Elucidating Interactions Between Ionic Liquids and Polycyclic Aromatic

Article pubs.acs.org/JPCC

Elucidating Interactions Between Ionic Liquids and Polycyclic Aromatic Hydrocarbons by Quantum Chemical Calculations Durgesh Wagle, Ganesh Kamath, and Gary A. Baker* Department of Chemistry, University of Missouri-Columbia, Columbia, Missouri 65211, United States S Supporting Information *

ABSTRACT: Using quantum mechanical calculations performed at the density functional level of theory, the present study explores the binding energetics, orbital energies, and charge transfer behavior accompanying sorption of 12 different ionic liquids (ILs) onto 6 archetypal polyaromatic hydrocarbons (PAHs). The ILs were based on combinations of three different onium cations (i.e., 1-butyl-3-methylimidazolium, 1-butylpyridinium, 1-butyl-1-methylpyrrolidinium) paired with four common anions, that is, bromide, tetrafluoroborate, hexafluorophosphate, and bis(trifluoromethylsulfonyl)imide. In general, the size of the anion as well as interaction of the butyl side chain present on the cation with the paired anion exerted significant influence over the cation ring orientation with respect to the PAH surface. A smaller highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) energy band gap was observed for pyridinium-based ILs upon adsorption on the PAH surface in comparison to imidazolium and pyrrolidinium analogs, hinting at stronger interactions between PAHs and pyridinium ILs. Of the 12 ILs investigated, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide displays the least favorable free energy of adsorption with PAHs whereas PAH interactions with 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide are the most favored thermodynamically. Charges determined from a Mulliken population analysis were consistent with charge transfer (CT) from the IL to the PAH. On the contrary, charges determined via electrostatic potential using the more reliable grid based analysis method (i.e., CHELPG) suggested the reverse direction of CT from the PAH to the IL. The direction of the CT occurring from the HOMO of the PAH to the LUMO of the IL, as shown by CHELPG analysis, is consistent with the physical location of the orbitals and the negative shift in the Fermi energy level observed for the IL−PAH complex. A more favorable enthalpy of adsorption for ILs onto a PAH is observed with an increase in the size of the PAH considered. The free energy of adsorption, however, does not change significantly with an increase in the PAH surface area. The adsorption of an IL on the PAH surface leads to a small change in the entropy of the adsorbate/adsorbent system. The thermochemistry computed at variable temperature indicates a significant increase in the free energy of adsorption (i.e., a less favorable adsorption) as temperature rises. Additionally, decomposition of the entropic contribution suggests a greater contribution from translational and rotational entropies upon cooling, again consistent with stronger association at lower temperatures. Overall, the thermochemical analyses suggest an entropically driven process of desorption of an IL from the PAH surface, generally leading to fairly weak interactions between ILs and ordinary PAHs under normal laboratory conditions. due to radiative recombination of holes and excitons.12 Graphene, a related nanocarbon (some, but not all, C-dots are apparently graphitic in nature) holding superlative properties, including huge surface area, charge carrier mobility of 15 000 cm2 V−1 s−1, high thermal stability, and unparalleled mechanical strength, has had a revolutionary impact on nanotechnology within the past couple years.13,14 It is imperative to find cheaper and practical routes for the industrial scale production of graphene. Large scale production of graphene frequently implies, in one form or another, graphene

1. INTRODUCTION Carbon-based materials have gained popularity because of their unique and flexible properties and accessibility from numerous synthetic routes, finding ubiquitous application in nanotechnology.1,2 Fluorescent carbon quantum dots (C-dots), for instance, have generated keen interest among researchers for their promising potential in biological imaging and nanomedicine.3,4 C-dots are primarily composed of sp2 carbon atoms and may be produced by several means, including laser ablation,5−7 arc discharge,8 and electrochemical oxidation.9,10 The photoluminescence of C-dots has been attributed to the presence of surface energy traps that become emissive due to surface passivation and subsequent stabilization.11 The currently accepted mechanism for luminescence emission is © 2013 American Chemical Society

Received: October 31, 2012 Revised: January 31, 2013 Published: February 2, 2013 4521

