A simple
strategy to avoid the formation of polyaryl layer during
the functionalization of carbon surface by diazonium electroreduction
is presented. The approach proposes to directly act on the polymerization
mechanism by the use of a radical scavenger. The kinetic gap between
the surface coupling and the multilayer formation is exploited to
prevent the growth of the layer without interfering with the grafting.
The well-known 4-nitrobenzenediazonium electrografting was used to
demonstrate the possibility of reaching a monolayer surface coverage
with an excess of DPPH (2,2-diphenyl-1-picrylhydrazyl). Experimental
conditions were varied to validate the efficiency of the grafting
limitation and the radical capture was confirmed by isolation of the
aryl radical/DPPH coupling product.
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 monitored 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. Differences 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.
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