Charge Transport Dynamics and Redox Induced Structural Changes within Solid Deposits of a Ruthenium Dimer

Solid particles of a ruthenium dimer, [(Ru(bpy)2)2 bpzt-]3+, have been abrasively attached to macroelectrodes and microelectrodes, where bpy is 2,2‘-bipyridine and bpzt- is 3,5-bis(pyrazin-2-yl)-1,2,4-triazole. The voltammetry of these solids deposits is unusually ideal in NH4PF6-containing aqueous solutions, and the response is characterized by semi-infinite linear diffusion for scan rates between approximately 50 and 2000 mV s-1. SEM imaging reveals that sparse films of solid particles (1−10 μm in diameter) are efficiently transformed into microcrystals by voltammetric cycling. The charge transport diffusion coefficient, D CT, has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, D CT increases from 2.4 to 3.6 × 10-10 cm2 s-1 as the NH4PF6 concentration is increased from 0.1 to 2.0 M, while, for oxidation of the deposit, D CT increases from 1.1 to 3.9 × 10-10 cm2 s-1 over the same concentration range. The maximum D CT observed would correspond to an electron self-exchange rate constant of 1.1 × 105 M-1 s-1. Despite the smaller electron-transfer distance expected within the solid, this apparent self-exchange rate constant is more than 2 orders of magnitude smaller than that typically found for ruthenium bis-bpy complexes in solution. This observation suggests that ion rather than electron transfer may limit homogeneous charge transport through these solid deposits.