Molecule–Metal Bond of Alternant versus Nonalternant Aromatic Systems on Coinage Metal Surfaces: Naphthalene versus Azulene on Ag(111) and Cu(111)

Abstract

Interfaces between polycyclic -electron systems and metals play prominent roles in organic or graphene-based (opto)electronic devices, in which performance-related parameters depend critically on the properties of metal/semiconductor contacts. Here, we explore how the topology of the -electron system influences the bonding and the electronic properties of the interface. We use azulene as a model for nonalternant pentagonheptagon (5-7) ring pairs and compare it to its isomer naphthalene, which represents the alternant 6-6 ring pair. Their coverage-dependent interaction with Ag(111) and Cu(111) surfaces was studied with the normal-incidence X-ray standing wave (NIXSW) technique, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, UV and X-ray photoelectron spectroscopy (UPS, XPS), and density functional theory (DFT). Coveragedependent adsorption heights and spectroscopic data reveal that azulene forms shorter interfacial bonds than naphthalene and engages in stronger electronic interactions with both surfaces. These differences are more pronounced on Cu. Increasing coverages lead to larger adsorption heights, indicating bond weakening by intermolecular repulsion. The extensive DFT calculations include dispersive interactions using: (1) the DFT-D3 scheme, (2) the vdW surf correction based on DFT-TS, (3) a Many-Body Dispersion (MBD) correction scheme, and (4) the D3 surf scheme. All methods predict the adsorption heights reasonably well with an average error below 0.1 Å. The stronger bond of azulene is attributed to its nonalternant topology, which results in a reduced HOMO-LUMO gap and brings the LUMO energetically close to the Fermi energy of the metal, causing stronger hybridization with electronic states of the metal surfaces.

Formation of Highly Ordered Molecular Porous 2D Networks from Cyano‐Functionalized Porphyrins on Cu(111)

“…Furthermore, we interpret that the commonly observed T‐type and π–π intermolecular stacking of porphyrin derivatives are not in line with the relatively large distance of peripheral cyano groups . For the same reasons, we also rule out the known binding interaction for cyano groups, namely, via hydrogen bonds and dipolar coupling …”

The nuts and bolts of core-hole constrained ab initio simulation for K-shell x-ray photoemission and absorption spectra

“…This approach retains the accurate description of relative transition energies for each atomic center, but additionally improves not only the description of the chemical shift of each atom but also the absolute transition energies. We have employed such a hybrid approach in all our previous work on metal-organic interfaces and organic molecules, where we have achieved good agreement with experimental data [34][35][36][37]. In principle this method can also be employed on the basis of XTP instead of TP to calculate the KS-energies of the unoccupied states, as we will show below.…”

First-principles calculations of hybrid inorganic–organic interfaces: from state-of-the-art to best practice

“…Spuriously large unit cells particularly affect the adsorbate geometry: For (mostly) upright standing molecules, the coverage and packing motif crucially determines their tilt angle, 147 but also for simple, weakly interacting, flat-lying molecules, a dependence of the adsorption height on the molecular coverage has been reported. 148,149 Moreover, a surprisingly large number of molecules shows coveragedependent re-orientations, phase transitions, or conformational and chemical changes. 62,[150][151][152][153][154][155][156][157][158][159][160] Even if the "single molecule in a supercell" approach is the best suited model (e.g., because the molecules repel each other on the surface), finding the correct geometry (i.e., adsorption site, orientation, conformation, etc.)…”