Highly Controllable Ring–Chain Equilibrium in Quadruply Hydrogen

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Highly Controllable Ring−Chain Equilibrium in Quadruply Hydrogen Bonded Supramolecular Polymers Tangxin Xiao,† Xiaoqing Feng,† Shuyang Ye,† Yangfan Guan,† Shao-Lu Li,† Qi Wang,† Ya Ji,† Dunru Zhu,‡ Xiaoyu Hu,† Chen Lin,† Yi Pan,† and Leyong Wang*,† †

Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ State Key Laboratory of Material-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210093, China S Supporting Information *

ABSTRACT: Electron-rich dioxynaphthalene (DNP) group bridged bifunctional ureidopyrimidinone (UPy) derivatives (L1, L2, and L3) were synthesized, which could form small cyclic monomers, oligomers, or linear supramolecular polymers at certain concentration in solution, to achieve a highly controllable ring−chain equilibrium self-assembling supramolecular system. The ring−chain equilibrium of these supramolecular monomers constructed by different lengths of oligo(ethylene oxide) (oligoEO) chain as spacers were investigated by a combination of techniques, such as 1H NMR, DOSY, single-crystal X-ray diffraction, and viscometry. The experiment results demonstrated that there exists intramolecular π−π stacking interaction between DNP group and intramolecularly dimerized UPy motif in the monomeric cyclic form of these supramolecular monomers, and the strength of this π−π stacking interaction directly depends on the length of the oligoEO chain. Furthermore, strong intramolecular π−π stacking interaction was found to promote self-assembly favorable for intramolecularly cyclic monomerization, leading to a great increase of critical polymerization concentration (CPC). Monomer L1a with the shortest length of oligoEO chain is present as an exclusive type of intramolecularly hydrogen-bonded assembly, namely the cyclic monomers, over a broad concentration range (1.6−500 mM) in solution. Single crystal structure of the cyclic monomer L1b, which is an analogue of L1a, was thoroughly studied. The CPC values of monomer L2 and L3 with longer oligoEO chain are ca. 70 and 23 mM, respectively. However, L2 and L3 could perform selective cyclization over the entire concentration range in solution after threading into the tetracationic cyclobis(paraquat-p-phenylene)cyclophane (CBPQT4+) driven by host−guest interaction, which provides another supramolecular strategy to control ring−chain equilibrium. The combined results may provide new insights into the ring−chain equilibrium and offer valuable information on the understanding of the correlation between supramolecular assistance and polymerizability.

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INTRODUCTION Tremendous examples of precise and concise self-assembly processes are exhibited in nature, such as the formation of the cell cytoskeleton and DNA’s replication and transcription procedures.1 Inspired by this, scientists have been encouraged to explore self-assembly processes in order to get in close proximity to nature through artificial synthetic platform; thus, a huge number of highly complicated and ingenious architectures or molecular machines have been fabricated.2 By combining supramolecular self-assembly notion with conventional polymer chemistry, scientists have constructed various kinds of supramolecular polymers3 with special properties, such as easy processability,4 biocompatibility,5 stimuli-responsiveness,6 and self-healing.7 During the procedure of design and synthesis, scientists gradually recognize that the supramolecular polymerization mechanism is of great importance to direct the growth of monomers and affect the properties of the resulting supramolecular polymers.8 Among the supramolecular poly© 2012 American Chemical Society

merization mechanisms, the most important one is the ring− chain equilibrium mechanism,8f,9 which is usually employed to describe a wide range of supramolecular polymers constructed by metal−ligand coordination,10 hydrogen bonding,11 ionic interaction,12 or host−guest interaction.13 The theory of ring− chain mechanism is characterized by the fact that linear oligomers and polymers are in equilibrium with their cyclic counterpart in solution. Of particular interest is the existence of a critical polymerization concentration (CPC), below which the cyclic assemblies are predominant species and above which the concentration of cyclic species remains constant and excess monomers mainly produce linear species.8f,11b In general, viscometry, 1H NMR, and DOSY are used to determine the CPC of the ring−chain equilibrium in supramolecular polymer systems. Recently, our group has exploited a novel photoReceived: November 29, 2012 Published: December 10, 2012 9585

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Figure 1. Representations of the ring−chain equilibrium of DNP bridged bifunctional UPy derivatives.

