Supramolecular Architecture of Molecular-Level-Ordered 1,1

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Supramolecular architecture of molecular-levelordered 1,1’-ferrocenedicarboxylic acid with poly(4-vinylpyridine) for bulk magnetic coupling Hong-Joon Lee, Won-Jeong Shin, Nakheon Sung, Wonbin Kim, Beong Ki Cho, Mohammad Changez, and Jae-Suk Lee ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00123 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on February 4, 2019

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ACS Applied Polymer Materials

Supramolecular Architecture of Molecular-Level-Ordered 1,1’-Ferrocenedicarboxylic acid with Poly(4-vinylpyridine) for Bulk Magnetic Coupling

Hong-Joon Lee,1 Won-Jeong Shin,1 Nakheon Sung,1 Wonbin Kim,1 Beongki Cho,1,2 Mohammad Changez,1,3* and Jae-Suk Lee1*

1

School of Materials Science and Engineering, 2Grünberg Center for Magnetic Nanomaterials,

Gwangju Institute of Science and Technology (GIST), 123 Cheomdangwagi-ro, Buk-gu, Gwangju 61005, Korea 3

Department of Basic Sciences, College of Applied Sciences, A’Sharqiyah University, Ibra

400, Oman *Corresponding Authors. E-mail: [email protected]; [email protected]

Abstract Molecular-level ordering provides a powerful approach to enhancing the properties of materials. However, the precise arrangement of molecules in a bulk material is a considerable challenge. To overcome such limitations, hydrogen bonding directed self-assembly has drawn a lot of attention due to its facile nature in controlling molecular-level order. In this study we report ordering of the magnetic Fe centers achieved through hydrogen bonding between poly(4vinylpyridine) (P4VP, MW 60 kDa) and 1,1’-ferrocenedicarboxylic acid (FDA). Co-dissolving P4VP and FDA in dry methanol leads to P4VP-FDA showing unprecedented degree of order for both FDA and the polymer chain. Such an event of mutual assistance between a dicarboxylic acid and a high molecular weight polymer chain in building the ordered supramolecular architecture is rare. FDA is uniformly distributed in a ordered polymer matrix, with each Fe center in P4VP-FDA linked at the molecular-level through polymeric bridges in

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a face-centered cubic structure. The P4VP-FDA in bulk form show a large enhancement of magnetic moment with a paramagnetic resonance and asymmetric current-voltage characteristics similar to the properties of electrode-FDA-electrode architecture. Keywords: Molecular-level ordering, Hydrogen-bonded self-assembly, Supramolecular architecture, Magnetic Resonance Imaging, Poly(4-vinyl pyridine), Ferrocene dicarboxylic acid

Introduction Beyond molecular chemistry based on the covalent bond lies supramolecular chemistry based on molecular interactions.1 These noncovalent interactions lead to a spontaneous and reversible organization of molecules into ordered structures. A self-assembled structure has a higher order than the isolated components, be it a shape, a property, or a function that the self-assembled entity may manifest. For example, biological molecules self-assemble to endow living cells of its basic functions. Self-assembly of lipids leading to formation of membranes, the formation of double helical DNA through hydrogen bonding of the individual strands, and the assembly of proteins to form quaternary structures are nature’s demonstration of self-assembly as the very basis of life and living. Hydrogen bonds of types X−H⋅⋅⋅A in the solid state2 can have varied strength with the bond dissociation energy in the range 0.2–40 kcal mol−1 depending on the environment. Hydrogen bond and other noncovalent interactions build supramolecular aggregates of organic molecules3 in molecular crystals,4,5 design complex architecture of proteins,6 peptides,7 and enzyme mimics,8 with well-designed applications, such as, semiconductors,9 catalyst support,10 and tumor targeted imaging.11 Hydrogen-bonded self-assembly at the solid-liquid interface has been shown in controlling layer structure,12,13 often in competition with van der Waals


