Photoredox catalysis using infrared light via triplet fusion upconversion.

Abstract

Recent advances in photoredox catalysis have made it possible to execute a multitude of challenging synthetic transformations, polymerizations, and surface modifications 1,2,3. In all cases, these transformations require ultraviolet or visible light stimuli. The use of visible light irradiation has intrinsic challenges. For example, the penetration of visible light is very low through most reaction media, leading to problems in large scale reactions. Moreover, reactants can compete with the photocatalysts for incident light absorption, limiting scope. Near infrared (NIR) radiation can overcome many of these fundamental problems. NIR light is known to have a much higher penetration depth through a variety of media, notably biological tissue4. Here, we demonstrate a variety of photoredox transformations under infrared radiation by utilizing the photophysical process of triplet fusion upconversion. This generates molecules in their singlet excited state that can perform single electron transfer (SET), serving as photocatalysts or photoinitiators. We illustrate that this is a general strategy applicable to a wide range of photoredox reactions. We tune the upconversion components to adjust the output light, accessing both orange light and blue light from low-energy infrared (IR) light, by pair-wise manipulation of the sensitizer and annihilator. We further demonstrate that the annihilator itself can be used as a photocatalyst, simplifying the reaction. This approach allows us to execute catalysis through several barriers that are impenetrable by visible light, expanding the landscape of photocatalysis to a variety of new materials and environments. The results demonstrated here allow high-energy transformations from low-energy IR light with all the benefits that the latter affords.