Imidazolium

Article pubs.acs.org/JPCC

How Does the Solvent Modulate Shuttling in a Pillararene/ Imidazolium [2]Rotaxane? Insights from Free Energy Calculations Ying Liu,† Christophe Chipot,‡,§,∥ Xueguang Shao,†,⊥,#,& and Wensheng Cai*,†,⊥,& †

Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin 300071, China Laboratoire International Associé Centre National de la Recherche Scientifique et University of Illinois at Urbana−Champaign, Unité Mixte de Recherche No. 7565, Université de Lorraine, B.P. 70239, 54506 Vandœuvre-lès-Nancy, Cedex, France § Theoretical and Computational Biophysics Group, Beckman Institute, and ∥Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States ⊥ Tianjin Key Laboratory of Biosensing and Molecular Recognition, Tianjin 300071, China # State Key Laboratory of Medicinal Chemical Biology, Tianjin 300071, China & Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China ‡

S Supporting Information *

ABSTRACT: Pillararene-based [2]rotaxanes have gained notoriety since the synthesis of the first pillar[5]arene in 2008. The marked propensity of pillararenes to bind cationic groups is often utilized to prepare functional host−guest complexes. Interestingly enough, the interaction of pillararenes with cationic groups is modulated by the nature of the solvent. The molecular mechanism that underlies binding, examined experimentally, remains, however, partially understood. In the present contribution, the solvent-controlled motion in a [2]rotaxane composed of a 1,4-diethoxypillar[5]arene (P[5]) ring threaded onto an hydrogen-bond donor imidazolium axle was investigated in eight different solvents. Apart from the polarity, the hydrogen-bond-accepting ability of the solvent was considered with particular care. In environments featuring hydrogen-bond acceptors, the P[5] tends to include the alkyl chain at one end of the axle, staying away from the cationic imidazolium unit at the other end of it. Inclusion is primarily driven by the favorable interaction of the alkyl chain with the P[5], alongside the hydrogen-bonding interaction of the imidazolium moiety with the solvent. However, in a low-polarity solvent, devoid of hydrogen-bond acceptors, the P[5] binds favorably the imidazolium moiety and the neighboring methylene groups, resulting in hydrogen bonds established between the imidazolium moiety and the P[5], and the unusual C−H···π interaction of the methylene groups adjacent to the imidazolium moiety with the benzene rings of the P[5]. The present results have important bearings on the design of artificial molecular machines formed by pillararenes and cationic moieties.

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INTRODUCTION Artificial supramolecular assemblies with fascinating molecular structures and tunable properties play a crucial role in the fields of chemistry, biomedical science, and materials science.1 Rotaxanes are a type of interlocked compounds, which are composed of a linear molecule and a macrocycle threaded onto it. In general, the macrocycle component of a rotaxane can shuttle in response to environmental changes, including light irradiation,2,3 pH variation,4 temperature,5 solvent,6,7 and other control elements.8,9 The trigger-responsive property of rotaxanes makes them critical molecular switch in a variety of nanoscale objects, such as molecular memory,10 membrane transporters,11,12 nanovalves,13,14 and product lines,15 offering applications ranging from chemical manufacturing to biological engineering. Pillararenes, a new class of macrocyclic hosts, are formed by hydroquinone rings linked mutually through their para© 2016 American Chemical Society

positions by means of methylene bridges, resulting in unique pillar architectures. 16 The highly symmetrical structure endowed with a π-rich cavity renders pillararenes ideal hosts in the design of rotaxanes and pseudorotaxanes with alkyl chain derivatives and electron-poor guests, including pyridinium cations,5 viologens,17 imidazolium cations,6,18,19 and other cationic species.20 Experiments show that the interaction of pillararenes and cationic compounds can be modulated by the polarity of solvent.6,20 For example, the monofunctionalized pillar[5]arene synthesized by Ogoshi et al.20 can form a selfinclusion complex in chloroform with the cationic part of its octyltrimethylammonium arm incorporated into its cavity. However, 1H NMR spectra reveal that in acetone dissociation Received: January 27, 2016 Revised: March 5, 2016 Published: March 7, 2016 6287