dx.doi.org/10.1021/jp310787t | J. Phys. Chem. C 2013, 117, 4521−4532

The Journal of Physical Chemistry C

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exfoliation from graphite.15 This cost-effective and scalable approach typically involves chemically modified graphite (e.g., graphite oxide, GO), but pristine graphite is preferred as it gives better conductivity and fewer defects.16 One approach to the liquid-phase exfoliation of graphite entails ultrasonic agitation of graphite particles dissolved in a judiciously selected solvent to produce pristine monolayer or few-layered graphene sheets. This requires graphene stabilization in solution, which can be accomplished by using appropriate organic solvents or ionic liquids (ILs), whose surface tension is matched to the surface energy of graphene, in order to minimize the enthalpic cost of mixing.17 Although liquid-phase direct exfoliation of graphite using organic solvents offers several advantages, the process yields a low-concentration colloidal suspension of graphene.17−19 ILs, on the other hand, have gained incredible popularity as potentially eco-friendly alternatives to organic solvents and represent another alluring possibility.20 Significantly, typical ILs have surface tensions in close proximity to that of graphene, a key aspect for efficient exfoliation of graphene sheets from graphite.15,21 Indeed, high yields of graphene were recently obtained based on the direct exfoliation of graphite into few-layered graphene sheets using ILs.22,23 This advance is made all the more pertinent by the demonstration that IL−graphene composites hold potential in advanced solar cell,24 supercapacitor,25 and electrochemical applications.26,27 Likewise, C-dots dispersed into ILs display high stability and enhanced photoluminescence due to surface passivation by components of the IL.28 Substantial progress in IL research makes available a vast toolbox of ILs with countless cation− anion combinations seeking to satisfy the requirements of diverse and demanding chemical processes and applications.20 In these endeavors, it is important to identify the key microscopic interactions subject to synthetic control that modulate their molecular-level behavior. During the past decade, extensive investigations, both experiments29,30 and simulations,31,32 have been devoted to better understanding ILs as an emergent class of task-tailored fluid. Although there has been significant computational focus on the IL−graphene interface, most of the work to date has aimed primarily, and in most cases exclusively, at imidazolium-based ILs.33−35 The quantum mechanical and molecular dynamics calculations indicate preference for a specific orientation of the ion pair on the PAH surface and implicate a charge transfer (CT) process between the ions and the PAH, suggestive of a strong interaction.21,36 In this research, we explore the sorption behavior of 12 representative ILs on the surface of six model polyaromatic hydrocarbons (PAHs), that is, naphthalene, anthracene, phenanthrene, pyrene, perylene, and coronene. The ILs studied comprise cations from the imidazolium, pyridinium, and pyrrolidinium families, paired to bromide [Br]−, tetrafluoroborate [BF4 ] −, hexafluorophosphate [PF6 ] −, and bis(trifluoromethylsulfonyl)imide [Tf2N]−. The computationally tractable model PAHs presented in Figure 1 serve as surrogates to better understand how changes in IL ion structure modulate interactions with the PAH (i.e., “nanographene”) surface. Model compounds are frequently used as templates to understand complex phenomena such as sorption because of the decreased computation time required by these molecules and the large amount of experimental data available for such compounds in terms of condensed phase properties, crystal geometries, and vibrational frequencies.37 Toward this, the adsorption enthalpies, free energies, entropies, and interaction

Figure 1. Schematic diagram showing the PAHs and ILs studied in this work. PAHs: (a) naphthalene, (b) anthracene, (c) phenanthrene, (d) pyrene, (e) perylene, (f) coronene. Cations: (g) 1-butyl-3-methylimidazolium, (h) 1-butylpyridinium, (i) 1-butyl-1-methylpyrrolidinium. Anions: (j) bromide, (k) tetrafluoroborate, (l) hexafluorophosphate, and (m) bis(trifluoromethylsulfonyl)imide.

energies of ILs with these model “surfaces” will be determined. While benzene can be thought of as the fundamental building block for C-dots and graphene (and, indeed, any graphitic surface),13 fused ring effects will start to become accessible only by studying model compounds larger than benzene, such as naphthalene and anthracene. Since previous Raman studies have shown that graphene sheets at an edge site have energetic differences due to edge effects on the graphene basal plane,38,39 zigzag edges will be investigated on the basis of the model PAHs naphthalene, anthracene, pyrene, and coronene (Figure 1a,b,d,f). Meanwhile, the model PAHs phenanthrene and perylene (Figure 1c,e) will provide an arm-chair edge illustrative of the fundamental unit structure for an “infinitely long” edge resembling graphene and small C-dots. The structures of the different cations and anions included in our analysis are given in Figure 1 (g−m). In this study, we report on the geometry, thermochemistry, orbital energies, and charge transfer capabilities of 12 illustrative ILs variously adsorbed onto the surfaces of six different PAHs. This work serves as a tool to illuminate the behavior of adsorbed ions at an atomistic level, providing important insights into the process of surface passivation of C-dots and other graphitic nanosurfaces. Ultimately, it is our aim that the approach taken here be extended to enable an elucidation of the parameters responsible for efficient large-scale production of stable colloidal suspensions of graphene and inorganic graphene analogs in ILs as exfoliating solvents. 4522



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2. COMPUTATIONAL DETAILS Initial optimization of the geometries of ILs and PAHs (Figure 1) was performed at the Hartree−Fock (HF) level of theory using Pople’s medium 6-31G(d,p) basis set. These optimized structures obtained from HF/6-31G(d,p) were further optimized at density functional theory (DFT) and the hybrid functional expand (B3LYP) with Pople’s medium 6-311G basis set. All of the calculations were carried out without symmetry restrictions in the singlet ground state. The absence of imaginary frequencies in the calculated vibrational frequencies of the optimized structures ensured stable structures. The optimized structures were tested for the stability of their wave functions and any negative eigenvalues resulting from transition state structures or higher order saddle points using “stable” and “opt=calcfc” keywords, respectively. Orbital energy calculations, Mulliken charge analysis, and electrostatic potential charge analysis were each performed at the same level of theory on the optimized structures. Basis set superposition error (BSSE) correction was performed using the counterpoise (CP) procedure of Boys and Bernadi.40 All calculations reported herein were performed using the Gaussian 09 program package.41 Compositions of molecular orbitals and density of states (DOS) spectra were calculated using the AOMix program.42 Enthalpy (H), free energy (G), and interaction energy (E) for the adsorption of an IL on the PAH surface at 298.15 K was calculated using eq 1 ΔX = ΔXILs + PAH − (ΔXILs + ΔXPAH)

Figure 2. Reference angles used to describe various ionic liquid cation−anion geometries in this study, where “A−” represents a specific atom on the anion interacting with the cation. Certain carbon and hydrogen atoms have been removed for clarity. (a) ∠C2−Ha····A− indicates the angle between the plane defined by the imidazolium ring and the atom on the interacting anion. (b) ∠N3−C2−Ha····A− is the dihedral angle between the plane of the imidazolium ring and the atom on the associated anion. (c) ∠C2−Ha····A− defines the angle between the plane of the pyridinium ring and the atom of the anion interacting with the cation. (d) ∠C6−N−C2····A− is the dihedral angle between the plane of the pyridinium ring and a particular atom of the anion in question. No such angles are assigned to the [Pyrr14]+ cation given the nonplanarity of the pyrrolidinium ring.

