ИСТИНА

Ionic liquids (ILs) are perspective media for various fields, including catalysis, extraction, biotechnologies and energetics. The investigation of the stabilization and transport of excess electrons, generated in ILs with the light or ionizing irradiation, may lead to a better understanding of dynamics and structure of these unusual media and is of great interest for the series of special applications. Previously we have presented the first EPR evidence of a physically trapped electron in a pyrrolidinium-type ionic liquid at low temperature [1]. Here we report comparative EPR studies of the low-temperature radiolysis of ILs with pyrrolidinium and piperidinium-type cations. N-metyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl) imide (P13+NTf2-) and N-metyl-N-propylpiperidinium bis(trifluoromethanesulfonyl) imide (PP13+NTf2-) (Kanto Chemical Co., Inc) were used without additional purification. ILs were placed into SK-4B glass ampoules, which gave no background EPR signal after irradiation. Then the samples were evacuated at 423 K and irradiated with X-rays (30 keV) at 77 K. EPR spectra were measured at 77 K using an X-band (9.4GHz) spectrometer with a 100 kHz high-frequency modulation (SPIN, St. Petersburg, Russia). A high-pressure arc mercury lamp (250 W) equipped with a series of filters was used for photobleaching experiments. The EPR spectra of the irradiated ILs show a superposition of a broad multiplet signal and a sharp singlet signal. The former signal may be attributed to the alkyl-type radicals from the cation. The latter signal is clearly seen at low microwave power and shows remarkable saturation with increasing the microwave power level. Such a saturation behavior is a typical characteristic of the signals of trapped electrons in low-temperature organic glasses. The optic spectra of irradiated ILs in the visible region reveal an absorption band with the λmax ≈ 500 nm, which may be presumably attributed to some kind of “hole” species. Photolysis with near IR light (λ > 700 nm) results in significant decay of the singlet signal without any other changes in the EPR spectrum pattern. Difference EPR spectrum shows that the bleached species is characterized by a narrow singlet signal, which is attributed to a physically trapped electron. Photolysis of P13+NTf2- with the light with λ = 450 – 600 nm during 10 minutes results in complete disappearance of the narrow singlet signal, attributed to the trapped electron in the EPR spectrum. At the same time, intensity ratio of the components in the EPR spectrum is slightly changed and the absorption band in the visible region decayed to a certain extent. Further photolysis at λ = 450 – 600 nm during 20 more minutes gave no changes in the EPR and optical spectra of the ILs samples. Such a behavior under photolysis may be explained by the recombination of trapped electron and the “hole”. The slow kinetics of trapped electron decay at 77 K is in agreement with the assumption of tunneling mechanism. In summary, our results demonstrate the EPR evidence of physically trapped electron in pyrrolidinium- and piperidinium-type glassy ionic liquids at low temperature. This work was supported by the Russian Foundation for Basic Research (project no. 14-03-31978). References [1] Saenko E.V., Takahashi K., Feldman V.I. EPR Evidence for a Physically Trapped Excess Electron in a Glassy Ionic Liquid // J. Phys. Chem. Lett. 2013. V. 4. P. 2896–2899.