A series of AB 2 -type second-order nonlinear optical (NLO) polyaryleneethynylenes: using different end- capped spacers with adjustable bulk to achieve high NLO coefficients† Wenbo Wu, a Cheng Ye, b Jingui Qin a and Zhen Li * a In this article, according to the concept of “suitable isolation group” , four new polyaryleneethynylenes containing azo-chromophore moieties derived from the same AB 2 -type monomer were prepared through a typical Sonogashira reaction successfully, in which different isolation groups in different size (from H to carbazole) were introduced to the periphery of the hyperbranched polymers as end-capped moieties via highly efficient “click chemistry” reaction under copper(I) catalysis. Due to the different end- capped groups, these hyperbranched polymers (P1–P4) exhibited different NLO properties accordingly, realizing the adjustment of the properties of hyperbranched polymers through the structural design. Among them, P4, with carbazole as “suitable isolation group” , demonstrated the best performance with the SHG coefficient of 113.8 pm V 1 and the onset temperatures for decays in d 33 value (T onset ) of 132 C. Introduction Due to their unique architectural and functional features, dendritic macromolecules (i.e., dendrimers and hyper- branched polymers) are poised to make signicant contribu- tions in several areas of physical and biological science and engineering. 1 Especially, hyperbranched polymers, with similar properties to dendrimers such as good solubility and low viscosity, have attracted much attention in recent years because of their easy synthetic accessibility. Typically, the one- pot synthesis allows for their production in large quantities and their application on an industrial scale. 2 Thanks to the great efforts of scientists, many different synthetic approaches were explored for the synthesis of hyperbranched polymers, such as the polymerization of AB n -type monomers (when n is 2 or greater), 3 copolymerization of A 2 and B n (when n is not less than 3) 4 and so on. Because A 2 -type and B 3 -type monomers were much more stable and easier to be obtained than AB n - type monomers, most of the hyperbranched polymers reported so far took an “A 2 +B 3 ” approach. However, to construct hyperbranched polymers with the controlled structure more like dendrimers, it might be better to prepare hyperbranched polymers derived from AB 2 -type monomers. Recently, partially based on the excellent work of scientists on “click chemistry” reactions 5 (which have many remarkable features, such as nearly quantitative yields, mild reaction conditions, broad tolerance toward functional groups, low susceptibility to side reactions, and simple product isolation), by modifying the synthetic procedure and adding end-capped groups, we have successfully prepared a series of soluble azo- chromophore-containing AB 2 -type hyperbranched poly- triazoles, which could act as new second-order nonlinear optical (NLO) polymeric materials (one kind of materials with the promise of performance and cost improvements related to telecommunications, computing, embedded network sensing, THz wave generation and detection, and many other applica- tions 6 ), with very exciting results such as enhanced NLO effects. 7 To pursue the relationship between the structure and properties of this AB 2 -type hyperbranched polytriazoles and tackle the challenge in the NLO eld (how to efficiently translate the large mb values of the organic chromophores into high macroscopic NLO activities of polymers), 8 we have intro- duced different sizes of the end-capped groups to the periphery of the hyperbranched polymers as isolation moieties to adjust their solubility, processability, and NLO properties (see Chart S1 in ESI†), according to the concept of “suitable isolation group”. 7b However, due to the high reactivity of azide–alkyne cycloaddition, the AB 2 -type “click chemistry” polymerization was a little difficult to be controlled, and before end-capping, the hyperbranched polymer intermediate HP–N 3 (Chart S2 in ESI†) was not so stable: it would be not soluble in any solvent aer about one week due to the large amount of unreacted azido groups in the periphery. Thus, it is very important to a Department of Chemistry, Wuhan University, Wuhan 430072, China. E-mail: lizhen@ whu.edu.cn; [email protected]; Fax: +86 027 68755363 b Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China † Electronic supplementary information (ESI) available: Some of our previous studies on NLO hyperbranched polymers; NMR, TGA and UV-vis spectra of polymers P1–P4. See DOI: 10.1039/c3py00003f Cite this: Polym. Chem., 2013, 4, 2361 Received 2nd January 2013 Accepted 1st February 2013 DOI: 10.1039/c3py00003f www.rsc.org/polymers This journal is ª The Royal Society of Chemistry 2013 Polym. Chem., 2013, 4, 2361–2370 | 2361 Polymer Chemistry PAPER Downloaded by McMaster University on 18 March 2013 Published on 01 February 2013 on http://pubs.rsc.org | doi:10.1039/C3PY00003F View Article Online View Journal | View Issue
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PolymerChemistry
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aDepartment of Chemistry, Wuhan University
whu.edu.cn; [email protected]; Fax: +86bInstitute of Chemistry, The Chinese Academ
† Electronic supplementary informationstudies on NLO hyperbranched polymepolymers P1–P4. See DOI: 10.1039/c3py00
Cite this: Polym. Chem., 2013, 4, 2361
Received 2nd January 2013Accepted 1st February 2013
DOI: 10.1039/c3py00003f
www.rsc.org/polymers
This journal is ª The Royal Society of
A series of AB2-type second-order nonlinear optical(NLO) polyaryleneethynylenes: using different end-capped spacers with adjustable bulk to achieve highNLO coefficients†
Wenbo Wu,a Cheng Ye,b Jingui Qina and Zhen Li*a
In this article, according to the concept of “suitable isolation group”, four new polyaryleneethynylenes
containing azo-chromophore moieties derived from the same AB2-type monomer were prepared
through a typical Sonogashira reaction successfully, in which different isolation groups in different size
(from H to carbazole) were introduced to the periphery of the hyperbranched polymers as end-capped
moieties via highly efficient “click chemistry” reaction under copper(I) catalysis. Due to the different end-
capped groups, these hyperbranched polymers (P1–P4) exhibited different NLO properties accordingly,
realizing the adjustment of the properties of hyperbranched polymers through the structural design.