(1)

where X = H, G, and E. The entropy (S) of adsorption was calculated at 298.15 K from eq 2

ΔS =

(ΔH − ΔG) 298.15

(2)

3. RESULTS AND DISCUSSION 3.1. Optimized Molecular Structures. The optimized structures and energies of the six model PAHs were obtained at the B3LYP level of theory using the 6-311G basis set. The calculated C−H bond lengths of naphthalene (1.076 Å), anthracene (1.076 Å), phenanthrene (1.075 Å), pyrene (1.075 Å), perylene (1.075 Å), and coronene (1.076 Å) are in accordance with the value of 1.072 Å obtained for benzene optimized at the HF/3-21G level of theory.36 The computed diameter for coronene at 9.6 Å is in fair agreement with the previously reported diameter of 10.0 Å for coronene calculated at B3LYP/6-311G by Ghatee et al.36 The bond lengths computed for the PAH compounds at B3LYP/6-311G level of theory and basis set are consistent with the reported theoretical calculations of Pauling bond orders by Herndon.43 The cation− anion interaction energies of 82.76 and 76.17 kcal mol−1 computed at B3LYP/6-311g for [bmim][BF4] and [bmim][PF6] ILs, respectively, are in excellent agreement with the corresponding cation−anion interaction energies of 82.52 and 76.81 kcal mol−1 at B3LYP/6-311G** reported by Zhao et al.,44 strengthening the credibility of our preliminary calculations. The structure, orientation, and dynamics of adsorbed IL species at a PAH surface are critical to wholly understanding the interactions involved in sorption processes.36 For all 12 ILs investigated, it was observed that the anion interacts with the

positive electrostatic surface present on the hydrogen atoms at the edge of the PAH, whereas the cation interacts with the negative electrostatic surface above and below the plane of the PAH. This outcome, pointing to the selective affinity of the cations and anions for different regions of the electrostatic surface of a PAH, is consistent with the results of molecular dynamics simulations for benzene with imidazolium-based ILs studied by Hanke et al.45 Certainly, in this previous study, the cation was seen to interact with benzene from above and below the plane of benzene although the anion interacted with the edge of the benzene plane. For [bmim][Br] and [bmim][Tf2N] sorbed on a PAH, interaction between the cation’s butyl side chain and the anion of the IL resulted in a tilting of the cation ring with respect to the PAH plane. In the cases of [bmim][BF4] and [bmim][PF6], the larger distance between the butyl chain and the anion allowed a more parallel orientation of the cation ring with respect to the PAH surface with an interplanar distance of 3.9− 4.2 Å, suggesting the electrostatic nature of the cation−πinteraction between the positive electrostatic surface of the cation and the negative electrostatic surface of the PAH. This interaction is similar to the cation−π-stacking observed by Cao et al.46 between free base porphyrin and an imidazolium cation in which the positive electrostatic surface of the imidazolium 4523



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Figure 3. The optimized structure of [C4Py][BF4] interacting with the six model PAHs at the B3LYP/6-311g level of theory. Dark gray is used to color carbon, light gray for hydrogen, cyan for fluorine, blue for nitrogen, and pink for boron.

of the [C4Py]+ cation ring with respect to the PAH plane. A representative plot of the interactions of [C4Py][BF4] with several PAHs is presented in Figure 3. Again, butyl chain interaction with the anion tilts the cation ring with respect to the plane of the PAH, whereas in the absence of this interaction a parallel stacking of the cationic ring on the PAH basal plane results, accompanied by weak interactions of the cation ring carbons with carbon atoms of the PAH (also see Figures S5−S7 in the Supporting Information). We note that in all of our simulations of IL adsorption on a PAH surface, the anion interacts to a varying degree with the edge plane hydrogen atoms of the PAH, a feature discussed in further detail at the end of this section. These hydrogen atoms on the cation are observed to lie in closer proximity to PAH carbon atoms when the cation ring is skewed in a nonparallel orientation, suggesting stronger interaction between the tilted cation ring and the PAH surface (refer to the entries in Supporting Information Table S1). In contrast to the interactions observed for imidazolium- and pyridinium-based ILs with PAHs, optimized geometries for a pyrrolidinium IL on a PAH surface indicate that the cation has a stronger affinity for its anionic partner than for the PAH. Precisely, the anion interacts strongly with the hydrogen atoms on the α-carbons on the cation. This is likely due to large partial positive charges of +0.26 to +0.28 present on these hydrogen atoms, as determined by a Mulliken population analysis (for optimized structures of [Pyrr14][X] (X = Br−, BF4−, PF6−, Tf2N−) interacting with model PAHs, refer to Supporting Information Figures S8−S11). The butyl side chain of the [Pyrr14]+ cation is directed away from the PAH surface,