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interactions.14 Interests in the field of supramolecular chemistry spanning over three decades continues to grow.15,16 The high degree of order in the self-assembled motifs have been imaged in finer details by scanning tunneling and atomic force microscopy showing the robust nature of this noncovalent interaction. High resolution imaging of a variety of supramolecular nanostructure, clusters, chains, sheets on substrates have been studied.17,18 Interest in determining the nature of hydrogen bond continues unabated still with more methods to quantitatively map the bond.19 Like small organic molecules synthesis of supramolecular polymers is achieved adopting several strategies, such as, chain extension of telechelic polymers using reactive hydrogen bonding synthon,20 cooperative stacking of hydrogen bonded pairs,21 molecular-recognition directed self-assembly from

chiral

components,22

and

interaction between self-

complementary23 and hetero-complementary monomers.24 Fascinating syntheses of the supramolecular polymers and copolymers25 demonstrate the strength of multiple hydrogen bonding interactions. Several other non-covalent interactions used in supramolecular polymerization are aromatic donor-acceptor interaction, such as, a molecular tweezer,26 molecular recognition,27 host-guest

interactions28 involving macrocyclic hosts like cucurbiturils, crown ethers,

cryptands, and cyclodextrins.29,30 Supramolecular polymers based on metal coordination31 led to materials with interesting properties32 and useful applications.33 Functional supramolecular polymers34,35 have shown great promise with applications ranging from air-stable field-effect trasistor,36 optoelectronic device,37 lithium ion battery38 to tissue engineering,39 injectable delivery system,40 self-healing property41 suitable for use as electronic skin.42 In general, the highly ordered structures lead to a synergetic effect, such as improved photoconductivity,43 charge carrier enhancement,44 and surface‐enhanced Raman scattering (SERS) performance.45



In our work, we demonstrated long-range molecular-scale ordering through quaternization leading to covalent bonding.46 Poly(2-vinyl pyridine) (P2VP) was treated with 1,4-dibromobutane, resulting in crosslinking of adjacent polymer chains by quaternization of pyridine nitrogen. We further demonstrated in situ formation of molecular-scale ordered polyaniline (PANI) films through coordinate covalent bonding with Zn ions.47 Crystalline thin films of PANI network crosslinked by Zn coordination were directly fabricated using solventvapor thermal annealing technique. Furthermore, we succeeded in using ionic bonding to synthesize crystalline conjugated polymers using two-monomer-connected precursors.48 The crosslinking of molecules in monomer or oligomer effectively suppressed the directional freedom of polymers during the polymerization, inhibiting chain entanglements and bringing in long range molecular order. Herein, we report an ordered supramolecular aggregate from hydrogen-bonded selfassembly of poly(4-vinylpyridine) (P4VP) and 1,1’-ferrocene dicarboxylic acid (FDA) in a polar solvent. Supramolecular polymers usually are random and entangled coils. Rarely these can be formed by self-assembly to yield shape-persistent and highly ordered structures. The use of strong and directional interactions between P4VP and FDA can achieve not only rich dynamic behavior but also high degrees of internal order that are not known in ordinary polymers. Furthermore, the supramolecular architecture of Fe ions in an ordered array in large domain might enhance magnetic property by the synergetic effect. Details of studies analyzing results of HRTEM, XRD, XAS and magnetic moment measurements are presented.

Results and Discussion Co-dissolving P4VP and FDA in dry methanol leads to self-assembly of FDA with P4VP through hydrogen bonding as seen in cocrystals49 affording uniform spatial distribution of the FDA molecules (Figure 1). The homo polymer P4VP (Mn = 60,000) and FDA in 1:1


