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Pyridine-Functionalized Luminescent Organoboron Quinolate Block Copolymers as Versatile Building Blocks for Assembled Nanostructures Fei Cheng,† Edward M. Bonder,‡ Shadwa Salem,† and Frieder Jak̈ le*,† †

Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States Department of Biological Sciences, Rutgers University-Newark, 195 University Avenue, Newark, New Jersey 07102, United States

‡

S Supporting Information *

ABSTRACT: Novel luminescent boron quinolate block copolymers with pyridine functionalities were prepared via RAFT polymerization and employed as building blocks for assembled nanostructures. In selective solvents, the block copolymers selfassembled into vesicles, spherical and extended branched aggregates. Quaternization reactions were performed to modify the solubility of the block copolymers and to prepare a cross-linked polymeric gel. As polymeric Lewis bases, the pyridine-functionalized block copolymers are able to form supramolecular complexes with metal ions and Lewis acids. Coordination to ZnCl2 in chloroform led to spherical micelles or larger aggregates. The supramolecular coassembly of PS-b-PBQPy-1 and zinc meso-tetraphenylporphyrin (ZnTPP) in cyclohexane as a selective solvent for PS resulted in extended branched structures with controllable emission properties. Energy transfer from the boron chromophores to the ZnTPP moieties was observed in these supramolecular complexes.

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INTRODUCTION Luminescent organoboron polymers have drawn much attention due to their diverse applications in materials chemistry.1 Conjugated polymers containing luminescent boron chromophores have been used successfully, for example, as active layers in electronic devices and as chemical sensors for anions and neutral electron-donating compounds.2 On the other hand, chain growth polymers decorated with boron (or other) chromophores in side chains or as terminal groups hold great potential for the development of luminescent self-assembled nanostructures, responsive materials, and biological imaging applications.3−5 A variety of different boron-containing chromophores have been developed over the years; organoboron 8-hydroxyquinolates display desirable characteristics such as easy largescale synthesis, excellent stability, and tunable emission colors.6,7 In recent reports, we introduced a series of novel organoboron quinolate monomers and discussed their conversion to welldefined luminescent block and star copolymers via RAFT5,8 polymerization.9−11 For example, amphiphilic block copolymers with poly(ethylene oxide) (PEO) and poly(N-isopropylacrylamide) (PNIPAM) were prepared and found to form luminescent micellar solutions in water with long-term chemical and colloidal stability.9,11 A poly(styrene-alt-maleic anhydride) (P(St-alt-MAh)) block copolymer served as a platform for post-polymerization modification with azobenzene as a second chromophore, and the corresponding azobenzene-modified polymer underwent solventdependent self-assembly in basic solution.9 To further extend the applications of boron quinolate block copolymers, it is desirable to incorporate other responsive/ © 2013 American Chemical Society

reactive moieties into the block copolymer structure. Pyridinefunctionalized polymers are highly useful building blocks for assembled nanostructures and composite materials, with polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) as one of the most widely studied systems.4,12,13 Antonietti and co-workers examined in detail the scaling relationships between the aggregation number, corona dimension, and the block lengths for the self-assembly of PS-b-P4VP in toluene.14 PS-b-P4VP is also important as a polymeric ligand (Lewis base), pH-responsive polymer, and H-bonding acceptor; hence, this block copolymer has been utilized widely for the preparation of polymer/inorganic hybrid materials, responsive materials, and supramolecular complexes.15,16 In view of the excellent photophysical properties of organoboron quinolate chromophores and the versatility of pyridinecontaining polymers in materials chemistry, we decided to pursue two new types of organoboron quinolate block copolymers as analogues to PS-b-P4VP: In one case the block copolymer features P4VP in combination with a PS block that is modified with a luminescent borane chromophore (A), while in the second structure the PS block is kept as is and the pyridyl unit is modified with the borane chromophore (B). These pyridinefunctionalized organoboron quinolate block copolymers are then used as building blocks for self-assembled nanostructures and supramolecular complexes. Received: February 12, 2013 Revised: March 25, 2013 Published: April 12, 2013 2905

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Chart 1. Conceptual Illustration of Boron ChromophoreFunctionalized PS-b-P4VP Derivatives

Scheme 1. Synthesis of Pyridine-Functionalized Boron Quinolate Block Copolymers and Corresponding Monomer Structuresa

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RESULTS AND DISCUSSION Synthesis of Block Copolymers. The synthesis of the boron quinolate monomer BQ and the corresponding homopolymer PBQ was reported in previous publications.9,11 The new pyridine-functionalized boron quinolate monomer BQPy was prepared using similar methods, and its structure was verified by multinuclear NMR spectroscopy and high-resolution MALDI mass spectrometry. RAFT polymerization was chosen for the block copolymer synthesis because of its proven good control of boron quinolate monomer conversion and effectiveness in chain extension. As shown in Scheme 1, the synthesis of PBQ-b-P4VP was accomplished by using PBQ as a macro-CTA for the controlled polymerization of 4VP. PS-b-PBQPy, on the other hand, was prepared by controlled polymerization of the novel monomer BQPy in the presence of narrow PS macroCTAs. The corresponding homopolymer, PBQPy, was also synthesized via RAFT polymerization and serves as a reference for 1H NMR comparison, molecular weight, and photophysical characterization. The formation of PBQPy-b-PS via a reverse polymerization sequence relative to that used for PS-b-PBQPy was not explored but is likely to be successful also given that BQPy is a styrene-derived monomer.9 GPC was used to analyze the molecular weight of the block copolymers and corresponding macro-CTA precursors (Figure 1). The homopolymer PBQ was characterized in THF17 and PBQ-bP4VP in DMF with 0.2% w/v [Bu4N]Br to minimize column interactions with the pyridyl groups. The block copolymer PS-bPBQPy and the corresponding PS precursors were analyzed in a solvent mixture of THF/pyridine = 95/5. The GPC traces revealed reasonably narrow profiles for the block copolymers, and in the case of PS-b-PBQPy chain extension was clearly evident from the shorter column retention time in comparison to the homopolymer precursor. According to our previous studies, the molecular weights of boron quinolate homopolymers are usually underestimated by a factor of 2−3 when using GPC detection with narrow PS standards.9,11,18 Therefore, we also determined the absolute molecular weights of the block copolymers by estimating the mass fraction of the boron chromophores, using as reference the absorptivity of the corresponding homopolymers PBQ and PBQPy. (This method is much more accurate than 1H NMR analysis due to signal overlap in the 1H NMR data.) The molecular weight data of all the homopolymers and block copolymers from GPC and UV−vis analysis are summarized in Table 1. The polymers were also analyzed by multinuclear NMR spectroscopy. All polymers show a single peak with a chemical shift of 7.6 ppm in the 11B NMR, which confirms the presence of the boron quinolate chromophores.9,11,18 The boron quinolate blocks generally show very broad signals in the aromatic region of the 1H NMR spectra and a characteristic singlet at ca. 1.2 ppm due to the tert-butyl groups. A representative comparison of the 1H and 11B NMR

a For PBQ-b-P4VP: [4VP]/[PBQ-CTA]/[AIBN] = 3080/1/0.46; dioxane, 70 °C, 8 h. For PS-b-PBQPy-1: [BQPy]/[PS-1-CTA]/ [AIBN] = 50/1/0.20; dioxane, 80 °C, 5 h. For PS-b-PBQPy-2: [BQPy]/[PS-2-CTA]/[AIBN] = 48/1/0.24; dioxane, 70 °C, 8 h.

