Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids

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Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids Kamal B. Dhungana, Fei Wu, and Claudio Javier Margulis J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07499 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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The Journal of Physical Chemistry

Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids Kamal B. Dhungana, Fei Wu, and Claudio J. Margulis∗ Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA E-mail: [email protected]

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Abstract The study of ionic liquids that may be compatible with the type of radiation chemistry events occurring in nuclear separation processes is a topic of high current interest. In this article we focus on two ionic liquids based on the benzylpyridinium cation. This cation has been proposed as able to capture either an excess electron or hole without undergoing fast dissociation. Shkrob, Wishart and collaborators (J. Phys. Chem. B., 117, 46, 2013) have indicated that the stabilization is likely in the form of dimers in solution with the excess electron localized on adjacent pyridinium rings and the excess hole on phenyl rings. Our first-principles dynamical studies support these ideas but present a more nuanced view of the time-dependent behavior that is likely to occur at short time for systems at roomtemperature.

Introduction Ionic liquids (ILs) have received significant attention over the past two decades 1–7 for different types of practical applications including radiation chemistry. One area of particular interest is their use in nuclear separation techniques 8–10 as a replacement for other conventional solvents or diluents. In this area, the goal is to find specific combinations of cations and anions that can resist radiation damage and that have the proper polarity characteristics to play well in combination with other process components. Pioneering work on excess electrons and holes in ILs has been carried out independently or in collaboration by the groups of Shkrob, Wishart, Takahashi, as well as others. 11–34 Most relevant to our current study is reference 11 on the radiation resistance of cations based on the benzylpyridinium family. Radiation can cause damage both to cation and anion subcomponents, but anions are often reported as more susceptible. 16 Nonetheless, Wishart, Shkrob and collaborators have described several anions which appear to be quite robust and do not easily undergo rup2 ACS Paragon Plus Environment

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ture including saccharinate, 15 dicyanamide, 1,1,2,3,3,-pentacyano-propenide, and 1,2,3,4,5pentacyanocyclopentadienide. 16 Shkrob and colleagues 11 have also suggested that the 1benzyl-pyridinium (BzPy+ ) family of cations is unique in that both excess positive and negative charge corresponding to oxidized and reduced states can safely localize over BzPy+ dimers without fragmentation. The excess electron was predicted to localize on pyridinium moieties forming what the authors describe as a π-electron sandwich dimer preventing the elimination of benzyl arms; whereas the excess hole was predicted to form a dimer that prevents fragmentation by localizing on phenyl moieties. 11 Many of these conclusions were derived in glassy matrices at low temperature or via gas phase DFT calculations; however, we will show that DFT calculations in the condensed phase for the most part support these findings but present a more dynamical picture of localization and delocalization. To connect with prior experimental studies we focus on the BzPy+ /NTF− 2 system. Although we are unaware of prior studies, we also focus on the BzPy+ /DCA− system because dicyanamide has been shown to be robust upon radiation exposure and tends to form liquids that are of low viscosity which have generated considerable interest for energy applications.

Computational Methods The results presented in the first part of this article stem from ab-initio molecular dynam+ − ics (md) simulations for an excess electron/hole in liquid BzPy+ /NTF− 2 and BzPy /DCA

using the SIESTA package. 35–37 Except where otherwise noted, we considered periodically + − replicated boxes of 8 ion pairs (320 and 240 atoms for BzPy+ /NTF− 2 and BzPy /DCA

respectively). The Initial configuration for our ab-initio md simulations were derived from classical md simulations (See Fig. 1). Classical md simulations were performed using the GROMACS simulation package 38,39 and available OPLS-based force field parameters taken from references 40, 41, 42, 43, 44, 45 and 46. Because charges and the dihedral angle coefficients for CA-CA-CT-N and CA-N-CT-CA in the cation were not available in the liter-



