Structured Semifluorinated Polymer Ionic Liquids for Metal

Jun 12, 2013 - Structured semifluorinated polymer ionic liquids (FPILs) contain a flexible hyperbranched polyether core connected with a covalently at...
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Structured Semifluorinated Polymer Ionic Liquids for Metal Nanoparticle Preparation and Dispersion in Fluorous Compartments Kristina Schadt, Benjamin Kerscher, Ralf Thomann, and Rolf Mülhaupt* Freiburg Materials Research Center (FMF) and Institute for Macromolecular Chemistry, Albert-Ludwigs-University of Freiburg, Stefan-Meier-Str. 31, 79104 Freiburg i. Br., Germany S Supporting Information *

ABSTRACT: Structured semifluorinated polymer ionic liquids (FPILs) contain a flexible hyperbranched polyether core connected with a covalently attached shell of imidazolium cations and perfluorinated alkyl chains at their periphery. In a facile synthesis, alkylation of tosylated poly(3-ethyl-3-hydroxymethyloxetane) (PEHO) converts 1-(n-1H,1H,2H,2H-perfluorooctyl)imidazole into hyperbranched FPILs. They form fluorous compartments for the preparation and effective dispersion of Ag and Au nanoparticles. Only in the presence of the nonfluorinated (PIL) and the semifluorinated polymeric ionic liquids (FPIL), Ag and Au cations are reduced in N,N-dimethylacetamide (DMAC) to form stable metal particle dispersions with average nanoparticle sizes varying between 2 and 13 nm. In the absence of the hyperbranched PEHO core, the corresponding low molecular weight ILs fail to afford stable nanoparticle dispersions. In contrast to the corresponding nonfluorinated PIL, FPIL enables to produce both spherical and flaky uniformly dispersed nanoparticles as a function of the FPIL content. This is attributed to the self-assembly of FPIL at nanoparticle interfaces. Designing nanostructured FPILs and FPIL-mediated metal nanoparticle dispersion is of interest to fluorous multiphase catalysis in fluorous compartmentalized ionic systems without requiring the use of fluorinated solvents.

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amphiphiles, dendritic and hyperbranched PILs offer unique opportunities regarding easy design of multifunctional architectures and matching polarity with different media. Moreover, micelle-like hyperbranched and dendritic PILs can assemble at surfaces and interfaces to produce ultrathin layers. While many dendritic PILs require tedious multistep syntheses,13 hyperbranched PILs with micelle-like topologies are readily available. Typical examples of hyperbranched PILs include polyethylenimines14 and ionenes such as alkylated lutidines.15 In contrast to hyperbranched PILs, containing IL moieties in the core and n-alkyl chains in their nonpolar shells, the incoporation of IL moieties as shell in the periphery of nonionic hyperbranched polymer cores can considerably lower both softening temperatures and viscosities. For example, polyglycidol was esterified with ω-bromoacyl chlorides and alkylated with tertiary amines to incorporate pyridinium or imidazolium groups into the shell.16 For example, rhodium nanoparticles were immobilized on hyperbranched polyelectrolytes and used as catalyst in a hydroformylation.17 In a recent advance, polyglycidol tosylates were used to alkylate 1methylimidazole to produce hyperbranched polyethers containing an imidazolium shell.18 In this synthetic strategy, poly(3-ethyl-3-hydroxymethyloxetane) tosylates were used to alkylate 1-(n-alkyl) imidazoles in order to produce PILs

anometer-scaled structuring of ionic liquids (ILs) by selfassembly of amphiphilic ILs, containing long alkyl chains, or by blending ILs together with various low and high molecular weight amphiphiles, respectively, represents a synthetic tool for preparing nano- and micrometer-sized compartments for synthesis and dispersion of nanoparticles. The diversified applications of compartmentalized ILs range from nanophase-separated reaction and sorption media to nanoreactors, phase transfer and dispersing agents, catalysts, and smart materials such as micellar transporters for temperature-switchable shuttling of ingredients between different phases.1−9 Since the assemblies of most low molecular weight ILs are shear-, pH-, and temperature-sensitive, structured polymer ionic liquids (PILs) with micelle-like topologies and stable conformations are designed. The incorporation of polymer cores and backbones into ionic liquids enables processing typical for polymeric materials.10 Moreover, PILs afford significantly improved dispersion of metal nanoparticles. Apart from entropic and osmotic stabilizing effects typical for conventional polymer dispersing agents, PILs provide highly effective electrosteric stabilization owing to the presence of ionic groups in the polymer.11 Hence, PILs are recognized as effective dispersing agents used for nanoparticle preparation. For instance, Au-NP dispersions, stabilized by a linear polyvinylimidazolium PIL (poly[C10VIm−][Cl−]), containing imidazolium moieties as side chains, were produced by photopolymerization of 1-decyl-3-vinylimidazolium chloride in the presence of HAuCl4. The resulting Au-NP dispersions were used to prepare electrodes.10,12 In the families of PIL © 2013 American Chemical Society

