Understanding the Kinetics and Spectroscopy of Photoredox Catalysis and Transition-Metal-Free Alternatives

Abstract

Over the past decade, the field of photoredox catalysis has gained increasing attention in synthetic organic chemistry because of its wide applicability in sustainable free-radical-mediated processes. Numerous examples have shown that under carefully optimized conditions, efficient and highly selective processes can be developed through excitation of a photosensitizer using inexpensive, readily available light sources. However, despite all of these recent advancements, some generalizations and/or misconceptions have become part of the photoredox culture, and often many of these discoveries lack in-depth investigations into the excited-state kinetics and underlying mechanisms. In this Account, we begin with a tutorial for understanding both the redox properties of excited states and how to measure the kinetics of excited-state processes. We discuss the generalization of direct excitation of closed-shell species to generate more potent reductive or oxidative excited states, using the helium atom as a quantitative example. We also outline how to apply redox potentials to calculate whether the proposed electron transfer events are thermodynamically feasible. In the second half of our tutorial, we discuss how to measure the kinetics of excited-state processes using techniques such as steady-state and time-resolved fluorescence and transient spectroscopy and how to apply the data using Stern-Volmer and kinetic analysis. Then we shift gears to discuss our recent contributions to the field of photoredox catalysis. Our lab focuses on developing transition-metal-free alternatives to ruthenium and iridium bipyridyl complexes for these transformations, with the goal of developing systems in which the reaction kinetics is more favorable. We have found that methylene blue, a member of the thiazine dye family, can be employed in photoredox processes such as oxidative hydroxylations of arylboronic acids to phenols. Interestingly, we were able to demonstrate that methylene blue is more efficient for this reaction than Ru(bpy)3Cl2, which upon further examination using transient spectroscopic techniques we were able to relate to the reductive quenching ability of the aliphatic amine. Recently we were also successful in applying methylene blue for radical trifluoromethylation reactions, which is discussed in detail. Finally, we have also demonstrated that common organic electron donors, such as α-sexithiophene, can be used in photoredox processes, which we demonstrate using the dehalogenation of vic-dibromides as a model system. This is a particularly interesting system because well-defined, long-lived intermediates allowed us to fully characterize the catalytic cycle. Once again, through an in-depth kinetic analysis we were able to gain valuable insights into our reaction mechanism, which demonstrates how powerful a tool proper kinetic analysis can be in the design and optimization of photoredox processes.

Customizing Photoredox Properties of PXX‐based Dyes through Energy Level Rigid Shifts of Frontier Molecular Orbitals

“…The substrate‐dependent photoreactivity of the diverse PXX derivatives engendered a mechanistic study, by means of which we could disclose more in depth the chemical properties of the photogenerated radical intermediates during their transformation. Stern–Volmer analysis (Table ) showed that the fluorescence quenching rates observed for derivatives PXXMI and PXXDI with PhCOCH 2 Br ( k q =2.2×10 8 and 4.5×10 8 m −1 s −1 , respectively) are considerably lower than those determined with DIPEA ( k q =4.2×10 9 and 5.8×10 9 m −1 s −1 ) in CH 3 CN. This suggests that, under illumination, PXXMI .− and PXXDI .− are likely formed through oxidation of DIPEA into DIPEA .+ (Scheme ).…”

Micellized Tris(bipyridine)ruthenium Catalysts Affording Preparative Amounts of Hydrated Electrons with a Green Light‐Emitting Diode

“…for conciseness. This comparative study pays heed to recently voiced concerns that mechanistic and kinetic studies on photoredox catalytic systems are much too scarce …”

Reductive Amination by Photoredox Catalysis and Polarity‐Matched Hydrogen Atom Transfer

“…[1] Recently, a-aminoalkyl radical intermediates have received considerable attention, in particular as one-electron photooxidation products of amines. [1] Recently, a-aminoalkyl radical intermediates have received considerable attention, in particular as one-electron photooxidation products of amines.…”