Control of Diffusion and Conformation Behavior of Methyl

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Control of Diffusion and Conformation Behavior of Methyl Methacrylate Monomer by Phenylene Fin in Porous Coordination Polymers Masayoshi Takayanagi,†,‡ Srimanta Pakhira,† and Masataka Nagaoka*,†,‡,§ †

Graduate School of Information Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan Core Research for Evolutional Science and Technology, Japan Science and Technology Agency, Honmachi, Kawaguchi 332-0012, Japan § ESICB, Kyoto University, Kyodai Katsura, Nishikyo-ku, Kyoto 615-8520, Japan ‡

S Supporting Information *

ABSTRACT: Radical polymerization in nanoscale channels of porous coordination polymers has been explored as a promising technique to control the properties of product polymers. In particular, tacticity of poly(methyl methacrylate) was successfully controlled by utilizing onedimensional channels of [M2(L)2TED]n type of porous coordination polymers by changing the dicarboxylate ligand L. Toward understanding the atomistic mechanism of this tacticity control, we computationally studied the behavior of methyl methacrylate monomers in the onedimensional channels by molecular dynamics simulations. Smooth monomer diffusion in the direction along the channel was shown as expected from the channel shape. In addition, we confirmed that the monomers can pass through the narrow apertures between the channels and can diffuse slowly in the direction perpendicular to the channel. In the channels, the ratio of the s-trans and s-cis conformations of the monomers is different from that in monomer liquid. We found two factors affecting these behaviors: the host−guest electrostatic interactions and the anisotropic shape of the channels by the planar and nonplanar dicarboxylate ligands (L). These factors will contribute to understanding of the tacticity modification mechanism by different ligands from the atomistic point of view.

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histamine11 or the complex of Li+ ions and polyethylene glycol (PEG). 12 PEGs accommodated in the framework of [Zn2(BDC)2TED]n exhibited unusual dependence of the thermal transition temperature on the polymer length.13 The crystalline sponge method utilized well the host−guest interactions to determine structures of guest organic compounds by X-ray method without crystallization.14,15 Radical polymerization of vinyl monomers accommodated in channels of PCPs can control the properties of product polymers.16−23 These indicate the promising functions of guest-accommodated PCPs. Understanding the transfer (or diffusion) behavior of guest molecules inside PCPs are important for efficient use of guestaccommodated PCPs. For example, if the guest transfer in onedimensional channels of PCP is limited only in the direction along the channels, full filling of the channels with the guest is not possible since some of the channels should be plugged by impurity or crystal defects. The guest diffusion behaviors have been experimentally and computationally studied by observing

INTRODUCTION Porous coordination polymers (PCPs) or metal organic frameworks (MOFs) are composed of metal cations and organic ligands and extensively studied because of their promising functions derived from their porous frameworks. Many kinds of PCP frameworks with different sizes, surface property, and topology of channels or pores have been synthesized with various combinations of metals and ligands.1−4 Such channels accommodate guest molecules, and, when appropriate combinations of host PCP and guest molecules are chosen, one can achieve efficient functions of gas storage, separation, sensing, catalysis, radical polymerization, and so on. Guest accommodation in the framework of PCP is considered as a hybridization approach to improve the functions of PCPs.3 A prominent example is the CO2 sensing by utilizing the breathing behavior, or the framework deformation by guest inclusion,5−8 of the framework of [Zn2(BDC)2TED]n (BDC = 1,4-benzenedicarboxylate, TED = triethylenediamine) with guest luminescent distyrylbenzene molecules.9 Another prominent example is the explosive detection by using luminescent PCP.10 High proton and ionic conductivity was achieved by the accommodation of the © XXXX American Chemical Society

Received: September 24, 2015 Revised: November 19, 2015

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DOI: 10.1021/acs.jpcc.5b09332 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. (a) MD simulation model composed of 12 × 3 × 3 PCP framework and 292 MMA monomers. The PCP framework and MMA monomers are drawn in gray thin lines and thick bonds with different colors for each molecule, respectively. Hydrogen atoms were omitted for simple visualization. (b) Narrow aperture of the PCP connecting adjacent channels. (c) Window of the PCP along the c-axis direction channels.

