Conjugated Star Polymers from Multidirectional Suzuki–Miyaura

Article pubs.acs.org/Macromolecules

Conjugated Star Polymers from Multidirectional Suzuki−Miyaura Polymerization for Live Cell Imaging Christoph S. Fischer, Christian Jenewein, and Stefan Mecking* Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78464 Konstanz, Germany S Supporting Information *

ABSTRACT: Comparative Suzuki−Miyaura coupling polymerization (SMCP) studies with initiators generated in situ from 4iododiphenylether (1), 4,4′-diiododiphenylether (2), 1,6-bis(4iodophenoxy)hexane (3) and 1,4-bis(4-iodophenoxy)cyclohexane (4) result in bidirectional growth for 2, versus preferred monodirectional growth for 3 and 4 with their aliphatic spacers, in agreement with a “chain walking” of the metal along the aromatic system. Multidirectional controlled SMCP was initiated on a 1,6,9,14-tetra(4-iodophenoxy)-N,N′-(2,6-diisopropylphenyl) terrylene-3,4:11,12-tetracarboxidiimide (TDI-ArI4) near-infrared fluorescent dye as a tetrafunctional core. MALDI−TOF characterization and end group analysis reveal that four-armed polyfluorene-star-terrylenediimide (PF-star-TDI) polymers are formed with narrow molecular weight distributions (e.g., Mw/Mn = 1.29 at Mn = 1.8 × 104 g mol−1 from GPC). An efficient energy transfer results in a virtually exclusive NIR emission from the core upon excitation of the stars’ polyfluorene arms. Consequently, “doping” of self-stabilized nanoparticles (NP) from polyfluorene−poly(ethylene glycol) (PF−PEG) diblock copolymers with PF-star-TDI dye afforded particles with a bright emission in the red and NIR regime upon excitation at λexc = 380 nm. The utility of the “doped” particles for cell imaging was demonstrated by differentiation experiments with J774 macrophages and NIH-3T3 mouse fibroblast cells. In cocultures of both cell types, they can be differentiated by their emission color via confocal fluorescence microscopy due to their size-dependent uptake.

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INTRODUCTION

In extension of a purely fundamental interest in understanding the SMCP mechanism, mechanistic insights can be helpful to further broaden the synthetic scope of this method beyond linear polymers. Designing polymerization initiators that allow for a synthesis of multiarm star-polymer structures requires a consideration of the type of mechanism operative: For a “reactive chain end” mechanism it would be sufficient to mimic polymer chain ends by short di- or trimers attached to an arbitrary multiarm linker. On the other hand, for a star-polymer synthesis by a “metal-π-coordination-mechanism” the initiator structure of choice should be composed of multiple functional groups on which polymerization can be initiated linked together by an aromatic core segment enabling the Pd-center to walk over this core. Aside from a fundamental interest in multiarm structures,11 particularly for conjugated polymer systems a star-structure with a functional acceptor-dye “core” and multiple light-absorbing polymer “arms” is attractive as an efficient and practically useful energy transfer system owing to the multiple light harvesting antennas perfectly surrounding the acceptor-dye core.12 Here, we report for the first time on a multidirectional SMCP chain growth, also allowing for mechanistic conclusions and providing access to conjugated star polymers with favorable

A high degree of control over conjugated polymer structures in terms of molecular weight,1 polydispersity,2 regioregularity,3 and specific end groups4 is desirable; however, conventional step growth polycondensation syntheses of conjugated polymers display significant shortcomings in these respects. These can be overcome by controlled polymerization techniques which possess characteristics of chain growth reactions. Controlled Pd-catalyzed Suzuki−Miyaura coupling polymerization (SMCP), first demonstrated by Yokozawa and co-workers,5 has been applied to obtain e.g. polythiophenes,6 polyphenylenes5 or polyfluorenes5 of precisely adjustable molecular weights with heterobifunctional chain termini,7,8 and additionally stands out in terms of functional group tolerance.7,9 Different mechanisms for the observed transformation of a classical polycondensation reaction into a chain growth type polymerization have been suggested. On the one hand a “reactive chain end” effect is discussed, according to which polymer chain ends should display a notably higher reactivity in comparison to monomer molecules and thus ultimately lead to the living behavior of SMCP. On the other hand a “metal-π-coordination” effect has been proposed. This suggests that the controlled character of SMCP relies on a preferred intramolecular oxidative addition based on πcoordination of the metal center to the conjugated chain, resulting in “chain walking” over the last incorporated monomer unit to the chain end.10 © XXXX American Chemical Society

