Nematic reactive mesogens

Fig. 8.8 Schematic of a crosslinked polymer network. The nematic reactive mesogens have photopolymerisable groups at each end of an extended aromatic core with semiconducting properties. The mesogens are deposited as a thin film by solution processing. They are polymerised and crosslinked either thermally or on irradiation with ultraviolet light...

Researchers at the University of Hull undertook comparisons between model liquid crystalline compounds and related reactive mesogens, which form nematic mesophases using two different conjugated cores containing the fluorene moiety, see Chaps. 7 and 8. In one study, based on a biphenyl-bithiophene-fluorene(compound HI), core time of flight measurements were carried out in the nematic phase of the model compound, the nematic phase of the diene substituted RM and on the crosslinked compound at room temperature [34], They found a large drop in hole mobility between the model compound and the nematic phase of the RM from 8 X 10 " to 1.5 X 10 cm s There was a small recovery in hole mobility... 

We described in 2001 the synthesis of fluorene trimers, pentamers and oligomers with pendant acrylate units [61] (Fig. 7.18). The fluorene reactive mesogens exhibit broad mesophases, for example, the pentamer has a nematic phase between —10 and 123°C, which makes it ideally suited for orientation experiments. A 25 nm thick layer of the pentamer deposited onto rubbed polyimide exhibits a photoluminescence orientation ratio of 25 1. This ratio decreases to 9 1 for an analogous layer of... 

In 1972 the photovoltaic effect was first demonstrated in devices with nematic liquid crystals by means of ionic conduction [36]. Although electronic charge transport was widely researched in these materials [37, 38], it was not until 2006 that electronic conduction was first applied to photovoltaics in nematics [39]. A novel approach based on reactive mesogens was used to create a D-A bilayer with a distributed interface. Reactive mesogens are polymerisable equivalents of small molecule LCs, but with two additional polymerisable groups, one at each end of a flexible aliphatic spacer attached to the aromatic core. Chapters 2 and 5 discusses charge transport in these materials. Figure 8.8 illustrates the photopolymerisation of such molecules. 

A liquid crystal composite approach was used to provide a distributed interface to vertically separate D and A films in an OPV device [39, 47]. The concept is illustrated in Fig. 8.9. A nematic polymer network with a porous surface with sub-micron scaled grooves and electron-donating properties was prepared by pho-topolymerising a thin film containing a blend of the reactive mesogen, FTl, which has a fiuorene-thiophene structure, and a non-polymerisable analogue, compound FT2. Photopolymerisation leads to phase separation of the polymerised and the non-polymerised materials, the latter of which is removed by washing in a suitable solvent [39]. The chemical structures of the nematic materials discussed here are... 

Nematic photo-crosslinkable polymers [108] and reactive monomer mixtures [109] can be easily oriented in a magnetic field. Due to their diamagnetism the mesogens usually orient parallel to the magnetic field in the LC phase. 

LCEs forming nematic, cholesteric, and smectic phases have been synthesized, with various chemical structures comprising of both side-chain and main-chain types. Because of the multitudes of preparation techniques, I will here focus on our synthetic methods [7]. Reactive cyanobiphenyl monoacrylate mesogens were mixed with diacrylate cross-linkers to produce a photoinitiated side-chain acrylate NE (Fig. 10.2). As it will be described later, the LCE fixes the liquid crystal orientation at the time of cross-linking, and it can be said that this orientation... 

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