2,3,4-Trihydroxy benzonitrile-based liquid crystals: Fiber forming room temperature nematic phases

Shanker G., Bindushree A., Chaithra K., Pratap P., Gupta R.K., Achalkumar A.S., Yelamaggad C.V.

Technologically significant chiral nematic (N*) phase is a helicoidal superstructure, that is generally formed by the spontaneous self-assembly of entire chiral mesogens (mostly calamitics) or by the LC mixtures where a chiral dopant is being added to the host nematic (N) phase. However, the stabilization of the N* phase at room temperature solely by chiral mesogens is highly challenging where the molecular designing is rather crucial, and thus scarcely reported. Working in this direction, we have rationally designed, synthesized and characterized new chiral materials namely, non-symmetric dimers derived from cholesterol. Notably, as expected, some of these dimers exhibit room temperature N* phase over a wide thermal range. The exclusive occurrence of N* phase has been evidenced unequivocally with the help of polarized optical microscopy (POM) and differential scanning calorimeter (DSC). The chiroptical property of the phase has been studied by recording circular dichroism (CD) spectra as a function of temperature where the intense bisignate bands (change of sign within the band) were observed. Notably, one of the dimers forms stable gel in an aprotic solvent such as hexadecane. Thus, our study reveals a new molecular architecture to stabilize the technologically important N* phase at ambient temperature.

Wang K., Jirka M., Rai P., Twieg R.J., Szilvási T., Yu H., Abbott N.L., Mavrikakis M.

Two series of new hydroxy tail-terminated cyanobiphenyl compounds are described. The 4′-ω-hydroxyalkynyl-4-cyanobiphenyl compounds (1a–1g) were synthesised as the key intermediates to the 4′-?ω-hyd...

Alaasar M., Tschierske C.

A new type of supramolecular liquid crystalline complexes formed by intermolecular hydrogen bond formation between rod-like 4-(4ʹ-pyridylazophenyl)-4ʺ-alkoxybenzoates as proton acceptors and 4-[5-(...

Walker R., Pociecha D., Abberley J.P., Martinez-Felipe A., Paterson D.A., Forsyth E., Lawrence G.B., Henderson P.A., Storey J.M., Gorecka E., Imrie C.T.

The spontaneous formation of a chiral structure in a mixture consisting of achiral components is attributed to the formation of a hydrogen-bonded complex between the unlike components.

Alaasar M., Poppe S., Tschierske C.

ABSTRACT New five-ring hockey-stick liquid crystalline materials with 4-bromoresorcinol as the central core unit and an azobenzene-based side arm were synthesised and their mesophase behaviour was investigated by polarising optical microscopy, differential scanning calorimetry, X-ray diffraction and under a triangular wave electric field. Additional structural modification was done by introducing a lateral fluorine atom in the terminal ring of one of the side arms. It is found that regardless of the alkyl chain length or the lateral fluorine substitution, all of the prepared materials are liquid crystalline exhibiting nematic phases composed of cybotactic clusters of the SmC-type (NCybC) in addition to a monotropic SmC phase for the longest homologue. GRAPHICAL ABSTRACT

Paterson D.A., Xiang J., Singh G., Walker R., Agra-Kooijman D.M., Martı́nez-Felipe A., Gao M., Storey J.M., Kumar S., Lavrentovich O.D., Imrie C.T.

The liquid crystal nonsymmetric dimer, 1-(4-butoxyazobenzene-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl) hexane (CB6OABOBu), shows enantiotropic twist-bend nematic, NTB, and nematic, N, phases. The NTB phase has been confirmed using polarized light microscopy, freeze fracture transmission electron microscopy, and X-ray diffraction. The helicoidal pitch in the NTB phase is 18 nm. The NTB-N (TNTBN) and N-I (TNI) transition temperatures are reduced upon UV light irradiation, with the reduction in TNTBN being much larger than that in TNI. An isothermal, reversible NTB-N transition may be driven photochemically. These observations are attributed to a trans-cis photoisomerization of the azobenzene fragment on UV irradiation, with the cis isomers stabilizing the standard nematic phase and the trans isomers stabilizing the NTB phase. The dramatic changes in TNTBN provide evidence that the transition between the normal nematic and twist-bend nematic with spontaneous breaking of chiral symmetry is crucially dependent on the shape of molecular dimers, which changes greatly during the trans-cis isomerization.

