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

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, 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 deposited 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 from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D-app, is independent of the perchlorate concentration for 0.1 < [ClO4-] < 1.0 M (pH 6.6) at 3.14 0.11 x 10(-9) cm(2) s(-1). Moreover, D-app is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining 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-determining step for homogeneous charge transport iS not the availability of charge-compensating counterions or protons; but the dynamics of electron self-exchange between H2Q and Q. We have used the Dahms-Ruff formalism to determine 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 approximately 2.0 and less than -0.3,respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The 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.