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We present the prototype of a ferroelectric tunnel junction (FTJ), which is based on a self-assembled monolayer (SAM) of small, functional molecules. These molecules have a structure similar to those of liquid crystals, and they are embedded between two solid-state electrodes. The SAM, which is deposited through a short sequence of simple fabrication steps, is extremely thin (3.4 ± 0.5 nm) and highly uniform. The functionality of the FTJ is ingrained in the chemical structure of the SAM components: a conformationally flexible dipole that can be reversibly reoriented in an electrical field. Thus, the SAM acts as an electrically switchable tunnel barrier. Fabricated stacks of Al/Al2O3/SAM/Pb/Ag with such a polar SAM show pronounced hysteretic, reversible conductance switching at voltages in the range of ±2–3 V, with a conductance ratio of the low and the high resistive states of up to 100. The switching mechanism is analyzed using a combination of quantum chemical, molecular dynamics, and tunneling resistance calculation methods. In contrast to more common, inorganic material-based FTJs, our approach using SAMs of small organic molecules allows for a high degree of functional complexity and diversity to be integrated by synthetic standard methods, while keeping the actual device fabrication process robust and simple. We expect that this technology can be further developed toward a level that would then allow its application in the field of information storage and processing, in particular for in-memory and neuromorphic computing architectures.

Thermal stability of several representative self-assembled monolayers (SAMs) with phosphonic acid (PA) anchoring group on Al2O3 substrates was studied by in situ high resolution X-ray photoelectron...

The absorption properties of a dye molecule depend on its orientation relative to the light source. Thus the ability to predict how well a particular dye aligns with a liquid crystal host will impr...

A straightforward access to a hitherto unknown C3 -symmetric tricyclic triol both in racemic and enantiopure forms has been developed. Treatment of 7-tert-butoxynorbornadiene with peroxycarboxylic acids provided mixtures of C1 - and C3 -symmetric 3,5,7-triacyloxynortricyclenes via transannular π-cyclization and replacement of the tert-butoxy group. By refluxing in formic acid, the C1 -symmetric esters were converted to the C3 -symmetric formate. Hydrolysis gave diastereoisomeric triols, which were separated by recrystallization. Enantiomer resolution via diastereoisomeric tri(O-methylmandelates) delivered the target triols on a gram scale. The pure enantiomers are useful as core units of dopants for liquid crystals.

We investigated the influence of molecular packing on the optical and electrical properties of the liquid crystalline dye 4,7-bis[5-(2-fluoro-4-pentyl-phenyl)-2-thienyl]-2,1,3-benzothiadiazole (FPPTB). FPPTB is crystalline at room temperature, exhibits a nematic phase at temperatures above 149 °C and is in an isotropic melt at temperatures above 230 °C. Solution processed FPPTB films were subject to thermal annealing through these phase transition temperatures and characterized with X-ray diffraction and polarized optical microscopy. Cooling FPPTB films from the nematic and isotropic phases increased crystal domain size, but also induced local structural variations in the molecular packing of crystalline FPPTB. The decrease in long-range order was correlated with an increase in short-range π-π interactions, leading to changes in molecular aggregation which persisted even when the FPPTB films were cooled to room temperature. Annealing-induced changes in molecular aggregation were confirmed with optical spectroscopy. The carrier mobility in FPPTB films increased over 2 orders of magnitude from (2.2 ± 0.4) × 10-5 cm2 V-1 s-1 in as-spun films to μ = (5.0 ± 0.8) × 10-3 cm2 V-1 s-1 in films cooled from the isotropic melt. We discuss the relationship between thermal stability and high carrier mobility values in terms of the interplay between long-range molecular order and increased π-π interactions between molecular pairs in the FPPTB film.

A key challenge in the field of organic electronics is predicting how chemical structure at the molecular scale determines nature and dynamics of excited states, as well as the macroscopic optoelectronic properties in thin film. Here, the donor–acceptor dyes 4,7-bis[5-[4-(3-ethylheptyl)-2,3-difluorophenyl]-2-thienyl]-2,1,3-benzothiadiazole (2,3-FFPTB) and 4,7-bis[5-[4-(3-ethylheptyl)-2,6-difluorophenyl]-2-thienyl]-2,1,3-benzothiadiazole (2,6-FFPTB) are synthesized, which only differ in the position of one fluorine substitution. It is observed that this variation in chemical structure does not influence the energetic position of the molecular frontier orbitals or the ultrafast dynamics on the FFPTB backbone. However, it does result in differences at the macroscale, specifically regarding structural and electrical properties of the FFPTB films. Both FFPTB molecules form crystalline films at room temperature, whereas 2,3-FFPTB has two ordered smectic phases at elevated temperatures, and 2,6-FFPTB does not display any liquid crystalline phases. It is demonstrated that the altered location of the fluorine substitution allows to control the electrostatic potential along the molecular backbone without impacting molecular energetics or ultrafast dynamics. Such a design strategy succeeds in controlling molecular interactions in liquid crystalline phase, and it is shown that the associated molecular order, or rather disorder, can be exploited to achieve ambipolar transport in FFPTB films., Advanced Electronic Materials, 7 (8), ISSN:2199-160X

