Directional Transport of Photogenerated Electrons to Reduction Sites in Covalent Organic Frameworks by Microenvironment Modulation for CO2 Photoreduction

Abstract

AbstractAs the transfer of photogenerated electrons to CO2 directly determines its reduction performance, it is important to boost the local photogenerated electron density at the reaction site. Herein, a new strategy is demonstrated to evaluate the local photogenerated electron density at the reaction site by modulating the microenvironment of the reaction site at a molecular level based on 3 rationally designed relatively electron‐deficient (ED) and electron‐rich (ER) type COFs. Expectedly, En‐COF‐TAPB‐TDOEB exhibits high photogenerated electron density at the reduction site due to the presence of an additional electric field, whose polarization direction is consistent with that of the basic unit of COFs. As comparison, photogenerated electrons in Am‐COF‐TAPB‐TFB and Pr‐COF‐TFPB‐TAB mostly distribute in benzene and triphenylene, respectively, due to the delay in exciton dissociation and the presence of an electric field with an inverse direction compared with that of the basic unit of the COFs. Consequently, En‐COF‐TAPB‐TDOEB shows superior CO2 photoreduction efficiency to Am‐COF‐TAPB‐TFB and Pr‐COF‐TFPB‐TAB. Further mechanism investigation demonstrates the influence of the polar microenvironment on the excited state and charge separation pathways of the π‐system in En‐COF‐TAPB‐TDOEB, which convincingly confirms that the ability of directional transport of photogenerated electrons to reduction sites in photocatalysts is directly correlated to their activity in CO2 reduction.