Synthesis and characterization of a range of heterocyclic liquid crystalline materials incorporating the novel thiophene-pyrimidine moiety

In this structure/property study six 2-(4-alkyloxyphenyl)-5-thiophen-2-ylpyrimidines and thirty-six 2-(4-alkyloxyphenyl)-5-(5-alkylthiophen-2-yl) pyrimidines were synthesized to evaluate the effect of the introduction of heterocyclic rings on the liquid crystalline (LC) behaviour of mesogens. The ‘bent’ nature of the structure of the 2-(4-alkyloxyphenyl)-5-thiophen-2-yl-pyrimidines, due to the presence of the thiophene ring, was further exaggerated in the 2-(4-alkyloxyphenyl)-5-(5-alkylthiophen-2-yl) pyrimidines by the incorporation of an alkyl group at the 5-position in the thiophene ring. This simple change in structure led to a remarkable change in the LC behaviour of the mesogens. From a series of mesogens having high melting points and exhibiting monotropic nematic phases, the incorporation of the alkyl group gave a series of three-ring mesogens with relatively low melting points (60–80°C), and exhibiting a range of tilted smectic phases, including SmI, SmC and SmCalt phases.