Among them, P4, with carbazole as “suitable isolation group”, demonstrated the best performance with
the SHG coefficient of 113.8 pm V�1 and the onset temperatures for decays in d33 value (Tonset) of 132 �C.
Introduction
Due to their unique architectural and functional features,dendritic macromolecules (i.e., dendrimers and hyper-branched polymers) are poised to make signicant contribu-tions in several areas of physical and biological science andengineering.1 Especially, hyperbranched polymers, withsimilar properties to dendrimers such as good solubility andlow viscosity, have attracted much attention in recent yearsbecause of their easy synthetic accessibility. Typically, the one-pot synthesis allows for their production in large quantitiesand their application on an industrial scale.2 Thanks to thegreat efforts of scientists, many different synthetic approacheswere explored for the synthesis of hyperbranched polymers,such as the polymerization of ABn-type monomers (when n is 2or greater),3 copolymerization of A2 and Bn (when n is not lessthan 3)4 and so on. Because A2-type and B3-type monomerswere much more stable and easier to be obtained than ABn-type monomers, most of the hyperbranched polymersreported so far took an “A2 + B3” approach. However, toconstruct hyperbranched polymers with the controlledstructure more like dendrimers, it might be better toprepare hyperbranched polymers derived from AB2-typemonomers.
, Wuhan 430072, China. E-mail: lizhen@
027 68755363
y of Sciences, Beijing 100080, China
(ESI) available: Some of our previousrs; NMR, TGA and UV-vis spectra of003f
Chemistry 2013
Recently, partially based on the excellent work of scientistson “click chemistry” reactions5 (which have many remarkablefeatures, such as nearly quantitative yields, mild reactionconditions, broad tolerance toward functional groups, lowsusceptibility to side reactions, and simple product isolation),by modifying the synthetic procedure and adding end-cappedgroups, we have successfully prepared a series of soluble azo-chromophore-containing AB2-type hyperbranched poly-triazoles, which could act as new second-order nonlinearoptical (NLO) polymeric materials (one kind of materials withthe promise of performance and cost improvements related totelecommunications, computing, embedded network sensing,THz wave generation and detection, and many other applica-tions6), with very exciting results such as enhanced NLOeffects.7 To pursue the relationship between the structure andproperties of this AB2-type hyperbranched polytriazoles andtackle the challenge in the NLO eld (how to efficientlytranslate the large mb values of the organic chromophores intohigh macroscopic NLO activities of polymers),8 we have intro-duced different sizes of the end-capped groups to the peripheryof the hyperbranched polymers as isolation moieties to adjusttheir solubility, processability, and NLO properties (see ChartS1 in ESI†), according to the concept of “suitable isolationgroup”.7b However, due to the high reactivity of azide–alkynecycloaddition, the AB2-type “click chemistry” polymerizationwas a little difficult to be controlled, and before end-capping,the hyperbranched polymer intermediate HP–N3 (Chart S2 inESI†) was not so stable: it would be not soluble in any solventaer about one week due to the large amount of unreactedazido groups in the periphery. Thus, it is very important to
further design a more convenient synthetic route to AB2-typehyperbranched NLO polymers.
