Thermal, Magnetic, and Luminescent Properties of Dendronized Ferrite Nanoparticles Saı ¨wan Buathong, † Diane Ung, † T. Jean Daou, † Corinne Ulhaq-Bouillet, † Genevieve Pourroy, † Daniel Guillon, † Lyubomira Ivanova, ‡ Ingolf Bernhardt, ‡ Sylvie Be ´gin-Colin,* ,† and Bertrand Donnio* ,† Institut de Physique et Chimie des Mate ´riaux de Strasbourg, UMR 7504 CNRS-UniVersite ´ Louis Pasteur, BP43, 23 rue du Loess, F-67034 Strasbourg Cedex 2, France, and Laboratory of Biophysics, Saarland UniVersity, House A2.4, P.O. Box 151150, 66041 Saarbruecken, Germany ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: May 6, 2009 The design, synthesis, and some properties of magnetic nanocrystals of ferrite functionalized by luminescent prodendritic ligands are described. Multipotent hybrids resulting from the coupling of an inorganic nanocore (metallic, alloy, or oxide nanoparticle) embedded in an organic shell currently elicit sustained research activity in various areas of materials science and notably in the development of numerous, potential applications, for example, as relevant building blocks for self-assembled arrays of nanoparticles with magneto-luminescence properties, as magnetically movable luminescent probes, or as platforms for biomedical applications. In this study, we have essentially focused on the covalent functionalization of γ-Fe 2 O 3 magnetic nanoparticles, with an average diameter of 40 nm by engineered luminescent, liquid-crystalline oligo(phenylene vinylene)-based prodendritic ligands. The grafting rate of the various ligands on the oxide surface and the thermal stability and behavior of the hybrids have been studied in detail and are reported here. The dendronized nanoparticles exhibit room temperature ferrimagnetic behavior, as do the parent naked particle. Furthermore, the grafting of covalent organic chromophores confers to the ensemble luminescent properties that can be tuned by the structure of the luminophore. However, despite the liquid crystalline character of the luminescent ligands, none of the corresponding hybrids showed mesomorphic properties, a result attributed to the large size discrepancy between the nanocrystalline core and the ligand. Introduction Single, multifunctional, core-shell nanoparticles constituted of a multivalent metallic, alloy, or oxide nanocrystalline nucleus and an organic coating, as active entities able to accomplish various and specific tasks, will be more and more present in the future nanotechnologies with a particular emphasis in connecting one structure to several functions by design. 1 The concept of bottom up functionalization opens new routes to the design and synthesis of such novel types of hybrid nanomate- rials, in which the multiscale hierarchisation of the functions (mainly magnetic, optic, and/or electronic properties located in the inorganic core, and structuring and directing, coordinating, and chemical and physicochemical properties associated to the organic shell) and the synergy between the intrinsic properties of the elementary parts may play an important role through cooperative effects. 2-4 The stabilization of these nano-objects, their propensity to self-assemble and self-organize into low-dimensional, periodic nanostructures, and the main characteristics of the final device will be conditioned by this key approach and the choice of the initial components. 1,2 The various/needed functions can be intimately controlled by the multivalent hard core structural characteristics (controlled size and size distribution, morphology) 5,6 and provided by their designed structure (function, chemical composition, for example, heterostructures, core-shells, etc.) 7-9 on the one hand and, on the other hand, by the use of the adequate organic moiety linked onto the core surface. The later consists of smart, protective, structuring molecules giving supplementary functionalities (e.g., chemically active mol- ecules, 10 polymers, 11 dendrimers, 12 or biomolecules 13 ), leading to the formation of intricate hybrid organic-inorganic particles. Moreover, such core-shell nano-objects coated by a hydro- phobic layer (usually alkyl groups are used to passivate the nanocrystal surface) can be further self-assembled into su- pracrystals (by slow solvent evaporation 14 ) or into binary nanoparticles superlattices (BNPS, by controlling the size of the different NPs 15 ) mainly through steric and van der Waals interactions. The rational construction of various types of hybrid materials, narrowly disperse in diameter, with predefined and controlled organic coatings governs their possible self-organiza- tion, in a reliable manner, either in the bulk (1D, 2D, 3D low- dimensionality self-organizations, for example, liquid crystalline mesophases) or into ordered, periodic particles arrays (planar array of 1D chains, 16 regular and 2D and 3D networks, superlattices, 6,15 and supracrystals 14 ) that will influence the future nanostructured materials’ properties. The search for other functions of such types of nanostructures in biology and medical technologies 9,13,17 is also an important trend. Magnetic particles 18 are particularly promising in this respect, with numerous applications in catalysis, 19 magnetic fluids, 20 data storage 18 and transport, contrast agents for MRI, 21 and so on. In this work, we describe the synthesis of novel organic- inorganic hybrid dendronized nanoparticles presenting magnetic and luminescent functionalities (Chart 1). In particular, this study is focused on the controlled covalent functionalization of γ-Fe 2 O 3 magnetic nanoparticles, with an average diameter of 39 nm and with acicular shape, by engineered luminescent, * To whom correspondence should be addressed. E-mail: bdonnio@ ipcms.u-strasbg.fr (B.D.); [email protected] (S.B.-C.). † UMR 7504 CNRS-Universite ´ Louis Pasteur. ‡ Saarland University. J. Phys. Chem. C 2009, 113, 12201–12212 12201 10.1021/jp902046d CCC: $40.75 2009 American Chemical Society Published on Web 06/11/2009 Downloaded by UNIV LOUIS PASTEUR STRASBOURG 1 on July 10, 2009 Published on June 11, 2009 on http://pubs.acs.org | doi: 10.1021/jp902046d
Transcript
Thermal, Magnetic, and Luminescent Properties of Dendronized Ferrite Nanoparticles
Saıwan Buathong,† Diane Ung,† T. Jean Daou,† Corinne Ulhaq-Bouillet,† Genevieve Pourroy,†
Daniel Guillon,† Lyubomira Ivanova,‡ Ingolf Bernhardt,‡ Sylvie Begin-Colin,*,† andBertrand Donnio*,†
Institut de Physique et Chimie des Materiaux de Strasbourg, UMR 7504 CNRS-UniVersite Louis Pasteur, BP43,23 rue du Loess, F-67034 Strasbourg Cedex 2, France, and Laboratory of Biophysics, Saarland UniVersity,House A2.4, P.O. Box 151150, 66041 Saarbruecken, Germany
ReceiVed: March 2, 2009; ReVised Manuscript ReceiVed: May 6, 2009
The design, synthesis, and some properties of magnetic nanocrystals of ferrite functionalized by luminescentprodendritic ligands are described. Multipotent hybrids resulting from the coupling of an inorganic nanocore(metallic, alloy, or oxide nanoparticle) embedded in an organic shell currently elicit sustained research activityin various areas of materials science and notably in the development of numerous, potential applications, forexample, as relevant building blocks for self-assembled arrays of nanoparticles with magneto-luminescenceproperties, as magnetically movable luminescent probes, or as platforms for biomedical applications. In thisstudy, we have essentially focused on the covalent functionalization of γ-Fe2O3 magnetic nanoparticles, withan average diameter of 40 nm by engineered luminescent, liquid-crystalline oligo(phenylene vinylene)-basedprodendritic ligands. The grafting rate of the various ligands on the oxide surface and the thermal stabilityand behavior of the hybrids have been studied in detail and are reported here. The dendronized nanoparticlesexhibit room temperature ferrimagnetic behavior, as do the parent naked particle. Furthermore, the graftingof covalent organic chromophores confers to the ensemble luminescent properties that can be tuned by thestructure of the luminophore. However, despite the liquid crystalline character of the luminescent ligands,none of the corresponding hybrids showed mesomorphic properties, a result attributed to the large sizediscrepancy between the nanocrystalline core and the ligand.
