Functional Liquid-Crystalline Polymers for Ionic and Electronic Conduction - Liquid Crystalline Functional Assemblies and Their Supramolecular Structures - page 162

Fig. 13 a Charge carrier hopping between chromophores connected to the side chain, and
the effect on molecular alignment of the introduction of mesogenic groups. b Resulting
energy level distribution of the chromophores. Molecular alignment should result in nar-
rower distribution of molecular orbital energy levels
mobilities in these columnar and smectic phases are on the order of 10 -4 to
10 -1 cm 2 V -1 s -1 , which are several magnitudes larger than those of an elec-
tronic carrier in amorphous polymer semiconductors and of ions in isotropic
liquids of organic compounds with low molecular weight (typically on the
order of 10 -7 to 10 -5 cm 2 V -1 s -1 ). It should be noted that layer or colum-
nar structures in which molecules align and stack are indispensable for fast
electronic carrier transport, in contrast to the nematic phase which exhibits
ionic conduction [86, 87] or relatively slow electronic charge carrier trans-
port [88-90]. In fact, carrier mobility has clear relationships with molecular
order within smectic layers or columns as shown in Fig. 15. Carrier mo-
bility was on the order of 10 -4 cm 2 V -1 s -1 in the smectic A or C phases
of 2-phenylnaphthalene and dialkylterthiophene derivatives where no long
range order existed within the smectic layer. However, it was on the order of
10 -3 cm 2 V -1 s -1 in the smectic B or F phases, which had a hexagonal bond
order within the smectic layers- It increased to around 10 -2 cm 2 V -1 s -1 in the
smectic E or G phases, where there was long range order in the molecular
position within the layers [91]. The same tendency was also observed in the
various columnar phases of the triphenylene derivatives [92].
A remarkable characteristic of carrier transport property in columnar and
smectic phases is that carrier mobility is independent of temperature and
electric field above room temperature [80, 83, 85]. A clear reason for this has
not yet been confirmed although some models based on small polaron theory
have been proposed [93].