Syntheses and Controllable Self-Assembly of Luminescence Platinum

Article pubs.acs.org/Macromolecules

Syntheses and Controllable Self-Assembly of Luminescence Platinum(II) Plane−Coil Diblock Copolymers Nijuan Liu, Yongyue Wang, Chen Wang, Qun He, and Weifeng Bu* Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou City, Gansu Province, China S Supporting Information *

ABSTRACT: Polystyrenes have been synthesized with terminal groups of luminescence square-planar platinum(II) complexes, which can be conceptually regarded as plane−coil diblock copolymers. They can self-assemble into spherical micelles with a core of polystyrenes and a corona of planar platinum(II) complexes in the chloroform/ methanol mixture solvents, while free-standing bilayered sheets form in the toluene/methanol mixture solvents. With increasing methanol content in the latter case, where the solvent quality was highly deteriorated for polystyrene blocks, the sheetlike nanostructures are readily transformed into spherical micelles. Pt···Pt and/or π−π stacking interactions between the planar platinum(II) blocks contributed significantly to these aggregate morphologies and their evolutions, leading to remarkable luminescence enhancements. This work represents a novel approach to design and synthesize luminescence platinum(II) plane−coil diblock copolymers, in which the planar platinum(II) complexes are for the first time regarded as an individual block for the creation of block copolymers and micellelike aggregates in solution.

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spacing of ca. 3.4 Å.35−43 The latter value is indicative of intermolecular Pt···Pt and/or π−π interactions. This class of platinum(II) complexes shares typical molecular architectures and sizes with those conjugated macrocycles and aromatic discs as described above.16−19 Moreover, the Pt···Pt and/or π−π stacking interactions are really designable and can induce controllable spectroscopic and luminescence properties.35−43 Consequently, for the first time, the [Pt(bzimpy)Cl] + complexes can be conceptually regarded as an individual planar block to explore phosphorescence plane−coil diblock copolymers. The [Pt(bzimpy)Cl]+ complex can be blended into methacrylate polymers and show vapochromic and mechanochromic properties.44 Metallosupramolecular polymers containing [Pt(Me2bzimpy)Cl]+ (Me2bzimpy = 2,6-bis(N-methylbenzimidazol-2′-yl)pyridine) units are synthesized by reacting Pt(DMSO)2Cl2 with ditopic polymer ligands ended with Me2bzimpy and have been used as precursors to prepare platinum nanoparticles.25 Recently, we prepared luminescence polymeric hybrids by combining the [Pt(bzimpy)Cl]+ based complexes with negatively charged block copolymers. They can self-assemble to form spherical and wormlike micelles and vesicles in solution, leading to remarkable luminescence enhancements.45,46 In a recent preliminary report,47 we

INTRODUCTION Block copolymers comprise chemically distinct homopolymer chains connected by covalent bonds.1−15 Depending on the chain stiffness, diblock copolymers are categorized into three types: coil−coil,1−6 rod−coil,7−11 and rod−rod.10,12−15 They can self-assemble to generate hierarchical nanostructures in selective solvents, typically including spherical and cylindrical micelles and bilayered vesicles.1−15 Nowadays, the concept of rod−coil block copolymers has been extensively expanded. For example, organic π-conjugated oligomers with relatively small block sizes are commonly utilized as rigid building blocks to create rod−coil block copolymers, leading to highly available nanoassemblies with electrooptical functions.7−9 Certainly, the coil blocks are typical polymers with high molecular weights. Similarly, conjugated shape-persistent macrocycles or discshaped planes even with smaller planar sizes of 1−3 nm have been regarded as optoelectronic building blocks for creating rod−coil block copolymers and fabricating functional nanostructures.16−19 All of these nanostructures have resulted in various potential applications, such as patterning fabrication, drug delivery, fluorescence sensors, and nanoreactors.1−25 On the other hand, square-planar platinum(II) complexes exhibit intriguing spectroscopic and luminescent properties.26−34 They tended to form Pt···Pt and/or π−π stacking interactions, leading to controllable phosphorescence in the aggregate states.26−28 The chloroplatinum(II) complexes with 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy, [Pt(bzimpy)Cl]+) are planar with a flat size of 1.2 × 1.0 nm2 and an interplanar © XXXX American Chemical Society

Received: January 23, 2017 Revised: March 15, 2017

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DOI: 10.1021/acs.macromol.7b00171 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Synthetic procedures are illustrated for Sn-Pt-I (a) and Sn-Pt-II (b). These plane−coiled diblock polymers can self-assemble into micelles (c) and bilayered sheets (d) in the chloroform/methanol and toluene/methanol mixture solvents, respectively. N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA) were obtained from J&K Chemical. Bzimpy48 and 4-hydroxy-2,6-bis(Nmethylbenzimidazol-2′-yl)pyridine49 were synthesized as described in the previous literatures. 1 H and 13C NMR spectra were recorded on a JNM-ESC400 spectrometer, during which the samples were dissolved in dchloroform (CDCl3) and tetramethylsilane was used as an internal standard. High-resolution electrospray ionization mass spectra (ESIMS) were obtained on a Bruker APEX II FT-MS mass spectrometer. Gel permeation chromatography (GPC) plots were recorded on a Waters 1515 instrument and determined the values of number-average molecular weight (Mn), weight-average molecular weight (Mw), and polydispersity index (PDI) of the bzimpy-terminated polystyrenes (SnL-3 and Sn-L-7). Polystyrene was used as a calibration standard, and tetrahydrofuran (THF) was the eluent at a flow rate of 1.0 mL/min. The column temperature was kept at 35 °C. Elemental analyses of plane−coil diblock copolymers were performed with an Elementar VarioELcube. UV−vis absorption spectra were recorded by using a SHIMADZU UV-2550 spectrophotometer. Luminescence measurements were made on a Hitachi-7000 spectrofluorometer with a xenon lamp as the excitation source (150 W). Dynamic light scattering (DLS) measurements were performed on a Brookhaven BI-200SM spectrometer. Transmission electron microscopy (TEM) images were performed with an FEI Tecnai F30 operating at 300 kV or a JEM-2100 operating at 200 kV. Scanning electron microscopy (SEM) measurements were performed on a field emission Hitachi S-4800. Before the imaging, thin gold layer (ca. 4 nm) was coated on the specimen by using a Hitachi E-1045 ion sputter. Fluorescence microscopy

