Letter pubs.acs.org/macroletters
Photoresponsive Supramolecular Polymer Networks via Hydrogen Bond Assisted Molecular Tweezer/Guest Complexation Zongchun Gao,† Yifei Han,† Shuhan Chen,† Zijian Li,† Huijuan Tong,‡ and Feng Wang*,† †
Key Laboratory of Soft Matter Chemistry, iChEM, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ‡ Department of Chemistry and Materials Engineering, Hefei University, Hefei, Anhui 230601, People’s Republic of China S Supporting Information *
ABSTRACT: Noncovalent molecular recognition exerts significant impact on the structure and functionality of selfassembled materials. In this work, we have developed a novel strategy toward supramolecular polymer networks, with the utilization of molecular tweezer/guest complexation as the cross-linkages. Intermolecular O−H···N hydrogen bond is embedded in bis[alkynylplatinum(II)] terpyridine molecular tweezer/trans-azobenzene recognition motif, which could not only enhance the cross-linking strength, but also endow photoresponsiveness to the supramolecular network assemblies. Moreover, supramolecular polymer networks display intriguing singlet oxygen generation capability due to the inherent incorporation of organoplatinum(II) units. Hence, it offers a new avenue toward supramolecular materials via the combination of π-functionality, processability, and stimuli-responsiveness in an elaborate manner.
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donor−acceptor interactions.13 More interestingly, the pyridine group on 1 could act as the hydrogen bond accepting site. By embedding hydrogen bond donating unit on the arene guests, the binding affinity between molecular tweezer receptor and the complementary guest can be enhanced to a large extent.14 Herein, we sought to develop novel supramolecular polymer networks, by taking advantage of hydrogen−bond assisted molecular tweezer/guest complexation as the noncovalent linkages. Specifically, homoditopic monomer 2 and telechelic homotetratopic poly(ε-caprolactone) (PCL) monomer 3 have been designed (Scheme 1). Their A2/B4-type complexation is primarily on the basis of molecular recognition between molecular tweezer receptor 1 and trans-azobenzene guest 4a (Scheme 1). As compared with the previously reported arene guests such as pyrene and anthracene,13 4a features with ease of synthesis and interesting photoresponsive characters.15 Nevertheless, its relatively small π-surface limits efficient donor− acceptor interactions with the alkynylplatinum(II) terpyridine pincers on 1. On this account, hydroxyl unit is attached on 4a. It is expected to form intermolecular O−H···N hydrogen bond with 1 and thereby maintains sufficient noncovalent binding strength. Stimuli-responsiveness of the resulting supramolecular polymer networks is further envisaged by modulating molecular tweezer/guest complexation strength on the molecular level. For the synthesis of 3 (Scheme S2), the first step involved anionic ring-opening polymerization of ε-caprolactone, with the
upramolecular polymer networks representing the crosslinkages of polymer chains by noncovalent bonds have emerged as a fascinating class of macromolecular materials.1 They can be formed and disrupted by external stimuli and, thereby, display intriguing adaptive and self-healing properties.2 Generally, the macroscopic properties of supramolecular network assemblies rely heavily on the fundamental molecular recognition behaviors. In addition to hydrogen bond and metal−ligand recognition systems, host−guest complexation represents a suitable choice for the construction of supramolecular polymer networks.3 In this regard, a variety of macrocycle-based host−guest motifs have been employed up to now.4−7 However, much less attention has been paid to the utilization of molecular tweezer/guest complexation as the noncovalent cross-linkages.8 It is worthy to note that acyclic molecular tweezer displays enhanced solubility and easier derivatization than the macrocyclic counterparts. Additionally, by modulating the structural parameters of molecular tweezer receptor, various types of guests (such as planar,9 tubular,10 and spherical species11) can be reversibly encapsulated into its cavity by external stimuli. Hence, it potentially serves as an efficient and versatile strategy for the fabrication of supramolecular polymer networks. We are especially interest ed in implement ing organoplatinum(II) species into molecular tweezer receptor, in light of their intriguing photophysical and photochemical properties.12 Due to the square planar geometry and positive charged character of the pincer units, preorganized bis[alkynylplatinum(II)] terpyridine molecular tweezer 1 (Scheme 1) is prone to encapsulate electron-rich arene via electron © XXXX American Chemical Society
