Photoinitiated electron collection in polyazine chromophores coupled to water reduction catalysts for solar H2 production

The efficient conversion of solar energy to transportable, combustible fuels and chemical feedstock remains an important challenge for synthetic chemists. This daunting task requires a delicate balance of light absorption, electron transfer, charge separation, and catalysis of bond breaking and bond forming reactions. This review highlights our recent studies of supramolecular systems coupling Ru(II) or Os(II) polyazine chromophores with directional charge transfer excited states to catalytically active cis-RhCl2 or cis-PtCl2 metal centers. Comparison to similar supramolecules is provided to highlight design constraints. These supramolecules absorb strongly in the low energy visible and exhibit emissive Ru → BL 3MLCT excited states. Systems with catalytic Rh(III) undergo photoinitiated electron collection (PEC) at the Rh center via Ru or Os → Rh metal-to-metal charge transfer (MMCT) state, while those with cis-PtCl2 moieties form charge-separated (CS) excited states. Electrochemical studies provide a means for understanding relative orbital energetics as well as predicting the existence of a charge-separated state or the propensity for photoinitiated electron collection. Emission spectroscopy including time resolved measurements provides support of these interstate dynamics. Detailed kinetic analysis by photophysical studies indicates formation of the 3MMCT states and 3CS states occurs via intramolecular electron transfer with rate constants on the order of 106–107 s−1. The complexes are active photocatalysts which absorb lower energy visible light, and provide high quantum efficiency and long term stability for the reduction of water to hydrogen. Catalytic activities are modulated by component modification and system parameters and are often dependent upon driving forces for intramolecular electron transfer, reductive excited state quenching, and component modification.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (226 K)Download as PowerPoint slideHighlights► Photoinitiated electron collection. ► Supramolecules coupling Ru-based chromophores to cis-RhCl2 centers can collect two electrons at the Rh using visible light, possessing metal-to-metal excited states. ► Systems with cis-PtCl2 units form a charge separated excited state. ► Electrochemical potentials, absorption and emission spectroscopies, and excited state electron transfer kinetics are discussed and compared with sub-unit modification. ► The application as photochemical catalysts for the reduction of water to hydrogen is highlighted.