THE SYNTHESIS AND TRANSITION-TEMPERATURES OF SOME 4'-ALKYL-4-CYANO-3-FLUOROBIPHENYLS AND 4'-ALKOXY-4-CYANO-3-FLUOROBIPHENYLS

The preparation of 4'-alkyl- and 4'-alkoxy-4-cyano-3-fluorobiphenyls by four different procedures is described and discussed; the best method involves palladium(0)-catalysed coupling of arylboronic acids and 4-bromo-(or 4-iodo-)-2-fluorobenzonitrile. The effects on transition temperatures of fluoro substitution ortho to the terminal cyano group in biphenyls are compared with the effects in other terminal cyano compounds and in mesogens without a terminal cyano group. The effect of the ortho fluoro substituent in 4-cyanobiphenyls is particularly large and is probably due to the severe disruption of antiparallel correlations; the depressions of the smectic A and nematic phase stabilities are similar which suggests that, as for compounds with alkyl or alkoxy terminal groups, the fluoro substituent at the 3-position has a tendency to enhance smectic character, but this is offset by the molecular broadening it causes.