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 λmax = 564 nm.
Emission energies decrease from λmax = 650 nm to
885 nm across the series. One-mode Franck–Condon analysis of
room-temperature emission spectra are used to calculate key excited
state properties, including excited state redox potentials. The impacts
of ligand changes on visible light absorption, excited state reduction
potentials, and Ru3+/2+ potentials are assessed in the
context of preparing low energy light absorbers for application in
dye-sensitized photoelectrosynthesis cells.