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diffusion constants. While the three-dimensional isotropic diffusion was observed in isoreticular PCPs such as IRMOF1,24−27 the anisotropic diffusion along the channel direction was observed in anisotropic framework PCPs with one-dimensional channels.28−34 Moreover, the spatial distribution, adsorption site, and conformation of guest molecules in the framework of PCPs are also important for the PCP functions, especially catalysis. The mechanism of the PCP-channel-promoted polymerization of acidic acetylenes involves the acid−base interaction between the guest molecule and the surface-exposed oxygen atoms of the PCP.35,36 Recently, transient multistep guest adsorption in channels of metal-macrocycle framework with multiple adsorption sites was analyzed by in situ singlecrystal X-ray diffraction.37 In this paper, we have computationally studied the molecular behavior of guest methyl methacrylate (MMA) monomers in the one-dimensional channels of [M2(L)2TED]n (M = Zn or Cu, L = dicarboxylate ligands, and TED = triethylenediamine) by molecular dynamics (MD) simulations. Uemura et al. utilized the channels of [M2(L)2TED]n for MMA polymerizations and succeeded in controlling properties of product poly(methyl methacrylate) (PMMA) such as conversion rate, molecular weight, and tacticity by changing the dicarboxylate ligand L.18,19 However, atomistic mechanism for the PMMA property modification by the different ligand is not known and still elusive. One of the possible factors for the property control of PMMA is the polarity of the ligands L in PCP because the most significant tacticity change was obtained with 2,5dimethoxy-1,4-benzendicaboxylic acid (DMBDC), which corresponds to the substitution of the two hydrogen atoms of BDC with polarized −OCH3 groups.19 Another possible factor is the equilibrium conformation of the ligand in the PCP. X-ray crystallographic structures of [Cu2(L)2TED]n reveals planar and nonplanar equilibrium structures depending on the substituent at the benzene ring hydrogen atoms in BDC.38 To investigate the atomistic mechanism, MD simulation is a powerful tool because it can simulate dynamics of each molecule in the PCP channels and can provide atomistic information.

COMPUTATIONAL METHODS

Simulation Models. We used General AMBER Force Field (GAFF)39 version 1.4 for MMA monomers with modification of the dihedral parameters (Table S1, Supporting Information). The atomic charges were determined by CHelpG method at the B3LYP/6-311G(d,p) level of density functional theory (DFT) calculation at the most stable conformation. The atom names, atom types in GAFF, and atomic charges of the monomer are shown in Figure S1. We prepared three [Zn2(BDC)2TED]n PCP framework models, MD_PCP, MD_PCPzeroCharge (PCP framework with no atomic charges), and MD_PCP45deg (PCP framework with 45° rotated BDC ligands). For MD_PCP, we used force field parameters including the atomic charges reported in our previous work,33 which were based on the parameters of DMOF-1 (DMOF-1 stands for [Zn2(BDC)2TED]n) in ref 8 (Figure S2 and Table S2). For MD_PCPzeroCharge, all the atomic charges of the PCP framework atoms were modified to be zero. For MD_PCP45deg, the dihedral term for the rotation of the BDC benzene ring was modified to shift the equilibrium angle from 0° (or 180°) to ±45° (or ±135°) (Table S3). All host− guest interactions were described with Lennard−Jones interactions with the Lorentz−Berthelot combination rule and Coulomb interactions with the atomic point charges. The initial PCP framework structure was retrieved from the Cambridge Structural Database (CCDC-238860).40 The unit cell size was 10.9 × 10.9 × 9.8 Å along the a-, b-, and c-axes. We built 12 × 3 × 3 PCP framework model with the periodic boundary condition. According to the adsorbed monomer amount, 2.7 molecules per unit cell, in the polymerization experiment,18 292 MMA monomers were included in the PCP channels (Figure 1a). We also prepared a liquid MMA monomer system (MD_liquid) composed of 261 monomers with the periodic boundary condition as a reference system. All the quantum chemical and DFT calculations were executed by Gaussian09 program package.41 All the molecular images were produced by VMD 1.9.1 program.42 MD Simulation Procedure. All MD simulations were executed using AMBER12 PMEMD program43 under periodic boundary conditions. The PMEMD program was modified to B

