Hierarchical Assembly of 2D Covalent Organic Frameworks into Janus Optical Devices

Abstract Assembly of 2D molecules into hierarchical structures opens new avenues for creating multifunctional materials, yet fabricating the optical materials with multiscale structural features, and extending functions in a controllable way remains a challenge. Here, an asynchronous liquid depositing strategy is introduced that enables the asymmetrical growth of 2D covalent organic frameworks (COFs) on a graphene substrate. This results in Janus composite (JC) materials with independently controlled mesoscopic features, such as the COF layer thickness (100–300 nm), and its nanoflake length (0–200 nm), and thickness (0–30 nm). The JCs exhibit impressive light coupling capabilities and display a wide range of angle‐independent structural colors. These colors can be dynamically and reversibly changed in 10 ms when exposed to volatile organic compounds (VOCs). They also demonstrate remarkable tailorability to form both random and ordered patterns which shows excellent information processing potential. These unique properties make JCs highly suitable as novel responsive optical devices for collecting and processing information about their surroundings, as demonstrated by visual and real‐time VOC monitoring and localization in both open and confined spaces. It is believed that the controlled assembly of 2D components into composites with well‐organized hierarchical structures will facilitate the future development of extraordinary materials with extensive applications.