Discotic liquid crystals as quasi-one-dimensional electrical conductors

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 (sigma(parallel to)) to increase by a factor of 10(7) or more relative to that in undoped samples; sigma(parallel to) 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, sigma(parallel to)/sigma(perpendicular to) greater than or equal to 10(10), 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, sigma(perpendicular to), 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.