Light-Driven Redox Activation of CO2- and H2-Activating Complexes in a Self-Assembled Triad

We report a self-assembled triad for artificial photosynthesis composed of a chromophore, carbon-dioxide reduction catalyst, and hydrogen-oxidation complex, which is designed to operate without conventional sacrificial redox equivalents. Excitation of the zinc-porphyrin chromophore of the triad results in ultrafast charge transfer between a tungsten-alkylidyne donor and a rhenium diimine tricarbonyl acceptor, producing a charge-separated state that persists on the time scale of tens of nanoseconds and is thermodynamically capable of the primary dihydrogen and carbon dioxide binding steps for initiating the reverse water-gas shift reaction. The charge-transfer behavior of this system was probed using transient absorption spectroscopy in the visible, near-infrared, and mid-infrared spectral regions. The behavior of the triad was compared with that of the zinc-porphyrin-rhenium-diimide dyad; the triad was found to have a significantly longer charge-separated lifetime than other previously reported porphyrin-rhenium diimine compounds.