Non-conventional multi-arm ester liquid crystal derived from cholic acid: synthesis,thermal and optical behaviour

ABSTRACT

With cholic acid as the core, five multi-arm ester liquid crystals were synthesised with different terminal substituents. B₁–B₅ were mesogenic arms which were linked to multifunctional chiral core cholic acid. The effect of electron-withdrawing and electron-donating groups on the mesogenic behaviour of the compounds was discussed. All products were structurally well characterised by elemental analysis, ¹HNMR, and FT-IR. The phase behaviours were investigated by means of polarised optical microscopy and differential scanning calorimetry. B₂, B₄, B₅ and C₂, C₄, C₅ and D₂, D₄, D₅ exhibited typical nematic texture and belonged to thermotropic enantiotropic nematic liquid crystals. B₃, C₃ and D₃ also displayed typical nematic texture of thermotropic monotropic nematic liquid crystal. B₁, C₁ and D₁ did not have mesogenic performance. Wider mesogenic range for 81.6°C of C₅ whose terminal substituent is nitro on heating cycle while 121.1°C on cooling cycle. The results indicated that terminal substituents have a pretty important effect on the mesogenic phase and range of multi-arm liquid crystal compounds. The mesomorphic behaviour of compounds with polar groups terminally substituted performance much better than those without polar groups. In this ester multi-arm liquid crystal system, electron-withdrawing groups terminally substituted behave better than those electron-donating groups substituted.     

Adiabatic calorimetry of the metallomesogen purple cobalt stearate Co(O2CC17H35)2

The heat capacity of the metallomesogen purple cobalt stearate Co(O₂CC₁₇H₃₅)₂ has been measured by adiabatic calorimetry at temperatures between 16 and 420?K. This compound exhibits two crystalline phases (low temperature Cr2 and high temperature Cr1 phases), mesophase (M phase), and isotropic liquid (I phase). A third crystalline phase Cr3, which is entirely metastable with respect to all the others, is suggested by DSC studies. The Cr2-to-Cr1, Cr1-to-M, and M-to-I phase transitions occurred at 362.1, 380.9, and 400.4?K, respectively. The enthalpy and entropy gains at these phase transitions were determined. The mesophase is either smectic A or nematic.     

Nematic liquid crystals in frequency and amplitude modulated electric fields

In this paper we study peculiarities of behaviour of a nematic liquid crystal (NLC) in an electric field consisting of two harmonics with different amplitudes and frequencies. The most interesting result is the experimental observation of high frequency, stabilization of a low frequency electrohydrodynamic instability in the NLC. A method of measurement of the one frequency instability threshold has been proposed. The beating regime of the two frequency electric field was also studied. The theory developed provides an explanation for all the experimental results.     

Nematic nanocomposites with enhanced optical birefringence

The proper performance of electro-optical devices utilising liquid crystals (LCs) requires materials with high diffraction efficiency, i.e. with high optical/dielectric anisotropy, low threshold voltage and fast switching. One can achieve increase of dielectric anisotropy by using chemical synthesis or mixing LC materials. However, in most cases, this causes an increase in the threshold voltage and switching times. Therefore obtaining materials with high dielectric anisotropy and keeping threshold voltage and switching times low is a challenging task. We achieved promising results by making binary mixtures of a polar nematic LC 4'-hexyl-4-biphenylcarbonitrile (HBPCN) with low percentage (1–10% by weight) gold nanoparticles. We report that for the mixtures with 1% and 2% gold the dielectric anisotropy increases by 100% and the birefringence by about 50% of their values for pure nematic. We also report that the increase of the dielectric anisotropy in the mixtures only slightly affects threshold voltage and switching times. We propose that this increase is caused by cluster formation in the mixtures.     

Main-chain liquid crystalline copolymers: synthesis and comparative study of the mesomorphic properties and non-linear optical properties

Linear main-chain polymers (P1, P2 and P3) have been synthesised and characterised. The polymers were designed to understand the rigidity of the mesogen’s linking groups and its effect on transition temperature. The liquid crystalline behaviour of these polymers was investigated using polarised optical microscopy and differential scanning calorimetry. All polymers exhibited phase transition on heating and cooling runs. The mesophase stability decreases with the decrease of rigidity of the second mesogen (P2>P1>P3). Similarly, the enthalpy and entropy changes are dependent on the rigidity of the second mesogen (Mesogen B). Polymer P1 undergoes photoisomerisation under UV-visible light irradiation and it attains a photo-stationary state at 275 s. This provides an opportunity to investigate the third-order non-linear optical properties. The non-linear behaviour of polymer P1 in chloroform presents a distinct deviation in a different concentration, where polymer P1 also exhibits a negative non-linear refractive index. Thus, polymer P1 can be used as a potential candidate for optical device applications such as optical limiters.     

