Time-Resolved Electron Paramagnetic Resonance Study of

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Time-Resolved Electron Paramagnetic Resonance Study of Photoexcited Fullerenes in Ionic Liquids Ivan V Kurganskii, Michael Yu. Ivanov, and Matvey V. Fedin J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b04000 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 18, 2018

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The Journal of Physical Chemistry

Time-Resolved Electron Paramagnetic Resonance Study of Photoexcited Fullerenes in Ionic Liquids

Ivan V. Kurganskii,a,b Mikhail Yu. Ivanov,a,b Matvey V. Fedin a,b* a

International Tomography Center SB RAS, 630090, Novosibirsk, Russia

b

Novosibirsk State University, 630090, Novosibirsk, Russia

Abstract Molecular-level properties of ionic liquids (ILs) draw an increasing interest. Several informative experimental approaches for investigation of nano/miscrostructuring phenomena and local viscosity/rigidity of ILs use probe molecules sensitive to microenvironment along with suitable detection techniques. In this work we for the first time investigate capabilities of photoexcited triplet fullerenes to probe local properties of ILs, with Time-Resolved Electron Paramagnetic Resonance (TR EPR) being a sensitive detection tool. We have selected C60 and its derivative phenyl-C61-butyric acid methyl ester (PCBM) as probes and ILs [Bmim]BF4 and [C10mim]BF4 as solvents. C60 and PCBM demonstrate different sensitivities to microenvironment in ILs. Spin dynamics of photoexcited C60 is strongly contributed by pseudorotation of the Jahn-Teller axis, making its use as a probe for microenvironment challenging. This behavior is strongly suppressed in PCBM, which, in addition, is more soluble in ILs than C60. The in-depth analysis of variabletemperature 2D TR EPR data shows that spectral shapes are sensitive to the restricted mobility of PCBM in ILs. In this way, the information on local environment and heterogeneities in ILs can be obtained by TR EPR. PCBM usefully complements the other spin probes previously implemented for EPR studies in ILs. It is larger in size and, in addition, allows high-sensitivity TR EPR measurements up to a room temperature, which is an important improvement for characterization of heterogeneities in room-temperature ILs.

*

[email protected] (to whom the correspondence should be addressed)

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1. Introduction Due to their outstanding physico-chemical properties, ionic liquids (ILs) find growing number of applications in various areas of modern science.1-4 Lot of applications relate to their low toxicity and thus high relevance for green chemistry. ILs are actively used in catalysis, fuel science, biotechnology, biomedicine, etc.5-8 Their properties are highly tunable, because the structures of cations and anions can be adjusted chemically in a broad range, providing means for designing optimum ILs for each particular application. In addition, in many cases cations and anions of different ILs are interchangable giving rise to a number of new combinations of structures and functionalities. One of the properties of ILs drawing an increasing interest is their ability for selforganization and formation of nanostructures.9-21 Such phenomena in many ways resemble processes occurring in biological membranes; they can also be used for drug delivery and other biomedical applications.8 Formation of heterogeneities might influence the pathways and yields of chemical reactions, including catalytic ones, and the scope of potential applications of such inherent self-organization phenomena might expand furthermore as new areas are being reached by ILs. Effects of nanostructuring in ILs were intensively investigated both theoretically and experimentally during past decade.9-21 It has been realized that polar moieties and non-polar alkyl chains of neat ILs tend to segregate in different nanodomains, resulting in a formation of nanosized micelle-like structures. When water-IL mixtures are studied, the size of heterogeneities increases to a micrometer scale and can be detected by electron microscopy.17 However, detecting heterogeneities in neat ILs on a nanometer scale is more challenging. Most of the experimental studies were performed using X-ray or neutron diffraction/scattering.9 Another type of investigations involves molecular probes dissolved in ILs, e.g. fluorescent19 or paramagnetic molecules detected by optics or Electron Paramagnetic Resonance (EPR),22-34 respectively. On the one hand, using probe molecules causes uncertainty that local nanoenvironment around the probe is exactly the same as that in “unperturbed” IL. On the other hand, most of the imaginable applications of ILs will necessary include their interactions with other molecules. Therefore, if probe molecules model some reactants/solutes in the target application, their use is more than appropriate and might provide important complementary information. Continuous Wave (CW) EPR of nitroxide spin probes was successfully used in the studies of physical properties of ILs over past ten years.22-32 In most cases, these studies addressed phase transitions and microviscosity issues in ILs monitored via mobility of the dissolved nitroxide. Conclusions on the localization of nitroxide in polar or non-polar domains were also derived, including those obtained in high-field pulse EPR experiments. Recent applications of pulse EPR 2