DOI: 10.1021/acs.jpcc.6b00852 J. Phys. Chem. C 2016, 120, 6287−6293

Article

The Journal of Physical Chemistry C

benzene, diethyl ether, DMSO, acetone, isobutanol, and 2butanol, resulting in eight distinct molecular assemblies. A chloride ion was placed in each solvent box, 10 Å away from the rotaxane, to ensure electric neutrality. A soft harmonic potential was used to prevent the counterion from approaching the rotaxane. Molecular Dynamics Simulations. All the atomistic MD simulations described herein were performed using the parallel, scalable MD program NAMD 2.10 with the CHARMM General Force Field (CGenFF).21,22 The ability of this force field to describe hydrogen bonds as well as π−π and C−H···π interactions has been carefully examined and validated in a preliminary investigation.23 In principle, the results are quantitatively correct for interactions involving hydrogen bonds, albeit only semiquantitatively so for the description of π−π and C−H···π interactions. Since the present work aims at investigating the various effects that the solvent exerts on shuttling, force-field inaccuracies are anticipated to not change the overall picture of the study. DMSO was described by means of the all-atom model proposed by Strader and Feller,24,26 which offers a reasonable reproduction of the experimental liquid properties. In the case of chloroform, the rigid model of Dietz and Heinzinger (DH model) was used.25,26 The parameters representing the other six solvents were taken from CGenFF.21,22 Langevin dynamics was applied to control the temperature at 298 K, and the pressure was maintained at 1 atm employing the Langevin piston method.27 Covalent bonds involving hydrogen atoms were constrained to their equilibrium length by means of the SHAKE/RATTLE algorithms.28,29 Long-range electrostatic forces were evaluated using the particle mesh Ewald scheme,30 and a smoothed 12 Å spherical cutoff was applied to truncate short-range van der Waals and electrostatic interactions. The r-RESPA multiple time-step algorithm31 was utilized to integrate the equations of motion with a time step of 2 and 4 fs for short- and long-range interactions. The MD trajectories were visualized and analyzed with VMD.32 Free Energy Calculations. The free energy profiles that characterize shuttling in the [2]rotaxane were generated using the multiple-walker adaptive biasing force (MW-ABF) algorithm,33,34 an improved importance-sampling approach aimed at achieving ergodic sampling on the basis of the ABF algorithm35−39 implemented within the collective variables module40 of NAMD. In each MW-ABF calculation, six walkers were spawned and the force samples accrued by each walker were combined every 1000 steps. The transition coordinate, ξ, was defined as the projection onto the z-axis of the Euclidian distance between the center of mass of the P[5] and that of the dodecyl chain of the axle (see Figure 1B). The axle of the rotaxane was first oriented parallel to the z-axis. A weak harmonic potential with a force constant of 1.0 kcal/(mol Å2) was then enforced on the carbon atoms located at both ends and at the center of the axle, thereby preventing it from deviating from the z-axis. As shown in Figure 1B, the transition pathway extends from −8 to +8 Å. The instantaneous values of the force were stored in bins 0.1 Å wide. Eight independent MW-ABF simulations, corresponding to the [2]rotaxane immersed in the different solvents, were carried out. The total simulation time amounted to 300 ns for each system investigated here, representing an aggregate time of 2.4 μs.

of the guest moiety from the host took place (see Figure 1A). Besides, Huang and co-workers6 designed a [2]rotaxane

Figure 1. (A) Solvent-dependent structural change of a monofunctionalized pillar[5]arene. (B) [2]Rotaxane formed by a 1,4diethoxypillar[5]arene (P[5]) and an axle molecule composed of an imidazolium unit, a dodecyl carbamic chain, and two 3,5-dimethylphenyl moieties (stoppers).