Figure 1. GPC-RI traces for (A) PBQ (dashed line, THF at 1.0 mL/min) and PBQ-b-P4VP (solid line, DMF with 0.2% w/v [Bu4N]Br at 0.5 mL/min) and (B) PS-1 (dashed line) and PS-b-PBQPy-1 (solid line, THF/pyridine = 95/5 at 1.0 mL/min).

spectra of homopolymer PBQPy and the corresponding block copolymer PS-b-PBQPy-1 is provided in Figure 2. For PBQ-b-P4VP the signals of the boron quinolate block are hardly seen due to the much longer P4VP block (Figure S10). The photophysical data of all monomers and the corresponding polymers are summarized in Table 2. Consistent with previously 2906

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Table 1. Molecular Weight Data of Pyridine-Functionalized Boron Quinolate Block Copolymers and the Corresponding Macro-CTA Precursors polymera

Mn,GPCb

PDIGPCb

Mn,MALLSc

PBQ m PBQ m-b-P4VPn PBQPym PSm-1 PSm-b-PBQPyn-1 PSm-2 PSm-b-PBQPyn-2

8710 44400 3420 3840 9990 8130 11440

1.28 1.27 1.28 1.18 1.23 1.16 1.25

27600 g g 3570 g 9470 g

Mn,UVd 110 640

17 450 13 110

m, nGPCb,e 22 22, 339 7 35 35, 13 76 76, 7

m, nUVe, f 70, 790

32, 30 89, 8

a

For the block copolymers, the block sequence corresponds to the polymerization sequence. bDetermined by GPC-RI analysis relative to narrow PS standards. cDetermined by GPC-MALLS analysis. dBased on UV−vis analysis and absolute molecular weight data for the respective homopolymer precursor. Molecular weight analysis by 1H NMR integration was prevented by signal overlap. em and n refer to the degree of polymerization of the first and second block, respectively. fm is based on the GPC-MALLS data. gNot measured.

fluorescence quencher,5 thereby affecting the quantum yield for the polymers having shorter PBQPy chain lengths (n < 10). Block Copolymer Self-Assembly in Solution. As analogues of PS-b-P4VP, the self-assembly of PBQ-b-P4VP and PS-b-PBQPy was examined in different selective solvents. Similar to PS, the borane-functionalized PBQ block in PBQ-b-P4VP is hydrophobic and exhibits good solubility in organic solvents such as THF, chloroform, and toluene. In toluene as a selective solvent for the PBQ block, PBQ-b-P4VP formed vesicles (Figure 3A) because the P4VP block is much longer than the PBQ block (m = 70, n = 790). A dynamic light scattering (DLS) measurement revealed a hydrodynamic diameter (⟨Dh⟩) of 76 ± 19 nm (Figure 3B), which is in good agreement with TEM observations. In contrast, the borane-functionalized PBQPy block in PS-bPBQPy shows similar solubility as P4VP. For example, PBQPy is well soluble in MeOH and insoluble in low-polarity solvents, such as cyclohexane. In methanol, the block copolymer PS-bPBQPy-1 formed polydisperse spherical aggregates (Figure 3C), which are expected to consist of a PS core and a PBQPy shell. The ⟨Dh⟩ of the block copolymer aggregates in methanol was determined to be 168 ± 50 nm (Figure 3D). In cyclohexane, reverse self-assembled aggregates with a PBQPy core and a PS shell were expected to form. The TEM image in Figure 3E shows that the block copolymer gave rise to small aggregates, which further assembled to give extended branched structures, because cyclohexane is only a moderately good solvent for the PS shell. A DLS measurement gave ⟨Dh⟩ = 1169 ± 147 nm (Figure 3F), which is in agreement with the formation of larger assemblies. Quaternization of Pyridine-Functionalized Boron Quinolate Block Copolymers. Pyridinium compounds are wellknown as effective germicidal agents, and with appropriate substituents on the pyridine ring or the nitrogen atom, pyridinium compounds have proven useful in a range of other applications, including cosmetics, pharmaceuticals, gene delivery, and phase transfer catalysis.19 We decided to explore the pyridine-functionalized boron quinolate block copolymers as precursors for new luminescent polymeric pyridinium materials. Quaternization reactions were carried out in chloroform by addition of an excess of methyl triflate, and the products were purified by precipitation in hexanes (Scheme 2). The quaternized block copolymers PBQ-b-[P4VPMe]OTf and PS-b[PBQPyMe]OTf exhibited good solubility in highly polar solvents, including water. The methylation of the 4VP moieties in PBQ-b-P4VP was quantitative as evidenced by 1H NMR analysis of the product (PBQ-b-[P4VPMe]OTf) in DMSO-d6, which showed the expected downfield shift in the pyridyl resonances and the appearance of the methyl signal at 4.2 ppm

Figure 2. 1H NMR spectra of PBQPy and PS-b-PBQPy-1 in CDCl3. The corresponding 11B NMR spectra are shown as insets. Residual MeOH solvent is indicated with an asterisk.

Table 2. Photophysical Data of Boron Quinolate Monomers and Polymers samplea

λabs (nm)

λem (λexc) (nm)

ΦF

BQ PBQ-b-P4VP PBQ-b-[P4VPMe]OTf BQPy PBQPy PS-b-PBQPy-1 PS-b-PBQPy-2 PS-b-[PBQPyMe]OTf

395 393 310, 390 (sh) 406 406 406 406 360, 398

506 (395) 505 (395) 509 (390) 519 (406) 519 (406) 519 (406) 519 (406) 499 (398)

0.20 0.19 0.19 0.23 0.14 0.20 0.13 0.09

a

PBQ-b-[P4VPMe]OTf and PS-b-[PBQPyMe]OTf were studied in DMF and all other samples in THF.

reported data for PBQ and related copolymers,9,11,18 the absorption and emission spectra of PBQ-b-P4VP show maxima at 393 and 505 nm, respectively, and the fluorescence quantum yield is ΦF = 0.19. The new monomer BQPy and the corresponding homopolymers and block copolymers all show slightly red-shifted absorption and emission maxima at 406 and 519 nm, respectively. This effect is attributed to the larger conjugated π-system in the pyridylquinolate ligand.6 The quantum yield of ΦF = 0.20 for PS-b-PBQPy-1 is similar to that of the monomer BQPy (ΦF = 0.23). However, the quantum yields of PBQPy and PS-b-PBQPy-2 are comparatively lower (ΦF = 0.14 and 0.13), possibly because the CTA at the chain end acts as a 2907

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Figure 3. TEM image (A) and number-averaged size distribution histogram (B) of PBQ-b-P4VP aggregates in toluene. TEM image (C) and numberaveraged size distribution histogram (D) of PS-b-PBQPy-1 aggregates in methanol. TEM image (E) and number-averaged size distribution histogram (F) of PS-b-PBQPy-1 aggregates in cyclohexane.