Received: March 14, 2013 Revised: May 25, 2013 Published: June 12, 2013 4799

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chains on nanoparticle size and shape as well as dispersion stability.

containing a hyperbranched polyether core, an inner imidazolium shell with variable counterions, and an outer alkyl shell with variable n-alkyl chain length. Such onion-like PILs serve as very robust compartmentalized ionic systems for preparation, transport, and dispersion of various nanoparticles including functionalized graphenes.19 Little is known on synthesis and application of semifluorinated hyperbranched polymer ionic liquids (FPILs), in particular with respect to creating fluorous compartments for nanoparticle syntheses. As compared to fluorine-free n-alkyl-substituted ILs, the incorporation of perfluoroalkyl chains can markedly improve thermal and oxidative stabilities, reduce surface energy, and enhance oxygen and carbon dioxide solubility.20 In principle, it is feasible that nanoparticles dispersed in such fluorous compartments can serve as fluorous biphase catalyst systems which do not require the addition of fluorinated solvents. This concept was first demonstrated for nanoparticles encapsulated in perfluoroetherfunctionalized PAMAM dendrimers.21 During recent years, significant progress has been made in the synthesis and application of low molecular weight fluorinated ILs (FILs) substituted with perfluorinated alkyl chains.22 As compared to the corresponding fluorine-free n-alkyl-substituted ILs, carbon dioxide solubility of FILs substantially increased with increasing chain length of the perfluorinated alkyl substituent.23,24 Similar to low molecular weight FILs nonionic semifluorinated dendrimers with a fluorinated shell were reported to transport CO2 molecules.25 The presence of a perfluoroalkyl shell in the periphery of a dendritic core renders semifluorinated dendrimers soluble in various fluorinated solvents.26 This is paralleled by significantly increased oxygen solubility.20 Fluorinated solvents are of interest in the aerobic oxidation of organometallic compounds.27 In a recent advance, nonionic dendritic fluoroalcohols containing polyglycidol cores were claimed to function as very effective catalysts for alkene epoxidation with hydrogen peroxide.28 Owing to their low surface energy, semifluorinated nonionic hyperbranched polymers comprising a polyglycidol core and perfluoroalkyl shell migrate to the surface of poly(methyl methacrylate) and self-assemble to produce in situ ultrathin nanocoatings at the polymer surface.29 The self-assembly of nonionic dendronized semifluorinated Janus dendrimers was reported to produce vesicular columns useful as reverse thermal actuators.30 Furthermore, the self-assembly of phosphonium FILs was used to create superhydrophobic coatings.31 One of the first dendritic FPILs was prepared by protonation of amineterminated dendritic poly(propylene imine) and poly(amidoamine) with a fluorinated alkane carboxylic acid. As a function of ionic bond formation and microsegregation of the dendritic branches and the fluorinated peripheral chains, it was possible to control the formation of liquid crystalline phases.32 Yet, in view of the envisioned application as compartmentalized fluorous ionic media for nanoparticle synthesis, the covalent attachment of the perfluorinated alkyl chains via hydrolytically stable linkers is highly desirable. Herein we report on a synthetic strategy for the preparation of nanostructured FPILs, containing a flexible hyperbranched fluorine-free polyoxetane core, an inner imidazolium shell with tosylate counterions, and an outer shell consisting of covalently attached 1H,1H,2H,2H-perfluorooctyl groups. Such hyperbranched FPILs are employed as fluorous ionic reaction compartments for the preparation of silver and gold nanoparticles. Fluorine-free and semifluorinated PILs are compared in order to understand the influence of the perfluoroalkyl