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ature, we computed the charges using the CHelpG protocol implemented in the Gaussian09 package 47 (G09) at the MP2 level of theory using the cc-PVTZ basis set and we fitted those dihedral angles using the protocol suggested in reference 40 (see Figs. S1 and S2 in the SI). The full set of charges, and the fitted dihedral coefficients for the cation are provided in the Supporting Information (Fig. S1, Tables S1 and S2). The computed densities from classi+ − 3 cal md with 8 ion pairs for BzPy+ /NTF− 2 and BzPy /DCA are 1.526 and 1.128 gm/cm

at 300 K respectively. There is no experimental value to contrast against in the case of 3 48 BzPy+ /DCA− , but the experimental value for BzPy+ /NTF− in 2 is 1.52 gm/cm at 298 K,

excellent agreement with our findings. Non-relativistic Troullier-Martins pseudopotentials and the Perdew, Burke, and Ernzerhof (PBE) implementation of the generalized gradient approximation (GGA) were used in our DFT calculations. The ATOM program, part of the SIESTA package, was used to generate the pseudopotentials. We used the double-ζ plus polarization basis set with an energy shift of 25 meV, and as in prior studies, 27,30 only the Γ point was used to sample the Brillouin zone in reciprocal space. Unless noted otherwise, the Mesh cutoff and the electronic temperature were set to 300 Ry and 100 K respectively. Transferring the classically equilibrated system onto the DFT potential must be done carefully as preferred bond lengths and angles may be slightly different; for this, we followed the protocol described in our previous publication. 27 DFT calculations in the condensed phase are expensive and long simulations on the order of 100 ps are simply prohibitive. With the intention of exploring the condensed phase in the presence of an excess electron or hole for time scales beyond a few picoseconds, we have used the density-functional tight-binding methodology (DFTB). 49,50 For this we used the dftb+ simulation package 51 were DFTB3 –the latest development of the original DFTB– is implemented which better accounts for possible hydrogen bonding. 52,53 The 3ob set of parameters, 54–56 specific for DFTB3, was used for our simulations in conjunction with the Grimme corrections to van der Waals interactions. 57,58 Consistent with our DFT calcula-


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tions, only the Γ point was considered for sampling the Brillouin zone and the electronic temperature was set to 100 K.

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Figure 1: Classically equilibrated simulation box containing 8 periodically replicated ion + − pairs for (a) BzPy+ /NTF− 2 and (b) BzPy /DCA . In validating the semi-empirical method against full DFT methodology we found that for the IL based on DCA− , the semi-empirical methodology reproduced results reasonably well including structural features and the density of state. Instead, we have less confidence in the DFTB parameters for the S atom in the case of NTF− 2 . Specificaly, we optimized the cation (BzPy+ ) and the anions (DCA− and NTF− 2 ) in the gas phase using three different methods, MP2 with the cc-PVTZ basis set in G09, DFT using SIESTA and DFTB using dftb+. In general, bond lengths involving atoms in BzPy+ and DCA− (Table S3 and S5) obtained from all three methods are found to match reasonably well. But Table S4 shows that this is not the case for the N-S bond length which is significantly overestimated by DFTB. When computing cationic and anionic projected density of states (PDOS) in the condensed phase + − for BzPy+ /NTF− 2 and BzPy /DCA (see Fig. S6 in the SI) in comparison with DFT results

(see Figs. 2a and b) we found those to match reasonably well in the case of BzPy+ /DCA− − + but not in the case of BzPy+ /NTF− 2 . The PDOS for BzPy /NTF2 obtained using DFT

shows that both HOMO and LUMO bands are for the most part cationic in nature with some anionic contribution to the HOMO band. Instead, DFTB predicts that the HOMO band is of mixed cationic/anionic nature with the highest occupied portion being anionic (see Fig. 5 ACS Paragon Plus Environment


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S6 a). DFTB therefore predicts that an excess hole should distribute on anions instead of cations which is opposite to our DFT results as well as predictions in reference 11. Based on the significant N-S bond length mismatch and the band structure differences, we suspect that the Slater-Koster parameters for S based on DFTB3 may still require improvement to adequately describe the electronic structure of NTF− 2.

