FULL PAPER DOI: 10.1002/asia.201402410
Hierarchical Self-Assembly of Supramolecular Hydrophobic Metallacycles into Ordered Nanostructures Jing Zhang,[a] Riccardo Marega,[b] Li-Jun Chen,[a] Nai-Wei Wu,[a] Xing-Dong Xu,[a] David C. Muddiman,[c] Davide Bonifazi,*[b, d] and Hai-Bo Yang*[a]
Abstract: We describe herein the hierarchical self-assembly of discrete supramolecular metallacycles into ordered fibers or spherical particles through multiple noncovalent interactions. A new series of well-defined metallacycles decorated with long alkyl chains were obtained through metal–ligand interactions, which were capable of aggregating into ordered fibroid or spherical nanostructures on the surface,
mostly driven by hydrophobic interactions. In-depth studies indicated that the morphology diversity was originated from the structural information encoded in the metallacycles, including Keywords: metallacycles · noncovalent interactions · nanostructures · self-assembly · supramolecular chemistry
Introduction
molecular building blocks into the more complex functional architectures (the “bottom-up” strategy)[3] has attracted tremendous attention owing to their wide applications in materials science.[4] Indeed, the precise morphological control at different scales (e.g., from molecular to supramolecular and from the nanometer to sub-micrometer range) is of pivotal importance in the fields of nanoscience and nanotechnology, as materials morphology plays an essential role in determining the functionality of the final devices. Nowadays, efforts have been devoted toward the construction of the ordered nanostructures through supramolecular self-assembly by means of noncovalent interactions.[5] Among the widely used weak interactions, metal–ligand coordination has proven to be one of the most efficient driving forces in constructing well-ordered complexes.[6] Many nanoscale morphologies such as spherical structures have been formed through coordination bonds from functional coordination polymers.[7] In this respect, some of us have recently reported the construction of fibroid morphologies from platinum– acetylide xerogel,[8] whereas Wang and co-workers[9] have described the formation of sub-micrometer-scale monodisperse spherical materials by means of coordination-induced self-assembly (CISA) at room temperature. Moreover, by exploiting Pt N and Pt O interactions,[10] the construction of nanoscale functionalized polygons and polyhedra obtained through the CISA approach have been reported recently.[11] According to both “directional bonding” and “symmetry interaction” models, the shape of an individual 2D polygon is usually determined by the value of the turning angle within its angular components.[6a, 12] For example, the combination of two ditopic 1208 donors with two suitable 608 acceptors will yield a molecular rhomboid.[13]
Hierarchical self-assembly (HSA) is a stepwise process in which the components are brought together in a precisely defined way at different scales by multiple noncovalent interactions, whereby the assemblies produced in one step are seeds for the subsequent level of organization.[1] HSA of small molecules that contain specific information into nanosized objects of precisely determined shape, structure, and function is widely utilized in natural and unnatural systems.[2] In particular, the design and self-assembly of small [a] J. Zhang, L.-J. Chen, N.-W. Wu, X.-D. Xu, Prof. Dr. H.-B. Yang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry East China Normal University 3663 N. Zhongshan Road Shanghai, 200062 (P.R. China) Fax: (+ 86) 21-6223-5137 E-mail: [email protected] [b] Dr. R. Marega, Prof. Dr. D. Bonifazi Namur Research College (NARC) and Department of Chemistry University of Namur, Rue de Bruxelles 61, 5000 Namur (Belgium) Fax: (+ 32) 081-725433 E-mail: [email protected] [c] Prof. Dr. D. C. Muddiman W. M. Keck FT-ICR Mass Spectrometry Laboratory and Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695 (United States) Fax: (+ 1) 919-513-0084 [d] Prof. Dr. D. Bonifazi Department of Chemical and Pharmaceutical Sciences and INSTM UdR Trieste, University of Trieste 34127 Trieste (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402410.
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the number of alkyl chains and their spatial orientation. Interestingly, the morphology of the metallacycle aggregates could be tuned by changing the solvent polarity. These findings are of special significance since they provide a simple yet highly controllable approach to prepare ordered and tunable nanostructures from small building blocks by means of hierarchical self-assembly.
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On the basis of the models mentioned above, a predesigned supramolecular hexagon can be self-assembled in two different ways, namely, through (3+3)- or (6+6)-cyclization routes. The (3+3) route follows the combination of two complementary ditopic molecular modules, each of which incorporate a 1208 angle between the active coordination sites,[14] whereas (6+6) assemblies can be obtained from the interaction of six units that bear a 1208 spacing between the coordination sites and six linear units (which contain two coordination sites oriented 1808 from each other).[15] Encouraged by the versatility of the CISA method, we envisioned that a bispyridyl unit equipped with alkyl long chains could lead to the formation of a new family of supramolecular metallacycles with rhomboid (2+2) or hexagonal (3+3 and 6+6) superstructures when combined with the appropriately designed PtII acceptors. It should be noted that the introduction of a 3,4,5-trisACHTUNGRE(alkoxy)phenyl substituent into a conjugated planar skeleton is favorable to the formation of highly ordered nanostructures such as liquid crystals and gels by means of the hydrophobic interaction of alkyl chains.[16] Thus we anticipate that such supramolecular metallacycles decorated with different-numbered hydrophobic alkyl long chains might aggregate into ordered nanostructures through HSA under the proper conditions, as also recently reported by Stang et al.[17] Herein, we present the preparation and characterization of a new family of metallacycles with different shapes and sizes along with different-numbered alkyl long chains by means of CISA. By a proper selection and combination of different angular units, rhomboidal or hexagonal metallacycles decorated with hydrophobic alkyl chains were successfully prepared (> 99 %). Owing to the different geometrical properties and alkyl chain density, the resulting metallacycles exhibited distinct self-organization patterns on different surfaces, for example, SiACHTUNGRE(111) or mica, thus yielding various nanostructures with ordered sizes and shapes. For instance, rhomboidal complex 5 formed a fibrous morphology on the surface, whereas hexagonal complexes 6 and 7 generated nanospheres under the same conditions. Moreover, by changing the polarity of the solvents, the morphology of the rhomboidal metallacycle aggregates can be tuned from spherical to fibroid. This finding is of particular significance since it provides a new path to precisely control the shape and size of the nanosized structures by simply changing the molecular modules or the polarity of the solvent, thus providing an opportunity to gain a better understanding on how control self-organization of organometallic skeletons at large scale.
Hai-Bo Yang, Davide Bonifazi et al.
with hydrophobic alkyl chains in hand, the self-assembly of a new family of metallacycles with different shapes and sizes along with multiple alkyl chains was investigated. Recent research results have indicated that the addition of functional groups, such as crown ethers, ferrocenes, and Frchet-type dendrons at the vertex of the 1208 building block enabled the preparation of novel functionalized hollow molecular assemblies.[20] Thus, the introduction of long alkyl chains to the vertex of a 1208-shaped building block will not hinder the self-assembly process into the metallacycles functionalized with the alkyl chains. Stirring the mixture of predesigned 1208 precursor 1 and 60, 120, or 1808 di-PtII acceptor 2, 3, or 4 in CH2Cl2 with a ratio of 1:1, respectively, resulted in the formation of the (2+2) rhomboidal 5, the (3+3) hexagon 6, and the (6+6) hexagon 7 with the peripheral alkyl chains at the donor vertexes in excellent yield (> 99 %, Scheme 1). Multinuclear NMR (1H and 31P) spectroscopic analysis of the reaction mixtures revealed the formation of discrete, highly symmetric species. Each 31P{1H} NMR spectrum of molecules 5, 6, and 7 displayed a sharp singlet (d = 12.7 ppm for 5, d = 13.5 ppm for 6, and d = 13.3 ppm for 7) that was shifted upfield from the starting platinum acceptors 2, 3, and 4 by approximately d = 6.5, 6.1, and 6.2 ppm, respectively
Results and Discussion Synthesis Newly designed 1208 donor precursor 1 can be easily prepared by means of an etherification reaction of 3,5-bis(4ethynlpyridinyl)phenol[18] with 1-bromomethyl-3,4,5-tris(dodecyloxy)benzene.[19] With the 1208 precursor substituted
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Scheme 1. Molecular structures of modules 1–4, as well as a graphical representation of the rhomboidal metallacycle 5 and hexagonal metallacycles 6 and 7.
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formation of Pt N coordination bonds (see the Supporting Information). The structures of rhomboidal and hexagonal complexes 5, 6, and 7 were further confirmed by mass spectrometry. In this study, owing to the high molecular weight and relatively weak PtII N bonds formed between a soft metal and a hard ligand, it was difficult to obtain intense mass signals. Usually, the cold-spray ionization time-of-flight (CSI-TOF) MS technique allows the assemblies to remain intact during the ionization process while obtaining the high resolution required for isotopic distribution.[22] For instance, the CSITOF-MS spectrum of rhomboid 5 revealed two peaks at m/z 2040.04 and 1339.36, which corresponded to [M 2 ONO2 ]2 + and [M 3 ONO2 ]3 + (Figure 3a), respec-
Figure 1. Partial 31P NMR (161.9 MHz, CD2Cl2, 298 K) spectra of a) 608 di-PtII acceptor 2 and rhomboidal metallacycle 5, b) 1208 di-PtII acceptor 3 and hexagonal metallacycle 6, and c) 1808di-PtII acceptor 4 and hexagonal metallacycle 7.
