Conducting Polymer with Orthogonal Catechol and ... - ACS Publications

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Conducting Polymer with Orthogonal Catechol and Disulfide Anchor Groups for the Assembly of Inorganic Nanostructures Benjamin Klöckner, Kerstin Niederer, Ana Fokina, Holger Frey,* and Rudolf Zentel* Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany S Supporting Information *

ABSTRACT: To combine several inorganic components with organic material in a controlled special and permanent manner still remains a difficult issue. Two specifically functionalized block copolymers were synthesized separately and combined in a second step. A heterofunctional poly(ethylene glycol) (PEG) block copolymer bearing a single amino unit, a short PEG spacer, and multiple catechol functionalities was obtained via anionic ringopening polymerization (AROP). Using the reversible addition− fragmentation chain transfer (RAFT) radical polymerization technique, a semiconducting block copolymer with carbazole side groups was obtained. The second polyacrylate block contained reactive ester groups and was polymerized onto this hole conducting block. By substitution of the reactive esters with the amino functional PEG-catechol block copolymer and cysteamine methyl disulfide, a hole conducting polymer material with two orthogonal anchor groups for the coating of CdSe QDs, and also for TiO2, was obtained. TEM images show that upon coating of both materials we were able to obtain QDs homogeneously distributed at the surface of TiO2 nanoparticles. This spatial assembly is a consequence of the special directing features of the copolymer, possessing two orthogonal anchor groups combined in one material.

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INTRODUCTION The combination of various organic and inorganic materials is a desirable goal in many sectors of research to achieve the desired multifunctional materials. An example is quantum dot sensitized hybrid solar cells. They find great interest, but their efficiencies are often still low due to a lack of control of the nanomorphology of the inorganic/organic hybrid active composites.1 Generally, in hybrid solar cells the acceptor material and sensitizer are composed of inorganic nanocrystals, while the donor material consists of an organic semiconducting polymer or a semiconducting small molecule.2−6 These materials need to be assembled properly to create optoelectronic systems, combining the advantages associated with both types of materials. So from a general point, conditions for a macroscopically homogeneous, but locally demixed morphologies are desirable.1 A local phase separation should facilitate the creation of a percolated system and thus enable efficient charge carrier transport of both charges to the electrodes. On the other hand, a macroscopically homogeneous distribution of the nanocrystals in the organic material should create a large interface area for efficient exciton separation. Concerning percolation, this is especially a problem of the usually sphere like quantum dots (QDs) because for efficient percolation onedimensional (1D) structures like nanorods are much more efficient.7 Thus, concepts to combine sphere-like QD absorbers with 1D electron transport material-like TiO2 nanorods become very attractive. This requires, however, concepts to place the absorbing QDs in intimate contact to the electron transporting material. The discovery of organic semiconducting materials © XXXX American Chemical Society

enabled the development of organic electronic devices, for instance in organic photovoltaics (OPVs) or for organic lightemitting diodes (OLEDs) and organic field effect transistors (OFETs).8−12 An extended π-conjugated system capable of conducting charge carriers, holes, or electrons is crucial for application in electronic devices.13 Two types of polymer systems can be used as donor materials in hybrid solar cells, either main chain conjugated polymers or polymers with semiconducting moieties in their side chain. Poly(3-hexylthiophene) (P3HT) is the most prominent main chain conjugated polymer donor material currently applied in solar cells,2 but also other semiconducting polymers increase in interest.14,15 In the case of side chain conjugated polymers, a monomer unit (styrene, methacrylate, norbornene) with an additional conjugated functional moiety can be polymerized.16−20 Via controlled radical polymerization techniques, it is possible to generate polymers with reproducible molecular weight and narrow molecular weight distribution. For hole transport, a widely used side chain conjugated polymer is poly(9-vinylcarbazole) (PVK). This material possesses a high-lying highest occupied molecular orbital (HOMO), which corresponds to all p-type semiconductors. In a previous work, some of the authors presented PVK derivative polymers with tunable optical properties for quantum dot light-emitting diodes (QLEDs).21,22 Depending on the substituents of the carbazole Received: January 28, 2017 Revised: May 2, 2017

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DOI: 10.1021/acs.macromol.7b00217 Macromolecules XXXX, XXX, XXX−XXX

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candidate for the easy creation of percolated paths for electron and hole transportation, imaginable in solar cell systems. To the best of our knowledge, the assembly of two different inorganic nanostructures by one polymer material with semiconducting properties has never been explored before. The coating capabilities of the respective anchor group were studied as well as the assembly of the three materials after combination via ligand exchange procedures in general.

