Article pubs.acs.org/Macromolecules
Dual-Stimuli-Responsive Fluorescent Supramolecular Polymer Based on a Diselenium-Bridged Pillar[5]arene Dimer and an AIE-Active Tetraphenylethylene Guest Yan Wang,† Ming-Zhe Lv,† Nan Song,† Zeng-Jie Liu,† Chunyu Wang,†,‡ and Ying-Wei Yang*,† †
International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China ‡ State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China S Supporting Information *
ABSTRACT: We report a new strategy to construct a multi-stimuliresponsive fluorescent supramolecular polymer by the strong host−guest interactions between a diselenium bond-bridged pillar[5]arene dimer and an AIE-active tetraphenylethylene (TPE)-containing neutral guest bearing two imidazole terminal binding sites. The resulting supramolecular polymer shows a remarkable fluorescence emission decrease at low concentration. Significantly, the diselenium bonds introduced into the supramolecular polymer serve as redox-responsive building blocks. Upon addition of reductants, the supramolecular polymer depolymerized owing to the cleavage of the covalent diselenium bonds in the system. On the other hand, competitive guests such as adiponitrile, which bind strongly with pillar[5]arenes, could lead the disassembly of the polymer to oligomers without breaking any covalent bonds in the system. These two types of depolymerization approaches can both result in the fluorescence intensity recovery of the system to a certain extent, which will hopefully enrich the methodology toward the construction of smart supramolecular polymeric materials with different properties.
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molecular chemistry.32−38 Among them, the most popular one, i.e., pillar[5]arene, contains five hydroquinone units linked through methylene bridges at the 2- and 5-positions forming a pillar-shaped “hollow” structure.39 Benefiting from the unique characteristics of pillararenes, such as rigid structures, different sizes of cavity, and easy functionalization, a series of pillararenebased supramolecular functional systems40−42 have been reported and applied in various fields of research including nanovalves,43,44 MOFs,45 gas sorption,46 controlled release systems,47−49 sensing and detection,50−54 and biological related applications.55,56 Among them, most reported supramolecular systems were fabricated by using functional single-pillar[n]arene as host. While another important class of pillararene derivatives, that is, bridged pillararene dimers 57,58 or tetramers,59 are rarely reported and applied in supramolecular functional materials, but they already start receiving more and more attention owing to their advantages in constructing supramolecular polymers with distinct functions. Herein, we report an efficient synthetic method to construct new fluorescent and dual-stimuli-responsive supramolecular polymer system based on pillar[5]arene (Scheme 1). A
INTRODUCTION Different from the traditional macromolecular polymers polymerized by the formation of covalent bonds between monomers, supramolecular polymers are formed with the aid of supramolecular host−guest noncovalent interactions, either before1 or after2−10 the polymerization. Supramolecular polymer-based materials often exhibit interesting properties, such as specific recognition,11−13 self-healing,14,15 and stimuli responsiveness.16−23 Although more and more supramolecular polymers with distinctive functions have been designed and constructed in recent years,24 it is still highly demanded to fabricate novel supramolecular polymers with multifunctions for extensive applications. Dynamic bond, with its reversible cleavage and formation at specific conditions, has attracted much attention in the fields of dynamic combinatorial chemistry (DCC)25 and stimuli-responsive supramolecular systems.26 Compared with disulfide bond containing molecules which have been widely studied and applied,27 diselenium bond containing molecules possess faster redox responsiveness.28 Consequently, introducing diselenium bonds to supramolecular functional systems has been a hot topic in the past decades in terms of the fabrication of smart functional materials.29,30 On the other hand, pillar[n]arenes (n = 5−15, pillararenes, or pillarenes for short), first reported by Ogoshi et al. in 2008,31 are a new class of macrocyclic host compounds in supra© XXXX American Chemical Society
Received: May 15, 2017 Revised: July 12, 2017
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in the fabrication of linear supramolecular functional polymer. Diselenium bonds can serve as redox-responsive building blocks on supramolecular polymer skeleton. Then, we designed and synthesized a tetraphenylethylene (TPE) containing neutral guest with an imidazole site at each terminal, namely TPE-(Im)2, as a potential guest linker. Relying on the host− guest interactions of pillar[5]arene cavity and imidazole moiety, TPE-(Im)2 participates in the copolymerization process and endows the resulting linear supramolecular polymer with luminescent property. Such a fluorescent supramolecular polymer system exhibits fluorescent enhancement in response to two different external triggers, that is, redox and competitive binding agent, under two totally different depolymerization mechanisms: (1) external redox stimulus reduces the diselenium bonds, destroying the polymer by cleaving the Se−Se covalent bonds; (2) the introduction of competitive binding agent, which can bind stronger with pillar[5]arene cavity, robs the diselenium-bridged pillar[5]arene dimer off the TPE-(Im)2⊂SeSe-(P5)2 complexation, disassembling the system without the breakage of any covalent bonds. We envision that this strategy may facilitate the evolution of novel fluorescent functional materials.
