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

Abstract

This is the first report of the 77 K triplet metal-to-ligand charge-transfer ((3)MLCT) 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 (3)MLCT excited states of Ru-MDA complexes are either similar to or lower than those of the lowest energy metal-centered excited states ((3)MC(X(Y))), that the barrier to internal conversion at 77 K is large compared to kBT, and that the (3)MC(X(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 (3)MC excited states are very large and are in both Ru-ligand bonds along a single Cartesian axis for each different (3)MC excited state, nominally resulting in (3)MC(X(Y)), (3)MC((X)Y), and (3)MC(Z) lowest energy metal-centered states. The (3)MC(X(Y)) and (3)MC((X)Y) states appear to be the pseudo-Jahn-Teller distorted components of a (3)MC((XY)) state. The (3)MC(X(Y)) states are distorted up to 0.5 Å in each H3N-Ru-NH3 bond along a single Cartesian axis in the pentaammine and trans-tetraammine complexes, whereas the (3)MC(Z) states are found to be dissociative. DFT modeling of the (3)MLCT excited state of [Ru(NH3)5(py)](2+) indicates that the Ru center has a spin density of 1.24 at the (3)MLCT energy minimum and that the (3)MLCT → (3)MC(Z) crossing is smooth with a very small barrier (<0.5 kcal/mol) along the D3N-Ru-py distortion coordinate, implying strong (3)MLCT/(3)MC excited state configurational mixing. Furthermore, the DFT modeling indicates that the long-lived intermediate observed in earlier flash photolysis studies of [Ru(NH3)5py](2+) is a Ru(II)-(η(2)(C═C)-py) species.