Artificial Photosynthesis: From Nanosecond Electron Transfer to Catalytic Water Oxidation

Abstract

Human society faces a fundamental challenge as energy consumption is projected to increase due to population and economic growth as fossil fuel resources decrease. Therefore the transition to alternative and sustainable energy sources is of the utmost importance. The conversion of solar energy into chemical energy, by splitting H2O to generate molecular O2 and H2, could contribute to solving the global energy problem. Developing such a system will require the combination of several complicated processes, such as light-harvesting, charge separation, electron transfer, H2O oxidation, and reduction of the generated protons. The primary processes of charge separation and catalysis, which occur in the natural photosynthetic machinery, provide us with an excellent blueprint for the design of such systems. This Account describes our efforts to construct supramolecular assemblies capable of carrying out photoinduced electron transfer and to develop artificial water oxidation catalysts (WOCs). Early work in our group focused on linking a ruthenium chromophore to a manganese-based oxidation catalyst. When we incorporated a tyrosine unit into these supramolecular assemblies, we could observe fast intramolecular electron transfer from the manganese centers, via the tyrosine moiety, to the photooxidized ruthenium center, which clearly resembles the processes occurring in the natural system. Although we demonstrated multi-electron transfer in our artificial systems, the bottleneck proved to be the stability of the WOCs. Researchers have developed a number of WOCs, but the majority can only catalyze H2O oxidation in the presence of strong oxidants such as Ce(IV), which is difficult to generate photochemically. By contrast, illumination of ruthenium(II) photosensitizers in the presence of a sacrificial acceptor generates [Ru(bpy)3](3+)-type oxidants. Their oxidation potentials are significantly lower than that of Ce(IV), but our group recently showed that incorporating negatively charged groups into the ligand backbone could decrease the oxidation potential of the catalysts and, at the same time, decrease the potential for H2O oxidation. This permitted us to develop both ruthenium- and manganese-based WOCs that can operate under neutral conditions, driven by the mild oxidant [Ru(bpy)3](3+). Many hurdles to the development of viable systems for the production of solar fuels remain. However, the combination of important features from the natural photosynthetic machinery and novel artificial components adds insights into the complicated catalytic processes that are involved in splitting H2O.

Recent progress in homogeneous multielectron transfer photocatalysis and artificial photosynthetic solar energy conversion

“…While modelling important functions of photosynthetic reaction centers such as light-absorption across the visible spectrum, excitation energy transfer and subsequent generation of charge separated states with high quantum yields has already reached a considerable level within the last decades [18], the development of complete artificial photosynthetic model systems capable of converting light energy into solar fuels still remains a major challenge [18,19]. One of the most serious problems encountered is to couple photoinduced charge separated states to functional building blocks acting as efficient catalysts for multielectron transfer (MET) and proton coupled electron transfer (PCET) steps, which are required to achieve permanent energy storage in sufficiently stable chemical bonds (Scheme 1).…”

Real Time Quantification of Ultrafast Photoinduced Bimolecular Electron Transfer Rate: Direct Probing of the Transient Intermediate

“…PET has been used to concur some of the greatest challenges of human survival, like energy harvesting from sunlight and mimicking our sources for food. 13, 14 As anticipated, during the 20th and 21st century there have been extensive studies in the literature on PET due to its potential to meet the energy demand of the universe. 15−17 The knowledge of PET reactions enables Thomas J. Meyer and his group to explore 18 the solar energy conversion process.…”

Fate of methanol under one-pot artificial photosynthesis condition with metal-loaded TiO2 as photocatalysts

“…Even in the dark and without any catalyst (Pd n -TiO 2 ), formation of MF and DMM in small amounts was observed, regardless of the nature of the carrier gas (Table SI- 3). This indicates that there also exist slower noncatalytic thermal pathways from formaldehyde and methanol to methoxymethanol, from methoxymethanol to DMM, and from methoxymethanol to MF at room temperature.…”