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 k(dis)=3 mol(-1) dm(3) 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.