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

敏化三重态-三重态湮灭上转换在非相干和低功率光激发下将两个低能量输入光子转换成一个较高能量输出光子。这种双光子过程成为一种有前途的策略，应用于生物成像，临床治疗，太阳能电池和电子器件。特别地，基于湮灭的光子上转换在生物学中吸引了越来越多的关注，因为它允许使用低能量的可见光或近红外光用于化学反应，否则需要高能光子的能量输入，例如UV光。然而，到目前为止，用于上转换的光敏剂几乎完全依赖于贵金属，如铂、钌、铱或锇，以及镧系元素如铕或镱。近年来，进行更可持续的化学的愿望促进了由地球上丰富的金属（例如铁、铜或铬）制成的光活性配位络合物的实质性发展。这些络合物被认为是如上所述的基于贵金属和稀土金属的公知络合物的合适替代物。近年来，光活性稀土金属配合物的研究取得了显著进展。然而，到目前为止，只有少数基于这些金属的配合物已被用作基于湮灭的上转换的光敏剂，并且在大多数情况下，它们的上转换性能迄今为止仍低于由贵金属制成的敏化剂。几乎没有尝试使用地球丰富的金属络合物敏化的光子上转换用于低能量光的光催化反应。因此，C. Wang建议开发基于地球上丰富的金属的新型光敏剂，用于基于湮灭的光子上转换，然后可以促进低能量光的能量需求光催化反应。将这些光活性化合物掺入纳米结构中将使它们适合于在环境条件下的应用。所提出的光敏剂和策略可能在照明、能量转换和催化等多方面具有更可持续的应用潜力。

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.