Testing the triple network structure of the cubic Im3̄m (I) phase by isomorphous replacement and model refinement

This study provides further tests of the validity of the triple network structure of the body-centred Im3m (I) cubic phase in polycatenar (phasmidic) liquid crystals, proposed by Zeng et al., Nat. Mater., 2005, 4, 562. New hemiphasmid compounds were synthesised, two of which (6 and 7) display the above phase, while 8 shows the hexagonal columnar phase. Comparison of 3-d electron density maps based on small-angle X-ray diffraction of two compounds with different proportions of high and low electron density regions (aliphatic and aromatic) helped with phasing the X-ray reflections of the cubic structure. This variant of the isomorphous replacement technique gave further support to the triple network structure proposed earlier. Furthermore, fitting calculated diffraction intensities of geometrical models to the observed ones confirmed that the oscillating segment cross-section of two of the three networks (the "octahedral networks") are genuine structural features rather than artefacts. A rational explanation is proposed for the variable thickness of the aromatic network segments. The study has highlighted complexities of molecular packing in bicontinuous phases of thermotropic liquid crystals that are not encountered in related phases in lyotropics or block copolymers.