Introduction
Single, multifunctional, core-shell nanoparticles constitutedof a multivalent metallic, alloy, or oxide nanocrystalline nucleusand an organic coating, as active entities able to accomplishvarious and specific tasks, will be more and more present inthe future nanotechnologies with a particular emphasis inconnecting one structure to several functions by design.1 Theconcept of bottom up functionalization opens new routes to thedesign and synthesis of such novel types of hybrid nanomate-rials, in which the multiscale hierarchisation of the functions(mainly magnetic, optic, and/or electronic properties located inthe inorganic core, and structuring and directing, coordinating,and chemical and physicochemical properties associated to theorganic shell) and the synergy between the intrinsic propertiesof the elementary parts may play an important role throughcooperative effects.2-4
The stabilization of these nano-objects, their propensity toself-assemble and self-organize into low-dimensional, periodicnanostructures, and the main characteristics of the final devicewill be conditioned by this key approach and the choice of theinitial components.1,2 The various/needed functions can beintimately controlled by the multivalent hard core structuralcharacteristics (controlled size and size distribution, morphology)5,6
and provided by their designed structure (function, chemicalcomposition, for example, heterostructures, core-shells, etc.)7-9
on the one hand and, on the other hand, by the use of the
adequate organic moiety linked onto the core surface. The laterconsists of smart, protective, structuring molecules givingsupplementary functionalities (e.g., chemically active mol-ecules,10 polymers,11 dendrimers,12 or biomolecules13), leadingto the formation of intricate hybrid organic-inorganic particles.Moreover, such core-shell nano-objects coated by a hydro-phobic layer (usually alkyl groups are used to passivate thenanocrystal surface) can be further self-assembled into su-pracrystals (by slow solvent evaporation14) or into binarynanoparticles superlattices (BNPS, by controlling the size ofthe different NPs15) mainly through steric and van der Waalsinteractions. The rational construction of various types of hybridmaterials, narrowly disperse in diameter, with predefined andcontrolled organic coatings governs their possible self-organiza-tion, in a reliable manner, either in the bulk (1D, 2D, 3D low-dimensionality self-organizations, for example, liquid crystallinemesophases) or into ordered, periodic particles arrays (planararray of 1D chains,16 regular and 2D and 3D networks,superlattices,6,15 and supracrystals14) that will influence the futurenanostructured materials’ properties. The search for otherfunctions of such types of nanostructures in biology and medicaltechnologies9,13,17 is also an important trend. Magnetic particles18
are particularly promising in this respect, with numerousapplications in catalysis,19 magnetic fluids,20 data storage18 andtransport, contrast agents for MRI,21 and so on.
In this work, we describe the synthesis of novel organic-inorganic hybrid dendronized nanoparticles presenting magneticand luminescent functionalities (Chart 1). In particular, this studyis focused on the controlled covalent functionalization ofγ-Fe2O3 magnetic nanoparticles, with an average diameter of39 nm and with acicular shape, by engineered luminescent,
† UMR 7504 CNRS-Universite Louis Pasteur.‡ Saarland University.
J. Phys. Chem. C 2009, 113, 12201–12212 12201
10.1021/jp902046d CCC: $40.75 2009 American Chemical SocietyPublished on Web 06/11/2009
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mesomorphic oligo(phenylene vinylene)-based dendrimers, andon the characterization of the magnetic, luminescent, and thermalproperties of these hybrid nanostructures. Maghemite (γ-Fe2O3)NPs were preferentially used to avoid magnetite (Fe3O4)stoichiometric instability (the oxidation of the surface can disturbthe physicochemical characterization after functionalization).These 39 nm NPs present room temperature ferrimagneticbehavior. In this perspective, the organic molecules, premisesof dendritic architectures as ideal candidates to protect theparticles,12 were designed to be soluble in organic medium, topossess the ability to coordinate strongly the oxide surface, toexhibit luminescent properties, and to be mesogenic to stimulatetheir tendency to self-organize into predictable ordered andperiodic 2D and 3D arrays. Oligo(phenylene vinylenes) (OPV)p-based oligomers22 were selected as the functional groups to becombined with the oxides for their interesting electrical, optical(absorption/emission, nonlinear optics, luminescence), and op-toelectronic properties,23,24 chemical versatility, and also as aprobe to quantify the degree of the particle coverage by UVspectroscopy. Two types of organic architectures were consid-ered (Chart 1): linear oligo(phenylene vinylenes) (OPV)p,referred thereafter to as G0-OPVp, and pro-dendritic oligomerswith a tapered shape (referred thereafter to as G1-OPVp). Thesechromophores were equipped at the focal point by a phosphonicacid function to bind firmly to the surface of the oxide particlesand at the other extremity by three dodecyloxy fragments tomotivate the self-organization of the hybrids into liquid crystal-line mesophases, to enhance their solubility in a wide range oforganic solvents, and improve their stability by reducinginterparticle aggregation and surface’s oxidation. Recent studieshave demonstrated that the use of the phosphonic acid whichpresents stronger affinity to oxide surfaces25 is more convenientthan the commonly used carboxylic acid function which exhibitsweaker interactions.4,26
Synthesis and Characterization of the Ferrite Moiety
The synthesis of magnetite nanoparticles with an average sizeof 39 nm has recently been reported.27 First, the 10-12 nmsized Fe3-δO4 nanoparticles were produced by coprecipitationof ferrous Fe2+ and ferric Fe3+ ions at 70 °C by rapid additionof tetramethylhydroxylamine (N(CH3)4OH) at constant pH (pH) 10) under argon. Then, in the second step, the solutioncontaining small nanoparticles was introduced in an autoclaveunder argon, sealed, and heated for 24 h at 250 °C; thishydrothermal growth process yielded the magnetite Fe2.95O4
nanoparticles with an average size of 39 ( 5 nm.27 The as-formed NPs were separated by magnetic decantation, followedby the removal of supernatant, washed thoroughly with deion-ized water, and dried by lyophilization. Finally, maghemiteγ-Fe2O3 NPs were obtained by air oxidation of the magnetiteNPs at 300 °C for 24 h. The morphology of the obtained NPswas characterized by transmission electron microscopy (TEM,Figure 1, top) and scanning electron microscopy (SEM, Figure1, bottom). Electronic micrographs and BET measurementsconfirm that the oxidation step has not altered the sizedistribution of acicular particles, with an average diameter of39 ( 5 nm.
