Excited States of New Ru(II) Complexes: Controlling Reactivity

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In this project funded by the Chemical Structure, Dynamic & Mechanism B Program of the Chemistry Division, Professor Claudia Turro of the Department of Chemistry and Biochemistry at The Ohio State University, in collaboration with Professor Kim Dunbar (Texas A&M University) and Professor Randolph Thummel (University of Houston), aims at understanding the steps that take place at very early times (femtosecond to picosecond) after a transition metal complex absorbs a photon of light. These studies are important in applications that include solar energy conversion and photodynamic therapy. The project also includes the design of new complexes that release a wider range of potential drugs more efficiently, as well as those that exhibit dual-reactivity. These complexes will be useful for the photo-release of "chemical payloads" when exposed to light and to effect cell death via different mechanisms. The project combines ultrafast laser spectroscopy, synthesis, and computation, and is therefore well suited to the education of scientists at all levels. This collaborative team is also well positioned to provide the highest level of education and training for students underrepresented in science. Ruthenium (II) complexes have been widely used in numerous areas of science, including medicine, engineering, and materials science. Despite the fact that [Ru(bpy)3]2+ (bpy = 2,2´-bipyridine) is the prototypical inorganic compound for use in these areas, the ultrafast excited state dynamics and electronic structure of related complexes remain, in most cases, poorly understood. For the majority of these applications, deactivation through the low-lying ligand-field (3LF) metal-centered dd states is undesirable, since it reduces the emission and lifetime of the 3MLCT (metal-to-ligand charge transfer) excited state. In sharp contrast, the application of photoinduced drug release is aimed at increasing the population of the 3LF state(s) with Ru-L dissociative character. The major focus of the project is to understand the basic principles that govern photoinduced ligand exchange with the ultimate goal of being able to design new systems with improved ligand exchange yields with the primary elements being to effect efficient photorelease of strong ligands, use low energy visible and near-IR light, and to involve multiple pathways following excitation. The latter goal provides the potential to exhibit increased biological activity by virtue of the production of two reactive species that act via vastly different mechanisms, thus producing a "drug cocktail" contained in a single molecule. Moreover, the basic knowledge gained regarding photoinduced ligand exchange will also aid in the design or complexes that require the ligand exchange process to be eliminated in order to increase the lifetime of the 3MLCT state.