Synthesis and characterisation of novel discotic liquid crystals based on triphenylene and phthalocyanine cores
The discovery of discotic liquid crystals in 1977 by Chandrasekhar has led to an ever
increasing interest in the preparation of new discotic mesogens. Research on the
potential application of such systems has, however, been held back mostly by synthetic
and purification difficulties to obtain enough of the required pure materials.
The work described in this thesis (Chapter 2) concerns the synthesis of macrodiscotic
structures which led to the formation novel discotic liquid crystalline systems. Nonplanar
substituted phthalocyanines have been described by Simon and were found to be
mesogenic. We expected that planarising these systems (formation of triphenylene)
would have the effect of changing both the absorption (shift to longer wavelength due
to extra conjugation) and the mesogenic properties (due to formation of large discs).
The syntheses of triphenylenophthalocyanines were achieved using palladium
catalysed aryl-aryl cross-coupling as key step in the syntheses. The end materials were
characterised and tend to be mesogenic.
After the successful synthesis of the triphenylenophthalocyanines, we were in a
position to further extend the n-system of our phthalocyanines and the molecules
targeted were perylenophthalocyanines (Chapter 3). The syntheses were achieved
through a cyc1oaddition reaction as key step. In this case, however, none of the
perylenophthalocyanines were liquid crystalline. 3: 1 phthalocyanines were also
prepared and tend to be non-mesogenic.
A senes of mixed hexyloxy-Ihexylthio-triphenylenes has also been synthesised to
determine the structural features governing the formation of helical mesophases shown
by HHTI (Chapter 4). It has been shown that four sulphide substituents (plus two
alkoxides) are required to induce the additional, more ordered phase. Furthermore, the
position of substitution is crucial for its formation. Also, the synthesis and
characterization of a series of triphenylenes has been described in which 2, 4 or 6 of the
alkyloxy chains of parent hexa(hexyloxy)triphenylene are replaced by heptyl chains
(giving mixed alkyl-alkyloxy triphenylenes). This series provides a direct comparison
to the derivatives which gives to the rare helical phase. Finally, the synthesis of a few
novel triphenylene molecules expected to be on the boundaries between the traditional
rod-shaped and disc-shaped structure were achieved. Some of these compounds formed