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EXPERIMENTAL SECTION

Materials. Unless otherwise stated, all reagents were used as received. Cesium hydroxide monohydrate, THF, ethylene oxide, benzene, DMSO (abs.), 2-cyano-2-propyldodecyl trithiocarbonate (CTA), and CaH2 were obtained from Sigma-Aldrich. N,N-Dibenzylaminoethanol was purchased from TCI Chemicals. THF was dried over sodium prior to use. 2,2-Azobis(2-methylpropionitrile) (AIBN) was purchased from Acros and recrystallized from diethyl ether prior to use. Cysteamine methyl disulfide and quantum dots with CdSe core, CdxZn1−xS shell (core diameter 4 nm, total diameter 16 nm with oleic acid surface ligands), were synthesized according to the literature.25,43 Deuterated DMSO-d6 was purchased from Deutero GmbH. Characterization. 1H NMR spectra at 300 and 400 MHz, 13C spectra at 100 MHz, 19F spectra, and DOSY measurements were recorded on a Bruker Avance III HD 300 and a Bruker Avance III HD 400. The spectra are referenced internally to residual proton signals of the deuterated solvent. For all polymers, SEC was performed in THF with polystyrene as external and toluene as an internal standard to calculate the molecular weights. Both a refractive index detector (G 1362A RID, Jasco) and a UV/vis detector (UV-2075 Plus, JASCO) were used to detect the polymer. TGA measurements were performed at a PerkinElmer Pyris 6 TGA under nitrogen flow. Heating rate was 10 °C min−1 from 50 to 700 °C. Infrared spectroscopy was performed on a Jasco FT/IR-4100 with an ATR sampling accessory (MIRacle, Pike Technologies) using 16 scans per measurement. IR spectra were analyzed using Spectra Manager 2.0 (Jasco). TEM measurements were performed on a Tecnai G2 Spirit (FEI) at 120 kV with 2 × 2 k camera. Synthesis. Catechol Acetonide Glycidyl Ether (CAGE); 2,2Dimethyl-1,3-benzodioxole-5-propanyl-1-glycidyl Ether. The CAGE monomer was synthesized as described elsewhere.36 In brief, via phase transfer reaction 2,2-dimethyl-1,3-benzodioxole-5-propanol was reacted with epichlorohydrin. For the two phases benzene and a KOH in water (40 wt %) solution were used. After purification via extraction and column chromatography the product was obtained as a yellow liquid. N,N-Dibenzylpoly(ethylene glycol-block-catechol acetonide glycidyl ether), Bn2N-P(EG-b-CAGE). To synthesize block copolymers of EO and CAGE, N,N-dibenzyl-2-aminoethanol (1 equiv) was dissolved in 10 mL of benzene in a 250 mL Schlenk flask. Cesium hydroxide monohydrate (0.9 equiv) was added, and the mixture was stirred at 70 °C for 1 h. To remove benzene and water, the flask was evacuated (10−3 mbar) at 70 °C overnight. 30 mL of dry THF was distilled in the cold into the Schlenk flask, and 1−2 mL of absolute DMSO was added to dissolve the initiator. EO was first distilled in the cold into a graduated ampule and subsequently into the 250 mL Schlenk flask. The mixture was heated to 70 °C and stirred for 2 days. Subsequently, potassium naphthalide (0.5 M in THF) was titrated to the solution to ensure all chain ends are deprotonated and CAGE was added via syringe. The mixture was stirred for another day, and 1 mL of methanol was added to stop the reaction. Precipitation was performed in cold diethyl ether. Mn,SEC: 4700 g mol−1, Mw/Mn = 1.09. 1H NMR (300 MHz, DMSO-d6): δ [ppm] = 7.36−7.22 (10H, m, 1), 6.64−6.52 (br, aromat, 2), 3.72−3.29 (br, polyether backbone), 2.48 (br, CH2, 3), 1.69 (br, CH2, 4), 1.57 (br, CH3, 5). Removal of the Benzyl Protective Groups H2N-P(EG-b-CAGE). The N,N-dibenzylpoly(ethylene glycol-block-catechol acetonide glycidyl ether) was dissolved in methanol, and palladium hydroxide on activated charcoal (10%) was added. 60 bar of hydrogen was pressed on the reaction vessel, and the mixture was stirred for 4 days. The solution was filtered, and the polymer was precipitated in cold diethyl

Figure 1. Schematic presentation of a quantum dot sensitized hybrid solar cell in which the electron transport occurs via TiO2 nanorods.

