The Rational Design of Reducing Organophotoredox Catalysts Unlocks Proton-Coupled Electron-Transfer and Atom Transfer Radical Polymerization Mechanisms

Abstract

Photocatalysis has become a prominent tool in the arsenal of organic chemists to develop and (re)imagine transformations. However, only a handful of versatile organic photocatalysts (PCs) are available, hampering the discovery of new reactivities. Here, we report the design and complete physicochemical characterization of 9-aryl dihydroacridines (9ADA) and 12-aryl dihydrobenzoacridines (12ADBA) as strong reducing organic PCs. Punctual structural variations modulate their molecular orbital distributions and unlock locally or charge-transfer (CT) excited states. The PCs presenting a locally excited state showed better performances in photoredox defunctionalization processes (yields up to 92%), whereas the PCs featuring a CT excited state produced promising results in atom transfer radical polymerization under visible light (up to 1.21 Đ, and 98% I*). Unlike all the PC classes reported so far, 9ADA and 12ADBA feature a free NH group that enables a catalytic multisite proton-coupled electron transfer (MS-PCET) mechanism. This manifold allows the reduction of redox-inert substrates including aryl, alkyl halides, azides, phosphate and ammonium salts (Ered up to -2.83 vs SCE) under single-photon excitation. We anticipate that these new PCs will open new mechanistic manifolds in the field of photocatalysis by allowing access to previously inaccessible radical intermediates under one-photon excitation.