Matthias Bremer

Nematic Liquid Crystals for Active Matrix Displays: Molecular Design and Synthesis

Substances forming calamitic mesophases have been known for more than 100 years but only the recent, rapid advance in active matrix liquid crystal display (AM-LCD) technology helped these materials to achieve the crucial position in flat panel display technology they hold today. Due to their high contrast, large viewing angle, and rapid switching times, modern AM-LCDs offer a superior picture quality even compared to conventional cathode ray tubes. Their flatness, low weight, and low energy consumption render them the technology of choice for all kinds of portable devices. Some of the future promises of AM-LCD technology are centered around the development of liquid crystalline materials for the different subtypes of active matrix applications. This development is aimed, on the one hand, towards improved electrooptical and viscoelastic properties; on the other hand, the increasing performance of LCDs leads to extremely stringent reliability demands on the liquid crystals. Responding to these high standards of performance and quality, most liquid crystals for contemporary AM-LCD applications are multiply fluorinated compounds with very high purities, as is typical for materials used in the electronics industry. The synthesis of these superfluorinated materials (SFMs) often requires specialized methods, which, in several cases, had to be introduced for the first time into the canon of industrial production. The immense market pressure, as well as the rapid advance of AM-LCD technology on the side of the display manufacturers, urges an increasing pace of the materials development. This demand for new materials can no longer be fulfilled by conventional trial-and-error approaches. As in the pharmaceutical industry, in the search for new, superior liquid crystals, the purely empirical methods are increasingly supported by a rational design based on computational methods.

The TV in Your Pocket: Development of Liquid‐Crystal Materials for the New Millennium

New liquid crystals with very low viscosity, good mesophase behavior, and high reliability are necessary to achieve the breakthrough from flat computer monitors to large displays for television. Fluorine plays a decisive role not only because of the polarity it induces in organic molecules but also because of its low polarizability and weak propensity for ion solvation. In addition, subtle stereoelectronic effects in fluorine-containing liquid crystals influence material properties and allow these to be tuned to some extent to achieve the desired outcome. Some fairly sophisticated chemistry is required that is normally ruled out in the specialty chemicals industry because of cost. The television display market is now entering a phase of saturation. The broad availability of the internet has led to an ever increasing tendency for mobile products. Tablet PCs and smartphones require touch-panel functionality and low power consumption. New LCD modes with high-performance liquid crystals and additional components, such as polymerizable materials, can be used in such products.

Nematische Flüssigkristalle für Aktiv-Matrix-Displays: Design und Synthese

Substanzen, die calamitische Mesophasen bilden, kennt man schon seit mehr als 100 Jahren, doch erst der rasche Fortschritt in der Technik der Aktiv-Matrix-Flussigkristalldisplays (AM-LCDs) fuhrte dazu, dass sie ihre heutige Bedeutung in der Flachbildschirmtechnologie erlangen konnten. Auf Grund ihres hohen Kontrastes, des weiten Blickwinkels und der kurzen Schaltzeiten erreichen moderne AM-LCDs eine Bildqualitat, die haufig sogar der von konventionellen Kathodenstrahlrohren uberlegen ist. Geringer Platzbedarf, geringes Gewicht und geringer Energieverbrauch machen sie zur Technologie der Wahl fur viele tragbare elektronische Gerate. Einige der Zukunftsperspektiven der AM-LCD-Technologie konzentrieren sich auf die Entwicklung flussigkristalliner Substanzen fur die verschiedensten Aktiv-Matrix-Anwendungsbereiche. Zum einen zielt diese Entwicklung auf verbesserte elektrooptische und viskoelastische Eigenschaften ab, zum anderen fuhrt die zunehmende Leistungsfahigkeit der LCDs zu auserst strengen Anforderungen an die Qualitat der Materialien. Um diesen gerecht zu werden, sind die meisten Flussigkristalle fur AM-LCD-Anwendungen heutzutage mehrfach fluorierte Verbindungen von hohem Reinheitsgrad, wie er in der Elektronikindustrie typisch ist. Fur die Synthese dieser so genannten SFMs (superfluorinated materials) bedarf es oft spezieller Methoden, die in einigen Fallen erst auf den Masstab industrieller Produktion ubertragen werden mussten. Der ungeheure Marktdruck und der rasche Fortschritt in der AM-LCD-Technologie bei den Displayherstellern drangt geradezu auf ein schnelleres Tempo bei der Entwicklung dieser Substanzen. Dieser Forderung kann man mit dem konventionellen Vorgehen, das den Erfolg haufig dem Zufall uberlasst, nicht mehr gerecht werden. Wie in der pharmazeutischen Industrie werden bei der Suche nach neuen, besseren Flussigkristallen die rein empirischen Methoden vermehrt durch ein rationales Design auf der Grundlage von Rechenverfahren unterstutzt.