Received: November 13, 2014 Revised: January 16, 2015

A

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Scheme 1. General Fluorene Polymerization Procedure Using SMCP Initiators Generated in Situ (only monodirectional growth shown)

Scheme 2. Synthesis of TDI-star-PF Using an in Situ Initiator Generation Approach on Tetrafunctional Organic Initiator Precursor TDI-ArI4

(Table 1, entry 1-1) displays only one series of signals in the MALDI−TOF MS (Supporting Information, Figure S2), separated by the mass difference of one fluorene repeat unit. The signals correspond to polyfluorenes (PF) exhibiting a 4phenoxyphenyl starting chain end derived from the organic initiator precursor and a proton on the terminal chain end, originating from the acidic quenching after polymerization (Scheme 1, procedure A). The degree of polymerization ( DPn ) of ca. 9 determined by MALDI−TOF MS agrees reasonably well with gel permeation chromatography (GPC) results.16 A narrow molecular weight distribution (Mw/Mn) of 1.18 from GPC (polymer obtained from precipitation in acetone) underlines the controlled nature of the chain growth polymerization mechanism. This data (entry 1-1) clearly corroborates that polymer chain propagation is exclusively initiated on the designated organic initiator precursors. No chains lacking the corresponding (PhOC6H4−) starting chain end derived from the organic precursor are obtained. In situ 1H NMR however demonstrated that even after prolonged polymerization times at a 1:1 ratio of organic to inorganic initiator precursor, only about 50% of the aryl iodides (Ar−I) of 1 are converted (Supporting Information, Figure S3). To ensure bidirectional growth on the bifunctional organic initiator precursor 4,4′-diiododiphenylether 2, the polymerization was therefore first initiated with a 2-fold excess of Pd relative to Ar−I. This indeed resulted in

emission characteristics as underlined by demonstration of low energy visible light live cell imaging.

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RESULTS AND DISCUSSION Polymerization on Model Organic Initiator Precursors. Controlled chain growth SMCP is commonly initiated by addition of a well-defined, isolated initiator of the general structure [(Ar)PtBu3Pd(X)] (where Ar = aryl and X = halide) to AB-type bromo-boronic monomers. In order to simplify the synthetic procedure, an in situ initiator generation approach was chosen (Scheme 1).13 A tetrahydrofuran (THF) solution of an aryl iodide as an organic initiator precursor, combined with PtBu3 and [Pd(dba)2] as an inorganic initiator precursor was kept at room temperature for 1 h, and added to the 2-bromo-7pinacol boronic acid ester fluorene monomer solution without further purification. Rylene dyes such as terrylene diimide (TDI) are known starting materials for the stepwise synthesis of multiarm structures.14 Following the above approach tetra(4-iodophenoxy)-substituted terrylene diimide (TDI-ArI4)15 could potentially serve as a suitable organic initiator precursor with multiple initiation sites, and incorporated as a core in TDI-star-PF by multidirectional SMCP (vide inf ra, Scheme 2). To optimize the reaction conditions, 4-iododiphenylether (1) was chosen as an organic initiator precursor because of its simplicity and structural similarity to the TDI-ArI4. The obtained polymer B

