Excited state electron transfer from cobalt coordination compounds anchored to TiO2

There exists an urgent need for stable, earth-abundant inorganic coordination compounds and materials that efficiently harvest sunlight and initiate electron transfer reactions that produce electrical power and/or chemical fuels. It was recently discovered that Co(I) coordination compounds do indeed harvest large fractions of solar photons and efficiently transfer electrons to the acceptor states of TiO2 from metal-to-ligand charge transfer excited states. In some cases the electron transfer quantum yield was ∼46%. Remarkably, and unlike other first row transition metal compounds, the unfilled d-orbitals do not quantitatively quench the excited states and allow for efficient excited state electron transfer. A novel feature of the Co(II/I∗) electron transfer chemistry is a large inner-sphere contribution that results from a change in the coordination number. This short review summarizes the most recent findings that suggest new opportunities for solar energy conversion with first-row transition metal compounds.

Graphical abstractCobalt(I) macrocycles harvest large fractions of solar photons and efficiently transfer electrons to the acceptor states of TiO2 from metal-to-ligand charge transfer excited states. The presence of axial ligands in solutions dramatically alters the kinetics and thermodynamics of electron transfer processes.Figure optionsDownload full-size imageDownload as PowerPoint slide