Self-Assembled Rigid Monolayers of 4‘-Substituted-4-mercaptobiphenyls on Gold and Silver Surfaces

We have prepared, characterized, and studied self-assembled monolayers (SAMs) of rigid 4‘-substituted-4-mercaptobiphenyls on planar Au(111) and Ag(111) substrates. These monolayers show lower contact angles than their corresponding alkanethiolate SAMs due to the polar nature of the aromatic moieties. Quantitative analysis of infrared spectroscopy, using Eulerian angle, allowed estimation of the average molecular orientation on a surface. In cases where such analysis is possible, it was found that the biphenyl planes are tilted away from the surface normal toward by 14°, 20°, and 12°, for NO2, C(O)CH3, and CO2Et, respectively, and rotate around the 1,4 axis of the ring at 30°, 15°, and 30° on gold, respectively. For SAMs on silver, the tilt is 8°, 21°, and 11°, respectively, and the rotation angle is 32°, 10°, and 31°, respectively, with errors ≤±2°. Scanning tunneling microscopy studies indicate that these plate-shaped molecules form incommensurate structures on Au(111) surfaces.