Article pubs.acs.org/JPCC
Molecular-Level Studies on Dynamic Behavior of Oligomeric Chain Molecules in Porous Coordination Polymers Takashi Uemura,*,‡,+ Gosuke Washino,‡ Susumu Kitagawa,*,‡,⊥ Hirohide Takahashi,§ Aiko Yoshida,§ Kunio Takeyasu,§ Masayoshi Takayanagi,+,# and Masataka Nagaoka*,+,# ‡
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan + CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ⊥ Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan § Laboratory of Plasma Membrane and Nuclear Signaling, Graduate School of Biostudies, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan # Department of Complex Systems Science, Graduate School of Information Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan S Supporting Information *
ABSTRACT: The quest for porous coordination polymers (PCPs) has been the subject of intense research because of their unique porous functions. Although considerable attention has been paid to the behavior of gases and low-molecular-weight compounds in PCPs, the study of adsorption, diffusion, and interaction of macromolecules, including oligomers and polymers, in PCPs at the molecular level has been limited to date. Here, we have studied the dynamic behavior of oligomeric chain molecules, both theoretically and experimentally, in [Zn2(bdc)2(ted)]n (1; bdc = 1,4-benzenedicarboxylate, ted = triethylenediamine). Molecular dynamics simulations revealed that the motion of linear oligomeric chains, such as oligo(ethylene glycols) or paraffins, is composed of smooth transfer along the large channels and slow transfer to the adjacent channels by penetrating the narrow apertures of 1. Moreover, this anisotropic behavior depends on the polarity of the guest chains because of the Coulomb interactions with the polarized aperture surface. Atomic force spectroscopy was also performed, for the first time, in an effort to probe directly the host−guest unbinding forces occurring at the molecular level in PCPs. Specific interactions of oligomers immobilized on an atomic force microscope cantilever with the PCP nanopores could be detected. This was likely caused by desorption of the guest molecules from the PCP pores, which agrees with the molecular behavior shown by the theoretical study. A quantitative understanding of macromolecular behavior in PCPs at the molecular level will provide important guidelines for the development of functional host−guest nanocomposites.
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properties residing in PCPs have been determined.4−7 In contrast to bulk experiments, analysis of the guest molecule behavior at the molecular level in PCPs not only would contribute to reducing the amount of sample and analysis time required but also would be critical to understanding the subtle interplay of PCPs and guest molecules. Thus, a study of the guest molecule behavior, such as the permeability and diffusivity in PCPs at the molecular level, would provide insight into the performance of PCPs and would allow the rational design of well-defined PCP structures with significant functionalities.8−10 Although a large number of papers have reported on the behavior of gaseous molecules and low-molecular-weight compounds in PCPs, studies on the behavior of large
INTRODUCTION There has been tremendous interest in the preparation of porous coordination polymers (PCPs, also known as metal− organic frameworks, MOFs) based on the self-assembly of metal ions and bridging organic ligands because of their functional properties. They are useful for their storage, separation, exchange, and catalysis functionalities.1−3 The advantageous features of PCPs are their controllable pore size, shape, and surface properties that can be tuned readily by changing the combination of their components, which can allow selective guest accommodation and unique host−guest properties. For the practical use of PCPs, an understanding of guest molecule adsorption, diffusion, and release at the molecular level is very important. In the conventional analysis of the guest molecule behavior in PCPs, a wide variety of measurements have been employed on bulk host and guest molecules. In these measurements, the individual behaviors of guest molecules cannot be distinguished, but the average guest © 2015 American Chemical Society
Received: June 25, 2015 Revised: August 19, 2015 Published: August 25, 2015 21504
DOI: 10.1021/acs.jpcc.5b06083 J. Phys. Chem. C 2015, 119, 21504−21514
Article
The Journal of Physical Chemistry C
Figure 1. Nanochannel structure of 1 along the (a) c- and (b) b-axes.
tems;36−38 thus, this technique will contribute to understanding of the single-molecule behavior of guest molecules accommodated in PCPs. The host PCP used in this work, [Zn2(bdc)2(ted)]n (1; bdc = 1,4-benzenedicarboxylate, ted = triethylenediamine), has large channels (7.5 × 7.5 Å2) along its c-axis and narrow apertures (5.0 × 3.5 Å2) along its a,b-axes (Figure 1).39 Not only gaseous and small molecules but also linear macromolecules can be accommodated in the nanopores of 1.11,12,39,40 Here, we study the dynamic guest molecule behavior of linear chain molecules in 1. Anisotropic permeation and transfer of these molecules occurred, depending on the crystal axis direction that was revealed by advanced MD simulations. Fast transfer of the chain molecules was observed along the large channels, but the molecules could also transfer slowly through the narrow apertures that were comparable in thickness to the dimensions of the chains. Interestingly, this anisotropic behavior depends on the polarity of the guest molecules because of their Coulomb interactions with the framework of 1. Single-molecule analysis using AFM showed the specific and anisotropic interactions between 1 and the guest chain molecules, which could not be analyzed using conventional experiments on bulk samples. In this work, the combination of theoretical and experimental approaches was able to probe deeply into the molecular-level dynamics of linear macromolecules in PCPs.
macromolecules, such as polymers and oligomers, as guest molecules in PCPs have been limited.11,12 It is not clear how such large molecules can permeate, diffuse, and interact in the channels of PCPs because of the large conformational requirements of macromolecules in occupying the micropores of PCPs (0.05 Å−3. To visualize the sliding motion of the OEG, one oxygen atom of the OEG is drawn as large nontransparent red balls at 140 and 150 ns and as small balls every 0.5 ns from 140 to 150 ns.
occasionally slid along the b-axis. The distribution of oxygen atoms in OEG was located at the narrow apertures. This can be explained not only by the van der Waals (vdW) radius of oxygen atoms (1.6837 Å) being smaller than that of the CH2 groups (1.9080 Å for carbon and 1.3870 Å for hydrogen atoms), but also by the electrostatic potential field of the framework of 1 (Figure 5). A positive potential was observed at the narrow apertures, which were composed of bdc and ted organic ligands. The positively charged ted ligands (the sum of
Figure 5. Electrostatic potential generated by the framework of 1. This was calculated by the PME Electrostatics utility in VMD program48 and averaged over the 200 ns MD trajectory at 300 K. Regions lower than −4kBT/e and higher than +4kBT/e are drawn in red and blue, respectively. 21509
DOI: 10.1021/acs.jpcc.5b06083 J. Phys. Chem. C 2015, 119, 21504−21514
Article
The Journal of Physical Chemistry C
Figure 6. MD snapshot of C43 molecules in the framework of 1 at 300 K. Atoms of C43 molecules satisfying the condition 21.896 Å < a < 32.844 Å are drawn in thick bonds for simple visualization.
hydroxysuccinimidyl esters at the carboxyl ends. Under these conditions, the linear molecules were immobilized sparsely by an amide bond on the surface of the cantilever.51 Single crystals of 1 were prepared, and only high-quality crystals were chosen from many candidate crystals. These were stored in superdehydrated DMF before the AFM measurements. We identified the crystal faces of 1 using polarized light irradiation, where different light transmission properties were observed depending on the crystal faces (Figure S16). Further confirmation of the crystal faces was performed from X-ray powder diffraction measurements (Figure S17). Surface imaging on 1 using AFM peak force tapping ensured that both the (100) and (001) surfaces of the PCP crystal were flat enough for the force spectroscopy (Figure 8). In these imaging measurements, the PCP crystals were found to have molecularly flat surfaces with a height difference of
