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 ((MLCT)-M-3) emission spectra of pentaammine-MDA-rutheniurn(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 (MLCT)-M-3 excited states of Ru MDA complexes are either similar to or lower than those of the lowest energy metal-centered excited states ((MCX(Y))-M-3), that the barrier to internal conversion at 77 K is large compared to k(B)T, and that the (MCX(Y))-M-3 excited states are weakly bound. The [Ru(NH3)(5)py](2+) complex is an exception to the general pattern: emission has been observed for the [Ru(ND3)(5)(d(5)-py)](2+) complex, but its lifetime is apparently very short. DFT modeling indicates that the excited state distortions of the different (MC)-M-3 excited states are very large and are in both Ru ligand bonds along a single Cartesian axis for each different (MC)-M-3 excited state, nominally resulting.in (MCX(Y))-M-3, (MC(X)Y)-M-3, and (MCZ)-M-3 lowest energy metal-centered states. The (MCX(Y))-M-3 and (MCX(Y))-M-3 states appear to be the pseudo-Jahn Teller distorted components of a (MCX(Y))-M-3 state. The (MCX(Y))-M-3 states are distorted up to 0.5 angstrom in each H3N-Ru-NH3 bond along a single Cartesian axis in the pentaammine and trans-tetraammine complexes, whereas the (MCZ)-M-3 states are found to be dissociative. DFT modeling of the (MLCT)-M-3 excited state of [Ru(NH3)(5)(py)](2+) indicates that the Ru center has a spin density of 1.24 at the (MLCT)-M-3 energy minimum and that the (MLCT)-M-3 -> (MCZ)-M-3 crossing is smooth with a very small barrier (<0.5 kcal/mol) along the D3N-Ru-py distortion coordinate, implying strong (MLCT)-M-3/(MC)-M-3 excited state configurational mixing. Furthermore, the DFT modeling indicates that the long-lived intermediate observed in earlier flash photolysis studies of [Ru(NH3)(5)py](2+) is a Ru-II-(eta(2)(C=C)-py) species.