Fig. 2

Dependence of IE on charge−quadrupole interactions along the π−π-stacking geometry: a Sketch of the film structure of ZnPc, F₄ZnPc and F₁₆ZnPc, showing the difference in molecular orientation between two different substrates. b UPS spectra of thin films of ZnPc (top) and F₈ZnPc (bottom) in edge-on orientation (substrate: p-doped BPAPF on silver) and face-on orientation (substrate: gold). The different quadrupole moments of ZnPc and F₈ZnPc cause electrostatic shifts in opposite directions from the gas-phase IE (IE₀, obtained with DFT) to the IE value of thin films (obtained with UPS). c The difference in IE between edge-on and face-on orientation in UPS experiments (violet squares) scales with the calculated quadrupole component perpendicular to the molecular plane (Q_π). The green solid line shows the difference in the charge−quadrupole interaction energy E_Q between both orientations, as obtained from simulation. d IE of F_nZnPc on p-doped BPAPF, obtained by subtracting the substrate spectrum from the superimposed spectra (see Supplementary Fig. 6a), changes strongly during the formation of the first monolayer in edge-on orientation due to charge−quadrupole interactions along the π−π-stacking direction. The slight changes of IE from 3 to 20 nm are also observed in simulation and can be explained by increased interactions with other quadrupole components than Q_π. IE_s (dotted lines) is attributed to molecules with E_Q ≈ 0, being 0.6 eV smaller than IE₀