New unsymmetrically substituted ferrocenyl-phosphonium ionic liquids (ILs) [FcPR2R']NTf2 are synthesized by two or three step syntheses starting from ferrocene, Fc = (C5H5)Fe(C5H4); R = Me, (n)Bu, (n)Hex, Ph; R' = Me, (n)Pr, (n)Bu, Ph; NTf2 = N(SO2CF3)2. The selective synthesis of alkyl phosphines FcPR2via a Friedel-Crafts phosphorylation is highlighted as an alternative for the standard protocol commonly used for ferrocenyl arylphosphines involving lithiation of FcH followed by phosphorylation. The influence of the P-substituents on thermal stability, electrochemical potential, chemical shift, and UV-Vis absorption behavior of the ILs is studied. The phosphonium group acts both as an ionic tag and as an electron-withdrawing substituent directly bound at the Cp-ring position. Therefore the title compounds are attractive for further studies to use them as tunable redox mediators for (photo)electrochemical devices such as dye sensitized solar cells (DSSCs) or redox flow batteries.
Redox-active
ionic liquids are of interest for various applications
in electrochemistry, for example, as electrochemically active anions
and cations in supercapacitors, as redox mediators in dye-sensistized
solar cells and for overcharge protection in batteries. Due to the
chemical variability of redox-active ionic liquids, their electrochemical
properties can be easily tuned. Here, we investigate the electrochemical
kinetic properties of four ionic liquids containing sulfonium and
phosphonium cations, with ferrocenyl substituents directly attached
to the onium center. The redox-active ionic liquids are dissolved
in the electrochemically innocent ionic liquid [EMIm]TFSI. The results
are compared to the electrochemical kinetic properties of free ferrocene
standard. We obtain precise values for the heterogeneous rate constant k
0 by means of a multispectrum fit of impedance
spectra measured at different overpotentials. Diffusion coefficients
are derived from a convolution analysis of cylic voltammograms. The
redox active cations exhibit lower k
0 values
and lower diffusion coefficients than ferrocene, but the k
0 values are only weakly dependent on the chemical structure
of the cations. Furthermore, we observe no correlation between k
0 and the hydrodynamic radius of the redox active
cations. We offer an explanation for these observations based on kinetic
barriers caused by the structure of the electrochemical double layer.
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