Owen R. Lozman

Discotic liquid crystals 25 years on

Abstract Disc-shaped objects can be induced to form columnar assemblies whether their scale is tens of Angstroms (molecular), tens of nanometers (macromolecular or supramolecular), hundreds of nanometers (colloidal) or tens of microns (‘manufactured’ platelets). The last couple of years have seen rapid progress in the development of conducting columnar systems and in controlling the orientation of discotic mesophases. The first serious commercial development has also emerged. Fuji Film Company has perfected and marketed optical compensating films based on cross-linked nematic discogens with controlled hybrid orientation.

Photoconducting liquid crystals

Abstract Major recent advances : ‘High’ mobility photoconduction in the columnar mesophases of disc-shaped (discotic) liquid crystals in which the charge carriers are holes or electrons was discovered in 1995. Prior to this photoconduction in liquid crystals was attributed to photo-generated ions and associated with ‘low’ mobilities. Over the last 7 years our understanding of the mechanism of carrier generation and transport in these novel, self-assembling systems has progressed to the point where we are able to design and manufacture organic semi-conductors with well-defined electronic and physical properties. Serious commercial devices incorporating conducting liquid crystals are finally on the horizon.

Temperature-independent hole mobility in discotic liquid crystals

Experimental measurements are presented of the hole mobilities of four conjugated discotic systems, forming columnar liquid crystals, as a function of temperature. The measurements cover the crystalline/glassy phase and mesophase of these materials. It is a remarkable fact that the mobility is almost independent of temperature in the range 30 °C–170 °C. Various explanations of a weak temperature dependence exist and these are explored. They include the small polaron of Holstein in the nonadiabatic limit and the effect of the dynamic disorder present in the system.

Enhanced electronic transport properties in complementary binary discotic liquid crystal systems

Abstract The electronic transport properties of the molecular stacks in discotic liquid crystals (DLCs), have generated much recent interest. In particular, among a certain class of these DLCs, the triphenylenes, some derivatives exhibit high hole mobilities as demonstrated by the time of flight transit signals obtained in transient photoconduction experiments. The stacks have also been shown to exhibit one-dimensional transport behaviour. If the DLCs are to find application as new electronic materials it is desirable to improve their electronic transport properties. In this paper we demonstrate that the hole mobility and range are greatly increased as a result of the ordering imposed in complementary binary mixtures formed by addition of a large core discogen to the triphenylene molecules.

Helical geometry and liquid crystalline properties of 2,3,6,7,10,11-hexaalkoxy-1-nitrotriphenylenes

The single-crystal X-ray structure of 2,3,6,7,10,11-hexaethoxy-1-nitrotriphenylene confirms earlier calculations of the molecular geometry and shows that the α-nitro substituent imparts a helical twist to the triphenylene nucleus. Within the crystal, the molecules are arranged in tilted columns on an oblique lattice, and along the columns the molecular dipoles are arranged antiferroelectrically, with a molecule–molecule dipole–dipole interaction of ca. −5.5 kJ mol−1. It is suggested that this interaction may be important in stabilising the mesophase. Eleven other 2,3,6,7,10,11-hexaalkoxytriphenylenes and their mononitro derivatives (with side chains ranging from –OC2H5 to –OC16H33) have been prepared and characterised. Most of the α-nitrated compounds give enantiotropic columnar mesophases that show wider mesophase ranges than the precursor aryl ethers. For the systems with side chains from –OC4H9 to –OC11H23, α-nitration increases the mesophase range (relative to the simple aryl ether), and for the systems with –OC12H25, –OC14H29, and –OC16H33, α-nitration induces liquid crystal behaviour (the simple aryl ethers analogues being non-mesogenic). Although it had previously been claimed that 2,3,6,7,10,11-hexapropoxytriphenylene is not mesogenic, we found that it exhibits a plastic columnar phase between 100 and 175 °C.

