Charge‐carrier transport in organic materials is the fundamental physical process behind devices such as laser printers. Charge‐carrier mobility data are presented for hexabutyloxytriphenylene (H4T), which exhibits an unusually high charge‐carrier mobility, which can be traced to the formation of a plastic discotic phase. The Figure shows the normal discotic hexagonal texture with six‐fold symmetry observed for H4T at 144°C; this symmetry disappears at lower temperatures (see also the cover). magnified image

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The hole transport properties of ester substituted hexaalkyloxytriphenylenes are investigated and compared with the materials without ester substituents. The high hole mobilities of the recently investigated discotic liquid crystals of the hexaalkyloxytriphenylene-type are restricted to the very small temperature range of their mesophase. It is extended substantially by substitution of one ester group, as this hinders crystallization and results in glass formation. It is found that the substitution of an ester group alters the temperature and Ðeld dependence of the mobility completely. In the ester substituted compounds the mobility k is not independent of temperature but follows a ln k P 1/T 2 law. We attribute this to the dipole moment of the ester group which causes random Ñuctuations in the local electric Ðeld and leads to disorder dominated charge carrier hopping as the prevailing transport mechanism.

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