Metal-Coordination-Induced Fusion Creates Hollow Crystalline

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Metal-Coordination-Induced Fusion Creates Hollow Crystalline Molecular Superstructures Maria di Gregorio, Priyadarshi Ranjan, Lothar Houben, Linda J.W. Shimon, Katya Rechav, Michal Lahav, and Milko E. van der Boom J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03055 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Metal-Coordination-Induced Fusion Creates Hollow Crystalline Molecular Superstructures Maria Chiara di Gregorio,† Priyadarshi Ranjan,† Lothar Houben,‡ Linda J. W. Shimon,‡ Katya Rechav,‡ Michal Lahav,†,* and Milko E. van der Boom†,* †

Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, 7610001 Rehovot, Israel.

ABSTRACT In this work, we report the formation of superstructures assembled from organic tubular crystals mediated by metal-coordination chemistry. This template-free process involves the crystallization of molecules into crystals having a rectangular and uniform morphology, which then go on to fuse together into multi-branched superstructures. The initially hollow and organic crystals are obtained under solvothermal conditions in the presence of a copper salt, whereas the superstructures are subsequently formed by aging the reaction mixture at room temperature. The mild thermodynamic conditions and the favorable kinetics of this unique self-assembly process allowed us to ex-situ monitor the superstructure formation by electron microscopy, highlighting a pivotal and unusual role for copper ions in their formation and stabilization.



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INTRODUCTION In living organisms, crystalline materials commonly self-assemble into organized superstructures as a means of achieving essential structural, mechanical, and optical properties. Interesting examples are (i) photonic crystal assemblies that modulate the skin color of chameleons1 and color-changing fishes2,3 or (ii) the complex scaffolds that confer outstanding mechanical rigidity to organisms, such as Euplectella sp.4 Inspired by nature, the fabrication of man-made crystalline superstructures has become an intriguing strategy for improving the performance of materials and for widening the range of their properties.5 Superstructures were designed from metal-organic frameworks (MOFs),6,7 polymers8,9 and both inorganic and organic nanoparticles.5 Applications include catalysis,10 photonic devices,11-15 solar cells,16 and biomedical systems.17 Diverse non-covalent forces have been used to generate superstructures, including the functionalization of metallic nanoparticle surfaces with DNA.18 In forming such superstructures, the specificity of the interactions between the complementary base pairs results in directed aggregation. Deciphering the complex formation of molecular superstructures is challenging. Their parameter space is extremely large and their construction and features are affected by many factors, including (i) the molecular geometry and dimensions,19 (ii) the morphology and uniformity of the constitutive crystals,20 and (iii) the forces driving the threedimensional assembly of the constitutive crystals.21 The generation of initial crystals of uniform size and shape is essential for the subsequent formation of superstructures by fusion. To control crystal morphology, several modulators, surfactants, and capping ligands have been used by others as additives.22–25 Crystals can have heterogeneous surfaces, surface charges, as well as a net dipole moment26 – factors that can contribute to the formation of superstructures. Incorporating organic or metallo-organic crystals into ordered superstructures is in itself of great fundamental interest. To date, very little is known about the factors that allow the design of such materials. Here, we present a unique metal-mediated fusion process for fabricating superstructures from individual and uniform hollow crystals. Our alternative strategy involves the use of a metal salt that plays a remarkable dual role in the step-wise production of superstructures. This unprecedented process is guided by the formation and



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stabilization of hollow organic crystals at elevated temperatures by the metal ions, followed by the subsequent metal-mediated fusion of these constitutive crystals, at room temperature, into micro-scale multi-branched superstructures. These remarkable superstructures have the same composition and structure as the initially formed organic crystals. Though not incorporated into the lattice of the superstructures, the copper salt is an essential component in their formation, as its accumulation at the fusion junctions is needed for the fusion to occur. Over time, the superstructures slowly evolve and merge into even larger entities. Their hollow structures offer the possibility of molecular inclusion within their supramolecular cavities, thus potentially serving, as delivery systems or chemical reactors. RESULTS AND DISCUSSION As the molecular building block for the creation of superstructures, we chose to use TPEPA (1,3,5,7-tetrakis(4-(pyridyl-4’-yl-ethynyl)phenyl] adamantine) due to its poor solubility in common organic solvents (Figure 1A). We envisioned here that the pyridine moieties of TPEPA would generate a crystal surface that interacts with a metal salt to induce uniformity and stability and, at the same time, would facilitate the fusion of the crystals into larger superstructures by coordination chemistry. The poor solubility of this organic ligand diminishes the formation of a metal-organic framework (MOF). Similar but soluble organic ligands in combination with metal salts such as Cu(NO3)2 form MOFs as has been reported by us27,28 and others.29,30 Some of these Cu-based MOFs are most likely formed by a cascade of complex fusion processes.27 Crystal surfaces of ligands isostructural to TPEPA are terminated with pyridine moeities.31 The formation of the superstructures involves a two-step crystallization process: (i) a solvothermal reaction to form the constitutive organic crystals, which is followed by (ii) a cascade of crystal fusion processes at room temperature. The solvothermal treatment of TPEPA at 105°C with a four-fold excess of Cu(NO3)2 in a chloroform/DMF solution for a period of 2 days resulted in the formation of uniform and hollow crystals.



