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Solvent and Structure Effects on the Shuttling in Pillar[5]arene/Triazole Rotaxanes Shuangshuang Wang, Xueguang Shao, and Wensheng Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07279 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Journal of Physical Chemistry

Solvent and Structure Effects on the Shuttling in Pillar[5]arene/Triazole Rotaxanes Shuangshuang Wang,a Xueguang Shao,a,b,c,d and Wensheng Cai,*,a,b,d

a

Research Center for Analytical Sciences, College of Chemistry, Nankai University, Tianjin, 300071,

China

b

Tianjin Key Laboratory of Biosensing and Molecular Recognition, Tianjin, 300071, China

c

State Key Laboratory of Medicinal Chemical Biology, Tianjin 300071, China

d

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin, 300071, China

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ABSTRACT:

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In the present contribution, a pillararene-based rotaxane, formed by a pillar[5]arene (P[5]) and a

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dumbbell-shaped thread composed by four 1,2,3-triazole moieties alternatively linked by three methylene

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moieties and thus leading to two kinds of stations (the C-ended and N-ended ones), was investigated at the

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atomic level. The effect of the linkers on shuttling in CHCl3 was investigated by building four rotaxane

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models with different lengths of methylene groups. The free-energy profiles delineating the shuttling of the

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P[5] along the thread revealed that the shuttling rate varied regularly with the length (n) of the methylene

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moieties and exhibited the slowest value for the rotaxane (n = 5). Decomposition of the free-energy

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profiles into free-energy contributions suggested that electrostatic interactions constitute the main driving

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force responsible for shuttling. Moreover, the stability of C-ended station is found to be much lower than

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the N-ended station in each rotaxane, which can also be ascribed to the electrostatic interactions of P[5]

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with the stations. To investigate the effect of the solvent, the shuttling movement of the rotaxane (n = 4) in

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DMSO was also studied and compared to that in CHCl3. The shuttling barrier in DMSO decreased

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significantly, which can be attributed to its higher polarity and the formation of H-bonds between DMSO

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and the triazole units. Therefore, the polarity of the solvent and its hydrogen-bond acceptor ability can

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affect the shuttling rates of the rotaxanes. The present results provide understanding of the shuttling

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mechanism of the molecules formed by pillararenes and triazole moieties and are expected to serve in the

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design of pillararene-based molecular machines.

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INTRODUCTION

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Molecular devices and machines, designed to achieve specific functions, have received much attention1-5

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in nanoscience and nanotechnology. Switchable, mechanically interlocked molecules have facilitated the

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fabrication of these devices because of their ability to control the relative movement of each component.

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Rotaxanes,6-8 a type of these interlocked species, are formed by sliding a macrocycle onto an axle molecule,

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followed by the attachment of large groups (stoppers) at both termini of the axle molecule. Numerous

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external stimuli, such as temperature changes,9 pH changes,10 and solvent changes,1,2 can be responsive for

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the macrocycle in a rotaxane shuttling between two or more sites.11-13 Furthermore, a slight disparity in the

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structure of a rotaxane could lead to different site-exchange rates of the macrocycle.14 These features play

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important roles in estimating their functions as molecular shuttles and switches.15-17

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Pillararenes,18 composed of hydroquinone rings linked by methylene bridges at para-positions, are a

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novel class of macrocyclic hosts with a symmetric structure with an overall pillar-like shape. The unique

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architectures, which are endowed with a -rich cavity and a hydrogen-bonding accepting ability from

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multiple hydroxyl groups, have afforded pillararenes as useful platforms for the construction of rotaxanes

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with alkylamines, viologen and pyridinium derivatives, and imidazolium axle.18-20 The pillararenes with

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the hydroxyl groups at both sides are easy to synthesize, which can strengthen the recognition of the guest

17

molecules. A pillararene-based molecular shuttle designed by Ogoshi et al.21 served as a paradigm for a

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rotaxane reflecting the features of a pillararene. The rotaxane was formed by a 1,4-diethoxypillar[5]arene

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(P[5]) host and a dumbbell-shaped thread composed of four 1,2,3-triazole moieties with alternating

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linkages with methylene moieties (n = 4) as stations and two 2,6-diisopropyl-phenyl moieties as stoppers

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(see Figure 1). The rotaxane possessed two N-ended linkers, defined as the methylene linker between 3



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1-subsitituted 1,2,3-triazole moieties, and one C-ended linker, defined as the methylene linker between

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4-substituted 1,2,3-triazole moieties.