Scheme 2. Quaternization of Pyridine-Functionalized Boron Quinolate Block Copolymers

copolymers remained unchanged, indicating the excellent stability of the boron chromophores (Figures S10 and S11). The photophysical data of the quaternized block copolymers were acquired in DMF as a good solvent for both blocks and compared to those of the precursor polymers (Figure 5 and Table 2). New absorption bands attributed to formation of the pyridinium groups appeared at 310 and 360 nm, respectively, for PBQ-b-[P4VPMe]OTf and PS-b-[PBQPyMe]OTf. The boron chromophores in PBQ-b-[P4VPMe]OTf showed essentially the same absorption and emission profiles as the PBQ-b-P4VP precursor in THF. In contrast, the absorption and emission maxima of the quaternized boron chromophores of PS-b[PBQPyMe]OTf were blue-shifted to 398 and 499 nm, respectively, due to the electron-withdrawing effect of the pyridinium moieties. 6,20 The quantum yield for PS-b[PBQPyMe]OTf in DMF was measured to be ΦF = 0.09, about half that of the precursor PS-b-PBQPy-1 (in THF). The quaternization reaction is not only useful for postmodification but also a powerful tool for nanostructured material fabrication. The Chen group developed a quaternization crosslinking method to prepare polymeric nanoparticles.21 They dissolved a PS-b-P4VP block copolymer and 1,4-dibromobutane as a cross-linker in a common solvent, DMF. Two bromoalkyl units from one cross-linker molecule reacted with the pyridyl units from different block copolymer chains slowly at room temperature, eventually leading to core-cross-linked polymeric nanoparticles. Because a relatively long unreactive PS block was used, core−core coupling could be prevented. When we added 1,4-dibromobutane to PBQ-b-P4VP with its short unreactive PBQ block and much longer cross-linkable P4VP block (molar ratio of C4H8Br2/4VP = 2.0), the solution in DMF became increasingly viscous and gelation occurred over 6 days. Figure 6A shows the photographs of the resulting cross-linked block copolymer gel in natural light and upon UV irradiation (365 nm). The gel showed the same absorption and emission color as the

(Figure S10). For PS-b-PBQPy, the feasibility and selectivity of the pyridylquinolate quaternization were first examined on the model compound M-BQPy (Figure 4). Smooth conversion to the methylated species M-[BQPyMe]OTf without degradation of the boron quinolate chromophore was observed. The NMR signals of the pyridine moiety shifted downfield from 7.61 and 8.71 ppm to 8.37 and 9.04 ppm, respectively, and the methyl signal of the pyridinium group appeared at 4.37 ppm (Figure 4). For polymer PS-b-[PBQPyMe]OTf the corresponding peaks of the pyridinium moieties were observed at almost identical chemical shifts, and the quaternization was quantitative based on 1 H NMR integration. The 11B NMR signals of both block 2908

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Figure 4. Expansions of the 1H NMR spectra of model compound M-BQPy, quaternized M-[BQPyMe]OTf, and quaternized block copolymer PS-b-[PBQPyMe]OTf in DMSO-d6.

Figure 6. (A) Photographs of PBQ-b-P4VP cross-linked gel under ambient and 365 nm UV light. (B) SEM image of PBQ-b-P4VP crosslinked gel.

ZnCl2 as the metal ion source because Zn2+ is known to readily coordinate to pyridine ligands, and as a closed-shell d10 ion, it is not optically active itself. Experimentally, 50 μL of a ZnCl2 solution in methanol (12.1 mg/mL) was added to 20 mL of a chloroform solution containing 5.0 mg of PBQ-b-P4VP under stirring. As shown in Figure 7A, the resulting mixture exhibited strong light

Figure 5. UV−vis absorption and fluorescence spectra of boron quinolate block copolymers. (A) PBQ-b-P4VP (THF, λexc = 395 nm) and PBQ-b-[P4VPMe]OTf (DMF, λexc = 406 nm). (B) PS-b-PBQPy-1 (THF, λexc = 390 nm) and PS-b-[PBQPyMe]OTf (DMF, λexc = 398 nm).

block copolymer precursor. Different from the example of corecross-linked PS-b-P4VP polymeric nanoparticles discussed above, in our case, the nonreactive PBQ block is much shorter than the P4VP block and thus not long enough to stabilize small particles. Therefore, the extended cross-linking reaction of the P4VP blocks led to gel formation. Figure 6B shows the surface morphology of the highly cross-linked gel sample. Supramolecular Metal Complexes. Pyridine-functionalized block copolymers are also useful as polymeric ligands (Lewis bases) for polymer/inorganic hybrid material fabrication.12,16,22 In solution, the pyridine units can coordinate to metal cations or Lewis acidic metal complexes, forming supramolecular complexes that can then further organize into higher assembled nanostructures. We chose

Figure 7. (A) Number-averaged size distribution histogram of PBQ-bP4VP/ZnCl2 complex (red) and PBQ-b-P4VP (black) in chloroform/ methanol mixture (v/v = 400/1) (inset: photograph of a solution of PBQ-b-P4VP/ZnCl2 complex showing strong light scattering in the path of a green laser). (B) TEM image of PBQ-b-P4VP/ZnCl2 complex (inset: photograph of a solution of PBQ-b-P4VP/ZnCl2 complex under 365 nm UV irradiation).

scattering, indicating the formation of colloidal particles. The ⟨Dh⟩ of the supramolecular assembly of PBQ-b-P4VP/ZnCl2 was 2909

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177 ± 73 nm based on DLS measurements. A control experiment with PBQ-b-P4VP in chloroform/methanol mixture (v/v = 400/1) in the absence of ZnCl2 gave ⟨Dh⟩ = 11.2 ± 3.1 nm, which is indicative of single polymer chains. Single chains are expected because the trace amount of selective methanol in the mixture is not sufficient for block copolymer aggregation to occur. We conclude that the coordination between the P4VP blocks and Zn2+ is responsible for the observed aggregation behavior. The TEM image of PBQ-b-P4VP/ZnCl2 complex (Figure 7B) showed highly branched structures with much larger feature sizes in comparison to the DLS measurement, which may be a result of further aggregation on the copper grid upon solvent evaporation. The absorption and emission spectra of PBQ-b-P4VP/ZnCl2 complex were almost identical to those of the block copolymer itself (Figure S3). Similarly, the PS-b-PBQPy-1/ZnCl 2 complex formed extended aggregates with ⟨Dh⟩ = 496 ± 204 nm (Figure 8A,B).

Besides the propensity for nanostructure formation, BQPybased polymers also have potential as chemical sensors and responsive luminescent materials. Novel boron quinolate− pyridine−metal complex structures can be generated through coordination interaction. To explore this possibility, we chose to study the complex formation of zinc meso-tetraphenylporphyrin (ZnTPP) and PS-b-PBQPy-1. Porphyrins are attractive because of their exceptional photochemical and photophysical properties and potential applications in the areas of sensors, electronics, photosensitized solar cells, and light-emitting devices.23 A particularly interesting aspect is that the Q-bands of ZnTPP overlap with the emission of PBQPy polymers as illustrated in Figure S4. Energy transfer between boron quinolate and ZnTPP should therefore be possible, allowing for emission color tuning. We decided to study the structures and photophysical properties of PS-b-PBQPy-1/ZnTPP complex in CH2Cl2 as a good solvent for both blocks and in cyclohexane, which is a selective solvent for the PS block. On the basis of the reported binding constants for the complex formation between (molecular) pyridine and ZnTPP of 6900 M−1 in CH2Cl2 and 25 100 M−1 in cyclohexane,24 in both solvents partial complexation of the pyridyl pendant groups in PS-b-PBQPy-1 with ZnTPP is expected as illustrated in Figure 9.

