Photophysics of Doubly Reduced Quinacridone Dyes: Toward Redox Reactivity Activated by Red Light

Super-reductive photocatalysis is an emerging approach to broaden the conventional organic reactivity by exploiting reduced forms of a parent photocatalyst (PC) as the actual photoactive super-reductant. Recent reports have highlighted the advantages of using closed-shell, doubly reduced PC^2- with respect to open-shell, singly reduced PC^•-, with the former having longer excited-state lifetime and enlarged reductive power. It is thus important to expand the families of super-reductants that fit these features. In this work, we investigate doubly reduced N,N-dialkylated derivatives of quinacridone (QA) industrial pigment as potential super-reductants, exploiting the use of low-energy red light. These give the possibility of electrochemically generating the radical anion and dianion forms, with the latter being characterized by an emissive excited state with nanosecond lifetimes (compared to ps lifetime of the radical anion excited state) and reductive power up to -3.5 V vs Saturated Calomel Electrode, ca. 0.9 V enhanced with respect to the radical anion excited state. The super-reductant activity of the bis-reduced species is demonstrated in the challenging reductive deamination of arylammonium derivatives. All-in-one, these findings will inspire the design of closed-shell super-redox photocatalysts based on industrial pigments, thus impacting sectors involving organic reactivity, photocatalysis, and sustainable chemistry.