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have suggested the localization of nitroxides in low-density heterogeneities formed by non-polar alkyl chains of ILs.35 However, the information on nanoscale heterogeneities provided, in particular, by CW EPR was very limited. The analysis of nitroxides’ mobility in the liquid state of IL did not indicate simultaneous occurrence of different local environments, whereas in the frozen state the nitroxides were immobile. In our recent studies, an alternative approach has been developed that employs TimeResolved (TR) EPR36 of photoexcited triplet molecules.37,38 The molecules of Zntetraphenylporphyrin (ZnTPP) were dissolved in methyl-imidazolium based ILs,37 the solution was shock-frozen and investigated at variable temperatures. By means of laser excitation, ZnTPP molecules convert to transient paramagnetic (triplet) states, whose spin polarization evolves on the timescale of microseconds. The shapes and time dependences of TR EPR spectra indicated the localization of ZnTPP molecules in two microenvironments characterized by different mobility of ZnTPP. In comparison with CW EPR, TR EPR on triplet molecules appears to be more sensitive to inherent heterogeneities in ILs. Interestingly, the application of the same approach to dimethylimidazolium based ILs in the following work revealed interaction between ZnTPP and this type of ILs, resulting in noticeable changes of zero-field splitting (ZFS) tensor of the triplet state.38 Thus, there are several sources of information on heterogeneities in ILs provided by TR EPR of photoexcited triplets. At the same time, only one type of triplet probe (photoexcited ZnTPP) was implemented so far.37,38 Fullerenes represent one of the most appealing families having photoexcited triplet states with very long polarization lifetimes even at room temperatures. They have been broadly studied by different EPR techniques in the past. 39-48 The dimensions of fullerenes (C60, C70 and derivatives) are larger compared to ZnTPP and similar porphyrins, ZFS parameters of the triplet states are drastically different; in addition, decent manifestations of Jahn-Teller (pseudorotation) phenomena are well-known in fullerenes.39-43 All these factors assume that fullerenes in conjunction with TR EPR detection might have different sensitivity to local properties (in particular heterogeneities) of ILs compared to porphyrins. In this work we for the first time investigate fullerens (C60 and its derivative PCBM) in ILs using TR EPR. We discuss pros and cons of using fullerenes vs. porphyrins in ILs by this probe-based approach.

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2. Experimental Ionic

liquids

1-butyl-3-methylimidazolium

tetrafluoroborate

([Bmim]BF4),

1-decyl-3-

methylimidazolium tetrafluoroborate ([C10mim]BF4), as well as fullerenes С60 and PCBM (phenylC61-butyric acid methyl ester) were purchased from Sigma-Aldrich and used without additional purification. The samples were prepared using previously developed and validated procedure.35,37,38 Namely, the powder of C60 or PCBM was mixed with the corresponding IL and exposed to ultrasonic bath in the closed Eppendorf tube. In all cases it was not possible to dissolve all powder, therefore the precipitate was separated by centrifuging. The dissolved fraction was placed into EPR sample tube and exposed to 3-5 freeze-pump-thaw cycles. Next, the solution in the quartz tube was evacuated (10-2 Torr pressure) with simultaneous heating at 75 oC for 3 hours to reduce the amount of remaining water down to ~200 ppm (m/m) and to eliminate the remaining oxygen. The concentration of the dissolved fullerene was estimated using UV-vis optical spectra for PCBM in [C10mim]BF4, being maximal for all solutions in ILs, and was found to be ca. 6·10-5 M. EPR measurements were performed using a homemade continuous wave/TR EPR setup based on an X-band Bruker EMX spectrometer (9 GHz) equipped with an N2-cooled temperature control system (T ∼ 80−300 K). In all variable temperature experiments the sample was first shockfrozen in liquid nitrogen and then transferred into the probe and equilibrated with its temperature for several minutes. A Nd:YaG laser LOTIS-TII with the excitation wavelength of 532 nm was used. All measurements used as low microwave power as possible to reach conditions where kinetic decay is power-independent and thus is fully determined by longitudinal relaxation49; however, since the kinetic decays were extremely long at low temperatures, slight influence of the microwave power artificially shortening the observed kinetics at long times and low temperatures could not be totally excluded. All simulations of CW and TR EPR spectra were done using EasySpin,50 whereas TR EPR kinetics were simulated using the numerical solution of modified Bloch equations and/or mono- and biexponential analysis at times long compared to initial buildup of the kinetics.

3. Results and Discussion 3.1. TR EPR spectra of C60 and PCBM in ILs We have selected two fullerenes in order to investigate their sensitivity to heterogeneities and nanotructuring in ILs: C60 and its derivative PCBM (phenyl-C61-butyric acid methyl ester) shown in Scheme 1. 4


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Scheme 1. Chemical structures of C60 and PCBM probes, as well as the ILs used in this work.

C60 appears to be most suitable probe molecule due to its high symmetry and thus, potentially, high sensitivity to various possible distortions introduced by surrounding medium of IL. However, unfortunately, the solubility of C60 is poor in many ILs, as was outlined in previous work.51 For instance, we did not succeed to dissolve C60 in concentrations sufficient for TR EPR experiment in such well-known ILs as [Bmim]BF4 and [Bmim]PF6. However, the use of the cation with longer alkyl chains in [C10mim]BF4 allowed reaching feasible concentrations and measuring reliable TR EPR spectra (Fig. 1).

Figure 1. TR EPR spectra of C60 (left) and PCBM (right) measured at 90-100 K in various frozen solvents and ILs (indicated on the plot). All spectra were measured at time delays corresponding to the maximum of the kinetic curve. Red curves show simulations using the parameters described in the text.

