A New Strategy for Improvement of Photophysical Properties in Ruthenium(II) Polypyridyl Complexes. Synthesis, Photophysical and Electrochemical Characterisation of Six Mononuclear Ruthenium(II) Bisterpyridine Type Complexes

The synthesis and characterization of six ruthenium(II) bistridentate polypyridyl complexes is described. These were designed on the basis of a new approach to increase the excited-state lifetime of ruthenium(II) bisterpyridine-type complexes. By the use of a bipyridylpyridyl methane ligand in place of terpyridine, the coordination environment of the metal ion becomes nearly octahedral and the rate of deactivation via ligand-field (i.e., metal-centered) states was reduced as shown by temperature-dependent emission lifetime studies. Still, the possibility to make quasilinear donor-ruthenium-acceptor triads is maintained in the complexes. The most promising complex shows an excited-state lifetime of τ = 15 ns in alcohol solutions at room temperature, which should be compared to a lifetime of τ = 0.25 ns for [Ru(tpy)₂]²⁺. The X-ray structure of the new complex indeed shows a more octahedral geometry than that of [Ru(tpy) ₂]²⁺. Most importantly, the high excited-state energy was retained, and thus, so was the potential high reactivity of the excited complex, which has not been the case with previously published strategies based on bistridentate complexes.