Electron Transport and Counterion Relaxation Dynamics in Neat Ferrocenated Imidazolium Ionic Liquids

The electron transport properties of 12 imidazolium ionic liquids (ILs) in which ferrocene groups have been attached to the imidazolium center by different linkers have been investigated. Cobalticenium is alternatively attached in another ionic liquid. The electron transport is measured in the neat (undiluted) ILs, which are very concentrated and viscous, so that electron transport (“electron diffusion”, D E ) occurs by electron self-exchange (hopping) reactions of the Fc +/0 couple. The ionic conductivities are also measured and converted into counterion diffusion coefficients D ION assuming that the small counterions (PF 6 − or BF 4 − ) carry most of the ionic current. Converting the electron diffusion rate into an electron exchange rate constant, k EX gives only an apparent rate constant value, and electron transport is instead demonstrably controlled by the values of D ION, as expected from a previously introduced transport model called ion atmosphere relaxation. The fluctuational rate of counterion motions around a donor−acceptor pair governs formation of a precursor complex within which electron transfer takes place. In keeping with the model, values of k EX and D ION vary in a parallel fashion, and indeed D E and D ION are nearly numerically equal. Activation barriers for ionic conductivity and electron transport are also equal. Log−log plots of k EX vs D ION are linear with slopes of one, and all of the imidazolium transport data fall onto a common correlation line. The generality of the ion atmosphere relaxation explanation for electron transport control is shown by a log−log coplot of the imidazolium IL data with previous data for PEG-based ILs, where the results also all fall onto a common correlation line.