Controllable Supramolecular Polymerization Promoted by Host

Dec 5, 2016 - As shown in Figure 1a, almost all the signals of the protons on MOED moieties (Hd, He, Hf, Hg and Hh) exhibited upfield shift after the ...
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Controllable Supramolecular Polymerization Promoted by HostEnhanced Photodimerization Yuetong Kang,† Zhengguo Cai,† Zehuan Huang, Xiaoyan Tang, Jiang-Fei Xu,* and Xi Zhang* Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: In this letter, we report a new method of controllable supramolecular polymerization, taking advantage of host-enhanced photodimerization. The low-molecular-weight supramolecular oligomers were formed by noncovalent complexation between cucurbit[8]urils (CB[8]) and the bifunctional monomers (DBN) with Brooker’s merocyanine moiety (MOED) on either end. Interestingly, when irradiated with UV light, the supramolecular oligomers could transform into supramolecular polymers with high molecular weight. The molecular weight of supramolecular polymers could be controlled by varying the irradiation time. It is highly anticipated that this work can enrich the methods on the modulation of supramolecular polymerization.

S

In this letter, we will introduce a new method to fabricate high-molecular-weight supramolecular polymers, which is promoted by host-enhanced photodimerization. As shown in Scheme 1, the bifunctional monomer (noted as DBN) containing a dimethylphenyl moiety in the middle and a stilbazolium group on either end was synthesized. The stilbazolium groups (MOED) on DBN shared the molecular

upramolecular polymers refer to polymeric arrays in which monomeric units are connected by noncovalent interactions in a directional and reversible manner.1−14 The noncovalent nature endows supramolecular polymers with attractive properties,15−26 such as reversibility27−30 and stimuliresponsiveness,31−35 which are not easily realized in their covalent counterparts, rendering them as desirable candidates for the fabrication and processing of dynamic materials. Although the research on supramolecular polymers has achieved significant development in the past decades, there are plenty of rooms for further investigation on new approaches of controllable supramolecular polymerization. In general, there are two approaches of fabricating supramolecular polymers. One approach is to design and synthesize bifunctional monomers by covalent synthesis, and then to fabricate supramolecular polymers by supramolecular polymerization of the bifunctional monomers.36−42 To realize effective supramolecular polymerization, unfavorable cyclization and dimerization should be avoided. Various strategies have been reported to achieve the goal, for example, the fine molecular design of ABBA-type monomers,43 the employment of bulky linkers44 or long linkers45 between the functional moieties for noncovalent linkage, and the noncovalent reinforcement on the rigidity of bifunctional monomers in a manner of selfsorting.46,47 The other approach is to fabricate supramonomers by noncovalent synthesis, then to fabricate supramolecular polymers by covalent polymerization of supramonomers. The advantage of this approach is to convert not easily controllable supramolecular polymerization into easily tractable covalent polymerization.48−50 In other words, one may rely on suitable methods of covalent polymerization for controlling the formation of supramolecular polymers. © XXXX American Chemical Society

Scheme 1. Schematic Illustration of Light-Promoted Supramolecular Polymerization of DBN@CB[8]

Received: November 15, 2016 Accepted: December 2, 2016

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ACS Macro Letters structure with Brooker’s merocyanine,51,52 and MOED groups in this system could function as both the guest motifs for cucurbit[8]uril (CB[8]) encapsulation and the reactive sites for the covalent photodimerization.53−55 The host−guest complexation between MOED groups and CB[8] could drive the formation of cyclic supramolecular oligomers which was probably ascribed to the flexibility of the bifunctional monomer. When irradiated with UV light, the MOED pair encapsulated in CB[8] cavity could undergo [2 + 2] photodimerization to yield a rigid cyclobutanyl moiety.56,57 In this way it might avoid the cyclization and dimerization of the bifunctional monomers, two unfavorable factors for supramolecular polymerization. As a result, it could favor the linear supramolecular polymerization, leading to the formation of supramolecular polymers with high molecular weight. We investigated the host−guest complexation of DBN and CB[8] by 1H NMR and UV−vis spectroscopy. As shown in Figure 1a, almost all the signals of the protons on MOED

parameter (α = k2/k1 = 4K2/K1) is around 3.67, suggesting that this supramolecular dimerization process is positively cooperative.58 To study if the host−guest complexation between DBN and CB[8] could be used to fabricate supramolecular polymers with high molecular weight, diffusion-ordered NMR spectroscopy (DOSY) was utilized to measure the diffusion coefficients of different species. As shown in Figure 2a, the diffusion

Figure 1. (a) Partial 1H NMR spectra of DBN@CB[8] and DBN. [DBN] = 2.0 mM; (b) ITC data and fitting curve of DBN (0.50 mM) titrating into CB[8] (0.05 mM), solvent: NaAc/HAc (50 mM) aqueous buffer with pH = 4.75. Figure 2. (a) DOSY spectra and diffusion coefficients of monomer, oligomer before and after 365 nm light irradiation for 40 min. (b) Diffusion coefficients of oligomer (2.0 mM) vs irradiation time.