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RESULTS AND DISCUSSION Synthesis of Semifluorinated Polymer Ionic Liquid PEHO-C8F13ImOTs and Polymer Ionic Liquid PEHOC8ImOTs. As illustrated in Scheme 1, the nanometer-scale Scheme 1. Synthesis of Hyperbranched Semifluorinated and the Corresponding Fluorine-Free Polymer Ionic Liquids with Hyperbranched Polyoxetane Core and Substituted Imidazolium Tosylate Shell

structured hyperbranched fluorine-containing PEHO-C8F13ImOTs with molecular architectures resembling micelles was prepared in a facile three-step synthesis, similar to that previously reported for the preparation of fluorine-free structured PILs.19 In the first step, 3-ethyl-3-hydroxymethyloxetane was polymerized by cationic ring-opening polymerization to produce hydroxy-functional poly(3-ethyl-3-hydroxymethyloxetane) (PEHO-OH) as flexible and very robust polyether core, following synthetic procedures reported by Pencek and co-workers.33 According to the NMR spectroscopic end group analysis of trifluoroacetylated PEHO (PEHO-TFA), the calculated hydroxyl group number was 1.00 (1H NMR) and the degree of branching 49% investigated by 13C-inverse-gated NMR. The number-average molecular mass (Mn) of the resulting PEHO-OH was determined, by size exclusion chromatography measurements of the PEHO-TFA, to be 1700 g/mol with a polydispersity Mw/Mn of 1.9. The thermal characterization revealed a glass transition temperature of 34 °C (DSC) and a thermal stability up to 290 °C (TGA). In the second step, PEHO-OH was tosylated by reacting it with tosyl chloride to produce PEHO-OTs with a degree of substitution of 95%, as determined by 1H NMR spectroscopy. In the third step, PEHO-OTs was used to alkylate 1-(n-1H,1H,2H,2Hperfluorooctyl)imidazole and the corresponding fluorine-free 1(n-octyl-)imidazole, respectively, to produce both the fluorinecontaining PEHO-C8F13ImOTs, containing perfluorohexyl groups in the outer shell, and the corresponding fluorine-free PEHO-C8ImOTs, both of which contain tosylate as counteranion. 4800

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fluorine-free PIL. The Debye rings correspond to the Ag surfaces 111, 200, 220, and 311, and the ratios of these Debye ring radii are in accord with the literature.34 Obviously, the presence of perfluoroalkyl groups promotes the formation of smaller and better dispersed Ag-NPs in the case of the same concentrations. The Ag content and the Ag/ imidazolium molar ratio (Ag/Im) were varied in order to examine their influence on the average Ag-NP size. The results are listed in Table 1. At low Ag/Im molar ratios up to 0.07 spherical Ag-NPs with an average diameter varying between 3 and 13 nm, as determined by TEM image analyses, resulted in the presence of both the PIL and the FPIL. In the case of the nonfluorinated PIL, higher Ag/Im molar ratios afforded exclusively spherical particles. In contrast, in the presence of fluorinated FPIL at an Ag/Im molar ratio of 0.09 it was possible to produce flaky particles. The Ag-NP size increased in both cases with increasing Ag/Im molar ratio. Moreover, the average Ag-NP size was also affected by both the (F)PIL content and the precursor concentration. At constant precursor concentration, increasing (F)PIL concentration decreased the average Ag-NP size. However, at constant (F)PIL concentration, increasing the precursor concentration increased the Ag-NP size. However, it is not possible to make such a direct comparison of the resulting particle sizes in the presence of the nonfluorinated PEHO-C8ImOTs and the fluorine-containing PEHO-C8F13ImOTs. Interestingly, also the shape of the Ag-NPs is influenced by the reaction parameters. While the fluorine-free PEHO-C8ImOTs addition afforded the formation of exclusively spherical particles, the addition of fluorine-containing PEHO-C8F13ImOTs gave spherical and also flaky Ag-NPs, especially when using higher Ag/Im molar ratios. There are several reports in the literature on the formation of flaky particles, but the origins of this change in morphology are not yet well understood.35,36 The reaction conditions were reported to influence the shape of the resulting nanoparticles.37 The formation of flaky particles improves the specific surface area38 and can improve electrical conductivity.39 They were used as active ingredient of antimicrobial films.40 Most likely, fluorinecontaining PEHO-C8F13ImOTs assemble at the Ag-NP surfaces and influence the growth of the Ag-NPs. Since fluorine-containing PEHO-C8F13ImOTs has a higher glass transition temperature than the nonfluorinated PEHOC8ImOTs, it is likely that fluorine-containing PEHOC8F13ImOTs forms different assemblies at the Ag-NP interface. Preparation of Gold Nanoparticles (Au-NPs). Gold nanoparticles (Au-NPs) were prepared in DMAC by chemical reduction of HAuCl4 with NaBH4 at room temperature in presence of the fluorine-containing PEHO-C8F13ImOTs or nonfluorinated PEHO-C8ImOTs. Again the precursor was dissolved in a solution of fluorine-containing PEHO-C8F13ImOTs or PEHO-C8ImOTs in DMAC under argon and stirred for an hour prior to the addition of the reducing agent NaBH4. Within a few minutes the color of the solution changed from brown, respectively orange, to ruby. TEM and SAED attested the formation of zerovalent Au-NPs. The Debye rings of the corresponding electron diffraction patterns could be subscripted very close to the reported data of the gold cell parameters which indicated the formation of zerovalent metal nanoparticles.41 As illustrated in Figure 3, in DMAC no stable Au-NP dispersions were formed and precipitation of Au occurred in the absence of (F)PIL and also in the presence of

