The photophysics of photoredox catalysis: a roadmap for catalyst design

Abstract

Recently, the use of transition metal based chromophores as photo-induced single-electron transfer reagents in synthetic organic chemistry has opened up a wealth of possibilities for reinventing known reactions as well as creating new pathways to previously unattainable products. The workhorses for these efforts have been polypyridyl complexes of Ru(ii) and Ir(iii), compounds whose photophysics have been studied for decades within the inorganic community but never extensively applied to problems of interest to organic chemists. While the nexus of synthetic organic and physical-inorganic chemistries holds promise for tremendous new opportunities in both areas, a deeper appreciation of the underlying principles governing the excited-state reactivity of these charge-transfer chromophores is needed. In this Tutorial Review, we present a basic overview of the photophysics of this class of compounds with the goal of explaining the concepts, ground- and excited-state properties, as well as experimental protocols necessary to probe the kinetics and mechanisms of photo-induced electron and/or energy transfer processes.

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development

“…However, Stern–Volmer quenching is insufficient to distinguish between energy- and electron-transfer processes. Transient absorption spectroscopic studies may be able to address this question 51 and provide a stronger basis for further development. Importantly, the mechanistic difference may strongly bias efforts in catalyst development to improve the efficiency of these reactions.…”

Photosensitized, energy transfer-mediated organometallic catalysis through electronically excited nickel(II)

“…At the same time, these metallophotocatalysts have also found widespread use as photosensitizers to facilitate energy transfer with a diverse range of organic substrates (14–17). Indeed, these studies stand as a direct analogy to the abundance of organic excited-state reactions that can be accessed via organocatalytic photosensitization (e.g., with benzophenone), as popularized in part by Schenck, Turro, and Hammond and colleagues in the 1950s and 1960s (18–20).…”

A Fully Automated Continuous‐Flow Platform for Fluorescence Quenching Studies and Stern–Volmer Analysis

“…Once in their excited states, photocatalysts decay to their ground energy levels following either a radiative or non‐radiative process (according to the kinetic constant k 0 , which is a property of every chromophore) 3b. In other words, a photocatalyst dissipates the energy acquired through light absorption by either emitting light or heat.…”

“…To determine the rate at which an organic substrate can quench the excited state of a photocatalyst, a Stern–Volmer analysis can be conducted 3b, 6. In this case, the emission of a photocatalyst is measured at increasing concentration of the quencher 7.…”

“…According to the Stern–Volmer relationship (Scheme 4), plotting the fraction of emission (i.e., photons emitted in absence of quencher over photons emitted in presence of quencher) against the concentration of quencher yields a linear relation, also known as Stern–Volmer plot (Scheme 4 B) 3b. If the lifetime of the excited state ( τ 0 ) of the photocatalyst is known, the quenching rate constant K q can be derived from the slope of the Stern–Volmer plot.…”