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Controlling the Molecular Self-Assembly into Nanofibers of Amphiphilic Zinc(II) Salophen Complexes Ivan Pietro Oliveri,† Salvatore Failla,‡ Graziella Malandrino,*,† and Santo Di Bella*,† †

Dipartimento di Scienze Chimiche, Università di Catania, I-95125 Catania, Italy Dipartimento di Ingegneria Industriale, Università di Catania, I-95125 Catania, Italy

‡

S Supporting Information *

ABSTRACT: The synthesis, characterization, and aggregation properties in the solid state of a series of amphiphilic Zn(salophen) Schiff-base complexes are presented, through a combined FE-SEM/XRD approach. It is found that these complexes self-assemble into nanofibers depending on the solvent used to prepare the solutions. Thus, fibrous aggregates are obtained from solutions of weak and volatile Lewis base solvents, either by drop-casting or by complete solvent evaporation, whereas, in the case of noncoordinating solvents, where oligomeric aggregates are already present in solution, no formation of nanofibers is observed. The length of side alkyl groups and their degree of interdigitation lead to a 2D columnar square structure in the case of the complex with the short 4-ethyloxy substituents, whereas complexes having longer 4-alkyloxy chains are characterized by a lamellar structure. Bundles of twisted nanofibers are formed by further interactions and interdigitation of the outside alkyl groups of each nanofiber. A simple model, which describes the mechanism of formation and structure of these nanofibers, is presented.

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coordination to the ZnII ion, accompanied by sizable changes of optical absorption, fluorescence,15,21,32−38 and second-order nonlinear optical properties.39 To further probe into the nature and mechanism of aggregation of these complexes in the solid state, in this paper, we have investigated a series of Zn(salophen) Schiff-base complexes, having alkoxy substituents as lateral groups in the salicylidene rings (Chart 1), and studied their supramolecular structure through a detailed combined, field emission scanning electron microscopy/X-ray diffraction (FE-SEM/XRD) study. It is found that these complexes self-assemble into nanofibers depending on the coordinating or noncoordinating nature of

INTRODUCTION The molecular aggregation and control of the supramolecular architecture is a widely explored field of research, involving both fundamental1−5 and application aspects.6−11 In the case of dipolar chromophores, aggregation generally occurs via noncovalent bonds, by dipolar or π−π stacking interactions, or a combination of them.1−5 An additional possibility is offered by transition-metal complexes in which molecular aggregation can occur through metal−ligand coordination.12 In this view, tetracoordinated ZnII Schiff-base complexes possess unique peculiarities. They, in fact, are Lewis acidic species13−15 that saturate their coordination sphere by coordinating auxiliary Lewis bases or, in their absence, are stabilized through intermolecular Zn···O axial interactions,16 thus allowing for a different control of the supramolecular architecture. Therefore, a variety of molecular aggregates,17−20 supramolecular assemblies,21−24 and nanostructures25−28 have been found. In this last regard, MacLachlan and co-workers have investigated a series of analogous mono- and dinuclear Zn II Schiff-base complexes able to form nanofibrillar structures,29−31 whose aggregation is dependent on the nature of substituents in the ligand framework. We have recently investigated a series of amphiphilic bis(salicylaldiminato)ZnII Schiff-base complexes and demonstrated that these species always form aggregates in solution of noncoordinating solvents.13−15 The degree and type of aggregation are related to the nature of the bridging diamine. In coordinating solvents or in the presence of coordinating species, a complete deaggregation occurs, because of the axial © 2013 American Chemical Society

Chart 1

Received: April 17, 2013 Revised: June 13, 2013 Published: July 8, 2013 15335

dx.doi.org/10.1021/jp4038182 | J. Phys. Chem. C 2013, 117, 15335−15341

The Journal of Physical Chemistry C

Article

Figure 1. FE-SEM images at different magnifications of 1 (a,b), 2 (c,d), and 3 (e,f), deposited by casting onto a Si(100) substrate from 5.0 × 10−4 M THF solutions.

