Thioethylpyrrole Monolayers on Gold. A Spectroscopic Study in Ultrahigh Vacuum

Abstract

The adsorption of 3-(2-thioethyl)pyrrole (3-TEP) and 1-(2-thioethyl)pyrrole (1-TEP) on gold has been studied in ultrahigh vacuum with infrared reflection absorption spectroscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption mass spectroscopy. Depositions were made from liquid phase at atmospheric pressure, from gas phase in ultrahigh vacuum, and from gas phase at atmospheric pressure. Both molecules were found to chemisorb to the gold surface regardless of the deposition method. Most of the 3-TEP, however, broke up, losing the pyrrole ring as it adsorbed on the surface. Both monolayers were sensitive to oxygen and oxidized quickly if stored in air. Deposition of 1-TEP from solution or vapor in air gave a higher desorption peak temperature than deposition from gas phase in UHV. The masses of the desorbing fragments that were detected from the heated monolayers were smaller than those seen upon introducing 1-TEP gas into the UHV system, revealing that a partial decomposition of the adsorbed molecules occurred prior to or at desorption. Some samples were electrochemically cycled to positive potentials to investigate the stability of the monolayers. A large fraction of the 1-TEP and 3-TEP stayed on the surface when cycled to 0.8 V, but some oxidation, fragmentation, and desorption were seen at higher potentials.

Thiol-Modified Pyrrole Monomers: 3. Electrochemistry of 1-(2-Thioethyl)pyrrole and 3-(2-Thioethyl)pyrrole Monolayers in Propylene Carbonate

“…A more thorough report of the XPS results is given elsewhere. 27 These results show that whereas the same elements were present after oxidation past the oxidation peaks as before, a considerable amount of material had been lost. Some of this lost material appears to be due to desorption of 1-TEP, as seen from the lower S and N content.…”

“…Normally, R is between 0.3 and 0.7 and is usually assumed to be 0.5. Assuming R to be approximately the same for all three and a process that consumes a total of 7 electrons (based on the amount of charge, assuming half a monolayer coverage, 27 considering the slope ratio, and requiring an integer number), one could roughly estimate a ratio of 2:4:1 for the number of electrons in each peak. If the same packing density found by other authors is used, 4 × 10 -10 mol/cm 2 , then one obtains 260 µC/cm 2 as the charge that should have been obtained, which it was at 10 mV/s.…”

“…Studies of the monolayers in an ultrahigh vacuum are presented elsewhere. 27 Note on terminology: We do not refer to the adsorbed layers as self-assembled monolayers, or SAMs, since they are disorganized, as shown in paper 2. Although the TEPs spontaneously adsorb onto the gold surface, they do not form well-packed "assembled" structures.…”

Thiol-Modified Pyrrole Monomers: 2. As-Deposited Monolayers of 1-(2-Thioethyl)pyrrole and 3-(2-Thioethyl)pyrrole

“…The pressure in the analysis chamber was approximately 3 × 10 -10 mbar and the temperature of each sample was 28 °C when recording the spectra. Full details of this study are presented elsewhere …”

“…The electrochemical behavior of 1-TEP and 3-TEP monolayers and the growth of polypyrrole (PPy) over these monolayers are reported in papers 3 and 4. Further details of the IRAS, thermal desorption, and XPS studies are discussed in another paper …”