units, it was also possible to adjust the HOMO level of the polymers to achieve maximum efficiencies in a hybrid LED device.21,22 One prominent QD candidate for hybrid solar cells is cadmium selenide (CdSe) which possesses an absorption in the visible.23,24 Such CdSe-QDs possess nowadays mostly a core− shell morphology to enhance their stability.25−27 To combine such efficient absorbers with rod-like acceptors and electron donors, multifunctional polymers become attractive. Besides their electron donor (hole conducting) properties, they can possess anchor groups which enable an exchange of the initial stabilizing surfactants on the nanoparticle with the polymer.1,28,29 Molecules with disulfide functionalities are excellent coating materials for unsaturated zink (Zn) centers which are often found in the shell of QDs.19 Electron acceptor materials, e.g., TiO2 nanorods, can be targeted with the catechol functionality.30,31 Thus, polymers with orthogonal anchor groups could offer a way to link two types of inorganic nanoparticles together and coat themat the same timewith a conducting donor material. In this context especially the combination of CdSe and TiO2 nanorods gets attractive as it combines efficient light absorption with quick charge transfer and good electron transport.32−35 Recently, the Frey group established a protected catechol functional monomer for the anionic ring-opening polymerization (AROP).36 Multicatechol functional poly(ethylene glycol) (PEG) copolymer architectures were developed. PEG possesses a highly flexible aliphatic backbone and is soluble in water as well as various organic solvents.37,38 It was already used in solar cell devices as polymer electrolyte as well as a supplement in the anode buffer layer of polymer solar cells, leading to improved conductivity in this layer.39−42 In this work, we present the synthesis of a new polymer material with two orthogonal anchor groups for the different types of inorganic nanomaterials. The polymer combines a semiconducting side-chain polymer block with disulfide anchor units for the coating of CdSe QDs in a second block. Additionally, a multicatechol functional PEG-based block copolymer was attached to the anchor block. It was chosen (i) as an anchor for the coating of TiO2 and (ii) to form an electrically insulating layer between the electron transporting TiO2 nanorods and the hole conducting polymer to reduce charge recombination. The combined polymer is a promising B


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Macromolecules ether. Yields: quantitative. 1H NMR (300 MHz, DMSO-d6): δ [ppm] = 6.64−6.52 (br, aromat, 1), 3.72−3.29 (br, polyether backbone), 2.48 (br, CH2, 2), 1.69 (br, CH3, 3), 1.57 (br, CH3, 4). Removal of the Catechol Protecting Groups. H2N-Poly(ethylene glycol-block-catechol-glycidyl ether) (P1). 200 mg of the catecholcontaining copolymer was dissolved in 20 mL of 1 M HCl in methanol and stirred under reduced pressure (400 mbar, 40 °C) for 6 h. The solvents were removed in a vacuum, and the polymer was precipitated from methanol in cold diethyl ether. The copolymers had the appearance of sticky viscous, slightly brown materials. The samples were stored under an argon atmosphere at 6 °C. Yield: quantitative. Mn,NMR: 4400 g mol−1. 1H NMR (300 MHz, DMSO-d6): δ [ppm] = 6.64−6.52 (br, aromat, 1), 3.72−3.29 (br, polyether backbone), 2.40 (br, CH2, 2), 1.69 (br, CH3, 3). Synthesis of 9-(4-Vinylbenzyl)-9H-carbazole. This monomer was synthesized as described elsewhere.21 In brief, carbazole was reacted with 4-vinylbenzyl chloride under basic conditions. After precipitation in cold water and recrystallization from acetone the product was obtained as colorless needles. RAFT Radical Polymerization of the Hole Conductor (HCP). After dissolving 2-cyano-2-propyldodecyl trithiocarbonate (chain transfer agent, CTA), AIBN, and appropriate amounts of the styrene− carbazole (SC) monomer in dry dioxane, three freeze−pump−thaw cycles were performed to degas the mixture. The solution was heated to 70 °C for 48 h. To stop the reaction, the flask was cooled down by liquid nitrogen, and the polymer was precipitated into hexanes three times. The remaining solutions containing an excess of the monomer were collected, and the unreacted monomer was recovered for use in future reactions. The polymer was dried under vacuum for 24 h, giving light yellow powder (yield: 51%). Mn,SEC: 13 800 g mol−1, Mw/Mn = 1.21. 1H NMR (400 MHz, CDCl3): δ [ppm] = 8.13−7.88 (br, 2H, carbazole), 7.38−6.81 (br, 6H, carbazole), 6.59−6.26 (br, 2H, benzyl), 6.26−5.73 (br, 2H, benzyl), 5.30−4.67 (br, 2H, benzyl-CH2), 1.77− 1.33 (m, 1H, backbone), 1.19−0.65 (br, 2H, backbone). RAFT Polymerization of the Reactive Ester Block on the Hole Conductor Block (P2). The obtained hole conducting polymer (HCP) was dissolved in dry dioxane together with the desired amount of pentafluorophenyl acrylate (RE) monomer and AIBN. After three freeze−pump−thaw cycles the flask was filled with nitrogen and the solution was heated to 70 °C for 72 h. The flask was cooled down with liquid nitrogen, and the formed polymer and excess monomer were first precipitated into hexanes. For purification, the polymer was repeatedly precipitated into hexanes. After drying under vacuum for 24 h the block copolymer was obtained as a white powder (yield: 30%). Mn,SEC: 18 100 g mol−1, Mw/Mn = 1.32. 1H NMR (400 MHz, CDCl3): δ [ppm] = 8.13−7.88 (br, 2H, carbazole), 7.38−6.81 (br, 6H, carbazole), 6.59−6.26 (br, 2H, benzyl), 6.26−5.73 (br, 2H, benzyl), 5.30−4.67 (br, 2H, benzyl-CH2), 3.18−2.99 (m, 1H, PFPA backbone), 2.61−2.38 (m, 2H, PFPA backbone), 1.77−1.33 (m, 1H, backbone), 1.19−0.65 (br, 2H, backbone). 19F NMR (400 MHz, CDCl3): δ [ppm] = −155 (2F), −159 (1F), −164 (2F). Removal of the CTA End Group. The block copolymer P1 was dissolved in dry dioxane. 70 equiv of AIBN was added, and the mixture was stirred at 75 °C for 24 h. The reaction solution was cooled down and precipitated into hexane three times. The polymer appeared as white powder. Yields: quantitative. Coupling of P1 (H2N-Poly(ethylene glycol-block-catechol glycidyl ether)) to P2 (Hole Conductor Pentafluorophenyl Ester Block Copolymer) (P3a). P2 and 2 equiv of P1 as well as 4 equiv of triethylamine were dissolved in dioxane and stirred for 24 h at room temperature. Dioxane was removed by nitrogen flow, and the resulting polymer was redissolved in chloroform. After dialysis for 3 days in chloroform, the polymer was obtained as a brownish, waxy solid. Mn,SEC: 12 200 g mol−1, Mw/Mn = 1.27. 1H NMR (400 MHz, CDCl3): δ [ppm] = 8.13−7.88 (br, 2H, carbazole), 7.38−6.81 (br, 6H, carbazole), 6.59−6.26 (br, 2H, benzyl and br, aromat, catechol), 6.26− 5.73 (br, 2H, benzyl), 5.30−4.67 (br, 2H, benzyl-CH2), 3.70−3.25 (br, polyether backbone), 3.18−2.99 (m, 1H, PFPA backbone), 2.61−2.38 (m, 2H, PFPA backbone), 1.77−1.33 (m, 1H, backbone), 1.19−0.65

