Synthesis and Luminescence of POSS-Containing Perylene Bisimide

Mar 26, 2012 - A novel well-defined amphiphilic fluorescent polymer containing asymmetric perylene bisimide was designed and synthesized by combining ...
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Synthesis and Luminescence of POSS-Containing Perylene BisimideBridged Amphiphilic Polymers Fanfan Du, Jiao Tian, Hu Wang, Bin Liu, Bangkun Jin, and Ruke Bai* CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, 230026, People’s Republic of China S Supporting Information *

ABSTRACT: A novel well-defined amphiphilic fluorescent polymer containing asymmetric perylene bisimide was designed and synthesized by combining reaction of perylene anhydride with amino functional polyhedral oligomeric silsesquioxane (POSS) and atom transfer radical polymerization (ATRP) of N-isopropylacrylamide (NIPAM). All the intermediate and final products were characterized by NMR, Fourier transform infrared spectroscopy (FT-IR), elemental analyses, and gel permeation chromatograph (GPC). Self-assembly of the amphiphilic polymers was investigated in aqueous solution and POSS-containing hybrid nanoparticles were obtained and characterized by dynamic laser light scattering (DLS) and transmission electron microscopy (TEM). The novel hybrid nanoparticles exhibit attractive high red fluorescence at 645 nm due to the significant effect of the bulky POSS moieties. Moreover, based on the thermoresponsive PNIPAM coronas, the fluorescence intensity of the self-assembled hybrid nanoparticles can be further enhanced and tuned by changing temperature.

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intermolecular π-stacking caused by their polar or nonpolar but extensively π-conjugated structures.17−20 Perylene bisimides (PBIs), the well-known fluorescent dyes, display exceptional chemical, thermal and photochemical stability, strong absorption for visible light, near unity fluorescence quantum yields and low toxicity.21,22 Red excimer-like emissions are usually observed for weakly interacting π-stacked PBI molecules in solution. However, these attractive red emissions are generally strongly quenched by π-stacking in aggregate state.23−26 Strong red fluorescence in the aggregate state could be succeeded with the elimination of this self-quenching. The incorporation of sterically demanding groups onto PBIs can reduce the intermolecular interactions and maintain the luminescence in the aggregate state simultaneously. Würthner and co-workers reported a series of amphiphiles based on PBI having different ratios of hydrophobic and hydrophilic coil segments.27 Aggregation in water of these amphiphile PBIs resulted in the formation of micelles or bilayer vesicles with broad fluorescence band at 600−800 nm.28 Liu and co-workers demonstrated that the steric hindrance of β-cyclodextrin can increase the π−π stacking distance of perylene chromophore and reduce the intermolecular interactions.29 An amphiphilic perylene-cyclodextrin conjugate was synthesized and the resulting aggregates were successfully employed as solid-state fluorescence sensing for organic amines.30

INTRODUCTION Recently, research on fluorescent nanoparticles has gained considerable attention due to their potential applications as probes and sensors for biological imaging and sensing.1−3 Such particles are very interesting due to their small size while their fluorescent properties can be tuned by varying their size and composition. Much focus has been placed on the development of efficient, inexpensive, stable, and tunable nanoparticles, such as quantum dots/rods,4 silica nanoparticles,5 metal nanoparticles,6 carbon nanomaterials,7 and up-converting nanophosphores.8 However, applications of these robust materials are hampered by the toxicity of the inorganic component and sometimes difficulties with respect to surface modification.9 In another approach, organic fluorescent particles have been constructed from vesicles and micelles in which fluorescent dyes are incorporated.10 Such systems can be easily chemically modified to enhance their binding affinity toward specific tissue and analytes. Many organic dyes have been utilized as emitters of light-emitting nanoparticles with various colors.9,11−14 Among these nanoparticles, red fluorescent organic nanoparticles that emit long enough wavelength (emission maximum wavelength λmax > 610 nm), where the influence of the main tissue absorbing components, oxy- and deoxyhemoglobin (λmax < 600 nm) is minimal,15 have been dedicated immense attention in biological imaging technologies.16,17 However, only few red fluorescent organic dyes are suitable for the application. Conventional red fluorescent organic dyes are highly susceptible to concentration quenching and become either weakly emissive or even not emissive at all in aggregate state, due to either attractive dipole−dipole interactions or © 2012 American Chemical Society

