MARCUS · Mapping Reaction Pathways Using Transient Ultrafast Spectroscopies: Kinetic and Mechanistic Investigation of Photoredox Catalysed Reactions

Mapping Reaction Pathways Using Transient Ultrafast Spectroscopies: Kinetic and Mechanistic Investigation of Photoredox Catalysed Reactions

Catalysts form the backbone of chemistry and biology because of their ability to enhance and drive chemical reactions without being consumed in the process. In recent decades, a new class of catalysis, termed as photoredox catalysis, has gained popularity amongst synthetic chemists. The term photoredox catalysis is made up of photo - which refers to a photon of light promoting a catalyst to its electronically excited state, and redox - referring to the catalyst then initiating a sequence of chemical reactions by either donating or accepting an electron, i.e., reduction or oxidation from its excited state. The reason photoredox catalysis has become so popular is because it presents an opportunity to perform chemical reactions in a sustainable and environmental friendly way: (i) the chemistry can be driven by sunlight or cheap LED sources; (ii) the catalytic cycles are efficient and catalyst loading is small, allowing straightforward purification; and (iii) the chemistry works under mild conditions. In 2019 alone, more than 700 publications reported employing photoredox processes to drive different types of chemistries including but not limited to the synthesis of industrial polymers, pharmaceutical drugs and their precursors, and organic molecules. As the publications reporting newer photoredox applications are exploding in numbers, studies investigating their mechanistic underpinnings have become more important than ever to drive this emerging field further. The main objective of this project was to study the mechanistic and kinetic details of the modus operandi of these photocatalysts (PC) using laser based spectroscopic methods. A photoredox cycle involves multiple sequential steps from the ultrafast photoexcitation of the catalyst (10-15 seconds or femtoseconds) to the reaction completion and recovery of the PC which happens on much slower time scales (microseconds to milliseconds). Using ultrafast laser pulses (10-15 s) and experiments which spanned more than 10 orders of magnitude in time (femtoseconds to milliseconds), our objective was to probe each of these steps from start to completion, thereby revealing their role in controlling the catalytic process. To this end, using our laser facilities at the University of Bristol and at the Rutherford Appleton Laboratory, we have mechanistically investigated many different photoredox cycles. We were able to track several short-lived species, some previously unobserved, using time-resolved absorption spectroscopies. The outcomes from these studies have shed light on kinetic and mechanistic details in these reactions and have allowed us to refute or support the mechanisms proposed by synthetic chemists. The results from these studies will benefit synthetic and theoretical chemists alike. While the understanding of the mechanistic pathways can help synthetic chemists in designing more robust catalysts, the high quality experimental spectroscopic data can greatly help computational chemists in building better theoretical models by comparing their predictions to our data.

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