Transport columnar discotics

In contrast it was shown in the past years that discotic liquid crystals like hexapenty-loxytriphenylene (HPT) 23 have carrier mobilities up to 10 cm A s in the liquid crystalline I o phase [10]. Here, the disc-shaped molecules are ideally stacked above each other and therewith allow a fast carrier transport. The discotic hexathioether 24 with a highly ordered helical columnar (H) phase exhibits mobilities up to 10 cm A s, which are almost as high as in organic single crystals [11]. 

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... 

Bleyl, I., Erdelen, C., Schmidt, H.W., Haarer, D. One-dimensional hopping transport in a columnar discotic liquid-crystalline glass. PWlos. Mag. B 79,463-475 (1999)... 

Zhang, Y.-D., et al. Columnar discotic liquid-crystalline oxadiazoles as electron-transport materials. Langmuir 19(16), 6534-6536 (2003). doi 10.1021/la0341456... 

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... 

PAH chemistry is of practical as well as theoretical interest. PAHs can be regarded as well defined subunits of graphite, an important industrial material, which is so far not totally understood at the macroscopic level. In this context, it is our aim to delineate the molecular size at which the electronic properties of PAHs converge to those of graphite. Furthermore, alkyl substituted derivatives of hexabenzocoronene (HBC) form discotic mesophases and, therefore, provide opportunities for materials which allow one-dimensional transport processes along their columnar axis [83,84]. Their application for photovoltaics and Xerox processes is also of current interest. 

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. 

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... 

In 1993, a research group at the University of Bayreuth confirmed fast electronic transport in the discotic columnar phase of hexaalkoxytriphenylene using the time-of-flight method [80], which made a strong impact on liquid... 

Understanding the organization of disc-shaped mesogens in columnar phases is of practical interest because of their useful optoelectronic applications [168-170]. It will be instructive to explore the role of fluctuations in influencing transport properties of thermotropic discotic liquid crystals—in particular, in the columnar phase. 

The most obvious family of compounds able to induce columnar phases is that constituted by the macrocyclic compounds which often possess a discoid molecular shape. The ability of macrocyclic systems to stack and to self-assemble into columnar mesophases confers on these systems the properties required for potential utilization in molecular devices based on photonics, electronics, or low-dimensional ionic transport. The search for new discotic mesogens is thus still of current interest, in order to improve their chemical, thermal, and photophysical stabihty, and new families of discogenic materials appear regularly. 

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... 

The unique transport properties of columnar phases of discotic liquid crystals (well... 

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]. 

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