composed of a 1,4-diethoxypillar[5]arene (P[5]) host and an imidazolium axle (see Figure 1B). The 1H NMR spectra for this supramolecular assembly indicate that the P[5] binds the imidazolium moiety in chloroform and gradually moved away to include the methylene groups with increasing DMSO ratio in the binary solvent, showing a remarkable dependence of the binding site on the axle to the solvent polarity as well. Yet, the mechanism that underlies the solvent-controlled behavior has been only marginally considered in the aforementioned experimental investigations. The main thrust of the present work is to investigate the effect of the solvent on the molecular motion of the pillar[5]arene/imidazolium [2]rotaxane. Toward this end, eight solventswhich can be divided into three groups, namely high polarity with hydrogen-bond acceptors, low polarity with hydrogen-bond acceptors, and low polarity without hydrogen-bond acceptorswere considered. All-atom molecular dynamics (MD) simulations combined with free energy calculations were performed, and the potentials of mean force (PMFs) characterizing shuttling in these eight solvents were determined. To shed light on the role of the solvent and the intermolecular interactions at play, free energy contributions were determined by decomposing the total free energy into physically meaningful individual components. Put together, the present contribution provides results that allow us to understand the characteristic of pillararene/cation interactions and how these interactions are modulated by the solvent. These theoretical findings are envisioned to help design the desired nano-objects formed by pillararenes and cations, upstream from costly wet-chemistry syntheses.

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COMPUTATIONAL METHODS Molecular Models. The molecular model of a [2]rotaxane formed by a P[5] host and a dumbbell-shaped axle containing an imidazolium unit, a dodecyl carbamic chain, and two 3,5dimethylphenyl moieties was constructed, as described in Figure 1B. The geometry of the rotaxane was optimized using a conjugate-gradient algorithm and subsequently immersed in a pre-equilibrated solvent box of chloroform, dichloromethane, 6288


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RESULTS AND DISCUSSION

Free Energy Profiles. The free energy profiles characterizing shuttling of the P[5] along the axle in the eight solvents are gathered in Figure 2. It can be observed that the PMFs fall into three categories reflecting the nature of the solvent, namely mildly polar, polar, and diethyl ether.

Figure 3. Snapshots of the [2]rotaxane at the inflection points of the PMF. (A) ξ = −6.2 Å, (B) ξ = −4.0 Å, and (C) ξ = +4.0 Å. For clarity, the counterion and solvent molecules are not shown. Snapshots of the [2]rotaxane wherein ξ corresponds to −8.0 and +6.5 Å are shown in Figure S2.

Figure 2. Free energy profiles delineating the shuttling process along ξ: (A) in chloroform, (B) in dichloromethane, (C) in benzene, (D) in diethyl ether, (E) in DMSO, (F) in acetone, (G) in isobutanol, and (H) in 2-butanol. The error bars correspond to the statistical error of the free energy calculation, i.e., the precision.

to the polar solvents also drives the P[5] toward the right end of the axle. For the other six solvents employed in our simulations, there are no available experimental data suitable for direct comparison. However, the experimental observations in the DMSO/chloroform binary solvent with different ratios can be used as references.6 The P[5] was found to bind the imidazolium moiety in chloroform, gradually moving away to include the methylene groups with an increasing DMSO ratio in the binary solvent. This dependence of the molecular motion of the P[5] to the solvent polarity is also reflected in our calculations. A comparison of the free energy profiles in acetone and DMSO can be found in Figure S1 of the Supporting Information. One can reasonably conclude that the P[5] includes the imidazolium unit in low-polarity solvents and binds the methylene groups in polar solvents. However, the P[5] in diethyl ether would not bind the imidazolium moiety. As shown in Figure 2D, the global minimum emerging at +4.0 Å indicates that the P[5] includes the alkyl chain on the right alkyl station of the axle. Since diethyl ether is a nonpolar solvent, of polarity similar to that of benzene,41 the distinct behavior in the former environment, compared to the latter, implies that other factors at play contribute to the binding propensity of the P[5] to the axle aside from the solvent polarity. Free Energy Decomposition. To investigate the physical origin of the propensity of the P[5] to bind a different section of the axle, the net free energy change embodied in the PMFs was decomposed into physical contributions. Decomposition was achieved by (i) partitioning the instantaneous force acting along the model transition coordinate into P[5]−axle and P[5]−solvent contributions and (ii) binning, averaging, and integrating the components of the instantaneous force