On the other hand, in addition to “click chemistry” reac-tions, there is another famous reaction of the terminal alkynegroup, namely the Sonogashira coupling reaction, which was apalladium-catalyzed reaction between terminal alkynes and arylor vinyl halides, rst reported in 1975.9 In comparison with“click chemistry”, this reaction has a much lower chemicalreaction rate, and could be easily controlled through the reac-tion temperature, concentrations, catalyst and so on, during thepolymerization procedure. Thus, a new AB2-type nitro chromo-phore based monomer 3 with two terminal alkynes and one aryliodide group, was designed and synthesized successfully in thiswork. Considering that the triazoles formed during “clickchemistry” polymerization could act as “suitable isolationgroups”10 to enhance the NLO activities in our previous work,7
we introduced a phenyl ring, which has the similar size of tri-azole ring, to the monomer 3, and the phenyl rings could beused as isolation groups in the target hyperbranched polymerssimilar to triazole rings used before. As we know, there were alarge amount of end groups in the periphery of hyperbranchedpolymers, and these groups could affect the properties (i.e.,solubility, processability, chemical reactivity and stability, glasstransition temperature, etc.) of hyperbranched polymers to alarge degree.11 For P1, half of the chromophores and a largeamount of terminal alkynes, which could be further modied bythe powerful “click reaction”, were in the periphery. Accordingto the concept of a “suitable isolation group” (for a givenchromophore moiety and a given linkage position, there shouldbe a suitable isolation group present to boost its microscopic bvalue to possibly increase the macroscopic NLO propertiesefficiently),10 different end-capped spacers with different bulk,which could act as isolation groups for the chromophores in theperiphery of hyperbranched polymers, could affect their NLOproperties to some extent. Therefore, in this paper, to furtherimprove the NLO activities as well as other optical and physicalproperties, we would like to use azido group containing isola-tion spacers with different sizes to react with the terminalalkynes in the periphery of P1 to yield more hyperbranchedpolymers containing different end-capped groups (Scheme 1).The nal obtained four hyperbranched polymers (P1–P4)exhibited different NLO properties, in accordance with thedifferent end-capped groups. Furthermore, carbazole in theperiphery of these hyperbranched polymers could act as a“suitable isolation group”, and led to the best performance ofP4, with the SHG coefficient of 113.8 pm V�1 and the onsettemperatures for decays in d33 value (Tonset) of 132 �C. Herein,we would like to present the syntheses, characterization, andproperties of these new hyperbranched polymers in detail.
Results and discussionSynthesis
As mentioned in the introduction part, possibly due to the lackof reasonable synthetic routes, there are few examples con-cerning AB2-type hyperbranched polymers compared to muchmore hyperbranched polymers derived from A2 + B3 type
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monomers in the literature,6,12,13 no matter that their structurewas more like a tree (this point should be benecial to achievehighmacroscopic NLO effects). Even in our successful examplesof AB2-type hyperbranched polytriazoles via “click chemistry”,the monomers containing both azide and alkyne groups have tobe stored in a refrigerator.7 On the contrary, the Sonogashiracoupling reaction could be conducted under the catalysis ofPd(0), such as tetrakis(triphenylphosphine)palladium(Pd(PPh3)4), that means the terminal alkyne group and aryliodide group could be stable in one compound under normalconditions. As shown in Scheme 1, the AB2-type monomer 3containing azo-chromophore in this work could be easilyprepared using an esterication reaction and was very stableunder room temperature.
Different from the copolymerization of A3 and B2 monomers,in which there is the possibility of the formation of insolublegel, the procedure of AB2 polymerization would not form thenetwork structure. Thus, through the typical Sonogashiracoupling reaction, catalyzed by Pd(PPh3)4, PPh3, and CuI,chromophore 3 underwent the AB2 polymerization to afford anew hyperbranched polymer P1 with a large amount of terminalalkynes in the periphery for the further functionalization insatised yield (87.9%). Then, as demonstrated in Scheme 1, P2–P4 were prepared by the “click chemistry” reaction between P1and the azido-containing compounds 4–6, conveniently. All ofthese polymers and monomer were very stable, and could bestored under normal conditions, different from our previousprepared AB2-type NLO polytriazoleHP–N3.7However, the use ofthe mother hyperbranched polymer P1 with terminal alkynes inthe periphery instead of azido groups in HP–N3, caused theproblem of how to determine the end point of the reactionbetween the mother hyperbranched polymer and the azidocompounds, since the absorption peak of terminal alkynegroups in the FT-IR spectrum is not so strong as that of azidoone, and the absorption peak of terminal alkyne could not beobserved in the post-functional reaction. Thus, in the syntheticprocedure for hyperbranched polymers P2–P4, we could onlycontrol the reaction conditions and reaction time, the same asfor our previous cases of the syntheses of HP1–HP6,7b andidentify their structure aer purication. Moreover, it should bepointed out that the size of end-capped groups was adjustedfrom small (H in P1) to larger (carbazole in P4), thus, we couldinvestigate the inuence of different end-capped isolationgroups with different size on the properties of the resultanthyperbranched polymers at the same level, because all thepolymers were derived from the same mother hyperbranchedpolymer P1.
Characterizations
The prepared polymers were characterized by spectroscopicmethods, and all gave satisfactory spectral data (see Experi-mental section and ESI† for detailed analysis data). Asmentioned above, the reaction progress could not be monitoredby the FT-IR spectrum. Luckily, aer the post-processingprocedure, much information could be obtained from the FT-IRspectra of the puried polymers. As shown in Fig. 1, the
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Scheme 1 Synthesis of hyperbranched polymers P1–P4.
Fig. 1 The FT-IR spectra of polymer P1–P4 and their monomer M1.
Fig. 2 The 1H NMR spectra of the AB2-type monomer M1, the mother polymerP1 and the polymer P4 with carbazole as end-capped group.
