Mechanistic Investigation of a Photocatalyst Model Reveals Function by Perylene-Like Closed Shell Super-Photoreductant Capable of Reducing Unactivated Arenes.

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 system-a 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 two-electron, 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.