Polymer Chemistry

Polymer Chemistry

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DOI: 10.1039/C8PY01070F

COMMUNICATION Photo-driven morphological transformations of supramolecular polymers actuated by an overcrowded alkene switch Received 00th January 20xx, Accepted 00th January 20xx

Wen-Jing Liang, Jing-Jing Yu, Qi Zhang, Chang-Shun Ma, Zhao-Tao Shi, Da-Hui Qu *

DOI: 10.1039/x0xx00000x www.rsc.org/

A novel light-responsive supramolecular polymer was demonstrated from an overcrowded alkene switch (OAS) mounted with two rigid 2-ureido-4-pyrimidinone (UPy) groups. By strong quadruple hydrogen bonding interactions, trans OAS-Upy could form well-defined nanospheres in chloroform, which would be transformed into worm-like assemblies after UV irradiation. Moreover, the inverse process of this system could be achieved by exposing to the heating.

responsive unit is an intriguing project owing to the remarkable structural change when isomerized, which might provide the supramolecular polymer with more possibilities to undertake morphological transition, even in dimension. However, to the best of our knowledge, most of studies have 16 focused on the switching of polymer functions and sol-gel 17 conversion . Very few of them can observe the morphological transformations of supramolecular polymers based on a molecular switch in a dimensional scale.

Supramolecular polymers are polymeric arrays of monomer 1 building blocks , which are held together by reversible non2 covalent interactions, such as hydrogen bond , 3 4 donor−acceptor interac;on , metal-coordination bond , and halogen bond5. A variety of supramolecular polymers have been designed and developed with highly dynamic nature, which are widely used in advanced soft materials, including self-healing6, self-adapting7, drug release system8, actuators9, and stimuli-responsive smart materials10. Typically, supramolecular polymers formed by the 2-ureido-4pyrimidinone (UPy) building blocks, which can dimerize by directional quadruple hydrogen bonding with an extremely large binding constant (Ka = 6 × 107 M-1 in CHCl3), have been greatly developed11.

Herein, we modified two rigid Upy groups on both ends of the OAS units (OAS-Upy), in which the Upy groups provide quadruple hydrogen bond building block, and the middle OAS unit performs as a photoswitch (Scheme 1a). The trans OASUpy could form linear supramolecular polymers in chloroform, which can further fold and assemble into the well-defined nanospheres by the secondary π-π stacking interaction from central OAS units (Scheme 1b). Upon irradiation of UV light, the structure of OAS-Upy can be isomerized from relaxed trans configuration to overcrowded cis configuration, which could break the secondary stacking formed by π-π stacking interactions, as a result, the zero-dimensional nanospheres would transform into one-dimensional worm-like polymers (Scheme 1b). Furtherly, the depolymerization of nanospheres can be achieved due to the presence of competitive molecule, namely 2,7-diamido-1, 8-naphthyridine derivatives (Napy), which can form an exceptionally strong quadruply hydrogenbonded complex with tautomeric form of Upy groups18 (Scheme S2). We hope that this work would pave the way for better understanding of the assembly and depolymerization process of supramolecular polymers.

Overcrowded alkene switches (OAS), which possess efficient photoisomerization performance and distinctive spatial configuration, have been received extensive attention because of their potential applications in the areas such as asymmetric catalysis12, ion recognition13, chirality switching14 and nanocontainers15. Combining supramolecular polymers with photo-

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The detailed synthesis route of the target OAS-Upy molecule is shown in Scheme S1 (ESI†). Firstly, diamino-substituted OAS derivatives were obtained through a Buchwald–Hartwig 19 coupling reaction . After a mild reaction between diaminosubstituted OAS derivatives and blocked isocytosine imidazolides, the target OAS-Upy was obtained with yield of 1 65%. The structure of OAS-Upy was well characterized by H 1 1 NMR spectroscopy, H- H COSY NMR spectrum, 2D Roesy NMR spectrum and high-resolution electrospray ionization (HR-ESI) mass spectrometry (Figure S1-S3, S12-S14).

shifted downfield (Δδ = 0.09 ppm, Δδ = 0.08 ppm, respectively), owing to the deshielding effect from 1 naphthalene units. Interestingly, the H NMR signals of NH hydrogens H9, H10, H11 from urea bond and pyrimidone had no significant chemical shift changes, which illustrated that the photoisomerization of OAS-Upy could not break up the hydrogen bonding between Upy groups. The reversibility of OAS-Upy photoisomerization was further confirmed by o heating the cis isomers at 50 C for 30 minutes (Figure 1c), 95% of cis OAS-Upy could return to the original trans state.

