Using an isolation chromophore to further improve the comprehensive performance of nonlinear optical (NLO) dendrimers† Wenbo Wu, a Qi Huang, a Guohua Xu, b Can Wang, a Cheng Ye, c Jingui Qin a and Zhen Li * a By the combination of divergent and convergent approaches, through a powerful “click chemistry” reaction, a new series of dendrimers (G1-NS-TB to G3-NS-TB) were conveniently prepared with satisfactory yields, in which two types of chromophores with a regular AB structure were used to further improve their comprehensive performance. Due to the presence of an isolation chromophore, its perfect 3D structure and the isolation effect of the interior triazole rings, G3-NS-TB exhibited a very large NLO coefficient with a d 33 value as high as 247 pm V 1 and d 33(N) value of 60 pm V 1 . Introduction Due to their unique architectural and functional features, dendritic macromolecules (i.e., dendrimers and hyperbranched polymers) are poised to make signicant contributions in several areas of physical and biological science and engi- neering. 1,2 In comparison with hyperbranched polymers, dendrimers are defect-free and perfect monodisperse macro- molecules with a regular structure, which could be controlled easily by changing the core, dendron or periphery. 3 Also, their special three dimensional (3D) highly branched structure can bring out some special properties, including nanometer size, multivalent character, the modularity of the assembly, high solubility, and low viscosity, making them competitive candi- dates for applications in a variety of elds, including catalysis, biology and materials science. 1–4 In the eld of material science, the development of organic nonlinear optical (NLO) materials is motivated by their prom- ising performance and cost improvements related to telecom- munications, computing, embedded network sensing, terahertz wave generation and detection, and many other applications. 5 Recently, dendrimers have been extended to be applied into this area 6 to partially solve the major problem that hinders the rapid development of this eld: how to efficiently translate the high mb values of the organic chromophores into large macroscopic NLO activities of the corresponding polymers. Their special 3D spatial separation of the chromophore moieties endows the dendrimers with a favorable site isolation effect, which could minimize the strong intermolecular electrostatic interactions among chromophore moieties with high dipole moment, and thus enhance the macroscopic optical nonlinearities, according to the site isolation principle. 7 Based on the excellent work of pioneering scientists in the NLO eld, as well as our previous work on suitable isolation groups, 8 we also prepared a new series of NLO dendrons G1 to G5 (Schemes S1–S3†) and den- drimers G1-TPA to G3-TPA (Chart S1 and Schemes S4 and S5†), through a “double-stage” method. 9 These dendrons and den- drimers demonstrated that accompanying the increase of the loading density of the chromophore moieties, the tested NLO performance was increased, indicating that the frequently observed asymptotic dependence of EO activity on the number density of chromophore moieties may be overcome through rational design, in accordance with the prediction of Sullivan and co-workers. 10 This also conrmed our idea of using the formed triazole rings as suitable isolation groups to enhance the macroscopic NLO effect. 9a,11 Furthermore, the d 33 values of dendrimer G3-TPA was up to 246 pm V 1 , which was a new record by using simple azo chromophore. Recently, the “click chemistry” reaction, with nearly quantitative yields and simple product isolation, was used to further improve the synthesis of these dendrimers, and achieve success. 12 On the other hand, nearly all the previously used isolation groups were normal groups without any donor–acceptor (D–A) structure, which could decrease the effective concentration of the NLO chromophore moieties in NLO materials. In 2012, we used a chromophore with a lower mb value as the isolation group for another chromophore having a higher mb value, to achieve high poling efficiency as a result of the decreased strong a Department of Chemistry, Wuhan University, Wuhan 430072, China. E-mail: lizhen@ whu.edu.cn; [email protected]; Fax: +86 027 68755363 b Wuhan Institute of Physics and Mathematics, The Chinese Academy of Sciences, Wuhan, 430071, China c Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China † Electronic supplementary information (ESI) available: Some NLO materials and their NLO activities. Characterizations of dendrimers G1-NS-TB to G3-NS-TB. See DOI: 10.1039/c3tc00007a Cite this: J. Mater. Chem. C, 2013, 1, 3226 Received 2nd January 2013 Accepted 21st March 2013 DOI: 10.1039/c3tc00007a www.rsc.org/MaterialsC 3226 | J. Mater. Chem. C, 2013, 1, 3226–3234 This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry C PAPER Downloaded by Cape Breton University on 26/04/2013 09:36:00. Published on 22 March 2013 on http://pubs.rsc.org | doi:10.1039/C3TC00007A View Article Online View Journal | View Issue
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Journal ofMaterials Chemistry C
PAPER
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aDepartment of Chemistry, Wuhan University
whu.edu.cn; [email protected]; Fax: +86bWuhan Institute of Physics and Mathema
Wuhan, 430071, ChinacInstitute of Chemistry, The Chinese Academ
† Electronic supplementary information (their NLO activities. Characterizations ofDOI: 10.1039/c3tc00007a
Cite this: J. Mater. Chem. C, 2013, 1,3226
Received 2nd January 2013Accepted 21st March 2013
DOI: 10.1039/c3tc00007a
www.rsc.org/MaterialsC
3226 | J. Mater. Chem. C, 2013, 1, 32
Using an isolation chromophore to further improve thecomprehensive performance of nonlinear optical (NLO)dendrimers†
By the combination of divergent and convergent approaches, through a powerful “click chemistry”
reaction, a new series of dendrimers (G1-NS-TB to G3-NS-TB) were conveniently prepared with
satisfactory yields, in which two types of chromophores with a regular AB structure were used to further
improve their comprehensive performance. Due to the presence of an isolation chromophore, its perfect
3D structure and the isolation effect of the interior triazole rings, G3-NS-TB exhibited a very large NLO
coefficient with a d33 value as high as 247 pm V�1 and d33(N) value of 60 pm V�1.
