EPR Evidence for a Physically Trapped Excess Electron in a Glassy Ionic Liquid Elizaveta V. Saenko, † Kenji Takahashi, ‡ and Vladimir I. Feldman* ,† † Department of Chemistry, Lomonosov Moscow State University, Moscow 119991 Russia ‡ Institute of Science and Engineering, Kanazawa University, Kanazawa 920-1192 Japan ABSTRACT: The fate of excess electron produced in room temperature ionic liquids is an attractive and important topic, and its kinetics and optical absorption spectrum have been studied using pulse radiolysis and laser photolysis. However, there has been no clear evidence for existence of the solvated electron in EPR experiments. Here we report that an EPR spectrum of irradiated glassy pyrrolidinium-based ionic liquid at 77 K reveals a sharp singlet signal (line width of 0.47 mT), which can be attributed to a physically trapped (solvated) electron. The EPR signal shows easy microwave power saturation and is effectively bleached with red light (>600 nm). The present results have important implications for the characterization of trapped or solvated electrons in ionic liquids and better understanding of their structure and reactivity. SECTION: Spectroscopy, Photochemistry, and Excited States D uring the past two decades, room-temperature ionic liquids (RTILs, or simply ILs) have been extensively studied as prospective media for various applications, including catalysis, extraction, biotechnology, and energy conversion. 1 The investigations of trapping and transport of excess electrons generated by light or ionizing radiation in ILs may contribute to better understanding of the structure, dynamics, and electronic properties of these unusual media and, in particular, of their radiation chemistry and photochemistry. An important basic issue is concerned with existence and properties of physically trapped (solvated) electrons in ILs. In 2003, Wishart and Neta first reported a transient IR optical absorption with maximum at 1400 nm in the pulse irradiated methyltributylammonium bis[(trifluoromethyl)sulfonyl]imide (R 1444 NTf 2 ), which was attributed to solvated electron. 2 Similar-type transient absorp- tions in the near-IR region were also found for other kinds of ILs. 3-6 It is worth noting that the reported optical absorption characteristics of solvated electrons are close to those known for solvated electrons in nonpolar and weakly polar liquids. 7,8 The identification was supported by the electron scavenging experiments. 4-6 More recently, the solvation dynamics evidenced by a blue shift of the absorption spectrum in a subnanosecond time domain was examined from experimental and theoretical viewpoints. 9-12 Meanwhile, both structure of the species referred to as “solvated electron” in ILs, and solvation and reaction dynamics of excess electrons are still far from detailed understanding. Despite the advantages of high sensitivity and high time resolution, one weak point of absorption spectroscopy for the studies of the radiation-induced intermediates is lack of structural details, in particular, under the conditions of possible overlapping of absorption spectra from different species. In other words, it does not tell much on the issue, what is “solvated electron” in ILs. An alternative tool is using EPR spectroscopy, which is particularly applicable to the studies of glassy media at low temperatures. Under the conditions of low- temperature experiment the excess electrons may be trapped and clearly detectable in the EPR spectra, even in the presence of large amounts of other paramagnetic species. It is well- known that the trapped electrons (or solvated electrons, often described as cavity-type species) in nonpolar or weakly polar glasses yield relatively narrow characteristic singlet signals in the EPR spectra. 7 However, to the best of our knowledge, such signals were not found in any irradiated glassy ILs. Recently, Shkrob et al. reported the results of EPR studies of paramagnetic intermediates produced by irradiation in a series of glassy ILs at low temperatures. 13-16 Regarding the species originating from excess electrons, detailed studies were carried out for the imidozalinium-type ILs. 14,15 The conclusion was that the electrons attached to the cations resulted in formation of the dimeric radical cations, which could be responsible for the IR absorption observed in optical spectra. However, this interpretation strongly depends on the cation nature and it may be not applicable to other type ILs (e.g., ammonium or pyrrolidinium type). In fact, these authors claimed 16 that the failure to observe the signals from physically trapped electrons might be explained by experimental factors. Bearing in mind these reasons, we have re-examined the initial stages of the radiation-induced processes in a number of ILs by a low- temperature EPR spectroscopy. Here we report the first EPR Received: June 22, 2013 Accepted: August 5, 2013 Published: August 5, 2013 Letter pubs.acs.org/JPCL © 2013 American Chemical Society 2896 dx.doi.org/10.1021/jz401292e | J. Phys. Chem. Lett. 2013, 4, 2896-2899