Organic heterostructures are a central part of a manifold of (opto)electronic devices and serve a variety of functions. Particularly, molecular monolayers on metal electrodes are of paramount importance for device performance as they allow tuning energy levels in a versatile way. However, this can be hampered by molecular exchange, i.e., by interlayer diffusion of molecules toward the metal surface. We show that the organic−metal interaction strength is the decisive factor for the arrangement in bilayers, which is the most fundamental version of organic−organic heterostructures. The subtle differences in molecular structure of 6,13-pentacenequinone (P2O) and 5,7,12,14-pentacenetetrone (P4O) lead to antithetic adsorption behavior on Ag(111): physisorption of P2O but chemisorption of P4O. This allows providing general indicators for organic−metal coupling based on shifts in photoelectron spectroscopy data and to show that the coupling strength of copper-phthalocyanine (CuPc) with Ag(111) is in between that of P2O and P4O. We find that, indeed, CuPc forms a bilayer when deposited on a monolayer P4O/Ag(111) but molecular exchange takes place with P2O, as shown by a combination of scanning tunneling microscopy and X-ray standing wave experiments.

We present a systematic X-ray standing wave (XSW) study of the πconjugated organic compound 2H-tetraphenylporphyrin (2HTPP) and copper(II)tetraphenylporphyrin (CuTPP) on Cu(111) at room and low temperatures. We exploit the feature of thermally activated self-metalation of 2HTPP to CuTPP to study the influence of the central metal atom on the bonding distance of the molecule to the substrate surface. Comparison between the average adsorption distances of the carbon and nitrogen atoms of 2HTPP reveals a distorted molecule with the nitrogen atoms being closer to the surface than the carbon ones on average. Additionally, the measured positions of the two chemically inequivalent types of nitrogen atoms (iminic and aminic) of 2HTPP indicate a distorted porphyrin ring. After the chemical reaction from 2HTPP to CuTPP at 500 K, no change of the adsorption distance of the carbon skeleton is seen, but the entire molecule becomes flattened. Despite the changes upon metalation, adsorption distances for both molecules show a strong interaction with the substrate in comparison to similar π-conjugated molecules.

We review the binding and energy level alignment of π-conjugated systems on metals, a eld which during the last two decades has seen tremendous progress both in terms of experimental characterization as well as in the depth of theoretical understanding. Precise measurements of vertical adsorption distances and the electronic structure together with ab initio calculations have shown that most of the molecular systems have to be considered as intermediate cases between weak physisorption and strong chemisorption. In this regime, the subtle interplay of different effects such as covalent bonding, charge transfer, electrostatic and van der Waals interactions yields a complex situation with different adsorption mechanisms. In order to establish a better understanding of the binding and the electronic level alignment of π-conjugated molecules on metals, we provide an up-to-date overview of the literature, explain the fundamental concepts as well as the experimental techniques and discuss typical case studies. Thereby, we relate the geometric with the electronic structure in a consistent picture and cover the entire range from weak to strong coupling.

We present a comprehensive study of the complex interface between perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and the (111) surfaces of the three coinage metals. The specific structural, electronic, and chemical properties of the interface rendered by the different substrate reactivities are monitored with low-energy electron diffraction (LEED), x-ray standing waves (XSW), and ultraviolet and x-ray photelectron spectroscopy (UPS and XPS). In particular, the balance between molecule-substrate and molecule-molecule interactions is considered when interpreting the core-level spectra of the different interfaces. By presenting additional adsorption distances of the unsubstituted perylene, we showthat the molecular functionalization via end groups with acceptor character facilitates the charge transfer from the substrate but it is not directly responsible for the associated short adsorption distances, demonstrating that this frequently assumed correlation is not necessarily correct

We investigated the structural and electronic properties of vacuum sublimed 7,8,15,16-tetraazaterrylene (TAT) thin films on Au(111), Ag(111), and Cu(111) substrates using inverse photoemission spectroscopy, ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and the x-ray standing wave (XSW) technique. The LEED reveals a flat adsorption geometry of the monolayer TAT on these three substrates, which is in accordance with the XSW results. The molecules are slightly distorted in monolayers on all three substrates with the nitrogen atoms having smaller averaged bonding distances than the carbon atoms. On Ag(111) and Cu(111), chemisorption with a net electron transfer from the substrate to the adsorbate takes place, as evidenced by UPS and XPS. Combining these results, we gain full insight into the correlation between electronic properties and interface geometry.