Figure 1. 1H NMR spectrum (400 MHz, 10 mM, CDCl3, 298 K). (a) trans OAS-Upy before UV irradiation; (b) trans OAS-Upy after UV irradiation for 60 min; (c) the cis OAS-Upy after standing at 50 oC water bath for 30 min. Scheme 1. (a) The chemical structure and photochemically and thermally induced reversible trans-cis isomerization of OAS-Upy; (b) Graphical representation for the morphological transformations of trans/cis OAS-Upy in chloroform solution.

Concentration-varied 1H NMR spectroscopy was employed to characterize the concentration-dependent supramolecular polymerization of OAS-Upy in chloroform (Figure S5). As the monomer concentration increased from 1 mM to 100 mM, the major signals of OAS-Upy were gradually broaden with no chemical shift changes, suggesting that the supramolecular polymers were in majority at high concentration. The photoisomerization process of the OAS-Upy was detected by 1H NMR (Figure 1). Before the UV irradiation, the OAS-Upy stayed at stable trans form, after irradiation with UV (365 nm) for 60 minutes under the ice bath, trans OAS-Upy converted into cis OAS-Upy, the photostationary state (PPS) was confirmed from variable-illumination time versus 1H NMR spectroscopy (Figure S4). Meanwhile 1H NMR signal peaks had undergone significant changes (Figure 1b), the partial 1H NMR signals of the methyl hydrogen H8 and tertiary carbon hydrogen H7 in OAS unit were obviously shifted upfield (Δδ = −0.64 ppm, Δδ = −0.45 ppm, respectively), which was attributed to the disappearance of the deshielding effect from naphthalene units, the efficiency of trans-to-cis isomerization was calculated as 55% via NMR integration. In addition, the partial 1H NMR signals of aromatic hydrogens H1, H3 were

UV-Vis absorption and fluorescence spectra were used to further confirm the photoisomerization process of OAS-Upy. The diluted chloroform solution of OAS-Upy was exposed to UV light (365 nm) for 15 minutes, the UV-vis absorption spectra showed that the maximum absorption wavelength redshifted from 380 nm to 425 nm (Figure 2a), at the same time, the solution changed from colorless to yellow green. Furthermore, the fluorescence spectra showed a remarkable fluorescence quenching at 435 nm (Figure 2b). All results are 15 consistent with previous reports , confirming the efficient photo-responsiveness of OAS unit.

Figure 2. (a) UV-vis spectral changes from trans OAS-Upy to cis OAS-Upy obtained after irradiation with UV light for 15 min (0.01 mM in CHCl3); (b) Fluorescence spectral changes from trans OASUpy to cis OAS-Upy obtained after irradiation with UV light for 15 min (λex = 380 nm, 0.01 mM in CHCl3).

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In order to study the properties of the supramolecular polymers formed by trans OAS-Upy, variable-concentration viscosity experiments were performed in chloroform at 298 K. The double-logarithmic plot of specific viscosity (ηsp) versus concentration of trans OAS-Upy showed a linear curve with a slope of 1.33 at the whole concentration range (Figure S6), which indicated trans OAS-Upy could assemble into supramolecular polymers at a very low concentration, the result was consistent with the result in variable-concentration 1 H NMR experiment. Notably, trans OAS-Upy solution still borne a low viscosity even at an extremely high concentration (200 mM). This is different from the previous reported Upy functionalized supramolecular polymers, which could form a well-defined gel at high concentrations17a, 17c, 20. A reasonable explanation is that OAS units were rigid enough and maintained an overcrowded nonplanar configuration, which would prevent the OAS-Upy from assembling more closely to form a networked gel structure, eventually formed a discrete supramolecular polymer microstructure.

by π-π stacking interaction from the central OAS units (Scheme 1). Upon the UV light (365 nm) for 15 min, interestingly, the nanospheres were disappeared, and transformed into a large number of one-dimensional worm-like polymers (Figure 3c). The width of the polymers was estimated to be about 80 nm, which was also consistent with DLS data of cis OAS-Upy. This result could be interpreted as that the trans configuration transformed into more crowded and distorted cis configuration due to the photoisomerization of OAS units, inhibiting the intermolecular π-π stacking interaction. To further track this process, the CHCl3 solution of cis OAS-Upy 0 was heated under darkness at 50 C for 30 min, some circular aggregates were clearly observed in TEM pictures (Figure 3d), which ascribed that partial cis OAS-Upy reversibly returned to the original trans state. The broken π-π stacking interaction was reformed, and the worm-like polymers reassembled into more concentrated nanoring. The whole process of morphological changes could also be observed by higher resolution TEM images (Figure S10).

