Richard J. Bushby

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.

Mechanism of quasi‐one‐dimensional electronic conductivity in discotic liquid crystals

Recently, it has been shown that a new class of quasi‐one‐dimensional conductors can be created by doping discotic liquid crystals with appropriate oxidants. This paper reports the elucidation of the mechanism of conduction in these new materials. In particular, the ac conductivity of 2,3,6,7,10,11‐hexahexyloxytriphenylene (HAT6) doped with the Lewis acid AlCl3, has been measured as a function of frequency (10−3–107 Hz), and temperature in its crystalline solid (K), hexagonal discotic liquid crystal (Dho), and isotropic liquid (I) phases. In all three phases the conductivity is independent of frequency at low frequencies, but shows a power law dependence on frequency [σ(ω)∼ωs, s∼0.7–0.8] at higher frequencies. This behavior is characteristic of charge carrier transport by a hopping mechanism. The conductivity data have been analyzed in terms of the Scher and Lax theory to obtain the parameters describing this process. In macroscopically aligned K and Dho phases, the conductivity measured along the column ...

Liquid crystals that affected the world: discotic liquid crystals

The main applications and potential applications of discotic liquid crystals are reviewed. At present their most important application is in the manufacture of optical compensating films; films that increase the angle of view in liquid crystal displays. Their use as lubricants and lubricant additives also seems to be developing into a significant area. Progress in the development of other applications is reviewed, including field effect transistors, light-emitting devices, photovoltaic devices and the electronic nose. This is a relatively young field and, as with any new class of materials, it takes time to find applications where their unique properties present a real advantage. However, discotic liquid crystals are already having a significant impact.

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.

Very high time-of-flight mobility in the columnar phases of a discotic liquid crystal

1,4,8,11,15,18,22,25-Octaoctylphthalocyanine shows the highest time-of-flight (“long-range”) hole mobility so far reported for the columnar phase of a discotic liquid crystal. Unlike most other high long-range mobility columnar discotics, there is no clear evidence from x-ray diffraction of high order.

One-dimensional electronic conductivity in discotic liquid crystals

Abstract The discotic mesogen 2,3,6,7,10,11-hexa-hexyloxytriphenylene (HAT6) forms a columnar hexagonal phase which is an electrical insulator. Doping with 1 mol% of the Lewis acid AlCl 3 converts it to a p-type semiconductor with the preferred direction of conduction being along the axes of the columns. The electrical conductivity is envisaged to arise from the migration of positive holes created in the π-electron band of the triphenylene stack. The behaviour is established by electrical conductivity measurements, which show anisotropy, and ESR lineshapes which are consistent with Dysons theory of resonance absorption by conduction electrons.

Device applications of charge transport in discotic liquid crystals

We discuss some of the unique structural and electronic properties of discotic liquid crystals, particularly the nature of the ‘band structure’, and of charge injection from metal electrodes. Attempts have been made to use conducting discotic liquid crystals in electroluminescent devices and in gas sensors. The latter are created by spin-coating a thin film of discotic liquid crystals onto an array of interdigitated electrodes. We discuss the temperature dependence of the conductivity of these new systems, analyse the mechanism of charge diffusion and the effect of adsorbed gases.

Effects of side-chain length on the charge transport properties of discotic liquid crystals and their implications for the transport mechanism

Four homologous mesogens of the hexakis(n-alkoxy)triphenylene series (HATn, n= 4,6,9,11), have been doped with the one electron oxidant NOBF4, and the electrical conductivity measured as a function of frequency in the hexagonal columnar Dh phase. The limiting low-frequency conductivities both parallel, σ‖, and perpendicular, σ⊥, to the column axes decrease exponentially with increasing side-chain length. The value of σ‖ decreases by three orders of magnitude in going from n= 4 to n= 11. This can be understood in terms of an increasing degree of disorder in the packing of molecules in the columns. The decrease of σ⊥ by some two orders of magnitude can be attributed to the increase in the distance between the columns.

Designing new lyotropic amphiphilic mesogens to optimize the stability of nematic phases

Abstract The factors which govern the stability of lyotropic amphiphilic nematic phases are delineated and then used to design mesogens which give rise to stable NC and ND phases on dissolution in water. The synthesis and phase behaviour of novel discoid amphiphiles, designed to form NC phases, are described.

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.