cation sits atop the negative electrostatic surface of the nitrogen atoms in the underlying porphyrin ring. The calculated interplanar distances are provided in Table S1 of the Supporting Information. The electron deficient N1−C2−N3 region of the imidazolium ring is a three-centered four-electron system that interacts with the anion through a C2−Ha bond.47 A change in this cation−anion interaction is seen upon adsorption at the PAH surface. This is monitored by observing a change in the plane of the cation ring with respect to the anion (A−) upon sorption onto the PAH surface, the two angles under consideration being (a) the angle defined by C2− Ha(cation)···A−(anion) and (b) the C4−N3− C2(cation)···A−(anion) dihedral angle, respectively (see Figure 2a,b).36 A significant change in these angles upon adsorption is often an indicator of reduced cation−anion interaction and increased ion−PAH interaction. In [bmim][Br] and [bmim][Tf2N], a negligible change in these angles is seen upon adsorption of IL onto the PAH. This indicates strong cation− anion interaction/association (refer to Supporting Information Table S1). Conversely, in [bmim][BF4] and [bmim][PF6], the change in the C4−N3−C2(cation)···A− (anion) dihedral angle is about 9−30° with the cation ring being parallel to the PAH plane and the butyl chain flanking the PAH. This suggests an increased cation−PAH interaction, leading to significant perturbation of the cation−anion orientation due to adsorption on the PAH surface (compare Figures S1−S4 in the Supporting Information). Similar to the case of imidazolium ring interactions with the PAH surface, in pyridinium based ILs we find that interactions of the butyl chain with the anion also influence the orientation 4524



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symptomatic of unfavorable interactions. On average, the cation ring lies at a distance of 3.0−3.2 Å from the PAH surface, signifying weak van der Waals interactions between the nonaromatic [Pyrr14]+ ring and the PAH surface. In line with this, no significant change in the dipole moment of [Pyrr14]+ based ILs is seen upon adsorption on the PAH surface with the exception of [Pyrr14][Tf2N]. This insignificant change in dipole moment upon adsorption onto the PAH is consistent with retention of the original cation−anion geometry, a situation reflecting the negligible charge transfer between a PAH and the pyrrolidinium-based IL. Viewed from the perspective of the anion interacting with the PAH surface, these DFT calculations reveal a preferential mobilization of anions to the edge atoms of the PAH, implying an interaction electrostatic in nature (i.e., between the negatively charged anion and the positive electrostatic surface of the PAH). With anion−PAH interaction distances of around 3.1−3.2 Å, bromide has the weakest interaction with PAHs in comparison to F···PAH and O···PAH interactions distances of 2.4−2.6 Å for [BF4]−/[PF6]− and [Tf2N]− anions, respectively. 3.2. Orbital Energies and Density of States Calculations. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) present within an interacting system are key determinants of the chemical stability of the system.48 The energy gap between the HOMO and the LUMO dictates the molecular electrical transport properties such as the electron carrier and mobility.49 The size of the HOMO−LUMO energy gap depends on the relative magnitudes of the respective orbital energies. For instance, a decrease in the energy of the HOMO simultaneous with an increase in the energy of the LUMO, increases the HOMO− LUMO gap while, conversely, an increase in the HOMO energy and a decrease in the LUMO energy yields a reduced HOMO−LUMO gap. Figure 4 summarizes the HOMO− LUMO gaps of the 12 ILs studied upon adsorption to the six model PAH compounds. The entries of Supporting Information Table S2 illustrate the reduction in the HOMO−LUMO gap resulting from IL adsorption on coronene. Such a reduction in band gap is seen in all cases, as shown in Figure 4, although the bromidecontaining ILs experience only minor decreases. These results are consistent with the findings of Ghatee et al. wherein a similar decrease in the HOMO−LUMO gap was reported for IL adsorption on circumcoronene.36 As can be seen in Figure 4a, only a negligible reduction in the HOMO−LUMO gap (∼0.1 eV) is implicated for [bmim][Br] upon adsorption on coronene. In contrast, for the other [bmim]+ ILs, initially large band gaps showed a significant drop upon interacting with coronene; that is, drops of 0.1073 au (2.91 eV), 0.1094 au (2.97 eV), and 0.0551 au (1.50 eV) for [bmim][BF4], [bmim][PF6], and [bmim][Tf2N], respectively (Supporting Information Table S2). This band gap reduction is due to a rise in the energy of the HOMO orbital of the IL− PAH complex resulting from the contribution of the 3p orbitals present on the carbon atom of the coronene and a small decrease in the energy of LUMO orbital of the IL−PAH complex. The magnitude of the band gap decrease implies the strength of interaction between the IL and the PAH. That is, the larger the decrease in the band gap, the stronger the interaction between the orbitals, leading to a more stable IL− PAH complex. Figure 4b shows the HOMO−LUMO band gap for the pyridinium based ILs upon adsorption on coronene. For the

Figure 4. Plots of the HOMO−LUMO gap for (a) imidazolium-, (b) pyridinium-, and (c) pyrrolidinium-based ILs before and after adsorption onto a PAH. Green symbols/lines are used to denote [Br]−, blue for [BF4]−, magenta for [PF6]−, and black for the [Tf2N]− anion. Solid lines are guides for the eye to show HOMO energy trends while dashed lines trace the LUMO orbitals.

