Electrochemical and vibrational spectroscopic characterization of self-assembled monolayers of 1,1 -disubstituted ferrocene derivatives on gold

Abstract

Self-assembled monolayers (SAMs) of 1,1'-disubstituted ferrocene derivatives, i.e., 1,1'-bis(11-mercaptoundecyl)ferrocene (1) and 1-decyl-1'-(11-mercaptoundecyl)ferrocene (2), on a gold surface were prepared, and their structural and electrochemical properties were characterized by reflection-absorption infrared spectroscopy (RAIRS), ellipsometry, cyclic voltammetry (CV), and subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS), From the RAIR spectral features, both molecules, 1 and 2, were found to chemisorb on gold as thiolates after deprotonation. The peak positions of the methylene stretching modes indicated that the alkyl chains of 1 and 2 assume disordered structures on the gold surface. The thicknesses of the monolayers 1 and 2 on gold were determined by ellipsometry to be 2.27 +/- 0.10 and 2.30 +/- 0.10 nm, respectively. In the CV experiments, symmetric redox peaks were identified at; ca. 0.32 and 0.30 V versus saturated calomel electrode (SCE) for the SAMs of 1 and 2, respectively. The surface coverage values determined from the CV of 1 and 2 were 3.4 x 10(-10) and 3.2 x 10(-10) mol/cm(2), respectively. Both SAMs at full-coverage limits were stable in neutral (0.2 M NaClO4) as well as in acidic (0.2 M HClO4) medium, suggesting that hardly any decomposition of the ferricinium cation occurred for the SAMs prepared from disubstituted ferrocene derivatives. In particular, the SAMs of 1 were stable irrespective of the surface coverage, displaying the bonding capability of dithiols to gold; the submonolayer of 2 was slightly unstable in neutral medium. The SNIFTIR spectral data suggested that the alkyl chain of the two SAMs takes a more upright orientation when the ferrocene moiety is oxidized to a ferricinium cation.