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Table 1. Polymerization Results with Mono- and Bis(4-iodophenoxy) Organic Initiator Precursors (Aryl Iodide = Ar−I)a

entry

initiator precursor

Ar−I:Pd0

1-1 1-2 1-3e 1-4 1-5 1-6 1-7 1-8

1 2 2 2 2 2 3 4

1:1 0.5:1 1:1 2:1 5:1 10:1 10:1 10:1

Mn (Mw/Mn) [g mol‑1]b 6300 6600 5100 3900 5300 4500 4400 4900

(1.18) (1.18) (1.14) (1.11) (1.12) (1.13) (1.15) (1.19)

H/H:H/Br:H/Ic

bidirectionality [%]

d

50:35:15

→

f

65:20:15 60:25:15 60:25:15 40:0:60 10:0: 90

→ → → → →

− 85 >85f 85 85 85 40 10

Polymerization conditions: [monomer] = 5 mol L−1 in THF/water (25:1), ratio monomer: Ar−I = 10:1, ratio Pd0:PtBu3 = 1:2, tinitiator generation = 60 min, Tinitiator generation = 25 °C, tpolymerization = 30 min, and Tpolymerization = 0 °C. bDetermined by GPC vs polystyrene standards. Polymer obtained from precipitation in acetone. cObserved termination patterns estimated from MALDI−TOF of isolated polymer. d99% PhOC6H4/H termination (Supporting Information, Figure S2). eAddition of monofunctional tolyl end-capper after tpol = 30 min. End-capping reaction time 60 min. f85% of all chains double-end-capped, 5% mono-end-capped as estimated by MALDI−TOF. a

Figure 1. Absorption spectra of polyfluorenes 1-1 and 1-2 (Table 1). Degrees of polymerization calculated from absorption maxima (DPabs) were determined by comparison with reported data.17

bidirectional chain growth (Table 1, entry 1-2) as evidenced by (i) the polymer end-groups accessible from MALDI−TOF MS and (ii) comparative UV−vis absorption measurements. (i): The major signals of the MALDI−TOF MS of the isolated polymer of entry 1-2 correspond to polyfluorenes initiated from

2 displaying H/H or H/Br chain termini. The amount of chains containing the mass of a residual iodine atom (H/I) was quantified by MALDI−TOF MS to as little as 15%. Only these latter signals represent chains that propagated exclusively monodirectional and have thus retained the second Ar−I of C

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Figure 2. Typical MALDI−TOF MS of TDI-star-PF. * refers to termination patterns H/H/Br/I, H/Br/Br/I and Br/Br/Br/I. # refers to H/H/H/H plus the mass of one additional fluorene repeat unit. The insets (A and B) correspond to the depicted molecule with k + l + m + n = 9 and k + l + m + n = 15, respectively, and different end group patterns.

(Table 1, entry 1-8) as evidenced by MALDI−TOF MS as well as 1H NMR spectroscopy (Supporting Information, Figures S7−S9). Overall, these experiments can be rationalized by the following mechanistic picture: If the Pd center detaches fully from the growing chain upon reductive elimination and formation of a new carbon−carbon bond, it would be expected to undergo oxidative addition into the aryl halide bond with the smallest barrier of activation for this process. As oxidative addition is favored for aryl iodides relative to the bromides by ca. 5 kcal/mol,19,20 this would result in a (near) complete consumption of all Ar−Is (= initiation sites of the organic initiator precursors) in the early stages of the polymerization reaction until exclusively aryl bromides (= monomers and chain ends) remain - independent of the ratio of Ar−I : Pd. This consideration is in agreement with the results for entries 1-2 to 1-6 (Table1), however only partially for the organic initiator precursor 3 featuring a flexible aliphatic spacer (Table 1, entry 1-7). Finally, the findings with precursor 4 with a rigid aliphatic spacer (Table 1, entry 1-8) contradict such a picture, also proven to be incorrect when considering the notable amount of unreacted Ar−I of initiator precursor 1 in the NMR experiment (vide supra). If a “metal-π-coordination” is considered, however, the Pd center would be expected to remain associated with the propagating chain upon reductive elimination, possibly “walking” along adjacent aromatic segments and thus ultimately from chain end to chain end. The two Ar−Is of organic precursor 2 are bridged by a single oxygen atom and thus in a physical proximity that may allow for migration of the Pd metal center from one aromatic system to the other. In the hexyl-linked bis(4-iodophenoxy) initiator precursor 3 the Ar−Is are separated by a notably longer and

