Surface-Enhanced Raman Spectroscopic Study of 1,4-Phenylene Diisocyanide Adsorbed on Gold and Platinum-Group Transition Metal Electrodes

Self-assembled monolayers of 1, 4-phenylene diisocyanide (PDI) were formed on Au and Pt-group transition metals and examined by surface-enhanced Raman spectroscopy under controlled applied potential. On all of the metals examined, PDI adsorbs in an edge-on manner, with one NC group bound to the surface and the other pointing away from the surface. The N−C stretching frequency (ν_NC) suggests that depending on the metal, PDI adsorbs on different binding sites:  terminal sites on Au, both terminal and bridging on Rh and Pt, and predominantly 3-fold hollow sites for Pd. This binding site preference can be understood in terms of the difference in d-band center energy and d-orbital filling among the metals. The applied potential affects the N−C bonding differently as inferred from the potential dependence of ν_NC. On Au, Rh, and Pd, the ν_NC increases linearly with the applied potential, yielding a Stark tuning slope, dν_NC/dE, of 25, 12, and 10 cm^-1/V, respectively. On Pt, the ν_NC is nearly independent of the applied potential. On all of the metals studied, the frequencies of benzene ring vibration modes are not dependent on the applied potential, consistent with the edge-on orientation in which the ring does not directly interact with the surface. Several ring vibrations are, however, sensitive to the nature of metal substrate due to different binding sites involved. The ability of the free NC group to function as an anchoring point is demonstrated by the attachment of gold nanoparticles on PDI-covered Au and Pd. The study provides useful NC−metal bonding information for isocyanide-based molecular electronic developments.