Thermosensitive Amphiphilic Janus Dendrimers ... - ACS Publications

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Thermosensitive Amphiphilic Janus Dendrimers with Embedded Metal Binding Sites. Synthesis and Self-Assembly Xin Liu† and Ivan Gitsov*,†,‡ †

Department of Chemistry, State University of New York - ESF, Syracuse, New York 13210, United States The Michael M. Szwarc Polymer Research Institute, Syracuse, New York 13210, United States

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ABSTRACT: A family of amphiphilic diblock Janus dendrimers were synthesized by chemoselective “click” coupling of poly(benzyl ether) dendrons, PBE, and poly(ether−ester), PEE, dendrons based on three different ethylene glycol spacers. The well-defined defect-free structure of these dendrimers was proven by MALDI-TOF. In aqueous solutions these dendrimers were able to self-assemble into vesicles with diameter ranging from 140 nm to 3.6 μm or solid micelles (10 to 14 nm diameter). The form of these assemblies was affected by the spacer length between branching points and by the generations of both dendrons. The ethylene glycol moieties in the PEE dendrons induced notable thermosensitivity with sharp transitions observed for all members of the series. The presence of two triazole groups at each branching point was further exploited to fabricate palladium bearing nanomaterials with potential application in catalysis.

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INTRODUCTION Dendrimers, as a unique class of macromolecules, have been intensely investigated for over four decades. Because of their highly functional terminal and inner groups, they have been utilized in catalysis,1−3 drug delivery,4−6 and bioimaging.7,8 Dendrimers composed of two dendron wedges of different hydrophilicity/hydrophobicity, adequately named “Janus dendrimers”, are interesting macromolecules combining in a unique way the properties of the constituting blocks.9−11 Compared to amphiphilic copolymers, their dendritic counterparts, Janus dendrimers, are more tunable on the molecular level to further affect their aggregation behavior, including dendron generation, density of branching, and peripheral groups. In an earlier study, Percec et al. elegantly displayed the selfassembling versatility of this class of dendrimers, which formed various supramolecular structures including vesicles, cubosomes, disks, and others.12 In another research, mono- and disaccharide decorated Janus dendrimers formed vesicles, described as “glycodendrisomes”, and were proven to be biologically active.13,14 More recently, fluorinated and fluorescent dye conjugated Janus dendrimers were prepared by the same group for medicinal and biological research.15 An interesting class of amphiphilic Janus dendronized polymers was reported by Schlüter et al.16 Synthesis of Janus dendrimers composed of first-generation Percec-type dendron and up to second generation of bis-MPA-based polyester dendron were synthesized via copper-catalyzed alkyne−azide cycloaddition (CuAAC).17,18 These dendrimers formed stable and sizetunable vesicles by the facile ethanol injection method. © XXXX American Chemical Society

Janus dendrimers also found application as catalysts. Fréchet-type poly(benzyl ether), PBE, and Tomalia-type poly(amido-amine), PAMAM, based Janus dendrimers were synthesized and utilized as soluble support in Suzuki coupling reactions.19,20 This study was motivated by these publications and our previous work21 on the synthesis of new poly(ether−ester) dendrimers. Herein we report the creation of new family of amphiphilic Janus dendrimers composed of a poly(benzyl ether), PBE, and poly(ether−ester), PEE, dendrons containing three ethylene glycols with different chain lengths. Dendrimers up to third generation were built by covalent “click” chemistry coupling of the complementary functional groups at the “focal” point of each dendron. The macromolecules formed could selfassemble into vesicles or micelles in aqueous solutions depending on their hydrophobic−hydrophilic balance. The resulting supramolecular structures were characterized by dynamic light scattering (DLS), optical phase contrast microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Interestingly, the micelles exhibited generation and concentration dependent lower critical solution temperature (LCST). Furthermore, the dendrimers were utilized to fabricate palladium nanocomposites via the chelating triazole rings embedded at each branching unit. Received: April 2, 2018 Revised: June 6, 2018

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DOI: 10.1021/acs.macromol.8b00700 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Poly(ether−ester) Dendrons with Different Ethylene Glycol Spacers

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N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDTEA, 99%), Cu(I) bromide (CuBr, 98%), propargyl bromide (80% in toluene), N,N′-dicyclohexylcarbodiimide (DCC, 99%), 4-(dimethylamino)-

EXPERIMENTAL SECTION

Materials. 2-Bromoethanol (95%), 2-(2-chloroethoxy)ethanol (95%), 2-[2-(2-chloroethoxy)]ethanol (96%), NaH (95%), B


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Macromolecules

Alternate Method A. The CuSO4/THTPA complex was used instead of pure CuSO4. The complex was prepared by mixing the ligand to copper salt in water at 4:1 ratio (w/w). Method B. A desired amount of CuBr was placed into a Schlenk flask. The flask was vacuumed and filled with argon in three cycles. An acetone solution containing the organic azide, alkyne, and PMTDA was placed into another Schlenk flask. The above solution was degassed by the freeze−thaw process in three cycles. After warming to room temperature, the solution was transferred with a syringe into the flask containing CuBr. The reaction mixture was stirred at room temperature, and the reaction progress was monitored by MALDITOF or TLC. Alternate Method B. CuBr2 and Cu(0) powder were placed into a round-bottom flask. Then an acetone solution of the organic azide, alkyne, and PMDTA was added. The solution turned into deep blue and kept the same color upon completion of the reaction. General Procedure for Acetonide Protecting Group Removal on PEE Dendrons. PEE dendron was dissolved in 2 mL of DCM followed by addition of 2 mL of TFA. The mixture was stirred at room temperature for 1 h. The solvents were removed under reduced pressure. The residue was dissolved in MeOH. NaHCO3 was added to neutralize the remaining TFA. The solution was filtered and passed through a short silica column to remove the water and the salts. The deprotected PEE dendrons are obtained as viscous oils after evaporating the solvents and used freshly for Janus dendrimer synthesis. Compound 3a. To a round-bottom flask were added 2bromoethanol (2.88 g, 23 mmol) and isopropylidene-2,2-bis(methoxy)propionic acid (2, 3 g, 17.2 mmol) followed by 20 mL of dry DCM. To the above suspension were added DCC (4.26 g, 20.6 mmol) and DMAP (0.63 g, 5 mmol). The reaction mixture was stirred at room temperature for 24 h. The resulting dicyclohexyl urea (DCU) was removed by vacuum filtration. The product was obtained as colorless oil by passing the concentrated reaction mixture through a silica column (10:1 hexanes:ethyl acetate gradually decreasing to 8:1). Yield: 3.65 g (75%). 1H NMR (300 MHz, CDCl3): δ 4.46 (t, 2H, J = 6 Hz), 4.22 (d, 2H, J = 11.8 Hz), 3.67 (d, 2H, J = 11.8 Hz), 3.55 (t, 2H, J = 6 Hz), 1.44 (s, 3H) 1.39 (s, 3H), 1.22 (s, 3H). 13C NMR (300 MHz, CDCl3): δ 173.9, 98.12, 65.96, 63.94, 42.05, 28.70, 24.91, 22.33, 18.59. Compound 4a. 3a (3 g, 10.6 mmol) was dissolved in 15 mL of DMSO in a round-bottom flask. To the above solution, NaN3 (2.067 g, 31.8 mmol) was added. The reaction mixture was stirred at room temperature for 12 h. Then it was diluted with 30 mL of water and extracted three times with 20 mL of ethyl acetate. The combined organic layers were twice washed with brine. The product was obtained as a colorless oil by evaporating the solvent. Yield: 2.47 g (95%). 1H NMR (CDCl3, 300 MHz): δ 4.34 (t, 2H, J = 6 Hz), 4.22 (d, 2H, J = 11.8 Hz), 3.68 (d, 2H, J = 11.8 Hz), 3.51 (t, 2H, J = 6 Hz), 1.45 (s, 3H) 1.41 (s, 3H), 1.23 (s, 3H). 13C NMR (300 MHz, CDCl3): δ 173.78, 98.10, 65.93, 63.64, 49.80, 41.99, 24.83, 22.41, 18.47. Compound 6a. 5 (1.8 g, 8.57 mmol) and 2-bromoethanol (1.29 g, 10.32 mmol) were dissolved in 15 mL of dry DCM in a round-bottom flask. To the above solution were added DCC (2.13 g, 10.33 mmol) and DMAP (0.33 g, 2.7 mmol). The reaction mixture was stirred at room temperature for 24 h. Then the DCU was removed by vacuum filtration. The product was purified by silica column chromatography (10:1 ethyl acetate:hexanes gradually changed to 8:1). Yield: 1.92 g (71%) of light yellow oil. 1H NMR (CDCl3, 300 MHz): δ 4.44 (t, 2H, J = 6.1 Hz), 4.18 (d, 4H, J = 3.3 Hz), 3.69 (s, 4H), 3.53 (t, 2H, J = 3.3 Hz), 2.44 (s, 2H), 1.28 (s, 3H). 13C NMR (CDCl3, 75 MHz): δ 175.3, 78.7, 76.4, 72.4, 71.4, 60.6, 41.1, 27.1, 15.6. SG1-Br. Click method B: 4a (2.433 g, 10 mmol), 6a (1.44 g, 4.5 mmol), and PMDTA (0.52 g, 1.5 mmol) were dissolved in 20 mL of acetone into a round-bottom flask then placed into an ice path and degassed three times by freezing and thawing cycles. To the above solution, CuBr (0.214 g, 1.5 mmol) was added. The flask was sealed with a rubber stopper. The reaction was warmed to room temperature and stirred overnight. Then the dark green reaction mixture was

