New Azo-Chromophore-Containing Hyperbranched Polytriazoles

New Azo-Chromophore-Containing Hyperbranched Polytriazoles

(Parte 1 de 3)

New Azo-Chromophore-Containing Hyperbranched Polytriazoles

Derived from AB2 Monomers via Click Chemistry under Copper(I) Catalysis

Zhong’an Li,† Gui Yu,‡ Pan Hu,† Cheng Ye,‡ Yunqi Liu,‡ Jingui Qin,† and Zhen Li*,†

Department of Chemistry, Hubei Key Laboratory on Organic and Polymeric Opto-Electronic Materials, Wuhan UniVersity, Wuhan 430072, China, and Organic Solids Laboratories, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China

ReceiVed NoVember 1, 2008; ReVised Manuscript ReceiVed January 9, 2009

ABSTRACT: By modifying the synthetic procedure, the previous reported impossible approach was successfully utilized to construct new azo-chromophore-containing hyperbranched polymers (HP1 and HP2) from AB2 monomers through click chemistry reactions with the aid of copper(I) catalysis. The two polymers were soluble in organic solvents and well characterized. Thanks to the advantages of the hyperbranched polymeric structure, the two polymers demonstrated good NLO properties, making them promising candidates for the practical applications.

Introduction

In recent years, the “click” chemistry has aroused much interest among researchers because of its remarkable features such as nearly quantitative yields, mild reaction conditions, broad tolerance toward functional groups, low susceptibility to side reactions, and simple product isolation. Especially, the copper-catalyzed Huisgen cycloaddition, also termed as the Sharpless “click” reaction, is the typical example and has been successfully applied to macromolecular chemistry, affording polymericmaterialsvaryingfrom blockcopolymers,dendrimers, to complex macromolecular structures.1-4 However, as to the hyperbranched polymers, the reported examples are still scarce, and most of them took an A2 + B3 approach,5a in which two monomers containing terminal azido and yne groups were needed to construct the hyperbranched polymers through the click chemistry reaction. And it was reported that the Cu(I)- catalyzed click polymerizations of the AB2 monomers failed to yield soluble polymers due to the self-oligomerization.5b

On the other hand, on the basis of the work reported in the literature,6-1 we have tried to explore some approaches to partially solve one of the major problems encountered in optimizing organic second-order nonlinear optical (NLO) materials:12 to efficiently translate the large values of the organic chromophores into high macroscopic NLO activities of polymers, according to the site isolation principle.13 Also, our obtained results demonstrated that the introduction of the NLO chromophoremoietiesto the system of hyperbranchedpolymers would lead to novel NLO polymerswith very large macroscopic NLO effects, partially due to the advantages of hyperbranched polymers, such as the globular structure, the three-dimentional spatial separation of the chromophore moieties.14 And in comparisonwith dendrimers,it was much convenientto prepare NLO hyperbranched polymer with one-pot synthesis. But the synthetic methods we used before were based on the normal

A2 + B3 approach. To construct hyperbranched polymers with the structure more like dendrimers to achieve higher macro- scopic NLO effects, we would like to prepare new NLO hyperbranched polymers from AB2 monomers. Considering the remarkable features of the click chemistry reactions mentioned above, it should be ideal to prepare these new polymers via click chemistry.

Thus, partially based on our previous work on the syntheses of linear polymers with the usage of click chemistry reactions,12b,c,f we successfullyobtained new azo-chromophore- containing hyperbranched polymers HP1 and HP2 from AB2 monomers (Scheme 1) by modifying the synthetic procedure, confirmingthat the self-oligomerizationof AB2 monomerscould be avoided. The two polymers were easily soluble in common organic solvent and well characterized.The tested NLO properties demonstrated that the NLO values were up to 124.4 pm/V; thus, coupled with their convenient synthesis and high thermal stability of the NLO activities, HP1 and HP2 could be good candidates for the practical photonic applications. Herein we would like to present the synthesis and characterization of these new hyperbranched polymers in detail.

Experimental Section

Materials and Instrumentation. Chromophores 4 and 5 were prepared as reported previously.12f All the other reagents were used as received. 1H NMR and 13C NMR spectra were measured on a Varian Mercury300 spectrometer using tetramethylsilane (TMS; δ ) 0 ppm) as internal standard. The Fourier transform infrared (FTIR) spectra were recorded on a PerkinElmer spectrometer-2 in the region of 3000-400 cm-1 on NaCl pellets. UV-vis spectra were obtained using a Schimadzu UV-2550 spectrometer. Gel permeation chromatography (GPC) was used to determine the molecular weights of polymers. GPC analysis was performed on a Waters HPLC system equipped with a 2690D separation module and a 2410 refractive index detector. Polystyrene standards were used as calibration standards for GPC. DMF was used as an eluent, and the flow rate was 1.0 mL/min. Thermal analysis was performed on a NETZSCH STA449C thermal analyzer at a heating rate of 10 °C/min in nitrogen at a flow rate of 50 cm3/min for thermogravimetricanalysis(TGA).The thermaltransitionsof the polymerswere investigated using a METTLER differential scanning calorimeter DSC822e under nitrogen at a scanning rate of 10 °C/min. The thickness of the films was measured with an Ambios Technology XP-2 profilometer.

