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

Efficient transfer of charge carriers through a fast transport pathway is crucial to excellent photocatalytic reduction performance in solar-driven CO₂ reduction, but it is still challenging to effectively modulate the electronic transport pathway between photoactive motifs by feasible chemical means. In this work, we propose a thermally induced strategy to precisely modulate the fast electron transport pathway formed between the photoactive motifs of a porphyrin metal-organic framework using thorium ion with large ionic radius and high coordination number as the coordination-labile metal node. As a result, the stacking pattern of porphyrin molecules in the framework before and after the crystal transformations has changed dramatically, which leads to significant differences in the separation efficiency of photogenerated carriers in MOFs. The rate of photocatalytic reduction of CO₂ to CO by IHEP-22(Co) reaches 350.9 μmol·h⁻¹·g⁻¹, which is 3.60 times that of IHEP-21(Co) and 1.46 times that of IHEP-23(Co). Photoelectrochemical characterizations and theoretical calculations suggest that the electron transport channels formed between porphyrin molecules inhibit the recombination of photogenerated carriers, resulting in high performance for photocatalytic CO₂ reduction. The interaction 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, and theoretical calculations. These results provide a new method to regulate the efficient separation and migration of charge carriers in CO₂ reduction photocatalysts and will be helpful to guide the design and synthesis of photocatalysts with superior performance for the production of solar fuels.