The synthesis and properties of certain heterocyclic mesogens

Tools

Saint, S., 1998. The synthesis and properties of certain heterocyclic mesogens. PhD, Nottingham Trent University.

Preview

Text
10290090.pdf - Published version
Download (23MB) | Preview

Official URL: http://gateway.proquest.com/openurl?url_ver=Z39.88...

Abstract

This thesis reports the synthesis and properties of three-ring thiophene- and pyrimidine-based liquid crystals in order to evaluate their usefulness as potential additives in commercial liquid crystal mixtures. Confidentiality restricts the reporting of actual measurements evaluated during the mixture investigations. Two homologous series, namely: 5-n-alkanoyl-5"-n-alkyl-2,2':5',2"-terthienyls (series I) and their corresponding reduced homologues 5,5"-di-n-alkyl-2,2';5',2"-terthienyls (series II) were prepared.

Members of series I exhibit the SA phase alone whereas members of series II exhibit the CrG phase. Unfortunately, when incorporated in commercial mixtures, members of series I and II cause photochemical degradation. The thiophene strategy was abandoned and new work focused on three- ring pyrimidine-based compounds.

Twelve series of fluorinated three-ring pyrimidine-based compounds were successfully prepared via palladium-catalysed boronic acid cross-coupling technology utilising 5-bromo-2- iodopyrimidine (41) as a key intermediate. However, initially the first pyrimidine compound, i.e., 2-(4- n-hexyloxyphenyl)-5-(2,3-difluoro-4-n-heptylphenyl)pyrimidine (38c) was prepared utilising the classical ring closure/condensation technique which was extremely troublesome.

(series IIIa-c) 2-(4-n-alkoxyphenyl)-5-(2,3-difluoro-4-n-alkylphenyl)pyrimidines
(series IV) 2-(2,3-difluoro-4-/!-alkoxyphenyl)-5-(4-n-alkylphenyl)pyrimidines
(series V) 2-(4-n-alkylphenyl)-5-(2,3-difluoro-4-n-alkoxyphenyl)pyrimidines
(series VI) 2-(2,3-difluoro-4-n-aIkylphenyl)-5-(4-n-alkoxyphenyl)pyrimidines
(series VII) 2-(4-n-alkylphenyl)-5-(2,3-difluoro-4-n-alkylphenyl)pyrimidines
(series VIII) 2-(2,3-difluoro-4-n-alkylphenyl)-5-(4-n-alkylphenyl)pyrimidines
(series IX) 2-(4-n-alkoxyphenyl)-5-(2-fluoro-4-n-alkylphenyl)pyrimidines
(series X) 2-(2-fluoro-4-n-alkoxyphenyl)-5-(4-n-alkylphenyl)pyrimidines
(series XI) 2-(4-n-alkylphenyl)-5-(2-fluoro-4-n-alkoxyphenyl)pyrimidines
(series XII) 2-(2-fluoro-4-n-alkyIphenyl)-5-(4-n-alkoxyphenyl)pyrimidines
(series XIII) 2-(4-n-alkylphenyl)-5-(2-fluoro-4-n-alkylphenyl)pyrimidines
(series XIV) 2-(2-fluoro-4-n-alkylphenyl)-5-(4-n-alkylphenyl)pyrimidines

The mesomorphic properties are dependent upon; the number of fluoro-substituents; nature of the terminal groups and; disposition of the pyrimidine ring within the central molecular core. High order crystal smectic phases are not observed and the phase types are restricted to N, SA and Sc- Generally, dialkyl compounds do not exhibit the Sc phase.