Supramolecular Self-Assembly of p-conjugated Hydrocarbons via 2D Cooperative CH/p Interaction

Qing Li*, Chengbo Han**, Scott R Horton*, Miguel Fuentes-Cabrera*, Bobby G. Sumpter*, Wenchang Lu**, Jerry Bernholc**†, Petro Maksymovych*, and Minghu Pan*

^{*Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge,Tennessee
**Center for High Performance Simulation and Department of Physics, North Carolina State University, Raleigh, North Carolina
†Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge,Tennessee}

Achievement
This work demonstrates a viable pathway toward deterministic control over weak attractive and repulsive interactions between aromatic molecules on a metal surface. There are three key elements in this achievement:

I. CH/p hydrogen bonding among 2D aromatic hydrocarbons on a metal surface is definitively observed for the first time

II. Cooperative (multicentric) CH/p interactions are found to enable supramolecular self-assembly of “magic” chiral molecular clusters with almost perfectly uniform size-distribution

III. State-of-the-art first-principle calculations in combination with tunneling spectroscopy are used to reveal emergent electronic properties of supramolecular assemblies

Significance
Supramolecular self-assembly on well-defined surfaces provides access to a multitude of nanoscale architectures, including clusters of distinct symmetry and size. The driving forces underlying supramolecular structures generally involve both graphoepitaxy and weak directional nonconvalent interactions. Here we show that functionalizing a benzene molecule with an ethyne group introduces attractive interactions in a 2D geometry, which would otherwise be dominated by intermolecular repulsion. Furthermore, the attractive interactions enable supramolecular self-assembly, wherein a subtle balance between very weak CH/p bonding and molecule-surface interactions produces a well-defined “magic” dimension and chirality of supramolecular clusters. The nature of the process is corroborated by extensive scanning tunneling microscopy/spectroscopy (STM/S) measurements and ab initio calculations, which emphasize the cooperative, multi-center character of the CH/p interaction. This work points out new possibilities for chemical functionalization of p-conjugated hydrocarbon molecules that may allow for the rational design of supramolecular clusters with a desired shape and size.

Credit
This work will be published in ACS Nano (DOI: 10.1021/nn203952e). The research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. The work at NCSU was supported by DOE grant DE-FG02-98ER45685. The computations were performed using the resources of the CNMS and the National Center for Computational Sciences at Oak Ridge National Laboratory.