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Title: Synthesis and evaluation of reactive bimesogens for optical compensating films
Author: Pandya, Ritesh V.
ISNI:       0000 0001 3466 4778
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2008
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Optical compensating films form a key component of modem day flat screen displays. In this application, materials with negative birefringence and negative dispersion are good candidates. Such materials, however, are rare. This thesis describes our investigation into polymerisable and non-polymerisable structural analogues of a lead cyclohexyl tolane bimesogen originally reported by Thrumes et al. It was reported to exhibit negative birefringence. New versatile synthetic procedures are described which provide access to a series of novel materials containing the same core structure of cyclohexylethyl tolane. The most successful synthetic route towards the formation of these structural analogues used the initial attachment of a methoxyphenyl moiety to the cyclohexyl fraction to obtain the cyclohexylethyl tolane precursor via formation of 1,3-dithiane derivatives. Palladium catalysed aryl-alkyne crosscoupling was then employed as second key step. Thermal investigation (polarising optical microscopy and DSC analyses) of the series of synthesised tolane materials revealed that the minimum structural feature required to exhibit mesophase behaviour was the trans-4-pentylcyclohexylethyl moiety as its absence resulted in failure to achieve a LC state. Optical experiments to evaluate birefringence were performed using formulations of the tolane materials doped into a nematic LC host such as RM 257 or ZLI-4792. The birefringence measurement results were unexpected and disappointing as all materials exhibited positive birefringence. This led to re-examination of Thrumes' original compound. The pure, synthesised material was also found to exhibit positive birefringence. Its mesophase behaviour was also found to be different to the original report. The original design principle (introduction of a polarisable functional group perpendicular to molecular long axis) was therefore probed by appending the polarisable acetylene group to a simple cyclohexyl anisole derivative (monomesogen). This material exhibited the expected negative birefringence, albeit of low magnitude, indicating that the design principle is sound but alignment of the bimesogen structures does not occur efficiently in the manner predicted.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available