The XRD patterns of the Fe2.95O4 nanoparticles display theX-ray peaks characteristic of a ferrite spinel (Figure 2, top) andconfirmed the absence of hematite and iron hydroxides. Afteroxidation of the magnetite, the observation of the X-ray peaks
CHART 1: Schematic Representation of theDendronized Maghemite Nanoparticles and ChemicalStructure (and nomenclature) of the Organic MoietiesGi-OPVp
Figure 1. TEM (top) and SEM (bottom) micrographs of the 39 nm-sized γ-Fe2O3 NPs obtained after air oxidation at 300 °C.
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(210) and (211) confirms the formation of the maghemite aswell as the lattice parameter, which corresponds to that ofmaghemite (0.8346 nm, JCPDS file 39-1346). Another wayto prove the formation of the maghemite phase can be achievedby IR spectroscopy, as both magnetite and maghemite can beeasily differentiated on symmetry grounds. Wavenumbersassigned to spinel structure are in the 800-400 cm-1 range.The IR spectra of stoichiometric magnetite display one peak ataround 570 cm-1, while the IR spectra of maghemite (Fe3+
A-[Fe3+
5/301/3]BO42-) are more complicated due to the sensitivity
of IR analyses to vacancies ordering: the higher the order, thelarger the number of lattice absorption bands between 800 and200 cm-1 (Figure 2, bottom). The Fe-O bands of the oxidizednanoparticles are characteristic of a partial disordered spinelmaghemite structure.4
Synthesis and Thermal Behavior of the Organic Moiety
In general, the most efficient and versatile methods for thesynthesis of OPV-based compounds involve palladium-based-catalyzed Heck coupling reactions between styrene and ha-lobenzene basic building modules. We specifically elaborateda strategy based on such a cross-coupling reaction to make useof multipotent building blocks to avoid synthetic redundancyand to limit the number of steps (i.e., molecular modules that
can be employed repeatedly at various stages of the synthesis).This approach was moreover facilitated due to the readyavailability of most of the starting materials. The reactionschemes are shown below (Schemes 1 and 2). All the intermedi-ates were characterized by 1H NMR spectroscopy only, and thefinal products by 1H and 13C NMR and MALDI-TOF spec-troscopies and elemental analysis. Details are given in theSupporting Information.
The preparation of the linear OPV2 and OPV3 phosphonicdiethyl esters (3, 10) and corresponding phosphonic acid (4,11) derivatives were obtained by a convergent synthetic routein three and six steps (Scheme 1), respectively, starting from3,4,5-tridodecyloxystyrene, 1, one of the key compounds of ourprocedure, prepared as previously described in four steps fromthe commercially available methyl 3,4,5-trihydroxybenzoate.28
This compound was coupled to iodo-4-bromobenzene, accordingto the Heck cross-coupling reaction using Pd(OAc)2 as cata-lyst, to afford the bromostilbene module 2 as the major product.The diethyl arylphosphonate 3 was prepared from the bromideprecursory derivative 2 by a convenient palladium(0)-catalyzedphosphonation reaction with diethyl phosphite in triethylamine.29
The resulting diethyl ester was then treated by trimethylsilylbromide (TMSBr) followed by hydrolysis in methanol togenerate the desired phosphonic acid (4, G0-OPV2) in goodyields. The preparation of G0-OPV3 (11) involved the trans-formation of the oligophenylenevinylene (OPV) hydroxylderivative (8) into the trifluomethanesulphonate ester (9), a betterleaving group and easier to prepare than the homologousbromide precursory.30 The module 2 was therefore reacted withthe silyl-protected p-hydroxyl styrene (6) and, after deprotectionof the hydroxy group by TBAF (7), afforded the alcoholic OPVderivative, 8, which was first transformed into the sulfonicderivative (9), and subsequently into the phosphonate diethylester (10), using the same reaction conditions as for 3. Finally,ester 10 was treated by TMSBr followed by hydrolysis inmethanol to yield the phosphonic acid 11 (G0-OPV3).
The preparation of the branched oligomers 21-24 followeda similar convergent synthetic route, also based on repeatedpalladium-catalyzed Heck cross-coupling reactions (Scheme 2).The modular brick 12 was prepared directly from the cross-coupling of 3,4,5-tridodecyloxystyrene (1) and 4-bromobenzylicalcohol. Then, 12 can either be converted into the methylbro-mide stilbene derivative (13) by direct bromination or followinga succession of standard reactions (12f 14f 15f 16f 17)into the corresponding methylbromide OPV3 analogous com-pound (17). The two OPV derivatives were then coupled to 3,5-dihydroxybromobenzene (18) to afford the bromide-precursoryprodendritic systems (19, 20: X ) Br). The prodendriticarylphosphonic diethyl ester homologues (21 and 22: X )PO(OEt)2) were prepared by the Pd(0)-catalyzed phosphonationreaction with diethyl phosphite in triethylamine, as above for 3and 10. Hydrolysis in basic and acidic conditions provided thedesired G1 branched phosphonic acids (23 and 24: X )PO(OH)2); TMSBr was not used here as cleaving all the methylether junctions to yield the precursory arms 13 and 17,respectively.
