One-Step Formation of Bifunctionnal Aryl/Alkyl Grafted Films on Conducting Surfaces by the Reduction of Diazonium Salts in the Presence of Alkyl Iodides

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH₂CH₂C₆F₁₃ and IC₆H₁₃) and their reaction with surfaces takes place at low driving force (∼−0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN₂⁺), at a potential where the latter is reduced. By comparison to the direct grafting of ICH₂CH₂C₆F₁₃, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl–alkyl groups (Ar = 3-CH₃–C₆H₄, 4-NO₂–C₆H₄, and 4-Br–C₆H₄, R = C₆H₁₃ or (CH₂)₂–C₆F₁₃). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.