(br, 2H, backbone). 19F NMR (400 MHz, CDCl3): δ [ppm] = −155 (2F), −159 (1F), −164 (2H). Modification with Cysteamine Methyl Disulfide (P3). The remaining pentafluorophenyl esters were replaced by adding 32 equiv of cysteamine methyl disulfide (cdiS) and 60 equiv of triethylamine to P3a dissolved in dioxane. The solution was stirred at room temperature for 24 h before the dioxane was removed by reduced pressure. The solid was redissolved in chloroform for 3 days of dialysis. The resultant material was dark brown colored and of a waxy consistence. Mn,SEC: 11 200 g mol−1, Mw/Mn = 1.33. 1H NMR (400 MHz, CDCl3): δ [ppm] = 8.13−7.88 (br, 2H, carbazole), 7.38− 6.81 (br, 6H, carbazole), 6.59−6.26 (br, 2H, benzyl and br, aromat, catechol), 6.26−5.73 (br, 2H, benzyl), 5.30−4.67 (br, 2H, benzylCH2), 3.70−3.25 (br, polyether backbone), 2.75−2.48 (m, 7H, disulfide chain), 1.77−1.33 (m, 1H, backbone), 1.19−0.65 (br, 2H, backbone). 19F NMR (400 MHz, CDCl3): δ [ppm] = no signal. Remark: The two coupling steps can also be performed in a facile one-pot reaction to facilitate the procedure. Coating of TiO2 Nanoparticles. 10 mg of TiO2 nanoparticles (mixture of rutile and anatase; average width: 75 nm; average length: 296 nm) was dispersed in THF by sonication. After 10 min, 5 mg of the functionalized polymer P3 was added, and the mixture was sonicated for further 20 min. Subsequently, the mixture was stirred at 50 °C under a nitrogen atmosphere overnight. To remove an excess of polymer the solution was centrifuged (4500 rpm) for 1 h, and the supernatant colorless solution was exchanged with fresh THF three times. Coating of the QDs. Polymer P3 (5.2 mg) and quantum dots (QD, red, CdSe core, core diameter 4 nm, CdxZn1−xS shell, total diameter 16 nm, oleic acid ligands, 5.2 mg) were separately dispersed in chloroform (each 50 μL) and subsequently combined. The reaction mixture was sonicated for 1 h, and hexanes (1.5 mL) were added to precipitate the functionalized QDs. The precipitate was dispersed in chloroform (250 μL), sonicated for one additional hour, and left at room temperature overnight. Hexanes (1.5 mL) were added for a second time to remove the remaining QDs which were not coated with P3. Again, the precipitate was dispersed in chloroform and sonicated for 1 h to obtain the QD−polymer hybrid solution. Coating of the QDs with the Polymer−TiO2 Hybrids. TiO2− polymer hybrids (1.8 mg) and quantum dots (1.8 mg) were separately dispersed in chloroform by sonication and subsequently combined. The mixture was sonicated for 1 h, and hexanes (1.5 mL) was added to precipitate TiO2−QD−polymer hybrids. The precipitate was redispersed in chloroform (300 μL), sonicated for an additional 1 h, and left at room temperature overnight. Afterward, hexanes (1.5 mL) were added to remove unfunctionalized QDs, and the precipitate formed was again dispersed in chloroform and sonicated for 1 h. After addition of hexanes (1.5 mL) the solid was dried and redispersed in the appropriate amount of chloroform to obtain a TiO2−QD−polymer hybrid solution. TEM Preparation. QD−polymer−, TiO2−polymer−, and TiO2− QD−polymer hybrid solutions were diluted in an appropriate amount of chloroform to achieve concentrations in between 18 and 50 μg mL−1 (TiO2/polymer = 50 μg mL−1; QD/polymer = 20 μg mL−1; TiO2/QD/polymer = 18 μg mL−1). 10 μL of this solution was directly applied on the TEM grid. A tissue was used to carefully remove most of the solvent. After complete evaporation of the solvent, TEM images were taken.

