Letter pubs.acs.org/macroletters
CdSe/CdSConjugated Polymer Core−Shell Hybrid Nanoparticles by a Grafting-From Approach Tjaard de Roo, Steffen Huber, and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany S Supporting Information *
ABSTRACT: Hybrid particles consisting of II−VI semiconductor quantum dots and conjugated polymers are increasingly relevant, but access is limited by the usual stepgrowth nature of polymer formation. We report on a graftingfrom approach by controlled Pd(II)-mediated polymerization to yield CdSe/CdS nanocrystals with a defined number of polyfluorene chains grown from their surface, as concluded from MALDI-TOF analysis and quantitative end-capping. Further studies underline the importance of matching the monomers’ and the surface-bound initiators’ reactivity.
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Scheme 1. Grafting-from Approach from Pd(II)Functionalized Nanocrystals
ybrid nanoparticles consisting of II−VI semiconductor nanocrystals and organic semiconducting polymers are fundamentally and practically relevant. A decisive feature is the possibility for charge or energy transfer between the organic and inorganic part, combined with their colloidal nature. Possible fields of application are photovoltaics and light emitting devices1−3 or bioimaging.4,5 For the synthesis of hybrid particles with a well-defined shell of polymer chains attached to the core’s surface, grafting-from approaches are generally the method of choice. They allow for a covalent attachment of polymers with defined chain length and high graft densities.6 Grafting-from techniques are, however, incompatible with step growth polymerizations, the common mechanism for conjugated polymer syntheses. Thus, to date only a grafting-through approach has been reported for II-VIinorganic/organic semiconductor hybrid nanoparticles, by which Emrick et al.7 obtained CdSe nanoparticles functionalized with oligo(phenylene vinylene) tri- and tetramers and by which Holder et al.8 yielded CdSe nanocrystals functionalized with polyfluorene. We now report on a controlled grafting-from polymerization approach to yield CdSe/CdS nanocrystals with polyarylene chains grown from their surface (Scheme 1). Chain growth type polymerizations are possible by advanced Suzuki-Miyaura- and Kumada-coupling polymerization protocols.9−11 The palladium-catalyzed Suzuki-Miyaura polymerization is compatible with a larger scope of functional groups12,13 and, in particular, solvents, as it does not require organo-metallic monomers as the Kumada coupling polymerization. As a further consequence, no magnesium salts are left behind in the final product, which appears beneficial for optoelectronic properties.6 High quality CdSe/CdS quantum dots (QDs) with a mean size of 8 nm were obtained by a modified previously reported synthesis,14 resulting in QDs with 47% quantum yield with a maximum intensity at 636 nm and a narrow full width at half© XXXX American Chemical Society
maximum of 30 nm. During the CdS shell synthesis, the major ligand present is the L-type ligand oleyl amine, which allows for an effective displacement by the X-type ligand (4-halophenyl)phosphonic acid. The amount of ligand that can bind to the QDs was determined by 31P NMR. Nanoparticle-bound species give rise to broad 31P NMR signals, whereas nonbound species result in sharp signals. Successive portions of (4-halophenyl)phosphonic acid were added to the QDs until a sharp 31P NMR signal was observed (Figure S3). According to these NMR experiments and considering the ligand/QD concentration (QD concentration determined by the method of Yu15), a single nanocrystal can be functionalized with approximately 350 (4-iodophenyl)phosphonic acid ligands. This translates to 1.80 ligands/nm2, assuming spherical particles, and is consistent with reported values.16 The quantum yield and emission maximum is unaltered after functionalization, underlining the excellent photostability of the QDs and the effectiveness of the ligand exchange process, as unoccupied surface sides would result in a deterioration of the optical properties. Received: April 28, 2016 Accepted: June 9, 2016
