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

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII 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 of3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 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 NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into1MLCT absorption bands and ultrafast intersystem crossing to the3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate3MLCT relaxation to the3MC state. The3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest3MLCT 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 NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.