Crystal Dimers Research Articles

Chiral Lanthanide Complexes with l- and d-Alanine: An X-ray and Vibrational Circular Dichroism Study.

A whole series of [Ln(H2O)4(Ala)2]26+ dimeric cationic lanthanide complexes with both l- and d-alanine enantiomers was synthesized. The single-crystal X-ray diffraction at 100 and 292 K shows the formation of two types of dimers (I and II) in crystals. Between the dimer centers, the alanine molecules behave as bridging (μ2-O,O’-) and chelating bridging (μ2-O,O,O’-) ligands. The first type of bridge is present in dimers I, while both bridge forms can be observed in dimers II. The IR and vibrational circular dichroism (VCD) spectra of all l- and d-alanine complexes were registered in the 1750–1250 cm−1 range as KBr pellets. Despite all the studied complexes are exhibiting similar crystal structures, the spectra reveal correlations or trends with the Ln–O1 distances which exemplify the lanthanide contraction effect in the IR spectra. This is especially true for the positions and intensities of some IR bands. Unexpectedly, the ν(C=O) VCD bands are quite intense and their composed shapes reveal the inequivalence of the C=O vibrators in the unit cell which vary with the lanthanide. Unlike in the IR spectra, the ν(C=O) VCD band positions are only weakly correlated with the change of Ln and the VCD intensities at most show some trends. Nevertheless, this is the first observation of the lanthanide contraction effect in the VCD spectra. Generally, for the heavier lanthanides (Ln: Dy–Lu), the VCD band maxima are very close to each other and the mirror reflection of the band of two enantiomers is usually better than that of the lighter Lns. DFT calculations show that the higher the multiplicity the higher the stability of the system. Actually, the molecular geometry in crystals (at 100 K) is well predicted based on the highest-spin structures. Also, the simulated IR and VCD spectra strongly depend on the Ln electron configuration but the best overall agreement was reached for the Lu complex, which is the only system with a fully filled f-shell.

Collective excitations and field induced anti-ferroelectric modes in chiral liquid crystal dimer

Collective excitations and field influence are investigated in tilted or orthogonal layered liquid crystal smectic (Sm) phases in a chiral hydrogen bonded (HB) calamitic dimer 10OBA:M*SA:10OBA by low-frequency dielectric spectroscopy. Phase transition temperatures, hysteresis, and phase thermal ranges are determined. The hysteresis in SmC* reveals the storage device suitability. The magnitude of the tilt angle in SmC* infers moderate viewing angle. The growth of primary order parameter tilt angle θ(T) infers long range interaction in SmC*. The growth of ferroelectric (FE) relevant secondary order parameter polarization Ps(T) infers strong layer-over-layer helical coupling of transverse dipole moment μt. Dispersion (loss vs permittivity) in 1D SmC*, 3D orthogonal SmBcryst, and 3D hexagonal tilted SmG crystal phases infers two distinct collective modes, viz., high frequency soft mode (SM) and low frequency Goldstone mode (GM). The dielectric strength of the SM in the SmC* phase confirms the FE Curie–Weiss behavior. The GM persists down to low temperature 3D LC phases. The AF order in SmC* is smeared by high temperature thermal fluctuations. A bias field resolves the GM in SmC* to reveal antiferroelectric (AF) modes. AF order manifested as GM1 and GM2 is explained by distinct Ps helices with different relaxation frequencies fR1 and fR2. GM1 and GM2 in 3D SmBcryst and SmG phases are explained by quenching by crystal field. HB LC dimer with a just shifted chiral center promoted the occurrence of field induced AF modes. AF P-switching in SmC* infers the gray scale mode. Trends of dielectric parameters, relaxation frequency, loss maximum, activation energy, dielectric strength, distribution parameter, etc., are discussed with respect to utility in appliances.