pyridine (98%), sodium azide (NaN3, 99%), trifluoracetic acid (TFA, 99%), sodium ascorbate (NaAsc, 98%), and tris(3-hydroxylpropyltriazolylmethyl)amine (THPTA, 95%) were purchased from SigmaAldrich. Copper powder (spherical APS, 10 μm) was purchased from Alfa Aesar. Copper(II) sulfate (anhydrous), CuSO4 (98.5%), was purchased from Fisher. All reagents were used as received. Tetrahydrofuran, THF, and acetone were dried over 4 Å molecular sieves prior to use. All other solvents were used without further treatment. Second- and third-generation Fréchet-type PBE dendrons, isopropylidene-2,2-bis(methoxy)propionic acid (2, see Scheme 1), 2-bis(propargyl)propionic acid (5, see Scheme 1), and triethylene glycol PEE dendrons (LG1, LG2, and LG3, see Scheme 1) were synthesized according to the literature.21 Methods. The NMR spectra were recorded at room temperature on a 600 MHz Bruker AVANCE instrument using CDCl3 or acetoned6 as solvents. Chemical shifts are reported in ppm relative to the solvent signal. The MALDI-TOF measurements were performed on a Bruker Autoflex III with smart ion source equipped with a Nd:YAG laser (266 and 355 nm) using positive-reflect mode. The samples were prepared by mixing the analyte with the matrix (2,5-dihydroxylbenzoic acid) in THF and then deposited and dried on a Bruker ground steel MALDI target plate. The vesicles and micelles were prepared by an injection method.12−14 Briefly the procedure is executed as follows: 50 μL of dendrimer solution in acetone (20 mg/mL) was injected into 1 mL of nanopure water followed by 10 s vortex. Vesicles formed by FM1 Janus dendrimers were obtained using film hydration in the following procedure: dendrimer thin film was formed by evaporating on rotary evaporator 1 mL of dendrimer methanol solution (1 mg/mL) in a round-bottom flask and then adding 1 mL of water to the film followed by 20 min ultrasonication. Optical microscopy analysis was performed on Nikon Optiphot using phase contrast objective lenses. The samples were prepared by casting a droplet of solution on a concavity glass slide and subjected to microscope analysis immediately. The SEM samples were prepared by depositing the solution on a piece of clean glass and then dried overnight at room temperature. The dried sample was sputter coated by Au/Pd on a Denton Vacuum Desk V. The SEM analyses were accomplished ona JEOL JSM-IT100 at 10−15 kV. The TEM samples were prepared by casting a droplet of solution onto a carbon-coated grid and then blotted after 5 min. The grid was stained by 1% uranyl acetate solution. The samples were visualized by a JEOL-2000EX at 80 kV. The images were captured by negative film and digitalized by an EPSON V800 scanner. Turbidity analysis was performed on Beckman DU640B UV/vis spectrometer by recoding the transmittance at 500 nm. The critical micelle concentration was determined using pyrene as the probe. The pyrene emission fluorescence spectrum was recorded on a Horiba Fluorolog-3 spectrometer with excitation at 334 nm and 0.1 s integration time. Dynamic light scattering analysis was executed on a Malvern Zetasizer ZS equipped with a 633 nm laser source and a backscattering detector at 173°. Data were processed by a CONTIN procedure. Synthesis of poly(ether−ester) (PEE) dendrons was accomplished using our previously published procedure with small modifications. The synthetic pathway is presented in Scheme 1. “Click” Chemistry Syntheses. Method A. Organic azide and alkyne reagents were dissolved in THF. CuSO4 in water was added to the above solution followed by NaAsc. The reaction mixture turned deep brown upon the NaAsc addition, then the color quickly disappeared. The mixture was left stirring at room temperature, and the reaction progress was monitored by MALDI-TOF or TLC. Upon completion, the mixture was concentrated by rotary evaporation, and products were isolated by silica column chromatography. C


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24H). 13C NMR (CDCl3,151 MHz): δ 173.82, 145.05, 123.46, 98.19, 72.06, 65.99, 64.77, 62.86, 62.58, 49.16, 49.01, 48.33, 42.12, 26.11, 21.23, 21.02, 18.27, 17.80. MS (MALDI-TOF, positive mode) calculated for C171H255BrN42O60 [M] = 3938.06, found [M + H]+ m/z = 3939.361 and [M + Na]+ m/z = 3962.431. SG3-N3. The same method as for SG1-N3 was adopted using SG3Br instead (0.4 g, 0.1 mmol), NaN3 (0.5 g, 7.6 mmol), and DMSO (5 mL). Yield: 0.364 g (92%) of light yellow very viscous oil. 1H NMR (CDCl3, 600 MHz): δ 7.78 (s, 8H), 7.74 (s, 2H), 7.73 (s, 4H), 4.68 (t, 16H, J = 5.4 Hz), 4.63−4.60 (m, 16H), 4.58−4.56 (m, 40H), 4.49 (t, 12H, J = 5.4 Hz), 4.24 (t, 2H, J = 5.9 Hz), 4.15 (d, 16H, J = 12.1 Hz), 3.65−3.63 (m, 20H), 3.59−3.55 (m, 24H), 3.44 (t, 2H, J = 5.9 Hz), 1.44 (s, 24H), 1.37 (s, 24H), 1.20 (s, 3H), 1.13 (s, 12H), 1.12 (s, 6H), 1.09 (s, 24H). 13C NMR (CDCl3, 151 MHz): δ 173.82, 173.80, 145.05, 144.87, 123.52, 123.46, 98.19, 72.16, 72.06, 65.99, 64.82, 64.76, 64.71, 63.34, 62.86, 62.59, 49.85, 49.16, 49.015, 48.41, 48.36, 48.33, 42.12, 41.06, 30.90, 26.12, 21.22, 18.27, 17.86, 17.80. MS (MALDI-TOF, positive mode) calculated for C171H255N45O60 [M] = 3900.18, found [M + H]+ m/z = 3901.151 and [M + Na]+ m/z = 3923.017. MG1, MG2, and MG3 were synthesized following a similar procedure as for SG dendrons. 2-(2-Chloroethoxy)ethanol was used instead of 2-bromoethanol as the spacer. Compound 3b. The DCC coupling method is the same as for 3a using 2 (2.5 g, 14.3 mmol), 2-(2-chloroethoxy)ethoxyethanol (3.6 g, 2.2 mmol), DCC (4.43 g, 2.2 mmol), DMAP (0.5 g, 4 mmol), and dry DCM 30 mL. Colorless oil. Yield: 2.49 g (62%). 1H NMR (CDCl3, 600 MHz): δ 4.34 (t, 2H, J = 4.9 Hz), 4.22 (d, 2H, J = 11.7 Hz), 3.78 (t, 2H, J = 5.7 Hz), 3.76 (t, 2H, J = 4.7 Hz), 3.67 (d, 2H, J = 11.7 Hz), 3.64 (t, 2H, J = 5.7 Hz), 1.45 (s, 3H), 1.41 (s, 3H), 1.24 (s, 3H). 13C NMR (CDCl3, 151 MHz): δ 174.15, 98.09, 71.22, 69.04, 65.97, 63.70, 42.74, 41.87, 24.37, 22.93, 18.67. Compound 4b. 3b (2.49 g, 8.8 mmol) was dissolved in 10 mL of DMSO followed by an addition of 3 g of NaN3. The mixture was stirred overnight in an oil bath at 70 °C. The mixture was diluted by 20 mL of water and extracted with 20 mL of ethyl acetate three times. The combined organic layer was washed with brine twice and dried over Na2SO4. The product was obtained by evaporating the solvent. Colorless oil. Yield: 2.27 g (89%). 1H NMR (CDCl3, 600 MHz): δ 4.32 (t, 2H, J = 4.8 Hz), 4.21 (d, 2H, J = 11.8 Hz), 3.73 (t, 2H, J = 4.8 Hz), 3.69 (t, 2H, J = 4.9 Hz), 3.66 (d, 2H, J = 11.8 Hz), 3.38 (t, 2H, J = 4.8 Hz), 1.44 (s, 3H), 1.40 (s, 3H), 1.23 (s, 3H). 13C NMR (CDCl3, 151 MHz): δ 174.17, 98.06, 70.04, 69.04, 65.95, 63.78, 50.72, 41.85, 24.38, 22.89, 18.62. Compound 6b. Same procedure was followed as for 3b using 5 (2.2 g, 10.5 mmol), 2-(2-chloroethoxy)ethoxyethanol (2.64 g, 15.7 mmol), DCC (3.23 g, 15.7 mmol), DMAP (0.5 g, 4 mmol), and dry DCM 25 mL. Light yellow oil was obtained. Yield: 2.15 g (65%). 1H NMR (CDCl3, 600 MHz): δ 4.30 (t, 2H, J = 4.9 Hz), 4.17 (d, 4H, J = 2.3 Hz), 3.77 (t, 2H, J = 5.8 Hz), 3.74 (t, 2H, J = 4.7 Hz), 3.68 (d, 4H, J = 4.7 Hz), 3.64 (t, 2H, J = 5.7 Hz), 2.43 (t, 2H, J = 2.3 Hz), 1.25 (s, 3H). 13C NMR (CDCl3, 151 MHz): δ 174.05, 79.67, 74.33, 71.76, 71.23, 69.12, 63.70, 58.66, 48.05, 42.73, 17.83. MG1-Cl. Click method A: 4b (2.27 g, 7.9 mmol), 6b (1.18 g, 3.75 mmol), CuSO4 (50 mg), NaAsc (100 mg), and 20 mL of THF/H2O (4:1, v/v). Colorless viscous oil. Yield: 2.46 g (74%). 1H NMR (CDCl3, 600 MHz): δ 7.68 (s, 2H), 4.61 (s, 4H), 4.53 (t, 4H, J = 5.2 Hz), 4.29 (t, 4H, J = 4.7 Hz), 4.25 (t, 2H, J = 4.7 Hz), 4.17 (d, 4H, J = 11.8 Hz), 3.88 (t, 4H, J = 5.1 Hz), 3.73 (t, 2H, J = 5.4 Hz), 3.71−3.66 (m, 6H), 3.65 (s, 4H), 3.64−3.62 (m, 4H), 3.60 (t, 2H, J = 5.4 Hz), 1.44 (s, 6H), 1.37 (s, 6H), 1.20 (s, 3H), 1.15 (s, 6H). 13C NMR (CDCl3, 151 MHz): δ 174.26, 174.11, 145.02, 123.57, 98.07, 72.11, 71.18, 69.38, 69.08, 69.03, 65.98, 64.91, 63.59, 63.58, 50.21, 48.32, 42.85, 41.90, 25.18, 22.13, 18.53, 17.91. MS (MALDI-TOF, positive mode) calculated for C39H63ClN6O15 [M] = 890.41, found [M + H]+ m/z = 891.591 and [M + Na]+ m/z = 913.598. MG1-N3. Same procedure as 4b using MG1-Cl (2.46 g, 2.76 mmol), NaN3 (1 g, 15.3 mmol), and 20 mL of DMSO. Colorless viscous oil. Yield: 2.25 g (91%). 1H NMR (CDCl3, 600 MHz): δ 7.68 (s, 2H), 4.62 (s, 4H), 4.54 (t, 4H, J = 5.1 Hz), 4.30 (t, 4H, J = 4.8