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Figure 1. From constitutive TPEPA crystals to hierarchical superstructures. (A) scheme representing the consecutive metal-mediated formation of constitutive TPEPA crystals and hollow superstructures thereof by fusion. (B) Left: Scanning electron microscopy (SEM) image of constitutive TPEPA crystals formed after the solvothermal reaction. The inset shows a representative terminus. Center: SEM images recorded after several days of aging of the TPEPA crystals in the product solution. Right: SEM images, recorded after 12 days of aging, showing the presence of superstructures and their hollow termini.

Combining TEM diffraction data of the constitutive crystals with the X-ray diffraction structural analysis of the superstructures revealed that these materials comprise the organic TPEPA ligand and have the same crystallographic structures (vide infra). The morphologies of the constitutive and intermediate crystals, and the superstructures have been visualized by scanning electron microscope (SEM) imaging (Figure 1B). When left at room temperature in the mother liquid, these rectangular TPEPA crystals undergo a series of consecutive fusion processes to generate multi-branched superstructures. In this



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process, the copper salt is vital for linking the crystals at the fusion junctions. The copper salt plays a structural and stabilizing role in the formation of these uniform crystals: the solvothermal treatment of TPEPA at 105°C in the absence of Cu(NO3)2 results in the formation of a mixture of non-uniform crystals (Figure S1). Addition of Cu(NO3)2 after solvothermal treatment also did not result in multi-branched structures (Figure S2). To learn about the formation of the superstructures we carried out ex-situ timedependent SEM studies starting directly after the solvothermal reactions. The data collected during 20 days show the generation and evolving of superstructures (Figure 2). The initially formed crystals were found to be morphologically homogeneous in the presence of Cu(NO3)2 with a rectangular geometry (w ´ h ´ l = ~0.83 µm ´ ~0.42 µm ´ ~14.5 µm; Figures 2 Chart 1A, S3). A decrease in the number of initially formed rectangular crystals is observed with a concurrent increase in the number of these crystals assembled into superstructures, as presented graphically in Figure 2 Chart 1B,C. The length of the subunits of the superstructures is comparable to that of the initially formed crystals (at least up to 3 days at room temperature). For longer reaction times, the radius of the superstructures gradually increases (Figure 2 Chart 1D). The constitutive crystals undergo lengthening during this period (Figures S3, S5A-S10A), arguing against an assembly process in which these crystals slowly dissolve as one would expect for nucleation growth of the superstructures.



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Figure 2. Ex-situ time-dependent SEM studies showing the formation of the superstructures. Chart 1: (A) Length distribution (14.56 ± 3.95 µm) of the constitutive TPEPA crystals obtained after the solvothermal reaction. This data is extracted from the time-dependent ex-situ SEM imaging shown in Figure 1, S3A. (B) Number of notaggregated TPEPA crystals as a function of the aging time. (C) Number of subunits assembled in a superstructure, (D) radius of the superstructures. Chart 2: (A-E) Timedependent ex-situ SEM images showing the fusion of constitutive TPEPA crystals into superstructures. The right column shows the structural changes of the termini.

