Twin-peaks absorption spectra of excess electron in ionic liquids

Twin-peaks absorption spectra of excess electron in ionic liquids

Radiation Physics and Chemistry 100 (2014) 32–37 Contents lists available at ScienceDirect Radiation Physics and Chemistry journal homepage: www.els...

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Radiation Physics and Chemistry 100 (2014) 32–37

Contents lists available at ScienceDirect

Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Twin-peaks absorption spectra of excess electron in ionic liquids Raluca M. Musat a, Takafumi Kondoh b, Yoichi Yoshida b, Kenji Takahashi a,n a b

Institute of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki 567-0047, Japan

H I G H L I G H T S  þ  The solvated electron (esol ) in the ionic liquids P14 /NTf2 was investigated.  þ  Pulse radiolysis shows a twin-peak absorption spectrum for the esol in P14 /NTf2 .  The absorption spectrum of the hole was measured using scavenging experiments.

art ic l e i nf o

a b s t r a c t

Article history: Received 5 December 2013 Accepted 4 March 2014 Available online 19 March 2014

The solvated electron in room temperature ionic liquids (RTILs) has been the subject of several investigations and several reports exist on its nature and absorption spectrum. These studies concluded that the solvated electron exhibits an absorption spectrum peaking in the 1000–1400 nm region; a second absorption band peaking in the UV region has been assigned to the hole or dication radicals simultaneously formed in the system. Here we report on the fate of the excess electron in the ionic liquid þ /NTf2 using nanosecond pulse 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, P14 radiolysis. Scavenging experiments allowed us to record and disentangle the complex spectrum þ /NTf2 . We identified a bi-component absorption spectrum, due to the solvated electron, measured in P14 the absorption maxima located at 1080 nm and around 300 nm, as predicted by previous ab-initio molecular dynamics simulations for the dry excess electron. We also measured the spectra using different ionic liquids and confirmed the same feature of two absorption peaks. The present results have important implications for the characterization of solvated electrons in ionic liquids and better understanding of their structure and reactivity. & 2014 Elsevier Ltd. All rights reserved.

Keywords: Ionic liquid Pulse radiolysis Excess electron Trapped electron Dry electron Solvation dynamics

1. Introduction The successful use of Room Temperature Ionic Liquids (RTILs) in industrial and technological applications depends profoundly on the understanding of their basic chemistry, and the primary chemical reactions dominating it. Many experimental and theoretical studies focused on the solvation dynamics of an excess electron in these fascinating liquids (Kobrak, 2007; Margulis et al., 2011; Molins i Domenech et al., 2012; Wang et al., 2009, 2010; Wishart et al., 2012). The electron solvation depends on the þ Abbreviations: RTILs, Room Temperature Ionic Liquids; N1444 /NTf2 , methyltribuþ tylammonium bis (trifluoromethylsulfonyl) amide; P14 /NTf2 , 1-butyl-1þ methylpyrrolidinium bis (trifluoromethylsulfonyl) imide; P14 /CF3SO3 , N-butyl-Nþ methyl-pyrrolidinium trifluoromethanesulfonate; P14 /(C2F5)3PF3 , 1-butyl-4methyl-pyrrolidinium trifluorotris(1,1,2,2,2-pentafluoroethyl) phosphate; DEME þ /BF4 , N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate; DEMMA þ /NTf2 , N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis (trifluoromethane-sulfonyl) imide n Corresponding author. Tel.: þ 81 76 234 4828. E-mail address: [email protected] (K. Takahashi).

http://dx.doi.org/10.1016/j.radphyschem.2014.03.013 0969-806X/& 2014 Elsevier Ltd. All rights reserved.