described the fabrication of luminescence plane−coil supramolecular block copolymers by electrostatic combination of [Pt(Me2bzimpy)Cl]+ with sulfonate terminated polystyrenes and their self-assembly into vesicles in solution. Herein, we report on the syntheses of two series of luminescence planar platinum(II)−polystyrene diblock copolymers, Sn-Pt-I and SnPt-II (Figure 1a,b), and their self-assembly into spherical micelles (Figure 1c) in the chloroform/methanol mixture solvents and bilayered sheets (Figure 1d) in the toluene/ methanol mixture solvents. Methanol was a selective solvent for the planar platinum(II) cations and a poor solvent for polystyrene. Therefore, the inside cores are filled with polystyrenes, and the coronas are occupied by planar [Pt(bzimpy)Cl]+ complexes. Both morphologies and sizes of the micellelike aggregates are characteristic of block copolymer self-assembly in solution. Among these aggregates, Pt···Pt and/ or π−π stacking interactions occur together with notable luminescence enhancements.

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EXPERIMENTAL SECTION

General Considerations. Styrene and cuprous bromide (CuBr) were purchased from Sinopharm Chemical. Before use, they were purified under reduced pressure and via washing with glacial acetic acid, respectively. Other commercially available solvents and chemicals were directly used without any further purification. 1-Bromododecane, 11-bromo-1-undecanol, 2-bromo-2-methylpropanoyl bromide, B


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Macromolecules measurements were performed with a Nikon Eclipse 80i. All measurements were performed at 20 °C except for GPC under the temperature of 35 °C. Synthetic Procedures and Characterization Details. L-1. Anhydrous powdered K2CO3 (0.35 g, 2.5 mmol) was added into an N,N-dimethylformamide (30 mL) mixed solution of 2,6-bis(Nmethylbenzimidazol-2′-yl)-4-hydroxypyridine (0.36 g, 1.0 mmol) and 11-bromo-1-undecanol (0.3 g, 1.2 mmol). The resulting suspension was stirred at 80 °C for 24 h. Upon cooling to room temperature, the solvent was removed under reduced pressure. The crude product was dissolved in 20 mL of CH2Cl2, and the insoluble impurity was removed by filtration. By evaporating CH2Cl2 to dryness, the residue was subjected to column chromatography (SiO2, CH2Cl2/MeOH, 50/ 1, v/v) and then L-1 was isolated with a yield of 80% (0.4 g). 1H NMR (CDCl3, 400 M Hz): δ: 1.26−1.36 (m, 12H, CH2), 1.51 (m, 2H, CH2), 1.57 (m, 2H, CH2), 1.85 (m, 2H, CH2), 3.65 (t, J = 4.0 Hz, 2H, CH2), 4.24 (m, 8H, NCH3, ArOCH2), 7.36 (m, 4H, Ar), 7.47 (d, J = 8.0 Hz, 2H, Ar), 7.88 (d, J = 8.0 Hz, 2H, Ar), 7.93 (s, 2H, Ar). 13C NMR (CDCl3, 100 M Hz): δ 166.62, 150.92, 150.36, 142.36, 137.07, 123.48, 122.76, 120.03, 111.73, 109.87, 68.60, 62.77, 32.75, 32.48, 29.38, 29.31, 29.24, 29.02, 28.77, 25.71, 25.68. ESI-MS: m/z calcd for [C32H39N5O2 + H]+: 526.3177. Found: 526.3156. L-2. 2-Bromo-2-methylpropanoyl (0.25 mL, 2 mmol) was added dropwise into a CH2Cl2 (20 mL) mixed solution of Et3N (0.14 mL, 1 mmol) and L-1 (0.52 g, 1 mmol) over 1 h, and then the obtained solution was further stirred at room temperature for 24 h. The reaction mixture was washed three times with 30 mL of 10% HCl aqueous solution. The CH2Cl2 layer was collected and dried over MgSO4. The white MgSO4 was removed by filtration. By evaporating CH2Cl2 to dryness, the residue was subjected to column chromatography (SiO2, CH2Cl2/MeOH, 200/1, v/v), and then L-2 was isolated with a yield of 83% (0.56 g). 1H NMR (CDCl3, 400 M Hz): δ: 1.23−1.38 (m, 12H, CH2), 1.49 (m, 2H, CH2), 1.68 (m, 2H, CH2), 1.85 (m, 2H, CH2), 1.93 (s, 6H, CH3), 4.16 (t, J = 4.0 Hz, 2H, CH2), 4.25 (m, 8H, NCH3, ArOCH2), 7.36 (m, 4H, CH), 7.47 (d, J = 8.0 Hz, 2H, Ar), 7.88 (d, J = 8.0 Hz, 2H, Ar), 7.93 (s, 2H, Ar). 13C NMR (CDCl3, 100 M Hz): δ 171.69, 166.68, 150.77, 150.26, 142.11, 136.98, 123.59, 122.90, 119.99, 111.93, 109.91, 68.68, 66.14, 55.99, 32.51, 31.08, 30.74, 29.42, 29.20, 29.13, 28.83, 28.29, 25.83, 25.75. ESI-MS: m/z calcd for [C36H34BrN5O3 + H]+: 674.2676. Found: 674.2700. S156-L-3. All procedures were performed under an atmosphere of Ar. CuBr (0.035 g, 0.24 mmol), L-2 (0.1622 g, 0.24 mmol), and PMDETA (0.1 mL, 0.48 mmol) were dissolved in styrene (5.5 mL, 0.048 mol) in a round-bottom flask. The feeding molar ratio between styrene and L-2 was controlled at 200:1. The mixture was degassed by three freeze−pump−thaw cycles, sealed under vacuum, and placed in an oil bath preheated at 110 °C for 2 h, during which the solution gradually became viscous. After cooling the reaction mixture to room temperature, it was opened to air and THF (5 mL) was added. Then the THF solution was passed through a neutral alumina column to remove the copper catalyst. The filtrate was concentrated to 10 mL and then precipitated into 200 mL of methanol for three times. The solids were subsequently collected via vacuum filtration and then dried under vacuum to afford 3.4 g (68% yield) as a white powder. GPC: Mn = 16 900, PDI = 1.04 (Table 1). S67-L-3, S197-L-3, and S283-L-3 were synthesized by the similar procedures, but with the feeding molar ratios between styrene and L-2 at 100:1, 250:1, and 400:1, respectively. The Mn and PDI values were determined to be (7600, 1.14), (21 200, 1.18) and (30 200, 1.14) by GPC measurements (Table 1). S156-Pt-I. Red solid K2PtCl4 (0.03 g, 0.072 mmol) was added to a DMSO/toluene (40 mL/20 mL) mixture of S156-L-3 (1 g, 0.06 mmol). The reaction mixture was heated for 15 days at 90 °C, and then an orange red solution was produced. The solvent was removed under reduced pressure. The resulting crude product was precipitated and purified in a THF−methanol mixture, and then an orange red solid was isolated with a yield of 95% (0.97 g). The plane−coil diblock polymers of S67-Pt-I, S197-Pt-I, and S283-Pt-I were prepared by using the similar procedure, but with different polymer ligands. All of the isolated yields were larger than 95%.

