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Triple Phase Boundary Photovoltammetry: Resolving Rhodamine B Reactivity in 4-(3-Phenylpropyl)-Pyridine Microdroplets



Rhodamine B base is employed as a photoactive redox system in a liquid | liquid | electrode triple phase boundary electrochemical experiment. Microdroplets of the water-immiscible 3-(4-phenylpropyl)-pyridine (PPP) containing rhodamine B are deposited onto a basal plane pyrolytic graphite electrode surface which is then immersed into an aqueous solution containing 0.5 M phosphate buffer solution (pH 12). In cyclic voltammetry experiments, it is shown that dark reduction of rhodamine B to leuco-rhodamine B in PPP occurs in two steps—both one electron processes—and the re-oxidation occurs in a single two-electron process. Voltammetric signals are consistent with Na+ transfer (for the first reduction step) and proton transfer (for the second reduction step) coupled to the electron transfer. A proton-driven disproportionation reaction (with kdis=3 mol−1 dm3 s−1 at pH 12) is observed for the one-electron reduced intermediate. In the presence of light, a new photocatalysed oxidation process for the two-electron conversion of leuco-rhodamine B to rhodamine B is observed. Photoexcitation of rhodamine B is shown to trigger an effective photo-comproportionation mechanism. Quantitative insights into the dark and photomechanisms are obtained by comparison of rhodamine B and rhodamine B octadecylester and by applying an approximate numerical simulation procedure based on DigiSim.