Aromatic versus aliphatic thiols on Au(111) surface: a DFT exploration of adsorption registry and electronic structure

Self-assembled monolayers (SAMs) on metal surfaces have inspired many interesting applications, such as chemical and biological sensors, molecular electronics, magnetism and protective coatings. In this respect, SAMs having different head and tail groups have been synthesised which allow the modification of the material properties by tuning intermolecular, monolayer-metal and/or monolayer-solvent interactions. In this respect, we investigate the adsorption of thiols having aromatic, p-mercaptobenzonitrile (pPhCN) or aliphatic, 2-azidoethanthiol (N3S), tail groups on Au(111) surface by means of Density Functional Theory (DFT). Monomer and dimer adsorption configurations of both pPhCN and N3S molecules as well as the modification of the electronic structures upon adsorption are studied. Our results show that different adsorption mechanisms are observed for the molecules under consideration. While monomer and dimer structures of the pPhCN prefer to adsorb laterally on the surface, for N3S vertical arrangement of the molecules enhances the molecular immobilisation. Although, dominant contributions to the adsorption energy of the laterally adsorbed pPhCN are through S-Au chemical bond, phenyl ring-surface and cyano N-surface interactions, pi-pi stacking of the rings contributes to the stabilisation of the complex in addition to the S-Au chemical bond in the case of vertical attachment. For N3S, on the other hand, only S-Au chemical bond determines the adsorption strength. S-Au interactions result in broad molecular orbital redistribution of the S atoms of both molecules, due to the rehybridisation of S and Au states. Density of cyano-N and cyano-C states present in the pPhCN is affected by the orientation of the rings with respect to each other. In the case of 2N3S, instead, the distribution of the tail group molecular orbitals is not modified by the adsorption.