As expected, on the basis of their molecular structure,31 thephosphonic chromophores exhibited mesomorphic properties asdeduced jointly by POM, DSC, and XRD methods; only G1-OPV2 was devoid of mesomorphism and was obtained as aroom temperature amorphous solid (Table 1). For the G0mesomorphic samples, homogeneous, highly birefringent opticaltextures, exhibiting developable domains alongside large ho-meotropic zones in some cases could be observed, suggesting
Figure 2. Top: XRD pattern of the 39 ( 5 nm maghemite particles(a) and after oxidation of magnetite nanoparticles (b). Bottom: Infraredspectra of as-prepared magnetite NPs and of the maghemite nanopar-ticles after the oxidation step at 300 °C in the Fe-O wavenumber range.
Properties of Dendronized Ferrite Nanoparticles J. Phys. Chem. C, Vol. 113, No. 28, 2009 12203
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columnar-type of mesophases (in agreement with their poly-catenar structure, because a smectic-like arrangement, the otherpossibility, is excluded on the basis of noncompensation of themolecular areas).32 All the mesophases can be considered asdisordered, with the presence in the wide angle region of theX-ray diffraction (XRD) patterns of only one broad and diffusesignal, centered at around 4.5 Å, distance reflecting the liquid-like state of the molten alkyl chains. In the low angle region,two sharp and intense signals with the spacing ratio 1:�3 wereobserved for G0-OPV3, reflecting the 2D hexagonal arrange-ment of the columns; for G0-OPV2 and G1-OPV3, only onesharp and intense reflection was observed, whereas for G1-OPV2, broad reflections were observed instead. Thus, from theseexperiments it can be concluded that the two linear chro-mophores G0 exhibit a Colh phase, between 87 and 147 °C (G0-OPV2) and between 149.5 and about 200 °C (G0-OPV3). Theincrease of the transition temperatures is obviously connectedto the increasing length of the OPV segment, as already observedin other systems.33,34 Increasing generation is detrimental tomesomorphism for G1-OPV2, while the mesophase of G1-OPV3 (a Colh phase between 74 and 180 °C) is destabilizedwith respect to the zeroth generation homologous compound,G0-OPV3. This can be attributed to the increasing flexibilityand conformational freedom of the dendrimers with respect tothe pendant linear segments; this is in agreement with theabsence of mesophase for G1-OPV2, where the anisotropy ofthe rigid segment is not sufficient to compensate for theconformational disorder. The phase lattice parameters increaseconcomitantly with the increasing length of the chromophoreand with increasing generation (a ) 42 Å, G0-OPV2 f a )54.1 Å, G0-OPV3 f a ) 58.3 Å, G1-OPV3). The mode oforganization in the Colh phase is similar in the three cases,namely, about 5 and 7.5 molecular equivalents of the flat-taperedG0-OPV1 and G0-OPV3, respectively, and about 4 molecular
equivalents of G1-OPV3 in conical-like conformation, aggregateinto supramolecular discs 4.5 Å thick on average, with thephosphonic acid moiety pointing toward the center of the disk;the discs then stack on top of each other to generate the columns,which further self-organize into a hexagonal lattice.
Functionalization of the Iron Oxide Particles
Because the preparation of the oxides did not necessitate theuse of surfactants, the surface functionalization of the ferritewas carried out at room temperature via an efficient and directgrafting procedure. Typically, 100 mg of γ-Fe2O3 NPs weredispersed homogenously by sonication in 50 mL of THF for20 mn, prior to the addition of a defined quantity of the organicmolecules, also dissolved in 50 mL of THF. The mixture wassonicated (20 min) and mechanically stirred for 12 h. The as-prepared maghemite nanoparticles are not stable in suspensionin THF, which is an aprotic polar solvent, but from the firstminutes of grafting and with help of the ultrasonic treatment,the formation of suspensions of grafted NPs is observed. Thehigher the amount of added molecules, the denser the suspen-sion. The mechanical stirring is performed to favor the adsorp-tion kinetics.
The same procedure of NP functionalization was used forall the different chromophores (G0-OPV2, G0-OPV3, G1-OPV2, and G1-OPV3), and for each series, the concentrationof added organic molecules was varied from 5 to 300 mg ofmolecules per gram of NP. The coated particles were isolatedby magnetic decantation or by centrifugation (10000 rpm) andwashed thoroughly with THF. The suspension of graftednanoparticles has been submitted to three successive washingsteps in order to ensure the complete elimination of nongraftedorganic molecules in the suspension; although, after the firstrinsing, no more free molecules could be observed in the mother
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solutions as attested by thin layer chromatography and UVspectroscopy analyses.
The total amount of grafted organic molecules onto theparticles was evaluated by two different but complementarymethods. The first method allowed to quantify indirectly by UVtitration, using the Beer-Lambert law, the amount of nongraftedligands (and therefore what was grafted since the initialconcentration of organic species is known), recovered in themother solutions after successive rinsing procedures; in this case,the UV absorption spectrum of the washing solutions wascompared to the UV absorption curve obtained as a function ofthe chromophores’ concentrations. The second method gavedirectly the quantity of grafted molecules on the nanoparticlesby thermogravimetric measurements because the first decom-position event, observed in the range of 250-300 °C, wasattributed to the degradation of the organic parts only.
Characterization of Grafted Nanoparticles
Microstructural Characterization. Physico-chemical char-acterizations (IR, TEM, SEM, DTA, TGA, XRD) performed
on the particles before and after functionalization showed strongevidence of the successful grafting of the organic moleculesonto the surface of γ-Fe2O3 NPs. SEM micrographs (Figure 3,top, γ-Fe2O3 NP@G0-OPV3, shown as a representative ex-ample) confirmed that the morphology and the size and shapedistributions of the particles were not altered by the organiccoating as previously observed (Figure 1).4,26 TEM observationsrevealed the presence of a homogeneous and amorphous“organic” monolayer all around the particles (Figure 3, middleand bottom; see additional images in the Supporting Informa-tion). This layer, having thickness maxima varying between 3and 5 nm, is in good agreement with the molecular structuresradius of the grafted moieties (G0-OPVp and G1-OPVp); thetight packing of the organic molecules forces their orientationsnormal or quasi-normal to the nanoparticle surface.
