Role of Edge Groups in Defining the Stability of Aromatic Molecules Under Electron Irradiation

Abstract

AbstractElectron irradiation is known to be an important physical tool in tuning the properties of self-assembled organic molecules. Here, we use X-ray photoelectron spectroscopy (XPS) measurements to study the effect of electron bombardment on the structural properties of dithiol aromatic molecules, where the sulfur atoms are either in direct conjugation with phenyl rings (case of Biphenyl-4,4′-dithiol, BPN) or separated by a methylene group (case of 5,5′-bis(mercaptomethyl)-2,2′-bipyridine, BPD). The former molecule shows enhanced stability against the electron irradiation, whereas the presence of the saturated CH2 group results in considerable reduction of both carbon and sulfur contents in the XPS spectra after irradiation (around 5%). Qualitative description of the experimental results is given through bond distance-dependent total energy calculations and structural and electronic structure analysis within density functional theory. The simulation results show that the binding energy of the thiol unit to the molecule decreases by more than 15% by including the CH2 group. This effect becomes even more pronounced when extra electrons are injected to the system. The simulation results predict the easy removal of the edge group of the BPD molecules upon irradiation as compared to BPN SAMs. Our findings show the importance of the oligomeric units in altering the properties of thiol-terminated molecular self-assemblies by electron irradiation.