the bisiodophenoxy initiator precursor. (ii): Polymers of entries 1-1 and 1-2 display almost identical molecular weights and molecular weight distributions (Mn = 6.3 × 103 g mol−1 (Mw/ Mn = 1.18) and Mn = 6.6 × 103 g mol−1 (Mw/Mn = 1.18) determined by GPC). However, the corresponding UV−vis absorption maxima of entries 1-1 and 1-2 correlate to different oligofluorenes of ten and six repeat units, respectively.17 This corroborates that the conjugation of the polymer of entry 1-2 is divided into two segments separated by the aryl ether oxygen group, in agreement with the anticipated bidirectional growth on initiatior precursor 2 (Figure 1). Additionally, the reaction of both Ar−I groups of 2 was verified by addition of 4tolylboronic acid as an end-capper after sufficient polymerization time (Scheme 1, procedure B), yielding almost 90% of di-endcapped product as evidenced by MALDI−TOF MS. This further clearly corroborates the conversion of both Ar−Is during the course of the polymerization reaction.18 (Table 1, entry 1-3; Supporting Information, Figure S4) In additional polymerization experiments with bis(iodoaryl) initiator precursor 2 the ratio of Ar−I to Pd0 of 0.5:1 (Table 1, entry 1-2) was increased stepwise to 10:1 (Table 1, entry 1-4 to 1-6). Remarkably the polymer termination pattern remains virtually unaffected and PFs obtained from initiator precursor 2 are structurally identical, independent from the relative amount of organic to inorganic initiator precursor applied. In contrast, polymerization with initiator precursor 3 featuring a linear hexyl spacer separating both Ar−Is under otherwise identical reaction conditions was monodirectional to about 2/3 when using substoichiometric amounts of Pd (Table 1, entry 1-7). Similarly, polymerization initiated by 4 with a more rigid cyclohexyl-spacer separating the two 4-iodophenoxy substituents displayed almost exclusively monodirectional chain growth D

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Macromolecules very flexible spacer. Consequently, a geometrical arrangement in which the two arenes are physically close enough to one another to allow for migration of the Pd center from one to the other is possible, though not very frequent. The rigid cyclohexyl spacer of bis(4-iodophenoxy) organic precursor 4 on the other hand does not adopt conformations allowing any such close physical proximity of the two separate aromatic segments. Overall, this mechanistic picture agrees with the experimental findings: Bidirectional growth can be observed on 2, whereas polymerization on 3 proceeds preferably monodirectional and exclusively monodirectional on 4. Synthesis of Functional-Core Multi-Armed Star-Polymer. Encouraged by these findings and considerations the synthesis of four-armed star-polyfluorenes using the organic initiator precursor TDI-ArI415 was studied under these optimized polymerization conditions (Scheme 2). The molecular weight distributions of Mw/Mn < 1.3  at molecular weights (Mn) ranging from 1.0 × 104 g mol−1 to 1.8 × 104 g mol−1 (as determined by GPC) depending on the polymerization time  remain narrow, thus corroborating the reasonably controlled character of the polymerization (polymer obtained from precipitation in acetone). MALDI−TOF MS signals can be assigned to TDI functionalized by multiple repeat units of 9,9-dioctylfluorene and the respective chain terminal atoms. In agreement with the expected four-armed structure, four terminating groups can be distinguished by MALDI−TOF MS (Figure 2) and will be noted in the following as for example H/Br/Br/I for a TDI-star-PF with one hydrogen, two bromine and one iodine termini (note that iodine represents an unreacted Ar−I and is therefore technically not an end-group located on the terminus of a polymer chain). The TDI-star-PFs obtained via acidic quenching (procedure A in Scheme 2) exhibit a very high degree of Ar−I conversion, reflected by only 7% to 25% of star-structures with residual ArIs (as determined by MALDI−TOF) when using a 1:1 or 4:1 ratio of Ar−I : Pd0, respectively. Note that even in these molecules only one of the four sites is unreacted. The high level of 50 to 90% of Br/Br/Br/I, H/Br/Br/Br and Br/Br/Br/Br terminations21 (also depending on the Ar−I to Pd0 ratio) clearly demonstrates that the majority of TDI cores are converted to three- and four-armed star polymers. This starnature was further verified by addition of a suitable arylboronic acid as an end-capper after sufficient polymerization times (procedure B in Scheme 2). With 4-hydroxymethylbenzeneboronic acid 4-fold end-capping is 75% complete after 60 min of end-capping time as evidenced by 1H NMR spectroscopy (Supporting Information, Figure S10). Similarly three- and four-fold end-capped species represent the major MALDI− TOF MS signals from end-capping with tolylboronic acid (Supporting Information, Figure S12). Fluorescence Emission. A mixture of PF and free TDI molecules in an appropriate solvent such as THF is not expected to exhibit a notable TDI emission upon excitation of the PF, because energy transfer is limited by poor spectral overlap (Supporting Information, Figure S13) and lack of spatial proximity. The fluorescence emission is consequently limited to the PF emission band between 400 and 500 nm (Figure 3A, blue line). By contrast, TDI-star-PF in solution features a notable energy transfer to the TDI resulting in a prominent emission band at 700 nm (Figure 3, A, green line). This enhanced energy transfer efficiency can also be estimated by comparison of the ratios of fluorescence quantum yields of TDI-emission (Φ>650 nm) over the total quantum yield (Φtotal),

