Charge transport, columnar discotics

The most extensive series of experiments using the PR-TRMC technique in recent years has been directed towards an understanding of the charge transport properties of discotic materials. In these materials the individual molecules, consisting of an aromatic core with several peripherally substituted alkyl chains, columnarly stack with the columns packed in a well-organized, two-dimensional lattice. The types of aromatic cores investigated are illustrated in Fig. 5. 

Discotic LC, based on the molecules having disc-like shapes [22, 23, 74], have attracted much attention in recent years owing to their inherent ability to form self-organized columnar structures, which can provide superior charge transport ability... 

We start by reminding ourselves that columnar discotic liquid crystals are comprised of disordered stacks (1-dimensional fluids) of disc-shaped molecules arranged on a two-dimensional lattice (Fig. 1) [1]. This structure imparts novel properties to these materials from which applications are likely to stem. One such property is the transport of charge along the individual molecular stacks [2-7]. The separation between the aromatic cores in, for example, the hexa-alkoxytriphenylenes (HATn), the archetypal columnar discotic mesogen, is of the order of 0.35 nm, so that considerable overlap of n orbitals of adjacent aromatic rings is... 

As discussed in Chaps. 3 and 4, (columnar) discotic liquid crystals are oriented in columns separated by molten aliphatic chains and, consequently, they can conduct charge efficiently along the channels in one dimension. The organization of the different phases is described elsewhere [19, 20] and the efficiency of charge transport can be directly related to the short intermolecular spacing and order of different types of mesophase, with few exceptions [21]. For example, hole mobility is higher in ordered, rather than disordered, columnar phases and even higher in helically-ordered phases where molecular rotation is suppressed about the columnar axis [22], Some mesomorphic derivatives of hexabenzocoronene, for example hexaphenyl-substituted hexabenzocoronene (HBCn, see Table 8.2 for chemical structures of all discotic materials discussed here) have hole mobilities... 

In general, for side chain liquid-crystalline polymers, macroscopic molecular alignment is not easy and therefore clear evidence of electronic charge carrier transport was confirmed first in liquid crystals with low molecular weight. In the 1990s, fast electronic conduction was verified in discotic columnar phases of triphenylene derivatives [79,80] and hexabenzocoronene derivatives [81,82] as well as smectic phases of 2-phenylbenzothiazole [83, 84] and 2-phenylnaphthalene derivatives [85], as shown in Fig. 14. Carrier... 

Charge carrier transport in mesophases is quite unique compared to that in amorphous and crystalline materials. The mobility hardly depends on temperature at room temperature and above as shown in Fig. 2.6. Furthermore, it hardly depends on electric field either, as shown in Fig. 2.7. This behavior is not limited to one particular class of liquid crystals, but is probably a general characteristic of charge carrier transport in mesophases above room temperature. In fact, this behavior is also found in discotic columnar mesophases [11, 39,40,42]. 

US with a great benefit for device applications requiring large-areas. For the discotic columnar phases, there are few reports of the effect of structural defects on charge carrier transport. Unlike the smectic mesophases, where two dimensional transport takes place, it is likely that structural defects in a column may affect the carrier transport properties seriously, because a carrier has to pass along a column without detouring to adjacent columns. From this point of view, relatively low mobility in the columnar ordered phase of triphenylene derivatives, where the intermolecular distance is as small as 3.5 A, may be explained by the structural defects or disorder of molecular alignment in the columns as described above. 

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