Synthesis and Mesomorphic Properties of Novel Bent-shaped Naphthyl Diketones

The synthesis and liquid-crystalline properties are reported for novel naphthyl-based diketones incorporating variant terminal chains and lateral fluoro- substitution. Newly prepared materials exhibit a broad temperature range of the nematic phase. The study demonstrates how subtle structural modifications can be exploited to alter the efficiency of molecular packing and consequently the thermal behaviour. This work is licensed under a Creative Commons Attribution 4.0 International License

Induced smectic phase in binary mixture of twist-bend nematogens

The investigation of liquid crystal (LC) mixtures is of great interest in tailoring material properties for specific applications. The recent discovery of the twist-bend nematic phase (NTB) has sparked great interest in the scientific community, not only from a fundamental viewpoint, but also due to its potential for innovative applications. Here we report on the unexpected phase behaviour of a binary mixture of twist-bend nematogens. A binary phase diagram for mixtures of imino- linked cyanobiphenyl (CBI) dimer and imino- linked benzoyloxy-benzylidene (BB) dimer shows two distinct domains. While mixtures containing less than 35 mol % of BB possess a wide temperature range twist-bend nematic phase, the mixtures containing 55–80 mol % of BB exhibit a smectic phase despite that both pure compounds display a Iso–N–NTB–Cr phase sequence. The phase diagram shows that the addition of BB of up to 30 mol % significantly extends the temperature range of the NTB phase, maintaining the temperature range of the nematic phase. The periodicity, obtained by atomic force microscopy (AFM) imaging, is in the range of 6– 7 nm. The induction of the smectic phase in the mixtures containing 55–80 mol % of BB was confirmed using polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction. The origin of the intercalated smectic phase was unravelled by combined spectroscopic and computational methods and can be traced to conformational disorder of the terminal chains. These results show the importance of understanding the phase behaviour of binary mixtures, not only in targeting a wide temperature range but also in controlling the self-organizing processes

Temperature Dependence of the Electroclinic Effect in the Twist-Bend Nematic Phase

Funding Information: This research was funded by the Croatian Science Foundation (Grant No. IP-2019-04-7978); by the Agence Nationale pour la Recherche ANR (France) through Grant BESTNEMATICS, No. ANR-15-CE24-0012; by the French-Croatian bilateral program COGITO; by the Université de Picardie Jules Verne, Amiens, France. Publisher Copyright: © 2023 by the authors.Peer reviewedPublisher PD

Fréedericksz-Like Transition in a Biaxial Smectic- A Phase

The two main classes of liquid-crystal (LC) phases of rodlike molecules are nematics, where the rods align in the same direction (the nematic director n), and smectics, where the rods not only are aligned but also form layers. The electro-optic effects in LC devices that are a backbone in today’s display industry mainly use the Fréedericksz transition, which is the bulk reorientation of a surface-anchored nematic by an electric field. Conventional (uniaxial) smectics do not present a Fréedericksz transition, because, due to their layered structure, the director reorientation would distort the layers, which would cost too much energy. In a worldwide ongoing effort to extend the variety of LC compounds suitable for applications in the display industry, bent- shaped molecules have recently raised much attention, since they present multiple new LC phases with unusual properties. In this paper, we report on a structural and electro-optic study of the LC phases of a bent-shaped dimer. On cooling from the isotropic liquid, this compound shows a usual nematic (N), a twist- bend nematic (NTB), and a biaxial smectic-A phase (Sm Ab). Quite surprisingly, contrary to usual smectics, Sm Ab presents a remarkable electro-optic response, with low ( < 4 V) voltage threshold, no reorganization of the smectic layers, and low ( < 1 ms) response time (i.e., 30 times faster than the N phase at higher temperature). We interpret this unexpected electro-optic effect as a Fréedericksz transition affecting the secondary director m of the Sm Ab, and we model it by analogy with the usual Fréedericksz transition of the n director of the uniaxial N phase. Indeed, a Fréedericksz transition affecting only m in this biaxial fluid smectic does not alter its layered structure and costs little energy. From the point of view of applications, thanks to its low relaxation time, this “biaxial” Fréedericksz transition could be exploited in electro-optic devices that require fast switching

Nematic Twist Bend Textures under different anchoringconditions

International audienceNematic Liquid Crystals (CL) dominate the market for high-resolution display devices.Unfortunately, their relatively slow response times (> 1 ms) greatly limit their use in thedevelopment of more efficient screens (3D screens) and components (switches, couplers,filters). In this context, newly discovered Twist-Bend Nematic (TBN) similar to the usualnematic phases, but spontaneously deformed at the level of a few nanometers, due to thecurved shape of the molecules, are very promising [1]. In fact, the more rigid structure ofTBN makes it possible to achieve ultra-short electrical field response times (In this paper we present an experimental study of a newly synthesized TBN the: N,N0 -Bis(4,40-cyanobiphenymethylidene)-nonane-1,9-diamine (CBI-9-ICB), under well controlledanchoring and confinement conditions.In the experiment, the compound is inserted by capillarity in a cell made by two parallel glassplates with few microns gap. Different anchoring conditions have been examined. We showthat rubbed polyvinyl alcohol (PVA) and Dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (DMOAP) treated substrates give different anchoring and texture fromthat given by usual nematic as 4-Cyano-4'-pentylbiphenyl (5CB). In Figure 1 is represented atypical TBN texture observed under Polarizing Optical Microscope in a cell withhomogeneous planar anchoring obtained with rubbed PVA. Stripes parallel to the anchoringorientation having a wavelength proportional to the cell thickness are measured. The sametexture is also found in rubbed Polyimide substrates. Currently, two models are proposed toexplain this texture. The first is related to the Helfrich-Hurault instability due to the presenceof TBN pseudo-layers [2]. The second is based on the peculiar elasticity of the TBN directorfield [3]. Although it is not clear how these models could reproduce the observed lineardependence of pattern periodicity as a function of the cell gap. In Figure 2 is reported theTBN texture for silane treated glass plates that gives, for ordinary nematic, homeotropic (i.e.perpendicular to the plate) anchoring. The resulting anchoring for TBN is surprisingly planardegenerated as one can infer from the presence of half-strength disclinations. A mosaictexture with homogeneously oriented domains reminiscent of SmC mosaic also appears.Such textures will be discussed in the framework of the elastic theory of TBN.References:[1] C. Meyer, G. R. Luckhurst and I. Dozov, Phys. Rev. Lett., 2013,111 (6), 0031-9007.[2 P. K. Challa,V. Borshch,O. Parri, Phys. Rev. Lett., 2014, 89 (6), 1539-3755.[3] V. P. Panov, M. Nagaraj, J. K. Vij, Phys. Rev. Lett., 2010, 105 (16), 0031-9007.<BR