chromic method14 to investigate the ring−chain equilibrium mechanism by introducing a dithienylethene unit into the monomers incorporating two ureidopyrimidinone (UPy) motif that was discovered by Meijer et al. in 1997 and was often used as an ideal candidate to study ring−chain equilibrium due to its high association constant (Kassn > 107 in chloroform) and synthetic accessibility.15 Meijer et al. have thoroughly studied the ring−chain equilibrium of bifunctional UPy molecules with a series of methyl-substituted alkyl spacers.11b In the production of covalent polymers, ring formation is usually considered as byproduct which should be prevented as much as possible. However, optimization of a high yield of one specific aggregate with well-defined size and structure in selfassembled systems is one of the goals of synthetic research on supramolecular chemistry.16 In this context, an increasing number of well-defined cyclic oligomers based on bifunctional UPy monomers have been developed for different application due to their unusual three-dimensional structures and for different application. Scientists mainly focused on two ways to construct such stable UPy-based structures by employing rigid preorganized building blocks17 or flexible linkers with conformational bias as spacers.18 Mendoza et al. have constructed a cyclic dimer and tetramer by using calix[4]arene spacer17d and 3,6-carbazolyl cores,17a respectively. Moreover, Meijer et al. have fabricated a very stable cyclic dimer by using flexible alkyl linkers through enantioselective cyclization of

racemic supramolecular polymers.18b In many cases, the cyclic dimers are the smallest assemblies, and some of them could persist over a wide range of concentration in solution or even in the solid state.17e,18a,b However, fabrication of highly stable cyclic monomers (the simplest assemblies) of bifunctional UPys is still a big formidable challenge for chemists, which might be due to the following two reasons: (1) when short linkers (fewer than 14 atoms18c) between bifunctional UPys are employed, it is very difficult to cyclize intramolecularly since the two UPy units associate in an antiparallel fashion and stable cyclic dimers are usually obtained;18b,c (2) when longer flexible spacers (more than 14 atoms) between bifunctional UPys are utilized, there usually exists a CPC in self-assembly process, resulting in the occurrence of ring-opening polymerization of cyclic monomers. Therefore, chemists have been put into a dilemma with the aim of achieving the stable cyclic monomers of bifunctional UPys. In 2006, Aliev et al. reported highly stable cyclic dimers based on bifunctional UPys with relatively long and flexible linkers.18a The selective cyclization should be ascribed to a combination of different noncovalent interactions, including hydrogen bonding, parallel stacking, and hydrophobic shielding. The strategy of using additional noncovalent interaction except quadruple hydrogen bonding to induce the selective cyclization of bifunctional UPys gave us great inspiration. In our research, one of the goals is to fabricate the highly stable cyclic monomer of bifunctional UPys with 9586

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supramolecular assistance, which could exist in solution over a broad concentration range and in the solid state. No matter how chemists would like to optimize the yield of a well-defined cyclic structure in self-assembled systems or reduce the CPC in supramolecular polymer to prevent the cyclic structure, understanding the parameters related to the ring−chain equilibrium is useful and imperative for controlling the product distribution. In our own laboratory,19 UPy-based functional supramolecular polymers have been continuously produced employing orthogonal self-assembly strategy.20 When exploring dynamic [2]catenanes2d,21 interlocked by electrondeficient macrocycle tetracationic cyclobis(paraquat-pphenylene)cyclophane22 (CBPQT4+) and electron-rich 1,5dioxynaphthalene (DNP) group bridged bifunctional UPy molecules, we surprisely found that there exists π−π stacking interaction between the aromatic unit and intramolecularly dimerized UPy plane in the cyclic form of individual L2 (Figure 1).23 According to the literature, the π−π stacking interaction can not only reinforce the hydrogen bonds involved in multiple H-bonded arrays but also promote the dimerized UPy motif to form one-dimensional stacks.24 Consequently, it strongly implies that this π−π stacking interaction could possibly be employed to control the supramolecular polymerization process of bifunctional UPys. Herein, we provide a detailed study to demonstrate the relationship of the π−π stacking interaction and the ring−chain equilibrium of DNP bridged bifunctional UPy derivatives in supramolecular system. It was recognized that the strength of the π−π stacking interaction in the cyclic monomers was inversely proportional to the distance between DNP plane and intramolecularly dimerized UPy plane which was directly determined by the length of oligo(ethylene oxide) (oligoEO) chain as spacers in the derivatives, so we designed L1a with shorter spacer (n = 1) that would possess stronger π−π stacking interaction in its cyclic form and L3 with longer spacer (n = 3) that would have weaker π−π stacking interaction (Figure 1). The stronger π−π stacking interaction involved in the cyclic monomer is, the more stable architecture it has. Thus, we suppose that, compared to L2, L1a would possess a larger CPC value while L3 would have a relatively smaller CPC value during supramolecular polymerization process. Indeed, the final experimental results verified our hypothesis, but what tremendously surprised us was that L1a possessed a very huge CPC value larger than 500 mM, leading to a highly stable cyclic monomer over this concentration range in solution. The stability and morphology of the assemblies of L1a, L2, and L3 were found to be greatly influenced by their subtle structural variation of spacers and different polarity of solvents used. Moreover, in an attempt to gain further insight into cyclic monomer formation, we have successfully obtained and analyzed the crystal structure of the methyl-substituted UPy analogue L1b (n = 1). To the best of our knowledge, it is the first time to realize a highly stable cyclic monomer based on bifunctional UPys that could persist both in solution over a wide concentration range and in the solid state. On the basis of these interesting discoveries, we have focused on ring−chain equilibrium controlled by the strength of intramolecular π−π stacking interaction to (1) systematically analyze the supramolecular polymerization process of DNP bridged bifunctional UPy derivatives L1a, L2, and L3, (2) thoroughly study the parameters of the cyclic monomers in the solid state, which would provide great meaningful information to ring−chain equilibrium, and (3) investigate the self-assembly behavior of