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The 1H NMR spectra of the rotaxane revealed that the shuttling processes of the P[5]s were solvent

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sensitive. Moreover, the macrocycle preferred to stay on the N-ended linker, rather than stay on the

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C-ended linker. Considering this observation, the following questions should be addressed: how do the

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solvents affect the shuttling of the rotaxane? What gives rise to the stability difference between the

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N-ended and C-ended linkers? In addition, does shortening or extending the linkers affect the shuttling of

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the rotaxane?

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Figure 1. Rotaxanes formed by a P[5] and an axle molecule composed of four 1,2,3-triazole moieties

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linked alternately by methylene groups with same lengths (n = 3, 4, 5, 6) and two 2,6-diisopropyl-phenyl

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moieties as stoppers. The four similar rotanxanes only differ in the lengths of their methylene groups (n = 3,

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4, 5, 6). The rotaxanes have three stations, and station 1 and station 3 are N-ended stations, while station 2

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is a C-ended station. Translocation of the P[5] between the two stations are denoted as S12 (from station 1

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to station 2) and S21 (from station 2 to station 1).

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To cogently shed light on these questions, four models with different lengths of methylene groups

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(see Figure 1) were built and investigated in chloroform (CHCl3) by all-atom molecular dynamics (MD)

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simulations combined with free-energy calculations. The free-energy contributions extracted from the

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potentials of means force (PMFs) were analyzed to interpret the physical origin of shuttling by dividing the

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total free energy into physically meaningful individual components. To investigate the effects of solvents,

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the shuttling of the rotaxane (n = 4) was also studied in DMSO for comparison with that in CHCl3.

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Together, the present contribution provided results that allow us to understand the characteristics of the

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pillararene/triazole interactions and the effect of these interactions on the shuttling of the rotaxanes. These

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theoretical findings are envisioned to help reasonably design the desired controllable molecular shuttles

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formed by pillararenes and triazoles.

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SIMULATION DETAILS

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Molecular Models. The molecular models of the rotaxanes formed by a P[5] host and a

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dumbbell-shaped axle composed of four 1,2,3-triazole moieties linked by methylene groups with different

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lengths and two 2,6-diisopropyl-phenyl moieties were constructed, as described in Figure 1. Four

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rotanxanes only differed in the lengths of their methylene groups (n = 3, 4, 5, 6). The geometries of the

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four molecular assemblies were energy minimized and then immersed independently in a periodic box of

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CHCl3 using the Solvate module of the visualization program VMD.22 For investigating the effect of the

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solvent on shuttling, the rotaxane (n = 4) was also immersed in a periodic box of DMSO to compare with

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CHCl3. During the MD simulations, the backbones of the rotaxanes were softly restrained to their extended

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conformations to avoid spurious folding of the methylene chains. For each solvated system, a 20-ns 5



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equilibrium MD simulation was performed prior to the free-energy calculations.

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Molecular Dynamics Simulations. All the atomistic MD simulations described here were performed

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using the parallel, scalable MD program NAMD 2.12,23 with the CHARMM 36 general force field

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(CGenFF).24 Visualization and analysis of the MD trajectories were performed with the VMD program.22

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The parameters for DMSO were taken from ref 25. The rigid model of Dietz and Heinzinger (DH model)26

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was used to represent CHCl3, which was merged into the CGenFF. Langevin dynamics was used to control

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the temperature at 298 K, and the Langevin piston method was applied to maintain the pressure at 1 atm.27

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Covalent bonds involving hydrogen atoms were constrained to their equilibrium length employing the

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SHAKE/RATTLE algorithms.28,29 The r-RESPA multiple time step algorithm30 was employed to integrate

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the equations of motion with a time step of 2 and 4 fs for short- and long-range interactions, respectively.

