Ground- and Excited-State Structure and Spin State of a Nickel-Bipyridine Photocatalyst Revealed by X-ray Absorption Spectroscopy

Photo-assisted catalysis using Ni complexes is an emerging field for cross-coupling reactions in organic synthesis. However, the mechanism by which light enables and enhances reactivity of these complexes often remains elusive. Although optical techniques have been widely used to study the ground and excited states of photocatalysts, they lack the specificity to interrogate the electronic and structural changes at specific atoms. Herein we report metal-specific studies using static and transient Ni L- and K-edge X-ray absorption spectroscopy of a prototypical Ni photocatalyst, (dtbbpy)Ni(o-tol)Cl (dtb = 4,4-di-tert-butyl, o-tol = ortho-tolyl). We discovered that the ground state of this complex has a mixed-spin character of ~70/30% singlet/triplet. Furthermore, we confirm that the long-lived (~5 ns) excited state is a tetrahedral metal-centered triplet state. These results pave the way for the future design of Ni-bipyridine based photocatalysts by, for example, judiciously tuning the electronic and geometric properties of the ligands with the goal of increasing excited-state lifetimes and quantum yields of reactive species.