The synthesis and properties of substituted heterocyclic mesogens

• Main Author: O'Donovan, Jacqueline Patricia
• Published: Kingston University 1993
• Subjects: 547; Chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.385770

The initial aim of this work was to synthesise novel pyrazine based esters where the pyrazine ring occupies a central position within the ester molecule. Four novel series of pyrazine based esters were prepared. (1) 5-Phenylpyrazin-2-yl 4-n-alkoxyphenylcarboxylates. (2) 5-(4[sup]/-n-Propylphenyl)pyrazin-2-yl trans-4-n-alkylcyclohexylcarboxylates. (3) 5-(4[sup]/-Ethoxy-3[sup]/-fluorophenyl)pyrazin-2-yl 4-n-alkyloxyphenylcarboxylates. (4) 5-(3[sup]/-Fluoro-4[sup]/-methoxyphenyl)pyrazin-2-yl 4-n-alkoxyphenylcarboxylates. Comparison of the liquid crystal properties of the pyrazine based esters with the analogous phenyl esters was carried out. For comparison with the pyrazine ester series (2) and (3) the analogous phenyl ester series (5) and (6) were prepared. (5) 4-(4[sup]/n-Propylphenyl)phenyl trans-4-n-alkylcyclohexylcarboxylates and, (6) 4-(4[sup]/-Ethoxy-3[sup]/-fluorophenyl)phenyl 4-n-alkyloxyphenylcarboxylates. In general the substitution of the central phenyl ring in the ester molecules with a pyrazine ring resulted in: (i) A depression in the nematic thermal stability. (ii) A depression in the smectic thermal stability to a greater extent than that of the nematic stability. (iii) A general reduction in the melting points. Two ring pyrazine based systems containing a terminal chlorine were synthesised i.e. (7). The analogous terminal bromine compounds of series (8) were also prepared for comparison purposes. (7) 5-( 4-Chlorophenyl)-2-( n-alkoxy)pyrazines. (8) 5-(4-Bromophenyl)-2-(n-alkoxy)pyrazines. Series (7) showed a greater smectic thermal stability than series (8). Synthesis of series (7) and (8) also produced the N-alkylated isomers of series (7) and (8), the 1-n-alkyl-5-(4-chlorophenyl)-pyrazin-2-ones and the 1-n-alkyl-5-(bromophenyl)-pyrazin- 2-ones respectively. Three series of terphenyl analogues which contain a terminal pyrazine ring were prepared using the appropriate 5-(4-bromophenyl)-2-(n-alkoxy)pyrazines as the precursors. (9) 2-n-Alkoxy-5-[4-(4[sup]/-n-propylphenyl)phenyl]pyrazines. (10) 2-n-Alkoxy-5-[ 4-( 4[sup]/-fluorophenyl)phenyl]pyrazines. (11) 2-n-Alkoxy-5-[4-(4[sup]/-n-butoxy-2[sup]/,3[sup]/-difluorophenyl)phenyI]pyrazines. A selection of pyrazine based ethynyl compounds were also prepared. These compounds exhibited very high birefringence values, and were highly smectogenic. The preparation of these novel pyrazine based compounds resulted in a number of patent applications being made by Merck U.K. Ltd.