Results For a neat IL, the cationic and anionic projected density of states provides an accurate indication of whether a dry excess electron or hole will initially delocalize over cationic, anionic or mixed states. By “dry”, as opposed to solvated, we refer to a state in which excess kinetic energy has been mostly dissipated but the local solvent environment has not yet had any time to respond to the presence of the excess positive or negative charge. Figures 2a + − and b show the calculated cationic and anionic PDOS for BzPy+ /NTF− 2 and BzPy /DCA

respectively. Both HOMO and LUMO bands are majorly contributed by cations in the case of BzPy+ /NTF− 2 , but smaller anionic contribution exists and the extent of it fluctuates along simulation. Instead, the HOMO band is anionic and the LUMO band cationic in the case of BzPy+ /DCA− . Without further analysis this implies that for BzPy+ /NTF− 2 , both the dry excess electron and hole should initially delocalize over BzPy+ . Instead, in the case of BzPy+ /DCA− , a dry excess hole should delocalize over the anions whereas a dry excess electron over cations. We insist on the word “delocalize” because we have consistenly found in prior studies 27,30 that any form of localization requires some degree of relaxation response to the presence of the excess positive or negative charge and this takes time. In our calculations, an excess electron (hole) is in the spin-up (spin-down) channel. We therefore represent singly occupied states with an excess electron or hole by SOMO↑ or + − SOMO↓ respectively. For BzPy+ /NTF− at 0 fs before any possible 2 and BzPy /DCA

solvent reorganization, Figs. 3(a), (b), (c) and (d) show SOMO↑ and SOMO↓ in the presence


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of an excess electron or excess hole. These figures show that in the case of BzPy+ /NTF− 2, the dry excess electron and hole are for the most part distributed among different cations, whereas in the case of BzPy+ /DCA− , the excess electron and hole are distributed among different cations and anions respectively. These delocalization patterns are consistent with the PDOS plotted in Fig. 2. Perhaps the most interesting initial finding is that consistent with prior predictions, 11 in the case of BzPy+ /NTF− 2 , the excess electron delocalizes over pyridinium rings whereas the excess hole delocalizes over phenyl rings. We also find that a dry excess electron in the case of BzPy+ /DCA− delocalizes over pyridinium rings.

Figure 2: Cationic and anionic projected density of states for neat (a) BzPy+ /NTF− 2 and + − (b) BzPy /DCA . The Fermi energy is set to zero.

Dynamics in the several picoseconds regime As in our prior studies, 27,30 we follow the early time evolution of excess electron and hole states in these two liquids by computing time-dependent Mulliken charges of the ions as a function of time. Whereas Mulliken charges are not the most precise representation of individual ion charges, they prove very useful in identifying which ions appear to have excess negative or positive charge. Mulliken charges in our DFT calculations always show noninteger values; this is expected for non-minimal and non-orthogonal basis sets. 27,30 Instead, in our DFTB calculations where basis sets are much smaller, values are closer to unity (vide infra). + − For each ion in BzPy+ /NTF− 2 and BzPy /DCA , Fig. 4 shows the time evolution of the



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Figure 3: For BzPy+ /NTF− 2 , (a) SOMO↑ occupied by an excess electron and (b) SOMO↓ occupied by an excess hole. For BzPy+ /DCA− , (c) SOMO↑ occupied by an excess electron and (d) SOMO↓ occupied by an excess hole. The blue bounding box represents the unit cell for the periodic system and everything outside the unit cell is a periodic image.