Table 1. Molecular Characteristics of Sn-L-3, Sn-L-7, Sn-Pt-I, and Sn-Pt-II sample

initiator

Mna

Mw/Mna

Mn,NMRb

sample

Mnc

S67-L-3 S156-L-3 S197-L-3 S283-L-3 S140-L-7 S269-L-7 S388-L-7

L-2 L-2 L-2 L-2 L-6 L-6 L-6

7600 16900 21200 30200 15400 28800 41100

1.14 1.04 1.18 1.14 1.17 1.23 1.27

7700 18600 22500 40000 15200 35200 47300

S67-Pt-I S156-Pt-I S197-Pt-I S283-Pt-I S140-Pt-II S269-Pt-II S388-Pt-II

7900 17200 21400 30400 15700 29000 41400

a

The values of Mn, Mw, and Mw/Mn (PDI) were determined by GPC. The values of Mn,NMR were calculated by 1H NMR spectroscopy. The Mn values of platinum(II) plane−coil diblock copolymers were obtained by adding the molecular weight of PtCl2 with the corresponding polymer ligands. b

L-4. To a solution of bzimpy (0.62 g, 2 mmol) and KOH (0.34 g, 6 mmol) in 2-butanone (30 mL) that was already stirred at 80 °C for 30 min, 1-bromododecane (0.50 g, 2 mmol) was added. The resulting mixture was further stirred at 80 °C for 24 h. After removing the solvent, the crude product was dissolved in 20 mL of CH2Cl2, and the insoluble impurity was removed by filtration. By evaporating CH2Cl2 to dryness, the residue was subjected to column chromatography (SiO2, petroleum ether/ethyl acetate, 2/1, v/v), and then L-4 was isolated as a white solid with a yield of 30% (0.29 g). 1H NMR (400 M Hz, CDCl3, ppm), δ: 0.86 (t, J = 8.0 Hz, 3H, CH3), 0.98−1.28 (m, 18H, CH2), 1.36−1.41 (m, 2H, CH2), 3.85 (t, J = 7.6 Hz, 2H, NCH2), 7.16−7.19 (m, 1H, Ar), 7.35−7.39 (m, 1H, Ar), 7.42−7.44 (m, 2H, Ar), 7.45−7.46 (m, 1H, Ar), 7.64 (t, J = 7.6 Hz, 1H, Ar), 7.87 (dd, J1 = 7.6 Hz, J2 = 0.8 Hz, 2H, Ar), 7.90 (d, J = 8.4 Hz, 1H, Ar), 7.99−8.02 (m, 1H, Ar), 8.16 (dd, J1 = 8.0 Hz, J2 = 0.8 Hz, 1H, Ar), 13.01 (s, 1H, NH). 13C NMR (100 MHz, CDCl3, ppm), δ: 14.10, 22.64, 26.39, 28.82, 29.24, 29.30, 29.45, 29.58, 31.84, 43.63, 111.07, 111.61, 119.56, 120.09, 121.71, 123.00, 123.15, 123.80, 124.90, 134.95, 135.59, 137.57, 142.35, 144.38, 147.94, 149.33, 150.24, 150.93. ESI-MS: m/z calcd for [C31H37N5 + H]+: 480.3122. Found: 480.3121. L-5. Anhydrous powdered K2CO3 (0.35 g, 2.5 mmol), 11-bromo-1undecanol (0.31 g, 1.2 mmol), and KI (0.17 g, 1.0 mmol) were added into a 2-butanone solution (30 mL) of L-4 (0.48 g, 1.0 mmol). The resulting suspension was stirred at 80 °C for 24 h. After cooling to room temperature and then removing the solvent, the crude product was dissolved in 20 mL of CH2Cl2, and the insoluble impurity was removed by filtration. By evaporating CH2Cl2 to dryness, the residue was subjected to column chromatography (SiO2, petroleum ether/ ethyl acetate, 1/1, v/v), and then L-5 was isolated as a viscous liquid with a yield of 49% (0.32 g). 1H NMR (400 M Hz, CDCl3, ppm), δ: 0.86 (t, J = 8.0 Hz, 3H, CH3), 0.98−1.28 (m, 32H, CH2), 1.48−1.55 (m, 2H, CH2), 1.72−1.73 (m, 4H, CH2), 3.63 (t, J = 6.4 Hz, 2H, OCH2), 4.71 (q, J = 7.6 Hz, 4H, NCH2), 7.32−7.38 (m, 4H, Ar), 7.45−7.48 (m, 2H, Ar), 7.86−7.89 (m, 2H, Ar), 8.06 (t, J = 7.6 Hz, 1H, Ar), 8.30−8.33 (m, 2H, Ar). 13C NMR (100 MHz, CDCl3, ppm), δ: 13.97, 22.50, 25.56, 28.89, 28.95, 29.08, 29.12, 29.16, 29.21, 29.34, 31.71, 32.63, 44.75, 62.33, 110.18, 120.06, 122.58, 123.33, 125.31, 136.01, 138.00, 142.54, 149.78, 149.97. ESI-MS: m/z calcd for [C42H59N5O+H]+: 650.4792. Found: 650.4797. L-6. 2-Bromo-2-methylpropanoyl (0.089 mL, 0.72 mmol) was added dropwise into a mixture of Et3N (0.10 mL, 0.72 mmol) and L-5 (0.23 g, 0.36 mmol) in 10 mL of CH2Cl2 over 1 h. This mixture was stirred at 25 °C for 12 h. And then, the reaction mixture was washed three times with 20 mL of 10% HCl aqueous solution. The CH2Cl2 layer was dried over MgSO4 and removed by filtration. By evaporating CH 2 Cl 2 to dryness, the residue was subjected to column chromatography (SiO2, CH2Cl2/MeOH, 25/1, v/v), and then L-6 was isolated as a viscous liquid with a yield of 91% (0.26 g). 1H NMR (400 M Hz, CDCl3, ppm), δ: 0.86 (t, J = 8.0 Hz, 3H, CH3), 0.99−1.27 (m, 32H, CH2), 1.61−1.63 (m, 2H, CH2), 1.71−1.73 (m, 4H, CH2), 1.91 (s, 6H, CH3), 4.13 (t, J = 6.8 Hz, 2H, OCH2), 4.70 (T, J = 6.4 Hz, C


