Electrografting via Diazonium Chemistry: The Key Role of the Aryl Substituent in the Layer Growth Mechanism Thibaud Menanteau, † Maryle ̀ ne Dias, † Eric Levillain, † Alison J. Downard, ‡ and Tony Breton* ,† † MOLTECH-Anjou, Universite ́ d’Angers, UMR CNRS 6200, 2 Boulevard Lavoisier, 49045 Angers, France ‡ MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand *S Supporting Information ABSTRACT: A series of diazonium salts bearing different para substituents was used to functionalize glassy carbon (GC) and pyrolyzed photoresist film (PPF) under electrografting conditions in the absence and presence of the radical scavenger diphenyl-1-picrylhydrazyl (DPPH). Depositions were moni- tored by electrochemical quartz crystal microbalance (EQCM) and the grafted layers were analyzed by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). DPPH was used to selectively suppress film growth by radical coupling and thereby to reveal the existence of secondary mechanisms involved in the polymerization. Differ- ences in grafting behaviors between various diazonium ion derivatives can be explained by the influence of the para substituent’s electronic properties on the electrophilic aromatic substitutions of diazonium ions on already grafted aromatic groups. ■ INTRODUCTION The use of diazonium salts is now a recognized route to attach functionalities 1 or sustainably change surface properties of carbon 2, 3 and various metals. 4−6 The flexibility of the implementation and the stability of the materials has led to an increasing interest of the scientific community for various applications. 7−11 It is now well-known that the grafting process rests on the production of a highly reactive aryl radical at the substrate−solution interface by reduction of the corresponding diazonium ion. 2,12 The subsequent coupling of this radical to the surface generates a covalently linked molecular layer. 3,13 However, in most cases, when no strategy is used to control the radical process, the aryl radical also reacts on already grafted aryl groups, leading to the formation of a polyaryl layer. 14,15 The homogeneity of the layer is generally quite low depending on the substrate roughness and the deposition conditions but its thickness is typically between 1 and 20 nm. 16 Several studies have underlined the steric effect of the arylsubstituent on the grafting efficiency of diazonium ions. 17−20 When bulky substituents are used, the surface coverage is lowered and the polymerization is limited. This aspect has been exploited to generate monolayer or near monolayer films adapted to nanotechnology uses. 21 In contrast, electronic activation/deactivation effects of the substituents have been significantly less investigated. This lack of studies can be explained by the fact that the mechanism is generally considered to be radical 14 and consequently poorly dependent on the activation/deactivation effect of the arylsubstituents. However, the existence of secondary electrophilic mechanisms has been tentatively proposed to explain some experimental results such as the presence of azo links in the layer. Azo groups appear to be incorporated in films under all deposition conditions. 22−24 The real impact of this type of mechanism on the layer growth and the consequences on the film composition remains difficult to evaluate and has most often been neglected. In this work, a series of diazonium salts with para substituents which are activating or deactivating for electro- philic addition was used to investigate the variable grafting behaviors of diazonium salts. A radical scavenger, 2,2-diphenyl- 1-picrylhydrazyl (DPPH), was used to selectively suppress the radical polymerization 25,26 and to assess the role of nonradical mechanisms in the layer formation. ■ EXPERIMENTAL SECTION Products and Reagents. 4-Nitrobenzenediazonium tetra- fluoroborate (D-NO 2 , Aldrich), 4-methoxybenzenediazonium tetrafluoroborate (D-OCH 3 , Aldrich), 2,2-diphenyl-1-picrylhy- drazyl (Aldrich), tetrabutylammonium hexafluorophosphate (Aldrich) and acetonitrile (HPLC grade, Carlo Erba) were used as received. Received: December 23, 2015 Revised: February 5, 2016 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.5b12565 J. Phys. Chem. C XXXX, XXX, XXX−XXX