Influence of Merocyanine Molecular Dipole Moments on the Valence Levels in Thin Films and the Interface Energy Level Alignment with Au(111)

Abstract

We study the effect of increasing permanent dipole moment of four dipolar merocyanine dyes on the energy level alignment with Au(111) and the energy levels in thin films with ultraviolet (UV) photoemission spectroscopy. Preceding X-ray diffraction experiments showed that these dipolar molecules form centrosymmetric dimeric units in their bulk crystal structure. The effect of dimerization was also seen in a splitting of the highest occupied level of bilayers and thin films of all four molecules. This split into two components due to intermolecular electronic coupling between two merocyanines is estimated to be 0.2–0.3 eV. Monolayer films on Au(111) did not exhibit such splitting due to a different adsorption structure. A significant ultraviolet radiation sensitivity of the dyes near the metal surface was noticed. Unlike other radiation-sensitive compounds, the reason for molecule damage is related to the secondary electrons produced in the metal substrate during photoelectron spectroscopy measurements and not to the UV radiation itself.