Controlling the Molecular Self-Assembly into Nanofibers ofAmphiphilic Zinc(II) Salophen ComplexesIvan Pietro Oliveri,† Salvatore Failla,‡ Graziella Malandrino,*,† and Santo Di Bella*,†
†Dipartimento di Scienze Chimiche, Universita di Catania, I-95125 Catania, Italy‡Dipartimento di Ingegneria Industriale, Universita di Catania, I-95125 Catania, Italy
*S Supporting Information
ABSTRACT: The synthesis, characterization, and aggregation properties inthe solid state of a series of amphiphilic Zn(salophen) Schiff-base complexesare presented, through a combined FE-SEM/XRD approach. It is found thatthese complexes self-assemble into nanofibers depending on the solvent usedto prepare the solutions. Thus, fibrous aggregates are obtained from solutionsof weak and volatile Lewis base solvents, either by drop-casting or bycomplete solvent evaporation, whereas, in the case of noncoordinatingsolvents, where oligomeric aggregates are already present in solution, noformation of nanofibers is observed. The length of side alkyl groups and theirdegree of interdigitation lead to a 2D columnar square structure in the case ofthe complex with the short 4-ethyloxy substituents, whereas complexes havinglonger 4-alkyloxy chains are characterized by a lamellar structure. Bundles oftwisted nanofibers are formed by further interactions and interdigitation ofthe outside alkyl groups of each nanofiber. A simple model, which describes the mechanism of formation and structure of thesenanofibers, is presented.
■ INTRODUCTION
The molecular aggregation and control of the supramoleculararchitecture is a widely explored field of research, involvingboth fundamental1−5 and application aspects.6−11 In the case ofdipolar chromophores, aggregation generally occurs via non-covalent bonds, by dipolar or π−π stacking interactions, or acombination of them.1−5 An additional possibility is offered bytransition-metal complexes in which molecular aggregation canoccur through metal−ligand coordination.12
In this view, tetracoordinated ZnII Schiff-base complexespossess unique peculiarities. They, in fact, are Lewis acidicspecies13−15 that saturate their coordination sphere bycoordinating auxiliary Lewis bases or, in their absence, arestabilized through intermolecular Zn···O axial interactions,16
thus allowing for a different control of the supramoleculararchitecture. Therefore, a variety of molecular aggregates,17−20
supramolecular assemblies,21−24 and nanostructures25−28 havebeen found. In this last regard, MacLachlan and co-workershave investigated a series of analogous mono- and dinuclearZnII Schiff-base complexes able to form nanofibrillarstructures,29−31 whose aggregation is dependent on the natureof substituents in the ligand framework.We have recently investigated a series of amphiphilic
bis(salicylaldiminato)ZnII Schiff-base complexes and demon-strated that these species always form aggregates in solution ofnoncoordinating solvents.13−15 The degree and type ofaggregation are related to the nature of the bridging diamine.In coordinating solvents or in the presence of coordinatingspecies, a complete deaggregation occurs, because of the axial
coordination to the ZnII ion, accompanied by sizable changes ofoptical absorption, fluorescence,15,21,32−38 and second-ordernonlinear optical properties.39
To further probe into the nature and mechanism ofaggregation of these complexes in the solid state, in thispaper, we have investigated a series of Zn(salophen) Schiff-basecomplexes, having alkoxy substituents as lateral groups in thesalicylidene rings (Chart 1), and studied their supramolecularstructure through a detailed combined, field emission scanningelectron 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
Received: April 17, 2013Revised: June 13, 2013Published: July 8, 2013
the involved solvent, while the length of side alkyl groups andtheir degree of interdigitation influence the structure ofnanofibers. A picture of the structure of these nanofibers ispresented.
■ EXPERIMENTAL SECTION
Materials and General Procedures. Zinc acetatedihydrate, 2,4-dihydroxybenzaldehyde, 1-iodoethane, 1-iodode-cane, and 1-bromohexadecane (Aldrich) were used as received.o-Phenylenediamine (Aldrich) was purified by crystallizationfrom aqueous 1% sodium hydrosulphite. Column chromatog-raphy was performed on silica gel 60 (230−400 mesh) eluting
with cyclohexane and EtOAc. TLC was performed using silicagel 60 F254 plates with visualization by UV and standardstaining. Chloroform (Aldrich) stabilized with amylene was usedto prepare solutions of 1−3.
