An efficient and controlled synthesis of persulfonylated G1 dendrimers via click reaction

Abstract

This work presents the first report, we believe, on controlled synthesis of clickable dendrimers using aromatic and heteroaromatic cores and persulfonylated dendrons. Owing to poor thermal stability of persulfonylated dendrons, CuAAC reaction conditions were standardized at room temperature. Click reactions went smoothly with Cu(PPh₃)₃Br complex in the presence of tridentate chelating ligands and produced copper-free G1 dendrimers in excellent yield. This methodology has now unveiled the peripheral decoration of high generation POPAM dendrimers with persulfonyl groups in a one-step click reaction.

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Introduction

Synthesis of bio-inspired molecules has fascinated recent research activities. Among these, synthesis of macromolecules with a biologically active scaffold is of paramount interest.^1–3 Furthermore, recent years have witnessed unprecedented growth of research in the area of dendrimer synthesis for their application in the fields of biomedical science^4,5 and nanoscience.^6–8 Consequently, development of dendrimers with well-defined size and shape is of eminent interest because macromolecular dendritic species generally suffer from structural imperfections and polydispersity.⁹ However, the synthesis of dendritic molecules is an apparently straightforward iterative process,^10,11 yet production of high generation dendrimers in good yield with high purity is challenging. To overcome this issue, click reactions are being used to produce magnificent yields of dendrimers.^12,13 Azide- and alkyne-based click reactions incorporate a 1,2,3-triazole ring in the branching positions of the dendrimers. Since triazoles are efficient ligands for Cu(I) complexation, selection of catalyst and ligand is a challenging task.^14,15 Moreover, functionalization of dendrimers at the periphery via N,N-bis-sulfonylation with various sulfonyl chlorides has been developed as a controlled and selective method, but difficulty arose with peripheral p-nitro benzenesulfonyl (p-Ns or nosyl) functional groups, which convert back into the corresponding amines.^16–18 No doubt owing to commercial availability of oligo-amines, POPAM and PAMAM dendrimers, these have often been decorated at their periphery with diverse groups, with the aims of introduction of a specific function and consecutive dendrimer growth.¹⁹ This was usually restricted to complete functionalization at each of the peripheral amino groups. Therefore, such functionalization often lacks selectivity and gives a monodisperse product.^9,16 Despite this, the groundwork on sulfone-based clickable dendrimers has not yet been reported in the literature and its synthetic exploration is crucial.

Results and discussion

In this preliminary work we have synthesized alkyne-functionalized aromatic and heteroaromatic cores (Scheme 1) and azide-functionalized persulfonylated termini (Scheme 2). Subsequently, the obtained azide functionalized dendrons and alkyne functionalized cores were stitched together via Cu(I)-catalyzed Azide–Alkyne Cycloaddition (CuAAC) reaction. Since p-nosyl amines are thermally unstable, the reaction conditions for the click reaction have been standardized at ambient temperature. Although mere triethylamine is capable of stabilizing the copper(I) active species even in reactions carried out ‘in water’, the selection of appropriate ligand and solvent is of consequence in the synthesis of macromolecues.^20,21 Table 1 shows the screening of copper catalysts and nitrogen-containing ligands/bases. As the triazole ring constructed in the click reaction itself has the capability to form stable copper complexes in the reaction medium, the nitrogen-containing ligand/base used as an additive must have higher affinity towards stable copper complexation than the triazole ring generated in the reaction. It was observed that in the presence of CuI and monodentate ligands the yield of D2 was very low while D6 was negligible. Lower yield of both the dendrimers revealed that the generated dendrimers formed more stable copper complexes than DIPEA. Therefore, we checked the yield of dendrimers under the same conditions by adding CuI from 0.3 mol% to 10 mol%, but yield was not improved satisfactorily. Subsequently, use of tridentate chelating ligands such as tris(benzyltriazolylmethyl)amine (TBTA), N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDTA), tris(2-benzimidazolylmethyl)amine (BimH)₃, tris(2-pyridylmethyl)amine (TPMA) and tris[2-(dimethylamino)ethyl]amine (Me₆TREN) produced up to 80–85% yield with only 0.3 mol% of CuI.^22,23 It was observed that use of less hindered ligands lowered the yield of D8 while hindered ligands improved the yield as well as lowered the reaction time. After screening the ligands with CuI, click reaction was tested with Cu(PPh₃)₃Br. It is well known in the literature that PPh₃ improves the solubility of the catalyst in the reaction solvent (i.e. DCM); therefore it allowed comparatively lower copper loadings. Consequently, dendrimers obtained with Cu(PPh₃)₃Br/TPMA or Me₆TREN are highly pure and copper-free.

Scheme 1 Synthesis of alkyne-terminated clickable core molecules 1a, 2a and 3a–c.

Scheme 2 Synthesis of azide-functionalized persulfonylated dendrons 5a–c.

Table 1 Screening of catalysts and additives for CuAAC reaction of dendron and core molecules

S. No.	Catalyst (0.3 mol%)	Solvent	Additive (0.3 mol%)	Time (h)	Yield^a (%) D2/D8
a Isolated yield is reported. Since loss of sulfonyl group was common in these reactions, ^1H NMR did not predict the exact percentage conversion of the products.
1	CuSO4·5H2O	THF/H2O	NaAsc	24	Trace
2	CuI	THF	DIPEA	24	10/trace
3	CuI	DCM	DIPEA	24	30/5
4	CuI	DCM	DBU	24	23/trace
5	CuI	DCM	TBTA	8	55/20
6	CuI	DCM	PMDTA	8	65/60
7	CuI	DCM	(BimH)3	8	50/32
8	CuI	DCM	TPMA	3	85/82
9	CuI	DCM	Me6TREN	3	85/80
10	Cu(PPh3)3Br	DCM	None	24	35/trace
11	Cu(PPh3)3Br	DCM	DIPEA	8	50/30
12	Cu(PPh3)3Br	DCM	PMDTA	4	90/88
13	Cu(PPh3)3Br	DCM	TPMA	1	98/95
14	Cu(PPh3)3Br	DCM	Me6TREN	1	99/97

