Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited-State Decay

Square-planar Ni^II complexes are interesting as cheaper and more sustainable alternatives to Pt^II luminophores widely used in lighting and photocatalysis. We investigated the excited-state behavior of two Ni^II complexes, which are isostructural with two luminescent Pt^II complexes. The initially excited singlet metal-to-ligand charge transfer (¹ MLCT) excited states in the Ni^II complexes decay to metal-centered (³ MC) excited states within less than 1 picosecond, followed by non-radiative relaxation of the ³ MC states to the electronic ground state within 9-21 ps. This contrasts with the population of an emissive triplet ligand-centered (³ LC) excited state upon excitation of the Pt^II analogues. Structural distortions of the Ni^II complexes are responsible for this discrepant behavior and lead to dark ³ MC states far lower in energy than the luminescent ³ LC states of Pt^II compounds. Our findings suggest that if these structural distortions could be restricted by more rigid coordination environments and stronger ligand fields, the excited-state relaxation in four-coordinate Ni^II complexes could be decelerated such that luminescent ³ LC or ³ MLCT excited states become accessible. These insights are relevant to make Ni^II fit for photophysical and photochemical applications that relied on Pt^II until now.