L1a, L2, and L3 with the electron-deficient macrocycle CBPQT4+ molecule by host−guest interaction. By carefully designing the monomer structure especially with different lengths of spacers, combined with the assistance of supramolecular interaction, intramolecular π−π interaction, or intermolecular host−guest interaction, the distribution of ring and chain could be highly controlled. This would provide us a better understanding of the correlation between monomer architecture, its cyclization and polymerization, and noncovalent interaction assistance, which allows tuning the CPC to control ring−chain equilibrium for the development of “smart materials” capable of responding to external stimuli.

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RESULTS AND DISCUSSION Monomer Synthesis. We have designed and successfully synthesized a series of DNP bridged bifunctional UPy compounds, which can be employed as perfect modeling supramolecular monomers to study the ring−chain equilibrium of hydrogen-bonded supramolecular polymers. The key difference of these compounds is the different length of the oligoEO chain as spacer, which could determine the strength of intramolecular π−π stacking interaction in cyclic monomer, while the strength of intramolecular π−π stacking interaction could determine the CPC of the supramolecular monomer. This perfectly controllable supramolecular systerm is very difficult to realize in previously reported systems which did not have DNP or other aromatic rings as π donor group located at the bridged postion. The synthesis of the bifunctional UPys employed in this system is straightforward and based on previously reported methods.23 Coupling of the bifunctional amine terminated DNP bridged ethylene oxides M1−M3 with 1,1′-carbonyldiimidazole (CDI) activated alkyl-substituted pyrimidinone resulted in the desired DNP bridged bifunctional UPy molecules L1−L3 (Scheme 1). L1a, with a flexible 1ethylpentyl substituent at the 6-position of pyrimidinone, is Scheme 1. Synthesis of L1−L3

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the cyclic monomer concentration against the total monomer concentration. The plotted data demonstrated that the concentration of the cyclic monomer was steadily increasing (slope is nearly 1), and then it reached a plateau as the total monomer concentration increased, where a CPC of ca. 23 mM of L3 was obtained (Figure S4, Supporting Information). Monomer L2 was investigated by 1H NMR spectroscopy in a similar manner in CDCl3 solution. Based on concentrationdependent 1H NMR spectroscopy, monomer L2 was also found to perform the ring−chain mechanism-based polymerization process (Figure S1). A CPC of ca. 70 mM of L2 was also obtained from the plotted data by 1H NMR spectroscopy (Figure S4), which was much bigger than L3. This is due to the shorter length of the oligoEO chain in L2 compared to L3, resulting in much stronger intramolecular π−π stacking interaction between DNP unit and dimerized UPy motif in the cyclic monomer of L2. That is to say, the structure of cyclic form of L2 is much more stable than L3 because of the stronger π−π stacking, which leads to a relatively higher CPC. After we have recognized that stronger π−π stacking interaction leads to higher CPC, we started thinking what would happen if the length of the oligoEO chain in L2 became shorter. Would it form a much more stable cyclic monomer structure with very high CPC or much possibly form a stable cyclic dimer structure as the smallest assembly or form linear polymer because of the failure of intramolecular cyclization in solution? As a consequence, we designed and successfully synthesized monomer L1a, which possesses a shorter spacer with n = 1 to investigate its self-assembly behavior. The preliminary study taken by 1H NMR spectroscopy and 2D NOESY suggested that the highest possibility of the selfassembly of L1a was to form an extraordinarily stable cyclic monomer with a CPC larger than 500 mM. In 1H NMR spectra no chemical shift change and only one set of signals were observed for L1a upon increasing its concentration from 16 to 500 mM, reflecting that L1a was in the form of an extremely stable cyclic monomer in solution (Figure 3). Furthermore, proton signals of L1a were still very sharp even at a concentration of 500 mM, which also provided direct evidence that no polymerization process occurred. In order to verify that