Syntheses of new 'large core' discogens based on the triphenylene, azatriphenylene and hexabenztrinaphthylene nuclei

We have synthesized new discogens based on polyphenylated triphenylene, polyphenylated azatriphenylene and hexabenztrinaphthylene. A procedure for the extension of the aromatic core by Suzuki coupling is described, which minimizes competitive reductive dehalogenation.

CPI induction of liquid crystal behaviour in triphenylenes with a mixture of hydrophobic and hydrophilic side chains

2,3,6,7,10,11-Hexasubstituted triphenylenes have been synthesized that contain a mixture of hydrophobic (C6H13O) and hydrophilic (CH3OCH2CH2OCH2CH2O) side chains. At one extreme HAT6 (1a) (six hydrophobic chains) shows thermotropic behaviour and at the other TP6EO2M (1e) (six hydrophilic chains) shows lyotropic behaviour. Of the triphenylenes with a mixture of hydrophobic and hydrophilic side chains, only the triphenylene with one hydrophilic side chain and five hydrophobic side chains (1b) gives a thermotropic columnar phase. None of the others show liquid crystal behaviour. However, all of these triphenylenes form binary 1:1 compounds when mixed with PDQ9 (2a) and with PTP9 (2b). These CPI (complimentary polytopic interaction) stabilized compounds give thermotropic hexagonal columnar phases over wide temperature ranges.

The stability of columns comprising alternating triphenylene and hexaphenyltriphenylene molecules: variations in the structure of the hexaphenyltriphenylene component

Previous investigations of complementary polytopic interaction (CPI) columnar mesophases, in which the columns are built up of alternating hexaalkoxytriphenylene (HAT) and hexaphenyltriphenylene (PTP) molecules, concentrated mainly on the effect of variations in the structure of the HAT component. This investigation is concerned with the effect of variations in the structure of the PTP component and, in particular, variations in the position of an alkoxy side chain in the phenyl ring. Stable columnar mesophases are obtained when a hexyloxy substituent is placed in the meta‐ or para‐position but not in the ortho‐position. In the case of the meta‐ and para‐substituted systems, the two‐component CPI columnar phases are stable over a considerably larger temperature range than the one‐component HAT systems. The evidence suggests that unfavourable PTP/PTP stacking is as much a driving force for the formation of these mixed stacks as is favourable PTP/HAT stacking, but both need to be explained in terms of the sum of atomically dispersed van der Waals and coulombic interactions. On cooling from the isotropic into the Colh phase, the columnar phase based on a 1:1 mixture of hexakis(hexyloxy)tripenylene and the meta‐hexyloxy‐substituted PTP gives an unusual texture consisting of ‘viking‐axe’‐shaped structures.

DESIGNING BETTER COLUMNAR MESOPHASES

The conductivity, photoconductivity and mesophase ranges of hexaalkoxytriphenylene (HAT)-based discotic liquid crystals are all increased by adding one equivalent of {hexakis(4-nonylphenyl)dipyrazino[2,3-f:2′3′-h]quinoxalene} (PDQ9). For some triphenylene derivatives, addition of PDQ9 induces liquid crystal behaviour, for HAT-based homopolymers the alignment characteristics are improved and for some HAT-based block copolymers, addition of PDQ9 induces microphase separation. The bonding interaction between the HAT and PDQ components is described as a CPI (complementary polytopic interaction) and is best modelled as an atom by atom sum of van der Waals and multipolar terms. Although strong it does not detectably perturb the electronic structure of either component and there is no charge-transfer.

Molecular Engineering the Phototransport Properties of Discotic Liquid Crystals

This paper explores how charge carrier mobilities in columnar discotic liquid crystals may be enhanced. The hole mobility and its temperature dependence has been measured in a variety of triphenylene based DLCs using the time of flight method in the mesophase and where possible in the glassy (or crystalline) phase. A model using the Holstein small polaron is fitted to the results and the resulting polaron self energy and bandwidth are found. Where trapping is observed it shows a tendency to trap at a faster rate as electric field is increased. This is discussed in terms of dimensionality of the carrier transport. Some transient data is fitted to the Noolandi multiple trapping model and trapping rates and detrapping rates using that model are recovered. Their variation with electric field is discussed.