Complex Molecules on a Flat Metal Surface: Large Distortions Induced by Chemisorption Can Make Physisorption Energetically More Favorable

Normally, chemisorpiton will give stronger bonding a surface than physisorption. However, if chemisorpiton induces large distortions of the adsorbate that are energetically costly, then physisorption may become more favorable. We illustrate this second, anomalous behavior through theoretical modeling of a 2,2':6',2-terpyridine (terpy) molecule on a flat metal surface. Terpy exhibits rich intramolecular ring-ring interactions mainly due to the N heteroatoms; in addition, the single C-C bond linking rings allow the molecules to be twisted into quite different conformations. On a Cu(111) surface, the flat physisorption of terpy is energetically more favorable than its strongest chemisorption in a twisted and strongly distorted conformation. We may expect, in general, that intramolecular ring-ring interactions that control distortions like molecular twisting can become important in the adsorption of molecules with N-heterocyclic rings linked by twistable bonds.