diluted by 40 mL of water and extracted three times with 20 mL of ethyl acetate. The combined organic layer was dried over Na2SO4, concentrated, and loaded onto a silica column (eluents: ethyl acetate:MeOH = 30:1 gradually decreased to 20:1). The product was obtained as light yellow viscous oil. Yield: 2.37 g (65%). 1H NMR (CDCl3, 600 MHz): δ 7.79 (s, 2H), 4.67 (t, 4H, J = 5 Hz), 4.64 (s, 4H), 4.56 (t, 4H, J = 5 Hz), 4.40 (t, 2H, J = 5.6 Hz), 4.14 (d, 4H, J = 11.7 Hz), 3.68−3.66 (m, 8H), 3.49 (t, 2H, J = 5.6 Hz), 1.45 (s, 6H), 1.38 (s, 6H), 1.22 (s, 3H), 1.09 (s, 6H). 13C NMR (CDCl3, 151 MHz): δ 173.8, 173.76, 145.35, 123.29, 98.18, 71.99, 64.93, 63.78, 53.86, 53.42, 49.16, 48.33, 31.71, 30.88, 29.28, 28.82, 26.17, 21.12, 17.86. MS (MALDI-TOF, positive mode) calculated for C33H51BrN6O12 [M] = 802.71. Found [M + H]+ m/z = 803.979, [M + Na]+ m/z = 825.982. SG1-N3. SG1-Br (2 g, 2.48 mmol, 1 equiv) was dissolved in 15 mL of DMSO in a round-bottom flask, and then NaN3 (0.48 g, 7.44 mmol, 3 equiv) was added. The reaction mixture was stirred at room temperature for 12 h and then was diluted with 40 mL of water and extracted three times with 20 mL of ethyl acetate. The combined organic layers were washed with brine and dried over Na2SO4. Removal of solvents yielded the product as colorless viscous oil. Yield: 1.71 g (90%). 1H NMR (CDCl3, 600 MHz): δ 7.78 (s, 2H), 4.67 (t, 4H, J = 5 Hz), 4.63 (s, 4H), 4.57 (t, 4H, J = 5 Hz), 4.26 (t, 2H, J = 5.1 Hz), 4.17−4.12 (m, 8H), 3.68−3.64 (m, 8H), 3.46 (t, 2H, J = 5.1 Hz), 1.45 (s, 6H), 1.38 (s, 6H), 1.22 (s, 3H), 1.09 (s, 6H). 13C NMR (CDCl3, 151 MHz): δ 174.00, 173.82, 145.33, 123.31, 98.21, 71.94, 66.00, 64.93, 63.29, 62.85, 49.87, 49.16, 48.32, 42.14, 30.9, 26.16, 21.12, 18.23, 17.85. MS (MALDI-TOF, positive mode) calculated for C33H51N9O12 [M] = 765.82, found [M−N2]+ m/z = 740.860, [M + H]+ m/z = 766.860 and [M + Na]+ m/z = 788.840. SG2-Br. Click method B: SG1-N3 (1.5 g, 1.96 mmol), 6a (0.295 g, 0.93 mmol), CuBr (30 mg), PMDTA (60 μL), and 3 mL of acetone. Eluents for silica column chromatography: ethyl acetate:methanol = 11:1 gradually decreasing to 8:1. Very viscous oil. Yield: 1.01 g. (58%). 1H NMR (CDCl3, 600 MHz): δ 7.79 (s, 4H), 7.72 (s, 2H), 4.68 (t, 8H, J = 5.2 Hz), 4.62−4.61 (m, 8H), 4.58−4.56 (m, 16H), 4.5 (t, 4H, J = 5.2 Hz), 4.37 (t, 2H, J = 6.1 Hz), 4.15 (d, 8H, J = 11.8 Hz), 3.65 (s, 8H), 3.63 (s, 4H), 3.60−3.57 (m, 8H), 3.49 (t, 2H, J = 6.1 Hz), 1.44 (s, 12H), 1.37 (s, 12H), 1.21 (s, 3H), 1.14 (s, 6H), 1.09 (s, 12H). 13C NMR (CDCl3, 151 MHz): δ 173.80, 173.75, 171.08, 145.11, 145.04, 123.43, 123.37, 98.18, 72.12, 72.07, 65.98, 64.85, 64.76, 63.81, 62.83, 62.57, 60.35, 49.15, 49.01, 48.38, 48.33, 42.11, 28.94, 26.09, 21.22, 21.01, 18.26, 17.88, 17.79. MS (MALDI-TOF, positive mode) calculated for C79H119BrN18O28 [M] = 1848.82, found [M + H]+ m/z = 1849.243 and [M + Na]+ m/z = 1871.282. SG2-N3. The dendron focal point bromide substitution reaction was the same as SG1-N3 using SG2-Br instead (1.07 g, 0.58 mmol), NaN3 (1 g, 15 mmol), and DMSO (10 mL). Product was isolated as colorless very viscous oil. Yield: 0.979 g (92%). 1H NMR (CDCl3, 600 MHz): δ 7.79 (s, 4H), 7.72 (s, 2H), 4.68 (t, 8H, J = 5.2 Hz), 4.62−4.60 (m, 8H), 4.58−4.56 (m, 16H), 4.50 (t, 4H, J = 5.2 Hz), 4.25 (t, 2H, J = 6 Hz), 4.15 (d, 8H, J = 12 Hz), 3.65 (s, 8H), 3.63 (s, 4H), 3.59−3.56 (m, 8H), 3.45 (t, 2H, J = 6 Hz), 1.44 (s, 12H), 1.37 (s, 12H), 1.21 (s, 3H), 1.14 (s, 6H), 1.09 (s, 12H). 13C NMR (CDCl3, 150 MHz): δ 173.81, 145.10, 145.06, 123.43, 123.37, 98.20, 72.07, 65.99, 64.83, 64.77, 63.32, 62.84, 62.57, 60.36, 49.85, 49.15, 49.00, 48.37, 48.33, 42.12, 41.05, 26.10, 21.22, 21.02, 18.26, 17.87, 17.79, 14.19. MS (MALDI-TOF, positive mode) calculated for C79H119N21O28 [M] = 1809.94, found [M−N2]+ m/z = 1783.828 [M + H]+ m/z = 1810.749 and [M + Na]+ m/z = 1832.781. SG3-Br. Click method B: SG2-N3 (0.7 g, 0.38 mmol), 6a (0.061 g, 0.18 mmol), CuBr (10 mg), PMDTA (30 μL), and acetone (5 mL). Eluents for silica column chromatography: 10:1 ethyl acetate:methanol gradually changed to 4:1. Product was obtained as light yellow very viscous oil. Yield: 0.43 g (59%). 1H NMR (CDCl3, 600 MHz): δ 7.78 (s, 8H), 7.74 (s, 2H), 7.73 (s, 4H), 4.68 (t, 16H, J = 5.4 Hz), 4.62−4.61 (m, 16H), 4.58−4.55 (m, 40H), 4.50 (t, 12H, J = 5.4 Hz), 4.37 (t, 2H, J = 5.9 Hz), 4.15 (d, 16H, J = 12.1 Hz), 3.65−3.63 (m, 20H), 3.59−3.55 (m, 24H), 3.49 (t, 2H, J = 5.9 Hz), 1.44 (s, 24H), 1.37 (s, 24H), 1.20 (s, 3H), 1.14 (s, 12H), 1.13 (s, 6H), 1.09 (s, D


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Macromolecules Hz), 4.26 (t, 2H, J = 4.8 Hz), 4.18 (d, 4H, J = 11.8 Hz), 3.89 (t, 4H, J = 4.9 Hz), 3.70−3.63 (m, 16H), 3.37 (t, 2H, J = 4.8 Hz), 1.45 (s, 6H), 1.38 (s, 6H), 1.21 (s, 3H), 1.16 (s, 6H). 13C NMR (CDCl3, 151 MHz): δ 174.29, 174.12, 145.06, 123.57, 98.08, 72.10, 70.00, 69.39, 69.10, 69.07, 65.99, 64.92, 63.64, 63.60, 60.37, 50.72, 50.21, 48.32, 41.92, 25.17, 22.15, 21.03, 18.54, 17.90. MS (MALDI-TOF, positive mode) calculated for C39H63N9O15, [M] = 897.98, found [M + H]+ m/z = 898.993 and [M + Na]+ m/z = 921.012. MG2-Cl. Click method A: MG1-N3 (2 g, 2.2 mmol), 6b (0.33 g, 1 mmol), CuSO4 (30 mg), NaAsc (60 mg), and 20 mL of THF/H2O (4:1, v/v). Compound was isolated as colorless viscous oil. Yield: 1.676 g (76%). 1H NMR (CDCl3, 600 MHz): δ 7.69 (s, 6H), 4.61 (s, 4H), 4.60 (s, 8H), 4.54 (t, 8H, J = 5.4 Hz), 4.51 (t, 4H, J = 5.1 Hz), 4.29 (t, 8H, J = 4.8 Hz), 4.25 (t, 2H, J = 4.8 Hz), 4.22 (t, 4H, J = 4.8 Hz), 4.18 (d, 8H, J = 11.7 Hz), 3.89 (t, 8H, J = 5 Hz), 3.86 (t, 4H, J = 5.2 Hz) 3.73 (t, 2H, J = 6 Hz), 3.69 (t, 10H, J = 4.8 Hz), 3.66−3.59 (m, 26H), 1.45 (s, 12H), 1.38 (s, 12H), 1.20 (s, 3H), 1.18 (s, 6H), 1.16 (s, 12H). 13C NMR (CDCl3, 151 MHz): δ 174.12, 144.90, 123.65, 98.08, 72.23, 72.15, 71.18, 69.37, 69.08, 69.03, 65.99, 64.94, 64.86, 63.60, 63.48, 53.40, 50.21, 48.37, 48.30, 42.90, 41.91, 25.17, 22.18, 18.55, 17.91. MS (MALDI-TOF positive mode) calculated for C93H147ClN18O35 [M] = 2111.74, found [M + H]+ m/z = 2112.549 and [M + Na]+ m/z = 2134.560. MG2-N3. Same procedure as 4b using MG2-Cl (1.676 g, 0.79 mmol), NaN3 (1 g, 15.4 mmol), and 20 mL of DMSO. Colorless viscous oil. Yield 1.429 g (85%). 1H NMR (CDCl3, 600 MHz): δ 7.69 (s, 6H), 4.61 (s, 4H), 4.60 (s, 8H), 4.54 (t, 8H, J = 5.2 Hz), 4.51 (t, 4H, J = 5.2 Hz), 4.30 (t, 8H, J = 5 Hz), 4.25 (t, 2H, J = 5 Hz), 4.22 (t, 4H, J = 4.6 Hz), 4.19 (d, 8H, J = 11.7 Hz), 3.89 (t, 8H, J = 5.1 Hz), 3.86 (t, 4H, J = 5.1 Hz), 3.69 (t, 10H, J = 4.6 Hz), 3.68−3.61 (m, 26H), 3.37 (t, 2H, J = 5 Hz), 1.45 (s, 12H), 1.38 (s, 12H), 1.21 (s, 3H), 1.18 (s, 6H), 1.16 (s, 12H). 13C NMR (CDCl3, 151 MHz): δ 174.12, 144.90, 123.65, 98.08, 72.15, 71.94, 69.37, 69.08, 65.99, 64.94, 64.87, 63.60, 63.48, 50.72, 50.21, 48.30, 41.91, 25.17, 22.18, 18.55, 17.91. MS (MALDI-TOF positive mode) calculated for C93H147N21O35 [M] = 2119.31, found [M−N2]+ m/z = 2094.313 and [M + H]+ m/z = 2120.338. MG3-Cl. Click method A: MG2-N3 (1 g, 0.47 mmol), 6b (0.073 g, 0.2 mmol), CuSO4 (10 mg), NaAsc (20 mg), and 10 mL of THF/ H2O (4:1, v/v). Yield: 0.566 g (54%). 1H NMR (CDCl3, 600 MHz): δ 7.69 (s, 14H), 4.61 (s, 4H), 4.60 (s, 16H), 4.59 (s, 8H), 4.54 (t, 16H, J = 5.1 Hz), 4.51 (t, 12H, J = 5.1 Hz), 4.29 (t, 16H, J = 4.8 Hz), 4.24 (t, 2H, J = 5 Hz), 4.21 (t, 12H, J = 4.8 Hz), 4.18 (d, 16H, J = 11.8 Hz), 3.89 (t, 16H, J = 5.1 Hz), 3.86 (t, 12H, J = 5 Hz), 3.73 (t, 2H, J = 5.1 Hz), 3.69 (t, 20H, J = 4.6 Hz), 3.66−3.60 (m, 54H), 1.45 (s, 24H), 1.38 (s, 24H), 1.20 (s, 3H), 1.18 (s, 12H), 1.17 (s, 6H), 1.16 (s, 24H). 13C NMR (CDCl3, 151 MHz): δ 174.14, 124.44, 98.08, 72.21, 72.15, 70.00, 69.10, 64.20, 63.58, 63.51, 57.73, 53.42, 48.21, 42.91, 41.93, 41.04, 36.69, 30.90, 29.28, 25.34, 22.01, 18.54, 17.98. MS (MALDI-TOF, positive mode) calculated for C200H313ClN42O75 [M] = 4538.37, found [M + H]+ m/z = 4539.890, [M + Na]+ m/z = 4562.092 and [M + K]+ m/z = 4578.101. MG3-N3. Same procedure as compound 4b using MG3-Cl (0.566 g, 0.12 mmol), NaN3 (0.5 g, 7.7 mmol), and 10 mL of DMSO. Yield: 0.493 g (87%). 1H NMR (CDCl3, 600 MHz): δ 7.88 (s, 4H), 7.81 (s, 8H), 7.77 (s, 2H), 4.65 (s, 4H), 4.63 (s, 8H), 4.63 (s, 16H), 4.58 (t, 28H, J = 4.8 Hz), 4.29 (t, 16H, J = 4.8 Hz), 4.24 (t, 2H, J = 4.7 Hz), 4.21 (t, 12H, J = 4.8 Hz), 4.18 (d, 16H, J = 11.8 Hz), 3.92−3.88 (m, 26H), 3.70 (t, 20H, J = 4.8 Hz), 3.67−3.61 (m, 54H), 3.37 (t, 2H, J = 4.8 Hz), 1.45 (s, 24H), 1.37 (s, 24H), 1.20 (s, 3H), 1.18 (s, 12H), 1.17 (s, 6H), 1.15 (s, 24H). 13C NMR (CDCl3, 151 MHz): δ 174.14, 124.44, 98.08, 72.21, 72.15, 70.00, 69.10, 64.20, 63.58, 63.51, 57.73, 50.72, 48.21, 41.93, 41.04, 36.69, 30.90, 29.28, 25.34, 22.01, 18.54, 17.98. MS (MALDI-TOF, positive mode) calculated for C200H313N45O75 [M] = 4546.94, found [M + H]+ m/z = 4547.586 and [M + Na]+ m/z = 4569.882. FG2-A. To a solution of FG2-OH (1.5 g, 2 mmol), was added under argon NaH (0.145 g, 6 mmol) followed by propargyl bromide (0.436 g, 4 mmol). The reaction mixture was stirred for 6 h at room temperature before quenched by water. The mixture was extracted