The authors thank the European Research Council for and Advanced Grant and DO’H acknowledges the Royal Society for a Wolfson Merit Award. This paper explores the synthesis of a series of prototype negative dielectric liquid crystalline (LC) compounds which contain fluorinated cyclohexane motifs. The series are analogues and differ only in that they contain between one to four fluorine atoms. The stereochemistry is such that all of the fluorines/C-F bonds are on one face of the cyclohexane ring. This follows from the recent recognition that cyclohexanes with fluorines orientated in one direction, perpendicular to the ring have a strong molecular dipole, a characteristic that might be an advantage in the design of negative dielectric materials. However it is found that the increased polarity, particularly with two or more oriented C-F bonds, leads to higher melting points and poorer solubility in test matrix formulations, relative to hydrocarbon liquid crystals. This arises due to electrostatic intermolecular interactions between the polarised cyclohexyl and aryl rings. The study highlights that in order to take advantage of these polar cyclohexane motifs for liquid crystal design, appropriate scaffolds are required which are free of aromatic rings and contain peripheral solubilising motifs. Postprint Postprint

Fluorine is an essential component of most liquid crystalline materials as they are used in LCD devices. The reasons for this are manifold: fluorination limits the coordination ability to cationic impurities and thus improves reliability of the LCD, it provides a custom-tailored molecular dipole moment with the right orientation, and it can stabilize certain conformations and rigidify flexible alkyl chains.

A protocol for the preparation of a series of liquid crystals is presented, where the structures carry a central propyleneoxy motif carrying two vicinal threo-C–F bonds. The negative dielectric anisotropy (−Δɛ) of the resultant liquid crystals has been explored. The stereoelectronic relationship between the vicinal C–F bonds was anticipated to orientate the fluorine atoms gauche to each other when the propylene chain is extended. This is shown to be the case. This orientation of the C–F bonds generates a net dipole perpendicular to the molecular axis, a prerequisite for the design of dielectrically negative liquid crystals. However the molecules adopt a conformation where one C–F bond reinforces the net molecular dipole, and the other has a counter effect, thus the introduction of the motif has an almost neutral effect on dielectric anisotropy (Δɛvirt) of these candidate liquid crystals. However, introduction of the difluoro motif raises the melting points of the liquid crystals and increases their conformational rigidity.

Using a simple synthetic protocol, heterohexacene analogues with a quadrupolar distribution of partial charges are readily available. In contrast to most other acenes, these compounds crystallize with a slipped-stack, brickwork-like packing which is mainly controlled by electrostatic interactions. This type of packing offers an advantage for organic semiconductors, because it allows more isotropic charge transport compared to the “herring bone” stacking observed for other acenes.

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.

Neue Flussigkristalle mit sehr niedriger Viskositat, gutem Mesophasenverhalten und hoher Zuverlassigkeit ermoglichen den Durchbruch vom flachen Computermonitor zum grosformatigen LCD-Fernseher. Fluor spielt hierbei eine entscheidende Rolle, nicht nur, weil es Polaritat in organischen Molekulen induziert, sondern auch wegen seiner niedrigen Polarisierbarkeit und seiner schwach ausgepragten Tendenz zur Ionensolvatation. Zusatzlich sind in fluorierten Flussigkristallen subtile stereoelektronische Effekte am Werk und ermoglichen eine Feinabstimmung der gewunschten Eigenschaften. Dafur ist eine aufwendige Chemie erforderlich, wie sie sich normalerweise in der Spezialchemie allein schon aus Kostengrunden verbietet. Der Markt fur Fernseh-Displays tritt heute in eine Phase der Sattigung ein. Bedingt durch die weite Verbreitung des Internets gibt es jedoch einen neuen Trend zu mobilen Geraten. Tablet-PCs und Smartphones benotigen beruhrungsempfindliche Bildschirme und niedrigen Stromverbrauch. Neue LC-Modi mit hocheffizienten Flussigkristallen und zusatzlichen Komponenten, z. B. polymerisierbaren Materialien, werden in solchen Produkten verwendet.

The function of liquid crystals for ubiquitous liquid crystal display (LCD) technology depends critically on the presence of fluorine. Fluorine and fluorinated groups shape the mesogenic profile, viscoelastic, and electrooptic properties as well as technical reliability. The industrial design of liquid crystalline materials for display applications is guided by quantum chemical methods. Many rather “exotic” fluorinated groups—such as hypervalent sulfur fluorides—found their first practical application through LCD technology. The technical synthesis of fluorinated liquid crystals for display applications often posed significant challenges for industrial chemistry: typical lab scale reactions such as directed ortho -lithiation or oxidative fluorodesulfuration had to be transferred from lab to multiton-scale. In order to provide multigram quantities of new liquid crystals for application-oriented testing, direct fluorination with elemental fluorine as well as fluorination of carbonyl compounds using sulfur tetrafluoride is routinely used.

