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 mum 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, DCT increases from 2.4 to 3.6 x 10(-10) cm(2) 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 x 10(-10) cm(2) s(-1). over the same concentration range. The maximum DCT observed would correspond to an electron self-exchange rate constant of 1.1 x 10(5) 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.