Nitrophenyl groups in diazonium-generated multilayered films: which are electrochemically responsive?

Various nitrophenyl-containing organic layers have been electrografted to glassy carbon surfaces using diazonium chemistry to elucidate the extent by which the layer structure influences the solvent (i.e., acetonitrile) accessibility, electroactivity, and chemical reactivity of the films. For most of these films, cyclic voltammetric and impedance spectroscopy measurements show that the electron-transfer process at the electrode is facile and independent of film thickness and structure. This is consistent with the occurrence of self-mediated electron transfers throughout the film with nitrophenyl groups serving as redox stations. Importantly, this behavior is seen only after the first potential sweep, the effect of which is to increase the porosity of the layer by inducing an irreversible desorption of nonchemisorbed material along with a reorganization of the film structure. From a kinetic point of view, the radical anions of surface-attached nitrophenyl groups are reactive toward the residual water present in acetonitrile. Thin layers (thickness of 1 to 2 nm) containing redox-active groups only in the outer part of the layer are protonated two to three times as fast as groups located in a more hydrophobic but still solvent-accessible inner layer. Hence, kinetic measurements can detect small differences in the layer environment. Finally, a deconvolution of the cyclic voltammetric response of an electrode grafted from 4-nitrobenzenediazonium discloses that roughly 25% of the overall signal can be attributed to the presence of 4-azonitrophenyl moieties introduced during the electrografting process.