Long-lived excited states open the door to earth-abundant metal photocatalysts

Long lived, high-energy excited states can give cobalt (III) complexes photocatalytic properties previously thought impossible, two US groups have shown. The findings could allow earth-abundant metal-based catalysts to replace precious metals for many important reactions and even to catalyse reactions not previously thought feasible.

Some complexes of precious metals such as ruthenium and iridium are extremely good photocatalysts because, when they absorb visible light, they enter long-lived ‘charge-transfer’ excited states that facilitate electron or energy transfer. But the electronic band structures of first-row transition metals like iron make such states unstable as they tend to decay into short-lived ‘ligand-field’ excited states. Efforts to use these metals in catalysis have largely focused on stabilising the charge-transfer states, but this often requires highly specialised ligands. ‘It becomes a trade-off: is it really a cheap alternative if you have to use really expensive ligands?’ notes Thomas Auvray of the University of Birmingham, UK.