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

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 (CF 3 ) 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.