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DOI: 10.1021/acsmacrolett.6b00323 ACS Macro Lett. 2016, 5, 786−789
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ACS Macro Letters
2.7.18 Accordingly, a true molecular weight of 5800 g/mol is estimated corresponding to a degree of polymerization (DP) of 15, which matches well with the molecular weight of the polyfluorene species observed by MALDI-TOF MS. Sacrificial nonphosphonic acid functionalized species are found in the supernatant of the precipitated hybrid particles (Figure S5), but are easily separated off by the centrifugation step. They are probably a result of the reaction of residual [Pd(PtBu3)2], or Pd(0) species from side reactions, with monomer, generating nonbound chains. The marginal presence of phenylphosphonic acid functionalized polyfluorene in the supernatant indicates a strong binding of the surface-grown polymer chains to the inorganic core. To further confirm that the polymer is grafted from the nanocrystal surface, we terminated the growing chains by addition of the end-capper 2-[3,5-bis(trifluoromethyl)phenyl]4,4,5,5-tetramethyl-1,3,2-dioxaborolane (Scheme 1). Analysis of the polymer isolated after separating and destroying the hybrid particles reveals the end-capping to be effective, as the predominant species is a polyfluorene with phenylphosphonic acid as initiating chain-end and 3,5-bis(trifluoromethyl)phenyl as terminating chain-end (Figure S6). Only minor amounts of proton-terminated polymer (with a phenylphosphonic acid initiating end-group) is observed, demonstrating that the chainends were still active as the end-capper was introduced. From the amount of polymer stripped off from the hybrid particles by destruction of the inorganic core and considering the QD concentration, we estimate that one single quantum dot is functionalized with around 50 polymer chains. This indicates that only around 15% of surface bound (4bromophenyl)phosphonic acid react with [Pd(PtBu3)2] and form a surface bound initiator species. Assuming a significant decrease in reactivity of the nanoparticle-bound bromo aryl, 15% are reasonable considering the yields of around 50% observed for solution syntheses of these three-coordinate complexes. The synthesized hybrid particles feature a bright photoluminescence composed of polyfluorene and QD fluorescence (Figure 2, red dashed spectrum). The QDs’ intensity maximum is unaltered, indicating that the basic polymerization conditions
A surface bound initiator was formed (Scheme 1) by heating (4-bromophenyl)phosphonic acid functionalized QDs and [Pd(PtBu3)2] (0.75 equiv vs 1 equiv of (4-bromophenyl)phosphonic acid) in toluene to 75 °C for 4 h. The color of the dispersion turned from bright red to orange/yellow, the latter being the characteristic color of three-coordinate Pd(II) complexes.12,13,17 Formation of Pd black was not observed. These reaction conditions parallel solution syntheses of several functionalized three-coordinate Pd(II) initiators for SuzukiMiyaura chain growth polymerization, which in solution typically result in yields of around 50%.12,13 Without further isolation of the Pd(II) functionalized nanocrystals, polymerization was initiated by injection into a monomer solution containing monomer, CsF, and 18 crown 6 in THF/H2O (SI). The molar ratio of monomer to the Pd(0) source was set to approximately 10:1. The polymerization was quenched by addition of methanol. This also results in the precipitation of the hybrid particles, which can be easily collected by centrifugation. Dispersed in toluene, the hybrid particles are colloidally stable for months. For analysis of the potentially surface-bound polymer by matrix-assisted laser desorption/ ionization−time of flight mass spectrometry (MALDI-TOF MS), NMR, and gel permeation chromatography (GPC), the hybrid particles were redispersed in toluene and mixed with concentrated HCl, thereby destroying the inorganic core. Figure 1 depicts the MALDI-TOF mass spectrum of
Figure 1. MALDI-TOF mass spectrum of isolated polyfluorene obtained after quenching the polymerization with methanol, collecting the hybrid particles by centrifugation, and destroying the inorganic core by addition of concentrated HCl.