Figure 5. (a) Density of states for the adsorption of [C4Py][BF4] on coronene. The red profile represents [C4Py][BF4] alone, the blue spectrum represents coronene, and the green line indicates the result for [C4Py][BF4] adsorbed on coronene. 4525



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Figure 6. Comparison between the atomic charges obtained using CHELPG versus Mulliken charge analysis before and after adsorption of [C4Py][BF4] onto the coronene surface, both at B3LYP/6-311g. (a) Schematic indicating charge transfer between the HOMO orbital of coronene and the LUMO of [C4Py][BF4], specifying the magnitude of the HOMO−LUMO energy gap involved. (b,c) Atomic charges on the coronene and [C4Py][BF4] atoms derived from Mulliken analysis. (d,e) The corresponding charges obtained using CHELPG charge analysis. The black line connects the atomic charges before adsorption and the red profile shows charges on the coronene−[C4Py][BF4] complex.

band gap primarily results from a rise in the HOMO energy in IL−PAH complex with little change in the LUMO energetics. Orbital decomposition analysis again indicates that the rise in the energy of the HOMO is primarily due to contribution from

nonhalide pyridinium based ILs, adsorption on coronene results in smaller band gaps by 2.87, 3.13, and 1.21 eV for [C4Py][BF4], [C4Py][PF6], and [C4Py][Tf2N], respectively (see Supporting Information, Table S2). This decrease in the 4526



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Table 1. Cation, Anion, and Coronene Mulliken Charges q(e) before and after Adsorption of the IL onto Coronene, Calculated at the B3LYP/6-311g Level of Theory cation

anion

coronene

IL on coronene

before adsorption

after adsorption

before adsorption

after adsorption

after adsorption

[bmim][Br] [bmim][PF6] [bmim][BF4] [bmim][Tf2N] [C4Py][Br] [C4Py][PF6] [C4Py][BF4] [C4Py][Tf2N] [Pyrr14][Br] [Pyrr14][PF6] [Pyrr14][BF4] [Pyrr14][Tf2N]

0.741445 0.837004 0.867112 0.894594 0.712283 0.858813 0.878492 0.921241 0.763108 0.816858 0.866894 0.92153

0.770064 0.85104 0.883101 0.928808 0.754797 0.849923 0.885655 0.927524 0.784316 0.828723 0.869995 0.958243

−0.741445 −0.837004 −0.867112 −0.894594 −0.712283 −0.858813 −0.878492 −0.921241 −0.763108 −0.816858 −0.866894 −0.92153

−0.729512 −0.818167 −0.855947 −0.935128 −0.715578 −0.811819 −0.87131 −0.914476 −0.754434 −0.80622 −0.848435 −0.957298

−0.040552 −0.032873 −0.027154 0.006319 −0.039219 −0.038104 −0.014345 −0.013047 −0.029882 −0.022502 −0.02156 −0.000945

Table 2. Cation, Anion, and Coronene CHELPG Charges q(e) upon Adsorption, Calculated at the B3LYP/6-311g Level of Theory cation

anion

coronene

IL on coronene

before adsorption

after adsorption

before adsorption

after adsorption

after adsorption

[bmim][Br] [bmim][PF6] [bmim][BF4] [bmim][Tf2N] [C4Py][Br] [C4Py][PF6] [C4Py][BF4] [C4Py][Tf2N] [Pyrr14][Br] [Pyrr14][PF6] [Pyrr14][BF4] [Pyrr14][Tf2N]

0.752183 0.87903 0.881908 0.957159 0.709834 0.887858 0.888988 0.947764 0.756148 0.889155 0.878152 0.893156

0.71084 0.79004 0.79004 0.806473 0.69435 0.79208 0.80043 0.81128 0.73269 0.802342 0.798543 0.898306

−0.752183 −0.87903 −0.881908 −0.957159 −0.709834 −0.887858 −0.888988 −0.947764 −0.756148 −0.889155 −0.818152 −0.893156

−0.71644 −0.85273 −0.85273 −0.866064 −0.69256 −0.85464 −0.868716 −0.899454 −0.738333 −0.857742 −0.871397 −0.96025

0.0056 0.0627 0.0627 0.059587 −0.001789 −0.06256 0.06829 0.088169 0.005643 0.0554 0.072854 0.061946

dominates the HOMO, resulting in no significant narrowing of the band gap in the complex. An evaluation of the relative change in the HOMO−LUMO (band) gap can also be performed using density of states (DOS).50 DOS is a powerful technique utilized to probe variations in the electronic structure of a system as a result of changes in energetics due to molecular interactions. It is defined by the number of energy levels per interval of energy that are available for the electrons to occupy. A representative DOS plot for the adsorption of [C4Py][BF4] on coronene is given in Figure 5. A high value for the DOS at a particular energy interval suggests that multiple states are available for the occupation of the electrons, while a zero DOS indicates that no states are available for the electrons to be occupied at that particular energy interval. Further, the nonavailability of states for occupation by an electron at a particular energy interval represents the existence of a band gap. The states having lower energies with respect to the band gap are known as occupied states whereas states with higher energies relative to the band gap are unoccupied states.50 To illustrate, the DOS spectrum in Figure 5 shows that, in the energy interval between −5.0 and −6.25 eV, there are two states available for occupation of an electron while no states are available in the −3.75 to −5.0 eV interval (i.e., this latter interval is the band gap). The DOS spectra for all 12 ILs evaluated reveal very large band gaps prior to adsorption, as shown in the Supporting Information Figure S12. The unoccupied (LUMO) states of the

