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Polytriazoles with Aggregation-Induced Emission Characteristics: Synthesis by Click Polymerization and Application as Explosive Chemosensors
Anjun Qin,†,‡ Jacky W. Y. Lam,‡ Li Tang,†
Cathy K. W. Jim,‡ Hui Zhao,† Jingzhi Sun,† and Ben Zhong Tang*,†,‡
Department of Polymer Science and Engineering, Key Laboratory of Macromolecular Synthesis and Functionalization of the Ministry of Education of China, Zhejiang UniVersity, Hangzhou 310027, China, and Department of Chemistry, The Hong Kong UniVersity of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
ReceiVed NoVember 4, 2008 ReVised Manuscript ReceiVed January 19, 2009
The copper-catalyzed 1,3-dipolar cycloaddition of alkynes with azides is a typical example of “click” reaction.1 Since the reaction enjoys the advantages of high efficiency and regiose- lectivity and requires mild reaction conditions and simple purification procedures, it has become a versatile synthetic tool with applicability in diverse areas including bioconjugation and surface modification.2 The click reaction has also been utilized in polymer science with emphasis on the functionalization of preformed polymers through postpolymerization approaches.3
The effort of developing the reaction into a new polymerization technique, however, has met with only limited success. The polymerization reactions of arylene diazides and arylene diynes catalyzed by copper(I) species were sluggish, taking as long as 7-10 days to finish.4 The products often precipitated from the reaction mixtures even at the oligomer stage or became insoluble in common organic solvents after purification, unless very long alkyl chains, such as n-dodecyl groups, were attached to the arylene rings. The prepared polytrizoles were nonluminescent in the solid state, although their dilute solution emitted UV light, suggesting that the polymer emission has been quenched by the aggregate formation. If the polytriazoles are to be utilized as light-emitting materials, this issue must be properly tackled because luminophores are commonly used as solid films in their practical applications.5
We have recently discovered an intriguing phenomenon of aggregation-induced emission (AIE): a series of nonemissive molecules such as tetraphenylethene (TPE) and hexaphenylsilole as well as their derivatives are induced to emit efficiently by aggregate formation. The AIE effect greatly boosts the fluo- rescence quantum yields (ΦF) of the molecules by up to 3 orders of magnitude, turning them from faint luminophores into strong emitters. Thanks to their unique AIE characteristics, the molecules have been found to serve as chemosensors, bioprobes, stimuli-responsive nanomaterials, and active layers in the construction of efficient organic light-emitting diodes.6 Among the AIE luminophores, the TPE system has received much attention because of its facile preparation, ready functionaliza- tion, good photostability, and high photoluminescence (PL) efficiency.7
For practical applications, these low molecular weight lumi- nophores have to be fabricated into solid films by expensive
techniques such as vacuum vapor deposition processes, which are not well suited to the manufacture of large-area, flat-panel devices. One way to surmount this processing disadvantage is to synthesize high molecular weight polymers, which can be readily fabricated into large-area films by simple macroscopic processing techniques such as spin coating and static casting. However, polymers with efficient light emissions in the ag- gregate or solid state are rare because aggregation of the polymer chains commonly quenches light emission.8,9 In this paper, we report a group of new TPE-containing polytriazoles (P3) synthesized from the click polymerization of diyne (1) with diazides (2; Scheme 1). The light emission of the polymers is dramatically enhanced, instead of being quenched, by aggregate formation.
The diyne and diazide monomers, namely 1,4-bis(propargy- loxy)benzene (1), 1,2-bis[4-(azidohexyloxy)phenyl]-1,2-diphe- nylethene (2a), and 1,2-bis[(4-azidomethyl)phenyl]-1,2-diphe- nylethene (2b), were synthesized according to Scheme S1.10
The reactions proceeded smoothly, and the desired monomers were obtained in good yields. The monomer structures were confirmed by spectroscopic analyses (see Supporting Informa- tion for detailed characterization data).10
We attempted to transform the monomers to their polymers by 1,3-dipolar polycycloaddition. We have previously succeeded in the catalyst-free polycycloaddition of bis(aroylacetylene)s with diazides by simple heating.11 No high molecular weight polymers, however, were obtained when 1 and 2 were refluxed in THF for 110 h (Table 1, no. 1). This is due to the low reactivity of diyne 1 because it contains no election-withdrawing groups.11,12 Although the monomers could be polymerized by
* To whom correspondence should be addressed: Ph +852-2358-7375; Fax +852-2358-1594; e-mail email@example.com.
† Zhejiang University. ‡ The Hong Kong University of Science & Technology.