(Figure 1). This change, as well as the decrease in coupling of flanking 195Pt satellites (DJ 178 Hz for 5, DJ = 147 Hz for 6, and DJ = 157 Hz for 7) is consistent with the back-donation from Pt transition-metal atoms. Additionally, in the 1H NMR spectrum of each assembly, pyridinecentered protons exhibited downfield shifts (a-HPy, d 0.07– 0.31 ppm; b-HPy, d = 0.40–0.54 ppm) as a consequence of the coordination reaction of the pyridine-N atom with the PtII metal center. Notably two doublets for a- and b-hydrogen nuclei on the pyridine rings of the rhomboidal complex 5 were observed in the 1H NMR spectrum, which could have resulted from the hindered rotation about the Pt N (pyridyl) bond in a small-sized metallacycle (Figure 2).[20c, 21] Fur-
Figure 3. Theoretical (top) and experimental (bottom) CSI-TOF MS spectra of rhomboidal metallacycles a) 5 and b) 6.
tively. These peaks were isotopically resolved, and they are in agreement with the calculated pattern. Moreover, in the case of hexagon 6, the peaks at m/z 1561.3 and 1219.2, which corresponded to [M 4 OTf ]4 + and [M 5 OTf ]5 + , respectively (Figure 3b), were observed, and their isotopic resolutions were in excellent agreement with the theoretical isotopic pattern. It should be noted that, in the case of 7, because of the high molecular weight (13 049.52 Da), it was extremely difficult to obtain its mass signal even under CSITOF-MS conditions. With considerable effort, however, one peak that corresponded to [M 11 OTf ]11 + (m/z 1038.00) was observed in the ESI-TOF-MS spectrum of 7 and its isotopic resolution matches that which was derived from calculations (Figure 4), thus supporting the formation of the large hexagonal metallacycle 7. All attempts to obtain the crystal structure of rhomboidal complex 5 and hexagonal metallacycles 6 and 7 have been unsuccessful to date. Therefore, to gain more structural information, the semiempirical molecular orbital method (PM6) was employed to optimize the geometrical properties
Figure 2. Partial 1H NMR (400 MHz, CD2Cl2, 298 K) spectra of 1208 donor 1, and metallacycles 5, 6, and 7.
ther characterization with two-dimensional spectroscopic techniques (1H,1H COSY and NOESY) are in agreement with the formation of highly symmetric metallacycles decorated with alkyl chains (see the Supporting Information). For example, the spatial interactions between the a-H proton on pyridine and PEt3 protons were observed in the NOESY spectrum of each complex, which authenticated the
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Self-Assembly on the Surface As a consequence of the presence of long alkyl chain subunits at the alternate corners of the obtained metallacycles, we envisioned that these macromolecules could have a tendency to aggregate into ordered nanostructures under proper solvent conditions. To allow the self-assembly process to occur, we dissolved the metallacycles in CH2Cl2 and then slowly added subsequent aliquots of CH3OH to reach a 1:1 ratio, ultimately reaching a final concentration of 4.4 10 4 m. The solution was then exposed to n-hexane vapors at 273 K, thus allowing for a gradual, time-dependent solvent composition change. After 84 h, it was found that the metallacycles were still dissolved in the solution without any evidence of further evolution of the self-organization (see also the dynamic light scattering (DLS) measurements in the Supporting Information). However, drop-casting small aliquots of the solution that contained the hydrophobic building block 1 onto the freshly cleaned SiACHTUNGRE(111) surfaces (for SEM imaging) yielded the entangled nano-to-microstructured infinite mesh (see the Supporting Information). By repeating the same procedure starting from the solutions that contained rhomboidal metallacycle 5 drop-cast onto SiACHTUNGRE(111) surfaces or onto carbon-coated Cu grids (for TEM analysis), the fiberlike morphologies with diameters on the order of 100–300 nm and lengths spanning from 500 nm to 5 mm were observed in both SEM and TEM images (Figure 6a and the
Figure 4. Theoretical (dot) and experimental (black line) ESI-TOF MS spectrum of 7. The theoretically most abundant isotope is centered at 1037.91400, whereas the experimental value was found at 1038.00491.
of all metallacycles (Figure 5). The optimized structure featured a roughly planar ring surrounded by flexible long alkyl chains subunits at alternate corners. Complex 5 resembled a well-defined rhombus that featured an approximately 2.3 1.3 nm cavity, whereas metallacycles 6 and 7 featured a hexagonal macrocycle with an internal radius of 1.5 and 2.5 nm and an outer radius of 4.0 and 5.0 nm, respectively.
Figure 6. SEM images of SiACHTUNGRE(111) surfaces after drop-casting of solutions (CH2Cl2/CH3OH 1:1 under n-hexane diffusion after 84 h) containing metallacycles a) 5, b) 6, and c) 7. Inset: Representative TEM images of Cu grids prepared by drop-casting a solution containing the appropriate metallacycles.
Figure 5. Geometrical structures of metallacycles a) 5, b) 6, and c) 7 as optimized by the PM6 semiempirical molecular orbital method. The dashed (red) ellipse highlights hydrophobic portions, whereas the full (blue) one surrounds the hydrophilic parts.
Supporting Information). Furthermore, by employing solutions that contained different-sized hexagonal metallacycles 6 and 7, round-shaped particles with a diameter of approximately 200 nm for 6 ((180 43) nm from a count of 106 particles) and approximately 400 nm for 7 ((370 130) nm, from a count of 103 particles) were obtained (Figure 6b– c and the Supporting Information). Similar spheroidal nanoparticles (Figure 6b–c, inset) were also observed in TEM images, which were in good agreement with the SEM observations. Any attempt to analyze these nanostructures by means of X-ray diffraction (XRD) failed, since during the mechanical transfer of the dried powdered material from the Si surfaces to an XRD sample holder, the morphologies underwent decomposition (see SEM images and NMR spectroscopic characterization in the Supporting Information). To further verify the hypothesis that all morphologies were formed during the solvent evaporation, we have tried to de-
To gain further insight into the structures of the obtained metallacycles, their diffusion coefficient (D) was determined by two-dimensional diffusion-ordered 1H NMR (2D DOSY) spectroscopic experiments (see the Supporting Information). According to the Einstein–Smoluchowski relation, at a fixed temperature and solution properties (viscosity) an increase in the hydrodynamic radius (which is strictly connected to size and thus molecular weight) determines the reduction of the D value.[23] 2D DOSY NMR spectroscopic measurements for metallacycles 5, 6, and 7 (CD2Cl2, 293 K) showed the expected trend in D values, with the higher D value for 5 (3.39 10 10 m2 s 1), an intermediate D value for 6 (1.51 10 10 m2 s 1), and the lower D value for 7 (0.74 10 10 m2 s 1). This trend is in agreement with the structural features suggested by the mass analysis and the computational models.
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was clearly found that the obtained nanostructures were generated on the surface during the solvent evaporation, with the latter dictating both morphology and size distribution. We then assessed how the time-dependent change in the solvent composition (CH2Cl2/CH3OH volumetric ratio) under n-hexane diffusion affected the morphology of the nanomaterials. With this aim, we sampled the solutions at different times (from 0.5 to 84 h) and drop-cast it onto SiACHTUNGRE(111) surfaces. Interestingly, for solutions that contained rhomboidal metallacycle 5, some spherical objects appeared on the surface after just 2–4 h, whereas from 6 to 48 h a gradual change from spherical to the fibrillar objects was clearly observed. From 48 to 84 h the ratio of the fibrillar to spherical nano-objects was maximized (Figure 8e and f). However, for the solution of hexagonal metallacycles 6 and 7, the spherical nanoparticles could be found at the early stages (2–4 h), with a sharpening of their spherical structure occurring after 48 h (from 48 to 84 h, see the Supporting Information). Since a solvent polarity change was expected during the n-hexane diffusion, it was reasonable to investigate how the polarity affected the morphologies obtained from complexes
posit the aforementioned solutions by spin-coating onto the muscovite (mica) surfaces. Indeed, by setting different rotational speeds of the substrate (range 1–8 krpm) and keeping constant the solution composition and the deposition volume (5 mL), it was possible to investigate influence of the solvent evaporation on the morphology evolution. In particular, we have found that the rotational speed dramatically influenced the morphology as imaged by atomic force microscopy (AFM) or SEM techniques. Indeed, by depositing the solution at 1 krpm (relatively slow solvent evaporation conditions), the rhomboidal metallacycle 5 led to the formation of the same fibrillar morphology as that obtained after drop-casting (Figure 7a, d and the Supporting Information). However, at 5 krpm only spheroidal nanoparticles were observed for 5 (Figure 7g). When employing hexagonal metallacycles 6 and 7, the spin-coating procedure generated the similar round-shaped morphologies and size distributions as indicated by SEM imaging of the drop-casted materials (Figure 7b, c and e, f, respectively, and the Supporting Information). Nevertheless, increasing the rotational speed to 8 krpm determined a reduction in the average diameters for both metallacycles 6 and 7 (about 100 and 150 nm for 6 and 7, respectively; see Figure 7h, i). From these observations it
Figure 7. AFM images (topography channel) of the mica surfaces after spin-coating of solutions (CH2Cl2/CH3OH 1:1 under n-hexane diffusion after 84 h) containing metallacycles 5, 6, or 7. a, d) Metallacycle 5 deposited at 1 krpm, at lower and higher magnification, respectively. b, e) Metallacycle 6 deposited at 5 krpm, at lower and higher magnification, respectively. c, f) Metallacycle 7 deposited at 5 krpm, at lower and higher magnification, respectively. g) Metallacycle 5 deposited at 5 krpm. h) Metallacycle 6 deposited at 8 krpm. i) Metallacycle 7 deposited at 8 krpm. Images were scaled in the height range according to the color intensity reported to the right of each picture.