Mandle R.J., Voll C.C., Lewis D.J., Goodby J.W.

ABSTRACTDespite the relationship between molecular structure and the occurrence of the twist-bend nematic phase being partially understood, very little is known about how these relationships are manifested for ether-linked bimesogens. In this article, we report several novel ether-linked bimesogens that exhibit the NTB phase and explore how the thermal properties of these materials are largely governed by the angle between the two aromatic, carbocyclic or rigid cyclic units.

Wang Y., Singh G., Agra-Kooijman D.M., Gao M., Bisoyi H.K., Xue C., Fisch M.R., Kumar S., Li Q.

The nanostructured heliconical twist-bend nematic (Ntb) phase is a new condensed phase of matter with unique properties. Here we present the first example of an achiral hybrid bent-core liquid crystal trimer that at temperatures below the conventional N phase exhibits the Ntb phase from approximately 80 °C down through room temperature. The Ntb phase has a helical structure with a period of ~19 nm.

Takabatake S., Shikata T.

The formation of anti-parallel dimers of nCBs was evidently confirmed by dielectric and fluorescence experiments even in isotropic cyclohexane solution.

Jansze S.M., Martínez-Felipe A., Storey J.M., Marcelis A.T., Imrie C.T.

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.

Francescangeli O., Vita F., Samulski E.T.

The molecular clustering observed in the fluid nematic phase of nonlinear liquid crystal molecules underlies exaggerated field effects that portend unique technological advances in next-generation liquid crystal displays. However, the detailed nature of the molecular organization within the clusters and the temporal and spatial persistence of the organization remain unclear. Herein we review the evolution of structural studies of this unique nematic phase. The mounting experimental evidence points to a converging picture of the microscopic nature of this relatively new class of liquid crystals.

Shanker G., Prehm M., Nagaraj M., Vij J.K., Weyland M., Eremin A., Tschierske C.

AbstractSeveral series of bent‐core mesogens derived from 3,5‐diphenyl‐1,2,4‐oxadiazole with or without lateral groups and with different length terminal chains at both ends, and polycatenar molecules with three to six alkoxy chains are synthesized and their mesomorphic behaviour is investigated by polarizing microscopy, differential scanning calorimetry, X‐ray diffraction (XRD), dielectric, electro‐optical and second‐harmonic generation (SHG) experiments. Most compounds exhibit broad regions of skewed cybotactic nematic (NcybC) and tilted smectic (SmC) phases with a strong tilt of the aromatic cores (up to 63°), but non‐tilted SmA and NcybA phases are also observed for a compound that has only one terminal chain. The XRD patterns of the nematic phases of most of the compounds investigated indicate a 2D periodicity with short correlation length in the magnetically aligned samples. This is of importance for the general interpretation of the small‐angle XRD splitting patterns typically observed for aligned samples of bent‐core nematic phases. In most nematic phases one current peak is observed in the half period of an applied electric field, though no coherent signal is found in the SHG experiments. Based on additional electro‐optical and dielectric results, the nematic phases are considered to be cybotactic nematic phases with local polar order, and show a dielectric reorientation of the polar domains. Only chiral nematic phases (NcybC*), but not blue phases, are obtained for compounds with one or two chiral (3S)‐3,7‐dimethyloctyloxy tail(s).

Mandle R.J., Davis E.J., Archbold C.T., Cowling S.J., Goodby J.W.

A detailed microscopy study of the mesophases of 1,11-di-(1′′-cyanobiphenyl-4-yl)undecane (CB11CB) was undertaken, with an emphasis on attempting to relate the recent helical “twist-bend” model of the NTB phase to the observed optical textures. Our ability to draw a freestanding film indicates that the phase is unlikely to be nematic and possesses long range periodicity. No electrooptic response could be detected in the NTB phase, with or without addition of a chiral dopant although dielectric breakdown and space charge were observed at high voltage leading to field dependent pulsing of the sample. No optical rotation associated with a macroscopic helical structure could be discerned in the NTB phase. However, this observation could be taken to be in keeping with the extremely short pitch of a proposed “twist-bend” model.

Bhattacharyya S.S., Galerne Y.