2D diffusion-ordered 1H NMR spectroscopy (DOSY) is a common method to evaluate the size distribution of supramolecular aggregates. The DOSY experiment of trans OAS-Upy was performed in CHCl3 at 20 mM, which showed a single set of signals (Figure S7), the diffusion coefficient (D) was calculated to be about 1.99×10-9. Upon the UV (365 nm) irradiation for 60 min, the partial trans OAS-Upy transformed into cis OAS-Upy, the diffusion coefficient (D) was calculated to be about 5.49×10-9 (Figure S8), which was larger than that of trans OAS-Upy. This result demonstrated that the supramolecular polymers formed by trans OAS-Upy have been transformed into smaller aggregates after irradiation with UV light. Dynamic light scattering (DLS) experiments were also used to measure the size of aggregates formed by OAS-Upy in chloroform, as shown in the Figure 3a, trans OAS-Upy displayed a uniform distribution at a concentration of 1×10-5 M, the average hydrodynamic diameter (Dh) value was about 400 nm, indicating the formation of well-defined supramolecular aggregates. Upon UV (365 nm) irradiation for 15 min, the size of aggregates dramatically decreased from 400 nm to about 100 nm, which was consistent with the 2D DOSY. The variation process of sizes could be detected by DLS versus continuous illumination (Figure S9). The morphologies of the OAS-Upy aggregates were investigated via transmission electron microscopy (TEM), as shown in the Figure 3b, a large number of uniformly distributed spherical aggregates with an average diameter of about 400 nm could be observed, which showed that trans OAS-Upy could self-assemble into spherical supramolecular polymers at low concentrations (1×10-5 M). The sizes of the nanospheres were consistent with DLS data of trans OAS-Upy. The formed nanospheres can be attributed to the hierarchical self-assembly: trans OAS-Upy firstly formed a linear supramolecular polymer by intermolecular quadruplex hydrogen bonding interaction from Upy groups, and then the discrete linear polymers further assembled into nanospheres

Figure 3. (a) DLS data of trans OAS-Upy and cis OAS-Upy (0.01 mM in CHCl3); (b) TEM images of trans OAS-Upy aggregates; (c) TEM images of trans OAS-Upy aggregates after UV (365 nm) irradiation for 15 min; (d) TEM images of cis OAS-Upy after heating (500C) under darkness for 30 min, all the TEM tested onto copper grid (1 × 10−5 M in CHCl3).

2,7-diamido-1,8-naphthyridine derivatives (Napy), which can selectively form strong complexes with 2-ureido-4[1H]pyrimidinone (UPy) units through quadruple hydrogen bonding between ADDA and DAAD arrays (Scheme S2). To further confirm the presence of hydrogen bonding, a control experiment was performed by using Napy as a competitor for the Upy groups. With gradual addition of Napy monomers, the quadruple hydrogen bonding system formed by trans OAS-Upy 1 could be easily disturbed. H NMR spectroscopy clearly

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confirmed this process (Figure S11). The H NMR signals of NH hydrogens H9, H10 from urea bond were split into two peaks, meanwhile, the NMR signals of NH hydrogens H11 from Pyrimidone was significantly shifted upfield (Δδ = −0.62 ppm), which indicated the formation of a new quadruple hydrogen bond by Upy-Napy complexes. In addition, all of the original broad signal peaks from trans OAS-Upy dramatically became sharp after adding Napy monomers, indicating the efficient depolymerization.

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Conclusions In summary, we successfully synthesized a rigid 2-ureido-4pyrimidinone (UPy) groups functionalized overcrowded alkene switch (OAS), which could form well-defined nanospheres in chloroform through intermolecular quadruple hydrogen bonding and π-π stacking interactions. Significantly, larger nanospheres can gradually transform into smaller worm-like polymers after exposure to the UV light (365 nm), actuated by the reversible trans–to-cis photoisomerization of OAS-Upy. Furthermore, after adding the competing molecule 2,7diamido-1,8-naphthyridines (Napy) monomers to the above system, hydrogen-bonded arrays between OAS-Upy units are destroyed by stronger complementary hydrogen bonds formed by Upy-Napy complexes, subsequently, the trans OASUpy supramolecular polymers depolymerize furtherly. We envision that this process could provide a new strategy to construct photo-responsive polymer soft nanomaterials, especially in the precise control of morphological dimensions.

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Acknowledgments We thank the Research Center of Analysis and Test of East China University of Science and Technology for help on the material characterization. We appreciate Prof. Li-Hui Zhou for her strong support in TEM tests. This work was supported by NNSF/China (grants 21790361, 21672060, 21788102), the Fundamental Research Funds for the Central Universities (grants WJ1616011, WJ1213007, 222201717003), and the Programme of Introducing Talents of Discipline to Universities (B16017).

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There are no conflicts to declare.

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Polymer Chemistry


A novel light-responsive supramolecular polymer was actuated by an overcrowded alkene switch. Trans OAS-Upy could form well-defined nanospheres in chloroform, which would be transformed into worm-like assemblies after UV irradiation.



DOI: 10.1039/C8PY01070F