title: Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer.
abstract: The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)32+photosensitizers and two peripheral triarylamine (TAA) donors was investigated by transient IR and UV-vis spectroscopies in the presence of 0.2 M p-toluenesulfonic acid (TsOH) in deaerated acetonitrile. In ∼15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH2) via reversible intramolecular electron transfer from the two TAA units (τ = 65 ps), followed by intermolecular proton transfer from TsOH (τ ≈ 3 ns for the first step). Although the light-driven accumulation of reduction equivalents occurs through a sequence of electron and proton transfer steps, the resulting photoproduct decays via concerted PCET (τ = 4.7 μs) with an H/D kinetic isotope effect of 1.4 ± 0.2. Moreover, the reoxidation of AQH2seems to take place via a double electron transfer step involving both TAA+units rather than sequential single electron transfer events. Thus, the overall charge-recombination reaction seems to involve a concerted proton-coupled two-electron oxidation of AQH2. The comparison of experimental data obtained in neat acetonitrile with data from acidic solutions suggests that the inverted driving-force effect can play a crucial role for obtaining long-lived photoproducts resulting from multiphoton, multielectron processes. Our pentad provides the first example of light-driven accumulation of reduction equivalents stabilized by PCET in artificial molecular systems without sacrificial reagents. Our study provides fundamental insight into how light-driven multielectron redox chemistry, for example the reduction of CO2or the oxidation of H2O, can potentially be performed without sacrificial reagents.
authors.affiliation: Department of Chemistry, University of Basel , St. Johanns-Ring 19, 4056 Basel, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Kuss-Petermann
authors.affiliation: Department of Chemistry, University of Zurich , Winterthurerstrasse 190, 8057 Zurich, Switzerland.
authors.forename: Margherita
authors.initials: M
authors.lastname: Orazietti
authors.affiliation: Department of Chemistry, University of Basel , St. Johanns-Ring 19, 4056 Basel, Switzerland.
authors.forename: Markus
authors.initials: M
authors.lastname: Neuburger
authors.affiliation: Department of Chemistry, University of Zurich , Winterthurerstrasse 190, 8057 Zurich, Switzerland.
authors.forename: Peter
authors.identifier: http://orcid.org/0000-0003-1106-6032
authors.initials: P
authors.lastname: Hamm
authors.affiliation: Department of Chemistry, University of Basel , St. Johanns-Ring 19, 4056 Basel, Switzerland.
authors.forename: Oliver S
authors.identifier: http://orcid.org/0000-0002-0739-0553
authors.initials: OS
authors.lastname: Wenger