Introduction
Due to their unique architectural and functional features,dendritic macromolecules (i.e., dendrimers and hyperbranchedpolymers) are poised to make signicant contributions inseveral areas of physical and biological science and engi-neering.1,2 In comparison with hyperbranched polymers,dendrimers are defect-free and perfect monodisperse macro-molecules with a regular structure, which could be controlledeasily by changing the core, dendron or periphery.3 Also, theirspecial three dimensional (3D) highly branched structure canbring out some special properties, including nanometer size,multivalent character, the modularity of the assembly, highsolubility, and low viscosity, making them competitive candi-dates for applications in a variety of elds, including catalysis,biology and materials science.1–4
In the eld of material science, the development of organicnonlinear optical (NLO) materials is motivated by their prom-ising performance and cost improvements related to telecom-munications, computing, embedded network sensing, terahertzwave generation and detection, and many other applications.5
Recently, dendrimers have been extended to be applied into thisarea6 to partially solve the major problem that hinders the rapiddevelopment of this eld: how to efficiently translate the highmb values of the organic chromophores into large macroscopic
, Wuhan 430072, China. E-mail: lizhen@
027 68755363
tics, The Chinese Academy of Sciences,
y of Sciences, Beijing 100080, China
ESI) available: Some NLO materials anddendrimers G1-NS-TB to G3-NS-TB. See
26–3234
NLO activities of the corresponding polymers. Their special 3Dspatial separation of the chromophore moieties endows thedendrimers with a favorable site isolation effect, which couldminimize the strong intermolecular electrostatic interactionsamong chromophore moieties with high dipole moment, andthus enhance the macroscopic optical nonlinearities, accordingto the site isolation principle.7 Based on the excellent work ofpioneering scientists in the NLO eld, as well as our previouswork on suitable isolation groups,8 we also prepared a newseries of NLO dendrons G1 to G5 (Schemes S1–S3†) and den-drimers G1-TPA to G3-TPA (Chart S1 and Schemes S4 and S5†),through a “double-stage” method.9 These dendrons and den-drimers demonstrated that accompanying the increase of theloading density of the chromophore moieties, the tested NLOperformance was increased, indicating that the frequentlyobserved asymptotic dependence of EO activity on the numberdensity of chromophore moieties may be overcome throughrational design, in accordance with the prediction of Sullivanand co-workers.10 This also conrmed our idea of using theformed triazole rings as suitable isolation groups to enhancethe macroscopic NLO effect.9a,11 Furthermore, the d33 values ofdendrimer G3-TPA was up to 246 pm V�1, which was a newrecord by using simple azo chromophore. Recently, the “clickchemistry” reaction, with nearly quantitative yields and simpleproduct isolation, was used to further improve the synthesis ofthese dendrimers, and achieve success.12
On the other hand, nearly all the previously used isolationgroups were normal groups without any donor–acceptor (D–A)structure, which could decrease the effective concentration ofthe NLO chromophore moieties in NLO materials. In 2012, weused a chromophore with a lower mb value as the isolationgroup for another chromophore having a higher mb value, toachieve high poling efficiency as a result of the decreased strong
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electronic interactions, fortunately, the increased NLO effectwas realized (Scheme S6†).13a Also, the isolation chromophoreshave given good results in NLO hyperbranched polymers(Scheme S7†) and NLO dendrons,13 if the main chromophoremoieties were in the periphery and the two types of chromo-phore moieties had a regular AB structure. Also, the resultsshowed that the utilization of isolation chromophore moietiescould further improve their optical transparency and stability.Thus, this useful method should also work in NLO dendrimerswith a global-like structure, just like G1-TPA to G3-TPA.However, the relative studies are very scarce, despite the factthat we have designed several NLO dendrimers with differenttopological structures (these dendrimers were based on theimproved topological structure9c,12 or Ar–ArF self-assemblyeffect12,14). Since the d33 value of G3-TPA was already very large,we wonder if the utilization of an “isolation chromophore”could lead to a larger NLO coefficient.