Received: January 15, 2012 Revised: March 9, 2012 Published: March 26, 2012 3086

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Scheme 1. Synthesis Procedure of the PBI-Bridged Amphiphilic Polymer

received. Tetrahydrofuran (THF, Shanghai Chemical Reagent Co., >99%) was purified by refluxing over sodium and distillation prior to use. Triethylamine (NEt3, Shanghai Chemical Reagent Co., >99%) was distilled over potassium. N-Isopropylacrylamide (NIPAM) was purified by recrystallization from a mixture of benzene and n-hexane (1/3, v/v). Copper(I) bromide (CuBr) was purified by stirring overnight over CH3COOH at room temperature followed by washing the solid with ethanol, acetone, and diethyl ether, and drying at 40 °C in vacuo for 1 day. Tris(2-aminoethyl) amine (TAEN) was purchased from Fluka and used as received. Tris(2-(dimethylamino)ethyl)amine (Me6TREN) was prepared according to literature procedures.48 All other chemical agents were used as received unless otherwise noted. Synthesis of N-Hydroxyethyl-3,4:9,10-perylenetetracarboxylic3,4-anhydride-9,10-imide (3). Perylene-3,4:9,10-tetracarboxylic acid monoanhydride monopotassium carboxylate 2 (0.9 g, 2 mmol), prepared according to the procedure reported by Tröster,49 was suspended in water (30 mL) at room temperature. 2-Aminoethanol (0.6 g, 10 mmol) dissolved in 5 mL of water, was added dropwise to the suspension of 2. The mixture was stirred for 3 h at room temperature. Acetone (100 mL) was added into the resulting red solution to induce precipitation. The mixture was allowed to stand overnight, and the resulting brick red precipitate was obtained by vacuum filtration and washed with acetone. The filter residue was resuspended in hydrochloric acid (10%), heated to 90 °C, kept at this temperature for 1 h. The product was then isolated by vacuum filtration, washed with water, and dried under vacuum at 60 °C overnight to yield 0.9 g (1.8 mmol, 90%) of 3 as a brick red solid. IR (KBr): ν = 3500, 1764, 1717, 1693, 1653, 1591, 1504, 1403, 1374, 1321, 1236, 1171, 1152, 1013, 851, 807, 736, 635 (Figure S1, Supporting Information). Anal. Calcd for C26H13NO6: C, 71.72; H, 3.00; N, 3.22. Found: C, 68.87; H, 3.31; N, 3.09. Synthesis of N-Hydroxyethyl-N′-propylheptakis(isobutyl) POSS3,4:9,10-Perylenetetracarboxylic Bisimide (4). To a stirred suspension of 3 (0.34 g, 0.75 mmol) and imidazole (1 g, 15 mmol) in 50 mL of dimethylacetamide (DMAc) was added POSS-NH2 (1.75 g, 2 mmol). The reaction mixture was stirred at 120 °C under nitrogen for 8 h. After cooling to room temperature, diethyl ether (100 mL) was added, followed by washing with hydrochloric acid (10%, 100 mL × 3). The organic layer was dried over anhydrous Na2SO4 and filtered. After removing all the solvents, the residues were further purified on a silica gel column using chloroform as eluent, affording 4. The product was obtained as a bright red solid (0.78 g, yield: 80%). 1H NMR (CDCl3, δ, ppm): 8.61 (d, 2H, Ar−H), 8.54 (d, 2H, Ar−H), 8.42 (m, 4H, Ar−H), 4.50 (t, 2H, β-CH2), 4.20 (t, 2H, α-CH2), 4.08 (t, 2H, αCH2), 1.85 (m, 7H, SiCH2CH), 1.66 (m, 2H, SiCH2CH2), 0.95 (m, 42H, CH(CH3)2), 0.59 (m, 16H, SiCH2) (Figure S2, Supporting