Comparative DFT Study of Parallel and Antiparallel Conformation of 5CB and 6CB Liquid Crystal Dimers

An investigation of the interaction of 5CB and 6CB liquid crystals along with their dimer configurations in different conformations. The total energy, thermal energy, HOMO-LUMO Gap, dipole moment, polarizability, constant volume heat capacity, entropy, zero-point energy (ZPE), and enthalpy of the different configurations of 5CB and 6CB liquid crystal dimers are affected during the molecular interaction. The different interaction properties of 5CB, 6CB, and 5CB-6CB dimers studied in parallel and antiparallel conformation with the help of density functional theory method lc-blyp by NWChem software package. The 5CB and 6CB liquid crystal dimers have the least dipole moment and negative entropy in the antiparallel conformation. The cross conformation of 5CB liquid crystal dimer (5CB-5CB) has the least isotropic polarizability; however, the parallel conformation of 5CB liquid crystal dimer has the highest isotropic polarizability. The isotropic polarizability is minimum in the antiparallel conformation of 6CB liquid crystal dimer (6CB-6CB), while isotropic polarizability is maximum in the parallel conformation of 6CB liquid crystal dimer. The 5CB liquid crystal dimer and 6CB liquid crystal dimer have minimum negative interaction energy in the antiparallel conformation, but 5CB-6CB liquid crystal dimer has positive interaction energy for all the possible conformations.

Twist-Bend Nematogenic Supramolecular Dimers and Trimers Formed by Hydrogen Bonding

A selection of novel non-symmetric supramolecular liquid crystal dimers and trimers formed by hydrogen-bonding have been prepared and their phase behaviour characterised by polarised optical microscopy, X-ray diffraction and temperature-dependent Fourier-transform infrared spectroscopy. We mix the bent twist-bend nematogen 4-[6-(4’-cyanobiphenyl-4-yl) hexyloxy]-benzoic acid (CB6OBA) with a series of small stilbazole-based compounds 4-[(E)-2-(n-alkoxyphenyl)ethenyl]pyridines (nOS) of varying terminal chain length (n) to obtain the CB6OBA:nOS complexes. Complexes with n ≤ 7 exhibit nematic and twist-bend nematic behaviour, followed on cooling by a smectic C phase for n = 4–7, and finally, a hexatic-type smectic X phase for n = 3–7. Mixtures with n = 8–10 exhibit a smectic A phase below the conventional nematic phase, and on further cooling, a biaxial smectic Ab phase and the same hexatic-type SmX phase. Supramolecular trimers, CB6OBA:CB6OS and CB6OBA:1OB6OS, formed between CB6OBA and dimeric stilbazoles [(E)-2-(4-{[6-(4’-methoxy[1,1’-biphenyl]-4-yl)hexyl]oxy}phenyl)ethenyl]pyridine (1OB6OS) or 4-[(E)-4’-(6-{4-[(E)-2-(pyridin-4-yl)ethenyl]phenoxy}hexyl)[1,1’-biphenyl]-4- carbonitrile (CB6OS), exhibit nematic and twist-bend nematic phases, and are the first hydrogen-bonded trimers consisting of unlike donor and acceptor fragments to do so.

Structures of human galectin-10/monosaccharide complexes demonstrate potential of monosaccharides as effectors in forming Charcot-Leyden crystals

The galectins are a family of β-galactoside-specific animal lectins, and have attracted much attention as novel regulators of the immune system. Galectin-10 is well-expressed in eosinophils, and spontaneously forms Charcot-Leyden crystals (CLCs), during prolonged eosinophilic inflammatory reactions, which are frequently observed in eosinophilic diseases. Although biochemical and structural characterizations of galectin-10 have been done, its biological role and molecular mechanism are still unclear, and few X-ray structures of galectin-10 in complex with monosaccharides/oligosaccharides have been reported. Here, X-ray structures of galectin-10 in complexes with seven monosaccharides are presented with biochemical analyses to detect interactions of galectin-10 with monosaccharides/oligosaccharides. Galectin-10 forms a homo-dimer in the face-to-face orientation, and the monosaccharides bind to the carbohydrate recognition site composed of amino acid residues from two galectin-10 molecules of dimers, suggesting that galectin-10 dimer likely captures the monosaccharides in solution and in vivo. d-Glucose, d-allose, d-arabinose, and D-N-acetylgalactosamine bind to the interfaces between galectin-10 dimers in crystals, and they affect the stability of molecular packing in crystals, leading to easy-dissolving of CLCs, and/or inhibiting the formation of CLCs. These monosaccharides may serve as effectors of G10 to form CLCs in vivo.