Liquid Crystals Based on Hypervalent Sulfur Fluorides: Pentafluorosulfuranyl as Polar Terminal Group

The somewhat “exotic” pentafluorosulfuranyl functionality is one of the strongest electron-withdrawing groups with a purely inductive effect. Owing to its chemical stability and the resulting high dipole moment of its aromatic derivatives, this group was crucial to the design and synthesis of a new class of highly polar liquid crystals (such as 1) for application in active matrix displays. A computational model was developed with the aim to understand and predict the electrooptic properties of liquid crystals based on hypervalent sulfur fluorides.

Calculation of Optical and Dielectric Anisotropy of Nematic Liquid Crystals

The optical birefringence (Δn) and dielectric anisotropy (Δe) of nematic liquid crystals were calculated using the well-known theories of Vuks and Maier-Meier, respectively. The molecular dipole moments and polarizabilities needed for the estimation were taken from semiempirical AM1 calculations. The calculated values are in good agreement with the experimental results for the investigated compounds having different core structures and polar substituents.

New Liquid-Crystal Materials for Active Matrix Displays with Negative Dielectric Anisotropy and Low Rotational Viscosity

Liquid-crystal single materials with large negative dielectric anisotropies, high clearing temperatures and extremely low rotational viscosities were identified. Using these materials, the rotational viscosities for a broad range of mixture specifications were significantly reduced. Thus, these mixtures are found suitable for the improvement of the switching time of liquid-crystal displays using vertical alignment (VA) modes.

1,1,6,7-Tetrafluoroindanes: improved liquid crystals for LCD-TV application

New materials based on the 1,1,6,7-tetrafluoroindane skeleton were synthesized via ortho-metallation and intramolecular Heck cyclization followed by an oxidative fluorination procedure. The materials offer improved properties over liquid crystals currently employed in flat panel LCD-TVs.

Investigation of high Δε derivatives of the [closo-1-CB9H10]− anion for liquid crystal display applications

Two series of polar compounds 3[n] and 4[n] with longitudinal dipole moments ranging from 10 to 17 D were synthesized and investigated as additives to two nematic hosts, ClEster and CinnCN. Compounds 3[n] do not exhibit liquid crystalline behavior and have limited compatibility with nematic hosts, but still are effective additives for increasing dielectric anisotropy, Δe, of liquid crystalline host materials. On the other hand, esters 4[n] typically exhibit nematic behavior and form stable solutions in ClEster up to 5 mol%. In this concentration range, the binary solutions exhibit a linear dependence of dielectric parameters and the extrapolated Δe values range from <18 (4[3]l, μ = 11.3 D) to 70 (4[3]c, μ = 14.1 D and 4[3]k, μ = 17.2 D). Analysis of the dielectric data with the Maier–Meier formalism using DFT-calculated α and μ parameters gave apparent order parameter values Sapp in a range of 0.51–0.68 and Kirkwood parameters g in a range 0.55–0.78 for esters 4[n].

Novel liquid-crystal materials with negative dielectric anisotropy for TV application

— The development of materials for vertically aligned (VA) active-matrix display technology is briefly reviewed and the newest generation of liquid crystals for this application is disclosed. These new materials possess a unique combination of high dielectric anisotropy Δe and low rotational viscosity γ1 with good clearing temperatures TNI for fast-switching LC mixtures targeted for TV application.

23.1: Invited Paper: New Materials for Polymer-Stabilized Blue Phase

Compared to other liquid crystalline mesophases, the Blue Phase exhibits extremely fast switching via induced birefringence by an external field. However, the pure Blue Phase is characterized by a rather narrow temperature range on the order of a few K. For display applications, one very promising way to broaden the stable temperature range is achieved by polymer-stabilization of the Blue Phase. A crucial prerequisite for efficient stabilization is the optimal material selection/matching of reactive mesogens RMs and the chiral host. New material concepts will be introduced and discussed.