Published: September 09, 2011 r2011 American Chemical Society 19379 dx.doi.org/10.1021/jp206927w | J. Phys. Chem. C 2011, 115, 19379–19385 ARTICLE pubs.acs.org/JPCC Electrochemistry of Ferrocene-Functionalized Phosphonium Ionic Liquids Joshua E. F. Weaver, †,§ Daniel Breadner, ‡ Fanguo Deng, ‡ Bala Ramjee, †,|| Paul J. Ragogna, ‡ and Royce W. Murray* ,† † Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599, United States ‡ Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 3K7, Canada b S Supporting Information ’ INTRODUCTION In recent years ionic liquids have begun to move from the realm of novel chemical materials to materials with many practical applications. These uses are centered on the unique property of being both innately conducting and liquid and are often associated with electrochemical applications. 1 Practical applications now include use as electrolytesolvents in batteries, fuel cells, and capacitors. Some recent studies have examined the behavior of various solutes in ionic liquids, 2 molecular force fields of ionic liquids, 3 hydrogen oxidation in ionic liquids at a platinum electrode surface, 4 density estimation of ionic liquids, 5 the applicability of Marcus theory to ionic liquid redox processes, 6 and the evaluation of new materials such as tetraalkylphospho- nium polyoxometalate ionic liquids. 7 This group has investigated the electrochemical properties of various classes of ionic liquid materials that have included redox active polyether hybrids, DNA complexes, and imidazolium systems. 817 Redox functionalized ionic liquids inherently offer unique opportunities for investigations of electron transfer and mass transport in melt phases, when used in their undiluted forms where electron hopping dominates. These materials often have melting points below 100 °C and are sufficiently ionically conductive as to permit quantitative voltammetry in the undi- luted state. They can be designed to incorporate a variety of redox species, either appended to or as a counterion 8 of a molecular ionic liquid. The present study is of the redox active ferrocenated phosphonium ionic liquids shown in Figure 1, describing their synthesis and electron transport properties. Our previous studies have focused on investigating and under- standing the rates of mass and electron transport in semisolid media, choosing as model media, redox functionalized ionic liquids. With very high redox site concentrations, such as ferrocene in the 12 M range, the neat melt materials are concurrently both electron and ionically conductive. An impor- tant property of the undiluted redox ionic liquids is that the spacing between redox sites is small, allowing facile electron self- exchange (hopping). The viscosities of the neat materials are substantial, so that physical diffusion of redox sites is greatly retarded, and except for redox couples with nominally very slow electron transfers, 12 becomes slower than the transport of electrochemical charge by electron hopping. The electron hop- ping process is initiated by electrochemical voltammetry, which generates a mixed valent layer at the electrode interface. As originally pointed out by Buttry and Anson, 18 the general expression for the combined electron hopping and physical diffusion processes, that is, the apparent diffusion (D APP ), is the transport summation of the DahmsRuff equation: 19,20 D APP ¼ D PHYS þ D E, TRANS ¼ D PHYS þ k EX, APP δ 2 C 6 ð1Þ Received: July 20, 2011 Revised: August 23, 2011 ABSTRACT: The synthesis of and voltammetry in undiluted form of several ferrocene-functionalized phosphonium ionic liquids is reported. Electron transport in the mixed valent diffusion layer around an electrode occurs primarily by Fc +/0 electron self-exchange reactions, as opposed to physical diffu- sion of the ferrocenated phosphonium species. The apparent diffusion coefficients for electron transport and for the coun- terions of the ionic liquid, and in particular their activation energy barriers, are similar to one another, evidence for the control of rates of electron transport by relaxation of the counterion atmosphere. For a net transport of electronic charge to accompany electron transfer, counterion displacement must occur to relax the charge displacement associated with the electron transfer. The ferrocenated phosphonium ionic liquids thus behave in a manner like that of previously investigated redox ionic liquids based on combinations of redox groupings with imidazolium and with short- chain polyether functionalities.