The influence of oligo(ethylene glycol) side chains on the self-assembly of benzene-1,3,5-tricarboxamides in the solid state and in solution

Abstract

Substituted benzene-1,3,5-tricarboxamides (BTAs) 1–4 comprising polar tetraethyleneglycol (tetraEG) and/or apolar (R)-3,7-dimethyloctyl side chains were synthesised and their self-assembly in the solid state and in solution was investigated. While BTA 1 (comprising 3 apolar side chains) shows helical columnar packing via threefold α-helical type intermolecular hydrogen bonding in the solid state and up to high dilutions in alkane solution (10⁻⁵ M), helical columnar order is only preserved for asymmetric BTA 2 (comprising 1 polar and 2 apolar side chains) in the solid state and in a concentrated alkane solution (10⁻² M). The association constant K_ass is reduced by a factor of 10⁷ by introducing one polar tetraEG chain into the BTA. A further increase in the number of polar tetraEG chains attached to BTA core results in the complete loss of intermolecular hydrogen bond formation in the solid state and in solution. Moreover, for the polar BTAs 3–4, comprising 2 or 3 polar tetraEG chains, no self-assembly in water occurs because of the lack of hydrophobic shielding. We propose that tetraEG side chains interfere with the intermolecular hydrogen bonds, weakening the stacking behaviour of these asymmetric derivatives and drastically lowering the association constant due to competing intramolecular hydrogen bonding interactions. In contrast, one methoxyethyl unit does not affect the stability of the aggregation of BTAs (K_ass = 3 × 10⁷ M⁻¹) showing that more than one EG unit is required to disrupt the self-assembly of BTAs.