Developing the Concept of Attractor Groups to Make Short-Lived Excited States of First-Row Transition Metal Complexes More Amenable to Photocatalysis

• Applicant: Dr. Florian Döttinger
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 541250497

Although impressive progress has been achieved in sustainable photoredox catalysis, contributions of novel, widely applicable concepts are highly demanded. In the context of sustainability especially 1st row transition metal catalysts are of interest which, however, commonly exhibit short-lived excited states. In this regard photoredox catalysts based on, for example, iron(III) are discussed in the literature. As result of their short-lived states, efficient catalytic transformations are significantly hampered in many cases. To counteract this issue high concentrations of the photoredox catalyst (PC), the sacrificial reagent (Q) and the substrate, as well as intense light irradiation are commonly utilized. However, such strategies are counterproductive within the idea of a sustainable and economical implementation of these comparably low-cost PCs. The applicant therefore proposes a widely applicable concept based on the pre-association of the Q to the PC. The approach especially aims to broaden the scope of applicable 1st row transition metal-based PCs. The fundamental idea consists of tethering an Attractor Group (AG) to the PC. The resulting association is expected to increase the interaction probability between PC and Q and can thereby cause a drastic enhancement in catalytic efficiency. On the basis of known kinetic models, an increase of the excited-state quenching rate constant of about four orders of magnitude is expected, which would achieve a boost in reaction efficiency by a factor of up to 5000. On a fundamental basis, the approach is likely be best suited for photocatalysts with particularly short excited state lifetimes. The novel approach shall be realized by following an interdisciplinary strategy. Possible associative interactions inherent to known Qs are identified. By carefully selecting complementary AGs and throughout their synthetic attachment, several novel derivatives of known PCs with short-lived excited states are prepared. Subsequently, detailed photophysical measurements are conducted in order to precisely characterize the attraction as well as the enhancement of the interaction probability between Q and PC (e.g. by time-resolved laser spectroscopy and nuclear magnetic resonance). In this context, fundamental kinetic models like “dynamic quenching” are important. In addition, quantum chemical calculations are consulted to support experimental findings and to grant deeper insight. Of high relevance is the demonstration of the applicability of the novel approach in photoredox catalysis. Hydrodehalogenation reaction and the photocatalytic splitting of water to hydrogen are investigated as model reaction. The project extends further by examining the impact of the molecular linker between PC and AG, as well as the simultaneous attachment of multiple AGs.

• DFG Programme: WBP Fellowship
• International Connection: Switzerland
• Host: Professor Dr. Oliver S. Wenger