The stochiometric nature of the 4A2E-PDA-COF was also verified by solid-state NMR (ssNMR) (Figure , B, including assignments following ref ), where no significant aldehyde peak was observed at 190 ppm. At the same time ssNMR showed the characteristic peak of the quaternary carbon of the methyl ester (1) at 165 ppm and the methyl ester carbon (5) can be assigned to the peak at 50 ppm for the 4A2E-PDA-COF. The methyl ester peak is thus shifted downfield from the methyl group attached to the 4A2E linker core, which can be found at 18 ppm. These measurements strongly indicate a stoichiometric structure as there are no dangling aldehyde groups or amine groups are detectable, and an intact methyl ester group. Additionally, dangling amines from a PDA core would not lead to the formation of a two-periodic structure and would unlikely, but has been observed before under special cirumstances. To determine the porosity of the synthesized COFs, nitrogen sorption isotherms were measured at 77 K. The N2 gas adsorption isotherms showed a type I isotherm with expected steep uptake at low pressures (Figure ), as even the "larger" trigonal pores in a kgm net are below 1.4 nm as seen from the structure simulations. The resulting BET surface showed a moderate area of 622 m 2 /g, which suggest an overall porous structure. It is known that the stacking can lead to symmetry reductions or even complete changes of a PXRD pattern through the change in symmetry. Therefore, we considered again the kgm structural model, and started building models with different stacking modes, to test if the stacking can explain the absence of the 100 peak in the PXRD and the TEM images. Slip stacking only leads to peak broadening or small peak splitting through apparent symmetry reduction, but cannot explain the absence of one significant reflection. In AB stacking where one hexagonal pore stacks on top of a triangular pore, while one trigonal pore remains open, leads to a dramatic reduction, but not to a complete loss off intensity of the 100 peak in the simulated pattern. When simulating the ABC-stacking of the kgm net layers of the COF we obtained a good match with the simulated XRD pattern, which was also validated by Pawley refinement of the ABC-stacked structure. The ABC stacking leads to a change of the symmetry to 𝑅3 ̅ , the symmetry has a 3-fold screw axis, which leads to systematic absences in the X-Ray diffraction. The reflection conditions lead to a complete absence of the 100 reflection. Additionally, this structural model can explain the previously unexplained reflex at 7.85° 2θ (d=11.33 Å), which corresponds to the 101 reflection. The periodicity corresponding to the 101 reflections in the PXRD can also be observed in some crystal orientations in the TEM images (Figure ). Explaining the presence of this reflection is important, as in typical hexagonal COFs with small parameters of c < 5 Å, no reflections are expected between the "first" and "second" reflection in a hexagonal unit cell, which normally correspond to the 100 and 110 reflections, respectively. In the 𝑅3 ̅ space group of the ABC stacked 4A2E-PDA-COF also requires the presence of three layers in one unit cell. While reflections such as 001 are not observed due to systematic absences (only 000l: l = 3n are allowed), the 101 reflection is allowed as it fulfills the hkil: -h+k+l = 3n reflection condition. Therefore, the ABC stacked model of the 4A2E-PDA-COF explains the absent and additional reflections in the PXRD fully, and matches well with the crystallites seen in cryo-TEM.