Bottom-up assembly of molecular wagons on a surface.

The bottom-up assembly of molecular building blocks, carrying specific functions, is a promising strategy for the construction of nanomachines. In this study we show how molecules with a mechanical function, i.e., being equipped with wheels, can be connected in a controlled way directly on a surface. By choosing suitable building blocks, assembled dimers and wagon trains can be formed, whereas the length of the chains can be limited by using a heterogeneous mixture of molecules. By using low temperature scanning tunneling microscopy, the chemical nature of the intermolecular connection is determined as a metal-ligand bond, which is stable enough to maintain the wagon train structure at room temperature. The intermolecular bonds can be controllably changed from trans to cis configurations thereby achieving bond angles of almost 90°.