Nontrivial differentiation between two identical functionalities within the same molecule studied by STM

Physisorbed monolayer films of small alkyl-substituted bisurea derivatives based on the structure R-1-NHCONH-R-2-NHCONH-R-1 with R-1 = C12H25 and R-2 = C9H18 or C12H24 have been imaged on graphite (HOPG) at the solution-substrate interface using scanning tunneling microcopy (STM). They form well-ordered two-dimensional monolayers, which are imaged with submolecular resolution. The number of carbon atoms (odd or even) in the alkyl spacer between both urea groups determines the molecular conformation giving rise to an odd-even effect. The nonlinear conformation of these molecules could easily be visualized in the STM images. The position of the urea moieties can clearly be assigned. Lamellae of these compounds are remarkably stable, which allows for study of monolayer defects in detail. Surprisingly, sometimes urea groups reveal contrast variation within a monolayer. For R-2 = C12H24, the contrast of the urea groups differs within one molecule. On the other hand, the contrast of both urea groups for R-2 = C9H18 is the same within a molecule, but also for this compound, contrast variation is observed within a monolayer. For both compounds, this contrast variation is correlated with the orientation of the molecules within the monolayer.