Polyhedral oligomeric silsesquioxane (POSS) has attracted a great deal of attention in the materials field because of the unique nanoscale cage-shaped structure and good solubility in organic solvents.31 It has been incorporated into conjugated organic/polymer luminescent materials to improve solubility and maintain the luminescence in the aggregate state simultaneously.32−41 Furthermore, according to the previous works, POSS shows high stability under biological environment and little cytotoxicity.42,43 Thus, POSS is a suitable scaffold to realize the multifunctional materials for biological applications.44−46 Incorporation of POSS group into PBI should not only enhance the fluorescence quantum yields of the aggregate state, but also endow novel biological applications of corresponding assemblies. However, much less attention has been paid to this aspect,47 and to the best of our knowledge, there is no report on the successful preparation and applications of fluorescent polymers based on POSS-containing PBI dyes. In this work, we report on the synthesis of well-defined amphiphilic polymer possessing an asymmetric PBI as a bridge between a POSS moiety and a PNIPAM chain. The preparation procedure is shown in Scheme 1. All the intermediate and final products were characterized by NMR, Fourier transform infrared spectroscopy (FT-IR), elemental analyses, and gel permeation chromatograph (GPC). The optical properties of the polymer in solution were measured by UV−vis spectroscopy and photoluminescence (PL) spectroscopy. Waterdispersible hybrid nanoparticles were then prepared by selfassembly of the amphiphilic polymers and characterized by dynamic laser light scattering (DLS) and transmission electron microscopy (TEM). It was found that the nanoparticles exhibit strong red fluorescence with a maximum at 645 nm. Moreover, further experimental results show that the fluorescence intensity of the self-assembled hybrid nanoparticles can be further enhanced and tuned by changing temperature owing to the incorporation of a thermal responsive PNIPAM chain in the amphiphilic polymer.

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EXPERIMENTAL SECTION

Materials. Perylene-3,4:9,10-tetracarboxylic dianhydride and 2bromoisobutyryl bromide were obtained from J&K. Aminopropylheptakis (isobutyl) POSS (POSS-NH2, Hybrid Plastics, 97%) was used as 3087

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Information). 13C NMR (CDCl3, δ, ppm): 164.2, 163.2, 134.8, 134.2, 131.6, 131.2, 129.4, 129.2, 126.2, 126.1, 123.4, 123.3, 123.0, 61.5, 43.1, 40.5, 25.8, 24.0, 22.6, 9.9 (Figure S3, Supporting Information). IR (KBr): ν = 3443, 2957, 2871, 1693, 1660, 1464, 1344, 1234, 1111, 837, 744, 482 (Figure S1, Supporting Information). Anal. Calcd for C57H82N2O17Si8: C, 53.02; H, 6.35; N, 2.17. Found: C, 52.89; H, 6.40; N, 2.14. Synthesis of POSS-PBI-Br. NEt3 (0.2 g, 2 mmol) and 4 (0.65 g, 0.5 mmol) dissolved in anhydrous THF (30 mL) were respectively added into a flask, then 2-bromoisobutyryl bromide (1.0 g, 2 mmol) was added dropwise at 0 °C. The mixture was warmed to room temperature and stirred overnight. After being filtered, diethyl ether (100 mL) was added, followed by washing with distilled water three times. The organic solution was dried over anhydrous Na2SO4 and filtered. After removing all the solvents, the residues were further purified on a silica gel column using chloroform as eluent to afford POSS-PBI-Br. The product was obtained as a bright red solid (0.59 g, yield: 82%). 1H NMR (CDCl3, δ, ppm): 8.67 (d, 4H, Ar−H), 8.58 (d, 4H, Ar−H), 4.59 (m, 4H, α-CH2, β-CH2), 4.20 (t, 2H, α-CH2), 1.85 (m, 7H, SiCH2CH), 1.66 (m, 2H, SiCH2CH2), 0.95 (m, 42H, CH(CH3 ) 2), 0.59 (m, 16H, SiCH 2) (Figure S4, Supporting Information). 13C NMR (CDCl3, δ, ppm): 171.6, 163.4, 163.1, 134.9, 134.4, 131.5, 131.3, 129.5, 129.3, 126.5, 126.4, 126.0, 123.5, 123.2, 123.0, 122.9, 63.2, 55.8, 39.0, 30.7, 23.8, 22.5, 22.4, 9.8 (Figure S5, Supporting Information). IR (KBr): ν = 3443, 2955, 2871, 1730, 1699, 1660, 1594, 1556, 1464, 1441, 1402, 1345, 1229, 1110, 836, 810, 744, 484 (Figure S1, Supporting Information). Anal. Calcd for C61H87N2BrO18Si8: C, 50.87; H, 6.05; N, 1.95. Found: C, 50.77; H, 6.06; N, 1.94. Synthesis of POSS-PBI-PNIPAM. POSS-PBI-PNIPAM was prepared by ATRP. Polymerization of NIPAM was performed using POSS-PBIBr as an initiator, CuBr as a catalyst and Me6TREN as a ligand. Typically, CuBr (14 mg, 0.1 mmol) and POSS-PBI-Br (144 mg, 0.1 mmol) were added to a round-bottom flask fitted with a rubber septum and pump-filled with nitrogen three times. The mixture was subsequently deoxygenated, and inhibitor-free NIPAM (3.40 g, 30 mmol), DMF (3 mL), 2-propanol (1 mL), THF (2 mL), and Me6TREN (23 mg, 0.1 mmol) were added to the round-bottom flask under nitrogen. The solution was further deoxygenated by three freeze−pump−thaw cycles before being kept in 25 °C, and the solution was maintained at this temperature for 7 h. After the polymerization was stopped, the reaction mixture was diluted with THF, and exposed to air. The reaction mixture was passed through a basic alumina column to remove the copper catalyst. After removing the solvents by a rotary evaporator, the residues were dissolved in THF and precipitated into an excess of ethyl ether. The above dissolution−precipitation cycle was repeated twice. The final product as a red solid was dried in a vacuum oven at room temperature in a yield of 1.70 g (58%). Mn,GPC = 11.1 kDa, Mw/Mn = 1.31). The DP of PNIPAM block was determined to be 245 by 1H NMR analysis in CDCl3 (Figure 1). The product was denoted as POSS-PBIPNIPAM245. N,N′-Propylheptakis (isobutyl) POSS-perylene Tetracarboxylic Acid Bisimides (POSS-PBI-POSS). POSS-NH2 (2.36 g, 2.7 mmol), 3,4:9,10-perylenetetracarboxylic dianhydride (0.47 g, 1.2 mmol), imidazole (1.0 g) and N,N-dimethylacetamide (25 mL) were vigorously stirred under nitrogen at 140 °C for 6 h. After the reaction, CH2Cl2 (100 mL) was added, and the mixture was washed by dilute hydrochloric acid for three times. The organic solution was dried with sodium sulfate and the salts were subsequently filtered off. The filtrate was rotary concentrated and further purified on a silica gel column using CHCl3 as eluent to afford POSS-PBI-POSS as bright-red solid (2.24 g, yield:89%). 1H NMR (CDCl3, δ, ppm): 8.69 (d, 4H, Ar− H), 8.63 (d, 4H, Ar−H), 4.21 (t, 4H, α-CH2), 1.84 (m, 18H, SiCH2CH, SiCH2CH2), 0.95 (m, 84H, CH(CH3)2), 0.59 (m, 32H, SiCH2) (Figure S6, Supporting Information). 13C NMR (CDCl3, δ, ppm): 163.3, 140.8, 134.6, 131.4, 129.4, 126.5, 123.4, 123.0, 42.9, 25.7, 23.9, 22.5, 22.4, 9.8 (Figure S7, Supporting Information). IR (KBr): ν = 2956, 2873, 1700, 1660, 1595, 1464, 1443, 1403, 1346, 1252, 1230,

