Crystal Engineering of Supramolecular 1,4‐Benzene Bisamides by Side‐Chain Modification – Towards Tuneable Anisotropic Morphologies and Surfaces

Abstract Benzene bisamides are promising building blocks for supramolecular nano‐objects. Their functionality depends on morphology and surface properties. However, a direct link between surface properties and molecular structure itself is missing for this material class. Here, we investigate this interplay for two series of 1,4‐benzene bisamides with symmetric and asymmetric peripheral substitution. We elucidated the crystal structures, determined the nano‐object morphologies and derived the wetting behaviour of the preferentially exposed surfaces. The crystal structures were solved by combining single‐crystal and powder X‐ray diffraction, solid‐state NMR spectroscopy and computational modelling. Bulky side groups, here t‐butyl groups, serve as a structure‐directing motif into a packing pattern, which favours the formation of thin platelets. The use of slim peripheral groups on both sides, in our case linear perfluorinated, alkyl chains, self‐assemble the benzene bisamides into a second packing pattern which leads to ribbon‐like nano‐objects. For both packing types, the preferentially exposed surfaces consist of the ends of the peripheral groups. Asymmetric substitution with bulky and slim groups leads to an ordered alternating arrangement of the groups exposed to the surface. This allows the hydrophobicity of the surfaces to be gradually altered. We thus identified two leitmotifs for molecular packings of benzene bisamides providing the missing link between the molecular structure, the anisotropic morphologies and adjustable surface properties of the supramolecular nano‐objects.

For all substances, melting and sublimation points, as determined by DSC and TGA measurements, are too close to each other to be distinguished properly. Therefore, we provide the temperature of 5 % mass loss in Table S1. Mass spectroscopy: Mass spectra were recorded on a Finnigan MAT 8500 spectrometer (Thermo Fisher Scientific) (EI, 70 eV) using direct injection mode. -6-

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Crystallographic information of the single crystal structure solutions  A preferred orientation of the 4 th order using spherical harmonics [1] has been applied to all models except the Pawley fit.
The single crystal solution of 2A yielded a cell in the space group P21/n, however, the R-values are relatively poor. This is explained by the habitus of the crystal. The compound crystalizes in very thin platelets leading to a limited number of strong reflexions. Indexing the PXRD led to a similar but slightly different cell in the space group P21. The absence of the glide plane leads to a doubling of the asymmetric unit. The single crystal data was refined in this space group as well and the obtained model placed in the cell obtained from PXRD. This model was geometry optimised by force field and DFT methods to correct for the now incorrect bond lengths and angles. The geometry optimized model could be refined with a good agreement with Rietveld methods. Then, the glide plane was found again and the Rietveld refinement could be redone with the space group P21/n yielding even better R-values ( Figure S3). This shows that although the R-values result in an A-Alert in the CheckCIF report, the structure solution arising from the single crystal is correct and the data quality arises from the desired and consequently engineered morphology as a thin platelet.

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Different coordination of the two series Figure S9. Hydrogen bond pattern of series 1 (left) and series 2 (right) in the so-called Etter notation [2] . The big black dots resemble molecules and the lines resemble hydrogen bonds. The three small dots represent propagation of the pattern in this direction.

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Different global packing of 2A compared to 2B and 2C. Figure S10. Difference in the global packing of 2A (left) and series 2B and 2C (right). In the sketch on the bottom the black lines resemble the orientation of the top layer and the grey lines resemble the 2 nd layer. In red, the symmetry element that determines the space group is depicted.

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Structure models of 2A in the structure type of series 1 Figure S11. Possibilities of packing 2A in the structure of 1A.
To investigate the driving force that leads to the different structure types models of all molecules were built in the structure of the respective molecule of the other series. Since the molecules of series 1 are symmetric, there is only one way to place them into the unit cells of series 2. For the asymmetric molecules of series 2, different options arise as depicted for 2A in Figure S11. These options are: Model 1: The space symmetry is reduced from P1 " to P1 while maintaining the original metric. One side chain of 1A is then replaced by a t-Bu group. The resulting unit cell consists of one molecule leading to a model where t-Bu and the CF groups are facing each other at the interlayer gap.
Model 2: The c-axis is doubled creating two molecules within the unit cell. The molecules are placed in a way that similar side chains, the t-Bu groups and the fluorinated alkyl chains from neighbouring molecules face each other.
Model 3: The a-axis is doubled and the CF chains are replaced by t-Bu in a way that they are alternating along the a-axis. The unit cell consists of two molecules.
Model 4: The b-axis is doubled and the CF chain is replaced by t-Bu in a way that they are alternating along the b-axis. The unit cell consists of two molecules.
Model 5: Finally, a random combination of all models is possible. Here, however, the unit cell increases dramatically.
For models 1, 2 and 4 the local binding situations around the NH×××O hydrogen bonds is similar. In all cases, both the NH and the carbonyl function are terminated by the same side chain. Thus, for the comparison between packing pattern 1 and 2, both models will lead to the similar results. Models 3 shows a different local environment. Here, the NH and the carbonyl function are terminated by different side chains. Therefore, DFT calculations were restricted to models 1 and 3. They lead to very similar energies per molecules. Doubling one unit cell axis results in an alternating pattern of longer (2.4 Å) and shorter (2.1 Å) hydrogen bond lengths due to the additional degrees of freedom as the molecules are now able to rotate against each other. Model 5 is a superposition of the other models and will show similar results. Thus, for the discussion within the manuscript the smallest model with only one molecule in the asymmetric unit was chosen for the comparison with molecules of series 1 calculated in packing pattern 2, in order to keep the number of variable parameters similar.

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Scanning electron microscopy of self-assembled platelets of symmetric 1,4-benzene bisamides 1B-C and asymmetric 1,4-benzene bisamides 2B-C For the preparation of SEM samples from dispersions, a drop was cast on a clean silicon wafer.
The supernatant solvent was removed with a filter paper and the sample was dried at ambient conditions. Subsequently, the sample was sputtered with a platinum layer with a layer thickness of 1.3 nm using a Cressington 208HR sputter coater. SEM measurements were performed with a field emission scanning electron microscope (Zeiss LEO 1530) using an accelerating voltage of 3 kV.

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AFM of self-assembled platelets of both series AFM measurements were performed using a Veeco dimension 3100 atomic force microscope equipped with a NanoScope IV controller. Bruker OTESPA-R3 silicon cantilevers were used in tapping mode. Square images were captured with 512 lines and 512 points per line and evaluated using Bruker NanoScope Analysis software (version 1.40). Prior to analysis, the images were flattened (1 st order). To analyze the layer terraces, small sections from crystals' top surfaces were used. A step analysis, which considers many parallel lines to reduce noise in z direction, gave a profile of the terraces along the x axis. From this profile, tilts were removed using the evaluation software. Parallel lines were fitted to the different terrace levels and the lines' distance was measured to obtain the terrace heights.