Optically induced band shifts in infrared spectra of mixed self-assembled monolayers of biphenyl thiols

Optically induced band shifts in reflection absorption infrared spectra (RAIR) of mixed self-assembled monolayers (SAMs) are reported. SAMs of 4-trifluoromethyl-4- mercaptobiphenyl dispersed in 4′-methyl-4-mercaptobiphenyl on gold exhibit significant shifts of the spectral envelope of the symmetric CF₃ stretch and the b_1u skeletal mode. A linear relationship is observed between the band positions, the band intensity, and the surface CF₃ group concentration. To rationalize the observed changes in band positions and shapes, a classical electromagnetic theory approach is used, providing an excellent agreement between theory and experiments. The simplified RAIR signature of 4′-trifluoromethyl-4-mercaptobiphenyl on gold is a unique example of the surface selection rule.