Non-symmetric liquid crystal dimers containing an isoflavone moiety

The first series of liquid crystal dimers to contain an isoflavone-based moiety has been prepared, the alpha-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-omega-(4-methoxyazobenzene-4'-oxy)alkanes. All eight members of this homologous series exhibit an enantiotropic nematic phase. The nematic-isotropic transition temperatures and associated entropy changes exhibit a dramatic dependence on the length and parity of the flexible spacer. This characteristic behaviour for a homologous series of liquid crystal dimers has been interpreted in terms of how the flexible alkyl spacer controls the average molecular shape. A comparison of the transitional properties of this series with those of the symmetric parent dimers, the alpha,omega-bis(4-methoxyazobenzene-4'-oxy) alkanes, reveals that replacing a methoxyazobenzene unit by the isoflavone-based group reduces the melting point but increases the nematic-isotropic transition temperature. Three other non-symmetric dimers are reported: 1-(4'-methoxy-3-phenyl-4H-1-benzopyran-4-one-7-oxy)-9-(4-cyanobiphenyl-4'-oxy) nonane, 1-(4'-methoxy- 3-phenyl-4H-1-benzopyran-4-one-7-oxy)-10-(4-nitroazobenzene-4'-oxy) decane and cholesteryl 11-(4'-methoxy- 3-phenyl-4H-1-benzopyran-4-one-7-oxy) undecanoate. The former two show enantiotropic nematic behaviour while the latter exhibits an enantiotropic chiral nematic phase. Again, when compared with the parent symmetric dimer, a reduction in melting point is observed while the clearing temperature is increased. The trend in the melting points is accounted for in terms of the difficulties associated with packing the non-symmetric dimers into a crystalline lattice compared to the symmetric dimers. The physical significance of the increase in the clearing temperatures is not yet clear.