The degrees of PEHO-OTs conversion are 0.87 for the reaction with the fluorine-free imidazole and 0.92 for the semifluorinated imidazole as determined by 1H NMR spectroscopy. The molecular weight was calculated from PEHOOTs and the degree of modification to be 5900 g/mol for the fluorine-free PEHO-C8ImOTs and 8100 g/mol for the fluorine-containing PEHO-C8F13ImOTs. While the semifluorinated PEHO-C8F13ImOTs had a glass transition temperature of 62 °C, the glass transition of the corresponding fluorine-free PEHO-C8ImOTs was significantly lower with 8 °C owing to the higher molecular weight of the fluorinecontaining FPIL. According to thermogravimetric analysis, the incorporation of the 1H,1H,2H,2H-perfluorooctyl groups marginally affect thermal stability, which is still up to 320 °C. Synthesis of Silver Nanoparticles (Ag-NPs). The AgNPs were synthesized by reduction of AgNO3 with NaBH4 in N,N-dimethylacetamide (DMAC) under an argon atmosphere at room temperature in the presence of PIL or FPIL. Since increasing fluorine content can drastically increase oxygen solubility, all samples were carefully degassed prior to their use as compartmentalized systems. Silver nitrate was added to a DMAC solution of fluorine-containing PEHO-C8F13ImOTs and the fluorine-free PEHO-C8ImOTs, respectively. After stirring for 1 h to ensure complete penetration of the salt into the hyperbranched polymer ionic liquids, the reducing agent NaBH4 was added. Thereby the color of the solution changed within a few minutes from brown, respectively orange, to yellow-brown for PEHO-C8ImOTs (cf. Figure 1b) and red

Figure 1. Ag mirror in DMAC (a) and Ag nanoparticles in the presence of nonfluorinated PEHO-C8ImOTs (b) and fluorinecontaining PEHO-C8F13ImOTs (c).

for fluorine-containing PEHO-C8F13ImOTs (cf. Figure 1c). In the presence of both polymer ionic liquids stable dispersions were formed. However, as is apparent from Figure 1a, silver precipitated and formed a silver mirror at the flask surface when polymer ionic liquids were absent. As verified by transmission electron microscopy (TEM) images and selected area diffraction (SAED) analyses displayed in Figure 2, Ag-NPs were formed in the presence of both fluorine-containing and

Figure 2. TEM images and SAED pattern of Ag-NPs, prepared in the presence of nonfluorinated PEHO-C8ImOTs (a, Ag-7@PIL) and fluorine-containing PEHO-C8F13ImOTs (b and c, Ag-4@FPIL). 4801

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Table 1. Synthesis of Ag-NPs in the Presence of Nonfluorinated PEHO-C8ImOTs (PIL) and Fluorine-Containing PEHOC8F13ImOTs (FPIL) Ag-1@FPIL Ag-2@FPIL Ag-3@FPIL Ag-4@FPIL Ag-5@PIL Ag-6@PIL Ag-7@PIL Ag-8@PIL

c(Ag)a [mg/mL]

c((F)PIL)b [mg/mL]

