Three-Dimensional Bicarbazole-Based Covalent Organic Frameworks as Efficient Yeager-Type Photocatalysts for H2O2 Generation in a Two-Phase System

Photocatalytic reduction of oxygen to hydrogen peroxide (H2O2) represents an attractive solar-to-chemical conversion pathway. Nevertheless, it remains a significant challenge to achieve efficient H2O2 photosynthesis while simultaneously mitigating photocorrosion of the catalysts owing to the presence of superoxide radicals (•O2-) and the accumulation of photoexcited holes (h+). In the present work, a 1,2,3,4-tetrahydroisoquinoline (THIQ)-water two-phase system was developed to achieve high-efficiency and durable production of H2O2 by suppressing •O2- intermediates and rapidly consuming h+. The •O2--free direct two-electron oxygen reduction reaction (2e- ORR) to H2O2 was accomplished on special three-dimensional (3D) covalent organic framework (COF) metal-free photocatalysts consisting of bicarbazole units (BCTB) as electron donors and thiazole (BT) or triazine (TAPT) as electron acceptors (COF-BCTB-BT or COF-BCTB-TAPT). The unique structures endow them with a high H2O2 production rate in the water phase of ∼33.2 mmol gcat.-1 h-1 over COF-BCTB-BT. In the other organic phase, the photoexcited h+ was also efficiently consumed by semidehydrogenation of THIQ (THIQ-SDR) to 3,4-dihydroisoquinoline (DHIQ). Theoretical calculations revealed a Yeager-type four-step direct 2e- ORR mechanism over two COFs, with a lower energy barrier of *O-O to *O-OH for COF-BCTB-BT. A four-step mechanism of the THIQ-SDR to DHIQ was also suggested. This work provides an impressive Yeager-type two-phase H2O2 photosynthesis strategy over high-efficiency 3D bis-heterocyclic COF photocatalysts, effectively suppressing both •O2- formation and h+ accumulation.