Measurements. Elemental analyses were performed on aCarlo Erba 1106 elemental analyzer. Optical absorption spectrawere recorded at room temperature with a Varian Cary 500UV−vis-NIR spectrophotometer. ESI mass spectra wererecorded with a Finnigan LCQ-Duo ion trap electrospraymass spectrometer (Thermo). X-ray diffraction (XRD, θ−2θ)patterns were recorded in grazing incidence mode (0.5°) on aBruker-AXS D5005 θ−θ X-ray diffractometer, using a Goebel
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 MTHF solutions.
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mirror to parallel Cu−Kα radiation, λ = 1.5418 Å, operating at40 kV and 30 mA. The film surface morphology was examinedby FE-SEM using a ZEISS SUPRA VP 55 microscope. Samplesfor XRD measurements were obtained by drop-casting fromsolutions of complexes 1−3 at different concentrations ontocleaned Si(100) substrates. Samples were subsequentlysputtered with gold to avoid charging effects before FE-SEManalysis. Powder samples were obtained by the completeevaporation of the solvent from solutions of complexes 1−3.FE-SEM analyses were carried out on powders stuck on carbontape and sputtered with gold.Syntheses. Syntheses of 4-ethoxy-2-hydroxybenzaldehyde,
4-hexadecyloxy-2-hydroxybenzaldehyde, and [N,N-bis(4-decy-loxy-2-hydroxybenzylidene)-1,2-phenylene-diaminato]ZnII (2)were previously reported.14
[N,N-Bis(4-ethoxy-2-hydroxybenzylidene)-1,2-phenylene-diaminato]ZnII (1). To a solution of 4-ethoxy-2-hydroxyben-zaldehyde (1.00 mmol) in ethanol (20 mL) was added 1,2-phenylenediamine (0.500 mmol) under stirring. The mixturewas heated at reflux with stirring for 1 h, under a nitrogenatmosphere. To the solution so-obtained was added zincacetate dihydrate (0.1095 g, 0.500 mmol), and the mixture washeated at reflux with stirring for 1h, under a nitrogenatmosphere. After cooling, the precipitated product wascollected by filtration, washed with ethanol, and dried. Yellowpowder (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,
■ RESULTSFE-SEM images of structures obtained by drop-casting fromTHF solutions of 1−3 onto Si(100) substrates are shown inFigure 1.They indicate the formation of fibrous nanostructures. The
morphology of these nanostructures is almost independentfrom the concentration of the cast solution. Actually, thedifferences between the FE-SEM images obtained by drop-casting 1.0 × 10−3 and 5.0 × 10−4 M THF solutions areconsistent with the different amounts of material deposited onthe substrate. Thus, the structures obtained by casting moredilute THF solutions are better defined and less dense thanthose obtained from 1.0 × 10−3 M THF solutions (see FigureS1 in the Supporting Information).Further details of these molecular aggregates can be gained
from FE-SEM images of 1−3 at higher magnification (Figure1b,d,f). They indicate an essentially identical shape, in whichnanofibers of 40−60 nm in width are twisted together, forming
bundles of nanofibers, ∼100−300 nm wide. However, slightdifferences in the bundles of nanofibers can be observed onpassing from 1, having the ethyl alkyls, to 2, having the decylalkyls. In particular, thicker bundles of nanofibers are observedin the latter case. Instead, on passing from 2 to 3, noappreciable differences in the thickness of bundles of nanofibersare observed.The same nanofiber samples of 1−3, deposited by casting on
Si(100) substrates, were analyzed by X-ray diffraction measure-ments recorded in grazing incidence mode. XRD patterns andrelated d-spacings are reported in Figure 2 and Table 1.