Figure 1. 1H NMR spectrum of POSS-PBI-PNIPAM245 in CDCl3. 1113, 837, 809, 744, 564, 482, and 433 (Figure S8, Supporting Information). Self-Assembly of POSS-PBI-PNIPAM. Typically a solution of POSSPBI-PNIPAM (5 mg) in THF (0.5 mL) was rapidly injecting into water (50 mL) under ultrasonic condition at 25 °C and a micellar solution was obtained by further ultrasonic treatment for 5 min. Characterization. Nuclear Magnetic Resonance Spectroscopy (NMR). All NMR spectra were recorded on a Bruker AVANCE II spectrometer (resonance frequency of 400 MHz for 1H) operated in the Fourier transform mode. CDCl3 and TMS were used as the solvent and internal standard, respectively. Fourier Transform Infrared Spectroscopy (FT-IR). Fourier transform infrared (FT-IR) spectra were recorded on a Bruker VECTOR22 IR spectrometer and were collected at 64 scans with a spectral resolution of 4 cm−1. Elemental Analyses. Elemental analyses were performed on a VARIO ELIII C, H, and N analyzer. Gel Permeation Chromatography (GPC). Molecular weights and molecular weight distributions were determined by gel permeation chromatography (GPC) equipped with a Waters 1515 pump, a Waters 2414 RI detector, and Waters UV/RI detectors (set at 30 °C). It used a series of three linear Styragel columns HR3, HR4, and HR6 at an oven temperature of 45 °C. The eluent was THF at a flow rate of 1.0 mL/min. A series of low-polydispersity polystyrene (PS) standards were employed for the GPC calibration. Ultraviolet−Visible Spectroscopy (UV−Vis). UV−vis spectra of solutions were recorded with a UV−vis TU-1901 spectrometer. Photoluminescence (PL) Spectroscopy. Emission spectra of solutions were measured by an F-4600 PL Spectrophotometer. Quantum yields in solution were determined twice using Rhodamine B in ethanol (5.0 μg/mL) as the standard and the measurements were averaged. Temperature-dependent fluorescence spectra were acquired on a LS 55 luminescence spectrometer. The PL quantum yield of POSS-PBI-POSS in the solid state was determined as absolute values in an integrating sphere. Transmission Electron Microscopy (TEM). TEM observations were conducted on an H-7650 transmission electron microscope at an acceleration voltage of 100 kV. The sample for TEM observations was prepared by placing 10 μL of micellar solution on copper grids. Dynamic Laser Light Scattering (DLS). A commercial spectrometer (ALV/DLS/SLS-5022F) equipped with a multitau digital time correlator (ALV5000) and a cylindrical 22 mW UNIPHASE He−Ne laser (λ0 = 632 nm) as the light source was employed for DLS measurements. Scattered light was collected at a fixed angle of 90° for duration of 10 min. 3088