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Additional support for a crystal fusion mechanism has been obtained by prolonging the solvothermal reaction from 2 to 5 days: much longer TPEPA crystals (96.6 µm ± 42.1

µm) were formed after 5 days of heating. We observed that, after aging at room temperature, superstructures formed with dimensions resembling these larger crystals (Figure S4). The time-dependent SEM images shown in Figure 2, Chart 2 indicate that during the first 3 days at room temperature the superstructures are step-wise assembled in an asymmetric fashion by fusion of the initially formed crystals obtained under solvothermal conditions (Figure S5). The structures become more dense after 5 days (Figure S6). Fully formed and ordered superstructures are observed at 10-12 days of aging (Figures S7, S8). After 15 days, these fused crystals slowly lose their regularity and begin to fuse (Figures S9, S10). With time, the superstructures are also undergoing a morphological change at their termini: from the irregular spike-type profile resembling the edges of the initial crystals to well-defined rectangular shapes. With aging, the hollow termini are eventually filled-in and the cuboid faces experience non-symmetric elongation Figure 2, Chart 2, Figures S5-S10. This morphological change indicates that a secondary crystallization pathway is operating and becomes competitive with crystal fusion when the concentration of the initial formed crystals is low. The nature of the junctions of the intermediate superstructures formed after 3 days reveals mechanistic information about their development. A series of SEM images show the constitutive TPEPA crystals preferentially fuse edge-to-face or end-to-face (Figures 3 Chart 1 and S11). The edge-to-face fusion results in both symmetrical and asymmetrical scaffolds. However, the end-to-face scaffolds still preserve the spike-type profile of the constitutive crystals. Once this first junction is formed, additional TPEPA crystals are likely added to it.



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Figure 3. Formation and structure of the junctions. Chart 1: (A) SEM images and corresponding zoom-ins showing the fusion of constitutive TPEPA crystals into superstructures after 3 days. Edge-to-face and end-to-face fusions. (B) Fusion of multiple TPEPA crystals to generate the multi-branched structures. Chart 2: (A) Energy dispersive X-ray spectra (EDS; SEM) of a constitutive TPEPA crystal after the solvothermal reaction (top). Energy dispersive X-ray spectra (EDS; TEM) of a superstructure after 12 days of aging at room temperature (bottom). The asterisks denote the substrates (Si, O and Ni). (B) Right: SEM backscattered images taken after 2 (left) and 12 (right) days of aging. (C) SEM secondary electron image (left) and mapping of copper by EDS-SEM (right) of a superstructure after 20 days of aging at room temperature. Chart 3: (A) X-ray photoelectron (XPS) spectra of superstructures formed after 12 days at room temperature: (top) Cu 2p and (bottom) N 1s region. Experimental data are reported in grey; envelopes are reported in red and blue. The asterisks denotes the satellite peaks of Cu 2p.(B,C) Optical microscopy images of the structures analyzed by MicroRaman. The circles indicate the regions where the laser was focused for spectrum collection. (D) MicroRaman spectra of a constitutive TPEPA crystal (green) and of a superstructure formed after 5 days of aging at room temperature (red and blue). Both the initial crystals and the superstructures contain copper as indicated by energy dispersive spectroscopy (EDS) analysis (Figure 3 Chart 2 A). Backscatter electron SEM images show the presence of extremely bright regions localized at the junctions, which is due to the copper salt (Figure 3 Chart 2 B). The presence and the location of copper is confirmed by EDS mapping (Figure 3 Chart 2 C). X-ray photoelectron (XPS) measurements indicate that the copper ions retained their oxidation state (+2) as judged by the binding energy of 933.8 eV for Cu 2p3/2 and the satellite bands (939–947 eV and 960–965 eV) (Figure 3 Chart 3 A).32-34 We observed a signal at 407 eV which is attributed to the counterions (NO3-).32,33 The Npyr band appears at 398-405 eV and is coherent with both uncoordinationed and coordinated vinylpyridine groups.32-34 The Ntot/Cu2+ ratio of 10.9 indicates that these superstructure contain a significant amount of copper, however it is a fraction of the used starting materials. Chlorine is also observed by EDS measurements (Figure 3 Chart 2 A). These observations are in very good agreement with the XPS data of superstructures after 12 days of aging showing the presence of Cl- (Figure S12). No other chlorine species were observed. The binding energies (Cl- 2p3/2 = 198 eV and Cl- 2p1/2 = 199.2 eV) are consistent with the formation of the copper chloride salt, (NH4)CuCl3, as indicated by PXRD (vide infra).32,33 This salt can