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Short-range van der Waals and electrostatic interactions were truncated using a smoothed 12.0 Å spherical

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cutoff, and long-range electrostatic forces were evaluated by means of the particle-mesh Ewald method.31

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Free-Energy Calculations. The free-energy profiles characterizing shuttling in the rotaxanes were

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generated using the multiple-walker adaptive biasing force (MW-ABF) algorithm32,33 which is an

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improved, importance-sampling approach aimed at achieving ergodic sampling based on the ABF

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algorithm,33-37 implemented within the collective variables module (Colvar)38 of NAMD. The model

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reaction coordinate, , was defined as the projection onto the z-axis of the Euclidian distance between the

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center of mass of the P[5] and that of the methylene group in the middle of the axles (see Figure 1). The

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axles of the rotaxanes were first oriented parallel to the z-axle. A weak harmonic potential with a force

20

constant of 1.0 kcal/(mol Å2) was then enforced on the carbon atoms of all the methylene groups in the

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axle of the rotaxanes (n = 4-6), thereby preventing them from deviating from the z-axis. For the rotaxane 6


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(n = 3), a restraining force of 2.0 kcal/(mol Å2) was enforced but only on the carbon atoms of the

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methylene groups connected to the stoppers and in the middle of the three methylene chains. As shown in

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Figure 1, since the lengths of the methylene groups in these rotaxanes are different, the spans of the

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transition pathways explored were altered accordingly. Therefore, the transition pathways extended from

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-8.5 Å  ξ  8.5 Å for the rotaxane with n = 3, -11 Å  ξ  11 Å for n = 4, -12 Å  ξ  12 Å for n = 5, and

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-14 Å  ξ  14 Å for n = 6. Instantaneous values of the force were accrued in bins 0.1 Å wide. The

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variation of the free energy, ΔG(ξ), was determined by integrating the average force acting on ξ. All the

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MW-ABF simulations were carried out in CHCl3 or DMSO at 298 K. The total simulation time amounted

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to 0.63, 0.65, 0.72 and 0.81 μs for the rotaxanes (n = 3, 4, 5 and 6), respectively, in CHCl3 and 1.01 μs for

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the rotaxane (n = 4) in DMSO, representing an aggregate time of 3.82 μs.

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RESULTS AND DISCUSSION

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Effect of the Lengths of the Linkers on Shuttling. The free-energy profiles characterizing the

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shuttling of the P[5] along the axles of the rotaxanes (n = 3, 4, 5, 6) in CHCl3 are gathered in Figure 2. It

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can be observed that these profiles have three apparent valleys separated by two barriers, except that of the

16

rotaxane (n = 3), which illustrates that the rotaxanes have three stations: station 1 and 3 for the N-ended

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linkers and station 2 for the C-ended linker. Since the structures of the all rotaxanes are symmetric, the two

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stations, station 1 and 3, have the same stability, leading to the P[5] shuttling between them. It can also be

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seen that in each rotaxane, the P[5] located on the N-ended stations (station 1 and 3) is more stable than on

20

the C-ended one (station 2). This result agrees well with experimental observations.21 Moreover, for the

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rotaxanes (n ≥ 4), the free-energy differences between N- and C-ended stations in the free-energy barriers 7



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of S12 and S21 are all approximately 3.8 kcal/mol, indicating that the stability discrepancy between the two

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stations will not be changed by lengthening or shortening the linkers providing the lengths of the two

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linkers are the same.