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modified the monomer force field parameters (Figure S1 and Table S1). Using these fitted parameters, we executed MD simulations of MMA monomers in the framework of the [Zn2(BDC)2TED]n PCP (MD_PCP) and liquid MMA monomer (denoted as MD_liquid) as reference. Parallel Alignment of Two MMA Monomers along One-Dimensional Channels. We investigated the framework dynamics in MD_PCP before focusing on the MMA monomers. During the MD simulation, the framework structure was almost stable with the exception of TED ligand rotation every several picoseconds. This is consistent with the crystal structure of [Cu2(BDC)2TED]n PCP.38 Thus, the framework kept the shape of the narrow apertures along the a- and b-axes directions (Figure 1b) and the windows along the c-axis direction (Figure 1c). These apertures and windows are important for the behavior of the guest monomers. The trajectory of MD_PCP provides information on the behavior of MMA monomers in the PCP framework. We found that two monomers tend to align parallel in the channels (Figure 3a). Density distribution of O1 and C5 atoms of

handle covalent bonds across the periodic boundary box to correctly simulate the periodically connected PCP framework. Integration time step was 2 fs with the exception of 1 fs for the MD simulations at 1500 K. The SHAKE algorithm was used to constrain the bond distances including hydrogen atoms. The procedure of MD simulations for the three PCP systems, MD_PCP, MD_PCPzeroCharge, and MD_PCP45deg, are as follows. First, an NVT (constant volume and temperature condition) MD simulation at 1500 K was executed for 1 ns with positional restraint on the PCP framework to randomize the distribution of MMA monomers. Then an NVT MD simulation at 343 K for 100 ps with the restraint and an additional NVT MD simulation at 343 K for 1 ns without the framework restraint were executed to equilibrate the system. Finally, a production NVT MD at 343 K, which is an experimental temperature of the polymerization,18,19 was executed for 300 ns (MD_PCP), 100 ns (MD_PCP zeroCharge ), or 50 ns (MD_PCP45deg). These MD simulation lengths were determined to calculated diffusion constants. MD simulations of the MD_liquid were performed as follows. First, an NVT MD simulation at 1500 K was executed for 1 ns to completely randomize the distribution of MMA monomers. Then the system was equilibrated by an NPT (constant pressure and temperature condition) MD simulation at 1 atm and 343 K for 5 ns. Finally, a production NPT MD was executed at 1 atm and 343 K for 50 ns.

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RESULTS AND DISCUSSION Conformational Energy of MMA Monomer. At first, we executed the quantum chemical calculations of an MMA monomer to prepare the molecular mechanics (MM) force field parameters focusing on the dihedral rotation. At the B3LYP/6-311G(d,p) level of DFT optimization, we found strans and s-cis conformations (Figure 2). The relative

Figure 3. (a) Typical snapshot of MMA monomers in the channel of the PCP. (b) Density distribution of O1 (cyan) and C5 (red) atoms of monomers calculated by time-averaging the MD_PCP trajectory. Locations with density >0.005 Å−3 are drawn by solid surfaces.

monomers reveals the one-dimensional distribution along the caxis direction channels (Figure 3b). The positively charged C5 atoms (the sum of the atomic charges of the CH3 group is +0.222e) are localized at the corners of windows, or in the vicinity of the carboxylate groups of BDC, which is identical to the methane sorption site,49 and the negatively charged O1 atoms (−0.516e) are localized to avoid the windows. Anisotropic Diffusion of MMA Monomers in OneDimensional Channels. Next, diffusion behavior of MMA monomers were analyzed. As predicted from the abovedescribed one-dimensional distribution, monomers smoothly diffuse in the c-axis direction along the channels. Moreover, they can also transfer to the adjacent channel by passing through narrow apertures (Figure S3). To calculate diffusion constants of monomers of each direction (Dx, where x = a, b, or c), the mean square displacement of each direction (MSDx) must satisfy a condition of a slope close to 1 in log(MSDx) to log(t) plot.50 Since all the MSDx satisfied the condition (Figure S4), we calculated Dx = gradient(MSDx)/2 by the least-squares fit of MSDx curves in the linear behavior time range51 as