AbstractAfter a short review on the physics of pulled threads and their mechanical properties, the paper reports and discusses the strand elongation of disordered columnar phases, hexagonal or lamella‐columnar, of small molecules or polymers. The mechanical properties appear to be relevant to the length of the columns of molecules compared to the thread length, instead of the usual correlation length. If, taking the entanglement effect into account, the column length is short, the strand exhibits rather fluid‐like properties that may even look nematic‐like at the macroscopic scale. The Plateau–Rayleigh instability breaks the thread shortly thereafter. However, because the hydrodynamic objects are the columns instead of the molecules, the viscosity is anomalously large. The observations show that the strands in the columnar phases are made of filaments, or fibrils, which are bundles of columns of molecules. This explains the grooves and rings, which are observed on the antenna or bamboo‐like strand profiles. On pulling a strand, the elongation stress eventually exceeds the plasticity threshold, thus breaking the columns and the filaments. As a result, cracks, more exactly, giant dislocations are formed. These change the strand thickness by steps of different birefringence colors. Interestingly, the addition of a solute may drastically change the effective viscosity of the columnar phase and its mechanical properties. Some solutes, such as alkanes, exhibit lubricant and detangling properties, whereas others such as triphenylene, are antilubricant.

Malkondu S., Kocak A.

Novel liquid crystal (LC) trimers with a flexible tris(2-aminoethyl)amine (TREN) core and cyanobiphenyl arms have been designed. These non-traditional mesogens form nematic mesophase over a broad temperature range depending on the alkyl chain length. Their chemical structures were confirmed by the combination of 1 H NMR, 13 C NMR, APT, COSY and FT-IR spectra. The mesophases were characterized by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). Thermogravimetric analysis showed that LC trimers are highly thermal stable both in the range of the mesophase and above the clearing point.

Alkali M., Iliş M., Micutz M., Pasuk I., Gabriela Popescu R., Cătălin Marinescu G., Cîrcu V.

D S.K., Shetty A., B. S R., M V., G S., Alaasar M., Hegde G.

In this paper, we discuss the synthesis and characterization of 2,3,4-tris[

Zhang M., Liao R., Zhang A., Fang Y., Song X., Xie M., Zhang C., Feng Y., Yu H.

Ranjitha B.S., Alassar M., Shanker G.

We report on the design and synthesis of new fluorescent liquid crystal dimers with high chirality. The molecular design involves attaching two cholesteryl moieties to a resorcinol molecule, resulting in a structure with reduced symmetry and a bent angle of 120 degrees, accompanied by a lateral cyano group. The influence of reduced symmetry, chirality, and bent angle on the mesophase of 4-cyano-1,3-phenylene bis(n-cholesteryloxy)alkanoates dimers are assessed. The synthesized dimers are characterized using standar spectroscopic techniques. The mesomorphic properties of these chiral dimers are evaluated through polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction measurements. Remarkably, all the dimers exhibit either the N*/ SmA phase, or both. Self-assembly into H-aggregation with fluorescent nature using UV-visible and fluorescence spectrophotometry studies. On further development, chiral fluorescence could be utilized for real-time visualization and tracking of cholesterol distribution within cell membranes, as well as in lipid-protein interactions.

Ranjitha B.S., Kumar Behera P., Achalkumar A.S., Hegde G., Singh S., Shanker G.

We synthesized novel thiacyanine chromonic liquid crystals (CLCs) and structurally characterized using NMR and mass spectrometry. The impact of distinct substitution at the para position of aromatic counter anions, aliphatic counter ion chain length, and varied spacer parity of thiacyanine dyes on CLC formation is investigated. Liquid crystal properties of the synthesized dyes are characterized by polarizing optical microscopy (POM) and X-ray diffraction (XRD) studies. Dyes exhibit nematic (N), lamellar (Lα), columnar rectangular (Colr), and columnar oblique (Colob) CLCs at different concentrations in the water. Electronic absorption spectra reveal Scheibe aggregation in all the dyes. Cylicvoltametry studies confirm redox behaviour in TC-1a and TC-5e dyes. Chromonic LCs hybrid nano-materials are synthesized using sol–gel method. Scanning electron microscopy employed to confirm nano tubular fiber structure of the hybrid nanomaterilals. .

Ashirbaev S.S., Brás N.F., Frei P., Liu K., Moser S., Zipse H.

