Engineering Metal-Phenolic Network Materials through Compositional Tuning of Phenolic Molecules

Metal-phenolic coordination between polyphenols and metal ions is a well-established chemistry for preparing supramolecular assemblies. The selection of building blocks for assembly can influence the physicochemical properties of the assembled structures (e.g., morphology, permeability, and functionality). Herein, the compositional engineering of commercially available tannic acid, a commonly used polyphenol in assembly, is demonstrated to tailor the properties of metal-phenolic network (MPN) capsules. Through purification via preparative high-performance liquid chromatography, the composition of commercial tannic acid (cTA)-which consists of a mixture of phenolic compounds-is refined into "purified" tannic acid (pTA) that consists mainly of large molecular weight species. Assembling pTA with Fe^II ions, mediated by the oxidation of Fe^II to Fe^III, yielded MPN capsules with thicker (≈3×) films, increased stiffness (by ≈70%), and reduced film permeability (≈3×) compared with MPN capsules prepared with cTA and Fe^II. Molecular dynamics simulations suggest that these property differences are due to the different interaction energies between TA and metal ions, which are influenced by the presence of small molecular weight phenolic species, e.g., gallic acid in cTA. This study highlights a strategy to tailor the properties of MPN materials through tuning the interaction energies and reaction kinetics of phenolic building blocks.