On‐Liquid Surface Synthesis of Crystalline 2D Polyimine Thin Films

On‐water surface synthesis has emerged as a powerful approach for constructing thin‐layer, crystalline 2D polyimines and their layer‐stacked covalent organic frameworks. This is achieved by directing monomer preorganization and subsequent 2D polymerization on the water surface. However, the poor compatibility of water with many organic monomers has limited the range of accessible 2D polyimine structures. Herein, the on‐liquid surface synthesis of crystalline 2D polyimine films from a water‐insoluble, C 3 ‐symmetric monomer previously deemed incompatible with aqueous systems is reported. In situ grazing incidence X‐ray scattering reveals a stepwise evolution of monomer adsorption, preorganization, and 2D polymerization assisted by the fluorinated surfactant monolayer, leading to the formation of large‐area, face‐on‐oriented 2D polyimine films. Notably, a pronounced lattice expansion from 3.4 nm in the monomer assembly to 5.3 nm in the 2D polyimine framework is observed, highlighting the templating effect of the preorganized monomers in defining the final crystallinity. The representative 2DPI‐TCQ‐DHB is obtained as free‐standing thin film with well‐defined hexagonal pores, mechanical robustness, and a negatively charged surface (zeta potential: −58.8 mV). Leveraging these structural characteristics, it is integrated 2DPI‐TCQ‐DHB films into osmotic power generators, achieving a power density of 16.0 W m −2 by mixing artificial seawater and river water, surpassing most nanoporous 2D membranes.