Thermal, Magnetic, and Luminescent Properties of Dendronized

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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-Fe2O3 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 nanomaterials, 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 * 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.

adequate organic moiety linked onto the core surface. The later consists of smart, protective, structuring molecules giving supplementary functionalities (e.g., chemically active molecules,10 polymers,11 dendrimers,12 or biomolecules13), leading to the formation of intricate hybrid organic-inorganic particles. Moreover, such core-shell nano-objects coated by a hydrophobic layer (usually alkyl groups are used to passivate the nanocrystal surface) can be further self-assembled into supracrystals (by slow solvent evaporation14) or into binary nanoparticles superlattices (BNPS, by controlling the size of the different NPs15) 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-organization, in a reliable manner, either in the bulk (1D, 2D, 3D lowdimensionality 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 supracrystals14) that will influence the future nanostructured materials’ properties. The search for other functions of such types of nanostructures in biology and medical technologies9,13,17 is also an important trend. Magnetic particles18 are particularly promising in this respect, with numerous applications in catalysis,19 magnetic fluids,20 data storage18 and transport, contrast agents for MRI,21 and so on. In this work, we describe the synthesis of novel organicinorganic hybrid dendronized nanoparticles presenting magnetic and luminescent functionalities (Chart 1). In particular, this study is focused on the controlled covalent functionalization of γ-Fe2O3 magnetic nanoparticles, with an average diameter of 39 nm and with acicular shape, by engineered luminescent,

10.1021/jp902046d CCC: $40.75  2009 American Chemical Society Published on Web 06/11/2009

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CHART 1: Schematic Representation of the Dendronized Maghemite Nanoparticles and Chemical Structure (and nomenclature) of the Organic Moieties Gi-OPVp

mesomorphic oligo(phenylene vinylene)-based dendrimers, and on the characterization of the magnetic, luminescent, and thermal properties of these hybrid nanostructures. Maghemite (γ-Fe2O3) NPs were preferentially used to avoid magnetite (Fe3O4) stoichiometric instability (the oxidation of the surface can disturb the physicochemical characterization after functionalization). These 39 nm NPs present room temperature ferrimagnetic behavior. In this perspective, the organic molecules, premises of dendritic architectures as ideal candidates to protect the particles,12 were designed to be soluble in organic medium, to possess the ability to coordinate strongly the oxide surface, to exhibit luminescent properties, and to be mesogenic to stimulate their tendency to self-organize into predictable ordered and periodic 2D and 3D arrays. Oligo(phenylene vinylenes) (OPV)pbased oligomers22 were selected as the functional groups to be combined with the oxides for their interesting electrical, optical (absorption/emission, nonlinear optics, luminescence), and optoelectronic properties,23,24 chemical versatility, and also as a probe to quantify the degree of the particle coverage by UV spectroscopy. Two types of organic architectures were considered (Chart 1): linear oligo(phenylene vinylenes) (OPV)p, referred thereafter to as G0-OPVp, and pro-dendritic oligomers with a tapered shape (referred thereafter to as G1-OPVp). These chromophores were equipped at the focal point by a phosphonic acid function to bind firmly to the surface of the oxide particles and at the other extremity by three dodecyloxy fragments to motivate the self-organization of the hybrids into liquid crystalline mesophases, to enhance their solubility in a wide range of organic solvents, and improve their stability by reducing interparticle aggregation and surface’s oxidation. Recent studies have demonstrated that the use of the phosphonic acid which presents stronger affinity to oxide surfaces25 is more convenient than the commonly used carboxylic acid function which exhibits weaker interactions.4,26

Figure 1. TEM (top) and SEM (bottom) micrographs of the 39 nmsized γ-Fe2O3 NPs obtained after air oxidation at 300 °C.

Synthesis and Characterization of the Ferrite Moiety The synthesis of magnetite nanoparticles with an average size of 39 nm has recently been reported.27 First, the 10-12 nm sized Fe3-δO4 nanoparticles were produced by coprecipitation of ferrous Fe2+ and ferric Fe3+ ions at 70 °C by rapid addition of tetramethylhydroxylamine (N(CH3)4OH) at constant pH (pH ) 10) under argon. Then, in the second step, the solution containing small nanoparticles was introduced in an autoclave under argon, sealed, and heated for 24 h at 250 °C; this hydrothermal growth process yielded the magnetite Fe2.95O4 nanoparticles with an average size of 39 ( 5 nm.27 The asformed NPs were separated by magnetic decantation, followed by the removal of supernatant, washed thoroughly with deionized water, and dried by lyophilization. Finally, maghemite γ-Fe2O3 NPs were obtained by air oxidation of the magnetite NPs at 300 °C for 24 h. The morphology of the obtained NPs was characterized by transmission electron microscopy (TEM, Figure 1, top) and scanning electron microscopy (SEM, Figure 1, bottom). Electronic micrographs and BET measurements confirm that the oxidation step has not altered the size distribution of acicular particles, with an average diameter of 39 ( 5 nm. The XRD patterns of the Fe2.95O4 nanoparticles display the X-ray peaks characteristic of a ferrite spinel (Figure 2, top) and confirmed the absence of hematite and iron hydroxides. After oxidation of the magnetite, the observation of the X-ray peaks