The TR EPR spectrum of C60 in [C10mim]BF4 is noticeably different from that measured at common organic solvents toluene and o-terphenyl at similar temperatures (100 K). Previous studies 5



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of C60 in toluene have demonstrated how pseudorotation of the Jahn-Teller distortion is manifested in the spectrum shape vs. temperature.41 If at liquid helium temperatures (~4-20 K) canonical Zcomponents are clearly resolved, at higher T>60 K they become essentially smoothed out and poorly visible.41 At the same time, the spectrum of C60 in [C10mim]BF4 at 100 K clearly shows canonical Z-components due to the zero-field splitting (ZFS), being generally similar to the spectrum measured previously in frozen toluene at 4 K.41 Thus, the observation of resolved perpendicular (X,Y) and parallel (Z) components of ZFS tensor in IL [C10mim]BF4 at 100 K indicates that pseudorotation of C60 is suppressed, so that the Jahn-Teller elongation remains essentially static in the molecular frame. This observation implies that C60 molecules are decently influenced by the IL matrix, which hinders even such subtle structural changes as pseudorotation. Indeed, simulation of TR EPR spectrum of C60 in [C10mim]BF4 allowed good agreement without taking into account any pseudorotation phenomena (Fig.1, left, red trace). At the same time, TR EPR spectrum of C60 in toluene or o-terphenyl could not be reasonably simulated in the absence of pseudorotation. Note that the experimental TR EPR spectrum in toluene (Fig.1, left) is one-to-one similar to the echo-detected spectrum in toluene at 80 K reported in Fig.6 of Ref.41, which was successfully simulated assuming pseudorotation and ZFS parameters |D|=307 MHz and |E|=0.41 Remarkably, simulation of TR EPR spectrum of C60 in [C10mim]BF4 yielded |D|=278 MHz and |E|=42 MHz (zero-field populations of triplet sublevels were taken as |Pz-Px|/|Pz-Py|=0.7). The obtained rhombicity of the ZFS tensor (E≠0) is significant, and this is also evident from the spectral shape and reduced distance between absorptive and emissive X,Y peaks in [C10mim]BF4 compared to common solvents (Fig.1, left). The value of |E|=42 MHz is much higher than previously reported values |E|=15 MHz 41 and |E|=20 MHz

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(also in toluene glass) at 4-5 K, where pseudorotation is

nearly inactive. Thus, in addition to significantly suppressed pseudorotation for C60 in [C10mim]BF4, the molecular geometry becomes more distorted in this IL. Unfortunately, at temperatures higher than 100 K TR EPR intensity of C60 in [C10mim]BF4 drops down and investigation becomes problematic. This mainly owes to the low solubility of C60 in ILs and accelerated relaxation at higher temperatures. PCBM was chosen as second probe for TR EPR because its solubility in ILs was found to be much higher compared to C60 due to the presence of functionalizing phenyl and butyric acid methyl ester groups. Being an advantage for dissolvation, at the same time such functionalization lowers the inherent symmetry of the fullerene, being generally an unwanted by-product for our purposes. Indeed, TR EPR spectrum of PCBM reflects clear ZFS pattern even in common organic solvents toluene and o-terphenyl (Fig.1, right), both due to the lowering of the symmetry and the absence of isotropic pseudorotation. The spectrum can be well simulated without account of pseudorotation 6


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using |D|=293 MHz, |E|=11 MHz and |Pz-Px|/|Pz-Py|=0.7 (Fig.1, right), the values which are close to those used in literature previously.46 Some pseudorotation phenomena are still possible in the plane perpendicular to the elongated axis, but they must be considerably less efficient compared to isotropic pseudorotation in C60. TR EPR spectra of PCBM in ILs are very similar to the corresponding spectra measured in common organic solvents (Fig.1, right). However, closer inspection shows that the peaks at canonical Z-orientations are slightly broader in ILs compared to toluene and o-terphenyl. This indicates that some dynamics is enhanced in ILs compared to common solvents. In order to gain more understanding of these trends, below we study the kinetic dependences of TR EPR.

3.2. TR EPR kinetics of C60 and PCBM: general trends Figure 2 shows TR EPR kinetics of C60 and PCBM in corresponding ILs and common organic solvents.

Figure 2. TR EPR kinetics of C60 (top) and PCBM (bottom) measured at 90-100 K in various frozen solvents and ILs (indicated on the plot). Note that the curves in toluene and o-terphenyl coincide so well that it is not easy to see both. Black lines show simulations using the parameters reported in Table 1. All curves were measured at the maximum of absorption (B~343 mT).

In general, all kinetic curves shown in Figure 2 are described by biexponential decay and can be analyzed using a function I(t)=k⋅exp(-t/τslow) + (1-k)⋅exp(-t/τfast) at times longer than initial buildup of the kinetics (Table 1). Here τslow and τfast refer to the characteristic decay times of slow and fast exponentials, k is the contribution of slow exponential, and (1-k) – of fast exponential (0 ≤ k ≤ 1).

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For C60 in common solvents the contribution of fast exponential is maximum and its decay time τfast is shortest, resulting in overall shortest TR EPR decays. We assume that the reason behind this is Jahn-Teller dynamics (pseudorotation), which has been previously evidenced in glasses at similar temperatures.41 Notably, the contribution of these processes is much smaller in [C10mim]BF4 (k is larger, Table 1), being in agreement with conclusion proposed in previous section that pseudorotation of C60 is suppressed in [C10mim]BF4. The lengthening of the TR EPR decay in [C10mim]BF4 vs. common solvents is also observed for PCBM (Fig. 2, bottom). However, this effect is found only for [C10mim]BF4, whereas the decay in [Bmim]BF4 is close to that in common solvents. Comparing the absolute values given in Table 1, we find that τslow is approximately the same for both triplets (C60 and PCBM) in any solvent; most likely, this long enough time of ca. 100 microseconds is limited by a microwave power effect artificially shortening the observed kinetics. We assign systematically smaller τfast values for C60 vs. PCBM to pseudorotation, which is less efficient in PCBM (again, in agreement with the conclusions of previous section). Finally, higher k values in [C10mim]BF4 observed for both triplets might originate from suppression of pseudorotation, as well as to the effect of long nonpolar alkyl chains. Table 1. Parameters of biexponential fitting of the kinetic curves shown in Fig. 2. The experimental data were approximated by a function I(t)=k⋅exp(-t/τslow) + (1-k)⋅exp(-t/τfast). The accuracy of each value is estimated as 20%. Solvent [C10mim]BF4 toluene o-terphenyl [C10mim]BF4 [Bmim]BF4 toluene o-terphenyl

