Exploiting Quantum Phenomena in Earth Abundant Photocatalysts

Project Summary:

Photoinduced reactions are crucial to all life on Earth: from the primary steps of vision or photosynthesis, to photocatalysts in chemical synthesis. This latter category of reaction forms the focus of our investigations. We seek to gain a fundamental molecular level understanding of how light absorption by photosensitiser molecules makes them potent chemical reagents, and enables greater control and more sustainable chemical synthetic pathways. Our recent work in this area includes using low-cost photocatalysts to convert greenhouse gases, such as CO2, into valuable feedstocks; and novel organometallic photosensitisers that incorporate Earth-abundant metals.

Proust’s law, a fundamental principle of Chemistry, states that the properties of a chemical substance are independent of the method of preparation. For photoinduced transformations this prevailing wisdom is often violated when reactions proceed on excited states with different spin configurations (e.g. singlet or triplet states). Seminal examples where the spin of the electronic states dramatically change the outcome of a photochemical reaction include: the pyrimidine photodimerization reaction in DNA, where the deleterious transformation can lead to cancer, is enhanced by the availability of both singlet and triplet channels; photoinduced radical reactions in cryptochrome receptors in bird’s eyes that underpin the magnetic field sensitivity of migratory bird navigation.

Curiously, the spin-sensitivity of these reactions cannot be explained using classical theories and requires new experimental techniques to understand how quantum spin effects control them. Our research focuses on photoinduced ligand–ligand charge transfer in novel photocatalysts. Upon light irradiation, they form a pair of charge-transfer states with triplet and singlet spin configurations. The reactivity of the two spin-states are different, and the quantum correlations mean they also react in a non-statistical manner. The influence of quantum effects on the photocatalytic activity will be explored using cutting-edge ultrafast spectroscopic techniques. These advances will provide insights into spin-dependent reactivity, and the role of spin states in novel photocatalytic systems, ultimately enabling new photochemically driven syntheses.

Visitor Biography:

Stephen Bradforth is Professor of Chemistry at the University of Southern California. His lab designs experiments to understand how the inter-connected motions of molecules, and their quantum mechanical correlations, impact chemical reactions in complex environments. These include frequently encountered environments such as the solution reaction media for chemical synthesis, biochemical environments, water-air interfaces or in functional molecular materials. His research applies ultrafast laser techniques to address contemporary challenges such as UV damage to DNA, photochemistry, radical pairs, the mechanisms for solar energy conversion and photocatalysis as well as condensed phase ionization. His honors include a Packard Fellowship in Science and Engineering, the Research Corporation Cottrell Scholarship and STAR award, and the ACS Senior Experimental

Physical Chemistry Award. He is Fellow of the American Physical Society, the Royal Society of Chemistry and the American Association for the Advancement of Science.

At USC, Bradforth has served as department chair, divisional dean for Natural Sciences and most recently as Executive Vice Dean for the Dornsife College of Letters, Arts and Sciences. He serves as Director of one of five Association of American Universities department demonstration projects in the US to improve faculty evaluation and incentive structures in STEM departments so as to accelerate reform in undergraduate science instruction.

Bradforth earned his Ph.D. in Chemistry from the University of California, Berkeley in 1992 after completing his undergraduate education at Cambridge University in 1987 where he read Natural Sciences.