Abstract: The orientation and structural characterization of the ultrathin film of azobenzene-containing amphiphilic compound, C₁₂AzoNaph(1,4)C₆N⁺Br^-, were studied in the present study. The compound can form a stable monolayer with sodium dextrin sulfate(SDS) by means of electrostatic interaction. Fourier-transform infrared(FT-IR) and near-infrared surface-enhanced Raman scattering(NIR-SERS) spectroscopies were used to study the orientation and characterize the structure of the Langmuir-Blodgett(LB) film and the dipping film. The FT-IR spectra indicate that the alkyl tail is nearly perpendicular to the substrate surface without any aggregation and adopts largely trans-zigzag conformation in the LB film. The NIR-SERS spectra demonstrate that the chromorphoric part in C₁₂AzoNaph(1,4)C₆N⁺Br is also nearly perpendicular to the surface of silver substrate both in the dipping film and the LB film. A new "sandwiched system" model was designed to investigate the orientation and structural characterization of the chromophoric part in the multi-monolayer LB films on the non-SERS active substrate. The SERS mechanism of the "sandwiched system" is discussed in the present paper.

Key words: Molecular orientation, LB film, NIR-SERS spectroscopy