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The scope of this reaction was then explored for various dendrons and cores (Scheme 3). The use of alkyne-terminated simple cores (1a, 2a and 3a) and hypercores (3b and 3c) did not affect the reaction rate, except for the bis-nosyl dendron (5c), which showed poor reactivity perhaps due to having the least stability under the reaction conditions. To our knowledge, this is the first time that persulfonylated clickable dendrimers have been designed, and structural purity of the G1 dendrimers was fully confirmed by means of NMR, GPC and MALDI-TOF spectrometry. From the ¹H and ¹³C NMR spectra it is clear that all dendrimers possessed almost symmetrical structures in their solution phase (see ESI†). Although a dendrimer is often described as a polymer with excellent PDI (as low as 1.01), this is obtained only after extensive purification. The structural heterogeneity that arose from undesired side-reactions during synthesis directly affected the weight-average (M_w) molecular weight, and PDI of the dendrimers. Therefore, GPC analysis was performed after purification by column chromatography. GPC analysis (Fig. 1) determined a well-defined molecular structure for all the dendrimers, with a low polydispersity ranging from 1.01 to 1.03. The M_w of the dendrimers increased from D1 = 1239 to D12 = 2880 and their retention time varied from 21.00 to 19.85 minutes.

Scheme 3 Synthesized persulfonylated dendrimers D1–12.

Fig. 1 GPC curves of D1–D6 (upper) and D7–D12 (lower) after purification.

Matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry is frequently used for dendrimer characterization as coupling to time-of-flight analyzer commonly offers a broad mass range. Nevertheless, MALDI-MS may point to the presence of large amounts of defects, because they may be formed during laser irradiation.²⁴ The wavelength of the light required for photocleavage of sulfonamides falls in the UV range (i.e. 254 nm), which is far below the wavelength of the N₂ laser of the MALDI instrument (i.e. 337 nm). But photochemical cleavage of the S–N bonds cannot be ignored, because UV spectra of the dendrons show absorption around 330–336 nm and thus they may absorb light energy from the MALDI laser. To show clear evidence of the synthetic advantage of the catalysts, we tried to isolate the dendrimers with and without copper complex. To our great delight, purification of D4 obtained using CuI catalyst delivered the D4 with copper. The structurally perfect dendrimer appears with quite intense signals along with loss of SO₂ from the parent ion with quite low intensity (Fig. 2, left). Interestingly, the copper-bound D4 shows an intense signal at [M + Cu + H]⁺ and a second intense signal at [M +2Cu + H]⁺ followed by low intensity signals at [M + Cu ​− Ts + H]⁺and [M + 2Cu − 3Ts + H]⁺ (Fig. 2, middle). The tentative mechanism for the loss of SO₂ and tosylsulfinic acid is shown in Fig. 2 (right). The most striking result coming from MALDI is the photochemical cleavage of the S–N bonds. The copper-free dendrimer loses only SO₂, keeping other parts of the molecule connected to each other through intramolecular ipso substitution in a concerted mechanism, while the copper-bound dendrimer goes for detosylation through 1,2-elimination reactions.¹⁷

Fig. 2 MALDI-TOF mass spectra of D4 without copper complex (left) and with copper complex (middle) and tentative mechanism for the loss of SO₂ (right upper) and Ts (right lower).

Conclusions

Overall, it can be concluded that we have developed an efficient and highly controlled set of reaction conditions for the synthesis of peripheral persulfonylated dendrimers via CuAAC reaction at room temperature. The yield of persulfonylated dendrimers in CuI-catalyzed reactions was very low or in trace amounts with DIPEA and DBU, but addition of tridentate chelating ligands improved the yield to some extent as well as reduced the reaction time. Therefore, it can be estimated that the in situ generated dendrimers worked as competitive ligands for the copper center. Finally, we found Cu(PPh₃)₃Br with TPMA or Me₆TREN to be an excellent catalyst for the production of copper-free dendrimers. Further, GPC analysis revealed that all dendrimers are monodisperse, with PDI = 1.01–1.3. Moreover, MALDI evinced the molecular perfection of the dendrimers with intense signals at [M + H]⁺. After successful development of peripheral persulfonylated dendrimers with high molecular perfection, now decoration of higher generation POPAM dendrimers at the periphery should be easy. Consequently, we are now developing higher generation POPAM-based persulfonylated clickable dendrimers using this methodology.

Experimental section

General considerations

All reagents and solvents used were of pure analytical grade. The glassware used in the experiments was carefully cleaned and oven dried. Completion of reaction was checked by commercially available thin layer chromatography (TLC) from Merck, made up of silica gel 60/Kieselguhr F₂₅₄ precoated on aluminum sheets (thickness 0.2 mm). Column chromatography was performed using Merck silica gel (100–200 mesh). Visualization of spots on TLC plates was accomplished with UV light. Product yield refers to either crude or isolated yield after column chromatography. ¹H and ¹³C NMR spectra were recorded on 300 or 500 and 75 or 125 MHz spectrometers, respectively, using the internal standard tetramethylsilane (TMS). Chemical shifts are given in parts per million, and J values are in hertz. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed with Bruker ultrafleXtreme, equipped with smart beam technology and 2,5-dihydroxybenzoic acid (DHB) matrix. The number-average molecular weight (M_n) and polydispersity index (M_w/M_n) were determined in THF at 40 °C with a flow rate of 0.5 mL min⁻¹ on two polystyrene gel columns. The columns were calibrated against seven polystyrene standard samples. Electronic absorption spectra were obtained in air-equilibrated solvents at room temperature.

Synthesis of core molecules

Compounds 1a, 2a, 3a, 3b, 3c, iia and iib were synthesized according to literature methods.^25–30

1,3,5-Tris(prop-2-yn-1-yloxy)benzene (1a). In a 100 mL round bottomed flask, phloroglucinol (1.0 g, 7.93 mmol) and K₂CO₃ (3.3 g, 23.79 mmol) were dissolved in DMF and stirred for 10 min at room temperature. Afterwards, propargyl bromide (2.5 mL, 27.75 mmol) was added and stirring was continued for a further 24 h. Completion of reaction was checked with TLC. After completion of reaction, the reaction mixture was poured into a beaker containing ice-cold water with continuous stirring and kept at 0–5 °C for precipitation. After an appropriate amount of precipitate had been obtained, it was filtered and dried in air. White solid; yield 1.90 g (99%); IR (ν_max/cm⁻¹) 3394, 3165, 2214, 1694, 1584; ¹H NMR (300 MHz, CDCl₃) δ_H 6.27 (s, 3H, Ar-H̲), 4.64 (s, 6H, –O–CH̲₂), 2.53 (s, 3H, alkyne–CH̲); ¹³C NMR (75 MHz, CDCl₃) 160.6, 94.30, 74.9, 48.2; chemical formula: C₁₅H₁₂O₃; elemental analysis: calculated C, 74.99; H, 5.03; found C, 74.94; H, 5.06.