with cyclohexane and EtOAc. TLC was performed using silica gel 60 F254 plates with visualization by UV and standard staining. Chloroform (Aldrich) stabilized with amylene was used to prepare solutions of 1−3. Measurements. Elemental analyses were performed on a Carlo Erba 1106 elemental analyzer. Optical absorption spectra were recorded at room temperature with a Varian Cary 500 UV−vis-NIR spectrophotometer. ESI mass spectra were recorded with a Finnigan LCQ-Duo ion trap electrospray mass spectrometer (Thermo). X-ray diffraction (XRD, θ−2θ) patterns were recorded in grazing incidence mode (0.5°) on a Bruker-AXS D5005 θ−θ X-ray diffractometer, using a Göebel

the involved solvent, while the length of side alkyl groups and their degree of interdigitation influence the structure of nanofibers. A picture of the structure of these nanofibers is presented.

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EXPERIMENTAL SECTION

Materials and General Procedures. Zinc acetate dihydrate, 2,4-dihydroxybenzaldehyde, 1-iodoethane, 1-iododecane, and 1-bromohexadecane (Aldrich) were used as received. o-Phenylenediamine (Aldrich) was purified by crystallization from aqueous 1% sodium hydrosulphite. Column chromatography was performed on silica gel 60 (230−400 mesh) eluting 15336



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bundles of nanofibers, ∼100−300 nm wide. However, slight differences in the bundles of nanofibers can be observed on passing from 1, having the ethyl alkyls, to 2, having the decyl alkyls. In particular, thicker bundles of nanofibers are observed in the latter case. Instead, on passing from 2 to 3, no appreciable differences in the thickness of bundles of nanofibers are observed. The same nanofiber samples of 1−3, deposited by casting on Si(100) substrates, were analyzed by X-ray diffraction measurements recorded in grazing incidence mode. XRD patterns and related d-spacings are reported in Figure 2 and Table 1.

mirror to parallel Cu−Kα radiation, λ = 1.5418 Å, operating at 40 kV and 30 mA. The film surface morphology was examined by FE-SEM using a ZEISS SUPRA VP 55 microscope. Samples for XRD measurements were obtained by drop-casting from solutions of complexes 1−3 at different concentrations onto cleaned Si(100) substrates. Samples were subsequently sputtered with gold to avoid charging effects before FE-SEM analysis. Powder samples were obtained by the complete evaporation of the solvent from solutions of complexes 1−3. FE-SEM analyses were carried out on powders stuck on carbon tape and sputtered with gold. Syntheses. Syntheses of 4-ethoxy-2-hydroxybenzaldehyde, 4-hexadecyloxy-2-hydroxybenzaldehyde, and [N,N-bis(4-decyloxy-2-hydroxybenzylidene)-1,2-phenylene-diaminato]ZnII (2) were previously reported.14 [N,N-Bis(4-ethoxy-2-hydroxybenzylidene)-1,2-phenylenediaminato]ZnII (1). To a solution of 4-ethoxy-2-hydroxybenzaldehyde (1.00 mmol) in ethanol (20 mL) was added 1,2phenylenediamine (0.500 mmol) under stirring. The mixture was heated at reflux with stirring for 1 h, under a nitrogen atmosphere. To the solution so-obtained was added zinc acetate dihydrate (0.1095 g, 0.500 mmol), and the mixture was heated at reflux with stirring for 1h, under a nitrogen atmosphere. After cooling, the precipitated product was collected by filtration, washed with ethanol, and dried. Yellow powder (70%). C24H22N2O4Zn (467.85): calcd C, 61.61; H, 4.74; N, 5.99; found C, 61.73; H, 4.79; N, 5.51. ESI-MS: m/z = 937 ([(M)2 + H]+, 100%). 1H NMR (500 MHz, DMSO-d6, TMS): δ = 1.33 (t, 3JHH = 7.0 Hz, 6H; CH3), 4.03 (t, 3JHH = 7.0 Hz, 4H; OCH2), 6.14 (dd, 3JHH = 8.5 Hz, 4JHH = 2.5 Hz, 2H; ArH), 6.17 (d, 4JHH = 2.5 Hz, 2H; ArH), 7.27 (d, 3JHH = 8.5 Hz, 2H; ArH), 7.29 (m, 2H; ArH), 7.79 (m, 2H; ArH), 8.86 (s, 2H; CHN). [N,N-Bis(4-hexadecyloxy-2-hydroxybenzylidene)-1,2phenylene-diaminato]ZnII (3). Complex 3 was prepared with the same procedure used for complex 1. Yellow powder (40%). C52H78N2O4Zn (860.60): calcd C, 72.57; H, 9.14; N, 3.26; found C, 71.61; H, 9.11; N, 3.10. ESI-MS: m/z = 1722 ([(M)2 + H]+, 100%). 1H NMR (500 MHz, DMSO-d6, TMS): δ = 0.84 (t, 3JHH = 7.0 Hz, 6H; CH3), 1.23−1.40 (m, 50H; CH2), 1.69− 1.79 (m, 6H; CH2), 3.96 (t, 3JHH = 6.5 Hz, 4H; OCH2), 6.13 (dd, 3JHH = 9.0 Hz, 4JHH = 2.5 Hz, 2H; ArH), 6.16 (d, 4JHH = 2.5 Hz, 2H; ArH), 7.28 (m, 4H; ArH), 7.78 (m, 2H; ArH), 8.86 (s, 2H; CHN).