Figure 2. Conformations of MMA monomer calculated at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p) level of calculations. Carbon, oxygen, and hydrogen atoms are drawn in cyan, red, and white, respectively. The relative energy is shown in kcal/mol. The dipole moment (μ) by the MM force field is shown in Debye (D).

conformational energy difference and the rotational energy barrier between the s-trans and s-cis conformations with the dihedral angle restraint at 90° were calculated at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311G(d,p) level of theory. The most stable conformation was s-trans. The relative conformational energy of the s-cis conformation was +0.28 kcal/mol and the rotational energy barrier was +5.04 kcal/mol. The obtained conformations and energies are consistent with previous DFT calculations.44−48 By fitting the conformational energy difference and the rotational energy barrier in the conformations optimized by the MM force field parameters, we C

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The Journal of Physical Chemistry C summarized in Table 1 (see Supporting Information for details). It should be noted that the long 300 and 100 ns Table 1. Diffusion Constants (Dx) of MMA Monomers with the Width of 95% Confidential Intervals in Parentheses Dx (10−10 m2 s−1) MD_liquid MD_PCP MD_PCPzeroCharge MD_PCP45deg

average of Da and Db

Dc

31.6 (0.5) 0.026 (0.006) 0.047 (0.011) 0.84 (0.13)

32.0 (0.7) 5.3 (0.9) 10.3 (1.7) 1.26 (0.23)

Figure 4. Autocorrelation function of dihedral angle rotation of MMA monomers.

MD simulation was necessary for MD_PCP and MD_PCPzeroCharge, respectively, to achieve the slope condition in the a- and b-axes. In the case of MD_liquid, as expected, Dx is isotropic as Da = Db = Dc = 3.2 × 10−9 m2 s−1. Meanwhile, in the case of MD_PCP, Dx is obviously anisotropic as the average of Da and Db is 2.6 × 10−12 m2 s−1 and Dc is 5.3 × 10−10 m2 s−1. The two orders of magnitude difference between Dc and Da (Db) qualitatively indicates the anisotropic diffusion behavior of MMA monomers with the smooth diffusion along the c-axis and the slow interchannel diffusion along the a- and b-axes. The Dx anisotropy is consistent with those of benzene molecules in the channels of [Zn2(BDC)2TED]n measured by the NMR method, 1 × 10−11 m2 s−1 along the channels and 3 × 10−13 m2 s−1 perpendicular to the channels.32 Conformational Behavior of MMA Monomers in PCP Channels. The conformation of MMA monomers also provides information on the monomer behavior. In the case of MD_liquid, the conformational ratios of s-trans and s-cis were 62.3% and 37.7% (Table 2). The s-trans ratio is similar to,

that the monomer conformational change occurs more slowly in the PCP channels than in liquid. Table 3. Relaxation Time of Autocorrelation Function of MMA Dihedral Angle Rotation

s-transa s-cisb

MD_PCP

MD_PCPzeroCharge

MD_PCP45deg

62.3% 37.7%

59.7% 40.3%

61.7% 38.3%

62.2% 37.8%

Dihedral angle of MMA monomer ϕ (C3−C2−C4−O1) satisfies 90° < ϕ < 270°. bϕ satisfies 0° < ϕ < 90° or 270° < ϕ < 360°.

a

but a little smaller than, the ratio 60.2% predicted from the Boltzmann factor exp(−ΔE/kBT) with the conformational energy difference ΔE = 0.28 kcal/mol and the temperature T = 343 K. In the case of MD_PCP, the s-trans ratio is 59.7% with the 2.6% decrease compared to the MD_liquid. We can analyze the conformational dynamics of MMA monomers by calculating the autocorrelation function (ACF) of the monomer dihedral angle ϕ(C3−C2−C4−O1). Since we only focus on the conformational changes between s-cis and strans, we first projected ϕ into binary as ⎧ 0 90° < ϕ < 270° f (ϕ) = ⎨ ⎩1 otherwise