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absorption bands associated with the nitro groups and carbonylgroups in their FT-IR spectra were at about 1338, 1517 cm�1 and1720 cm�1 respectively, showing the chromophore was stableduring the Sonogashira polymerizations and “click chemistry”end-capped reaction. Also, it was easily seen that the peak at3270 cm�1, derived from the ^C–H stretching vibrations, dis-appeared in P2–P4, indicating that the end-capped reaction wasconducted completely, thanks to the powerful “click chemistry”reaction.
The same information could be also obtained in their NMRspectra. The 1H NMR spectra of the AB2-type monomer 3, the
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mother polymer P1 and P4 with carbazole moieties as end-capped groups, conducted in the solvent of chloroform-d, wereshown in Fig. 2 as an example, while all the NMR spectra are
a Determined by GPC in THF on the basis of a polystyrene calibration.b Glass transition temperature (Tg) of polymers detected by the DSCanalyses under argon at a heating rate of 10 �C min�1. c The 5%weight loss temperature of polymers detected by the TGA analysesunder nitrogen at a heating rate of 10 �C min�1.
Table 2 The maximum absorption of polymers (lmax, nm)a
a The maximum absorption wavelength of these polymer solutions withthe concentrations xed at 0.02 mg mL�1.
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given in Fig. S1–S10 (ESI†). The chemical shis of chromophore3 were consistent with the proposed structure, with their cor-responding protons marked in the structure of Fig. 2. Aerpolymerization, all the peaks showed an inclination of signalbroadening apparently, and the peak around 2.0 ppm
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associated with the protons of C^CH was still present specially,indicating that there were still a large amount of terminalalkynes in the periphery of P1. Thus, these terminal alkynescould be further used to adjust its chemical structure. However,in the 1H NMR spectrum of P4, the peak of C^CH disappeared,coupled with some new peaks associated with the protons ofend-capped groups (for example: the broad peak of –NCH2– ataround d ¼ 4.1), this indicated that all the terminal alkynegroups had already reacted with azido-containing compound toyield isolation groups in the periphery of hyperbranched poly-mers. However, it was a pity that the peak associated with theproton of triazoles was very difficult to differentiate from thebroad peaks of the other ArH of P4, as shown in Fig. 2. On theother hand, it is well known that nearly quantitative yields andlow susceptibility to side reactions are two important features of“click chemistry” reactions,5 meaning that when catalyzed byCu(I), the reaction between azido group and terminal alkynecould only yield a 1,2,4-triazole ring. In this manuscript, theend-capped reaction was conducted under catalysis of CuBr/PMDETA, a famous catalytic system for “click chemistry” reac-tions. Therefore, if we could nd the evidence to conrm thatthe terminal alkyne in the periphery disappeared and the end-capped group was introduced into these polymers successfullyaer the end-capping reaction, the successful “click chemistry”reaction could be conrmed indirectly.7,14 As mentioned above,in the FT-IR spectra and 1H NMR spectra, aer end-cappedthrough the “click chemistry” reaction, the characteristic peaksof terminal alkyne both disappeared, conrming the successfulend-capped “click chemistry reaction”. However, in comparisonwith the 1H NMR spectra of monomer 3, there were some peaksat d ¼ 7.2–7.5 appearing in P1, which was very difficult to beassigned. Thus, two model molecules 7 and 8 (ref. 15) weresynthesized (their synthetic route was shown in Scheme S1 inthe ESI,† and their original NMR spectra were shown inFig. S11–S14, ESI†), and their 1H NMR spectra of ArH part wasshown in Fig. 3. Aer Sonogashira reaction, some peaks with asimilar chemical shi to the new peaks in the spectrum of P1appeared, which could be assigned to the protons of phenyl-ethynyl groups. Thus, the new signals of P1 should be the peaksof the phenylethynyl groups from the Sonogashira reaction,indicating the successful polymerization again.
On the other hand, in the 1H NMR spectrum of P4 (Fig. 2),there are two peaks both assigned to the protons of –NCH2–
from end-capped carbazole groups: one is centered at about4.05, and the other one is centered at about 4.10. This should becaused by their different external environments. Here, the onlypossibility was that one of them was from the linear unit (L),and the other one was from the terminal unit (T). From the 1HNMR spectrum of the model compound 10 (Fig. S15, ESI,† itssynthetic route is shown in Scheme 2), it could be easily seenthat the peaks centered at about 4.10 ppm were from the T unit.Thus, the degree of branching (DB) could be calculated fromthese peaks. Asmentioned above, because of the powerful “clickchemistry” reaction, all the terminal alkyne groups were end-capped, as conrmed by FI-IR and NMR spectra, thus, thereshould be only three structural units in P4: one type of dendriticunit (D), one type of L unit and one type of T unit, and their
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a The best poling temperature. b Film thickness. c Second harmonic generation (SHG) coefficient. d The nonresonant d33 values calculated by usingthe approximate two-level model. e Order parametersF¼ 1� A1/A0, A1 and A0 are the absorbance of the polymer lm aer and before corona poling,respectively. f The loading density of the effective chromophore moieties. g The onset temperature for decay in d33 values.
