biostudies-literatureOgawa TSwiss National Science FoundationFonds National de la Recherche LuxembourgStaatssekretariat f?r Bildung, Forschung und InnovationJapan Society for the Promotion of Science21948-21960https://www.ebi.ac.uk/biostudies/studies/S-EPMC9732883biostudies-literatureUnknown144(48)Square-planar Ni^II complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic Pt^II 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 ³MLCT states in square-planar Ni^II 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 Ni^II center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into ¹MLCT absorption bands and ultrafast intersystem crossing to the ³MLCT excited state, the latter relaxes onward into a metal-centered triplet state (³MC). A torsional motion of the tridentate ligand and a Ni^II-carbon bond elongation facilitate ³MLCT relaxation to the ³MC state. The ³MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar Ni^II complexes with enhanced photophysical properties. The longest ³MLCT 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^II complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.Journal of the American Chemical SocietyMolecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.PMC97328832000202020.0027202160473200021_20732914583224Pfund BPrescimone AOgawa TWenger OSSinha NfalseMolecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.Square-planar Ni^II complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic Pt^II 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 ³MLCT states in square-planar Ni^II 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 Ni^II center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into ¹MLCT absorption bands and ultrafast intersystem crossing to the ³MLCT excited state, the latter relaxes onward into a metal-centered triplet state (³MC). A torsional motion of the tridentate ligand and a Ni^II-carbon bond elongation facilitate ³MLCT relaxation to the ³MC state. The ³MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar Ni^II complexes with enhanced photophysical properties. The longest ³MLCT 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^II complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.2022-01-01T00:00:00Z2022 Dec2025-04-20T03:34:23.247Z2025-04-20T03:34:23.247ZS-EPMC97328833641778210.1021/jacs.2c08838