Structure- and solvent-triggered influences in the self-assembly of polyoxometalate–steroid conjugates

Abstract

We report that supramolecular structures formed by polyoxometalate–steroid conjugates in a self-assembly process can be greatly influenced by molecular structures and solution components. Specifically, we deliberately selected three steroids (e.g. cholic acid, dehydrocholic acid, cholesterol) and an organically modified Anderson-type POM cluster to construct conjugates with the molecular structure of steroid–POM–steroid, then carried out careful research on their supramolecular self-assembly in toluene–DMF, CH₂Cl₂–CH₃OH and acetone, respectively. Upon investigation of the TEM and AFM characterizations, we found that the three POM–steroid conjugates displayed unexpectedly different supramolecular structures in the self-assembly process, and only the POM–Chol conjugate could fabricate well-defined fibrous structures in different solutions. Based on the TEM, AFM, FT-IR and XRD characterizations and discussion, a mechanism of the hierarchical self-assembly process of the POM–cholesterol conjugate was also proposed. Moreover, we explained on a molecular level why the morphologies of the POM–Chol conjugate in different solvents were better than those of the POM–Ca conjugate and POM–Dhca conjugate.