PROJECT SUMMARY/ABSTRACT Synthetic photochemistry is a powerful tool for biomedical research. The small-molecule therapeutics that constitute the core of modern molecular medicine occupy a relatively narrow segment of chemical diversity space. Photochemistry offers a new capability that can expand the range of chemical structures that can be investigated for their biological activity and their potential as new life-saving drugs. Specifically, reactions that are driven by light can use the energy of a photon to produce highly reactive intermediates that react in ways that are not accessible by other means. Our group's research focuses on the discovery of reaction methods that can control the outcomes of photochemical processes in predictable ways. We have a long-standing interest in the development of mechanistically novel catalytic processes that are directly applicable to the synthesis of complex bioactive compounds. We are also interested in the use of external stimuli that can control or divert the intrinsic reactivity of open-shelled photogenerated intermediates towards desirable synthetic goals. Finally, we are deeply committed to elucidating the mechanisms by which photochemical transformations occur. The next phase of our research will continue to investigate these broad themes. We propose investigations into new strategies for controlling the stereochemistry of diverse photochemical reactions. We will study a new approach towards the generation of carbocationic intermediates from photoredox activation. Finally, we will develop a new class of photoreactions that take advantage of the direct photochemistry of base metal coordination complexes. These studies will result in new, generalizable strategies for the controlled photochemical synthesis of complex organic molecules.