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

Ogawa, Tomohiro and Sinha, Narayan and Pfund, Björn and Prescimone, Alessandro and Wenger, Oliver S.. (2022) Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes. Journal of the American Chemical Society, 144 (48). pp. 21948-21960.

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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 of 3MLCT 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 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 NiII-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 NiII 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 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.
Faculties and Departments:05 Faculty of Science > Departement Chemie > Chemie > Anorganische Chemie (Wenger)
UniBasel Contributors:Wenger, Oliver and Sinha, Narayan and Ogawa, Tomohiro and Pfund, Björn and Prescimone, Alessandro
Item Type:Article, refereed
Article Subtype:Research Article
Publisher:American Chemical Society
Note:Publication type according to Uni Basel Research Database: Journal article
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Last Modified:08 Jun 2023 08:34
Deposited On:15 Dec 2022 07:39

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