Dynamic supramolecular polymers based on benzene-1,3,5-tricarboxamides : the influence of amide connectivity on aggregate stability and amplification of chirality

N-Centred benzene-1,3,5-tricarboxamides (N-BTAs) composed of chiral and achiral alkyl substituents were synthesised and their solid-state behaviour and self-assembly in dilute alkane solutions were investigated. A combination of differential scanning calorimetry (DSC), polarisation optical microscopy (POM) and X-ray diffraction revealed that the chiral N-BTA derivatives with branched 3,7-dimethyl-octanoyl chains were liquid crystalline and the mesophase was assigned as Colho. In contrast, N-BTA derivatives with linear tetradecanoyl or octanoyl chains lacked a mesophase and were obtained as crystalline compounds. Variable-temperature infrared spectroscopy showed the presence of threefold, intermolecular hydrogen bonding between neighbouring molecules in the mesophase of the chiral N-BTAs. In the crystalline state at room temperature a more complicated packing between the molecules was observed. Ultraviolet and circular dichroism spectroscopy on dilute solutions of N-BTAs revealed a cooperative self-assembly behaviour of the N-BTA molecules into supramolecular polymers with preferred helicity when chiral alkyl chains were present. Both the sergeants-andsoldiers as well as the majority-rules principles were operative in stacks of N-BTAs. In fact, the self-assembly of N-BTAs resembles closely that of their carbonyl (C=O)-centred counterparts, with the exception that aggregation is weaker and amplification of chirality is less pronounced. The differences in the self-assembly of N- and C=O-BTAs were analysed by density functional theory (DFT)calculations. These reveal a substantially lower interaction energy between the monomeric units in the supramolecular polymers of NBTAs. The lower interaction energy is due to the higher energy penalty for rotation around the Ph??NH bond compared to the Ph-CO bond and the diminished magnitude of dipole–dipole interactions. Finally, we observed that mixed stacks are formed in dilute solution when mixing N-BTAs and C=O BTAs.