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Photoredox catalysis driven by visible light has improved chemical synthesis by enabling milder reaction conditions and unlocking distinct reaction mechanisms. Despite the transformative impact, visible-light photoredox catalysis remains constrained by the thermodynamic limits of photon energy and inefficiencies arising from unproductive back electron transfer, both of which become particularly pronounced in thermodynamically demanding reactions. In this work, we introduce an organic photoredox catalyst system that overcomes these obstacles to drive chemical transformations that require super-reducing capabilities. This advancement is accomplished by coupling the energy of two photons into a single chemical reduction, whereas inefficiencies from back electron transfer are mitigated through a distinct proton-coupled electron transfer mechanism embedded in the catalyst design. The super-reducing capabilities of this organic catalyst system are demonstrated through efficient application in a broad scope of challenging arene reductions. 

Benzo[ghi]perylene monoimides (BPIs) have recently been employed as organic photocatalysts for challenging reductions. In probing their function, we identify a thermal degradation product involving imide ring opening, and this in turn motivates the development and synthesis of a high-symmetry model systema benzo[ghi]perylene diester (BPDE)whose structural simplicity is useful for mechanistic exploration relevant to the broader photocatalyst class. Using electrochemical and spectroscopic tools, we probe both the singly and doubly reduced states of BPDE and report the generation of [BP-H]−, a twoelectron, one-proton activated closed-shell super-reductant. This catalytically active species, after visible photon absorption, operates from its singlet excited state, where the motions of the added proton are coupled to an electron transfer event, which enables direct reduction of inert substrates like benzene and fluorobenzene. Traditional Birch chemistry on benzene has been previously realized only by solvated electrons or electrochemistry. The function of this model system uncovered in these mechanistic explorations suggests modes of operation for this photocatalyst class that will enable future optimizations.