AbstractFlavonoids are known to covalently modify amyloidogenic peptides by amination reactions. The underlying coupling process between polyphenols and N‐nucleophiles is assessed by several in vitro and in silico approaches. The coupling reaction involves a sequence of oxidative dearomatization, amination, and reductive amination (ODARA) reaction steps. The C6‐regioselectivity of the product is confirmed by crystallographic analysis. Under aqueous conditions, the reaction of baicalein with lysine derivatives yields C−N coupling as well as hydrolysis products of transient imine intermediates. The observed C−N coupling reactions work best for flavonoids combining a pyrogallol substructure with an electron‐withdrawing group attached to the C4a‐position. Thermodynamic properties such as bond dissociation energies also highlight the key role of pyrogallol units for the antioxidant ability. Combining the computed electronic properties and in vitro antioxidant assays suggests that the studied pyrogallol‐containing flavonoids act by various radical‐scavenging mechanisms working in synergy. Multivariate analysis indicates that a small number of descriptors for transient intermediates of the ODARA process generates a model with excellent performance (r=0.93) for the prediction of cross‐coupling yields. The same model has been employed to predict novel antioxidant flavonoid‐based molecules as potential covalent inhibitors, opening a new avenue to the design of therapeutically relevant anti‐amyloid compounds.

Ranjitha B.S., Alassar M., Shanker G.

Rabari M., Solanki S., Prajapati A.K.

One new homologous series of liquid crystalline λ-shaped tris azo trimers containing azobenzene moiety as a rigid core and ether linkages were synthesized by the reaction of various n-butylazobenzene having bromoalkoxy tail with magneson-I dye respectively. Their right structure was confirmed by FT-IR, 1H NMR, 13C NMR spectroscopy, mass spectrometry and elemental analysis. On the polarising optical microscope (POM), their thermotropic properties were examined, and additional studies were conducted using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). All the trimers with an even number of flexible spacers (n = 2 and 4) show excellent liquid crystalline properties as an enantiotropic nematic phase with greater mesophase length and good thermal stability. The introduction of odd-membered methylene disrupts the orderly stacking of layers and results in a change in mesogenic behaviour. Their structure-property relationships, the influence of different flexible spacers, the mesophase ranges and the photo-isomerisation of liquid crystalline trimers were described. A view of their photochromic behaviour was gained by UV–Visible spectroscopy. The 6-31G (d, p) basis set is combined with the B3LYP functional, which is used for computational studies of λ-shaped tris azo trimers. This approach allows for the determination of various quantum mechanical descriptors that can help us comprehend the mesogenic nature of molecules in computational studies. The results of the Gaussian calculation showed that the target compounds have low energy differences between HOMO-LUMO levels and good chemical reactivity. The examination of ESP and optical characteristics also yielded important information about the electronic structure and potential uses of these compounds.

Hobbs J., Reynolds M., Krishnappa Srinatha M., Shanker G., Mattsson J., Nagaraj M.

We present a detailed investigation of the phase behaviour, molecular relaxation dynamics, rheology and dielectric properties of two cyanobiphenyl-based liquid crystal (LC) tripods, differing only in the length of their spacer units (6 or 9 carbons). These LC tripods combine properties of low molecular weight LCs and LC polymers, resulting in a range of advantageous properties including a wide nematic range (ΔT>90 K) and a large birefringence (0.3 at T−TNI=-80 K for the 6 spacer tripod). Using broadband dielectric relaxation spectroscopy, calorimetry, oscillatory and steady state shear rheology, we identified four molecular relaxation processes: the structural (α) relaxation (defining the glass transition temperature, Tg); the δ relaxation (reorientation of the mesogen unit around its short axis); the β relaxation (reorientation around its long axis); and the γ relaxation (internal tripod arm fluctuations). The β and γ relaxations follow Arrhenius temperature dependencies in the glass, whereas the α and δ relaxations merge above, but near Tg, where they follow a non-Arrhenius VFT behaviour. For higher temperatures, the two relaxations separate and for T>T⁎, where T⁎ marks a dynamic crossover, the α relaxation transitions from VFT to Arrhenius behaviour. For the two tripods, ratios of T⁎/Tg=1.14 and T⁎/Tg=1.13 were observed respectively, consistent with the ratio observed for many side-chain LC polymers (and other LC systems), and consistent with the ratio where a dynamic crossover is typically observed also for non-LC glass-formers. However, for non-LC systems, the transition to Arrhenius behaviour happens at significantly higher temperatures relative to Tg and the dynamic crossover at T⁎ is typically observed as a transition between two different VFT behaviours. We argue that for the tripods these differences arise from differences in the molecular relaxation mechanisms induced by the LC order. Finally, for temperatures where δ and α relaxations are separated, we find that ion conductivity decouples from the α relaxation, instead following the δ relaxation; this demonstrates that the ion transport properties for the tripods can be tuned by the design of the tripod mesogen arm.

