Charge-Transfer States and Two-Photon Photochemistry of ${\text{C}\text{u} (\text{N}\text{N})_2}^+$ Systems

Abstract

Because Cu(I) has a d¹⁰ electronic configuration, the metal-centered absorption bands tend to occur in the UV spectral region. But because the Cu(II) oxidation state is quite accessible, metal- to-ligand charge-transfer (CT) absorption bands appear in the visible spectrum when the ligands present low-lying acceptor orbitals. For example, Cu(I) complexes involving chelating heteroaromatic ligands (NN ligands) such as bipyridine (bpy) or 1, 10-phenanthroline (phen) or one of their derivatives are brightly colored. In fact, several different CT excited states are accessible in these systems. One reason is that the ligands present 2 low-lying acceptor levels which Orgel (1961) designated as χ and ψ orbitals. (They transform as symmetric and antisymmetric orbitals, respectively, under rotation about the two-fold axis (Fig. 1).) Further complexity arises in ${\text{C}\text{u} (\text{N}\text{N})_2}^+$ systems because the d shell of the metal is split into at least four sublevels. Finally, there is some evidence that a charge-transfer-to-solvent state can be populated when a high intensity pulsed laser source is used.