Ag(0) [wt %]c

Ag/Im ratio [mol/mol]d

0.2 0.2 0.3 0.4 0.2 0.4 0.4 0.9

10 12 12 16 12 12 14 19

1.3 8.0 1.3 1.6 8.0 2.1 1.8 3.0

0.06 0.05 0.07 0.09 0.03 0.08 0.07 0.11

dTEM [nm] 9 3 13 8 6 9 8 10

± ± ± ± ± ± ± ±

2 1 5 3e 5 5 2 3

a

c(Ag) stands for the concentration of the silver salt. bc((F)PIL) stands for the concentration of the (F)PIL. cAg(0) [wt %] indicates mass Ag(0) to mass Ag(0) and mass (F)PIL. dAg/Im molar ratio indicates moles of Ag per mole of imidazolium groups of PEHO-C8ImOTs/PEHOC8F13ImOTs. eThe TEM micrographs show flaky particles.

Figure 3. Au precipitate in DMAC (a) and Au-NPs in the presence of nonfluorinated PEHO-C8ImOTs (b), fluorine-containing PEHOC8F13ImOTs (c), and C8-IL (d).

Figure 4. TEM images of Au-NPs prepared in DMAC in the presence of nonfluorinated PEHO-C8ImOTs (a, Au-6@PEHO-C8ImOTs) and fluorine-containing PEHO-C8F13ImOTs (b, Au-2@PEHO-C8F13ImOTs).

the similar low molecular weight 1-methyl-3-octylimidazolium tosylate (C8-IL), which does not contain a PEHO core but has the same composition as the PIL shell. The influences of the Au concentration, Au/Im molar ratio, and (F)PIL content on AuNP size are listed in Table 2. The TEM micrographs of Au-NPs in Figure 4 showed the existence of spherical particles with an average diameter varying between 4 and 13 nm in the presence of the (F)PIL. The NPs were distributed homogenously in the presence of fluorine-free PEHO-C8ImOTs, whereas in the presence of fluorinecontaining PIL PEHO-C8F13ImOTs the resulting Au-NPs were dispersed within nanometer-scaled compartments. Moreover, the Au-NP sizes are controlled by varying the precursor concentration. At constant (F)PIL concentration the Au-NP size decreased with increasing precursor concentration. Although the direct comparison of nanoparticle formation in the presence and the absence of flour in the PILs is difficult, it is

obvious that only in the presence of the fluorine-containing PIL PEHO-C8F13ImOTs the Au-NPs are dispersed in nanometerscaled compartments.

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CONCLUSIONS The alkylation of N-alkyl- and N-perfluoroalkylethyl-substituted imidazoles with poly(3-ethyl-3-hydroxymethyloxetane) tosylate represents a very versatile synthetic route to new families of semifluorinated and the corresponding fluorine-free nanostructured polymer ionic liquids with dimensions and molecular architectures resembling those of micelles. A characteristic feature is the presence of a flexible hyperbranched polyether core with covalent attachment of an inner imidazolium cation shell and of semifluorinated alkyl or n-alkyl chains at their

Table 2. Synthesis of Au-NP in the Presence of C8-IL, PEHO-C8ImOTs (PIL), and Fluorine-Containing PEHO-C8F13ImOTs (FPIL) c(Au)a [mg/mL] Au@C8IL Au-1@FPIL Au-2@FPIL Au-3@FPIL Au-4@PIL Au-5@PIL Au-6@PIL Au-7@PIL

1.0 0.2 0.4 0.6 1.0 0.4 0.4 1.6

c((F)PIL)b [mg/mL] 15 10 10 10 10 10 15 15

e

Au(0) [wt %]c

Au/Im ratio [mol/mol]d

3.4 1.1 2.2 3.5 5.7 2.2 1.5 5.5

0.06 0.03 0.07 0.11 0.12 0.04 0.03 0.12

dTEM [nm] 5 13 6 4 7 12 9 13

± ± ± ± ± ± ± ±

3f 4 2 2 2 4 4 5

a

c(Au) stands for the concentration of the gold salt. bc((F)PIL) stands for the concentration of the (F)PIL. cAu(0) [wt %] indicates mass Au(0) to mass Au(0) and mass F(PIL). dAu/Im molar ratio indicates moles of Au per mole of imidazolium groups of PEHO-C8ImOTs/PEHOC8F13ImOTs. ec stands for the concentration of the IL 1-methyl-3-octylimidazolium tosylate. fNot representative because the most precipitated on the bottom of the flask and could not be analyzed with TEM. 4802