surrounding medium, the interactions with the solvent molecules, and their collective motions (Maroncelli, 1993; Schwartz and Rossky, 1994). This transient extremely reactive species (Buxton et al., 1988; Hart, 1964) is involved in many reactions, and its characterization is essential to many important applications involving charge transfer, radical reactions and polarons (Kavarnos and Turro, 1986; Langen et al., 1995; Nguyen et al., 2011). Comprehension of the charge transfer, and the solvent-mediated charge transport mechanisms in RTILs is of great importance to energy storage, (MacFarlane et al., 2007; de Souza et al., 2003; Zakeeruddin and Grätzel, 2009) nuclear waste retreatment, (Bradley et al., 2004; Srncik et al., 1993) solar cells development (Matsumoto et al., 2001; Papageorgiou et al., 1996) and other technological applications (Jin et al., 2007; Kashyap et al., 2013; Liang et al., 2012; Samanta, 2010; Shkrob et al., 2007; Sun et al., 2012). Despite the importance of the excess electron and of the extended number of studies that focused on this topic, the excess electron solvation dynamics, spectra and its reactivity in RTILs is far from being fully understood. Pulse radiolysis and photolysis are powerful tools for the investigation of highly reactive transient

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species and fast reducing reactions. Several studies investigating the chemistry of RTILs using these techniques have been reported (Behar et al., 2002; Funston and Wishart, 2005; Grodkowski and Neta, 2002; Grodkowski et al., 2003; Katoh et al., 2007; Wishart and Neta, 2003). These studies indicate that the evolution of the electron depends on the type of ionic liquid. There has been no clear experimental evidence for the existence of solvated electron in RTILs until 2003 (Wishart and Neta, 2003). After this study several investigations have been performed. However the solvated electron has not been observed in imidazolium based ionic liquids because the excess electron reacts immediately with the imidazolium cation. In the imidazolium based ionic liquids, the radiation induced electrons rapidly attach to the imidazolium ring cation, (Behar et al., 2001; Kondoh et al., 2009) before their full solvation occurs. The opposite is observed in ammonium ionic liquids, where the electron is rapidly solvated and does not react with the cation (Katoh et al., 2007). Because of the high viscosity of RTILs, (Jacquemin et al., 2006; Rodriguez and Brennecke, 2006) it is expected that all physical and chemical processes occur slower (Ito et al., 2004; Maroncelli et al., 2012; Paul and Samanta, 2007; Vieira and Falvey, 2007; Weingärtner, 2008; Wulf et al., 2007). Thus, in the fate of the electron (solvation process), the pre-solvated (dry) electron reactions will play an important role. The electron solvation is significantly slower than in water and takes place in about 4 ns in methyltributylammonium bis (trifluoromethylsulfonyl) amide, þ N1444 /NTf2 (viscosity is 0.7 Pa s) (Wishart and Neta, 2003) and 270 ps in 1-methyl-1-butyl-pyrrolidinium bis(trifluoromethylsulþ fonyl)amide P14 /NTf2 (viscosity is 0.07 Pa s) (Wishart et al., 2012). In other words, it will take a few hundred picoseconds to a few nanoseconds to form fully solvated electrons in RTILs. Therefore there is a high probability that the excess dry electron (presolvated electron) will react before the electron solvation is completed. In this paper, we report the absorption spectrum and kinetics of the solvated electron in 1-butyl-1-methylpyrrolidinium bis (trifluorþ omethyl-sulfonyl) imide, P14 /NTf2 based on nanosecond pulse

