Screening‐Effect‐Driven Growth of Large‐Area High‐Quality COF Films with Wavelength‐Dependent Photochromism

ABSTRACT Competing heterogeneous and homogeneous nucleation severely restricts scalable, high‐yield synthesis of high‐quality covalent organic framework (COF) films, as bulk homogeneous nucleation generates particulate impurities and cuts film yield below 5% with exorbitant fabrication costs. Here we propose a screening‐effect‐mediated nucleation control strategy to fully suppress homogeneous nucleation by overlapping monomer depletion layers around substrate nuclei, implemented via a custom batch micro‐slit reactor that tunes inter‐substrate distance, polymerization kinetics, and surface wettability to achieve exclusive heterogeneous film growth. This approach affords high‐crystallinity COF films with ∼90% intrinsic yield and minimal particle contamination, applicable to 12 COF structures and capable of fabricating >1 m 2 films at drastically reduced costs (6.0–27.5 USD/m 2 ). The impurity‐free films display pronounced wavelength‐dependent photochromism from selective 350–500 nm absorption, enabling high‐resolution optical patterning for anti‐counterfeiting and sensing. This work provides a scalable, low‐cost route for COF membrane production and establishes a universal kinetic framework to decouple dual nucleation processes in 2D crystalline porous materials.