twice by DCM (30 mL). The crude product was purified by silica column chromatography (hexanes:DCM 1:1 gradually increased to 1:3). Product was obtained as light yellow viscous oil. Yield: 1.23 g (79%). 1H NMR (CDCl3, 600 MHz): δ 7.44 (d, 8H, J = 7.3 Hz), 7.40 (t, 8H, J = 7.7 Hz), 7.35 (t, 4H, J = 7.3 Hz), 6.71 (d, 4H, J = 2.2 Hz), 6.63 (d, 2H, J = 2.2 Hz), 6.60 (t, 2H, J = 2.2 Hz), 6.56 (t, 1H, J = 2.2 Hz), 5.06 (s, 8H), 5.00 (s, 4H), 4.58 (s, 2H), 4.19 (d, 2H, J = 2.3 Hz), 2.48 (t, 1H, J = 2.3 Hz). 13C NMR (CDCl3, 151 MHz): δ 160.16, 159.97, 139.70, 139.27, 136.79, 128.54, 127.96, 127.51, 106.95, 106.41, 101.67, 101.62, 74.64, 71.38, 70.12, 69.97, 67.93, 57.06. MS (MALDI-TOF, positive mode) calculated for C52H46O7, [M] = 782.93, found [M + Na]+ m/z = 804.942 and [M + K]+ m/z = 820.933. FG3-A. Synthesized in a similar fashion as FG2-A using FG3-OH (1.5 g, 0.94 mmol), propargyl bromide (0.2 g, 1.83 mmol), NaH (0.1 g, 4.2 mmol), and 5 mL of dry THF. Yield: 0.905 g (59%). 1H NMR (CDCl3, 600 MHz): δ 7.43 (d, 16H, J = 7.4 Hz), 7.38 (t, 16H, J = 7.6 Hz), 7.33 (t, 8H, J = 7.4 Hz), 6.70 (s, 8H), 6.69 (s, 4H), 6.64 (s, 2H), 6.59 (s, 4H), 6.58 (s, 1H), 6.56 (s, 2H), 5.04 (s, 16H), 4.99 (s, 12H), 4.56 (s, 2H), 4.17 (d, 2H, J = 2.3 Hz), 2.46 (t, 1H, J = 2.3 Hz). 13C NMR (CDCl3, 151 MHz): δ 160.19, 160.08, 160.02, 139.78, 139.28, 139.25, 136.81, 128.58, 128.00, 127.56, 106.98, 106.49, 106.42, 101.66, 79.61, 74.73, 71.42, 70.13, 70.03, 57.11. MS (MALDI-TOF, positive mode) calculated for C105H94O15 [M] = 1631.93, found [M + Na]+ m/z = 1656.605 and [M + Cu]+ m/z = 1698.600. Janus Dendrimer F2S1. Click method A was adopted. 25 mg of deprotected SG1-N3 and 50 mg of FG2-A were dissolved in 1 mL of THF. To the above solution were added 5 mg of CuSO4 in 100 μL of H2O followed by 10 mg of NaAsc in 150 μL of H2O. The reaction mixture was stirred at room temperature for 30 min. The product was purified by silica column chromatography (ethyl acetate:MeOH = 12:1 gradually decreased to 10:1). Product was obtained as viscous oil. Yield: 0.01 g (18.8%). 1H NMR (acetone-d6, 600 MHz): δ 8.04 (s, 1H), 8.02 (s, 2H), 7.48 (d, 8H, J = 7.6 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.33 (t, 4H, J = 7.6 Hz), 6.78 (d, 4H, J = 2.1 Hz), 6.67 (d, 2H, J = 2.2 Hz), 6.65 (t, 2H, J = 2.3 Hz), 6.59 (t, 1H, J = 2.3 Hz), 5.12 (s, 8H), 5.05 (s, 4H), 4.70−4.67 (m, 6H), 4.65 (s, 2H), 4.54 (s, 2H), 4.50− 4.47 (m, 8H), 3.71−3.67 (m, 8H), 3.56−3.52 (m, 3H), 1.10 (s, 6H). 13 C NMR (acetone-d6, 151 MHz): δ 174.55, 173.41, 160.21, 160.01, 139.92, 137.40, 128.39, 127.74, 127.60, 106.56, 106.46, 101.21, 101.03, 71.92, 71.53, 69.70, 69.49, 64.80, 64.40, 62.65, 62.49, 50.34, 48.93, 48.80, 48.19, 29.66, 17.25, 16.52. MS (MALDI-TOF, positive mode) calculated for C79H89N9O19, [M] = 1468.63, found [M + Na]+ m/z = 1492.222. Janus Dendrimer F2M1. Click method B. 32 mg of deprotected MG1-N3, 60 mg of FG2-A, 3 mg of CuBr, 6 μL of PMDTA, and 1 mL of acetone were used. Yield: 0.021 g (31%). 1H NMR (acetone-d6, 600 MHz): δ 7.97 (s, 1H), 7.92 (s, 2H), 7.48 (d, 8H, J = 7.6 Hz), 7.39 (t, 8H, J = 7 Hz), 7.33 (t, 4H, J = 7 Hz), 6.78 (s, 4H), 6.68 (s, 2H), 6.64 (s, 2H), 6.60 (s, 1H), 5.12 (s, 8H), 5.07 (s, 4H), 4.64 (s, 2H), 5.57−5.54 (m, 12H), 4.19 (t, 4H, J = 4.7 Hz), 4.16 (t, 2H, J = 4.5 Hz), 3.90 (t, 6H, J = 4.7 Hz), 3.70 (s, 8H), 3.67 (t, 4H, J = 4.3 Hz), 3.40 (t, 2H, J = 4.3 Hz), 1.13 (s, 3H), 1.12 (s, 6H). 13C NMR (acetone-d6, 151 MHz): δ 174.94, 173.74, 160.84, 160.21, 160.01, 144.39, 141.15, 139.94, 137.39, 128.39, 127.74, 127.60, 123.96, 123.89, 106.56, 106.44, 101.22, 101.07, 72.01, 71.53, 69.70, 69.49, 69.14, 69.11, 68.68, 68.64, 64.78, 64.48, 63.39, 63.30, 63.12, 54.60, 50.11, 49.81, 49.76, 48.16, 40.28, 17.37, 16.62. MS (MALDI-TOF, positive mode) calculated for C85H101N9O22 [M] = 1600.78, found [M + H]+ m/z = 1601.640 and [M + Na]+ m/z = 1622.931. Janus Dendrimer F2L1. Click method B. 121 mg of deprotected LG1-N3, 201 mg of FG2-A, 10 mg of CuBr, 20 μL of PMDTA, and 2 mL of acetone were used. Yield: 0.064 g (29%). 1H NMR (acetone-d6, 600 MHz): δ 7.98 (s, 1H), 7.92 (s, 2H), 7.48 (d, 8H, J = 7.5 Hz), 7.39 (t, 8H, J = 7.6 Hz), 7.33 (t, 4H, J = 7.5 Hz), 6.79 (d, 4H, J = 2.2 Hz), 6.68 (d, 2H, J = 2.2 Hz), 6.65 (t, 2H, J = 2.2 Hz), 6.60 (t, 1H, J = 2.2 Hz), 5.13 (s, 8H), 5.07 (s, 4H), 4.64 (s, 2H), 4.57−4.54 (m, 12H), 4.20 (t, 4H, J = 4.9 Hz), 4.15 (t, 2H, J = 5 Hz), 3.90−3.87 (m, 6H), 3.70 (t, 8H, J = 5 Hz), 3.64 (t, 4H, J = 5 Hz), 3.61 (d, 4H, J = 1.5 Hz), 3.59−3.57 (m, 10H), 1.15 (s, 9H). 13C NMR (acetone-d6, E