moieties (Hd, He, Hf, Hg and Hh) exhibited upfield shift after the addition of CB[8], suggesting that DBN was encapsulated into the cavity of CB[8]. It should be pointed out that there was almost no change of the protons of benzene moiety (Ha) in the middle of DBN before and after host−guest complexation, suggesting that CB[8] did not bind on the benzene moiety. Since our experiments were conducted in acidic NaAc/HAc buffer solution (50 mM, pH = 4.75), the phenol moieties in DBN were generally protonated, rendering DBN as a cation due to the intrinsic positive charge of the pyridium moieties. Considering that CB[8] has eight carbonyl groups on both portals, it should favor its affinity to cationic parts. Besides, as shown in Figure S2, the maximum absorption wavelength of MOED moieties shifted bathochromically with the increase of CB[8] content, indicating the change on microenvironment of MOED moieties. Therefore, we can propose that the MOED moieties of DBN are preferentially encapsulated by CB[8]. The host−guest complexation of DBN and CB[8] was further quantitatively characterized by isothermal titration calorimetry (ITC). As shown in Figure 1b, the binding ratio between DBN and CB[8] was confirmed to be 1:1, indicating that two MOED moieties combined with one CB[8] stoichiometrically. By fitting ITC data with a sequential binding model, the binding constants were estimated to be K1 = 8.20 × 105 M−1 and K2 = 7.52 × 105 M−1, respectively. The interaction

coefficient of DBN (2.0 mM) in HAc/NaAc buffer was 2.64 × 10−10 m2/s. After addition of equivalent CB[8] into the DBN solution (noted as DBN@CB[8]), the diffusion coefficient decreased to 1.09 × 10−10 m2/s. This can be ascribed to the formation of polymeric species. However, these polymeric species are considered as supramolecular oligomers because their apparent degree of polymerization (DP) is estimated to be no more than 15 according to the Stokes−Einstein equation. The structural flexibility of DBN may have constrained the supramolecular oligomers from growing into linear supramolecular polymers with high molecular weight. We wondered if supramolecular polymerization could be promoted by UV irradiation. To answer this question, we irradiated the aqueous solution of the supramolecular oligomer (DBN@CB[8]) mentioned above with UV light. As shown in Figure 2b and Table S1, after exposing the DBN@CB[8] sample to UV light for 20 min, the diffusion coefficient decreased significantly to 5.03 × 10−11 m2/s, with the apparent DP around 144. When the UV irradiation was extended to 40 min, the diffusion coefficient further dropped to 3.94 × 10−11 m2/s, corresponding to the apparent DP around 300. Within 40 min, the longer the UV irradiation time, the higher the apparent 1398

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ACS Macro Letters

observe that the reaction extent of DBN generally matched with the formation of the supramolecular polymers. What is the mechanism behind the controllable supramolecular polymerization promoted by UV irradiation? We inferred that before UV irradiation, DBN@CB[8] generally formed supramolecular oligomers, which was ascribed to the flexibility of DBN and the dynamic nature of the host−guest complexation. According to the low fluorescence emission of DBN@CB[8] (shown in Figure S3), the supramolecular oligomers could be cyclic in most cases, in which almost all MOED groups were encapsulated in CB[8] and became fluorescence quenched. After UV irradiation, the paired MOED groups encapsulated in CB[8] undertook photodimerization, generating cyclobutanyl structure with high rigidity, which favored linear elongation to obtain high-molecular-weight supramolecular polymers. As the reaction process of photodimerization correlates to the irradiation time, we can control the molecular weight of supramolecular polymers by controlling UV irradiation time. To confirm the above mechanism, we relied on the asymmetrical flow field flow fractionation (AsF-FFF) to characterize the molecular weights of supramolecular polymers. As displayed in Table 1, there were almost no clear signals for

DP. When the irradiation time was over 40 min, the diffusion coefficient derived from DOSY data nearly remained still, suggesting that photodimerization may have come to an end probably because of the nearly complete consumption of reactive MOED moieties. Therefore, the supramolecular polymerization can be promoted by UV irradiation, and moreover, the molecular weight of supramolecular polymers can be adjusted by varying the irradiation time. As a control, when exposing DBN itself under UV light for 40 min without the addition of CB[8], the diffusion coefficient was just around 2.42 × 10−10 m2/s. As shown in Figure 2a and Table S2, there is no big difference in the diffusion coefficient before and after UV irradiation, though the fluctuation after UV irradiation became larger than before. Such a control experiment supports the claim that the combined effect of the host−guest complexation and UV irradiation is responsible for fabricating the supramolecular polymers with high molecular weight. To understand if the UV irradiation induced covalent dimerization between the paired MOED groups in the cavity of CB[8], we utilized UV−vis spectroscopy to answer this question. As shown in Figure 3a and Figure S4, before UV

Table 1. Molecular Weights and Polydispersity (PDI) of Supramolecular Polymers with Different UV Irradiation Timesa irradiation time (min)

Mw (kDa)

Mn (kDa)

PDI

0 5 10 20 40 60 60 (+1 equiv ADA)