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RESULTS AND DISCUSSION Polymer Synthesis. In the following, we introduce a novel polymer system which enables simultaneous QD functionalization with TiO2 nanoparticles and a semiconducting polymer. To this end, we developed a block copolymer with hole conducting properties, which contains an additional block with two orthogonal anchor groups for QD and TiO2 coating. The architecture of the block with the anchor groups to TiO2 and CdSe was thereby chosen to ensure a good coating and to C


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Figure 2. Polymer synthesis: (A) synthesis of the amino functional poly(ethylene glycol-block-catechol glycidyl ether) copolymer (P1); (B) synthesis of the hole conductor-reactive ester block copolymer (P2); (C) coupling of the P1 with P2 and cysteamine methyl disulfide.

catechol acetonide glycidyl ether (CAGE) was polymerized for another day onto the living PEG chain ends. The composition of the blocks was determined by 1H NMR spectroscopy. A comparison of the signal intensity of the benzyl groups at the initiator at 7.36−7.22 ppm with the respective signals of the catechol units at 6.64−6.52 and 1.57 ppm as well as the signals of the polyether backbone at 3.72−3.29 ppm permits to determine the precise amount of the respective monomer in the polyether block (Figure 3, bottom spectrum). The obtained average block copolymer composition was 70 units of EO and 6 units of CAGE. After cleavage of the orthogonal protecting groups via hydrogenation of the benzyl groups and acidic hydrolysis of the acetal moiety, the polymers possess a single amino functionality for further reaction with a reactive ester. The second block possesses multiple catechol functionalities for coating of the TiO2 nanorods. The presence of multiple catechol functionalities is preferred to a coating with only a single catechol unit, ensuring sufficient multiple interaction with the TiO2 nanoparticles and therefore the desired association of the two types of materials. The PEG spacer, on the other hand, was chosen to provide a certain distance between TiO2 and the semiconducting units of polymer P2. This distance should subsequently enable the incorporation of QDs between TiO2 particles and the semiconducting polymer. All reaction steps can clearly be monitored via 1H NMR spectroscopy (Figure 3).