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Table 1. ATRP Results of NIPAM Initiated by POSS-PBI-Bra polymer f

POSS-PBI-PNIPAM58 POSS-PBI-PNIPAM145g POSS-PBI-PNIPAM254g

time (h)

conversionb (%)

Mn,thc (KDa)

Mn,NMRd (KDa)

Mn,GPCe (KDa)

PDIe

3 5 7

26 40 67

10.2 15.0 24.1

8.0 17.8 29.1

3.2 8.6 11.1

1.39 1.32 1.31

a Polymerization conditions: [NIPAM]0/[POSS-PBI-Br]0/[CuBr]0/[Me6TREN]0 = 300/1/1/1, [NIPAM]0 = 5 M in DMF/2-propanol/THF (3/1/ 2, v/v/v), temperature = 25 °C. bDetermined by 1H NMR. cCalculated from the equation: Mn,th = Conversion ×300 × 113 + 1439. dCalculated from the equation: Mn,NMR = 113× 16A3.92/A0.52 + 1439, where A3.92 and A0.52 are the integral values of the peaks at 3.92 and 0.52 ppm. eDetermined by GPC. fPOSS-PBI-PNIPAM58 was purified by dialysis in water. gPurified by precipitated into an excess of ethyl ether.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Amphiphilic Polymers. Scheme 1 illustrates the synthetic route of the amphiphilic polymer comprising of one perylene bisimide moiety, one POSS moiety, and one PNIPAM chain. The synthesis of POSS-containing ATRP initiator includes mainly three steps: first, synthesis of N-hydroxyethyl-3,4:9,10-perylenetetracarboxylic monoanhydride monoimide 3 was achieved from the reaction of 2 with 2-aminoethanol; second, POSScontaining asymmetric PBI 4 with a hydroxy functional group was prepared by incorporating POSS moiety into 3; third, esterification of compound 4 with 2-bromoisobutyryl bromide was carried out to produce the ATRP initiator (POSS-PBI-Br). The products were characterized by FT-IR spectra. As shown in Figure S1, Supporting Information, the successful synthesis of 3 can be confirmed by the sharp band at 1764 cm−1 of anhydride and 1693 cm−1 of imide,50 as well as the corresponding hydroxy band at 3500 cm−1. After the reaction of 3 with POSS-NH2, the complete consumption of anhydride can be confirmed by the disappearance of the characteristic anhydride absorption peak at 1764 cm−1 in the spectrum of 4. Moreover, the bands at 2800−3000 cm−1 of C−H vibration stretching on the eight pendant alkyl arms of POSS, and the Si−O−C stretching band of the cubic cores of POSS at 1111 cm−1 both indicate that the POSS moiety is incorporated into perylene chromophore.41 The esterification of 4 with an excess of 2-bromoisobutyryl bromide affords POSS-PBI-Br and typical absorption peak of ester at 1730 cm−1 can be observed in the spectrum of POSSPBI-Br. We noticed that PBI 3 has poor solubility, however, both PBI 4 and POSS-PBI-Br show good solubilities in common organic solvents due to the incorporation of POSS group. Thus, the structures of PBI 4 and POSS-PBI-Br can be well determined by NMR spectroscopy. 1H NMR and 13C NMR spectra of 4 and POSS-PBI-Br are shown in Figures S2−S4 (Supporting Information), and all the peaks have been well designated. The PBI protons of both 4 and POSS-PBI-Br present a simple pattern of sharp signals located in the region of 8.67−8.39 ppm in CDCl3, indicating that PBI groups exist in the monomeric form (or at most a very low aggregated form) at a high concentration of 1.0 × 10−2 M.29 In order to obtain well-defined amphiphilic polymers, ATRP of NIPAM was carried out using POSS-PBI-Br as initiator in the presence of CuBr/Me6TREN catalyst in ternary mixed solvents of DMF, 2-propanol, and THF at 25 °C. The ternary mixed solvent was chosen as the solvent for ATRP based on the following reasons. First, 2-propanol can form hydrogen bonds with the amide groups of the monomers and the polymers to reduce the interaction between the catalyst and the propagating chain ends.51 Second, THF can enhance the solubility of POSSPBI-Br and increase the initiator efficiency. From the polymerization results listed in Table 1, we can see that the

