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 CO2 photoreduction, achieving a remarkable CH4 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 CO2 photoreduction from both thermodynamic and kinetic perspectives.