Unlocking the ConPeT Mechanism: Correspondence on “Catalytic Asymmetric Redox‐Neutral [3 + 2] Photocycloadditions of Cyclopropyl Ketones with Vinylazaarenes Enabled by Consecutive Photoinduced Electron Transfer”
Marco Villa, Andrea Fermi, Francesco Calogero, Andrea Gualandi, Paola Franchi, Marco Lucarini, Barbara Ventura, Pier Giorgio Cozzi, P. Ceroni
Abstract
Abstract Recently, in a communication to this journal, Qiao, Jang, and coworkers described an asymmetric photoredox reaction promoted by TADF cyanoarene photocatalysts (specifically 4DPAPN (3,4,5,6‐tetrakis(diphenylamino)phthalonitrile)). The authors claimed that the high reduction potential required for the reaction in acetonitrile was achieved by the radical anion of the photocatalyst in its excited state, which initiated the reaction. This mechanism is usually named consecutive photoinduced electron transfer (ConPeT), in which two photons are involved: the first one to excite the photocatalyst and generate the radical anion 4DPAPN •– and the second photon to promote 4DPAPN •– to its excited state * 4DPAPN •– . Employing ultrafast transient absorption spectroscopy, here we report that, although two photons are indeed involved in this transformation, the excited state * 4DPAPN •– is short‐lived, not emissive, and not quenched by the organic substrate employed in the reaction, opposite to what was claimed by the authors. The photocatalyst in the excited state * 4DPAPN •– can generate a solvated electron that is able to reduce the substrate involved in this chemistry. It is worth noting that a different photochemical mechanism is likely to be operative in CH 2 Cl 2 , where solvated electrons are much less stabilized and reduction of the solvent might occur.