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molar ratio of pyridine to carboxylic acid groups are dissolved in anhydrous methanol at room temperature. The solution is mixed for 48 hours during which the color of the solution slowly changed from orange to black without any precipitation (Figure 1b). The resulting product is spin-coated or drop-cast on various substrates and solvent-vapor annealed. The samples are characterized by Fourier-transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), current-voltage (I-V) measurements and superconducting quantum interference device (SQUID) measurements. After solvent-vapor annealing, the P4VP-FDA film shows a smooth surface morphology (Figure S1, Supporting Information). The FT-IR spectra of P4VP-FDA (Figure S2, Supporting Information) show the pyridine ring bands to shift to lower frequencies.50 The characteristic stretching band of P4VP at 1595 cm-1 shifts to 1588 cm-1. The FDA carbonyl band shifts from 1672 to 1689 cm-1. Such band shifts are ascribed to the hydrogen bonding interactions between the pyridine nitrogen of P4VP and the carboxylic acid group of FDA.51 The interaction is further confirmed by change in N1s signals in the XPS spectra (Figure S3, Supporting Information). The peak corresponding to P4VP-FDA shifts to a higher binding energy region and becomes broader.52 For the O1s signals, the peaks for the hydroxyl and carbonyl groups of P4VP-FDA are observed together53 and slightly shifted (Figure S4, Supporting Information), indicating a change in hydrogen bonding strength compared to the interaction between only FDA molecules. The HRTEM and fast Fourier transform (FFT) images of P4VP-FDA measured by high-voltage electron microscopy (HVEM) are shown in Figure 2. The long-range subnanometer ordering is distinctly visible in the HRTEM images, with a periodicity of 0.455 nm corresponding to the inter-planar spacing (d spacing) (Figure 2a). Furthermore, we obtained



HRTEM images from three different zone axes (Figure 3). The resulting FFT patterns of all the images correspond to the face-centered cubic (fcc) structure (close-packed crystal structure) (Figure 3b,c,e,f).54 Figure 2b shows the Fourier-mask-filtered FFT image of Figure 2a, where each d spacing relates to the {220} plane of the [1̅11] zone in the fcc crystal. From the theoretical ratio of d spacings between the fcc lattices,54 the (100) d spacing is 1.28 nm, corresponding to lattice parameter a. Such a lattice spacing in the [100] direction is also observed at the [023] zone axis (Figure 3d,e). Since the HRTEM and electron diffraction measurements are represented by and based on the heaviest atom, 48,55,56 i.e., Fe(II) of ferrocene, we conclude that ferrocene forms face-centered cubic lattice, as shown in Figure 2c, with a lattice parameter a of 1.28 nm. The XRD pattern of P4VP-FDA also correlates well with the HRTEM results. No crystalline peak is observed in the XRD pattern of the P4VP due to its amorphous nature. However, XRD spectra of P4VP-FDA show a few peaks (Figure 4a) commensurate with a crystalline structure. The characteristic peaks are correlated to the measured lattices via HRTEM. The crystalline peaks at 2 theta values of 13.05 (200), 18.6 (220), 24.2 (222), 26.42 (400), and 32.3 (420), corresponding to a fcc lattice geometry with lattice parameter of 1.28 nm. The primary observed lattices of (200) and (400) are the second and fourth reflections of lattice parameter a (1.28 nm). Pristine FDA possesses triclinic crystal packing (see SEM, HRTEM images, and XRD of the FDA control film in Figure S5, Supporting Information). Furthermore, we also measured the difference of crystallinity of P4VP-FDA (Figure S6, Supporting Information) by varying the ratio of FDA. We find that the ratio of crosslinker (mole of COOH) to polymer (mole of 4-vinyl pyridine unit) is very crucial for crystallization, the 1:1 molar ratio of FDA (COOH) to 4VP being the most effective. In our previous work, we observed ~ 0.27 nm periodicity of the line pattern in P2VP, being the distance between consecutive carbon atoms containing the pyridine (Py) units