the 3p orbitals present on the carbon atom of coronene. And once again, [C4Py][Br] is a peculiar case in that there is a tiny increase in the band gap by roughly 0.10 eV upon adsorption to coronene. The density of states features of [C4Py][Br] (Figure S12) reveal that the small increase in the band gap may be attributed to a contribution from the 3p orbital of [Br]− at ca. −5 eV on the occupied side (HOMO states), signifying a weak interaction of the IL with coronene. Proceeding to the pyrrolidinium set, [Pyrr14][Br] is again an outlier; that is, as before, there is no appreciable change in the band gap following sorption to coronene, as can be seen in Figure 4c. For [Pyrr14][BF4], [Pyrr14][PF6], and [Pyrr14][Tf2N], the large increase in the HOMO coupled with a substantial decrease in the LUMO yields sizable overall band gap reductions of 4.55, 4.01, and 2.04 eV, respectively (refer to Table S2 in the Supporting Information). Taken together, these observations reveal that bromidecontaining ILs consistently show insignificant band gap changes following adsorption to any PAH studied. This anomalous behavior appears to derive from a contribution from the 3p orbital of [Br]−, an outcome derived from orbital decomposition analysis of the PAH-sorbed product using AOmix software. Overall, these orbital decomposition calculations indicate that for [Br]− containing ILs, the HOMO overlap orbital contributions of the IL−PAH complex comprise 60.7 and 36.7% contributions from the 3py and 3pz orbital of bromide, respectively. In other words, the bromide ion 4527



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Figure 7. Enthalpy of adsorption for (a) imidazolium, (b) pyridinium, and (c) pyrrolidinium IL sorption onto the six model PAHs. The green line is used for [Br]− anion, blue for [BF4]− anion, magenta for [PF6]−, and black for [Tf2N]−. The size of the PAH increases from left (naphthalene, 128.17 Da) to right (coronene, 300.35 Da).

Figure 8. Free energy of adsorption for (a) imidazolium, (b) pyridinium, and (c) pyrrolidinium IL sorption onto the six model PAHs. The green line is used for [Br]− anion, blue for [BF4]− anion, magenta for [PF6]−, and black for [Tf2N]−. The size of the PAH increases proceeding from left to right.

IL−coronene complexes possess slightly negative values in comparison to the unoccupied states of the IL prior to adsorption, indicating stability in the IL−coronene complexes. In the DOS profile for coronene, the band gap is located below zero with certain virtual orbital energies assuming negative values, indicating the stability of the PAH system. Supporting Information Figure S12 also demonstrates a negative shift in the Fermi energies of the DOS profile for IL−coronene relative to the profiles of the IL or coronene individually, implying that the adsorption is accompanied by charge transfer from coronene to the adsorbed IL species. This observation is similar to the Fermi energy shifts computed for the adsorption of nitric oxide on a ruthenium surface by Neyman et al.51 Interestingly, a Mulliken population analysis suggests charge transfer from the IL to the PAH, just the reverse of the direction for charge transfer predicted by the more reliable Fermi energy criterion.51 This result illustrates the recognized shortcomings of Mulliken analysis in the case of extended basis

sets,52,53 whereas charges from electrostatic potentials using a grid based method (CHELPG) are harmonious with the shift in Fermi energy as we detail in the following discussion. 3.3. Elucidating Charge Transfer During IL−PAH Interaction. CT is a process in which a fraction of electronic charge is transferred either within a molecule or intermolecularly between species of a complex. The associated CT complex generally involves an association of two or more molecules or a mobilization of charge between disparate regions of a molecule, wherein one portion acts as electron donor and another plays the role of acceptor. The CT occurs via molecular orbitals or frontier orbitals and generally proceeds in the direction from HOMO to LUMO.54 As a result, the direction of CT relies on the physical location of these orbitals within the CT complex. A recent study of the IL−graphene interface by Rao et al. suggests that CT might be instrumental for tailoring the properties of graphene and related materials such as graphene 4528



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Figure 10. Temperature-dependent enthalpy, free energy, and entropy of adsorption for the 12 different ILs at the coronene surface.

charges present on atoms of coronene and [C4Py][BF4] as a result of adsorption is given in Figure 6 panels b and c, respectively. In addition, quantitative estimates of the charges on the individual ions of [C4Py][BF4] and on coronene prior and subsequent to adsorption are tabulated within Table 1. Upon adsorption of an IL to a PAH surface, the overall charge on the PAH becomes negative, indicating CT from the IL to the PAH. The CT in imidazolium- and pyridinium-based ILs is more pronounced (i.e., the CT leads to a greater charge difference in the PAH) than for pyrrolidinium-based ILs when paired with the anions [Br]− and [PF6]−. However, according to a Mulliken analysis, the pyrrolidinium IL shows the greatest tendency for CT to the PAH in the [Br]− analog. The magnitude and the direction of the CT from an imidazolium IL to the PAH based on the Mulliken population analysis is consistent with the results of Ghatee et al. who also suggested that CT occurred from the IL to coronene or circumcoronene.36 Notably, this result conflicts with CHELPG calculations; this forms the subject of the discussion below. Partial charges were also determined for the IL−PAH complex using CHELPG analysis based on the representation of the electrostatic potential determined at the van der Waals surface.53 The CHELPG charges computed at the HF and B3LYP levels of theory differ in important ways from the Mulliken charge analysis. A comparative study of Mulliken and CHELPG charges predicted for water and formamide systems

