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

Abstract

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 g_cat.^–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.