Figure 2. XRD patterns for nanofibers of 1−3 obtained by casting from 1.0 × 10−3 M THF solutions. The asterisks refer to the 10 reflection peak.

Table 1. Spacing (d) for Nanofibers of 1−3 Derived from the 10 Reflection Peak of Nanostructures Obtained by Casting and Powders (in Parentheses) sample

2θ (deg)

d (Å)

1 2 3

5.55 (5.68) 3.00 (3.35) 2.40 (2.47)

15.91 (15.54) 29.42 (26.35) 36.78 (35.76)

The XRD patterns of the nanofibers of 1−3 show broad peaks due to the assembled structures. The reflection angles do not change with the concentration of the cast solution. In fact, the diffraction patterns of the samples obtained from 1.0 × 10−3 and 5.0 × 10−4 M THF solutions remain almost identical (see, for example, for 1, Figure S1 in the Supporting Information). Conversely, on switching from 1 to 2 and 3, a distinct effect of the side alkyl groups in the XRD patterns is observed. In particular, the position of the first diffraction peak moves toward lower angles and, consequently, the d-spacing progressively increases on increasing the length of alkyl groups (Table 1). The peaks at the lowest angles may be associated with the 10 reflection of the self-assembly. The broad profile associated with this reflection can be related to some disordering in the self-assembled structure. More interestingly, analogous, but sharper, XRD patterns are observed even from powder samples of 1−3 obtained from their THF solutions by complete evaporation of the solvent (Figures 3 and 4; Figure S2 in the Supporting Information). In particular, the diffraction pattern of 1 shows a set of diffraction peaks at 2θ = 5.68, 8.42, 12.30, and 13.50° corresponding to d = 15.54, 10.49, 7.19, and 6.55 Å, with a ratio of almost 1:√2:2:√5, consistent with a 2D columnar square structure with a lattice constant of 15.54 Å. Instead, the diffraction pattern of 2 shows a set of diffraction peaks at 2θ = 3.35, 6.74,

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RESULTS FE-SEM images of structures obtained by drop-casting from THF solutions of 1−3 onto Si(100) substrates are shown in Figure 1. They indicate the formation of fibrous nanostructures. The morphology of these nanostructures is almost independent from the concentration of the cast solution. Actually, the differences between the FE-SEM images obtained by dropcasting 1.0 × 10−3 and 5.0 × 10−4 M THF solutions are consistent with the different amounts of material deposited on the substrate. Thus, the structures obtained by casting more dilute THF solutions are better defined and less dense than those obtained from 1.0 × 10−3 M THF solutions (see Figure S1 in the Supporting Information). Further details of these molecular aggregates can be gained from FE-SEM images of 1−3 at higher magnification (Figure 1b,d,f). They indicate an essentially identical shape, in which nanofibers of 40−60 nm in width are twisted together, forming 15337



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Figure 3. Comparison of XRD patterns of 1 obtained by casting from a THF solution (red line), and a powder sample obtained from a THF solution by complete evaporation of the solvent (black line).