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Macromolecules 151 MHz): δ 174.83,173.73, 173.70, 160.22, 160.01, 144.42, 144.38, 141.23, 139.94, 137.40, 128.39, 127.75, 127.61, 123.93, 123.82, 106.53, 106.43, 101.21, 101.05, 71.99, 71.47, 70.23, 70.16, 70.14, 69.70, 69.49, 69.23, 69.20, 68.72, 68.68, 64.93, 64.58, 63.53, 63.33, 63.09, 50.23, 49.75, 48.15, 17.39, 16.56. MS (MALDI-TOF, positive mode) calculated for C93H113N9O25 [M] = 1732.94, found [M + Na]+ m/z = 1755.306. Janus Dendrimer F2S2. Click method B. 146 mg of deprotected SG2-N3, 126 mg of FG2-A, 10 mg of CuBr, 20 μL of PMDTA, and 2 mL of acetone were used. Yield: 0.075 g (35%). 1H NMR (acetone-d6, 600 MHz): δ 8.09 (s, 4H), 8.07 (s,1H), 8.00 (s, 2H), 7.48 (d, 8H, J = 7.2 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.32 (t, 4H, J = 7.4 Hz), 6.79 (d, 4H, J = 2.2 Hz), 6.68 (d, 2H, J = 2.2 Hz), 6.64 (t, 2H, J = 2.2 Hz), 6.59 (t, 1H, J = 2.2 Hz), 5.12 (s, 8H), 5.06 (s, 4H), 4.70 (t, 8H, J = 5.3 Hz), 4.69−4.66 (m, 8H), 4.54 (s, 2H), 4.52 (s, 12H), 4.50−4.46 (m, 14H), 4.15 (t, 8H), 3.69−3.64 (m, 16H), 3.58−3.53 (m, 12H), 1.10 (s, 12H), 1.09 (s, 9H). 13C NMR (acetone-d6, 151 MHz): δ 174.65, 173.51, 173.47, 160.21, 160.01, 144.49, 144.44, 141.12, 139.93, 137.40, 128.40, 127.75, 127.63, 124.33, 124.17, 124.13, 106.57, 106.48, 101.25, 101.07, 72.09, 71.93, 71.51, 69.72, 69.52, 64.42, 63.25, 62.72, 62.67, 62.54, 50.35, 48.96, 48.80, 48.27, 48.23, 29.44, 17.32, 17.30, 16.61. MS (MALDI-TOF, positive mode) calculated for C119H149N21O35 [M] = 2433.61, found [M + Na]+ m/z = 2454.829. Janus Dendrimer F2M2. Click method B. 52 mg of deprotected MG2-N3, 55 mg of FG2-A, 5 mg of CuBr, 10 μL of PMDTA, and 2 mL of acetone were used. Yield: 0.028 g (38.3%). 1H NMR (acetoned6, 600 MHz): δ 7.97 (s, 1H), 7.95 (s, 4H), 7.91 (s, 2H), 7.48 (d, 8H, J = 7.3 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.33 (t, 4H, J = 7.3 Hz), 6.77 (d, 4H, J = 2.2 Hz), 6.69 (d, 2H, J = 2.2 Hz), 6.64 (t, 2H, J = 2.2 Hz), 6.60 (t, 1H, J = 2.2 Hz), 5.12 (s, 8H), 5.07 (s, 4H), 4.64 (s, 2H), 4.58−4.54 (m, 28H), 4.19 (t, 8H, J = 4.8 Hz), 4.16−4.14 (m, 6H), 3.92 (t, 16H, J = 5.5 Hz), 3.90−3.87 (m, 6H), 3.71−3.67 (m, 24H), 3.64−3.60 (m, 18H), 1.14 (s, 21H). 13C NMR (acetone-d6, 151 MHz): δ 174.87, 173.72, 173.70, 160.21, 160.02, 144.36, 141.17, 139.95, 137.40, 128.40, 127.75, 127.63, 123.97, 123.92, 106.56, 106.46, 101.23, 101.08, 72.09, 71.99, 71.54, 69.71, 69.51, 69.15, 69.13, 68.66, 64.82, 64.49, 63.40, 63.33, 63.12, 50.15, 49.79, 48.19, 48.16, 29.42, 28.39, 17.41, 16.62. MS (MALDI-TOF, positive mode) calculated for C133H177N21O42 [M] = 2741.98, found [M + Na]+ m/z = 2764.692. Janus Dendrimer F2L2. Click method B. 104 mg of deprotected LG2-N3, 102 mg of FG2-A, 11 mg of CuBr, 20 μL of PMDTA, and 3 mL of acetone were used. Yield: 0.061 g (43.8%). 1H NMR (acetoned6, 600 MHz): δ 7.99 (s, 1H), 7.94 (s, 4H), 7.92 (s, 2H), 7.48 (d, 8H, J = 7.5 Hz), 7.39 (t, 8H, J = 7.8 Hz), 7.33 (t, 4H, J = 7.4 Hz), 6.79 (d, 4H, J = 2.2 Hz), 6.68 (d, 2H, J = 2.2 Hz), 6.65 (t, 2H, J = 2.2 Hz), 6.60 (t, 1H, J = 2.2 Hz), 5.12 (s, 8H), 5.07 (s, 4H), 4.64 (s, 2H), 4.57−4.53 (m, 28H), 4.21 (t, 8H, J = 4.9 Hz), 4.17−4.14 (m, 6H), 3.91−3.85 (m, 22H), 3.73−3.68 (m, 18H), 3.65 (t, 10H, J = 4.7 Hz), 3.62−3.57 (m, 48H), 1.16−1.14 (m, 21H). 13C NMR (acetone-d6, 151 MHz): δ 174.85, 173.74, 160.22, 160.02, 144.42, 144.39, 141.18, 139.94, 137.40, 128.41, 127.76, 127.62, 123.97, 123.88, 123.83, 106.54, 106.45, 101.22, 101.06, 72.04, 71.99, 71.49, 70.23, 70.16, 69.72, 69.50, 69.24, 69.21, 68.72, 68.70, 64.93, 64.58, 63.55, 63.34, 63.12, 50.24, 49.78, 48.16, 17.43, 16.59. MS (MALDI-TOF, positive mode) calculated for C147H205N21O49 [M] = 3050.36, found [M + Na]+ m/z = 3072.006. Janus Dendrimer F3M2. Click method A. 89 mg of deprotected MG2-N3, 104 mg of FG3-A, 10 mg of CuSO4, 20 mg of NaAsc, and 2 mL of THF/H2O (4:1, v/v) were used. Yield: 0.029 g (17.8%). 1H NMR (acetone-d6, 600 MHz): δ 7.94 (s, 1H), 7.92 (s, 4H), 7.89 (s, 2H), 7.46 (d, 16H, J = 7.4 Hz), 7.38 (t, 16H, J = 7.8 Hz), 7.31 (t, 8H, J = 7.3 Hz), 6.77 (s, 12H), 6.68 (d, 2H, J = 1.9 Hz), 6.64−6.61 (m, 8H), 5.10 (s, 16H), 5.05 (s, 12H), 4.62 (s, 2H), 4.56−4.52 (m, 28H), 4.19 (t, 8H, J = 4.6 Hz), 4.14 (t, 6H, J = 4.6 Hz), 3.90 (t, 8H, J = 4.9 Hz), 3.59 (t, 10H, J = 5.4 Hz), 3.73−3.66 (m, 24H), 3.62−3.59 (m, 18H), 1.13 (s, 21H). 13C NMR (acetone-d6, 151 MHz): δ 173.81, 163.43, 160.21, 144.40, 139.94, 139.84, 137.60, 137.39, 128.39, 127.75, 127.60, 123.92, 106.52, 106.46, 101.26, 71.99, 69.70, 69.10,

68.65, 65.20, 65.02, 64.47, 63.41, 63.12, 54.04, 50.17, 49.85, 49.82, 48.16, 17.48, 16.59. MS (MALDI-TOF, positive mode) calculated for C189H225N21O50 [M] = 3590.58, found [M + K]+ = 3629.329. Janus Dendrimer F3L2. Click method A. 92 mg of deprotected LG2-N3, 101 mg of FG3-A, 9 mg of CuSO4, 20 mg of NaAcs, and 2 mL of THF/H2O were used. Yield: 0.033 g (21%). 1H NMR (acetone-d6, 600 MHz): δ 7.96 (s, 1H), 7.93 (s, 4H), 7.89 (s, 2H), 7.46 (d, 16H, J = 7.3 Hz), 7.38 (t, 16H, J = 7.8 Hz), 7.31 (t, 8H, J = 7 Hz), 6.77 (s, 12H), 6.67 (d, 2H, J = 1.8 Hz), 6.63−6.61 (m, 7H), 5.10 (s, 16H), 5.06 (s, 12H), 4.62 (s, 2H), 4.58−4.51 (m, 28H), 4.21 (t, 9H, J = 4.9 Hz), 4.16−4.13 (m, 6H), 3.89 (t, 8H, J = 4.9 Hz), 3.86 (t, 6H, J = 5.4 Hz), 3.73−3.69 (m, 18H), 3.65−3.56 (m, 63H), 1.15 (s, 21H). 13C NMR (acetone-d6, 151 MHz): δ 174.87, 160.86, 160.21, 144.34, 139.79, 137.37, 128.39, 127.75, 127.60, 123.85, 106.45, 101.26, 71.99, 70.23, 70.15, 69.70, 69.57, 69.23, 68.69, 67.16, 64.97, 64.59, 63.56, 63.15, 54.04, 49.77, 48.16, 29.69, 29.42, 17.45, 16.60. MS (MALDI-TOF, positive mode) calculated for C203H253N21O57 [M] = 3899.35. found [M + K]+ = 3938.126. Janus Dendrimer F2S3. Click method B alternate. 35 mg of deprotected SG3-N3, 40 mg of FG2-A, 4 mg of CuBr2, 6 μL of PMDTA, 5 mg of Cu(0) powder, and 1 mL of acetone were used. Yield: 0.021 g (49.2%). 1H NMR (acetone-d6, 600 MHz): δ 8.10 (s, 8H), 8.07 (s, 1H), 8.03 (s, 4H), 8.00 (s, 2H), 7.48 (d, 8H, J = 7.4 Hz), 7.39 (t, 8H, J = 7.4 Hz), 7.33 (t, 4H, J = 7.4 Hz), 6.79 (d, 4H, J = 2.3 Hz), 6.68 (d, 2H, J = 2.3 Hz), 6.64 (t, 2H, J = 2.3 Hz), 6.59 (t, 1H, J = 2.3 Hz), 5.13 (s, 8H), 5.06 (s, 4H), 4.72−4.69 (m, 28H), 4.67−4.65 (m, 6H), 4.54−4.45 (m, 64H), 4.14 (t, 14H, J = 5.7 Hz), 3.70−3.64 (m, 36H), 3.58−3.54 (m, 30H), 1.11 (s, 24H), 1.09 (s, 12H), 1.08 (s, 3H), 1.07 (s, 6H). 13C NMR (acetone-d6, 151 MHz): δ 174.63, 173.51, 160.20, 159.97, 144.50, 144.46, 140.06, 137.40, 128.41, 127.76, 127.64, 124.33, 124.17, 106.50, 101.25, 72.05, 71.94, 69.72, 69.53, 64.64, 64.42, 62.70, 62.55, 50.37, 48.98, 48.83, 48.24, 29.42, 28.38, 17.34, 17.31, 16.61. MS (MALDI-TOF, positive mode) calculated for C199H269H45O67 [M] = 4363.59, found [M + Na]+ m/z = 4387.646. Janus Dendrimer F2M3. Click method B alternate. 39 mg of deprotected MG3-N3, 32 mg of FG2-A, 3 mg of CuBr2, 4 μL of PMDTA, 5 mg of Cu(0) powder, and 2 mL of acetone were used. Yield: 0.022 g (47.8%). 1H NMR (acetone-d6, 600 MHz): δ 7.97 (s, 1H), 7.94 (s, 8H), 7.92 (s, 4H), 7.91 (s, 2H), 7.48 (d, 8H, J = 7.3 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.33 (t, 4H, J = 7.3 Hz), 6.79 (d, 4H, J = 2.2 Hz), 6.69 (d, 2H, J = 2.2 Hz), 6.65 (t, 2H, J = 2.2 Hz), 6.61 (t, 1H, J = 2.2 Hz), 5.13 (s, 8H), 5.09 (s, 4H), 4.64 (s, 2H), 4.58−4.53 (m, 60H), 4.20 (t, 16H, J = 4.7 Hz), 4.17−4.15 (m, 14H), 3.93−3.87 (m, 42H), 3.72−3.68 (m, 48H), 3.64−3.60 (m, 42H), 1.146−1.134 (m, 45H). 13C NMR (acetone-d6, 151 MHz): δ 174.86, 173.74, 160.21, 160.02, 144.37, 139.94, 137.39, 128.42, 128.39, 127.78, 127.64, 127.59, 123.96, 106.58, 106.48, 106.37, 101.24, 72.08, 72.01, 69.72, 69.53, 69.16, 69.13, 68.66, 64.95, 64.48, 63.43, 63.16, 50.19, 49.81, 48.17, 17.45, 17.42, 16.63. MS (MALDI-TOF, positive mode) calculated for C230H330N45O82 [M] = 5023.40, found [M + Cu]+ m/z = 5087.861. Janus Dendrimer F3M3. Click method A alternate. 43 mg of deprotected MG3-N3, 40 mg of FG3-A, 2 mg of CuSO4, 8 mg of THTPA, 5 mg of NaAsc, and 1.5 mL of THF/H2O (4:1 v/v) were used. Yield: 0.028 g (47%). 1H NMR (acetone-d6, 600 MHz): δ 7.95 (s, 9H), 7.93 (s, 4H), 7.91 (s, 2H), 7.46 (d, 16H, J = 7.3 Hz), 7.37 (t, 16H, J = 7.6 Hz), 7.31 (t, 8H, J = 7.3 Hz), 6.77 (d, 12H, J = 2.1 Hz), 6.69 (d, 2H, J = 2.2 Hz), 6.64−6.61 (m, 7H), 5.10 (s, 16H), 5.06 (s, 12H), 4.62 (s, 2H), 4.58−4.51 (m, 62H), 4.19 (t, 16H, J = 4.8 Hz), 4.16−4.12 (m, 14H), 3.98 (t, 12H, J = 5.5 Hz), 3.91 (t, 16H, J = 5 Hz), 3.89−3.84 (m, 14H), 3.71−3.67 (m, 48H), 3.63−3.59 (m, 42H), 1.14−1.12 (m, 45H). 13C NMR (acetone-d6, 151 MHz): δ 174.92, 173.75, 160.21, 160.11, 144.37, 144.36, 139.86, 137.38, 128.40, 127.76, 127.62, 123.97, 106.52, 106.47, 101.28, 72.08, 72.02, 69.71, 69.58, 69.13, 68.66, 64.72, 64.62, 64.49, 63.43, 63.14, 50.15, 49.81, 48.20, 48.17, 29.42, 28.39, 17.46, 17.43, 16.67. MS (MALDITOF, positive mode) calculated for C285H377N45O90 [M] = 5873.38, found [M + K]+ m/z = 5912.680. F


