Designing a New Two‐Dimensional Molecular Layout by Hydrogen Bonding

Abstract

[a] Supramolecular structures formed by the self-assembly of functional molecular building blocks are a promising class of materials for future technologies. [1][2][3] Hydrogen bonding is a key ingredient in their fabrication, as it provides high selectivity and directionality. Hydrogen bonding has recently been used to prepare both two-dimensional (2D) and linear nanostructures at surfaces by molecular beam epitaxy. [4][5][6][7][8] Herein, we show that molecular films can evolve with time into a close-packed arrangement induced by the activation of an original 2D hydrogen-bond network consisting of H···Cl bonds. Scanning tunneling microscopy (STM) experiments combined with density functional calculations were performed on chlorine-zincphthalocyanines (ZnPcCl 8 ; see Figure 1 for structure) adsorbed onto Ag(111). We demonstrate that the location of the chlorine atoms on the phenyl rings predetermines the final 2D molecular network.The adsorption of large, flat metal-phthalocyanine (MPc) molecules on both metallic and semiconductor surfaces is well documented. [9][10][11][12][13] For this class of molecules, self-assembly is governed by the subtle balance between molecule-molecule and molecule-surface interactions, which can be tuned by the modification of the surrounding atoms of the phthalocyanine. [14] A large variety of arrangements at surfaces, depending on the nature of the central atom and the surrounding atoms of the molecule, has been reported. Intermolecular interactions influence the way molecules organize themselves on top of a surface. They can either be electrostatic in nature, including both multipolar static and dynamical Van der Waals contributions, or arise from orbital overlapping, or both. ZnPcC 8 , which is obtained from the substitution of chlorine atoms for eight of the sixteen surrounding hydrogen atoms, was chosen because it is capable of organizing itself into compact molecular arrangements mainly governed by hydrogen bonds. Phthalocyanines are well-known electron acceptors and their interactions with a metallic surface result from partial charge transfer from the surface electronic states to the lowest unoccupied molecular orbital (LUMO) of p type.[15] Thus, depending on the strength of the overlapping, the symmetry of the host surface may, more or less, influence the way molecules adsorb and organize themselves on this surface. Because of its moderate reactivity and oxidation potential (that is, its electron-donor character), Ag metal seems a good choice as a substrate with little influence on the molecular arrangement, because of its low surface energy. Looking for minimal substrate-molecule interactions, Ag (111) Herein is presented a combined experimental and theoretical approach to the molecular packing of ZnPcCl 8 molecules deposited onto Ag(111) surfaces. STM was performed at room temperature, and theoretical calculations were based on density functional theory (DFT) within the framework of the projector-augmented wave (PAW) method.[16] The evolution of the three different 2D...

First-principle study of the hydrogen bonds in a thin film of phthalocyanine molecules

“…Experiments [20,19] showed that the two-dimensional arrangement of ZnPcCl 8 molecules undergoes an evolution at room temperature, from a low density phase (0.33 molecules/nm 2 ) to a more compact phase (0.44 molecules/ nm 2 ). The corresponding two-dimensional unit cells are represented in Fig.…”

“…The transition occurs by the activation in two steps of a H Á Á Á Cl bond network. In a previous letter [20], the results obtained from DFT calculations on the two-dimensional arrangements of ZnPcCl 8 , considering the experimental cell parameters, were briefly presented.…”

“…In Section 4, the two-dimensional hydrogen bond network of the compact arrangement of ZnPcCl 8 is investigated through the topology of the electron density. This section details and supplements the preliminary results reported in a previous letter [20]. An estimate of the binding energy of each intermolecular contact is given through the study of model complexes made of two dichloro-diiminoisoindoline molecules.…”

“…Experiments [19] showed that molecular films consisting in chlorine-zinc-phthalocyanine (ZnPcCl 8 ) molecules adsorbed onto Ag(1 1 1) surface, evolve in time towards a close-packed organization. This final two-dimensional arrangement is induced by the activation of an unusual hydrogen bond network, consisting in weak Cl Á Á Á H-C bonds [20]. The phthalocyanine molecules are well-known electron acceptors and interactions with a metallic surface result from partial charge transfer from surface electronic states to the lowest unoccupied molecular orbital (LUMO), of p-type [21,22].…”

Bicomponent Supramolecular Packing in Flexible Phthalocyanine Networks

“…[15,16] 2D crystallization of this compound occurred from a low-density to a compact packing through two phase transitions. Similar results have been obtained by Samuely et al with phenoxy-substituted phthalocyanines.…”