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-dimethyloctanoyl chains were liquid crystalline and the mesophase was assigned as Col(ho). 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-and-soldiers 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 N-BTAs. 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.