The rotating optical disc-ring electrode consists of a transparent disc surrounded by a concentric ring electrode. Light is shone through the central disc to drive a photoredox system. The current from the photogenerated product is measured downstream on the ring electrode. The convective diffusion equation for this system is solved and analytical solutions are presented for the collection of a stable product. The theory takes into account the bleaching of the solution. Experiments performed on the iron-thionine system are found to be in good agreement with the theory.
A general synthetic strategy for the preparation of disubstituted thionines is described. The Lauth hydrogen sulfide and the Bernthsen thiosulfate methods have been explored for the preparation of disulfonated thionines (DST's) with negative results for the former (less than 1% yield) and very modest results for the latter (5% yield). A new method based on the nucleophilic coupling of a p-phenylenediamine with the synthetic equivalent of an aniline has resulted in considerable improvement, regarding particularly the suppression of byproducts (15-20% yield). The relatively low yields obtained still with the new route are an indication that thionation and ring closure of diphenylamines are difficult when electron-withdrawing groups are present. The new route has enabled the unambiguous structural characterization of two isomeric DST's known as DST-1 and DST-2 to be 4,6-and 2,6-DST, respectively. The 470-MHz
Results are reported on the dark electrochemistry and photoelectrochemistry of thionine (TH) and two of its disulfonated derivatives (2,6-DST and 4,6-DST) on the title electrode (RODRE) that clearly indicate that coating of the electrode with a thin layer of polymeric thionine is crucial for measurement of sizable photoinduced ring currents. The parent TH does not require the polymeric coat since, due to its adsorption and aggregation properties, a One to two monolayer film of aggregated molecules spontaneously coats the electrode. The advantage of using, in addition to the lipophilic TH, the hydrophilic DST's rests in the lack of adsorption properties for the latter since, otherwise, their photophysical, photochemical, and electrochemical properties are quite similar, the presence of the sulfonates representing only a second-order effect. These findings are consistent and help to explain the recently published observation that the measured efficiency of the parent iron-thionine system drops by a factor of 19 or more when semitransparent gold electrodes coated with thick polythionine layers are utilized in experimental photogalvanic cells. The key to these results has been the application to this problem of an optical rotating electrode, the RODRE, which combines two different surfaces on the same electrode: the transparent disk surface, used to illuminate the photogalvanic solution, and the Pt metal ring surface, used to detect stable electroactive species photochemically generated in the disk zone. In the RODRE, the ring detector is effectively maintained in the dark and, thus, photocurrents arising from species in solution can be easily separated from photocurrents arising from species in the coat. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-06 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 128.255.6.125 Downloaded on 2015-06-06 to IP
The rotating optical disk ring electrode (RODRE) has been successfully applied to study interfacial charge transfer phenomena in colloidal
TiO2
particles. In this electrode, light enters the cell through a transparent quartz disk and photogenerated species are detected at a metal (Pt) ring surrounding the transparent disk. Measurement of dark currents as a function of rotation speed allows calculation of the heterogeneous electron transfer rate constant and the average number of conduction‐band electrons per particle. Illumination of the RODRE provides, from the dependence of the photocurrent on light intensity and rotation speed, an estimate of the stability of photogenerated conduction‐band electrons, the quantum yield of photoelectron generation, and the photoelectron lifetime. The results reported clearly illustrate that the RODRE is a useful and convenient technique for studying charge transfer phenomena in colloidal suspensions of semiconductor particles. The advantage of the disk ring technique (RODRE) over the alternative disk electrode technique (ORDE) is that, for the RODRE, there is no need to use any coating material to make the ring electrode conducting, while, for the ORDE, a semitransparent layer of
normaln‐SnO2
is applied to make the transparent surface conducting. The nature of this coat imposes limitations in the kind of semiconductor materials that can be studied.
The kinetics of the oxidation with ferric ion of 2,6-and 4,6-disulfonated leucothionines has been examined by stopped-flow techniques. Ortho-sulfonation of the dye form of thionine (D) has led to complex kinetics that can be interpreted in terms of association or ion-pair formation between ferric ion and the fully reduced leuco form (L) of the dye. The resulting effect is that association of the reagents, instead of facilitating the oxidation of L, becomes a trap since the final stages of the oxidation are a relatively slow process. Return to the colored form D is viewed to take place mostly by electron exchange between nonassociated species. The behavior of
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