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–π interactions. 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.