Synthesis of a Three-Layer Organic Thin Film Prepared by Sequential Reactions in the Absence of Solvents

Abstract

We report the synthesis of a trilayer organic thin film on Au prepared via vapor-phase coupling of surface-confined 11-mercaptoundecanoic acid (MUA), hexamethylenediamine (HMDA), and 4-(trifluoromethyl)benzoyl chloride (TFMBC). The surface is prepared by activating the MUA monolayer with vapor-phase trifluoroacetic anhydride, which yields anhydrides at the solid/vapor interface. Next, the surface is exposed to vapor-phase HMDA to form an amine-terminated bilayer linked to MUA via both covalent amide bonds and electrostatic acid/base interactions. The bilayers are then capped with vapor-phase TFMBC to form a third covalently bound fluorinated layer. The reaction products are confirmed by Fourier transform infrared−external reflection spectroscopy. The spectroscopic data are supported by ellipsometric measurements and contact-angle goniometry, which show modulation in both the film thickness (1.2 to 2.1 nm) and contact angle (15 to 106°) as the film progresses from the MUA monolayer to the final trilayer product. X-ray photoelectron spectroscopic analysis indicates the presence of terminal trifluoromethyl (CF3) functional groups in the final product. Films prepared using the same reactants in organic solvents have properties very similar to those prepared in the vapor phase. Comparison of the C−F stretching regions of trilayer-film spectra with that of a bilayer film prepared using a simple amide-coupling reaction between activated aminothiophenol and TFMBC (which we assume goes to completion) suggests that the trilayer films cover 30−40% of the Au surface.