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and the raw product was purified by column chromatography. The product was obtained as a yellowish solid (13.5 g, 50%). Synthesis of the Semifluorinated and Nonfluorinated Polymer Ionic Liquids. The synthesis of hyperbranched PEHO and the functionalization to tosylated PEHO (PEHO-OTs) as well as the functionalization to trifluoracetylated PEHO (PEHO-TFA) were done according to the literature.15 Prior to the functionalization to the FPIL and the PIL, the PEHO-OTs was dried under vacuum at 80 °C. Synthesis of the Semifluorinated Polymer Ionic Liquid PEHO-C8F13ImOTs. PEHO-OTs and 1-(n-1H,1H,2H,2H-perfluorooctylimidazole (1.70 equiv with respect to tosylgroups) were mixed under an argon atmosphere. The mixture was stirred at 120 °C for 10 days. The mixture was dissolved in methanol. The FPIL was isolated by precipitation in toluene twice. This afforded the product in yields of 75%−88%. Synthesis of the Polymer Ionic Liquid PEHO-C8ImOTs. PEHO-OTs and 1-octylimidazole (1.70 equiv with respect to tosylgroups) were mixed under an argon atmosphere. The mixture was heated to 120 °C for 7 days. The mixture was dissolved in CHCl3. The PIL was isolated by precipitation in toluene twice. This afforded the product in yields of 74%−83%. Detailed information on the PEHO derivatives and the PIL and FPIL is found in the Supporting Information. Synthesis of 1-Methyl-3-octylimidaziolium Bromide. 1-Octyl bromide (3.50 mL, 3.90 g, 22.0 mmol, 1.10 equiv) was added dropwise to a solution of 1-methylimidazole (1.60 mL, 1.60 g. 22.0 mmol) in toluene (20 mL). The mixture was refluxed for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure. The raw product was purified by drying at 100 °C for 2 days in vacuum. The product was obtained as a colorless viscous liquid (5.00 g, 90%). Synthesis of the Ionic Liquid 1-Methyl-3-octylimidazolium Tosylate (MOImOTs).43 1-Methyl-3-octylimidazolium bromide (1.50 g, 5.42 mmol, 1.00 equiv) was dissolved in water (0.5 mL). pTsOH·H2O (1.03 g, 5.42 mmol, 1.00 equiv) was added slowly. The resulting mixture was stirred at room temperature for 4 h. The solvent was removed under reduced pressure. After drying at 60 °C in vacuum, the product was obtained as a viscous yellowish liquid (1.98 g, 99%). Synthesis of Ag and Au Nanoparticles. The procedures for the synthesis of Ag and Au nanoparticles which are encapsulated in a hyperbranched polymer are adopted with slight variations from those reported in the literature for dendrimers.44 The metal nanoparticles were prepared using AgNO3 or HAuCl4 as metal precursor and sodium borohydride as reducing agent. Fluorine-containing PEHO-C8F13ImOTs or nonfluorinated PEHO-C8ImOTs was dissolved under argon in DMAC, and the precursor salt for silver nanoparticles, AgNO3, was added. After 1 h, NaBH4 (3 equiv) was added to the mixture of the silver salt and PEHO-C8F13ImOTs or PEHO-C8ImOTs while stirring. The reaction mixture turned yellow-brown within a few minutes after addition of NaBH4. The stirring was continued for an hour to complete the reaction. For TEM observations, solutions containing Ag nanoparticles were coated on a TEM grid and the solvent was removed under vacuum. The reference without polymer ionic liquid was conducted in analogy. Within a few minutes after addition of NaBH4 a silver mirror was found on the vessel wall. The preparation of gold nanoparticles was conducted in analogy to the silver nanoparticles. An aqueous solution containing 50 wt % was used as precursor. After the addition of an excess NaBH4 to the mixture of salt and PEHO-C8F13ImOTs or PEHO-C8ImOTs the color of the solution turned to ruby within a few minutes. The solutions containing Au nanoparticles were analyzed as described before. The reference without polymer ionic liquid was conducted in analogy. Within a few minutes after addition of NaBH4 gold precipitated and was found on the bottom of the vessel. Detailed information can be found in the Supporting Information. Reduction of HAuCl4 in Presence of the IL MOImOTs. MOImOTs (124 mg, 0.4 mmol) was dissolved in DMAC (8 mL)

