Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar Ni complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic Pt luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of MLCT states in square-planar Ni complexes by creating coordination environments that seemed particularly well adapted to the 3d valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the Ni center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into MLCT absorption bands and ultrafast intersystem crossing to the MLCT excited state, the latter relaxes onward into a metal-centered triplet state (MC). A torsional motion of the tridentate ligand and a Ni-carbon bond elongation facilitate MLCT relaxation to the MC state. The MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar Ni complexes with enhanced photophysical properties. The longest MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar Ni complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.