PhD defense of Chloé Magne

To replace fossil fuels, pathways for global transition to more sustainable energy systems are needed for the conversion and storage of solar energy into chemical bonds. The utilization of solar energy for multi-electron chemical conversions is a central issue in artificial photosynthesis. While natural photosynthesis accomplishes this through a highly organized process using light harvesting and multi-electron redox chemistry, the realization of this process in synthetic molecular systems remains an outstanding challenge. More specifically, the rapid production, separation, and accumulation of multiple charges from sunlight is still limited by recombination losses and photon utilization efficiency. Conventional molecular light-harvesting schemes are fundamentally limited at the one-photon-one-exciton level. This results in suboptimal quantum efficiency and less effective charge storage. This thesis explores bio-inspired singlet fission (SF) as a molecular strategy to address these limitations and achieve multi-electron photochemistry under visible-light excitation.

Singlet fission, a spin-allowed process of exciton multiplication, transforms one photoexcited singlet state (S1) into two triplet states (T1), potentially doubling the maximum quantum yield for the formation of charge carriers from absorbed photons. Yet, the most characteristic SF materials discovered to date can only function in the form of a solid-state crystalline film and are not suitable for molecular-scale-controlled photocatalytic systems, due to their limited solubility and chemical tunability. In this work, we developed molecular design strategies that allow SF in solution by elucidating how structural disorder, exciton coupling, and energy landscape control can be used to generate long-living states and finally combine them with light-driven redox processes.

Natural photosynthetic light-harvesting structures served as inspiration to initiate studies on carotenoid-based assemblies as a functional model. Inspired by the structural organization of natural chromophores, this thesis first demonstrates that supramolecular disorder can be harnessed as a –        -functional design element rather than being considered a defect. Artificial lycopene aggregates were shown to undergo efficient SF from S1-like states, producing triplet states with microsecond lifetimes. Time-resolved spectroscopy and structural analysis revealed that heterogeneous aggregate domains create energetic gradients that promote triplet separation and limit triple-triplet annihilation recombination. This bio-inspired idea opens up functional disorder as an exciting approach to enhanced long-living exciton generation in molecular materials.

Exploiting this fact, a second class of systems based on perylene tetracarboxylate (PTC) dimers demonstrated the first molecular-level observation of singlet fission in homogeneous aqueous solution without aggregates. These excited dimers, stabilized by alkali cations in water, display a strong excitonic coupling while conserving conformational flexibility. Spectroscopic investigations identified two competing pathways: (1) a singlet fission process leading to quantum yields exceeding 100%, and (2) a charge-transfer pathway generating long-living radical ion pairs. This achievement further expanded SF to green and polar environments, which is crucial for solar chemical applications.

The insights obtained in this thesis support the development of antenna-catalyst molecular triads able to quasi-simultaneously transfer multiple holes for oxidative transformations. Future efforts will focus on combining SF antennas with molecular catalysts to drive two- and four-electron reactions, including alcohol oxidation and water oxidation. Controlling supramolecular disorder and excitonic coupling by rational molecular design will be the key to developing sustainable SF-driven molecular photocatalysts.  In conclusion, we have demonstrated that bio-inspired exciton multiplication can be achieved in molecular solutions in water, and have provided a roadmap for realizing such an SF process for environmentally friendly solar energy conversion.