2,4,6-Tris(prop-2-yn-1-ylthio)-1,3,5-triazine (2a). This was synthesized according to the method used in the synthesis of 1a. But phloroglucinol was replaced by thiocyanuric acid. Yellow solid; yield 2.00 g (97%); IR (ν_max/cm⁻¹) 3447, 3176, 2208, 1698, 1583; ¹H NMR (300 MHz, CDCl₃) δ_H 3.98 (s, 6H, –S–CH̲₂), 2.22 (s, alkyne–CH̲); ¹³C NMR (75 MHz, CDCl₃) 178.3, 80.3, 71, 29.2; chemical formula: C₁₅H₁₂S₃; elemental analysis: calculated C, 49.46; H, 3.11; N, 14.42; S, 33.01 found C, 49.45; H, 3.10; N, 14.42; S, 33.00.

N²,N⁴,N⁶-Tri(prop-2-yn-1-yl)-1,3,5-triazine-2,4,6-triamine (3a). In a round bottomed flask a 1 : 1 mixture of THF and methanol (20/20 mL) was cooled down to 0–5 °C, then cyanuric chloride (3.0 g, 16.27 mmol) was dissolved by slow addition so that the temperature of the solution remained at 0–5 °C. To a conical flask containing THF at 0–5 °C, propargyl amine (3.30 mL, 16.27 mmol) was added. Then the solution of propargyl amine in THF was added to the cyanuric chloride solution in a dropwise fashion. The reaction mixture obtained was treated with a solution of NaHCO₃ (4.10 g, 16.27 mmol) in THF (10 mL) at 0–5 °C to neutralize the solution. Then it was stirred at room temperature for 1 hour with subsequent refluxing for 1.5 hours. After confirmation of the completion of the reaction with TLC, solvent was evaporated at reduced pressure. The obtained crude solid was re-dissolved in THF and undissolved impurities were filtered out. Afterwards, THF was evaporated from the filtrate and pure compound (3a) was obtained. Yellow solid; yield 3.9 g (99%); mp 147–149 °C; IR (ν_max/cm⁻¹) 3426, 3338, 2853, 1586, 1475; ¹H NMR (300 MHz, DMSO_d6) δ_H 8.25 (s, 3H, –NH̲), 3.98 (s, 6H, –N–CH̲₂), 2.49, (s, 3H, alkyne–CH̲); ¹³C NMR (75 MHz, DMSO_d6) 165.4, 82.3, 72.1, 29.2; chemical formula: C₁₅H₁₅N₃; elemental analysis: calculated C, 59.99; H, 5.03; N, 34.98; found C, 59.92; H, 5.05; N, 34.97.

1-Nitro-4-(prop-2-yn-1-yloxy)benzene (iia). 4-Nitrophenol (2.0 g, 12.89 mmol) was reacted with propargyl bromide (1.9 mL, 21.57 mmol) and K₂CO₃ (2.17 g, 25.79 mmol) in DMF (20 mL) according to the synthetic procedure of 1a. Pure compound iia was isolated as a crystalline light-yellow solid. Yield 2.5 g (98%); IR (ν_max/cm⁻¹) 3290, 2980, 2112, 1524, 1330; ¹H NMR (300 MHz, CDCl₃) δ_H 8.23 (d, 2H, J = 9.3 Hz, Ar-H̲), 7.15 (d, 2H, J = 9 Hz, Ar-H̲), 4.82 (s, 2H, –CH̲₂), 2.58, (s, 1H, alkyne–CH̲); ¹³C NMR (75 MHz, CDCl₃) 150.8, 139.2, 136.1, 125.2, 119.0, 115.6, 74.5, 72.5, 57.0; chemical formula: C₉H₇NO₃; elemental analysis: calculated C, 61.10; H, 3.95; N, 7.90; found C, 61.02; H, 3.98; N, 7.91.

4-Nitro-1,2-bis(prop-2-yn-1-yloxy)benzene (iib). 3,4-Dihydroxycatechol (2 g, 12.89 mmol) was reacted with propargyl bromide (3.5 mL, 28.37 mmol), and K₂CO₃ (2.17 g, 25.79 mmol) in DMF (20 mL) according to the synthetic procedure of 1a. Pure compound iib was isolated as a light-yellow solid. Yield 2.8 g (98%); mp = 107 °C; IR (ν_max/cm⁻¹) 3298, 2986, 2118, 1521, 1335; ¹H NMR (300 MHz, CDCl₃) δ_H 7.98 (d, 1H, J = 9 Hz, Ar-H̲), 7.28 (s, 1H, Ar-H̲), 7.15 (d, 1H, J = 8.6 Hz, Ar-H̲), 4.90 (s, 4H, CH̲₂), 2.62, (s, 2H, alkyne CH̲); ¹³C NMR (75 MHz, CDCl₃) 152.7, 139.2, 136.1, 135.1, 128.2, 119.0, 118.3, 112.6, 109.6, 105.8, 72.5, 59.2, 57.0; chemical formula: C₁₂H₉NO₄, elemental analysis: calculated C, 62.34; H, 3.92; N, 6.06; found C, 62.31; H, 3.91; N, 6.04.

