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 (kr) and non-radiative (knr) 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, knr 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.