The XRD patterns of the nanofibers of 1−3 show broadpeaks due to the assembled structures. The reflection angles donot 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 ofthe side alkyl groups in the XRD patterns is observed. Inparticular, the position of the first diffraction peak movestoward lower angles and, consequently, the d-spacingprogressively increases on increasing the length of alkyl groups(Table 1). The peaks at the lowest angles may be associatedwith the 10 reflection of the self-assembly. The broad profileassociated with this reflection can be related to somedisordering in the self-assembled structure.More interestingly, analogous, but sharper, XRD patterns are
observed even from powder samples of 1−3 obtained fromtheir THF solutions by complete evaporation of the solvent(Figures 3 and 4; Figure S2 in the Supporting Information). Inparticular, the diffraction pattern of 1 shows a set of diffractionpeaks 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 almost1:√2:2:√5, consistent with a 2D columnar square structurewith a lattice constant of 15.54 Å. Instead, the diffractionpattern of 2 shows a set of diffraction peaks at 2θ = 3.35, 6.74,
Figure 2. XRD patterns for nanofibers of 1−3 obtained by castingfrom 1.0 × 10−3 M THF solutions. The asterisks refer to the 10reflection peak.
Table 1. Spacing (d) for Nanofibers of 1−3 Derived from the10 Reflection Peak of Nanostructures Obtained by Castingand Powders (in Parentheses)
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10.30, and 13.81° corresponding to d = 26.35, 13.10, 8.58, and6.41 Å. The last three peaks can be associated with higher-orderreflections of the 10 peak observed at 2θ = 3.35°, thussuggesting a lamellar organization. Analogously to 2, a lamellarstructure can be also deduced from powder samples of 3(Figure S2 in the Supporting Information). As the well-definedXRD patterns for powder samples of 1−3 allowed assigningtheir structure, considering that XRD patterns obtained fromcast samples show broad peaks whose reflection angles are fullycomparable to those obtained from powder samples (Figures 3and 4; Figure S2 in the Supporting Information), we canassume 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, suchas acetonitrile (ACN). In fact, FE-SEM images of structuresobtained by drop-casting from ACN solutions of 1−3 ontoSi(100) substrates show the formation of fibrous nanostruc-tures, whose XRD patterns are comparable to those observedfrom THF solutions (see, for example, 2, Figures S5 and S6 inthe Supporting Information).In contrast with the results obtained from THF or ACN
solutions, the behavior of 2 and 3 in chloroform solutions isvery different. Actually, FE-SEM images of samples obtained bycasting from chloroform solutions do not show any definednanostructure, but rather a flat structure of the material
deposited over the substrate (see, for example, 2, Figure S7 inthe Supporting Information). Moreover, the XRD pattern ofsample 2 deposited by casting shows a unique peak at 5.15°(Figure S8 in the Supporting Information), exactly correspond-ing to one of the reflections found in the very complexdiffraction pattern of the powder sample obtained from aCHCl3 solution by complete evaporation of the solvent. Thesefindings, together with the FE-SEM image of the drop-castedfilm, point to the formation of an oriented structure.Unfortunately, it is not possible to assign this reflection andhence the film orientation, since no crystal structure is availablefor 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 XRDpatterns and FE-SEM images of samples deposited by castingfrom these solutions indicate almost identical results to thoseobserved from pure THF (see, for example, 2, Figure S9 in theSupporting Information).
Figure 3. Comparison of XRD patterns of 1 obtained by casting froma THF solution (red line), and a powder sample obtained from a THFsolution by complete evaporation of the solvent (black line).
Figure 4. Comparison of XRD patterns of 2 obtained by casting froma THF solution (red line), and a powder sample obtained from a THFsolution by complete evaporation of the solvent (black line).
Figure 5. High-magnification FE-SEM images of powder samples of 1(a) and 2 (b) obtained from THF solutions by complete evaporationof the solvent.
