Broadly versus Barely Variable Complex Chromophores of Planar Nickel(II) from κ3-N,N′,C and κ3-N,N′,O Donor Platforms

This work reports on complex chromophores of planar nickel(II) of the general formula [(kappa(3)-N,N',C/O)Ni-ER], which combines a kappa 2-N,N'-coordinate aromatic N-heterocycle with a pendant phenyl/phenol donor and the variable coligands ER (= carboxylato, phenolato, amido, and arylthiolato). The absorption properties of the planar complex chromophores vary broadly depending on the interplay of the three constituting components. Increasing the manifold of pi*-orbitals at the N-heterocyclic moiety through pi-extension along 1,10-phenanthroline, tetrahydrodipyridophenazine, and dipyridophenazine has only a small impact on the energies of charge transfer type transitions across the visible (vis) to near-infrared (NIR) spectral region and is due to an inversion among spatially distinct but energetically close unoccupied molecular orbitals. Substantial absorptivity across the visible spectral region rather associates with the increasing capability among carboxylato, phenolato, amido, and arylthiolato coligands to engage in Ni-O/N/S pi-bonding. This adds a mechanism for electronic coupling of coligand RE- and pi*-orbitals at the N-heterocycle fragment through occupied d orbitals at nickel, enabling light-induced charge transfer. Aryl-S- performs best in this regard, but {Ni-O/N/S}-to-(N-heterocycle-pi*) charge transfer efficacy depends on donor-acceptor orbital alignment and appears to be kinetically hindered except for amido coligands owing to polar effects. Irrespective of the nature of coligand RE-, the choice of the pendant phenyl/phenol side arm rules overall absorptivity by setting the energy of occupied frontier molecular orbitals.