molecular weights of POSS-PBI-PNIPAM are well-controlled and the molecular weight distributions are relatively narrow. The structure and molecular weight of the polymers were characterized by 1H NMR spectroscopy and GPC. In Figure 1, aromatic proton peaks of PBI appear at about 8.6 ppm and methine and methylene proton peaks at 1.5−2.8 ppm are ascribed to the backbone of PNIPAM. The signal at 3.92 ppm belongs to the protons of pendant groups of PNIPAM, and the characteristic peaks of POSS moieties appear at 0.87 and 0.52 ppm. The GPC of the polymers depicts monomodal distributions of polymeric species and a clear shift to higher molecular weight with increasing conversion (Figure 2).

Figure 2. Evolution of molecular weight with monomer conversion measured by GPC.

The absorption and PL spectra of POSS-PBI-PNAPAM245 in THF solution (0.1 g/L) are shown in Figure 3. In the visible region, three pronounced absorptions are observed respectively

Figure 3. UV−vis absorption and fluorescence spectra (λexcit. = 455 nm) of POSS-PBI-PNIPAM245 in THF (0.1 g/L) at 20 °C. Inset: Picture of THF solution of POSS-PBI-PNIPAM245 (0.1 g/L) under a hand-held UV lamp (λexcit. = 365 nm). 3089

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at 455, 485, and 521 nm, corresponding to the 0−2, 0−1, and 0−0 electronic transitions of the PBI units. Notably, the intensity of the 0−0 band is higher than that of the 0−1 band, this phenomenon indicates that the Franck−Condon factors favor the lower (0−0) excited vibronic state, and implies the absence of significant interaction between the PBI units.52−54 The fluorescence spectrum of POSS-PBI-PNIPAM245 displays three bands at 535, 568, and 620 nm. They match well with the absorption spectra and there was good mirror-image symmetry between the absorption and fluorescence spectra. These results demonstrate that the PBI fluorophores are in a nonaggregated state.21 The PL quantum yield of POSS-PBI-PNIPAM245 in dilute THF solution was determined to be above 95%. Morphology and Fluorescence of Hybrid Nanoparticles Self-Assembled from POSS-PBI-PNIPAM in Aqueous Solution. The newly synthesized amphiphilic polymers contain highly hydrophobic POSS units and PBI species as well as thermoresponsive water-soluble PNIPAM segments.55,56 In aqueous solution, the amphiphilic nature causes them self-assemble into core−shell micellar structures consisting of hydrophobic POSS cores and hydrophilic PNIPAM coronas at room temperature. Stable micellar solution of POSS-PBI-PNIPAM245 in aqueous solution was successfully prepared by rapidly injecting a solution of POSS-PBIPNIPAM245 in THF (0.5 mL, 10 g/L) into 50 mL water under ultrasonic condition at room temperature. However, at the same condition, POSS-PBI-PNIPAM58 and POSS-PBIPNIPAM145 precipitated in water due to their shorter hydrophilic PNIPAM chains. This result indicates that design of polymer structures is very important for the fromation of stable polymer particles. Figure 4 shows typical plots of the hydrodynamic radius distribution, f(Rh), for the hybrid nanoparticles self-assembled