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be formed from reactions between the solvents and copper ions.35-40 The absence of Na+ and K+ peaks excludes the presence of common impurities (e.g., NaCl, KCl). Micro-Raman measurements of the constitutive TPEPA crystals and the superstructures subunits show nearly identical spectra (Figure 3 Chart 3 B-D). However, measurements performed at the junctions of the superstructures show significant shifts both in the bands related to the pyridine (1000-1030 cm-1 and 1590-1606 cm-1 ranges) and in the triple bond modes (2215-2225 cm-1). These shifts are indicative of different compositions and molecular interactions, possibly with copper ions. To further characterize and correlate the structure of the initial crystals with the subunits of the superstructures, we carried out both focused ion beam (FIB)-SEM imaging and HAADF STEM measurements (Figure 4). FIB-SEM analysis along the longitudinal direction of the initial crystal reveals that not only the spike-like termini are hollow, but almost the entire crystals (Figure 4, Chart 1). The heart of the structure is more solid suggesting that these crystals grow from this core with widening of the cavity towards the termini. The faces of these hollow crystals consist of a layered structure (Figure S3F). This observation indicates a non-simultaneous growth of the crystal planes within the layers along the longitudinal direction resulting in the spike-like termini (Figure S3B-E). The growth of the laminae belonging to a particular face is also not contemporaneous, resulting in groups of layers with different lengths. The fused crystals are also hollow. FIB-SEM analysis across and along the longitudinal direction of these structures shows both the appearance of hollow structures and their gradient (Figure 4, Chart 2). The presence of cavities for more developed superstructures is shown by the electron density profile obtained by high angle annular dark field (HAADF) STEM (Figure 4, Chart 3). Measurements performed at different areas on the subunits are showing a hollow structure. HAADF STEM images show consistently with the initial structures, cavities within the core and a hollow structure at the termini. Deconvolution of the HAADF STEM profiles of the superstructure indicates the presence of copper on the crystal surface which is in good agreement with EDS mapping. The structural similarities between the initial crystals and the superstructures are in agreement with a fusion model.



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Figure 4. Cross-sections of TPEPA crystals and superstructures. Chart 1: FIB-SEM images of a constitutive TPEPA crystal formed after the solvothermal reaction. Chart 2: (A,B) Scanning electron microscopy (SEM) images of superstructures after 15 days of aging before (left) and after (right) focused ion beam (FIB) milling. Chart 3: (A) TEM image of a superstructure. (B) High-angle annular dark-field scanning transmission electron microscopy (HAADF S-TEM) image of a superstructure. For (A) and (B): The HAADF S-TEM profiles related to the transversal scans are highlighted as green and red lines. The images were obtained after 15 days of aging at room temperature



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The X-ray crystal structure of a subunit of a superstructure aged for 25 days shows p-p stacked TPEPA molecules that form continuous channels (q = 5.4 Å × 5.4 Å) (Figure 5 Chart 1). These channels formed along the c axis (longitudinal direction) of the porous structure (15.8% of the unit-cell volume is void, the porosity was calculated by Mercury by the contact surface model). There is no incorporation of copper ions into the crystal lattice by coordination with the pyridine moieties of TPEPA. The nitrogen atoms of the pyridine moieties are located at the edges of the unit cell of the crystal structure, indicating that the crystal surfaces, including those of the channels (plane 001), can bind copper ions as observed by HAADF STEM (Figure 4, Chart 3C). The estimated accessible internal surface for a spherical probe with a radius equal to the Cu2+ ionic radius (0.73 Å) is 18.1% of the lattice volume. High-resolution TEM of the rectangular, hollow TPEPA crystals along the longitudinal direction immediately after the solvothermal reaction showed that the crystal plane spacing of 1.32 nm matches the 020 plane distance inferred by the crystallographic data of the superstructure subunit (Figure 5 Chart 2). The experimental TEM diffraction and the diffraction pattern simulated on the basis of the X-ray crystallographic data are nearly identical (98%) (Figure 5 Chart 3). Powder XRD measurements on the initial TPEPA crystals and the superstructures after 12 day of aging are in good agreement with the XRD pattern generated from the single crystal analysis (Figure S13). These observations indicate that the TPEPA crystals indeed constitute the building blocks of the superstructures. Additional peaks with 2q values of 11.6, 12.5 and 23.3 are compatible with d spacings of Cu salts, such as Cu2(NO3)(OH)3 and (NH4)CuCl3.41,42 Cu2(NO3)(OH)3 was observed by PXRD when solubilizing Cu(NO3)2 in DMF/CHCl3 (3:1 v/v).43 The formation of the other salt is consistent with the EDS and XPS data (Figure 3 Chart 2 A; Figure S12). The elemental composition (weight %) of the superstructure from duplicate elemental analysis is: C, 50.96; H, 6.88; N, 8.87; Cl, 7.16; Cu 3.60. The amount of oxygen is estimated

as

22.6%.