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þ radiolysis studies. The solvated electron absorption spectrum in P14 / NTf2 exhibits two peaks, one in the NIR and one in the UV region. The absorption in the UV region is overlapped with the absorption peak of the hole and, using electron and hole scavengers, it was possible to separate the contributions arising from these two species. For comparison, we also investigated the spectrum of the solvated electron in three other ionic liquids: N-butyl-N-methyl-pyrrolidinium trifluoroþ/ methanesulfonate, P14 CF3SO3 , 1-butyl-4-methyl-pyrrolidinium triþ fluorotris(1,1,2,2,2-pentafluoroethyl) phosphate, P14 /(C2F5)3PF3 , and N,N,diethyl-N-methyl-N-(2-methoxyethyl) ammonium tetrafluoroborate, DEME þ /BF4 , and observed the same behavior. Our results confirm the theoretical predictions of Margulis et al. (2011) about the general trends to be expected at short time after the injection of an excess electron in RTILs. According to these molecular dynamics simulations, the transient UV spectrum of the excess dry electron is characterized by two bands, a broad band at low energies (above 1000 nm) due to transitions between electronic states with similar character on ions of the same class but in different locations of the liquid, and another weaker band at high energies (around 400 nm) due to transitions in which the electron is promoted to electronic states of different character, in some cases on counter ions. Our results recreate a bimodal absorption spectrum of the solvated electron at long time delays, when the solvation process has completed, although the exact nature of the origin of these transitions deserves further scrutiny. Our results confirm the complex nature of the solvated electron in RTILs, and underline the importance of further investigations of this specie for a full understanding of its chemistry and of the stability of RTILs in radiation filled environments.

2. Materials and methods The ionic liquids used in this study (Fig. 1) were purchased from Kanto Chemical. Co. Ltd. and used without further purification. The samples were dried and degased at 80 1C for 12 h in a hightemperature vacuum desiccator in order to remove any organic volatile impurities and moisture. The samples were subsequently

þ þ þ Fig. 1. Molecular structure of the ionic liquids used in this study: (a) P14 /NTf2 , (b) P14 /CF3SO3 , (c) P14 /(C2F5)3PF3 and (d) DEME þ /BF4 .

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bubbled with Ar, H2 or N2O gas for 30 minutes. The sample cells are 1 cm path-length quartz cells. The formation and decay of the solvated electron was followed using the nanosecond pulse radiolysis system on the L-band linear accelerator (LINAC) at the Institute of Scientific and Industrial Research (ISIR) Osaka University. The excitation electron beam had an energy of 28 MeV and duration of 8 ns. A Xe flash lamp was used as the analysis light. A monochromator with cut filters was used to select the analyzing light wavelength. The probing light was passed through a monochromator and detected using Si or InGaAs photodiodes. The formation of the solvated electrons and reactions with electron scavengers were measured. The experimental set-up is presented elsewhere (Asano et al., 2008). All experiments were conducted at room temperature. The typical dose in these experiments was evaluated from similar experiments performed on bulk water and measurements of the solvated electron production at 720 nm. In the calculation we assumed that the molar extinction coefficient of the hydrated electron at 720 nm is 18600 L mol  1 cm  1,(Fielden and Hart, 1967) and G-value of hydrated electron is 2.7  10  7 mol J  1 (Mozumder, 1999). The estimated absorbed dose per pulse in water was 108 Gy. Considering the high doses employed in this type of experiments, it is very important to avoid any cumulative effects due to degradation of the sample by the electron beam's ionizing radiation that will affect the observed kinetics. Contributions from byproducts might lead to an increase in the decay rate of the transient species present in solution. Our experiments are single shot experiments and the sample is renewed for each wavelength during the experimental data acquisition, and these degradation effects are thus avoided.

3. Results and discussion

that the electron solvation occurs rapidly. We have tried to fit this set of data using the Gaussian function. However this fit lacks accuracy, and we had to try a different fit; more details about the fitting procedure are presented in the Supporting Information. It is known that the shape of the absorption spectrum of the solvated electron in different solvents is asymmetrical, generally with the high energy tail differing from one type of solvent to another (Jou and Freeman, 1977; Siano, 1969). For example the low-energy side might be determined mainly by the distribution of ground state (trap) energies, while the high-energy side might contain an overlapping series of excited states and a continuum. We have therefore globally fitted the data using two overlapping functions: a log-normal curve for the low energy lying band and a Gaussian curve for the high energy band. As seen in Fig. 2, the log-normal function appears to fit the data very well. Thus, we were able to confirm the location of the low energy band from the fits of the data, at 1070 nm. Details about the fitting procedure are available in the Supporting Information. The reaction kinetics of the solvated electron was determined from the decay of its optical absorption at 900 nm. The decay þ profile of the pulse radiolysis of P14 /NTf2 saturated with Ar observed at 900 nm and 300 nm are presented in Fig. 3(a). The decay of the optical absorption at 900 nm can be fitted by a single exponential function with a time constant of 280 ns, similar to the þ lifetime of the solvated electron in N1444 /NTf2 , (Wishart and Neta, 2003) and in trimethylpropylammonium bis (trifluoromethylsulþ fonyl)imid, N1113 /NTf2 (Kondoh et al., 2009). In the UV region, we observe a broad absorption band and the decay in this region is bi-exponential with short-lived component with a lifetime of 270 ns and a long-lived component (lifetime longer than 450 ns). This bi-exponential character of the band indicates that there are two chemical species absorbing in this region, as has been previously suggested by Wishart and Neta (2003).

