Restricting Conformational Space: A New Blueprint for Electrically Switchable Self-Assembled Monolayers.

Tunnel junctions comprising self-assembled monolayers (SAMs) from liquid crystal-inspired molecules show a pronounced hysteretic current-voltage response, due to electric field-driven dipole reorientation in the SAM. This renders these junctions attractive device candidates for emerging technologies such as in-memory and neuromorphic computing. Here, the novel molecular design, device fabrication, and characterization of such resistive switching devices with a largely improved performance, compared to the previously published work are reported. Those former devices suffer from a stochastic switching behavior limiting reliability, as well as from critically small read-out currents. The present progress is based on replacing Al/AlOx with TiN as a new electrode material and as a key point, on redesigning the active molecular material making up the SAM: a previously present, flexible aliphatic moiety has been replaced by a rigid aromatic linker, thereby introducing a molecular "ratchet". This restricts the possible molecular conformations to only two major states of opposite polarity. The above measures have resulted in an increase of the current density by five orders of magnitude as well as in an ON/OFF conductance ratio which is more than ten times higher than the individual scattering ranges of the high and low resistance states.

Synthesis of organic liquid crystals containing selectively fluorinated cyclopropanes.

This paper describes the synthesis of a series of organic liquid crystals (LCs) containing selectively fluorinated cyclopropanes at their termini. The syntheses used difluorocarbene additions to olefin precursors, an approach which proved straightforward such that these liquid crystal candidates could be efficiently prepared. Their physical and thermodynamic properties were evaluated and depending on individual structures, they either displayed positive or negative dielectric anisotropy. The study gives some guidance into effective structure-property relationships for the design of LCs containing selectively fluorinated cyclopropane motifs.

C3 -Symmetric Tricyclo[2.2.1.02,6 ]heptane-3,5,7-triol.

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.

Fluorinated liquid crystals: evaluation of selectively fluorinated facially polarised cyclohexyl motifs for liquid crystal applications.

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.

Hypervalent sulfur fluorides and the design of liquid crystals.

Hypervalent sulfur fluorides are providing functional materials - such as liquid crystals - with unique physicochemical properties and new geometric structural elements.

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.

Crystal design using multipolar electrostatic interactions: A concept study for organic electronics.

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.

The preferred conformation of erythro- and threo-1,2-difluorocyclododecanes.

Cyclododecane adopts a square-like structure with corner and edge CH(2) groups. In this study erythro- and threo-1,2-difluorocyclododecanes were prepared to explore whether the two vicinal C-F bonds, with different relative configurations, preferably locate at corner/edge or edge/edge locations. Conformational analysis comparing the diastereoisomers was explored by using a combination of (19)F{(1)H} NMR spectroscopy, computational studies and, in the case of the threo isomer, X-ray structural analysis. In the lowest energy conformers for both diastereoisomers the vicinal C-F bonds are located corner/edge, rather than edge/edge. These structures avoid placing a C-F bond endo into the ring, and appear to benefit from C-CHF-C angle widening, which relaxes 1,4-H,H transannular interactions.

Conductance Switching in Liquid Crystal-Inspired Self-Assembled Monolayer Junctions.

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.

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.

The multi-vicinal fluoroalkane motif: an examination of 2,3,4,5-tetrafluorohexane stereoisomers.

Three unique diastereoisomers of 2,3,4,5-tetrafluorohexane have been prepared, compounds intermediate between hexane and perfluorohexane in their degree of fluorination, and they show very different conformational behaviour and physical properties.

Liquid crystals carrying stereodefined vicinal difluoro- and trifluoro- alkyl motifs.

The synthesis of the stereospecifically fluorinated difluoro- and trifluoro- rac-3 and rac-4 is described where the fluorine atoms are positioned adjacent/vicinal to each other and the physical characteristics of these candidate liquid crystals including negative dielectric anisotropy are measured and rationalised.

Interplay between Long-Range Crystal Order and Short-Range Molecular Interactions Tunes Carrier Mobility in Liquid Crystal Dyes.

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.

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.

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.

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.

Difluorooxymethylene-Bridged Liquid Crystals: A Novel Synthesis Based on the Oxidative Alkoxydifluorodesulfuration of Dithianylium Salts.

Insertion of a difluorooxymethylene bridge into the mesogenic core structure of phenylcyclohexane-based liquid crystals results in a class of materials that exhibits a surprising and unprecedented improvement of essentially all application-relevant properties. A novel synthetic procedure allows the convenient large-scale preparation of a variety of compounds based on α,α-difluoroalkyl ether structures.

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.