Molecular Structure of Monolayers from Thiol-Terminated Polyimide Model Compounds on Gold. 1. A Spectroscopic Investigation

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.