The liquid crystalline behaviour of certain analogous biphenyl and 2- and 4-phenylpyridine systems

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Lacey, D., 1977. The liquid crystalline behaviour of certain analogous biphenyl and 2- and 4-phenylpyridine systems. PhD, Nottingham Trent University.

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Abstract

In order to compare the mesomorphic properties of certain biphenyl and 2- and 4-phenylpyridine systems, seven homologous series of esters were studied, namely the:

(i)4-biphenylyl 4"-n-alkoxybenzoates;
(ii) 4"-n-alkoxyphenyl biphenyl-4-carboxylates;
(iii) 2'-fluoro-4-biphenylyl 4"-n-alkoxybenzoates;
(iv) 2'-chloro-4-biphenylyl 4"-n-alkoxybenzoates;
(v) 4-(4'-pyridyl)phenyl 4"-n-alkoxybenzoates;
(vi) 4-(2'-pyridyl)phenyl 4"-n-alkoxybenzoates;
(vii) N-oxides of 4-(4'-pyridyl)phenyl 4"-n-alkoxybenzoates;

The 4-(4'-n-alkoxyphenyl)pyridines and their N-oxides were also investigated.

The 4-biphenylyl 4''-n-alkoxybenzoates and the 4"-n-alkoxyphenyl biphenyl-4-carboxylates provided the foundation on which subsequent work was based. These isomeric esters have almost the same N-I transition temperatures but only in the former series are smectic properties observed. An explanation for this behaviour is offered.

The effect of replacing -CH= in the 2- and the 4-position of the biphenyl system by a hetero-N atom was studied by measurement of the transition temperatures of the 4-(2'-pyridyl)phenyl and 4-(4'pyridyl)phenyl 4"-n-alkoxybenzoates. The effect of the terminal N-atom of the 4-pyridylphenyl esters is to enhance the smectic properties, whereas the effect of the lateral N-atom of the 2-pyridylphenyl esters is to lower both the m.p. and the N-I transition temperatures. This latter effect is attributed to broadening of the molecule due to twisting about the inter-ring bond. For this reason the 2'-fluoro- and -chloro-4-biphenylyl 4-n-alkoxy-benzoates were investigated. In both series of halogeno-substituted biphenyl esters twisting about the 1,1'-bond results in substantial lowering of both the m.p. and the N-I transition temperature, although several of the esters exhibited monotropic mesophases .

4'-Alkyl- and 4'-alkoxy-4-cyanobiphenyls'are low melting nematogens that are used extensively in electro-optical display devices. In order to compare the effect on mesophase thermal stability of the cyano-group and the heterocyclic N-oxide function, the 4-cyano-4'-biphenylyl 4"-n-alkoxybenzoates and the N-oxides of the 4-(4'-pyridyl)phenyl 4"-n-alkoxybenzoates were studied. The latter series is essentially smectogenic but problems arose in the measurement of their mesophase transition temperatures because of the inherent thermal and photo sensitivity of these compounds. The N-oxides of the 4-(4'-n-alkoxy- phenyl)pyridines were also investigated as a type of heterocyclic 'analogue' of the 4-alkoxy-4'-cyanobiphenyls. The compounds were not as low melting (73.5-97.5°) as was hoped and showed smectic properties, some of which were monotropic. These compounds were affected by prolonged exposure to light.

Miscibility studies, optical microscopy and differential thermal analysis were used to investigate the various types of mesophase. Suggestions to explain the trends of the mesomorphic thermal stabilities of the various homologous series are offered, along with some group efficiency orders.