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Abstract The many applications of photon upconversion—conversion of low‐energy photons into high‐energy photons—raises the question of the possibility of “electron upconversion”. In this Review, we illustrate how the reduction potential can be increased by using the free energy of exergonic chemical reactions. Electron (reductant) upconversion can produce up to 20–25 kcal mol−1of additional redox potential, thus creating powerful reductants under mild conditions. We will present the two common types of electron‐upconverting systems—dissociative (based on unimolecular fragmentations) and associative (based on the bimolecular formation of three‐electron bonds). The possible utility of reductant upconversion encompasses redox chain reactions in electrocatalytic processes, photoredox cascades, design of peroxide‐based medicines, firefly luminescence, and reductive repair of DNA photodamage.
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The Photoredox Paradox: Electron and Hole Upconversion as the Hidden Secrets of Photoredox Catalysis
Although photoredox catalysis is complex from a mechanistic point of view, it is also often surprisingly efficient. In fact, the quantum efficiency of a puzzlingly large portion of photoredox reactions exceeds 100% (i.e., the measured quantum yields (QYs) are >1). Hence, these photoredox reactions can be more than perfect with respect to photon utilization. In several documented cases, a single absorbed photon can lead to the formation of >100 molecules of the product, behavior known to originate from chain processes. In this Perspective, we explore the underlying reasons for this efficiency, identify the nature of common catalytic chains, and highlight the differences between HAT and SET chains. Our goal is to show why chains are especially important in photoredox catalysis and where the thermodynamic driving force that sustains the SET catalytic cycles comes from. We demonstrate how the interplay of polar and radical processes can activate hidden catalytic pathways mediated by electron and hole transfer (i.e., electron and hole catalysis). Furthermore, we illustrate how the phenomenon of redox upconversion serves as a thermodynamic precondition for electron and hole catalysis. After discussing representative mechanistic puzzles, we analyze the most common bond forming steps, where redox upconversion frequently occurs (and is sometimes unavoidable). In particular, we highlight the importance of 2-center-3-electron bonds as a recurring motif that allows a rational chemical approach to the design of redox upconversion processes.
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- Award ID(s):
- 2102579
- PAR ID:
- 10587673
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- Volume:
- 146
- Issue:
- 40
- ISSN:
- 0002-7863
- Page Range / eLocation ID:
- 27233 to 27254
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/jacs.4c10422
