Discotic liquid crystals as quasi–one–dimensional electrical conductors

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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’.