k

τslow / µs C60 0.57 90 0.29 90 0.24 100 PCBM 0.91 120 0.54 100 0.64 100 0.59 110

τfast / µs 4.7 2.6 2.7 15 13 13 15

The fact that C60 was poorly soluble in [Bmim]BF4 but could be dissolved in reasonable amounts in [C10mim]BF4 clearly emphasizes the role of longer alkyl chains of cation. Therefore, the dissolved C60 molecules are likely localized in non-polar nanodomains of the segregated alkyl chains. However, other types of localization upon solvation cannot be excluded. One anticipates the same trends for PCBM. Indeed, we observe that TR EPR kinetics of PCBM is slower in [C10mim]BF4 compared to [Bmim]BF4. Very similar observations were recently done for the nitroxides dissolved in series of ILs, where the slowest transverse relaxation (T2) was found in 8


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[C10mim]BF4, apparently due to a more distant localization of radical from the charged moieties.35 In order to analyze the possibility of one vs. two types of solvation and local environments, below we perform the detailed analysis of variable-temperature 2D TR EPR data for PCBM.

3.3. Analysis of temperature-dependent 2D TR EPR data Our previous studies on application of TR EPR in ILs used ZnTPP (Zn-tetraphenylporphyrin) molecule as a probe.37,38 This molecule exhibits biexponential TR EPR decay in glassy states of both common organic solvents and ILs. We have demonstrated that careful analysis of 2D TR EPR data (the dependence of the spectrum shape vs. time delay after laser flash) allows disentangling two contributions in ILs, whereas there is only one in common organic solvents. The two contributions in ILs were assigned to the two local environments of ZnTPP governing different mobility of a probe, and one of the environments referred to the localization in micelle-like cavities formed by alkyl chains. In this work we use fullerene probes of larger volume and potentially different interactions with ILs, therefore we attempt performing similar analysis and test possibility of disentangling 2D TR EPR data into two contributions in ILs. TR EPR kinetics of C60 and PCBM is biexponential, both in common organic solvents and in two studied ILs (Fig.2 and Table 1). The reason for biexponentiality, especially in case of C60, is unclear. The assignment of one exponential to the relaxation of polarization and another one to the triplet state decay is unlikely, because the lifetime of triplet states of fullerenes at cryogenic temperatures reach ~0.4-50 ms (for C60 and C70 at 9 K, respectively

52

), i.e. are noticeably longer

than the observed kinetics. In addition, as we will see below, the TR EPR spectrum at long time delays is also polarized. Another possibility might occur from the specifics of electron relaxation in three-level system.53 Indeed, analytical solutions for the kinetic equations indicate the possibility of biexponential relaxation with characteristic decay times differing by a factor of 3.53 However, such ratio is not at all close to that observed experimentally in our case (Table 1). Moreover, most common polarization mechanisms leading to symmetric emission/absorption or absorption/emission spectra imply that only one exponential contributes to the relaxation.53 One more possibility to explain biexponential behaviour is the involvement of low-lying excited states leading to OrbachAminov relaxation mechanism in addition to direct relaxation. However, in case of C60 no such closely-lying states are known. Thus, the origin of biexponentiality observed in TR EPR decays of C60 (as well as PCBM) is not identified. As an intermediate suggestion, we suppose that long exponential could be artificially shortened by the microwave field, that is why all τslow values in Table 1 are very close. 9



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At the same time, certain trends can be obtained by analyzing 2D TR EPR data at (relatively) short times vs. temperature. As was mentioned above, low solubility of C60 in ILs did not allow us such measurements; however, in case of PCBM we succeeded to obtain TR EPR spectra and kinetics in [C10mim]BF4 up to a room temperature. For comparison, the data in oterphenyl were also obtained and analyzed, because being a common organic solvent o-terphenyl retains its glassy state up to T~243 K.

Figure 3. TR EPR spectra (top) and kinetics (bottom) of PCBM in [C10mim]BF4 measured at 90 K (a), 200 K (b) and 295 K (c). Time delays τDAF for TR EPR spectra are indicated for each trace; green vertical lines guide the eye for the transformation of spectral shapes and peak positions (top). TR EPR kinetics are plotted for (X,Y) and Z positions of both absorptive and emissive parts of the spectrum (bottom).

Figure 3 shows the dependence of spectral shape of PCBM in [C10mim]BF4 on time delay after the laser flash (τDAF) (top panels). The build-up time of the TR EPR kinetics at all temperatures was exceedingly shorter than the characteristic decay time; therefore, for simplicity, we denote τDAF=0 as absolute maximum of each kinetic curve. The low-temperature (90 K) spectrum at τDAF=0 shows well resolved parallel (Z) and perpendicular (X,Y) components of the ZFS tensor. However, at longer τDAF delays Z-peaks decay faster relative to X,Y-peaks (Fig.3a, top), what is also reflected in faster TR EPR kinetics measured at Z-peaks (Fig.3a, bottom). Such behavior can be assigned to anisotropic relaxation caused by a modulation of ZFS, which should be more efficient for Z-components relative to X,Y-components. 10


