Electron Self-Exchange in the Solid-State: Cocrystals of Hydroquinone and Bipyridyl Triazole

Abstract

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 pi-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 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 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 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.

Optical and electrochemical properties of copoly(aryl ether)s consisting of alternate 2,5‐distyrylbenzene and electron‐transporting oxadiazole or triazole derivatives

“…Comparing with those spectra in CHCl 3 , the absorption and emission reveal red‐shift result from the protonation of imine nitrogen in DIOXD, OXD, or TAZ fluorophores. The protonation usually leads to change in optical properties such as fluorescence quench, absorption or emission shift, and spectra broadening 42–44. Monkman et al had demonstrated that the protonation of nitrogen has an important structural effect: intramolecular hydrogen bonding to the adjacent oxygen atom in the alkoxy substituent planarizes backbones of the molecules, which leads to red‐shifts of both absorption and emission spectra 34.…”

Electroanalytical chemistry for the analysis of solids: Characterization and classification (IUPAC Technical Report)

“…Additionally, SSEAC methods provide information on the reactivity of solid compounds, both from a thermodynamic as well as from a kinetic point of view. Available examples include nucleation, growth, redistribution, and dissolution processes in fullerenes [165] and metal-organic frameworks [159,160], electrochemical lattice reconstruction in hexacyanometallates [166], electron self-exchange in the solid state [167], and formation of solid inclusion complexes [168].…”

“…Additionally, an electrocatalytic effect exerted by the solid materials on selected electrochemical processes can be utilized to obtain information on the composition of solids [166][167][168][169][210][211][212], including organic and pharmaceutically active compounds [213][214][215][216][217]. Here, the electrochemical oxidation/reduction process of an auxiliary species in solution phase is catalytically enhanced at sample-mod- ified electrodes relative to that occurring at unmodified electrode.…”

Voltammetric behaviour, homogeneous charge transport dynamics and electrocatalytic properties of an Os2+ functionalised pyrrole monomer

“…Previously researchers [44] 2) where n is the number of electrons involved, D APP is the apparent diffusion coefficient describing homogeneous charge transport through the film, C EFF is the effective concentration of the Os 2+ centres in the film, and r is the radius of the microelectrode.…”