Singlet Exciton Fission: A New Concept for Catalytic Two-Electron Redox Processes

In recent years, photoredox catalysis has emerged as a powerful tool in synthetic organic chemistry. The generation of reactive intermediates trough single-electron transfer from an excited photocatalyst has enabled researchers to develop a plethora of novel reactions. Yet, numerous redox processes in nature, organic synthesis and transition metal catalysis require the concerted transfer of two electrons. Inspired by recent work in the field of molecular electronics and photovoltaics, we propose to use intramolecular singlet exciton fission as a novel approach to two-electron photoredox chemistry. The process of splitting one singlet exciton into two coupled triplets affords two unpaired excited electrons and two electron holes that can be harnessed for electron transfer reactions. Based on preliminary results from the group of PROF. TOMISLAV ROVIS, we will initially investigate two-electron oxidation of organic molecules utilizing a photocatalyst capable of intramolecular singlet fission in combination with molecular oxygen as terminal oxidant. A salient feature of this approach is the upconversion of a mild oxidant (O2) to a highly oxidizing species using the energy of visible light. Later, we will investigate the photocatalyzed two-electron oxidation of transition metals. Specifically, we aim establish singlet exciton fission as a novel approach for the generation of otherwise hard to access, high energy copper(III) species from stable copper(I) precursors. If successful, this will allow us to develop novel copper-catalyzed couplings that are exceedingly mild and do not require stoichiometric amounts of strong oxidants. We believe that these studies will be of fundamental scientific value, as they will establish a novel approach towards multi-electron redox chemistry.