prevent interparticle cross-linking, according to the results of Meuer44 and Zorn.45,46 In the final step, it becomes feasible to link CdSe QDs to TiO2 nanorods with a hole conducting polymer (HCP) to obtain a TiO2−QD−polymer triple hybrid system. The organic hole conducting polymer21,22 was chosen according to its positive properties for the preparation of CdSe LEDs,21 in which case it led to an improved charge carrier balance.21 Regarding its use for potential solar cells, it allows for hole transfer from the excited QDs to the polymer (HOMO polymer P2: −5.78; LUMO polymer P2: −2.32 eV; HOMO QD: −6.5 eV; LUMO QD: 4.2 eV).21,47 The catechol was be incorporated into the nonconducting PEG units to limit charge recombination between the electron transporting, negatively charged TiO2 nanorods and the hole conducting carbazole units. Figure 2 illustrates the individual routes of the synthesis of the components of the complex functional polymer structure. Step A describes the synthesis of the multi catechol-functional TiO2 anchor polymer (P1). In a one-pot, two-step reaction via anionic ring-opening polymerization this hetero-multifunctional building block was obtained. For this purpose, the cesium salt of N,N-dibenzyl-2-aminoethanol was used as an initiator, possessing a latent amino group. First, ethylene oxide (EO) was polymerized for 2 days to ensure full conversion, obtaining a PEG block as intermediate. Subsequently, after addition of potassium naphthalide, to ensure all chain ends are still active, D


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Figure 3. 1H NMR spectra of the sequence of reaction steps leading to P1. (1) spectrum of the original aminobenzyl and catechol protected block copolymer; (2) amino functional catechol protected block copolymer (after removal of the benzyl protection group); (3) amino functional, catechol functional block copolymer (after removal of the acetonide protecting group).

Figure 4. 1H NMR spectrum of the complex copolymer P3.

Figure 7). The polystyrene backbone provides chemical and electrochemical robustness. We used the semiconducting carbazole-based side chains due to their low HOMO level and their already known suitability as hole conductors.17,48−50 The low-lying HOMO level of the HCP reduces the energy offset for more efficient hole injection.51 Furthermore, in the case of QD-based light-emitting diodes (QLEDs), it was found that the morphology has an enormous influence on the device

The polyether block copolymer exhibited a narrow and monomodal molecular size distribution (Mw/Mn = 1.09; see Figure 7). In step B, the synthesis of the hole conducting vinyl polymer with an additional reactive ester block is described (P2). Both blocks were synthesized via reversible addition−fragmentation chain transfer (RAFT) radical polymerization, leading to moderate, monomodal size distributions (Mw/Mn = 1.32; E


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Figure 5. DOSY 2D NMR spectrum of P3, demonstrating one diffusing species as a proof for the successful coupling of P1 and P2 as well as cyteamine methyl disulfide.

Figure 6. IR spectra of the polymers and the successive reaction steps. The left IR spectra: whole spectral range. Right: zoom-in of the interesting areas for the reaction progress; bottom (blue, green): C−O−C vibration of the polyether in the combined polymer. Top (blue): new CO amide band after complete substitution of the reactive esters.

inorganic nanomaterials, especially QDs and TiO2 (P2). To determine the composition of the two blocks, SEC and 1H NMR spectroscopy were conducted. After the synthesis of the second block the degree of polymerization of the two blocks can be determined via 1H NMR spectroscopy (Figure S1). To achieve the desirable properties of hole conduction as well as a sufficient length of the reactive ester block for attachment of

characteristics and potential for improvement. A homogeneous distribution of the QDs within the polymer matrices leads to an improved charge transport balance.52 For the purpose of an efficient attachment and dispersion of QDs and TiO2 particles in the polymer matrix, we synthesized an additional reactive pentafluorophenyl ester block onto the HCP. This block was employed for introducing further anchor functionalities to coat F