Figure 8. (A) Number-averaged size distribution histograms of PS-bPBQPy-1/ZnCl2 complex (red) and PS-b-PBQPy-1 (black) in chloroform/methanol mixture (v/v = 500/1). (B) TEM image of PS-bPBQPy-1/ZnCl2 complex. (C) Number-averaged size distribution histograms of PS-b-PBQPy-2/ZnCl2 complex (red) and PS-b-PBQPy2 (black) in chloroform/methanol mixture (v/v = 400/1) (inset: photograph of a solution of PS-b-PBQPy-2/ZnCl2 complex showing strong light scattering in the path of a green laser). (D) TEM image of PS-b-PBQPy-2/ZnCl2 complex (inset: photograph of a solution of PS-b-PBQPy-2/ZnCl2 complex under 365 nm UV irradiation).

Figure 9. Structure of supramolecular complexes formed from PS-b-PBQPy-1 and ZnTPP.

We added varying amounts of a ZnTPP solution in CH2Cl2 to a solution of PS-b-PBQPy-1 in CH2Cl2. The mass ratios of ZnTPP/PS-b-PBQPy-1 were set to 0.1, 0.2, 0.5, and 1.0, respectively, corresponding to ZnTPP/pyridine molar ratios of 0.085, 0.17, 0.43, and 0.85. Under these conditions the polymer chains are moleculary dissolved and partial binding of ZnTPP to the pyridine functionalities occurs (vide inf ra). Cyclohexane as a PS-selective solvent was then added dropwise to the mixture under stirring and the lower boiling solvent CH2Cl2 was allowed to evaporate. In cyclohexane, the PBQPy/ZnTPP complex is expected to aggregate, while the PS block should sustain the solubility of the supramolecular assemblies. Indeed, analysis of the cyclohexane solutions by TEM and DLS revealed the formation of supramolecular coassemblies of PS-b-PBQPy-1/ ZnTPP complex. Similar to the self-assembly of PS-b-PBQPy-1 in cyclohexane (see Figure 3E), the PS-b-PBQPy-1/ZnTPP complexes formed assemblies that most likely consist of a

To study the block length effect on the complex morphology, PS-b-PBQPy-2 with its relatively longer PS block was also used for Zn2+ complexation. The ⟨Dh⟩ of PS-b-PBQPy-2/ZnCl2 complex was determined to be 73 ± 27 nm (Figure 8C). Different from the complexes with relatively longer pyridinecontaining blocks, the TEM image of PS-b-PBQPy-2/ZnCl2 complex (Figure 8D) showed spherical assemblies with a core that likely consists of PBQPy/ZnCl2 complex and a PS shell. Extended aggregates were not observed because of the presence of a relatively longer PS block. As a result of pyridine coordination to Zn2+ ions, the absorption and emission spectra of PS-b-PBQPy-2/ZnCl2 complex (λabs = 404, λem = 512 nm; Figure S3) were slightly blue-shifted relative to those of the free block copolymer, but not as much as for the fully methylated product PS-b-[PBQPyMe]OTf.20 2910

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PBQPy/ZnTPP complex core and a PS shell; these assemblies then further aggregate into branched structures due to the moderate solubility of PS in cyclohexane (Figure 10A−D). Based on DLS measurements, the number-averaged diameters of these supramolecular coassemblies were all around 1000 nm (Figure 10E−H). It should be kept in mind that these data correspond neither to the sizes of the initial particles of PS-b-PBQPy-1/ ZnTPP complexes nor to the branched structures observed by TEM, but represent the statistically averaged hydrodynamic diameters of branched aggregates in cyclohexane.

Photographs of solutions of ZnTPP (CH2Cl2), PS-b-PBQPy-1 aggregates (cyclohexane), and PS-b-PBQPy-1/ZnTPP complex aggregates (cyclohexane) under natural and UV light are displayed in Figures 11A and 11B, and the corresponding photographs for PS-b-PBQPy-1/ZnTPP complex solutions in CH2Cl2 are shown in Figure S6. Because of the poor solubility in cyclohexane, solutions of ZnTPP were only examined in CH2Cl2 where they show a weak but distinct red emission. In the absence of ZnTPP, both the molecularly dissolved block copolymer in CH2Cl2 and the PS-b-PBQPy-1 aggregate solution in cyclohexane

Figure 10. (A−D) TEM images of PS-b-PBQPy-1/ZnTPP complex at mass ratios of 0.1, 0.2, 0.5 and 1.0; (E−H) Number-averaged size distribution histograms of PS-b-PBQPy-1/ZnTPP complex at mass ratios of 0.1 (⟨Dh⟩ = 867 ± 108 nm), 0.2 (⟨Dh⟩ = 1132 ± 103 nm), 0.5 (⟨Dh⟩ = 986 ± 139 nm), and 1.0 (⟨Dh⟩ = 1189 ± 151 nm). The corresponding molar ratios of Zn/Py are 0.085, 0.170, 0.43, and 0.85, respectively.

Figure 11. (A, B) Photographs of PS-b-PBQPy-1/ZnTPP supramolecular coassembly solutions under natural light and UV irradiation at 365 nm. From left to right: ZnTPP in CH2Cl2, PS-b-PBQPy-1 aggregates in cyclohexane, PS-b-PBQPy-1/ZnTPP complex aggregates in cyclohexane (concentration of PS-bPBQPy-1 = 0.05 mg/mL; mass ratios ZnTPP/PS-b-PBQPy-1 = 0.1, 0.2, 0.5 and 1.0; molar ratios ZnTPP/Py = 0.085, 0.17, 0.43 and 0.85; λexc = 416 nm). (C, D) Absorption and fluorescence spectra (λexc = 416 nm) of PS-b-PBQPy-1/ZnTPP supramolecular aggregates in cyclohexane (samples with large ZnTPP content were further diluted with cyclohexane). The new bands that are attributed to pyridine-complexed ZnTPP are indicated with arrows. 2911

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block copolymer self-assembly upon addition of cyclohexane as a block-selective solvent.

