Scanning Electrochemical Microscopy. 43. Investigation of Oxalate Oxidation and Electrogenerated Chemiluminescence across the Liquid-Liquid Interface ² Fre ´ de ´ ric Kanoufi, ‡,§ Ce ´ line Cannes, ‡ Yanbing Zu, ‡ and Allen J. Bard* ,‡ Department of Chemistry and Biochemistry, The UniVersity of Texas at Austin, Austin, Texas 78712, and Laboratoire EnVironnement et Chimie Analytique, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France ReceiVed: March 7, 2001; In Final Form: June 15, 2001 To treat the problem of oxalate oxidation in an aqueous phase by a Ru(III) species (mediator) generated in an immiscible nonaqueous phase (benzonitrile) at a scanning electrochemical microscope (SECM tip), the theory of the feedback mode of the SECM is extended to include both finite heterogeneous electron transfer (ET) kinetics at the substrate and homogeneous decomposition of the mediator. As for the classical electrochemical (EC) scheme, a zone diagram is constructed showing pure ET heterogeneous kinetic control, pure homogeneous kinetic (C) control (insulating behavior), and mixed EC kinetic control. The model allows interpretation of the anomalous approach curves obtained for oxalate oxidation by a ruthenium(III) coordination complex at the benzonitrile (BN)-water interface and allows calculation of the rate constant for the ET at the liquid-liquid interface. Attempts are made to relate these rates to the homogeneous rate constant in terms of Marcus theory. The second ET (oxidation of CO 2 • - ) at the liquid-liquid interface generates an emitting excited state of the ruthenium species. The electrogenerated chemiluminescence process is related to the current crossing the interface. Introduction Electron transfer (ET) reactions at the liquid-liquid interface have been extensively investigated over the past 20 years 1 owing, in part, to interest in the liquid-liquid interface as an intermediate case between homogeneous and heterogeneous sites for ET. 2 Several different models have been developed to depict the ET at the interface between two immiscible electrolyte solutions (ITIES). 3-6 Among the various techniques used to probe ET at an ITIES (e.g., cyclic voltammetry 1 or voltfluorometry 7-9 in a four- electrode cell, voltammetry in a thin-layer cell 10 ), the scanning electrochemical microscope (SECM) in its feedback mode has been shown to be particularly useful in the characterization of ET kinetics. 11-19 The SECM technique has the advantage of easy separation of the contribution of electron transfer and ion transfer processes and does not require a four-electrode arrange- ment with an externally applied potential across the interface. Typically in these experiments, a stable redox active species, for example an oxidant, is generated at an ultramicroelectrode (UME) tip located in one liquid phase and the tip is approached to the second liquid that contains another redox species, a reductant, that can transfer an electron to the tip-generated oxidant. The tip feedback current depends on the rate of this electron transfer. In most studies this oxidant is part of a stable redox couple, thus simplifying the treatment of the kinetics and mechanism of the system. However, the SECM technique can also be used to investigate the kinetics of irreversible ET at an ITIES, such as the reduction of dibromocyclohexane by vitamin B 12 . 20 Such studies of electrochemical catalysis are of interest in characterizing syntheses in microemulsions. Moreover, in principle, they should provide interesting information by comparing the ET rates to their homogeneous analogues in terms of ET theory. Previous reports from this laboratory have shown that oxidation of a luminophor such as RuL 2 L′ 2+ in the presence of a coreactant, such as the oxalate dianion, leads to the formation of an excited state [electrogenerated chemilumines- cence (ECL)], by homogeneous reaction in the aqueous phase, 21,22 either at a polymer-modified electrode, 23 or at an ITIES. 24 This latter study qualitatively demonstrated ECL at the ITIES and proposed that its occurrence was evidence of Marcus inverted region behavior. This paper is a more detailed and quantitative study of this type of ET and ECL reaction. Based on the observation of ECL at the BN-water interface and on the homogeneous 22 or electrochemical 25 oxidation of oxalate, a mechanism of the ECE type, depicted in Scheme 1 and schematically presented in Figure 1 in the SECM config- uration, can be invoked for the SECM monitored oxalate ² Part of the special issue “Royce W. Murray Festschrift”. ‡ The University of Texas at Austin. § ESPCI. SCHEME 1 8951 J. Phys. Chem. B 2001, 105, 8951-8962 10.1021/jp0108667 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/10/2001