Pigment-acceptor-catalyst triads for photochemical hydrogen evolution

In order to solve the problems of global warming and shortage of fossil fuels, researchers have been endeavoring to achieve artificial photosynthesis: splitting water into H₂ and O₂ under solar light illumination. Our group has recently invented a unique system that drives photoinduced water reduction through "Z-scheme" photosynthetic pathways. Nevertheless, that system still suffered from a low turnover number (TON) of the photocatalytic cycle (TON=4.1). We have now found and describe herein a new methodology to make significant improvements in the TON, up to around TON=14-27. For the new model systems reported herein, the quantum efficiency of the second photoinduced step in the Z-scheme photosynthesis is dramatically improved by introducing multiviologen tethers to temporarily collect the high-energy electron generated in the first photoinduced step. These are unique examples of "pigment-acceptor-catalyst triads", which demonstrate a new effective type of artificial photosynthesis. Electron harvesting: Photo-hydrogen-evolving molecular devices showing substantially improved turnover numbers have been developed by introducing multiviologen tethers into a [PtCl₂(2,2′-bipyridine)]-based moiety serving as a light-harvesting and H₂-evolving center (see scheme). The improved photocatalytic performance is attributed to the rapidly regenerating character of the pigment due to intramolecular electron transfer from the pigment to the electron reservoirs.