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[−CH(Py)−CH2−CH(Py)−] of the polymer backbone.46 From TEM and XRD analysis, we observe a periodicity of ~ 0.27 nm, which is correlated with the (420) lattice (Figure 4a-c). Thus, the (420) d spacing is consistent with pyridine repeating units in the P4VP backbone, forming hydrogen bonds with FDA. In addition, the (420) line pattern provides clear evidence for the self-assembled orientation of the P4VP chain (Figure 4c,d). This indicates that the polymer backbone is parallel to the [420] direction, maintaining ordering of FDA with the P4VP chains. In forming the hydrogen bond, the P4VP polymer chain may adopt syndiotactic conformation46 for effective ordering of P4VP-FDA. Ferrocene is an organometallic compound with an Fe(II) center sandwiched between two cyclopentadiene (Cp) rings through covalent bonds. The ligand field from the Cp rings is controlled by spin pairing energy, and as a result, ferrocene shows diamagnetic properties with an (e2g)4 (a1g)2 electronic configuration.57,58 In the P4VP-FDA supramolecular assembly, FDA59 acts as a single molecule magnet (SMM). The SMMs are multi-center transition metal complexes and behave as exchange-coupled clusters in which paramagnetic transition metal (or rare-earth) ions are linked by bridges, providing anisotropic exchange coupling.60-64 The organic ligands of SMMs strongly encapsulate the magnetic centers, coupled via exchange interactions, and shield inter-cluster exchange from the surrounding environment. The SMMs on designed substrates, in which metals are attached to each other at a molecular distance in monolayer, show extraordinary magnetic properties.65,66 However, achieving the same properties in bilayer or multilayer systems as those in a monolayer have been unsuccessful due to magnetic and structural disorder. Hydrogen-bonded clusters of ferrocene carboxylic acid, ferrocene dicarboxylic acid, ferrocenyl acetylene have been studied by STM showing dimers and cyclic five- and six-membered ring structures.67-70 The sub-nanometer-level ordered P4VP-FDA shows significantly increased magnetization compared to that of pristine FDA. Figure 5a shows an isothermal magnetization



at T = 2 K in a field range of - 50 kOe  H  50 kOe. The expanded plot of the magnetization curve in the low field region is presented in Figure 5b, where magnetization of pristine FDA is also plotted for comparison. The magnitude of P4VP-FDA magnetization considerably increases (Figure 5a), and the magnetization hysteresis indicates a paramagnetic resonance (Figure 5b).71,72 For ferrocene-based materials, such interactions typically occur in molecular solids or charge transfer salts that have regular alternating structures of ferrocene-based donors and hydrocarbon-based acceptors (D+A−D+A−…).58,71 It is rational to conclude from the magnetization curve, together with the HRTEM images, that P4VP-FDA also forms alternating FDA (donor) and pyridine (acceptor) units at the sub-nanometer scale. The color change of P4VP-FDA from orange to black can be explained by the change of electron density from this sub-nanometer-level alternating system between FDA and pyridine. In other solvent systems, the solution color is brownish (Figure S7, Supporting Information). For further analysis, XAS is used to study pristine FDA and ordered FDA in a polymer matrix (Figure 6). Pristine FDA and P4VP-FDA show characteristics transition metal L2 and L3 spectral bands. The Fe2p edge features of FDA are similar to those in a previously reported work.57 In the XAS spectra, the main L3 region band of FDA corresponds to the 2p3/2 edge of the 3d transition within the metal. The peak at 710 eV is mainly related to Fe 3d xz and Fe 3dyz molecular orbitals (MOs) with π*(Cp) ligand contribution for the lowest unoccupied molecular orbital (LUMO) and LUMO+1, respectively, showing a lower-energy LUMO in comparison to normal ferrocene due to the Fe 3dxz with a π*(Cp−COOH) ligand (where Cp−COOH is the cyclopentadiene carboxylic acid). The satellite band at 710.9 eV is assigned to Fe 3dx2 -y2 and Fe 3dyz with a π*(Cp-COOH) ligand, involved in nonbonding interactions between Cp and −COOH. The peak at 712.3 eV for FDA is associated with the Fe 3dxz MO with a π*(Cp) ligand without contribution from −COOH and its electronic environment. However, the peaks for pristine FDA at 710 and 710.9 eV changed in the ordered P4VP-FDA system. The original


