Unveiling the Different Emission Behavior of Polytriazoles Constructed

Mar 7, 2018 - State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology...
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Unveiling the Different Emission Behavior of Polytriazoles Constructed from Pyrazine-Based AIE Monomers by Click Polymerization Ming Chen,†,§ Lingzhi Li,† Haiqiang Wu,† Lingxiang Pan,‡ Shiwu Li,‡ Bairong He,‡ Haoke Zhang,†,§ Jing Zhi Sun,† Anjun Qin,*,†,‡ and Ben Zhong Tang*,†,‡,§ †

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China ‡ State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou 510640, China § Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Centre for Tissue Restoration and Reconstruction, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *

ABSTRACT: Polymers with aggregation-induced emission (AIE) characteristics have aroused tremendous interest because of their potential applications in large-area flexible display and luminescent self-assembling, and as stimuliresponsive and porous materials. However, the design of AIE-active polymers is always not as easy as that of small molecules because their properties are hard to predict. In some cases, the polymers prepared from the AIE-active monomers show the aggregation-caused quenching (ACQ) instead of AIE effect. To understand the structure−property relationship of the polymers constructed from the AIE monomers, in this paper, two pyrazine-containing AIE monomers were utilized to construct luminescent polymers by click polymerization. The photophysical property investigation indicates that the polytriazole containing tetraphenylpyrazine units is AIE-active, whereas that bearing 2,3-dicyano-5,6diphenylpyrazine units suffers from the ACQ effect. Through systematical investigation, the cause for such difference was unveiled. Thus, this work provides a useful guidance for further design of AIE-active polymers. KEYWORDS: aggregation-induced emission, structure−property relationship, tetraphenylpyrazine, polytriazole, click polymerization

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INTRODUCTION

during the operation, which in turn discounts the performance of devices. At the same time, scientists have devoted great efforts on overcoming the difficulty via chemical approaches. For example, the polymers with topological structures instead of the linear ones were designed and synthesized to suppress the close-packing of fluorophoric units in the solid state.3 In addition, bulky groups like polyhedral oligomeric silsesquioxane (POSS) were also incorporated into the polymers to prevent the π−π stacking of fluorophoric units.4 These strategies have greatly improved the device performance; however, the complicated molecular design and tedious synthetic procedures of polymers have to be involved. Thus, limited success was achieved. It is well-known that aggregation of a fluorophore is a natural process. It will be nice if the aggregation plays a positive instead of negative role in enhancing the emission of a fluorophore.

In comparison to low-mass organic molecules, polymeric materials possess unique viscoelasticity, which makes them easy to fabricate into thin-solid films and use as the flexible electronics or functional materials with versatile properties.1 Over the last decades, a wide variety of conjugated polymers, such as poly(9,9-dialkylfluorene)s (PFOs), poly(p-phenylenevinylene)s (PPVs), poly(p-phenylene) (PPPs), and polyacetylenes (PAs), have been developed, which boost the rapid development of polymeric light-emitting diodes (PLEDs), etc.2 Nevertheless, the weary but burning issue on the excimer formation in the solid films of these emissive conjugated polymers caused by their planar structures generally decreases their emission efficiency, that is, they are suffering from the aggregation-caused quenching (ACQ) effect. To alleviate this harmful effect, various strategies have been adopted. One straightforward method is to dope the polymer as guest into a host to avoid fluorophoric unit aggregation.1a However, the concentration of these units is hard to accurately control in the technological process. Meanwhile, the phase separation inevitably takes place in the doped films © XXXX American Chemical Society

Special Issue: AIE Materials Received: February 23, 2018 Accepted: February 26, 2018

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DOI: 10.1021/acsami.8b03178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Synthesis of Pyrazine-Containing Polytriazoles by the Cu(I)-Catalyzed Click Polymerization

DCDPP, TPP, and their containing polytriazoles as well as the model compounds, we drew a conclusion that the molecular conformation of the chosen AIE units plays a crucial role in determining the emission behavior of the resultant polymers. The partially planar structures of AIE units in the polymers enable them to form the excimers with ease, which readily quench the emission, while, no such intermolecular interaction was found in nonplanar AIE unit containing polymers, which endows them with AIE activity. Thus, this work provides a general guidance for the design of AIE polymers bearing AIE units in their main-chains.