are light yellow in color and strongly green-emissive. With increasing amounts of added ZnTPP the emission color changed gradually from green to yellow to red in CH2Cl2 as a common solvent (Figure S6), indicative of dual emission from the boron quinolate and ZnTPP chromophores. In contrast, only the red emission from ZnTPP was observed for all the PS-b-PBQPy-1/ ZnTPP complex solutions as shown in Figure 11B. Our observations suggest that energy transfer from the boron quinolate chromophores to the ZnTPP molecules is favored in cyclohexane. This phenomenon is likely a direct result of aggregate formation, and a contributing factor could be the incorporation of “uncomplexed” ZnTPP into the rigid core of the nanostructures. We further verified our findings by performing UV−vis absorption and fluorescence measurements. In cyclohexane, uncomplexed PS-b-PBQPy-1 showed an absorption maximum at 416 nm, and in agreement with literature data,25 ZnTPP absorbs at 419 nm (strong Soret band), 547 nm (β band), and 585 nm (α band). Although for the PS-b-PBQPy-1/ZnTPP mixtures quantitative intensity analysis of the spectra was somewhat compromised by the strong scattering of light from the large supramolecular aggregates (∼800−1000 nm), the axial coordination of block copolymer-bound pyridyl units to ZnTPP was clearly evident from the spectral data (Figure 11C). A new set of red-shifted absorptions at about 564 and 603 nm for the Q bands and at 428 and 442 nm for the Soret band are in good agreement with literature data25 for the pyridine complex of ZnTPP. Consistent with prior reports, the relative intensity of the longer wavelength Q-band (α band) increased significantly.26 The samples to which larger quantities of ZnTPP were added contained more and more uncomplexed ZnTPP based on the increasing intensity of the bands at 547 and 585 nm. This is expected, given that only partial complexation to pyridine should occur in the concentration range of the measurements based on the known binding constant for pyridine and ZnTPP in cyclohexane;24 moreover, complexation is likely limited by steric constraints and neighboring groups effects in the polymer chain. Importantly, all our data are consistent with the copolymer structure proposed in Figure 9 (x ≤ 0.5). The binding constant for pyridine complexation to ZnTPP in CH2Cl2 is relatively smaller,25 and indeed, the UV−vis data of solutions of PS-bPBQPy-1 and ZnTPP are consistent with a lower degree of complexation at similar concentrations (Figure S5). This implies that supramolecular Zn−pyridine complexation is enhanced in cyclohexane, while block copolymer self-assembly is triggered by addition of cyclohexane to the mixture in CH2Cl2. Looking at the corresponding emission profiles, uncomplexed PS-b-PBQPy-1 emits at 518 nm in cyclohexane, while ZnTPP shows emission bands at 598 and 643 nm upon excitation at 547 nm (see Figure S4). The emission spectra of the PS-b-PBQPy-1/ ZnTPP aggregates in cyclohexane upon excitation at 416 nm are displayed in Figure 11D. Even at the very low molar ratio of ZnTPP/pyridine = 0.085, the characteristic emission from ZnTPP (598 and 643 nm) rather than that from the boron quinolate chromophore (519 nm) was observed, indicating almost complete energy transfer from boron quinolate to ZnTPP. With increasing molar ratio, the emission of ZnTPP weakened and disappeared at a molar ratio of 0.85, presumably due to self-quenching of ZnTPP in the supramolecular assemblies. Importantly, energy transfer is not as efficient in CH2Cl2 as the solvent, where dual emission from the boron quinolate and ZnTPP chromophores is evident (Figure S6), underlining the importance of metal ligand complexation and

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CONCLUSIONS RAFT polymerization proved to be an excellent method for the controlled synthesis of pyridine-functionalized luminescent boron quinolate block copolymers. The new amphiphilic block copolymers PBQ-b-P4VP and PS-b-PBQPy served as versatile building blocks for nanostructure formation. The pyridine functionalities lend themselves to quaternization, which led to facile adjustment of the solubility characteristics of the block copolymers and, more importantly, allowed us to prepare a luminescent cross-linked polymer gel. The pyridine-functionalized block copolymers also form supramolecular complexes with metal ions and Lewis acids. The addition of ZnCl2 to the block copolymers in chloroform induced the formation of spherical micelles or even larger aggregates as a result of pyridine coordination. The supramolecular coassembly of PS-b-PBQPy-1 and the porphyrin chromophore ZnTPP in cyclohexane resulted in extended branched structures with controllable emission properties. Energy transfer from the boron quinolate chromophores to ZnTPP was facilitated in these complexes. We conclude that boron quinolate block copolymers that are equipped with pyridine functionalities are powerful building blocks for the generation of luminescent and multifunctional nanostructures with potential applications in biological imaging and more generally as tunable and responsive fluorescent materials.

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EXPERIMENTAL SECTION

General Methods. The 499.9 MHz 1H, 125.7 MHz 13C, and 160.4 MHz 11B NMR spectra were recorded on a Varian Inova 500 MHz spectrometer. The spectrometer was equipped with a boron-free probe, and the 11B NMR spectra were acquired using boron-free quartz NMR tubes. The 1H and 13C NMR spectra were referenced internally to the solvent peaks and the 11B NMR spectra externally to BF3·Et2O (δ = 0) in C6D6. The following abbreviations are used: Q = 8-hydroxyquinolate, Ph = phenyl, St = styryl, Vi = vinyl, Py = pyridyl, d = doublet, dd = double doublet. High-resolution MALDI-MS (benzo[α]pyrene matrix) data were obtained on an Apex Ultra 7.0 Hybrid FTMS (Bruker Daltonics).

GPC-RI analyses were performed in THF (1.0 mL/min), THF/ pyridine (95/5 v/v; 1.0 mL/min), or DMF with 0.2% w/v [Bu4N]Br (0.50 mL/min) using a Waters Empower system equipped with a 717plus autosampler, a 1525 binary HPLC pump, a 2487 dual λ absorbance detector, and a 2414 refractive index detector. Three styragel columns (Polymer Laboratories; two 5 μm Mix-C and one 10 μm Mix-D), which were kept in a column heater at 35 °C (THF, THF/pyridine), or a set of two poly(vinyl alcohol) columns (Shodex Asahipak; one 5 μm GF-510 HQ and one 9 μm GF-310 HQ) at 65 °C (DMF) were used for separation. The columns were calibrated with narrow polystyrene standards (Polymer Laboratories, Varian Inc.). The dynamic light scattering (DLS) measurements were performed at 25.0 ± 0.1 °C with a Malvern Zetasizer Nano-ZS instrument, equipped with a 4 mW, 633 nm He−Ne laser, and an Avalanche photodiode detector at an angle of 173°. 2912

