Exploring Intra- and Intermolecular [2+2] Cycloadditions Enabled by Triplet-Triplet Annihilation

Abstract

Despite the recent development of photon upconversion techniques, they are limited by requirements of high excitation intensities, and weak absorptions of visible light. However, triplet-triplet annihilation (TTA) is a promising upconversion approach because of its use of low-intensity light and tunable molecules. As very little application of TTA upconversion is seen in organic synthesis, herein I investigate the implementation of TTA combined with Förster resonance energy transfer (FRET) to promote UV-catalyzed reactions with blue light. Initially, I looked into intramolecular [2+2] photocycloadditions and was able to optimize the reaction conditions to achieve significant product yield and conversion. To verify the energy transfer steps in the TTA/FRET process, I conducted mechanistic studies by observing stereoselectivity between the singlet and triplet excited states of the cycloaddition reaction. Although the stereochemical experiments were concluded to not be a strong diagnostic of the TTA mechanism, quantum yield studies and Stern-Volmer experiments gave significant evidence in support of the mechanism. Given sufficient mechanistic evidence, I was able to expand the scope of TTA upconversion to intermolecular [2+2] photocycloadditions, developing optimized conditions for the reaction. Since concurrent electron transfer of the photocatalyst reducing the substrate was observed, Stern-Volmer experiments and cyclic voltammetry were carried out for the direction of further reaction optimization.