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Figure 2. Free-energy profiles delineating the shuttling process along ξ in CHCl3; n is the length of the

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methylenes for each linker; the error bars correspond to the statistical error of the free-energy calculation.39

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By comparing the free-energy profiles of the different rotaxanes, the barrier for shuttling from station 1

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to station 2 (S12) first rapidly increases with a gradual lengthening of the linkers, and that of the rotaxane (n

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= 5) reaches the highest value, while a slight decrease is observed for the rotaxane (n = 6). In other words,

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the shuttling rate of the P[5] in the rotaxane (n = 5) is the slowest. For the rotaxane (n = 3), the length of

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the linkers is too short to form a relatively stable station for the P[5], leading to very low barriers 8


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(approximately 3.6 kcal/mol) and thus a fast shuttling rate.

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In light of the above observations, it is reasonable to deduce that (1) although the structures of the

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N-ended stations are similar to that of the C-ended one, the stabilities of the corresponding structures of the

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rotaxanes are different from each other, and the former is more stable; (2) the lengths of the linkers have a

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marked impact on the free-energy barriers, which affect the shuttling rates.

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Free-Energy Decomposition. To investigate the physical origin of these observations, the net

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free-energy change embodied in each free-energy profile was divided into physical contributions. The

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breakdown was achieved by partitioning the instantaneous force acting along the model transition

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coordinate into the P[5]-axle contribution and binning, averaging, and integrating the force. The results are

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gathered in Figure 3, indicating that the P[5]-axle interactions were the main contributions to the barriers

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of the PMFs.

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The P[5]-axle interactions were further decomposed into van der Waals (P[5]-axle-vdw) and

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electrostatic (P[5]-axle-elec) contributions. The P[5]-axle-vdw for the all rotaxanes generally feature a

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wide valley, which is because the size of the P[5] cavity is suitable to include the linear molecules. In

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contrast, the P[5]-axle-elec for the all rotaxanes have two barriers and show similar tendencies as the

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free-energy profiles, which illustrates that electrostatic interactions determine the stable binding site of the

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P[5] and constitute the main driving force responsible for shuttling.

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Figure 3. Breakdown of the total free-energy profiles into P[5]−axle contributions, which were further decomposed into electrostatic (P[5]-axle-elec) and van der Waals (P[5]-axle-vdw) contributions, for the shuttling processes of the rotaxanes (n = 3, 4, 5, 6) in CHCl3.

The P[5]-axle-elec contributions feature three valleys, which correspond respectively to the three stations, and the two side ones are far lower than the middle one, suggesting that these contributions are the primary cause of the observation that the P[5] located on the N-ended linkers are more favorable for structural stability than that located on the C-ended one in each rotaxane. For further understanding, electrostatic interactions of P[5] with different stations in the rotaxanes with different n were computed independently. Figure 4 shows the free-energy components corresponding to the electrostatic interactions

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of P[5] with station 1 (P[5]-NN-elec), and station 2 (P[5]-CC-elec) for the rotaxanes (n = 3-6), using the same method for Figure 3. Here, station 1 or 2 includes the N- or C-ended linker and the triazole groups at both ends. Therefore, in the calculation of the P[5]-NN-elec and P[5]-CC-elec contributions, in addition to P[5], only the atoms of the linker and the triazole groups at both ends were selected. According to the results shown in Figure 4, it can be seen that, regardless of the length of the linkers, the P[5]-NN-elec interactions are stronger than the P[5]-CC-elec interactions, which is mirrored in the P[5]-axle-elec components in Figure 3. In other words, this is because of the methylene chains connecting to different positions on the triazole rings, leading to different charge distributions of station 1 and 2 (see the Supporting Information), and thus resulting in a great difference between the P[5]-NN-elec and the P[5]-CC-elec interactions.

Figure 4. Electrostatic interactions of the P[5] with station 1, including the methylene units and the triazole 11



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units of both ends (P[5]-NN-elec), colored by the blue line and station 2 (P[5]-CC-elec) colored by the red

2

line along  for the rotaxanes (n = 3, 4, 5, 6).

