Self-assembled monolayers of cyanoterphenyl terminated alkyl disulfides studied by vibrationally and electronically doubly-resonant sum-frequency generation spectroscopy

Self-assembled monolayers (SAMs) of asymmetric (LC-monomer) and symmetric (LC-dimer) cyanoterphenyl terminated alkyl disulfides were investigated by doubly-resonant sum-frequency generation (DR-SFG) with the help of DFT calculation. In addition to the cyano stretching band, intense bands of the terphenyl were observed due to electronic resonance. Similar coverages on both SAMs were obtained by RAIRS and XPS; i.e., the number density of the terphenyl on LC-monomer SAM was about half of that on the dimer, though the DR-SFG signal of the monomer exceeded that of the dimer. These findings are explained by the difference in the orientational distributions of the terphenyls.