π-plane size-dependent photocatalytic activity of radical-doped diimide covalent organic frameworks

Abstract

Rylene diimides (RDIs) are a type of electron-deficient organic dye exhibiting strong visible light absorption capabilities and photochemical properties that depend on the size of their π-conjugated plane. They are versatile organic monomers used for constructing covalent organic frameworks (COFs) and for investigating structure-property relationships in organic synthesis. In this study, three diimide-based COFs, PDI-COF, NDI-COF, and PBI-COF, with successively larger π-plane sizes, were constructed through condensation of 4,4',4''-(pyridine-2,4,6-triyl)trianiline (PTT) with pyromellitic dianhydride (PDA), 1,4,5,8-naphthalenetetracarboxylic dianhydride (NDA), or 3,4,9,10-perylenetetracarboxylic dianhydride (PTA), respectively. The obtained COFs present not only well-defined crystallinity but also reactive organic radicals under ambient conditions. Upon exposure to light, all three COFs efficiently produce superoxide radical anions that promote the photocatalytic selective C-C and C-P coupling of tetrahydroisoquinoline. Combined experimental and theoretical studies demonstrated that the obtained COFs displayed prominent π-plane size-dependent photocatalytic activity in the order PBI-COF > NDI-COF > PDI-COF, consistent with the decreasing size of their electron-deficient π-conjugated planes.