Properties of Dendronized Ferrite Nanoparticles

Figure 2. Top: XRD pattern of the 39 ( 5 nm maghemite particles (a) and after oxidation of magnetite nanoparticles (b). Bottom: Infrared spectra of as-prepared magnetite NPs and of the maghemite nanoparticles after the oxidation step at 300 °C in the Fe-O wavenumber range.

(210) and (211) confirms the formation of the maghemite as well as the lattice parameter, which corresponds to that of maghemite (0.8346 nm, JCPDS file 39-1346). Another way to prove the formation of the maghemite phase can be achieved by IR spectroscopy, as both magnetite and maghemite can be easily differentiated on symmetry grounds. Wavenumbers assigned to spinel structure are in the 800-400 cm-1 range. The IR spectra of stoichiometric magnetite display one peak at around 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, the larger the number of lattice absorption bands between 800 and 200 cm-1 (Figure 2, bottom). The Fe-O bands of the oxidized nanoparticles are characteristic of a partial disordered spinel maghemite structure.4 Synthesis and Thermal Behavior of the Organic Moiety In general, the most efficient and versatile methods for the synthesis of OPV-based compounds involve palladium-basedcatalyzed Heck coupling reactions between styrene and halobenzene basic building modules. We specifically elaborated a strategy based on such a cross-coupling reaction to make use of multipotent building blocks to avoid synthetic redundancy and to limit the number of steps (i.e., molecular modules that

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12203 can be employed repeatedly at various stages of the synthesis). This approach was moreover facilitated due to the ready availability of most of the starting materials. The reaction schemes are shown below (Schemes 1 and 2). All the intermediates were characterized by 1H NMR spectroscopy only, and the final products by 1H and 13C NMR and MALDI-TOF spectroscopies and elemental analysis. Details are given in the Supporting Information. The preparation of the linear OPV2 and OPV3 phosphonic diethyl esters (3, 10) and corresponding phosphonic acid (4, 11) derivatives were obtained by a convergent synthetic route in three and six steps (Scheme 1), respectively, starting from 3,4,5-tridodecyloxystyrene, 1, one of the key compounds of our procedure, prepared as previously described in four steps from the commercially available methyl 3,4,5-trihydroxybenzoate.28 This compound was coupled to iodo-4-bromobenzene, according to the Heck cross-coupling reaction using Pd(OAc)2 as catalyst, to afford the bromostilbene module 2 as the major product. The diethyl arylphosphonate 3 was prepared from the bromide precursory derivative 2 by a convenient palladium(0)-catalyzed phosphonation reaction with diethyl phosphite in triethylamine.29 The resulting diethyl ester was then treated by trimethylsilyl bromide (TMSBr) followed by hydrolysis in methanol to generate the desired phosphonic acid (4, G0-OPV2) in good yields. The preparation of G0-OPV3 (11) involved the transformation of the oligophenylenevinylene (OPV) hydroxyl derivative (8) into the trifluomethanesulphonate ester (9), a better leaving group and easier to prepare than the homologous bromide precursory.30 The module 2 was therefore reacted with the silyl-protected p-hydroxyl styrene (6) and, after deprotection of the hydroxy group by TBAF (7), afforded the alcoholic OPV derivative, 8, which was first transformed into the sulfonic derivative (9), and subsequently into the phosphonate diethyl ester (10), using the same reaction conditions as for 3. Finally, ester 10 was treated by TMSBr followed by hydrolysis in methanol to yield the phosphonic acid 11 (G0-OPV3). The preparation of the branched oligomers 21-24 followed a similar convergent synthetic route, also based on repeated palladium-catalyzed Heck cross-coupling reactions (Scheme 2). The modular brick 12 was prepared directly from the crosscoupling of 3,4,5-tridodecyloxystyrene (1) and 4-bromobenzylic alcohol. Then, 12 can either be converted into the methylbromide stilbene derivative (13) by direct bromination or following a succession of standard reactions (12 f 14 f 15 f 16 f 17) into the corresponding methylbromide OPV3 analogous compound (17). The two OPV derivatives were then coupled to 3,5dihydroxybromobenzene (18) to afford the bromide-precursory prodendritic systems (19, 20: X ) Br). The prodendritic arylphosphonic diethyl ester homologues (21 and 22: X ) PO(OEt)2) were prepared by the Pd(0)-catalyzed phosphonation reaction with diethyl phosphite in triethylamine, as above for 3 and 10. Hydrolysis in basic and acidic conditions provided the desired G1 branched phosphonic acids (23 and 24: X ) PO(OH)2); TMSBr was not used here as cleaving all the methyl ether junctions to yield the precursory arms 13 and 17, respectively. As expected, on the basis of their molecular structure,31 the phosphonic chromophores exhibited mesomorphic properties as deduced jointly by POM, DSC, and XRD methods; only G1OPV2 was devoid of mesomorphism and was obtained as a room temperature amorphous solid (Table 1). For the G0 mesomorphic samples, homogeneous, highly birefringent optical textures, exhibiting developable domains alongside large homeotropic zones in some cases could be observed, suggesting