The free energy profiles computed in low-polarity solvents such as chloroform, dichloromethane, and benzene are very similar, revealing two well-defined minima separated by a barrier (see Figure 2A−C). The global minimum of the PMF is found at about −6.2 Å, corresponding to the thermodynamically stable state, wherein the P[5] includes the imidazolium unit and its neighboring methylene groups (see Figure 3A). This result agrees well with the experimental observation that the imidazolium part and adjacent methylene groups in chloroform are ensconced of the P[5] ring.6 The structure of the [2]rotaxane corresponding to the local minimum at +4.0 Å shows that the P[5] resides over the methylene groups adjacent to the carbamic stopper (see Figure 3C). A free energy barrier of about 7.5 kcal/mol needs to be overcome for the translocation of the P[5] from the imidazolium unit to the methylene moiety at the other end of the axle. In stark contrast, the profiles determined in polar solvents, including DMSO, acetone, isobutanol, and 2-butanol, are suggestive of a different scenario. As can be observed in Figure 2E−H, the global minimum emerges at approximately +4.0 Å, thus indicating that in polar solvents the P[5] tends to stay away from the imidazolium moiety and to include the methylene chain instead (see Figure 3C). This result agrees reasonably well with the experimental observation that in DMSO the P[5] ring is on average located on the methylene groups, close to the carbamic stopper.6 Since the alkyl chain is strongly solvophobic, it follows that the interaction of the former with the P[5] cavity is more favorable than that with the high-polarity solvent. Besides, the affinity of the cationic moiety 6289


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The Journal of Physical Chemistry C independently. Evidently, the solvent plays an indirect role by modulating the interactions of the different parts, which should be mirrored in the free energy components. The free energy components that we report here result from an approximate decomposition of the free energy into physically meaningful components. The resulting free energy components, namely in chloroform, DMSO, benzene, and diethyl ether, are gathered in Figure 4the PMF in dichloromethane being similar to that in chloroform and that in DMSO resembling that in acetone, isobutanol, and 2-butanol.

The hydrogen bonds established between the three C−H donors of the imidazolium unit and the acceptor oxygen atoms of the P[5] were examined. As can be seen in Figure 6,

Figure 6. Evolution of the average number of the hydrogen bonds formed between the C−H donors of the imidazolium unit and the acceptor oxygen atoms of the P[5] in chloroform (results in other seven solvents are similar). The hydrogen-bonding criteria are (i) the angle C−H···O > 135° and (ii) the distance C···O < 3.5 Å. The hydrogen bonds of the inset have been highlighted in green.

hydrogen bonds formed as the P[5] approaches the imidazolium station. Two hydrogen bonds can be observed at best, wherein the imidazolium C(2)−H and C(4)−H act as donors. Moreover, the multiple C−H···π interactions of the methylene groups adjacent to the imidazolium moiety with the benzene rings of the P[5] are envisioned to play an important role in the binding of the former to the P[5] (see Figure 7). As

Figure 4. Decomposition of the total free energy profile into van der Waals P[5]−axle (P[5]−A), electrostatic P[5]−A, and P[5]−solvent (P[5]−S) contributions for the shuttling (A) in chloroform, (B) in DMSO, (C) in benzene, and (D) in diethyl ether.

