In research area C, we will investigate such assembly and solvation effects in photoreactions and photocatalysis with the aim of developing a more general picture and rational use of dispersion interactions in photochemical synthesis.
Introduction. The distance and relative orientation between a substrate molecule, to be activated for a reaction by energy or electron transfer, and the light-absorbing organic photocatalyst are crucial for the success and efficiency of photocatalytic transformations. In intermolecular reactions, such substrate-photocatalyst interactions controlling the assembly are not well defined and are rarely designed. Despite all recent achievements in applying organic photocatalysts in synthesis, the prediction of a suitable catalyst from thermodynamic data remains a challenge. Based on redox potential data from electrochemistry and excitation energies derived from optical spectra, excited state redox potentials can be estimated and used to predict the free energy of electron transfer to or from a given substrate. However, despite several organic photocatalysts fulfilling the thermodynamic criteria of a reasonably exergonic electron transfer in the excited state typically only few are competent in a given synthetic transformation. Screening of a series of photocatalysts is necessary to identify a working catalyst. The kinetics of photoinduced electron transfer (PET) or energy transfer depend on the molecular orientations of and distance between photocatalyst and reactant. This becomes particularly relevant for excited radical ions with doublet state lifetimes in the picosecond regime. A productive electron transfer cannot occur via diffusion controlled encounters and requires a preorganization of photocatalyst and reactant before excitation. Likewise, the assembly of reactants on the 2D surface of heterogeneous photocatalysts may be rate-determining for successful reactions, but little information is available. If the interactions between organic photocatalysts and reactants are better known and even become predictable, higher efficiencies and better selectivities in chemical photocatalysis with organic photocatalysts can be achieved.
Summary of the project. We aim to determine non-covalent substrate-photocatalyst interactions for several reaction types and elucidate their effect on the course of the reaction. With this insight, we will gain control over the assembly, which will be leveraged to improve the efficiency and selectivity of challenging photocatalytic transformations. We divide the project in three work packages (WPs) based on the type of organic photocatalyst and its intermolecular interaction with the substrate. WP 1 aims to control the assembly of in situ generated radical ion organic photocatalysts with substrates. In WP 2, we use anionic organic photocatalysts and their counter ions for assembly control with substrates. WP 3 focusses on photocatalyst – substrate assembly at interfaces (liquid-liquid, liquid-gas) and 2D-surfaces of heterogeneous carbon nitride photocatalysts.
A. Chatterjee, B. König ‘Birch-Type Photoreduction of Arenes and Heteroarenes by Sensitized Electron Transfer’ Angew. Chem. Int. Ed. 2019, 58, 14289-14294.
Q.-Y. Meng, T. E. Schirmer, A. L. Berger, K. Donabauer, B. König ‘Photocarboxylation of Benzylic C-H bonds‘ J. Am. Chem. Soc. 2019, 141, 11393-11397.
M. Schmalzbauer, T. D. Svejstrup, F. Fricke, P. Brandt, M. J. Johansson, G. Bergonzini, B. König ‘Redox-neutral Photocatalytic C-H Carboxylation of Arenes and Styrenes with CO2’ Chem 2020, 6, 2658-2672.
M. Schmalzbauer, M. Macron, B. König ‘Excited State Anions in Organic Transformations’ Angew. Chem. Int. Ed. 2021, 60, 6270 – 6292.
A. Savateev, I. Ghosh, B. König, M. Antonietti ‘Photoredox catalytic organic trans-formations using heterogeneous carbon nitrides’ Angew. Chem. Int. Ed. 2018, 57, 2-14.
I. Ghosh, T. Ghosh, J. I. Bardagi, B. König ‘Reduction of aryl halides by consecutive visible light-induced electron transfer processes’ Science 2014, 346, 725-728.
I. Ghosh, B. König ‘Chromoselective Photocatalysis: Controlled Bond Activation through Light-Color Regulation of Redox Potentials’ Angew. Chem. Int. Ed. 2016, 55, 7676-7679.
J. Haimerl, I. Ghosh, B. König, J. Vogelsang, J. M. Lupton ‘Single-molecule photoredox catalysis’ Chem. Sci. 2019, 10, 681-687.
M. Giedyk, R. Narobe, S. Weiß, D. Touraud, W. Kunz, B. König ‘Photocatalytic activation of alkyl chlorides by assembly-promoted single electron transfer in microheterogenous solutions’ Nat. Catal. 2020, 3, 40-47.
I. Ghosh, J. Khamrai, A. Savateev, N. Shlapakov, M. Antonietti, B. König ‘Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes’ Science 2019, 365, 360-366.
Organic Chemistry, Synthesis
The central goal of this project is to develop an ultrasensitive NMR tool for the detection and structural characterization of electron donor acceptor complexes in photocatalysis. Further goals are the detection, analysis and modulation of electron transfer processes and their impact in photocatalysis. In the third work package the development of a setup for the investigation of reactions with heterogeneous photocatalysts is envisioned.
Organic Chemistry Spectroscopy
Within project C3 we aim to unlock the potential of fluorinated alcohols on photoinduced electron transfer reactions with particular focus on polyene cyclizations. Our goal will be reached by taking advantage of the unique ability of F alcohols to form supramolecular structures in solution. Embedding photocatalysts within these H-bonding networks lead to distinct self-assembled catalytic entities that allow to efficiently control reactivity and selectivity. These multifunctional, enzyme-like properties will be harnessed in terpene cyclization cascades, mimicking the concept that nature beautifully orchestrates in biocatalytic ring-closing events.
Organic Chemistry, Synthesis
Complementary state-of-the-art electronic structure methods are combined with excited state dynamics to realize an accurate multiscale approach applicable to key processes in photocatalytic systems. Linear scaling methods are used to benchmark the extent of the quantum region needed for structural and dynamical calculations, while multiconfigurational methods capture detailed electronic structure changes. Selected examples of the CRC will be investigated aiming for an atomistic picture in direct comparison to NMR and transient spectroscopy. The insights are expected to stimulate optimization strategies for 2nd generation photocatalysts with improved performance.
Theoretical Chemistry, Theory
Enzymes are tremendously efficient and selective protein-based catalysts that have been optimized by nature over billions of years of evolution. They are in increasing demand to complement traditional chemical catalysis in industry to facilitate more sustainable processes. However, the development of artificial enzymes for reactions that go beyond nature’s synthetic repertoire remains a major challenge.
Photoexcitation has emerged as a powerful strategy to trigger new-to-nature activities, so far mainly in the context of flavin- and nicotinamide-dependent enzymes. In this project, we utilize an additional class of redox enzymes, carrying a pyrroloquinoline quinone (PQQ) cofactor, to promote light-driven radical chemistry. Importantly, the chiral protein environment may induce stereoselectivity by controlling the fate of reactive radical species, which are generated by photoinduced electron transfer. We aim to optimize these novel photoenzymes by directed evolution, which requires iterative cycles of mutagenesis and screening for the best performing variants.
Biochemistry, Protein Chemistry