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SCHEME 1: Preparation of G0-OPV2 and G0-OPV3a

a Reagents and conditions: (a) Pd(OAc)2, ToP, NEt3, 80°C; (b) (EtO)2P(OH), Pd(PPh3)4, C6H6/NEt3(12/1,5); (c) (i) (CH3)3SiBr, CH2Cl2, (ii) MeOH; (d) Ph3PCH3Br, t-BuOK, THF; (e) Pd(OAc)2, ToP, xylene, NEt3, 110 °C; (f) TBAF, THF; (g) (CF3SO2)2O, CH2Cl2, pyridine.

columnar-type of mesophases (in agreement with their polycatenar structure, because a smectic-like arrangement, the other possibility, is excluded on the basis of noncompensation of the molecular areas).32 All the mesophases can be considered as disordered, with the presence in the wide angle region of the X-ray diffraction (XRD) patterns of only one broad and diffuse signal, centered at around 4.5 Å, distance reflecting the liquidlike state of the molten alkyl chains. In the low angle region, two sharp and intense signals with the spacing ratio 1:3 were observed for G0-OPV3, reflecting the 2D hexagonal arrangement of the columns; for G0-OPV2 and G1-OPV3, only one sharp and intense reflection was observed, whereas for G1OPV2, broad reflections were observed instead. Thus, from these experiments it can be concluded that the two linear chromophores G0 exhibit a Colh phase, between 87 and 147 °C (G0OPV2) and between 149.5 and about 200 °C (G0-OPV3). The increase of the transition temperatures is obviously connected to the increasing length of the OPV segment, as already observed in other systems.33,34 Increasing generation is detrimental to mesomorphism for G1-OPV2, while the mesophase of G1OPV3 (a Colh phase between 74 and 180 °C) is destabilized with respect to the zeroth generation homologous compound, G0-OPV3. This can be attributed to the increasing flexibility and conformational freedom of the dendrimers with respect to the pendant linear segments; this is in agreement with the absence of mesophase for G1-OPV2, where the anisotropy of the rigid segment is not sufficient to compensate for the conformational disorder. The phase lattice parameters increase concomitantly with the increasing length of the chromophore and with increasing generation (a ) 42 Å, G0-OPV2 f a ) 54.1 Å, G0-OPV3 f a ) 58.3 Å, G1-OPV3). The mode of organization in the Colh phase is similar in the three cases, namely, about 5 and 7.5 molecular equivalents of the flat-tapered G0-OPV1 and G0-OPV3, respectively, and about 4 molecular

equivalents of G1-OPV3 in conical-like conformation, aggregate into supramolecular discs 4.5 Å thick on average, with the phosphonic 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 the use of surfactants, the surface functionalization of the ferrite was carried out at room temperature via an efficient and direct grafting procedure. Typically, 100 mg of γ-Fe2O3 NPs were dispersed homogenously by sonication in 50 mL of THF for 20 mn, prior to the addition of a defined quantity of the organic molecules, also dissolved in 50 mL of THF. The mixture was sonicated (20 min) and mechanically stirred for 12 h. The asprepared maghemite nanoparticles are not stable in suspension in THF, which is an aprotic polar solvent, but from the first minutes of grafting and with help of the ultrasonic treatment, the formation of suspensions of grafted NPs is observed. The higher the amount of added molecules, the denser the suspension. The mechanical stirring is performed to favor the adsorption kinetics. The same procedure of NP functionalization was used for all the different chromophores (G0-OPV2, G0-OPV3, G1OPV2, and G1-OPV3), and for each series, the concentration of added organic molecules was varied from 5 to 300 mg of molecules per gram of NP. The coated particles were isolated by magnetic decantation or by centrifugation (10000 rpm) and washed thoroughly with THF. The suspension of grafted nanoparticles has been submitted to three successive washing steps in order to ensure the complete elimination of nongrafted organic molecules in the suspension; although, after the first rinsing, no more free molecules could be observed in the mother


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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.

