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Dooshaye Moonshiram: Electronic and Structural Configurations of Earth-Abundant Water Splitting Catalysts and Spin Crossover Complexes
The solar light-driven splitting of water for hydrogen fuel production is a promising alternative to fossil fuels due to their rapid depletion and concomitant environmental pollution. The design of light-driven devices composed of organic, inorganic or hybrid materials that can mimic natural photosynthetic processes is extremely desirable. An essential component of such systems is the light-harvesting chromophore, analogous to the photosynthetic pigments, which can absorb the energy of the incident photons. Consequently, the light energy is converted into an electronically excited state for the creation of a charge-separated state that helps to generate the required thermodynamic driving force for subsequent catalytic reactions.
Commonly used molecular photosensitizers traditionally contain precious and scarce 4d or 5d transition metals such as Platinum, Ruthenium, Rhenium or Iridium. However, over the past decades, a range of noble metal-free photosensitizers based on earth-abundant metals such as Copper, Chromium, and Zinc appeared, with the aim to bring these light-harvesting molecules into more practical applications. A systematic series novel homo- and heteroleptic Cu(I) photosensitizers based on tetradentate 1,10-phenanthroline ligands of the type X^N^N^X containing two additional donor moieties in the 2,9-position (X = SMe or OMe) were designed. Time-resolved X-ray absorption spectroscopy in the picosecond time scale, coupled with time-dependent density functional theory calculations, provided in-depth information on the excited state electron configurations. For the first time, a significant shortening of the Cu-X distance and a change in the coordination mode to a pentacoordinated geometry is shown in the excited states of the two homoleptic complexes. These findings are important with respect to a precise understanding of the excited state structures and a further stabilization of this type of photosensitizers. This talk will further demonstate the reaction pathways of several cobalt and nickel-based hydrogen evolving complexes, examined in unprecedented detail with picosecond time resolution when coupled with copper and ruthenium-based photosensitizers. Results shown will enable the rational design of molecular hydrogen-evolving photocatalysts that can perform beyond the current microsecond time scale, and suggest ways in which the ligand structures can be adjusted to facilitate protonation and catalytic efficiency.
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