Theoretical investigation of macrodipoles in supramolecular columnar stackings

Supramolecular columnar assemblies are known to form intrinsic macrodipoles, which play an important role in intercolumnar interactions and govern the self-assembly on the mesoscale. A prominent class that provides this feature are trisamide derivatives, namely, 1,3,5-benzenetrisamides and 1,3,5-cyclohexanetrisamides. The understanding of how subtle changes in the chemical structure influence the columnar order and consequently the macrodipole formation is of fundamental interest. Here we report on the theoretical investigation of trisamide derivatives and how the formed macrodipole is related to the properties of the columnar aggregates. Calculations were carried out on a semiempirical level using the PM6 approximation, which is able to treat weak interactions like hydrogen bonding and dispersion forces with a sufficient accuracy. We have compared the influence of a benzene core with a cyclohexane core on the macrodipole formation. It was revealed that columnar aggregates based on 1,3,5-cyclohexanetrisamides have much higher dipole moments than those formed with aromatic cores. A cooperative effect was found during aggregation, as longer aggregates show stronger hydrogen bonding, thereby facilitating the addition of the next molecule. We have also investigated the influence of the amide connection on the strength of the formed macrodipole. The trends observed for the macrodipole strength correlate with the calculated heat of formation. If the amide groups are inverted, the strength of the macrodipole is reduced and the negative heat of formation is increased. HOMO-LUMO gaps were correlated with the inverse of the dipole moment per monomer unit, thus indicating that the macrodipole might act as a perturbation to the supramolecular assemblies.