A self-assembled monolayer film (SAM) of 2,3,7,8,12,13,17,18-octaethylporphinatozinc formed on a gold surface premodified with a SAM of 4-pyridinethiol was studied by means of surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), and ultraviolet−visible (UV−vis) spectroscopies. The SERS spectrum of the porphyrin monolayer on the pyridine-modified Au surface shows bands only from the underlying pyridine group. The frequency shift and change in the relative intensity of the bands due to the pyridine group in the SERS spectra imply indirectly the binding of the metalloporphyrin to the pyridine group in the SAM. This is further supported by the SEIRA spectra of the porphyrin−pyridine SAM, in which vibrational bands arising from both the porphyrin moiety and the pyridyl group appear clearly. In addition, the UV−vis spectra also suggest the formation of the porphyrin−pyridine SAM and the weak porphyrin−porphyrin interactions in the SAM.

The molecular orientation and/or packing for monolayers formed by adsorption of (4-mercaptophenyl)phthalimide (4-MPP), a thiol-terminated polyimide model compound, onto gold substrates, was determined using surface-enhanced Raman scattering (SERS), reflection-absorption infrared spectroscopy (RAIR), X-ray photoelectron spectroscopy (XPS), ellipsometry, and electrochemistry. SERS, RAIR, and XPS showed that 4-MPP molecules were chemisorbed onto the gold surface through the thiol groups to form an oriented monolayer. For adsorption times less than about 2 h, the ellipsometric thickness of the films increased as a function of time, but for longer times the ellipsometric thickness reached a limiting value of about 11.5 Å. For adsorption times greater than about 2 h, the surface density was about 5.2 × 10 -10 mol/cm 2 , corresponding to a molecular area of 32 Å 2 /molecule. Considering the size of the 4-MPP molecule compared to that of an alkanethiol molecule, it was concluded that the adsorbed molecules were highly packed on the gold surface. The orientation of adsorbed 4-MPP molecules was determined quantitatively using infrared spectroscopy. It was found that the molecules were oriented with a vertical configuration in which the molecular axes tilted away from the surface normal by about 21°. There was no preferred rotation angle for the imide ring in 4-MPP. The thickness and orientation determined by infrared spectroscopy were in excellent agreement with results obtained from molecular dynamics, which are presented in the following paper.