Nickel(II) Analogues of Phosphorescent Platinum(II) Complexes with Picosecond Excited‐State Decay
Tomohiro Ogawa, Oliver S. Wenger
Abstract
Abstract 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.