Article

Macromolecules

Figure 1. MALDI-TOF spectra of SG1-Br (A), SG2-Br (B), and SG3-Br (C) dendrons; see Experimental Section for details (S = short spacer).

Scheme 2. “Click” Synthesis of Janus Dendrimers F2S1 (n = 1), F2M1 (n = 2), and F2L1 (n = 3) Using Second-Generation Poly(benzyl ether) Dendrons

used. Yield: 0.035 g (39.8%). 1H NMR (acetone-d6, 600 MHz): δ 8.07 (s, 4H), 7.97 (s, 1H), 7.91 (s, 2H), 7.48 (d, 8H, J = 7.3 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.33 (t, 4H, J = 7.3 Hz), 6.79 (d, 4H, J = 2.3 Hz), 6.68 (d, 2H, J = 2.3 Hz), 6.64 (t, 2H, J = 2.3 Hz), 6.60 (d, 1H, J = 2.3 Hz), 5.13 (s, 8H), 5.07 (s, 4H), 4.70 (t, 8H, J = 4.8 Hz), 4.64 (s, 2H), 4.57−4.53 (m, 20H), 4.50 (t, 8H, J = 4.8 Hz), 4.16 (t, 6H, J = 4.83), 3.96−3.88 (m, 6H), 3.70−3.58 (m, 36H), 1.13 (s, 9H), 1.11 (s, 12H). 13 C NMR (acetone-d6, 151 MHz): δ 174.55, 160.22, 144.63, 141.19, 139.96, 137.40, 128.40, 127.75, 127.62, 124.26, 106.57, 106.45, 101.22, 72.07, 71.96, 71.61, 69.71, 69.51, 69.14, 68.66, 64.83, 64.50, 63.40, 62.53, 54.04, 50.38, 49.82, 48.93, 48.14, 31.34, 22.35, 17.41, 17.36, 16.53. MS (MALDI-TOF, positive mode) calculated for C125H161N21O38 [M] = 2565.77, found [M + Na]+ m/z = 2588.912. “Gradient” Janus Dendrimer with Mixed Spacers FSSM2. Click method B: 54 mg of deprotected SSM2-N3, 58 mg of FG2-A, 5 mg of CuBr, 6 μL of PMDTA, and 3 mL of acetone were used. Yield: 0.031 g (40.2%). 1H NMR (acetone-d6, 600 MHz): δ 8.06 (s, 1H), 7.97 (s, 2H), 7.95 (s, 4H), 7.48 (d, 8H, J = 7.3 Hz), 7.39 (t, 8H, J = 7.7 Hz), 7.33 (t, 4H, J = 7.3 Hz), 6.79 (d, 4H, J = 2.2 Hz), 6.68 (d, 2H, J = 2.2 Hz), 6.64 (t, 2H, J = 2.2 Hz), 6.60 (d, 1H, J = 2.2 Hz), 5.13 (s, 8H),

Janus Dendrimer F3L3. Click method A alternate. 32 mg of deprotected LG3-N3, 37 mg of FG3-A, 2 mg of CuSO4, 8 mg of THTPA, 5 mg of NaAsc, and 1.5 mL of THF/H2O (4:1 v/v) were used. Yield: 0.018 g (41.8%). 1H NMR (acetone-d6, 600 MHz): δ 7.97 (s, 3H), 7.95 (s, 8H), 7.93 (s, 3H), 7.91 (s, 1H), 7.46 (d, 16H, J = 7.3 Hz), 7.38 (t, 16H, J = 7.8 Hz), 7.32 (t, 8H, J = 7.4 Hz), 6.77 (d, 12H, J = 1.8 Hz), 6.68 (d, 2H, J = 1.8 Hz), 6.64−6.61 (m, 7H), 5.10 (s, 16H), 5.06 (s, 12H), 4.62 (s, 2H), 4.60−4.51 (m, 62H), 4.21 (t, 18H, J = 5 Hz), 4.18−4.15 (m, 12H), 3.93−3.85 (m, 38H), 3.73− 3.67 (m, 36H), 3.66−3.65 (m, 144H), 1.17−1.14 (m, 45H). 13C NMR (acetone-d6, 151 MHz): δ 176.94, 174.89, 173.77, 160.23, 160.13, 144.39, 137.39, 128.41, 127.76, 127.62, 123.90, 106.47, 101.28, 72.01, 70.23, 70.16, 69.71, 69.57, 69.25, 68.70, 68.00, 67.16, 64.84, 64.58, 63.57, 63.13, 50.23, 49.79, 48.17, 29.42, 28.38, 27.14, 25.26, 21.93, 17.44, 16.63. MS (MALDI-TOF, positive mode) calculated for C315H437N45O105 [M] = 6534.17, found [M + K]+ m/ z = 6574.165. “Gradient” Janus Dendrimer with Mixed Spacers FMMS2. Click method A: 61 mg of deprotected MMS2-N3, 52 mg of FG2-A, 5 mg of CuSO4, 10 mg of NaAsc, and 2 mL of acetone/H2O (4:1 v/v) were G


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Figure 2. MALDI-TOF spectra of second-generation Janus dendrimers (see Experimental Section for details).

Figure 3. Molecular structures of “gradient” Janus dendrimers and their MALDI-TOF spectra (see Experimental Section for details). Diethylene glycol spacers (M) are colored in purple, and ethylene glycol spacers (S) are colored in green. H


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Figure 4. MALDI-TOF spectrum showing the presence of an oxidative product formed during F2L1 synthesis.

Figure 5. Optimization of reaction conditions for synthesis of F3L3 monitored by MALDI-TOF: (A) CuSO4, NaAsc, THF/H2O 4:1, 30 min; (B) CuBr/PMDTEA, acetone, 45 min; (C) CuBr2/PMDTEA, Cu(0) powder, acetone, 12 h; (D) CuBr2/PMDTEA, Cu(0) powder, acetone, 20 h; (E) CuSO4/THPTA, NaAsc, THF/H2O 4:1, 40 min; (F) CuSO4/THPTA, NaAsc, THF/H2O 4:1, 8 h.

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5.06 (s, 4H), 4.68 (t, 2H, J = 5.2 Hz), 4.65 (t, 6H, J = 5.2 Hz), 4.57 (t, 8H, J = 5.2 Hz), 4.54 (s, 2H), 4.52 (s, 12H), 4.48 (t, 2H, J = 5.2 Hz), 4.45 (t, 4H, J = 5 Hz), 4.19 (t, 8H, J = 4.9 Hz), 3.99 (t, 6H, J = 5.4 Hz), 3.91 (t, 8H, J = 5.2 Hz), 3.70 (s, 16H), 3.67 (t, 8H, J = 4.8 Hz), 3.58−3.53 (m, 12H), 1.13 (s, 12H), 1.09 (s, 3H), 1.09 (s, 9H). 13C NMR (acetone-d6, 151 MHz): δ 174.93, 173.46, 160.21, 160.01, 144.43, 144.26, 141.14, 139.94, 137.40, 128.40, 127.75, 127.63, 124.15, 124.07, 123.99, 106.56, 106.48, 101.24, 101.06, 72.10, 71.94, 71.50, 69.72, 69.52, 69.12, 68.65, 64.66, 64.56, 64.41, 63.26, 63.13, 62.63, 50.13, 49.79, 48.77, 48.28, 48.24, 17.31, 16.66. MS (MALDITOF, positive mode) calculated for C128H167N21O39 [M] = 2608.16, found [M + Na]+ m/z = 2631.515.