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warmed up to 50 °C for another 3 h. The reaction was worked up with CH2Cl2/10% aqueous NH4[HCO3], and the organic extracts were dried over MgSO4. The solution was concentrated and then subjected to column chromatography on silica gel with hexanes/ethyl acetate as the eluent. The monomer was obtained as a yellow solid. Yield: 1.22 g, 60%. 11B NMR: δ = 11.9 ppm (w1/2 = 400 Hz). 1H NMR (499.895 MHz, CDCl3): δ = 8.76 (d, 3 J = 5.5 Hz, 2H, Py-H2,6), 8.67 (d, 3J = 5.0 Hz, 1H, Q-H2), 8.55 (d, 3J = 8.5 Hz, Q-H4), 7.71 (d, 3J = 8.0 Hz, 1H, Q-H6), 7.67 (dd, 3J = 5.0 and 8.5 Hz, 1H, Q-H3), 7.47 (d, 3J = 8.5 Hz, 2H, Ph), 7.43 (d, 3J = 6.0 Hz, 2H, Py-H3,5), 7.40 (d, 3J = 8.0 Hz, 2H, Ph), 7.35 (d, 3J = 8.0 Hz, 2H, Ph), 7.30 (d, 3J = 8.0 Hz, 2H, Ph/St), 7.26 (d, 3J = 8.0 Hz, 1H, Q-H7), 6.71 (dd, 3J = 11.0, 17.5 Hz, 1H, Vi), 5.72 (d, 3J = 17.5 Hz, 1H, Vi), 5.18 (d, 3J = 11.0 Hz, 1H, Vi), 1.31 (s, 9H, CMe3). 13C NMR (125.698 MHz, CDCl3): δ = 159.6, 150.6, 150.0, 146.2, 140.0, 138.0, 137.4, 137.1, 136.6, 133.9, 132.4, 132.0, 126.4, 125.7, 124.8, 124.5, 123.6, 123.2, 113.2, 110.0, 34.6, 31.6, B−C not observed. UV− vis in THF: λmax = 406 nm, ε = 4140 cm−1 M−1. Fluorescence in THF: λem = 519 nm (λexc = 406 nm), Φ = 23%. High-resolution MALDI-MS (negative mode, benzo[α]pyrene): m/z 468.2359 ([M]−, 100%, calcd for 12 C321H2911B14N216O 468.2367). Synthesis of Model Compound M-BQPy. In a glovebox, a solution of BBr3 (117 mg, 0.466 mmol) in 5 mL of CH2Cl2 was prepared in a flamedried Schlenk flask and cooled to −20 °C. A precooled (−20 °C) solution of 1-trimethylstannyl-4-tert-butylbenzene (277 mg, 0.933 mmol) in 5 mL of CH2Cl2 was added dropwise under stirring. The mixture was allowed to warm up to room temperature and stirred for 3 h. Then a solution of 5-pyridyl-8-methoxyquinoline (110 mg, 0.466 mmol) in 5 mL of CH2Cl2 was added dropwise under stirring. After 2 h, the Schlenk flask was removed from the glovebox, and the crude product was subjected to column chromatography on silica gel with CH2Cl2 as the eluent, followed by crystallization in a hexanes/CH2Cl2 mixture. The product was obtained as yellow crystals. Yield: 160 mg, 69%. 11B NMR: δ = 11.7 ppm (w1/2 = 430 Hz). 1H NMR (499.895 MHz, DMSO-d6): δ = 9.17 (d, 3J = 5.0 Hz, 1H, Q-H2), 8.73 (d, 3J = 8.5 Hz, 1H, Q-H4), 8.71 (d, 3J = 5.5 Hz, 2H, Py-H2,6), 7.92 (dd, 3J = 8.5 and 5.0 Hz, 1H, Q-H3), 7.82 (d, 3J = 7.9 Hz, 1H, Q-H6), 7.61 (d, 3J = 5.5 Hz, 2H, Py-H3,5), 7.31 (d, 3J = 8.0 Hz, 4H, Ph), 7.28 (d, 3J = 7.9 Hz, 1H, Q-H7), 7.25 (d, 3J = 8.0 Hz, 4H, Ph), 1.23 (s, 18H, CMe3). 1H NMR (499.895 MHz, CDCl3): δ = 8.76 (d, 3J = 5.5 Hz, 2H, Py-H2,6), 8.70 (d, 3J = 5.0 Hz, 1H, Q-H2), 8.54 (d, 3J = 8.5 Hz, Q-H4), 7.70 (d, 3J = 8.0 Hz, 1H, Q-H6), 7.67 (dd, 3J = 5.0 and 8.5 Hz, 1H, Q-H3), 7.43 (d, 3J = 8.0 Hz, 4H, Ph), 7.43 (2H, overlapped, Py-H3,5), 7.33 (d, 3J = 8.0 Hz, 4H, Ph), 7.26 (d, 3J = 8.0 Hz, 1H, Q-H7), 1.31 (s, 18H, CMe3). 13C NMR (125.698 MHz, CDCl3): δ = 159.7, 150.6, 149.9, 146.2, 143.4 (broad B−C), 140.0, 138.0, 136.9, 133.9, 131.9, 126.4, 124.7, 124.4, 123.5, 123.0, 109.9, 34.6, 31.6. UV−vis in THF: λmax = 407 nm, ε = 5510 cm−1 M−1. Fluorescence in THF: λem = 520 nm (λexc = 407 nm), Φ = 24%. High-resolution MALDI-MS (negative mode, benzo[α]pyrene): m/z 498.2822 ([M]−, 100%, calcd for 12 C341H3511B14N216O 498.2837). Quaternization of M-BQPy with Methyl Triflate: Synthesis of M-[BQPyMe]OTf. To a solution of M-BQPy (50.0 mg, 0.107 mmol) in 2 mL of chloroform at 0 °C was added dropwise a solution of methyl triflate (35.0 mg, 0.213 mmol) in 1 mL of chloroform. After stirring for 2 h, the solvent was removed under vacuum, and the crude product was recrystallized from a solution in CH2Cl2/hexanes. The product was obtained as yellow needle-like crystals. Yield: 58 mg, 86%. 11B NMR: δ = 13.0 ppm (w1/2 = 850 Hz). 1H NMR (499.895 MHz, DMSO-d6): δ = 9.26 (d, 3J = 5.1 Hz, 1H, Q-H2), 9.04 (d, 3J = 6.3 Hz, 2H, Py-H2,6), 8.84 (d, 3J = 8.5 Hz, 1H, Q-H4), 8.37 (d, 3J = 6.3 Hz, 2H, Py-H3,5), 8.08 (d, 3 J = 8.5 Hz, 1H, Q-H6), 8.04 (dd, 3J = 8.5 and 5.1 Hz, 1H, Q-H3), 7.39 (d, 3J = 8.0 Hz, 1H, Q-H7), 7.31 (d, 3J = 8.0 Hz, 4H, Ph), 7.26 (d, 3J = 8.0 Hz, 4H, Ph), 4.37 (s, 3H, NMe), 1.24 (s, 18H, CMe3). 1H NMR (499.895 MHz, CDCl3): δ = 8.83 (d, 3J = 5.0 Hz, 2H, Py-H2,6; overlap with 1H, Q-H4), 8.73 (d, 3J = 4.9 Hz, 1H, Q-H2), 8.05 (d, 3J = 4.5 Hz, 2H, Py-H3,5), 7.84 (d, 3J = 8.0 Hz, 1H, Q-H6), 7.83 (dd, not resolved, Q-H3), 7.38 (d, 3J = 8.0 Hz, 4H, Ph), 7.32 (d, 3J = 8.0 Hz, 4H, Ph), 7.25 (d, 3J = 8.0 Hz, 1H, Q-H7), 4.48 (s, 3H, NMe), 1.29 (s, 18H, CMe3). 13C NMR (125.698 MHz, CDCl3): δ = 162.5, 155.2, 150.2, 145.4, 142.6 (br, B−C), 141.1, 138.0, 136.7, 136.4, 131.9, 127.4, 126.2, 125.4, 124.9, 118.5, 110.3, 48.3, 34.6, 31.6. UV−vis in THF: λmax = 401 nm, 363 nm, ε = 13 940 cm−1 M−1. Fluorescence in THF: λem = 498 nm (λexc = 401 nm),