solutions as attested by thin layer chromatography and UV spectroscopy analyses. The total amount of grafted organic molecules onto the particles was evaluated by two different but complementary methods. The first method allowed to quantify indirectly by UV titration, using the Beer-Lambert law, the amount of nongrafted ligands (and therefore what was grafted since the initial concentration of organic species is known), recovered in the mother solutions after successive rinsing procedures; in this case, the UV absorption spectrum of the washing solutions was compared to the UV absorption curve obtained as a function of the chromophores’ concentrations. The second method gave directly the quantity of grafted molecules on the nanoparticles by thermogravimetric measurements because the first decomposition event, observed in the range of 250-300 °C, was attributed to the degradation of the organic parts only. Characterization of Grafted Nanoparticles Microstructural Characterization. Physico-chemical characterizations (IR, TEM, SEM, DTA, TGA, XRD) performed

on the particles before and after functionalization showed strong evidence of the successful grafting of the organic molecules onto the surface of γ-Fe2O3 NPs. SEM micrographs (Figure 3, top, γ-Fe2O3 NP@G0-OPV3, shown as a representative example) confirmed that the morphology and the size and shape distributions of the particles were not altered by the organic coating as previously observed (Figure 1).4,26 TEM observations revealed the presence of a homogeneous and amorphous “organic” monolayer all around the particles (Figure 3, middle and bottom; see additional images in the Supporting Information). This layer, having thickness maxima varying between 3 and 5 nm, is in good agreement with the molecular structures radius of the grafted moieties (G0-OPVp and G1-OPVp); the tight packing of the organic molecules forces their orientations normal or quasi-normal to the nanoparticle surface. IR spectra of naked γ-Fe2O3 NP and NP coated with G0OPV3, chosen as representative examples, are given in Figure 4. IR investigations of all the grafted NPs revealed the presence of organic moieties. Asymmetric and symmetric C-H stretching vibrations were visible in the IR spectra: three peaks appeared

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TABLE 1: Mesomorphic Properties of the Phosphonated Dendritic Ligands (i ) 0, 1; 2; p ) 2, 3) Gi-OPVp G0-OPV2

mesomorphisma G 56 (-28.7) Cr 87 (35.0) Colh 147 (0.7) I

dexp/Åb

Ic

[hk]d

dtheo/Åb,e

parametersb,e

36.35 4.5

VS (sh) VS (br)

10 hch

36.35

46.85 27.05 4.5

VS (sh) W (sh) VS (br)

10 11 hch

46.85 27.05

40.0 4.5

VS (br) VS (br)

hmol hch

T ) 120 °C a ) 42 Å S ) 1525 Å2 Colh-p6mm N)5 T ) 160 °C a ) 54.1 Å S ) 2535 Å2 Colh-p6mm N ) 7.5 T ) 80-160 °C amorphous

50.5 4.5

VS (sh) VS (br)

10 hch

Tdec 245 °C G0-OPV3

Cr1 101.0 (13.7) Cr2 149.5(32.0)Colh200.0 (5.5) I Tdec 270 °C

G1-OPV2

G 41.5 (0.5) MX 175 (-) I

G1-OPV3

Tdec 270 °C G 73.7 (0.4) Colh 180 (-) I Tdec 260 °C

50.5

T ) 120 °C a ) 58.3 Å S ) 2945 Å2 Colh-p6mm N)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 in kJ · °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 the experimentally measured and theoretically calculated diffraction spacings (see table footnote e). The distances are given in Å. c Intensity of the reflections: VS: very strong, W: weak; br and sh stand for broad and sharp reflections, respectively. d [hk] are the Miller indices of the reflections; hch stands for the diffuse scattering corresponding to the molten alkyl chains, hmol is a shortly correlated distance corresponding to the molecular 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.

in the 2800-3000 cm-1 region, which correspond to the vibrations νas(CH3) at 2960 cm-1, νas(CH2) at 2920 cm-1, and νas(CH2) at 2850 cm-1, respectively. In the 900-1400 cm-1 region, a large number of weak peaks, corresponding to the vibrations of the phosphonate and of the OPVs’ aromatic groups were also observed, although, the nature of the coordination mode of the phosphonate onto the surface particle could not be interpreted on the basis of these data alone. The lattice absorption bands in the range 800 and 500 cm-1 are due to the Fe-O bonds in a maghemite phase with a partial vacancies ordering.4 Quantitative Aspects of Grafting. The grafting rate of the phosphonated ligands on the NPs surface and its dependence on the added amount of organic molecules and the molecule architecture were evaluated using two different techniques and compared. The first method consisted in following the weight variation of functionalized NPs with temperature, directly related to the quantity of grafted organic moiety. Thermogravimetric analyses showed weight loss at around 250-350 °C (Figure 5, top). As no weight variation was observed below 500 °C for the naked 40 nm sized γ-Fe203 NPs, this weight loss was evidently attributed to the organic moiety degradation and is directly related to the quantity of grafted molecules on the NPs surface, 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 molecules on 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), corresponding to a grafting rate of 40 mg for G1-OPV2, 53 mg for G1-OPV3, 54 mg for G0-OPV2, and 68 mg for G0-OPV3 per gram of NP (Table 2). A rough estimation of the molecular density at the surface gives 1.5 and 1.3 molecule · nm-2 for both G0 derivatives and 0.46 and 0.53 molecules · nm-2 for the dendritic G1 derivatives. With the hypothesis that the molecular

