Covalent Grafting and Nanostructuration of Porphyrins on Lignin Improve Sustainable Blue‐LED Synthesis of Dialkylimines

Covalent grafting of porphyrins on kraft and enzymatic hydrolytic lignin by Mitsunobu reaction afforded a panel of novel heterogeneous photocatalysts useful for the blue-LED synthesis of dialkylimines by oxidative coupling of aromatic amines. The catalytic performance of photocatalysts was improved by nanostructuration via sustainable solvent-shift procedure. Compared to previously reported encapsulated systems, covalent grafting limited porphyrin leaching and enhanced electronic transfer processes. Optical band gap and singlet oxygen quantum yield revealed that the acetylation of lignin modulated the excited-state dynamics reducing non-radiative deactivation pathways. The synthesis of dialkylimines from aromatic amines occurred in high yield, kraft lignin being the best support. The possibility to reuse photocatalysts for more runs further improved the sustainability of the system. Overall, covalent grafting was an effective strategy to control stability, excited-state behavior, and catalytic efficiency of porphyrins in lignin-based photocatalytic systems.