polyfluorene isolated from CdSe/CdS/polyfluorene hybrid particles. The two intense signal sets can be assigned to polyfluorene species carrying the phenylphosphonic acid initiating chain-end and a proton (green downward triangles) or a bromine (blue triangles) terminating chain-end, respectively. According to GPC (vs polystyrene standards), the molecular weight of the polymer released from the inorganic nanoparticles amounts to Mn 15800 g/mol with a polydispersity index (PDI) of 2.7 (Figure S4). GPC of polyfluorene vs polystyrene standards overestimates the molecular weight by a factor of
Figure 2. Black solid line: Absorption spectrum of CdSe/CdS/ polyfluorene hybrid particles. Red dashed line: Photoluminescence spectrum of CdSe/CdS/polyfluorene hybrid particles, excited at 400 nm. 787
DOI: 10.1021/acsmacrolett.6b00323 ACS Macro Lett. 2016, 5, 786−789
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ACS Macro Letters do not affect the band gap/size of the QDs. In the absorption spectrum of the CdSe/CdS/polyfluorene hybrids (Figure 2, black solid line), a local maximum at 437 nm can be observed. This peak is characteristic for beta-phase polyfluorene and is the result of an increased conjugation length of the polymer chains.19 This additional absorption band has also been observed by Tkachov et al.20 for polyfluorene grafted from silica particles. It can be assumed, that bending and twisting of the polymer chains is reduced because of the high grafting density,20 resulting in the absorption band at 437 nm. The treatment of a highly diluted hybrid particle dispersion with conc. HCl leads to a strong increase in polyfluorene emission (Figure S8) as a result of the removal of the inorganic core, alluding to effective energy transfer from the polymer to the inorganic core in the intact particles. This energy transfer has been observed before for similar hybrid particles in single particle microphotoluminescence measurements.13,21 We furthermore investigated the polymerization from QDs functionalized with (4-iodophenyl)phosphonic acid. Solution polymerization with in situ systems based on aryl iodides, [Pd(dba)2] and PtBu3 are well established. Due to the higher reactivity of the aryl iodide versus the brominated monomer, the initiator can be formed in the monomer solution at an initial stage of the polymerization.22,23 An attempted polymerization from the iodo-functionalized QDs did not result in the formation of any polymer, hinting at a drastically reduced reactivity of the aryl iodide when being bound to the surface of a nanoparticle. However, formation of exclusively phenylphosphonic acid functionalized polyfluorene (Figure S9) can be observed when the (4-iodophenyl)phosphonic acid functionalized QDs, [Pd(dba)2] and PtBu3 mixture, is heated to 50 °C for 2 h before addition of the monomer. The amount of polymer formed was, however, significantly lower (approximately 1/10), and higher temperatures during initiator synthesis led to Pd black formation. Different than in solution polymerization, for surface-initiated polymerization, the more robust bromo-aryl initiators clearly appear advantageous at this point. Decisive factors and further directions are identified by studies of thiophene polymerization. Surface-initiated polymerization of 2-(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)-4,4,5,5tetramethyl-1,3,2-dioxaborolane was performed. According to MALDI-TOF MS, polymerization of the thiophene monomer exclusively results in nonfunctionalized polymer species with H/H, H/Br and to a small extent Br/Br terminated chain-ends (Figure S10). The corrected molecular weight24 determined by GPC is 3000 g/mol (DP of 15) with a PDI of 1.2 (Figure S11). In order to explain the different behaviors of the two monomers in surface-initiated Suzuki-Miyaura chain growth polymerizations, we investigated their respective reactivity toward oxidative addition with [Pd(PtBu3)2]. The reaction of 1 equiv of the respective AB monomer with 1 equiv of [Pd(PtBu3)2] in THF-d8 was separately monitored by 31 P NMR spectra recorded over time. For both monomers, the product of oxidative addition of [Pd(PtBu3)2] into the C−Br bond was observed and characterized by NMR spectra (Figures S12−S15). Figure 3 depicts the conversion of [Pd(PtBu3)2] with 2-(5-bromo-3-(2-ethylhexyl)thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (red dots) and with 2-(7-bromo9,9-dioctyl-9H-fluoren-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (black squares) toward the oxidative addition product versus time. From these NMR experiments, we conclude that
Figure 3. Conversion of 2-(7-bromo-9,9-dioctyl-9H-fluoren-2-yl)4,4,5,5-tetramethyl-1,3,2-dioxaborolane (black squares) and 2-(5bromo-3-(2-ethylhexyl)thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (red dots) with [Pd(PtBu3)2] in THF-d8 at 40 °C vs time.
