Theoretical and experimental correlations between solution phase and gas phase photoredox-reactivity of molecular vanadium oxide clusters

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Molecular metal oxide anions, so-called polyoxometalates (POMs) are versatile photoredox catalysts for important organic transformations including alcohol oxidations, epoxidations and C H-activations. Although POM photoreactivity is empirically well established, there is a fundamental lack of understanding of the mechanisms, which control the electronic structure of the POM photocatalyst in the excited state, the mode of charge separation upon photoexcitation and the effects of structural and chemical catalyst modification on the overall photoreactivity. Further, photochemical key processes such as substrate-catalyst pre-association and catalyst / solvent-interactions are not well understood. Knowledge-driven development of new POM-based photoredox-processes is therefore far from trivial.This project will address these challenges and aim at providing fundamental understanding of the structural and electronic features which control the photoredox-activity of polyoxovanadates (POVs). POVs are ideal POM photoredox catalyst models as they combine well-established photooxidative activity towards organic substrates (focus: alcohols as model bio-feedstock) with predictable (photo-)chemical tunability and visible light absorption. To understand the solution- and gas-phase processes which control the photooxidation steps, model reactions between selected substrates and POVs will examine substrate-catalyst pre-aggregation, photooxidation and product selectivity in solution and in the gas phase. The experimental studies will be performed using solution-phase photooxidation analyses (Streb group) as well as model studies in the gas phase using pump-probe fragmentation action spectroscopy, i.e. high-resolution electrospray ionization mass spectrometry coupled with femtosecond laser spectroscopy (Riehn group). Theoretical investigations of substrate-catalyst interactions, charge-distribution and relocalization upon photoexcitation as well as photooxidation reaction mechanisms will be performed using high-level, customized density functional theory calculations (Jacob group). The project will provide significant progress by (i) delivering tuneable visible light-active POV photooxidation catalysts; (ii) providing experimental and theoretical understanding of the photoredox-mechanism of these catalysts and (iii) demonstrate their performance for the selective conversion of model substrates into value-added products. For the priority program SPP2102, the project adds access to an important class of molecular photocatalysts as well as a unique combination of time-resolved experimental and theoretical expertise to investigate complex photoreactions in solution and the gas phase. In a potential 2nd funding phase, the project would study the theoretical prediction of high activity POV catalysts, their synthetic development and their use for the photooxidation of biogenic polyol substrates (e.g. glycols) to value-added products as carbonyls or carboxylic acids.