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porting the surface-confined solvent polarity-induced nanostructuring process (see the Supporting Information). A further investigation of guest incorporation by using the obtained nanomaterials was carried out. We tried to add a fluorescent dye to the solution (CH2Cl2/CH3OH 1:1) that contained different metallacycles. One could envisage that under those conditions the chosen dye, rhodamine B (RhoB), might be incorporated into the nanostructures and impart strong emissive properties. Unfortunately, no fluorescent nanofibers or nanospheres were observed by confocal microscopy upon drop-casting the solutions onto glass coverslips. However, SEM images of the deposited material prepared by using different concentrations of Rho-B (2.2 10 5, 2.2 10 4, and 2.2 10 3 m) again confirmed the influence of the solvent polarity in inducing the nanostructuring of the metallacycle assemblies (Figure 9). In particular, the self-or-
Figure 8. SEM images of the time-dependent morphology of 5 (from CH2Cl2/CH3OH at 1:1 ratio under n-hexane diffusion up to 84 h) in the self-assembly process after drop-casting onto SiACHTUNGRE(111) surfaces a) after 30 min, b) after 2 h, c) after 4 h, d) after 6 h, e) after 48 h, and f) after 84 h.
5, 6, and 7 after drop-casting. Thus n-hexane vapor was diffused into vials that contained the solutions of the metallacycles in pure CH2Cl2 or CH3OH, respectively, then the morphological evolution was evaluated after 84 h. By removing CH3OH, the rhomboidal metallacycle 5 formed spheroidal aggregates, whereas the hexagonal metallacycles 6 and 7 did not lead to any regular nanostructured morphologies (see the Supporting Information). In addition, if both CH3OH and n-hexane solvents were not used, only a porous matrix could be observed on the surface (see the Supporting Information). This finding was most likely promoted by the fast CH2Cl2 evaporation. These references studies all proved the necessity of using a polar solvent such as CH3OH to drive the nanostructuring process under solvent evaporation conditions, whereas the major role of CH2Cl2 was essentially limited to the solubilization of metallacycles (5, 6, and 7). It should be noted that rhomboidal metallacycle 5 was revealed to be the most versatile module to provide either fibrillar or spheroidal morphologies by varying the polarity of the solutions. Thus, complex 5 was dissolved in different solutions that contained CH2Cl2 and CH3OH under n-hexane diffusion conditions, with volumetric ratios changing from 1:7 (the most polar mixture) to 7:1 (the least polar). It was observed that, according to the initial solution polarity, the dominant morphologies were fibers (CH2Cl2/CH3OH 1:7), fibers and some nanospheres (CH2Cl2/CH3OH 1:3, 3:5, 5:3, 3:1), and nanospheres only (CH2Cl2/CH3OH 7:1), again sup-
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Figure 9. SEM images of the Rho-B concentration-dependent morphology on Si surfaces of a, d, g) 5; b, e, h) 6; and c, f, i) 7 from CH2Cl2/CH3OH at 1:1 ratio under n-hexane diffusion after 84 h.
ganization of the rhomboidal metallacycle 5 was dramatically affected by both Rho-B and the employed protocols (6 or 84 h; see the Supporting Information), for which fibers or spheroidal structures were formed under different ratios (Figure 9a, d, g and the Supporting Information). It turned out that the higher Rho-B concentrations affected the morphology formation in the same manner as that of the solvent polarity (e.g., CH2Cl2/CH3OH 5:3 or 7:1 ratios) or faster solvent evaporation conditions (5 krpm versus 1 krpm). However, the morphologies that stemmed from the deposition of solutions that contained metallacycles 6 and 7 were not affected by the Rho-B concentration; the latter modulated only a significant increase in the average size distribution after 84 h (spheres reaching the micrometer range; see Figure 9b, c, e, f, h, i and the Supporting Information). By comparing the structural features of building blocks 1, 2, 3, and 4, it was clearly found that donor 1 was a strongly
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apolar unit, whereas the Pt complex-based angular acceptors 2, 3, and 4 displayed a more hydrophilic character. When comparing metallacycles 5, 6, and 7, one could note that the Pt atom centers were differently exposed toward the solvent in the different complexes (Figure 5). Specifically, rhomboidal metallacycle 5 showed well-exposed and solvent-accessible metal centers (orthogonal to apolar module 1), whereas in hexagonal metallacycles 6 and 7 the hydrophilic Pt atoms were progressively less solvent-accessible as a consequence of the burying effect of the aliphatic chains that surrounded the metallacycle scaffolds. It is therefore very likely that the metallacycle polarity decreases when going from rhomboidal metallacycle 5 to hexagonal metallacycle 7. Consequently, the solvation properties during either the drop-casting or spin-coating procedures changed, thus dictating the final morphology and size distribution of the observed nanomaterials. This should explain why under n-hexane diffusion conditions, rhomboidal metallacycle 5 formed small nanospheres from a CH2Cl2 solution or under high Rho-B concentrations, whereas long fibers were only observed when using more polar mixtures (CH2Cl2/CH3OH) or low Rho-B concentrations. Considering the faster evaporation rate of CH2Cl2 than that of CH3OH under spin-coating conditions, the speed-dependent morphologies of solvent evaporation as observed through AFM measurements supported this hypothesis. For the less polar hexagonal metallacycles 6 and 7, which never formed fibers on the surfaces, the solvophobic effect was the pivotal issue that governed the formation of the nano- or microspheres. This would explain why faster spin-coating depositions and high Rho-B concentrations had opposite effects in modulating the average size distribution of the self-organized spheres.
phology of the metallacycle aggregation could be tuned by changing the solvent polarity during the surface deposition. These findings are of special significance since this study not only enriches the library of functional metallacycles but also provides a simple yet controllable approach to the formation of ordered nanoscale soft materials. Further understanding and application of these novel supramolecular nanostructures in materials science are now under investigation.
Experimental Section Materials All reagents and solvents were purchased from commercial sources. DMF was distilled from CaH2 and degassed under N2 for 30 min before use. Reactions were performed in standard glassware under an inert N2 atmosphere. All air-sensitive reactions were carried out under an Ar atmosphere. 3,5-Bis(4-ethynlpytidinyl)phenol and 1-bromomethyl-3,4,5tris(dodecyloxy)benzene were prepared following the procedures reported below. SiACHTUNGRE(111) wafers were provided courtesy of Prof. Sorin Melinte from Universit Catholique de Louvain (Belgium), whereas mica sheets were purchased from Ted Pella (quality grade V1). Characterizations 1
H, 13C, and 31P NMR spectra were recorded with a Bruker 400 MHz spectrometer at 298 K. The 1H and 13C NMR spectroscopic chemical shifts are reported relative to residual solvent signals, and 31P NMR spectroscopic resonances are referenced to an internal standard sample of 85 % H3PO4 (d = 0.0 ppm). 1H,1H COSY, 1H,1H NOESY, and 1H,1H DOSY spectra were recorded with a Bruker 500 MHz spectrometer at 293 K. SEM images were obtained with a Jeol 7500F microscope. The drop-cast SiACHTUNGRE(111) surfaces were deposited with a thin film of Au prior to analysis. AFM measurements were carried out in air at 293 K with a Nanoscope V Multimode 8 instrument (Veeco, USA). Silicon tips on nitride levers (T = 0.55–0.75 mm; k = 0.4 Nm 1, Veeco, USA) operated at a frequency in the range of 50–90 kHz were used to image the surfaces through ScanAsyst-air mode. The samples were prepared by spin-coating 5 mL of the same solution employed for the other microscopic observations onto a mica substrate. TEM images were obtained with a JEOL JEM-2100 instrument.
