Influence of Mesoionic Carbenes on Electro- and Photoactive Ru and Os Complexes: A Combined (Spectro-)Electrochemical, Photochemical, and Computational Study.

In recent years, mesoionic carbenes (MICs) are finding increasing use as building blocks of electro- and photoactive metal complexes. We present here a series of RuII and OsII polypyridine complexes where one or two pyridyl moieties of the well-known tris(bipyridine) analogues are replaced by MICs. We probe the structural, electrochemical, UV-vis-NIR/electron paramagnetic resonance spectroelectrochemical, and photophysical properties of these complexes as a function of the number of MICs in them. Insights from theoretical studies are used to describe the electronic structures of the various redox states. Additionally, electron flux density calculations provide an idea of the flow of electron densities in the excited states of these molecules. This is the first time that such electron flux density calculations are used to probe the excited state properties of transition metal complexes. Our results conclusively prove that the incorporation of MICs into Ru/Os-polypyridyl complexes has a profound influence on the ground and the excited state redox potentials, the position of the emission bands, as well as on the lifetimes of the excited states. These observations might thus be useful for the generation of novel photocatalysts and photosensitizers for dye-sensitized-solar-cells based on MICs.