Monovalent Cation–Phenolic Crystals with pH-Driven Reversible Crystal Transformation

The conversion of renewable plant polyphenol to advanced materials with tailorable properties and various functions is desirable and challenging. In this work, monovalent cation-phenolic crystals contained K⁺ or Na⁺ ions were synthesized by using plant polyphenol as an organic source in alkaline solution. The crystal structure was resolved, showing a laminar crystal structure with M⁺ as connecting nodes. The morphologies (e.g., rod-like and spindle-shaped) and chemical compositions of crystals could be tuned by changing the cations. Interestingly, these polymer crystals exhibited a pH-driven reversible crystal transformation. They transformed into their protonated crystalline form under acidic conditions (e.g., pH 2) and went back to the cation-bound crystalline form in alkaline solutions. Furthermore, the crystals proved excellent antioxidants and heavy metal ion adsorbents.