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peak at 710 eV in FDA is split into two peaks at 709 and 709.9 eV, showing a shoulder peak in the P4VP-FDA system. Furthermore, the peak at 710.9 eV shifts to higher energy by 0.2 eV in P4VP-FDA, indicating that the presence of hydrogen bonding between the −COOH groups and pyridine nitrogen atoms effected the π* conjugation between Cp and −COOH. This revitalized the π* orbital degeneracy and changed the spectral features. Finally, the area ratio of the L3/(L2+L3) edge also remarkably increased in P4VP-FDA compared to pristine FDA, indicating an increase in total amount of d character in the unoccupied orbitals.72 In general, ligands with electron-withdrawing groups increase the electron-nuclear coulomb attraction between the metal center and ligand due to a decrease of the electron density on the metal. In P4VP-FDA, the electron-withdrawing substituent on Cp is −COOH∙∙∙Py. This increased the electron-nuclear coulomb attraction between Fe(II) and Cp−COOH∙∙∙Py that changed the MO characteristics from delocalization of the Fe 3d orbitals by the π*(Cp) LUMO of the new substituent.57 Thus, magnetic ordering and coupling of P4VP-FDA are rationally expected from the increased electron-nuclear coulomb attraction with delocalization of the Fe 3d orbitals alternating between FDA/pyridine on the molecular level, which is similar to the case for monolayer SMMs.61,62 This increased electron-nuclear coulomb attraction is likely the origin for the significantly improved magnetization with a hysteresis curve at 2K (Figure 5b). Although P4VP is an electrical insulator the I-V curve of ordered P4VP-FDA shows asymmetric characteristics similar to single FDA molecules directly linked to metal electrodes (Figure S8, Supporting Information).73 For comparison, in the case of P2VP-FDA, I-V curve is not observed ascribed to its amorphous nature, which is caused by steric constraints between the ortho-position of nitrogen and FDA molecules (Figure S9, Supporting Information). However, the asymmetric I-V curve of ordered P4VP-FDA supports the experimental findings from HVEM and the model (Figure 2c) in which the FDA molecules arrange at the subnanometer level in a precise donor-acceptor arrangement. Therefore, in P4VP-FDA, each Fe



center is linked by a bridge (Cp−COOH∙∙∙Py) with the potential to provide anisotropic exchange coupling. The P4VP-based acceptors encapsulate the Fe centers at the sub-nanometer level for the exchange interactions, shielding the exchange between coupled clusters. Each Fe center can be considered one bit in the resulting ordered FDA structure.

Conclusion We describe a simple, one-step approach to a molecular-level alternating ordered structure of P4VP and FDA through intermolecular hydrogen bonding interaction. There is a significant degree of cooperativity that mutually assisted both P4VP and FDA to generate a highly organized structure that is reminiscent of a rare synergetic effect. Precisely organized FDA molecules in fcc arrangement in a bulk material results in paramagnetic resonance observed for the first time. This close-packed alternating structure increased the electronnuclear coulomb attraction between Fe and the polymeric ligand via delocalization of the Fe 3d orbitals and MOs, which enhanced the magnetic coupling. Such enhancement of magnetic properties provides a model for synthesis of bulk magnetic materials. The approach is very simple and can be applied to other small organic molecules and polymers.

Experimental Section Materials: P2VP (Mn = 35,000), P4VP (Mn = 60,000) and FDA are purchased from Sigma Aldrich. All materials are used as received without further purification. Anhydrous methanol from Aldrich is utilized as a solvent. Self-assembly of P4VP-FDA by hydrogen bonding: P4VP (0.05 g) is dissolved in 2 mL anhydrous methanol at room temperature with 0.66g FDA. The molar ratio of pyridine groups to the carboxylic acid groups is 1:1. This solution is mixed for 48 hours at room temperature without any stirring. After 48 hours, the films are deposited


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on silicon wafers and conductive Au- or Pt-coated Si wafers by spin coating or drop casting. The films are annealed under methanol vapor at room temperature for 1 hour.