the reactivity of the thiophene monomer is significantly higher toward oxidative addition. In a surface-initiated polymerization, the combination of residual Pd(0) and a high monomer concentration leads to the formation of mainly nonfunctionalized polythiophene. By contrast, the reactivity of the fluorene monomer is balanced, in that it allows for efficient chain growth but does not primarily compete with initiator formation. Hence, surface initiation by the preformed surface bound initiator complex is strongly dominating, and primarily polymer with phenylphosphonic acid chain-ends, originating from the initiator, is obtained. The difference between the formation of CdSe/CdS/ polyfluorene hybrid particles and the formation of a physical mixture of CdSe/CdS and polythiophene can also be elaborated from transmission electron microscopy (TEM) images. TEM samples were prepared directly from the isolated nanocrystals dispersed in toluene (Figure 4). The inorganic
Figure 4. (Left) TEM image: CdSe/CdS/polyfluorene hybrids. (Right) TEM image: Physical mixture of CdSe/CdS and polythiophene.
nanocrystals are randomly distributed over the grid in case of the molecular CdSe/CdS/polyfluorene hybrids (Figure 4, left image). The interparticle distance is large and mostly in the range of >10 nm, which is in agreement with the polyfluorene graft layer. Phase separation between the polymer and the inorganic nanocrystals is prevented by the direct binding of the polymer to the QD surface. This is not the case for the CdSe/ CdS/polythiophene sample, which is a physical mixture according to MALDI-TOF MS (Figure S10). Phase separation 788
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(14) Negele, C.; Haase, J.; Budweg, A.; Leitenstorfer, A.; Mecking, S. Macromol. Rapid Commun. 2013, 34, 1145−1150. (15) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854−2860. (16) Gomes, R.; Hassinen, A.; Szczygiel, A.; Zhao, Q.; Vantomme, A.; Martins, J. C.; Hens, Z. J. Phys. Chem. Lett. 2011, 2, 145−152. (17) Stambuli, J. P.; Weng, Z.; Incarvito, C. D.; Hartwig, J. F. Angew. Chem., Int. Ed. 2007, 46, 7674−7677. (18) Grell, M.; Bradley, D. D. C.; Long, X.; Chamberlain, T.; Inbasekaran, M.; Woo, E. P.; Soliman, M. Acta Polym. 1998, 49, 439− 444. (19) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Macromolecules 1999, 32, 5810−5817. (20) Tkachov, R.; Senkovskyy, V.; Horecha, M.; Oertel, U.; Stamm, M.; Kiriy, A. Chem. Commun. 2010, 46, 1425−1427. (21) Negele, C.; Haase, J.; Leitenstorfer, A.; Mecking, S. ACS Macro Lett. 2012, 1, 1343−1346. (22) Zhang, H.-H.; Xing, C.-H.; Hu, Q.-S. J. Am. Chem. Soc. 2012, 134, 13156−13159. (23) Fischer, C. S.; Jenewein, C.; Mecking, S. Macromolecules 2015, 48, 483−491. (24) Liu, J.; Loewe, R. S.; McCullough, R. D. Macromolecules 1999, 32, 5777−5785.
and agglomeration, leading to the formation of dense nanocrystal packings, can be observed in the TEM image (Figure 4, right image). In summary, we have demonstrated that hybrid nanoparticles consisting of a II−VI semiconductor nanocrystal core and a covalently attached conjugated polymer shell can be generated through a grafting-from approach. For the case studied of CdSe/CdS/polyfluorene hybrid particles, growth of the polymer chains from the nanocrystal surface allows for high grafting densities and precise end-capping to generate bifunctional polymer chains. Our findings reveal that even more reactive initiators are a key to further enhance the utility of the method, both in terms of monomer scope and to achieve high grafting densities without generating sacrificial dissolved polymer. We are currently pursuing this guideline.
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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/acsmacrolett.6b00323. All experimental procedures, as well as 1H, 19F, 31P, 2DNMR spectra, additional MALDI-TOF MS spectra, photoluminescence spectra, and TEM images (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by the DFG (SFB767) is gratefully acknowledged. The authors thank Silke Müller for MALDITOF MS measurements and Lars Bolk for GPC measurements.
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REFERENCES
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