Ranjitha B.S., Sandhya Kumari D., Shetty A., Shanker G., Alaasar M., Pashameah R., Hegde G.

Three new series of photo responsive dimers bearing different terminal functional groups (-CN, -COOEt, -OMe) and variable aliphatic spacers have been synthesized and investigated in detail. The molecular structures of the new materials were proved using different spectroscopic techniques and their liquid crystal self-assembly was characterized using differential scanning calorimetry (DSC), polarized light microscope (POM) and X-ray diffraction (XRD). Moreover, their photo switching behaviour was investigated in details in solution. The results revealed that, almost all members of the dimeric materials are mesomorphic exhibit either nematic or both smectic A and nematic phases. Under UV illumination the materials show interesting photo-switching properties, reaching a photo-stationary state in 40 s and a thermal back-relaxation time of approximately 35 h in solution. Finally, a fabricated device authenticates the potential of the reported materials for optical storage devices.

Bashir N., Hameed S., Muhammad K., Vieira A.A., Bechtold I.H., Hameed S.

Shanker G., Srinatha M.K., Ocak H.

Shanker G., Kumar K.R., Paul B.

Shanker G., Srinatha M.K., Sandhya Kumari D., Ranjitha B.S., Alaasar M.

• Disk-shaped liquid crystalline material synthesized using multi-component reactions. • A novel green synthetic approach saves time, energy, and column chromatography purification. • Frozen nematic glassy state. We report the synthesis of disk-shape liquid crystalline materials using multi-component reactions (MCR). Two new series, 3,9-di(3′,4′,5′-tris-alkyloxy)phenyl-2,3,4,8,9,10-hexahydronaphtho[1,2-e:5,6-e']bis([1,3]oxazine) and 3,10-di((3′,4′,5′-tris-alkyloxy)phenyl)-2,3,4,9,10,11-hexahydronaphtho[1,2-e:4,3-e']bis([1,3]oxazine) synthesized by coupling either 2,3-dihydroxy naphthalene or 2,6-dihydroxy naphthalene with 3,4,5-tris- O -alkylated anilines in a solvent free condition. The molecular structure of the new materials was confirmed using 1 H NMR, 13 C NMR and CHN elemental analysis. The phase behavior investigated using polarizing optical microscope (POM) and differential scanning calorimetry (DSC) experiments. All the materials exhibit room temperature discotic nematic (N D ) during the cooling cycles as confirmed by X-ray diffraction studies. This synthetic approach provides multiple advantages such as high atom economy (AE), short reaction time, gentle reaction condition, simple workup, and do not require column chromatography purification, additionally this could pave a new way in the synthesis of liquid crystalline compounds.

Reppe T., Dressel C., Poppe S., Eremin A., Tschierske C.

AbstractControlling the spatial organization of π‐conjugated molecules in soft systems is of importance for the design of new materials with application relevant optimized properties. Here, soft self‐assembly of functional multi‐chained π‐conjugated 5,5′‐diphenyl‐2,2′‐bithiophenes is investigated. Focus is on molecules with a 3,5‐disubstitution pattern at one end and no or only a single chain at the other end. By elongation of the single terminal chain, a transition from lamellar phases to helical networks is observed. With lowering the temperature, the lamellar phase types change from smectic via hexatic to crystalline. In some cases, different kinds of orders are combined in a single uniform lamellar structure. Moreover, a transition from uniform (synclinic) to alternating (anticlinic) tilt in adjacent layers is observed with decreasing chain length or rising temperature, due to a change of the molecular conformation from tuning fork like to Y‐shaped. For compounds terminated with a long chain, helical self‐assembly of the π‐conjugated rods in networks takes place, leading either to a racemic (, gyroid) or a spontaneous mirror symmetry broken (I23) bicontinuous cubic phase and a chiral isotropic liquid phase (Iso1[*]) as well. This work establishes rules for controlling the self‐assembly of functional π‐conjugated rods in soft matter and fluids.