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As illustrated in Figure 3, the P[5]-axle-elec first increases and then decreases with a gradual

5

lengthening of the linkers, showing a very small difference between n = 4 and 5. However, the free-energy

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barriers of the rotaxane (n = 4) is obviously lower than that of the rotaxane (n = 5). According to Figure 3,

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the reason is that the van der Waals interactions of the P[5] and the axle weaken the contribution of the

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P[5]-axle-elec to the total free-energy barriers. Moreover, Figure 4 indicates that the decrease of the

9

shuttling barrier (S12) for the rotaxane (n = 6) compared to the one (n = 5) can be ascribed to the decrease

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of the electrostatic interaction of the P[5] with station 1. This decrease is due to the length of the linker in

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the rotaxane (n = 6) being too long for the two triazole groups at both ends of the linker to form strong

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interactions with the ethoxy groups at both tori of the P[5], leading to an overall decrease of the

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electrostatic interaction of the P[5] with station 1. It can be concluded that the matching of the length

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between the linker and the P[5] is important for the stability of the station. Furthermore, according to the

15

trend in Figure 2, it can be inferred that the barriers of the rotaxanes with longer linkers (n = 7, 8) may

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decrease slightly as compared to the rotaxane (n = 6).

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Analysis of the Hydrogen-Bonding and C−H···π Interactions. It has been reported that the CH group and

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N atoms of a triazole ring exhibit geometric preferences, which allows the ring to possess the ability of

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forming hydrogen-bonding interactions.40 The inductive effects due to the electron-withdrawing N atoms

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make the C(5)−H an effective hydrogen-bond (H-bond) donor and N(2) and N(3) effective H-bond

21

acceptors (see Figure 5a). These atoms can form H-bonds with the P[5], which bears acceptor O atoms and 12


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C−H donors, particularly those formed between the C−H donors of the triazole units and the acceptor O atoms of the P[5] (C−H···O) and between the acceptor N atoms of the triazole units and the C−H donors of the P[5] (C−H···N). The schematic illustration of the H-bonds is shown in Figure 5b. Therefore, to further analyze the P[5]-axle-elec of the rotaxanes, the hydrogen-bonding (H-bonding) interactions of the P[5] and the axles are discussed.

Figure 5. (a) Schematic illustration of the triazole unit wherein the C(5)−H acts as a hydrogen-bond donor and N(2) and N(3) act as hydrogen-bond acceptors. (b) The hydrogen bonds formed between (i) the C−H donors of the triazole unit and the acceptor O atoms of the P[5] (C−H···O) and (ii) the acceptor N atoms of the triazole unit and the C−H donors of the P[5] (C−H···N). The hydrogen-bonding criteria are (i) the 13



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angle C−H···O(N) > 135° and (ii) the distance C···O(N) < 3.5 Å. The hydrogen bonds of (b) are highlighted in orange.

Figure 6. Evolution of the average number of (i) the C−H···O H-bonds (colored black), (ii) the C−H···N H-bonds (colored red) and (iii) the total intramolecular H-bonds formed between the P[5] and the axle (colored magenta) in the rotaxanes (n = 3, 4, 5, 6).

The evolution of the average number of the intramolecular H-bonds by analyzing the trajectories is shown in Figure 6. At first glance, H-bonds are formed between the P[5] and the axle when the P[5] includes the methylene-chain groups, and the C−H···O H-bonds are more abundant than the C−H···N ones, and thus, the former bonds are the main H-bonding contributors for the all the rotaxanes. As the P[5] deviates from the methylene chains, the H-bonds are broken gradually and reach nearly no interaction, 14


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while the triazole units are encapsulated in the P[5] cavity. This indicates that the destruction of the

2

H-bonds contributes to the barriers of the P[5]-axle-elec. For the rotaxane (n = 6), the number of H-bonds

3

is far less than that of the other rotaxanes. The fragment structures of the rotaxanes at the left minimum

4

points of the free-energy profiles are shown in Figure 7. We can see that with the methylene chains of the

5

rotaxanes lengthening gradually, the bond lengths of the H-bonds increase accordingly. Therefore, due to

6

the long methylene chains of the rotaxane (n = 6), the number of H-bonds is greatly reduced when the P[5]

7

locates in the middle of the former, leading to a sudden drop in the curves.

