Proton-Coupled Electron Transfer in a Ruthenium(II) Bipyrimidine Complex in Its Ground and Excited Electronic States

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 126; no. 27; pp. 4349 - 4358
• Main Authors: Drummer, Matthew C., Weerasooriya, Ravindra B., Gupta, Nikita, Askins, Erik J., Liu, Xiaolin, Valentine, Andrew J. S., Li, Xiaosong, Glusac, Ksenija D.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.07.2022
• Subjects: A: Structure, Spectroscopy, and Reactivity of Molecules and Clusters; Excited states; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ligands; Metal to ligand charge transfer; Reaction mechanisms; Solvents
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.2c02255

Proton-coupled electron transfer (PCET) was studied for the ground and excited electronic states of a [Ru­(terpy)­(bpm)­(OH2)­(PF6)2] complex, Ru-bpm. Cyclic voltammetry measurements show that the Ru­(II)-aqua moiety undergoes PCET to form a Ru­(IV)-oxo moiety in the anodic region, while the bpm ligand undergoes PCET to form bpmH2 in the cathodic region. The photophysical behavior of Ru-bpm was studied using steady-state and femtosecond transient UV–vis absorption spectroscopy, coupled with density functional theory (DFT) calculations. The lowest-lying excited state of Ru-bpm is described as a (Ru → bpm) metal-to-ligand charge-transfer (MLCT) state, while the metal-centered (MC) excited state was found computationally to be close in energy to the lowest-energy bright MLCT state (MC state was 0.16 eV above the MLCT state). The excited-state kinetics of Ru-bpm were found via transient absorption spectroscopy to be short-lived and were fit well to a biexponential function with lifetimes τ1 = 4 ps and τ2 = 65 ps in aqueous solution. Kinetic isotope effects of 1.75 (τ1) and 1.61 (τ2) were observed for both decay components, indicating that the solvent plays an important role in the excited-state dynamics of Ru-bpm. Based on the pH-dependent studies and the results from prior studies of similar Ru-complexes, we hypothesize that the 3MLCT state forms an excited-state hydrogen-bond adduct with the solvent molecules and that this process occurs with a 4 ps lifetime. The formation of such a hydrogen-bond complex is consistent with the electronic density accumulation at the peripheral N atoms of the bpm moiety in the 3MLCT state. The hydrogen-bonded state 3MLCT decays to the ground state with a 65 ps lifetime. Such a short lifetime is likely associated with the efficient vibrational energy transfer from the 3MLCT state to the solvent.