P[5]−Axle Interactions. The P[5]−axle interactions were decomposed into van der Waals and electrostatic contributions. The former are comparable for all solvents and feature a broad valley, which stems from the favorable binding of the methylene groups of the axle to the P[5] cavity. Conversely, the electrostatic P[5]−axle interaction for each solvent reveals a significant barrier and sharply decreases as the P[5] approaches the imidazolium unit. It has been reported that the CH groups of an imidazole ring exhibit geometric preferences, strongly suggestive of hydrogenbonding interactions.42,43 The inductive effects due to the presence of two electron-withdrawing nitrogens being covalently bonded to C2 make the C(2)−H an effective hydrogen bond donor. The presence of only one nitrogen covalently bonded to C4 and C5 reduces the acidity of the C(4)−H and C(5)−H donors compared to C(2)−H (see Figure 5).42

Figure 7. Three methylene groups that exhibit C−H···π interactions with the benzene rings of the P[5] as it resides over the imidazolium station.

Table 1. Average H···Ring Center Distancea no. d/Å

CaH1 2.76

CaH2 2.89

CbH1 2.91

CbH2 2.80

CcH1 3.52

CcH2 3.34

a

The average distance between the hydrogen atoms of the methylene groups in Figure 7 and the nearest benzene, while −6.5 ≤ ξ ≤ −5.7 Å.

shown in Table 1, within the interval spanning ca. −6.5 ≤ ξ ≤ −5.7 Å, the average distance between the hydrogen atoms bonded to Ca and Cb in Figure 7, and the centroid of the nearest benzene ring is less than 3.0 Å, which meets the requirement for a C−H···π interaction.44,45 From the above results, one can conclude that (i) the hydrogen bonds formed between the imidazolium C−H donors and the acceptor

Figure 5. Schematic illustration of the imidazolium unit wherein the C(2)−H, C(4)−H, and C(5)−H act as hydrogen-bond donors. 6290


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Axle−Solvent Interactions. To quantify the effect of the solvent on shuttling, the imidazolium−solvent and the methylene−solvent interactions were computed independently. The corresponding free energy contributions are depicted in Figure 9A,B. The depth of the valley in Figure 9A represents

oxygen atoms of the P[5] and (ii) the C−H···π interactions of the methylene groups adjacent to the imidazolium moiety with the benzene rings of the P[5] contribute to the sharp decrease of the electrostatic P[5]−axle interaction as the P[5] approaches the imidazolium unit, thus constituting the main driving force for the host−guest inclusion between the P[5] and the imidazolium unit in chloroform, dichloromethane, and benzene. P[5]−Solvent Interactions. As illustrated in Figure 4, the P[5]−solvent free energy component, irrespective of the environment, possesses a broad valley. The average depths of the valleys, relative to the left-hand region of the curves, are measured to be ca. −6.4, −22.5, −8.3, and −16.8 kcal/mol in chloroform, DMSO, benzene, and diethyl ether, respectively. In other words, as the P[5] translocates from the imidazolium unit to the methylene chain close to the right stopper, the P[5]− solvent interaction drops more abruptly in diethyl ether and DMSO compared to chloroform and benzene. Similar P[5]− solvent profile can also be found for isobutanol (see Figure S3). From this point, one may wonder why the P[5]−solvent component in diethyl ether resembles that in DMSO and isobutanol. A reasonable explanation is that hydrogen bonds can be established between the imidazolium unit and these three solvents, which are all hydrogen-bond acceptors. The intermolecular hydrogen bonds established between the C−H group of the imidazolium unit and the oxygen atoms of acetone, DMSO, isobutanol, and diethyl ether have been monitored as a function of the transition coordinate (see Figure 8). As can be observed, hydrogen bonds are formed in the

Figure 9. Interaction of the solvents with (A) the imidazolium unit and (B) the methylene chain close to the carbamic moiety.