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Mulliken charge when either an excess electron or an excess hole is present in the system. For example, in the case of BzPy+ /NTF− 2 when in the presence of an excess electron, Fig. 4a shows that all anions have similar negative Mulliken charge. The opposite is true for cations where fluctuations in the charge across ions is large. This is consistent with the excess electron being captured by the cations. At very short time, it would appear that the excess charge is delocalized (the dry electron state), but already on a subpicosecond time scale, a small subset of cations start to share the largest portion of the excess negative charge. The smaller group of two or more cations that share this largest portion of the excess negative charge is not always the same and we observe localization and delocalization events. Figs 5a and b, exemplify this delocalization and localization often on pairs of pyridinium rings. Other examples of periods dominated by excess electron localization on what apparently are cationic pairs are shown in Fig. S3 of the SI. Such structures are very reminiscent of the predictions by Shkrob and coworkers in reference 11. An important difference to take into account is that our study is at room temperature and on the picosecond time scale whereas the EPR studies in reference 11 are done on a frozen or supercooled matrix at what for all purposes is infinite time; the accompanying first principles calculations in reference 11 are for a static system and in the gas phase. At room temperature, for a dynamical system in the liquid-state, deviations from the idealized “sandwich-like” dimeric state are of course expected. In the case of the excess hole, Fig. 4b highlights that initial delocalization evolves into localization on a smaller subset of cations. Here also Figs. 5c and d show localization and delocalization of SOMO↓. Fig. 5c in particular is somewhat reminiscent of the “sandwichlike” dimeric state predicted by Shkrob, Wishart and coworkers. 11 Even though the hole is for the most part on phenyl rings on the cations, Fig. 4b shows that during brief subpicosecond intervals some of the excess positive charge can leak onto NTF− 2 ; we see this for example from Fig. S4 in the SI. On the time scale of our DFT calculations this behavior is transient with the excess hole returning to the cations.



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Similarly, the excess electron in the case of BzPy+ /DCA− localizes and delocalizes over cations (Fig. 4c), whereas the excess hole localizes and delocalizes over DCA− (Fig. 4d). For BzPy+ /DCA− at two different times, SOMO↑ for the excess electron and SOMO↓ for the excess hole are shown in Fig. S5 of the SI. The excess electron is always found on pyridinium rings whereas the excess hole is always localized on DCA− anions. Interestingly the localized hole appears to occur in the form of a dimer radical anion.

+ − Figure 4: Mulliken charges for all ions in BzPy+ /NTF− 2 and BzPy /DCA in the presence of either an excess electron or hole as a function of time. (a) and (b) are for the excess electron and hole in BzPy+ /NTF− 2 respectively. (c) and (d) are for the excess electron and + − hole in BzPy /DCA respectively.

The Optical Spectrum of BzPy+ /NTF− 2 Reference 11, provides the pulse radiolysis transient absorption spectrum of BzPy+ /NTF− 2 at approximately 50 ns after pulse excitation. The spectrum is characterized by a broad and intense band between 600 nm and 1500 nm as well as transitions that grow in intensity below 500 nm. Following the same protocol detailed in reference 30 and to make contact with experiments, we also computed from snapshots of our ab-initio md simulations corre10 ACS Paragon Plus Environment

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Figure 5: SOMO↑ (excess electron) and SOMO↓ (excess hole) in BzPy+ /NTF− 2 at different times: (a) SOMO↑ at 2500 fs, (b) SOMO↑ at 5100 fs (c) SOMO↓ at 2000 fs and (d) SOMO↓ at 3600 fs.



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sponding optical spectra. Before directly comparing experimental and computational results, several caveats must be brought to the attention of the readers. First, the experimental spectrum results from the absorption of an ensemble of excess electrons and holes in the same pot, whereas we can only produce from our trajectories early transient spectra either of a single excess electron or hole. Hence, our spectra lack statistical averaging as well as possible electron-hole interactions. Second, the computational spectra are at a few picoseconds and should be different from the experimental spectrum at 50 ns where significant solvent reorganization and solvation will have taken place. Last, our spectra do not include the contribution from cavity electrons or perfectly overlapping radical dimers that could potentially form at 50 ns due to relaxation and solvation. With all these caveats in mind, for selected snapshots along our trajectories with an excess electron or hole, Fig. 6 shows the computed optical spectra of the spin channel containing the excess electron or hole respectively. As can be gleaned from this figure, at all times the excess electron and hole systems have large absorption below 500 nm. This is because in our calculations, around this energy is when the liquid band-gap can be crossed and a myriad transitions –not just from the excess hole or electron– but also from the bulk electrons in the liquid become accessible. The most significant feature in the transient spectrum at 50 ns in reference 11 between 600 nm and 1500 nm is also present in our simulations –most prominently for the excess hole– . In some cases, we observe the same types of transitions occurring even at lower energies below 1500 nm. Transitions of the excess electron in the 600 nm to 1500 nm region also occur but in our study they are of low intensity. However, we do not discard the possibility that at longer times not accessible to our study, the presence of cavity electrons or perfectly formed “sandwich-like” dimeric structures may enhance the intensity of excess electron transitions. We notice that several but not all snapshots along the excess hole simulation give rise to this large intensity absorption peak. This is to be expected as the real transient spectrum is an ensemble average over the transitions of many holes but we only simulate one. Before