Figure 4. Comparison of XRD patterns of 2 obtained by casting from a THF solution (red line), and a powder sample obtained from a THF solution by complete evaporation of the solvent (black line).

10.30, and 13.81° corresponding to d = 26.35, 13.10, 8.58, and 6.41 Å. The last three peaks can be associated with higher-order reflections of the 10 peak observed at 2θ = 3.35°, thus suggesting a lamellar organization. Analogously to 2, a lamellar structure can be also deduced from powder samples of 3 (Figure S2 in the Supporting Information). As the well-defined XRD patterns for powder samples of 1−3 allowed assigning their structure, considering that XRD patterns obtained from cast samples show broad peaks whose reflection angles are fully comparable to those obtained from powder samples (Figures 3 and 4; Figure S2 in the Supporting Information), we can assume the same structure even for the cast samples of 1−3. FE-SEM images of powder samples clearly show the formation of well-defined bundles of nanofibers for 2 and 3 (Figure 5; Figures S3 and S4 in the Supporting Information), while 1 shows a distinct nanoribbon morphology (Figure 5). Analogous results, in terms of morphology and XRD patterns, are observed using a different volatile solvent, such as acetonitrile (ACN). In fact, FE-SEM images of structures obtained by drop-casting from ACN solutions of 1−3 onto Si(100) substrates show the formation of fibrous nanostructures, whose XRD patterns are comparable to those observed from THF solutions (see, for example, 2, Figures S5 and S6 in the Supporting Information). In contrast with the results obtained from THF or ACN solutions, the behavior of 2 and 3 in chloroform solutions is very different. Actually, FE-SEM images of samples obtained by casting from chloroform solutions do not show any defined nanostructure, but rather a flat structure of the material

Figure 5. High-magnification FE-SEM images of powder samples of 1 (a) and 2 (b) obtained from THF solutions by complete evaporation of the solvent.

deposited over the substrate (see, for example, 2, Figure S7 in the Supporting Information). Moreover, the XRD pattern of sample 2 deposited by casting shows a unique peak at 5.15° (Figure S8 in the Supporting Information), exactly corresponding to one of the reflections found in the very complex diffraction pattern of the powder sample obtained from a CHCl3 solution by complete evaporation of the solvent. These findings, together with the FE-SEM image of the drop-casted film, point to the formation of an oriented structure. Unfortunately, it is not possible to assign this reflection and hence the film orientation, since no crystal structure is available for this compound. Solutions of 2 and 3 in different mixtures of CHCl3/THF (70:30, 50:50, 30:70, v/v) were also analyzed. Both XRD patterns and FE-SEM images of samples deposited by casting from these solutions indicate almost identical results to those observed from pure THF (see, for example, 2, Figure S9 in the Supporting Information). 15338



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Scheme 1. Sketch of Intermolecular Zn···O Interactions Forming 1-D Chains (Top). Cross-Sectional and Axial Representation of Nanofibers (Bottom) in a Columnar Square (Left) or a Lamellar (Right) Structurea

a

The distance, d, can be related to the spacing derived from the XRD patterns (Table 1).

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DISCUSSION Aggregates of complexes 1−3, obtained by casting from related THF solutions on Si(100) substrates, exhibit a fibrous nanostructure. Even if these complexes in THF solution are present as monomeric adducts having the solvent axially coordinated,14 nanofibers are stabilized through intermolecular Zn···O interactions upon solvent evaporation. In other words, the low Lewis basicity of THF40 and its high volatility favor the removal of the coordinated solvent, leaving a vacant coordination site around the ZnII ion that is then saturated by intermolecular Zn···O interactions. Thus, the molecular selfassembly through intermolecular Zn···O interactions, further assisted by π−π stacking interactions between the salicylidene and the phenylene rings, leads to more stable structures with respect to the monomeric 1−3·solvent adducts. Accordingly, the formation of these fibrous aggregates is independent from the concentration of the cast solution. Moreover, nanostructures are observed even in powder samples obtained from THF solutions of 1−3 upon the complete evaporation of the solvent. In contrast, these nanostructures are not observed in the samples obtained from cast solutions of complexes 2 and 3 in the noncoordinating chloroform solvent. This can be reasonably explained considering that, in chloroform solution, these complexes have been characterized as oligomeric aggregates,14 stabilized through intermolecular Zn···O interactions involving the phenolic oxygen atoms of the ligand framework, thus fulfilling the coordination sphere of the ZnII ion. Therefore, these aggregates do not further organize into