the free energy gain due to the favorable interaction of the imidazolium moiety with the solvents, as the P[5] departs from the former. It can be observed that the free energy gain in DMSO, isobutanol, and diethyl ether is greater than that in benzene and chloroform, which can be ascribed to the favorable hydrogen-bonding interactions of the imidazolium unit and the solvents possessing acceptor oxygen atoms. Not too surprisingly, the methylene−solvent free energy components decrease sharply in DMSO and isobutanol, as the P[5] approaches the carbamic stopper (see Figure 9B); that is to say, the inclusion of the methylene chain to the P[5] in these two solvents is extremely favorable on account of solvophobic interactions. The methylene−solvent interactions, however, are comparable in low-polarity solvents, such as benzene, chloroform, and diethyl ether. In a nutshell, the properties and the polarity of the solvents can modulate shuttling in the [2]rotaxane by acting on imidazolium−solvent and methylene−solvent interactions.

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CONCLUSIONS In this contribution, the [2]rotaxane designed by Huang and co-workers6 was utilized as a model to investigate the solventcontrolled inclusion involving pillararenes and cationic moieties. In general, the P[5] includes the methylene groups in polar solvents and translocates to bind the imidazolium moiety in low-polarity environments. However, the free energy profile determined in the low-dielectric diethyl ether indicates that the P[5] tends to include the methylene groups of the axle. Though the polarity of diethyl ether is very similar to that of benzene, the behavior of the [2]rotaxane in the two solvents is appreciably different. Analysis of the trajectories shows that this result can be ascribed to the hydrogen-bond-donating ability of the imidazolium unit. Hydrogen bonds formed between the imidazolium ring and the acceptor oxygen atoms of diethyl ether, pushing the P[5] to the right end of the axle. It follows that the solvent polarity and its hydrogen-bond accepting ability, the solvophobicity of the alkyl chain, the C−H···π and hydrogen-bonding interaction of the axle with the P[5], modulate the molecular motion of the [2]rotaxane. Furthermore, one can generalize that the C−H groups covalently bonded to positively charged moieties exhibit a clear preference toward hydrogen-bonding interactions. Solvent-controlled

Figure 8. Evolution of the average number of intermolecular hydrogen bonds formed between the imidazolium unit and the oxygen atoms of (A) acetone, (B) DMSO, (C) isobutanol, and (D) diethyl ether. The hydrogen-bonding criteria are (i) the angle C−H···O > 135° and (ii) the distance C···O < 3.5 Å.

range of ξ values from −4.0 to +8.0 Å, as the P[5] moves away from the imidazolium ring to the alkyl chain. In light of these observations, one may conclude that in environments featuring acceptor oxygen atoms the translocation of the P[5] to include the methylene groups at the right end of the axle is driven by favorable imidazolium−solvent interactions. Diethyl ether is a low-polarity solvent with hydrogen-bond acceptors. It can interact favorably with both the alkyl chain and the imidazolium unit. According to the free energy profile in Figure 2D, the interaction of the solvent with the imidazolium moiety is more favorable than that with the alkyl chain. The local minimum found around −4.0 Å is envisioned to stem from the competition between these two interactions. 6291


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dissociation of the monofunctionalized pillar[5]arene from an octyltrimethylammonium ion20 can therefore be rationalized in terms of a hydrogen-bonding interactions of the cationic arm and the acceptor oxygen atoms of the solvent (see Figure 1A). The results reported here disentangle the question as to how the solvent modulates pillararenes/cation interactions and, hence, pave the way for the efficient design and construction of related molecular machines controlled by the solvent.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00852. Discussion on the compatibility of the force fields; comparison of the free energy profiles in acetone and DMSO; snapshots of the [2]rotaxane wherein ξ corresponds to −8.0 and +6.5 Å; decomposition of the free energy profile in isobutanol (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (W.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is supported by National Natural Science Foundation of China (No. 21373117), MOE Innovation Team (IRT13022) of China, and Natural Science Foundation of Tianjin, China (No. 13JCYBJC18800). The CINES, Montpellier, France, and the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) are gratefully acknowledged for provision of generous amounts of CPU time. The Cai Yuanpei program is also appreciatively acknowledged for its support of the international collaboration between the research groups of C.C. and W.C.

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