Supramolecular Self-Assembly Driven by Electrostatic Repulsion: The 1D Aggregation of Rubrene Pentagons on Au(111)

Abstract

At present, organic molecules are among the best candidate "building blocks" for the construction of self-assembling nanoscale devices based on metal substrates. Control of the formation of specific patterns in the submonolayer regime is usually achieved by appropriate choice and/or functionalization of the adsorbates. The effect of this intervention, though, is limited by the typically short-range character of the bonding. We present here a theoretical study on the system rubrene/gold to show that substrate-induced molecular charging can instead determine the assembly on larger scales. DFT calculations and electrostatic considerations are used to discuss the charge transfer at the metal/organic interface. This allows rationalization of previous puzzling experimental results and, in particular, of the unusual molecular gap broadening upon adsorption observed in STS spectra. The self-assembly process is further studied by means of classical molecular dynamics simulations. The charged adsorbates are modeled as mutually repulsive standing dipoles, with van der Waals interactions intervening at short distances. The striking resemblance between the experimental STM images and the results of our MD simulations shows that this simple model is able to capture the key effects driving the assembly in this system. The competition between long-range repulsive interactions and short-range attractive forces leads to characteristic and easily recognizable 1D patterns. We suggest that experimental evidence of the presence of similar patterns in other metal/organic systems can provide crucial information on the electronic level alignment at the interface, that is, on the occurrence of charge-transfer processes between metal and organic adsorbates.

Temperature-dependent self-assembly of NC–Ph5–CN molecules on Cu(111)

“…An interaction driven by a charge transfer from/to the substrate can be the origin of the repulsion between the chains, as already reported for other one dimensional molecular systems. 25,[48][49][50] A surface state mediated interaction, observed for different adatoms on Cu(111) 51,52 or for pentacene molecules on Cu(110), 53 seems not to be pertinent here, since we find a coverage-dependent average distance between the chains.…”

Coverage‐Dependent Self‐Assembly of Rubrene Molecules on Noble Metal Surfaces Observed by Scanning Tunneling Microscopy

“…[54] In the present case, a possible explanation for the formation of the low-coverage self-assembled phases (and specifically of the chains of pentagonal supermolecules) is charge transfer from the highest occupied molecular orbital to the gold surface, leading to the formation of dipoles. [23,55] As a consequence, short-range attractive intermolecular forces compete with long-range repulsive forces due to dipolar interactions, and this leads to the observed formation of dispersed assemblies. This process is determined by the specific relation between the work function of gold and the electronic structure of rubrene.…”

Charge Reorganization Energy and Small Polaron Binding Energy of Rubrene Thin Films by Ultraviolet Photoelectron Spectroscopy

“…[22] However, recently, (sub)monolayers of rubrene on Au (111) are reported to formdepending on the specific preparation conditions-manifold supramolecular structures of essentially flat lying molecules as evidenced by scanning tunneling microscopy (STM). [23][24][25][26] Con sequently, the mutual supramolecular stabilization might lead to a conformational transition on a shorter length scale or even to the concomitant occurrence of rubrene in a twisted and a planar conformation in the monolayer on Au (111) or other inert substrates like graphite. In a recent STM study of rubrene on Bi (001) such a coexistence was actually experimentally shown for monolayer coverage.…”