Fig. 4 (A) Comparison of the d33 values of the polymers; (B) comparison of thecalculated d33 values, which were obtained by using the tested d33 values of thepolymers by dividing the concentrations of the active chromophore moieties ofthe polymers; (C) comparison of the calculated d33(N) values of the polymers,according to the approximate two-level model; (D) comparison of the F values ofthe polymers. All the above used P1 as the reference.
Fig. 5 Decay curves of the SHG coefficients of polymers P1–P4 as a function ofthe temperature.
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chemical structure and characteristic peaks of 1H NMR spectrawere shown in Fig. 2. According to the integration area of thethree resonance peaks marked as 1, 2 and 3 in Fig. 2, as well asthe number of characteristic protons in each units, thefollowing relationships hold for the molar fractions of the threestructural units (fD, fL and fT) could be found:
fL/2fT ¼ 1/1.56
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(fL + 2fT)/(2fL +2fT + 2fD) ¼ (1 + 1.56)/4.82
fL + fT + fD ¼ 1
From the above equations, the molar fractions of these threestructural units could be calculated as 0.415 for fL, 0.324 for fTand 0.261 for fD. Thus, the DB value of P4, could be calculatedas:
DB ¼ 2fD/(2fD + fL) ¼ 0.557
This value was similar to the theoretical value of AB2-typehyperbranched polymer (0.5), as proposed by Frey et al.16 Andthe slightly higher DB value may be caused by the errors of theintegration in the NMR spectra. Since these hyperbranchedpolymers were derived from the samemother polymer, we couldsurmise that all of these hyperbranched polymers demon-strated the same DB values as 0.557, although we could notcalculate the values of P1–P3 from their 1H NMR spectradirectly.
The molecular weights of polymers were determined by gelpermeation chromatography (GPC) with THF as an eluent andpolystyrene standards as calibration standards, which areshown in Table 1 and the experimental section. Here, it must bepointed out that due to their very different hydrodynamic radiusfrom normal linear polymers, the GPC analysis using linearpolystyrenes as calibration standards oen underestimates themolecular weights of hyperbranched polymers, with differencesas big as�40 times being reported in the literature.17 Therefore,the actual molecular weights of these hyperbranched polymersP1–P4 thus should be much higher than those given in Table 1.However, the changing trend in the tested GPC results might beconsistent with the actual molecular weights. Here in thispaper, the weight-average molecular weights (Mw) of P1 weretested as 5100, with polydispersity indexes (PDI) of 1.82. Aerthe end-capping reaction, the molecular weights of P2–P4 werea little higher, withMws lying in the range 6000–7100. Moreover,the increasing trend of the molecular weights was consistentwith the size of the end-capped groups: P2 was the lowest one,while P4 was the highest.
All the polymers were thermally stable, as shown in Fig. S17(ESI†), their degradation temperatures (Td) were all higher than280 �C, with the 5% weight loss temperature of the polymers islisted in Table 1. As they are derived from the same
intermediate P1, their TGA thermograms were nearly the same.The glass transition temperature (Tg) of the polymers were alsoinvestigated and summarized in Table 1. P3 and P4 demon-strated much higher Tg values (up to 155 �C) than P1, con-rming the importance of the introduced peripheral groups.Generally, the high Tgs would lead to better thermal stabilitiesof their NLO effects, which should be an advantage of suitableperipheral groups of hyperbranched polymers.
All of these polymers have good solubility in common polarorganic solvents, such as chloroform, THF,DMF, andDMSO, andtheir solutions could be easily spin-coated into thin solid lms.Therefore, it was convenient to test their NLO and other proper-tiesbasedon thesolutions andthinlms, suchas theirmaximumabsorption wavelength (lmax, which could demonstrate theoptical transparency of NLO materials). To research their activi-ties, their UV-vis absorption spectra in different solvents(Fig. S18–S23†) were tested, and the maximum absorption wave-lengths for the p–p* transition of the azo moieties weresummarized inTable 2.P4with carbazolemoieties as end-cappedgroups showed a little blue-shied maximum absorption wave-length in comparison with P1–P3, both in solutions and lms.This should be attributed to the presence of lots of carbazolemoieties, which connected the primary interface between thehyperbranched polymers and the environment to demonstratebetter site-isolation effect than others. Thus, in this series ofpolymers, the carbazole one might be the suitable isolationgroup, which could be further indicated in their NLO activities.
NLO properties
According to the concept of “suitable isolation groups”,10 theNLO coefficients of these polymers might be quite different. Weprepared their poled thin lms to evaluate their NLO activities.The most convenient technique to study the second-order NLOactivity is to investigate the second harmonic generation (SHG)processes characterized by d33, an SHG coefficient. The methodfor the calculation of the SHG coefficients (d33) for the poledlms has been reported in our previous papers. From theexperimental data, the d33 values of these polymers werecalculated at 1064 nm fundamental wavelength (Table 3). Tocheck the reproducibility, we repeated the measurements threetimes and got the same results.