surface area is governed by the alkyl chain cross-section area of the ligand projected normal to the ferrite surface (ca. 22-24 Å2 per chain), a complete coverage of the NP surface is obtained with the G0 derivatives in agreement with earlier results,4 while only 67 and 88% of the NP surface is covered with the dendritic molecules G1-OPV2 and G1-OPV3, respectively. Thus, the grafting with linear structures (or slightly conical) seems more efficient than that with dendritic ones. The grafting rates can concomitantly be determined by UV spectroscopy. All the Gi-OPVp chromophores present an absorption band in the UV-vis domain that can be utilized to estimate the quantity of nongrafted molecules in the washing solutions using the Beer-Lambert law. The difference between the initial added quantity of organic molecules and the quantity recuperated after functionalization (nongrafted molecules) in the washing solutions gives the quantity of adsorbed molecules on the NPs surface. In Figure 6 is represented the isotherm absorption for the different molecules. However, this methodology is less reliable at high concentration due to possible intermolecular interactions that may occur and disturb the UV band intensity. Nevertheless, it is possible to give a first estimation of the grafting rate by determining the saturation threshold at the surface: such a value corresponds to the apparition of free molecules in the medium. The extrapolation line, shown in Figure 6, gives the grafting rate (Table 2) corresponding to 1.4 and 1.5 molecules · nm-2 for G0-OPV2 and G0-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 for the evaluation of the grafting rate values of G0-OPVp, the measurements of the dendritic systems being seemingly less reliable that those of the linear systems (Table 2). Anyway, the architecture of the organic molecules influences this rate and leads to an a priori complete surface coverage for the linear structure (high grafting density), while this coverage is only partial for the dendritic structures. This is likely due to steric


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Figure 5. Loss weight variations for the functionalized particles determined by TGA as a function of temperature (300 mg of ligand per 1 g of NP (top) and as a function of added molecules (bottom).

Figure 3. SEM micrograph of the 39 nm sized γ-Fe2O3 NP@G0OPV3 (top). TEM micrographs showing amorphous and homogeneous layers for the 40 nm γ-Fe2O3 NPs functionalized (middle) with OPV3G0 and (bottom) with G1-OPV3 molecules.

TABLE 2: Maximum Quantities of Grafted Chromophores on the Particles and Molecular Density Determined by UV and TGA mg of grafted chromophore per g of NP

number of molecules per nm2

Gi-OPVp

UV

TGA

UV

TGA

G0-OPV2 G1-OPV2 G0-OPV3 G1-OPV3

60 20-30 70 30-40

54 40 68 53

1.4 0.3-0.4 1.5 0.3-0.4

1.5 0.46 1.3 0.53

constraints and, consequently, to the oxide surface accessibility and packing density. Whereas the G0 derivatives, which mainly exist in one single conical-like conformation, can readily reach the surface of the oxides and pack efficiently, this is less likely for the G1 dendrimers, more bulky and existing in several nonsuitable conformations. Properties of the Grafted Nanoparticles

Figure 4. IR spectra of the 40 nm sized γ-Fe2O3 NPs naked (bottom) and functionalized with G0-OPV3 (top). Inset zoom in the region between 3000 and 2800 cm-1.

Thermal Stability. The thermal stabilities of the various recovered powders (naked and coated γ-Fe2O3 NPs) and of molecules were analyzed by TDA/TGA, which showed supplementary evidence for the presence of organic molecules and of their strong interaction with the γ-Fe2O3 NPs surface. Before functionalization, the TGA thermogram of the naked γ-Fe2O3 NPs (Figure 7, top, trace a), except a small weight loss due to dehydration below 150 °C, did not record any weight variation

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Figure 6. UV spectroscopy titration curves for the determination of the grafting rate.

Figure 8. TDA curves of functionalized NPs as a function (top) of added organic moieties (G0-OPV3) and (bottom) as a function of the molecular architecture (300 mg per g of NP).