− 86.3 151 197 262 265 −

− 43.4 93.3 86.3 140 158 −

− 1.99 1.62 2.28 1.87 1.68 −

a

[DBN@CB[8]] = 2.0 mM for UV irradiation and diluted to 1.0 mM for AsF-FFF measurements in all cases; detected wavelength at 237 nm. “” means that the molecular weight is too low to be detected.

the sample of the supramolecular oligomer (DBN@CB[8]), indicating that its molecular weight was too low to be detected by this method. With UV irradiation upon DBN@CB[8], the supramolecular oligomers were transformed into supramolecular polymers, as indicated by AsF-FFF data. The molecular weight of the supramolecular polymers increased fast within the first 10 min, and then it grew gradually with irradiation time. After 60-min 365 nm irradiation, the weight-average molecular weight of the supramolecular polymers reached 265 kDa. The increase in molecular weight of the supramolecular polymers accompanied by UV irradiation, confirming the promotion effect of photodimerization on supramolecular polymerization. It should be pointed out that the photodimerization did not need to be 100% for promoting supramolecular polymerization, and there were still unreacted MOED groups as well as noncovalent linkages within the resultant supramolecular polymers. To confirm this statement, we added 1.0 equiv adamantanamine (ADA) into the supramolecular polymer solution. If the linkages within the supramolecular polymers were all covalent, the molecular weight of the supramolecular polymers would be generally maintained. While if there were noncovalent linkages remained in the backbone of supramolecular polymers, 1 equiv ADA would suffice to break almost

Figure 3. (a) UV−vis absorbance changes of oligomer as irradiation time increased. (The curve shows at t = 0 min, 30 and 60 min. The sample under irradiation was 2.0 mM, and was then diluted to 0.10 mM for UV−vis characterization.) (b) Absorbance at 390 nm (left axis, black line, square dot) and 237 nm (right axis, blue curve, triangle dot) vs irradiation time.

irradiation, DBN@CB[8] complexes absorbed maximally at ∼405 nm, corresponding to the stilbazolium structure of MOED moieties. After 40 min UV irradiation, the absorbance at ∼405 nm decreased significantly, along with the increase of the absorbance at 237 nm, implying that MOED moieties with relatively large conjugated structure were transforming to cyclobutane derivatives with small aromatic structures. As shown in Figure 3b, it displayed that the photodimerization occurred fast at the first 15 min, then gradually slowed down, and tended to reach the end after 40 min. Herein we could 1399

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all the noncovalent linkages due to the competitively strong binding affinity between ADA and CB[8], thus, rendering harsh decrease in molecular weight of the supramolecular polymers. As measured by AsF-FFF, the resultant degraded species were not detectable, meaning that the molecular weights of the degraded species were lower than 10 kDa. Thus, after 365 nm irradiation, the resultant high-molecular-weight species were still supramolecular polymers rather than completely covalentattached polymers. As indicated by UV−vis data in Figures S5 and S6, the reaction extent of the photodimerization of MOED moieties was around 86% after 60-min 365 nm irradiation. With some MOED moieties unreacted on their backbone, the supramolecular polymers remained the dynamic property, which could still be noncovalently disassociated by competitive replacement of ADA. Besides, the final degraded species by ADA replacement could be considered as ‘supramolecular macromonomers’ with covalent backbone and unreacted MOED end groups which could further undergo supramolecular polymerization via noncovalent linkage. In addition, the suprmaolecular polymers could also be disassociated by 254 nm irradiation, as it could trigger the split of the photoadducts of MOED moieties. As shown in Table S3, the diffusion coefficient of the resultant species was 1.37 × 10−10 m2/s, which was similar to that of the supramolecular oligomers (DBN@CB[8]), suggesting that the supramolecular polymers were disassociated after 254 nm irradiation. Therefore, the polymerization and depolymerization process can be controlled by irradiation with different wavelengths. In this letter, we have demonstrated a new method of controllable supramolecular polymerization promoted by hostenhanced photodimerization of Brooker’s merocyanine moieties encapsulated by CB[8]. The molecular weight of supramolecular polymers can be facilely controlled by varying the irradiation time. In principle, the same methodology could be used to fabricate not only linear supramolecular polymers but also branched or hyperbranched supramolecular polymers. Besides CB[8], other supramolecular hosts might also be employed to play similar games. In addition, we may transfer the solution process to liquid−solid interface, fabricating thin films by the interfacial supramolecular polymerization promoted by UV irradiation. It is highly anticipated that this work can enrich the methods on the modulation of supramolecular polymerization.

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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 (21434004, 21274076), and the NSFC Innovation Group (21421064). We thank Yunhao Bai and Dr. Hao Chen for the efforts on the preparation of CB[8]. We also appreciate Guoxiang Zhu, Dr. Ruilong Zong, and Prof. Yongfa Zhu for their kind provision of the 254 nm light source.

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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.6b00871.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xi Zhang: 0000-0002-4823-9120 Author Contributions †

These authors contributed equally (Y.K. and Z.C.). 1400

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