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Discotic (disk-shaped) molecules or molecular aggregates may form, within a certain temperature range, partially ordered phases, known as discotic liquid crystals, which have been extensively studied in the recent past. On the one hand, this interest was prompted by the fact that they represent models for testing energy and charge transport theories in organic materials. However, their long-range self-assembling properties, potential low cost, ease of processability with a variety of solvents and the relative ease of tailoring their properties via chemical synthesis, drove the attention of researchers also towards the exploitation of their semiconducting properties in organic electronic devices. This review covers recent research on the charge transport properties of discotic mesophases, starting with an introduction to their phase structure, followed by an overview of the models used to describe charge mobility in organic substances in general and in these systems in particular, and by the description of the techniques most commonly used to measure their charge mobility. The reader already familiar or not interested in such details can easily skip these sections and refer to the core section of this work, focusing on the most recent and significant results regarding charge mobility in discotic liquid crystals.

We report on the measurement of the photorefractive space-charge field in a chiral smectic C mesophase. The liquid crystal is intrinsically photoconducting and undoped. The space-charge field amplitude, obtained from a model connecting the induced refractive index modulation with the electric fields within the medium, is only 1% of the applied field. Nonetheless, high refractive index modulations Δn∼10[sup -3] can be obtained because of the high spontaneous birefringence of the mesophase. The use of the standard models for inorganic crystalline photorefractive materials would have given a space-charge field amplitude two orders of magnitude higher. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present results of the photorefractive properties of a chiral smectic A liquid-crystalline phase. In our approach, the mesophase is sandwiched between two photoconducting layers. In contrast to the effect due to the dielectric anisotropy typical of nematic phases, the refractive-index modulation is due to the electroclinic effect. We measured the grating spacing and the field dependence of the gain coefficient and obtained a net two-beam coupling gain of 600 cm-1 at 35 V/μm. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]