Crystal transformation from the incorporation of coordinate bonds into a hydrogen-bonded network yields robust free-standing supramolecular membranes

In this work, we report on the synthesis of a free-standing, macroscopic robust supramolecular membrane by introducing silver-nitrogen coordinate bonding into preorganized, supramolecular hydrogen-bonded cyanuric acid-melamine (CAM) crystals. With the assistance of ammonia, silver ions competitively replace two of the three hydrogen atoms from cyanuric acid resulting in the transformation from short CAM nanorods to long CAM-Ag nanofibers (length over 1000 μm), accompanied by tautomerization of cyanuric acid. The single crystal structure of the CAM-Ag nanofibers is solved in the space group P1, with the asymmetric unit containing eight silver atoms, four melamine and four cyanuric acid molecules, which generate 1D coordination polymer chains consisting of alternating melamine and dianionic cyanurate ligands linked via silver-nitrogen bonds. The presence of interchain hydrogen bonds results in the expansion of the supramolecular network into undulating 2D sheets, which then stack into a 3D network via a series of intersheet hydrogen bonds and π-πinteractions. Significantly, the CAM-Ag nanofibers spontaneously assemble into a free-standing membrane, with lateral size up to square centimeters and thickness of 30 μm. The membrane shows high flexibility and mechanical strength, owing to the improved flexibility of the CAM-Ag nanofibers with bonded chain structure, and can be reversibly and repeatedly bent over 90 degrees. Remarkably, the CAM-Ag membrane demonstrates distinct optical transmittance being shortwave IR transmissive but impenetrable to UV and visible light.