Highly Reactive Diazenyl Radical Species Evidenced during Aryldiazonium Electroreduction

Abstract

We report the experimental reassessment of the widely admitted concerted reduction mechanism for diazonium electroreduction. Ultrafast cyclic voltammetry was exploited to demonstrate the existence of a stepwise pathway, and real-time spectroelectrochemistry experiments allowed visualization of the spectral signature of an evolution product of the phenyldiazenyl radical intermediate. Unambiguous identification of the diazenyl species was achieved by radical trapping followed by X-ray structure resolution. The electrochemical generation of this transient under intermediate energetic conditions calls into question our comprehension of the layer structuration when surface modification is achieved via the diazonium electrografting technique as this azo-containing intermediate could be responsible for the systematic presence of azo bridges in nanometric films.

Selective Nsp2‐ and Csp2‐ Photografting of Au‐Surface by Aryldiazonium Salts and Arylazo Sulfonates

“…The photografting of arylazo sulfonates and diazonium salts to the gold surface must be discussed in the light of recently published, important results. [37] The authors demonstrate through fast scan cyclic voltammetry the existence of the diazenyl radical ArÀ N=N . (ArN 2 * )Ar-N = N * (ArN 2 * ) (Scheme 4a) which implies that the electrochemical reduction of diazonium salts is stepwise (Scheme 4b).…”

Harnessing Aryldiazonium Salts Reduction for Tailored Bifunctionalized Surface Engineering

“…The formation of nitrogen−substrate bonds and azo bridges (−N�N−) within the layer is usually observed 24,25 without knowing yet the mechanism involved, whether it comes from a coupling between grafted rings and diazoniums 26 or from the direct grafting of aryl diazenyl radicals. 27 Conventionally, a direct carbon contact with the substrate is established, which represents a definite advantage in terms of stability. 28−35 Three different pathways leading to bifunctional surfaces through the reduction of diazonium salts have been described in the literature (Scheme 1):…”

“…When generated in close proximity to a substrate, these radicals lead to the functionalization of the substrate through the deposition of an organic layer. The formation of nitrogen–substrate bonds and azo bridges (−NN−) within the layer is usually observed , without knowing yet the mechanism involved, whether it comes from a coupling between grafted rings and diazoniums or from the direct grafting of aryl diazenyl radicals . Conventionally, a direct carbon contact with the substrate is established, which represents a definite advantage in terms of stability. − …”

Nanometrology based control: taming radical grafting reactions with attoliter precision

“…pristine glass, metal surfaces or previously grafted layers), leading to non-linear addition rates which often develop into inhomogeneities. [16][17][18][19][20] Moreover, aryl-radicals are relatively stable and thus able to diffuse for at least a few tens of nanometers before reacting, 21,22 often leading to resolution loss and decreasing the fabrication precision. The benefits of developing controlled aryl-radical addition is however high, motivating considerable efforts towards methodologies for their generation 23,24 and surface addition control.…”