Diazonium Grafting Control through a Redox Cross‐Reaction: Elucidation of the Mechanism Involved when using 2,2‐Diphenylpicrylhydrazyl as an Inhibitor

Abstract

Investigation into the mechanism involved in the control of layer growth through the reduction of diazonium salts by using 2,2-diphenylpicrylhydrazyl (DPPH). This approach, previously assumed to be based on the radical capture of highly reactive aryl intermediates via the scavenging activity of DPPH, actually rests on redox cross reactions involving its electro-reduced form. A numerical approach was developed to model the proposed processes, involving a redox reaction between diazonium cations and the reduced species of DPPH as a central step. Extension of this approach was validated by using substituted benzoquinones instead of DPPH, leading to good control of the radical polymerization to give sub-monolayer surface coverage. Modulation of the surface coverage can be obtained by adjusting the concentration of the redox mediator and a near-monolayer can be prepared in this way. The difference in formal redox potential between the diazonium salt and the electro-reducible mediator was identified as a key parameter, and could be exploited to generalize the grafting control.

Self-limiting covalent modification of carbon surfaces: diazonium chemistry with a twist

“…23 Extensive work has been carried out in grafting monolayer films using radical inhibitors such as 2,2-diphenyl-1-picrylhydrazyl (DPPH) which prevent multilayer growth providing relatively homogeneous films. 24,25 Additionally, application of high overpotentials during the electrografting process has also been suggested as a general method to obtain monolayers. 26 While the aforementioned strategies have evolved within the electrochemistry domain, there have been parallel attempts to achieve efficient grafting onto various surfaces using chemical activation via the use of external reducing agents.…”

The Role of Aryldiazonium Chemistry in Designing Electrochemical Aptasensors for the Detection of Food Contaminants

“…Still, as commonly acknowledged, the major drawback of electrografting procedures is the high reactivity of electrogenerated aryl radicals, which react unselectively with the substrate and surface-grafted aryl groups. This usually leads to polyaryl layers with disordered structure and uncontrolled thickness, which are less suitable for building adequate sensing platforms [ 169 ]. The approaches reported so far to limit the formation of polyaryl layers have been reviewed elsewhere [ 35 ] and therefore only a brief inventory will be given here.…”

“…Another popular procedure to suppress the formation of polyaryl layers uses radical scavengers during the electrografting step to capture the excess aryl radicals [ 175 , 176 ]. Although early studies employing 2,2-diphenyl-1-picrylhydrazyl (DPPH) as radical scavenger suggested that DPPH couples with aryl radicals generated from diazonium reduction, a recent study established that DPPH acts more like a redox mediator than a radical trapping species [ 169 ].…”

Use of calixarenes bearing diazonium groups for the development of robust monolayers with unique tailored properties

“…In a more recent work, they actually demonstrated that the radical scavenger is more likely acting as a redox mediator than a radical trapping species. 51 Lastly, strategies that take benefit of pre-organized rigid structures have been smartly exploited for preparing monolayers. 11-20, 26, 52, 53 Especially, in this connection, we have recently developed an innovative strategy based on the direct grafting of calix [4]arene diazonium salts constrained in the cone conformation.…”