The synthesis and transition temperatures of some fluorinated terphenyls with alkenyl terminal chains

Abstract

A carbon-carbon double bond has been introduced into an aliphatic alkoxy terminal chain of 2,3- or 2′,3′- difluoroterphenyl systems. The effect of the position and nature (E/Z) of the double bond on the mesomorphic behaviour of these compounds in comparison with the parent systems has been studied. The smectic C thermal stabilities are reduced in all the cases. The introduction of a trans-double bond at the 2-position of the terminal group increased the melting points and clearing points for the compounds with difluoro substituents in the middle ring; this results in a decrease of the smectic C mesophase range but an increase in the nematic mesophase range. All the transition temperatures are reduced with a cis-double bond at the 3-position. Both smectic C and nematic mesophase ranges are decreased, but the reduction in the smectic C range is less than for the trans-2-ene terminal group. With the difluoro substituents in the end ring linked to the alkenyloxy chain, all the transition temperatures are decreased. The melting points are reduced more than S_C-S_A or S_C-N transition temperatures, which leads to an increase in the smectic C mesophase range. Additionally, a 2-(E)-double bond suppresses the smectic A mesophase while a 3-(Z)-double bond suppresses the nematic mesophase.