This

composition

indicates

a

formula

of:

C74.9H120N11.2Cl3.56Cu1.00O24.9. The TPEPA/Cu2+ ratio is ~1.2 assuming that all carbon content originates from the TPEPA (C62H44N4), The remaining elements are in agreement with the presence of H2O and the Cu salts. Inductively coupled plasma mass spectrometry



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(ICP-MS) measurements are in excellent agreement with the elemental analysis. A Cu concentration of 18 ppb (= 3.60 w/w%), was observed in samples containing 500 ppb of 12 days aged superstructures.

Figure 5. Comparison between the crystallographic structures of constitutive TPEPA crystals and a superstructure thereof. For the ORTEP drawing of TPEPA, see Figure S14. Chart 1: Crystallographic structure of a superstructure after 25 days of aging at room temperature. A needle-like crystal was separated from a superstructure. Chart 2: (A-C) TEM images of a constitutive TPEPA crystal isolated after the solvothermal reaction. The white lines and arrows highlight the crystal plane spacing of 1.32 nm along the

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longitudinal direction. (D) Representation of the 020 planes (distance 1.332 nm) obtained from the X-ray diffraction data of a superstructure. Chart 3: (A) TEM diffraction pattern of a constitutive TPEPA crystal. (B) X-ray based 0kl layer of the superstructure shown in Chart 1. CONCLUSION The aesthetically pleasing multi-branched superstructures we generated have inorganic44– 47

and organic46,48,49 analogs; however, we are not aware of similar multi-branched and

uniform architectures for molecular crystals formed by metal-mediated processes. Our observations negate the possibility that these superstructures are generated solely by commonly accepted dissolution-recrystallization and nucleation processes of the constitutive crystals. Dissolution-recrystallization and nucleation processes have been proposed in the literature for other multi-branched architectures. The shape of the termini of the struts of our crystals evolving over time - this cannot be explained by a crystal fusion mechanism. However, we think that a crystal fusion mechanism is in play here to generate the multi-branched structures, thus making our case closer to other examples of crystal attachment processes already reported for nano-,50,51 micro-,52 and millimetersized53-55 crystals. The crystalline subunits do not grow out of a common central nucleus. Our conclusion is based on the following: (i) the constitutive organic crystals closely resemble the subunits of the superstructures in terms of uniformity, morphology, and crystal lattice; (ii) we did not note any dissolution of these crystals; (iii) the structure of the junctions formed by the subunits of the superstructures indicate fusion rather than nucleation; (iv) the length of the initially formed organic crystals determines the superstructure formation; and (iv) the merging of superstructures by way of fusion shows that such processes are possible for these materials. The experimental data unambiguously show that the copper salt plays dominant and different roles in the formation of the constitutive crystals and the superstructures. The copper salt controls the formation and stabilizes the TPEPA crystals. Moreover, its presence at the junctions of the multi-branched structures indicates that the formation of relatively strong copper–pyridine bonds (BDE ~ 50 kcal/mol) is most likely driven by this fascinating assembly process, by overcoming the loss in entropy. Another interesting observation in our study is the room temperature reactivity of the constitutive organic



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crystals, which were obtained by heating. This apparent thermodynamic product continues to undergo a complex cascade of reactions and represents an example of the thermal preconditioning of a chemical process where reactants have multiple functions. ASSOCIATED CONTENT Supporting Information Experimental section, supplementary figures and ORTEP style figure of TPEPA. Crystallographic data are deposited at the Cambridge Crystallographic Date Centre (CCDC) as CCDC 1590454 AUTHOR INFORMATION Corresponding Authors * [email protected] * [email protected] ACKNOWLEDGEMENTS This research was supported by the Irving and Cherna Moskowitz Center for Nano and Bio-imaging at the Weizmann Institute of Science, the Israel Science Foundation (ISF), and the Helen and Martin Kimmel Center for Molecular Design. We thank Shira Hamami for her assistance in the organic synthesis. We thank Dr I. Pinkas, Dr. T. Bendikov, Dr. I. Cohen Ofri and Dr Y. Feldman for carrying out Raman, XPS, ICP-MS and powder XRD measurements respectively. M.E.vdB. holds the Bruce A. Pearlman Professional Chair in Synthetic Organic Chemistry. REFERENCES (1)

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