A Ten-Year Perspective on Twist-Bend Nematic Materials

The discovery of the twist-bend nematic phase (N_TB) is a milestone within the field of liquid crystals. The N_TB phase has a helical structure, with a repeat length of a few nanometres, and is therefore chiral, even when formed by achiral molecules. The discovery and rush to understand the rich physics of the N_TB phase has provided a fresh impetus to the design and characterisation of dimeric and oligomeric liquid crystalline materials. Now, ten years after the discovery of the N_TB phase, we review developments in this area, focusing on how molecular features relate to the incidence of this phase, noting the progression from simple symmetrical dimeric materials towards complex oligomers, non-covalently bonded supramolecular systems.     

Nematic liquid crystal gyroids as photonic crystals

ABSTRACT

We demonstrate nematic and cholesteric liquid crystal (LC) gyroids and show their photonic properties as photonic crystals by using numerical modelling. The LC gyroids are designed as composite optical materials, where we take one labyrinth of passages to be a solid dielectric, whereas the other (complementing) labyrinth of passages is taken to be filled by chiral or achiral nematic LC, with the intermediate gyroid surface imposing homeotropic (perpendicular) surface anchoring. The nematic inside the gyroid matrix is shown to exhibit a variety of possible orientational profiles which are characterised by complex networks of topological defects – from ordered, semi-ordered, to completely disordered. The diversity of possible nematic states is shown to lead to a rich structure of photonic bands, which can be tuned by the LC volume fraction and the cholesteric pitch, including control over full – direct and indirect – band gaps.     

Nematic liquid crystals in frequency and amplitude modulated electric fields

In this paper we study peculiarities of behaviour of a nematic liquid crystal (NLC) in an electric field consisting of two harmonics with different amplitudes and frequencies. The most interesting result is the experimental observation of high frequency, stabilization of a low frequency electrohydrodynamic instability in the NLC. A method of measurement of the one frequency instability threshold has been proposed. The beating regime of the two frequency electric field was also studied. The theory developed provides an explanation for all the experimental results.     

A Liquid Crystalline Oligomer Exhibiting Nematic and Twist‐Bend Nematic Mesophases

The twist‐bend nematic phase (N_TB) has been described as the structural link between the untilted uniaxial nematic phase (N) and the helical chiral nematic phase (N*). The N_TB phase exhibits phenomena of fundamental importance to science, that is, 1) the spontaneous formation of a helical pitch on the nanometer scale in a fluid and 2) the spontaneous breaking of mirror symmetry, leading to the emergence of chiral domains in an achiral system. In this Communication, we present a study on T4₉ [bis(4‐(9‐(4‐((4‐cyanobenzoyl)oxy)phenyl)nonyl)phenyl) 4,4′‐(nonane‐1,9‐diyl)dibenzoate], a liquid‐crystalline oligomer exhibiting the twist‐bend nematic phase, which has a molecular length that is of comparable dimensions to the sub‐10 nm pitch determined for CB9CB, and provide new insights into the differentiation between the nano‐ and macro‐science for self‐assembling supermolecular systems.     

Ultrafast,electric field-induced fingers in an anticlinic liquid crystal

Propagating fingers of synclinic liquid crystalline phase were observed to invade the anticlinic phase for applied electric fields E larger than a characteristic threshold field E_th. The front velocity was found to be highly non-linear in E, with enormous velocities of at least 10 cm s^-1, and perhaps as high as 400 cm s^-1 for the maximum applied field. These are by far the largest velocities ever observed for a liquid crystal. The results are discussed theoretically, including the possibilities of a field-dependent molecular interaction coefficient and shear thinning.     

