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

Periodic Reporting for period 1 - MARCUS (Mapping Reaction Pathways Using Transient Ultrafast Spectroscopies: Kinetic and Mechanistic Investigation of Photoredox Catalysed Reactions)

Reporting period: 2018-05-01 to 2020-04-30

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.
The work performed during the duration of the project involved four different facets : (1) a training period for the applicant to learn about the operation of ultrafast lasers, experimental methods, data analysis etc; (2) Performing experiments on different reaction systems and their data analysis throughout the project period; (3) Performing computational studies to support and explain the experimental data; (4) Preparation and editing of manuscripts for dissemination of the results in the form of publications in peer reviewed journals.
The main objective of this project was to study the mechanistic details of photoredox catalyzed reactions using ultrafast transient spectroscopies. We have successfully investigated many photoredox reactions such as photoredox based controlled polymerizations, and the synthesis of smaller organic molecules using photoredox catalysis. The outcomes of these studies have been reported in three peer reviewed publications in high impact journals (such as Nature Communications, Chemical Science) so far and more are in preparation. These publications which are all open access are widely accessible and have led to further collaborations with international groups. These results were further disseminated to the wider scientific community through participation in conferences where the applicant presented a talk or poster. In addition to this, the European Researcher’s night in 2018 and 2019 was used as a platform to talk to the general public about the project and how important the fundamental understandings emerging through laser-based spectroscopies are in driving chemistry further.
There are many different sectors which will benefit from the outcomes of this project. The first and direct benefit can be expected for the scientific community (most importantly, synthetic chemists) who can use the knowledge obtained from these studies to further their own research. This has already become visible as dissemination of these results in conferences and peer reviewed journals has led to international collaborations with the host’s laboratory. Secondly, teaching about this research both by the host in his undergraduate and postgraduate classrooms as well as by the applicant in her outreach activities has raised awareness and triggered interest for chemistry and STEM research in younger audiences. Thirdly, the open access form of the publications can enable students and researchers in poorer developing countries to use these results to their benefit, particularly since photoredox reactions can be set up without much capital investment. Fine chemical, agrochemical and pharmaceutical sectors are also expected to benefit from the outcomes of these studies. For example, Merck is already investing in designing new catalysts and LED sources for photoredox reactions, and one project undertaken with collaborators at the University of Bath has generated sponsorship from Syngenta. As the outcomes of this project shed light on the fundamental working principles of these reactions, these and other companies can use these outcomes as well as from other laboratories in designing more robust catalysts and optimizing their large-scale synthetic processes.
Ultrafast spectroscopies can shed light on the mechanistic underpinnings of photoredox catalysis