Electron self-exchange in the solid-state: Cocrystals of hydroquinone and bipyridyl triazole

Solid-state voltammetry, spectroscopy, and microscopy studies have been use d to probe the proton and electron conductivity within a self-assembled coc rystal, HQBpt. This crystallographically defined material contains 3,5-bis( pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are ct-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When depo sited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q , across a wide range of voltammetric time scales, electrolyte compositions , and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied fro m 1 to 7 triggers electrocrystallization and the extensive formation of rod like crystals. Fast scan rate voltammetry reveals that the homogeneous char ge transport diffusion coefficient, D-app, is independent of the perchlorat e concentration for 0.1 < [ClO4-] < 1.0 M (pH 6.6) at 3.14 <plus/minus> 0.1 1 x 10(-9) cm(2) s(-1). Moreover, D-app is independent of the perchloric ac id concentration for concentrations greater than approximately 2.0 M, maint aining a value of 4.81 +/- 0.07 x 10(-8) cm(2) s(-1). The observation that D-app is independent of the supporting electrolyte suggests that the rate-d etermining step for homogeneous charge transport iS not the availability of charge-compensating counterions or protons; but the dynamics of electron s elf-exchange between H2Q and Q. We have used the Dahms-Ruff formalism to de termine electron self-exchange-rate constants which are 2.84 +/- 0.22 x 10( 9) and 9.69 +/- 0.73 x 10(10) M-1 s(-1) for pH values greater than approxim ately 2.0 and less than -0.3,respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone se lf-exchange reactions in aqueous solution. These results indicate that hydr ogen bonds play an important role in supporting rapid electron transfer. Th e increase in D-app between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.