ChemInform Abstract: Excited State Decay of Cyclometalated Polypyridine Ruthenium Complexes: Insight from Theory and Experiment

Deactivation pathways of the triplet metal-to-ligand charge transfer (3MLCT) excited state of cyclometalated polypyridine ruthenium complexes with [RuN5C]+ coordination are discussed on the basis of the available experimental data and a series of density functional theory calculations. Three different complex classes are considered, namely with [Ru(N^N)2(N^C)]+, [Ru(N^N^N)(N^C^N)]+ and [Ru(N^N^N)(N^N^C)]+ coordination modes. Excited state deactivation in these complex types proceeds via five distinct decay channels. Vibronic coupling of the 3MLCT state to high-energy oscillators of the singlet ground state (1GS) allows tunneling to the ground state followed by vibrational relaxation (path A). A ligand field excited state (3MC) is thermally accessible via a 3MLCT → 3MC transition state with the 3MC state being strongly coupled to the 1GS surface via a low-energy minimum energy crossing point (path B). Furthermore, a 3MLCT → 1GS surface crossing point directly couples the triplet and singlet potential energy surfaces (path C). Charge transfer states either with higher singlet character or with different orbital parentage and intrinsic symmetry restrictions are thermally populated which promote non-radiative decay via tunneling to the 1GS state (path D). Finally, the excited state can decay via phosphorescence (path E). The dominant deactivation pathways differ for the three individual complex classes. The implications of these findings for isoelectronic iridium(III) or iron(II) complexes are discussed. Ultimately, strategies for optimizing the emission efficiencies of cyclometalated polypyridine complexes of d6-metal ions, especially RuII, are suggested.