The role of hydrogen bonding in the phase behaviour of supramolecular liquid crystal dimers

• Equimolar mixtures of octyloxybenzoic acid/pyridine-based compounds show smectic behaviour.
• Liquid crystalline behaviour linked to the shape of the hydrogen-bonded heterocomplex.
• Carbonyl IR stretching region shows the presence of free acid, acid dimers and heterocomplexes.
• 1:1 hydrogen-bonded heterocomplex is not formed quantitatively.
• Liquid crystalline behaviour must be accounted for in terms of competing equilibria.

The phase behaviour of equimolar mixtures of 4-octyloxybenzoic acid, OOBA, and either 1-(4-butylazobenzene-4′-oxy)-6-(4-oxypyridine)pentane, BuABO5OPyr, or 1-(4-butylazobenzene-4′-oxy)-6-(4-oxypyridine)hexane, BuABO6OPyr, is reported. OOBA exhibits enantiotropic smectic C and nematic phases, whereas BuABO5OPyr and BuABO6OPyr are not liquid crystalline. Both the OOBA/BuABO5OPyr and OOBA/BuABO6OPyr equimolar mixtures show an enantiotropic smectic A phase identified using X-ray diffraction and polarised light microscopy. The liquid crystallinty of the mixtures is attributed, in part, to the formation of a 1:1 hydrogen-bonded complex between OOBA and either BuABO5OPyr or BuABO6OPyr. Differences in the transitional properties of the mixtures are accounted for in terms of the average shapes of these 1:1 complexes. Temperature dependent FTIR spectroscopy shows, however, that the formation of the complex is not quantitative in the mixtures but instead a complex mixture exists over the complete temperature range studied, involving the 1:1 complex, both cyclic and open acid dimers, free acid and hence, free BuABOnOPyr molecules.

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