Using visible light to make molecules promises more sustainable practices in chemical synthesis, where photons supply the required energy instead of high temperature or pressure. Photoredox catalysis, combining light-driven and redox chemistry, offers unique reactivity and selectivity to conventional methods and transition metal catalysis. Molecular photocatalysts absorb light to form excited states that act as single-electron oxidants or reductants and provide mild access to open-shell radical intermediates. Over the last 15 years photoredox catalysis has become a versatile tool for synthetic chemists, enabling innovative method development in late-stage functionalisation, C-H activation and skeletal editing.
The energy of an absorbed photon typically sets intrinsic limits on the resulting chemical reactivity. Further limitations are imparted by the photocatalyst, which must expend some energy to convert light into chemical potential. These unavoidable energy losses underpin a reliance on costly precious metal catalysts that maximise the accessible redox potential but inefficiently utilize high energy photons. Multi-photon excitation is an emerging strategy that modestly overcomes these energy limits by combining multiple photons for a single reactive step, but at the expense of greatly diminished efficiency.
This presentation discusses recent work that represents a radically different approach to synthetic photochemistry. By decoupling the redox requirements from the light-harvesting catalyst, we instead exploit overlooked sacrificial additives and convert the free energy of their chemical bonds into redox potential. This approach makes it possible to surpass photon limits without sacrificing reactivity, and increases photochemical efficiency by promoting chain propagation where multiple products may be generated from a single photon.