The aggregation-induced emission (AIE), conceptually termed by us in 2001, could actively use the luminogen aggregation to enhance its emission.5 Exactly opposite to the conventional emitters, the AIE luminogens (AIEgens) are non- or weakly emissive in dilute solutions, but emit efficiently in the aggregate or solid states owing to the restriction of intramolecular motion (RIM), which includes restriction of intramolecular rotation (RIR) and restriction of intramolecular vibration (RIV).6 Under the guidance of RIM, scientists have designed and synthesized tremendous AIEgens with diversified architectures and versatile properties and applied them in the areas of optoelectronic devices, chemo- and biosensors, biological imaging, etc.7 Nevertheless, the AIE polymers are relatively rare, though they possess excellent film-forming ability and synergy interaction. Currently, the predominant strategy for the preparation of AIE polymers is to incorporate AIE moieties as pendants via postfunctionalization or as terminal groups via the radical polymerizations. Meanwhile, they also be prepared by direct polymerization of the monomers containing AIE moieties, such as tetraphenylethene (TPE) and silole as well as their derivatives.8 Unexpectedly, in some cases, the resultant polymers show the ACQ instead of AIE feature. Thus, the structure−property relationship of such polymers should be investigated in detail although such study has been reported for the low mass molecules.9 Our research group has been specializing in the exploitation of new AIEgens and development of their high-tech applications. Besides TPE and silole derivatives, a pyrazine based AIEgen, namely 2,3-dicyano-5,6-diphenylpyrazine (DCDPP), was developed in 2009 under the guidance of the RIR.10 Very recently, by replacing cyano groups of DCDPP by the phenyl rings, a new AIE core of tetraphenylpyrazine (TPP) was generated. The TPP possesses lots of charming merits, such as easy synthesis, facile modification, good stability, tunable emission colors, and diversified functionalities.11 Detailed analysis of the structures of DCDPP and TPP suggested that the former adopts a partially planar structure, whereas, the latter shows no such conformation. Such difference makes them ideal candidates to investigate the structure−property relationship of their containing polymers. In this paper, by utilizing DCDPP and TPP-containing diynes, we prepared DCDPP and TPP-containing polytriazoles by the robust and efficient Cu(I)-catalyzed azide−alkyne click polymerization.12 The photophysical property investigation shows that DCDPP-containing polytriazole suffers from the ACQ effect, whereas TPP-containing polymer retains the AIE activity. Through systematical investigation on the photophysical properties of

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RESULTS AND DISCUSSION Polytriazoles PI and PII with weight-average molecular weights (Mw) of 9500 and 16100 were readily prepared in the yields above 98% by the click polymerization of DCDPP and TPPcontaining diynes 1 and 2 and diazide 3 in the presence of organosoluble catalyst of Cu(PPh3)3Br (Scheme 1, Scheme S1, and Table S1). PI and PII are soluble in commonly used organic solvents, such as dichloromethane, chloroform, and tetrahydrofuran (THF), offering them to be processable. The thermal stabilities of PI and PII were investigated by thermogravimetric analysis (TGA) at a heating rate of 10 °C/min under nitrogen (Figure S1). Inherited from the excellent stability of DCDPP and TPP,10,11 the resultant PI and PII also exhibit high Td (temperature for 5% weight loss) of 362 and 389 °C, respectively. The higher Td of the latter is probably due to its higher content of aromatic units, whereas the presence of cyano groups in the former gives rise to higher residual at elevated temperature. The resultant polytriazoles were characterized by spectroscopic techniques, and satisfactory analysis data corresponding to their expected structures were obtained (Figures S2−S13). Herein, the FT-IR and 1H and 13C NMR spectra of PI and their corresponding monomers 1 and 3 are given as examples. The stretching vibrations of ≡C−H and CC in 1 at 3276 and 2107 cm−1 and N3 in 3 at 2098 cm−1 almost disappeared in the IR spectrum of PI after click polymerization, suggestive of the occurrence of the click polymerizaiton. More detailed structural information can be obtained from the NMR analysis. In the 1H NMR spectra, the signal at δ 3.25 and 3.26 assignable to the resonances of ethynyl group of 1 and CH2 group adjacent to the N3 of 3 are weakened remarkably in that of the polymer. Meanwhile, the proton in the phenyl ring at δ 7.50 in 1 was shifted to downfield because of the generated electron-withdrawing triazole rings. Similarly, in the 13C NMR spectra, the ethynyl carbons of 1 resonated at δ 81.1 and 83.1 B

