Controlling ground and excited state properties through ligand changes in ruthenium polypyridyl complexes.

The capture and storage of solar energy requires chromophores that absorb light throughout the solar spectrum. We report here the synthesis, characterization, electrochemical, and photophysical properties of a series of Ru(II) polypyridyl complexes of the type [Ru(bpy)2(N–N)]2+ (bpy = 2,2′-bipyridine; N–N is a bidentate polypyridyl ligand). In this series, the nature of the N–N ligand was altered, either through increased conjugation or incorporation of noncoordinating heteroatoms, as a way to use ligand electronic properties to tune redox potentials, absorption spectra, emission spectra, and excited state energies and lifetimes. Electrochemical measurements show that lowering the π* orbitals on the N–N ligand results in more positive Ru3+/2+ redox potentials and more positive first ligand-based reduction potentials. The metal-to-ligand charge transfer absorptions of all of the new complexes are mostly red-shifted compared to Ru(bpy)32+ (λmax = 449 nm) with the lowest energy MLCT absorption appearing at λ...