nanofibers upon evaporation of the noncoordinating solvent. However, cast samples of 2 obtained from mixtures of CHCl3/ THF exhibit fibrous nanostructures almost identical to those observed from pure THF. In these cases, even in a mixture with chloroform, the coordinating THF solvent being in large excess with respect to the complex, the latter will be present in solution as monomeric adducts having the solvent axially coordinated; hence, they behave as in pure THF. On the other hand, nanofibers of 1−3 are also found from related solutions of other volatile coordinating solvents with a low Lewis basicity,40 such as ACN. Overall, from SEM and XRD data, a representation of nanofiber formation upon casting is proposed in Scheme 1. Nanofibers are primarily composed of one-dimensional molecular chains formed by intermolecular Zn···O interactions, further stabilized by π−π stacking interactions between the salicylidene and the phenylene rings (Scheme 1, top). The presence of the alkyl side groups on the salicylidene rings in each molecular unit allows for secondary interactions. Thus, nanofibers are formed by interdigitation of the alkyl side groups of each molecular chain, leading to a 2D columnar square structure in the case of the 1 having the short 4-ethyloxy substituents (Scheme 1, bottom). Instead, 2 and 3, having longer 4-alkyloxy chains, are characterized by a lamellar structure (Scheme 1, bottom). Thus, the XRD-derived different d-spacing for nanofibers of 1−3 (Table 1) parallels the length of their side alkyl groups. Note that the spacing deduced from XRD for 1 (15.54 Å) is comparable with the length of its 15339



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molecular unit (15.8 Å) deduced from the (PM3) geometry optimization. The growth mechanism of these nanofibers can be investigated by analyzing aggregates produced by casting from dilute THF solutions in regions of low local complex concentration (e.g., for 2; Figure 6, and Figure S10 in the

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.M.), [email protected] (S.D.B). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the MIUR (PRIN2009A5Y3N9 and PRIN-20097X44S7_002 projects) and PRA (Progetti di Ricerca di Ateneo).