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Figure 7. Snapshots of fragments of the rotaxanes (n = 3, 4, 5, 6) at the left minimum points of the

11

free-energy profiles.

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On the other hand, the methylene groups can form C−H∙∙∙π bonds with the benzene rings of the P[5],

14

and these interactions could play an important role in the binding of the axle to the P[5]. The average 15



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number of the C−H···π bonds (π-bonds) in the rotaxanes along ξ is shown in Figure 8. Four π-bonds are

2

formed as the P[5] is located near the methylene groups, and no bonds are formed when the P[5] is located

3

at the triazole units. Moreover, upon increasing the length of the linkers, the space containing π-bonds

4

gradually broadens, which partly explains the discrepancy of the P[5]-axle-elec among the rotaxanes. From

5

an overall analysis of the results of the H-bonding and C−H∙∙∙π interactions for the rotaxanes (n = 4, 5), the

6

number of H-bonds of the former is more than that of the latter, while the space with C−H∙∙∙π bonds of the

7

latter is wider, leading to similar trends of the P[5]-axle-elec. For the rotaxanes (n = 3, 6), the barriers of

8

the P[5]-axle-elec of the former are low due to the narrow space with C−H∙∙∙π bonds. However, for the

9

latter, although the space with the C−H∙∙∙π bonds is widest, the minimum number of H-bonds lead to

10

reduced barriers of the P[5]-axle-elec.

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Figure 8. Evolution of the average number of C−H···π bonds in the rotaxanes (n = 3, 4, 5, 6).

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1 2

From the above results, one can conclude that (i) the overall electrostatic interactions between the P[5]

3

and the stations can mainly explain the free-energy profiles and the stability difference between the

4

C-ended station and the N-ended station; (ii) the matching of the length between the linker and the P[5] is

5

important for the stability of the stations.

6

With the increase of n, the contribution of entropy becomes significant. For the rotaxanes investigated in

7

this study, when the methylene linker is included by P[5], the conformation of this part is found to be

8

almost extended and rigid (See Figure S1 and S2 in the Supporting Information), due to the limited space

9

of the cavity of P[5], thus resulting in a loss of conformational entropy. The loss of entropy can be gained

10

again, when the P[5] moves away from the methylene chain. It means that the increased entropy facilitates

11

the shuttling of the P[5]. In our ABF simulations, the methylene linkers in the rotaxanes were softly

12

restrained. Therefore, it is difficult to estimate accurately the entropic contribution.

13

Effect of the Solvent on Shuttling. According to the above conclusion, the electrostatic interactions of

14

the P[5] and the axle are the main contributor to the barriers of the PMFs. Then, does the polarity of the

15

solvent effect the shuttling rates? To answer this question, the shuttling behavior of the rotaxane (n = 4) in

16

DMSO was studied by comparing with the results in CHCl3 to uncover the effect of the solvent on the

17

shuttling rate.

18

Free-Energy Profiles. The free-energy profiles delineating the shuttling processes of the rotaxane (n = 4)

19

along ξ in CHCl3 and DMSO are gathered in Figure 9. The free-energy barrier of S12 in DMSO is

20

remarkably lower than that in CHCl3, and thus, the shuttling rate is obviously faster, which is in agreement

21

with the experimental observation.21 On one hand, the polar DMSO solvent would weaken the electrostatic 17



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Page 18 of 24

interactions between the P[5] and the linkers; on the other hand, DMSO can form H-bonds with the triazole units.

