Shining a New Light on Photoredox Catalysis and Small Molecule Activation

This project will provide detailed and much-needed understanding of fundamental reaction mechanisms in the emerging field of photoredox catalysis. This will be used to achieve long-sought-after but highly challenging new chemical transformations. These include the unprecedented direct conversion of inert but abundant dinitrogen directly into useful nitrogen-containing compounds, bypassing existing industrial methods that require extreme conditions, enormous energy input and inefficient, multi-step procedures.

Chemical synthesis - the process by which simple and readily-available chemical precursors are transformed into progressively more complex, functional and valuable compounds - is a crucial scientific discipline and a key foundation of modern society, providing access to the almost innumerable synthetic chemicals encountered throughout every branch of industry. In recent years, one of the most dramatic and exciting developments in this area has been the emergence of so-called "photoredox catalysis" (PRC). PRC reactions use visible light as an abundant energy source to drive complex and challenging chemical reactions under very mild conditions, using easy- and safe-to-handle reagents, and with high selectivity. As a result, they are typically significantly less hazardous, less wasteful, and more sustainable than existing options. The development of this field is therefore a crucial goal of modern synthetic chemistry that promises significant improvements to the environmental impact of a wide variety of chemical processes.

In order to facilitate the application of PRC methods towards these challenging goals it is essential to understand the underlying mechanisms - that is, the individual reaction steps that combine to give rise to these complex reactions - as it is this understanding that provides the framework for further progress. Unfortunately, investigations into these mechanisms are significantly underdeveloped, creating a substantial barrier to further advances. As the demands on new PRC reactions become ever more stringent (in terms of yield, selectivity, complexity and, pertinently, substrates) this problem is becoming ever more acute.

This project will provide a powerful new method for investigating PRC reactions. By carefully isolating proposed intermediate states of the catalyst it will become feasible to investigate individual reaction steps in a precisely controlled manner. By directly interrogating the elementary reaction steps of the catalytic cycle it will be possible to clearly and unambiguously establish a comprehensive mechanistic picture of PRC processes. The resulting deep understanding of established catalysts and reactions will permit their rapid optimisation, alongside the development of entirely new and unprecedented transformations.

As a compelling example, PRC will be used to facilitate the direct transformation of dinitrogen (N2) using mild reagents and under mild conditions. N2 is one of the single most important feedstocks for the modern chemical industry, acting as an abundant source of nitrogen atoms. However, the transformation of the highly inert N2 molecule is notoriously challenging and must currently be performed using the century-old Haber-Bosch process, which generates NH3 under extremely high temperatures and pressures and has an enormous environmental footprint (being responsible for roughly 2% of total world energy consumption). This NH3 can then be further transformed into other nitrogen-containing compounds, which typically requires multiple reaction steps, further limiting overall efficiency. In contrast, PRC methods will allow N2 activation to be performed under much milder conditions. Moreover, they will allow N2 to be transformed not only into NH3, but also directly into other useful nitrogen-containing compounds (e.g. triarylamine hole transporters used in OLEDs), thus bypassing the need for the laborious multi-step procedures needed to produce them via NH3.