The commonly accepted structural requirement for liquid crystals is possession of a rigid central core with flexible (usually alkyl or alkoxy) or polar tails. Recently known is that the reversed structural arrangement, that is, two rigid mesogenic biphenyl terminal groups connected by a flexible alkoxy or alkyl chains, also exhibit liquid crystal properties. These new type of liquid crystal structures are known as "Siamese Twins".
To investigate the influence of the chiral center in the central flexible chain upon the liquid crystal properties of the Siamese twins, we synthesized 1,6-bis(4,4'-cyanobiphenyloxy)-3-methylhexane. We also synthesized 1,6-bis(4,4'-cyanobiphenyl)hexane and 1,6-bis(4,4'-acetylbiphenyl)hexane for the purpose of comparison.
1,6-Bis(4,4'-cyanobiphenyloxy)-3-methylhexane was prepared from 4-Hydroxy-4'-cyanobiphenyl and 3-methyl-1,6-hexanedibromide in 70% yield, and purified by recrystallization from ethyl acetate.
1,6-Bis(4,4'-cyanobiphenyl)hexane was prepared from 1,6-bis(4,4'-bromobiphenyl) hexane and cuprous cyanide in dry DMF. The product was isolated by column chromatography in 30% yield.
1,6-Bis(4,4'-acetylbiphenyl)hexane was prepared from 1,6-bis(biphenyl)hexane and acetic anhydride by Friedel-Craft acylation in dry carbon disulfide at 12℃. Column chromatography of the reaction mixture gave 20% yield of the titled compound.
Examination of these materials on the hot stage of polarizing microscope and DSC data revealed that 1,6-bis(4,4'-cyanobiphenyloxy)-3-methylhexane produced smectic. A phase rather than nematic phase of cholesteric phase. 1,6-Bis(4,4'-cyanobiphenyl)hexane and 1,6-bis(4,4'-acetylbiphenyl)hexane did not exhibit any liquid crystalline properties.
