Electroreduction of Viologen Phenyl Diazonium Salts as a Strategy To Control Viologen Coverage on Electrodes

A majority of the reported electrografting of aryldiazonium salts result in the formation of covalently attached films with a limited surface coverage of below 5 nmol·cm^-2. Herein, we report the preparation of higher-thickness redox-active viologen-grafted electrodes from the electroreduction of viologen phenyl diazonium salts, by either cyclic voltammetric (CV) sweeps or electrolysis using a fixed potential. Both of the methodologies were successfully applied for various conductive surfaces, including glassy carbon (GC), gold disc, indium tin oxide glass, mesoporous TiO₂ electrodes, and 3D compacted carbon fibers. A robust maximal viologen coverage, Γ_viologen = 9.5 nmol·cm^-2, was achieved on a GC electrode by CV electroreduction. Electroreduction held at a fixed potential at E_appl. = -0.3 V can fabricate viologen-grafted electrodes with Γ_viologen in the range of 0-37 nmol·cm^-2 in a controllable way, by simply adjusting the electrodeposition time t_appl.. Time-dependent Γ_viologen were found to be 10 nmol·cm^-2@2 min, 20 nmol·cm^-2@4.2 min, and 30 nmol·cm^-2@7 min. Furthermore, a TiO₂ electrode coupled with Γ_viologen of 140 nmol·cm^-2 exhibited electrochromic performance, with the color changing from pale yellow to blue and red brown.