Therefore, considering all the above points, we attempted todesign a new series of dendrimers, G1-NS-TB to G3-NS-TB(Chart S2, ESI†), in which NS means the nitro- and sulfonyl-based chromophores with regular AB structure, while TB meansthree branched. Excitingly, these dendrimers exhibited a verylarge NLO effect, i.e. the d33 value of G3-NS-TB was up to 247 pmV�1, and its d33(N) value was as high as 60 pm V�1. Also, both oftheir optical transparency and stability were improved. Herein,we present the syntheses, characterization and properties ofthese new dendrimers in detail.
Scheme 1 The synthesis of the core (G0-6N3-N-TB or G0-6N3-S-TB).
Result and discussionSynthesis
As shown in Schemes 1 and 2 and Table 1, these dendrimerswere conveniently prepared in good yields. Similar to ourprevious examples of G1 to G5 and G1-TPA to G3-TPA, the“double-stage”method, was also rationally designed in order toimprove the synthetic efficiency, in which the cores (Scheme 1)were obtained through a divergent approach, while the end-capped dendrons (Scheme 2) were obtained through a conver-gent approach. The core of the dendrimers was synthesizedthrough two steps: rst, G0-6Cl-N-TB or G0-6Cl-S-TB wasprepared via a “click chemistry” reaction between the threebranched core S2 and the chromophore containing two chlor-oethyl groups and one terminal alkyne group; then, through thesubstitution of the chloride groups by using NaN3 (N3
�) as anucleophilic reagent, G0-6N3-N-TB or G0-6N3-S-TB wasproduced with six azido groups for the further functionalizationby “click chemistry” in the periphery. Meanwhile, the end-cap-ped dendrons (G1-^-NS and G2-^-NS) were also synthesizedvia click chemistry. As shown in Scheme 2, this convergentsynthetic route fully utilized the advantage of the combinationof “click chemistry” and azo coupling reaction, with no need toprotect/deprotect some functional groups or include theconversion from one reactive group to another. Finally, thetarget dendrimers could be obtained conveniently via a “clickchemistry” reaction (Scheme 3). Even the preparation of G3-NS-TB with the large steric effect could proceed completely in about6 hours, as monitored by the FT-IR spectra (the disappearance
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of the peak centered at 2098 cm�1 associated with the azidogroups). Thus, the preparation of these dendrimers G1-NS-TB toG3-NS-TB was convenient, making their possible practicalapplications relatively cheap.
Characterization
The dendrimers were characterized by spectroscopic analysis,and all gave satisfactory data, in accordance with their expectedmolecular structures (see ESI for details†). Fig. S1–S26† showthe NMR spectra of the compounds in this paper. 1H NMRspectroscopy was a useful tool for illustrating the successfulsynthesis of the products by organic chemistry. In the divergentapproach, compared to the 1H NMR spectrum of chromophoreG0-2Cl-N, signals from the phenyl proton in the three branchedcore S2 at about 7.26 ppm as well as signals from the benzylproton at about 5.36 ppm appeared, while the signal of thereactive alkyne group (at 2.01 ppm) disappeared in the 1H NMRspectrum of G0-6Cl-N-TB (Fig. S3†), indicating the successfulsynthesis. In the 1H NMR spectrum of G1-6N3-N-TB (Fig. S5†),the original peak at 3.85 ppm of the six chloromethyl groups inG0-6Cl-N-TB disappeared, with the enlargement of the signal at3.58 ppm, conrming that the reaction of G0-6Cl-N-TB withsodium azide led to the quantitative formation of the desiredazidomethyl group. G0-6Cl-S-TB and G1-6N3-S-TB wereprepared in the same way as G0-6Cl-N-TB and G1-6N3-N-TB,thus their structure could be conrmed similarly. In the
a Determined by GPC in THF on the basis of a polystyrene calibration.b Mc was the calculated molecular weight. c Ref. 9c. d Ref. 9b.
Scheme 3 The synthetic route to dendrimers G1-NS-TB, G2-NS-TB, G3-NS-TB.
Fig. 1 The differences in the GPC results (A) and UV-vis spectra (B, film) ofdendrimers G1-TPA to G3-TPA and G1-NS-TB to G3-NS-TB. Mc was the calcu-lated molecular weight, while Mw was the one tested by GPC.
