Transient photoconduction in discotic liquid crystals

The hole transport properties of ester substituted hexaalkyloxytriphenylene s are investigated and compared with the materials without ester substituen ts. The high hole mobilities of the recently investigated discotic liquid c rystals of the hexaalkyloxytriphenylene-type are restricted to the very sma ll temperature range of their mesophase. It is extended substantially by su bstitution of one ester group, as this hinders crystallization and results in glass formation. It is found that the substitution of an ester group alt ers the temperature and field dependence of the mobility completely. In the ester substituted compounds the mobility mu is not independent of temperat ure 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 l ocal electric held and leads to disorder dominated charge carrier hopping a s the prevailing transport mechanism.