To gain additional orthogonal information about the crystal structure required for solving the COF's structure, we performed cryo-TEM. In the TEM the 4A2E-PDA-COF showed intergrown crystallites ranging in size from 100 to 150 nm (Figure S 8), which rendered single crystal electron diffraction structure determination unfeasible. However, owing to the high resolution achieved in the TEM images, we could observe these crystallites in various orientations, including alignment with the [001] direction, clearly revealing the hexagonal symmetry of the structure (Figure ). This observation confirmed a d-spacing of 16.9 Å in the fast Fourier transform (FFT) of the TEM images, matching well with the reflection observed in the PXRD at 5.22°, which can be assigned to the 110 reflection in a kgm model (Figure ). Additionally, FFT and real space images showed no indications of a 100 reflection, anywhere in the TEM images of the 4A2E-COF (Figure ). This corresponds to the absence of this feature in the PXRD. The observed periodicities in the TEM, coupled with the hexagonal symmetry, evidently deviate from a simple eclipsed kgm model. Next, we intended to exclude sub-stoichiometric structures, as these can lead to unexpected unit cells and can appear under specific synthesis conditions or when competing interactions are at play in the structure formation. When comparing the IR spectra of the 4A2E-linker molecules and the 4A2E-PDA-COF (Figure S 6), the strong band corresponding to the C=O stretching of the aldehyde at 1681 cm -1 disappears for the formed 4A2E-PDA-COF, while the C=O stretch vibration of the methyl ester group only shifts slightly from 1715 cm -1 to 1733 cm -1 for the linker and the COF, respectively. At the same time a vibration at 1612 cm -1 appears that can be assigned to the imine C=N stretch vibration. The characteristic amine vibrations from N-H stretch vibrations in the region 3100-3500 cm that are visible in the PDA linker completely disappear upon the formation of the 4A2E-PDA-COF.