Donor-Acceptor Ligand-to-Ligand Charge-Transfer Coordination Complexes of Nickel(II)

A family of charge-transfer chromophores comprising square-planar nickel(II) complexes with one catecholate donor ligand and one a-diimine acceptor ligand is reported. The nine new chromophores were prepared using three different catecholate ligands and three different a-diimine ligands. Single-crystal X-ray diffraction studies on all members of the series confirm a catecholate donor nickel(11) a-diimine acceptor electronic structure. The coplanar arrangement of donor and acceptor ligands manifests an intense ligand-to-ligand charge-transfer (LL'CT) absorption band that can be tuned incrementally from 650 nm (1.9 eV) to 1370 nm (0.9 eV). Electrochemical studies of all nine complexes reveal rich redox chemistry with two one-electron reduction processes and two one-electron oxidation processes. For one dye, both the singly reduced anion and the singly oxidized cation were prepared, isolated, and characterized by EPR spectroscopy to confirm ligand-localization of the redox processes. The optical and electrochemical properties of these new complexes identify them as attractive candidates for charge-transfer photochemistry and solar-energy conversion applications.