The effect of molecular structure on mesophase behaviour of non-symmetric liquid crystal dimers containing mandelic acid and fluorine group

ABSTRACT A series of non-symmetric dimers with the same chiral core, mandelic acid (MA), have been synthesised by varying the terminal group and the rigidity of the molecules, termed QBBMA-3F3BT, QBMA-3F3BT, QBMA-3F2BT, 3F3BMA-BBCN, 3F2BMA-BBCN and 3F1BMA-BBCN. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by Fourier transform infrared spectroscopy, 1H NMR, polarised optical microscopy (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and thermogravimetric analysis (TG). The six dimers displayed different optical activity (in THF). QBBMA-3F3BT and QBMA-3F3BT displayed enantiotropic chiral smectic A (SmA*) phase. QBMA-3F2BT displayed N phase. The Tm of QBMA-3F3BT is the highest among QBBMA-3F3BT, QBMA-3F3BT and QBMA-3F2BT, which indicated that molecular conformation played an important role on Tm besides molecular weight and molecular rigidity. 3F3BMA-BBCN and 3F2BMA-BBCN displayed chiral nematic (N*), which indicated that the mesogenic arm BBCN is beneficial to the formation of N* phase. The LC range of 3F2BMA-BBCN is wider, which may be due to that the similar length of the two LC arms is advantageous to the interaction between C≡N group and other molecules so as to stabilise the LC phase.

Birefringence and photoluminescence properties of diphenylacetylene-based liquid crystal dimers

We herein report phase transitions, mesomorphism, birefringence behavior and photoluminescence properties of symmetric liquid crystal (LC) dimers based on diphenylacetylene or tolane.

IMAGE ANALYSIS STUDIES IN RE-ENTRANT SMC PHASE ON HYDROGEN BONDED LIQUID CRYSTAL SA:10OBA

Ponte Academic JournalFeb 2020, Volume 76, Issue 2 IMAGE ANALYSIS STUDIES IN RE-ENTRANT SMC PHASE ON HYDROGEN BONDED LIQUID CRYSTAL SA:10OBAAuthor(s): K. Lakshmi Sarada ,A.V.N.Ashok Kumar, K. Mallika, S. Sreehari SastryJ. Ponte - Feb 2020 - Volume 76 - Issue 2 doi: 10.21506/j.ponte.2020.2.15 Abstract:Image analysis using MATLAB software for characterization of liquid crystalline (LC) phases and phase transition temperatures in hydrogen bonded liquid crystal dimer viz., Succinic acid (SA) (for n=10) with molecular formula nOBA:SA:nOBA is described. SmC, SmX, SmCRE and SmG phase variance is observed by optical textures through Polarized Optical Microscope and which are confirmed by Differential Scanning Calorimetry experimental technique. Influence of flexible end chain length and length of spacer moiety on the LC polymorphism is studied and the results are compared with reports on other HBLC dimers with similar type of molecular structure. Statistical parameters of LC textures based on gray scale intensity as a function of temperature are computed and discussed. Download full text:Check if you have access through your login credentials or your institution Username Password

Variable pitch hydrodynamic electro-optic gratings utilising bent liquid crystal dimers.

