Photocyclization by a triplet-triplet annihilation upconversion pair in water - avoiding UV-light and oxygen removal.

We present a formal [2 + 2]-cycloaddition of unsaturated ketones enabled by a green-to-ultraviolet triplet-triplet annihilation upconversion (TTA-UC) pair, using commercially available Ru(bpy) and pyrene as sensitizer and annihilator, respectively. In the developed protocol, visible light irradiation at = 520 nm allows for the reaction to proceed without the need for UV-light and the aqueous medium eliminates the need for oxygen removing protocols. Through this study, the application of the readily available upconversion pair is broadened to include cyclization reactions.