Photon upconversion sensitized by earth-abundant transition metal complexes

Sensitized triplet-triplet annihilation upconversion (sTTA-UC) converts two lower-energy absorbed photons into one emitting photon of higher-energy, and has become a popular approach for a wide range of applications. Current photosensitizers rely mostly on transition metal complexes made of expensive platinum group elements, such as palladium, platinum, and osmium, due to their strong absorption in the visible range, unity intersystem crossing, and long-lived triplet excited lifetimes. In recent years, fundamental breakthroughs have been made with photoactive complexes based on earth-abundant 3d metals including chromium, manganese, iron, cobalt, copper, and zinc, and 4d elements like zirconium and molybdenum. These novel complexes offer advantages, such as cost-effectiveness, sustainability, low toxicity, scalability for industrial use, and potential for innovative research in areas including catalysis and energy conversion, making them promising alternatives to noble metal-based photosensitizers in sTTA-UC and other fields. In this review, we delineate the recent advancements in sTTA-UC utilizing photoactive earth-abundant transition metal complexes. We explore their energy transfer mechanisms, evaluate their upconversion performance, discuss their applications, and outline the challenges and perspectives, aiming to offer insights for the development of novel photosensitizers based on earth-abundant metals for future research and applications.