The Role of Water in Carbon Dioxide Adsorption in Porphyrinic Metal-Organic Frameworks.

Capturing and converting CO through artificial photosynthesis using photoactive, porous materials is a promising approach for addressing increasing CO concentrations. Porphyrinic Zr-based metal-organic frameworks (MOFs) are of particular interest as they incorporate a photosensitizer in the porous structure. Herein, the initial step of the artificial photosynthesis is studied: CO sorption and activation in the presence of water. A combined vibrational and visible spectroscopic approach was used to monitor the adsorption of CO into PCN-222 and PCN-223 MOFs, and the photophysical changes of the porphyrinic linker as a function of water concentration. A shift in CO sorption site and bending of the porphyrin macrocycle in response to humidity was observed, and CO/HO competition experiments revealed that the exchange of CO with HO is pore-size dependent. Therefore, humidity and pore-size can be used to tune CO sorption, CO capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO photoreduction.