Curved poly- and heteroaromatic molecules present intriguing applications in supramolecular chemistry, for example to improve intermolecular convex-to-concave electronic coupling. Corannulene (C20H10), the simplest bucky-bowl fullerene fragment, displays the most intriguing behavior when deposited on crystalline surfaces due to its C5v symmetry that cannot match any crystal symmetry.
The adsorption of a monolayer of corannulene molecules on the Ag(111) surface is investigated by experimental and simulated scanning tunneling microscopy (STM) and core-level electron spectroscopic techniques, and by density functional theory (DFT) simulations. The molecules form a densely packed monolayer ordered into a 3x3 superlattice, on which is superimposed a long range coincidence pattern. Such an high density is only possible for molecules adsorbing tilted, as also evinced by the experimental STM. DFT simulations of the same were performed for the adsorbed molecule to determine the molecule-surface interaction and its electronic/spectral properties. While isolated molecules are computed to lie flat on the surface, corannulene islands are stabilized by intermolecular dispersion interactions into dense array of tilted molecules, in agreement with the experimental findings.
Near edge X-ray absorption fine structure spectroscopy (NEXAFS) with polarized light is used to distinguish the unoccupied molecular orbitals with pi and sigma symmetries from which the molecular tilt angle was determined. In analyzing the spectra, we demonstrate that the curved aromatic backbone introduces a non-trivial dependence of the NEXAFS spectral intensities on the adsorption angle that is assessed theoretically based on the transition potential approach. The symmetry of empty molecular orbitals and their symmetries are then investigated. Molecular tilting also produce a significantly different electronic screening of core-level excitations as probed by X-ray photoemission spectroscopy (XPS), resulting in initial-state effects to XPS and NEXAFS spectra highlighted by the simulations.