(1)

and then the ACF was calculated as ACF(t ) = ⟨f (ϕ(t ))f (ϕ(0))⟩

MD_PCP

MD_PCPzeroCharge

MD_PCP45deg

138 ps

212 ps

263 ps

165 ps

Two Factors of Host−Guest Interaction on MMA Monomer Behavior. Here we discuss the difference between MD_liquid and MD_PCP. We can point out two factors of the host PCP channels affecting the behavior of guest MMA monomers. The first factor is the electrostatic host−guest interaction by the polarized channel surface. The PCP framework is composed of metal cations and dicarboxylate ligands and thus the channel surface is polarized to interact with the guest monomers electrostatically. The second factor is the anisotropic shape of the one-dimensional channels. The shapes of the narrow apertures and large windows are significantly different (Figure 1b,c). First Factor: Host−Guest Electrostatic Interaction. To investigate the first factor by the host−guest electrostatic interaction, we additionally executed an MD simulation of MD_PCPzeroCharge in which atomic charges of all atoms constituting the PCP framework were zero and compared with the MD_PCP results. The distribution of monomers in MD_PCPzeroCharge is shown in Figure 5a. In both cases, the onedimensional distribution and the preference of parallel alignment of two monomers were common. However, in MD_PCPzeroCharge, the localization of O1 and C5 atoms observed in MD_PCP almost disappeared and there is a large overlap between the distributions of O1 and C5 (Figure 3a). With regard to the guest diffusion, the D x of MD_PCPzeroCharge are nearly 2 times larger than those of MD_PCP in all the three directions (Table 1). These differences indicate the effects of electrostatic interaction between the guest monomers and the host channel surface, which localize the monomer locations and slow down the monomer diffusion. The s-cis conformational ratio of MD_PCPzeroCharge is 2.0% smaller than that of MD_PCP (Table 2). Since the dipole moment of the s-cis conformation is 0.1 D larger than that of s-trans conformation (Figure 2), the electrostatic interaction should give additional stabilization in the s-cis conformation in MD_PCP. The relaxation time of the autocorrelation function of ϕ is larger in MD_PCPzeroCharge (Table 3), suggesting that the host−guest electrostatic interaction helps the conformational changes of monomers.

Table 2. Conformation Ratio of MMA Monomers MD_liquid

MD_liquid

(2)

The obtained ACFs were plotted in Figure 4 and the relaxation times (ACF(t) is 1/e) were tabulated in Table 3. A comparison of the ACF of MD_liquid and MD_PCP indicates D

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reaction toward isotactic and syndiotactic products were different depending on the conformation of the reactant MMA,45 the dependence of the product PMMA tacticity on different ligand L should be partly explained by the shift of the MMA conformation. In addition, the second factor of the diffusion anisotropy depending on the equilibrium dihedral angle of the ligand L can also contribute to the tacticity modification. In the planar ligand case, MMA monomers do not frequently pass through the narrow apertures between the channels and can approach the radical only from the direction along the channel. On the other hand, in the nonplanar ligand case, monomers can diffuse three-dimensionally and can approach the radical not only from the direction along the channel but also from the directions perpendicular to the channel through the open apertures. This difference of the approach directions can also contribute to the tacticity modification of the product PMMA. These two factors are consistent with the most significant tacticity modification in the L = DMBDC case19 because of the substitution by the polarized OCH3 groups and the nonplanar equilibrium structure.

Figure 5. (a) Density distribution of O1 and C5 atoms of MMA monomers calculated from MD_PCPzeroCharge. Drawing details are identical to Figure 3b, with the exception of the transparent drawing of the O1 density in the bc-axes drawing. (b) Density distribution calculated from MD_PCP45deg. (c) Narrow aperture and large window of the host framework in MD_PCP45deg. (d) Typical snapshot of MMA monomers in MD_PCP45deg.