RESULTS AND DISCUSSION Synthesis and Characterization. The hydrophilic dendrons were prepared following a strategy adopted from our previous work21 as shown in Scheme 1. In this study, two more repeating units, namely monoethylene glycol and diethylene glycol, are utilized to expand the poly(ether− ester) dendron family. The monoethylene glycol derivative 2bromoethanol is incorporated into the repeating unit to create “SG” dendrons (S denotes a short spacer). Since the only oxygen in this spacer is now part of the ester bonds, the “SG” dendrons would most probably be hydrophobic. On the other side, “MG” dendrons (M denotes a medium spacer) having diethylene glycol as spacer are expected to be more I


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Figure 6. Molecular structures of F2S1 (A), F2M1 (B), and F2L1 (C) and their 1H NMR spectra in acetone-d6. *H2O and acetone.

hydrophilic. “LG” dendrons (L denotes a long spacer) contain two “free” ethylene glycol moieties in each repeating unit and thus should be intrinsically hydrophilic. All the hydrophilic dendrons have defect-free structure as proven to by MALDITOF. An example is shown in Figure 1 (see also Supporting Information). The reaction pathway for the synthesis of the amphiphilic diblock dendrimers is shown in Scheme 2. Fréchet-type second- and third-generation poly(benzyl ether) dendrons with an OH group at their apex are functionalized with an alkyne group through a Williamson ether synthesis. Initially an attempt was made to synthesize the Janus dendrimers using PEE dendrons with protected hydroxyl groups, followed by a deprotection step. However, removing acetonide protection

under acidic conditions caused cleavage of benzyl ether linkage in the PBE dendron. Thus, the PEE dendrons are deprotected before coupling to the PBE dendrons. The deprotected PEE dendrons are coupled to excess of PBEs through “click” methods A or B. An excess of PBE dendron is used to guarantee the complete consumption of the corresponding PEE dendron, which is harder isolate from the target molecule. The unreacted nonpolar PBEs dendrons are relatively easy to remove through silica column chromatography. The reaction progress was monitored by MALDI-TOF. The purified products had single molecular weight distribution. An example for the second-generation diblock dendrimers is shown in Figure 2 (see Supporting Information for additional MALDI-TOF analyses). J


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Figure 7. Emission spectra of pyrene in water at different concentrations of F2L2 dendrimer. 1

Amphiphilic diblock dendrimers with gradient density distribution in the hydrophilic block (“gradient” Janus dendrimers) are synthesized with selective incorporation of S (short, green) and M (medium, purple) segments in the interior and/or periphery of the dendrons (Figure 3) and second-generation Fréchet dendron (F). In the FMMS2 dendrimer the poly(ether−ester) second-generation dendron is composed of two diethylene glycol and one ethylene glycol spacers from the center to the periphery. For the FSSM2 dendrimer the order is reversed: two ethylene glycol and one diethylene glycol spacers from the center to the periphery. Their supramolecular nanoconstructs might offer interesting opportunities for size-selective binding and release. It should be noted that coupling reactions involving dendrons with triethylene glycol (L) spacers could produce low yields because of side reactions. For example, oxidative coupling products were encountered during F2L1 synthesis (Figure 4). When the third-generation PEE (L) dendron is used, the “click” coupling could be problematic as well. The isolation of the target molecule from side products is difficult. Thus, optimization of the “click” reaction conditions is required to facilitate the purification. Figure 5 shows the MALDI-TOF spectra of F3L3 reaction products obtained at different conditions. It is first noticed that a peripheral fragment is lost under normal “click” conditions (CuSO4, NaAsc), as evidenced by the appearance of the [M-317] peak in Figure 5A. This problem could be solved by adding a copper ligand, either PMDETA or THPTA. If CuBr/PMDETA catalyst is used alone, a strict degassing process is necessary to prevent the oxidation of Cu(I) ion and the homocoupling of alkyne− alkyne. The reaction is completed in 45 min (Figure 5B). Another way to avoid the fragmentation is by using CuBr2/ PMDTEA complex as catalyst and Cu(0) powder as reducing reagent. It is found that degassing is not required for this method, although the reaction is much slower (Figure 5C,D). Another alternative is to use a CuSO4/THTPA complex as catalyst, NaAsc as reducing regent, and THF/H2O as solvents. The reaction requires shorter time to completion (Figure 5E,F). The disadvantage of this approach is the high cost of the THTPA ligand. NMR is used to characterize the chemical purity and structural integrity of the dendrimers formed. Representative

H NMR spectra are shown in Figure 6 for F2S1, F2M1, and F2L1 with the signals assigned. For example, in the spectrum of F2S1, the signals in the 7.9−7.8 ppm region are corresponding to the proton on the triazole ring. The peripheral benzene rings in the PBE dendron generate a set of signals from 7.5 to 7.3 ppm while the inner benzene rings produce signals from 6.8 to 6.5 ppm. The singlets at 5.62 and 5.07 ppm are from the protons on the benzyl ether bridges. The two sets of ethylene protons on each side of the ether linkage at the “focal” point of PBE dendrons, labeled as “l” and “k” show as singlets at ∼4.66 and 4.54 ppm. The monoethylene glycol protons appear as two sets of overlapped triplets at ∼4.68 and 4.48 ppm (c1, c2, i, and h in expanded spectrum). The methylene protons, adjacent to the triazole ring, are visible at 4.58 ppm as a singlet. The methylene protons by the branching point of bis-MPA yield signals at 3.68 and 3.54 ppm. The bis-MPA methyl groups are identified as a singlet at 1.10 ppm. In the case of F2M1 and F2L1, the proton signals from the PEE block are more clustered because more ethylene glycol units are incorporated. Still the signals could be reasonably assigned (Figure 6B,C). Self-Assembly Studies. In aqueous media the Janus dendrimers, constructed in this study, form either micelles or vesicles depending on the hydrophilic/hydrophobic ratio. It is found that those dendrimers bearing blocks of similar molecular weight tend to form vesicles, while dendrimers with a relatively large PEE dendrons are prone to assemble into micelles. The dendron generations also affect the aggregation leading to micellar or vesicular supermolecules. The fine structure of the PEE dendron (S, M, or L) further influences the size and the lower critical solution temperature (LCST) of the micelles. Pyrene is used as a probe to test the critical micelle concentration (CMC), since the fluorescent spectrum of this probe is sensitive to its microenvironment. The emission spectra of pyrene in contact with different concentrations of F2L2 dendrimers are recorded in water (Figure 7). As the concentration of dendrimer increases, the fluorescent intensity of pyrene is enhanced manifesting the onset of the selfassembly process. The other noticeable change is the increase of the relative intensity of the peak at 383 nm. The intensity ratio 383/373 nm is used to determine the CMC for several members of the Janus dendrimer family (Figure 8). The CMC

K


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Figure 9. TEM image of micelles formed by F2L2 (a) and F3L3 (b) stained by uranyl acetate.

examines the size of F2S1, F2M1, and F2L1, at first glance this rule does not apply. That is why the lower critical solution temperature (LCST) of the ethylene glycol moieties should also be considered to rationalize the effect of hydrophilic/ hydrophobic ratio. Except for F2S3, all other Janus dendrimer micelles exhibit thermosensitive properties due to the presence of oligo(ethylene glycol) moieties. The micelle solutions are transparent below and milky above LCST (inset in Figure 10). The LCST of these micelle systems are measured by the UV/vis spectroscopy turbidity test. The results are shown in Figure 10A. Transition temperatures have a narrow range, and the transition is completely reversible. Micrometer scale spherical particles are observed in the micelle solutions above LCST (Figure S3). The LCST of Janus micelles depends on both molecular structures and concentration. Since the aggregation happens between the micelles, the hydrophobic core has negligible effect on LCST. The structure of PEE dendrons as micelle corona is the main factor on LCST at fixed concentration. More oligo(ethylene glycol), OEG, moieties incorporated will increase the hydrophilicity of the entire micelle; thus, higher LCST is expected. If one examines the LCSTs of micelles prepared by Janus dendrimers containing M spacers, LCST increases in the following order: FMMS2 (3OEGs) < FSSM2 (4OEGs) < F2M2 (7OEGs) < F2M3 (15OEGs) < F3M3 (15OEGs). LCST of FMMS2 is below room temperature; thus, the micelle preparation is performed by injection of its acetone solution into excess of cold water. LCST also depends on the size of the PEE dendrons. For example, the LCST decreases as the PEE (L) dendron generation increases: F2L2 vs F3L3. Possibly, PEE L3 dendrons in the corona of the micelles play a significant role in lowering the LCST. This phenomenon is consistent with the observations reported for other oligo(ethylene glycol)-containing dendrimers.22−24 The LCST of these micelles are also tunable by adjusting the concentration (Figure 10B). The lower the concentration, the higher the LCST. Aggregation kinetic is responsible for this phenomenon. Aggregation happens faster at higher micelle concentrations. Taking into account the results from the observed LCST of “gradient density” dendrimer FMMS2, the three diethylene glycol moieties in F2M1 should be above their LCST at room temperature rendering the entire molecule more hydrophobic. That is why giant vesicles instead of micelles are formed (Table 1 and Figure 11). SEM and TEM analyses further reveal the unilamellar structure of these giant vesicles. This also explains why F2M1 giant vesicles need a specific preparation procedure, which involves hydration of the thin film formed by evaporating the solvent in the methanol solution of the dendrimer (methanol is good solvent for the PEE dendron and

Figure 8. Plot of I383/I373 ratio vs different dendrimer concentrations (F2L1, F2L2, F3L2, and F3L3 shown).

values of all dendrimer assemblies are summarized in Table 1 together with the particle size and distribution determined by DLS. The CMCs are at the microgram level, suggesting that the aggregates should be stable against dilution. Table 1. CMC of Janus Dendrimers, Aggregate Type, and Hydrodynamic Diameter sample

molecular mass (g/mol)

CMCa (μg/mL)

aggregate type

hydrodynamic diameterb (nm)

PDIb

F2S1 F2S2 F2S3 F2M1 F2M2 F2M3 F2L1 F2L2 F3M2 F3M3 F3L2 F3L3 FSSM2 FMMS2

1469 2433 4363 1601 2741 5023 1733 3050 3591 5873 3899 6534 2608 2565

89.9 NA 16.1 90 30.6 22.1 8.3 10.2 160 36 30 20 23 29

vesicle not stable micelle vesicle micelle micelle vesicle micelle vesicle micelle vesicle micelle micelle micelle

310 NA 10.78 3.59 μmc 10.91 11.92 163 10.95 2.38 μmc 14.02 148 14.07 10.04 10.1

0.224 NA 0.113 0.239c 0.083 0.097 0.013 0.089 0.204c 0.095 0.06 0.07 0.087 0.086

a

CMC of Janus dendrimer aggregations is measured by the pyrene probing method. bHydrodynamic diameter and PDI are determined by DLS analysis. cThe size and size distribution of vesicles formed by F2M1 and F3M2 are analyzed statistically from optical microscopy images of 30 vesicles.

