Discotic liquid crystals as quasi-one-dimensional electrical conductors

Abstract

We have studied the electrical conductivity of well-aligned samples of hexahexylthiotriphenylene (HHTT) and hexapentyloxyanthraquinone (HPA) in the pure, as well as doped, states. The former compound was doped with 0.62 mol % by weight of the electron acceptor, trinitrofluorenone (TNF), the latter with 0.60 mol % by weight of the electron donor, anthracene. In the columnar phases, doping causes the AC (1 kHz) conductivity along the columnar axis (σ ||) to increase by a factor of 10⁷ or more relative to that in undoped samples; σ || attains a value of 10^-2 S m^-1, which was the maximum measurable limit of our experimental set-up. On the other hand, in the isotropic phase, doping makes hardly any difference to the conductivity. The DC conductivity of doped HHTT exhibits an enormous anisotropy, σ _||/σ_⊥≥ 10¹⁰, which is seven orders of magnitude higher than that reported for any liquid crystalline system, and, to our knowledge, the largest observed in an organic conductor. Further, the perpendicular conductivity, σ_⊥, reaches the insulating regime. Thus the columnar phases behave nearly as one-dimensional conductors and may be described as 'molecular wires'. Thermoelectric power studies have also been carried out on the molecular wires that show a reversal of the sign on going from the p-type to the n-type system (positive for p-type and negative for n-type). This is in conformity with the expected nature of the charge carriers, namely, holes in HHTT + TNF (p-type) and electrons in HPA+anthracene (n-type). It is pointed out that since the doped systems behave nearly as one-dimensional conductors unusual physical properties might be anticipated. Finally, we discuss very briefly some further studies that are under way.We propose to adopt the four-probe method to increase the highest measurable conductivity to the metallic range (after eliminating the lead and contact resistances) with a view to finding metallic conductivity in discotic liquid crystals. We also intend to carry out Hall-effect studies and to explore the possibility of observing superconductivity by looking for the Meissner effect in magnetic susceptibility measurements.