Structure and Stability of 7-mercapto-4-methylcoumarin SAM on Gold: an Experimental and Computational analysis

Self-assembled monolayers of 7-mercapto 4-methylcoumarin (MMC) on a flat gold surface were studied by Molecular Dynamics (MD) simulations, reference-free grazing incidence X-ray fluorescence (GIXRF) and X-ray photoemission spectroscopy (XPS), to determine the maximum monolayer density and to investigate the nature of the molecule/surface interface. In particular, the protonation state of the sulfur atom upon adsorption was analyzed, since some recent literature presented evidences for physisorbed thiols (preserving the S-H bond), unlike the common picture of chemisorbed thiyls (losing the hydrogen). MD with a specifically tailored force field was used to simulate either thiol or thiyl monolayers with increasing number of molecules, to determine the maximum dynamically stable densities. This result was refined by computing the monolayer chemical potential as a function of the density with the Bennet Acceptance Ratio method, based again on MD simulations. The monolayer density was measured with GIXRF, which provided a quantitative estimate of the number of sulfur atoms on top of flat gold surfaces embedded in a solution of MMC, to allow the formation of a dense monolayer. The sulfur core level binding energies in the same monolayers were measured by XPS, fitting the recorded spectra with the binding energies proposed in the literature for free or adsorbed thiols and thiyls, to get insight on the nature of the molecular species present in the layer.