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

Drummer, Matthew C.; Weerasooriya, Ravindra B.; Gupta, Nikita; Askins, Erik J.; Liu, Xiaolin; Valentine, Andrew J. S.; Li, Xiaosong; Glusac, Ksenija D.

doi:10.1021/acs.jpca.2c02255

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

Journal Article · Wed Jun 29 00:00:00 EDT 2022 · Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

DOI:https://doi.org/10.1021/acs.jpca.2c02255· OSTI ID:1877688

Drummer, Matthew C. ^[1]; Weerasooriya, Ravindra B. ^[1]; Gupta, Nikita ^[1]; Askins, Erik J. ^[1]; ^[2]; Valentine, Andrew J. S. ^[2]; ^[2]; ^[1]

Univ. of Illinois, Chicago, IL (United States); Argonne National Lab. (ANL), Lemont, IL (United States)
Univ. of Washington, Seattle, WA (United States)

In this work, 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 $$τ_1$$ = 4 ps and $$τ_2$$ = 65 ps in aqueous solution. Kinetic isotope effects of 1.75 ($$τ_1$$) and 1.61 ($$τ_2$$) 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.

View Accepted Manuscript (DOE)

Research Organization:: Argonne National Laboratory (ANL), Argonne, IL (United States). Laboratory Computing Resource Center (LCRC)

Sponsoring Organization:: National Science Foundation (NSF); USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division

Grant/Contract Number:: AC02-06CH11357

OSTI ID:: 1877688

Journal Information:: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory, Journal Name: Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory Journal Issue: 27 Vol. 126; ISSN 1089-5639

Publisher:: American Chemical SocietyCopyright Statement

Country of Publication:: United States

Language:: English

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

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