Electronic conduction in nematic phase of small molecules

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 $\left(n{\text{-C}}_{14}{\text{H}}_{30}\right)$. 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 $\pi$-conjugate system of the core moiety. Thus, they provide a new insight into the conduction mechanism in fluidic materials.

Figure 1

Molecular structures of 2-($4^{\prime }$-octyloxyphenyl)-6-butoxybenzothiazole and 2-($4^{\prime }$-pentyloxyphenyl)-6-dodecylbenzothiazole (referred to as 8O-PBT-O4 and 5O-PBT-12, respectively).

Figure 2

(Color online) DSC chart of 8O-PBT-O4 obtained at a cooling rate of $10°\text{C}/\text{min}$ and a heating rate of $10°\text{C}/\text{min}$, and texture images obtained using a polarized optical microscope at (a) 110 and (b) $90°\text{C}$.

Figure 3

XRD patterns of 8O-PBT-O4: (a) nematic phase at $110°\text{C}$ and (b) smectic C phase at $90°\text{C}$.

Figure 4

Positive transient photocurrents measured by applying various voltages at $115°\text{C}$. The cell thickness was $10\mu \text{m}$. (a) Insufficiently purified 8O-PBT-O4, (b) moderately purified 8O-PBT-O4, and (c) highly purified 8O-PBT-O4.

Figure 5

Negative transient photocurrents in nematic phase measured at various applied voltages. The cell thickness was $4.8\mu \text{m}$. The arrows in the figure indicate transit times for fast and slow carriers.

Figure 6

Transient photocurrents for positive carriers in nondiluted and diluted 8O-PBT-O4 (moderately purified) containing 5 and 10 mol % $n$-tetradecane as a function of time in the nematic phase. The cell thickness was $10\mu \text{m}$.

Figure 7

Mobilities for positive carriers in nondiluted and diluted 8O-PBT-O4 containing 5 and 10 mol % $n$-tetradecane as a function temperature in the nematic phase. The cell thickness was $10\mu \text{m}$.