Perylene Bisimide-Based Systems for Solar-Driven Water Oxidation

Solar-driven water splitting is a promising method for generating "solar hydrogen," a clean, renewable, and carbon-neutral energy source. By mimicking key principles from natural photosynthesis, such as compartmentalization, the use of diverse light-harvesting pigments, and efficient charge separation, artificial systems can be designed to replicate the process of water oxidation. This Thesis project involves the development of PSII-inspired artificial quantasomes (QS), composed of bis-cationic perylene bisimides (PBI) antennas and a deca-anionic tetraruthenate polyoxometalate (Ru4POM) serving as water oxidation catalyst. This Thesis investigates novel perylene bismide assemblies, focusing on the strategic design and control of PBIs aggregation through covalent and supramolecular methodologies aimed at optimizing the organization of the quantasome assembly and enhancing photosynthetic efficiency. The PhD work has been developed along three main research lines: i. The introduction of hydrophilic polyethylene glycol (PEG) cross-linkers among the PBI building blocks to promote the pairing of the QS units while enhancing water harvesting and transport, functioning as artificial water channels. This precise hydration control directly translates into enhanced photocatalytic performance, up to 420% higher than that of the first-generation quantasome. ii. The covalent fixation of the QS supramolecular network was explored through photo-polymerization of styrene terminals on the PBI antennas, providing a novel strategy to reinforce the supramolecular architecture against operational stress experienced during oxygenic turnover. This modification resulted in marked improvements in oxygenic photocurrent and stability under high solar irradiance compared to its non-polymerized counterpart. iii. An in-depth analysis of the excited state dynamics of the QS architecture reveals that the unique hierarchical aggregation of PBIs with POMs significantly enhances their photophysical properties, facilitating ultrafast formation (in 140 fs) and long-lived charge-separated states. This charge separation is confirmed to stem from supramolecular order rather than catalytic redox activity, establishing a transformative design paradigm for bioinspired light-harvesting and catalytic systems. Notably, preliminary results for a new geometry of near-orthogonal PBI derivatives exhibit remarkable potential, showcasing a near-unity quantum yield for radical ion pair formation and long-lived charge-separated states, leading to a novel QS architecture that outperforms the former QS with a four-fold increase in the photocurrent output. Overall, these findings advance the development of quantasome-based photoanodes, offering opportunities to optimize efficiency through the tailored engineering of co-polymer residues, to enhance water harvesting and transport, and emphasize the significance of rigid molecular architectures, similar to the special pairs of photosynthetic reaction centers, in achieving efficient SB-CS.