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Macromolecules 4H, NCH2), 7.34−7.37 (m, 4H, Ar), 7.45−7.47 (m, 2H, Ar), 7.87− 7.90 (m, 2H, Ar), 8.06 (t, J = 8.0 Hz, 1H, Ar), 8.31 (d, J = 8.0 Hz, 2H, Ar). 13C NMR (100 MHz, CDCl3, ppm), δ: 14.08, 22.62, 26.60, 28.97, 29.16, 29.24, 29.30, 29.46, 29.99, 30.72, 31.83, 44.86, 55.95, 66.07, 110.31, 120.17, 122.77, 123.50, 125.52, 136.08, 138.15, 142.47, 149.77, 149.99, 171.65. ESI-MS: m/z calcd for [C46H64BrN5O2+H]+: 798.4316. Found: 798.4315. The Sn-L-7 polymers were synthesized according to the similar procedure for the Sn-L-3 series, but using L-6 as an ATRP initiator. The feeding molar ratios between styrene and L-6 were controlled at 100:1, 200:1, and 300:1, respectively. The Mn and PDI values were determined to be (15 400, 1.17), (28 800, 1.23) and (41 100, 1.27) by using GPC measurements (Table 1). The plane−coil diblock polymers of the Sn-Pt-II series were prepared by the similar procedure for the Sn-Pt-I series, but with the Sn-L-7 polymer ligands. Similarly, all of the isolated yields were larger than 95%.

ments (Table 1). Two series of chloroplatinum(II) plane−coil diblock copolymers, Sn-Pt-I and Sn-Pt-II (Figure 1a,b and Table 1), were synthesized by the coordination reactions of K2PtCl4 with Sn-L-3 and Sn-L-7 in a DMSO/toluene mixture solvent, respectively. To allow the full reaction of the polymer ligands, their molar ratios against K2PtCl4 were controlled at 1:1.2 and the reaction time was prolonged to 15 days. The plane−coil diblock polymers were isolated as orange solids by precipitating their crude products in THF with plenty of methanol. All of the yields were equal or larger 95% based on the corresponding polymer ligands. The high yields were consistent with the quantitative coordination reaction of K2PtCl4 with bzimpy-based ligands. After the coordination reaction, the proton resonances of bzimpy groups disappeared completely in the 1H NMR spectra, while the proton signals of polystyrenes remained (Figure 2c and Figures S10−S15). This situation was due to the formation of aggregates stemming from Pt···Pt and/or π−π stacking interactions of the planar platinum(II) blocks at a high concentration (30 mg/mL). Both Sn-Pt-I and Sn-Pt-II were further examined by elemental analyses. The results established compositions of the plane− coil diblock copolymers containing a few water molecules (Figure 1, Table 1, and Figure S1). This was reasonable, considering the hygroscopic blocks of planar [Pt(bzimpy)Cl]+ complexes.36,40−43 Absorption and Emission Spectra. All of the samples were further characterized by UV−vis absorption and luminescence spectra in both chloroform and toluene. According to previous spectroscopic behaviors of bzimpybased platinum(II) complexes,35−43 intense and structured bands from 310 to 390 nm were attributed to π → π* transitions of the bzimpy ligands (Figure 3a, Figures S16 and S17). Moderately intense metal-to-ligand charge-transfer (MLCT) transitions appeared from 410 to 500 nm. In the cases of Sn-Pt-II, shoulder bands near 545 nm became visible with increasing concentration and correspondingly were assigned to metal−metal-to-ligand charge-transfer (MMLCT) transitions that stemmed from intermolecular associations through Pt···Pt and π−π stacking interactions between the planar platinum(II) units covalently linked with the polystyrene blocks (Figure 3d, Figures S18 and S19). Upon excitation at 420 nm for the dilute chloroform solutions of Sn-Pt-I and Sn-Pt-II, weak vibronic-structured emissions appeared at 541 and 583 nm and at 561 and 605 nm, respectively (Figure 3 and Figures S16−S19). The progressional spacings (ca. 1300 cm−1) corresponded to vibrational stretching frequencies of the bzimpy ligands. Therefore, these emissions were assigned to metal-perturbed triplet intraligand charge-transfer excited states of the bzimpy ligands (3ILCT, π → π*). With increasing concentration, the emission at 600 nm was remarkably enhanced for the series of Sn-Pt-I (Figure 3b,c and Figure S16), while for the series of Sn-Pt-II, only the emission intensities increased significantly with almost same band positions (Figure S18). The band at 600 nm was thus assigned to the excimeric emission that originated from the formation of aggregates with increasing concentration. The driven force was believed to be the π−π stacking interactions between the planar platinum(II) blocks covalently connected with polystyrenes in the Sn-Pt-I series at higher concentrations. Upon dissolution of Sn-Pt-I in toluene, the excimeric emissions at 597 nm always dominated, and their intensities increased notably with increasing concentration (Figure S17). The difference was that in the case of Sn-Pt-II in toluene the

