Structure-property investigation of 2- and 3-thienylacrylates bearing laterally fluorinated azobenzene moieties

The synthesis, transition temperatures and structure-property relationships of a variety of thiophene-containing azobenzene esters derived from either 3-(2-thienyl)acrylic acid (series I, IV, VI, VIII, X and XII) or 3-(3-thienyl)acrylic acid (series II, V, VII, IX, XI and XIII) and appropriate fluoro- and non-fluoro-substituted 'azophenols' are reported. For comparative purposes, the non-heterocyclic counterparts, i.e. cinnamates (series III), were also prepared and are reported. All 70 final esters are mesomorphic, exhibiting the nematic phase alone. Their mesomorphic properties are dependent on the disposition of the terminal thiophene moiety. In general, 3-thienyl-substitution gives thermally more stable compounds than 2-thienyl-substitution. The influence of mono- (series IV, V, VI and VII) and di-lateral (series VIII, IX, X, XI, XII and XIII) fluoro-substitution on mesomorphic properties is investigated in detail. Lateral fluorination lowers mesophase thermal stability and its extent is dependent on the number and disposition of the lateral fluoro-substituents. Di-lateral fluorination across the long molecular axis is more detrimental to mesophase thermal stability than along the long molecular axis.