4-(Prop-2-yn-1-yloxy)aniline (iiia). In a 100 mL round bottomed flask compound iia (2 g, 11.29 mmol) was dissolved in 20 mL DCM/ethanol (1 : 1) mixture. Then a solution of SnCl₂·H₂O (3.52 g, 16.93 mmol) in 37% HCl was added and the reaction mixture was stirred at 40 °C for 4 h. After completion of the reaction, the mixture was poured into 300 mL of deionized water and extracted with CH₂Cl₂ (2 × 200 mL). The combined organic layers were washed with water followed by saturated NaHCO₃. The isolated organic layer was dried over anhydrous Na₂SO₄. Then solvent was removed under reduced pressure and the resulting solid residue was dried in air. A viscous yellow product was obtained. Yield 1.32 g (79%); IR (ν_max/cm⁻¹) 3490, 2880, 2115, 1330, 1208; ¹H NMR (300 MHz, CDCl₃) δ_H 6.85 (d, 2H, J = 8.6 Hz, Ar-H̲), 6.64 (d, 2H, J = 8.9 Hz, Ar-H̲), 5.62 (br, 2H, NH̲₂), 4.65 (s, 2H, –CH̲₂), 2.49, (s, 1H, alkyne–CH̲); ¹³C NMR (75 MHz, CDCl₃) 139.0, 137.1, 115.2, 112.6, 72.5, 70.1, 56.2; chemical formula: C₉H₉NO; elemental analysis: calculated C, 73.45; H, 6.16; N, 9.52; found C, 73.40; H, 6.18; N, 9.60.

3,4-Bis(prop-2-yn-1-yloxy)aniline (iiib). This was also synthesized by the same procedure as given for synthesis of compound iiia. It is a brown viscous product. Yield 1.8 g (98%); IR (ν_max/cm⁻¹) 3543, 3459, 3291, 2919, 1614, 1266; ¹H NMR (300 MHz, DMSO_d6) δ_H 10.10 (s, 2H, NH̲₂), 7.21 (d, 1H, J = 7.2 Hz, Ar-H̲), 7.04 (s, 1H, Ar-H̲), 6.96 (d, 1H, Ar-H̲), 4.87 (s, 4H, –CH̲₂), 3.62 (s, 6H, alkyne–CH̲); ¹³C NMR (75 MHz, DMSO_d6) 148.2, 147.3, 146.2, 125.4, 115.8, 114.9, 109.4, 56.3; chemical formula: C₁₂H₁₁NO₂; elemental analysis: calculated C, 71.63; H, 5.51; N, 6.96; found C, 71.65; H, 5.48; N, 6.98.

N²,N⁴,N⁶-Tris(4-(prop-2-yn-1-yloxy)phenyl)-1,3,5-triazine-2,4,6-triamine (3b). The synthetic procedure was the same as in 3a. But the refluxing time was 10–12 h. The reactants and product stoichiometry were as follows: compound iiia (2.25 g, 16.27 mmol) in THF (20 mL), cyanuric chloride (1.0 g, 5.42 mmol) in THF/methanol (10/10 mL) mixture and NaHCO₃ (2.25 g, 16.27 mmol). A solid product was obtained. Light brown solid; yield 2.7 g (96%); IR (ν_max/cm⁻¹) 3282, 3250, 2925, 2119, 1592; ¹H NMR (300 MHz, CDCl₃) δ_H 7.50 (d, 6H, J = 8.4 Hz, Ar-H̲), 7.44 (s, 3H, NH̲), 7.32 (d, 6H, J = 8.1 Hz, Ar-H̲), 4.70 (s, 6H, CH̲₂), 2.51 (s, 3H, alkyne–CH̲); ¹³C NMR (75 MHz, CDCl₃) 165.5, 148.2, 147.6, 146.2, 135.2, 121.9, 115.2, 79.4, 78.5, 56.0; chemical formula: C₃₀H₂₄N₆O₃; elemental analysis: calculated C, 69.76; H, 4.68; N, 16.27; found C, 69.78; H, 4.65; N, 16.22.

Compound 3c. Cyanuric chloride (1.0 g, 5.42 mmol) was reacted with compound iiib (3.27 g, 16.27 mmol) and NaHCO₃ (3.0 g, 16.27 mmol) in a THF/methanol (2 : 1) mixture according to the synthetic procedure of compound 3b. Pure compound 3c was isolated as a brown solid. Yield 3.0 g (81.5%); IR (ν_max/cm⁻¹) 3382, 3286, 2923, 2852, 1586, 1391; ¹H NMR (300 MHz, DMSO_d6) δ_H 9.95 (s, 3H, NH̲), 7.20 (d, 3H, J = 8.7 Hz), 7.15 (s, 3H), 7.04 (d, 3H, J = 8.7 Hz, Ar-H̲), 4.75 (s, 6H, CH̲₂), 4.54 (s, 6H, CH̲₂), 3.75 (s, 3H, CH̲), 3.51 (s, 3H, alkyne CH̲); ¹³C NMR (75 MHz, DMSO_d6) 163.8, 146.8, 143.4, 132.4, 114.8, 109.0, 104.8, 79.8, 78.9, 56.4; chemical formula: C₃₉H₃₀N₆O₆; elemental analysis: calculated C, 69.02; H, 4.46; N, 12.38; found C, 69.04; H, 4.50; N, 12.40.

Synthesis of dendrons

Mono/bis-sulfonylation and azidation were done according to literature methods.^31–33 Dendrons 5a–c were synthesized in three steps as shown in Scheme 2.

Step-1. Nosyl/tosyl chloride (1 eq.) was added to a stirred solution of 2-bromoethylamine hydrobromide (1 eq.) in pyridine and stirring was continued for the next 2–3 h at room temperature. After completion of the reaction, ice-cold water was added until precipitation was completed. Then the obtained precipitate was filtered and dried at room temperature.

Step-2. In this step, azidation of mono nosyl/tosyl products (4b/4a)^34,35 was done. For this purpose, 4b/4a (1 eq.) was dissolved in water/acetone mixture (1 : 4) and to it NaN₃ (1.5 eq.) was added. The reaction mixture was stirred at 60 °C for 6–7 h. After completion of the reaction, excess water was added at room temperature and then the reaction mixture was kept at 0 °C for 4 h. Thereupon, the crystalline precipitate was filtered and air dried.

Step-3. In this step persulfonylation was done. For this purpose the azide derivatives of mono nosyl/tosyl products (1 eq.) and TEA (4 eq.) were dissolved in dry DCM. In another beaker, sulfonyl chloride derivatives (4 eq.) were dissolved in DCM and added to the previous solution by stirring under argon atmosphere. After complete addition, the reaction mixture was refluxed for 3–4 h. After completion of the reaction, solvent was removed under reduced pressure and the desired products were purified by column chromatography.