Characterization of the B4 phase by dielectric and AFM measurements

Dielectric measurements on a sample consisting of banana-shaped molecules were carried out in a frequency range between 0.1 Hz and 10 MHz. The sample exhibited B₂ and B₄ phases. As usual, two ranges of relaxation were detected in the B₂ phase, the fast reorientation about the long axes of the molecules and a slow collective process. In the B₄ phase, only one dielectric active process at low frequencies was found; this does not differ from the low frequency relaxation of the B₂ modification. This relaxation is probably related to the dynamics of superstructures. Crystallization could be observed after keeping the sample for a longer time at higher temperature. Thus, it was possible to differentiate clearly between the crystalline and the B₄ phases. AFM investigations prove the existence of focal-conic domains and periodic superstructures in the B₄ phase; then do not appear in the crystalline state. For this reason the B₄ phase is regarded as different from a classical crystalline phase.     

Multiple Polar and Non-polar Nematic Phases

Liquid-crystal materials exhibiting up to three nematic phases are reported. Dielectric response measurements show that while the lower temperature nematic phase has ferroelectric order and the highest temperature nematic phase is apolar, the intermediate phase has local antiferroelectric order. The modification of the molecular structure by increasing the number of lateral fluorine substituents leads to one of the materials showing a direct isotropic-ferronematic phase transition.     

Photo-Responsive Behavior of Azobenzene Based Polar Hockey-Stick-Shaped Liquid Crystals

A study on the photoswitching behavior of azobenzene-based polar hockey-stick-shaped liquid crystals (HSLCs) has been presented. Two new series of five phenyl rings based polar HSLCs have been designed and synthesized. Solution state photoisomerization of the synthesized materials was investigated thoroughly via UV-visible and ¹H NMR spectroscopic techniques, whereas solid-state photochromic behavior was elucidated via physical color change of the materials, solid-state UV-visible study, powder XRD, and FE-SEM techniques. The materials exhibited decent photochromic behavior for different potential applications. The thermal phase behavior of the superstructural assembly has been characterized via polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and temperature-dependent small and wide-angle X-ray scattering (SAXS/WAXS) studies. Depending upon the length of the terminal alkyl chain, nematic (N) and partially bilayer smectic A (SmA_d) phases were observed. DFT calculations revealed the favorable anti-parallel arrangement of the polar molecules that substantiate the formation of SmA_d phase.     

Surface viscosity and reorientation process in an asymmetric nematic cell

The influence of surface viscosity and anchoring energy on the reorientation process of a nematic liquid crystal cell is theoretically investigated. The cell is a slab of thickness, d, whose limiting surfaces are characterised by different anchoring strengths and present easy directions parallel to the bounding surfaces, changing with time due to some external action. The exact space-time profile of the director angle is obtained by means of integral transform techniques and a Green function approach. From this formalism, the time dependence of the optical path difference is exactly determined and its behaviour is analysed in connection with the presence of surface viscosity and different anchoring energies. The problem is also exactly solved in the presence of a constant electric field. It is shown that the compatibility problem between the time derivative of the director field on the surface and in the bulk can be avoided.     

A Twist‐Bend Nematic Phase Driven by Hydrogen Bonding

The liquid crystalline phase behavior of 4‐[6‐(4′‐cyanobiphenyl‐4‐yl)hexyloxy]benzoic acid (CB6OBA) and 4‐[5‐(4′‐cyanobiphenyl‐4‐yloxy)pentyloxy]benzoic acid (CBO5OBA) is described. Both acids show an enantiotropic nematic phase attributed to the formation of supramolecular complexes by hydrogen bonding between the benzoic acid units. In addition, CB6OBA provides the first example of hydrogen bonding driving the formation of the twist‐bend nematic phase. The observation of the twist‐bend nematic phase for CB6OBA, but not CBO5OBA, is attributed to the more bent molecular shape of the complexes formed by the former, reinforcing the view that shape is a key factor in stabilizing this new phase. Temperature‐dependent FTIR spectroscopy reveals differences in hydrogen bonding between the two nematic phases shown by CB6OBA which suggest that the open hydrogen‐bonded complexes may play an important role in stabilizing the helical arrangement found in the twist‐bend nematic phase.     