Scheme 1. Graphical Representation of Diselenium BondBridged Pillar[5]arene Dimer (SeSe-(P5)2), TPE-Based Neutral Guest Linker (TPE-(Im)2), and the StimuliResponsive Supramolecular Fluorescent Polymers
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EXPERIMENTAL SECTION
Materials. All reagents were purchased from Aladdin and Energy Chemical Company and used without further purification. All solvents were obtained from commercial sources and used after drying treatments.
diselenium bond-bridged pillar[5]arene dimer, namely SeSe(P5)2, was synthesized and used as macrocyclic host compound
Scheme 2. Synthetic Routes to SeSe-(P5)2, DMP[5], (mSe)2, TPE-Im, and TPE-(Im)2
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Figure 1. 1H NMR spectra (300 MHz, CDCl3, 298 K) of (a) DMP[5] (2 mM), (c) TPE-Im (2 mM), and (b) their equimolar mixture (2 mM). the temperature was increased to 85 °C for 3 h to obtain P6 in a yield of 25%. At last, TPE-(Im)2 was synthesized through following process: A mixture of NaOH and imidazole was dissolved in dimethylformamide (DMF). After the mixture was heated at 60 °C for 2 h, P6 was added. The mixture was heated for another 2 h at 60 °C. Then brine was added to precipitate the crude product. After extraction and separation procedure, TPE-(Im)2 was eventually obtained in a yield of 92%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.606 (s, 2H), 7.159− 6.877 (m, 18H), 6.612 (d, 4H), 4.039 (t, 4H), 3.895 (t, 4H), 2.075− 1.895 (m, 4H), 1.816−1.636 (m, 4H). 13C NMR (126 MHz, CDCl3) δ 157.23, 144.21, 139.90, 139.39, 137.09, 136.51, 132.61, 131.33, 129.53, 127.67, 126.09, 118.74, 113.46, 77.30, 77.05, 76.79, 66.89, 46.76, 28.11, 26.29. HR-ESI-MS calcd for [TPE-(Im)2 + H]+ C40H40N4O2, 609.3224; found 609.3221.
Instrumentation. 1H or 13C NMR spectra were recorded on a Bruker Avance DMX 300 spectrophotometer or a Bruker Avance DMX 500 spectrophotometer with use of deuterated solvents as the lock and the residual solvent or TMS as the internal reference. 2D diffusion-ordered NMR spectroscopy and 2D nuclear Overhauser effect NMR spectroscopy were recorded on a Bruker Avance DMX 600 spectrophotometer. The fluorescence spectra were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer. High-resolution (HR) mass spectrometry experiments were performed on a Bruker Agilent1290-micrOTOF Q II instrument. Viscosity measurements were carried out in chloroform using a Cannon Ubbelohde semimicrodilution viscometer. All fluorescent photographs were taken on an inverted fluorescence microscope (IX78, Olympus, 50×, NA = 0.5). Synthesis of SeSe-(P5)2. SeSe-(P5)2 was synthesized according to a modified literature procedure.60 First, 4-methoxyphenol and 1,4dibromobutane were mixed in CH3CN under a nitrogen atmosphere, and the mixture was heated at 85 °C overnight under stirring to give 1(4-bromobutoxy)-4-methoxybenzene (P1) in a yield of 90%. Then, monofunctional pillar[5]arene derivative (P2) was synthesized through the co-oligomerization/cyclization of P1 and 1,4-dimethoxybenzene in a yield of 15%. After that, a solution of P2 in THF was added into an aqueous solution of Na2Se2 followed by heating at 50 °C for 10 h. SeSe-(P5)2 was obtained in a yield of 90% after the extraction and separation of the organic phase followed by column chromatography (silica gel, PE:DCM:EA = 15:10:1). 1H NMR (300 MHz, CDCl3, 298 K): δ 6.824−6.690 (m, 20H), 3.854−3.735 (m, 24H), 3.722−3.545 (m, 54H), 2.868 (t, 4H), 1.899−1.690 (m, 8H). 77Se NMR (95 MHz, CDCl3) δ 304.329. Synthesis of TPE-(Im)2. Some intermediates (P5 and P6 in the Supporting Information) for the synthesis of TPE-(Im)2 were prepared by modified literature procedures (see the Supporting Information for details).61 First, bis(4-(4-bromobutoxy)phenyl)methanone (P5) was prepared by substitution reaction between bis(4-hydroxyphenyl)methanone and 1,4-dibromobutane dissolved in EtOH under the condition of 85 °C and stirred overnight. The intermediate product was obtained with a yield of 87%. P6 was synthesized through the McMurry coupling reaction: TiCl4 and Zn were added into THF under ice bath and nitrogen atmosphere to prepare TiCl3 solution. Then P5 and benzophenone were added, and
