Triplet-triplet annihilation upconversion can generate UV light from visible light LEDs and visible light from infrared. Producing higher-energy photons at the location where they are needed is useful: some functional groups do not tolerate irradation with UV light, and visible light does not penetrate cells and polymers as deep as infrared light. In 1912, Ciamician had the vision of getting the industry to use light instead of heat to make chemical compounds. Light was regarded as an inexpensive, sustainable and renewable reagent to facilitate different chemical reactions. In the 20 th century, most photoinduced transformations used UV-light...