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

Andrew Collins 1Xiaohang Zhang 1Laurie M. Peter 1Jonathon J. Scragg 1Gary J. Blanchard 2Frank Marken 1

1. University of Bath, Department of Chemistry, Claverton Down, Bath BA2-7AY, United Kingdom
2. Michigan State University, Department of Chemistry (MSU), East Lansing, MI 48824, United States


The Rhodamine B base is employed as a redox system in a liquid|liquid electrochemical system. Microdroplets of 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 pH=12 phosphate buffer solution. In cyclic voltammetry experiments conducted in the dark it is shown that reduction of Rhodamine B to leuco-Rhodamine occurs in two steps, both are one-electron processes, and re-oxidation occurs in a single two-electron process. For the first reduction, voltammetric signals are consistent with the Na+ transfer coupled with the electron transfer, whilst for the second reduction the signals are consistent with a one-electron one-proton transfer. A chemical step occurs in the form of a proton driven disproportionation for the one-electron reduced species. This process has been shown to have a kdis= 3 mol-1 dm-3 s-1.

The presence of light facilitates a photocatalytic oxidation process which occurs in place of the two-electron oxidation observed in the dark scans. A conproportionation is triggered by the photo-activation of Rhodamine B, (kcon = 0.7/[Rhodamine B*] s-1 at pH=12 where [Rhodamine B*] denotes steady-state concentration of the photoexcited state). Quantitative insights into the dark and photo-mechanisms are obtained by comparison behaviour of Rhodamine B and Rhodamine B octadecylester and by applying numerical simulation using DigiSim.


Figure 1. Comparison of the photocatalysed oxidation (black) and dark (green) voltammogram for the Rhodamine B base at 0.01 V/s-1; 16 nl deposit, 0.28 M (Rhodamine B : PPP).

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Presentation: Short communication at SMCBS'2009 International Workshop, by Andrew Collins
See On-line Journal of SMCBS'2009 International Workshop

Submitted: 2009-09-03 20:41
Revised:   2009-09-09 18:10
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