Article pubs.acs.org/IC
Cite This: Inorg. Chem. 2017, 56, 13579-13592
Excited-State Decay Pathways of Tris(bidentate) Cyclometalated Ruthenium(II) Compounds Tyler C. Motley, Ludovic Troian-Gautier, M. Kyle Brennaman, and Gerald J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, United States
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ABSTRACT: 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 2phenylpyridine, 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 excitedstate 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.
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INTRODUCTION Ruthenium(II) polypyridyl compounds have been utilized in many applications spanning solar energy harvesting,1,2 photocatalysis,3,4 chemical sensing,5 and photodynamic therapy6,7 to name a few. This class of compounds is marked by their long (microsecond) excited-state lifetimes, photochemical and electrochemical stability, and tunability of their electronic structure through ligand modification. Tailoring these properties through design choices is made possible by the wellestablished, detailed understanding of the photophysics and photochemistry of ruthenium(II) polypyridyl compounds developed over the past 60 years.8−15 In the past decade, tris(bidentate) cyclometalated ruthenium(II) compounds, [Ru(N^N)2(C^N)]+, where N^N is a 2,2′-bipyridine (bpy) or substituted bpy ligand and C^N is the cyclometalating ligand 2phenylpyridine (ppy) or its derivatives, have received increasingly more attention because of the discovery of more mild synthetic procedures and overall solar conversion efficiencies of over 10% in dye-sensitized solar cells.16−19 © 2017 American Chemical Society
However, few detailed studies exist that characterize the excited-state photophysics of these compounds.20,21 Here, a systematic, electrochemical, and spectroscopic study of a series of [Ru(N^N)2(C^N)]+ compounds is reported. The dominant visible absorption feature of [Ru(bpy)3]2+, the prototypical ruthenium(II) polypyridyl compound, is a metalto-ligand charge-transfer (MLCT) band.9,22−25 Higher-energy transitions below 350 nm are ligand-centered π−π* transitions.9 Upon photoexcitation into the MLCT band, an electron is excited from a metal-centered t2g orbital to one of the π* orbitals on one of the bpy ligands. This transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) forms a 1MLCT state, which quickly undergoes intersystem crossing (