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convergent approach, the presence and absence of the signal ofthe phenyl proton at the para position to the amino group in thebenzene ring at about 6.7 ppm before (in G2-NS) and aer (inG1-^-NS and G2-^-NS) the azo coupling reactions in their 1HNMR spectra, demonstrated that the corresponding reactionswere successful. On the other hand, G1-NS-TB and G2-NS-TBcould be considered as the products of the reaction betweenG1-^-NS or G2-^-NS and S2. Similar to G0-6Cl-N-TB and G0-6Cl-S-TB, their structures could be conrmed easily. For thehighest generation dendrimer G3-NS-TB, due to its large
3228 | J. Mater. Chem. C, 2013, 1, 3226–3234
molecular weight, the peaks in its 1H NMR spectrum becamebroad, however, the characteristic peaks, such as those about3.1 ppm (–CH2S–), 5.3 ppm (–NCH2–), 6.54 (ArH) and so on, werestill present, indicating the successful preparation of G3-NS-TB.To further conrm the structure of the target dendrimers, theH–H COSY spectra of the three dendrimers (Fig. S23–S25†) andthe C–H COSY spectrum of G3-NS-TB (Fig. S26†) were tested,and the spectra were also in consistent with their structure inScheme 3. On the other hand, in their FT-IR spectra (Fig. S27†),all the dendrimers showed absorption bands associated withthe nitro groups at about 1518 and 1338 cm�1 and the sulfonylgroups at about 1140 cm�1, indicating the successful intro-duction of the two types of azo chromophore moieties into thesedendrimers. Also, the peak from the azido groups (at about 2098cm�1) disappeared in the spectra of the target NLO dendrimers,and a new peak from the carbonyl group appeared at 1718 cm�1.
Analysis of the dendritic growth by MALDI-TOF massmeasurements and gel permeation chromatography (GPC)
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(Table S1†) were performed for the products in each step, asshown in Schemes 1 and 2 and Table 1. MALDI-TOF-MS was agood method to conrm the exact molecular weights of thedendrimers. Fig. S28–S35 in ESI† showed MALDI-TOF-MSspectra of the prepared dendrimers, and all the experimentalresults were in good agreement with the expected molecularweights. The MALDI-TOF-MS spectra of G0-6N3-N-TB and G1-6N3-S-TB did not give clear signals, possibly caused by fragmentions arising due to the fragmentation of the unstable azidegroups during MALDI analysis. Unfortunately, we failed todetect any signals within the appropriate mass range for G2-NS-TB and G3-NS-TB. The possible reason might be that theirlarger molecular weight resulted in very low peak intensity.Additionally, GPC measurements were conducted. All the den-drimers showed narrow peaks in their GPC curves, with poly-dispersity indexes (PDI) less than 1.07, indicating that theproducts possessed monodispersed molecular weights. Here,the Mws of these dendrimers were determined to be 4380, 7540and 12 300, which can be compared to the calculated values of5364, 11 872 and 23 897, respectively. However, it should bepointed out that GPC analysis using linear polystyrenes ascalibration standards, oen underestimates the molecularweights of dendrimers with 3D branched structure and globularshape.15 Thus, if the difference between the tested and truevalues was higher, its hydrodynamic radius should be moredifferent from that of linear polymers, and its topologicalstructure should be much closer to spherical. Here, accompa-nying the increase in the generation of dendrimers, thebranched 3D topology improved, and the difference betweenthe measured and calculated values also increased (Fig. 1A andTable 2). In comparison with G1 to G5, the dendrimers G1-TPAto G3-TPA showed larger differences, indicating that the topo-logical structure became closer to ideal one aer modifying theshape of these series of dendrons to dendrimers.9c Furthermore,compared to G1-TPA to G3-TPA, G1-NS-TB to G3-NS-TBdemonstrated larger differences (Fig. 1A and Table 1), indi-cating that the dendrimers containing isolation chromophoreshould have more perfect topological structures, which wouldfurther affect their poling behavior and optical transparency.
Table 2 Physical and NLO results of dendrimers and polymers
a Glass transition temperature (Tg) of polymers detected by the DSCanalyses under argon at a heating rate of 10 �C min�1. b The bestpoling temperature. c Second harmonic generation (SHG) coefficient.d The nonresonant d33 values calculated by using the approximatetwo-level model. e Order parameter F ¼ 1 � A1/A0, A1 and A0 are theabsorbance of the polymer lm aer and before corona poling,respectively. f The loading density of the effective chromophoremoieties, both containing main chromophore and isolationchromophore.
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This might be another advantage of the isolation chromophoremoieties.
