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Photoconductivity Measurements in the Discotic Columnar Phase of a few Anthraquinone Derivatives

It has recently been shown that the structure of the columnar phase of discotic liquid crystals is suitable for fast transport of photogenerated charge carriers. Although the magnitude of the charge carrier mobility exhibited by these systems is still small compared to those in organic single crystals, the fluidity and the resulting self-healing nature of the liquid crystalline columnar phases hold promises for potential applications. In this paper we report on steady-state photoconductivity measurements carried out on three anthraquinone derivatives exhibiting the columnar phase. Upon illumination of the sample with UV light the current through the sample increases abruptly and reaches a photo-stationary value within a short span; the fastest observed response time is 17 ms. It was found that the anisotropy of the photoconductivity, defined as the ratio of the photocurrent along and perpendicular to the column axis, reaches a value of about 450. We also discuss the relation between the chemical structure of the molecule and the maximum value of the photocurrent observed.