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RESULTS AND DISCUSSION Host−Guest Interaction Study between SeSe-(P5)2 and TPE-(Im)2. To confirm whether there exists strong interactions between SeSe-(P5)2 and TPE-(Im)2 when mixed equimolarly in organic solvent, we chose DMpillar[5]arene (DMP[5])31 and TPE-Im possessing similar structures with SeSe-(P5)2 and TPE-(Im)2 as model compounds to investigate the host−guest complexation. First, the titration experiments of DMP[5] and TPE-Im were carried out by 1H NMR spectroscopy (Figure 1). Upon the addition of TPE-Im, new peaks (H1′ and H2′) appeared at −0.5 and −1.9 ppm, corresponding to the proton signals of the complexed alkyl chains (H1 and H2) of TPE-Im in pillararene cavity, and meanwhile, there exist uncomplexed H1 and H2 remaining in the same chemical shift positions. These results indicate the exchange rate of complexation process was slow on the NMR time scale,62 which is in consistency with the related literature report.63 The presence of the signals in the chemical shift region of negative ppm indicates there exists strong shielding effect. Besides, the signals of proton Ha broadened and shifted upfield after complexation due to the shielding effect of the electron-rich cavity of pillararenes, whereas the signal of proton C
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tration. In the low concentration range, the slopes approximated unity, demonstrating the presence of cyclic or linear oligomers in dilute solution that was concentration-independent. When the concentration exceeded the critical polymerization concentration (CPC; at approximately 50 mM), a sharp increase in viscosity was observed with the slope of 1.96, indicating the formation of a linear supramolecular polymer with increasing size. 2D diffusion-ordered NMR spectroscopy (DOSY) experiments were further employed to investigate the polymerization in solutions (Figure S30). The measured weight-average diffusion coefficient (D) of TPE-(Im) 2 decreased from 1.11 × 10−9 to 7.08 × 10−10 m2 s−1 with the addition of 1 equiv of SeSe-(P5)2, suggesting that the monomers of SeSe-(P5)2 and TPE-(Im)2 self-assembled into larger polymeric structures gradually. Fluorescence emission experiments were performed to investigate the fluorescent intensity changes during the polymerization. As illustrated in Figure 3, black curve represents the fluorescent intensity of 10 μM TPE-(Im)2 in chloroform. The fluorescence intensity decreased sharply with the addition of SeSe-(P5)2 accompanied by the formation of linear supramolecular polymers. This phenomenon can be explained according to the restriction of intramolecular rotation (RIR) mechanism reported by Tang et al. in previous literature.64 In TPE-(Im)2, four phenyl rings are linked to a central rod through single bonds. In a dilute solution, the four phenyl rings can undergo active intramolecular rotations like four paddles oaring, which serve as a nonradiative transition channel for electrons from the excited states to the ground states. When TPE-(Im)2 gradually combined into supramolecular polymers with SeSe-(P5)2 by host−guest interactions, linear polymer chains’ spontaneous inflections and rotations make intramolecular rotations still active; the two paddles of TPE-(Im)2 are attached to large “loads” and one TPE-(Im)2 unit connected to another to undergo a harder innate rotation. These factors lead to a larger nonradiative relaxation channel from the excited states to the ground state and decreased fluorescence intensity dramatically. To further prove this explanation, mixtures of TPE-Im and SeSe-(P5)2, TPE-(Im)2 and DMP[5], and TPE-(Im)2 and (mSe)2 were
Hb broadened and shifted downfield. Moreover, the NMR titration experiment indicated that the signals of DMP[5] at 3.77 and 3.64 ppm were split into two sets with the addition of TPE-Im: one was the signal for the uncomplexed DMP[5], and the other upfield shifted one was assigned to the resonances of DMP[5] complexed with TPE-Im (Figure S24). The host−guest interactions between DMP[5] and TPE-Im were further proved by 2D nuclear Overhauser effect NMR spectroscopy (NOESY). In Figure S25, Ha and H3 correspond to the proton signal for benzene ring of DMP[5] and imidazole substituent of TPE-Im, respectively. The obvious cross-peaks of Ha and H3 indicated that the proton on the benzene ring of DMP[5] is close to the proton on imidazole of TPE-Im, further suggesting that there exist strong host−guest interactions between pillar[5]arene cavity and the imidazole substituent guest. Formation of Supramolecular Polymers. The formation of supramolecular polymer was first investigated by viscosimetry. As shown in Figure 2, the polymerization of SeSe-(P5)2
Figure 2. Specific viscosity (298 K) of 1:1 mixture of TPE-(Im)2 and SeSe-(P5)2 in CHCl3 versus the monomer concentration.