The dendrimers were thermally stable (Fig. S36†), and theirdegradation temperatures (Td) were above 240 �C, and the Tdvalue for G3-NS-TB was even as high as 281 �C. This was a goodresult for NLO materials, since the temperature for the realapplication was usually lower than 200 �C. The glass transitiontemperatures (Tg) of the dendrimers were also investigated byusing differential scanning calorimetry (DSC) (Table 2). Similarto G1 to G5 and G1-TPA to G3-TPA, the growth of the NLOdendrimers also resulted in the increase of the glass transitiontemperature (Tg).
Optical transparency is an important parameter for NLOmaterials. As demonstrated in their UV-vis spectra (Fig. S37–S42and Table S2, ESI†), these dendrimers exhibited apparent blue-shied absorption compared to G1-TPA to G3-TPA (Fig. 1B, themaximum absorption of G3-NS-TB was only 446 nm, while themaximum absorption of G3-TPA was 470 nm), indicating thatthese dendrimers have excellent optical transparency, whichcould be benecial to practical application in photonics elds.This encouraging result should be mainly caused by two things:one is that the dendrimers G1-NS-TB to G3-NS-TB should havemore perfect topological structure as GPC results suggested;another one is that the mb value of the sulfonyl-based chromo-phore was lower than that of the nitro-based one, thus, itsoptical transparency should be better than the nitro-based one,according to the two-level model.
NLO properties
G1-NS-TB to G3-NS-TB exhibited good lm-forming ability, andtheir poled lms were prepared for the evaluation of their NLOactivities. The convenient technique to study the second-orderNLO activity was to investigate the second harmonic generation(SHG) processes characterized by d33, an SHG coefficient. Thetest procedure was similar to that reported previously,8,9,11–13
and from the experimental data, the d33 values were calculatedat the 1064 nm fundamental wavelength (Table 2). Similar to G1to G5 and G1-TPA to G3-TPA, the d33 values increased from G1-NS-TB (195 pm V�1) to G3-NS-TB (247 pm V�1) (Fig. 2A),
Fig. 2 The d33 values (A) and d33(N) values (B) of different dendrimers. The blackline: G1-TPA to G3-TPA; the red line: G1-NS-TB to G3-NS-TB.
Fig. 3 Decay curves of the SHG coefficients of dendrimers G1-NS-TB to G3-NS-TB as a function of the temperature.
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accompanying the increase in generation number and theloading density of the chromophore moieties. This should beascribed to the more perfect 3D structure of the high generationdendrimers and the isolation effect of the exterior benzenemoieties and the interior triazole rings, which could decreasethe interactions among the polar chromophore moieties andenhance the poling efficiency, according to the concept of a“suitable isolation group”.8,9 Once again, this phenomenon alsoindicated that the frequently observed asymptotic dependenceof EO activity on chromophore number density may be over-come through rational design, in accordance with the predic-tion of Sullivan and co-workers.10 Considering that thesedendrimers all consisted of a simple azo chromophore (usually,its d33 value could not be higher than 100 pm V�1), it should bea very exciting result. Furthermore, the highest d33 value for azo-chromophore based NLO materials was 257 pmV�1,14 and thevalue of 247 pmV�1 for G3-NS-TB was very close to that value.
On the other hand, it seemed that the d33 values of G3-NS-TBand G3-TPA were nearly the same, and the isolation chromo-phore did not affect the NLO coefficient. However, this was notthe case. Since the lms of the dendrimers still had someabsorption at the wavelength of 532 nm (the doubled frequencyof the 1064 nm fundamental wavelength), the true or actualNLO properties of these dendrimers should be much smallerthan the tested values, due to the resonant enhancementeffect.16 Thus, according to the two-level model, their true NLOcoefficients (d33(N) values) were calculated, with the results lis-ted in Table 2. As a result of their better optical transparency,the true NLO coefficient, d33(N) values, of these dendrimers weremuch higher than those of G1-TPA to G3-TPA (Fig. 2B). Forexample, the d33(N) value of G1-NS-TB was 45 pm V�1, alreadyhigher than that of G3-TPA (43 pm V�1). And the d33 (N) value ofG3-NS-TB was even as high as 60 pm V�1.
In the thin lms, it is nearly impossible to make all thechromophore moieties point in the same direction upon poling.Thus, to further explore the alignment of the chromophoremoieties in these dendrimers, we measured their orderparameter (F) (Table 2 and Fig. S43–S45†). The trend of the F
values was the same as their NLO coefficients, conrming thatthe higher generation and increasingly spherical shape couldlead to the better alignment of the chromophore moieties underthe poling process, thus, to better NLO performance.