Scheme 1. Synthetic Route to Polytriazoles P3 via Click Polymerization of Diyne 1 with Diazides 2
Table 1. Click Polymerization of Diyne 1 with Diazides 2a
no. monomer polymer catalyst time (h) Mwb PDIb
1 1 + 2a (catalyst-free) 110 1500 1.1 trace 2 1 + 2a CuSO4/SAc 1 insoluble ndd 3 1 + 2a P3a Cu(PPh3)3Br 12 36600 1.7 73.1 4 1 + 2b Cu(PPh3)3Br 12 insoluble ndd 5 1 + 2b P3b Cu(PPh3)3Br 2.5 12100e 2.1e 71.8
a Carried out in THF at 60 °C under nitrogen unless otherwise specified;  )  ) 0.1 M. b Weight-average molecular weight (Mw) and polydispersity index (PDI ) Mw/Mn) estimated by gel permeation chroma- tography (GPC) in THF on the basis of a polystyrene calibration. c Carried out in a THF/water mixture (5:1, v/v); SA ) sodium ascorbate. d nd ) not determined. e Soluble fraction in THF. The polymer P3b is completely soluble in polar solvents such as DMF and DMSO.
1421Macromolecules 2009, 42, 1421-1424
10.1021/ma8024706 CCC: $40.75 2009 American Chemical Society Published on Web 02/04/2009
using CuSO4/sodium ascorbate (SA) in a THF/water mixture under “standard” click reaction conditions,1 the resultant polymer was insoluble. The insolubility is possibly caused by the low solvating power of the aqueous mixture to the resultant polytriazole as well as the formation of polymer complexes due to the coordination of the Cu ions with the nitrogen-rich heteroatom rings.13,14
We then investigated the click polymerization in organic media using an organosoluble catalyst of Cu(PPh3)3Br.
Delightfully, polymerization of 1 and 2a in THF at 60 °C for 12 h produced P3a with a high molecular weight in a high yield (Table 1, no. 3). Under the same reaction conditions, the click polymerization of 1 and 2b gave insoluble P3b, probably due to the higher activity of 2b. Shortening the reaction time helped to solve the problem and enabled us to obtain soluble P3b in a high yield. Thus, Cu(PPh3)3Br in THF works much better than CuSO4/SA in THF/water mixture for the diyne-diazide click polymerization. Moreover, as Cu(PPh3)3Br is less likely to dissociate in the organic media, cross-linking reaction and hence the solubility problem associated with polymer complex forma- tion can be overcome. P3a is soluble in common organic solvents such as THF, chloroform, DMF, etc. Its cousin with shorter methylene spacer (P3b) is partially soluble in THF but can be completely dissolved in polar solvents such as DMF and DMSO.
P3 and P3b were characterized spectroscopically, and both the polymers gave satisfactory analysis data corresponding to their expected molecular structures (see Supporting Information for details).10 Our previous studies have revealed that the 1H NMR spectra of polytriazoles in DMSO-d6 are better resolved than those in CDCl3.
11,14 We thus measured the spectra of P3 in DMSO-d6 (Figures S1 and S2).
10 The resonances of the ethynyl protons of diyne 1 and the methylene protons adjacent to the azido group of diazide 2b occur at δ 3.62 and 4.46, respectively, which almost disappeared in the polymer spectrum, indicative of the high molecular weight of the polymer. A new peak associated with the resonance of the proton of the 1,4- triazole unit appears at δ 8.26. These results testify that 1 has indeed undergone click polymerization with 2 and that its acetylenic triple bonds have been transformed to the triazole rings of P3 by the cycloaddition with the azide groups of 2. The 13C NMR spectra (Figures S3 and S4)10 of the monomers and polymers substantiate the conclusions drawn from the 1H NMR spectra: the alkyne and azide units have been transformed to triazole rings. All the polytriazoles are thermally stable. As can be seen from the thermogravimeteric analysis curves shown in Figure S5,10 P3a and P3b lose 5% of their weights at temperatures of 363 and 343 °C, respectively, indicative of their high resistance to thermolysis.
The TPE unit is AIE active.7 Will the AIE effect be preserved in the polymers containing the TPE moieties? To answer this question, we investigated the PL behaviors of P3a and P3b in the solution and aggregate states. The PL spectrum of a diluted THF solution of P3a is almost a flat line parallel to the abscissa (Figure 1A); that is, the polymer is virtually nonluminescent when molecularly dissolved in its good solvent. However, when large amounts of water (>60 vol %) are added into the solution, strong PL peaks are observed at 480.5 nm under identical measurement conditions. As water is a nonsolvent of P3a, the polymer chains must have aggregated in the solvent mixtures with high water contents. P3a is thus induced to emit by aggregation; in other words, it is AIE active. The similar phenomenon is observed in the P3b system: its dilute solution in DMF is nonemissive, but its aggregates in the DMF/water
mixtures are highly luminescent (Figure S6).10 Less amount of water (∼10 vol %) is needed to cause the aggregate formation of P3b and to activate its AIE process (Figure 1B), owing to the lower solubility of P3b associated with its more hydrophobic and rigid molecular structure.