Conclusion
Synthesis of Building Block 1
In conclusion, we have described the preparation of nanoscale fibroid and spherical aggregates of coordination metallacycles 5, 6, and 7 by means of hierarchical self-assembly. The structure of rhomboidal and hexagonal metallacycles 5, 6, and 7 were confirmed by one-dimensional multinuclear (1H and 31P) and two-dimensional (1H,1H COSY, NOESY, and DOSY) NMR spectroscopy, mass spectrometry, and elemental analysis. Specifically, it was found that all metallacycles self-organized on SiACHTUNGRE(111) substrates and mica to form ordered nanofibers or nanoparticles upon solvent evaporation. For example, rhomboidal metallacycle 5 aggregated into nanosized fibers, evolving from spheroidal objects, whereas hexagonal metallacycles 6 and 7 both formed nanoparticles under the same conditions. All nanostructures were distinct from the morphology of ligand 1, which formed an infinite entangled mesh under the same conditions. The difference in the number of the alkyl chain and their orientation dictated the hydrophilic/lipophilic character of each metallacycle, thus leading to the formation of different morphologies at the nano- and microscale level depending on preparative conditions. Indeed, it was found that the mor-
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In a Schlenk flask, NaH (45 mg, 1.11 mmol) was dissolved in fleshly distilled and degassed DMF (3.5 mL), and then under an N2 atmosphere, 3,5-bis(4-ethynlpytidinyl)phenol[18] (100 mg, 0.34 mmol) was carefully introduced into the reaction flask. The mixture was stirred for 30 min at (269 mg, RT. 1-Bromomethyl-3,4,5-tris(dodecyloxy)benzene[19] 0.37 mmol) was then added under N2. The reaction was continued at 60 8C for another 12 h and then quenched by water (10 mL), extracted with CH2Cl2 (2 10 mL); then the organic layers were combined and extracted with brine (3 40 mL) and dried (MgSO4). The solvent was removed by evaporation on a rotary evaporator. The residue was purified by column chromatography on silica gel (acetone/CH2Cl2 1:10) to give the target product as a light yellow solid with a yield of 62.4 %. Rf = 0.39 (dichloromethane/acetone 20:1); m.p. 70–74 8C; 1H NMR (CDCl3, 400 MHz): d = 8.62 (d, J = 3.6 Hz, 4 H), 7.38 (br, 5 H), 7.18 (s, 2 H), 6.62 (s, 2 H), 5.00 (s, 2 H), 3.98–3.97 (m, 6 H), 1.82–1.73 (m, 6 H), 1.47–1.26 (m, 54 H), 0.88–0.86 ppm (m, 9 H); 13C NMR (CDCl3, 100 MHz): d = 158.5, 153.4, 149.8, 138.2, 130.9, 128.0, 125.5, 123.6, 119.1, 106.1, 92.5, 87.2, 73.4, 70.7, 69.2, 31.9, 30.3, 29.7, 29.6, 29.4, 29.3,26.1, 22.6, 14.0 ppm; elemental analysis calcd (%) for C63H90N2O4 : C 80.55, H 9.66, N 2.98; found: C 80.43, H 9.87, N 2.84; CSI-TOF MS of target product: m/z calcd for C63H91N2O4 : 938.69 [M+H] + ; found: 939.70.
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Synthesis of Rhomboidal Metallacycle 5
Ying Tung Education Foundation (no. 131014), and the Program for Changjiang Scholars and Innovative Research Team in University. D.B. acknowledges the European Union through the ERC Starting Grant “COLORLANDS” project, the FRS-FNRS (FRFC contract no. 2.4.550.09), the Science Policy Office of the Belgian Federal Government (BELSPO-IAP 7/05 project), the “TINTIN’’ ARC project (09/14-023), the Service Public de Wallonie through the 2013 Program of Excellence (FLYCOAT project), and the MIUR through the FIRB ”Futuro in Ricerca“ (”SUPRACARBON’’, contract no. RBFR10DAK6). We thank Prof. Hongwei Tan and Miss Quan-Jie Li (Beijing Normal University) for their help with molecular simulation. R.M. thanks the FRS-FNRS for his postdoctoral fellowship.
A solution of compound 1 (5.44 mg, 0.00582 mmol) in CH2Cl2 (1.5 mL) was added dropwise to a solution of compound 3 (6.78 mg, 0.00583 mmol) in CH2Cl2 (1.5 mL) under continuous stirring. The reaction mixture was stirred overnight at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %; 1H NMR (CD2Cl2, 400 MHz): d = 9.29 (d, J = 5.6 Hz, 4 H), 8.78 (s, 4 H), 8.69 (d, J = 5.6 Hz, 4 H), 7.94 (dd, J = 1.6 and 5.6 Hz, 4 H), 7.77 (dd, J = 1.2 and 5.6 Hz, 4 H), 7.64 (s, 2 H), 7.58–7.55 (m, 12 H), 7.36 (d, J = 1.2 Hz, 4 H), 6.66 (s, 4 H), 5.04 (s, 4 H), 3.99–3.90 (m, 12 H), 1.82–1.77 (m, 12 H), 1.47– 1.25 (m, 156 H), 1.16–1.08 (m, 72 H), 0.87–0.83 ppm (m, 18 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 12.74 ppm (s, 1JACHTUNGRE(Pt,P) = 2702.1 Hz); elemental analysis calcd (%) for C202H316N8O20P8Pt4 : C 57.70, H 7.57, N 2.66; found: C 57.57, H 7.79, N 2.52; CSI-TOF MS: m/z: 2040.04 [M 2 NO2 ]2 + , 1339.37 [M 3 NO2 ]3 + .
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Synthesis of Hexagonal Metallacycle 6 A solution of compound 1 (15.99 mg, 0.0170 mmol) in CH2Cl2 (2 mL) was added dropwise to a solution of compound 2 (22.84 mg, 0.0170 mmol) in CH2Cl2 (2 mL) under continuous stirring. The reaction mixture was stirred for 3 h at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %. 1H NMR (CD2Cl2, 400 MHz): d = 8.68 (d, J = 5.6 Hz, 12 H), 7.81 (d, J = 6.0 Hz, 12 H), 7.58 (s, 3 H), 7.54–7.48 (m, 24 H), 7.31 (s, 6 H), 5.01 (s, 6 H), 3.98–3.89 (m, 18 H), 1.97–1.64 (m, 18 H), 1.46–1.24 (m, 234 H), 1.56–1.08 (m, 108 H), 0.86– 0.83 ppm (m, 27 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 13.50 ppm (s, 1JACHTUNGRE(Pt,P) = 2647.1 Hz); elemental analysis calcd (%) for C306H474F18N6O33P12Pt6S6·2 CH2Cl2 : C 52.76, H 6.87, N 1.20; found: C 52.45, H 7.24, N 1.25; CSI-TOF MS: m/z: 1561.26 [M 4 OTf ]4 + , 1219.23 [M 5 OTf ]5 + . Synthesis of Hexagonal Metallacycle 7 A solution of compound 1 (3.09 mg, 0.00329 mmol) in CH2Cl2 (1 mL) was added dropwise to a solution of compound 3 (4.07 mg, 0.00329 mmol) in CH2Cl2 (1 mL) under continuous stirring. The reaction mixture was stirred for 1 h at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %; 1H NMR (CD2Cl2, 400 MHz): d = 8.68 (d, J = 5.2 Hz, 24 H), 7.81 (d, J = 5.6 Hz, 24 H), 7.60 (s, 6 H), 7.33 (s, 12 H), 7.05 (s, 24 H), 6.66 (s, 12 H), 5.04 (s, 12 H), 4.01–3.92 (m, 36 H), 1.83–1.71 (m, 36 H), 1.49–1.09 (m, 684 H), 0.88 ppm (t, J = 6.4 Hz, 54 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 13.30 (s, 1JACHTUNGRE(Pt,P) = 2718.3 Hz); elemental analysis calcd (%) for C570H948F36N12O60P24Pt12S12 : C 52.33, H 7.30, N 1.28; found: C 52.56, H 7.45, N 1.30; MS (ESI): m/z: 1038.00 [M 11 OTf]11 + . Preparation of Nanostructures For each assembly, the precursors were dissolved in CH2Cl2 (0.25 mL) inside a small glass vial and the resulting solution was diluted by slow addition of MeOH (ratio 1:1 unless otherwise stated) to afford a precursor concentration of 0.44 mm. The receptacle that contained such solution was then introduced into a bigger vial containing n-hexane (2.5 mL), and the resulting system was isolated with a stopper from the external environment and kept at 273 K, thereby allowing for gradual solvent evaporation from the small vial. After the desired time (for the proper CH2Cl2/ MeOH ratio inside the small vial, t = 84 h), one drop of the solution in the small vial was deposited onto a SiACHTUNGRE(111) or Cu grid for SEM and TEM, or the solution (5 mL) was spin-coated at the desired rotational speed (between 1 and 8 krpm) over freshly peeled mica for AFM analysis.
