Columnar Liquid Crystalline Semiconductors
The advantages and disadvantages of the various methods used to study semiconducting discotic liquid crystals are surveyed. Comprehensive tables are provided of the charge-carrier mobilities of discotic liquid crystals. Interpretations of these mobilities
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Columnar Liquid Crystalline Semiconductors Richard J. Bushby and Daniel J. Tate
3.1 Introduction In a typical discogen the aromatic core of the molecule is surrounded by an annulus of alkyl chains. The most common liquid crystal phases that they form are columnar in which the molecules are stacked one on top of another like a pile of coins (Fig. 3.1). The columns are arranged on a regular two-dimensional lattice. Whereas there may be short-range positional order of the discs along the column, there is no correlation of the positions of the discs between the columns and thus, no threedimensional order. The alkyl chains are fluid and disordered and a static image like that shown in Fig. 3.1, fails to convey the highly dynamic nature of the phase. For the Colh phase of the ethers of triphenylene shown in Fig. 3.1 the interconversion of the trans-gauche conformations of the alkyl chains occurs with a frequency of the order of 1010 s1 , the rotation of the aromatic cores has a time constant of the order of 108 s1 [1] and the exchange of discs between neighboring columns with a frequency of the order of 105 s1 [2]. Like most other molecular materials, pure discotic liquid crystals are insulators because the intrinsic concentration of charge carriers is very low. To make them conducting, holes or electrons have to be injected. This can be achieved by chemical doping, radiolysis, photolysis or (at sufficiently high potentials) from the surface of an electrode. The holes or electrons that are injected are essentially localized on the aromatic cores so that each column shown in Fig. 3.1 acts like an insulated molecular wire. The charge carriers can hop from the aromatic core of one molecule to the aromatic core of another along the stack, but R.J. Bushby () School of Chemistry, University of Leeds, Leeds, LS2 9JT UK e-mail: [email protected] D.J. Tate Organic Materials Innovation Centre, School of Chemistry, University of Manchester, Manchester, M13 9PL UK e-mail: [email protected] R.J. Bushby et al. (eds.), Liquid Crystalline Semiconductors, Springer Series in Materials Science 169, DOI 10.1007/978-90-481-2873-0 3, © Springer ScienceCBusiness Media Dordrecht 2013
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R.J. Bushby and D.J. Tate
Fig. 3.1 Molecular structure and schematic of the conducting columnar Colh phase of a HATn triphenylene-based discogen 1 (n is the number of carbons in each side-chain. Hence in HAT5 there are six pentyloxy side-chains)
the columns are insulated from each other by the sheath of alkyl chains. For the Colh phase of HATn, the hopping frequency is >109 s1 . An important consequence of the highly dynamic, fluid nature of columnar phases is that defects that appear within a given ‘molecular wire’ rapidly disappear: the column self-heals. However, there are other advantages in using columnar liquid crystalline semiconductors. Liquid crystals are partially ordered molecular materials whose properties, including their conduction properties, lie between those of (fully ordered) crystalline and (fully disordered) amo
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