and TPE-(Im)2 monomers exhibited a noticeable viscosity transition and was characterized by a change in slope in the double-logarithmic plots of specific viscosity versus concen-
Figure 3. Fluorescence emission spectra of (a) the host−guest supramolecular polymer system in CHCl3 at various concentrations of TPE-(Im)2 and SeSe-(P5)2 pairs (λex = 265 nm; λem = 372 nm; slit widths: ex 5 nm; em 5 nm; 25 °C, [TPE-(Im)2] = 10 μM, fixed for all traces; [SeSe-(P5)2] = 0, 2, 4, 6, 8, 10, 20, and 40 μM from top to bottom). (b) Plot of fluorescent intensity at 372 nm versus the molar ratio of TPE-(Im)2 and SeSe-(P5)2. D
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Macromolecules performed as contrast groups (control experiments) to investigate fluorescent intensity changes. Briefly, TPE-(Im)2 and P6 are endowed with typical AIE characteristics (Figure S26) benefiting from the TPE cores, which exhibited fluorescence intensity increase upon increasing the water fraction. The mixtures of TPE-Im and SeSe-(P5)2 and TPE(Im)2 and DMP[5] were all manifest as fluorescence intensity decrease (Figure S28), similar to the result of TPE-(Im)2 with the addition of SeSe-(P5)2. On the other hand, the fluorescent intensity of the mixture of TPE-(Im)2 and (mSe)2 remained almost constant, illustrating that the existence of diselenium dynamic bond has no contribution to the changes of fluorescent intensity. In addition, the concentration effects have been investigated to determine whether the difference in supramolecular polymer concentration results in various degree of fluorescence quenching. Figure 3 and Figure S29 show the fluorescence quenching of supramolecular polymer systems at different fixed concentrations of TPE-(Im)2 ([TPE-(Im)2] = 1, 5, and 10 μM) with various ratios of concentrations of SeSe(P5)2. The fluorescence emission results indicated that no matter how low the concentration is, fluorescence quenching phenomenon occurs as long as TPE-(Im)2 complexed with SeSe-(P5)2 to form linear AABB supramolecular polymers. Interestingly, higher concentration of the resulting supramolecular polymers (referring to the initial fixed concentration of TPE-(Im)2) led to larger degree of fluorescence quenching due to the formation of more stable supramolecular polymer systems at higher host−guest concentrations. Stimuli Responsiveness of Supramolecular Polymer. With the formation of the supramolecular polymer through the host−guest interactions between SeSe-(P5)2 and TPE-(Im)2, many Se−Se bonds were introduced into the polymer chains. The supramolecular polymer is expected to undergo dissociation process in the existence of redox or competitive guests, featuring dual-stimuli responsiveness. Stimuli responsiveness is a vital property for the construction of smart functional materials.65−67 DL-dithiothreitol (DTT), one of the most widely used reductants, was selected to break the diselenium bonds in the above supramolecular polymers. DOSY experiments (Figure S31) showed that the weight-average diffusion coefficient (D) of the supramolecular polymers at concentration of 70 mM was 0.95 × 10−10 m2 s−1. However, the value of D increased remarkably to 1.98 × 10−10 m2 s−1 upon addition of 0.4 equiv of DTT. Moreover, the proton signal of methylene adjacent to Se atom disappeared with the addition of DTT, which suggests the breakage of diselenium bond at the polymer chains accompanying the formation of low-molecular-weight segments. The 77Se NMR spectra have been obtained to determine the chemical environment changes after the addition of DTT. As shown in Figure S32, the only one single peak at ca. 304 ppm of the supramolecular polymer transformed into a double peak upon the addition of DTT, indicating the chemical environments of Se atom changed; that is, the diselenium bonds were broken down in the presence of reductants. Furthermore, fluorescence emission experiments indicated the fluorescence intensity increased after addition of DTT into the supramolecular polymer solutions (Figure 4, blue curve). This phenomenon can be explained that the cleavage of polymer chains and the formation of oligomers result in the innate rotation of TPE-(Im)2 units independent from adjacent (TPE(Im)2)-(SeSe-(P5)2) moieties, which significant reducing the rotational resistance from neighbors.
