Jonathan Clements

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 ...

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.

Nematic–lamellar tricritical behavior and structure of the lamellar phase in the ammonium pentadecafluorooctanoate (APFO)/water system

2H NMR spectroscopy has been used to map a high‐resolution (±0.04 K) phase diagram for the ammonium pentadecafluorooctanoate(APFO)/heavy water system. It is qualitatively similar to that for the CsPFO/heavy water system. In particular, it exhibits a discotic micellar nematic phase N+D intermediate to an isotropic micellar solution phase I and a lamellar phase for weight fractions of APFO between 0.395 (φ=0.278) and 0.589 (φ=0.455) and temperatures between 292.10 and 338.10 K. The N+D to lamellar transition crosses over from second to first order behavior at a tricritical point similar to the superfluid transition in 3He/4He mixtures. X‐ray scattering experiments shows there to be no dramatic change in the structure of the micelle at the I to N+D and the N+D to lamellar transitions. Nematic order parameters (orientational order parameters of the discoidal micelles) have been calculated from electrical conductivity measurements. Their variation with temperature in the nematic phase and across the nematic to...

Transient Photoconductivity and Dark Conductivity in Discotic Liquid Crystals

A comparative study of transient photoconductivity and dark conductivity in the discotic liquid crystal hexahexyloxytriphenylene (HAT6) is reported. In both experiments the transport is dispersive, and the carrier mobilities are essentially identical demonstrating that the presence of the counterions in the chemically doped conductors does not significantly perturb the mobility.

Electron transport across metal/discotic liquid crystal interfaces

Electron transport across micron thick films of columnar hexagonal discotic liquid crystal phases homeotropically aligned between metal electrode surfaces has been studied both experimentally and theoretically. These molecules are unique in their combination of charge transport along individual molecular columns with liquidlike self-organization. Typical of organic insulators, a high resistance Ohmic regime is evident at fields of less than 0.05 MV cm−1, due to a low concentration of chemical impurities (n<109 cm−3), and a space-charge injection regime at higher fields. Breakdown fields are reasonably high: in hexakis(hexyloxy)triphenylene they reach ∼5 MV cm−1 at room temperature. Our results show that triphenylene-based discotics form an excellent class of highly ordered optically transparent insulators. At high temperatures and high fields the current is injection controlled and exhibits typical tunneling and space charge limited, nonlinear I–V characteristics. Dramatic jumps in injection currents are ...

Discotic Liquid Crystals

Abstract When considering the possibility of commercially viable applications for discotic liquid crystals it is worth noting that in the case of calamitic liquid crystals some 80 to 90 years elapsed between their discovery in 1888 and their first application in displays. Since discotic mesophases were only discovered in 1977 [ll, it is not surprising that the first applications are yet to be realized.

One dimensional carrier trapping in the crystalline phase of a columnar liquid crystal

A study of charge transport in the crystalline phase of the columnar liquid crystal, hexakis(n-hexyloxy)triphenylene, HAT6, is reported. A simple model is introduced and fitted to observations of photocurrent transient decays in the crystalline phase of HAT6. From these a quasi one-dimensional trapping process is inferred with a field dependent trapping rate determined by carrier drift at high fields and controlled by carrier diffusion in the limit of zero field. The detrapping rate is found to be independent of field indicative of a neutral barrier. The trap concentration is found from fits of the transients to the model. The mobility of holes in the crystalline phase is found and compared to the time-of-flight mobility previously found for HAT6 in the discotic mesophase. Also found is the field dependence of the generation rate in the presence of an absorbing electrode adjacent to the photocreation site.

Characterization of the cationic species formed in p-doped discotic liquid crystals

UV–VIS absorption spectroscopy has been used to characterize the cationic species produced by doping the discotic liquid crystal 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6,1) with aluminium trichioride. A comparison of the spectrum for the conducting doped liquid crystal with those obtained by oxidising HAT6 in solution establishes that the product in both systems is one in which a single charge is confined to a single molecule, i.e. the radical cation HAT6˙+ rather than a non-covalently bound dimer (HAT6)2˙+.