Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore-Imidazole-Phenol Complex: Stepwise Oxidation and Concerted Reduction

Gerald F. Manbeck, Etsuko Fujita, Javier J. Concepcion

Research output: Contribution to journalArticle

33 Citations (Scopus)

Abstract

Proton-coupled electron transfer (PCET) reactions were studied in acetonitrile for a Photosystem II (PSII)-inspired [Ru(bpy)2(phen-imidazole-Ph(OH)(tBu)2)]2+, in which Ru(III) generated by a flash-quench sequence oxidizes the appended phenol and the proton is transferred to the hydrogen-bonded imidazole base. In contrast to related systems, the donor and acceptor are strongly coupled, as indicated by the shift in the RuIII/II couple upon phenol oxidation, and intramolecular oxidation of the phenol by Ru(III) is energetically favorable by both stepwise and concerted pathways. The phenol oxidation occurs via a stepwise ET-PT mechanism with kET = 2.7 × 107 s-1 and a kinetic isotope effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was characterized as adiabatic with λDA = 1.16 eV and 280 < HDA < 540 cm-1 consistent with strong electronic coupling and slow solvent dynamics. Reduction of the phenoxyl radical by the quencher radical was examined as the analogue of the redox reaction between the PSII tyrosyl radical and the oxygen-evolving complex. In our PSII-inspired complex, the recombination reaction activation energy is <2 kcal mol-1. The reaction is nonadiabatic (VPCET ≈ 22 cm-1 (H) and 49 cm-1 (D)) and concerted, and it exhibits an unexpected inverse KIE = 0.55 that is attributed to greater overlap of the reactant vibronic ground state with the OD vibronic states of the proton acceptor due to the smaller quantum spacing of the deuterium vibrational levels.

Original languageEnglish
Pages (from-to)11536-11549
Number of pages14
JournalJournal of the American Chemical Society
Volume138
Issue number36
DOIs
Publication statusPublished - Sep 14 2016

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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