Molecular-based Hybrid Materials for Artificial Photosynthesis: Exploring new Horizons in Sustainable Energy Solutions

Humanity is currently facing an increasing occurrence of extreme weather conditions and climate events, which are directly associated to global warming and the unlimited use of fossil fuels as energy source. The scientific community is thus attempting to face climate change by focusing on finding new sustainable alternatives. Solar energy is particularly interesting and promising in this regard, since it represents an almost unlimited energy source and is clean, sustainable and safe. The direct conversion of solar energy into chemical bonds, such as liquid or gas fuels, allows an easy storage and transportation and can lead to the achievement of the net zero emission objective by 2050. In this doctoral thesis, the focus of the research is kept on the process of artificial photosynthesis via photo(electro)catalytic devices, where the solar radiation is harvested into absorbing organic materials and, through the interaction with specific molecular catalysts, transferred into green fuels via water oxidation reaction (WOR) to dioxygen and CO₂ reduction reaction (CO2RR) to hydrocarbon products.

In the first part of the thesis, we report the synthesis of a novel hybrid material based on a Covalent triazine-based framework (CTF) structure bearing dangling pyridyl groups that allow the anchoring of a Ru-based water oxidation catalyst (WOC) via covalent bonding. The assembly can carry out efficiently light-induced water oxidation (WO) at neutral pH, reaching values of TOFs and TONs of 17 h^-1 and 220, respectively, using sodium persulfate as a sacrificial electron acceptor.

In the second part of this work, the anchoring of discrete Co-based molecular catalysts on organic polymeric semiconductors via covalent bonding is presented, and the generated molecular hybrid materials are deeply studied for CO₂ photoreduction. The best molecular hybrid material achieves efficient and selective photoreduction of CO₂ to CO in aqueous buffer, giving high production rates in the range of 458 μmol g^-1 h^-1 and turnover numbers above 550 in 48 h, with no deactivation and no detectable H₂.

Finally, carbon nitride (CN) photoanodes are used for photoelectrochemical water oxidation by anchoring highly active molecular catalysts using two straight-forward strategies. The first case presents a Ru-based polymer interacting with the CN electrode by CH-πstacking, which enables a strong connection and an efficient electronic communication. The photoelectrochemical measurements were performed under 1 sun irradiation at neutral pH, exhibiting 89% faradaic efficiency for oxygen evolution, with TONs in the range of 3300 and TOFs of 0.4 s^–1, and good stability up to 5 h. In the second case, a Ru molecular complex was grafted to the CN photoanode via amidation reaction. The new system, due to the covalent bonding, achieves TONs and TOFs of 2 x 10⁴ and 0.6 s^-1, respectively, improving the values obtained in the first case described.

In summary, this doctoral thesis presents a range of new organic (photo)anodes and photocatalysts, rationally designed and functionalized with highly active molecular catalysts, with the aim of developing eco-friendlier artificial photosynthetic devices.

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