Mapping Pathways in Photo-Catalytic Cycles using Ultrafast Spectroscopy

Catalysts are widely used in reactions which produce chemicals for a variety of everyday applications including pharmaceuticals and advanced materials such as polymers. They enhance the rates at which the products form, and their use can avoid harsh process conditions such as high temperatures. Photocatalysts that are activated by visible light are attracting attention because cheap light sources such as light emitting diodes (LEDs) can be used to drive useful chemical reactions. There is also growing interest in replacing photocatalysts containing transition metals with more sustainable organic compounds. Despite the recent and rapid development of photocatalytic cycles tailored to carry out specific chemical transformations, relatively little effort has been devoted to understanding the ways in which the photocatalysts work (their mechanisms of action) and the properties of the photocatalysts which should be optimized for greater efficiency. The proposed research will make detailed observations of the reactive species involved in catalytic cycles and their lifetimes, and in favourable cases will aim to observe every step in a full catalytic cycle from its initiation to its termination by recovery of the catalyst in its starting form.

The timescales for production and removal of the reactive intermediates are short, typically corresponding to femtosecond to picosecond intervals (less than one billionth of a second). The ultrafast lasers to be used in this research are capable of generating pulses of ultraviolet and infrared light short enough to take snapshots of the changing concentrations of these transient species. Consequently, the individual steps in a sequence of chemical reactions can be observed in a single set of measurements. Ultraviolet spectra are particularly informative about activated intermediates in excited electronic states, whereas infrared spectra provide specific information about the different molecules and radicals present at any particular time.

These unprecedented studies will use two ultrafast lasers, one located at the University of Bristol and the other at the Rutherford Appleton Laboratory (RAL). The Bristol laser will act as the workhorse system, profiling reaction intermediates and studying reactions up to times of 1.3 nanoseconds from initiation. The most interesting systems will then be studied using a laser system at RAL which has the unique capability to observe reactions over 11 orders of magnitude of time (from 100 femtoseconds to 10 milliseconds) in single sets of measurements. With this remarkable capability, we will capture every step in a photocatalytic cycle from start to finish for the first time. The rates at which each step occurs can then be interpreted to determine which properties of the photocatalyst, reactive substrate and surrounding solvent are most important for determining the efficiency of the reaction. Armed with new insights of this type, we will design novel photocatalytic cycles for important chemical reactions, such as those that form new bonds between carbon atoms (an essential structural feature of organic molecules), and test their performance using the methods adopted by organic chemists.

The benefits will be widespread. Organic chemists designing more efficient pathways to chosen target molecules, for example for medicinal applications, will have an extended palette of reactions at their disposal. This greater chemical control will also open up new classes of molecule that can be synthesized. The chemical and pharmaceutical industries rely on chemical synthesis to create new products such as drugs or advanced materials with properties tailored precisely to specific applications. They will draw upon the knowledge gained to refine existing industrial processes, and will also improve their understanding of how to develop new processes by activation of flowing samples of chemicals by illumination with cheap light sources.

Beyond the academic beneficiaries summarized above, we anticipate significant consequences of our research for the UK chemical and pharmaceuticals industries. These benefits will primarily derive from deeper mechanistic understanding of photocatalysed reactions leading to improved processes for preparation of fine chemicals and pharmaceuticals over a range of production scales, for example using photochemical flow reactors with sustainable organic photocatalysts. The new chemical procedures developed may also offer access to new or challenging chemical spaces, allowing wider exploration of applications of novel compounds. The longer term aspiration is that the fundamental research described here will have benefits to the economy and to society, the former through more efficient and sustainable industrial processes and the latter through consequent enhancements of health and quality of life.

The research will lead directly to highly trained and employable personnel, with skills spanning synthetic chemistry and compound characterization, catalysis, transient ultra-violet/visible and infra-red spectroscopy, use of lasers, quantitative spectroscopic data analysis, kinetic modelling, electronic structure calculations, and communication of results. Although the postdoctoral researchers appointed to the project will each develop a subset of these skills for their primary roles, we expect cross fertilization, as well as complementary training of postgraduate students joining the project through the Bristol Chemical Synthesis Centre for Doctoral Training. Dissemination of the research outcomes to the wider synthetic organic chemistry community will introduce new methods for the study of reaction mechanisms to this active field of research.

A substantial effort will be made to communicate the principles of the research (photocatalysis, sustainable chemistry, applications of spectroscopy) to non-specialist audiences such as school-age children through development of outreach activities and materials. This communication of our research will therefore benefit those studying chemistry at pre-university level, other interested groups, and the researchers who will train in, and contribute to, these science communication actions.