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

Abstract

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 axes is approximately 103 greater than that in the perpendicular direction. The conduction along the columns is identified with a single charge transport process in which the carriers hop between localized states (radical cations) associated with AlCl4−-counterions linearly distributed off-axis along the columns. The charge carrier diffusion coefficient is independent of the concentration of dopant and has the value D∥∼3.4×10−10 m2 s−1 in the Dho phase at 343 K. In the isotropic melt phase, the conductivity measurements reveal the involvement of two distinct processes. One of these is identified with charge migration along supramolecular ‘‘stacks’’ some 200 nm in length, whilst the other appears to be associated with carriers hopping between these stacks. The conductivity behavior of the unaligned Dho and K phases is very similar to that of the isotropic liquid phase implying that the mesoscopic structure of the latter phase is ‘‘frozen-in’’ as a ‘‘defect structure’’ on cooling. This defect structure then dominates the conductivity behavior of the unaligned Dho and K phases.

Observation of an anchoring transition in a discotic liquid crystal

“…Another reason for the continued interest in discotics lies in the potential applications of such systems. The type of orientation of the molecules inside a LC cell governs the characteristics of devices [1][2][3][4]. It has recently been shown [1,2] that the columnar phase structure of discotic liquid crystals is suitable for the fast transport of photogenerated charge carriers.…”

Effects of side-chain length on the magnetic response of discotic metallomesogens

“…These results indicate that the saturating magnetic field strength for the alignment increases with the side-chain length x, which may be attributed to the entropy increase with the side-chain length. It should be also noted that the saturating magnetic fields for the alignments of CoS10 and CoS12 are much lower than those for other DLCs (Boden et al, 1993;Eichhorn et al, 2003). This can be attributed to the constructive effects of the diamagnetic moments of the aromatic core and the paramagnetic moments of the central cobalt atom of CoS10 and CoS12 (Pate et al, 2002;.…”

“…The free energy of magnetic alignment for DLC systems can be expressed as ÁG align = À(1/2)Á (H Á n) 2 where Á is the anisotropy of magnetic susceptibility, H is an applied magnetic field, and n is the columnar director. Due to the large negative diamagnetic anisotropy of the aromatic core (Á < 0) of most discotic mesogens, the columnar directors align perpendicular to the applied magnetic field (Dekker, 1987;Boden et al, 1993).…”

Discotic liquid crystal-nanoparticle hybrid systems

“…A number of dopants, such as bromine, iodine, AlCl 3 , NOBF 4 , trinitrofluorenone and ferrocenium ions, have previously been dispersed in DLCs to study the electrical conducting behavior of these materials. [20][21][22][23][24] However, the most remarkable effect was observed in the case of trinitrofluorenone doping. When a triphenylene-based DLC, namely hexahexylthiotriphenylene (HHTT), was doped with a small amount (0.62 mol %) of trinitrofluorenone, the conductivity parallel to the columnar axis (s || ) increased by a factor of 10 7 or more.…”