Electrohydrodynamic instabilities (EHDI) in liquid crystals form uniform and continuously variable diffractive structures when subject to certain material and geometry determined conditions. A one-dimensional grating is one such diffractive structure, where the refractive index changes periodically in a direction parallel to the initial liquid crystal director. The period of this structure has been shown previously to vary continuously between the values of the cell gap and half-cell gap approximately, allowing continuous angular modulation of optical beams but with a limited angular range. In this work, the lower pitch limit is shown to also be governed in part by the ratio of the splay and bend elastic constants (k11/k33) of the liquid crystal. A host nematic liquid crystal with standard elastic constant ratios (k11/k33 < 1) is doped with odd-alkyl-spaced dimeric liquid crystal CB7CB, to create a liquid crystal mixture with a far higher elastic constant ratio (k11/k33 > 5) than for those previously used in literature EHDI studies. The EHDI gratings formed in this new mixture exhibit pitch lengths significantly below half-cell gap, allowing up to 50% wider angle continuous steering of light. This improves the potential for application in beamsteering and diffractive optical devices.

The influence of terminal chain on the formation of chiral nematic phase of non-symmetric liquid crystal dimers containing fluorinated units

In this study, we have synthesized four fluorine liquid crystals (LCs), termed FA-PD-UBA, FA-PD-PBA, FA-PD-ABA and FA-PD-BA. Through the research on thermal properties and mesophase type of the dimers, we can draw a conclusion that the non-symmetric structure type with flexible terminal chain of appropriate length, may help the dimers to form cholesteric fluorinated liquid crystals.

Spontaneous polarization, tilt angle, hysteresis and low-frequency relaxations in hydrogen bonded chiral liquid crystal dimer: M*SA:9OBA

Influence of low-frequency (2-Hz-10MHz) AC field on LC phases exhibited by chiral hydrogen bonded liquid crystal dimer, viz., M*SA:9OBA is investigated. Phase transition temperatures Tc determined by capacitance C(T) and loss factor TanΔ(T) agree with the earlier report. Induced surface polarization reflected through capacitance C(E) exhibited hysteresis in behavior SmC* phase. Growth of primary order parameter in SmC* phase investigated by tilt angle θ(T) inferred critical field exponent β1. Secondary order parameter investigated as spontaneous polarization Ps in SmC* inferred Mean field exponent β2. Dipole moment (μ) is resolved as longitudinal (μl) and transverse (μt) components. Reorientation of μ to the field analyzed. Relaxation studied at different temperatures in SmC*; SmBcryst and SmG phases inferred a low-frequency (LF fR∼100Hz) and another high frequency (HF fR∼100kHz) reorientation processes. LF relaxation in SmC* phase is influenced by dc bias field to suggest polarization helix formed by coupling of field μt. Threshold field (Vth) for polarization switching estimated from the data of field variation of loss εmax″(E). LF relaxation in SmC* is identified with Goldstone mode (GM) manifested as the suppressed ε″max(E) with field. HF relaxation in SmC* is identified with Soft Mode (SM) by its decreasing trend of fR towards TIC*. Activation energies (Ea) estimated from Arrhenius behavior of fR(T) agree with reports. Field influence on GM manifests through shift of loss ε″max(E) and fR(E). Activation energy Ea is estimated from the shift of fR(E) in terms of equivalent temperature plots. Dispersion, ε″ versus ε′ in LC phases infers a non-Debye distribution of relaxation times. Cole-Cole plots inferred large shift of static permittivity εo. Data of dielectric relaxation parameters, i.e., relaxation frequency fR, relaxation time τR, activation energy Ea, loss maxima ε″max, static permittivity (εo), high frequency permittivity ε∞, dielectric strength Δε and distribution (α) parameter are presented. Dielectric strength analyzed for SM in SmC* phase confirms Ferro-Electric Curie-Weiss behavior.

