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 b y four different procedures is described and discussed; the best metho d involves palladium(0)-catalysed coupling of arylboronic acids and 4- bromo-(or 4-iodo-)-2-fluorobenzonitrile. The effects on transition tem peratures of fluoro substitution ortho to the terminal cyano group in biphenyls are compared with the effects in other terminal cyano compou nds 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 s imilar which suggests that, as for compounds with alkyl or alkoxy term inal groups, the fluoro substituent at the 3-position has a tendency t o enhance smectic character, but this is offset by the molecular broad ening it causes.