Janus Covalent Organic Framework Films With Macroscopic Directional Carrier Migration for Switchable Gas Sensing.

Macroscopic directional migration of photogenerated carriers in covalent organic framework (COF)-based heterojunction films is typically hindered by random carrier diffusion and interfacial structural disorder. In this study, we report for the first time the construction of sequence-controlled Janus COF films from lattice-matched isoreticular COFs (COF-366/COF-366-OH). The intimate interfacial assembly and the energy-level offset induce a built-in electric field that drives electrons and holes to migrate directionally toward opposite sides across the entire film. This asymmetric charge distribution endows the two sides of the film with distinct redox activities, enabling switchable and highly sensitive detection of both oxidizing and reducing gases by simply flipping the film. Such a unique feature is unprecedented for a single film. Moreover, the response of Janus COF film surpasses that of single-component counterparts by 5.2-fold for NO2 and 5.3-fold for NH3. This work offers a general strategy for macroscopic directional carrier migration in COF films and paves a new route for advanced sensing materials.