Depoling experiments of dendrimers were conducted, inwhich the real time decays of their SHG signals were monitoredas the poled lms were heated from 35 to 150 �C in air at a rateof 4 �Cmin�1. Fig. 3 displays the decay of the SHG coefficient ofdendrimers as a function of temperature. Accompanying theincrease in generation number, the temperatures for decay werealso increased, indicating that the more perfect 3D macromo-lecular architecture might suppress the relaxation of theordered dipole alignment. Furthermore, the temperature for thedecay of G3-NS-TB was 107 �C, higher than that of G3-TPA(94 �C). Thus, in comparison with our previous case of anotherseries of dendrimers, G1-TPA toG3-TPA, nearly all aspects of theperformance, including NLO effects, optical transparency andstability of NLO coefficients, of G1-NS-TB to G3-NS-TB wereimproved to a large degree, making them promising candidates
3230 | J. Mater. Chem. C, 2013, 1, 3226–3234
for practical NLO applications, thanks to the introduction ofisolation chromophores.
Conclusions
In summary, by the combination of divergent and convergentapproaches, a series of dendrimers (G1-NS-TB, G2-NS-TB andG3-NS-TB), in which two different kinds of chromophoremoieties (the sulfonyl- and nitro-based ones) were arranged inan orderly fashion, were successfully obtained via the powerful“click chemistry” reaction. In comparison with our previouscase of another series of dendrimers (G1-TPA to G3-TPA), nearlyall aspects of the performance, including NLO effects, opticaltransparency and stability of NLO coefficients, were improved toa large degree, thanks to the introduction of isolation chro-mophores. Especially, the NLO effect (d33(N) value) of thehighest generation dendrimer G3-NS-TB, was even up to 60 pmV�1, which should be one of the highest values for a simple azochromophore. Thus, the utilization of an isolation chromo-phore might be a new method to further improve the macro-scopic NLO effects of polymeric materials.
ExperimentalMaterials
Tetrahydrofuran (THF) was dried over and distilled from K–Naalloy under an atmosphere of dry nitrogen. N,N-Dime-thylformamide (DMF) was dried over and distilled from CaH2.The azo chromophore G0-2Cl-N, G0-2Cl-S, G1-N, N,N-bis(2-azi-doethyl)aniline (S3) and diazonium salts S4 and S5 wereprepared as in our previous work.8,9 The dendrons G1-^-NS andG2-^-NS were prepared according to our previous work.13b
1,3,5-Tris(bromomethyl)benzene (S1) and N,N,N,N,N-penta-methyldiethylenetriamine (PMDETA) were purchased from AlfaAesar. All other reagents were used as received.
Instrumentation1H and 13C NMR spectra were measured on a VarianMercury300, Varian Mercury600 or Bruker ARX400 spectrom-eter using tetramethylsilane (TMS; d ¼ 0 ppm) as the internal
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standard. H,H-COSY and C,H-COSY spectra of high generationdendrimers were measured on a 600 MHz Bruker Avance IIINMR Spectrometer with a CryOProbe. The Fourier transforminfrared (FTIR) spectra were recorded on a PerkinElmer-2spectrometer in the region of 3000–400 cm�1. UV-visible spectrawere obtained using a Shimadzu UV-2550 spectrometer. Matrix-assisted laser desorption ionization time-of-ight mass spectrawere measured on a Voyager-DE-STR MALDI-TOF mass spec-trometer (MALDI-TOF MS; ABI, American) equipped with a 337nm nitrogen laser and a 1.2 m linear ight path in positive ionmode. Elemental analyses (EA) were performed using a CAR-LOERBA-1106 micro-elemental analyzer. Gel permeation chro-matography (GPC) was used to determine themolecular weightsof polymers. GPC analysis was performed on a Waters HPLCsystem equipped with a 2690D separation module and a 2410refractive index detector. Polystyrene standards were used ascalibration standards for GPC. THF was used as an eluent, andthe ow rate was 1.0 mL min�1. Thermal analysis was per-formed 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). The thermal transitions ofthe polymers were investigated using a METTLER differentialscanning calorimeter DSC822e under nitrogen at a scanningrate of 10 �C min�1. The thermometer for measurement of themelting point was uncorrected. The thickness of the lms wasmeasured with an Ambios Technology XP-2 prolometer.
Synthesis of core S2
1,3,5-Tris(bromomethyl)benzene (S1) (1.78 g, 5.0 mmol) andNaN3 (1.95 g, 30.0 mmol) were dissolved in 25 mL DMF. Thereaction was stirred for 12 hours at 80 �C, then treated with H2Oand extracted with CHCl3, washed with brine. The organic layerwas dried over anhydrous sodium sulfate. Aer removal of theorganic solvent, the crude product was puried by columnchromatography on silica gel using petroleum ether/chloroform(1/2, v/v) as eluent to afford colourless oil S2 (1.17 g, 95.9%). 1HNMR (300 MHz, CDCl3, 298 K) d (TMS, ppm): 4.38 (s, 6H,–CH2–), 7.24 (s, 3H, ArH). 13C NMR (75 MHz, CDCl3, 298 K) d(TMS, ppm): 53.9, 127.1, 136.6.