Figure 4. Fluorescence emission spectra (λex = 265 nm; λem = 372 nm; slit widths: ex 5 nm; em 5 nm; solvent: CHCl3; 25 °C; [TPE-(Im)2] = [SeSe-(P5)2] = 10 μM) of TPE-(Im)2 solution (black curve); supramolecular polymer consisted of 1:1 mixture of SeSe-(P5)2 and TPE-(Im)2 (purple curve); supramolecular polymer upon addition of 1 equiv of DTT (blue curve); supramolecular polymer upon addition of 50 equiv of adiponitrile (red curve). The inner photographs taken on a fluorescence microscope presented the fluorescent intensity of (a) supramolecular polymer in the presence of adiponitrile, (b) supramolecular polymer in the presence of DTT, and (c) supramolecular polymer in the absence of any stimuli.
On the other hand, dissociation of the supramolecular polymer by addition of competitive binding agents was also investigated. Herein, adiponitrile was selected as a representative competitive guest because it possesses strong binding affinity toward single pillar[5]arene to form a 1:1 host−guest complex as reported by Li et al.68 Compared with the addition of reductant as discussed above, the dissociative process does not destroy any covalent bonds, which represents a totally different depolymerization mechanism. Upon addition of adiponitrile into the supramolecular polymer solution, adiponitrile replaced TPE-(Im)2 to bind with SeSe-(P5)2 host in the form of 2:1 complexation to achieve the depolymerization process. Fluorescence experiments revealed a larger fluorescence intensity increase/recovery after addition of adiponitrile into the supramolecular polymer solutions (Figure 4, red curve) compared to the redox activated process (addition of DTT, blue curve), suggesting the release of “unloaded” TPE-(Im)2 and the extensive depolymerization of TPE-(Im)2⊂SeSe-(P5)2 supramolecular polymers. Fluorescent photographs also confirmed the above results. The inner photographs taken on a fluorescent microscope visually presented the difference in fluorescent intensity of the supramolecular polymer in the absence and presence of external stimuli. Compared to the presence of DTT, the fluorescence intensity recovered more remarkably when adiponitrile was introduced into the supramolecular polymer system. In control experiments, the fluorescence intensity both showed negligible changes upon the addition of DTT or adiponitrile into TPE-(Im)2 solution (Figure S33). The supramolecular polymerization via host− guest chemistry and depolymerization either by breaking the covalent bonds or destroying the noncovalent supramolecular interactions can result in the fluorescence intensity recovery of the system to different certain extents, representing a new step E
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toward the construction of smart supramolecular polymeric materials with optical properties.
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CONCLUSION In summary, we have successfully fabricated a linear supramolecular polymer from a diselenium bond-bridged pillar[5]arene dimer (SeSe-(P5)2) and an AIE-active compound with TPE core (TPE-(Im)2) via supramolecular assembly. Such fluorescent supramolecular polymer was endowed with dualstimuli responsiveness to reductant and competitive guest. In the presence of representative reductant or competitive guest, the polymer underwent depolymerization through the breakage of Se−Se covalent bonds on polymer skeleton or the dissociation of the host−guest complexation of SeSe-(P5)2 and TPE-(Im)2, resulting in the increase of solution fluorescent intensity. We envision that this dual-stimuli-responsive supramolecular polymer system based on diselenium-bridged bis(pillar[5]arene)s and emissive TPE guest may serve as a new member of smart functional materials and can be used in bioimaging and controlled optical materials.
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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/acs.macromol.7b01010. Scheme S1 and Figures S1−S33 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (Y.-W.Y.). ORCID
Ming-Zhe Lv: 0000-0001-5920-7434 Ying-Wei Yang: 0000-0001-8839-8161 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51673084, 51473061, and 51673079), the JLU Cultivation Fund for the National Science Fund for Distinguished Young Scholars, and the Fundamental Research Funds for the Central Universities for financial support.
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REFERENCES
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DOI: 10.1021/acs.macromol.7b01010 Macromolecules XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.macromol.7b01010 Macromolecules XXXX, XXX, XXX−XXX