UV−vis absorption data were acquired on a Varian Cary 500 UV−vis/ NIR spectrophotometer or a Agilent Technologies Cary 5000 UV−vis/ NIR spectrophotometer. The mass fraction of the boron quinolate block in the copolymers was determined by preparing solutions of known concentration and measuring the UV−vis absorbance at the absorption maximum of the respective boron chromophore. Assuming that the absorptivity of the individual chromophores is independent of the polymer architecture, we used the molar absorptivity of the homopolymers (ε394 = 2460 M−1 cm−1 per repeating unit of PBQ and ε406 = 4730 M−1 cm−1 per repeating unit of PBQPy) as the reference. The fluorescence data were acquired on a Varian Cary Eclipse fluorescence spectrophotometer or a Horiba Fluorolog-3 instrument equipped with a 450 W Xe source. Quantum yields were measured on the Varian Cary Eclipse fluorescence spectrophotometer with optically dilute solutions (A < 0.1). Anthracene was used as the standard, and the quantum yield of anthracene (0.33 in THF) was adopted from the Handbook of Photochemistry.27 Sample solutions were prepared using a microbalance (±0.1 mg) and volumetric glassware. The quantum yields were calculated by plotting a graph of integrated fluorescence intensity vs absorbance of at least four solutions with increasing concentration. The gradient of the graph is proportional to the quantum yield. Transmission electron microscopy (TEM) studies were conducted on a FEI Tecnai 12 electron microscope operated at 80 kV. One drop of polymer micelle solution was cast on a copper grid with a carbon coating, and the solvent was allowed to slowly evaporate under ambient conditions. Scanning electron microscopy was performed on a Hitachi S-4800 field emission scanning electron microscope (FE-SEM, Hitachi Co. Ltd. S-4800) at 3 kV. Materials. 1,4-Dioxane and THF were distilled from Na/ benzophenone prior to use. Azobis(isobutyronitrile) (AIBN) initiator was recrystallized from methanol. Styrene and 4-vinylpyridine were purified by passing through a neutral alumina column and then distilled under reduced pressure. 1-Trimethysilyl-4-tert-butylbenzene,6 4-trimethylstannylstyrene,28 5-bromo-8-methoxyquinolone,29 the boron quinolate monomer BQ,11 and the (macro)chain transfer agents (CTAs) benzyl dithiobenzoate (BDTP),30 PS-1 and PS-2,9 and PBQ11 were synthesized in analogy to literature procedures. All other solvents and chemicals were commercial products and used as received without further purification. Synthesis of 5-Pyridyl-8-methoxyquinoline. The 5-pyridyl-8methoxyquinoline ligand was prepared in analogy to a method reported by Anzenbacher.31 Under nitrogen protection, a Schlenk tube was loaded with 5-bromo-8-methoxyquinoline (3.87 g, 16.3 mmol), pyridylboronic acid (2.00 g, 16.3 mmol), Pd(PPh3)4 (875 mg, 0.757 mmol, 5 mol %), 50 mL of degassed toluene, and a mixture of degassed water/EtOH = 25 mL/25 mL. The tube was sealed and kept at 100 °C for 3 days. The mixture was extracted with CH2Cl2, and the combined CH2Cl2 solutions were washed with water and dried over MgSO4. The crude solution was concentrated on a rotary evaporator and subjected to silica gel column chromatography with hexanes/THF mixture as the eluent, followed by crystallization from a solution in hexanes/THF = 1/1. The product was obtained as white crystals. Yield: 2.39 g, 62%. 1H NMR (499.895 MHz, CDCl3): δ = 9.00 (d, 3J = 4.0 Hz, 1H, Q), 8.74 (d, 3J = 5.5 Hz, 2H, Py), 8.22 (d, 3J = 8.5 Hz, 1H, Q), 7.47 (d, 3J = 8.5 Hz, 1H, Q), 7.46 (dd, 3J = 4.0 and 8.5 Hz, 1H, Q), 7.41 (d, 3J = 5.5 Hz, 2H, Py), 7.15 (d, 3J = 8.0 Hz, 1H, Q), 4.17 (s, 3H, Me). 13C NMR (125.698 MHz, CDCl3): δ = 156.0, 150.1, 149.6, 147.6, 140.4, 133.6, 129.4, 127.8, 127.1, 125.2, 122.3, 107.3, 56.4. Synthesis of BQPy. In a glovebox, a solution of BBr3 (1.30 g, 5.17 mmol) in 15 mL of toluene was prepared in a flame-dried Schlenk flask and cooled to −20 °C. Then, a precooled (−20 °C) solution of 1-trimethysilyl-4-tert-butylbenzene (0.89 g, 4.31 mmol) in 15 mL of toluene was added dropwise under stirring. The mixture was allowed to warm up to room temperature and stirred for 6 h. After removal of the solvent under vacuum, the residue was redissolved in 5 mL of anhydrous CH2Cl2. A solution containing 4-trimethylstannylstyrene (1.15 g, 4.31 mmol) in 20 mL of CH2Cl2 was added dropwise under stirring at −20 °C. After 6 h, a solution of 5-pyridyl-8-methoxyquinoline (1.02 g, 4.32 mmol) in 20 mL of CH2Cl2 was added dropwise under stirring. The mixture was stirred for 3 h at room temperature and then 2913

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Φ = 44%. High-resolution MALDI-MS (positive mode, benzo[α]pyrene): m/z 513.3079 ([M]+, 100%, calcd for 12C351H3811B14N216O 513.3072). Synthesis of PBQ-b-P4VP. In a Schlenk tube PBQ-CTA (250 mg, 9.06 μmol; Mn, GPC = 8710 g/mol, PDI = 1.28, Mn,MALLS = 27 600 g/mol), AIBN (0.68 mg, 4.14 μmol), and 4VP (2.93 g, 27.9 mmol) were dissolved in 1.0 mL of dioxane ([4VP]/[PBQ-CTA]/[AIBN] = 3080/ 1/0.46). After three freeze−pump−thaw cycles, the tube was immersed in an oil bath at 70 °C for 8.0 h with stirring. The polymerization was terminated by placing the tube in liquid nitrogen. The polymer was precipitated three times by dropwise addition of a DMF solution to a 10-fold volume of diethyl ether. The product was obtained as a yellow powder after drying in high vacuum. Yield: 1.15 g (31% conversion assuming quantitative recovery). GPC-RI (DMF with 0.2% [Bu4N]Br): Mn,GPC = 44 400 g/mol, PDI = 1.27. The PBQ mass fraction is 0.25 based on UV−vis analysis using ε395 = 3370 cm−1 M−1. Synthesis of PBQPy. In a Schlenk tube, BQPy (350 mg, 0.747 mmol), benzyl dithiobenzoate (BDTB) (4.56 mg, 0.0187 mmol), and AIBN (0.77 mg, 0.0047 mmol) were dissolved in 1.5 mL of anisole ([BQPy]/ [CTA]/[AIBN] = 33.5/1/0.25). After three freeze−pump−thaw cycles, the Schlenk tube was immersed in an oil bath at 70 °C for 10 h. The crude product was precipitated twice by dropwise addition of a THF solution of the polymer to a 10-fold volume of diethyl ether. The solid was collected by filtration and dried under high vacuum, affording a yellow powder. Yield: 110 mg (30% conversion assuming quantitative recovery). 11B NMR: δ = 7.6 ppm (w1/2 = 650 Hz, CDCl3). 1H NMR (499.895 MHz, CDCl3): δ = 8.9−8.0 (3H), 8.0−5.5 (14H), 2.5−1.5 (3H, backbone), 1.2 (9H, tBu). 13C NMR (125.698 MHz, CDCl3): δ = 159.7, 150.5, 150.2, 146.1, 143.9, 140.2, 137.6, 136.9, 133.7, 131.9, 128.3, 127.1, 126.1, 124.7, 124.4, 123.8, 122.9, 109.4, 34.6, 31.6. GPC-RI (THF/pyridine = 95/5): Mn = 3420 g/mol, PDI = 1.28. UV−vis in THF: λmax = 406 nm, ε406 = 4750 cm−1 M−1. Fluorescence in THF: λem = 519 nm (λexc = 406 nm), Φ = 14%. Synthesis of PS-b-PBQPy-1. In a Schlenk tube, PS-1 macro-CTA (150 mg, 40 μmol; Mn,GPC = 3840 g/mol), BQPy (940 mg, 2.00 mmol), and AIBN (1.31 mg, 8.0 μmol) were dissolved in 5.0 mL of 1,4-dioxane ([BQPy]/[PS-1]/[AIBN] = 50/1/0.20). After three freeze−pump− thaw cycles, the Schlenk tube was immersed in an oil bath at 80 °C for 5 h. The polymerization was terminated by placing the tube in liquid nitrogen. The polymer was then precipitated three times from THF into a 10-fold volume of diethyl ether, followed by drying under high vacuum. The product was obtained as a yellow powder. Yield: 450 mg (32% conversion assuming quantitative recovery). GPC-RI (THF/ pyridine =95/5): Mn = 9990 g/mol, PDI = 1.23. UV−vis in THF: λmax = 406 nm. Fluorescence in THF: λem = 519 nm (λexc = 406 nm), Φ = 0.20. The PBQPy mass fraction is 0.78 based on UV−vis analysis using ε406 = 4750 cm−1 M−1. Synthesis of PS-b-PBQPy-2. In a Schlenk tube, PS-2 macro-CTA (79 mg, 8.3 μmol; Mn,GPC = 8130 g/mol, Mn,GPC = 9470 g/mol), BQPy (187 mg, 0.400 mmol), and AIBN (0.33 mg, 2.0 μmol) were dissolved in 1.0 mL of 1,4-dioxane ([BQPy]/[PS-2]/[AIBN] = 48/1/0.24). After three freeze−pump−thaw cycles, the Schlenk tube was immersed in an oil bath at 70 °C for 8 h. The polymerization was terminated by placing the tube in liquid nitrogen. The polymer was precipitated three times from THF into a 10-fold volume of MeOH, followed by drying under high vacuum. The product was obtained as a yellow powder. Yield: 152 mg (39% conversion assuming quantitative recovery). GPC-RI (THF/pyridine = 95/5): Mn = 11 440 g/mol, PDI = 1.25. UV−vis in THF: λmax = 406 nm. Fluorescence in THF: λem = 519 nm (λexc = 406 nm), Φ = 0.13. The PBQPy mass fraction is 0.38 based on UV−vis analysis using ε406 = 4750 cm−1 M−1. Quaternization of PBQ-b-P4VP with Methyl Triflate: Synthesis of PBQ-b-[P4VPMe]OTf. To a solution of PBQ-b-P4VP (50 mg, 0.36 mmol of pyridine units) in 3.0 mL of chloroform was added a large excess of methyl triflate (200 mg, 1.22 mmol) dropwise under stirring. A yellow suspension appeared within seconds. After stirring for 1 h at room temperature, the suspension was precipitated into a 10-fold volume of diethyl ether. After filtration, the product was dried under high vacuum. The product was obtained as a light yellow powder (57 mg). 1H NMR analysis in DMSO-d6 indicated quantitative quaternization of the pyridine moieties (N-Me appeared at 4.22 ppm; the pyridine signals shifted to 8.68 and 7.48 ppm; see Figure S10).

