Spontaneous organization of supramolecular rod-bundles into a body-centered tetragonal assembly in coil-rod-coil molecules

The synthesis and characterization of coil-rod-coil molecules of 4,4'-bis[4-methyloxypoly(propyleneoxy)propyloxy-4'-biphenyloxymethyl]biphenyl with a poly(propylene oxide) coil of 3 (7), 6 (8), 9 (9), 13 (10), 17 (11), and 22 (12) propylene oxide units are described. These molecules self-assemble into ordered structures that differ significantly on variation of the length of poly(propylene oxide) coil. Coil-rod-coil molecule 7 self-organizes into lamellar crystalline and bicontinuous cubic liquid crystalline assemblies, while 8 shows a hexagonal columnar liquid crystalline assembly. Remarkably, increasing the length of coil induces discrete supramolecular aggregates that self-assemble into a birefringent 3-D superlattice. The molecules 9 and 10 assemble into discrete supramolecular aggregates that spontaneously organize into a novel 3-D tetragonal lattice with a body-centered symmetry in the crystalline and melt states. Further increasing the length of coil as in the case of 11 and 12 induces only a body-centered tetragonal crystalline phase, while the liquid crystalline phase in these molecules is suppressed. X-ray diffraction experiments and density measurements showed that the aggregation of these molecules into a discrete supramolecular structure gives rise to aromatic rod bundles with hockey puck-like cylindrical shape encapsulated by phase-separated coil segments which results in the formation of oblate aggregates. This nonspherical oblate shape is believed to be responsible for the formation of a body-centered tetragonal phase. These results demonstrate that supramolecular structures, from 1-D lamellar to 3-D tetragonal superlattices, formed by the self-assembling process of molecular rods can be controlled in a systematic and predictive way by simple variation of the length of grafted coils.