Organic-Inorganic Supramolecular Photosynthesis: The Quantasome Vision

The use of solar-activated H₂O/CO₂ routines as a feed-stock for photosynthetic processes is nowadays a breakthrough concept for sustainable energy, solar fuels, green chemistry and food security. [1-2] The enormous potential of this vital cycle is far from being adequately exploited. An extraordinary research effort has been dedicated to elucidate the structural and mechanistic prerogatives of the natural oxygen evolving complex embedded within the photosystem II enzyme (PSII-OEC). [2] In the early studies, on Oxygenic Photosynthesis, the “quantasome hypothesis” led to seminal discoveries correlating the structure of natural photosystems with their complementary photo-redox functions.[1] Indeed, and despite the vast bio-diversity footprint, just one protein complex is used by Nature as the H₂O-photolyzer: photosystem II (PSII). Man-made systems are still far from replicating the complexity of PSII, showing the ideal co-localization of Light Harvesting antennas with the functional Reaction Center (LH-RC). A recent breakthrough in the field of artificial photosynthesis is the discovery of totally inorganic multi-redox clusters, as analogs of the PSII-OEC, with a common functional-motif, i.e. a redox-active, tetranuclear, metal-oxo core boosting H₂O oxidation to O₂ with unprecedented efficiency. Our vision points to a careful choice/design of the catalytic core, of its ligand set and of the surrounding nano-environment. We report herein a synthetic, spectroscopic and mechanistic study on the use multi-metal catalysts for water oxidation and their combined use with visible light sensitizers and carbon-based nanostructures (CNS). [3-4] In particular we will report on the design of multi-perylenebisimide (PBI) networks shaped to function by interaction with a polyoxometalate water oxidation catalyst (Ru₄POM). [5-7] Our results point to overcome the classical “photo-dyad” model, based on a donor-acceptor binary combination, with integrated artificial “quantasomes” formed both in solution and on photoelectrodes, showing a: (i) red-shifted, light harvesting efficiency (LHE&gt;40%), (ii) favorable exciton accumulation and negligible excimeric loss; (iii) a robust amphiphilic structure; (iv) dynamic aggregation into large 2D-paracrystalline domains. Photoexcitation of the PBI-quantasome triggers one of the highest driving force for photo-induced electron transfer applied so far. The outcome is a hybrid organic-inorganic nanomaterial,showing a hierarchical supramolecular structure with a striking resemblance to the natural plasmid membranes, enabling water splitting using low energy green photons at overpotentials as low as the natural protein. [5-7]<br/>References<br/>[1] Scheuring, S., Sturgis, J. N. Chromatic Adaptation of Photosynthetic Membranes. Science 309, 484–487 (2005);<br/>[2] Sartorel, A., Carraro, M., Toma, F. M., Prato, M., Bonchio, M. Shaping the beating heart of artificial photosynthesis: oxygenic metal oxide nano-clusters. Energy Environ. Sci. 5, 5592 (2012);<br/>[3] Piccinin, S.; Sartorel, A.; Aquilanti, G.; Goldoni, A.; Bonchio, M.; Fabris, S. Water oxidation surface mechanisms replicated by a totally inorganic tetraruthenium-oxo molecular complex. Proc. Natl. Acad. Sci. 110, 4917–4922 (2013)<br/>[4] Toma, F. M.; Prato, M.; Bonchio, M. et al. Efficient water oxidation at carbon nanotube–polyoxometalate electrocatalytic interfaces. Nature Chemistry 2, 826-831 (2010).<br/>[5] Bonchio, M.; Sartorel, A.; Prato, M. et al. Hierarchical organization of perylene bisimides and polyoxometalates for photo-assisted water oxidation. Nature Chemistry 11, 146-153 (2019).<br/>[6] Gobbo, P.; Bonchio, M.; Mann, S. et al. Catalytic processing in ruthenium-based polyoxometalate coacervate protocells Nature Commun 11, 41 (2020).<br/>[7] GobbatoT.; Rigodanza F.; Benazzi, E.; Prato, M.; Bonchio M. et al. J. Am. Chem. Soc.144,14021-14025 (2022).