The ABC-stacking of the kgm layers leads to the "closing" of the larger triangular pores with the smaller hexagonal pores with small pore apertures of only around 7 Å, which leads to a pearl-string pore structure, which is quite unusual for COFs (Figure ). The small pore apertures that are every third layer in the structure, imposed by the six phenyl methoxycarbonyl linkers protruding into the pore and nearly in contact with each other thereby dominate the adsorption properties and lead to only small micropores being observed in the structure. This highly crowded pore is exaggerated by the locking of the imine orientation away from the ortho-phenyl ring. This orientation was predicted by force field calculations and verified by the single crystal X-ray diffraction (SCXRD) structure of a model imine compound from aniline and a carboxy dialdehyde linker (Figure 3). This orientation can be attributed to steric repulsion with the phenyl ring. Interestingly the interaction between the imine and the phenyl ring reduces the number of conformers 14 of the imine bond, which can be used to improve the crystallinity of COFs by forcing it to point away from the phenyl rings. Similar strategies can be realized with intramolecular hydrogen bonding and ortho methoxy groups. To understand the preference for formation of the kgm structure over the sql structure in the 4A2E-PDA-COF we used force field simulations to look at the interactions within one pore in unconstrained molecular models (Figure ). In the hexagonal pore the methyl groups are in close contact but no out of plane bending is observed, which would be indicative of an overcrowded pore space. In the rhombic pore using also PDA, significant steric repulsion can be observed by the bulky functional groups bending out of each other's way (Figure ). Interestingly, this situation is reinforced by the conformational locking of the imine groups, which forces the bulky functional groups closer to each other. We theorized that this close contact leads to repulsion and thereby disfavors the sql structure when using the short phenylene diamine linker. The close contact of the methyl groups across the pore in the sql net is directly related to the length of the diamine linker, where a longer diamine might relieve some of the steric repulsion. Simulation of a benzidine based rhombic pore (Figure ) indeed showed less repulsion between the methyl groups despite close contact. To test this assumption experimentally, we synthesized the COF based on the tetraaldehyde and benzidine (Bz), which is significantly longer (N-N distance of 9.98 Å for Bz 43 vs. 5.66 Å for PDA , respectively), and therefore should lead to decreased repulsion between the ester groups. The benzidine based COF showed prominent reflexes in the PXRD at 4.60° and 6.44° with smaller broad and overlapping reflexes at 9.7°, 13.4°, 23.1° (Figure ). This pattern can be modelled by a sql based structure with a primitive unit cell (Z=1). Lattice parameters and the local symmetry match well with the observed crystallites in the TEM (Figure ). However, in the FFT additional periodicities can only be explained with a Z=2 superstructure, redefined with a = (110), b = (-110), c = (001) respective to the primitive unit cell. The additional periodicities of observed in the FFT then correspond to the 410, 520 and 610 reflections. Supercell periodicities are observed in multiple crystallites in the TEM (Figure S 9, Figure S 10). We therefore modeled the structure of the 4A2E-Bz-COF as a Z=2 unit cell in Materials Studio. Pawley refinement of the lattice parameters led to good agreement with the observed PXRD pattern (Figure , F), resulting in a unit cell with lattice parameters of: a = 27.5 Å, b =31.9 Å, c = 4.4 Å, α = 55.0°, β = 90°; γ = 86.6°. The deviation of γ from 90° dramatically improved the Pawley fit and matches with TEM images which showed γ = ~84°. The peak splitting at 6.44° indicated a symmetry reduction in the structure, which was well explained with slip-stacking perpendicular to the 4A2E linker in the structure. In sql lattice COFs, peak splitting can also be explained by a≠b structures, however these refinements all led to chemical unreasonably small values of a and b during refinement. Additionally, the N2-adsorption isotherm (Figure ) match the expected structure well. The calculated BET surface was 774 m²/g. The stoichiometric nature of 4A2E-Bz-COF was confirmed by IR spectroscopy and ssNMR (Figure S 7, Figure ), where no significant aldehyde peak was detected at 190 ppm. Simultaneously, ssNMR revealed the characteristic peak of the quaternary carbon of the methyl ester (1') at 165 ppm, and the methyl ester carbon (5') could be assigned to the peak at 52 ppm for 4A2E-Bz-COF (Figure S 11). The methyl ester peak is thus shifted downfield from the methyl group attached to the 4A2E linker core, which is evident at 17 ppm.