Thus, even after the TPE units have been knitted together by the triazole units in P3, their peripheral phenyl rings can still actively rotate in the solution state, making the polymers nonemissive. In the aggregate state, the intramolecular rotation is restricted, which blocks the nonradiative channel and turns on the light emission of the polymers.6,16 The aqueous mixtures are macroscopically homogeneous and visually transparent with no precipitates, suggesting that the polymer aggregates sus- pended in the mixture are in nano size.8 This is confirmed by particle size measurements (Figure S7).10 No signals are recorded for the P3 solutions in the pure solvents, proving that the polymers are genuinely dissolved. However, particles with sizes of ∼15-605 nm, with most being in the range of ∼62-241 nm, are detected in the THF/water and DMF/water mixtures of P3a and P3b with various water contents, verifying the formation of nanosized aggregates in the aqueous mixtures.
The changes in the ΦF values of P3 in the aqueous mixtures with different water fractions further confirm the AIE charac- teristics of the polymers. In the pure solvents, both P3a and P3b give negligibly small ΦF values (0.18% and 0.16%, respectively). The ΦF value of P3a remains almost unchanged until ca. 50% of water is added, while P3b starts to aggregate at a water fraction of 10%, as manifested by the sudden rise in the ΦF value in the aqueous mixture due to its poorer solubility (Figure 1B). At the water fraction of 90%, the ΦF values of
Figure 1. (A) PL spectra of P3a in the THF/water mixtures with different water fractions. Polymer concentration: 10 µM; excitation wavelength: 326 nm. (B) Variation in the quantum yields (ΦF) of P3a and P3b with water fraction in the THF/water mixture. The ΦF values were estimated using quinine sulfate in 0.1 N H2SO4 (ΦF ) 54.6%) as standard.
1422 Communications to the Editor Macromolecules, Vol. 42, No. 5, 2009
P3a and P3b reach ∼16.9 and 17.8%, which are about 94- and 110-fold higher than those in the pure solvents, respectively.
The efficient emissions of P3 in the aggregate state prompted us to explore their potential applications as chemosensors. Nitroaromatics such as 2,4-dinitrotoluene (DNT), 2,4,6-trini- trotoluene (TNT), and picric acid (PA; or 2,4,6-trinitrophenol) are warfare explosives, sensitive detection of which has antiter- rorism and homeland-security implications. Because of the commercial unavailability of DNT and TNT, we used PA as a model explosive in this work.17 The nanoaggregates of P3a and P3b in the THF/water and DMF/water mixtures with 90% water contents were used as explosive probes, respectively. The emissions of the nanoaggregates are weakened when PA is added (Figures 2A and S810). The emission quenching is observed at a concentration as low as 0.1 µg/mL or 0.1 ppm. When the concentrations of PA are below 10 and 30 µg/mL, the Stern-Volmer plots of P3a and P3b are linear and give quenching constants of 0.987 × 105 and 1.10 × 105 M-1, respectively (Figures 2B and S810).18 When the PA concentra- tion is further increased, the curves deviate from linearity and bend upward.
To gain an insight into the quenching mechanism, we measured the absorption spectrum of PA. The spectrum overlaps with the PL spectra of the polymer aggregates in wavelength region of ∼385-485 nm (Figure S9).10 This facilitates the energy transfer from the excited states of the polymers to the ground-state of PA, thus resulting in the fluorescence quench- ing.19 The self-absorption in the concentrated PA solutions have probably contributed to the “super-quenching” effect observed
in the high concentration region. In addition to the “wet” measurements, we tested whether the PA detection could also work in the solid state. The polymers were adsorbed onto filter papers.20 The paper films were then dipped into a toluene solution of PA (50 µg/mL) as well as pure toluene, which is a nonsolvent for both P3a and P3b. As shown in the inset of Figure 2B, the film of P3a displays strong PL upon photoex- citation, which becomes nonemissive after dipping into the PA solution. This demonstrates a prototype device using the AIE polymers for detecting explosives in real-world applications.
In summary, we successfully synthesized TPE-containing polytriazoles with high molecular weights in high yields by the Cu(I)-catalyzed click polymerization. The polymers are soluble and thermally stable (Td g 340 °C). Utilizing the novel AIE effect, the polytriazoles are used as chemosensors for the detection of explosives in the aggregate and solid states. This work offers a new synthetic route to functional polymers with “abnormal” but useful optical properties. The possibility of using these AIE-active polymers for other high-tech applications is currently under exploration in our laboratories.
Acknowledgment. This work was partially supported by the National Science Foundation of China (20634020 and 50703033), National Basic Research Program of the Ministry of Science & Technology of China (2009CB623605), and the Research Grants Council of Hong Kong (603008, 602707 and 602706). A.J.Q. thanks the support from the China Postdoctoral Science Foundation (20070420228), and B.Z.T. acknowledges the support from the Cao Guangbiao Foundation of Zhejiang University.
Supporting Information Available: Text describing the experimental procedures and giving the characterization data; 1H and 13C NMR spectra of monomers 1 and 2 and polymers P3a and P3b; TGA thermograms of P3; PL spectra of P3b in the absence and presence of PA in the DMF and DMF/water mixtures; histogram showing the particle size distributions of the polymer aggregates in the THF/water and DMF/water mixtures; and absorption spectrum of PA and emission spectra of P3 in the THF/water and DMF/water mixtures with 90% water fraction. This material is available free of charge via the Internet at http://pubs.acs.org.
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