For light-to-chemical energy conversion in a system for artificial photosynthesis (AP), the splitting of water into its constituent elements is the key energy-storing reaction. As in natural photosynthesis, an AP system relies on light absorption, charge separation, and catalysis of forming the two products. In this talk, the reductive side of water splitting and light-driven H2 formation catalyzed by Co and Ni complexes will be described. This process corresponds to the microscopic reverse of H2 activation, which for 4d and 5d complexes can proceed by either homolytic or heterolytic splitting of the H-H bond. For 3d metal complexes of interest as AP catalysts, hydrogen formation proceeds via a hetero-coupling step of proton and hydride. In some of these catalysts, redox-active ligands play a role in helping to store electronic charge upon reduction, while ligand hetero-atoms stabilize protons for ultimate coupling with a metal hydride intermediate. In this sense, the process is the reverse of H2 activation by Ru(II) catalysts studied by Halpern. The light absorbers in the systems described consist of water-solubilized semiconductor nanoparticles. An unrecognized factor in the effectiveness of these H2-generating systems involving quantum dot (QD) light absorbers regards interactions between the water-solubilizing capping agents used for the QDs and the metal complex catalysts. These interactions affect the nature of the actual catalyst for hydrogen generation.

Activation of hydrogen and the photogeneration of hydrogen: How Jack's mechanistic thinking informed artificial photosynthesis

256th National Meeting (2018)

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