Solar fuel production holds promise to combine light energy harvesting with fuel generation in a sustainable energy cycle. Toward this goal, molecular catalysts offer tailor-made approaches for H2 evolution and CO2 reduction, however long-term activity under catalytic conditions remains challenging. A selected route to address this is hosting molecular catalysts by metal-organic frameworks (MOFs). These porous coordination polymers provide innate porosities paired with tuneable physicochemical properties. Our recent work combines these compounds’ inherent features to design and synthesize molecular artificial photosystems hosted by MOFs, resulting in improved selective solar CO2 reduction, reaching state-of-the-art turnover numbers. We co-immobilized molecular catalysts and photosensitizers in a variety of MOF matrices, varying in pore sizes, cage structures, and optoelectronic properties. These defined MOF cages, acting as model nanoreactors, allowed us to evaluate reaction environment effects. Hereby revealing benefits upon matching the MOF’s cage size with molecular complex diameters to specifically control the number of molecules per cage. Such deliberate component assembly yielded improved molecular catalyst stability and synergistic effects between the host and the entrapped species. Further, such assemblies can be mired by diffusion-based mass transport limitations, which we circumvented by systematically reducing the MOF particle size. Crucially, by varying the MOFs’ pore sizes, we rationally controlled the molecular guest positioning (inside vs outside the cavity), which affected catalyst stability, electronic communication between guests, and consequently the apparent catalytic rates. Together, these studies provide a rational understanding of an optimized supramolecular layout of catalyst, photosensitizer, and host matrix, providing a blueprint for future system optimization toward application.

Metal-organic frameworks hosting molecular photosystems for solar fuel production

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.