periphery. In contrast to micellar systems, prepared by selfassembly of low molecular weight amphiphilic ionic liquids, such nanometer-scaled structured hyperbranched polymer electrolytes are multifunctional and very robust, even at elevated temperatures. Their polarity can be tuned to render them soluble in most organic solvents (see Supporting Information). Owing to the presence of semifluorinated alkyl groups at their periphery, such hyperbranched FPILs and their assemblies represent fluorous compartments which can be used as fluorous ionic compartments for chemical reactions, including nanoparticle formation and their dispersion. Only in the presence of PEHO-C8ImOTs and fluorine-containing PEHO-C8F13ImOTs the reduction of silver and gold cations in DMAC effects formation of stable dispersions of the corresponding nanometer-scaled metal particles. Without attachment of a polyether core, the similar low molecular weight alkylated imidazoles are ineffective under the same reaction conditions. The comparison of fluorine-free and fluorine-containing PIL with similar topology clearly demonstrated that both size and shape of the metal particles are influenced by the fluorine-containing PEHO-C8F13ImOTs content and process parameters. In addition to spherical nanoparticles at low fluorine-containing PEHO-C8F13ImOTs content, it was possible to produce flaky particles at high fluorine-containing PEHO-C8F13ImOTs concentrations and even nanoparticle dispersions in nanometer-scaled compartments. Although more research is required to examine these particle growth mechanisms, the self-assembly of fluorinecontaining PEHO-C8F13ImOTs via interactions of the perfluorinated chains at the nanoparticle interfaces plays an important role. In fact, FPIL are too small to serve as host for nanoparticles, but they readily assemble at nanoparticle interfaces to produce excellent steric stabilization. This selfassembly affords fluorous compartments for various catalytically active metal particles and complexes. High solubility in carbon dioxide and dissolution of oxygen could be exploited in various catalytic reactions including catalytic oxidation and carbon dioxide conversion reactions. On the basis of this versatile synthetic route, it should be possible to vary the chain length of the perfluorinated chains as well as the natures of counteranions. Such nanostructured FPILs could offer new opportunities for fluorous nanophase catalysis.

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METHODS

Materials and Methods. Added information to the synthesis described below as well as most experimental detail for nanoparticle syntheses and methods for characterization is found in the Supporting Information. Synthesis of 1-(n-1H,1H,2H,2H-Perfluorooctyl)imidazole. The preparation of 1-(n-1H,1H,2H,2H-perfluorooctyl)imidazole was adopted by a literature procedure.42 Potassium (1.03 g, 26.4 mmol, 1.20 equiv) was melted in dry toluene (40 mL) at 100 °C under argon. Imidazole (1.79 g, 26.4 mmol, 1.20 equiv) was added, and the solution was stirred at 100 °C for 2 h. After cooling to 60 °C 1H,1H,2H,2Hperfluorooctyl iodide (5.40 mL, 10.4 g, 22.0 mmol) was added, and the mixture was heated for 4.5 days at 110 °C. The solvent was removed under reduced pressure, and the raw product was purified by column chromatography. The product was obtained as a yellowish solid (2.16 g, 24%). Synthesis of 1-Octylimidazole. Imidazole (20.4 g, 30.0 mmol, 2.00 equiv) was dissolved in a mixture of methanol (20 mL) and 50% aqueous KOH solution (35 mL) and heated at 100 °C. 1-Octyl bromide (26.0 mL, 29.0 g, 15.0 mmol) was added dropwise during a period of 90 min. The reaction mixture was stirred at 100 °C for another 90 min. The solvent was removed under reduced pressure, 4803

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under argon at room temperature. HAuCl4 (6.6 μL, 7.6 mg, 23 μmol) was added and stirred for an hour. NaBH4 (2.6 mg, 69 μmol, 3.0 equiv) was added, and the solution was stirred for another hour. Within a few minutes a precipitate of Au(0) was formed.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of the nanoparticle syntheses, details of the characterization of the polyionic liquids, and further information on the solubility of the polyionic liquids. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.M.). Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ma400551e | Macromolecules 2013, 46, 4799−4804