Capturing and converting CO 2 through artificial photosynthesis using photoactive, porous materials is a promising approach for addressing increasing CO 2 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 2 sorption and activation in the presence of water. A combined vibrational and visible spectroscopic approach was used to monitor the adsorption of CO 2 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 2 sorption site and bending of the porphyrin macrocycle in response to humidity was observed, and CO 2/H 2O competition experiments revealed that the exchange of CO 2 with H 2O is pore‐size dependent. Therefore, humidity and pore‐size can be used to tune CO 2 sorption, CO 2 capacity, and light harvesting in porphyrinic MOFs, which are key factors for CO 2 photoreduction.
MOFs combined with photochemistry for CO 2 photoreduction have great potential for addressing the challenge of reducing CO 2 emissions. Herein, a combined visible‐infrared spectroscopic approach is reported to study the initial step of CO 2 photoreduction: adsorption of CO 2 in the presence of water into porphyrinic Zr‐MOFs. The findings will serve as guideline to tune adsorption site and photophysical properties of the photosensitizer.