þ The passage of high-energy electron beam through P14 /NTf2 induces excitation and ionization events that create a variety of initial products: electrons, ions, neutral radicals, etc:

P14 NTf 2

e–sol ; Hþ ; ðP14 Þ2þ ; ðNTf 2 Þ

ð1Þ

The solvated electrons may undergo geminate recombination, or reductive reactions with the solvent or the solute, while the radical dications may oxidize the solute or deprotonate or fragment. þ The measured absorption spectrum of Ar bubbled P14 /NTf2 is presented in Fig. 2. No spectral shift was observed in the spectrum up to 1 μs after the passage of the electron beam, which means

þ Fig. 2. Absorption spectrum measured in an Ar bubbled P14 /NTf2 sample. The experimental data are shown in markers, along with the fitted curves using two overlapping Gaussian functions (green line) and using a log-normal function for the low-energy band overlapped with a Gaussian function for the high energy band (orange line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. Experimental (markers) and fitted (lines) optical density decays at 300 nm þ (black) and 900 nm (red) in P14 /NTf2 saturated with Ar (a), N2O (b) and H2 (c). The þ decays at 900 nm in Ar saturated P14 /NTf2 (a) show the electron follows the firstorder process with a lifetime of 280 ns. Scavenging experiments (b), (c) confirm that the investigated species is indeed the solvated electron. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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By analogy to similar UV bands associated with the presence of the solvated electron in water (Nielsen et al., 1976) and amines, (Belloni et al., 1979) the authors assign the apparent absorption þ feature observed in N1444 /NTf2 around 350 nm to red-shifted þ transitions of the perturbed ionic solvent N1444 /NTf2 in the solvation shell surrounding the electron. The short component of þ the species absorbing in the UV that we observe in P14 /NTf2 has a lifetime similar to the one of the species absorbing in the NIR region, which may indicate that we are looking at the same species. For a proper assignment of these species, we performed scavenging experiments, bubbling the samples with N2O and H2. From water radiolysis studies, N2O is known to efficiently capture the solvated electron according to the following scheme: (Laverne and Pimblott, 1993; Takahashi et al., 2004)  esol þ N2 O-N2 O 