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As the temperature gets higher, this trend for TR EPR kinetics gradually changes to the opposite one, and at T=200 K TR EPR kinetics measured at X,Y-peaks decays faster compared to that at Z-peaks (Fig.3b, bottom). In addition, new splitting (marked by green vertical lines in Fig.3b, top) appears at long τDAF delays, whereas the rest of the spectrum transforms into isotropic-like broad line. Finally, at room temperature (295 K) this new splitting dominates over the TR EPR spectrum even at short τDAF values, and spectral features due to original ZFS become strongly smoothed (Fig.3c, top). The corresponding TR EPR kinetics become closely the same at all spectral positions (Fig.3c, bottom). Overall, within T=90-295 K, the characteristic decay time of TR EPR kinetics changes by approximately a factor of 500 (see SI for the values obtained by biexponential fitting). Our previous studies employing continuous wave EPR of dissolved nitroxides have shown that local environment of nitroxides in ILs (including [C10mim]BF4) softens around ~200 K and nitroxide undergoes isotropic rotation with rotational correlation times of a few nanoseconds,37 i.e. similar to that in viscous liquid solvents. PCBM is much larger compared to TEMPO nitroxide used in that study; however, softening of the local surrounding in frozen IL should also lead to a certain enhancement of the mobility. We suppose, that exactly this is happening at T around 200 K. First, the shape of the TR EPR spectrum at long τDAF delays becomes much more isotropic compared to τDAF=0, which is to be expected when the rotation (or pseudorotation) of molecule occurs during the recording of kinetic curve. Second, TR EPR kinetics at 200 K is markedly biexponential, where the characteristic decay times of the two exponentials differ by more than an order of magnitude (SI). The ‘isotropic-like’ shape of TR EPR spectrum is manifested at long exponential tail, therefore we assume that this long component refers to a slowly rotating molecule. Interesting, the new splitting in TR EPR spectrum appears at long τDAF near 200 K. This splitting is approximately twice smaller than that between absorptive and emissive X,Y-peaks at τDAF=0. At 295 K [C10mim]BF4 is no longer found in pure glassy state, but rather in amorphous melted state, and TR EPR spectra are dominated by new splitting already at τDAF=0, getting more isotropic at longer τDAF. This observation allows us to assign the new splitting to a melted-like phase of IL, which is already present at ~200 K (in quantity ~1/2 37) and is the main state at 295 K. Thus, the observation of two-component TR EPR spectrum at 200 K confirms the presence of two microenvironments allowing for high and low mobility in IL, which was proposed by us earlier.37 To confirm the above interpretation, we performed variable-temperature TR EPR study of PCBM in common organic solvent o-terphenyl. This solvent dissolves PCBM in high enough concentrations and forms glass at T below ~243 K, which is a glass transition point.54 At T>243 K 11



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o-terphenyl glass transforms into a viscous liquid which crystallizes on a timescale of tens of minutes even at room temperatures. Figure 4 shows similar set of TR EPR data as Figure 3, but for PCBM in o-terphenyl. At 80 K we observe very similar trend in o-terphenyl to that in [C10mim]BF4 (Fig.4a and Fig.3a). However, at 200 K no additional new splittings are found, and, in general, temporal evolution of TR EPR spectrum is similar to that at 80 K (Fig.4b). Some trend for isotropization at long τDAF values is visible (Fig.4b, top), and TR EPR kinetics at X,Y and Z components get close (Fig.4b, bottom), but to much smaller extent compared to PCBM in [C10mim]BF4 at 200 K. Only at 295 K the observed trends for TR EPR spectra and kinetics of PCBM in o-terphenyl become similar to those found in [C10mim]BF4 at 200 K. Most evidently, the TR EPR kinetics at X,Y and Z components show the same trend in melted o-terphenyl at 295 K as those in [C10mim]BF4 at 200 K, which partly resides in a melted-like state.

Figure 4. TR EPR spectra (top) and kinetics (bottom) of PCBM in o-terphenyl measured at 80 K (a), 200 K (b) and 295 K (c). Time delays τDAF for TR EPR spectra are indicated for each trace. TR EPR kinetics are plotted for (X,Y) and Z positions of both absorptive and emissive parts of the spectrum (bottom).

Thus, we conclude that the transformation of TR EPR spectrum into isotropic-like shape at long τDAF delays is a signature of the enhanced mobility (rotation or pseudorotation) of PCBM, resulting from either softening or melting of the surrounding matrix. Remarkably, the isotropic-like 12


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slow-relaxing components were also found by us previously in ILs using ZnTPP molecule as a probe.37 The two probe molecules, ZnTPP and PCBM, have noticeably different dimensions and shapes, yet they provide the concurrent information on local environments in ILs. It is also noteworthy that two microenvironments of PCBM are manifested in [C10mim]BF4 at 200 K by the onset of additional splitting (Fig.3b, top). Similar conclusions were previously derived using ZnTPP.37 We thus assume that the dimensions of micelle-like cavities in [C10mim]BF4 should be larger than each of the two probes implemented. Since the streched C10 alkyl chain is larger than the diameter of C60, this proposal is quite plausible. Thus, TR EPR of fullerene derivative PCBM provides similar type of information as that of ZnTPP, and having different dimensions/shapes they can fruitfully complement each other in studies of heterogeneities in ILs.