Figure 3. (A) Normalized fluorescence emission spectra of free TDI and PF-mixtures (blue line) and TDI-star-PF (green line) in solution (λexc = 380 nm). (B) Scheme illustrating absorption, energy transfer and emission of TDI-star-PF.

which amount to 0.01 versus 0.74 for PF/free TDI and TDIstar-PF, respectively. TDI-star-PF in Nanoparticle Dispersions. To further demonstrate the utility arising from these features of TDI-starPF, the latter was studied to improve the emission characteristics of conjugated polymers nanoparticles (CPNP).22 CPNPs combine a number of favorable characteristics such as high absorption coefficients and fluorescence quantum yields as well as tunable emission wavelengths. Unlike inorganic quantum dots,23 luminescent CPNPs are free of toxic metals like cadmium and are thus preferable for bioimaging in terms of toxicity.24 Emission wavelength of CPNPs within the nearinfrared (NIR) regime are advantageous due to a deeper penetration depth and a reduced interference with autofluorescence from biological tissue.25 However, PF, polyfluorenebenzothiadiazol and other common luminescent materials emit in the blue to green region of the visible light spectrum and the energy transfer from the polymer matrix to incorporated NIR dyes is limited. Different approaches for the efficient redshifting of conjugated polymer nanoparticle emission toward the NIR region have been developed, generally based on energy-transfer cascades in multicomponent mixtures.26 In contrast, TDI-star-PF based nanoparticles are simpler twocomponent systems, yet bright NIR-emitters as a result of the effective energy transfer characteristics due to the star structure. This favorably reduces the complexity of the system, and e.g. dye−polymer phase separation (“π-stacking”) is suppressed and E

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Figure 4. (A) Schematic representation of nanoparticle formation with PF−PEG diblock copolymers and TDI-star-PF. A THF solution of a PF− PEG diblock copolymer and TDI-star-PF (or for comparison free TDI) is rapidly injected into water to yield self-stabilized nanoparticles with TDI incorporated into the PF matrix. (B + C) Normalized fluorescence emission spectra of free TDI and PF-mixtures (blue lines) and TDI-star-PF (green lines) in nanoparticle dispersion (λexc = 380 nm).

