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Multivariate Synthetic Strategy for Improving Crystallinity of Zwitterionic Squaraine-Linked Covalent Organic Frameworks with Enhanced Photothermal Performance.

Two-dimensional covalent organic frameworks (2D COFs) offer a designable platform to explore porous polyelectrolyte frameworks with periodic ionic skeletons and uniform pore channels. However, the crystallinity of ionized 2D COF is often far from satisfactory as the electrostatic assembly of structures impedes the ordered layered arrangement. Here, a multivariate synthetic strategy to synthesize a highly crystalline squaraine (SQ)-linked zwitterionic 2D COF is proved. A neutral aldehyde monomer copolymerizes with squaric acid (SA) and amines in a controlled manner, resulting in the ionized COF with linkage heterogeneity in one tetragonal framework. Thus, the zwitterions of SQ are spatially isolated to minimize the electrostatic interaction and maintain the highly ordered layered stacking. With the addition of 85%-90% SA (relative to a total of aldehydes and SA), a fully SQ-linked zwitterionic 2D COF is achieved by the in-situ conversion of imine to SQ linkages. Such a highly crystalline SQ-linked COF promotes absorptivity in a full spectrum and photothermal conversion performances, and in turn, it exhibits enhanced solar-to-vapor generation with an efficiency of as high as 92.19%. These results suggest that synthetically regulating charge distribution is desirable to constitute a family of new crystalline polyelectrolyte frameworks.