Compound 4c. Compound 4a (2.5 g, 8.99 mmol) was reacted with sodium azide (1.8 g, 17.97 mmol) according to step 2. White crystalline solid; yield 2.0 g (92%); mp 180–185 °C; IR (ν_max/cm⁻¹) 3441, 3346, 2244, 1693, 1602, 1509, 1344; ¹H NMR (300 MHz, CDCl₃) δ_H 7.77 (s, 2H, J = 8.6 Hz, Ar-H̲), 7.34 (d, 1H, J = 8.8 Hz, Ar-H̲), 4.87 (s, 1H, –NH̲), 3.41 (t, 2H, J = 6.0 Hz, –CH̲₂), 3.11 (t, 2H, J = 6 Hz, –CH̲₂), 2.43 (s, 3H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 143.8, 136.6, 129.8, 127.0, 50.8, 42.2, 21.5; chemical formula: C₁₀H₇N₃O₃; elemental analysis: calculated C, 55.30; H, 3.25; N, 19.35; found C, 55.28; H, 3.26; N, 19.36.

Compound 4d. Compound 4b (2.5 g, 8.09 mmol) was reacted with sodium azide (1.8 g, 16.17 mmol) according to step 2. Yellow crystalline solid; yield 2.0 g (91%); mp 136–140 °C; IR (ν_max/cm⁻¹) 3449, 2952, 2107, 1594, 1363; ¹H NMR (300 MHz, CDCl₃) δ_H 8.36 (d, 2H, J = 8.4 HZ, Ar-H̲), 7.86 (d, 4H, J = 8.6 HZ, Ar-H̲), 5.04 (s, 1H, –NH̲), 3.55 (t, 2H, J = 6.0 Hz, –CH₂), 3.38 (t, 2H, J = 6 Hz, –CH₂); ¹³C NMR (75 MHz, CDCl₃) 150.2, 144.4, 128.4, 124.5, 51.1, 41.80; chemical formula: C₈H₉N₅O₄S; elemental analysis: calculated C, 35.42; H, 3.34; N, 25.82; S, 11.82; found C, 35.45; H, 3.35; N, 25.80; S, 11.80.

Dendron 5a. Azide derivative 4a (2.0 g, 8.32 mmol) was allowed to react with TEA (4.21 mL, 38.23 mmol) and p-toluenesulfonyl chloride (6.35 g, 33.29 mmol) in dry DCM (40 mL) according to step 3 of persulfonylation. A white crystalline product was isolated. Yield 3.0 g (92%); mp 93–98 °C; IR (ν_max/cm⁻¹) 2955.30, 2105.89, 1597.44, 1383.48; ¹H NMR (300 MHz, CDCl₃) δ_H 7.93 (d, 4H, J = 8.4 Hz, Ar-H̲), 7.36 (d, 4H, J = 8.1 Hz, Ar-H̲), 3.81 (t, 2H, J = 6.6, 6.5 Hz, –CH̲₂), 3.52 (t, 2H, J = 6.5, 6.9 Hz, –CH̲₂), 2.45 (s, 6H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 145.2, 136.3, 129.7, 128.3, 50.3, 46.9, 21.6; chemical formula: C₁₆H₁₈N₄O₄S₂; elemental analysis: calculated C, 48.72; H, 4.60; N, 14.20; S, 16.26; found C, 48.76; H, 4.65; N, 14.22; S, 16.20.

Dendron 5b. Azide derivative 4a (2.0 g, 8.32 mmol) was allowed to react with TEA (4.21 mL, 33.29 mmol) and p-nitrosulfonyl chloride (7.38 g, 33.29 mmol) in dry DCM (20 mL) according to step 3. A lemon-colored crystalline product was isolated. Yield 3.5 g (98%); mp 110–117 °C; IR (ν_max/cm⁻¹) 2924, 2110, 1528, 1345; ¹H NMR (300 MHz, CDCl₃) δ_H 8.47 (d, 2H, J = 8.6 HZ, Ar-H̲), 8.40 (d, 2H, J = 8.6 HZ, Ar-H̲), 7.95 (d, 2H, J = 8.4 Hz, Ar-H̲), 7.39 (d, 2H, J = 8.6 HZ, Ar-H̲), 3.85 (t, 2H, J = 6 Hz, –CH̲₂), 3.56 (t, 2H, J = 6 Hz, –CH̲₂), 2.48 (s, 3H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 150.6, 146.0, 135.4, 129.9, 128.5, 124.2, 50.4, 47.4, 21.7; chemical formula: C₁₅H₁₅N₅O₆S₂; elemental analysis: calculated C, 42.35; H, 3.55; N, 16.46; S, 15.07; found C, 42.37; H, 3.51; N, 16.50; S, 15.00.

Dendron 5c. Azide derivative (2.0 g 7.37 mmol) was allowed to react with TEA (3.7 mL, 29.49 mmol) and p-nitrosulfonyl chloride (6.6 g, 29.49 mmol) in dry DCM according to step 3. Yellow solid; yield 2.5 g, (79%); mp 83–85 °C; IR (ν_max/cm⁻¹) 2929, 2117, 1530, 1353; ¹H NMR (300 MHz, CDCl₃) δ_H 8.45 (d, 4H, J = 8.9 Hz, Ar-H̲), 8.33 (d, 4H, J = 8.6 Hz, Ar-H̲), 3.94 (t, 2H, J = 6 Hz, –CH̲₂), 3.62 (t, 2H, J = 6 Hz, –CH̲₂); ¹³C NMR (75 MHz, CDCl₃) 150.9, 144.1, 130.01, 124.4, 50.59, 48.15; chemical formula: C₁₄H₁₂N₆O₈S₂; elemental analysis: calculated C, 36.84; H, 2.65; N, 18.41; S, 14.05; found C, 36.80; H, 2.60; N, 18.45; S, 14.10.

Synthesis of dendrimers

The alkyne-terminated core molecules (1.0 eq.) and azide functionalized dendrons (1.1 eq. per alkyne group) followed by Cu(PPh₃)₃Br (0.1 mol% per alkyne group) as copper(I) catalyst and Me₆TREN (0.1 mol% per alkyne group) were dissolved in dry DCM and stirred at room temperature for 8–10 h. After confirming the completion of reaction by TLC, chloroform was added to the reaction mixture and washed with a saturated solution of NH₄Cl in water (10 mL) followed by brine (10 mL). The separated organic layer was dried over Na₂SO₄ and solvent was evaporated under reduced pressure to obtain the crude product. The desired products were obtained after purification by flash column chromatography.

