Energy Storage in the Outer Coordination Sphere of a Chromium(0) Complex Enhances Photophysical and Photochemical Properties

Luminescent 3d⁶ transition-metal complexes are rare because their excited states typically undergo nonradiative deactivation on time scales ranging from 100 fs to 1 ns. This complicates the development of first-row transition metal analogues of the commonly used 4d⁶ (RuII) and 5d⁶ (IrIII) luminophores and photocatalysts. Here, we report a Cr⁰ complex that exhibits triplet metal-to-ligand charge transfer (³MLCT) luminescence with a lifetime of 12 μs in solution at room temperature. This is 4 orders of magnitude longer than what has been previously achieved with FeII-based complexes, making it the longest luminescence lifetime observed in any 3d⁶ metal complex to date. This advance was made possible through the covalent attachment of 12 9-phenylanthracene units that serve as energy reservoirs in the second coordination sphere around a luminescent di(triisocyanide) Cr⁰ core. This molecular design retains the strong photoreducing power of the Cr⁰ core; however, the extended lifetime enables thermodynamically uphill elementary reaction steps, which are usually too slow to compete with the excited-state deactivation of 3d⁶ metal complexes. Transient absorption spectroscopy directly observes the photoinduced elementary reaction steps, and long-term irradiation experiments illustrate the advantages of our molecular design in a proof-of-principle reaction driven by red light. Overall, this work contributes to expanding the photophysical and photochemical performance limits of first-row transition metal complexes and opens new perspectives for applications in luminescence and photocatalysis.