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.