Development and Applications of Photon Upconversion in Organic Synthesis

Abstract

Triplet–triplet annihilation photon upconversion (TTA-UC) is a photophysical process in which the energy of two photons are combined into a single photon of higher energy. While this strategy has been implemented in applications ranging from bioimaging to solar energy conversion, its uses in synthetic organic chemistry have not been extensively developed. Here we report the application TTA-UC to produce singlet-state photoexcited intermediates that would otherwise require ultraviolet excitation. In this system, a visible light photosensitizer and a known TTA acceptor work cooperatively to harvest two visible photons and combine their excitations to generate a photon of near UV light in situ. We demonstrate that coumarins can collect this upconverted energy to furnish high-energy singlet excited-state intermediates that can participate in subsequent [2+2] cycloaddition events. We disclose mechanistic and spectroscopic evidence to support both the proposed TTA mechanism for photon upconversion and the singlet energy transfer step to access the excited-state reactivity of coumarins. These findings establish a proof-of-concept for the applicability of TTA-UC systems to mediate a classical UV photochemical transformation using low-energy visible light inputs. We further improve upon this method to newly characterized TTA-UC system that enable access to the excited-state of alpha-keto esters and their subsequent [2+2] cycloadditions.

Finally, a method and subsequent mechanistic study for the intramolecular arene alkylation with N-(acyloxy)phthalimides is presented. Spectroscopic, electrochemical, and computational details are provided in support of a closed-cycle catalytic process.