Transient photoconduction in discotic liquid crystals

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 field dependence of the mobility completely. In the ester substituted compounds the mobility mu is not independent of temperature but follows a ln mu proportional to 1/T-2 law. We attribute this to the dipole moment of the ester group which causes random fluctuations in the local electric field and leads to disorder dominated charge carrier hopping as the prevailing transport mechanism.