Shining a light on the photo-sensitisation of organic–inorganic hybrid polyoxometalates

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Abstract

Approaches to the visible light photo-sensitisation of organic–inorganic hybrid polyoxometalates are presented and future trends discussed.

Abstract

Finding new ways of using visible light (or, more specifically, solar irradiation) to drive commercially significant and/or challenging chemical processes is an ongoing research goal. Polyoxometalates (POMs) are discrete, metal–oxide clusters which are cheap, robust and easily synthesised but can also act as versatile molecular building blocks, allowing for astonishing variety in their structures and properties. In particular, the rich redox chemistry and inherent photo-activity of POMs makes them attractive for use in a variety of photochemical applications, however POMs characteristically only absorb strongly in the UV region. In this perspective, we discuss the various strategies which have been employed in order to sensitise POMs to visible light, with a particular focus on hybrid inorganic–organic POM species. We will discuss the two clear photo-activation mechanisms which have been developed to date and provide an outlook on some of the possible future directions of the field.

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Solar fuels via artificial photosynthesis.

Devens Gust, Thomas A Moore, Ana L Moore (2009)

Because sunlight is diffuse and intermittent, substantial use of solar energy to meet humanity's needs will probably require energy storage in dense, transportable media via chemical bonds. Practical, cost effective technologies for conversion of sunlight directly into useful fuels do not currently exist, and will require new basic science. Photosynthesis provides a blueprint for solar energy storage in fuels. Indeed, all of the fossil-fuel-based energy consumed today derives from sunlight harvested by photosynthetic organisms. Artificial photosynthesis research applies the fundamental scientific principles of the natural process to the design of solar energy conversion systems. These constructs use different materials, and researchers tune them to produce energy efficiently and in forms useful to humans. Fuel production via natural or artificial photosynthesis requires three main components. First, antenna/reaction center complexes absorb sunlight and convert the excitation energy to electrochemical energy (redox equivalents). Then, a water oxidation complex uses this redox potential to catalyze conversion of water to hydrogen ions, electrons stored as reducing equivalents, and oxygen. A second catalytic system uses the reducing equivalents to make fuels such as carbohydrates, lipids, or hydrogen gas. In this Account, we review a few general approaches to artificial photosynthetic fuel production that may be useful for eventually overcoming the energy problem. A variety of research groups have prepared artificial reaction center molecules. These systems contain a chromophore, such as a porphyrin, covalently linked to one or more electron acceptors, such as fullerenes or quinones, and secondary electron donors. Following the excitation of the chromophore, photoinduced electron transfer generates a primary charge-separated state. Electron transfer chains spatially separate the redox equivalents and reduce electronic coupling, slowing recombination of the charge-separated state to the point that catalysts can use the stored energy for fuel production. Antenna systems, employing a variety of chromophores that absorb light throughout the visible spectrum, have been coupled to artificial reaction centers and have incorporated control and photoprotective processes borrowed from photosynthesis. Thus far, researchers have not discovered practical solar-driven catalysts for water oxidation and fuel production that are robust and use earth-abundant elements, but they have developed artificial systems that use sunlight to produce fuel in the laboratory. For example, artificial reaction centers, where electrons are injected from a dye molecule into the conduction band of nanoparticulate titanium dioxide on a transparent electrode, coupled to catalysts, such as platinum or hydrogenase enzymes, can produce hydrogen gas. Oxidizing equivalents from such reaction centers can be coupled to iridium oxide nanoparticles, which can oxidize water. This system uses sunlight to split water to oxygen and hydrogen fuel, but efficiencies are low and an external electrical potential is required. Although attempts at artificial photosynthesis fall short of the efficiencies necessary for practical application, they illustrate that solar fuel production inspired by natural photosynthesis is achievable in the laboratory. More research will be needed to identify the most promising artificial photosynthetic systems and realize their potential.

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Heteropoly and Isopoly Oxometalates

Michael Thor Pope (1983)

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A fast soluble carbon-free molecular water oxidation catalyst based on abundant metals.

Qiushi Yin, Jeffrey Miles Tan, Claire Besson … (2010)

Traditional homogeneous water oxidation catalysts are plagued by instability under the reaction conditions. We report that the complex [Co4(H2O)2(PW9O34)2]10-, comprising a Co4O4 core stabilized by oxidatively resistant polytungstate ligands, is a hydrolytically and oxidatively stable homogeneous water oxidation catalyst that self-assembles in water from salts of earth-abundant elements (Co, W, and P). With [Ru(bpy)3]3+ (bpy is 2,2'-bipyridine) as the oxidant, we observe catalytic turnover frequencies for O2 production > or = 5 s(-1) at pH = 8. The rate's pH sensitivity reflects the pH dependence of the four-electron O2-H2O couple. Extensive spectroscopic, electrochemical, and inhibition studies firmly indicate that [Co4(H2O)2(PW9O34)2]10- is stable under catalytic turnover conditions: Neither hydrated cobalt ions nor cobalt hydroxide/oxide particles form in situ.

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Author and article information

Journal

Journal ID (publisher-id): ICHBD9

Title: Dalton Transactions

Abbreviated Title: Dalton Trans.

Publisher: Royal Society of Chemistry (RSC)

ISSN (Print): 1477-9226

ISSN (Electronic): 1477-9234

Publication date Created: 2018

Publication date (Electronic): 2018

Volume: 47

Issue: 15

Pages: 5120-5136

Affiliations

[1 ]GSK Carbon Neutral Laboratory for Sustainable Chemistry

[2 ]University of Nottingham

[3 ]Nottingham

[4 ]UK

[5 ]Faculty of Engineering

[6 ]School of Chemistry

Article

DOI: 10.1039/C8DT00400E

SO-VID: 48df4a76-c0ed-4798-b048-807e5dbdc953

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http://rsc.li/journals-terms-of-use

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