IR spectra of naked γ-Fe2O3 NP and NP coated with G0-OPV3, chosen as representative examples, are given in Figure4. IR investigations of all the grafted NPs revealed the presenceof organic moieties. Asymmetric and symmetric C-H stretchingvibrations were visible in the IR spectra: three peaks appeared
SCHEME 2: Preparation of G1-OPV2 and G1-OPV3a
a Reagents and conditions: (a-g) as in Scheme 1; (h) TMSBr, CHCl3; (i) MnO2, CH2Cl2; (j) BBr3, CH2Cl2; (k) K2CO3, acetone; (l) (i) NaOH,H2O, THF/MeOH, (ii) HCl.
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in the 2800-3000 cm-1 region, which correspond to thevibrations νas(CH3) at 2960 cm-1, νas(CH2) at 2920 cm-1, andνas(CH2) at 2850 cm-1, respectively. In the 900-1400 cm-1
region, a large number of weak peaks, corresponding to thevibrations of the phosphonate and of the OPVs’ aromatic groupswere also observed, although, the nature of the coordinationmode of the phosphonate onto the surface particle could not beinterpreted on the basis of these data alone. The latticeabsorption bands in the range 800 and 500 cm-1 are due to theFe-O bonds in a maghemite phase with a partial vacanciesordering.4
Quantitative Aspects of Grafting. The grafting rate of thephosphonated ligands on the NPs surface and its dependenceon the added amount of organic molecules and the moleculearchitecture were evaluated using two different techniques andcompared. The first method consisted in following the weightvariation of functionalized NPs with temperature, directly relatedto the quantity of grafted organic moiety. Thermogravimetricanalyses showed weight loss at around 250-350 °C (Figure 5,top). As no weight variation was observed below 500 °C forthe naked 40 nm sized γ-Fe203 NPs, this weight loss wasevidently attributed to the organic moiety degradation and isdirectly related to the quantity of grafted molecules on the NPssurface, that is, the grafting rate (Figure 5, top). In all cases,the weight loss increases with the quantity of added molecules,and the curves (Figure 5, bottom) rapidly reach a plateau,indicating a saturation threshold of grafted organic moleculeson the NPs surface. The weight loss maxima are 4, 5.3, 5.4,and 6.8% for NP@G1-OPV2, NP@G1-OPV3, NP@G0-OPV2,and NP@G0-OPV3, respectively (Figure 5, bottom), corre-sponding to a grafting rate of 40 mg for G1-OPV2, 53 mg forG1-OPV3, 54 mg for G0-OPV2, and 68 mg for G0-OPV3 pergram of NP (Table 2). A rough estimation of the moleculardensity at the surface gives 1.5 and 1.3 molecule ·nm-2 for bothG0 derivatives and 0.46 and 0.53 molecules ·nm-2 for thedendritic G1 derivatives. With the hypothesis that the molecular
surface area is governed by the alkyl chain cross-section areaof the ligand projected normal to the ferrite surface (ca. 22-24Å2 per chain), a complete coverage of the NP surface is obtainedwith the G0 derivatives in agreement with earlier results,4 whileonly 67 and 88% of the NP surface is covered with the dendriticmolecules G1-OPV2 and G1-OPV3, respectively. Thus, thegrafting with linear structures (or slightly conical) seems moreefficient than that with dendritic ones.
The grafting rates can concomitantly be determined by UVspectroscopy. All the Gi-OPVp chromophores present anabsorption band in the UV-vis domain that can be utilized toestimate the quantity of nongrafted molecules in the washingsolutions using the Beer-Lambert law. The difference betweenthe initial added quantity of organic molecules and the quantityrecuperated after functionalization (nongrafted molecules) in thewashing solutions gives the quantity of adsorbed molecules onthe NPs surface. In Figure 6 is represented the isothermabsorption for the different molecules. However, this methodol-ogy is less reliable at high concentration due to possibleintermolecular interactions that may occur and disturb the UVband intensity. Nevertheless, it is possible to give a firstestimation of the grafting rate by determining the saturationthreshold at the surface: such a value corresponds to theapparition of free molecules in the medium. The extrapolationline, shown in Figure 6, gives the grafting rate (Table 2)corresponding to 1.4 and 1.5 molecules ·nm-2 for G0-OPV2 andG0-OPV3 and 0.3-0.4 molecules ·nm-2 for the dendrons (i.e.,60% of NPs surface).
A fairly good agreement is obtained by the two methods forthe evaluation of the grafting rate values of G0-OPVp, themeasurements of the dendritic systems being seemingly lessreliable that those of the linear systems (Table 2). Anyway, thearchitecture of the organic molecules influences this rate andleads to an a priori complete surface coverage for the linearstructure (high grafting density), while this coverage is onlypartial for the dendritic structures. This is likely due to steric
TABLE 1: Mesomorphic Properties of the Phosphonated Dendritic Ligands (i ) 0, 1; 2; p ) 2, 3)
Gi-OPVp mesomorphisma dexp/Åb Ic [hk]d dtheo/Åb,e parametersb,e
G0-OPV2 G 56 (-28.7) Cr 87 (35.0) 36.35 VS (sh) 10 36.35 T ) 120 °CColh 147 (0.7) I 4.5 VS (br) hch a ) 42 Å
S ) 1525 Å2
Tdec 245 °C Colh-p6mmN ) 5
G0-OPV3 Cr1 101.0 (13.7) Cr2 46.85 VS (sh) 10 46.85 T ) 160 °C149.5(32.0)Colh200.0 (5.5) I 27.05 W (sh) 11 27.05 a ) 54.1 Å
4.5 VS (br) hch S ) 2535 Å2
Tdec 270 °C Colh-p6mmN ) 7.5
G1-OPV2 G 41.5 (0.5) MX 175 (-) I 40.0 VS (br) hmol T ) 80-160 °C4.5 VS (br) hch amorphous
Tdec 270 °CG1-OPV3 G 73.7 (0.4) Colh 180 (-) I 50.5 VS (sh) 10 50.5 T ) 120 °C
4.5 VS (br) hch a ) 58.3 ÅTdec 260 °C S ) 2945 Å2
Colh-p6mmN ) 4
a Abbreviations: Colh ) hexagonal columnar phase; Cri: crystalline phases; G: glassy phase; MX: amorphous solid; I: isotropic liquid.Temperatures are in °C. Values in parentheses correspond to enthalpy of transition (∆H in kJ ·mol-1) or glass transition (∆Cp inkJ · °C-1 ·mol-1); in some cases, this value could not be measured (-). Transitions are given from the second heat. b dexp and dtheo are theexperimentally measured and theoretically calculated diffraction spacings (see table footnote e). The distances are given in Å. c Intensity of thereflections: VS: very strong, W: weak; br and sh stand for broad and sharp reflections, respectively. d [hk] are the Miller indices of the re-flections; hch stands for the diffuse scattering corresponding to the molten alkyl chains, hmol is a shortly correlated distance corresponding to themolecular length. e dtheo and the mesophases parameters a and S are deduced from the following mathematical expression: the lattice parameter,a ) 2[Σhkdhk · (h2 + k2 + hk)1/2]/�3Nhk, where Nhk is the number of hk reflections, and the lattice area (i.e., columnar cross-section) S ) a231/2/2.For the columnar phases, it is convenient to define N, an equivalent molecular number (i.e., aggregation number) per slice, as N ) h ·S/Vmol;Vmol is the molecular volume estimated considering a density close to 1 by MW/0.6022; h is the intracolumnar repeating distance.
