Weak intermolecular H-bonds as a tool to design 2D self-organized molecular architectures: Tailoring a “Scottish Tartan” open network

We show here that weak intermolecular H-bonds (E = 2-3 kcal/mol) can be used as an efficient tool to design supramolecular self-organized 2D periodic networks. These weak attractive and directional forces can take place when the molecules bear weakly acidic hydrogen atoms (such as aromatic CH) and electron-attracting groups (such as NO2). We demonstrate the versatility of using weak H-bonds to tailor 2D networks with a simple model molecule, an adequately substituted benzene derivative bearing a nitro group, two aliphatic chains, but deprived of strongly acidic H atom (such as OH, NH, etc.). We show by means of scanning tunnelling microscopy (STM) that this compound self-assembles on Au(1 1 1) into a 2D open network through multiple intermolecular weak H-bonds between aromatic H-atoms and the O-atoms of the NO2 groups. The prominent role of weak H-bonds in the 2D arrangement of this molecule on gold is supported by ab initio calculations of intermolecular interaction energies (E = 3.1 kcal/mol) and H-bond lengths (L = 2.55 Å). Based on both experimental and theoretical results, we propose a 2D model where molecules first interlock to each other into quartets through eight weak H-bonds and then into quartets-of-quartets through interchain Van der Waals interactions, the final packing being reminiscent of a Scottish Tartan. It is worth noting that such an arrangement leaves a square-shape central cavity (1.1 × 1.1 nm2) in the unit cell thus providing opportunities towards guest-host systems of a new type.