Fluorinated chiral nematic liquid crystal dimers based on (S)-1-phenylethane-1,2-diol

ABSTRACTA series of symmetric liquid crystal (LC) dimers with the same chiral core (S)-1-phenylethane-1,2-diol ((S)-PE) have been synthesised, termed TBDA-(S)-PE, 3F3B-(S)-PE, 3F2B-(S)-PE, 1F3B-(S)-PE, 1F2B-(S)-PE, respectively. Chemical structures and LC properties of the five symmetric LC dimers were characterised by Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance hydrogen spectrometer (1H NMR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), X-ray diffraction (XRD) and polarised optical microscopy (POM). TBDA-(S)-PE displayed enantiotropic chiral smectic A (SmA*) phase while 3F3B-(S)-PE, 3F2B-(S)-PE, 1F2B-(S)-PE all displayed enantiotropic chiral nematic (N*) phase and 1F3B-(S)-PE displayed monotropic N* phase. The results indicated that the removal of the flexible spacers between the rigid mesogenic arm and the chiral core facilitated the formation of the N* phase. When the rigid mesogenic units are connected to the chiral core directly, the structure of the terminal fluorine group and the rigidity of the mesogenic unit played certain influence on the thermal properties of the LC dimers, but did not change the type of mesomorphic phase. Compared to 3F3B-(S)-PE and 1F3B-(S)-PE, 3F2B-(S)-PE and 1F2B-(S)-PE displayed wider LC ranges, respectively, suggesting molecular regularity had greater influence on LC-isotropic (I) transition temperature.

The Trials and Tribulations of Structure Assisted Design of KCa Channel Activators.

Calcium-activated K+ channels constitute attractive targets for the treatment of neurological and cardiovascular diseases. To explain why certain 2-aminobenzothiazole/oxazole-type KCa activators (SKAs) are KCa3.1 selective we previously generated homology models of the C-terminal calmodulin-binding domain (CaM-BD) of KCa3.1 and KCa2.3 in complex with CaM using Rosetta modeling software. We here attempted to employ this atomistic level understanding of KCa activator binding to switch selectivity around and design KCa2.2 selective activators as potential anticonvulsants. In this structure-based drug design approach we used RosettaLigand docking and carefully compared the binding poses of various SKA compounds in the KCa2.2 and KCa3.1 CaM-BD/CaM interface pocket. Based on differences between residues in the KCa2.2 and KCa.3.1 models we virtually designed 168 new SKA compounds. The compounds that were predicted to be both potent and KCa2.2 selective were synthesized, and their activity and selectivity tested by manual or automated electrophysiology. However, we failed to identify any KCa2.2 selective compounds. Based on the full-length KCa3.1 structure it was recently demonstrated that the C-terminal crystal dimer was an artefact and suggested that the “real” binding pocket for the KCa activators is located at the S4-S5 linker. We here confirmed this structural hypothesis through mutagenesis and now offer a new, corrected binding site model for the SKA-type KCa channel activators. SKA-111 (5-methylnaphtho[1,2-d]thiazol-2-amine) is binding in the interface between the CaM N-lobe and the S4-S5 linker where it makes van der Waals contacts with S181 and L185 in the S45A helix of KCa3.1.

Structure-property relationships in azobenzene-based twist-bend nematogens

ABSTRACTThe synthesis and characterisation of a new set of azobenzene-based non-symmetric liquid crystal dimers, the 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yl)hexanes (MeOB6OABX), that exhibit the twist-bend nematic phase, NTB, is described. The terminal substituents are methyl, methoxy, ethyl, butyl, butoxy, and nitrile. All six dimers exhibit both the NTB and conventional nematic, N, phases. The identification of the NTB phase was performed using polarised light microscopy and confirmed for binary mixtures with a standard twist-bend nematogen 1,7-bis-4-(4ʹ-cyanobiphenyl) heptane (CB7CB). The transitional behaviour of the MeOB6OABX dimers is compared with that of the corresponding ether-linked 1-(4-substitutedazobenzene-4ʹ-yloxy)-6-(4-methoxybiphenyl-4ʹ-yloxy)pentanes, MeOBO5OABX, all of which exhibit a conventional nematic phase. In addition, the nitrile-substituted MeOBO5OABCN shows the NTB phase. The behaviour of these non-symmetric dimers is also compared to that of the corresponding symmetric dimers. Differences in the transitional properties between these sets of new materials are accounted for in terms of not only molecular shape but also other factors including the strength of the mixed mesogen interaction.