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constraints and, consequently, to the oxide surface accessibilityand packing density. Whereas the G0 derivatives, which mainlyexist in one single conical-like conformation, can readily reachthe surface of the oxides and pack efficiently, this is less likelyfor the G1 dendrimers, more bulky and existing in severalnonsuitable conformations.
Properties of the Grafted Nanoparticles
Thermal Stability. The thermal stabilities of the variousrecovered powders (naked and coated γ-Fe2O3 NPs) and ofmolecules were analyzed by TDA/TGA, which showed supple-mentary evidence for the presence of organic molecules and oftheir strong interaction with the γ-Fe2O3 NPs surface. Beforefunctionalization, the TGA thermogram of the naked γ-Fe2O3
NPs (Figure 7, top, trace a), except a small weight loss due todehydration below 150 °C, did not record any weight variation
Figure 4. IR spectra of the 40 nm sized γ-Fe2O3 NPs naked (bottom)and functionalized with G0-OPV3 (top). Inset zoom in the regionbetween 3000 and 2800 cm-1.
Figure 3. SEM micrograph of the 39 nm sized γ-Fe2O3 NP@G0-OPV3 (top). TEM micrographs showing amorphous and homogeneouslayers for the 40 nm γ-Fe2O3 NPs functionalized (middle) with OPV3-G0 and (bottom) with G1-OPV3 molecules.
Figure 5. Loss weight variations for the functionalized particlesdetermined by TGA as a function of temperature (300 mg of ligandper 1 g of NP (top) and as a function of added molecules (bottom).
TABLE 2: Maximum Quantities of Grafted Chromophoreson the Particles and Molecular Density Determined by UVand TGA
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from 150 to 800 °C, while the TDA curve (Figure 7, bottom,trace a) presented a exothermic transition at 500 °C correspond-ing to the maghemite (γ-Fe2O3) to hematite (R-Fe2O3) phasetransformation.26,27,35,36
The TGA curve of the G0-OPV3 molecules (taken as arepresentative example) shows that their thermal decomposition
occurs in the temperature range 250-750 °C and in two stages:a large relative weight loss of about 55% between 250 to 300°C and a regular weight loss from 300 up to 750 °C (inset Figure7, top). The first weight loss may be attributed to the thermalpyrolysis of the alkyl chains (first plateau), as the contributionin wt % of alkyl chains in the whole molecule weight is about55.5%. The other features in the second stage (plateaus at ca.450 and 550 °C) may be attributed to the thermal decompositionof OPV units and of the phosphonate group. Moreover, theweight loss observed over a large range of temperature may bedue to the formation of molecular intermediates (phosphonatecompounds, etc.) and to different interactions between alkylchains or OPV units, leading to a distribution of thermaldecomposition temperatures.
The TGA curve of the γ-Fe2O3 NPs coated with G0-OPV3(taken as a representative example, Figure 7, top, trace b)exhibited a weight loss between 250-350 °C. This weight losscan clearly be attributed to the thermal decomposition of thegrafted organic species. Moreover, the percentage of weight losswas found to depend on the concentration and architecture ofadded organic molecules during the functionalization process(Figures 5 (top) and 8). The comparison of TGA curves of freemolecules and grafted molecules shows that the thermaldecomposition of molecules is modified when they coat nano-particles: their decomposition occurs at lower temperature. Asshown by the DTA curves of NPs coated with G0-OPV3, thedegradation of the organic moiety is exothermic with only twopeaks (Figure 7, bottom, trace b).
For all designed OPV derivatives, the organic decompositionof grafted NPs occurs according to two exothermic processes
Figure 6. UV spectroscopy titration curves for the determination ofthe grafting rate.
Figure 7. TGA (top) and TDA (bottom) curves recorded at 5 °C ·min-1
under air of (a) the 40 nm naked γ-Fe2O3 NPs and (b) the 40 nmγ-Fe2O3 NPs after functionalization with G0-OPV3 molecules at a initialadded concentration of 300 mg ·g-1 of NPs (inset: TGA curve of G0-OPV3 molecules).
Figure 8. TDA curves of functionalized NPs as a function (top) ofadded organic moieties (G0-OPV3) and (bottom) as a function of themolecular architecture (300 mg per g of NP).
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between 250-350 °C (Table 2). A two-step decomposition ofthe organic moiety has previously been reported in otherfunctionalized NPs systems from TGA derivatives and wasexplained by the presence of an organic molecular bilayer onthe particle surfaces or to two types of bonding on the NPsurface.37 In our system, however, TEM micrographs andquantitative analyses of grafting rates demonstrated that onemonolayer has been grafted. Moreover, the two peaks have alsobeen observed for NPs decorated with dendritic molecules (G1-OPVp), which do not possess the appropriate architectures toform bilayers. This two-steps degradation event already takesplace with NPs functionalized at low concentrations of addedmolecules, thus with a partial coverage of the surface. Finally,the first exothermic event observed during heat treatment ofungrafted molecules was attributed to the thermal decompositionof alkyl chains. This is consistent with results of Lee et al.38 onthermal studies on iron-oxy oleate precursor, which assignedan exothermic peak at 260 °C to the thermal decomposition ofoleate ligands.