Characterization of P4VP-FDA: The SEM image of the P4VP-FDA film spin-cast on a Si wafer is measured by field-emission SEM (FE-SEM, Hitachi S-4700). The FT-IR spectra of P4VP, FDA and P4VP-FDA are measured using a Perkin-Elmer FT-IR spectrometer (Spectrum System 2000) with potassium bromide pellets. The XPS spectra of drop-cast films are measured using an electron spectroscopy for chemical analysis (ESCA) instrument (VG Multilab 2000). For the XRD studies, thick films of FDA and P4VP-FDA are prepared via drop casting on Si wafers, and the XRD patterns are measured using a Rigaku D/max-2500 diffractometer with Cu-Kα radiation (λ = 1.54 Å) at 40 kV and 100 mA. HVEM (JEOL Ltd., JEM ARM 1300S) with a point resolution of 0.12 nm is used to observe the sub-nanometerlevel ordered structure of P4VP-FDA, and HRTEM images are recorded at 1250 kV (Gatan Inc., SP-US1000HV). Sample specimens are prepared by drop casting the samples on carboncoated copper grids (200 mesh, EM Science). The magnetization measurements of the samples are performed in a quantum design magnetic property measurement system (MPMS). XAS is performed at 80 K in the presence of an applied magnetic field (0.8 T) at beamline 7B1 XAS KIST of the Pohang Accelerator Laboratory (PAL). For XAS, the films are spin- and drop-cast on Au-coated SiO2/Si wafers.



Acknowledgement This research is supported by “Nobel Research Project” grant for Grubbs Center for Polymers and Catalysis funded by the GIST in 2018; and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (NRF-2018R1A2B6003616). We thank Seung Jo Yoo, Jin-Gyu Kim and Youn-Joong Kim for their assistance in the HVEM (JEMARM1300S) performed at the Korea Basic Science Institute (KBSI). We also thank Jae-Young Kim for the XAS experiment performed at the 2A Magnetic Spectroscopy beamline of the Pohang Light Source (PLS) in Pohang Accelerator Laboratory (PAL). This work is dedicated to late Professor Peter Grünberg.


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Figures

Figure 1. Schematic illustration showing molecular-level ordering of P4VP with FDA by hydrogen bonding. a) Chemical structures of P4VP and FDA. b) Color change of the solution during molecular-level ordering on hydrogen bonding. c) TEM image of the product showing a molecular-level lattice. Scale bar, 1 nm. The regions of dark contrast indicate ferrocene moieties. d) Schematic drawing of the hydrogen-bonded structure between P4VP and FDA.



Figure 2. Sub-nanometer scale ordering of hydrogen-bonded FDA with P4VP. a) HRTEM image of P4VP-FDA observed at the [1̅11] zone axis. b) Corresponding Fourier-mask-filtered FFT image of (a). c) Proposed arrangements of ferrocene molecules forming a fcc structure.


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Figure 3. HRTEM results of P4VP-FDA measured at different zone axes. a,d) HRTEM images measured along the [1̅14] (a) and [023] (d) directions. b,e) Corresponding FFT images of (a) and (d), respectively. c,f) Theoretical electron diffraction patterns of [1̅ 14] and [023] zone axes in the fcc crystal.



Figure 4. a) XRD pattern of the P4VP-FDA film, b) Chemical structure of the P4VP chain and the length of the repeating unit. c) HRTEM image of P4VP-FDA (Inset: FFT image of the white square box in the HRTEM image). d) Corresponding Fourier-mask-filtered HRTEM image from the yellow spots in the FFT image.


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Figure 5. Magnetic properties of FDA and P4VP-FDA. a) Isothermal magnetization of selfassembled FDA with P4VP at T = 2 K in a field range of –50 kOe  H  50 kOe. b) Expanded plot of (a) in the low magnetic field region. The same curve for pristine FDA is also plotted for comparison.



Figure 6. XAS analysis of the FDA and P4VP-FDA films. Spin-coated FDA and P4VP-FDA films (80 nm thick) and a drop-casted P4VP-FDA sample on Au-deposited SiO2/Si substrates for XAS measurement at 80 K and 0.8 T.


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Supramolecular Architecture of Molecular-Level-Ordered 1,1

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