As I described in Chap. 1, metal complexes bearing π-conjugated chelating ligands are valid for both application and novel properties ( Sect. 1.1). Their unique photophysics, derived from a formation of long-lived charge transfer excited state, is of much interest for dye-sensitized solar cell, light-emitting devices, and photocatalyst. Reversible redox activities of metal complexes are useful in nanotechnology applications such as in molecular electronics, photoelectronic functions, and molecular machines, where the function of muscle is well-imitated at the molecular level.