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CONCLUSION In this study, we have computationally investigated the behavior of guest MMA monomers in the [Zn2(BDC)2TED]n host PCP framework by MD simulations. We found anisotropic distribution and diffusion along the one-dimensional channels and analyzed the conformational ratio and dynamics of the monomers in the [Zn2(BDC)2TED]n framework. By analyzing the MD simulations of modified model systems of PCP frameworks with no atomic charges and nonplanar BDC ligands, we investigated two factors affecting the guest monomer behavior; the host−guest electrostatic interaction with the polarized PCP channel surface and the anisotropic shape of the one-dimensional PCP channel. These two factors should affect the tacticity of PMMA polymerized in the PCP framework with different dicarboxylate ligands. Our current analysis clarified the time scales of MMA monomer dynamics inside the PCP channels, the diffusion constants, and ACFs of the dihedral rotation. These provide us with information on what length of MD trajectory is necessary to analyze the monomer behaviors. On the basis of the information, we will in future quantitatively investigate the PMMA tacticity dependency on the dicarboxylate ligands L by preparing more realistic models with different ligands L and analyze the polymerization process including the MMA radicals.

Second Factor: Anisotropic Shape of One-Dimensional Channels. Next, the second factor by the anisotropic shape of the one-dimensional PCP channels was investigated by executing an MD simulation of MD_PCP45deg in which the stable dihedral angle of the BDC benzene ring was modified from 0 to 45°. This mimics the PCP framework with nonplanar dicarboxylate ligands.38 The monomer distribution in the modified framework (Figure 5b) was continuous in the a- and b-axes directions in contrast to the one-dimensional distribution in the original framework (Figure 3b). The anisotropy in the Dx values of the original framework almost disappeared in the modified framework with nearly isotropic values (Table 1). These changes are caused by the modification of the shape of the narrow apertures and windows, whose sizes are very different in the original framework (Figure 1b,c), but the difference of size is relatively small in the modified framework by the benzene ring rotation (Figure 5c). In addition, the conformational ratio (Table 2) and the relaxation time of the ACF (Table 3) of MD_PCP45deg are similar to those of MD_liquid, not to MD_PCP. These can be explained by the change of monomer alignment. Compared to the preference of the parallel alignment of two monomers in the original framework (Figure 3a), such alignment disappeared in the modified framework (Figure 5d). Therefore, the random alignment in MD_PCP45deg, which is similar to the liquid case, resulted in the conformation ratio and autocorrelation of ϕ similar to those in MD_liquid. Implications for PMMA Tacticity Dependence on PCP Framework Linkers. Both of the two factors must play important roles in the radical polymerization reaction in the [M2(L)2TED]n PCP channels16−23 by affecting the monomer behaviors. In particular, the MMA polymerization by utilizing the PCPs with different dicarboxylate ligand L, in which the hydrogen atoms of the BDC benzene ring are substituted by other atoms or groups, resulted in different tacticity of the product PMMA.19 The hydrogen substitution changes the charge distribution of the PCP channel surface and should affect the MMA monomers by the first factor of the host−guest electrostatic interactions. An important effect by the first factor should be the shift of the s-cis and s-trans conformational ratio. Because the reaction barriers of the MMA radical addition

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ASSOCIATED CONTENT

S Supporting Information *

Full citation for ref 41, computational details of diffusion analysis, force field parameters of MD simulations, snapshots of MMA monomers passing through a narrow aperture, plots of log(MSDx) to log(t), plot of MSDx to t, in MD_PCP, and diffusion constants with calculated errors. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09332. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. E

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Core Research for Evolutional Science and Technology (CREST) of the Japan Science Technology Agency (JST); by a Grant-in-Aid for Science Research from the Ministry of Education, Culture, Sport, Science and Technology (MEXT) in Japan; and by the MEXT program “Elements Strategy Initiative to Form Core Research Center” (since 2012), Japan. S.P. also acknowledges the support from Japan Society for the Promotion of Science (JSPS) as a JSPS postdoctoral fellowship for overseas researchers.

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DOI: 10.1021/acs.jpcc.5b09332 J. Phys. Chem. C XXXX, XXX, XXX−XXX