General Procedure for Synthesis of Chromophores M1 and M2. Diazonium salt 1 or 3 (1.0 equiv) and compound 2 (1.0 equiv) were dissolved in DMF at 0 °C. The reaction mixture was stirred for 12 h at 0 °C, then treated with H2O and extracted with CH2Cl2 (DCM),and washedwith brine.The organiclayer was dried

* Corresponding author: Ph 86-27-62254108; Fax 86-27-68756757; e-mail lizhen@whu.edu.cn

† Wuhan University. ‡ The Chinese Academy of Sciences.

10.1021/ma8025223 C: $40.75 2009 American Chemical Society Published on Web 02/13/2009

over Na2SO4. After removal the organic solvent, the crude product was purified by column chromatography on silica gel using DCM/

ethyl acetate (10/1) as eluent.

Chromophore M1. Diazonium salt 1 (135 mg, 0.41 mmol), compound 2 (95 mg, 0.41 mmol). The product M1 was obtained

Chromophore M2. Diazonium salt 3 (167 mg, 0.52 mmol) and compound 2 (121 mg, 0.52 mmol). The product M2 was obtained as a deep red solid (202 mg, 84.0%). IR (thin film), υ (cm-1): 2090

General Procedure for Synthesis of HP1 and HP2. HP1 and

HP2 were synthesized by the Cu-catalyzed 1,3-dipolar cycloadditions from chromophore M1 and M2, respectively. A typical experimental procedure for the preparation of HP1 is given below as an example. Chromophore M1 (56 mg, 0.12 mmol) was dissolved in DMF

(6 mL) under nitrogen, and then an aqueous solution of CuSO4 (150 µL, 0.04 M) and sodium ascorbate (NaAsc) (150 µL, 0.08

M) was dropped. After the reaction was stirred for 10 h at room temperature, the end-capped chromophore 4 (5 mg, 0.13 mmol) was added; at the same time, another batch of CuSO4 (50 µL, 0.04 M) and NaAsc (50 µL, 0.08 M) was also added. The reaction mixture continued to stir for 9 h. After that, a lot of methanol was poured into the mixture and then filtered. The obtained solid was further purified by reprecipitation from its DMF solution into

(THF, 0.02 mg/mL): λmax (nm): 445. HP2. Chromophore M2 (69 mg, 0.15 mmol), 5 (62 mg, 0.17 mmol). After the reaction was stirred 24 h, the end-capped group 5 was added for the next 9 h reaction.Red powder(101 mg, 80.8%).

155.3. UV-vis (THF, 0.02 mg/mL): λmax (nm): 464. Preparation of Polymer Thin Films. The polymers were dissolved in DMF (concentration ∼3 wt %), and the solutions were filtered through syringe filters. Polymer films were spin-coated onto indium-tin oxide(ITO)-coatedglasssubstrates,whichwere cleaned by N,N-dimethyformide, acetone, distilled water, and THF sequentially in ultrasonic bath before use. Residual solvent was removed by heating the films in a vacuum oven at 40 °C.

NLO Measurement of Poled Films. The second-order optical nonlinearity of the polymers was determined by in situ second harmonicgeneration(SHG) experimentusing a closed temperaturecontrolled oven with optical windows and three needle electrodes. The films were kept at 45° to the incident beam and poled inside the oven, and the SHG intensity was monitored simultaneously. Poling conditions were as follows: temperature: different for each polymer(Table 1); voltage:7.7 kV at the needle point; gap distance: 0.8 cm. The SHG measurements were carried out with a Nd:YAG laser operating at a 10 Hz repetition rate and an 8 ns pulse width at 1064 nm. A Y-cut quartz crystal served as the reference.

