Anti-Dissipative Solar Energy Conversion

• Applicant: Dr. Alejandro Cadranel
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550140216

The definitive replacement of fossil fuels by solar fuels requires to improve the conversion efficiencies that are available today. Notably, inspirational natural photosynthesis is not very efficient in terms of solar energy conversion to fuels. Photosynthetic chromophores dissipate one fourth of the absorbed light via internal conversion (IC) followed by vibrational relaxation (VR) to the lowest energy excited states. IC and VR shape a decay cascade for the excited state population, that reaches the lowest excited state in the sub-nanosecond timescale. Kasha’s rule, with its several formulations and interpretations, represents this scenario. Thus, this project pursues the retardation of dissipative VR/IC using high kinetic barriers to block them. This will be the key to trap high energy excited states, so they persist long enough to engage in bimolecular or long-range reactions and the absorbed photon energy can be exploited at its maximum, within an Anti-Kasha scenario. To this end, we will continue with our exploration of bimetallic ruthenium chromophores, which showed an anti-Kasha behavior in previous studies. We demonstrated that a high-energy excited state engages in bimolecular photoinduced electron transfer reactions with a sacrificial electron donor, mimicking catalyst activation in artificial photosynthesis. In fact, this reaction produced an anti-Kasha high-energy reduced complex, with a microsecond lifetime thanks to a high barrier for IC to the lowest-energy species. This saved 140 meV in comparison with a similar scheme that obeys Kasha’s rule. Systematic modifications in the chromophore’s backbone using electron accepting and electron withdrawing substituents, along with the exploration of osmium and iridium chromophores with similar structures, will be studied to derive structure-anti-Kasha behavior relations. The goal will be to maximize the amount of spared energy. The focus is on synthesis and characterization, which will include traditional photochemical techniques, spectroelectrochemistry, as well as ultrafast transient absorption spectroscopy from the femto- to microsecond timescales. Such a full-fledged investigation with tailored anti-Kasha chromophores will put us into a strategic position not only for translating our findings into the development of technological devices but also for conceptualizing additional basic investigations.

• DFG Programme: Research Grants
• International Connection: Belgium
• Partner Organisation: Fonds National de la Recherche Scientifique - FNRS
• Cooperation Partner: Professor Dr. Ludovic Troian-Gautier