Hexabenzocoronene liquid-crystalline phases

A new class of hexabenzocoronene derivatives, e.g. compounds (Scheme 7.13) can self-organize into liquid crystalline phases composed of molecular stacks that orient spontaneously parallel to the surface [177, 178]. Field effect transistors based on these materials show high charge-carrier mobilities, high on/off ratios, and low tum-on voltages. So far compound 44 exhibits the best field effect transistor properties achieved for a columnar discotic material. Polarized fight microscopy revealed that these materials tend to orient their columns parallel to the surface upon thermal annealing. 

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... 

Fig. 24. (a) Tetradecyl-modified hexabenzocoronenes to allow mlnmnar self-organized structure, (b) The mobility as a function of temperature, showing the effect of the phase transition from the crystalline solid to the hexagonally packed columnar liquid crystal at 65°C. Adapted from A. M. van de Craats, J. M. Warman, K. Mullen, Y. Geerts, and J. D. Brand, yldu Mater. 10,36(1998). 

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