Turning on Singlet Oxygen Generation by Outer-Sphere Microenvironment Modulation in Porphyrinic Covalent Organic Frameworks for Photocatalytic Oxidation

Singlet oxygen (1O2) is ubiquitously involved in various photocatalytic oxidation reactions; however, efficient and selective production of 1O2 is still challenging. Herein, we reported the synthesis of nickel porphyrin-based covalent organic frameworks (COFs) incorporating functional groups with different electron-donating/-withdrawing features on their pore walls. These functional groups established a dedicated outer-sphere microenvironment surrounding the Ni catalytic center that tunes the activity of the COFs for 1O2-mediated thioether oxidation. With the increase of the electron-donating ability of functional groups, the modulated outer-sphere microenvironment turns on the catalytic activity from a yield of nearly zero by the cyano group functionalized COF to an excellent yield of 98 % by the methoxy group functionalized one. Electronic property investigation and density-functional theory (DFT) calculations suggested that the distinct excitonic behaviors attributed to the diverse band energy levels and orbital compositions are responsible for the different activities. This study represents the first regulation of generating reactive oxygen species (ROS) based on the strategy of outer-sphere microenvironment modulation in COFs. The modulation of the outer-sphere microenvironment around catalytic metal sites in covalent organic frameworks (COFs) with diverse functional groups turns on the activity of 1O2-mediated photocatalytic oxidation of thioether with the increased electron-donating ability of the groups. The yields boost from nearly zero in the cyano-functionalized COF to 98 % in the methoxy-functionalized counterpart.image