Christopher D. McTiernan

Mechanistic Insights and Kinetic Analysis for the Oxidative Hydroxylation of Arylboronic Acids by Visible Light Photoredox Catalysis: A Metal-Free Alternative

The photocatalytic hydroxylation of boronic acids with methylene blue as photosensitizer proceeds with high efficiency. Detailed time-resolved studies of the relevant rate constants provide a clear mechanistic understanding of excited-state processes and guided the selection of the photocatalyst and the optimization of experimental conditions.

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

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.

Visible-Light Actinometry and Intermittent Illumination as Convenient Tools to Study Ru(bpy)3Cl2 Mediated Photoredox Transformations.

Photoredox catalysis provides many green opportunities for radical-mediated synthetic transformations. However, the determination of the underlying mechanisms has been challenging due to lack of quantitative methods that can be easily implemented in synthetic labs, where this research tends to be centered. We report here on the development, characterization and calibration of a novel actinometer based on the photocatalyst tris(2,2′-bipyridyl)ruthenium(II) chloride (Ru(bpy)3Cl2). By using the same molecule as the photocatalyst and the actinometer, we eliminate problems associated with matching sample spectral distribution, lamp-sample spectral overlap and other problems intrinsic to doing quantitative photochemistry in a laboratory that has little expertise in this area. In order to validate our actinometer system in determining the quantum yield of a Ru(bpy)3Cl2 photosensitized reaction, we test the Ru(bpy)3Cl2 catalyzed oxidation of benzhydrol to benzophenone as a model chain reaction. We also revive the rotating sector method by updating the technique for modern LED technologies and demonstrate how intermittent illumination on the timescale of milliseconds to seconds can help probe a chain reaction, using the benzhydrol to benzophenone oxidation to validate the technique. We envision these methods to have great implications in the field of photoredox catalysis, providing researchers with valuable research tools.

Polynuclear gold(I) complexes in photoredox catalysis: understanding their reactivity through characterization and kinetic analysis

The light mediated reduction of unactivated carbon–halogen bonds using polynuclear gold(I) complexes provides a mild and temporally controlled route to the generation of C–H, and C–C bonds. Through complete photophysical and electrochemical charaterization of a series of gold(I) complexes we have been able to achieve a more intimate understanding of the complexities surrounding the excited state processes and mechanism which underly these transformations, allowing for, and facilitating further and future optimizations and applications.

Electron transfer from the benzophenone triplet excited state directs the photochemical synthesis of gold nanoparticles

The rarely recognized electron donating ability of the benzophenone triplet excited state provides an unusual route for the photochemical synthesis of gold nanoparticles.

Two-Photon Excitation of a Plasmonic Nanoswitch Monitored by Single-Molecule Fluorescence Microscopy.

Visible-light excitation of the surface plasmon band of silver nanoplates can effectively localize and concentrate the incident electromagnetic field enhancing the photochemical performance of organic molecules. Herein, the first single-molecule study of the plasmon-assisted isomerization of a photochrome-fluorophore dyad, designed to switch between a nonfluorescent and a fluorescent state in response to the photochromic transformation, is reported. The photochemistry of the switchable assembly, consisting of a photochromic benzooxazine chemically conjugated to a coumarin moiety, is examined in real time with total internal reflection fluorescence microscopy in the presence of silver nanoplates excited with a 633 nm laser. The metallic nanostructures significantly enhance the visible light-induced performance of the photoconversion, which normally requires ultraviolet excitation. The resulting ring-open isomer is strongly fluorescent and can also be excited at 633 nm. These stochastic emission events are used to monitor photochromic activation and show quadratic dependence on incident power. The utilization of a single laser wavelength for both photochromic activation and excitation effectively mimics a pseudo two-colours system.

Ketorolac beats ketoprofen: lower photodecarboxylation, photohemolysis and phototoxicity

Ketorolac shows reduced photohemolytic activity and low phototoxicity against human skin fibroblasts when compared to ketoprofen. The low decarboxylation quantum yield together with the efficient non-radiative deactivation of the triplet and singlet excited states of ketorolac are believed to be responsible for this behaviour.