k ¼ 9:1  109 M  1 ðin waterÞ

N2 O  -N2 þ O  In Fig. 3(b), it can be seen that the absorption signal at 900 nm decreased completely within 20 ns due to the reaction of the solvated electron with N2O (lifetime of 13 ns). On the other hand, the decay kinetics at 300 nm shows fast and slow components dynamics and can be fitted by a bi-exponential function. The fast lifetime (13 ns) is the same as that determined from the decay kinetics of the solvated electron observed at 900 nm. Which means that in the UV region, the measured absorption if due not only to the hole but also due to the electron. To confirm the spectrum of the solvated electron in NIR and UV region, we also measured the spectrum of a N2O, H2 and Ar saturated samples. Fig. 4(a) shows the transient absorption spectrum in visible to UV region measured 14 ns after the passage of þ the electron pulse in the Ar saturated P14 /NTf2 . The small absorption band that may peak around 300 nm was previously assigned to the electron-deficient species – the neutral radical [NTf2], (Wishart, 2003) or the oxidized form of the cation (P14) þ 2 – produced by the detachment of the electron. As seen in Fig. 4(b), the disappearance of the visible to NIR band is observed upon N2O addition, indicating that we have correctly assigned this absorption to the solvated electron. At the same time, in Fig. 4(b) an increase of the band in the deeper UV region is observed when saturating the sample with N2O, which corresponds to an increase in the yield of the holes because the geminate recombination of the solvated electron–hole is suppressed. This increase overlaps with the decrease of the band around 300 nm, corresponding to N2O scavenging of electrons, making it difficult to discern it in Fig. 4(b). Fig. 4(b) also shows the absorption spectrum measured when the sample was saturated with H2. H2 is used as a hole scavenger, as it is known to undergo abstraction or addition reactions with radicals. Upon addition of the hole scavenger, we should be able to suppress the geminate recombination reaction between the electron and hole, and increase its lifetime. Indeed, we observe an increase in the yield of the solvated electron – optical density at 900 nm by 10%, and obtain lifetimes of 365 ns. A residual longlived contribution to the absorbance at 300 nm is observed, which means that we have not succeeded in scavenging all of the holes in þ this region because the limited solubility of H2 in P14 /NTf2 under the ambient conditions, and more experiments with better suited hole scavengers are required. Similar results were observed for the other RTILs investigated. þ þ Fig. 5 shows the measured absorbance spectra in P14 /CF3SO3 , P14 / (C2F5)3PF3 and DEME þ /BF4 . The Ar saturated samples display a two peak absorption spectrum and scavenging experiments using N2O allowed us to identify the spectrum of the solvated electron. As þ observed in P14 /NTf2 , and using the same fitting procedure, the solvated electron in these ionic liquids displays the same twin-peaks

þ Fig. 4. (a) Experimental and fitted absorbance spectra in P14 /NTf2 (black). Deconvoluted absorbance spectra of the electron (dashed line) and hole (dotted þ line) in P14 /NTf-2 extracted from scavenging experiments. The individual spectra have been offset for clarity; (b) Experimental (markers) and fitted (lines) absorþ bance spectra in P14 /NTf2 saturated with Ar (black), N2O (green) and H2 (orange) at 14 ns after the passage of ionizing radiation for Ar and H2, and 50 ns after the passage of ionizing radiation for N2O. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

features. Hence, the behavior of the solvated electron is independent on the nature of the anion. From these investigations we can speculate that the absorption due to the hole comes from cation related species: þþ P14 or Pn14. However, further studies are required to see what the role of the cation in the excess charge solvation process. Our result is in agreement with Margulis' theoretical calculation (Margulis et al., 2011) predicting that the dry excess electron displays a two peaks in the UV to NIR absorption spectrum. According to them, the broad band at low energies corresponds to what they call “translational electronic transitions”: upon photo-excitation in this energy range, the final state of the electron is almost indistinguishable from the original except for a spatial translation. The UV band corresponds to excitations that promote the electron to a state qualitatively different in character, sometimes localized on ions of the opposite charge. According to Margulis, the excess electrons' pattern of localization is defined by the relative alignment of cationic and anionic HOMO/LUMO gaps: the dry electrons are localized sometimes on anions and sometimes on cations, depending on the chemical nature of the ions. Our result indicate a similar bimodal behavior, however considering the different time delays at which our data was acquired and how the structure of the excess electron is changed upon solvation, the origin of these two absorption peaks requires further investigations. Considering the long lifetime of the solvated

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CTTS spectra (halides, pseudo-halides, etc.). These studies are extremely difficult because of the existence of several species absorbing in the same UV domain, their molar extinction coefficients are small and an accurate knowledge of their value is required to disentangle the real hydrated electron UV band. Despite their difficulty these studies deserve more scrutiny to acquire a better understanding of the structure of the solvated electron and for a better estimation of the interactions involved in a solvation shell.

