Fundamental roles of the Coordination Geometry in Transient Metallo-supramolecular Polymer Networks: Insights from the Stimuli-responsive Mussel-inspired Hydrogels

• Applicant: Mostafa Ahmadi, Ph.D.
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 491930291

Following the increasing recognition of the role of metal–ligand complexes in vital functions of biomaterials, chemists are encouraged to integrate them in complex structures to mimic functions like self-healing and stimuli-responsiveness. This is further promoted owing to the large flexibility in tuning their strength and stability by the mere choice of the metal ion or the ligand. Coordination geometry is another important aspect that is often overlooked due to the difficult characterization and inaccessible interconversion timescales. Despite that, this factor not only affects the strength and stability of transient bonds but many other features like photoluminescence. To this end, we have recently demonstrated that the coordination geometry preference of different transition metal ions in complexation with phenanthroline-functionalized tetra-arm poly(ethylene glycol) (tetraPEG) directly affects the structure and dynamics of the resulting metallo-supramolecular hydrogels. As such, Fe2+ with larger affinity for tris-complexes can create a stable gel at ligand-deficient conditions, in sharp contrast to Co2+ or Ni2+, which have a relatively larger affinity to form bis- and mono-complexes. Similarly, we have proved that heteroleptic complexes can contribute to gel formation by mixing phenanthroline- and terpyridine-functionalized tetraPEG precursors, depending on the coordination geometry preference of the utilized metal ion. As such, Co2+ with the natural ability to accommodate a Penta-coordinated complex can form a percolated network, which is mediated through heteroleptic complexation, in sharp contrast to Fe2+ or Ni2+, which prefer larger coordination numbers.Unfortunately, those coordination geometries are the intrinsic thermodynamic affinity of the studied systems and cannot be freely changed. In sharp contrast, mussel-inspired ligands, i.e., histidine and catechol, are capable of forming complexes with coordination geometries that not only can be freely changed by multiple external stimuli, like pH and the oxidation state of the metal ion, but are also distinguishable by several characterization techniques. Therefore, they can create a platform to systematically study the influence of the coordination geometry on the structure and dynamics of metallo-supramolecular polymer networks. To this end, we functionalize well-defined polymer precursors with mussel-inspired ligands and form transient networks upon the introduction of different transition metal ions, and employ pH as the coordination geometry regulator. We study network dynamics and structure at micro-, and macroscale by combining rheology/FRAP and DQ-NMR/light scattering techniques, respectively. We also evaluate the obtained results by combined DFT simulations and tube-based models. We expect this study to unlock the coordination geometry as a new paradigm for regulating properties of transient networks.

• DFG Programme: Research Grants
• International Connection: Belgium
• Co-Investigators: Professor Kay Saalwächter; Professor Sebastian Seiffert, Ph.D.
• Cooperation Partner: Professorin Evelyne Van Ruymbeke, Ph.D.