent-core liquid crystal

Spectroscopic evidences for the analysis of phase transition and hydrogen bonding in a newly synthesised four-ring bent-core liquid crystal.

Effect of methoxy group instead of polar group in the nematic phase of four-ring bent-core liquid crystals

Two achiral four-ring bent-core liquid crystals are investigated by using the optical polarizing microscope, electro-optical studies, and dielectric spectroscopy.

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.

Nematic phases in achiral unsymmetrical four-ring bent-core azo compounds possessing strongly polar cyano and nitro moieties as end substituents: Synthesis and characterization

Achiral unsymmetrical four-ring bent-core liquid crystals with strongly polar cyano and nitro moieties as substituents at one end and alkyloxy group (butyl to dodecyl) at the other end have been synthesized by a simple and straight forward synthetic method. The four phenyl rings are connected through azo, ester and imine linkages respectively and bent unit is derived from 3-aminobenzoicacid with 2-methyl substituent in the transverse direction of the molecule. The molecular structure characterization is consistent with elemental and spectroscopic analysis data. The materials thermal behaviour and phase characterization have been investigated by differential scanning calorimetry and polarizing optical microscopy. All the compounds exhibit enantiotropic nematic phase over a wide temperature range. The light induced cis–trans isomerization is also discussed.

Achiral unsymmetrical four-ring bent-core liquid crystals with a polar fluoro or chloro end substituent: synthesis and characterisation

Achiral unsymmetrical 2-methyl 3-amino-benzoic acid (2,6-substituted toluene)-derived four-ring bent-core liquid crystals with a polar fluoro or chloro substituent at one end and an alkyloxy group (butyl to dodecyl) at the other end have been synthesised by a simple and straightforward synthetic method. The molecular structure characterisation is consistent with the elemental and spectroscopic analysis data. The materials' thermal behaviour and phase characterisation have been investigated by differential scanning calorimetry and polarising optical microscopy. All the compounds exhibit an enantiotropic nematic phase over a wide temperature range.

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.