As expected, these AB2-type hyperbranched polymers P1–P4exhibited different d33 values, while the size of end-cappinggroups enlarged gradually. Similar to the triazole rings in HP1–HP6, the phenyl rings in the interior could act as suitableisolation groups in these hyperbranched polymers, and their d33values were relatively high. Among them, P1 demonstrated thelowest NLO active, with a d33 value of 58.7 pm V�1, since thereweren’t any isolation groups in its periphery. As the size of end-capping groups enlarged gradually, the d33 values of P2–P4 werealso enhanced, and the d33 value P4 was up to 113.8 pm V�1. Tostudy their NLO results more visually, we drew the curve (CurveA, Fig. 4), by using the d33 value of P1 as reference. Meanwhile,considering the effects of different molar concentrations of theactive chromophore moieties in the polymers, we usedthe tested d33 values dividing by the molar concentrations of the
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active chromophores and compared the results again with thoseof P1 as the reference (Curve B, Fig. 4). It was easily seen that thetrends of the two curves were the same, and carbazole moietieswere suitable isolation groups in comparison with other ones,proving our previous idea. On the other hand, the suitableisolation groups here, was also the suitable isolation groups inthe AB2-type hyperbranched polymers HP1–HP6 preparedthrough the “click chemistry”, which have similar chemicalstructures to P1–P4. However, the d33 value of P4 (113.8 pm V�1),was much higher than that of HP4 (89.1 pm V�1), indicatingthat in the interior of these nitro-chromophore based NLOhyperbranched polymers, the phenyl ring might be a moresuitable isolation group than the triazole ring. Coupled with itsmuch easier synthesis, P4 was a promising candidate for prac-tical NLO applications.
As there might be some resonant enhancement due to theabsorption of the chromophore moieties at 532 nm, the NLOproperties of P1–P4 should be smaller. As shown in Table 3, thecorresponding d33(N) values were calculated by using theapproximate two-level model. Also, we drew the curve, stillusing that of P1 as reference (Curve C, Fig. 4), and the trend wassimilar to the previous ones. To further explore the alignment ofthe chromophore moieties in these hyperbranched polymers,the order parameter (F) of the polymers (Table 3) was alsomeasured and calculated from the change of the UV-vis spectraof their thin lms before and aer poling under an electric eld(Fig. S24–S27, ESI†), according to the equation described inTable 3 (footnote 3). The tested F values were in good accor-dance with their d33 values, and carbazole was also conrmed asa suitable group in the periphery of these hyperbranchedpolymers (Curve D, Fig. 4).
The depoling experiments of these hyperbranched polymerswere conducted to investigate the dynamic thermal stabilities ofthe NLO activities of the polymers, in which the real time decaysof their SHG signals were monitored, as the poled lms wereheated from room temperature to 170 �C in air at a rate of 4 �Cmin�1. Fig. 5 displayed the decay of the SHG coefficient of P1–P4 as a function of temperature and the onset temperatures fordecays (Tonset) in their d33 values are also shown in Table 3.Since different end-capped groups in the periphery inuencedtheir Tgs, P1–P4 demonstrated different temporal stabilities,and the Tonset value of P4, which has the largest NLO coefficient,was up to 132 �C. This result showed a great improvement, asthe Tonset value of HP4 was only 109 �C.
Conclusions
In summary, an NLO polyaryleneethynylene P1 was prepared viatypical Sonogashira coupling reaction derived from an AB2-typemonomer. To further improve its NLO coefficient and stability,different isolation groups in different size were introduced to theperiphery as end-capped moieties, via the high efficient “clickchemistry” reaction under copper(I) catalysis, to yield P2–P4,according to the concept of “suitable isolation groups”. The NLOeffects of these polymers were different, in good accordance withthe different size of end-capped groups, and carbazole moietieswere demonstrated to be the suitable isolation groups.
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Furthermore, in comparison with our previous route to synthe-size this type of NLO hyperbranched polymers via “click chem-istry” and the obtained polymerHP4 with the best performance,P4demonstrated some large improvements: easier to synthesize;much larger d33 value and much better temporal stability,making it a good candidate for practical applications.
ExperimentMaterials
Tetrahydrofuran (THF) was dried over and distilled from K–Naalloy under an atmosphere of dry nitrogen. Triethylamine (Et3N)was distilled under normal pressure and kept over potassiumhydroxide. Dichloromethane (CH2Cl2, DCM) was dried overfrom CaH2 and distilled under normal pressure before use. N,N-Dimethylform amide (DMF) was dried over and distilled fromCaH2 under an atmosphere of dry nitrogen. Chromophore 1,azido-containing compounds 4–6 and compound 9 wereprepared as reported in our previous work.18 The model mole-cules 7 and 8 were prepared according to our previous work15
and their synthetic route is shown in Scheme S1.† 4-Iodo-benzoic acid (2) was purchased from Alfa-Aesar. All otherreagents were used as received.