Received: March 30, 2017 Accepted: April 26, 2017
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DOI: 10.1021/acsmacrolett.7b00241 ACS Macro Lett. 2017, 6, 541−545
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ACS Macro Letters
Scheme 1. Schematic Representation for the Formation of Supramolecular Polymer Networks via Hydrogen Bond Assisted Molecular Tweezer/Guest Complexation
employment of Sn(Oct)2 and pentaerythritol as the catalyst and initiator, respectively. The star-shaped PCL−OH underwent successive sulfonylation, azidation and copper(I)-catalyzed click reactions to provide the targeted monomer 3. For the final click reaction, its complete conversion can be validated by the vanishing of methylene 1H NMR signals on the intermediate PCL-N3 (3.27 ppm; Figure S1), together with the disappearance of azide stretching vibration band at 2088 cm−1 in FT-IR spectrum (Figure S3). Simultaneously, the triazole resonances emerge at 7.63 ppm. According to GPC experiments (Figure S2), number-average molecular weight (Mn) and dispersity (Đ) values of 3 are determined to be 9.7 kDa and 1.10, respectively. Molecular tweezer/guest recognition was first studied for the model system 1/4a. The noncovalent complexation is fastexchange on the 1H NMR time scale, as reflected by the appearance of only one set of signals. For a 1:1 mixture of 1 and 4a (Figure 1c−e), protons H5 on 1 shift downfield (Δδ = −0.31 ppm). Meanwhile, the terpyridine protons on 1 and aromatic protons on 4a exhibit significant upfield shifts (Δδ = 0.41, 0.36, 0.39, 0.41, 0.78, and 0.30 ppm for H1, H2, H3, H4, Ha, and Hd, respectively). Such phenomena support the presence of donor−acceptor interactions between electrondeficient terpyridine pincers on 1 and electron-rich azobenzene unit on 4a. In contrast, for guest 4b, 1H NMR resonances hardly change upon mixing with 1 (Figure 1a−c), denoting their negligible noncovalent complexation tendency. Accordingly, it suggests the crucial role of hydroxyl unit for strengthened binding affinity of 1/4a than that of 1/4b. Binding thermodynamics for complex 1/4a was further investigated. Specifically, 1H NMR Job’s plot experiment shows 1:1 binding stoichiometry between 1 and 4a (Figure S4−S5). Ka value for complex 1/4a is determined to be (5.89 ± 0.48) × 103 M−1 (Figure 2a), by nonlinear curve-fitting of the collected
Figure 1. Partial 1H NMR spectra (300 MHz, 298 K, CDCl3) of (a) 4b (4.00 mM), (b) 1:1 mixture of 1 and 4b (4.00 mM for each compound), (c) 1 (4.00 mM), (d) 1:1 mixture of 1 and 4a (4.00 mM for each compound), (e) 4a (4.00 mM), (f) 2 (10.0 mM), (g) 2:1 mixture of 2 and 3 (10.0 mM for 2), (h) 3 (5.00 mM). 1
H NMR resonances for H3 (Figure S6 and eq S1). In addition, isothermal titration calorimetry (ITC) measurements were performed, by progressive addition of 4a into the chloroform solution of 1 (Figure 2b). The negative ITC signal supports that noncovalent complexation between 1 and 4a is enthalpydriven. When fitting the exothermic isotherm data with one-site model, Ka value is determined to be (5.34 ± 0.46) × 103 M−1, which is highly consistent with the 1H NMR result. Density functional theory calculations were further performed to elucidate the noncovalent complexation structure of 1/4a (Figure 2c). For the optimized geometry, guest 4a is 542
DOI: 10.1021/acsmacrolett.7b00241 ACS Macro Lett. 2017, 6, 541−545
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ACS Macro Letters
Figure 3. (a) Specific viscosities of 2 (blue triangle), 3 (red circle), and a 1:2 mixture of 2/3 (black square) as a function of monomer concentration (CHCl3, 298 K). (b) Plots of [−ln (At/A0)] vs irradiation time (t) for the photo-oxidation of DMA, by varying the concentration of complex 2/3 as the photocatalyst (black square: 2.00 × 10−5 mol L−1; red circle: 6.70 × 10−6 mol L−1; blue triangle: 2.00 × 10−6 mol L−1).