Results and Discussion

Synthesis.The syntheticrouteof compounds1-3 was shown in SchemeS1, and the detailedprocedurewas similaras reported in the literature and our previous case.15 Then, under the normal azo coupling reaction conditions, especially at the low reaction temperature (0 °C), monomers M1 and M2 could be easily obtained with satisfied yields. From the structure of M1 and M2, it was easily seen that both of the azido and yne groups were present, and these two reactive groups possessed high reactivity and could react with each other at temperatures a little higher than room temperature. That is to say, if the two monomers, M1 and M2, were prepared through other ap-

Scheme 1

Table 1. Polymerization Results and Characterization Data a Determined by GPC in DMF on the basis of a polystyrene calibration. b The glass transition temperature (Tg) of polymers detected by the DSC analyses under argon at a heating rate of 10 °C/min. c The 5% weight loss temperature of polymers detected by the TGA analyses under nitrogen at a heating rate of 10 °C/min. d The best poling temperature. e Film thickness. f Second harmonic generation (SHG) coefficient. g The nonresonant d33 values calculated by using the approximate two-level model. h Order parameter Φ ) 1 - A1/A0; A1 and A0 are the absorbance of the polymer film after and before corona poling, respectively.

1590 Li et al. Macromolecules, Vol. 42, No. 5, 2009

proaches, perhaps we could not obtain the two monomers due to the high reactivity of azido and alkyne groups. Thus, the usage of the two diazonium salts, 1 and 3, possessed at least two advantages: (1) the followed azo coupling reaction could be conducted in pure organic solvent, avoiding the general used inorganic acids and the possible low solubility of the reaction starting materials in the mixed solvent of organic solvent and water under the normal azo coupling reactions; (2) the high reactivity of the diazonium salts assured the high yields of the azo coupling reaction at low temperature, at which both of the azido and alkyne groups are stable. Thus, the successful preparation of monomers containing both of the azido and alkyne groups might give light on the syntheses of other monomers with different functionalities. Actually, the obtained two monomers,M1 and M2, were relativelystableundernormal storageconditions:at 25 °C, they remainedunchangedfor nearly 1 month. So, for the preparation of other monomers containing both of the azido and alkyne groups, the reaction temperature might be raised to 25 °C, but not only 0 °C used in our case.

The polymerizationprocess of M1 and M2 was handled very carefully. As mentioned in the Introduction, the previous literatures have proposed that the Cu(I)-catalyzed click poly- merizations of the AB2 monomers failed to yield soluble polymers due to the self-oligomerization. After the successful utilization of the Sharpless “click” reaction in the linear polymeric systems,12b,c,f we considered that the “click” reaction might be controlled by adjusting the synthetic procedure. Thus, we attempted to prepare hyperbranched polymers from AB2 monomers through the Sharpless “click” reaction. Also, we thought that after the termination of the polymerization process there should be some unreacted azido groups remained, which could react with the remaining alkyne moieties, directly leading to the final products insoluble during the purification process; even the yielded polymer was soluble at the moment of the termination of polymerization. To avoid this possible crosslinking reaction, we would like to introduce some molecules with similar structure but only containing alkyne groups as monomers to react with the possible unreacted azido ones at the second stage of the polymerization process. This strategy was often used in the preparation of hyperbranched polymers through other reactions, for example, Suzuki reactions,16 though not being applied to the stuff of the click chemistry. Therefore, as described in the Experimental Section, chromophore 4 or 5 was added to the polymerization reaction to react with the unreacted azido moieties. At the same time, another batch of catalysts was added to aid the followed click reaction between the formed reactive hyperbranched polymer intermediate and chromophore 4 or 5, since the previous added catalyst might still coordinate with the azido and yne groups in the formed hyperbranched polymer intermediate. As presented in the Experimental Section, the second added batch of catalysts was not the same as the first one, but much less (1/3). Actually, we have tried to add the same quantity, but no soluble hyperbranched polymer was yielded and only insoluble cross-linked polymer was obtained. We thought that the possible reason might be the too high concentration of catalysts, which further accelerated the polymerization reaction of the reactive hyperbranched polymer intermediate, leading to the cross-linked insoluble polymer. Thus, we added less amounts of catalysts (1/3 of the original one). Fortunately, the click reaction worked well this time, and soluble hyperbranched polymers, HP1 and HP2, were prepared successfully. This was understandable. Under high concentrationof catalysts,not only the click reaction between the formed reactive hyperbranchedpolymer intermediate and chromophore 4 or 5 but also that in the reactive hyperbranched polymer intermediate itself were accelerated rapidly. And the latter one resulted in the unexpected insoluble products. However, while the concentration of catalysts decreased, the catalysts would first coordinate with chromophore 4 or 5 due to relatively higher concentration of chromophore 4 or 5 in comparison with the coordination points in the reactive hyperbranched polymer intermediate. After this, the concentration of catalysts was even lower, which would have little influence on the click reaction of the formed reactive hyperbranched polymer intermediate. Thus, with the decreased amount, the second batch of catalysts would mainly accelerate the click reaction between chromophore 4 or 5 and the formed reactive hyperbranched polymer intermediate to give soluble hyperbranched polymers with no azido groups.

(Parte 1 de 3)

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