Figure 9. Entropy of adsorption for (a) imidazolium, (b) pyridinium, and (c) pyrrolidinium IL sorption onto the six model PAHs. The green line is used for [Br]− anion, blue for [BF4]− anion, magenta for [PF6]−, and black for [Tf2N]−. The size of the PAH increases proceeding from left to right.

quantum dots (a member of the C-dot family) for semiconductor applications.55 In the current report, we have used Mulliken analysis and electrostatic surface potential as probes of the CT process between an IL and a PAH, both performed at the B3LYP/6-311g level of theory. An illustration of CT between the HOMO of coronene and LUMO orbitals located on the cation of [C4Py][BF4] is depicted in Figure 6a. The right-hand panel of Figure 6a displays the electrostatic surface of the orbitals for the complex. It should be clear that a negative electrostatic potential is located on the LUMO of [C4Py][BF4] while a positive electrostatic potential is present on the HOMO of coronene. This net CT occurring from the HOMO of coronene to the LUMO of [C4Py][BF4] results in some variation of the partial charges on certain atoms, as described in Figure 6 (panels b−e). According to Mulliken population analysis, the process of adsorption will primarily alter the charges on those atoms that are specifically involved in the interaction between an IL and a PAH. An example of the alteration produced in the partial 4529



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located on the [Br]− and the LUMO is located on the PAH surface, signifying CT from the IL to PAH. Similarly, in complexes involving [Pyrr14][Br] or [Pyrr14][Tf2N] with naphthalene, anthracene, phenanthrene, pyrene, or perylene, the HOMO is present on the anion and the LUMO resides on the PAH to again suggest CT from a species of the IL to the PAH. The situation is reversed for [Pyrr14][Br] and [Pyrr14][Tf2N] interaction with coronene. In these cases, the HOMO is situated on coronene with the LUMO on the anion of the ILs yielding CT from the PAH to the IL. In all remaining instances, CT likewise occurs from the HOMO of the PAH to the LUMO of the IL (i.e., [bmim][BF4], [bmim][PF6], [bmim][Tf2N], [C4Py][BF4], [C4Py][PF6], [C4Py][Tf2N], [Pyrr14][BF4], [Pyrr14][PF6]). However, Mulliken population analysis stipulates CT from the IL to the PAH for every system investigated, irrespective of the physical location of the HOMO and LUMO on the IL−PAH complex. For example, in the case of [C4Py][BF4] sorbed to coronene, the HOMO is located on coronene and the LUMO is localized on the IL. However, upon observing the charge variations in the PAH and IL predicted by Mulliken population analysis, one would erroneously be forced to conclude that CT must occur from the IL to the PAH, in clear defiance of where the orbitals are actually situated. What emerges is a lucid demonstration that CHELPG analysis may provide a more trustworthy and meaningful approach to elucidating CT, which lies in much better agreement with the physical location of the associated orbitals than does a Mulliken population analysis. 3.4. Describing the Thermochemistry of IL Sorption at a PAH Surface. Thermochemistry involves quantitative measures made in order to understand the energetics associated with a system, particularly energy and heat associated with chemical reactions and/or physical transformations. In this study, we have calculated the enthalpy, free energy, entropy, and interaction energy of 12 ILs present alone and when sorbed at the surface of several typical PAHs. An estimation of these quantities aids in understanding the feasibility of such a sorption (interaction) process. We have placed a special emphasis on the effects of different cation−anion combinations of the IL in order to attempt to deconvolve the energetics of adsorption of ILs onto model PAH surfaces. In the following sections, the discussion of our thermochemical results will be organized in terms of (i) PAH size and (ii) cation/anion combination. Figure 7 summarizes the enthalpy of adsorption for the 12 different ILs on the six PAH surfaces. On the whole, the enthalpy of adsorption of ILs gradually decreases with an increase in the size of the PAH. For a given IL, the most favorable adsorption enthalpies were typically witnessed for interaction with perylene. We propose that the “arm chair” edge of perylene provides a suitable cavity for accommodating the anion (or its interacting atom), leading to stronger interactions than for zigzag edged PAH compounds. This strong interaction of the anion with perylene is in excellent agreement with the smaller HOMO−LUMO gap observed for the IL−perylene complex, which is responsible for achieving maximal CT. Inspection of the thermodynamic quantities for these ILs interacting with different PAHs exposes the fact that reduction in the HOMO−LUMO band gap is loosely correlated with enthalpy of adsorption. Clearly, we understand that other factors are also responsible for modulating the strength of adsorption. In order to shed light on this, changes in the free energy (ΔG) and the entropy (ΔS) of adsorption were

Figure 11. Temperature-dependent translational, rotational, and vibrational contributions to the entropy of adsorption for the 12 ILs sorbed at the coronene surface.

