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

05 April 2022, Version 2
This content is a preprint and has not undergone peer review at the time of posting.

Abstract

Proton-coupled electron transfer (PCET) was studied for the ground and excited electronic states of a [Ru(terpy)(bpm)(OH2)(PF6)] complex, Ru-bpm. Cyclic voltammetry measurements show that the Ru(II)-aqua moiety undergoes PCET to form Ru(IV)-oxo moiety in the anodic region, while the bpm ligand undergoes PCET to form bpmH2 in the anodic 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 1=4 ps and 2=65 ps in aqueous solution. Kinetic isotope effect of 1.75 was 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 the 4 ps lifetime. The formation of such 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 short lifetime is likely associated with the efficient vibrational energy transfer from 3MLCT state to the solvent.

Keywords

ruthenium
polypyridyl
bipyrimidine
terpyridine
transient absorption spectroscopy
cyclic voltammetry
density functional theory
metal-to-ligand charge-transfer
metal-centered
short-lived excited state
PCET

Supplementary materials

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Additional details regarding experimental and computational methods. Supplemental figures, data, and results referenced in the main text.
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