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RESULTS AND DISCUSSION Synthesis and Characterization. As depicted in Figures 1a and 1b, the atom transfer radical polymerization (ATRP)50,51 initiators with bzimpy-based ligands (L-2 and L6) were respectively prepared by the simple acylation of L-1 and L-5 with 2-bromoisobutryl bromide. They were fully characterized by their 1H and 13C NMR spectra (Figure 2a and

Figure 2. 1H NMR spectra (400 MHz, CDCl3) of L-2 (a, 5 mg/mL), S67-L-3 (b, 30 mg/mL) and S67-Pt-I (c, 30 mg/mL). The ∗ and ∗∗ signals referred to the residual CHCl3 and H2O in CDCl3, respectively.

Figures S1−S9) and high-resolution ESI-MS. With the initiators of L-2 and L-6 in hand, two series of bzimpy terminated polystyrenes (Sn-L-3 and Sn-L-7) were synthesized by the ATRP of styrene, respectively (Figure 1a,b). Depending on the feeding molar ratios of styrene with initiators, the Mn values ranged from 7.6 to 41 kDa (Table 1), as revealed by GPC measurements. All of PDI values were smaller than 1.30, consistent with the controlled radical polymerization.50,51 The 1 H NMR spectra exhibited both the bzimpy end and polystyrene resonances (Figure 2b and Figures S10−S15). The proton signals of the bzimpy terminals shifted slightly toward the low field in comparison with those of the initiators, probably due to their polymer microenvironments. Since the polystyrene chains were completely end-functionalized with the bzimpy ligands, the Mn values were independently determined by 1H NMR spectroscopy, where the protons of −NCH3 and −NCH2− in the end ligands were used as internal standards. Within the experimental errors, the resulting Mn,NMR values agreed well with the Mn data obtained from GPC measureD


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Figure 3. UV−vis absorption and luminescence spectra of S67-Pt-I (a−c) in chloroform and S140-Pt-II (d−f) in toluene with increasing concentration (6.25 × 10−6, 1.25 × 10−5, 2.5 × 10−5, 5.0 × 10−5, 1.0 × 10−4, and 2 × 10−4 mol/L).

metal-perturbed 3ILCT emissions dominated at 561 nm under the dilute conditions. However, emission bands emerged at 613 nm and became dominant with increasing concentration (Figure 3e,f and Figure S19). Combining these with the aforementioned MMLCT absorption bands at 545 nm, we assigned the bands at 613 nm to a 3MMLCT excited state originating from intermolecular associations through Pt···Pt and π−π stacking interactions. These dominating excimeric and 3 MMLCT emission bands were consistent with the disappeared resonance signals of the platinum(II) blocks in the 1H NMR spectra obtained at high concentrations (vide supra). Methanol was added into the solutions of Sn-Pt-I and Sn-PtII with a final concentration of 0.33 mg/mL, where methanol was a poor solvent of polystyrene and a selective solvent for the cationic platinum(II) planes. The resulting solutions were subjected to luminescence spectral measurements. Both the chloroform and toluene solutions of S67-Pt-I showed moderate emission intensities at the excimeric band of 598 nm together with a weak 3ILCT shoulder at 542 nm (Figure 4). When 25 and 33 vol % methanol were added, the excimeric emissions were completely quenched and only very weak 3ILCT emission bands were detected at 542 and 588 nm. These spectral behaviors were consistent with the transparent solutions, where S67-Pt-I did not formed aggregates. Upon increasing the methanol content to 50% leading to poorer solvent quality for polystyrene blocks, the solutions were not transparent but became opaque, in which aggregates formed. At that moment, the emission intensities at 598 nm started to increase with a moderate extent (Figure 4). The further increasing methanol contents to 67, 75, and 90% resulted in remarkable enhancements of the excimeric emissions of S67-Pt-I in both chloroform/methanol and toluene/methanol mixture solvents (Figure 4). These enhanced excimeric emissions signified that there were strong π−π stacking interactions between the platinum(II) planes in the solutions of S67-Pt-I. Similar luminescence enhancements and evolutions were also observed in the other diblock copolymers of the Sn-Pt-I series (Figure S20). But, the luminescence intensities increased with less extent than those of S67-Pt-I under the same solvent conditions.

Figure 4. Emission spectra of S67-Pt-I in the chloroform/methanol (a, b) and toluene/methanol mixture solvents (c, d). The final concentrations were 0.33 mg/mL. With increasing the methanol contents, the emission intensities increased remarkably.

This situation was consistent with the lower contents of the planar platinum(II) complex with the increasing molecular weights in the plane−coiled diblock copolymers. When methanol was added into the solutions of S140-Pt-II with methanol volume ratios of 25 and 33%, the solutions were clearly transparent with almost ignorable 3ILCT emissions at 561 and 605 nm (Figure S21), similar to the situation of Sn-Pt-I as described above. Upon increasing the methanol contents to 50, 67, and 75%, the emissions were found to be very weak still. But the bands at 618 nm could be recognized together with a shoulder at 630 nm, both of which were accordingly assigned to the 3MMLCT excited states originating from intermolecular E


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Figure 5. SEM (a) and TEM (b−d) images displayed that S283-Pt-I self-assembled to form spherical micelles with a polystyrene core and a planar platinum(II) corona in the chloroform/methanol mixture solvent with a methanol volume ratio of 50%. The diameters of the spherical micelles of S283-Pt-I were determined by the Gaussian fit to be 25 ± 5 and 80 ± 15 nm (e). The core−corona nanostructure was further confirmed by HAADFSTEM imaging (f, g). Spherical micelles also formed with the increasing methanol contents to 75% (h) and 90% (i).