XPS analyses demonstrate that phosphorus stays always atthe surface of nanoparticles after the thermal treatments sug-gesting that the thermal decomposition of phosphonate groupsdoes not occur when phosphonate groups are grafted. Thesefacts demonstrate that the two exothermic peaks can not beattributed to two kinds of bonding or to the presence of amolecular bilayer.
Moreover, the temperature of the maghemite-hematite phasetransformation, at around 500 °C for naked nanoparticles, isshifted to higher temperature. This shifting has already beenobserved and attributed to the phosphonate-based coat, partiallydegraded but still present on NPs surface, which protects thesurface from oxidation and thus delays the phase transforma-tion.26 Nevertheless, depending on the grafted molecules, asecond weak peak may be observed in this temperature range.In Table S1 are reported the different temperatures of degrada-tion, recorded by TDA, for functionalized NPs with G0-OPV2,G0-OPV3, G1-OPV2, and G1-OPV3 (NP@G0-OPV2, NP@G0-OPV3, NP@G1-OPV2, and NP@G1-OPV3, respectively) andas a function of initial added molecules concentration. TD1 andTD2 represent, respectively, the decomposition temperatures ofthe organic molecules corresponding to the first and secondpeaks, TT1, the temperature of the maghemite-hematite trans-formation, and TD3, a temperature that we attributed to the finaldecomposition of the molecules. Indeed, the peak correspondingto TD3 is observed only with linear dendrons (G0) for whomthe grafting rate is the highest, and such peak has never beenobserved during studies of the thermal stability of phosphatedmagnetite nanoparticles.39
The use of a same batch of γ-Fe2O3 NPs for functionalizationallows comparing the thermal stability of grafted NPs as afunction of organic molecules nature and concentration. For allOPV-coated derivatives, the temperature dependence of TD1, TD2,and TT1 (and TD3 for G0 molecules) with added moleculeconcentration followed the same tendency: they first rapidlyincreased at low organic contents, followed by a suddentemperature stabilization from an added amount of moleculesof 80-100 mg g-1 NP. Indeed, as observed in Figure 5, fromthis amount, a saturation threshold of grafted molecules isreached. This fact confirms that the observed shifts are relatedto the grafting rate.
In Figure 8 (top) are represented TDA curves of NPsfunctionalized at different G0-OPV3 concentrations with cor-responding TGA curves (inset Figure 8, top). It appears clearthat the shift of the decomposition peaks TD1 and TD2 and the
increase of the loss weight (e.g., grafting rate) are related,indicating, consequently, an organic decomposition dependenceon molecule packing densities at NP surfaces. Indeed, whenthe amount of organic molecules grafted at the surface of NPincreases, the probability of van der Waals interactions betweenalkyl chains of molecules due to self-assembling increases.These interactions induce a shift of decomposition temperaturetoward higher values.37 That is consistent with results of Lee etal.38 and Shen et al.37b who observed an increase of thedecomposition temperature with the alkyl chain length of graftedmolecules. Similarly, Kataby et al.37a observed a shift to highertemperature of the decomposition temperatures of octadecanethiol at the surface of Fe2O3 nanoparticle when the concentrationin octadecane thiol increases.
The comparison of the organic moiety stability on NPs surfacefor different Gi-OPVp derivatives (Figure 8, bottom) confirmsthe strong dependence on the molecular architectures. As shownby the value of the TD1 temperatures (272, 286, 289, and 295°C for NPs coated with G1-OPV2, G1-OPV3, G0-OPV2, andG0-OPV3, respectively), layers made of G1 systems are lessdense than those with G0 and then the interactions between alkylchains are weaker inducing a lower shift. An additionalphenylene vinylene unit in the luminescent functional chain(OPV2 f OPV3) increases the degradation temperature. Theprobability of π-stacking between adjacent chromophoresincreases with the number of OPV units. Such interactionsshould favor the packing of molecules and therefore the vander Waals interactions between alkyl chains.
The TD2 temperatures (306, 325, 323, and 342 °C forNP@G1-OPV2, NP@G1-OPV3, NP@G0-OPV2, and NP@G0-OPV3, respectively) are also related to the molecular architec-ture, with a displacement of about 20 °C from G1 to G0architecture and from OPV2 to OPV3 molecules. Moreover,the relative intensity of TD2 peak is greater than TD1 peak forOPV3 molecules, indicating a correlation between TD2 and theOPV unit number in the functional chain. However, the organicmoiety decomposition in two steps has also been reported forγ-Fe2O3 NPs coated with straight alkylphosphonate andalkylcarboxylate37d chains covalently linked, suggesting that theorigin of the TD2 peak is likely due to intermolecular organizationon the surface rather than the presence of OPV units themselves.In this system, the presence of [OPV] groups could favor theassembly of OPV derivatives via anisotropic interactions. InFigure 8 (top) for NPs coated with G0-OPV3, one can alsonotice that for lower added concentration, the TD2 intensity islower than that of TD1 and an inversion operates for highconcentrations. This is in good agreement with the promotionof molecules self-assembly with the increase of their density atNPs surface.
The TD3 peak, which increases with the grafting rate, issupposed to be related also to the final decomposition of themolecules. One may also suggest that this decomposition maybe promoted by the phase transformation and should concernmainly the phosphonate residues.
In conclusion, the first decomposition stage is attributed tothe decomposition of alkyl chains. Their decomposition tem-perature increases with the grafting rate due to their self-assembling at the surface of NP (increase in the van der Waalsinteractions between alkyl chains with the grafting rate). TheG1 architecture does not favor van der Waals interactionsbetween alkyl chains, but an increase in the number of OPVunits favors the packing of molecules through π-stacking.Therefore, the probability of van der Waals interactionsincreases. The second stage of the decomposition would be
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attributed to the decomposition of OPV units, and similarly,the higher the probability of π-stacking, the higher the decom-position temperature. The lower temperatures observed withdendritic molecules is thus related to a lower facility of self-assembling for these molecules.
Concerning TT1, the temperature of the maghemite-hematitephase transformation, an average value of 505 °C is recordedfor the NPs capped by the dendrons and 525 °C for the NPscoated by G0-OPVp. As the shift of this temperature is due tothe phosphonate coating, the higher shift with linear moleculesthan with dendritic molecules is related to the higher graftingrate with the former molecules.