Dendrimer D1. Compound 1a (0.2 g, 0.83 mmol) was reacted with 5a (1.08 g, 2.75 mmol), Cu(PPh₃)₃Br (2.12 mg, 2.50 μmol), and Me₆TREN (700 μL, 2.62 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D1 was isolated by column chromatography with hexane/ethyl acetate (3/7). White solid; yield 1.15 g (98%); IR (ν_max/cm⁻¹) 3126, 3039, 2914, 2843, 1180; ¹H NMR (300 MHz, CDCl₃) δ_H 7.76 (s, 3H, triazole-H̲) 7.68 (d, 12H, J = 7.8 Hz, Ar-H̲), 7.12 (d, 12H, J = 7.8 Hz, Ar-H̲) 6.35 (s, 3H, Ar-H̲), 5.63 (t, 6H, J = 7.8 Hz, –CH̲₂), 4.78 (s, 6H, –CH̲₂), 4.21 (t, 6H, J = 7.2 Hz, –CH̲₂), 2.48 (s, 18H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 158.7, 145.3, 144.1, 135.8, 129.8, 128.2, 122.1, 98.1, 58.1, 49.2, 47.5, 29.2, 27.7, 21.1; SEC shows polydispersity 1.01; m/z MALDI-TOF MS: calculated for C₆₁H₅₇N₁₅O₂₁S₆ 1422.31, found 1423.67 [M + H]⁺.

Dendrimer D2. Compound 1a (0.2 g, 0.83 mmol) was reacted with 5b (1.17 g, 2.75 mmol), Cu(PPh₃)₃Br (2.12 mg, 2.50 μmol), and Me₆TREN (700 μL, 2.62 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D2 was isolated by column chromatography with hexane/ethyl acetate (1/3). Off-white solid; yield 1.2 g (95%); IR (ν_max/cm⁻¹) 3130, 3041, 2924, 2853, 1172; ¹H NMR (300 MHz, CDCl₃) δ_H 8.34 (d, 6H, J = 8.1 Hz, Ar-H̲), 8.06 (d, 2H, J = 8.4, Ar-H̲), 7.80 (d, 6H, J = 7.7 Hz, Ar-H̲), 7.79 (s, 3H, triazole-H̲), 7.36 (d, 6H, J = 7.8 Hz, Ar-H̲), 6.35 (s, 3H, Ar-H̲), 5.64 (t, 6H, J = 6.2 Hz, –CH̲₂), 5.08 (s, 6H, –CH̲₂), 4.31 (t, 6H, J = 6 Hz, –CH̲₂), 2.48 (s, 9H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 158.1, 150.7, 148.6, 146.1, 144.1, 139.2, 129.9, 126.9, 124.6, 106.2, 86.9, 66.8, 31.9, 29.6, 21.7; SEC shows polydispersity 1.01; m/z MALDI-TOF MS: calculated for C₆₁H₅₇N₁₅O₂₁S₆ 1515.22, found 1516.68 [M + H]⁺.

Dendrimer D3. Compound 1a (0.2 g, 0.83 mmol) was reacted with 5c (1.25 g, 2.75 mmol), Cu(PPh₃)₃Br (2.12 mg, 2.50 μmol) and Me₆TREN (700 μL, 2.62 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D3 was isolated by silica gel column chromatography with hexane/ethyl acetate (1/4). Yellow solid; yield 0.82 g (65%); IR (ν_max/cm⁻¹) 3426, 2924, 2853; 1596, 1480; ¹H NMR (CDCl₃, 300 MHz) δ_H 8.40 (d, 12H, J = 8.1 Hz, Ar-H̲), 8.24 (d, 12H, J = 8.1 Hz, Ar-H̲), 8.06 (s, 3H, triazole-H̲), 6.62 (s, 3H, Ar-H̲), 5.80 (t, 6H, J = 6.6 Hz, –CH̲₂), 5.10 (s, 6H, –CH̲₂), 4.91 (t, 6H, J = 6.1, –CH̲₂); ¹³C NMR (CDCl₃, 75 MHz) 159.6, 150.7, 146.3, 144.4, 134.9, 130.1, 129.6, 128.4, 124.4, 123.5, 98.6, 59.6, 49.0, 47.8, 29.6; SEC shows polydispersity 1.02; m/z MALDI-TOF MS: calculated for C₅₈H₅₀N₁₈O₂₆S₆ 1608.52, found 1609.52 [M + H]⁺.

Dendrimer D4. Compound 2a (0.2 g, 0.68 mmol) was reacted with 5c (0.90 g, 2.26 mmol), Cu(PPh₃)₃Br (1.92 mg, 2.06 μmol), and Me₆TREN (550 μL, 2.06 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D4 was isolated by column chromatography with hexane/ethyl acetate (1/7). Off-white solid; yield 1.01 g (98%); IR (ν_max/cm⁻¹) 2924, 2853, 1596, 1480; ¹H NMR (300 MHz, CDCl₃) δ_H 7.76 (d, 12H, J = 7.8 Hz, Ar-H̲), 7.54 (s, 3H, triazole-H̲), 7.31 (s, 12H, J = 7.5 Hz, Ar-H̲), 4.52 (t, 6H, J = 6.2 Hz, CH̲₂), 4.35 (t, 6H, J = 6.8 Hz, CH̲₂), 4.08 (s, 6H, CH̲₂), 2.49 (s, 18H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 178.7, 145.5, 135.8, 129.8, 128.2, 122.1, 49.2, 47.5, 29.2, 21.6; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₆₀H₆₃N₁₅O₁₂S₉ 1473.23, found 1474.75 [M + H]⁺.

