SELF-ASSEMBLED OMEGA -HYDROXYALKANETHIOL MONOLAYERS WITH INTERNAL FUNCTIONALITIES : ELECTROCHEMICAL AND INFRARED STRUCTURAL CHARACTERIZATIONS OF ETHER -CONTAINING MONOLAYERS

The structure of w-hydroxyalkanethiol monolayers containing an intemal ether group is probed by traditional surface infrared reflection-absorption spectroscopy and a novel electrochemical method based on the measured shifts in the potential of zero charge (pzc) upon introduction of the ether group. The analyses of the infrared and electrochemical data give complementary structural information. Vibrational bandwidth measurements indicate that these monolayers exhibit crystalline packing. From a comparison of the integrated infrared absorption intensities between monolayer and bulk samples of the thiols, an average tilt angle of 29" and an average twist angle of 40" are determined for the ether modified w-hydroxyalkanethiol monolayers. The pzc of the ether modified monolayer coated Au electrodes is observed to shift by 130 i 40 mV from that characteristic of an unmodified w-hydroxyalkanethiol monolayer. A simple dipole analysis of the magnitude of these pzc shifts, assuming a twist angle of 40", gives an average tilt angle of 23 & 8", in reasonable agreement with the infrared determination. In addition, the monolayers modified by an oxygen located at an even methylene site (from the S atom) are observed to give a positive pzc shift, while those located at odd sites give strictly negative pzc shifts. Such an evedodd effect on the sign of the pzc shift indicates that the S-C bond is directed away from the surface normal. This electrochemical approach to structural analysis is generally applicable to crystalline monolayers which contain a net static internal dipole.