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Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer


Kuss-Petermann, Martin; Orazietti, Margherita; Neuburger, Markus; Hamm, Peter; Wenger, Oliver S (2017). Intramolecular Light-Driven Accumulation of Reduction Equivalents by Proton-Coupled Electron Transfer. Journal of the American Chemical Society, 139(14):5225-5232.

Abstract

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)(3)(2+) 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 similar to 15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH(2)) via reversible intramolecular electron transfer from the two TAA units (tau = 65 ps), followed by intermolecular proton transfer from TsOH (tau approximate to 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 (tau = 4.7 mu s) with an H/D kinetic isotope effect of 1.4 +/- 0.2. Moreover, the reoxidation of AQH(2) seems 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 AQH(2). 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 CO2 or the oxidation of H2O, can potentially be performed without sacrificial reagents.

Abstract

The photochemistry of a molecular pentad composed of a central anthraquinone (AQ) acceptor flanked by two Ru(bpy)(3)(2+) 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 similar to 15% of all excited pentad molecules, AQ is converted to its hydroquinone form (AQH(2)) via reversible intramolecular electron transfer from the two TAA units (tau = 65 ps), followed by intermolecular proton transfer from TsOH (tau approximate to 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 (tau = 4.7 mu s) with an H/D kinetic isotope effect of 1.4 +/- 0.2. Moreover, the reoxidation of AQH(2) seems 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 AQH(2). 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 CO2 or the oxidation of H2O, can potentially be performed without sacrificial reagents.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
Dewey Decimal Classification:540 Chemistry
Language:English
Date:12 April 2017
Deposited On:09 Mar 2018 13:01
Last Modified:18 Apr 2018 11:49
Publisher:American Chemical Society (ACS)
ISSN:0002-7863
Funders:Swiss National Science Foundation, NCCR Molecular Systems Engineering, URRP LightChEC of the University of Zurich
OA Status:Hybrid
Publisher DOI:https://doi.org/10.1021/jacs.7b01605
Project Information:
  • : FunderSNSF
  • : Grant ID
  • : Project TitleSwiss National Science Foundation
  • : Funder
  • : Grant ID
  • : Project TitleNCCR Molecular Systems Engineering
  • : Funder
  • : Grant ID
  • : Project TitleURRP LightChEC of the University of Zurich

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