Summary

The generation of molecular hydrogen with visible light typically relies on multi-component systems and theabsorption of two photons per catalytic turnover. Single-molecule photocatalysts are highly desirable, but they suffer from very low hydrogen evolution rates. Based on our prior work on two-photon mechanisms for efficient photoreductions in water [1–4], we aim to explore novel hydrogen generation pathways via single-molecule catalysts (and inexpensive sacrificial electron donors) with the hydrogen atom as key species. The latter will be produced through direct proton reduction by hydrated electrons [2,4,5] or via an essentially unexplored proton-coupled electron transfer to the solvent.

All inherently different mechanistic possibilities, which will be elucidated by (time-resolved) optical spectroscopy [6–8], produce highly reactive species capable of attacking the photocatalyst. These undesired reactions limit the achievable turnover numbers in neat water, as initial studies revealed. The incorporation of the photocatalyst into micelles [4,8–10], either facilitated by Coulombic interactions or by preparing catalysts with hydrophobic alkyl chains, is expected to increase the system stability drastically. This is most likely due to (i) catalyst shielding by the micelle, thereby protecting it from the attack by reactive species [10], and (ii) suppressing catalyst aggregation [6] that might cause both unproductive quenching and harmful local heating during irradiation. The effects of the micellar environment and of an additional intramicellar electron donor as co-dopant (defect) on the stability and the performance of the photocatalytic activity will be studied in detail. The main objective of this project is to develop and understand micro-heterogeneous aqueous systems with tailor-made micelles as novel ”hydrogen factories”. The combination of several experimental techniques from different areas such as soft matter, supramolecular chemistry and photochemistry will pave the way for our ambitious goal.

[1] Pfund, B.; Steffen, D. M.; Schreier, M. R.; Bertrams, M.-S.; Ye, C.; Börjesson, K.; Wenger, O. S.; Kerzig, C. UV Light Generation and Challenging Photoreactions Enabled by Upconversion in Water. J. Am.
Chem. Soc. 2020, 142 (23), 10468–10476. https://doi.org/10.1021/jacs.0c02835.

[2] Kerzig, C.; Guo, X.; Wenger, O. S. Unexpected Hydrated Electron Source for Preparative Visible-Light Driven Photoredox Catalysis. J. Am. Chem. Soc. 2019, 141 (5), 2122–2127. https://doi.org/10.1021/jacs.8b12223.

[3] Kerzig, C.; Wenger, O. S. Sensitized Triplet–Triplet Annihilation Upconversion in Water and Its Application to Photochemical Transformations. Chem. Sci. 2018, 9 (32), 6670–6678.
https://doi.org/10.1039/C8SC01829D.

[4] Kerzig, C.; Goez, M. Combining Energy and Electron Transfer in a Supramolecular Environment for the “Green” Generation and Utilization of Hydrated Electrons through Photoredox Catalysis. Chem.Sci. 2016, 7 (6), 3862–3868. https://doi.org/10.1039/C5SC04800A.

[5] Kerzig, C.; Wenger, O. S. Reactivity Control of a Photocatalytic System by Changing the Light Intensity. Chem. Sci. 2019, 10 (48), 11023–11029. https://doi.org/10.1039/C9SC04584H.

[6] Neumann, S.; Wenger, O. S.; Kerzig, C. Controlling Spin-Correlated Radical Pairs with Donor–Acceptor Dyads: A New Concept to Generate Reduced Metal Complexes for More Efficient Photocatalysis. Chem. – Eur. J. 2021, 27 (12), 4115–4123. https://doi.org/10.1002/chem.202004638.

[7] Neumann, S.; Kerzig, C.; Wenger, O. S. Quantitative Insights into Charge-Separated States from One-and Two-Pulse Laser Experiments Relevant for Artificial Photosynthesis. Chem. Sci. 2019, 10 (21), 5624–5633. https://doi.org/10.1039/C9SC01381D.

[8] Bertrams, M.-S.; Kerzig, C. Converting p -Terphenyl into a Novel Organo-Catalyst for LED-Driven Energy and Electron Transfer Photoreactions in Water. Chem. Commun. 2021, 57 (55), 6752–6755. https://doi.org/10.1039/D1CC01947C.

[P9] Kerzig, C.; Hoffmann, M.; Goez, M. Resveratrol Radical Repair by Vitamin C at the Micelle-Water Interface: Unexpected Reaction Rates Explained by Ion-Dipole Interactions. Chem. – Eur. J. 2018, 24 (12), 3038–3044. https://doi.org/10.1002/chem.201705635.

[P10] Kohlmann, T.; Kerzig, C.; Goez, M. Laser-Induced Wurtz-Type Syntheses with a Metal-Free Photoredox Catalytic Source of Hydrated Electrons. Chem. – Eur. J. 2019, 25 (42), 9991–9996.https://doi.org/10.1002/chem.201901618.