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Macromolecules functional groups enabling the coating of QDs and TiO2, a block copolymer with 54 carbazole units and 10 units of reactive ester was prepared. The final combination of the two different polymer segments is shown in step C (Figure 2). First, a fraction of the reactive pentafluorophenyl esters was substituted by P2 to achieve the availability of the catechol (P3a), serving as TiO2 anchor groups. This reaction step was monitored by IR and NMR spectroscopy and revealed successful transformation of 20% of the pentafluorophenyl ester groups. 1H NMR measurements revealed a new signal at 3.65 ppm, corresponding to the polyether backbone of P2. At 3.09 ppm, the signal of the reactive ester backbone was still observed, indicating remaining reactive ester groups for the ensuing reaction step (Figure S2). This was confirmed via 19F NMR spectroscopy, which clearly showed nonconverted reactive pentafluorophenyl ester moieties (Figure S3). Additionally, DOSY measurements ensured complete coupling of the two polymers, showing only one polymer structure with a single diffusion coefficient. Additional signals correspond to traces of solvents (Figure S4). Eventually, cysteamine methyl disulfide was reacted with the remaining reactive ester units to form multiple anchor groups for the quantum dots. The disulfide moiety serves as an anchor of QDs (CdSe core, CDxZn1−xS shell) due to its high affinity to unsaturated Zn centers.22 Hence, this functionality is able to replace the pristine oleic acid ligands that stabilize the QDs normally.53 The reaction progress was once again monitored by IR and NMR spectroscopy. Because of the relative small amount within the whole polymer, the corresponding signals of the cysteamine methyl disulfide group in the 1H NMR spectrum are not significant and hardly visible at 2.75 and 2.48 ppm, respectively (Figure 4). Nevertheless, DOSY 2D NMR spectroscopy revealed successful attachment of the disulfide functionality to the polymers (Figure 5). This method is highly accurate, and in comparison to the DOSY 2D NMR measured after the combination of P1 and P2 shown in Figure S4 the signals of the cysteamine methyl disulfide units are found at 2.75−2.48 ppm. The whole spectrum only shows one diffusing species. This proves the successful covalent connection between all components. In case of the 19F NMR measurements, the spectrum only exhibits remnants of pentafluorophenol (Figure S5). However, after purification by precipitation or dialysis, the pure product shows no 19F signal at all (Figure S6). Via IR spectroscopy successful attachment of this QD anchor group to the complex polymer structure can be proven due to the complete disappearance of the CO band of the ester at 1783 cm−1 and the appearance of the CO band of the corresponding amide at a wavenumber of 1648 cm−1 (Figure 6). All reaction steps were monitored by SEC (Figure 7 and Table S1). Every particular curve appears monomodal with narrow to moderate size distribution (Mw/Mn = 1.09−1.33). The molecular weight changes with every reaction step. After coupling of the PEG-catechol polymer (P1) to the hole conductor polymer (P2) the molecular weight apparently decreased. SEC data showed a value for P3 of only Mn = 11 500 g mol−1, but we would theoretically expect a molecular weight of about Mn = 25 000 g mol−1 (calculated via 1H NMR spectroscopy) and lower elution times. We assume that the coiling behavior and the hydrodynamic radii change after the coupling of the two polymer species. Additionally, the

Figure 7. SEC curves of all reaction steps. Measured in THF, polystyrene standards (RI detector).

protecting groups of the catechol moieties of polymer P1 were removed before reaction with P2. As a consequence of this step the strong adhesion properties of the catechol moiety to any kind of surface emerge. Because of the interaction of the catechols with the column material of the SEC, the polymer stays longer on the stationary phase and shows delayed elution times, which results in the appearance of the polymer with lower molecular weight. After the attachment of the disulfide functionality the molecular weight decreases slightly. This is probably due to changed coiling behavior. Functionalization of TiO2 and QDs. The synthesized polymer P3 possesses two orthogonal anchoring functionalities. The disulfide functionality has the potential to coordinate to the QD surface, while the catechol units interact with TiO2. To prepare the targeted hybrid materials, first TiO2/polymer hybrids were prepared using a mixture of TiO2 nanorods (length of 200−400 nm, average 296 nm, mixture of rutil and anatase, see Figure S10) as well as polymer P3. The components were mixed in chloroform, sonicated, and stirred overnight. This functionalization was carried out in dilute solution to prevent interparticle cross-linking according to refs 44−46. To remove nonattached polymer, the TiO2−polymer hybrids were centrifuged, and the supernatant colorless solution was exchanged with fresh chloroform three times. After the purification steps, the hybrids were redispersed in chloroform and the obtained dispersion showed a slightly orange color, indicating linkage of the polymer with TiO2 via catechol groups (Figure S7).54 Via IR spectroscopy attachment of the polymer at the surface was confirmed (see Supporting Information). Figure S8 (left) demonstrates the differences between pristine TiO2 and TiO2 coated by P3, as new bands corresponding to the polymer were found. To examine the amount of polymer adsorbed to the nanoparticles, thermogravimetric analysis (TGA) was performed. TGA revealed successful coating with a polymer content of 6.5 wt % (Figure S9). TEM images were recorded from spin-coated solutions of the hybrids. They show individual nanoparticles in combination with small aggregates of a few nanoparticles (see Figure S10), which might have formed during the drying process. Large agglomerates were not observed. To verify the TEM results, the assemble average of the nanoparticle size was determined by a zeta-sizer (dynamic light scattering) in solution (CHCl3). These measurements give a hydrodynamic radius of 242 nm for the polymer-coated TiO2 nanorods (Figure S11). This excludes significant aggregation of G


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Macromolecules

inorganic components due to the two orthogonal moieties of the structure.