Acknowledgements H.-B.Y thanks the National Natural Science Foundation of China (grant nos. 21132005 and 21322206), the Key Basic Research Project of Shanghai Science and Technology Commission (no. 13JC1402200), the Fok
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Hierarchical Self-Assembly of Supramolecular Hydrophobic Metallacycles into Ordered Nanostructures Jing Zhang,[a] Riccardo Marega,[b] Li-Jun Chen,[a] Nai-Wei Wu,[a] Xing-Dong Xu,[a] David C. Muddiman,[c] Davide Bonifazi,*[b, d] and Hai-Bo Yang*[a]
Abstract: We describe herein the hierarchical self-assembly of discrete supramolecular metallacycles into ordered fibers or spherical particles through multiple noncovalent interactions. A new series of well-defined metallacycles decorated with long alkyl chains were obtained through metal–ligand interactions, which were capable of aggregating into ordered fibroid or spherical nanostructures on the surface,
mostly driven by hydrophobic interactions. In-depth studies indicated that the morphology diversity was originated from the structural information encoded in the metallacycles, including Keywords: metallacycles · noncovalent interactions · nanostructures · self-assembly · supramolecular chemistry
Introduction
molecular building blocks into the more complex functional architectures (the “bottom-up” strategy)[3] has attracted tremendous attention owing to their wide applications in materials science.[4] Indeed, the precise morphological control at different scales (e.g., from molecular to supramolecular and from the nanometer to sub-micrometer range) is of pivotal importance in the fields of nanoscience and nanotechnology, as materials morphology plays an essential role in determining the functionality of the final devices. Nowadays, efforts have been devoted toward the construction of the ordered nanostructures through supramolecular self-assembly by means of noncovalent interactions.[5] Among the widely used weak interactions, metal–ligand coordination has proven to be one of the most efficient driving forces in constructing well-ordered complexes.[6] Many nanoscale morphologies such as spherical structures have been formed through coordination bonds from functional coordination polymers.[7] In this respect, some of us have recently reported the construction of fibroid morphologies from platinum– acetylide xerogel,[8] whereas Wang and co-workers[9] have described the formation of sub-micrometer-scale monodisperse spherical materials by means of coordination-induced self-assembly (CISA) at room temperature. Moreover, by exploiting Pt N and Pt O interactions,[10] the construction of nanoscale functionalized polygons and polyhedra obtained through the CISA approach have been reported recently.[11] According to both “directional bonding” and “symmetry interaction” models, the shape of an individual 2D polygon is usually determined by the value of the turning angle within its angular components.[6a, 12] For example, the combination of two ditopic 1208 donors with two suitable 608 acceptors will yield a molecular rhomboid.[13]
Hierarchical self-assembly (HSA) is a stepwise process in which the components are brought together in a precisely defined way at different scales by multiple noncovalent interactions, whereby the assemblies produced in one step are seeds for the subsequent level of organization.[1] HSA of small molecules that contain specific information into nanosized objects of precisely determined shape, structure, and function is widely utilized in natural and unnatural systems.[2] In particular, the design and self-assembly of small [a] J. Zhang, L.-J. Chen, N.-W. Wu, X.-D. Xu, Prof. Dr. H.-B. Yang Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry East China Normal University 3663 N. Zhongshan Road Shanghai, 200062 (P.R. China) Fax: (+ 86) 21-6223-5137 E-mail: [email protected] [b] Dr. R. Marega, Prof. Dr. D. Bonifazi Namur Research College (NARC) and Department of Chemistry University of Namur, Rue de Bruxelles 61, 5000 Namur (Belgium) Fax: (+ 32) 081-725433 E-mail: [email protected] [c] Prof. Dr. D. C. Muddiman W. M. Keck FT-ICR Mass Spectrometry Laboratory and Department of Chemistry, North Carolina State University Raleigh, North Carolina 27695 (United States) Fax: (+ 1) 919-513-0084 [d] Prof. Dr. D. Bonifazi Department of Chemical and Pharmaceutical Sciences and INSTM UdR Trieste, University of Trieste 34127 Trieste (Italy) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402410.
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the number of alkyl chains and their spatial orientation. Interestingly, the morphology of the metallacycle aggregates could be tuned by changing the solvent polarity. These findings are of special significance since they provide a simple yet highly controllable approach to prepare ordered and tunable nanostructures from small building blocks by means of hierarchical self-assembly.
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On the basis of the models mentioned above, a predesigned supramolecular hexagon can be self-assembled in two different ways, namely, through (3+3)- or (6+6)-cyclization routes. The (3+3) route follows the combination of two complementary ditopic molecular modules, each of which incorporate a 1208 angle between the active coordination sites,[14] whereas (6+6) assemblies can be obtained from the interaction of six units that bear a 1208 spacing between the coordination sites and six linear units (which contain two coordination sites oriented 1808 from each other).[15] Encouraged by the versatility of the CISA method, we envisioned that a bispyridyl unit equipped with alkyl long chains could lead to the formation of a new family of supramolecular metallacycles with rhomboid (2+2) or hexagonal (3+3 and 6+6) superstructures when combined with the appropriately designed PtII acceptors. It should be noted that the introduction of a 3,4,5-trisACHTUNGRE(alkoxy)phenyl substituent into a conjugated planar skeleton is favorable to the formation of highly ordered nanostructures such as liquid crystals and gels by means of the hydrophobic interaction of alkyl chains.[16] Thus we anticipate that such supramolecular metallacycles decorated with different-numbered hydrophobic alkyl long chains might aggregate into ordered nanostructures through HSA under the proper conditions, as also recently reported by Stang et al.[17] Herein, we present the preparation and characterization of a new family of metallacycles with different shapes and sizes along with different-numbered alkyl long chains by means of CISA. By a proper selection and combination of different angular units, rhomboidal or hexagonal metallacycles decorated with hydrophobic alkyl chains were successfully prepared (> 99 %). Owing to the different geometrical properties and alkyl chain density, the resulting metallacycles exhibited distinct self-organization patterns on different surfaces, for example, SiACHTUNGRE(111) or mica, thus yielding various nanostructures with ordered sizes and shapes. For instance, rhomboidal complex 5 formed a fibrous morphology on the surface, whereas hexagonal complexes 6 and 7 generated nanospheres under the same conditions. Moreover, by changing the polarity of the solvents, the morphology of the rhomboidal metallacycle aggregates can be tuned from spherical to fibroid. This finding is of particular significance since it provides a new path to precisely control the shape and size of the nanosized structures by simply changing the molecular modules or the polarity of the solvent, thus providing an opportunity to gain a better understanding on how control self-organization of organometallic skeletons at large scale.
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with hydrophobic alkyl chains in hand, the self-assembly of a new family of metallacycles with different shapes and sizes along with multiple alkyl chains was investigated. Recent research results have indicated that the addition of functional groups, such as crown ethers, ferrocenes, and Frchet-type dendrons at the vertex of the 1208 building block enabled the preparation of novel functionalized hollow molecular assemblies.[20] Thus, the introduction of long alkyl chains to the vertex of a 1208-shaped building block will not hinder the self-assembly process into the metallacycles functionalized with the alkyl chains. Stirring the mixture of predesigned 1208 precursor 1 and 60, 120, or 1808 di-PtII acceptor 2, 3, or 4 in CH2Cl2 with a ratio of 1:1, respectively, resulted in the formation of the (2+2) rhomboidal 5, the (3+3) hexagon 6, and the (6+6) hexagon 7 with the peripheral alkyl chains at the donor vertexes in excellent yield (> 99 %, Scheme 1). Multinuclear NMR (1H and 31P) spectroscopic analysis of the reaction mixtures revealed the formation of discrete, highly symmetric species. Each 31P{1H} NMR spectrum of molecules 5, 6, and 7 displayed a sharp singlet (d = 12.7 ppm for 5, d = 13.5 ppm for 6, and d = 13.3 ppm for 7) that was shifted upfield from the starting platinum acceptors 2, 3, and 4 by approximately d = 6.5, 6.1, and 6.2 ppm, respectively
Results and Discussion Synthesis Newly designed 1208 donor precursor 1 can be easily prepared by means of an etherification reaction of 3,5-bis(4ethynlpyridinyl)phenol[18] with 1-bromomethyl-3,4,5-tris(dodecyloxy)benzene.[19] With the 1208 precursor substituted
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Scheme 1. Molecular structures of modules 1–4, as well as a graphical representation of the rhomboidal metallacycle 5 and hexagonal metallacycles 6 and 7.
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formation of Pt N coordination bonds (see the Supporting Information). The structures of rhomboidal and hexagonal complexes 5, 6, and 7 were further confirmed by mass spectrometry. In this study, owing to the high molecular weight and relatively weak PtII N bonds formed between a soft metal and a hard ligand, it was difficult to obtain intense mass signals. Usually, the cold-spray ionization time-of-flight (CSI-TOF) MS technique allows the assemblies to remain intact during the ionization process while obtaining the high resolution required for isotopic distribution.[22] For instance, the CSITOF-MS spectrum of rhomboid 5 revealed two peaks at m/z 2040.04 and 1339.36, which corresponded to [M 2 ONO2 ]2 + and [M 3 ONO2 ]3 + (Figure 3a), respec-
Figure 1. Partial 31P NMR (161.9 MHz, CD2Cl2, 298 K) spectra of a) 608 di-PtII acceptor 2 and rhomboidal metallacycle 5, b) 1208 di-PtII acceptor 3 and hexagonal metallacycle 6, and c) 1808di-PtII acceptor 4 and hexagonal metallacycle 7.