Figure 2. (a) 1H NMR chemical shift changes of protons H3 upon gradual addition of 4a into 1 (2.00 × 10−3 M). (b) ITC data for the consecutive injecting of 4a (8.00 mM in chloroform) into the chloroform solution of 1 (0.40 mM). (c) Optimized structure of 1/4a via density functional theory calculation.
states upon visible light irradiation. Considering that the functional groups are inherently embedded in supramolecular polymer networks 2/3, we turned to examine their singlet oxygen (1O2) generation capability, via the energy transfer from the excited alkynylplatinum(II) terpyridine units to the surrounding oxygen. In particular, when irradiating the mixture of 2/3 and 9,10-dimethylanthracene (DMA) with an OLED lamp (12 w, 460 nm), the DMA absorbance at 401 nm shows a dramatic decrease (Figure S10), indicating the photo-oxidation of DMA by 1O2 generated in situ. 1O2 production for complex 2/3 is highly concentration-dependent, as evidenced by the rate constant of 0.0155, 0.0118, and 0.00465 s−1 at the monomer concentration of 2.00 × 10−5, 6.70 × 10−6, and 2.00 × 10−6 mol L−1, respectively (Figure 3b). Next, stimuli-responsiveness of supramolecular polymer networks 2/3 was explored, by regulating noncovalent molecular tweezer/guest complexation strength. Azobenzene compound is well-known for reversible trans−cis transformation upon visible light irradiation. We tried to take advantage of this property in the current system. However, it failed to achieve reversible molecular tweezer/guest complexation. Such phenomena should be primarily ascribed to the presence of parahydroxyl unit on azobenzene, which speeds up the transition from cis-form to photostationary trans-form (Figure S11).17 As an alternative way, a photocleavable nitrobenzyl dimethyl ether moiety18 is attached to the hydroxyl unit of 4a. For the resulting compound 6 (Figure 4), deprotection proceeds smoothly upon UV light irradiation (365 nm), as reflected by the decrease of the benzylic 1H NMR resonances at 5.59 ppm (Figure S12). Due to the absence of intermolecular hydrogen bonds, compound 6 is unable to sandwich into the cavity of molecular tweezer 1. Upon UV light irradiation, the 1H NMR spectrum of 1/6 coincides very well with that of 1/4a (Figure S14). Hence, it proves that entity 1/6 undergoes phototriggered transition from “uncomplexed” to “complexed” states. Furthermore, photocleavable homotetratopic monomer 5 (Figure 4) was mixed with two equivalents of 2 in chloroform. On the basis of DOSY measurements, the diffusion coefficient for complex 2/5 is determined to be 1.25 × 10−9 m2 s−1 with the absence of light irradiation (Figure 4). Remarkably, it shows 5-fold decrease (2.51 × 10−10 m2 s−1) upon UV-light irradiation for 200 min. The results illustrate phototriggered size expansion of the supramolecular polymer networks. Moreover, thermal behaviors of the resulting assemblies are also influenced by the
encapsulated in the inner cavity of molecular tweezer receptor 1. The distances between the azobenzene moiety on 4a and the terpyridine pincers on 1 are determined to be 3.37 and 3.38 Å, respectively. Besides, intermolecular O−H···N hydrogen bond forms between the hydroxyl group on 4a and the pyridine unit on 1, as evidenced by the short H···N distance of 1.28 Å, together with the O−H···N angle of 165.4°. The exact role of intermolecular hydrogen bond for noncovalent complexation of 1/4a was further evaluated via solvent-dependent 1H NMR experiments (Figure S7). In particular, the azobenzene protons Ha move downfield (Δδ = −0.41 ppm) upon adding 5% methanol-d4 into the chloroform-d solution of 1/4a. The trend is contrary to that of noncovalent complexation process between 1 and 4a. Hence, it