Quaternization of PS-b-PBQPy with Methyl Triflate: Synthesis of PS-b-[PBQPyMe]OTf. To a solution of PS-b-PBQPy-1 (40 mg, 0.069 mmol of pyridine units) in 3.0 mL of chloroform was added a large excess of methyl triflate (80 mg, 0.49 mmol) dropwise under stirring. A yellow suspension appeared within seconds. After stirring for 1 h at room temperature, the suspension was precipitated into a 10-fold volume of diethyl ether/hexanes (v/v = 1/1). After filtration, the product was dried under high vacuum. The product was obtained as a yellow powder (38 mg). 1H NMR analysis indicated quantitative quaternization of the pyridine moieties (see Figure 4). Self-Assembly of PBQ-b-P4VP. (a) In toluene: The block copolymer PBQ-b-P4VP (1.9 mg) was dissolved in 1.9 mL of CH2Cl2. Toluene (10 mL) was added to the block copolymer solution dropwise under magnetic stirring. The CH2Cl2 solvent was allowed to evaporate slowly at room temperature while stirring for a period of 72 h. (b) In methanol: The block copolymer PS-b-PBQPy-1 (3.0 mg) was dissolved in 3.0 mL of CH2Cl2. Methanol (30 mL) was added to the block copolymer solution dropwise under magnetic stirring. The CH2Cl2 solvent was allowed to evaporate slowly at room temperature while stirring for a period of 72 h. Self-Assembly of PS-b-PBQPy-1. In cyclohexane: The block copolymer PS-b-PBQPy-1 (1.0 mg) was dissolved in 1.0 mL of CH2Cl2. Cyclohexane (20 mL) was added to the block copolymer solution dropwise under magnetic stirring. The CH2Cl2 solvent was allowed to evaporate slowly at room temperature while stirring for a period of 72 h. Quaternization Cross-Linking-Induced Gelation of PBQ-bP4VP in DMF. The block copolymer PBQ-b-P4VP (40 mg, 0.288 mmol of pyridine units) was dissolved in 2.0 mL of DMF, followed by addition of 1,4-dibromobutane (123 mg, 0.576 mmol). The mixture was stirred at room temperature, and gelation occurred over a period of 6 days. Coassembly of PBQ-b-P4VP and PS-b-PBQPy-1 with ZnCl2. (a) PBQ-b-P4VP (5.0 mg, 36 μmol of pyridine units) was dissolved in 20 mL of chloroform under stirring. A ZnCl2 solution (50 μL, 12.1 mg/mL in MeOH) was added under stirring. (b) PS-b-PBQPy-1 (5.0 mg, 8.6 μmol of pyridine units) was dissolved in 20.0 mL of chloroform under stirring. A ZnCl2 solution (40 μL, 10.0 mg/mL in MeOH) was added under stirring. The solutions were used directly for DLS and TEM analysis and further diluted with chloroform for acquisition of absorption and emission spectra. Coassembly of PS-b-PBQPy-1 with ZnTPP. The block copolymer PS-b-PBQPy-1 (4.0 mg, 6.9 μmol of pyridine units) was dissolved in 4.0 mL of CH2Cl2 under stirring. The block copolymer solution was divided equally into four vials. To the block copolymer solutions were added 0.4, 0.8, 2.0, and 4.0 mL of ZnTPP solution (0.25 mg/mL in CH2Cl2). Cyclohexane was added to the above mixtures dropwise under stirring. The volume of the solutions was set to be 20 mL CH2Cl2 (boiling point = 39.6 °C), allowed to partially evaporate under stirring at room temperature for 24 h, and then the volumes of solutions were reset to 20 mL by refilling with cyclohexane (boiling point = 80.7 °C). The evaporation/refill cycle was repeated three times, and the absence of CH2Cl2 was confirmed by 1H NMR analysis. The solutions were used directly for DLS and TEM analysis and further diluted with cyclohexane for acquisition of absorption and emission spectra. For comparison, the initial solutions were also diluted with CH2Cl2 to the same concentration and examined by UV−vis and fluorescence spectroscopy (see Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Additional GPC, UV−vis, and NMR data. 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]. Notes

The authors declare no competing financial interest. 2914

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant CHE-0956655. The SEM instrument used in these studies was acquired with partial support from the National Science Foundation (MRI-1039828). We thank Fang Guo for assistance with some of the UV−vis and fluorescence measurements, Jiawei Chen for mass spectrometry data collection, and Prof. John Sheridan for helpful discussions.

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dx.doi.org/10.1021/ma400310s | Macromolecules 2013, 46, 2905−2915