Supramolecular engineering comprises the design, synthesis and self-assembly of well-defined molecular modules into tailormade architectures. The incorporation of functional units in these molecular modules makes it possible to provide a preprogrammed function to the overall architecture and material, thus paving the way towards its technological application in a myriad of fields.[1] For this purpose, the last decade has witnessed an increasing interest towards the 3D engineering of supramolecular materials.[2–6] The colossal task of achieving full control over self-assembled systems however, is the ongoing endeavor of the supramolecular scientists and to date only few systems may be pre-programmed to undergo controlled self-assembly in 3D.[7–11] In contrast, 2D interfaces provide a simplified platform for supramolecular and crystal engineering, and the field has known an exciting growth.[12–15] These early attempts of 2D crystal engineering are exemplified in the pre-programmed molecular (self-) organization at surfaces and interfaces, a realm that holds per se a great potential for the generation of novel 2D nanoscale functional materials and devices [16] or devices with custom-made properties, such as charge injection,[17] transport [18, 19] and storage.[20] In this regard, the need for exploring ordered architectures at the molecular scale has made scanning tunneling microscopy (STM)[21] a widely employed, though extremely powerful tool to study supramolecular materials at interfaces with a submolecular resolution.[22–24] STM investigations provide electronic and thus chemical insights on the sub-nm scale.[25] The working principle of STM is the tunneling of electrons from a sharp scanning tip to a substrate. Since the tunneling current is proportional to the electron density of the molecule within an energy range,[26] the STM contrast in the so called “constant (tip) height” mode will appear brighter within electron-rich aromatic molecules than aliphatic groups. In addition, quantitative measurements on the electronic structure of the molecules can be obtained by means of scanning tunneling spectroscopy (STS), a mode which probes the elastic tunneling changes associated with the local density of states (LDOS).[27–29] This method was successfully employed to investigate on isolated molecules various phenomena such as charge-transfer,[30] rectification [16, 31] and switching.[32] The highest spatial resolution that can be attained by STM imaging allows one to gain detailed information on molecular interactions; thus it is a crucial tool to assist the design of molecular modules that undergo controlled self-assembly at interfaces at any desired condition (temperature, pressure and concentration) to form the chosen supramolecular motifs and ultimately functional materials.

Self-assembly at room temperature and atmospheric pressure is highly appealing, especially when working at the solid-liquid interface: in fact, thanks to the possibility of operating under equilibrium conditions and to mimic phenomena occurring in Nature, it represents a privileged playground where functional molecular systems can be elaborated. At such interface the selfassembly may be thermodynamically driven, making it possible to lay the foundation for 2D crystal engineering.[33, 34] In fact supramolecular engineering relies on the prediction of the thermodynamic state occupied by the grand majority of molecules