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 nm γ-Fe2O3 NPs after functionalization with G0-OPV3 molecules at a initial added concentration of 300 mg · g-1 of NPs (inset: TGA curve of G0OPV3 molecules).

from 150 to 800 °C, while the TDA curve (Figure 7, bottom, trace a) presented a exothermic transition at 500 °C corresponding to the maghemite (γ-Fe2O3) to hematite (R-Fe2O3) phase transformation.26,27,35,36 The TGA curve of the G0-OPV3 molecules (taken as a representative 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 Figure 7, top). The first weight loss may be attributed to the thermal pyrolysis of the alkyl chains (first plateau), as the contribution in wt % of alkyl chains in the whole molecule weight is about 55.5%. The other features in the second stage (plateaus at ca. 450 and 550 °C) may be attributed to the thermal decomposition of OPV units and of the phosphonate group. Moreover, the weight loss observed over a large range of temperature may be due to the formation of molecular intermediates (phosphonate compounds, etc.) and to different interactions between alkyl chains or OPV units, leading to a distribution of thermal decomposition 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 loss can clearly be attributed to the thermal decomposition of the grafted organic species. Moreover, the percentage of weight loss was found to depend on the concentration and architecture of added organic molecules during the functionalization process (Figures 5 (top) and 8). The comparison of TGA curves of free molecules and grafted molecules shows that the thermal decomposition of molecules is modified when they coat nanoparticles: their decomposition occurs at lower temperature. As shown by the DTA curves of NPs coated with G0-OPV3, the degradation of the organic moiety is exothermic with only two peaks (Figure 7, bottom, trace b). For all designed OPV derivatives, the organic decomposition of grafted NPs occurs according to two exothermic processes

Properties of Dendronized Ferrite Nanoparticles between 250-350 °C (Table 2). A two-step decomposition of the organic moiety has previously been reported in other functionalized NPs systems from TGA derivatives and was explained by the presence of an organic molecular bilayer on the particle surfaces or to two types of bonding on the NP surface.37 In our system, however, TEM micrographs and quantitative analyses of grafting rates demonstrated that one monolayer has been grafted. Moreover, the two peaks have also been observed for NPs decorated with dendritic molecules (G1OPVp), which do not possess the appropriate architectures to form bilayers. This two-steps degradation event already takes place with NPs functionalized at low concentrations of added molecules, thus with a partial coverage of the surface. Finally, the first exothermic event observed during heat treatment of ungrafted molecules was attributed to the thermal decomposition of alkyl chains. This is consistent with results of Lee et al.38 on thermal studies on iron-oxy oleate precursor, which assigned an exothermic peak at 260 °C to the thermal decomposition of oleate ligands. XPS analyses demonstrate that phosphorus stays always at the surface of nanoparticles after the thermal treatments suggesting that the thermal decomposition of phosphonate groups does not occur when phosphonate groups are grafted. These facts demonstrate that the two exothermic peaks can not be attributed to two kinds of bonding or to the presence of a molecular bilayer. Moreover, the temperature of the maghemite-hematite phase transformation, at around 500 °C for naked nanoparticles, is shifted to higher temperature. This shifting has already been observed and attributed to the phosphonate-based coat, partially degraded but still present on NPs surface, which protects the surface from oxidation and thus delays the phase transformation.26 Nevertheless, depending on the grafted molecules, a second weak peak may be observed in this temperature range. In Table S1 are reported the different temperatures of degradation, recorded by TDA, for functionalized NPs with G0-OPV2, G0-OPV3, G1-OPV2, and G1-OPV3 (NP@G0-OPV2, NP@G0OPV3, NP@G1-OPV2, and NP@G1-OPV3, respectively) and as a function of initial added molecules concentration. TD1 and TD2 represent, respectively, the decomposition temperatures of the organic molecules corresponding to the first and second peaks, TT1, the temperature of the maghemite-hematite transformation, and TD3, a temperature that we attributed to the final decomposition of the molecules. Indeed, the peak corresponding to TD3 is observed only with linear dendrons (G0) for whom the grafting rate is the highest, and such peak has never been observed during studies of the thermal stability of phosphated magnetite nanoparticles.39 The use of a same batch of γ-Fe2O3 NPs for functionalization allows comparing the thermal stability of grafted NPs as a function of organic molecules nature and concentration. For all OPV-coated derivatives, the temperature dependence of TD1, TD2, and TT1 (and TD3 for G0 molecules) with added molecule concentration followed the same tendency: they first rapidly increased at low organic contents, followed by a sudden temperature stabilization from an added amount of molecules of 80-100 mg g-1 NP. Indeed, as observed in Figure 5, from this amount, a saturation threshold of grafted molecules is reached. This fact confirms that the observed shifts are related to the grafting rate. In Figure 8 (top) are represented TDA curves of NPs functionalized at different G0-OPV3 concentrations with corresponding TGA curves (inset Figure 8, top). It appears clear that the shift of the decomposition peaks TD1 and TD2 and the

