We have reinvestigated the charge carrier transport properties in a liquid crystal of 2-(4-heptyloxyphenyl)-6-dodecylthiobenzothiazole (7O-PBT-S12), for which the electronic conduction was first established in rodlikeliquid crystals and for which the highest hole mobility in the smectic A (SmA) phase ever achieved wasreported. We found that 7O-PBT-S12 exhibited three crystal phases, one of which appeared in a limitedtemperature range of 10C just below the phase transition temperature from the SmA phase. In this crystalphase, nondispersive transient photohole currents were observed in time-of-flight experiments, and its holemobility was determined to be 8 x 10^-3 cm2/Vs, slightly higher than that reported previously in the SmAphase. For the SmA phase, however, the hole mobility was 1 x 10^-4 cm2/Vs. Furthermore, we establishedthe electron transport in the SmA phase of purified 7O-PBT-S12, whose mobility was the same as the holemobility in that phase. In order to confirm generality of the new findings in 7O-PBT-S12, we investigatedthe carrier transport properties of its derivative having a short hydrocarbon chain, 2-(4-heptyloxyphenyl)-6-butylthiobenzothiazole (7O-PBT-S4), and obtained comparable results. The present results correct a mistakein the previous report and give an idea of what a typical mobility in the SmA phase is. On the basis of theseresults, we discuss what determines the charge carrier mobility in smectic mesophases.
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