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Radicals in Ionic Liquids

Authors

  • Prof. Dr. Veronika Strehmel

    Corresponding author
    1. Department of Chemistry, Organic Chemistry and Institute for Coatings and Surface Chemistry, Hochschule Niederrhein, University of Applied Sciences, Adlerstrasse 32, 47798 Krefeld (Germany), Fax: (+49) 2151 822 4195
    • Department of Chemistry, Organic Chemistry and Institute for Coatings and Surface Chemistry, Hochschule Niederrhein, University of Applied Sciences, Adlerstrasse 32, 47798 Krefeld (Germany), Fax: (+49) 2151 822 4195
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Abstract

Stable radicals and recombination of photogenerated lophyl radicals are investigated in ionic liquids. The 2,2,6,6-tetramethylpiperidine-1-yloxyl derivatives contain various substituents at the 4-position to the nitroxyl group, including hydrogen-bond-forming or ionic substituents that undergo additional interactions with the individual ions of the ionic liquids. Some of these spin probes contain similar ions to ionic liquids to avoid counter-ion exchange with the ionic liquid. Depending on the ionic liquid anion, the Stokes–Einstein theory or the Spernol–Gierer–Wirtz theory can be applied to describe the temperature dependence of the average rotational correlation time of the spin probe in the ionic liquids. Furthermore, the spin probes give information about the micropolarity of the ionic liquids. In this context the substituent at the 4-position to the nitroxyl group plays a significant role. Covalent bonding of a spin probe to the imidazolium ion results in bulky spin probes that are strongly immobilized in the ionic liquid. Furthermore, lophyl radical recombination in the dark, which is chosen to understand the dynamics of bimolecular reactions in ionic liquids, shows a slow process at longer timescale and a rise time at a shorter timescale. Although various reactions may contribute to the slower process during lophyl radical recombination, it follows a second-order kinetics that does not clearly show solvent viscosity dependence. However, the rise time, which may be attributed to radical pair formation, increases with increasing solvent viscosity.

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