(Figure 1). This change, as well as the decrease in coupling of flanking 195Pt satellites (DJ 178 Hz for 5, DJ = 147 Hz for 6, and DJ = 157 Hz for 7) is consistent with the back-donation from Pt transition-metal atoms. Additionally, in the 1H NMR spectrum of each assembly, pyridinecentered protons exhibited downfield shifts (a-HPy, d 0.07– 0.31 ppm; b-HPy, d = 0.40–0.54 ppm) as a consequence of the coordination reaction of the pyridine-N atom with the PtII metal center. Notably two doublets for a- and b-hydrogen nuclei on the pyridine rings of the rhomboidal complex 5 were observed in the 1H NMR spectrum, which could have resulted from the hindered rotation about the Pt N (pyridyl) bond in a small-sized metallacycle (Figure 2).[20c, 21] Fur-
Figure 3. Theoretical (top) and experimental (bottom) CSI-TOF MS spectra of rhomboidal metallacycles a) 5 and b) 6.
tively. These peaks were isotopically resolved, and they are in agreement with the calculated pattern. Moreover, in the case of hexagon 6, the peaks at m/z 1561.3 and 1219.2, which corresponded to [M 4 OTf ]4 + and [M 5 OTf ]5 + , respectively (Figure 3b), were observed, and their isotopic resolutions were in excellent agreement with the theoretical isotopic pattern. It should be noted that, in the case of 7, because of the high molecular weight (13 049.52 Da), it was extremely difficult to obtain its mass signal even under CSITOF-MS conditions. With considerable effort, however, one peak that corresponded to [M 11 OTf ]11 + (m/z 1038.00) was observed in the ESI-TOF-MS spectrum of 7 and its isotopic resolution matches that which was derived from calculations (Figure 4), thus supporting the formation of the large hexagonal metallacycle 7. All attempts to obtain the crystal structure of rhomboidal complex 5 and hexagonal metallacycles 6 and 7 have been unsuccessful to date. Therefore, to gain more structural information, the semiempirical molecular orbital method (PM6) was employed to optimize the geometrical properties
Figure 2. Partial 1H NMR (400 MHz, CD2Cl2, 298 K) spectra of 1208 donor 1, and metallacycles 5, 6, and 7.
ther characterization with two-dimensional spectroscopic techniques (1H,1H COSY and NOESY) are in agreement with the formation of highly symmetric metallacycles decorated with alkyl chains (see the Supporting Information). For example, the spatial interactions between the a-H proton on pyridine and PEt3 protons were observed in the NOESY spectrum of each complex, which authenticated the
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Self-Assembly on the Surface As a consequence of the presence of long alkyl chain subunits at the alternate corners of the obtained metallacycles, we envisioned that these macromolecules could have a tendency to aggregate into ordered nanostructures under proper solvent conditions. To allow the self-assembly process to occur, we dissolved the metallacycles in CH2Cl2 and then slowly added subsequent aliquots of CH3OH to reach a 1:1 ratio, ultimately reaching a final concentration of 4.4 10 4 m. The solution was then exposed to n-hexane vapors at 273 K, thus allowing for a gradual, time-dependent solvent composition change. After 84 h, it was found that the metallacycles were still dissolved in the solution without any evidence of further evolution of the self-organization (see also the dynamic light scattering (DLS) measurements in the Supporting Information). However, drop-casting small aliquots of the solution that contained the hydrophobic building block 1 onto the freshly cleaned SiACHTUNGRE(111) surfaces (for SEM imaging) yielded the entangled nano-to-microstructured infinite mesh (see the Supporting Information). By repeating the same procedure starting from the solutions that contained rhomboidal metallacycle 5 drop-cast onto SiACHTUNGRE(111) surfaces or onto carbon-coated Cu grids (for TEM analysis), the fiberlike morphologies with diameters on the order of 100–300 nm and lengths spanning from 500 nm to 5 mm were observed in both SEM and TEM images (Figure 6a and the
Figure 4. Theoretical (dot) and experimental (black line) ESI-TOF MS spectrum of 7. The theoretically most abundant isotope is centered at 1037.91400, whereas the experimental value was found at 1038.00491.
of all metallacycles (Figure 5). The optimized structure featured a roughly planar ring surrounded by flexible long alkyl chains subunits at alternate corners. Complex 5 resembled a well-defined rhombus that featured an approximately 2.3 1.3 nm cavity, whereas metallacycles 6 and 7 featured a hexagonal macrocycle with an internal radius of 1.5 and 2.5 nm and an outer radius of 4.0 and 5.0 nm, respectively.
Figure 6. SEM images of SiACHTUNGRE(111) surfaces after drop-casting of solutions (CH2Cl2/CH3OH 1:1 under n-hexane diffusion after 84 h) containing metallacycles a) 5, b) 6, and c) 7. Inset: Representative TEM images of Cu grids prepared by drop-casting a solution containing the appropriate metallacycles.
Figure 5. Geometrical structures of metallacycles a) 5, b) 6, and c) 7 as optimized by the PM6 semiempirical molecular orbital method. The dashed (red) ellipse highlights hydrophobic portions, whereas the full (blue) one surrounds the hydrophilic parts.
Supporting Information). Furthermore, by employing solutions that contained different-sized hexagonal metallacycles 6 and 7, round-shaped particles with a diameter of approximately 200 nm for 6 ((180 43) nm from a count of 106 particles) and approximately 400 nm for 7 ((370 130) nm, from a count of 103 particles) were obtained (Figure 6b– c and the Supporting Information). Similar spheroidal nanoparticles (Figure 6b–c, inset) were also observed in TEM images, which were in good agreement with the SEM observations. Any attempt to analyze these nanostructures by means of X-ray diffraction (XRD) failed, since during the mechanical transfer of the dried powdered material from the Si surfaces to an XRD sample holder, the morphologies underwent decomposition (see SEM images and NMR spectroscopic characterization in the Supporting Information). To further verify the hypothesis that all morphologies were formed during the solvent evaporation, we have tried to de-
To gain further insight into the structures of the obtained metallacycles, their diffusion coefficient (D) was determined by two-dimensional diffusion-ordered 1H NMR (2D DOSY) spectroscopic experiments (see the Supporting Information). According to the Einstein–Smoluchowski relation, at a fixed temperature and solution properties (viscosity) an increase in the hydrodynamic radius (which is strictly connected to size and thus molecular weight) determines the reduction of the D value.[23] 2D DOSY NMR spectroscopic measurements for metallacycles 5, 6, and 7 (CD2Cl2, 293 K) showed the expected trend in D values, with the higher D value for 5 (3.39 10 10 m2 s 1), an intermediate D value for 6 (1.51 10 10 m2 s 1), and the lower D value for 7 (0.74 10 10 m2 s 1). This trend is in agreement with the structural features suggested by the mass analysis and the computational models.
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was clearly found that the obtained nanostructures were generated on the surface during the solvent evaporation, with the latter dictating both morphology and size distribution. We then assessed how the time-dependent change in the solvent composition (CH2Cl2/CH3OH volumetric ratio) under n-hexane diffusion affected the morphology of the nanomaterials. With this aim, we sampled the solutions at different times (from 0.5 to 84 h) and drop-cast it onto SiACHTUNGRE(111) surfaces. Interestingly, for solutions that contained rhomboidal metallacycle 5, some spherical objects appeared on the surface after just 2–4 h, whereas from 6 to 48 h a gradual change from spherical to the fibrillar objects was clearly observed. From 48 to 84 h the ratio of the fibrillar to spherical nano-objects was maximized (Figure 8e and f). However, for the solution of hexagonal metallacycles 6 and 7, the spherical nanoparticles could be found at the early stages (2–4 h), with a sharpening of their spherical structure occurring after 48 h (from 48 to 84 h, see the Supporting Information). Since a solvent polarity change was expected during the n-hexane diffusion, it was reasonable to investigate how the polarity affected the morphologies obtained from complexes
posit the aforementioned solutions by spin-coating onto the muscovite (mica) surfaces. Indeed, by setting different rotational speeds of the substrate (range 1–8 krpm) and keeping constant the solution composition and the deposition volume (5 mL), it was possible to investigate influence of the solvent evaporation on the morphology evolution. In particular, we have found that the rotational speed dramatically influenced the morphology as imaged by atomic force microscopy (AFM) or SEM techniques. Indeed, by depositing the solution at 1 krpm (relatively slow solvent evaporation conditions), the rhomboidal metallacycle 5 led to the formation of the same fibrillar morphology as that obtained after drop-casting (Figure 7a, d and the Supporting Information). However, at 5 krpm only spheroidal nanoparticles were observed for 5 (Figure 7g). When employing hexagonal metallacycles 6 and 7, the spin-coating procedure generated the similar round-shaped morphologies and size distributions as indicated by SEM imaging of the drop-casted materials (Figure 7b, c and e, f, respectively, and the Supporting Information). Nevertheless, increasing the rotational speed to 8 krpm determined a reduction in the average diameters for both metallacycles 6 and 7 (about 100 and 150 nm for 6 and 7, respectively; see Figure 7h, i). From these observations it
Figure 7. AFM images (topography channel) of the mica surfaces after spin-coating of solutions (CH2Cl2/CH3OH 1:1 under n-hexane diffusion after 84 h) containing metallacycles 5, 6, or 7. a, d) Metallacycle 5 deposited at 1 krpm, at lower and higher magnification, respectively. b, e) Metallacycle 6 deposited at 5 krpm, at lower and higher magnification, respectively. c, f) Metallacycle 7 deposited at 5 krpm, at lower and higher magnification, respectively. g) Metallacycle 5 deposited at 5 krpm. h) Metallacycle 6 deposited at 8 krpm. i) Metallacycle 7 deposited at 8 krpm. Images were scaled in the height range according to the color intensity reported to the right of each picture.