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discussing the type of transitions that for the excess hole give rise to high intensity contributions to this peak, the reader is asked to first consider Fig. 2 (a). SOMO↓ is the last orbital before crossing the band gap and transitions of the excess hole are to orbitals of lesser energy (equivalently one can think of transitions of lower energy electrons into SOMO↓). We see from Fig. 2 (a) that at energies consistent with this peak, transitions can occur either to cationic or anionic states and we observe both. Some of the high intensity transitions are due to strongly correlated changes in the orbitals of adjacent benzyl rings forming a radical cation dimer. For example, the hole can transition into a state in which a wave function node across rings disappears. Other cases include partial hole transfer onto close-by NTF− 2. Figure 7 shows one such example where the transition involves both partial hole transfer to NTF− 2 as well as the loss of a wave function node in the space between rings of the radical cation dimer. Our calculations predict yet another broad peak in the IR at lower energies (around 10,000 nm). This energy range which was not discussed in reference 11 may be the object of future studies.

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Figure 6: At different times along simulation, figure (a) shows the transient absorption spectra of the spin channel containing the excess electron, and (b) shows the transient absorption spectra of the spin channel containing the excess hole. In both cases, peaks of the opposite spin channel (not shown) are absent up to about 500 nm (approximately where the band gap is crossed) and closely resemble those shown here at higher energies (lower nm). The imaginary part of the dielectric constant 27 used to generate this plot has been broadened with a Gaussian function of 0.25 eV fwhm. To compute these spectra and generate the orbitals in Fig. 7 the electronic temperature was set to 25 K.



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Node Across Rings

No Node Across Rings

Figure 7: Example of states of the excess hole that can be connected via a high intensity transition at 1747 nm. SOMO↓ (on the left) shows most localization on two cations. The lower energy state into which the excess hole transitions upon excitation (right) shows most localization on the dimer as well as a nearby NTF− 2 . The figure shows that the transition involves the loss a node in the intervening space between rings of the sandwich-like radical cation dimer as well as partial hole transfer to a nearby NTF− 2.

Longer time Dynamics To make contact with experiments, it is important to go beyond the early picosecond regime. However, on systems with ∼ 300 atoms, condensed-phase DFT calculations become prohibitive. To extend our dynamics we explored the use of the density functional tight binding (DFTB) method. Initial configurations for our DFTB-based md simulations were taken from snapshots of our condensed phase DFT simulations at 5ps. Condensed-phase DFTB is much faster than condensed-phase DFT providing opportunity to study dynamical relaxation phenomena impossible to capture with the more expensive but more accurate method.

Because of the shortcomings we found with DFTB in the case of Sulfur within NTF− 2 described in the methods section, we focus almost exclusively on the BzPy+ /DCA− system. For BzPy+ /DCA− in the presence of the excess electron or excess hole, we extended our DFT molecular dynamics simulations using DFTB up to 200 ps. Mulliken charges for each ion as a function of time in the presence of an excess electron and hole are shown in Figs. 8a 14 ACS Paragon Plus Environment

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b. a.

