Abnormal Multistep Charge Transfer in a 2D Single-Crystalline Covalent Organic Framework Photocatalyst

Replicating the multiphoton and multielectron redox chemistry of natural photosynthesis remains a challenge for artificial photocatalysis. Herein, abnormal multistep charge transfer is discovered in a two-dimensional single-crystalline covalent organic framework (COF), namely USTB-35-Cu. Continuous rotation electron diffraction coupled with theoretical simulation resolves the crystal structure, containing imine-linked 5,10,15,20-tetrakis(4-aminophenyl)porphyrinato copper(II) (CuTAP) and tetraaldehyde-containing perylenediimide (TFPDI) layers in a staggered packing. After obtaining photoinduced donor−acceptor (D−A) charge-separated states, excited TFPDI anion radicals (A •− ) sequentially transfer photogenerated electrons to the surrounding porphyrin moieties (D). The TFPDI cation radicals (A •+ ) are formed due to fast charge delocalization, leading to the rearrangement of water hydrogen bonding for efficient proton-coupled electron transfer. Therefore, the CuTAP anion radicals enable eight-electron CO 2 photoreduction, achieving a remarkable CH 4 production rate of 188 μmol g −1 h −1 with a 99% selectivity. This work demonstrates the capability of single-crystalline COFs for coupling multistep charge transfer with multielectron CO 2 photoreduction from both thermodynamic and kinetic perspectives.