Peer Kirsch

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.

A NOVEL TYPE OF LIQUID CRYSTALS BASED ON AXIALLY FLUORINATED CYCLOHEXANE UNITS

Decidedly higher clearing temperatures and often also broader mesophases are exhibited by axially fluorinated cyclohexane derivatives 1 and 2 relative to their nonfluorinated analogues. A convenient method for the multigram synthesis of 1 and 2, which represent a promising new type of liquid crystals with positive or negative dielectric anisotropy, is presented.

Liquid crystals with multiple fluorinated bridges in the mesogenic core structure

Abstract A rational, building block oriented design approach gives rise to a new class of “super fluorinated” liquid crystals suitable for the newest generation of active matrix LCDs. These materials contain combinations of different fluorinated linking groups in their mesogenic core structure, resulting in a highly advantageous property profile including a broad nematic phase range, low rotational viscosity and high dielectric anisotropy. The large-scale synthesis of such complex materials became feasible only by some recent advances in the synthesis of α,α-difluoroether linkages.

DESIGN AND SYNTHESIS OF NEMATIC LIQUID CRYSTALS WITH NEGATIVE DIELECTRIC ANISOTROPY

Axially fluorinated cyclohexane derivatives are a promising new type of liquid crystals with negative dielectric anisotropy. Up to now, the technical application of these materials in LCDs was impeded by their strong tendency to form smectic B mesophases. We present a convenient synthetic pathway leading to new homologues carrying alkenyl side chains which induce nematic phases.

Combinatorial parallel synthesis and automated screening of a novel class of liquid crystalline materials

Combinatorial parallel synthesis has led to the rapid generation of a single-compound library of novel fluorinated quaterphenyls. Subsequent automated screening revealed liquid crystalline (LC) behaviour and gave qualitative relationships of molecular structures and solid state properties.

2-Trifluoromethyl-1,3-dithianylium triflate: a convenient ‘masked’ electrophilic pentafluoroethylation reagent

With 2-trifluoromethyl-1,3-dithianylium triflate for the first time the synthesis, isolation and full structural characterization of an α-perfluoroalkylcarbenium salt was achieved. The title compound can be easily obtained on a preparative scale. The thermally stable dithianylium salt in combination with fluorodesulfurization chemistry is a promising novel reagent for the electrophilic polyfluoroalkylation of organic substrates, demonstrated by the pentafluoroethylation of O-nucleophiles.

Diastereoselective Synthesis of 2,3,4,5,6-Pentafluoroheptanes

A stereocontrolled synthesis of alkanes containing five contiguous fluorine atoms is presented. The compounds were prepared by sequential fluorination of diastereoisomeric alcohol-diepoxides. The chemistry involved epoxide ring-opening with HF.NEt(3) and deshydroxyfluorination reactions of free alcohols with Deoxo-Fluor. The fluorination reactions were all highly stereospecific, with all five fluorines being incorporated in three sequential steps. Three different diastereoisomers of the 2,3,4,5,6-pentafluoroheptyl motif were prepared as heptane-1,7-diol derivatives, a structural format amenable for incorporation of the vicinal pentafluoro scaffold into larger molecular architectures.