Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds

The synthesis, electrochemistry, and photophysical characterization are reported for 11 tris­(bidentate) cyclometalated ruthenium­(II) compounds, [Ru­(N^N)2(C^N)]+. The electrochemical and photophysical properties were varied by the addition of substituents on the 2,2′-bipyridine, N^N, and 2-phenylpyridine, C^N, ligands with different electron-donating and -withdrawing groups. The systematic tuning of these properties offered a tremendous opportunity to investigate the origin of the rapid excited-state decay for these cyclometalated compounds and to probe the accessibility of the dissociative, ligand-field (LF) states from the metal-to-ligand charge-transfer (MLCT) excited state. The photoluminescence quantum yield for [Ru­(N^N)2(C^N)]+ increased from 0.0001 to 0.002 as more electron-withdrawing substituents were added to C^N. An analogous substituent dependence was observed for the excited-state lifetimes, τobs, which ranged from 3 to 40 ns in neat acetonitrile, significantly shorter than those for their [Ru­(N^N)3]2+ analogues. The excited-state decay for [Ru­(N^N)2(C^N)]+ was accelerated because of an increased vibronic overlap between the ground- and excited-state wavefunctions rather than an increased electronic coupling as revealed by a comparison of the Franck–Condon factors. The radiative (k r) and non-radiative (k nr) rate constants of excited-state decay were determined to be on the order of 104 and 107–108 s–1, respectively. For sets of [Ru­(N^N)2(C^N)]+ compounds functionalized with the same N^N ligand, k nr scaled with excited-state energy in accordance with the energy gap law. Furthermore, an Arrhenius analysis of τobs for all of the compounds between 273 and 343 K was consistent with activated crossing into a single, fourth 3MLCT state under the conditions studied with preexponential factors on the order of 108–109 s–1 and activation energies between 300 and 1000 cm–1. This result provides compelling evidence that LF states are not significantly populated near room temperature unlike many ruthenium­(II) polypyridyl compounds. On the basis of the underlying photophysics presented here for [Ru­(N^N)2(C^N)]+, molecules of this type represent a robust class of compounds with built-in design features that should greatly enhance the molecular photostability necessary for photochemical and photoelectrochemical applications.