Theoretical characterization of the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium dication complex

We present a theoretical study of the ground and the lowest triplet excited states of the tris-(1,4,5,8-tetraazaphenanthrene) ruthenium complex [Ru(tap)(3)](2+). Density functional theory (DFT) was used to obtain the relaxed geometries and emission energies (Delta-SCF), whereas time-dependent DFT (TD-DFT) was used to compute the absorption spectrum. Our calculations have revealed the presence of three low-lying excited-state minima, which may be relevant in the photophysical/photochemical properties of this complex. Two minima with similar energies correspond to the MLCT (3)A(2) and MLCT B-3 metal-to-ligand charge-transfer states, the first one corresponding to a D-3 structure, whereas the second is a slightly localized C-2 species. The third and lowest one corresponds to the metal-centered MC (3)A state and displays a pronounced C-2 distortion. We have examined for the first time the localized character of the excitation in the computed MLCT states. In particular, we have evaluated the pseudorotation barrier between the Jahn-Teller C-2 MLCT B-3 minima in the moat around the D-3 conical intersection. We have shown that the complex should be viewed as a delocalized [Ru3+(tap(-1/3))(3)](2+) complex in the lowest MLCT states, in agreement with subpicosecond interligand electron transfer observed by femtosecond transient absorption anisotropy study. Upper-bound estimates of the MLCT -> MC (3 kcal/mol) and MC -> MLCT (10 kcal/mol) activation energy barriers obtained from potential energy profiles in vacuum corroborate the high photoinstability of the MLCT states of the [Ru(tap)(3)](2+) complex.