DOI: 10.1021/acsami.8b03178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. PL spectra of (A) PI and (C) PII in THF/water mixtures with different water fraction ( f w). The excitation wavelengths for PI and PII are 366 and 348 nm, respectively. Polymer concentration (repeating unit): 1 × 10−5 M. Plots of relative intensity (I/I0) versus f w of the THF/water mixtures of (B) PI and (D) PII. I0 is the PL intensity in pure THF. Inset: Photographs of (B) PI and (D) PII in THF/water mixtures with f w from 0 to 90% taken under 365 nm via a UV lamp.

from 7.9 to 1.6% from the solution to the thin film states (Table S2). Thus, PI exhibits the typical ACQ effect though its containing DCDPP unit is AIE-active. Careful inspection of PL spectra of PI in THF/water mixtures with different f w, we found there is a ca. 10 nm bathochromic shift from the solution to the aggregate states. As a consequence, the cause of ACQ effect of PI was first ascribed to the formation of excimers, which thereby quenched the emission in the aggregate state. Different from the PL behavior of PI, PII is AIE active. As shown in Figures 1C, D, the THF solution of PII emits weak deep-blue light at 438 nm upon excitation at 348 nm. With addition of poor solvent of water, the PL intensity continually strengthened with the profiles remaining unchanged. The highest intensity of PII was recorded in the THF/water mixture with f w of 90%, which is about 18-fold higher than that in THF. Why do such huge different performance of PI and PII take place when the polymer main chains are subtly changed in their substituted groups? And how to further design the AIE-active polymers through rational choose of the AIE unit? These questions motivated us to unveil these puzzles, which are crucial for providing useful guidance to design and construct polymers with desired properties. We first tried to get answers from the single-crystal structures of DCDPP and TPP, the effective emission units in PI and PII. Through careful study of single crystal of DCDPP

are absent in that of the PI, which collectively indicated that the click polymerization successfully occurred. Similar results were obtained in the structural analysis of PII. After confirming the structures of PI and PII, we studied their photophysical properties. The UV−vis spectra measurement showed that PI and PII exhibit maximum absorption at 366 and 348 nm in THF with ε of 20 600 and 35 200 cm−1L mol−1, respectively (Figure S14). Compared to the DCDPP and TPP units, redder absorption of PI and PII about 28 and 10 nm, respectively, was observed due to the extension of the conjugation. Moreover, the redder absorption of PI than that of PII suggests that intramolecular charge transfer existed in the former because it contains strong electron-withdrawing cyano groups. This conclusion was further confirmed by the fact that PI exhibits more obvious solvatochromism and steeper plot of photoluminescence (PL) intensity versus solvent polarity than those of PII (Figures S15−S17). According to our previous results, DCDPP and TPP are both AIE-active, will the resultant PI and PII be AIE-active? To answer this question, we investigated their emission behaviors. When excited the THF solution of PI at 366 nm, it displays an intense sky-blue emission with a maximum peak at 478 nm (Figures 1A , B). With addition of poor solvent of water into the system, the emission is greadually quenched. Specifically, more than 85% of emission of PI is quenched at f w of 90%. Moreover, the absolute quantum yield (ΦF) sharply decreased C

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Figure 2. Diagram of evolution of photophysical behaviors of pyrazine-based molecules (DCDPP and TPP), model compounds (DCDPP-M and TPP-M) and polytriazoles (PI and PII). Inset: photographs of solution and film (from left to right) of each luminogen taken under irradiation of 365 nm UV light.

