Synthesis and investigation of triphenylene twins and dyads

Alsahli, Ahad (2020) Synthesis and investigation of triphenylene twins and dyads. Doctoral thesis, University of East Anglia.

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The work set out in this thesis focuses on the synthesis and investigation of triphenylene twins and diads. Since the discovery of discotic liquid crystals by Chandrasekhar in 1977, columnar systems have been much investigated due to their practical applications. However, the nematic phase which some discotic liquid crystals exhibit have been less thoroughly investigated, despite their importance in liquid crystal displays as optical compensating films. Our group has been particularly interested in such systems which have triphenylene at their cores. These are synthetically versatile and have molecular robustness, a useful attribute when they are used within devices. Numerous triphenylene twins have previously been synthesised by Cammidge et al. The current research therefore consists of three main phases: firstly, the synthesis of a series of diads linking two triphenylene units via alkyne bridges and an investigation of their liquid crystal properties, secondly, the synthesis of triphenylene-BODIPY hybrids and thirdly, the synthesis of a compound using diaminoisoindolene in a triphenylene core.

For the first phase of the work, twins were prepared through links at the 1,2-, 1,3- anmd 1,4-benzene sites plus through a 2,5-thiophene. Diiodobenzene and diiodothiophene were used as precursors to link between triphenylene units with Sonogashira coupling reactions used in the key steps. The difficulty in this series which arose was that the monoacetylene triphenylene starting material is reactive and tended to form a homo-coupling product, so purification proved challenging. However, the full characterisations have been obtained and liquid crystal investigation showed both columnar and nematic behaviour in 1,3-ditriphenylene benzene and 2,5-ditriphenylene thiophene. However, the linear diad 1,4-ditriphenylene benzene shows only columnar mesophase, and we rationalise this observation by noting that the separation between triphenylene units is appropriate for supporting columnar organisation. The diad 1,2-ditriphenylene benzene shows no liquid crystal behaviour, likely because the geometry and triphenylene spacing in the system prevents any efficient cofacial organisation.

The second phase of our work turned to explore the possibility of using (aza)BODIPY to synthesise symmetrical and unsymmetrical triphenylene-BODIPY hybrids. Precursor aminoisoindolenes were prepared again from monoacetylene triphenylene by reaction with ortho bromobenzamidine. The synthesis of a further diad series was undertaken by using a variety of aminoisoindolenes to produce aza-dipyrromethene product. The new twinned structures were characterised and none were found to display liquid crystal phases. Closer examination of their 1H NMR spectra, alongside molecular modelling, showed that the preferred arrangement in such structures was helical. This is an interesting and unusual arrangement but is unlikely to support liquid crystal behaviour.

The final phase sought to prepare a twinned, co-planar structure incorporating both triphenylene and azaBODIPY. Initially the aim was to couple diacetylene triphenylene with bromo benzamidine by the same strategy had been used earlier but it proved unsuccessful. The strategy was changed and the diacetylene triphenylene reacted to produce dibenzonitrile triphenylene, but further reaction was not attempted due to lack of time. However, the new material was found to show mesophase behaviour.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Chemistry
Depositing User: Chris White
Date Deposited: 21 Jul 2021 09:42
Last Modified: 21 Jul 2021 09:42

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