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porting the surface-confined solvent polarity-induced nanostructuring process (see the Supporting Information). A further investigation of guest incorporation by using the obtained nanomaterials was carried out. We tried to add a fluorescent dye to the solution (CH2Cl2/CH3OH 1:1) that contained different metallacycles. One could envisage that under those conditions the chosen dye, rhodamine B (RhoB), might be incorporated into the nanostructures and impart strong emissive properties. Unfortunately, no fluorescent nanofibers or nanospheres were observed by confocal microscopy upon drop-casting the solutions onto glass coverslips. However, SEM images of the deposited material prepared by using different concentrations of Rho-B (2.2 10 5, 2.2 10 4, and 2.2 10 3 m) again confirmed the influence of the solvent polarity in inducing the nanostructuring of the metallacycle assemblies (Figure 9). In particular, the self-or-
Figure 8. SEM images of the time-dependent morphology of 5 (from CH2Cl2/CH3OH at 1:1 ratio under n-hexane diffusion up to 84 h) in the self-assembly process after drop-casting onto SiACHTUNGRE(111) surfaces a) after 30 min, b) after 2 h, c) after 4 h, d) after 6 h, e) after 48 h, and f) after 84 h.
5, 6, and 7 after drop-casting. Thus n-hexane vapor was diffused into vials that contained the solutions of the metallacycles in pure CH2Cl2 or CH3OH, respectively, then the morphological evolution was evaluated after 84 h. By removing CH3OH, the rhomboidal metallacycle 5 formed spheroidal aggregates, whereas the hexagonal metallacycles 6 and 7 did not lead to any regular nanostructured morphologies (see the Supporting Information). In addition, if both CH3OH and n-hexane solvents were not used, only a porous matrix could be observed on the surface (see the Supporting Information). This finding was most likely promoted by the fast CH2Cl2 evaporation. These references studies all proved the necessity of using a polar solvent such as CH3OH to drive the nanostructuring process under solvent evaporation conditions, whereas the major role of CH2Cl2 was essentially limited to the solubilization of metallacycles (5, 6, and 7). It should be noted that rhomboidal metallacycle 5 was revealed to be the most versatile module to provide either fibrillar or spheroidal morphologies by varying the polarity of the solutions. Thus, complex 5 was dissolved in different solutions that contained CH2Cl2 and CH3OH under n-hexane diffusion conditions, with volumetric ratios changing from 1:7 (the most polar mixture) to 7:1 (the least polar). It was observed that, according to the initial solution polarity, the dominant morphologies were fibers (CH2Cl2/CH3OH 1:7), fibers and some nanospheres (CH2Cl2/CH3OH 1:3, 3:5, 5:3, 3:1), and nanospheres only (CH2Cl2/CH3OH 7:1), again sup-
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Figure 9. SEM images of the Rho-B concentration-dependent morphology on Si surfaces of a, d, g) 5; b, e, h) 6; and c, f, i) 7 from CH2Cl2/CH3OH at 1:1 ratio under n-hexane diffusion after 84 h.
ganization of the rhomboidal metallacycle 5 was dramatically affected by both Rho-B and the employed protocols (6 or 84 h; see the Supporting Information), for which fibers or spheroidal structures were formed under different ratios (Figure 9a, d, g and the Supporting Information). It turned out that the higher Rho-B concentrations affected the morphology formation in the same manner as that of the solvent polarity (e.g., CH2Cl2/CH3OH 5:3 or 7:1 ratios) or faster solvent evaporation conditions (5 krpm versus 1 krpm). However, the morphologies that stemmed from the deposition of solutions that contained metallacycles 6 and 7 were not affected by the Rho-B concentration; the latter modulated only a significant increase in the average size distribution after 84 h (spheres reaching the micrometer range; see Figure 9b, c, e, f, h, i and the Supporting Information). By comparing the structural features of building blocks 1, 2, 3, and 4, it was clearly found that donor 1 was a strongly
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apolar unit, whereas the Pt complex-based angular acceptors 2, 3, and 4 displayed a more hydrophilic character. When comparing metallacycles 5, 6, and 7, one could note that the Pt atom centers were differently exposed toward the solvent in the different complexes (Figure 5). Specifically, rhomboidal metallacycle 5 showed well-exposed and solvent-accessible metal centers (orthogonal to apolar module 1), whereas in hexagonal metallacycles 6 and 7 the hydrophilic Pt atoms were progressively less solvent-accessible as a consequence of the burying effect of the aliphatic chains that surrounded the metallacycle scaffolds. It is therefore very likely that the metallacycle polarity decreases when going from rhomboidal metallacycle 5 to hexagonal metallacycle 7. Consequently, the solvation properties during either the drop-casting or spin-coating procedures changed, thus dictating the final morphology and size distribution of the observed nanomaterials. This should explain why under n-hexane diffusion conditions, rhomboidal metallacycle 5 formed small nanospheres from a CH2Cl2 solution or under high Rho-B concentrations, whereas long fibers were only observed when using more polar mixtures (CH2Cl2/CH3OH) or low Rho-B concentrations. Considering the faster evaporation rate of CH2Cl2 than that of CH3OH under spin-coating conditions, the speed-dependent morphologies of solvent evaporation as observed through AFM measurements supported this hypothesis. For the less polar hexagonal metallacycles 6 and 7, which never formed fibers on the surfaces, the solvophobic effect was the pivotal issue that governed the formation of the nano- or microspheres. This would explain why faster spin-coating depositions and high Rho-B concentrations had opposite effects in modulating the average size distribution of the self-organized spheres.
phology of the metallacycle aggregation could be tuned by changing the solvent polarity during the surface deposition. These findings are of special significance since this study not only enriches the library of functional metallacycles but also provides a simple yet controllable approach to the formation of ordered nanoscale soft materials. Further understanding and application of these novel supramolecular nanostructures in materials science are now under investigation.
Experimental Section Materials All reagents and solvents were purchased from commercial sources. DMF was distilled from CaH2 and degassed under N2 for 30 min before use. Reactions were performed in standard glassware under an inert N2 atmosphere. All air-sensitive reactions were carried out under an Ar atmosphere. 3,5-Bis(4-ethynlpytidinyl)phenol and 1-bromomethyl-3,4,5tris(dodecyloxy)benzene were prepared following the procedures reported below. SiACHTUNGRE(111) wafers were provided courtesy of Prof. Sorin Melinte from Universit Catholique de Louvain (Belgium), whereas mica sheets were purchased from Ted Pella (quality grade V1). Characterizations 1
H, 13C, and 31P NMR spectra were recorded with a Bruker 400 MHz spectrometer at 298 K. The 1H and 13C NMR spectroscopic chemical shifts are reported relative to residual solvent signals, and 31P NMR spectroscopic resonances are referenced to an internal standard sample of 85 % H3PO4 (d = 0.0 ppm). 1H,1H COSY, 1H,1H NOESY, and 1H,1H DOSY spectra were recorded with a Bruker 500 MHz spectrometer at 293 K. SEM images were obtained with a Jeol 7500F microscope. The drop-cast SiACHTUNGRE(111) surfaces were deposited with a thin film of Au prior to analysis. AFM measurements were carried out in air at 293 K with a Nanoscope V Multimode 8 instrument (Veeco, USA). Silicon tips on nitride levers (T = 0.55–0.75 mm; k = 0.4 Nm 1, Veeco, USA) operated at a frequency in the range of 50–90 kHz were used to image the surfaces through ScanAsyst-air mode. The samples were prepared by spin-coating 5 mL of the same solution employed for the other microscopic observations onto a mica substrate. TEM images were obtained with a JEOL JEM-2100 instrument.