(CCDC 695762), we found that it adopts a partially propellerlike conformation: two phenyl rings twist out the plane of pyrazine center, whereas the two cyano groups nearly keep in the same plane (Figure S18A). It is thus reasonable that the pyrazine ring and cyano groups collectively provide a conjugation plane to facilitate the π−π stacking interaction. Indeed, this kind of interaction was found in the crystal of DCDPP with a distance of 3.259 Å (Figure S18B), which will lead to the formation of excimers. From the single crystal of TPP (CCDC 1031716), we can find that it possesses a propeller-like conformation. In solution, its peripheral phenyl rings can rotate vigorously against the central pyrazine ring to dissipate excited state energy, while the rotation can be restricted to turn on the emission in the aggregate state by means of multiple intermolecular C−H···π interactions.11a Moreover, such conformation makes it difficult to pack well in the aggregate and film states, and can effectively prevent the quenching effect caused by the formation of π−π stacking effect. Moreover, the time-resolved PL decay curves revealed that DCDPP and TPP possess excited state lifetime of 1.05 and 1.54 ns in the solutions and 2.25 and 1.23 ns in the film states, respectively (Figures S19 and S20, and Table S2). The longer lived excited state of DCDPP in aggregate state than that of TPP suggests that stronger intermolecular interaction is involved in the molecular packing, which thus gives rise to the formation of a weak excimer as observed in crystal.13 We next investigated the PL spectra of the model compounds of PI and PII, i.e., DCDPP-M and TPP-M, which were both synthesized by the Cu(I)-catalyzed click reaction (Scheme S2 and Figures S21−S23). The PL spectra show that DCDPP-M emits sky-blue light at near 475 nm with the ΦF values of 10.2 and 23.0% in the solution and film states, respectively (Figure 2 and Table S2). The obvious emission of DCDPP-M in THF solution is attributed to the twisted intramolecular charge transfer process, which has been reported before and also been found in

a series of other AIEgens that contain electron-donating and accepting groups.14−16 Meanwhile, the intramolecular motions of phenyl rings (rotation) and cyano groups (vibration) of DCDPP-M were suppressed vigorously in the film state, leading to the enhancement of emission. In contrast, TPP-M behaves a typical AIE feature (Figure 2 and Figure S24). Its THF solution is nonemissive with ΦF of only 0.4%. The emission is gradually intensified with addition of water, and the highest emission intensity was recorded in the THF/water mixture with f w of 90%, which is 20-fold higher than that in THF solution. Moreover, the ΦF of 10.0% was obtained in the film state, demonstrating a typical AIE activity. Besides the PL spectra measurement, we also measured the lifetime of the model compounds and polymers. The lifetime values of DCDPP, DCDPP-M and PI in their film states were recorded to be 2.63 and 3.16 ns, respectively (Table S2). The longer lifetime of PI is probably due to the enhanced interaction between the polymer chains, which promotes the formation of excimers and quenches the emission in the aggregate and film states.17 Meanwhile, the lifetime of TPP-M and PII was recorded to be 0.92 and 0.82 ns in their film states (Figure S20 and Table S2). This result implies that the intermolecular interaction is weakened from TPP to TPP-M and PII, which may give an explanation why TPP-M and PII have the similar ΦF (10.0 and 11.7%, respectively) but emit less efficiently than TPP (ΦF = 20.9%) in their film states. Additionally, the concentration-dependent lifetime experiments indicated that the values of PI nearly have no changes in THF with concentraction from 1 × 10−5 to 1 × 10−3 M, but increased subtly from 0.94 to 1.05 ns as the concentration increased from 1 × 10−3 to 1 × 10−2 M. For the solution of PII, the lifetime in 1 × 10−2 M is a little higher than that of 1 × 10−3 M. However, such value is much lower than in diluted solution (1 × 10−5 M). These results indicate that the excimers of PI are formed in higher concentration (Figure S25). D

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Figure 3. (A) Optimized molecular conformation of DCDPP-M obtained by the DFT/B3LYP/6-31G(d,p) basis set, where conjugation plane A and B resemble the wings of butterfly. (B) Proposed π−π stacking models when two butterfly get closer.

