Steric Effects in the Reaction of Aryl Radicals on Surfaces

Abstract

Steric effects are investigated in the reaction of aryl radicals with surfaces. The electrochemical reduction of 2-, 3-, 4-methyl, 2-methoxy, 2-ethyl, 2,6-, 2,4-, and 3,5-dimethyl, 4-tert-butyl, 3,5-bis-tert-butyl benzenediazonium, 3,5-bis(trifluoromethyl), and pentafluoro benzenediazonium tetrafluoroborates is examined in acetonitrile solutions. It leads to the formation of grafted layers only if the steric hindrance at the 2- or 2,6-position(s) is small. When the 3,5-positions are crowded with tert-butyl groups, the growth of the organic layer is limited by steric effects and a monolayer is formed. The efficiency of the grafting process is assessed by cyclic voltammetry, X-ray photoelectron spectroscopy, infrared, and ellipsometry. These experiments, together with density functional computations of bonding energies of substituted phenyl groups on a copper surface, are discussed in terms of the reactivity of aryl radicals in the electrografting reaction and in the growth of the polyaryl layer.

Shape‐Switchable Azo‐Macrocycles

“…[137] [138] Furthermore a diazonium cation can also attack an aryl radical already attached to the surface (c in Scheme 6). The radical forming electroreduction occurs preferentially at freely available electrode surfaces quickly covering remaining uncoated surface areas.…”

“…With appropriate experimental conditions homogenous monolayers can be grafted, which is rather difficult to achieve by traditional chemical methods only. [20][139]- [141] Recently, Pinson [137] studied the steric effects in the reaction of aryl radicals on surfaces. They have shown that it is possible to prevent multilayer formation by hindering different positions of the diazonium ions.…”

“…[137] (A) Sterical demanding substituents in 2,6-position hinder the grafting to the surface. (B) Sterical demanding substituents in 3,5-positions limited the growth of the organic layer by sterical effects and a monolayer is formed.…”

“…[137][138] Covalently grafting of a compact organic layer bearing aryl iodine moieties is important, especially for the subsequent modification by wet chemistry substituting iodine atoms attached to phenyl rings. Furthermore a sterically demanding group ortho to the iodide, in order to reduce multilayer formation, would hinder the subsequent cross coupling reaction.…”

“…The nature of the formed organic surface is reported to depend on several different parameters of the preparation, such as applied voltage, concentration of the diazonium salt, exposed time and the substrate itself. [137] The obtained film was thus characterized by CV, atomic force microscopy (AFM), contact angle (CA) measurements and X-ray photoelectron spectroscopy (XPS).…”