Other Applications

Measurements of conductivity and photoconductivity show that triphenylene mesophases behave as one-dimensional semiconductors.34, 35, 36

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1.5 Dimers

Recently the focus in synthesis of triphenylene derivatives has shifted to the formation of dimer or twinned compounds. A definition of dimer in relation to liquid crystals means two mesogenic groups that are linked by either a flexible or rigid spacer.37 The term ‘diad’ will be used to describe two triphenylene units linked by one spacer, while the term ‘twin’ will be used to describe two triphenylene units linked by two spacers (Figure 1.19). There are many examples of diads in the literature, however only a few examples exist of twinned triphenylene compounds.

Figure 1.19: Structures to show meaning of terms ‘diad’38 and twin’39

In 1995 the diad and triad in Figure 1.20 were reported,38 they both formed glassy mesophases that slowly crystallised at room temperature. The formation of these ordered glassy films shows that triphenylene dimers display some properties of polymers.

25 In 1999, Boden et al. did some further work on these diads and discovered more features of their liquid crystalline proprties.40 They found that the length of the spacer greatly influences the behaviour of the diad, if the spacer is too short then the diad will not be liquid crystalline (Figure 1.21). The effect on the formation of a glassy phase was also seen, with the lifetime of the glassy state decreasing as the length of the spacer increases.

Figure 1.21: DSC data for triphenylene diad above

A triphenylene diad linked by a phenylene dicarbamate unit was synthesised 35 (Figure 1.22), that also had good film forming properties, compared to the monomer.41 _{This was thought to}

be due to the prescence of the flexible spacer and the hydrogen bonds forming between diads. This compound was found to have application in OLEDs due to the charge carrier mobility and good hole transporting properties of the diad.

n = 3 Cr 81 I 5 Cr 98 I 7 Cr 69 I 8 Cr 58 Colh 91 I Tg 35 (ca. 18 hr) 9 Cr 72 Colh 92 I Tg 35 (ca. 15 hr) 10 Cr 50 Colh 104 I Tg 35 (ca. 11 hr) 12 Cr 68 Colh 107 I Tg 35 (ca. 12 hr) 16 Cr 41 Colh 84 I Tg 35 (ca. 3 hr)

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Figure 1.22: Triphenylene diad linked by a phenylene dicarbamate unit.41

In 2009 a range of diads linked by acetylene units were synthesised, the results showed that the dimers formed more stable, ordered columnar mesophases that the corresponing monomers and also improved semiconductive properties.42 They also found that the high symmetry of 36 enhanced the π-π interaction between the triphenylene cores which also improved the formation of ordered columnar mesophases.

Figure 1.23: Acetylene linked triphenylene diads.42

More recently a triphenylene diad has been prepared by click chemistry, linking the triphenylene units via a triazole.43 Neier et al. synthesised two diads, one linked by -CH2-

unit, and the other linked by a phenyl group; the idea being that the added aromatic group would aid the formation of columnar mesophases. However, on analysis of the diads they found that only the -CH2- linked dimer showed mesophase behaviour. The group postulated

27 the reason stated by Kumar that the rigidity of the extra aromatic ring reduces the flexibilty the spacer provides, making the stacking of the triphenylene cores more difficult.44

Figure 1.24: Triazole linked triphenylene diad.43

As these examples show, triphenylene diads show more ordered columnar mesophases over a wider temperature range than their counterpart monomers. The flexible spacer increases the order in the columnar mesophase by avoiding slippage from columns and fluctuation in the column. These properties led to the study being extended into the synthesis of triphenylene twins.

28 In 1998 the spiro twins 37 were synthesised,45 it was thought that the twist in the spiro centre may stop the formation of columnar mesophases. However, it was found that this was not the case, columnar mesophases were seen and the compounds shown to have potential application in photo conductivity.

Figure 1.25: Triphenylene spiro twins 45

Crown-ether linked triphenylene twin and triad were synthesised in 2010 by Cammidge et

al.39 They found that the twin compound 38 showed no mesophase behaviour, while the more flexible triad 39 showed columnar mesophases, with potential application in optoelectronic device applications.

29 Antiaromatic triphenylene twins bridged by acetylenes were also synthesised in 2010 by Cammidge et al.46 They originally followed a similar structure to the crown ether twin 38, linking the triphenylenes through the ortho-sites, however they found that the twinned product readily decomposed through pericyclic reactions. Therefore, they synthesised the twin linked through 3,6-positions which avoided the decomposition problems but kept the antiaromatic core.

Figure 1.27: Antiaromatic triphenylene twin linked by acetylene bridges

This twin was shown to form a stable, planar, board-like molecule that formed a nematic mesophase on heating. The crystal structure showed an interesting property in how the molecules are packed in the crystal, column formation is not possible as this would leave a free space from the void in the twin, therefore the twins stack in an overlapping formation, as can be seen in the cartoon illustration in Figure 1.28. This accounts for the observed nematic mesophase, and means that communication is possible both through the columns and also along the rows.

30 Another class of acetylene bridged triphenylene twins was synthesised in 2011 (Figure 1.29).47 The phenyl-substituted triphenylene twin formed well ordered 2D-structures, and was shown to be more conjugated in the ground state than in the excited state. This was postulated to be due to the constrained shape causing a bend in the diacetylene bridge.

Figure 1.29: Phenyl-substituted triphenylene twin linked by acetylene units.

The idea of creating a void region in the centre of the twin to force nematic behaviour lead to further functionalised twins to be targeted. In 2012, Cammidge et al. published the synthesis of triphenylene twins linked through thiophene bridges, structure 41.48 Thiophene was chosen as it would allow communication through the twin, and would form a less strained twin than other aryl groups, for example phenyl. The twin showed only a nematic mesophase, and was again a formally antiaromatic system, which showed in its strong Stokes-shifted fluorescence at around 500 nm.

31 Most recently, Cammidge et al. published the synthesis of a triphenylene twin linked by pyrrole.49 The twin was targeted as it was thought to have a porphyrin-like structure, and would therefore have similar properties such as the ability to coordinate metals. However, the crystal structure showed that the molecule adopts a strain-free configuration where the pyrroles face in opposite directions. It was found that the twins shared more properties with dipyrromethenes, although whereas dipyrromethenes are generally unstable in air/light, the triphenylene twin had good stability in both. The twin was again formally antiaromatic, however this time the absorption spectra were similar to the spectra for the triphenylene diads, suggesting that the effect of conjugation is localised leading to little antiaromatic behaviour being observed. The twin was stable beyond 300 o_{C therefore observation of any mesophase}

formation was not possible.