Finding photostable, first-row transition metal-based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II)... Show moreFinding photostable, first-row transition metal-based molecular systems for photocatalytic water oxidation is a step towards sustainable solar fuel production. Herein, we discovered that nickel(II) hydrophilic porphyrins are molecular catalysts for photocatalytic water oxidation in neutral to acidic aqueous solutions using [Ru(bpy)3]2+ as photosensitizer and [S2O8]2- as sacrificial electron acceptor. Electron-poorer Niporphyrins bearing 8 fluorine or 4 methylpyridinium substituents as electron-poorer porphyrins afforded 6-fold higher turnover frequencies (TOFs; ca. 0.65 min-1) than electronricher analogues. However, the electron-poorest Ni-porphyrin bearing 16 fluorine substituents was photocatalytically inactive under such conditions, because the potential at which catalytic O2 evolution starts was too high (+1.23V vs. NHE) to be driven by the photochemically generated [Ru(bpy)3]3+. Critically, these Ni-porphyrin catalysts showed excellent stability in photocatalytic conditions, as a second photocatalytic run replenished with a new dose of photosensitizer, afforded only 1–3% less O2 than during the first photocatalytic run. Show less
Plants harvest light energy and convert it into chemical energy. Light absorption by photosystems I and II (PSI and PSII) results in charge separations in their reaction centers (RCs), initiating a... Show morePlants harvest light energy and convert it into chemical energy. Light absorption by photosystems I and II (PSI and PSII) results in charge separations in their reaction centers (RCs), initiating a chain of redox reactions with PSI generating the reducing power for CO2 assimilation into sugars, and PSII oxidizing the ultimate electron donor, H2O, to O2. The rates of PSI and II must be balanced. If PSII releases more electrons than PSI can use, the intermediary pool of plastoquinone molecules and the primary acceptor quinone QA become fully reduced. PSII charge separation then leads to recombination, whereby the RC triplet state and hence singlet oxygen can be formed, causing damage to PSII that can decrease the photosynthetic yield. Photoinhibition results. Charge recombination to the triplet state may probably be avoided to some extent by a cyclic electron transfer (CET) pathway, short-circuiting the charge separation, which is somehow switched on when needed by cytochrome b559, an intrinsic subunit of PSII. Mechanism, quantitative significance, and even the very existence of CET continue to be subject to debate, however. This thesis describes studies on triplet generation, cytochrome b559, CET induction conditions, and other aspects of this elusive protection mechanism against photoinhibition. Show less
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