4. Conclusions

Fig. 5. Experimental (markers) and fitted (line) absorbance spectra in three þ þ different RTILs: (a) P14 /CF3SO3 , (b) P14 /(C2F5)3PF3 , (c) DEME þ /BF4 . The black lines correspond to the measured spectrum when the samples saturated with Ar and green to N2O saturated samples at 14 ns, 50 ns respectively, after the passage of ionizing radiation. The absorbance spectrum of the N2O saturated DEME þ /BF4 sample presented in (c) is acquired at 80 ns after the passage of ionizing radiation.

þ electron in P14 /NTf2 , we can conclude that it does not react with the cation, as is the case in imidazolium RTILs (Kondoh et al., 2009) and may localize near the anion NTf2 . þ The solvated electron in P14 /NTf2 has a transient absorption spectrum with a broad peak at 1080 nm, and a smaller component in the UV region, around 320 nm. The NIR peak is located at wavelengths significantly longer than the absorbance peak measured in polar solvents like methanol, 1-propanol (633 nm), (Jou and Freeman, 1977) or propane-1,2,3-triol (530 nm), (Lin et al., 2009) which means that the electron is weakly solvated/trapped; however it is located at higher energies than in methyltributylamþ monium bis(trifluoromethylsulfonyl)imide, N1444 /NTf2 , (1410 nm), (Wishart and Neta, 2003) and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, DEMMA þ / NTf2 , (1100 nm), (Kondoh et al., 2009) indicating a stronger þ solvation in P14 /NTf2 . Recently, we have become aware of a few studies on the solvated electron in aqueous solutions and ammonia that seem to indicate the existence of a second absorption band in the spectrum of the solvated electron in water and ammonia, below 200 nm and 180 nm respectively. Initially, Hart et al. showed that the UV absorbance in pulse irradiated water contained, in addition to the OH, H2O2 and H bands, a supplementary band with a maximum below 200 nm that should be assigned to the hydrated electron (Nielsen et al., 1976, 1969). This band was to be interpreted as an energy transition of those molecules that are involved in the solvation shell and are perturbed by the close presence of the delocalized electron charge, through a partial negative charge  transfer to the eaq . A decade later, Belloni et al. (1979, 1976) confirmed the existence of an UV band correlated and proportional to the broad IR band of metal-ammonia solutions, and that could be considered the red-shifted edge of the solvent band perturbed by the electron charge. The same UV weak band should therefore exist as well for all other solvated electrons and other

In this study we have investigated the solvated electron absorbance spectrum in four ionic liquids. Our findings confirm the bimodal behavior of the solvated electron in ionic liquids. Experimental investigations of Wishart and Neta suggested that þ the UV feature apparent in the ionic liquids N1444 /NTf2 is arising from red-shifted transitions of the perturbed ionic solvent in the solvation shell surrounding the electron. Previous computational predictions of Margulis et al. and experimental data suggest a double-peaks absorbance spectrum of the dry excess electron, with excitations in the high energy and low energy areas leading to translational and electronic transitions of the electron, respectively. Considering the long time delays at which we measure the þ transient spectrum in P14 /NTf2 , long after the solvation process is complete, it is surprising to find a similar bimodal behavior for the solvated electron, and the origin of the transitions giving rise to the two peaks requires further investigation. Investigations in four different ionic liquids show virtually no influence of the anion on the absorption spectrum of the solvated electron in RTILs. The solvated electron shows a relatively long lifetime, similar to þ what has been seen in N1444 /NTf2  . This long lifetime indicated that the electron does not readily react with the parent cation, and is probably localized on the anion. Scavenging experiments show an absorption spectrum for the hole peaking in the UV region, having a very long lifetime, outside of the probed time window. The exact nature of the hole still requires investigation.

Aknowledgements This work was supported by JSPS KAKENHI Grant numbers 23560918, 22110505 and 17073010. Raluca Musat would like to acknowledge support from the Japan Society for the Promotion of Science (Grant number PE12047) through the Postdoctoral Fellowship for North American and European Researchers.

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