Spectroelectrochemical studies of self assembled monolayers of biphenyl ethynyl thiols on gold electrodes

Adsorbate layers of 4-trimethylsilylethinyl-4′-thioacetyl-1,1′-biphenyl, 4-ethinyl-4′-thioacetyl-1,1′-biphenyl, and 4-trimethylsilylethinyl-1,1′-biphenyl-4′-thiol were formed on a roughened polycrystalline gold surface in contact with an acidic electrolyte solution. The degree of coverage of the SAMs on the gold electrode was examined with cyclic voltammetry. Results of surface enhanced Raman spectroscopy (SERS) suggest that the obtained SAMs adopt different adsorption orientations with respect to the gold surface depending on the terminal group. Vibrational modes assigned to the adsorbate–metal interaction of the molecule through the S atom or the CC group are found. Vibrational wavenumbers were calculated for the investigated biphenyl ethynyl thiols with DFT at the B3LYP level using the 6–31G* basis set. The data obtained from vibrational frequency calculations are used to assign vibrational bands obtained in infrared and Raman spectroscopy as well as in surface enhanced Raman spectroscopy of the studied molecules.