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Human-engineered systems capable of generating fuels from sustainable energy sources provide an approach to satiating modern societies' energy demands, with minimal environmental impact. Strategies to address this challenge for science and the imagination often draw inspiration from the biological process of photosynthesis that powers our biosphere and supplied the fossil fuels global economies rely on. In this context, the active sites of enzymes have inspired researchers to develop molecular complexes that capture key structural and functional principles of nature's catalysts. However, not all aspects of biological energy transducing systems are or should be targets of chemical mimicry in designing an artificial photosynthesis, and some of the more favorable properties associated with solid-state heterogeneous catalysts have motivated molecular based surface-modification strategies. In this presentation, I will discuss efforts from our research group to develop heterogeneous–homogeneous architectures that combine the form factors of their underpinning solid-state supports with molecular coatings, enabling cooperative control and tunability of physical properties. 

Chemical modification of semiconductor surfaces with electrocatalysts provides a strategy for developing integrated materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified photoelectrodes has not been as extensively elaborated. This work addresses kinetic parameters pertinent to heterogeneous-homogeneous catalysis at molecular-modified semiconductors. Photoelectrosynthetic hydrogen evolution using a cobalt porphyrin-modified gallium phosphide cathode is analyzed under variable scan rates, pH values, and light intensity, yielding information on the relationship between the external quantum efficiency, illumination conditions, and turnover frequency. 

Catalysts are central to energy conversion in biology and technology; they provide low-energy pathways for steering chemical transformations and are used in applications ranging from manufacturing fuels and fine chemicals to controlling the bioenergetic reactions essential to all living organisms. Accordingly, the study of homogeneous molecular catalysts, including porphyrins, has provided researchers significant insights regarding the mechanisms and structure−function relationships governing myriad catalytic processes, as well as design principles for further improving the performance of human-engineered catalysts. Our research group has recently reported on the favorable catalytic properties of piextended porphyrins for hydrogen evolution, demonstrating the promise of extended macrocycles as a design element and structural motif for preparing electrocatalysts. The pi-extended architecture provides an alternative strategy, compared to using electron-withdrawing or electron-donating functional groups, for adjusting the redox properties of a molecular catalyst and thus a promising avenue for catalyst design warranting further analysis.