resulting nonradiative energy dissipation pathways are thus reduced.7 In order to prepare stable PF nanoparticles, a THF solution of an amphiphilic polyfluorene-poly(ethylene glycol) (PF− PEG) diblock copolymer27 was rapidly injected into vigorously stirred water.7 TDI-star-PF, or for comparison free TDI molecules, can be incorporated into the hydrophobic core of the particles by addition to the initial polymer THF solution (Figure 4). By this “nanoprecipitation” technique particles with diameters as small as 20 nm, as evidenced by dynamic light scattering, were prepared. In order to optimize the particle brightness at wavelengths of λ > 650 nm, the weight percentage of TDI within the particle matrix was varied to yield PF−PEG nanoparticles containing 0.5 to 9.0 wt % of TDI dye. Higher dye loadings generally result in lower total fluorescence quantum yields ranging from 30 to 10% (Figure 5), but also in increasing PF-to-TDI energy transfer efficiencies (quantified by Φ>650 nm over Φtotal, Figure 5). These changes are most notable in the regime from 0.5 to 3.0 wt % of TDI incorporation. In comparison to the system with incorporated free TDI molecules, TDI-star-PF containing particles exhibit a notably better energy transfer efficiency, featuring values of Φ>650 nm over Φtotal as high as 0.86 (versus 0.60 for free TDI). This enhanced energy transfer is also reflected by superposition of the corresponding emission spectra (Figure 4, B and C) at variable dye concentrations. Consequently, even with the PF/ TDI combination with its poor spectral overlap (Supporting Information, Figure S13) a near complete donor−acceptor energy transfer is achieved owing to the beneficial structural prearrangement of dye and polymer in TDI-star-PF molecules.28 Live Cell Imaging. The utility of these self-stabilized29 CPNPs as biological markers in live cell experiments was

Figure 5. Total fluorescence quantum yields (Φtot, solid lines) and Φ>650 nm/Φtot (squares and circles for free TDI and TDI-star-PF respectively) of TDI−PF−PEG nanoparticles at different weight percentages of incorporated TDI dye. The ratio Φ>650 nm/Φtot is a measure of the energy transfer efficiency (λexc = 380 nm).

probed using two cell types, which differ in their uptake characteristics for different particle sizes. J774 macrophages readily internalize particles with diameters up to 3 μm, whereas particle uptake of NIH-3T3 mouse fibroblast cells via endocytosis is limited to diameters of 100 nm and smaller.30 Both cell types were incubated with red-emitting31 20 nm diameter particles, as well as with blue/NIR emitting 120 nm diameter particles individually, and with mixtures of the two different dispersions (Figure 6, parts A and B). The fluorescence labeled cells can be observed as fluorescence photomicrographs (Figure 6C and 7B) or by confocal fluorescence microscopy (Figure 7, parts A and C). As F

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Figure 6. (A) Emission spectra (λexc = 380 nm; left) and DLS trace (right) of red-emitting, perylene monoimide-doped PF-nanoparticles used for cellular imaging. (B) Emission spectra (λexc = 380 nm; left) and DLS trace (right) of blue and NIR-emitting, TDI-star-PF doped PF-nanoparticles used for cellular imaging. (C) Fluorescence micrographs of J 774 macrophages (top) and 3T3 fibroblasts (bottom) incubated with 20 nm diameter, red-emitting NPs (C1), 120 nm diameter, blue and NIR-emitting NPs (C2) and with both types of NPs (C3) (filter set: Supporting Information, Figure S15).