Synthesis of dendrimer G1-6Cl-N-TB
Chromophore G0-2Cl-N (494.3 g, 1.1 mmol), S2 (81.1 mg, 0.33mmol), CuSO4$5H2O (10 mol%), NaHCO3 (20 mol%), andascorbic acid (20 mol%) were dissolved in THF (5 mL)/H2O(1 mL) under nitrogen atmosphere in a Schlenk ask. Aer themixture was stirred at 25–30 �C under nitrogen atmosphere for8 hours, the reaction was stopped by addition of water, thenextracted with chloroform, and washed with brine. The organiclayer was dried over anhydrous magnesium sulfate and puriedby column chromatography using ethyl acetate/chloroform (2/1,v/v) as eluent to afford red solid G1-6Cl-N-TB (477.9 mg, 90.1%).IR (KBr), y (cm�1): 1510, 1338 (–NO2).
This journal is ª The Royal Society of Chemistry 2013
Hz, 3H, ArH), 7.8–8.0 (m, 9H, ArH). 13C NMR (75 MHz, CDCl3,298 K), d (TMS, ppm): 21.9, 28.3, 40.2, 53.0, 53.3, 68.7, 109.1,111.6, 116.3, 117.3, 121.3, 126.9, 136.9, 145.0, 146.5, 147.4,148.3, 149.5, 155.1. MS (MALDI-TOF) calcd for C72H75N21O9Cl6 +Na 1641.2; found 1641.1. Anal. calcd for C72H75N21O9Cl6: C,54.35; H, 4.75; N, 18.49. Found: C, 54.55; H, 4.81; N 18.32%.
Synthesis of dendrimer G1-6N3-N-TB
A Schlenk ask was charged with compoundG0-6Cl-N-TB (318.2mg, 0.20 mmol), NaN3 (156 mg, 2.4 mmol) and DMF (4 mL). Thereaction was allowed to stir at 80 �C for 12 h and then thesolution was poured into a lot of water. The precipitate wascollected and washed several times with water and methanoland then dried under vacuum to afford red solid G1-6N3-N-TB(300.0 mg, 92.0%). IR (KBr), y (cm�1): 2098 (–N3), 1511, 1339(–NO2).
The procedure was similar to the synthesis of dendrimer G1-6Cl-N-TB. Chromophore G0-2Cl-S (497.6 g, 1.1 mmol), S2 (81.1mg, 0.33 mmol). The crude product was puried by columnchromatography using pure ethyl acetate as eluent to affordorange solid G1-6Cl-S-TB (447.9 mg, 84.0%). IR (KBr), y (cm�1):1131 (–SO2–).
The procedure was similar to the synthesis of dendrimer G1-6N3-N-TB. Compound G0-6Cl-S-TB (360.0 mg, 0.225 mmol),NaN3 (175.5 mg, 2.7 mmol). G1-6N3-S-TB was obtained as anorange solid (298.2 mg, 80.8%). IR (KBr), y (cm�1): 2097 (–N3),1131 (–SO2–).
111.8, 121.5, 122.8, 125.9, 127.3, 128.9, 126.9, 138.2, 144.3,146.4, 149.8, 155.9. MS (MALDI-TOF) calcd for C72H78N36O6S3 +Na 1673; found 1674. Anal. calcd for C72H78N36O6S3: C, 52.74; H,4.79; N, 30.75. Found: C, 52.46; H, 4.62; N 30.47%.
Synthesis of dendrimer G1-^-NS
Diazonium salt S4 (224.2 mg, 0.675 mmol) and dendrimer G1-N(662.7 mg, 0.450 mmol) were dissolved in DMF (6 mL)/THF(3 mL) at 0 �C. The reaction mixture was stirred for 40 h at 0 �C,then treated with H2O and extracted with CHCl3, washed withbrine. The organic layer was dried over anhydrous sodiumsulfate. Aer removal of the organic solvent, the crude productwas puried by column chromatography on silica gel using pureethyl acetate as eluent to afford red solid G1-^-NS (684.4 mg,89.1%). IR (KBr), y (cm�1): 3289 (C^CH), 1716 (C]O), 1514,1338 (–NO2), 1140 (–SO2–).
The procedure was similar to the synthesis of dendrimer G1-6Cl-N-TB. Chromophore G1-^-NS (510.0 mg, 0.299 mmol), N,N-bis(2-azidoethyl)aniline (S3) (31.4 mg, 0.136 mmol). The crudeproduct was puried by column chromatography using THF/chloroform (1/1, v/v) as eluent to afford red solid G2-NS (339mg,77.6%). IR (KBr), y (cm�1): 1716 (C]O), 1514, 1338 (–NO2), 1139(–SO2–).