associations through Pt···Pt and π−π stacking interactions. The latter shoulders were much more pronounced in the toluene/ methanol mixture solvents (Figure S21). The lower energy band at 630 nm suggested that the Pt···Pt separation was somewhat shorter under the present solvent conditions. When 90% vol methanol was added, the emission intensity only increased moderately. However, the relative intensity of the shoulder at 630 nm decreased moderately. Similar luminescence enhancements and evolutions of 3MMLCT bands were observed in the cases of S269-Pt-II and S388-Pt-II, but with much less extent (Figure S21). Again, this was attributed to the increasing molecular weights and thus the lower contents of the platinum(II) complex in the plane−coiled diblock copolymers of Sn-Pt-II. In these cases, the luminescence was only enhanced slightly, which was in sharp contrast to the remarkable luminescence enhancements in the series of Sn-Pt-I under the same solvent conditions. The partially quenched luminescence in the present case was probably due to the platinum(II) complex−methanol and/or complex fluidity leading to the nonradiative decay of the excited states. The note was that these solutions were opaque, suggesting that aggregates formed actually. Platinum(II) Plane−Coil Diblock Copolymers SelfAssembled To Form Spherical Micelles in the Chloro-

form/Methanol Mixture Solvents. To reveal the mechanism of luminescence enhancements, all of the dispersions were cast onto carbon-coated copper grids or copper grids coated with a porous polymer membrane for both SEM and TEM observations. The solution of S283-Pt-I in the chloroform/ methanol mixture solvent with 50 vol % methanol content was highly luminescent and really opaque and thus was chosen as a representative sample to investigate the solution self-assembled behaviors of these plane−coil diblock copolymers. Both the SEM (Figure 5a) and TEM images (Figure 5b−d) revealed that S283-Pt-I self-assembled to form spherical aggregates under the present solvent condition. Two sizes were clearly identified at 25 ± 5 and 80 ± 15 nm, and their occurrence probabilities were estimated to 94 and 6%, respectively, by counting more than 500 nanoparticles (Figure 5e). Closer inspection of the TEM image revealed that smaller sized nanoparticles occupied a core−corona nanostructure with a darker corona and a gray core (Figure 5d). The corona thickness was estimated to be ca. 1.5 nm. This value agreed well with the planar size of the bzimpy-based platinum(II) complexes (1.2 × 1.0 nm2).35−43 Therefore, the darker corona was attributed to the planar platinum(II) block, while the inside was due to the polystyrene block. This is a typical micellar nanostructure (Figure 1c). To confirm this picture, the micelles were further examined by F


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micellar diameters achieved from the aforementioned SEM and TEM imaging. These larger populations were assigned to the aggregates of micelles. Such bigger micellar clusters have been captured in previous DLS studies on block copolymer micelles, which were believed to stem from larger solvophobic blocks or electrostatic repulsions between charged metallocomplexes emerging at the core−corona interface.52−54 Correspondingly, Dhs of the fast modes decreased from 92 to 76 to 62 nm. This picture agreed well with the decreasing tendency of the micellar diameter with the increase in methanol content as well addressed by the above microscopic observations. However, these small Dh values were still almost 4 times larger than the small micellar diameters obtained from the microscopic observations, respectively (vide supra). Meanwhile, the DLS signals of the smaller populations were much weaker than those of the larger ones. However, it should be pointed out that larger particles leads to much stronger DLS signal intensity than smaller particles.55 Accordingly, many more small particles only can scatter incident light more weakly than a small number of large particles.55 This explained well the rare observations of larger nanoparticles (the aggregates of micelle) in the microscopic images. As stated above, the remarkable enhancements of excimeric emissions originated from the strong π−π stacking interactions between the platinum(II) planes. Therefore, the fast modes were assigned to oligomeric micelles. The driven force was the π−π stacking interactions of the planar platinum(II) blocks among the different micelles. This assignment was supported by docking and fusing micelles that were commonly observed in the microscopic images (Figure 5 and Figures S23−S26). As reported previously, N∧N∧N ligand-based platinum(II) complexes associated into dimers in solution driven by Pt···Pt and/ or π−π stacking interactions and the equilibrium constants were in the range of 103−105 M−1.56−58 Here, the micelles contained a corona of the planar platinum(II) complexes. It was therefore reasonable that S283-Pt-I self-assembled to form oligomicelles and micellar aggregates in the solvents of weakened quality for polystyrene blocks. On the other hand, block copolymer micelles with large hydrophobic blocks are prone to form the aggregates of micelles in comparison with those with small hydrophobic blocks.52,53 Therefore, the large polystyrene blocks contributed significantly to the formation of the oligomicelles and micellar clusters besides the abovementioned π−π stacking interactions. Similarly, the plane−coil diblock copolymers of S156-Pt-I, S197-Pt-I, S283-Pt-I and Sn-PtII also self-assembled into spherical micelles with bimodal size distributions in the chloroform/methanol mixture solvents as revealed by both the TEM imaging (Figures S23−S26) and DLS measurements (Figures S27 and S28). With increasing methanol content, the diameters of the micelles decreased as shown in both the TEM images and DLS curves, which was consistent with the decreasing trend found in the case of S283Pt-I. Moreover, as suggested by the above luminescence spectral studies (Figure S21), the Pt···Pt and π−π stacking interactions also contributed to the bimodal spherical micelles in the case of Sn-Pt-II. Free-Standing Bilayered Sheets Formed and Evolved into Spherical Micelles in the Toluene/Methanol Mixture Solvents. Of difference was that when S283-Pt-I was dispersed in the toluene/methanol mixture solvent with a methanol volume ratio of 67%, free-standing sheets formed with a flat size up to micrometric scales (Figure 7a,b). As revealed by a magnified TEM image, the sheet was really

