## 3 Results and Discussion
### Investigated Systems  
To assess the possibility of probing bond dipoles by X-ray photoelectron spectroscopy, we simulated SAMs on Au(111) surfaces, as such systems have been studied extensively [71; 72; 74; 79; 127], which qualifies them as prototypical model systems. As molecular backbones we chose biphenyls with two molecules per (\(3\times \sqrt{3}\))rect surface unit cell arranged in a herringbone structure [85]. As docking groups we considered thiolates (system I) [71; 72; 74; 79], methyl thiolates (system II) [71; 72; 74; 79], pyridines (system III) [54; 128; 129; 130; 131; 132; 133], isocyanides (system IV) [39; 133; 134; 135] and cyanides (system V). The structures  
Figure 1: Schematics of the energy landscape of a metal–self-assembled monolayer (SAM) interface and an XPS experiment (disregarding the bias voltage applied between substrate and detector). The interface electronic structure is characterized by the work function, \(\Phi\), by the alignment of the frontier levels determining electron- and hole-injection barriers, EIB and HIB, and by the energetic positions of the core levels, here exemplarily denoted as \(\mathrm{E_{C1s}}\). The shift of the electrostatic energy due to the interface dipole, ID, determines the changes in the injection barriers, \(\mathrm{\Delta I}\)B, and the core level shifts, \(\mathrm{\Delta E_{C1s}}\). The work function change, \(\mathrm{\Delta\Phi}\), is additionally influenced by the energetic shift due to the tail group dipole, TD. Absorption of an X-ray photon with energy \(\mathrm{hv}\) creates a photoelectron, which at the position of the detector has a kinetic energy of \(\mathrm{E_{kin}}\). Additional positive potentials (not included in the plot) applied to the detector relative to the substrate increase the measured kinetic energy of the photoelectron. The top panel sketches the metal–SAM interface, whose properties are determined by the interface and the tail group dipole. In passing we note that the actual situation can be further complicated, e.g., when additional dipoles are contained along the backbones of the molecules [15; 17; 19; 20; 21; 22]. of all investigated systems can be seen in Figure 2. Furthermore, for the sake of comparison, we also simulated an upright-standing biphenyl layer not bearing a dedicated docking group (system VI), which represents a metastable structure, as the equilibrium configuration for the biphenyl-Au interface would very likely be flat-lying molecules. Due to different interfacial charge rearrangements and intrinsic dipole moments associated with the docking groups in these six systems, we expect distinct variations of the respective interface dipoles. Notably, in system VI, the only reason for an interface dipole would be Pauli-pushback [47; 136; 137]. To eliminate also that effect and to generate an essentially dipole-free interface, we also considered a system with the biphenyl layer shifted by 1 A further away from the substrate (system VIa). In passing we note that increasing the distance from the substrate by yet another 1 A does not change the spectrum (see Supplementary Materials). The electronic structure of system VIa is essentially identical to what is obtained when assuming vacuum-level alignment between an Au(111) surface and a free-standing biphenyl monolayer (the calculated work function reduction is on the order of 0.1 eV; for further information see Supplementary Material). Additionally, we considered two systems bearing electron accepting (system VII) and electron donating (system VIII) tail group substituents. These are known to change the work function without affecting the alignment of the frontier levels [27]. Figure 2: Front view of the unit cells investigated in the present study. The self-assembled monolayers are all derived from a biphenyl backbone and bear different docking groups (**Systems I** to **VI**) or different tail groups (**System VII** and **VIII**). The SAMs are bonded to an Au(111) metal slab consisting of 5 layers of gold atoms.