By examining the size data in Table 1, it could be easily concluded that the increase of hydrophilic to hydrophobic ratio will lead to a decrease of aggregates size. The extreme example is the vesicle to micelle transition when PEE dendron changes to higher generation. It is worth mentioning that increasing the generation of the PEE dendron also leads to a decrease in CMC and aggregation number. Therefore the micelle size is smaller in F3L3 than F2L2 (∼7 nm vs. 8.3 nm as observed in TEM, Figure 9). By comparing F2L1 and F3L2, the molecular mass ratio of PEE to PBE slightly increases from 1.21 to 1.39, which causes an average vesicle size decrease of 15 nm. The same rule also applied to another two vesicles forming dendrimers F2M1 and F3M2. However, if one L


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Figure 10. (A) LCST of Janus dendrimers. Inset: photograph of F3M3 solutions below LCST (left vial) and above LCST (right vial). (B) Concentration dependence of LCST for Janus dendrimer micelles.

hydration of the film and the formation of vesicles. Film hydration of dendrimer films prepared from other solvents such as THF and acetone fails to produce any well-defined structure because they are good solvents for both blocks. Injection of acetone solutions of F2S1, F2L1, F3M2, and F3L2 dendrimers into water results in quick vesicle formation, suggesting the hydration process is rapid and less entropy demanding. The bilayer thickness of the vesicles estimated by TEM analyses is approximately 4 nm for F2S1, F2M1, and F2L1 and approximately 4.5 nm for F3M2 and F3L2, indicating some dependence on the hydrophobic dendron size (generation). Deformed vesicles with a smaller size were also observed as ring-like structures in the SEM analysis of F2S1 (Figure 12a). The vesicular arrangement was further confirmed by TEM, which is normally considered as the most powerful validation technique for this structure type.25,26 As seen in Figure 12b, the vesicle inner cavity is surrounded by a thin wall and is slightly darker than the background, suggesting the vesicle is deformed before the staining process (the inner cavity should otherwise be lighter or at least similar to the background). By comparison, vesicles formed by F2L1 and F3L2 maintained their spherical shape after drying, as revealed by SEM (Figure 12c,e), indicating they are more robust. TEM study shows cavities of lighter gray color surrounded by a similar thin wall, confirming the assumption that the vesicles remain intact

Figure 11. Phase constrast optical microscopy images of vesicles formed by F2M1 (a) and F3M2 (b). SEM and TEM images of deformed F2M1 vesicles (c) and F3M2 vesicles (d); see Experimental Section for details.

nonsolvent for PBE dendron). F2M1 forms aggregates of ∼158 nm (measured by DLS) in methanol. The dendrimer “preorganizes” in the methanol solution which facilitates the M


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Figure 13. Aged F3L2 vesicles show shrinked membrane and holes, indicating a collapse (a). Micelles found in F3L2 solution aged over two months (b).

TEM (Figure 14). The F2L1−palladium complex is still able to form vesicular structures with the surface decorated by the

Figure 14. Typical TEM images of aggregates formed by dendrimer− palladium complexes in water F2L1−Pd (a) and F2L2−Pd (b); no staining is used. White spots in image (b) are caused by damage from electrons scattered by the palladium aggregates.

Figure 12. SEM images of of vesicles formed by F2S1 (a), F2L1 (c), and F3L2 (e) through dendrimer acetone injection method; TEM images of the vesicles prepared using the same procedure as (a) and (b): F2S1 (b), F2L1 (d), and F3L2 (f).

metal. The sizes of these metallovesicles are significantly larger than the original assemblies. Since the palladium salt is not soluble in water, the hydrophilic to hydrophobic ratio is decreased, leading to larger aggregates.28 Palladium is not evenly distributed on the vesicle surface. Small clusters are often observed. A similar phenomenon is also observed in F2L2−palladium. Remarkably this complex did not undergo any LCST transition up to 90 °C. Ethylene glycol moieties are probably closely associated with palladium clusters and thus are hampered to collapse upon heating.29,30 The palladiumdecorated vesicles are relatively short-lived for about one month, while the micelles are stable at least for half a year without observing precipitation and color change.

during staining (Figure 12d,e). Prolonged drying of sample with F2L1 vesicles under vacuum causes structural deformation and the uranyl acetate solution is able to stain the hydrophilic inner cavity (Figure S4). It is noticed that the lifetimes of these vesicles are relatively short in distinction to other types of Janus dendrimers with hydrophobic domains of long alkyl chains.12,17 This is not surprising considering the low interpenetration (entanglement) tendency of Fréchet dendrons compared to the alkyl chains. Giant vesicle solution prepared from F2M1 is stable at room temperature for approximately 2 weeks before large opaque agglomeration occurs. F3M2 vesicles are more stable than F2M1 by few more days. Smaller vesicles formed by F2L1 and F3L2 are stable for at least one month without precipitation. TEM study of F3L2 vesicle reveals that the bilayer membrane shrinks and becomes porous upon aging as seen in Figure 13a. Finally, the vesicle brakes into small micellar particles (Figure 13b). Similar vesicle degradation is also observed in F2L1 dendrimer solution (Figure S5). Palladium Binding and Self-Assembly. Dendrimers containing triazole rings are widely used as carriers of transition metals.27 As it was demonstrated in our previous study, PEE dendrons are able to bind efficiently palladium salts and catalyze Suzuki−Miyaura reactions at relatively mild, environmentally friendly conditions.21 Two dendrimers, F2L1 (vesicle forming) and F2L2 (micelle forming), are selected to test the metal-binding ability of the new Janus dendrimer family. The dendrimer−palladium complexes are prepared in DMSO, and then the resulting solution is injected into water to generate aggregates. These aggregates are further analyzed by

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CONCLUSIONS In summary, a series of new amphiphilic PEE-block-PBE Janus dendrimers are successfully synthesized by “click” chemistry. The optimized click reaction ensures the perfect branching structure up to the third generation. These dendrimers are able to form micelles or vesicles by facile organic solution injection into water revealed by microscope study. CMC’s of the dendrimers are low as evidenced by fluorescence measurements using pyrene as a probe, suggesting the aggregates are stable upon dilution. The vesicles have a relatively short lifetime. The micelles are thermosensitive, and their LCSTs are tunable by adjusting dendrimer concentration. Finally, these dendrimers are forming palladium complexes, which in turn are self-assembling into supermolecular structures in water. The investigations on the application of these dendrimers in catalysis and drug delivery are currently undergoing. N


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dendrimers into uniform dendrimersomes and other complex architectures. Science 2010, 328, 1009−1014. (13) Percec, V.; Leowanawat, P.; Sun, H.-J.; Kulikov, O.; Nusbaum, C. D.; Tran, T. M.; Bertin, A.; Wilson, D. A.; Peterca, M.; Zhang, S.; Kamat, N. P.; Vargo, K.; Moock, D.; Johnston, E. D.; Hammer, D. A.; Pochan, D. J.; Chen, Y.; Chabre, Y. M.; Shiao, T. C.; Bergeron-Brlek, M.; André, S.; Roy, R.; Gabius, H.-J.; Heiney, P. A. Modular Synthesis of Amphiphilic Janus Glycodendrimers and Their Self-Assembly into Glycodendrimersomes and Other Complex Architectures with Bioactivity to Biomedically Relevant Lectins. J. Am. Chem. Soc. 2013, 135, 9055−9077. (14) Sherman, S. E.; Xiao, Q.; Percec, V. Mimicking Complex Biological Membranes and Their Programmable Glycan Ligands with Dendrimersomes and Glycodendrimersomes. Chem. Rev. 2017, 117, 6538−6631. (15) Xiao, Q.; Rubien, J. D.; Wang, Z.; Reed, E. H.; Hammer, D. A.; Sahoo, D.; Heiney, P. A.; Yadavalli, S. S.; Goulian, M.; Wilner, S. E.; Baumgart, T.; Vinogradov, S. A.; Klein, M. L.; Percec, V. Self-Sorting and Coassembly of Fluorinated, Hydrogenated, and Hybrid Janus Dendrimers into Dendrimersomes. J. Am. Chem. Soc. 2016, 138, 12655−12663. (16) Bo, Z.; Rabe, J. P.; Schlüter, A. D. A Poly(para-phenylene) with Hydrophobic and Hydrophilic Dendrons: Prototype of an Amphiphilic Cylinder with the Potential to Segregate Lengthwise. Angew. Chem., Int. Ed. 1999, 38, 2370−2372. (17) Nummelin, S.; Selin, M.; Legrand, S.; Ropponen, J.; Seitsonen, J.; Nykanen, A.; Koivisto, J.; Hirvonen, J.; Kostiainen, M. A.; Bimbo, L. M. Modular Synthesis of Self-assembling Janus-dendrimers and Facile Preparation of Drug-loaded Dendrimersomes. Nanoscale 2017, 9, 7189−7198. (18) Nummelin, S.; Liljeström, V.; Saarikoski, E.; Ropponen, J.; Nykänen, A.; Linko, V.; Seppälä, J.; Hirvonen, J.; Ikkala, O.; Bimbo, L. M.; Kostiainen, M. A. Self-Assembly of Amphiphilic Janus Dendrimers into Mechanically Robust Supramolecular Hydrogels for Sustained Drug Release. Chem. - Eur. J. 2015, 21, 14433−14439. (19) Feng, Y.; He, Y.-M.; Zhao, L.-W.; Huang, Y.-Y.; Fan, Q.-H. A Liquid-Phase Approach to Functionalized Janus Dendrimers: Novel Soluble Supports for Organic Synthesis. Org. Lett. 2007, 9, 2261− 2264. (20) Liu, J.; Feng, Y.; He, Y.; Yang, N.; Fan, Q.-H. Janus dendritic phosphines: synthesis and application in Suzuki coupling reactions. New J. Chem. 2012, 36, 380−385. (21) Wang, L.; Kiemle, D. J.; Boyle, C. J.; Connors, E. L.; Gitsov, I. Click” Synthesis of Intrinsically Hydrophilic Dendrons and Dendrimers Containing Metal Binding Moieties at Each Branching Unit. Macromolecules 2014, 47, 2199−2213. (22) Aathimanikandan, S. V.; Savariar, E. N.; Thayumanavan, S. Temperature-Sensitive Dendritic Micelles. J. Am. Chem. Soc. 2005, 127, 14922−14929. (23) Li, W.; Zhang, A.; Chen, Y.; Feldman, K.; Wu, H.; Schlüter, A. D. Low toxic, thermoresponsive dendrimers based on oligoethylene glycols with sharp and fully reversible phase transitions. Chem. Commun. 2008, 0, 5948−5950. (24) Wu, W.; Driessen, W.; Jiang, X. Oligo(ethylene glycol)-Based Thermosensitive Dendrimers and Their Tumor Accumulation and Penetration. J. Am. Chem. Soc. 2014, 136, 3145−3155. (25) Liu, Y.; Yu, C.; Jin, H.; Jiang, B.; Zhu, X.; Zhou, Y.; Lu, Z.; Yan, D. A Supramolecular Janus Hyperbranched Polymer and Its Photoresponsive Self-Assembly of Vesicles with Narrow Size Distribution. J. Am. Chem. Soc. 2013, 135, 4765−4770. (26) Jiang, W.; Zhou, Y.; Yan, D. Hyperbranched polymer vesicles: from self-assembly, characterization, mechanisms, and properties to applications. Chem. Soc. Rev. 2015, 44, 3874−3889. (27) Huang, D.; Zhao, P.; Astruc, D. Catalysis by 1,2,3-triazole- and related transition-metal complexes. Coord. Chem. Rev. 2014, 272, 145−165 and references therein. (28) Hickey, R. J.; Haynes, A. S.; Kikkawa, J. M.; Park, S.-J. Controlling the Self-Assembly Structure of Magnetic Nanoparticles

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00700. Dendrimers and dendrons: MALDI-TOF spectra, SEM and TEM images, DLS traces, 1H and 13C NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 1-315-470-6851; Fax 1-3154706856 (I.G.). ORCID

Ivan Gitsov: 0000-0001-7433-8571 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported in part by The National Science Foundation (BIO-DBI 1531757). We thank Mr. Robert P. Smith from N.C. Brown Center for Ultrastructure Studies (State University of New York-ESF) for his assistance with the microscopy analyses. We also acknowledge Prof. Mathew M. May (Syracuse University) for providing access to DLS instrumentation.

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