Ultrafast Dynamics of a Red-Light-Activated Organic Photocatalyst in the Oxidative Hydroxylation of Phenylboronic Acid

Over the past few years, photoredox catalysis has led to significant transformations in modern synthetic chemistry. It has allowed the development of new synthetic pathways for the assembly of complex molecular scaffolds using light as a driving force. However, investigations of the ultrafast light-initiated mechanisms required for these reactions are relatively scarce. Here we follow the ultrafast dynamics of a red-light organic photocatalyst, N,N′-di-n-propyl-1,13-dimethoxyquinacridinium (nPr-DMQA+), in the aerobic oxidative hydroxylation of phenylboronic acid using transient absorption and time correlated single photon counting spectroscopy. Global target analysis supports a reaction mechanism that proceeds through the excited triplet state of nPr-DMQA+, leading to the generation of a superoxide anion and subsequent oxidative hydroxylation. The triplet pathway proposed here has relatively wide application in organic photocatalytic oxidative reactions including those using methylene blue and other organic dyes as catalysts. Observation of the ultrafast dynamics of nPr-DMQA+ as it acts as a catalyst can provide insights to improve the efficiency of oxidative hydroxylation reactions and the mechanisms of photoredox catalysis more broadly.