A combined spectroscopic, photophysical and theoretical (DFT) study of the electronically excited inorganometallic complexes [Ru(E)(E′)(CO)2(iPr-DAB) ] (E = Cl, Me, SnPh3, PbPh3; E′ = GePh3, SnR3, PbR3 (R = Me, Ph); iPr-DAB = N,N′-diisopropyl-1,4-diaza-1,3-butadiene): Evidence of an exceptionally long-lived 3σπ* excited state for [Ru(SnPh3)2(CO)2(iPr-DAB)]

The photophysical properties of the metal-metal bonded complexes [Ru(E)(E′)(CO)₂(iPr-DAB)] (E = Cl, E′ = SnPh₃, PbPh₃; E = Me, E′ = SnPh₃, PbPh₃; E = SnPh₃, E′ = SnMe₃, SnPh₃, GePh ₃; E = PbPh₃, E′ = PbMe₃, PbPh ₃, GePh₃; iPr-DAB = N,N′-diisopropyl-1,4-diaza-1,3- butadiene) have been studied. According to time-resolved emission, UV/vis and IR spectra, combined with density functional (DFT) MO calculations, the lowest excited state has triplet metal-to-ligand charge-transfer (³MLCT), triplet halide-to-ligand charge-transfer (³XLCT) or ³σ_(E-Ru-E′)π* character, depending on the nature and combination of the ligands E and E′. The ³σ_(E-Ru-E′)π* state is a bound state whose lifetime is strongly influenced by the ligands E and E′. An exceptionally long lifetime (264 μs at 80 K) is observed for the ³σ_(Sn-Ru-Sn)π* state of the symmetrically substituted [Ru(SnPh₃)₂(CO)₂(iPr-DAB)] complex. The reason for this long lifetime is the fact that the excited state of this complex is hardly distorted with respect to the ground state, owing to the delocalised character of the σ(Sn-Ru-Sn) bonding orbital, which mixes strongly with the π* orbital of the iPr-DAB ligand. This delocalisation is also responsible for the unusually high oscillator strength of the σ → π* electronic transition in the visible spectral region.