Photosensitizers based on complexes made of earth-abundant metals for photon upconversion and photocatalysis

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Sensitized triplet-triplet annihilation upconversion converts two low-energy input photons into one higher-energy output photon under non-coherent and low-power light excitation. This biphotonic process becomes a promising strategy for applications in photocatalysis, bioimaging, clinical treatments, solar cells, and electronic devices. Particularly, annihilation-based photon upconversion attracts increasing attention in photocatalysis, because it allows to use low-energy visible or near-infrared light for chemical reactions that otherwise requiring the energy input of high-energy photons, such as UV light. However, photosensitizers for upconversion rely almost exclusively on precious metals so far, such as platinum, ruthenium, iridium, or osmium, as well as lanthanides like europium or ytterbium. Since recent years, the desire of performing more sustainable chemistry promotes substantial developments towards photoactive coordination complexes made from earth-abundant metals, for example, iron, copper, or chromium. These complexes are considered as suitable substitutes of the well-known complexes based on precious and rare-earth metals, as mentioned above. Remarkable advances with photoactive earth-abundant metal complexes have been made recently. However, until now only a few complexes based on these metals have been used as photosensitizer for annihilation-based upconversion, and in most cases their upconversion performance has remained suboptimal to sensitizers made of precious metals so far. Almost no attempts have been made to use photon upconversion sensitized by earth-abundant metal complexes for photocatalytic reactions with low-energy light. Therefore, C. Wang proposes to develop new types of photosensitizers based on earth-abundant metals for annihilation-based photon upconversion, which can then promote energy-demanding photocatalytic reactions with low-energy light. Incorporation of these photoactive compounds into nanostructures will make them suitable for applications under ambient conditions. The proposed photosensitizers and strategies could have more sustainable potential for versatile applications in lighting, energy conversion, and catalysis.