Ground state in the novel dimer iridate Ba13Ir6O30 with Ir6+(5d3) ions

We have synthesized and studied a new iridate, Ba13Ir6O30, with unusual Ir oxidation states: 2/3 Ir6+(5d3) ions and 1/3 Ir5+(5d4) ions. Its crystal structure features dimers of face-sharing IrO6 octahedra, and IrO6 monomers, that are linked via long, zigzag Ir-O-Ba-O-Ir pathways. Nevertheless, Ba13Ir6O30 exhibits two transitions at TN1 = 4.7 K and TN2 = 1.6 K. This magnetic order is accompanied by a huge Sommerfeld coefficient 200 mJ/mole K below TN2, signaling a coexisting frustrated/disordered state persisting down to at least 0.05 K. This iridate hosts unusually large Jeff=3/2 degrees of freedom, which is enabled by strong spin-orbit interactions (SOI) in the monomers with Ir6+ ions and a joint effect of molecular orbitals and SOI in the dimers occupied by Ir5+ and Ir6+ ions. Features displayed by the magnetization and heat capacity suggest that the combination of covalency, SOI and large effective spins leads to highly frustrated ferrimagnetic ordering, possibly into a skyrmion crystal, a novelty of this new high-spin iridate.

Toward Understanding the Impact of Dimerization Interfaces in Angiotensin II Type 1 Receptor.

Angiotensin II type 1 receptor (AT1R) is a prototypical class A G protein-coupled receptor (GPCR) that has an important role in cardiovascular pathologies and blood pressure regulation as well as in the central nervous system. GPCRs may exist and function as monomers; however, they can assemble to form higher order structures, and as a result of oligomerization, their function and signaling profiles can be altered. In the case of AT1R, the classical Gαq/11 pathway is initiated with endogenous agonist angiotensin II binding. A variety of cardiovascular pathologies such as heart failure, diabetic nephropathy, atherosclerosis, and hypertension are associated with this pathway. Recent findings reveal that AT1R can form homodimers and activate the noncanonical (β-arrestin-mediated) pathway. Nevertheless, the exact dimerization interface and atomic details of AT1R homodimerization have not been still elucidated. Here, six different symmetrical dimer interfaces of AT1R are considered, and homodimers were constructed using other published GPCR crystal dimer interfaces as template structures. These AT1R homodimers were then inserted into the model membrane bilayers and subjected to all-atom molecular dynamics simulations. Our simulation results along with the principal component analysis and water pathway analysis suggest four different interfaces as the most plausible: symmetrical transmembrane (TM)1,2,8; TM5; TM4; and TM4,5 AT1R dimer interfaces that consist of one inactive and one active protomer. Moreover, we identified ILE2386.33 as a hub residue in the stabilization of the inactive state of AT1R.

Structural and functional study of FK domain of Fstl1.

Fstl1 is a TGF-β superfamily binding protein which involved in many pathological processes. The function of Fstl1 has been widely elucidated, but its structural characterization has not been explored. Here we solved the high-resolution crystal structure of FK domain of murine Fstl1, analyzed its unique characteristics, and investigated its contribution to the function of full-length Fstl1. We found that Fstl1-FK forms a stable dimer in both solution and crystal, which suggest that this protein may function as a dimer during its interaction with TGF-β, a molecule known to form dimer during activation process. We also found this FK domain is indispensable for the proper function of Fstl1 during the transduction of TGF-β signaling. These observations provide important insights into the understanding of Fstl1 and may facilitate the exploration of this molecule in clinical study.

