Organic Supramolecular Networks on Insulating Substrates: Versatility of the Hydrogen Bond

The study of the growth of extended supramolecular networks on insulating substrates which are stable at room temperature opens new vistas for the development of functional materials for applications in future nanodevices. In this work, we investigated the adsorption of especially synthesized organic molecules on three alkali halide surfaces (NaCl, KCl and RbCl) at room temperature. Our study is combining supramolecular chemistry, non-contact Atomic Force Microscopy (nc-AFM) imaging and theoretical calculations based on Density Functional Theory (DFT). The aim of this study is to structurally characterize the obtained supramolecular networks and to understand the influence of the substrate on the growth process. The molecule we used is 1,3,5-tri (4''-cyano-4, 4'-biphenyl) benzene (TCB) [Fig. 1.A]. The three arms of the molecule are terminated by polar CN groups which can provide both, electrostatic interaction to the ionic substrate [see also ref. 1] and hydrogen bonding to neighboring molecules [2] in order to develop two-dimensional highly ordered organic monolayers [Fig. 1.B]. Based on a careful evaluation of the experimental images and a theoretical study of the preferred adsorption geometry of TCB on these surfaces, we can determine a structural model of the organic layer and understand the details of the interactions involved in the film formation. The growth process is driven by a) molecule-substrate interaction which is the sum of van der Waals interaction (4 eV) and local electrostatic interaction between the cyano groups of the molecule and the cationic sites of the substrate (~ 0.2 eV per polar group), and b) intermolecular hydrogen bonding.