THE SINGLE PLUMBER S NIGHTMARE A LIQUID CRYSTALLINE ORGANIC FRAMEWORK

Several bicontinuous cubic liquid crystals are known, consisting of one to three interpenetrating infinite periodic networks of molecules, some with highly complex structures. This work reports a newly found such phase, being considered a liquid version of the widely used solid-state metal-organic and covalent-organic frameworks.

COMPLEX SYSTEMS AND BIOMEDICAL SCIENCES

78 ESRF

Metal-organic and covalent-organic frameworks are familiar, highly versatile solid-state materials. In the former, the organic network elements are connected by metal atoms and, in the latter, by covalent bonds. However, recent work utilising grazing-incidence small-angle X-ray scattering (GISAXS) data carried out at beamline BM28 has found that such a framework can be held together merely by hydrogen bonds and, moreover, be liquid crystalline, hence flexible and adaptive. This self-assembled framework is a bicontinuous cubic (Cubbi) mesophase, and is the last piece of the jigsaw of such phases shown in Figures 63a-f.

Cubbi phases consist of continua of chemically different and incompatible but linked components, and are found in amphiphile- solvent systems, in block and star copolymers, and in solvent-free thermotropic liquid crystals (TLC). They are of interest as potential isotropic, organic semi- and ionic conductors, as templates

for porous ceramics, as photonic materials and templates for protein crystallisation. The classic Cubbi phases contain two infinite interpenetrating networks and are known as double gyroid (DG, space group Ia 3d), double diamond (DD, Pn 3m) and plumber s nightmare (DP, Im 3m) (Figures 63a-c). In TLCs, two Cubbi phases dominate [1], the DG and a complex triple network (I23) phase [2]. For every double-network phase, there could be a single- network counterpart with the second network missing (Figures 63d-f). Indeed SG and SD are biophotonic, causing the colour of some butterflies and beetles. SD and a version of SG have recently also been found in TLCs [3,4], but the simplest of all, the SP, has not been reported until now in any system.

The simple cubic SP phase, space group Pm 3m, has now been obtained in the two compounds shown in Figure 64, each consisting of a π-conjugated aromatic rod with a glycerol at each end and with two long chains attached laterally. The model of the network structure is shown in Figure 64. The rod-like cores cluster in bundles of six to nine molecules lying along the edges of the cubic unit cell. They are held together by the hydrogen-bonding glycerol ends at 6-way octahedral junctions at the cell corners. The space between the segments of the single network is filled with the flexible side- chains. Being a liquid crystal, this soft structure is dynamic and the number and position of the molecules in a bundle fluctuate. It can be thought of as a plumber s nightmare structure with one of the two networks removed. The new liquid framework belongs to the family of rod- bundle phases, as do the thermotropic DD [5], SD [3], SG [4] and some DG [6].

Figure 63g helps understand the relationship between molecular architecture and the type of Cubbi phase. The curves show the rate of volume

Fig. 63: a-c) Double- and (d-f) single network bicontinuous cubic mesophases. g) Rate of increasing network volume with increasing radius of its cylindrical segments for different Cubbi phases and the columnar phase. When r = 1 the radius equals the inter-junction distance.