Surface-Enhanced Raman Scattering Study of 4-Biphenylcarboxylic Acid

The interfacial structures of biphenyl (BP), benzoic acid (BA), and 4-biphenylcarboxylic acid (BPCA) on a Au colloid monolayer surface were investigated using surface-enhanced Raman scattering (SERS). The absence of the C−H stretching vibrational mode in the SERS spectrum of BP indicated that the two benzene rings of BP were coplanar on the Au colloid surface, while their twisted structure was suggested in the bulk state. The downward frequency shift of the carboxylate stretching vibrational mode of BA implied the direct chemical adsorption of the carboxylate group onto the Au surface with a flat orientation. In BPCA, an absence of the C−H stretching vibrational mode and the red shift of the carboxylate stretching vibrational mode were observed. This fact strongly indicated that both the two benzene rings and the carboxylate group were adsorbed on the Au surface with flat orientation via their π-systems.