high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), in which the heavier components emerge brighter against the dark carbon background. As clearly shown in the corresponding HAADF-STEM images (Figure 5f,g), the corona was brighter than the inside, confirming a micellar nanostructure that contained a corona of the planar platinum(II) complexes and a core of the polystyrenes. Both the micellar diameter and corona thickness were completely consistent with those values obtained by bright field TEM imaging. When the methanol contents increased to 75 and 90%, micelles formed with average diameters of 20 and 18 nm, respectively (Figure 5h,i and Figure S22). Larger sized nanoparticles were also detected in the SEM and TEM images but with much smaller occurrence probabilities of ca. 1%. It was therefore concluded that the micellar diameter decreased with the increasing methanol volume ratio. Methanol is a typically poor solvent of polystyrene, while the planar platinum(II) cations can be selectively soluble in methanol. Therefore, the increasing methanol contents would weaken the solvent quality of polystyrene blocks and thus lead to more significant shrinkage of the polystyrene chains. Correspondingly, it was reasonable that the micellar diameters decreased gradually when the solvent quality of polystyrene blocks was worsened by stepwise increasing the methanol contents. Certainly, such chain compactness reduced the molecular area of the planar platinum(II) complex on the surface of micelles. Meanwhile, the solvent quality became better for the planar platinum(II) complex, leading to its much more mobility in the solvent mixtures. These collective features indicated that under poorer solvent conditions for polystyrene blocks π−π stacking interactions emerged more significantly between the planar platinum(II) blocks and thus induced the remarkable luminescence enhancements as already stated above. To confirm the formation of the micellar aggregates in the solution, the aforementioned dispersions were further subjected to DLS measurements. In the resulting DLS plots, two diffusive dynamic modes appeared in the case of S283-Pt-I (Figure 6),

Figure 6. DLS plots of S283-Pt-I dispersed in the chloroform/ methanol solvents with methanol volume ratios of 50%, 75%, and 90%.

corresponding to two different types of micellar nanostructures in solution. This bimodal pattern agreed well with two size distributions of the micelles found in the SEM and TEM images. Upon increasing the methanol contents from 50% to 75% to 90% in the chloroform solutions of S283-Pt-I, the slow modes occupied hydrodynamic diameters (Dhs) of 268, 272, and 227 nm, respectively, which were much larger than the G


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Figure 7. Bright-field TEM (a−d), HAADF-STEM (e−g), and SEM images (h, i) revealed that S283-Pt-I self-assembled into free-standing sheets in the toluene/methanol solvents with a methanol content of 67 vol % methanol. The sheets were really uniform and foldable. The thickness was determined to be 11 nm as marked by the arrows in the bright field TEM (d), HAADF-STEM (g), and SEM images (i).

Figure 8. Fluorescent microscopy images revealed that S283-Pt-I formed free-standing sheets in the toluene/methanol mixture solvent with a methanol content of 67 vol %.

by the HAADF-STEM imaging. As highlighted by the arrows, the white and straight lines were attributed to the areas of the platinum(II) block and occupied a thickness of 2 nm (Figure 7g). This assignment agreed well with the linear nature of the π−π stacking interactions between the platinum(II) planes.38 The in-between domain was accordingly assigned to the polystyrene block, where the observed width was 8 nm. This

uniform (Figure 7c). Furthermore, the fold of the sheet allowed catching its cross-sectional area, where the thickness was therefore determined to be 11 ± 2 nm (Figure 7d). As demonstrated in the HAADF-STEM image, the free-standing sheet was brighter, displaying that the planar platinum(II) complexes appeared homogeneously within the sheet (Figure 7e,f). Furthermore, the cross-sectional area was also captured H


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suggested that the sheetlike nanoassemblies were evolving to generate micelles at the present solvent condition. When the methanol content was further increased to 90 vol %, pure micellar phase was achieved finally (Figure S31). These micelles occupied diameters ranging from 25 to 80 nm (Figure 9 and Figure S31). The smaller sized aggregates were assigned to single micelles, while the larger ones were similarly attributed to oligomeric micelles (vide supra). Under this solvent condition, the solvophobic interactions of the polystyrene blocks were enough for the completely morphological transformation. Similarly, with the increasing methanol contents from 50% to 90%, the aggregate morphologies evolved from free-standing sheets to spherical micelles in the cases of S67-Pt-I (Figure S32), S156-Pt-I (Figure S33 and Figure 9c,d), and S197-Pt-I (Figure S34). Of note was that in the case of S156-Pt-I the micelles were docked together with a linear type coronas (Figure 9c), which was consistent with the remarkable luminescence enhancement originating from the linear π−π stacking interactions between the platinum(II) complexes.38 All of these morphologies and evolutions coincided with the gradual increase in those luminescence intensities under the same solvent conditions as addressed above (Figure 4 and Figure S20). However, when 67 vol % methanol was added into a toluene solution of S388-Pt-II, free-standing sheets coexisted with micelles (Figure 10a−c). The thickness of the sheet was estimated to be 12 nm by using the fold as highlighted by the arrows (Figure 10c). The sheet was found to be inherently uniform again (Figure 10d). Moreover, both S140-Pt-II and S269-Pt-II showed similar coexistence of sheetlike nanoassemblies with spherical micelles under the same solvent condition (Figures S35 and S36). The sheetlike assembly was further confirmed by the fluorescence microscopy. The red luminescence sheet was clearly observed in the case of S140-PtII, where the emission intensities were much stronger in the folding areas again (Figure 10e). Meanwhile, the spherical micelles were concomitant with the free-standing sheets. Here, the coronas of the micelles were clearly identified with a thickness of ca. 2 nm, which was consistent with the planar size of the platinum(II) complexes. The diameters ranged broadly from 30 to 260 nm, which was probably due to the intermediate state between the sheets and micelles. The concomitance of bilayered sheets with micelles was in sharp contrast to the pure sheet phase formed by Sn-Pt-I under the same solvent condition. This difference was presumably due to the stronger Pt···Pt and π−π stacking interactions between the platinum(II) planes in Sn-Pt-II than π−π stacking interactions in Sn-Pt-I, as addressed by the aforementioned emission spectra, although the luminescence was partially quenched in the former case (Figure S21). Eventually, they were converted to form spherical micelles when 90 vol % methanol was added (Figure 10f and Figures S37−S39), consistent with the situation occurring in the case of Sn-Pt-I.

