Comparison of the Charge-Transport Rate in Two Redox Levels of Polymers Derived from N,N′-Dibenzyl-4,4′-bipyridinium and N,N′-Dialkyl-4,4′-bipyridinium

Charge transport in electrode-confined polymers derived from N,N′-bis(p-(trimethoxysilyl)benzyl)-4,4′-bipyridinium dichloride, [(BPQ^2+/+/0)_n]_surf, and N,N′-bis((trimethoxysilyl)propyl]-4,4′-bipyridinium dibromide, [(PQ^2+/+/0)_n]_surf, has been studied. Specifically, a comparison of charge transport in the 2+/+ and +/0 redox systems of both polymers has been made. Interestingly, +/0 has an effective diffusion coefficient for charge transport, D_CT that is significantly greater than that for the 2+/+ redox level in H₂O/CH₃CN (9/1 by volume) 1.0 M LiCl. Platinum disk electrodes have been studied that have polymer thicknesses up to ∼2.5 µm in the 2+ state corresponding to a coverage of ∼7.5 × 10⁻⁷ mol of monomer units per cm². Up to a 30“40% decline in thickness is observed upon reducing the polymer from the 2+ to the 0 state. Values of the products D_CTC², where C is the concentration of monomer units in the + or 0 state of the polymer, have been determined by measuring the current for reduction of Ru(NH₃)₆³⁺ at polymer-coated rotating disks as a function of polymer coverage. The data support the conclusion that >20 mA/cm² can be maintained when the polymer thickness in the 2+ state is 1.0 µm, corresponding to a coverage of ∼2 × 10⁻⁷ mol/cm² of monomers per cm².