Mesomorphic polyacrylates containing isomeric methyl-substituted azobenzene mesogens

The synthesis and characterization are reported of two new series of structurally isomeric polyacrylates containing the azobenzene moiety spaced away from the backbone by a hexamethylene spacer and substituted in 4-position with linear alkoxy groups (C₁? C₆, C₁₀) and with a lateral methyl group in 2′- or 3′-position. Thermal optical and X-ray analyses of the prepared polymer samples, with molecular weight ranging from 50 × 10³ to 35 × 10⁴ Dalton (M?_w/M?_n = 2?3), provide evidences for the establishment of liquid crystalline behavior in the melt in a fairly broad range of temperature. In any case, quenching from the mesophase allows for the lock-in of the liquid crystalline structure in the glassy state. The presence of the lateral methyl-substitutents causes a drop as high as 50–100 K. in the stability of the mesophase with respect to the corresponding samples of the methyl-unsubstituted series. A parallel depletion of the smectogenic character is also observed. Typical even-odd effects on the isotropization temperature and relevant thermodynamic parameters have been detected along with the formation of interdigitated smectic structures.     

Self‐Assembly of Azobenzene Derivatives into Organogels and Photoresponsive Liquid Crystals

A new class of coil–rod–coil molecules with an azobenzene core was synthesized. They were found to form robust organogels in several organic solvents. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), FTIR spectroscopy, UV/Vis absorption spectroscopy, ¹H NMR spectroscopy, and X‐ray diffraction (XRD) revealed that in these organogels, the molecules self‐assembled into a nanofiber network with an H‐type aggregation mode under the joint effect of π–π stacking, intermolecular hydrogen bonding, and van der Waals forces. Interestingly, the incorporation of the azobenzene mesogene into the rigid core led to photoisomerizable liquid crystal materials, which exhibited quick responsiveness to light and temperature, along with the trans–cis transition stimulated by UV light and heating.     

Tuning helical twisting power and photoisomerisation kinetics of axially chiral cyclic azobenzene dopants in cholesteric liquid crystals

ABSTRACT

Significant attention has been paid to improve the helical twisting power (β) and Δβ between the two different isomers of axially chiral azobenzene dopants in cholesteric liquid crystals (CLCs); however, the correlations between the vales (β and Δβ) with the molecular structures as well as photoisomerisation kinetics are far from clear. In this study, a series of binaphthyl-azobenzene cyclic dopants R1–R3 with different lengths of alkoxy chain was synthesised, which exhibited photochemically reversible trans–cis isomerisation in both organic solvents and liquid crystal hosts. When doping into a nematic liquid crystal, dopant R2 with one linking alkoxy group showed the highest values of β and Δβ. The results revealed that the β value was related to the dihedral angle between two naphthyl planes and the miscibility between the dopants and the host molecule. Moreover, Δβ was also depended on the photoisomerisation quantum yields. With increasing length of alkoxyl chain, the photoisomerisation rate constant of dopants increased upon ultraviolet irradiation and decreased for the reverse process upon visible light irradiation either in isotropic acetonitrile or in CLCs. These results enable the precise tuning of the pitch and selective reflection wavelength of CLCs.     

Dual switchable six-ring bent-core liquid crystals with azo linkages exhibiting B1 and B2 mesophases

A new series of asymmetric bent-core compounds were synthesised using azo linkages with equal as well as unequal terminal alkyl chains and their photo and electrical switching properties investigated. The mesomorphic properties were characterised using polarised optical microscopy, X-ray diffraction and differential scanning calorimetry. The lower homologues of the series of compounds show a B₁ phase whereas higher homologues exhibit a B₂ phase. The B₂ phase shows an anticlinic-antiferroelectric switching behaviour during electro-optic measurements. The photo-switching properties of the azobenzene containing bent-core molecules were investigated using ultraviolet-visible spectroscopy. The trans to cis photo isomerisation was observed at 25 s whereas reverse processes took 14 h in chloroform.     

Photocontrolled manipulation of a microscale object: a rotational or translational mechanism

In this paper the photocontrolled manipulation of solid materials on the surface of a liquid crystalline thin film is described. Three different types of films namely cholesteric liquid crystal (ChLC), compensated nematic liquid crystal (NLC) and nematic LC were used. The rotational and translational manipulation of the microscale solid object was induced by irradiation of light and mode of manipulation (either translational or rotational) was changed by changing the isomer of the azobenzene compound used to make the film. Rotational motion of the object was observed on the ChLC and compensated NLC films containing chirally pure azobenzene compound. The direction of rotational motion was controlled either by changing the optical isomer of the chiral azobenzene or by changing the irradiating light (from ultraviolet to visible). When racemic mixture of the chiral azobenzene compound was used, a translational motion of the object was observed. Even though the direction of the translational motion can be controlled by controlling irradiation position, more facile and precise manipulation of the objects was possible by spatially controlled irradiation of Ar(+) laser and diode UV laser.