Magnetic Properties. The hysteresis loop of the naked 40nm-sized γ-Fe203 NPs (before functionalization) indicates aroom temperature ferrimagnetic behavior, as expected, with acoercive field (Hc) of 180 Oe and a saturation magnetization(Ms) of 67 emu/g (Figure 9), very close to that of bulkmaghemite (72 emu/g). After functionalization, the hysteresiscurves of the NPs decorated by G0-OPV3 at various quantitiesof added molecules (Figure 9) present also the same features,an opened hysteresis cycle and magnetization at saturation; themagnetic values recorded, Hc and Ms, are almost the same asfor the naked γ-Fe203 NPs. The slight difference in the saturationmagnetization, Ms, is not that relevant and may be simplyattributed to the incomplete extraction of the organic part inthe total weight calculation. The measurements performed forthe other samples (NPs decorated by G0-OPV2, G1-OPV2, andG1-OPV2) demonstrated the same magnetic behavior (ferri-magnetic), and no strong effect of the grafting was observedon the value of Hc and Ms. Thus, the intrinsic ferrimagneticbehavior of the 40 nm-sized γ-Fe203 NPs is conserved afterdirect (covalent) functionalization, and the grafting of organicmolecules does not affect their magnetic properties, as alreadyobserved with magnetite-based nanoparticles.4
Luminescent and Mesomorphic Properties. The normalizedemission spectra of the four used OPV derivatives are shownin Figure 10 (top). The G0-OPV2 and G1-OPV2 ligandsemission spectra were recorded at 330 nm wavelength excitationand those of G0-OPV3 and G1-OPV3 at 370 nm, correspondingto the maximum absorption of the chromophores, OPV2 andOPV3, respectively. As expected, supplementary OPV fragmentsinsertion into the molecule main functional chains fromOPV2fOPV3, leads to a shift of the emission bands by about45-48 nm toward lower energies (red shift). Moreover, a slightblue shift of 12 and 15 nm was observed from G0 to G1 systems(comparing those bearing the same chromophore OPV2 orOPV3). This additional effect of molecules architecture is
attributed to the phosphonic acid function localization. For theG0 systems, the coupling agent is directly linked to the OPVchains and thus contributes to the supplementary electrondelocalization along the chromophore, inducing a shift of theemission spectra to higher wavelengths. This is not the casefor the G1 systems, where the chromophore arms are decoupledfrom the phosphonic acid.
For the functionalized NP, before performing luminescentanalysis, the solution was substantially washed several times(THF) and the recuperated supernatants (centrifugation for 10mn at 14000 round.mn-1) were analyzed by UV to ensure thatno free molecules were present in solution, a crucial point forasserting that the observed luminescence is a property of thesole functionalized NP. The NP@Gi-OPV2 and the NP@Gi-OPV3 were respectively excited at 330 and 370 nm. For all of
Figure 9. Magnetic moment vs field at room temperature for nakedand capped (G0-OPV3) γ-Fe2O3 nanoparticles.
Figure 10. Emission spectra of the free chromophores (top) and ofthe particles functionalized by G0-OPVp (middle) and by G1-OPVp(bottom).
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them, the emission spectra exhibited bands at 392 nm, 400 nm,450 and 445 nm, demonstrating luminescent properties for thesedecorated γ-Fe203 NPs; the normalized recorded luminescentspectra of functionalized NPs and of their corresponding usedmolecules are superposed as shown in Figure 10 (middle andbottom). For the NPs functionalized by the linear ligands G0-OPV2 and G0-OPV3, a slight displacement of the emissionbands (with respect to the free ligands) toward highest energies(blue shift) was observed (Figure 10, middle). However fordendrons G1, this behavior did not occur, with the maximumof the emission occurring at the same values as for the freeG1-OPVp, showing that the grafting has no direct influence onthe luminescent properties for chromophores G1 (Figure 10,bottom). These blue shifts, of 20 and 10 nm, respectively, forG0-OPV2- and G0-OPV3 are in the same range than the redshift attributed previously to the phosphonic acid localization,suggesting that this displacement is due to the lost of couplingagent contribution for electron delocalization on OPV chain ashenceforward the phosphonate function serves to anchor themolecule on NPs surface.
Fluorescence microscopy gives a direct evidence of theluminescent properties of functionalized NPs. The image showedluminescent aggregates with an average diameter of about 300nm for NPs functionalized with G0-OPV3 (Figure 11) and G1-OPV3 (Supporting Information). Thus, the grafting of varioustypes of OPV dendrimeric derivatives equipped with a phos-phonic acid as coupling agent preserves their luminescentproperties even after their anchoring onto γ-Fe203 NPs surface.
Finally, despite the grafting of prodendritic mesogenic ligands,all almost exhibiting a Colh mesophase, none of the dendronizedparticles exhibit mesomorphic properties, likely due to the size/dimension incompatibility and densities between both theorganic and the inorganic parts.
Conclusions
The design, synthesis, and some physicochemical propertiesof novel organic-inorganic hybrid dendronized nanoparticlespresenting magnetic and luminescent functionalities have beendescribed. In particular, we focused on the covalent function-alization of 40 nm γ-Fe2O3 nanocrystals by engineered lumi-nescent oligo(phenylene vinylene)-based prodendritic ligands.The magnetic properties were not altered upon functionalizationas all dendronized nanoparticles exhibit room temperature
ferrimagnetic behavior. Moreover, the luminescent propertiesof the chromophores were still observed after grafting and werefurther tunable by the structure of the luminophore itself. Thegrafting rate of the various ligands on the oxide surface andtheir thermal stability and behavior have been studied in detailand were found to depend on the structure of the organic ligands.
Acknowledgment. The authors thank the CNRS, the Uni-versity de Strasbourg, the ANR DENDRIMAT, and the Thaigovernment for funding, Mr. D. Burger (TDA, TGA), Mr. C.Leuvrey (SEM), and Mr. A. Derory (SQUID) for their technicalexpertise, and Dr. M. Schmutz for some low-dose TEMexperiments (Institut Charles Sadron, Strasbourg).
Supporting Information Available: Information on theexperimental methods, the synthesis of the ligands and inter-mediates and their analytical characterizations, and additionalinformation (Table, TEM and fluorescence microscopy images).This material is available free of charge via the Internet at http://pubs.acs.org.
References and Notes
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Figure 11. Image of aggregates of functionalized NP (NP@G0-OPV3)observed by fluorescence microscopy.
Properties of Dendronized Ferrite Nanoparticles J. Phys. Chem. C, Vol. 113, No. 28, 2009 12211
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