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

Abstract

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, DCT, has been determined by systematically varying the voltammetric scan rate. For reduction of the deposits, DCT 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, DCT increases from 1.1 to 3.9 × 10-10 cm2 s-1 over the same concentration range. The maximum DCT 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.