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Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad

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Abstract

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis. Mapping these events in photophysical studies is an important step towards the development of efficient molecular photocatalysts. Three modular complexes comprised of a Ru-chromophore, an imidazole electron relay function, and a terpyridine unit as coordination site for a metal ion were synthesized and the light-induced electron transfer events studied by laser flash photolysis. In all cases, formation of an imidazole radical by internal electron transfer to the oxidized chromophore was observed. The effect of added base evidenced that the reaction sequence depends strongly on the possibility for deprotonation of the imidazole function in a proton-coupled electron transfer process. In the complex with MnII present as a proxy for a catalytic site, a strongly accelerated decay of the imidazole radical together with a decreased rate of back electron transfer from the external electron acceptor to the oxidized complex was observed. This transient formation of an imidazolyl radical is clear evidence for the function of the imidazole group as an electron relay. The implication of the imidazole proton and the external base for the kinetics and energetics of the electron trafficking is discussed.

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Scheme 1.
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Notes

  1. The significant absorption of the imidazole radical around 650 nm is responsible for the slow additional rise in the 605 nm kinetic traces (Fig. 1a) which is no longer purely attributable to the MV radical.

  2. Light-induced Mn oxidation in this kind of complexes has been demonstrated by EPR measurements in presence of irreversible electron acceptors.[17, 18] However, in these studies MnIII formation is not time-resolved and therefore the detection of Mn oxidation is not a proof for the occurrence of intramolecular ET.

  3. The same mechanism explains the absence of emission when the imidazole group is deprotonated in the GS.[14] The photoacid behavior is also the reason why this kind of complexes suffers from strong excited state quenching in aqueous solution.[19].

  4. The persistence of this wave at low concentrations of base could be related to oxidation by RuIII of the external imidazole coupled to deprotonation of the latter.

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Acknowledgements

This work has been supported by the French Infrastructure for Integrated Structural Biology (FRISBI) ANR-10-INSB-05-01 and by the Region Ile-de-France in the framework of C’Nano IdF. The Conseil Général d’Essonne (ASTRE) is acknowledged. The LABEX CHARMMMAT is also gratefully acknowledged.

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Authors and Affiliations

  1. Institut de Biologie Intégrative de La Cellule (I2BC), Université Paris Saclay, CEA, CNRS, 91191, Gif-sur-Yvette, France

    Philipp Gotico, Annamaria Quaranta, Sujitraj Sheth, Rajaa Farran & Winfried Leibl

  2. Institut de Chimie Moléculaire Et Des Matériaux d’Orsay (ICMMO), Université Paris Saclay, 91405, Orsay, France

    Christian Herrero, Zakaria Halime, Marie Sircoglou & Ally Aukauloo

  3. PhotoGreen Lab, Department of Chemistry, University of Pavia, 27100, Pavia, Italy

    Stefano Protti

  4. Department of Chemistry, University of Zürich UZH, Freiestrasse 3, CH-3012, Bern, Switzerland

    Reza Fallahpour

  5. Lebanese International University, Mazraa, Beirut, 146404, Lebanon

    Rajaa Farran

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  1. Philipp Gotico
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Pushing the limits of flash photolysis to unravel the secrets of biological electron and proton transfer - a topical issue in honour of Klaus Brettel.

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Gotico, P., Herrero, C., Protti, S. et al. Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad. Photochem Photobiol Sci 21, 247–259 (2022). https://doi.org/10.1007/s43630-021-00163-2

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