Supramolecular helical columns from the self-assembly of chiral rods

Abstract

Chiral-bridged rod molecules (CBRs) that consisted of bis(penta-p-phenylene) conjugated to an opened or closed chiral bridging group as a rigid segment and oligoether dendrons as flexible segments were synthesized and characterized. In the bulk state, both molecules self-assemble into a hexagonal columnar structure, as confirmed by X-ray scatterings and transmission electron microscopy (TEM) observations. Interestingly, these structures display opposite Cotton effects in the chromophore of the aromatic unit in spite of the same chirality (R,R) of the chiral bridging groups. The molecules were observed to self-assemble into cylindrical micellar aggregates in aqueous solution, as confirmed by light scattering and TEM investigations, and exhibit intense signals in the circular dichroism (CD) spectra, which are indicative of one-handed helical conformations. The CD spectra of each molecule showed opposite signals to each other, which were similar to those in the bulk. Notably, when the opened CBR was added to a solution of closed CBRs up to a certain concentration, the CD signal of the closed CBR was amplified. This implies that both molecules co-assemble into a one-handed helical structure because the opened chiral bridge is conformationally flexible, which is inverted to co-assemble with the closed CBR. These results demonstrate that small structural modifications of the chiral moiety can transfer the chiral information to a supramolecular assembly in the opposite way.