Charge mobility in discotic materials studied by PR-TRMC

The application of the pulse-radiolysis time-resolved microwave conductivity technique, PR-TRMC, to the determination of intracolumnar charge carrier mobilities within mesomorphic discotic materials is described. A review is given of the mobility values obtained for the large variety of materials which have been studied since the first results were reported in 1989. This includes peripherally substituted derivatives of triphenylene, phthalocyanine, porphyrin and hexabenzo-coronene. The influences of temperature, morphology and variations in the primary molecular structure are demonstrated and discussed. Both the mesomorphic and conductive properties are shown to be dramatically influenced by subtle changes in the peripheral alkyl chain structure or the core-to-chain coupling element, in addition to changes in the nature of the aromatic core itself. Mobilities as high as 1 cm 21 Vs are found in crystalline solids, and well in excess Of 0.1 cm(2)/Vs in columnar, liquid crystalline phases; values which approach those for electrons and holes in organic single crystals. The PR-TRMC mobility values are in reasonable agreement with values determined by the time-of-flight method (TOF) for the mesophase of triphenylene derivatives; the only class of discotic compounds for which the TOF method has been applied with success. In addition to its more general applicability to a range of core structures, the PR-TRMC method has the advantage that it does not require homeotropic alignment of the sample and even polycrystalline materials can be studied. A disadvantage is that only the sum of the charge carrier mobilities is measured; the sign of the major carrier cannot therefore be determined.