Studies of Thiophene and Substituted Thiophenes at Platinum(111) Electrodes by Vibrational Spectroscopy and Auger Spectroscopy: Monomers, Dimers, and Polymers

Abstract

[[abstract]]The adsorption behavior of various thiophenes from organic as well as aqueous solutions at well-defined Pt( 111) surfaces is examined in this study. The adsorbates studied include 3-thiophenecarboxylic acid (3TCA), 2-thiophenecarboxylic acid (XTCA), 3-thiopheneacetic acid (STAA), 2-thiopheneacetic acid (ZTAA), thiophene (TPE), 3-methylthiophene (3MT), 3,3’-dimethyL2,2’-bithiophene(3 3’DMBT), and 4,4’-dimethyL2,2’- bithiophene (44’DMBT). Packing densities (moles adsorbed per unit area) were measured for each compound by Auger spectroscopy. Surface vibrational spectra were obtained by electron energy loss spectroscopy (EELS) and were assigned by comparison with the IR spectra of the pure compounds. The Pt(ll1) surfaces used in this study were characterized by LEED. All the thiophenes studied are adsorbed with the ring plane nearly perpendicular to the platinum surface. Vibrational spectra of thiophenecarboxylic acids, and the dependence of adsorption on electrode potential, give valuable information about the adsorbate structure. The compounds 3TCA and 2TAA adsorb with pendant carboxylic acid moieties and give vibrational spectra that are noticeably dependent on the electrode potential. Adsorption at relatively positive potentials (+0.4 V vs Ag/AgCl) shows increased interaction of the carboxylic acid moieties with the metal surface compared with relatively negative potentials (-0.1 V vs Ag/AgCl), as evidenced by diminution of the intensities of bands due to 0-H and C=O stretching as well as shifts in the frequency and intensity of aromatic CC modes. Further evidence for the pendant nature of the carboxylic acid moieties in 3TCA and 2TAA is their reactivity with KOH. Auger spectroscopy shows that the pendant carboxylic acid functionality in 3TCA and 2TAA reacts with KOH to a larger extent than does STCA or 3TAA. The geometry of STCA and 3TAA allows the carboxylic acid to interact with the metal surface, even at relatively negative (-0.1 V vs Ag/AgCl) electrode potentials. This is borne out by low-intensity 0-H and C=O stretching vibrations, as well as relatively little reactivity with KOH (compared with STCA and 2TAA). The C/S elemental ratios based on Auger spectra are smaller than those expected on the basis of molecular formulas for all the thiophene derivatives studied. This result is due to desulfurization of the thiophenes at the Pt surface, occurring while the Pt is immersed in the thiophene solution, and is more pronounced on surfaces immersed at relatively positive potentials. The amount of desulfurization increases continuously with thiophene concentration in acetonitrile but reaches a maximum at about lo4 M in water. This study includes the first reported surface vibrational spectra of poly(3-methylthiophene) (P-3MT), as well as the first electropolymerized polymer fiis on well-characterized single-crystal metal surfaces. P-3W vibrational spectra are compared with the EELS spectra of the corresponding monomer and dimers (33’DMBT and 44’DMBT), and spectral evidence is presented for oxidative C-C bond formation in the electropolymerization of 3MT