Instrumentation1H and 13C NMR spectra were measured on a VarianMercury300 or Bruker ARX 400 spectrometer using tetrame-thylsilane (TMS; d ¼ 0 ppm) as internal standard. The Fouriertransform infrared (FT-IR) spectra were recorded on a Perki-nElmer-2 spectrometer in the region of 3000–400 cm�1. UV-visible spectra were obtained using a Shimadzu UV-2550 spec-trometer. EI-MS spectra were recorded with a Finnigan PRACEmass spectrometer. Elemental analyses (EA) were performed bya CARLOERBA-1106 microelemental analyzer. Gel permeationchromatography (GPC) was used to determine the molecularweights of polymers. GPC analysis was performed on a WatersHPLC system equipped with a 2690D separation module and a2410 refractive index detector. Polystyrene standards were usedas calibration standards for GPC. THF was used as an eluent,and the ow rate was 1.0 mL min�1. Thermal analysis wasperformed on NETZSCH STA449C thermal analyzer at a heatingrate of 10 �Cmin�1 in nitrogen at a ow rate of 50 cm3min�1 forthermogravimetric analysis (TGA) and the thermal transitionsof the polymers. The thickness of the lms was measured withan Ambios Technology XP-2 prolometer.
Synthesis of the AB2-type monomer chromophore 3
Chromophore 1 (347.6 mg, 0.80 mmol), 4-iodobenzoic acid (2)(396.8 mg, 1.60 mmol), 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide hydrochloride (EDC) (230 mg, 1.20 mmol), andcatalytic amount of 4-(N,N-dimethyl)aminopyridine (DMAP)were dissolved in dry CH2Cl2 (30 mL) and stirred at roomtemperature for 3 h, and then treated with saturated solution ofcitric acid and extracted with CH2Cl2, washed with saturatedsolution of citric acid and brine. Aer the removal of thesolvent, the crude product was puried by column
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chromatography on silica gel using ethyl ethyl acetate–chloro-form (1/20, v/v) as eluent to afford deep red solid (516.2 mg,97.2%). IR (KBr), y (cm�1): 3270 (C^C–H), 1715 (C]O), 1515,1332 (–NO2).
Amixture of AB2-type monomerM1 (456.8 mg, 0.687 mmol), 6.3mg copper iodide (CuI), 18.3 mg tetrakis(triphenylphosphine)palladium (Pd(PPh3)4), 13.3 mg triphenylphosphine (PPh3) wascarefully degassed and charged with argon. THF (18 mL)–Et3N(2 mL) was then added. The reaction was stirred for 3 days atroom temperature. The mixture was passed through a cottonlter and dropped into a lot of methanol. The precipitate wascollected, further puried by several precipitations of its THFsolution into acetone, and dried in a vacuum at 40 �C to aconstant weight. P1 was obtained as deeply red powder (324 mg,87.9%). Mw ¼ 5100, Mw/Mn ¼ 1.82 (GPC, polystyrene calibra-tion). IR (KBr), y (cm�1): 3270 (C^C–H), 1715 (C]O), 1515,1332 (–NO2).
General procedure for the synthesis of polymers P2–P4
A mixture of mother polymer P1 (1.0 equiv., calculated by thestructural units), azido-containing compounds (1.2 equiv.), andCuBr (1.00 equiv.) were dissolved in THF–DMF (2/1 in volume)(0.1 mmol mL�1 –N3 in THF) under nitrogen in a Schlenk ask,and then N,N,N,N,N-pentamethyldiethylenetriamine (PMDETA)(1.00 equiv.) was added. The mixture was stirred at roomtemperature for 12 h and then dropped into a large amount ofmethanol. The obtained polymer was ltered and washed with alot of acetone.
P2. Polymer P1 (64.4 mg), S5 (18.3 mg, 0.144 mmol), P2 wasobtained as a deep red powder (70 mg, 87.9%). Mw ¼ 6000, Mw/Mn ¼ 1.89 (GPC, polystyrene calibration). IR (KBr), y (cm�1):1720 (C]O), 1516, 1337 (–NO2).
Chromophore S3 (396 mg, 1.0 mmol), benzoic acid (183 mg, 1.5mmol), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydro-chloride (EDC) (383 mg, 2.0 mmol), and 4-(N,N-dimethyl)ami-nopyridine (DMAP) (24 mg, 0.20 mmol) were dissolved in 10 mLdry CH2Cl2 and stirred at room temperature for 3 h, and thentreated with saturated solution of citric acid and extracted withCH2Cl2, washed with a saturated solution of citric acid andbrine. Aer the removal of the solvent, the crude product waspuried by column chromatography on silica gel using ethylacetate–chloroform (1/20, v/v) as eluent to afford deep red solid(472 mg, 94.4%). 1H NMR (300 MHz, CDCl3, 298 K), d (TMS,ppm): 1.24 (t, J¼ 6.9 Hz, 3H, –CH3), 1.88 (m, 2H, –CH2–), 2.06 (s,1H, –C^CH), 2.30 (m, 2H, –CH2–), 3.50 (m, 4H, –NCH2–), 4.62(t, J ¼ 5.7 Hz, 2H, –O–CH2), 4.79 (t, J ¼ 5.7 Hz, 2H, –COOCH2–),6.70 (d, J ¼ 8.4 Hz, 2H, ArH), 7.40 (m, 2H, ArH), 7.57 (m, 1H,ArH), 7.71 (d, J¼ 8.7 Hz, 1H, ArH), 7.86 (d, J¼ 9.0 Hz, ArH), 7.92(d, 1H, ArH), 7.99–8.07 (m, 3H, ArH). 13C NMR (75 MHz, CDCl3,298 K), d (TMS, ppm): 12.33, 15.90, 25.96, 45.48, 49.20, 63.10,68.60, 69.42, 83.085, 110.67, 111.13, 117.42, 126.41, 128.30,129.70, 133.04, 144.06, 147.58, 147.74, 151.07, 154.71, 166.37.
