Influence of CH···N Interaction in the Self-Assembly of an Oligo(isoquinolyne-ethynylyne) Molecule with Distinct Conformational States

Molecular conformational flexibility can play an important role in supramolecular self-assembly on surfaces, affecting not least chiral molecular assemblies. To explicitly and systematically investigate the role of molecular conformational flexibility in surface self-assembly, we synthesized a three-bit conformational switch where each of three switching units on the molecules can assume one of two distinct binary positions on the surface. The molecules are designed to promote C–H···N type hydrogen bonds between the switching units. While supramolecular self-assembly based on strong hydrogen-bonding interactions has been widely explored, less is known about the role of such weaker directional interactions for surface self-assembly. The synthesized molecules consist of three nitrogen-containing isoquinoline (IQ) bits connected by ethynylene spokes and terminated by tert-butyl (tBu) groups. Using high-resolution scanning tunnelling microscopy, we investigate the self-assembly of the IQ-tBu molecules on a Au(111) surface under ultrahigh-vacuum conditions. The molecules form extended domains of brick-wall structure where the molecular backbones are packed regularly but without selection of specific molecular conformations. However, statistical analysis of the extended network demonstrates alignment/correlation for the orientations of the switching units indicating specific interactions. The primary interaction motifs in the structure are quantified from DFT calculations, showing that the brick-wall structure is indeed stabilized by two types of weak C–H···N bonds, involving either aromatic hydrogens on the IQ groups or nonaromatic hydrogens on the tBu groups. Analysis of the C–H···N interactions in the brick-wall structure explains the observed distribution and alignment of molecular conformations as well as the overall organization of the molecular surface structures.