Figure 8: DFTB-based Mulliken charge for all ions in BzPy+ /DCA− in the presence of (a) excess electron and (b) excess hole as a function of time. The insets in each figure show the charges of cations and anions for a short time interval (approximately 2 ps) when the excess electron and hole are mostly delocalized or localized. In the localized state, the charge of two ions is distinctly different from that of the rest. This is indicative of a dimer radical cation in the case of the excess electron and a dimer radical anion in the case of the excess hole. For each figure, delocalized states are shown on the left inset and localized states on the right. and b respectively. Insets highlight periods of significant localization or delocalization. We notice that this pattern of localization and delocalization of the excess electron and hole in BzPy+ /DCA− occurs throughout the whole trajectory and localization it is not always on the same ions. In these much longer trajectories when solvent can more effectively relax, the pattern of localization becomes more clear. The excess electron often tends to localize on pairs of nearby cations whereas the excess hole localizes on pairs of nearby anions. Figure 9 shows examples as a function of time of SOMO↑ for BzPy+ /DCA− . At 48.19 ps, the excess electron is distributed mostly over three different cations but at 99.22 ps, it is strongly localized on two close-by cations. Once again, this configuration supports predictions by Shkrob and collaborators. 11 The SOMO↓ for BzPy+ /DCA− in Fig. 10 highlights the hole distribution at 42.8 ps and 103.85 ps. Whereas the snapshot at 42.8 ps is significantly delocalized, the one at 103.85 ps clearly corresponds to a dimer radical anion. Because simulations based on DFTB are less expensive, in the case of BzPy+ /DCA− , we also run test studies with 32 ion pairs. Projected density of states as well as SOMO↑ and SOMO↓ at different times presented in the SI (Figs. S8, S9 and S10) support our findings 15 ACS Paragon Plus Environment


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based on the 8 ion pairs simulations. Even though for the case of BzPy+ /NTF− 2 we expect DFTB results to be inaccurate (see methods section), we report our findings for completion. At longer times the DFTB calculations show excess electron localization and subsequent fragmentation of NTF− 2 . We also observe hole distribution over anions (see Figs. S7).

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Figure 9: DFTB based SOMO↑ in the presence of an excess electron at (a) 48.19 ps and (b) 99.220 ps.

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Figure 10: DFTB based SOMO↓ in the presence of an excess hole at (a) 42.800 ps and (b) 103.85 ps.


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Conclusions We have studied the dynamics of an excess electron and an excess hole in ionic liquids based on the BzPy+ cation. We find that in the case of BzPy+ /NTF− 2 cations are able to support a radical state with either an excess positive or negative charge without undergoing fragmentation (at least on the time scales of our simulations). The excess electron state is associated with the pyridinium rings whereas the excess hole with the phenyl components. The early-time transient optical spectra for BzPy+ /NTF− 2 highlight the significant contribution of the hole to the broad peak above 600 nm. Transitions of the hole are associated with concerted changes in the wave function of adjacent benzyl rings as well as partial hole transfer to NTF− 2 . These findings match well the experimental observations in reference 11. We find that spatial localization occurs when the excess electron or excess hole are mostly distributed over BzPy+ pairs. This localization is dynamical, pairs are not always the same, and between localization events there are times in which several BzPy+ ions share the excess positive or negative charge. The situation for BzPy+ /DCA− is similar, except that the excess hole distributes over DCA− and when localization occurs it is by forming a dimer radical anion. In all cases, localized and delocalized configurations appear to be separated by small thermal energies and we speculate that this may be an early time mechanism for transport of the excess positive or negative charge without significant ionic diffusion. One should not discard the possibility that solvent relaxation at long time will result in long-lived localized states.

Acknowledgments The work presented here was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract No. DE-SC0001780. We also acknowledge the University of Iowa for a generous allocation of computational resources. CJM would like to thank Dr. James Wishart for insightful 17 ACS Paragon Plus Environment


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discussions.

Supporting Information Molecular structures, dihedral parameter fits, partial charges for BzPy+ , example snapshots for SOMO↑ and SOMO↓ in the presence of an excess electron and hole respectively, neat IL PDOS computed using the DFTB approach, Mulliken charges for BzPy+ /NTF− 2 in the presence of an excess electron and hole using the DFTB approach, bond distances computed from MP2, DFT and DFTB. For a system with 32 BzPy+ /DCA− pairs, neat liquid PDOS as well as examples of SOMO↑ and SOMO↓ in the presence of an excess electron and hole respectively using DFTB. This information is available free of charge at the ACS Publication Website.

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Graphical TOC Entry

Absorption(a.u.)

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Excess Electron and Hole in 1-Benzylpyridinium-Based Ionic Liquids

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