Synthesis of model molecule 8
Amixture of compound 7 (106mg, 0.21mmol), iodobenzene (34mg, 0.165 mmol), copper iodide (CuI) (5 mol%), tetrakis-(triphenylphosphine)palladium (Pd(PPh3)4) (3 mol%), and tri-phenylphosphine (PPh3) (5 mol%) was carefully degassed andcharged with argon. THF (9 mL) and Et3N (3 mL) were thenadded. The reaction was stirred for 2 days at room temperature,and then treated with a saturated solution of citric acid andextracted with CH2Cl2, washed with a saturated solution of citricacid and brine. Aer the removal of the solvent, the crudeproduct was puried by column chromatography on silica gelusing ethyl acetate–chloroform (1/35, v/v) as eluent to afford adeep red solid (38 mg, 40.0%). 1H NMR (300 MHz, CDCl3 298 K),
Compound 9 (26.9 mg, 0.050 mmol), compound 6 (32.2 mg,0.11 mmol), CuSO4$5H2O (10 mol%), NaHCO3 (20 mol%), andascorbic acid (20 mol%) were dissolved in THF (2.5 mL)–H2O(0.5 mL) under nitrogen in a Schlenk ask. Aer the mixturewas stirred at 25–30 �C under nitrogen atmosphere for 3 h, thereaction was terminated by the addition of water, then extractedwith chloroform, and washed with brine. The organic layer wasdried over anhydrous magnesium sulfate and puried bycolumn chromatography using ethyl acetate–chloroform (1/1,v/v) as eluent to afford a red solid (48.7 mg, 86.7%). 1H NMR(300 MHz, CDCl3 298 K), d (TMS, ppm): 0.92 (m, 4H, –CH2–),1.26 (m, 16H, –CH3), 1.82 (m, 8H, –CH2–), 2.00 (m, 4H, –CH2–),2.72 (t, J ¼ 7.2 Hz, 4H, –CH2–), 3.46 (s, br, 4H, –NCH2–), 4.1–4.3(m, 8H, –NCH2–), 4.59 (s, br, 2H, –OCH2–), 4.77 (s, br, 2H,–COOCH2–), 6.62 (d, J ¼ 9.0 Hz, 2H, ArH), 7.1–8.1 (m, 28H, ArHand C]CH). 13C NMR (75 MHz, CDCl3, 298 K), d (TMS, ppm):14.0, 22.8, 23.6, 26.2, 26.6, 26.8, 28.7, 30.0, 30.3, 38.6, 42.6, 49.9,50.4, 63.1, 68.0, 68.5, 108.5, 110.6, 111.3, 117.3, 118.7, 120.3,120.6, 122.6, 125.5, 126.3, 128.3, 128.7, 129.6, 130.8, 133.0,140.2, 144.1, 146.8, 147.5, 151.2, 154.7.
Preparation of polymer thin lms
The polymers were dissolved in THF (concentration � 4 wt%),and the solutions were ltered through syringe lters. Polymerlms were spin coated onto indium-tin-oxide (ITO)-coated glasssubstrates, which were cleaned by N,N-dimethylformide (DMF),acetone, distilled water, and THF sequentially in an ultrasonicbath before use. Residual solvent was removed by heating thelms in a vacuum oven at 40 �C.
NLO measurement of poled lms
The second-order optical nonlinearity of the polymers wasdetermined by in situ second harmonic generation (SHG)experiments using a closed temperature-controlled oven withoptical windows and three needle electrodes. The lms werekept at 45� to the incident beam and poled inside the oven, andthe SHG intensity was monitored simultaneously. Polingconditions were as follows: temperature, different for eachpolymer (Table 3); voltage, 7.5 kV at the needle point; gapdistance, 0.8 cm. The SHG measurements were carried out witha Nd:YAG laser operating at a 10 Hz repetition rate and an 8 nspulse width at 1064 nm. A Y-cut quartz crystal served as thereference.
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We are grateful to the National Science Foundation of China(no. 21034006), the National Fundamental Key ResearchProgram (2011CB932702), and academic award for excellentPhD Candidates funded by Ministry of Education of China (no.5052012203002) for nancial support.
Notes and references
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