Metal-to-Ligand Charge-Transfer Emissions of Ruthenium(II) Pentaammine Complexes with Monodentate Aromatic Acceptor Ligands and Distortion Patterns of their Lowest Energy Triplet Excited States

This is the first report of the 77 K triplet metal-to-ligand charge-transfer (3MLCT) emission spectra of pentaammine–MDA–ruthenium(II) ([Ru(NH3)5(MDA)]2+) complexes, where MDA is a monodentate aromatic ligand. The emission spectra of these complexes and of the related trans-[Ru(NH3)4(MDA) (MDA’)]2+ complexes are closely related, and their emission intensities are very weak. Density functional theory (DFT) calculations indicate that the energies of the lowest 3MLCT excited states of Ru–MDA complexes are either similar to or lower than those of the lowest energy metal-centered excited states (3MCX(Y)), that the barrier to internal conversion at 77 K is large compared to kBT, and that the 3MCX(Y) excited states are weakly bound. The [Ru(NH3)5py]2+ complex is an exception to the general pattern: emission has been observed for the [Ru(ND3)5(d5-py)]2+ complex, but its lifetime is apparently very short. DFT modeling indicates that the excited state distortions of the different 3MC excited states are very large a...