The effect of halogen substitution in self-assembled monolayers of 4-mercaptobiphenyls on noble metal substrates

Self-assembled monolayers (SAMs) formed from 4′-substituted 4-mercaptobiphenyls X-(C ₆H ₄) ₂SH (X-BPT, with X = I, Cl, and F) on polycrystalline (111) gold and silver substrates have been characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy and angle-resolved near-edge X-ray absorption fine structure spectroscopy. The X-BPT molecules were found to form highly oriented and densely packed SAMs on both substrates, with a smaller molecular inclination in the case of Ag. The experimental data show clear evidence for the charge transfer between the 4′-substituent and biphenyl moieties with the direction and extent of the transfer depending on the electronegativity of the halogen substituent. At the same time, no direct evidence of the charge transfer between the 4′-substituent and the thiolate group was observed. However, the substitution of the 4′-hydrogen by a halogen atom seems to affect the detailed packing arrangements of the SAM constituents.