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 rodlike liquid crystals and for which the highest hole mobility in the smectic A (SmA) phase ever achieved was reported. We found that 7O-PBT-S12 exhibited three crystal phases, one of which appeared in a limited temperature range of 10 degrees just below the phase transition temperature from the SmA phase. In this crystal phase, nondispersive transient photohole currents were observed in time-of-flight experiments, and its hole mobility was determined to be 8 x 10(-3) cm(2)/Vs, slightly higher than that reported previously in the SmA phase. For the SmA phase, however, the hole mobility was 1 x 10(-4) cm(2)/Vs. Furthermore, we established the electron transport in the SmA phase of purified 7O-PBT-S12, whose mobility was the same as the hole mobility in that phase. In order to confirm generality of the new findings in 7O-PBT-S12, we investigated the 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 mistake in the previous report and give an idea of what a typical mobility in the SmA phase is. On the basis of these results, we discuss what determines the charge carrier mobility in smectic mesophases.
We investigated charge-carrier transport in the nematic phase of small molecules such as 2-phenylbenzothiazoles by time-of-flight experiments, in which the conduction mechanism has been considered to be ionic. As a result, we established the hole and electron transports in the nematic phase of highly purified samples: we found that there were two transits, namely, fast and slow transits, in less pure samples; the slow transit was attributed to ionic conduction originating from trace amounts of impurities and the fast transit was attributed to electronic conduction whose attribution was elucidated by mobility changes in the diluted samples with a hydrocarbon of n-tetradecane ͑n-C 14 H 30 ͒. From these results, we conclude that the intrinsic conduction mechanism in the nematic phase of small molecules is ambipolar and electronic, irrespective of the size of the -conjugate system of the core moiety. Thus, they provide a new insight into the conduction mechanism in fluidic materials.
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