Thermally Induced Orderly Alignment of Porphyrin Photoactive Motifs in Metal-Organic Frameworks for Boosting Photocatalytic CO2 Reduction

Efficienttransfer of charge carriers through a fast transportpathway is crucial to excellent photocatalytic reduction performancein solar-driven CO2 reduction, but it is still challengingto effectively modulate the electronic transport pathway between photoactivemotifs by feasible chemical means. In this work, we propose a thermallyinduced strategy to precisely modulate the fast electron transportpathway formed between the photoactive motifs of a porphyrin metal-organicframework using thorium ion with large ionic radius and high coordinationnumber as the coordination-labile metal node. As a result, the stackingpattern of porphyrin molecules in the framework before and after thecrystal transformations has changed dramatically, which leads to significantdifferences in the separation efficiency of photogenerated carriersin MOFs. The rate of photocatalytic reduction of CO2 toCO by IHEP-22(Co) reaches 350.9 & mu;mol & BULL;h(-1)& BULL;g(-1), which is 3.60 times thatof IHEP-21(Co) and 1.46 times that of IHEP-23(Co). Photoelectrochemical characterizations and theoretical calculationssuggest that the electron transport channels formed between porphyrinmolecules inhibit the recombination of photogenerated carriers, resultingin high performance for photocatalytic CO2 reduction. Theinteraction mechanism of CO2 with IHEP-22(Co) was clarified by using in-situ electron paramagnetic resonance,in-situ diffuse reflectance infrared Fourier transform spectroscopy,in-situ extended X-ray absorption fine structure spectroscopy, andtheoretical calculations. These results provide a new method to regulatethe efficient separation and migration of charge carriers in CO2 reduction photocatalysts and will be helpful to guide thedesign and synthesis of photocatalysts with superior performance forthe production of solar fuels.