Selective Generation of Reactive Oxygen Species in Photocatalytic Oxidation by Tuning Porphyrin-Based COFs' Dimensionality

Regulating the selective generation of reactive oxygen species (ROS) is a significant challenge in the field of photocatalytic oxidation, with successful approaches still being limited. Herein, we present a strategy to selectively generate singlet oxygen (O-1(2)) and superoxide radicals (O-2(center dot-)) by tuning the dimensionality of porphyrin-based covalent organic frameworks (COFs). The transformation of COFs from three-dimensional (3D) solids to two-dimensional (2D) sheets was achieved through the reversible protonation of the imine bond. Upon irradiation, both bulk and thin-layer COF-367 can transfer energy to O-2 to generate O-1(2). However, thin-layer COF-367 exhibited a superior performance compared to its bulk counterpart in activating O-2 to form the O-2(center dot-) radicals via electron transfer. After excluding the influences of the band structure, O-2 adsorption energy, and frontier orbital composition attributed to the dimensionality of the COFs, it is reasonably speculated that the variance in ROS generation arises from the differential exposure ratios of the active surfaces, leading to distinct reaction pathways between the carrier and O-2. This study is the first to explore the modulation mechanism of COF dimensionality on the activation of the O-2 pathway, underscoring the importance of considering COF dimensionality in photocatalytic reactions.