Figure 9. Free-energy profiles delineating the shuttling process of the rotaxane (n = 4) along ξ in CHCl3 and DMSO. The error bars correspond to the statistical error of the free-energy calculation.39

H-Bonding Interactions between the Axle and Solvent. CHCl3 cannot form H-bonds, while DMSO bears an acceptor O atom and can form H-bonds (C−H···O) with the C−H donors of the triazole units. The evolution of the average number of intermolecular H-bonds formed between the solvents and the axle of the rotaxane (n = 4) by analyzing the trajectories is shown in Figure 10. Not too surprisingly, nearly no H-bonds are formed during the shuttling process in CHCl3. In DMSO, the solvent can form H-bonds with 18


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1

the triazole groups, and the maximum point of the H-bonding profile is almost two times as high as the

2

minimum one. According to the above analysis, the P[5] can form H-bonds with the triazole units when

3

the P[5] is located at the methylene-chain groups. Once the P[5] moves away from the stable binding sites,

4

the H-bonds will be broken gradually, which partly contribute to the free-energy barriers. In DMSO, the

5

solvent can form H-bonds rapidly with the triazole units, which offsets the loss of energy caused by the

6

destruction of the H-bonds between the P[5] and the axles during shuttling and lowers the free-energy

7

barriers. However, in CHCl3, the solvent cannot form H-bonds with the axles, and the energy loss cannot

8

be offset. To summarize, the polarity and the capability of forming H-bonds of the solvents together affect

9

the barriers and then modulate the shuttling of the rotaxane.

10 11

Figure 10. Evolution of the average number of intermolecular H-bonds formed between the solvents

12

(CHCl3 and DMSO) and the axle of the rotaxane (n = 4).

13 14

CONCLUSIONS

15

In this contribution, the [2]rotaxanes designed by Ogoshi et al.21 were utilized as models to investigate

16

the shuttling behaviors of P[5] affected by the structures of the rotaxanes and solvent. Comparing the 19



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Page 20 of 24

1

results of the free-energy calculations of the rotaxanes with linkers of different lengths, the free-energy

2

barriers first rapidly increased and then slightly decreased with the gradual lengthening of the linkers, and

3

that of the rotaxane (n = 5) reached the highest value. Therefore, the shuttling rate of the P[5] in such types

4

of rotaxanes could be modulated by the lengths of the linkers. Partitioning the PMFs into different

5

components revealed that the electrostatic interactions between the P[5] and the axles constitute the main

6

driving force responsible for shuttling. Moreover, the stability of the stations was very sensitive to the

7

position in the triazole rings that the methylene chains connect to, resulting from the significant difference

8

in electrostatic interactions between the P[5] and the stations. In addition, with the increase of n, the

9

increased conformational entropy of the methylene chain facilitates the shuttling of P[5]. Furthermore,

10

comparing the free-energy profiles of the rotaxane (n = 4) in CHCl3 with that in DMSO, the results

11

suggested that the barriers for shuttling in DMSO are remarkably lower than those in CHCl3, which can be

12

ascribed to the significant weakening of the hydrogen-bonding interactions of the P[5] with the axle in

13

DMSO. In conclusion, these factors, including the electrostatic interactions of the axle with the P[5], the

14

solvent polarity and its H-bond-accepting ability, together modulated the molecular motions of the

15

rotaxanes. The present results not only provide an understanding of the solvent modulation ability for

16

shuttling in the pillararene/triazole rotaxanes but also disentangle the effects of the structural modifications

17

on the motions. These new insights are expected to serve the efficient design and construction of molecular

18

machines.

19 20

Supporting Information

21

This material is available free of charge via the Internet at http://pubs.acs.org. 20


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1

Snapshots of the rotaxanes corresponding to the minimum points of the PMFs. Charge distributions of P[5]

2

and two stations in the axle molecule.

3 4

AUTHOR INFORMATION

5

Corresponding Author

6

* Email: [email protected].

7

Notes

8

The authors declare no competing financial interests.

9

ACKNOWLEDGMENTS

10

This study is supported by the National Natural Science Foundation of China (Nos. 21373117 and No.

11

21773125). The Special Program for Applied Research on Super Computation of the NSFC-Guangdong

12

Joint Fund (the second phase) under Grant No. U1501501 is gratefully acknowledged for providing

13

generous amounts of CPU time.

14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

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TOC Graphic

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Triazole

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