the nanoparticles and corresponds to results obtained with other polymers of a similar morphology of anchor and solubilizing block.44−46 To determine the coating behavior of the disulfide anchor toward QDs, hybrids of QDs with CdSe core and CdxZn1−xS shell and polymer P3 were prepared. As a consequence of their synthesis, the QDs are originally coated by oleic acid, which is substituted by ligand exchange due to coordination of the disulfide groups to the surface of the quantum dots. After the ligand exchange procedure, the QDs show altered solubility properties acquired from the polymers attached to their surface (Figure S12). In analogy to the procedure employed for TiO2, here also IR spectroscopy revealed successful coating by the polymer with the typical bands in the spectra, corresponding to P3 (Figure S8, middle). TEM measurements showed mostly individually dispersed QD-polymer hybrids. In analogy to refs 21 and 46 the QDs were not agglomerated but coated by the polymer (Figure S10). These results of a successful coating of the particular nanomaterial were promising for the next step of reactionthe combination of the two nanomaterials by coating with our new two anchor groups bearing polymer. First, the larger TiO2 nanorods were coated with P3. After purification of the hybrid material by removal of the unbound polymer and several centrifugation and redispersing steps, subsequently the CdSe QD were added to allow their incorporation into the hybrid. Again several precipitation and redispersing steps were performed ensuring to obtain the pure TiO2−QD−HCP hybrids. IR spectroscopy has been conducted to reveal the attachment of the polymer to both inorganic nanomaterials (Figure S8, right). Additionally, solubility experiments revealed stable suspensions of the hybrids in polar solvents like DMF (Figure S7, S12). The average size of the aggregates could, however, not be determined by dynamic light scattering in this case because of the high fluorescence of the CdSe QDs. TEM measurements demonstrate the targeted combination CdSe QDs and TiO2 by the polymer. Figure 8

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CONCLUSION In this work, we present a novel approach to polymers for the assembly of inorganic−organic hybrids consisting of different nanostructures, i.e., CdSe quantum dots and TiO2 nanorods. The polymerization methods employed, RAFT polymerization as well as AROP, lead to block copolymers with narrow molecular weight distributions and adjustable monomer contents. The radically synthesized, carbazole-derived polymer is already known to exhibit semiconducting properties and is frequently utilized as a hole conductor material.21 The additional second block, consisting of pentafluorophenyl reactive ester units, enables further functionalization via postpolymerization steps. The amino-functional PEG-based polymer with an additional multicatechol functional block was synthesized via AROP. Multiple catechol groups were used as TiO2 anchor groups. The introduction of the additional amino functionality was achieved via a protected amino-functional initiator. After removal of the protecting groups the polyether copolymer was reacted with the reactive ester block of the carbazole block copolymer. The polyether backbone is known to be highly flexible, rendering the polymer suitable for grafting processes, as well as to introduce a sufficient spacing between the hole conducting polymer segment with the coated QDs and TiO2. The permanent, covalent coupling of the two block copolymers resulted in a material with hole conductor properties and the capability to anchor at TiO2 nanoparticles via catechol moieties. Subsequently, the remaining reactive ester moieties were exchanged with cysteamine methyl disulfide to introduce an established anchor group for QDs. With this polymer material in hand, it was possible to coat each of the inorganic materials individually as well as both materials together in a complex hybrid structure. In case of the coating of both materials, we were able to obtain QDs distributed at the surface of TiO2 nanoparticles. This spatial assembly is due to a special feature of the polymer, i.e., the presence of two orthogonal anchor groups in one material. We believe that this material presents a promising platform for hybrid solar cells. Further studies of this concept for hybrid solar cell systems and their corresponding efficiency will be presented in a future work.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00217. Additional characterization data (Figures S1−S12) (PDF)

Figure 8. TEM pictures: assembly of TiO2 nanorods and QDs by coating with polymer P3. Round, darker gray particles: CdSe/ CdxZn1−xS quantum dots, large elongated light gray particles onto which the QDs are fixed: TiO2 nanorods. Left: scale bar 100 nm, right: higher magnification, scale bar 20 nm.

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shows two exemplary sections of the prepared sample. The small, spherical QDs are uniformly distributed over the whole surface of the TiO2 nanorods. Contrary, a control experiment proves that CdSe QDs and TiO2 do not exhibit any interaction without polymer coating. To sum up, the appropriate distribution of CdSe QDs onto TiO2 nanorods results from the successful coating of both materials by the hole conducting polymer P3. The images confirm that the complex polymer structure adds in a structure-directing manner, organizing both

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.F.). *E-mail: [email protected] (R.Z.). ORCID

Holger Frey: 0000-0002-9916-3103 Rudolf Zentel: 0000-0001-9206-6047 H


Article

Macromolecules Author Contributions

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B.K. and K.N. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.N. thanks the IRTG 1404 for financial support. K.N. and B.K. thank Philipp Daniel (Institute of Inorganic and Analytical Chemistry, Johannes Gutenberg University Mainz) for TEM measurements and for providing TiO2 nanoparticles as well as Wan Ki Bae (Korea Institute of Science and Technology, Seoul) for providing the quantum dots.

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