Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes

Coordination complexes of precious metals with the d(6) valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d(6) complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d(6) metal complex reported so far. The luminescence behaviour of the metal-to-ligand charge transfer excited states of these Cr(0) complexes is competitive with Os(II) polypyridines. With these Cr(0) complexes, the metal-to-ligand charge transfer states of first-row d(6) metal complexes become exploitable in photoredox catalysis, and benchmark chemical reductions proceed efficiently under low-energy red illumination. Here we demonstrate that appropriate molecular design strategies open up new perspectives for photophysics and photochemistry with abundant first-row d(6) metals.

Automated summary

Coordination complexes of precious metals with d(6) valence electron configuration are widely used in lighting, solar energy conversion, and photocatalysis. However, first-row transition metal complexes with similar properties have been elusive. In this study, we demonstrate that Cr(0) complexes, isoelectronic with the precious metals, exhibit higher photoluminescence quantum yields and excited-state lifetimes. The metal-to-ligand charge transfer states of these Cr(0) complexes show competitive luminescence behavior compared to Os(II) polypyridines. This opens up new possibilities for photophysics and photochemistry with abundant first-row d(6) metals.