Solvent induced helical aggregation in the self-assembly of cholesterol tailed platinum complexes

Three alkynylplatinum(ii) bipyridyl complexes in which two cholesterol groups are combined with a bipyridyl group via alkyl chains and amido bonds were designed and synthesized. The complexes have different lengths of ethylene glycol chains at the para-position of 1-phenylethyne. All three complexes can self-assemble to gel networks in DMSO, while only the morphology of 1a without an ether chain shows a well-defined right-handed helical structure in layer packing mode. However, 1c with long ethylene glycol chains forms perfect regular left-handed helical structures in aqueous ethanol solution while the volume percentage of water is less than 5% (v/v). As the ratio of water increases, the chirality changes from a left-handed helix to a right-handed helix and the packing mode alters from a monolayer structure to a hexagonal structure. As the ratio of water further increases to greater than 50% (v/v), the structure of the assembly finally transforms into bilayer vesicles. The process of the morphology transition is traced by circular dichroism spectra, powder X-ray diffraction, SEM and TEM images. The result indicates that a polar solvent (water) acts as a trigger to change the self-assembly of the chiral structures of the complex due to the strong hydrophobic interaction between cholesterol groups and the balance of the hydrophobicity and hydrophilicity of the solvent environment.

Tuning the self-assembly of rectangular amphiphilic cruciforms

The self-assembly of a series of nonionic amphiphilic cruciforms based on the 1,2,4,5-tetrakis(phenylethynyl)benzene (TPEB) skeleton, in which the peripheral substituents have been modified to modulate the morphology of the supramolecular structures, is reported. The presence of linear paraffinic and hydrophilic chains in TPEBs 1 and 2 gives rise to two-dimensional structures of high aspect ratio. In contrast, the incorporation of dendronized hydrophilic chains results in the formation of twisted ribbons in amphiphile 3 and impedes the organized self-assembly of TPEB 4. Theoretical calculations show that the self-assembly of these amphiphiles might be initiated with the formation of π-stacked dimeric units. Compound 2, which self-assembles into different morphologies depending on the solvent, interacts by π-stacking and also by the interdigitation of the peripheral decyl tails to generate bidimensional supramolecular structures. The steric demand exerted by the dendronized polar wedges in 3 and 4 strongly conditions their supramolecular organization. This steric demand together with the interdigitation of the decyl chains results in the self-assembly of cruciform 3 into helical aggregates. However, the lack of the paraffinic chains in 4 impedes this helical organization, and the formation of amorphous material is visualized. The joint experimental and theoretical study presented herein provides relevant guidelines for the modulated self-assembly of nonionic amphiphilic molecules.