The procedure was similar to the synthesis of dendrimerG1-^-NS. Dendrimer G2-NS (250 mg, 0.0686 mmol), diazoniumsalt (S5) (65.6 mg, 0.206 mmol). The crude product was puried
3232 | J. Mater. Chem. C, 2013, 1, 3226–3234
by column chromatography on silica gel using THF–chloroform(2/1) as eluent to afford red solid G2-^-NS (190.1 mg, 71.5%). IR(KBr), y (cm�1): 3289 (C^CH), 1717 (C]O), 1515, 1339 (–NO2),1142 (–SO2–).
A mixture of G0-6N3-S-TB (39.4 mg, 0.024 mmol), G0-^-N (104.3mg, 0.168 mmol), and CuBr (24 mg, 0.168 mmol) were dissolvedin 3.6 mL DMF under nitrogen in a Schlenk ask, thenN,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) (0.0372mL, 0.168 mmol) was added. Aer the mixture was stirred at 25–30 �C for 6 hours, the reaction was stopped by addition of water.The precipitate was puried by repeated precipitation of theCHCl3 solutions into ethyl acetate, and then ltered and washedwith ethyl acetate and methanol, dried in a vacuum at 40 �C to aconstant weight. G1-NS-TB was obtained as a red powder (94.0mg, 73.1%). IR (KBr), y (cm�1): 1716 (C]O), 1514, 1338 (–NO2),1140 (–SO2–).
A mixture of G0-6N3-N-TB (13.0 mg, 0.008 mmol), G1-^-NS(95.5 mg, 0.056 mmol), and CuBr (8 mg, 0.056 mmol) weredissolved in 4 mL DMF under nitrogen in a Schlenk ask, thenN,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) (0.0152mL, 0.056 mmol) was added. Aer the mixture was stirred at 25–30 �C for 6 hours, the reaction was stopped by addition of water.The precipitate was puried by repeated precipitation of THFsolutions into acetone, and then ltered and washed withacetone and methanol, dried in a vacuum at 40 �C to a constantweight. G2-NS-TB was obtained as red powder (62.4 mg, 65.8%).IR (KBr), y (cm�1): 1716 (C]O), 1514, 1339 (–NO2), 1142 (–SO2–).1H NMR (300 MHz, CDCl3, 298 K), d (TMS, ppm): 1.73 (s, br,
This journal is ª The Royal Society of Chemistry 2013
A mixture of G0-6N3–S-TB (5.74 mg, 0.0035 mmol), G2-^-NS(95.0 mg, 0.0245 mmol), and CuBr (3.5 mg, 0.0245 mmol) weredissolved in 4 mL DMF under nitrogen in a Schlenk ask, thenN,N,N,N,N-pentamethyldiethylenetriamine (PMDETA) (0.0067mL, 0.0245 mmol) was added. Aer the mixture was stirred at25–30 �C for 6 hours, the reaction was stopped by addition ofwater. The precipitate was puried by repeated precipitation ofthe THF solutions into acetone, and then ltered and washedwith acetone and methanol, dried in a vacuum at 40 �C to aconstant weight. G3-NS-TB was obtained as red powder (83.2mg, 95.5%). IR (KBr), y (cm�1): 1716 (C]O), 1514, 1339 (–NO2),1140 (–SO2–).
The dendrimers were dissolved in THF (concentration�4 wt%),and the solutions were ltered through syringe lters, and thelms were spin-coated onto indium-tin-oxide (ITO)-coated glasssubstrates, which were cleaned by N,N-dimethylformamide,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 measurement of poled lms was the same as our previouswork on G1 to G5 and G1-TPA to G3-TPA,9 in order to make thecomparison of these dendrimers convenient. The second-orderoptical nonlinearity of the dendrimers was determined by in situsecond harmonic generation (SHG) experiment using a closedtemperature controlled oven with optical windows and threeneedle electrodes. The lms were kept at 45� to the incidentbeam and poled inside the oven, and the SHG intensity wasmonitored simultaneously. Poling conditions were as follows:temperature: different for each polymer (Table 2); voltage: 7.5kV at the needle point; gap distance: 0.8 cm. The SHGmeasurements were carried out with a Nd: YAG laser operatingat a 10 Hz repetition rate and an 8 ns pulse width at 1064 nm. AY-cut quartz crystal served as the reference.
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Acknowledgements
We are grateful to the National Science Foundation of China(no. 21034006), and the National Fundamental Key ResearchProgram (2011CB932702) for nancial support.
Notes and references
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