J. Phys. Chem. C, Vol. 113, No. 28, 2009 12209 increase of the loss weight (e.g., grafting rate) are related, indicating, consequently, an organic decomposition dependence on molecule packing densities at NP surfaces. Indeed, when the amount of organic molecules grafted at the surface of NP increases, the probability of van der Waals interactions between alkyl chains of molecules due to self-assembling increases. These interactions induce a shift of decomposition temperature toward higher values.37 That is consistent with results of Lee et al.38 and Shen et al.37b who observed an increase of the decomposition temperature with the alkyl chain length of grafted molecules. Similarly, Kataby et al.37a observed a shift to higher temperature of the decomposition temperatures of octadecane thiol at the surface of Fe2O3 nanoparticle when the concentration in octadecane thiol increases. The comparison of the organic moiety stability on NPs surface for different Gi-OPVp derivatives (Figure 8, bottom) confirms the strong dependence on the molecular architectures. As shown by the value of the TD1 temperatures (272, 286, 289, and 295 °C for NPs coated with G1-OPV2, G1-OPV3, G0-OPV2, and G0-OPV3, respectively), layers made of G1 systems are less dense than those with G0 and then the interactions between alkyl chains are weaker inducing a lower shift. An additional phenylene vinylene unit in the luminescent functional chain (OPV2 f OPV3) increases the degradation temperature. The probability of π-stacking between adjacent chromophores increases with the number of OPV units. Such interactions should favor the packing of molecules and therefore the van der Waals interactions between alkyl chains. The TD2 temperatures (306, 325, 323, and 342 °C for NP@G1-OPV2, NP@G1-OPV3, NP@G0-OPV2, and NP@G0OPV3, respectively) are also related to the molecular architecture, with a displacement of about 20 °C from G1 to G0 architecture and from OPV2 to OPV3 molecules. Moreover, the relative intensity of TD2 peak is greater than TD1 peak for OPV3 molecules, indicating a correlation between TD2 and the OPV unit number in the functional chain. However, the organic moiety decomposition in two steps has also been reported for γ-Fe2O3 NPs coated with straight alkylphosphonate and alkylcarboxylate37d chains covalently linked, suggesting that the origin of the TD2 peak is likely due to intermolecular organization on the surface rather than the presence of OPV units themselves. In this system, the presence of [OPV] groups could favor the assembly of OPV derivatives via anisotropic interactions. In Figure 8 (top) for NPs coated with G0-OPV3, one can also notice that for lower added concentration, the TD2 intensity is lower than that of TD1 and an inversion operates for high concentrations. This is in good agreement with the promotion of molecules self-assembly with the increase of their density at NPs surface. The TD3 peak, which increases with the grafting rate, is supposed to be related also to the final decomposition of the molecules. One may also suggest that this decomposition may be promoted by the phase transformation and should concern mainly the phosphonate residues. In conclusion, the first decomposition stage is attributed to the decomposition of alkyl chains. Their decomposition temperature increases with the grafting rate due to their selfassembling at the surface of NP (increase in the van der Waals interactions between alkyl chains with the grafting rate). The G1 architecture does not favor van der Waals interactions between alkyl chains, but an increase in the number of OPV units favors the packing of molecules through π-stacking. Therefore, the probability of van der Waals interactions increases. The second stage of the decomposition would be

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Figure 9. Magnetic moment vs field at room temperature for naked and capped (G0-OPV3) γ-Fe2O3 nanoparticles.

attributed to the decomposition of OPV units, and similarly, the higher the probability of π-stacking, the higher the decomposition temperature. The lower temperatures observed with dendritic molecules is thus related to a lower facility of selfassembling for these molecules. Concerning TT1, the temperature of the maghemite-hematite phase transformation, an average value of 505 °C is recorded for the NPs capped by the dendrons and 525 °C for the NPs coated by G0-OPVp. As the shift of this temperature is due to the phosphonate coating, the higher shift with linear molecules than with dendritic molecules is related to the higher grafting rate with the former molecules. Magnetic Properties. The hysteresis loop of the naked 40 nm-sized γ-Fe203 NPs (before functionalization) indicates a room temperature ferrimagnetic behavior, as expected, with a coercive field (Hc) of 180 Oe and a saturation magnetization (Ms) of 67 emu/g (Figure 9), very close to that of bulk maghemite (72 emu/g). After functionalization, the hysteresis curves of the NPs decorated by G0-OPV3 at various quantities of added molecules (Figure 9) present also the same features, an opened hysteresis cycle and magnetization at saturation; the magnetic values recorded, Hc and Ms, are almost the same as for the naked γ-Fe203 NPs. The slight difference in the saturation magnetization, Ms, is not that relevant and may be simply attributed to the incomplete extraction of the organic part in the total weight calculation. The measurements performed for the other samples (NPs decorated by G0-OPV2, G1-OPV2, and G1-OPV2) demonstrated the same magnetic behavior (ferrimagnetic), and no strong effect of the grafting was observed on the value of Hc and Ms. Thus, the intrinsic ferrimagnetic behavior of the 40 nm-sized γ-Fe203 NPs is conserved after direct (covalent) functionalization, and the grafting of organic molecules does not affect their magnetic properties, as already observed with magnetite-based nanoparticles.4 Luminescent and Mesomorphic Properties. The normalized emission spectra of the four used OPV derivatives are shown in Figure 10 (top). The G0-OPV2 and G1-OPV2 ligands emission spectra were recorded at 330 nm wavelength excitation and those of G0-OPV3 and G1-OPV3 at 370 nm, corresponding to the maximum absorption of the chromophores, OPV2 and OPV3, respectively. As expected, supplementary OPV fragments insertion into the molecule main functional chains from OPV2fOPV3, leads to a shift of the emission bands by about 45-48 nm toward lower energies (red shift). Moreover, a slight blue shift of 12 and 15 nm was observed from G0 to G1 systems (comparing those bearing the same chromophore OPV2 or OPV3). This additional effect of molecules architecture is