Dendrimer D5. Compound 2a (0.2 g, 0.686 mmol) was reacted with 5b (0.96 g, 2.26 mmol), Cu(PPh₃)₃Br (1.92 mg, 2.06 μmol), and Me₆TREN (550 μL, 2.06 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D5 was isolated by column chromatography with hexane/ethyl acetate (1/4). Light-yellow solid; yield 1.00 g (92%); IR (ν_max/cm⁻¹) 3099, 2927, 2850, 1533, 1480, 1380, 1166; ¹H NMR (300 MHz, CDCl₃) δ_H 8.35 (d, 6H, J = 8.1 Hz, Ar-H̲), 8.08 (d, 6H, J = 8.4 Hz, Ar-H̲), 7.83 (d, 6H, J = 7.7 Hz, Ar-H̲), 7.53 (s, 3H, triazole-H̲), 7.35 (d, 6H, Ar-H̲), 4.52 (t, 6H, J = 7.2 Hz, CH̲₂), 4.36 (t, 6H, J = 7.2 Hz, CH̲₂), 4.15 (s, 6H, CH̲₂), 2.47 (s, 9H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 178.6, 150.7, 146.3, 144.4, 144.1, 134.9, 130.1, 129.6, 128.4, 124.4, 123.5, 49.0, 47.8, 29.6, 24.9, 21.7; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₅₇H₅₄N₁₈O₁₈S₉ 1566.13, found 1568.74 [M + H]⁺.

Dendrimer D6. Compound 2a (0.2 g, 0.68 mmol) was reacted with 5c (1.0 g, 2.26 mmol), Cu(PPh₃)₃Br (1.92 mg, 2.06 μmol), and Me₆TREN (550 μL, 2.06 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D6 was isolated by column chromatography hexane/ethyl acetate (1/8). Yellow solid; yield 0.70 g (62%); IR (ν_max/cm⁻¹) 3109, 2914, 2823, 1530, 1482, 1382, 1156; ¹H NMR (300 MHz, CDCl₃) δ_H 8.41 (d, 6H, J = 8.4 Hz, Ar-H̲), 8.20 (d, 6H, J = 8.4 Hz, Ar-H̲), 8.06 (s, 3H, triazole-H̲), 5.09 (t, 6H, J = 6 Hz, –CH̲₂), 4.48 (s, 6H, –CH̲₂), 4.37 (t, 6H, J = 6.2 Hz, –CH̲₂); ¹³C NMR (75 MHz, CDCl₃) 177.7, 155.4, 132.7, 131.1, 129.9, 126.9, 124.5, 54.6, 52.5, 42.7, 33.1; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₅₄H₄₅N₂₁O₂₄S₉ 1660.04, found 1660.97 [M + H]⁺.

Dendrimer D7. Compound 3a (0.2 g, 0.84 mmol) was reacted with 5a (1.10 g, 2.75 mmol), Cu(PPh₃)₃Br (2.35 mg, 2.53 μmol), and Me₆TREN (700 μL, 2.62 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D7 was isolated by column chromatography with hexane/ethyl acetate (1/9). Off-white solid; yield 1.0 g (83%); IR (ν_max/cm⁻¹) 3475, 3068, 2924, 2855, 1452, 1161; ¹H NMR (300 MHz, CDCl₃) δ_H 7. 85 (d, 12H, J = 8.4 Hz, Ar-H̲), 7.73 (s, 3H, triazole-H̲), 7.54 (s, 3H, –NH̲), 7.32 (d, 12H, J = 7.5 Hz, Ar-H̲), 4.53 (s, 6H, J = 6.2 Hz, CH̲₂), 4.37 (s, 6H, CH̲₂), 4.06 (t, 6H, J = 6 Hz, CH̲₂), 2.49 (s, 18H, CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 165.5, 148.3, 137.1, 129.1, 128.3, 124.8, 47.1, 31.9, 29.3, 21.6; SEC shows polydispersity 1.01; m/z MALDI-TOF MS: calculated for C₆₀H₆₆N₁₈O₁₂S₆ 1423.67, found 1423.98 [M + H]⁺.

Dendrimer D8. Compound 3a (0.2 g, 0.84 mmol) was reacted with 5b (1.10 g, 2.75 mmol), Cu(PPh₃)₃Br (2.15 mg, 2.53 μmol), and Me₆TREN (700 μL, 2.62 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D8 was isolated by silica gel column chromatography with MeOH/CHCl₃ (1/20). Light-yellow solid; yield 1.10 g (87%); IR (ν_max/cm⁻¹) 3475, 3065, 2918, 2856, 1524, 1452, 1350, 1161; ¹H NMR (300 MHz, CDCl₃) δ_H 8.44 (d, 6H, J = 8.1 Hz, Ar-H̲), 8.36 (d, 6H, J = 8.4 Hz, Ar-H̲), 7.83 (d, 6H, J = 7.8 Hz, Ar-H̲), 7.63 (s, 3H, triazole-H̲), 7.48 (d, 6H, J = 7.8 Hz, Ar-H̲), 7.36 (s, 3H, –NH̲), 4.52 (t, 6H, J = 6.8 Hz, –CH̲₂), 4.36 (s, 6H, –CH̲₂), 4.15 (t, 6H, J = 6.3 Hz, –CH̲₂), 2.47 (s, 9H, –CH̲₃);¹³C NMR (75 MHz, CDCl₃) 165.5, 151.7, 149.3, 146.4, 134.9, 130.1, 129.6, 128.4, 124.4, 123.5, 51.7, 50.0, 21.7; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₅₇H₅₈N₂₁O₁₈S₆ 1515.76, found 1516.58 [M + H]⁺.

Dendrimer D9. Compound 3b (0.2 g, 0.39 mmol) was reacted with 5a (0.50 g, 1.28 mmol), Cu(PPh₃)₃Br (1.08 mg, 1.16 μmol), and Me₆TREN (310 μL, 1.16 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D9 was isolated by column chromatography with hexane/ethyl acetate (3/17). Light-brown solid; yield 0.62 g (95%); IR (ν_max/cm⁻¹) 3426, 2924, 2853, 1596, 1480; ¹H NMR (300 MHz, CDCl₃) δ_H 8.34 (s, 3H, triazole-H̲), 8.13 (d, 12H, J = 10 Hz, Ar-H̲), 7.87 (s, 6H, J = 6.6 Hz, Ar̲-H̲), 7.39 (s, 3H, NH̲), 7.37 (d, 18H, J = 6.6 Hz, Ar-H̲), 5.24 (t, 6H, J = 6.2 Hz, –CH̲₂), 4.65 (s, 6H, –CH̲₂), 4.22 (t, 6H, J = 6 Hz, –CH̲₂), 2.46 (s, 18H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 164.1, 150.1, 148.3, 146.3, 144.3, 142.5, 135.0, 130.1, 129.7, 128.5, 124.5, 114.4, 70.4, 46.6, 36.6, 21.7; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₇₈H₇₉N₁₈O₁₅S₆ 1699.96, found 1700.04 [M + H]⁺.

