jz201494n_si_001.pdf (495.86 kB)
Kinetic Isotope Effects in Reductive Excited-State Quenching of Ru(2,2′-bipyrazine)32+ by Phenols
journal contribution
posted on 2015-12-16, 20:38 authored by Catherine Bronner, Oliver S. WengerElectron transfer (ET) from phenol molecules to a photoexcited
ruthenium(II) complex was investigated as a function of the para-substituent
(R = OCH3, CH3, H, Cl, Br, CN) attached to the
phenols. For phenols with electron-donating substituents (R = OCH3, CH3), the rate-determining excited-state deactivation
process is ordinary ET. For all other phenols, significant kinetic
isotope effects (KIEs) (ranging from 2.91 ± 0.18 for R = Br to
10.18 ± 0.64 for R = CN) are associated with emission quenching,
and this is taken as indirect evidence for transfer of a phenolic
proton to a peripheral nitrogen atom of a 2,2′-bipyrazine ligand
in the course of an overall proton-coupled electron transfer (PCET)
reaction. Possible PCET reaction mechanisms for the various phenol/ruthenium
couples are discussed. While 4-cyanophenol likely reacts via concerted
proton–electron transfer (CPET), a stepwise proton transfer–electron
transfer mechanism cannot be excluded in the case of the phenols with
R = Br, Cl, and H.