Electron donor-free photoredox catalysis via an electron transfer cascade by cooperative organic photocatalysts

Electron-donating sacrificial reagents are highly important for certain photo-redox reactions. However, the use of excessive amounts of sacrificial reagents, mostly amines, often leads to undesired side products and is especially troublesome for product purification. Herein, we take the light-induced electron transfer cascade process of natural photosystems as a role model and assemble organic photocatalysts into cooperative photocatalyst couples. The cooperative photocatalyst couples could undergo intermolecular electron transfer to facilitate the charge separation process and overcome the need for an extra electron donor. Time-resolved photoluminescence spectroscopy was conducted to precisely characterize the photo-excited dynamics within the cooperative photocatalyst couples. As a model photoredox reaction, the carbon-carbon formation reaction between heteroarenes and malonates, which usually requires electron-donating sacrificial reagents such as amines, was conducted to demonstrate the feasibility of the cooperative photocatalyst couples under visible light irradiation. A significant reaction conversion improvement from trace conversion for a single photocatalyst system to over 90% by cooperative photocatalyst couples was achieved.