Figure 10. Emission spectra of the free chromophores (top) and of the particles functionalized by G0-OPVp (middle) and by G1-OPVp (bottom).

attributed to the phosphonic acid function localization. For the G0 systems, the coupling agent is directly linked to the OPV chains and thus contributes to the supplementary electron delocalization along the chromophore, inducing a shift of the emission spectra to higher wavelengths. This is not the case for the G1 systems, where the chromophore arms are decoupled from the phosphonic acid. For the functionalized NP, before performing luminescent analysis, the solution was substantially washed several times (THF) and the recuperated supernatants (centrifugation for 10 mn at 14000 round.mn-1) were analyzed by UV to ensure that no free molecules were present in solution, a crucial point for asserting that the observed luminescence is a property of the sole functionalized NP. The NP@Gi-OPV2 and the NP@GiOPV3 were respectively excited at 330 and 370 nm. For all of


J. Phys. Chem. C, Vol. 113, No. 28, 2009 12211 ferrimagnetic behavior. Moreover, the luminescent properties of the chromophores were still observed after grafting and were further tunable by the structure of the luminophore itself. The grafting rate of the various ligands on the oxide surface and their thermal stability and behavior have been studied in detail and were found to depend on the structure of the organic ligands. Acknowledgment. The authors thank the CNRS, the University de Strasbourg, the ANR DENDRIMAT, and the Thai government for funding, Mr. D. Burger (TDA, TGA), Mr. C. Leuvrey (SEM), and Mr. A. Derory (SQUID) for their technical expertise, and Dr. M. Schmutz for some low-dose TEM experiments (Institut Charles Sadron, Strasbourg).

Figure 11. Image of aggregates of functionalized NP (NP@G0-OPV3) observed by fluorescence microscopy.

them, the emission spectra exhibited bands at 392 nm, 400 nm, 450 and 445 nm, demonstrating luminescent properties for these decorated γ-Fe203 NPs; the normalized recorded luminescent spectra of functionalized NPs and of their corresponding used molecules are superposed as shown in Figure 10 (middle and bottom). For the NPs functionalized by the linear ligands G0OPV2 and G0-OPV3, a slight displacement of the emission bands (with respect to the free ligands) toward highest energies (blue shift) was observed (Figure 10, middle). However for dendrons G1, this behavior did not occur, with the maximum of the emission occurring at the same values as for the free G1-OPVp, showing that the grafting has no direct influence on the luminescent properties for chromophores G1 (Figure 10, bottom). These blue shifts, of 20 and 10 nm, respectively, for G0-OPV2- and G0-OPV3 are in the same range than the red shift attributed previously to the phosphonic acid localization, suggesting that this displacement is due to the lost of coupling agent contribution for electron delocalization on OPV chain as henceforward the phosphonate function serves to anchor the molecule on NPs surface. Fluorescence microscopy gives a direct evidence of the luminescent properties of functionalized NPs. The image showed luminescent aggregates with an average diameter of about 300 nm for NPs functionalized with G0-OPV3 (Figure 11) and G1OPV3 (Supporting Information). Thus, the grafting of various types of OPV dendrimeric derivatives equipped with a phosphonic acid as coupling agent preserves their luminescent properties 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 dendronized particles exhibit mesomorphic properties, likely due to the size/ dimension incompatibility and densities between both the organic and the inorganic parts. Conclusions The design, synthesis, and some physicochemical properties of novel organic-inorganic hybrid dendronized nanoparticles presenting magnetic and luminescent functionalities have been described. In particular, we focused on the covalent functionalization of 40 nm γ-Fe2O3 nanocrystals by engineered luminescent oligo(phenylene vinylene)-based prodendritic ligands. The magnetic properties were not altered upon functionalization as all dendronized nanoparticles exhibit room temperature

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Thermal, Magnetic, and Luminescent Properties of Dendronized

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