Protonation effects on the structure and homogeneous charge transport dynamics of solid state osmium bis(bipyridyl)tetrazine chloride films

Mechanically attached solid state films of [Os(bpy)(2)-4-tet-Cl](ClO4) have been formed on platinum microelectrodes (where bpy is 2,2'-bipyridyl and 4-tet is 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine). Scanning electron microscopy reveals that films exist as an amorphous array of microscopically small particles. At relatively high scan rates, the voltammetric response of these films is reminiscent of that observed for an ideal reversible, solution phase redox couple. The film structure is not affected by voltammetric cycling in perchloric acid which protonates the unbound pyridine moiety of the tetrazine ligand. In contrast, the films become significantly more homogeneous when cycled in sodium perchlorate electrolyte and appear to become microcrystalline. Slow and fast scan linear sweep voltammograms have been used to provide an absolute determination of the fixed site concentration as 1.8 +/- 0.05 and 1.6 +/- 0.06 M and apparent diffusion coefficients of (6.4 +/- 0.3) x 10(-11) and (5.0 +/- 0.4) x 10(-11) cm(2) s(-1) in 1.0 M NaClO4 and HClO4 electrolytes, respectively. Under semiinfinite linear diffusion conditions, the voltammetric peak current varies in a sigmoidal manner with the electrolyte pH increasing with decreasing solution pH. This response provides an estimate of 3.5 +/- 0.1 for the pK(a) of the pyridine moiety of the tetrazine ligand within the solid which is approximately 0.8 pH units higher than that found in aqueous acetonitrile solution.