The effects of alkylthio chains on the properties of symmetric liquid crystal dimers

The synthesis and characterisation of four series of symmetric liquid crystal dimers containing either alkylthio or alkyloxy terminal chains are reported. The (E,E)-[pentane-1,5-diylbis(oxy-4,1-phenylene)]bis{N-[4-(alkyloxy)phenyl]methanimine}s, mO.O5O.Om and (E,E)-[hexane-1,6-diylbis(oxy-4,1-phenylene)]bis{N-[4-(alkyloxy)phenyl]methanimine}s, mO.O6O.Om series are nematogenic when the terminal chains are short whereas the higher homologues show the smectic A phase. The mO.O6O.Om series shows higher clearing temperatures regardless of the terminal chain length. The (E,E)-[pentane-1,5-diylbis(oxy-4,1-phenylene)]bis{N-[(alkylthio)phenyl]methanimine}s, mS.O5O.Sm series is solely nematogenic, and does not show liquid crystallinity when m >= 5. The (E,E)-[hexane-1,6-diylbis(oxy-4,1-phenylene)]bis{N-[(alkylthio)phenyl]methanimine}s, mS.O6O.Sm series, however, shows mesogenic behavior over the entire series, with the smectic C phase only emerging when the terminal chain is long, m >= 8. In general, for a given spacer length, the dimers containing alkyloxy chains show the highest clearing temperatures and the alkylthio chains the lowest. For the shortest terminal chains, however, the nematic-isotropic transition temperatures of the alkylthio materials are higher than those containing the alkyl chain. This behaviour is rather general and discussed in terms of steric and electronic interactions.