Figure 5. TEM image obtained for hybrid nanoparticles self-assembled from 0.1 g/L aqueous solution of POSS-PBI-PNIPAM245 at 25 °C.

state. Moreover, since DLS reports the intensity-average dimensions of nanoparticles in solution, it contains considerable contribution from the hydrophilic PNIPAM coronas. Therefore, the micelles sizes measured by DLS is often bigger than that measured by TEM.57,58 In addition, compared with the similar tadpole-shaped POSS-containing PNIAPM reported in literature, 57 POSS-PBI-PNIPAM is more prone to aggregation due to the highly hydrophobic property and effective intermolecular π-stacking of the PBI linkers. As a result, a nanoparticle self-assembled from POSS-PBI-PNIPAM245 contains much more polymer chains, this leads to the larger value of Rh. On the other hand, the cores of POSSPBI-PNIPAM nanoparticles should be more compact due to the strong π−π interaction of PBI groups. Thus, a huge difference in micelles sizes was observed in the measurements of DLS and TEM. The theoretical length of POSS-PBIPNIPAM could be calculated according to the molecular weight. For example, the calculated result shows that the theoretical length of POSS-PBI-PNIPAM245 in the unperturbed state is 65 nm (per repeat unit of PNIPAM is about 0.25 nm,59 the diameter of POSS molecule is 1.5 nm), which is much smaller than the value of Rh measured by DLS. As shown in Figure 6, the aqueous suspensions of POSS-PBIPNIPAM245 exhibit a strong excimer-like emission at 645 nm under UV illumination. The emission is very similar to the excimer-like emissions usually observed for weakly interacting π-stacked perylene molecules in solution. Compared with the emission in dilute solution, a redshift can be observed for the hybrid nanoparticles due to energy transfer to low-energy sites in the packed PBI moieties.25−27 The red shift is accompanied by reduced PL intensity but affords a relatively large stokes shift of 150 nm, which is useful to decrease the measurement error by excitation light and scattered light for a ideal probe.16,60,61 A few studies have been reported on the fluorescence properties of PBI aggregates to support the general rule-of-thumb that PBI aggregates tend to emit red fluorescence when separated by sterically demanding groups.27−30,62 However, the aggregate state emission ability of these examples remains a challenge and the quantum yields are relatively low. In the present case, the PL quantum yield for the assemblies in water was determined to be 27% at 20 °C, which did not provide in the literature. POSS is the vital component to realize the high fluorescence efficiency, because the steric hindrance of POSS prevents the

Figure 4. Hydrodynamic radius distributions obtained for hybrid nanoparticles self-assembled from 0.1 g/L aqueous solution of POSSPBI-PNIPAM245 at 25 °C.

from 0.1 g/L aqueous solution of POSS-PBI-PNIPAM245 at 25 °C. The intensity-average hydrodynamic radius was evaluated to be about 130 nm. TEM observations were performed to examine the actual morphologies of micellar aggregates formed from the same solution at 25 °C. Figure 5 clearly reveals that spherical hybrid nanoparticles are formed in water with diameters of around 10 nm. We noticed that there are huge differences in micelles sizes measured through DLS and TEM. In consideration of the soft nature of PNIPAM segments, TEM actually revealed the core dimensions of the micelles in the dry 3090

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Figure 6. Emission (λexcit. = 495 nm) spectra of hybrid nanoparticles self-assembled from 0.1 g/L aqueous solution of POSS-PBIPNIPAM245 at 20 °C. Inset: Picture of POSS-PBI-PNIPAM245 hybrid nanoparticles in pure water (0.1 g/L) under a hand-held UV lamp (λexcit. = 365 nm).

Figure 7. Temperature-dependent fluorescence spectra obtained for 0.1 g/L aqueous solution of POSS-PBI-PNIPAM245 (λexcit. = 495 nm). Inset: Temperature dependence fluorescence intensity at 645 nm.

