M. Blanca Ros

Bent-core liquid crystals in the route to functional materials

Several results on bent-core liquid crystals considered as new supramolecular functional materials with potential for applications are reviewed. Various topics are discussed where these materials present unique attributes with clear advantages over conventional mesogens. In particular the electrooptic responses of some mesophases are examined from the viewpoint of their possible use in electrooptic devices. Other issues are also treated such as nonlinear optical properties, flexoelectricity or the design of biaxial nematics from bent-core molecules. Future challenges and prospects for new possibilities with these materials are briefly outlined.

Banana-shaped liquid crystals: a new field to explore

The recent literature in the field of liquid crystals shows that “banana-shaped” mesogenic materials represent a bewitching and stimulating field of research that is interesting both academically and in terms of applications. Numerous topics are open to investigation in this area because of the rich phenomenology and new possibilities that these materials offer. The principal concepts in this area are reviewed along with recent results. In addition, new directions to stimulate further research activities are highlighted.

Breaking the Mold of Discotic Liquid Crystals

Small molecules of the indene and pseudoazulene type like 1 and 2 form columnar mesophases! These compounds do not possess peripheral flexible chains like conventional columnar liquid crystals. Instead, polarizable chlorine and sulfur atoms, as well as the polar cyano group, function as unusual soft regions between the rigid columns.

An Approach to the Design of Ferroelectric Liquid Crystals with Large Second Order Electronic Nonlinear Optical Susceptibility

Abstract Ferroelectric liquid crystal thin films in the Clark-Lagerwall surface-stabilized geometry exhibit well known spontaneous polar orientation of functional groups. The symmetry of the system thus allows the existence of bulk electronic second order nonlinear hyperpolarizability x(2) within thc context of the simple dipolar model. For all FLC materials examined to date, however, the magnitude of x(2) is small, presumably since the particular functional group arrays oriented along the polar axis possess small molecular hyperpolarizdbility β. Using the Boulder Model for the molecular origins of the polar order occurring in FLC films, it is possible to design materials with functionalized aromatic rings oriented along the polar axis of the film. Since such functional arrays may show respectable values of β, it should be possible to obtain FIX films (both low molecular weight and polymeric) with useful magnitude of x(2) using this approach. Results of initial experiments aimed at design of FLC materials...

Bent-core liquid crystals in a route to efficient organic nonlinear optical materials

The design, synthesis and nonlinear optical (NLO) response of some bent compounds, at both the molecular and macroscopic level, are reported. Based on a bent-core NLO-phore with a μβ0 in the range of 390 × 10−48 esu (1907 nm), the second order susceptibility tensor for second harmonic generation is evaluated at excitation wavelengths of 1064 and 1600 nm in a mesogenic material with a SmCP mesophase. The NLO coefficients are in the range 10–100 pm V−1. Thus it is deduced that a molecular design leading to bent-core liquid crystals with large β values is a correct strategy to achieve NLO-materials for bulk second order effects.

Liquid crystalline and nonlinear optical properties of bent-shaped compounds derived from 3,4'-biphenylene

The synthesis of different bent-core molecules derived from 3,4′-biphenylene bearing azo, azoxy, imine or ester linkages in their lateral structures is reported. Structure–activity relationships for their liquid crystalline behaviour are discussed. SmCP, USmCP and Colob mesophases are found depending on the type and number of these connecting units. The sequence ester ≈ azoxy > imine > azo can be proposed for the mesophase range, with significant differences observed in terms of mesopahase stabilization. SHG studies on these compounds give nonlinear coefficients in the range of 1–8 pm V−1. The molecular origin for these values is analyzed semi-quantitatively. It was concluded that the SHG performance of bent-core mesogens in general can still be increased substantially. An approach to improve the properties of these materials is briefly outlined.

The measurement of second‐harmonic generation in novel ferroelectric liquid crystal materials

We present the analysis of experimental results on second‐harmonic generation in a novel ferroelectric liquid crystal (FLC) material, o‐ nitroalkoxyphenyl biphenylcarboxylate 1, synthesized specifically for large nonlinear coefficients. All nonzero dij coefficients have been measured, with the largest being d22 = 0.6 ± 0.3 pm/V. Relative values of refractive indices are also given suggesting uniaxial optical properties with large birefringence (nz − nx≊ 0.20 at λ=1.064 μm). This material demonstrates the flexibility of FLC molecular synthesis to form thermodynamically stable macroscopically aligned materials for second‐order nonlinear optics applications.

Design and synthesis of ferroelectric liquid crystals. 15.1 FLC materials for nonlinear optics applications

Abstract We have recently reported the first FLCs designed specifically for large second order nonlinear optical (NLO) susceptibility χ(2); a series of o-nitro-1-methylheptyloxy biphenylbenzoates and phenyl biphenylcarboxylates. Properties of a stable room-temperature C* mixture of two such components (1:1 W316/W317 = MX5679) are presented. The nonlinear susceptibility, as evidenced by the Type 1 eeo deff for second harmonic generation (SHG) from 1,064 nm light for MX5679 is indeed large relative to other FLCs which have been evaluated for χ(2) (deff (ZLI3654) = 0.0016 pm/V, deff (SCE9) = 0.0037 pm/V, deff (MX5679) = 0.16 pm/V). The synthesis and some properties of second-generation FLCs designed for NLO applications is also described.

EIN NEUES BAUPRINZIP FUR DISKOTISCHE FLUSSIGKRISTALLE

Kleine Molekule vom Inden- und Pseudoazulentyp wie 1 bzw. 2 bilden kolumnare Mesophasen! Diese Verbindungen haben keine endstandigen flexiblen Ketten, wie sie sonst bei entsprechenden Mesogenen vorhanden sind, sondern polarisierbare Chlor- und Schwefelatome oder eine polare Cyanogruppe, die als ungewohnliche „weiche” Bereiche zwischen den saulenformigen Stapeln fungieren.

Liquid-crystalline hybrid materials based on [60]fullerene and bent-core structures

What a core-ker! By the appropriate combination of promesogenic bent-core structures and the C(60)  unit, lamellar polar liquid-crystal phases were induced. The supramolecular organization of the functional fullerene-based assemblies, the temperature range of the soft phase, the stabilization of the mesophase-like order at room temperature, and the molecular switching under an electric field can be tuned, depending on the molecular structure.