Dendrimer D10. Compound 3b (0.2 g, 0.39 mmol) was reacted with 5b (0.55 g, 1.28 mmol), Cu(PPh₃)₃Br (1.08 mg, 1.16 μmol), and Me₆TREN (310 μL, 1.16 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D10 was isolated by column chromatography with hexane/ethyl acetate (1/9). Light-brown solid; yield 1.2 g (96%); IR (ν_max/cm⁻¹) 3416, 2914, 2843, 1586, 1560, 1440; ¹H NMR (300 MHz, CDCl₃) δ_H 8.32 (d, 9H, J = 8.7 Hz, Ar-H̲), 8.13 (d, 6H, J = 8.6 Hz, Ar-H̲), 7.87 (d, 12H, J = 8.2 Hz, Ar-H̲), 7.87 (s, 3H, triazole-H̲), 7.36 (d, 6H, J = 7.8 Hz, Ar-H̲), 7.39 (s, 3H, –NH̲), 7.29 (d, 6H, J = 7.8 Hz, Ar-H̲), 5.22 (t, 6H, J = 6.2 Hz, –CH̲₂), 4.67 (s, 6H, –CH̲₂), 4.24 (t, 6H, J = 6 Hz, –CH̲₂), 2.46 (s, 18H, –CH̲₃); ¹³C NMR (75 MHz, CDCl₃) 164.1, 150.5, 145.2, 142.1, 137.4, 136.1, 129.6, 128.2, 127.8, 126.1, 124.6, 115.2, 48.2, 38.6, 21.7; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₇₅H₇₀N₂₁O₂₁S₆, 1791.33, found 1792.87 [M + H]⁺.

Dendrimer D11. Compound 3c (0.2 g, 0.30 mmol) was reacted with 5a (0.77 g, 1.94 mmol), Cu(PPh₃)₃Br (1.64 mg, 1.77 μmol), and Me₆TREN (480 μL, 1.80 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D11 was isolated by column chromatography with hexane/ethyl acetate (1/9). Off-white solid; yield 0.88 g, (98%); IR (ν_max/cm⁻¹) 3421, 2923, 2843, 1590, 1478; ¹H NMR (500 MHz, CDCl₃) δ_H 8.13 (d, 24H, J = 8.1 Hz, Ar-H̲), 8.13 (d, 24H, 7.5 Hz, Ar-H̲) 7.65 (s, 6H, triazole-H̲), 7.40 (d, 24H, J = 7.2 Hz, Ar-H̲), 7.39 (s, 6H, –NH), 7.32 (s, 3H, Ar-H̲), 7.18 (d, 3H, J = 7.5 Hz, Ar-H̲), 4.67 (t, 12H, J = 6 Hz, –CH̲₂), 4.42 (s, 12H, –CH̲₂), 4.13 (t, 12H, J = 6.2 Hz, –CH̲₂), 2.46 (s, 36H, –CH̲₃); ¹³C NMR (125 MHz, CDCl₃) 164.6, 150.0, 145.1, 142.1, 137.4, 136.1, 129.6, 128.2, 127.8, 126.1, 124.6, 119.9, 114.1, 106.0, 76.5, 43.3, 29.7 21.0; SEC shows polydispersity 1.03; m/z MALDI-TOF MS: calculated for C₁₃₅H₁₃₈N₃₀O₃₀S₁₂ 3044.20, found 3045.50 [M + H]⁺.

Dendrimer D12. Compound 3c (0.2 g, 0.30 mmol) was reacted with 5b (0.83 g, 1.94 mmol), Cu(PPh₃)₃Br (1.64 mg, 1.77 μmol), and Me₆TREN (480 μL, 1.80 μmol) in dry DCM (10 mL) according to the above procedure. Pure compound D12 was isolated by column chromatography with MeOH/CHCl₃ (1/20). Light-yellow solid; yield 0.90 g (94%); IR (ν_max/cm⁻¹) 3426, 2924, 2853, 1596, 1480; ¹H NMR (500 MHz, CDCl₃) δ_H 8.20 (d, 12H, J = 8.7 Hz, Ar-H̲), 8.13 (d, 12H, J = 7.4 Hz, Ar-H̲), 8.05 (d, 12H, J = 7.8 Hz, Ar-H̲), 8.02 (d, 3H, J = 7.5 Hz, Ar-H̲), 7.64 (s, 6H, Ar-H̲) 7.80 (s, 3H, triazole-H̲), 7.39 (d, 12H, J = 7 Hz, Ar-H̲), 7.38 (s, 6H, –NH̲), 7.27 (s, 3H, Ar-H̲), 7.16 (d, 3H, J = 7.5 Hz, Ar-H̲), 5.36 (t, 12H, J = 6. Hz, CH̲₂), 4.48 (s, 12H, –CH̲₂), 4.27 (t, 12H, J = 6.2 Hz, –CH̲₂), 2.48 (s, 18H, –CH̲₃); ¹³C NMR (125 MHz, CDCl₃) 164.8, 152.3, 143.8, 140.1, 137.1, 129.2, 128.2, 125.9, 120.5, 119.2, 110.2, 108.7, 108.6, 49.7, 42.3, 40.2, 25.8; SEC shows polydispersity 1.02; m/z MALDI-TOF MS: calculated for C₁₂₉H₁₂₀N₃₆O₄₂S₁₂ 3231.33, found 3232.34 [M + H]⁺.

Acknowledgements

We acknowledge DST India Grant SR/S1/381 OC-42/2012 for financial support. Department of Chemistry, Institute of Science of Banaras Hindu University is acknowledged for departmental facilities and TIFR-Mumbai for providing the facility for MALDI-TOF-MS. We are thankful to Prof. B. Ray, BHU, for his help in GPC analysis of dendrimers. S. K. R. thanks CSIR, New Delhi, for SRF.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR spectra of the synthesized compounds, MALDI-TOF of dendrimers, UV-spectra of dendrons and GPC data for dendrimers. See DOI: 10.1039/c6ra09929g

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