Synthesis and study of 2,7-disubstituted fluorenes in the search for mesogenic behaviour.

Abstract

A variety of 2,7-disubstituted fluorenes were prepared in order to investigate their mesogenic behaviour and evaluate their usefulness as possible components of mixtures in liquid crystal display devices. A series of 2-alkyl-7-cyanofluorenes were synthesised, some of which gave monotropic nematic phases. A ternary eutectic mixture of 2-alkyl-7-cyanofluorenes was prepared and the birefringence, viscosity and transition temperatures of the material were determined. Other measurements obtained for this material include the dielectric anisotropy, electro-optic threshold voltages and order parameter which were compared with those of an analogous mixture of 4,41-disubstituted biphenyls. 2-Pentyl-7-pentyloxyfluorene was also synthesised, but this compound displayed no mesogenic properties. Enantiotropic 2,7-disubstituted fluorene mesogens were synthesised by linking a phenyl ring to the fluorene core of the structure. The linkages used were -CE

CE

-" -COCE

-and ester moieties, and the transition temperaturesalong with some birefringence and viscosity data were determined for the mesogens. Attempted syntheses of 2,7-disubstituted fluorenone derivatives and 3,9-disubstituted-5,7-dihydrodibenz[c,e]oxepin are also described, but no mesogenic compounds were obtained.In addition, syntheses of hydrogenated fluorene derivatives were attempted with only partial success, again yielding non-mesogenic compounds. General aspects of liquid crystals are reviewed with special reference to nematogens useful in liquid displays.