Effects of Geometry and Electronic Structure on the Molecular Self-Assembly of Naphthyl-Based Dimers

Three new series of symmetric dimers containing a naphthoyloxybenzyl (NB), benzoyloxynaphthyl (BN), and naphthoyloxysalicyl (NS) mesogenic core linked to an alkylene spacer via an imino group were synthesized. The effects of the variant spacer parity as well as the variant core structure on the mesomorphic properties have been studied. The dimers having NB and BN mesogenic units display intercalated smectic structures regardless of the spacer parity. In contrast, bilayer smectic and Col_rec structures are observed for the NS core compounds with even and odd spacers, respectively. The influence of geometric and electronic factors on the mesomorphic behavior, in particular on the molecular packing within the smectic phase, is discussed based on conformational and dipolar considerations following DFT calculations using model molecules. The difference in self-organization of symmetric naphthyl-based dimers appears to be governed by the competition between geometric factors and dipole–dipole interactions between identical mesogenic units

Sensitivity of the N_TB phase formation to the molecular structure of imino-linked dimers

<p>Here we report on the synthesis and mesomorphic properties of a series of imino-linked dimeric molecules. In order to improve our understanding of the structure–N_TB phase correlations, we have studied the impact of geometric and electronic factors arising from varying mesogenic units, different spacer lengths and from the ratio (<i>n/m</i>) between the lengths of terminal chains (<i>n</i>) and spacer (<i>m</i>). From the perspective of the molecular geometry, the results show that the stability of the N_TB phase results from increasing effective molecular bending and with the broadening of the mesogenic unit, in particular near the spacer, and that the <i>n</i>/<i>m</i> ratio plays a substantial role in conjunction with the specific mesogenic unit. A computational study of the electronic properties shows that a broadening of the mesogenic core in the vicinity of the spacer is associated with an increased anisotropy of the electrostatic potential distribution. Within a given series of materials our study suggests that the incidence of the N_TB phase and its thermal stability are governed by the synergy of specific geometrical factors and the anisotropy of the electrostatic potential distribution of the mesogenic core.</p

Theoretical model for the Frank elastic moduli in the intercalated SmA_b phase of bent-shaped dimers

In our previous works we have shown that the elastic properties of the intercalated SmAb phase formed by bent-shaped dimers are governed by the nematic-like behaviour of the secondary director m that is associated with the projection of the molecular axes of the monomers on the plane of the smectic layer. From the experiment, the corresponding three Frank-like moduli K11m, K22m and K33m related to the secondary director demonstrate the usual behaviour of the Frank moduli of the nematics formed by rod-like molecules: monotonously increase with decreasing temperature. This is contrary to the temperature dependence of the elastic moduli for the primary director of N and NTB phases formed by bent-shaped dimers (for which the bend elastic constant decreases with temperature to zero). However, the values of the Frank-like moduli for SmAb were found to be smaller than their nematic-phase equivalents, and demonstrate a strong and unusual anisotropy, with K11m : K22m : K33m ratio being approximately 30 : 1 : 10. Here we present a theoretical model based on the assumption of the nematic-like order within the smectic layers that provides a qualitative explanation of the experimental results.</p

Temperature dependence of the electroclinic effect in the twist-bend nematic phase

The twist-bend nematic (NTB) phase of bent-shaped molecules has recently attracted much attention due to the spontaneous bend of its director field and the doubly-degenerate chirality of its heliconical structure. Despite intensive experimental and theoretical investigation worldwide, the main structural characteristics (pitch and conical angle) and elastic properties of the phase are still barely understood. This is mainly due to the difficulty in growing large single domains of the NTB phase, which prevents the application of the powerful electro-optical techniques developed for the nematic (N) phase. Moreover, the twist and bend distortions of the optic axis are forbidden by the pseudo-layered structure of the NTB phase, which makes its response to the field smectic-like instead of nematic-like. Therefore, the only macroscopic electric effect that can be observed deep in the NTB phase is the smectic-like “electroclinic” effect (ECENTB). Here, we achieve large monochiral NTB domains which remain uniform over a wide temperature range (20–60 °C) in thin (1.5 µm) planar cells, thus avoiding the so-called stripe- and rope-like textural instabilities. This allowed us to experimentally determine, using electro-optical measurements, the temperature dependence of the ECENTB response in four different NTB materials: namely the dimers CB7CB, CB9CB, CB6OCB, and BNA76. For all compounds, the thermal dependences of conical angle and pitch in the vicinity of the N-NTB transition follow the theoretically predicted power law behaviour. However, the agreement between the measured and predicted power law exponents remains only qualitative, which calls for improvement of the theoretical models.</p