The Webster Group and collaborators report the excited state (ES) linkage isomerization of a series of ruthenium complexes bearing a chelating triphenylphosphine-based sulfoxide ligand. The isomerization can be allowed (ΦS→O = 0.8) or forbidden (ΦS→O = 0.0) based on the remote [from ruthenium] substituents of the phosphine ligand, and this trend can be predicted via computational methods (DOI: 10.1021/jacs.8b05957). This behavior is not typically observed in transition metal complexes and presents the opportunity to have significant, predictable control over ES reactivity.
Controlling ES reactivity in a predictable manner is a central focus of photoscience. In collaboration with Professor Jeffrey Rack (University of New Mexico) and Professor Christopher Ziegler (University of Akron), we have studied a series of Ru-phosphine-sulfoxide complexes that exhibit drastic differences in excited state reactivity from a single remote [from ruthenium] substituent change. Naturally, understanding the reason for this change was of interest, so DFT was utilized to explore the subtle differences in these complexes. The S→O photoisomerization of the sulfoxide moiety is typically initiated by a MLCT excitation which essentially oxidizes the metal from Ru(II) to Ru(III). Time-dependent density functional theory (TD-DFT) demonstrates that for the p-H and p-CF3 containing complexes, a MLCT transition is indeed present in the ES. However, for the p-OCH3 complex, the ES is of ligand-to-ligand charge transfer (LLCT) character from a phosphine aryl ring to a bipyridine ligand. This work neatly demonstrates that by utilizing phosphine ligands, ES reactivity can be controlled and predicted according to logical principles.
G. K. Kosgei, D. Breen, R. W. Lamb, M. Y. Livshits, L. A. Crandall, C. J. Ziegler, C. E. Webster, J. J. Rack. "Controlling Photoisomerization Reactivity Through Single Functional Group Substitutions in Ruthenium Phosphine Sulfoxide Complexes" J Am Chem Soc, 2018, 140, 9819-9822.
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