Conclusions Investigation of heterogeneities in ILs using the dissolved probe molecules has a great potential, since it allows shedding light on dimensions and nature of such local nanoscale environments. In most types of potential applications of ILs such local environments of the solutes can eventually determine the routes of chemical and physical processes. Therefore, various types of probe molecules having different sizes, shapes, physico-chemical properties etc. should be available for each particular task. Previously, we have shown that spirocyclohexane-substituted nitroxides with pulse EPR detection35 and photoexcited porphyrins (ZnTPP) with TR EPR detection37,38 yield information on their local environments in ILs. Both of them have closely planar geometry, but the nitroxide is much smaller than ZnTPP. In this work we have for the first time tested another type of probe molecules for studying heterogeneities in ILs using TR EPR. Fullerene C60 and its derivative PCBM were suggested as test grounds, and a few imidazolium-type ILs as solvents. Fullerenes have much different dimensions compared to previously used nitroxides and ZnTPP, being ball-shaped molecules with diameter of roughly 1 nm; therefore, they were expected to usefully complement the above series of probes. We have found that C60 is not well suited for this application. First of all, its solubility in ILs was rather low; we tried several ILs of imidazolium series ([Bmim]BF4, [Bmim]PF6, and some others), and only [C10mim]BF4 with long alkyl chains could dissolve C60 in high enough concentrations for only low-temperature TR EPR measurements. In addition, we have found that inherent pseudorotation phenomena generally complicate the comparison with common solvents and interpretation of the obtained information. Although certain sensitivity to microenvironment in [C10mim]BF4 was

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manifested in suppression of pseudorotation and changes in zero-field splitting values, in general C60 is not recommended as photoexcited probe for ILs. At the same time, solubility of PCBM in ILs was found to be much higher, and pseudorotation phenomena less pronounced. We could use the main strengths of fullerenes compared to porphyrins (narrower spectrum, high polarization, slower electron relaxation) and perform TR EPR measurements in IL [C10mim]BF4 up to a room temperature with high enough signal-to-noise ratio. Similar to the previous studies with ZnTPP, the manifestations of two types of microenvironments were observed using PCBM, being characterized by relatively high and low mobility of the probe. This information could be obtained by in-depth analysis of the 2D TR EPR data, where the two contributions dominate at different time delays after the laser flash. In case of [C10mim]BF4 heterogeneities become detectable at temperatures above ~200 K, in agreement with our previous findings derived from CW EPR of nitroxides and TR EPR of ZnTPP.37 Thus, photoexcited triplet PCBM usefully complements the other smaller probes for EPR/TR EPR and can be fruifully implemented in future studies of heterogeneities in ILs.

Acknowledgement This work was supported by the Russian Science Foundation (No. 14-13-00826).

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.xxxxxxx. Extended TR EPR data for PCBM in [C10mim]BF4. Extended TR EPR data for PCBM in o-terphenyl. Simulation of TR EPR kinetics of PCBM in [C10mim]BF4 and oterphenyl. (PDF)

References (1) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev., 1999, 99, 20712083. (2) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev., 2011, 111, 3508-3576. (3) Dupont, J.; de Souza, R. F.; Suarez, P. A. Ionic Liquid (Molten Salt) Phase Organometallic Catalysis. Chem. Rev., 2002, 102, 3667-3692. (4) Special Issue on Ionic Liquids; Rogers, R. D., Voth, G. A., Eds.; Acc. Chem. Res., 2007, 40, 1077-1236. (5) Alvaro, M.; Ferrer, B.; Garcia, H.; Narayana, M. Screening of an Ionic Liquid as Medium for Photochemical Reactions. Chem. Phys. Lett., 2002, 362, 435-440. (6) Parvulescu, V. I.; Hardacre, C. Catalysis in Ionic Liquids. Chem. Rev., 2007, 107, 2615-2665.

