jp057393e_si_001.pdf (157.42 kB)
Surface-Enhanced Raman Spectroscopic Study of 1,4-Phenylene Diisocyanide Adsorbed on Gold and Platinum-Group Transition Metal Electrodes
journal contribution
posted on 2006-03-16, 00:00 authored by Scott M. Gruenbaum, Matthew H. Henney, Sachin Kumar, Shouzhong ZouSelf-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 (νNC) 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 νNC. On Au, Rh, and Pd, the νNC
increases linearly with the applied potential, yielding a Stark tuning slope, dνNC/dE, of 25, 12, and 10 cm-1/V, respectively. On Pt, the νNC 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.