4-(2-Methoxyphenyl)-5-(thiophen-2-ylmethyl)-2,4-Dihydro-1H-1,2,4-Triazole-3-Thione and Its Complex with Cadmium Chloride: Molecular and Crystal Structures

Compound 4-(2-methoxyphenyl)-5-(thiophen-2-ylmethyl)-2,4-dihydro-1H-1,2,4-triazole-3-thione (LН) and its complex with cadmium(II) C42.12H39.50N9O3.12S6Cl2Cd (I) are synthesized. Their molecular and crystal structures are determined by X-ray diffraction analysis (СIF files CCDC nos. 1835776 (LН) and 1547251 (I)). Free ligand LН is thione and in the crystal forms H-bonded centrosymmetric dimers by two hydrogen bonds N–H···S. Complex I has a distorted trigonal bipyramidal structure: the sulfur atoms lie in the base and the chlorine atoms occupy the axial positions. Compound LН manifests itself as a monodentate ligand (S) when forming complex I and retains the thione form. The molecule of complex I is stabilized due to strong intramolecular contacts N–H···Cl. The crystal packing of compounds LН and I is formed due to the intermolecular contacts C–H···π, H···S, H···O, and H···N.

Sulfur-linked cyanobiphenyl-based liquid crystal dimers and the twist-bend nematic phase

ABSTRACTThe synthesis and characterisation of two series of cyanobiphenyl-based liquid crystal dimers containing sulfur links between the spacer and mesogenic units, the 4ʹ-[1,ω-alkanediylbis(thio)]bis-[1,1ʹ-biphenyl]-4-carbonitriles (CBSnSCB), and 4ʹ-({ω-[(4ʹ-cyano[1,1ʹ-biphenyl]-4-yl)oxy]alkyl}thio)[1,1ʹ-biphenyl]-4-carbonitriles (CBSnOCB) are described. The odd members of both series show twist-bend nematic and nematic phases, whereas the even members exhibit only the nematic phase. An analogous cyanoterphenyl-based dimer, 34-{6-[(4ʹ-cyano[1,1ʹ-biphenyl]-4-yl)thio]-hexyl}[11,21:24,31-terphenyl]-14-carbonitrile (CT6SCB), is also reported and shows enantiotropic NTB and N phases. The transitional properties of these dimers are discussed in terms of molecular curvature, flexibility and biaxiality. The same molecular factors also influence the birefringence of nematic phases. Resonant X-ray scattering studies of the twist-bend nematic phase at both the carbon and sulfur absorption edges were performed, which allowed for the determination of critical behaviour of the helical pitch at the transition to the nematic phase, the behaviour was found to be independent of molecular structure. It was also observed that despite the different molecular bending angle and flexibility, in all compounds the helical pitch length far from the N-NTB transition corresponds to 4 longitudinal molecular distances.

Liquid crystal dimers containing Cholesteryl and Triazole-containing mesogenic units

ABSTRACT New liquid crystals categorised as cholesteryl dimers have been successfully synthesised through the reaction between cholesteryl 4-(prop-2-ynyloxy)benzoate moieties with n-azido(cholesteryloxy-carbonyl)alkane. All the dimers display enantiotropic mesophases. Whilst the odd-numbered dimers exhibit chiral nematic (N*), twisted grain boundary (TGB) and chiral smectic C (SmC*) phases, the even-numbered members from the same series show chiral smectic A and C. A detailed inspection on mesophase reveals that the chiral centres and the bent conformation of the odd-numbered members are essential for the induction of TGB phase. However, upon decreasing the temperature, the ratio of the transition temperatures (TSmC*-SmA*/TSmA*-I) is found to be 0.95, which indicate the second order transition according to the McMillan’s molecular theory. In addition, the X-ray diffraction study supports the presence of the smectic A phase on the even members rather than the N* by the appearance of the Bragg diffraction peaks at 190°C. A comparison study with the other analogues in which the cholesterol entity is substituted by azobenzene or biphenyl tails has been carried out to assess the relationship between the molecular structure and mesomorphic behaviour.