14


Page 15 of 23 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(7) Shkrob, I. A.; Marin, T. W.; Crowell, R. A.; Wishart, J. F. Photo- and Radiation-Chemistry of Halide Anions in Ionic Liquids. J. Phys. Chem. A, 2013, 117, 5742-5756. (8) Egorova, K. S.; Gordeev, E. G.; Ananikov, V. P. Biological Activity of Ionic Liquids and Their Application in Pharmaceutics and Medicine. Chem. Rev., 2017, 117, 7132-7189. (9) Greaves, T. L.; Drummond, C. J. Solvent Nanostructure, the Solvophobic Effect and Amphiphile Self-Assembly in Ionic Liquids. Chem. Soc. Rev., 2013, 42, 1096-1120. (10) Wang, Y.; Voth, G. A. Unique Spatial Heterogeneity in Ionic Liquids. J. Amer. Chem. Soc., 2005, 127, 1219212193. (11) Lopes, J.; Padua, A. A. H. Nanostructural Organization in Ionic Liquids. J. Phys. Chem. B, 2006, 110, 3330-3335. (12) Triolo, A.; Russina, O.; Bleif, H.-J.; Di Cola, E. Nanoscale Segregation in Room Temperature Ionic Liquids. J. Phys. Chem. B, 2007, 111, 4641-4644. (13) Russina, O.; Triolo, A.; Gontrani, L.; Caminiti, R. Mesoscopic Structural Heterogeneities in Room-Temperature Ionic Liquids. J. Phys. Chem. Lett., 2012, 3, 27-33. (14) Russina, O.; Triolo, A. New Experimental Evidence Supporting the Mesoscopic Segregation Model in Room Temperature Ionic Liquids. Faraday Discuss. 2012, 154, 97-109. (15) Ji, Y.; Shi, R.; Wang, Y.; Saielli, G. Effect of the Chain Length on the Structure of Ionic Liquids: from Spatial Heterogeneity to Ionic Liquid Crystals. J. Phys. Chem. B, 2013, 117, 1104-1109. (16) Seitkalieva, M. M.; Grachev, A. A.; Egorova, K. S.; Ananikov, V. P. Nanoscale Organization of Ionic Liquids and Their Interaction with Peptides Probed by 13C NMR Spectroscopy. Tetrahedron, 2014, 70, 6075-6081. (17) Kashin, A. S.; Galkin, K. I.; Khokhlova, E. A.; Ananikov, V. P. Direct Observation of Self-Organized WaterContaining Structures in the Liquid Phase and Their Influence on 5-(Hydroxymethyl)furfural Formation in Ionic Liquids Angew. Chem. Int. Ed., 2016, 55, 2161-2166. (18) Santos, C. S.; Annapureddy, H. V. R.; Murthy, N. S.; Kashyap, H. K.; Castner, E. W., Jr.; Margulis, C. J. Temperature-Dependent Structure of Methyltributylammonium Bis(trifluoromethylsulfonyl) Amide: X ray Scattering and Simulations. J. Chem. Phys., 2011, 134, 064501. (19) Funston, A. M.; Fadeeva, T. A.; Wishart, J. F.; Castner, E. W., Jr. Fluorescence Probing of TemperatureDependent Dynamics and Friction in Ionic Liquid Local Environment. J. Phys. Chem. B, 2007, 111, 4963-4977. (20) Annapureddy, H. V. R.; Kashyap, H. K.; De Biase, P. M.; Margulis, C. J. What is the Origin of the Prepeak in the X-ray Scattering of Imidazolium-Based Room-Temperature Ionic Liquids? J. Phys. Chem. B, 2010, 114, 16838-16846. (21) Hoffmann, V.; Lahiri, A.; Borisenko, N.; Carstens, T.; Pulletikurthi, G.; Borodin, A.; Atkin, R.; Endres, F. Nanostructure of the H-terminated p-Si(111)/Ionic Liquid Interface and the Effect of Added Lithium Salt. Phys. Chem. Chem. Phys., 2017, 19, 54-58. (22) Mladenova, B. Y.; Kattnig, D. R.; Grampp, G. Room Temperature Ionic Liquids Discerned via Nitroxyl Spin Probe Dynamics. J. Phys. Chem. B, 2011, 115, 8183-8198. (23) Mladenova, B.Y.; Chumakova, N. A.; Pergushov, V. I.; Kokorin, A. I.; Grampp, G.; Kattnig, D. R. Rotational and Translational Diffusion of Spin Probes in Room-Temperature Ionic Liquids. J. Phys. Chem. B, 2012, 116, 12295-12305. (24) NoeI, M. A. M.; Allendoerfer, R. D.; Osteryoung, R. A. Solvation in Ionic Liquids: an EPR Study. J. Phys. Chem., 1992, 96, 2391-2394. (25) Kawai, A.; Hidemori, T.; Shibuya, K. Polarity of Room Temperature Ionic Liquid as Examined by EPR Spectroscopy. Chem. Lett., 2004, 33, 1464-1465. (26) Evans, R. G.; Wain, A. J.; Hardacre, C.; Compton, R. G. An Electrochemical and ESR Spectroscopic Study on the Molecular Dynamics of TEMPO in Room Temperature Ionic Liquid Solvents. ChemPhysChem, 2005, 6, 1035-1039. (27) Strehmel, V.; Laschewsky, A.; Stoesser, R.; Zehl, A.; Herrmann, W. Mobility of Spin Probes in Ionic Liquids. J. Phys. Org. Chem., 2006, 19, 318-325. (28) Pergushov, V. I.; Chumakova, N. A.; Mel’nikov, M. Ya.; Grampp, G.; Kokorin, A. I. Structural and Dynamic Microheterogeneity of Ionic Liquid. Dokl. Phys. Chem., 2009, 425, 69-72.