Figure 4. Particle size distribution of aggregates of (A) PI and (B) PII suspended in THF/water mixtures with f w of 90 vol %. Polymer concentration (repeating unit): 1 × 10−5 M.

was recorded to be 230 and 133 nm, respectively (Figure 4). These results implied that PI is easy to form larger nanoparticles under the same condition because of its stronger intermolecular interaction. Afterward, we dropped PI and PII in THF and THF/water mixture (f w = 90%) onto the surface of silicon pellets, respectively. After the solvent is volatilized, we analyzed their morphologies by SEM. The results demonstrate that PI and PII from the THF solutions both form uniform spherical particles with diameter of ∼200 nm in a low solvent evaporation rate (Figure 5A, C), whereas the morphology of PI and PII generated from the THF/water mixtures is different from that of THF solutions. The aggregates of PI were formed abruptly without enough time for assembly after addition of water. As a result, a disordered but continuous pattern was observed because of its stronger interaction between polymer chains after solvent evaporation (Figure 5B). In contrast, PII still forms spherical particles with less uniform morphology because of being lack of such interaction (Figure 5D).

Since the model compounds and polytriazoles are constructed from the same AIE units, why do such different PL behaviors happen? We thus performed the theoretical calculation to have a deeper understanding. We optimized the geometrical conformation of DCDPP-M and TPP-M by DFT/B3LYP/ 6-31G(d,p) basis set.14 The results show that the two phenyl rings connected with the pyrazine ring are almost coplanar with the adjacent triazole rings, with their dihedral angles only 0.8 and 1.6°. Therefore, besides the dicyanopyrazine plane, there are another two conjugation planar structures in DCDPP-M, which could be vividly described like a butterfly. The two kinds of conjugation planes can be divided into A and B faces and dominate different areas of the wings of butterfly. With two butterflies “fly” close, multiple intermolecular stacking models, including A−B, A−A, and B−B packing interactions, could facilely occur, which provide a high possibility to form excimers to weaken or quench the emission (Figure 3). However, the calculation results showed that the propeller shape retained in the TPP-M, although the dihedral angles between triazole rings and adjacent phenyl rings are small (Figure S26), which makes the packing between the molecules much difficult and thus preserves the AIE-activity. Finally, we carried out the dynamic light scattering (DLS) and the scanning electron microscope (SEM) experiments to check whether the emission behaviors of polymers are related to their morphology. The DLS results indicated the sizes of particles of PI and PII formed in THF/water mixtures with f w = 90%

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CONCLUSION In conclusion, we prepared two emissive pyrazine-containing polytriazoles PI and PII by the powerful Cu(I)-catalyzed click polymerization of DCDPP and TPP-containing diynes and diazide. Although DCDPP and TPP both are AIE-active, the resultant polytriazoles behave totally different. PI suffers from the ACQ effect, whereas PII is AIE-active. Through systematical E

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Figure 5. SEM images of (A, B) PI and (C, D) PII films fabricated by drop-casting of their (A, C) THF solutions and (B, D) THF/mixtures (f w = 90%). Polymer concentration (repeating unit): 1 × 10−5 M.

Young Professionals and the Innovation and Technology Commission of Hong Kong (ITC−CNERC14S01). A.J.Q. and B.Z.T. thank the support from Guangdong Innovative Research Team Program (201101C0105067115).

investigation of the single crystal structures and packing model of DCDPP and TPP as well as the emission behaviors of model compounds of DCDPP-M and TPP-M, we could draw a conclusion that the ACQ effect of PI is caused by strong packing of the DCDPP unit in its backbones due to their partially planar structure. However, no such interaction was found in PII because of the propeller-shaped structure of TPP, which makes the AIE-activity preserved. Thus, this work provides a general guidance for further construction of AIE-active polymers by direct polymerization of AIE-active monomers.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03178. Synthesis and structure characterization of monomers, model compounds and polymers, TGA curve, UV−vis spectra, part PL spectra, PL decay curves, and tables of polymerization results and optical properties of luminogens (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] or [email protected] (A.J.Q.). *E-mail: [email protected] (B.Z.T.). ORCID

Ming Chen: 0000-0003-4071-6604 Jing Zhi Sun: 0000-0001-5478-5841 Anjun Qin: 0000-0001-7158-1808 Ben Zhong Tang: 0000-0002-0293-964X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21788102, 21525417, and 21490571), the National Program for Support of Top-Notch F

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DOI: 10.1021/acsami.8b03178 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX