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Chiral molecular walkers, standing on their ‘feet’ on an anisotropic surface, perform preferential unidirectional Brownian motion under the influence of an external oscillating field according to their orientation, conformation and chirality.

Various structural motifs such as 1D ribbons and their interconnections were recently seen in scanning tunneling microscopy experiments when para-terphenyl-meta-dicarbonitrile (pTmDC) molecules were deposited on the Ag(111) surface (Marschall, M.; et al. ChemPhysChem 2010, 11, 1446). By using ab initio density functional theory calculations, we systematically study the main building blocks that the two isomers of the pTmDC molecules can form in the gas phase, going from dimers to more complex structures involving trimers, chains, and various linkers. We show that the hydrogen bonding plays a decisive role in the formation of the different experimentally observed structures. We have also found that the energy barriers for the isomerization transition for a single molecule in the gas phase are always lower than 9.6 kJ/mol, proving that there must be equal amounts of both isomers on the surface initially at deposition.

Using a toolkit of theoretical techniques comprising ab initio density functional theory calculations, the nudged elastic band method and kinetic Monte Carlo (KMC) modeling, we investigate in great detail how para-terphenyl-meta-dicarbonitrile (pTmDC) molecules diffuse and isomerize to self-assemble on the Ag(111) surface. We show that molecules "walk" on the surface via a pivoting mechanism moving each of its two "legs" one at a time. We then identify a peculiar "under-side" isomerization mechanism capable of changing the molecules chirality, and demonstrate that it is fundamental in understanding the growth of hydrogen bonding assembles of ribbons, linkers, clusters and brickwall islands on the Ag(111) surface, as observed in recent scanning tunneling microscopy experiments (ChemPhysChem, 2010, 11, 1446). The discovered underlying atomistic mechanism of self-assembly may be behind the growth of other hydrogen bonding structures of chiral molecules on metal surfaces.