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 π-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 < [ClO 4 - ] < 1.0 M (pH 6.6) at 3.14 ± 0.11 × 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 × 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 H 2 Q and Q. We have used the Dahms−Ruff formalism to determine electron self-exchange rate constants which are 2.84 ± 0.22 × 10 9 and 9.69 ± 0.73 × 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.