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(1) Steed, J. W.; Turner, D. R.; Wallace, K. J. Core Concepts in Supramolecular Chemistry and Nanochemistry; Wiley: Chichester, U.K., 2007. (2) Lehn, J.-M. Supramolecular Chemistry; VCH: Weinheim, 1995. (3) Chen, Z.; Lohr, A.; Saha-Möller, C. R.; Würthner, F. SelfAssembled π-Stacks of Functional Dyes in Solution: Structural and Thermodynamic Features. Chem. Soc. Rev. 2009, 38, 564−584. (4) Beletskaya, I.; Tyurin, V. S.; Tsivadze, A. Yu.; Guilard, R.; Stern, C. Supramolecular Chemistry of Metalloporphyrins. Chem. Rev. 2009, 109, 1659−1713. (5) Würthner, F. Perylene Bisimide Dyes as Versatile Building Blocks for Functional Supramolecular Architectures. Chem. Commun. 2004, 1564−1579. (6) Zhao, Z.; Lam, J. W. Y.; Tang, B. Z. Tetraphenylethene: A Versatile AIE Building Block for the Construction of Efficient Luminescent Materials for Organic Light-Emitting Diodes. J. Mater. Chem. 2012, 22, 23726−23740. (7) Kim, H.-J.; Kim, T.; Lee, M. Responsive Nanostructures from Aqueous Assembly of Rigid-Flexible Block Molecules. Acc. Chem. Res. 2011, 44, 72−82. (8) Liu, H.; Xu, J.; Li, Y.; Li, Y. Aggregate Nanostructures of Organic Molecular Materials. Acc. Chem. Res. 2010, 43, 1496−1508. (9) Bhattacharya, S.; Samanta, S. K. Soft Functional Materials Induced by Fibrillar Networks of Small Molecular Photochromic Gelators. Langmuir 2009, 25, 8378−8381. (10) Elemans, J. A. A. W.; van Hameren, R.; Nolte, R. J. M.; Rowan, A. E. Molecular Materials by Self-Assembly of Porphyrins, Phthalocyanines, and Perylenes. Adv. Mater. 2006, 18, 1251−1266. (11) Würthner, F.; Yao, S.; Debaerdemaeker, T.; Wortmann, R. Dimerization of Merocyanine Dyes. Structural and Energetic Characterization of Dipolar Dye Aggregates and Implications for Nonlinear Optical Materials. J. Am. Chem. Soc. 2002, 124, 9431−9447. (12) Hui, J. K.-H.; MacLachlan, M. J. Metal-Containing Nanofibers via Coordination Chemistry. Coord. Chem. Rev. 2010, 254, 2363− 2390. (13) Consiglio, G.; Failla, S.; Finocchiaro, P.; Oliveri, I. P.; Di Bella, S. An Unprecedented Structural Interconversion in Solution of Aggregate Zinc(II) Salen Schiff-Base Complexes. Inorg. Chem. 2012, 51, 8409−8418. (14) Consiglio, G.; Failla, S.; Finocchiaro, P.; Oliveri, I. P.; Di Bella, S. Aggregation Properties of Bis(Salicylaldiminato)Zinc(II) Schiff-Base Complexes and Their Lewis Acidic Character. Dalton Trans. 2012, 41, 387−395. (15) Consiglio, G.; Failla, S.; Finocchiaro, P.; Oliveri, I. P.; Purrello, R.; Di Bella, S. Supramolecular Aggregation/Deaggregation in Amphiphilic Dipolar Schiff-Base Zinc(II) Complexes. Inorg. Chem. 2010, 49, 5134−5142. (16) Kleij, A. W. Zinc-Centred Salen Complexes: Versatile and Accessible Supramolecular Building Motifs. Dalton Trans. 2009, 4635−4639. (17) Bhattacharjee, C. R.; Das, G.; Mondal, P.; Prasad, S. K.; Rao, D. S. S. Novel Green Light Emitting Nondiscoid Liquid Crystalline Zinc(II) Schiff-Base Complexes. Eur. J. Inorg. Chem. 2011, 1418−1424.

Figure 6. FE-SEM images at different magnifications of 2 deposited by casting onto a Si(100) substrate from a 5.0 × 10−4 M THF solution.

Supporting Information). They clearly indicate that the fibrous aggregates originate from independent nuclei that self-assemble into nanofibers. Bundles of twisted nanofibers are presumably formed by further interactions and interdigitation of the outside alkyl groups of each nanofiber.

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CONCLUSIONS In this study, a rational approach to understanding the formation of nanofibers from a series of Zn(salophen) Schiffbase complexes, is presented. The formation of these nanofibers is related to the nature of the solvent used. Thus, fibrous aggregates of 1−3 are always obtained by drop-casting from solutions of volatile and weak Lewis base solvents, whereas, in the case of noncoordinating solvents, where oligomeric aggregates are already present in solution, no formation of nanofibers is observed. The formation of the nanostructures is independent from the method used, drop-casting or solvent evaporation, and the alkyl side chain length of complexes 1−3. Therefore, the driving force for the self-assembly is likely dominated by intermolecular Zn···O interactions. The length of side alkyl groups and their degree of interdigitation influence the structure of nanofibers, which self-assemble into a 2D columnar square or in a lamellar organization. Bundles of twisted nanofibers are conceivably formed by further interactions and interdigitation of the outside alkyl groups of each nanofiber. The present contribution, beyond being a fundamental study to rationalize and control the self-assembly of these complexes, envisages also interesting potential applications, as preliminary data indicate that these nanofibers could be used as unique supramolecular precursors for the fabrication of ZnO nanostructures.41

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Additional FE-SEM and XRD data. This material is available free of charge via the Internet at http://pubs.acs.org. 15340



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