at the HF and B3LYP level and various Pople basis sets revealed ample variation in the negative charge on oxygen ranging from −0.30 to −0.95 for Mulliken analysis and −0.60 to −0.90 for CHELPG.52,53 CHELPG charges are normally considered to be superior to Mulliken charges as they are less reliant on the underlying theoretical method and basis set used to compute the wave functions.53 It merits mention that one of the prescribed limitations of CHELPG charges lies in the estimation of charges on the atoms in large systems, wherein the atoms are embedded in the core of the molecule far removed from the point where molecular electrostatic potential is measured. However in the present study, since all of the atoms in the IL−PAH system under consideration reside on the surface, the CHELPG-computed atomic charges are reliable. The results in Table 2 and Figure 6d indicate an overall positive charge on the coronene atoms upon adsorption of [C4Py][BF4]. A correspondingly negative charge develops on the IL atoms upon adsorption on coronene (Figure 6e). This is indicative of CT from coronene to IL, an outcome in stark contrast to the direction of CT determined from Mulliken population analysis. Notably, the direction of the CT process suggested by CHELPG analysis is more physically meaningful, being consistent with the location of the HOMO and LUMO orbitals on the IL−PAH complex (refer to Figure 6a). For example, in the [bmim][Br] or [C4Py][Br] complexes with naphthalene, anthracene, or phenanthrene, the HOMO is 4530



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the IL corresponds to a stronger adsorption on the PAH surface. However, for pyridinium and pyrrolidinium ILs, a weak correlation exists between the degree of CT and the energetics of adsorption. The thermochemistry of adsorption indicates that the enthalpy of adsorption gradually decreases with an increase in the size of the PAH, without significant change in the free energy. The enthalpic and entropic values suggest tighter binding of the ILs with the arm chair edge as compared to zigzag edges of the PAH compounds. The investigation of thermochemistry at varying temperatures revealed that lowering the temperature leads to a decrease in the enthalpy and the free energy of adsorption, resulting in more favorable (albeit weak) IL−PAH interaction.

investigated as a function of PAH size. The results of this exercise are captured in Figures 8 and 9. The trends in Figure 8 indicate no clear trend in ΔG as the size of the PAH increases. A similar analysis of ΔS (Figure 9) indicates larger negative entropic values for ILs interacting with PAHs having arm-chair like edges (i.e., phenanthrene, perylene) in comparison to anthracene, pyrene and coronene, all of which contain zigzag edges. Clearly, there is an entropic penalty associated with preferred IL interaction at the arm-chair region that is only partially offset by the enthalpic gains involved in this interaction. Indeed, the negative entropies favor the process of desorption as temperature increases. It is noteworthy that, overall, [bmim][Tf2N] displays the least favorable interactions across all PAHs (in terms of ΔG) whereas [Pyrr14][Tf2N] interaction with PAH is ca. 7 kcal mol−1 more favorable, a remarkable observation considering that they have [Tf2N]− in common. To further explore this behavior, the effect of temperature on the thermochemical properties was studied at various temperatures, that is, 273, 298, 350, and 400 K. The results, shown in Figure 10, clearly indicate that ΔH and ΔG for adsorption of an IL on a PAH increase (become less favorable) as the temperature of the system rises. As might be expected, ΔS changes very little, although the entropy becomes somewhat less negative as temperature goes up. This affirms that IL−PAH interactions become more favorable at lower temperature with a more favorable enthalpy of adsorption at lower temperature. A further decomposition of entropy into rotational (ΔSr), translational (ΔSt), and vibrational (ΔSv) components is also possible. As Figure 11 reveals, ΔSt and ΔSr decrease as a function of temperature, but ΔSv increases with temperature. Plainly, ΔSv makes a higher contribution to the total entropy as this trend dictates the overall response of ΔS to temperature.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

This material contains 16 additional figures showing optimized geometries for ILs sorbed to various PAHs, additional DOS spectra, and supplemental CHELPG results, plus four tables collecting pertinent structural and thermochemical parameters involved in the process of IL adsorption onto model PAHs. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from an ACS PRF Grant (51865-DNI) to G.A.B. is gratefully acknowledged. The computational work was performed on the HPC resources at the University of Missouri Bioinformatics Consortium (UMBC).

4. CONCLUSIONS A molecular-level view of the IL/PAH interface has been presented to help elucidate key chemical interactions that are responsible for C-dot surface passivation and efficient exfoliation and dispersion of graphene using ILs. The geometry of the adsorbed ions at the PAH surface has been discussed at the B3LYP level of theory and 6-311G basis set. The cation predominantly interacts with the negative electrostatic surface above and below the plane of the PAH, whereas the anion has an affinity for the positive sites at the edge of the PAH. The interaction of the anion with the butyl chain tilts the cation ring with respect to PAH plane. When the cation is parallel to the PAH plane, an interplanar distance of 3.8−4.2 Å is indicative of electrostatic interaction between the negative electrostatic surface of the PAH and the cation ring in imidazolium and pyridinium ILs. The butyl chain on the cation prefers to orient away from the cation surface. The density of states structure of an IL on a PAH surface indicates significant reduction in the band gap of ILs upon adsorption. Pyridinium-based ILs show a smaller band gap as compared to their imidazolium and pyrrolidinium cationic counterparts. Imidazolium-based ILs have a smaller band gap compared to pyrrolidinium-based ILs when paired with [Br]− and [BF4]− anions and a larger gap when paired with [PF6]− and [Tf2N]−. The electrostatic potential charge analysis indicates CT from PAH to IL, thus forming a donor−acceptor complex. This suggests one potential strategy for C-dot surface passivation directed at an increased emissive quantum yield. For imidazolium ILs interacting with a PAH, an increase in CT from the PAH to

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REFERENCES

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