Highly fluorinated liquid crystals with unprecedented polarity were obtained by introducing a tetrahydropyran moiety into their mesogenic core structure. The new materials exhibit an unusually favourable combination of mesophase properties, excellent solubility and low rotational viscosity, which renders them highly attractive for application in fast-switching active matrix LCDs.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

The reaction of α-fluorostyrene (1a) with methyl phenyldiazoacetate in the presence of Rh2(OAc)4 gave a 64:36 mixture of diastereomeric monofluorinated cyclopropanes 5a and 6a. When using chiral RhII catalysts Rh2(TBSP)4 (7) and Rh2(DOSP)4 (8), the products were obtained enantioselectively in up to >99 % ee for both (1R,2R)-5a and (1R,2S)-6a. The fluorocyclopropane derivatives are of interest as dopants to induce chirality in liquid-crystal mixtures. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

A new class of liquid crystals with strongly negative dielectric anisotropy has been prepared by the exploitation of the somewhat “ambivalent” characteristics of the 1,3-dioxane structure with respect to its polarity and by the axial fluorination of this structure, which gives these compounds attractive mesogenic and electrooptic properties. The increased chemical stability of these materials compared with their carbocyclic analogues has been analyzed and explained by molecular modelling. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

The tetrahydropyran moiety has been identified as a highly advantageous addition to the toolbox for the design of nematic liquid crystals for LCD applications. A new synthetic procedure based on the Lewis acid catalysed ring opening of oxetanes by lithium iminoenolates followed by reductive dehydroxylation of the resulting hemiketal provides a convenient preparative access to this class of materials.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

New liquid crystals with a pentafluorosulfanyl function as their polar terminal group were prepared and fully characterized. The synthetic key step was the triethyl borane-catalyzed addition of SF 5 Br to a suitable olefin precursor, followed by dehydrobromination. The most interesting characteristic of the new materials is their combination of high polarity (Δ ɛ ) with very low birefringence (Δ n ), which is of particular importance for LCDs for mobile applications.

Liquid crystals that have ortho-fluorinated pentafluoro-λ6-sulfanyl terminal groups are the most polar materials that are still compatible with current active matrix LCD technology. The synthesis of the first examples of this class of substance has been achieved by direct fluorination. The physico-chemical characterization, supported by DFT calculations, revealed some surprising features of these new compounds: the bulky SF5 group is easily deformed and responds in a quite unusual way to the steric strain exerted by its ortho substituents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Several examples of highly fluorinated sulfimides and sulfoximides have been synthesized in order to assess their potential usefulness as functional materials, such as liquid crystals or as active components in molecular electronics. The polarity of liquid crystals with these types of functional groups was lower than expected, due to conformational effects. A new synthetic method provides a convenient access to N,S,S-triaryl sulfimides and sulfoximides, which were characterized by X-ray crystallography. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The hitherto elusive title compound was synthesized and characterized with the aim to study its potential as a carbon-based (“CF-type”) electrophilic fluorination reagent. A detailed discussion of its structure, properties and reactivity is based on experimental studies as well as on molecular modeling.

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.

A dramatic increase of the clearing temperatures of liquid crystals based on bis(cyclohexyl)ethane 1 by 50 to 70 K can be achieved by the perfluorination of the central ethylene link. Conformational analysis indicates that this effect is due to the increased rigidity of the mesogenic core structure and to the suppression of conformers with a bent shape. Materials based on bis(cylohexyl)tetrafluoroethane 2 might play a crucial role as materials for the next generation of active matrix LCDs with reduced power consumption.

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.

A significant reduction of the power consumption of a liquid crystal display can be achieved either by application of a lower driving voltage or – even more effectively – by use of a holographically structured reflective scattering type of display without colour filters. The first option requires strongly polar materials with extremely high reliability, the second one new types of liquid crystals with a birefringence as high as possible in order to optimize the scattering effect. A molecular modeling based method for the prediction of reliability parameters is presented.

The pentafluorosulfuranyl group is one of the strongest electron-withdrawing groups which act purely by an inductive effect. The strong dipole moment of its derivatives and its chemical stability make it highly attractive as a terminal group for polar nematic liquid crystals which are used in active matrix displays. A method for the prediction of the electrooptic parameters of pentafluorosulfuranyl based liquid crystals is presented.

Due to their octahedral symmetry, otherwise not available for carbon based chemistry, hypervalent sulfur fluorides are very attractive as structural building blocks for functional organic materials. Direct fluorination offers a convenient access to multigram quantities of highly stable bis(4-nitrophenyl)tetrafluorosulfurane. Guided by molecular modeling, a novel catalytic process was devised to isomerize the predominantly formed cis isomer to the pure trans isomer.

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.