Dual-mode ligand release from transition metal complexes: Towards photoselectivity by complementary synthetic and ultrafast UV-VIS-MIR spectroscopic approaches

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The activation of transition metal complexes by a specific interaction with light can spark a desired reaction temporally and spatially in a well-defined way, which is the basis of numerous applications in scientific fields ranging from engineering and synthetic chemistry to biomedical research. Photo-induced ligand release has emerged in the past years as an effective approach, commonly realized by a functional group serving as a monodentate ligand in a transition metal complex from which it can be photolyzed off. In the present proposal, this course of action will be extended to compounds with two non-equivalent or non-identical ligands which can be liberated upon electronic excitation, with the goal to unravel how and to which extent photoselectivity can be accomplished in such substances. Identification and characterization of the underlying dual-mode ligand release mechanisms from transition metal complexes will be pursued by advanced methods from synthetic organometallic and coordination chemistry, expedient simulation techniques, and versatile ultrafast UV-Vis-MIR spectroscopy. The studies will comprise a variety of systems with two leaving groups, initially carbonyl and nitrosyl, further along the project also incorporating one or two pyridine derivatives. The emphasis is on molybdenum and ruthenium complexes, with other metal centers also envisioned for a profound comparison. For deciphering the compounds’ suitability for dual-mode ligand release, the electronic properties will be rationally tuned by introducing orthogonal functional groups either on the ligands to be released or the coligand sphere to influence the leaving groups in an indirect yet predetermined way. The impact of these modifications is explored in comprehensive ultrafast photochemical studies, which will also disclose the wavelength-dependence of the incipient reaction in a time-resolved manner. Moreover, in an advanced approach for selective ligand release, labilized excite-state structures are generated by a first pump laser pulse and subsequently re-excited for ligand expulsion by a second one. In this way, the benefit of step-wise excitation for the goal of selective ligand release from compounds bearing two potential leaving groups is elucidated.As prospective achievement, the studies will demonstrate new reactivity patterns for photo-induced ligand release from transition metal complexes, providing a conclusive picture of how photoselectivity of dual-mode ligand release can be realized, in which systems and under which conditions.