Cofacial porphyrin organic cages. Metals regulating excitation electron transfer and CO2 reduction electrocatalytic properties

Abstract

Herein, we introduce a comprehensive study of the photophysical behaviors and CO₂ reduction electrocatalytic properties of a series of cofacial porphyrin organic cages (CPOC-M, M = H₂, Co(II), Ni(II), Cu(II), Zn(II)), which are constructed by the covalent-bonded self-assembly of 5,10,15,20-tetrakis(4-formylphenyl)porphyrin (TFPP) and chiral (2-aminocyclohexyl)-1,4,5,8-naphthalenetetraformyl diimide (ANDI), followed by post-synthetic metalation. Electronic coupling between the TFPP donor and naphthalene-1,4 : 5,8-bis(dicarboximide) (NDI) acceptor in the metal-free cage is revealed to be very weak by UV-vis spectroscopic, electrochemical, and theoretical investigations. Photoexcitation of CPOC-H₂, as well as its post-synthetic Zn and Co counterparts, leads to fast energy transfer from the triplet state porphyrin to the NDI unit according to the femtosecond transient absorption spectroscopic results. In addition, CPOC-Co enables much better electrocatalytic activity for CO₂ reduction reaction than the other metallic CPOC-M (M = Ni(II), Cu(II), Zn(II)) and monomeric porphyrin cobalt compartment, supplying a partial current density of 18.0 mA cm⁻² at −0.90 V with 90% faradaic efficiency of CO.