Charged and metallic molecular monolayers through surface-induced aromatic stabilization

Abstract

Large π-conjugated molecules, when in contact with a metal surface, usually retain a finite electronic gap and, in this sense, stay semiconducting. In some cases, however, the metallic character of the underlying substrate is seen to extend onto the first molecular layer. Here, we develop a chemical rationale for this intriguing phenomenon. In many reported instances, we find that the conjugation length of the organic semiconductors increases significantly through the bonding of specific substituents to the metal surface and through the concomitant rehybridization of the entire backbone structure. The molecules at the interface are thus converted into different chemical species with a strongly reduced electronic gap. This mechanism of surface-induced aromatic stabilization helps molecules to overcome competing phenomena that tend to keep the metal Fermi level between their frontier orbitals. Our findings aid in the design of stable precursors for metallic molecular monolayers, and thus enable new routes for the chemical engineering of metal surfaces.

Fingerprint of Fractional Charge Transfer at the Metal/Organic Interface

“…[8,9,11] Our results also show that for weakly bound systems, a single molecule approximation can be a reliable approach for the description of the occupied states of a physisorbed molecule, since these orbitals do not change upon adsorption. This is evidenced experimentally and theoretically.…”

“…[7] This observation supports the conclusion that the molecules are physisorbed on the surface. In fact, chemisorption would significantly modify the photoemission features of the F4PEN molecular assemblies close to the interface, [8,9] as also discussed for XPS above. We observe pentacenes.…”

“…[9] Resonance structures are stabilized on the surface through an initial metal-to-molecule charge transfer and rehybridization of suitable side groups, leading to an extended -electron system that is strongly coupled to the metal states. [8] This coupling, decreasing the molecular electronic gap, overcomes the competing phenomenon of Fermi-level pinning, and leads to substantially charged molecular monolayers, which, if the Fermi level comes to lie within a frontier molecular orbital, behave as metallic. The change from semiconducting to metallic nature of the organic material is suggested as a new route for the chemical engineering of metal surfaces.…”

“…Although organic/metal interfaces have been the focus of nearly two decades of investigations, [3][4][5][6][7] very recently the interest in this type of interface has enjoyed a renaissance with the flourishing of a body of work focused mainly on the interfaces between chemisorbed organic molecules and metals. [8][9][10][11] Photoemission features strongly depend on the strength of the molecule/substrate interaction, as demonstrated for a number of molecules on Ag(111). [9] These experiments show that the stronger the bond of the molecules with the substrate, the larger the effect of the charge transfer on photoemission is.…”

“…The change from semiconducting to metallic nature of the organic material is suggested as a new route for the chemical engineering of metal surfaces. [8] The present "state of the art" in the interpretation of the organic/metal and organic/organic interfaces, is based on the interface interaction strength, considering the complete range of interactions from physisorption of noble gases to strong chemisorption of -conjugated molecules. [11] The case of physisorption with and without charge transfer is typically examined within the integer electron charge transfer model, stating that physisorption on organic and passivated metal surfaces is possible, while weak chemisorption, with possible fractional charge transfer, occurs on non-reactive clean metal surfaces.…”