is apparent that intermolecular O−H···N hydrogen bond plays a vital role for the enhanced binding affinity of 1/4a than that of 1/4b. On this basis, we shed light on noncovalent complexation between monomers 2 and 3. According to 1H NMR experiments (Figure 1f−h), both terpyridine and azobenzene resonances on 2−3 shift upfield (Δδ = 0.26, 0.74, and 0.29 ppm for protons H1′, Ha′, and Hd′, respectively). Although the chemical shift changes for complex 2/3 are relatively slighter, their tendencies are fully consistent with those of the model system 1/4a. Such phenomena suggest that multivalent molecular tweezer/guest complexation takes place on the ends of 2 and 3, even with the presence of PCL chain. Size variation of the resulting supramolecular polymer networks 2/3 (2:1 mixture) was further examined via 2D diffusion-ordered NMR (DOSY) measurements. In detail, as the concentration increases from 2 mM to 10 mM for monomer 2, the measured diffusion coefficients drop from 1.23 × 10−9 m2 s−1 to 2.40 × 10−10 m2 s−1 (Figure S8). Besides, specific viscosity of complex 2/3 changes exponentially upon varying the monomer concentration (Figure 3a). In comparison, either monomer 2 or 3 shows a shallow viscosity growth.16 Hence, it demonstrates that size expansion of the supramolecular networks is originated from noncovalent molecular tweezer/guest complexation. It is widely known that alkynylplatinum(II) terpyridines undergoes intersystem crossing from singlet to triplet excited 543
DOI: 10.1021/acsmacrolett.7b00241 ACS Macro Lett. 2017, 6, 541−545
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ACS Macro Letters
Youth Innovation Promotion Association (2015365), Hefei University Research Project (14KY09ZR), and National Student Innovation Project (201311059007).
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Figure 4. (a) Schematic representation for the photodeprotection of 5 and 6. (b, c) DOSY spectra (400 MHz, CDCl3, 298 K, 10.0 mM for 2) of complex 2/5 before and after UV light irradiation (365 nm, 9 × 4 W, 200 min), respectively.
light irradiation. Briefly, at a scanning rate of 5 °C min−1, a clear differential scanning calorimetry (DSC) melting signal (Tm) at 49.6 °C is observed for the mixture of 2 and 5 (Figure S16). Upon photoexcitation, no obvious transition peak can be visualized. It is rationalized that, upon light-triggered formation of supramolecular networks, mobility of the parent PCL chain is dramatically restricted. In summary, we have successfully developed a novel fabrication approach toward supramolecular polymer networks, with the employment of molecular tweezer/guest recognition motif as the cross-linkages. Auxiliary O−H···N hydrogen bond is incorporated between bis[alkynylplatinum(II)] terpyridine molecular tweezer and the complementary trans-azobenzene guest. As a result, it not only enhances noncovalent crosslinking strength of the supramolecular networks, but also endows intriguing photoresponsiveness via the deprotection strategy. With the concurrent embedment of π-functionality, processability, and stimuli-responsiveness, the current work opens up a new avenue toward advanced supramolecular 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/acsmacrolett.7b00241. Full experimental details and characterization data (PDF).
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Feng Wang: 0000-0002-3826-5579 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21674106), the Fundamental Research Funds for the Central Universities (WK3450000001), CAS 544
DOI: 10.1021/acsmacrolett.7b00241 ACS Macro Lett. 2017, 6, 541−545
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DOI: 10.1021/acsmacrolett.7b00241 ACS Macro Lett. 2017, 6, 541−545
