  The fabrication of nanomaterials involves self-ordering processes of
functional molecules on inorganic surfaces. To obtain specific molecular
arrangements, a common strategy is to equip molecules with functional groups.
However, focusing on the functional groups alone does not provide a
comprehensive picture. Especially at interfaces, processes that govern
self-ordering are complex and involve various physical and chemical effects,
often leading to structures that defy chemical intuition, as we showcase here
on the example of a homologous series of quinones on Ag(111). From chemical
intuition one could expect that such quinones, which all bear the same
functionalization, form similar motifs. In salient contrast, our joint
theoretical and experimental study shows that profoundly different structures
are formed. Using a machine-learning-based structure search algorithm, we find
that this is due to a shift of the balance of three antagonizing driving
forces: adsorbate-substrate interactions governing adsorption sites,
adsorbate-adsorbate interactions favoring close packing, and steric hindrance
inhibiting certain otherwise energetically beneficial molecular arrangements.
The theoretical structures show excellent agreement with our experimental
characterizations of the organic/inorganic interfaces, both for the unit cell
sizes and the orientations of the molecules within. With a detailed examination
of all driving forces, we are further able to devise a design principle for
self-assembly of functionalized molecules. The non-intuitive interplay of
similarly strong interaction mechanisms will continue to be a challenging
aspect for the design of functional interfaces. Our agreement between theory
and experiment combined with the new physical insights indicates that these
methods have now reached the necessary accuracy to do so.