aromatic ring of PBI from close packing and reduces the intermolecular interactions. It is necessary to point out that a soft and nonchromogenic propylidene linkage between POSS and PBI is important to generate the strong red fluorescence for the hybrid nanoparticles. Recently, Cheng and co-workers reported a hybrid with two POSSs covalently attached to PBI via a rigid 1,4phenylene linkage (Figure S9, Supporting Information).47 The introduction of POSS seems to be ineffective in maintaining the aggregate state fluorescence. We consider that the quenching effect may be attributed to the rigid and electron-rich 1,4phenylene groups in the molecule.21 In order to confirm this assumption, we synthesized a similar compound, POSS-PBIPOSS (Figure S9, Supporting Information), with a propylidene linkage instead of 1,4-phenylene linkage between POSS and PBI. It was found that the fluorescence of POSS-PBI-POSS was not quenched in solution, or even in solid state and the solidstate PL quantum yield is about 46%. This result indicates that it is very important to design the structure of organic compounds and polymers for fluorescent materials. PNIPAM chain can undergo coil-to-globule phase transition upon its lower critical solution temperature (LCST).55,56 As shown in Figure S10, Supporting Information, with the increase of temperature from 20 to 50 °C, the stretching PNIPAM chains shrink and collapse on the surface of the nanoparticle core, leading to the decrease of Rh from 130 to 87 nm with the critical transition temperature at 31 °C. The temperaturedependent fluorescence spectra obtained for 0.1 g/L aqueous solution of POSS-PBI-PNIPAM245 are shown in Figure 7. With the temperature increasing from 20 to 50 °C, the PL quantum yields of the hybrid nanoparticles increase from ∼27% (20 °C) to ∼43% (50 °C) while the emission peaks remain almost unchanged. The LCST of the POSS-PBI-PNIPAM245 has been also determined in these experiments, and the lower aggregation temperature (∼31 °C) is similar to that of pure PNIPAM (32 °C).63 Moreover, the PNIPAM coronas display a well reversible coil-to-globule phase transition, and the change of fluorescence intensity of the hybrid nanoparticles is completely reversible (Figure 8). A mechanism was proposed for the reversible change of fluorescence intensity. In aqueous solution, POSS-PBIPNIPAM245 self-assemble into micelles with POSS as the

Figure 8. Fluorescent cycling of the hybrid nanoparticles (0.1 g/L in aqueous solution) at various temperature between 25 and 50 °C(λexcit. = 495 nm).

core and PNIPAM as the shell with PBI groups located in the middle layer between the core and the shell. Below the LCST, all PNIPAM chains in the shell of the micelles are stretched out and compact PBI aggregates with lower fluorescence form in the middle layer.27 However, above the LCST, the shrinking and collapsing of PNIPAM chains extend the space between the PBI groups, resulting in weaker π−π stacking interaction and higher fluorescence.64 Besides the thermoresponsive PNIPAM chain, the location of PBI in the polymer also contributes to the temperature-dependent fluorescence intensity. Because the size of POSS unit is comparable to the dimensions of a polymer segment in the solid or molten phase,43 the structure of POSS-PBI-PNIPAM is analogous to that of diblock copolymers with a fluorescent group at the junction between two blocks.64 Fluorescence response of amphiphilic polymers with a single fluorescent group at the middle of the polymer chains is quite different from that of polymers with a fluorescent group at the polymer chain end.65−67 Self-assembly of the latter forms micelles with fluorescent groups in the cores, and fluorescence intensity of the polymer solution has little dependence on environment temperature.64

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CONCLUSION In this paper, we present the design and synthesis of a novel amphiphilic polymer possessing a perylene bisimide bridge 3091

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between a POSS moiety and a PNIPAM chain. The target polymers were efficiently synthesized by ATRP with controlled molecular weight and low polydispersity, as characterized by 1H NMR spectroscopy and GPC. Hybrid nanoparticles were then fabricated via the self-assembly of the polymers in aqueous solution. Because of the incorporation of POSS moieties, the hybrid nanoparticles retain the luminescence in its aggregate state and show red emission with the PL peak at 645 nm and quantum yield of 27%. Moreover, based on the thermoresponsive PNIPAM coronas, the fluorescence intensity of the self-assembled hybrid nanoparticles can be further enhanced and tuned by changing temperature. This work provides a new strategy for design and preparation of red fluorescent nanoparticles with environmental responsiveness, which can find use in many applications such as biosensors, diagnostic nanoprobes, and targeted drug delivery.

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ASSOCIATED CONTENT

S Supporting Information *

Additional FT-IR, 1H NMR, and 13C NMR spectra, molecular structures, and hydrodynamic radius vs. temperature results. 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.

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ACKNOWLEDGMENTS We express thanks for the financial support from the National Natural Science Foundation (NNSF) of China (No. 20974104 and No. 21074120) and the Ministry of Science and Technology of China (NO. 2007CB936401).

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