Figure 7. (A) Brightfield (left), red detection channel (center), and NIR detection (right) confocal fluorescence microscopy images of fibroblast− macrophage coculture experiment incubated with both types of nanoparticles. (B, C) A modified sample preparation procedure was used to preserve spindle-shaped fibroblast structure: (B) fluorescence micrographs; (C, left) superposition brightfield/red detection channel; (C, right) superposition brightfield/NIR-detection confocal fluorescence images of macrophages/fibroblasts coculture incubated with both types of nanoparticles. G

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anticipated, macrophages take up both particle types as well as mixtures thereof, whereas the particle-uptake of fibroblasts is limited to the smaller, red-emitting particles (Figure 6C). In a coculture experiment both types of cells were seeded out in comparable concentrations together in one single well. The cells were incubated with a mixture containing both types of nanoparticle dispersions and imaged using a confocal fluorescence emission microscope (Figure 7A). Macrophages (1, 5, 8, 11) can be distinguished by their visibility in the red and in the NIR detection channel, while fibroblasts are observed exclusively in the red. By using a slightly modified sample preparation procedure that allows the fibroblasts to preserve their characteristic spindle shape this assignment is further verified: Spherical cells (macrophages) display a mixed blue and red emission color that appears almost white in the fluorescence micrographs (Figure 7B, cells 1, 4, 5 and 8), and can be distinguished in the red as well as in the NIR detection channel of the confocal fluorescence microscopes (Figure 7C, cells 1, 2, 4, 6, 8, 9 and 11). The more elongated fibroblasts are discernible by their exclusive red emission in the fluorescence micrograph (Figure 7B, cells 2, 3, 6, 7) and lack of signal intensity in the NIR detection channel (Figure 7C, cells 3, 5, 7 and 10). Further, biotin-labeled nanoparticles were obtained by using biotin-functionalized PEG in the synthesis of PF−PEG-block copolymers. These nanoparticles with surface biotin labels allowed specific membrane labeling of live MCF-7 cells following established protocols32 (experimental details and results see Supporting Information). Together, these experiments demonstrate the advantageous characteristics of these TDI-star-PF “doped” PF nanoparticles as biological markers due to their emission profile, stability, brightness, and variability in size as well as functionalizability on the surface.

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ASSOCIATED CONTENT

S Supporting Information *

All experimental procedures as well as 1H NMR, additional MALDI−TOF MS spectra, and fluorescence emission spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the DFG (Grant Me1388/7-1) and the Center of Applied Photonics is gratefully acknowledged. We thank Lars Bolk for GPC and Andreas Marquardt for MALDI− TOF measurements. The live cell imaging experiments were supported by Elisa May and the Bioimaging Center Konstanz. We thank Elisa May, and Marcus Groettrup and co-workers for supplying fibroblast, macrophage and breast cancer cell lines.

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ABBREVIATIONS PF, polyfluorene; TDI, terrylene diimide; CPNP, conjugated polymer nanoparticles; SMCP, Suzuki−Miyaura coupling polymerization; MALDI−TOF MS, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry; DLS, dynamic light scattering; Ar−I, aryl iodide; PEG, poly(ethylene glycol); NIR, near-infrared

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REFERENCES

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CONCLUSIVE SUMMARY

Suzuki−Miyaura coupling polymerization can be conducted in a multidirectional fashion from an appropriate initiator allowing for the synthesis of well-defined star polymers in a single step. End-group analysis and in particular studies of different difunctional initiators reveal that a given star molecule can be grown by a single active metal site, which can move over the entire molecule during chain growth. These insights are in line with a “chain-walking” mechanism via palladium-arene πinteractions. Other than well-documented “chain-walking” via a series of formation and cleavage of covalent bonds in Pd(II)catalyzed olefin polymerization to highly branched structures,33 experimental proof for such a mechanism as occurs here had been lacking. By virtue of the particular functional group tolerance of the catalysts employed here, this mechanism allows for a synthetically attractive one-pot approach to functionalized star polymers. The star structure results in efficient energytransfer characteristics to the core originating from the initiator. The resulting bright NIR emission can be employed beneficially for live cell imaging with conjugated polymer nanoparticles doped with small amounts of the star polymers, as demonstrated by differential uptake of different cell types. This multidirectional chain growth approach and its remarkable functional group tolerance is also promising for the growth of, for example, star-block copolymers or branched star-structures. H

DOI: 10.1021/ma502294n Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/ma502294n Macromolecules XXXX, XXX, XXX−XXX