15



Page 16 of 23

(29) Strehmel, V.; Rexhausen, H.; Strauch, P. Influence of Imidazolium Bis(trifluoromethylsulfonylimide)s on the Rotation of Spin Probes Comprising Ionic and Hydrogen Bonding Groups. Phys. Chem. Chem. Phys., 2010, 12, 19331940. (30) Strehmel, V. Radicals in Ionic Liquids. ChemPhysChem, 2012, 13, 1649-1663. (31) Akdogan, Y.; Heller, J.; Zimmermann, H.; Hinderberger, D. The Solvation of Nitroxide Radicals in Ionic Liquids Studied by High-Field EPR Spectroscopy. Phys. Chem. Chem. Phys., 2010, 12, 7874-7882. (32) Kattnig, D. R.; Akdogan, Y.; Bauer, C.; Hinderberger, D. High-Field EPR Spectroscopic Characterization of Spin Probes in Aqueous Ionic Liquid Mixtures. Z. Phys. Chem., 2012, 226, 1363-. (33) Kattnig, D. R.; Hinderberger, D. Temperature-Dependent Formation and Transformation of Mesostructures in Water−Ionic Liquid Mixtures. Chem. Asian J. 2012, 7, 1000−1008. (34) Kattnig, D. R.; Akdogan, Y.; Lieberwirth, I.; Hinderberger, D. Spin Probing of Supramolecular Structures in 1butyl-3-methylimidazolium Tetrafluoroborate/Water Mixtures. Mol. Phys. 2013, 111, 2723−2737. (35) Ivanov, M. Yu.; Krumkacheva, O. A.; Dzuba, S. A.; Fedin, M. V. Microscopic Rigidity and Heterogeneity of Ionic Liquids Probed by Stochastic Molecular Librations of the Dissolved Nitroxides. Phys. Chem. Chem. Phys. 2017, 19, 26158-26163. (36) Forbes, M. D. E.; Jarocha, L. E.; Sim, S.; Tarasov, V. F. Time-Resolved Electron Paramagnetic Resonance Spectroscopy: History, Technique, and Application to Supramolecular and Macromolecular Chemistry. Adv. Phys. Org. Chem. 2013, 47, 1−83. (37) Ivanov, M. Yu.; Veber, S. L.; Prikhod’ko, S. A.; Adonin, N. Yu.; Bagryanskaya, E. G.; Fedin, M. V. Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets J. Phys. Chem. B, 2015, 119, 13440-13449. (38) Ivanov, M. Yu.; Prikhod’ko, S. A.; Adonin, N. Yu.; Bagryanskaya, E. G.; Fedin, M. V. Influence of C2Methylation of Imidazolium Based Ionic Liquids on Photoinduced Spin Dynamics of the Dissolved ZnTPP Studied by Time-Resolved EPR Z. Phys. Chem., 2017, 231, 391-404. (39) Closs, G. L.; Gautam, P.; Zhang, D.; Krusi, P.J.; Hill, S. A.; Wasserman, E. Steady-State and Time-Resolved Direct Detection EPR-Spectra of Fullerene Triplets in Liquid Solution and Glassy Matrices - Evidence for a Dynamic Jahn-Teller Effect in Triplet C60. J. Phys. Chem. 1992, 96, 5228-5231. (40) Terazima, M.; Sakurada, K.; Hirota, N.; Shinoharat, H.; Saito, Y. Dynamics of C70 in the Lowest Excited Triplet State at Low Temperatures. J. Phys. Chem. 1993, 97, 5447-5450. (41) Bennati, M.; Grupp, A.; Mehring, M. Electron Paramagnetic Resonance Lineshape Analysis of the Photoexcited Triplet State of C60 in Frozen Solution. Exchange Narrowing and Dynamic Jahn–Teller Effect. J. Chem. Phys. 1995, 102, 9457-9464. (42) Ceola, S.; Franco, L.; Corvaja, C. Time Resolved EPR of Excited Triplet C60 Aligned in Nematic Liquid Crystals. J. Phys. Chem. B 2004, 108, 9491-9497. (43) Uvarov, M. N.; Kulik, L. V.; Bizin, M. A.; Ivanova, V. N.; Zaripov, R. B.; Dzuba, S. A. Anisotropic Pseudorotation of the Photoexcited Triplet State of Fullerene C60 in Molecular Glasses Studied by Pulse EPR. J. Phys. Chem. A 2008, 112, 2519-2525. (44) Uvarov, M. N.; Kulik, L. V.; Dzuba, S. A. Spin Relaxation of Fullerene C70 Photoexcited Triplet in Molecular Glasses: Evidence for Onset of Fast Orientational Motions of Molecules in the Matrix near 100 K. J. Chem. Phys. 2009, 131, 144501. (45) Filidou, V.; Mamone, S.; Simmons, S.; Karlen, S. D.; Anderson, H. L.; Kay, C. W. M.; Bagno, A.; Rastrelli, F.; Murata, Y.; Komatsu, K.; et.al. Probing the C60 Triplet State Coupling to Nuclear Spins Inside and Out. Phil. Trans. R. Soc. A 2013, 371, 20120475. (46) Franco, L.; Toffoletti, A.; Ruzzi, M.; Montanari, L.; Carati, C.; Bonoldi, L.; Po, R. Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend. J. Phys. Chem. C 2013, 117, 1554-1560. (47) Matsuoka, H.; Kotaki, T.; Yamauchi, S. Influence of C60 Aggregation on Pseudorotation in the Excited Triplet State Probed by Multifrequency Time-Resolved EPR. Appl. Magn. Reson. 2014, 45, 901–909. (48) Kraffert, F.; Steyrleuthner, R.; Albrecht, S.; Neher, D.; Scharber, M. C.; Bittl, R.; Behrends, J. Charge Separation in PCPDTBT:PCBM Blends from an EPR. J. Phys. Chem. C 2014, 118, 28482−28493.

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(49) Furrer, R.; Fujara, F.; Lange, C.; Stehlik, D.; Vieth, H.M.; Vollmann W. Transient ESR Nutation Signals in Excited Aromatic Triplet States. Chem. Phys. Lett. 1980, 75, 332-339. (50) Stoll, A.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR. J. Magn. Reson. 2006, 178, 42−55. (51) Martins, S.; Fedorov, A.; Afonso, C. A. M.; Baleizão, C.; Berberan-Santos, M. N. Fluorescence of Fullerene C70 in Ionic Liquids. Chem. Phys. Lett. 2010, 497, 43–47. (52) Wasielewski, M. R.; O’Neil, M. P.; Lykke, K. R.; Pellin, M. J.; Gruen, D. M. Triplet States of Fullerenes C60 and C70: Electron Paramagnetic Resonance Spectra, Photophysics, and Electronic Structures. J. Am. Chem. Soc. 1991, 113, 2774-2776. (53) Seidel, H.; Mehring, M.; Stehlik, D. Room-Temperature Kinetics of the Photoexcited Triplet State of Acridine in Fluorene Crystals as Obtained from Electron Spin Echo Studies. Chem. Phys. Lett. 1984, 104, 552-559. (54) Greet, R. J.; Turnbull, D. Glass Transition in o‐Terphenyl. J. Chem. Phys. 1967, 46, 1243-1251. _________

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Time-Resolved Electron Paramagnetic Resonance Study of

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