Conclusion
Synthesis of Building Block 1
In conclusion, we have described the preparation of nanoscale fibroid and spherical aggregates of coordination metallacycles 5, 6, and 7 by means of hierarchical self-assembly. The structure of rhomboidal and hexagonal metallacycles 5, 6, and 7 were confirmed by one-dimensional multinuclear (1H and 31P) and two-dimensional (1H,1H COSY, NOESY, and DOSY) NMR spectroscopy, mass spectrometry, and elemental analysis. Specifically, it was found that all metallacycles self-organized on SiACHTUNGRE(111) substrates and mica to form ordered nanofibers or nanoparticles upon solvent evaporation. For example, rhomboidal metallacycle 5 aggregated into nanosized fibers, evolving from spheroidal objects, whereas hexagonal metallacycles 6 and 7 both formed nanoparticles under the same conditions. All nanostructures were distinct from the morphology of ligand 1, which formed an infinite entangled mesh under the same conditions. The difference in the number of the alkyl chain and their orientation dictated the hydrophilic/lipophilic character of each metallacycle, thus leading to the formation of different morphologies at the nano- and microscale level depending on preparative conditions. Indeed, it was found that the mor-
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In a Schlenk flask, NaH (45 mg, 1.11 mmol) was dissolved in fleshly distilled and degassed DMF (3.5 mL), and then under an N2 atmosphere, 3,5-bis(4-ethynlpytidinyl)phenol[18] (100 mg, 0.34 mmol) was carefully introduced into the reaction flask. The mixture was stirred for 30 min at (269 mg, RT. 1-Bromomethyl-3,4,5-tris(dodecyloxy)benzene[19] 0.37 mmol) was then added under N2. The reaction was continued at 60 8C for another 12 h and then quenched by water (10 mL), extracted with CH2Cl2 (2 10 mL); then the organic layers were combined and extracted with brine (3 40 mL) and dried (MgSO4). The solvent was removed by evaporation on a rotary evaporator. The residue was purified by column chromatography on silica gel (acetone/CH2Cl2 1:10) to give the target product as a light yellow solid with a yield of 62.4 %. Rf = 0.39 (dichloromethane/acetone 20:1); m.p. 70–74 8C; 1H NMR (CDCl3, 400 MHz): d = 8.62 (d, J = 3.6 Hz, 4 H), 7.38 (br, 5 H), 7.18 (s, 2 H), 6.62 (s, 2 H), 5.00 (s, 2 H), 3.98–3.97 (m, 6 H), 1.82–1.73 (m, 6 H), 1.47–1.26 (m, 54 H), 0.88–0.86 ppm (m, 9 H); 13C NMR (CDCl3, 100 MHz): d = 158.5, 153.4, 149.8, 138.2, 130.9, 128.0, 125.5, 123.6, 119.1, 106.1, 92.5, 87.2, 73.4, 70.7, 69.2, 31.9, 30.3, 29.7, 29.6, 29.4, 29.3,26.1, 22.6, 14.0 ppm; elemental analysis calcd (%) for C63H90N2O4 : C 80.55, H 9.66, N 2.98; found: C 80.43, H 9.87, N 2.84; CSI-TOF MS of target product: m/z calcd for C63H91N2O4 : 938.69 [M+H] + ; found: 939.70.
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Synthesis of Rhomboidal Metallacycle 5
Ying Tung Education Foundation (no. 131014), and the Program for Changjiang Scholars and Innovative Research Team in University. D.B. acknowledges the European Union through the ERC Starting Grant “COLORLANDS” project, the FRS-FNRS (FRFC contract no. 2.4.550.09), the Science Policy Office of the Belgian Federal Government (BELSPO-IAP 7/05 project), the “TINTIN’’ ARC project (09/14-023), the Service Public de Wallonie through the 2013 Program of Excellence (FLYCOAT project), and the MIUR through the FIRB ”Futuro in Ricerca“ (”SUPRACARBON’’, contract no. RBFR10DAK6). We thank Prof. Hongwei Tan and Miss Quan-Jie Li (Beijing Normal University) for their help with molecular simulation. R.M. thanks the FRS-FNRS for his postdoctoral fellowship.
A solution of compound 1 (5.44 mg, 0.00582 mmol) in CH2Cl2 (1.5 mL) was added dropwise to a solution of compound 3 (6.78 mg, 0.00583 mmol) in CH2Cl2 (1.5 mL) under continuous stirring. The reaction mixture was stirred overnight at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %; 1H NMR (CD2Cl2, 400 MHz): d = 9.29 (d, J = 5.6 Hz, 4 H), 8.78 (s, 4 H), 8.69 (d, J = 5.6 Hz, 4 H), 7.94 (dd, J = 1.6 and 5.6 Hz, 4 H), 7.77 (dd, J = 1.2 and 5.6 Hz, 4 H), 7.64 (s, 2 H), 7.58–7.55 (m, 12 H), 7.36 (d, J = 1.2 Hz, 4 H), 6.66 (s, 4 H), 5.04 (s, 4 H), 3.99–3.90 (m, 12 H), 1.82–1.77 (m, 12 H), 1.47– 1.25 (m, 156 H), 1.16–1.08 (m, 72 H), 0.87–0.83 ppm (m, 18 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 12.74 ppm (s, 1JACHTUNGRE(Pt,P) = 2702.1 Hz); elemental analysis calcd (%) for C202H316N8O20P8Pt4 : C 57.70, H 7.57, N 2.66; found: C 57.57, H 7.79, N 2.52; CSI-TOF MS: m/z: 2040.04 [M 2 NO2 ]2 + , 1339.37 [M 3 NO2 ]3 + .
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Synthesis of Hexagonal Metallacycle 6 A solution of compound 1 (15.99 mg, 0.0170 mmol) in CH2Cl2 (2 mL) was added dropwise to a solution of compound 2 (22.84 mg, 0.0170 mmol) in CH2Cl2 (2 mL) under continuous stirring. The reaction mixture was stirred for 3 h at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %. 1H NMR (CD2Cl2, 400 MHz): d = 8.68 (d, J = 5.6 Hz, 12 H), 7.81 (d, J = 6.0 Hz, 12 H), 7.58 (s, 3 H), 7.54–7.48 (m, 24 H), 7.31 (s, 6 H), 5.01 (s, 6 H), 3.98–3.89 (m, 18 H), 1.97–1.64 (m, 18 H), 1.46–1.24 (m, 234 H), 1.56–1.08 (m, 108 H), 0.86– 0.83 ppm (m, 27 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 13.50 ppm (s, 1JACHTUNGRE(Pt,P) = 2647.1 Hz); elemental analysis calcd (%) for C306H474F18N6O33P12Pt6S6·2 CH2Cl2 : C 52.76, H 6.87, N 1.20; found: C 52.45, H 7.24, N 1.25; CSI-TOF MS: m/z: 1561.26 [M 4 OTf ]4 + , 1219.23 [M 5 OTf ]5 + . Synthesis of Hexagonal Metallacycle 7 A solution of compound 1 (3.09 mg, 0.00329 mmol) in CH2Cl2 (1 mL) was added dropwise to a solution of compound 3 (4.07 mg, 0.00329 mmol) in CH2Cl2 (1 mL) under continuous stirring. The reaction mixture was stirred for 1 h at RT. The solvent was removed under an N2 flow, then the product was collected. Yield: 99 %; 1H NMR (CD2Cl2, 400 MHz): d = 8.68 (d, J = 5.2 Hz, 24 H), 7.81 (d, J = 5.6 Hz, 24 H), 7.60 (s, 6 H), 7.33 (s, 12 H), 7.05 (s, 24 H), 6.66 (s, 12 H), 5.04 (s, 12 H), 4.01–3.92 (m, 36 H), 1.83–1.71 (m, 36 H), 1.49–1.09 (m, 684 H), 0.88 ppm (t, J = 6.4 Hz, 54 H); 31P NMR (CD2Cl2, 161.9 Hz): d = 13.30 (s, 1JACHTUNGRE(Pt,P) = 2718.3 Hz); elemental analysis calcd (%) for C570H948F36N12O60P24Pt12S12 : C 52.33, H 7.30, N 1.28; found: C 52.56, H 7.45, N 1.30; MS (ESI): m/z: 1038.00 [M 11 OTf]11 + . Preparation of Nanostructures For each assembly, the precursors were dissolved in CH2Cl2 (0.25 mL) inside a small glass vial and the resulting solution was diluted by slow addition of MeOH (ratio 1:1 unless otherwise stated) to afford a precursor concentration of 0.44 mm. The receptacle that contained such solution was then introduced into a bigger vial containing n-hexane (2.5 mL), and the resulting system was isolated with a stopper from the external environment and kept at 273 K, thereby allowing for gradual solvent evaporation from the small vial. After the desired time (for the proper CH2Cl2/ MeOH ratio inside the small vial, t = 84 h), one drop of the solution in the small vial was deposited onto a SiACHTUNGRE(111) or Cu grid for SEM and TEM, or the solution (5 mL) was spin-coated at the desired rotational speed (between 1 and 8 krpm) over freshly peeled mica for AFM analysis.
Acknowledgements H.-B.Y thanks the National Natural Science Foundation of China (grant nos. 21132005 and 21322206), the Key Basic Research Project of Shanghai Science and Technology Commission (no. 13JC1402200), the Fok
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