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

Abstract

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.

Photoactive Metal-to-Ligand Charge Transfer Excited States in 3d6 Complexes with Cr0, MnI, FeII, and CoIII

“…184 Recent work demonstrated that when this structural distortion is made more difficult by suitable molecular design (Figure 9b), then longer MLCT lifetimes are achievable. 137 Many different research avenues are currently pursued with photoactive Ni II complexes, 183,185−187 and much remains to be discovered yet concerning the basic photophysics and photochemistry of square-planar 3d 8 complexes. Luminescent and photochemically exploitable MLCT excited states could be one of the future targets, and this challenge seems even greater than in the case of 3d 6 complexes.…”

“…Many different research avenues are currently pursued with photoactive Ni II complexes, ,− and much remains to be discovered yet concerning the basic photophysics and photochemistry of square-planar 3d 8 complexes. Luminescent and photochemically exploitable MLCT excited states could be one of the future targets, and this challenge seems even greater than in the case of 3d 6 complexes …”

“…Luminescent and photochemically exploitable MLCT excited states could be one of the future targets, and this challenge seems even greater than in the case of 3d 6 complexes. 137 ■…”

“…Complexes of the type in Figure a are thermally stable variants of catalytically relevant reaction intermediates in cross-coupling reactions, − and time-resolved spectroscopic studies revealed that their MLCT excited states relax to an MC state within a few picoseconds, thereby undergoing a structural distortion from square-planar to tetrahedral geometry . Recent work demonstrated that when this structural distortion is made more difficult by suitable molecular design (Figure b), then longer MLCT lifetimes are achievable . Many different research avenues are currently pursued with photoactive Ni II complexes, ,− and much remains to be discovered yet concerning the basic photophysics and photochemistry of square-planar 3d 8 complexes.…”

“…184 (b) Longer MLCT excited state lifetimes are obtained by making the structural distortion from square-planar to tetrahedral more difficult 137. (c) Square-planar Ni II complex with an MLCT excited state lifetime of 48 ps in acetonitrile at room temperature 137.…”