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■ DISCUSSION
Aggregates of complexes 1−3, obtained by casting from relatedTHF solutions on Si(100) substrates, exhibit a fibrousnanostructure. Even if these complexes in THF solution arepresent as monomeric adducts having the solvent axiallycoordinated,14 nanofibers are stabilized through intermolecularZn···O interactions upon solvent evaporation. In other words,the low Lewis basicity of THF40 and its high volatility favor theremoval of the coordinated solvent, leaving a vacantcoordination site around the ZnII ion that is then saturatedby intermolecular Zn···O interactions. Thus, the molecular self-assembly through intermolecular Zn···O interactions, furtherassisted by π−π stacking interactions between the salicylideneand the phenylene rings, leads to more stable structures withrespect to the monomeric 1−3·solvent adducts. Accordingly,the formation of these fibrous aggregates is independent fromthe concentration of the cast solution. Moreover, nanostruc-tures are observed even in powder samples obtained from THFsolutions 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 inthe noncoordinating chloroform solvent. This can bereasonably explained considering that, in chloroform solution,these complexes have been characterized as oligomericaggregates,14 stabilized through intermolecular Zn···O inter-actions involving the phenolic oxygen atoms of the ligandframework, 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 thoseobserved from pure THF. In these cases, even in a mixture withchloroform, the coordinating THF solvent being in large excesswith respect to the complex, the latter will be present insolution as monomeric adducts having the solvent axiallycoordinated; hence, they behave as in pure THF. On the otherhand, nanofibers of 1−3 are also found from related solutionsof other volatile coordinating solvents with a low Lewisbasicity,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 thesalicylidene and the phenylene rings (Scheme 1, top). Thepresence of the alkyl side groups on the salicylidene rings ineach molecular unit allows for secondary interactions. Thus,nanofibers are formed by interdigitation of the alkyl side groupsof each molecular chain, leading to a 2D columnar squarestructure in the case of the 1 having the short 4-ethyloxysubstituents (Scheme 1, bottom). Instead, 2 and 3, havinglonger 4-alkyloxy chains, are characterized by a lamellarstructure (Scheme 1, bottom). Thus, the XRD-derived differentd-spacing for nanofibers of 1−3 (Table 1) parallels the lengthof their side alkyl groups. Note that the spacing deduced fromXRD for 1 (15.54 Å) is comparable with the length of its
Scheme 1. Sketch of Intermolecular Zn···O Interactions Forming 1-D Chains (Top). Cross-Sectional and Axial Representationof Nanofibers (Bottom) in a Columnar Square (Left) or a Lamellar (Right) Structurea
aThe distance, d, can be related to the spacing derived from the XRD patterns (Table 1).
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molecular unit (15.8 Å) deduced from the (PM3) geometryoptimization.The growth mechanism of these nanofibers can be
investigated by analyzing aggregates produced by casting fromdilute THF solutions in regions of low local complexconcentration (e.g., for 2; Figure 6, and Figure S10 in the
Supporting Information). They clearly indicate that the fibrousaggregates originate from independent nuclei that self-assembleinto nanofibers. Bundles of twisted nanofibers are presumablyformed by further interactions and interdigitation of the outsidealkyl groups of each nanofiber.
■ CONCLUSIONSIn this study, a rational approach to understanding theformation of nanofibers from a series of Zn(salophen) Schiff-base complexes, is presented.The formation of these nanofibers is related to the nature of
the solvent used. Thus, fibrous aggregates of 1−3 are alwaysobtained by drop-casting from solutions of volatile and weakLewis base solvents, whereas, in the case of noncoordinatingsolvents, where oligomeric aggregates are already present insolution, no formation of nanofibers is observed.The formation of the nanostructures is independent from the
method used, drop-casting or solvent evaporation, and the alkylside chain length of complexes 1−3. Therefore, the drivingforce for the self-assembly is likely dominated by intermolecularZn···O interactions. The length of side alkyl groups and theirdegree of interdigitation influence the structure of nanofibers,which self-assemble into a 2D columnar square or in a lamellarorganization. Bundles of twisted nanofibers are conceivablyformed by further interactions and interdigitation of the outsidealkyl 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 preliminarydata indicate that these nanofibers could be used as uniquesupramolecular precursors for the fabrication of ZnOnanostructures.41
■ ASSOCIATED CONTENT*S Supporting InformationAdditional FE-SEM and XRD data. This material is availablefree of charge via the Internet at http://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
This research was supported by the MIUR (PRIN-2009A5Y3N9 and PRIN-20097X44S7_002 projects) andPRA (Progetti di Ricerca di Ateneo).
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Figure 6. FE-SEM images at different magnifications of 2 deposited bycasting onto a Si(100) substrate from a 5.0 × 10−4 M THF solution.
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