Molecular modelling of self-assembled alkynyl monolayer structures — Unnatural symmetry units, surface bonding, and topochemical polymerization1

Geometric modelling techniques are used to map the potential energies of packing for self-assembled alkyl- and phenyl-backboned monolayers across a range of intermolecular separations. Natural packing distances of 4.2–4.4 Å produce less stable, more isotropic monolayers because of repulsive interchain contacts. Optimizations at unnatural surface densities found thin films of lower energy and higher symmetry existed at increased chain–chain separations. Head-group bonding is therefore identified as a force for controlling monolayer order. Analysis of the natural monolayer structures on a silicon dioxide surface determined the favourable head-group structures, and allowed the topochemical polymerization of p-bis(butadiynyl)benzene monolayers to be rationalized.Key words: self-assembled monolayers, molecular modelling studies, repeating symmetry units, thin-film order, topochemical polymerization.