Surface-enhanced Raman scattering study of 4-biphenylcarboxylic acid

The interfacial structures of biphenyl(BP), benzoic acid (BA), and 4-biphen ylcarboxylic acid(BPCA) on a Au colloid monolayer;surface were investigated using surface-enhanced Raman scattering (SERS). The absence of the C-H str etching 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 tw isted structure was suggested in the bulk state. The downward frequency shi ft of the carboxylate stretching vibrational mode of BA implied the direct chemical adsorption of the carboxylate group onto the Au surface with a fla t orientation. In BPCA, an absence of the C-H stretching vibrational mode a nd the red shift of the carboxylate stretching vibrational mode were observ ed. This fact strongly indicated that bath the two benzene rings and the ca rboxylate group were adsorbed on the Au surface with flat orientation via t heir pi -systems.