value together with both the coronas of the platinum(II) plane (2 + 2 = 4 nm) afforded the sheet a total thickness of 12 nm. Here, the sheet was typically bilayered, and the polystyrene chains were sandwiched covalently into the planar platinum(II) complexes as illustrated in Figure 1d. The sheetlike assemblies were further characterized by SEM imaging. During the measurement process, 4 nm thick gold layers were deposited onto both sides of the sheet to avoid electric charging and thus obtain high quality SEM images. The free-standing sheet was clearly observed, and the cross-sectional area was also captured (Figure 7h,i). The total thickness was therefore estimated to be 20 ± 3 nm. After removing the thickness of gold layers (8 nm), the naked sheet possessed a thickness of ca. 12 nm. This value agreed well with the aforementioned TEM observations. Moreover, the root-meansquare roughness of the sheet was determined to be 0.4 nm from a 100 nm square by atomic force microscopy imaging (Figure S29). This result suggested that the sheet surface was quite smooth in the observed area. The sheetlike aggregates were further confirmed by using fluorescent microscopy. As revealed in the corresponding fluorescent microscopy images, the red luminescence sheets appeared clearly (Figure 8), which was consistent with the excimeric emission of S283-Pt-I occurring at 600 nm under the present solvent condition. The red luminescence was much more significant for the folds than for the flat sheets (Figure 8b), which was due to the higher concentration of the planar platinum(II) block in the former area. In all of these microscopic images (Figures 7 and 8), the sheets were frequently folded and wrinkled, suggesting that the sheets formed in the solution rather than on the substrate during the solvent evaporation. Similarly, free-standing sheets were observed with the decreasing methanol content to 50% (Figure S30). The addition of 75 vol % methanol resulted in a coexistence of the sheet with the micelle (Figure 9a,b). This situation

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CONCLUSIONS In summary, we have synthesized and characterized two series of luminescence platinum(II) plane−coil diblock copolymers, Sn-Pt-I and Sn-Pt-II. With increasing concentration, these plane−coil diblock copolymers show up 3IL excimeric and 3 MMLCT emissions in chloroform and toluene as a result of intermolecular associations originating from Pt···Pt and/or π−π stacking interactions. They can self-assemble to form spherical micelles with a corona of planar [Pt(bzimpy)Cl] + Cl −

Figure 9. TEM images showed that free-standing sheets coexisted with spherical micelles in the toluene/methanol mixture solvents with a methanol volume ratio of 75% in the case of S283-Pt-I (a, b) and S156Pt-I (c, d). I


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Figure 10. TEM images displayed that free-standing sheets of S388-Pt-II coexisted with spherical micelles in the toluene/methanol mixture solvents with a methanol content of 67 vol % (a−d). A foldable sheet of S140-Pt-II was clearly observed with red luminescence (e). Spherical micelles of S269Pt-II formed when the methanol volume content increased to 90% (f).

block copolymers have been constructed by using metal− bipyridine coordination modes, and cylindrical and lamellar microstructures are recognized in their casting film.71−74 In all of these cases, metal−ligand coordination complexes are essentially utilized only as alternative connection modes for conventional block copolymers linked by covalent bonds. Recently, platinum(II) complexes have been ended with homopolymers25 or block copolymers.75 However, none of them are regarded as isolated blocks for the creation of block copolymers and their micellelike aggregates. From the opinion of molecular architectures, the planar [Pt(bzimpy)Cl]+Cl− complexes are highly similar to conjugated macrocycles and aromatic discs.16−19 In the present work, for the first time, this class of platinum(II) complexes has been regarded as an individual planar block to prepare luminescence plane−coil diblock copolymers. The resulting diblock copolymers can selfassemble controllably into hierarchical nanostructures with remarkable luminescence enhancements. This work represents a novel concept, in which the metallosupramolecular complexes can be utilized as isolated blocks, to design and synthesize functional block copolymers with desired nanostructures.

complexes and a polystyrene core in the chloroform/methanol mixture solvents. Of significance is that bilayered sheets form in the toluene/methanol mixture solvents with moderate methanol contents, where the central polystyrene layer is protected by the planar [Pt(bzimpy)Cl]+Cl− blocks. With increasing methanol content, where the solvent quality is highly weakened for polystyrene blocks, the sheetlike assemblies are readily converted to form spherical micelles with a core of polystyrenes and a corona of planar platinum(II) complexes. These morphologies and evolutions have been monitored by using the emission spectra, where remarkable luminescence enhancements are demonstrated in the case of Sn-Pt-I and the luminescence intensities increased moderately in the case of SnPt-II. Such emission spectral studies suggest that Pt···Pt and/or π−π stacking interactions emerge between the planar platinum(II) blocks and contributed significantly to the formation of the spherical micelles and bilayered sheets in the solvents of weakened quality for polystyrene blocks. The photophysical phenomena observed here were highly similar to aggregation-induced emission (AIE) or aggregationinduced emission enhancement (AIEE) recognized in propeller-shaped (macro)molecules.59−61 The underlying mechanism is restriction of intramolecular rotation when AIE or AIEE (macro)molecules aggregate in solution. In the present case, micellelike aggregates formed in the dispersions and induced the generation of Pt···Pt and/or π−π stacking interactions between the planar platinum(II) blocks, leading to remarkable luminescence enhancements. Therefore, the enhanced emissions found in this study can be regarded as a novel class of AIEE effects with a different luminescence enhancement mechanism. Linear metallosupramolecular block copolymers have been fabricated by metal−terpyridine,53,54,62−67 pyridylpalladium(II)SCS,68,69 and alkynylplatinum(II)−terpyridine coordination modes.70 They can self-assemble to form micelles in solution that are highly reversible under certain external stimuli. Starlike

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00171. H and 13C NMR spectra, additional fluorescence spectra, DLS plots, SEM and TEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(W.B.) E-mail [email protected]; Phone +86-931-8912265; Fax +86-931-8912582. J


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Weifeng Bu: 0000-0002-6213-2928 Author Contributions

N.L. and Y.W. contributed equally as co-first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21474044 and 21674044), the Fundamental Research Funds for the Central Universities (lzujbky-2015-k04 and lzujbky-2016-42), and the Open Project of State Key Laboratory of Supramolecular Structure and Materials of Jilin University (sklssm201601). The project was supported by Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences (201626).

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Syntheses and Controllable Self-Assembly of Luminescence Platinum

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