Static and time-resolved spectroscopic studies of low-symmetry Ru(II) polypyridyl complexes

The spectroscopic and electrochemical properties of a series of four Ru-II polypyridyl complexes are reported. Compounds of the form [Ru(dmb)(x)(dea)(3-x)](2+) (x = 0-3), where dmb is 4,4'-dimethyl-2,2'-bipyridine and dea is 4,4'-bis (diethylamino)-2,2-bipyridine, have been prepared and studied using static and time-resolved electronic and vibrational spectroscopies as a prelude to femtosecond spectroscopic studies of excited-state dynamics. Static electronic spectra in CH3CN solution reveal a systematic shift of the MLCT absorption envelope from a maximum of 458 nm in the case of [Ru(dmb)(3)](2+) to 518 nm for [Ru(dea)(3)](2+) with successive substitutions of dea for dmb, suggesting a dea-based chromophore as the lowest-energy species. However, analysis of static and time-resolved emission data indicates an energy gap ordering of [Ru(dmb)(3)](2+) > [Ru(dmb)(2)(dea)](2+) > [Ru(dea)312+ > [Ru(dmb)(dea)(2)](2+), at variance with the electronic structures inferred from the absorption spectra. Nanosecond time-resolved electronic absorption and time-resolved step-scan infrared data are used to resolve this apparent conflict and confirm localization of the long-lived (MLCT)-M-3 state on dmb in all three complexes where this;ligand is present, thus making the dea-based excited state unique to [Ru(dea)(3)](2+). Electrochemical studies further reveal the origin of this result, where a strong influence of the dea ligand on the oxidative Ru-II/III couple, due to pi donation from the diethylamino substituent, is observed. The electronic absorption spectra are then reexamined in light of the now well-determined excited-state electronic structure. The results serve to underscore the importance of complete characterization of the electronic structures of transition metal complexes before embarking on ultrafast studies of their excited-state properties.