highly soluble in CHCl3 but not suitable for crystal growth. In order to obtain the crystal by reducing the flexibility, we have designed and successfully synthesized the methyl-substituted analogue L1b, which is much more suitable for crystal growth. NMR Spectroscopy Studies. Ring−chain equilibria of the supramolecular monomers L1a, L2, and L3 were initially studied by 1H NMR spectroscopy in CDCl3 solution. Figure 2

Figure 2. Partial 1H NMR spectra (300 MHz, CDCl3, 298 K) of L3 at different monomer concentrations, from bottom to top: (a) 4, (b) 16, (c) 32, (d) 48, (e) 100, (f) 150, and (g) 200 mM. Signals from cyclic aggregates are labeled “c”, and signals from polymeric aggregates are labeled “p”.

shows part of the concentration-dependent 1H NMR spectra of L3, where the signals of the UPy protons and DNP protons are displayed. At low concentration ( 0.30 (Figure 5), indicating that the monomeric ring form of L1a is highly stable. While studying the 13C NMR of L1a at 200 mM in CDCl3, we were attracted by an interesting phenomenon that carbon signals corresponding to 1-ethylpentyl substituent attached to the 6-position of pyrimidinone were split into two sets of peaks, suggesting that L1a existed in a cyclic monomer form with some special structure in CDCl3 (Figure 6c and Figure S13). This might be rationalized to the chiral carbon in 1-ethylpentyl substituent, and two chiral centers would be integrated into dimerized UPys although the commercially available original reagent is racemic. However, according to the reported literature, only one set of peaks could be observed for the binding of two UPy units containing 1-ethylpentyl substituent via quadruple hydrogen bonding in CDCl3, implying that two-

Figure 6. Partial 13C NMR spectra (100 MHz, 298 K, 200 mM) of (a) L3 in CDCl3, (b) L1a in DMSO-d6, and (c) L1a in CDCl3. The asterisk symbol indicates the chiral carbon atom.

UPy binding moiety is undergoing quick dynamic exchange with others no matter for monofuctional or bifunctional UPys.14,25 For instance, in the spectrum of either L1a in its open form in DMSO-d6 that would destroy its stable cyclic monomer form (Figure 6b) or L3 as a supramolecular polymer in CDCl3 (Figure 6a), only one set of peaks without any carbon split were shown, indicating that UPy units within the assembly are free enough for dynamic exchanging and a racemic solution was obtained in the end. However, for L1a in CDCl3, the two UPy units within one monomer are not only dimerized but also tethered tightly by a linker simultaneously, leading to an extremely stable cyclic monomer structure that prevents the quick dynamic exchange of UPy. As a result, although the CDI activated pyrimidinone precursors are racemic compounds, the two chiral carbon centers of L1a are fixed in its highly stable cyclic monomer form, leading to two sets of enantiomers, (R,R)-L1a and (S,S)-L1a, (R,S)-L1a and (S,R)-L1a (Scheme S6). Since enantiomers showed the same signals in NMR spectrum while diastereoisomers exhibited different, the signals of these six carbon atoms attached to the chiral carbon were split into two sets of peaks in the 13C NMR spectrum. Conversely, this phenomenon also supported the formation of highly stable cyclic monomer of L1a again. X-ray Crystal Structure Analysis. So far, little has been known about the nature of a cyclic monomer of bifunctional UPys in the solid state. Encouraged by the highly stable 9589

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intermolecular π−π interaction produced from two dimerized UPy planes. This phenomenon was further verified by molecular packing in the solid state of L1a’s analogue L1b. As shown in Figure 8, each cyclic monomer of L1b lies in close

properties of L1a in solution, we synthesized its methylsubstituted UPy analogue L1b, which was suitable for crystal growth, to get deeper insight into the molecular parameters that govern supramolecular polymerization. Fortunately, we obtained a colorless single crystal by slow diffusion of cyclohexane to a solution of L1b in 1,2-dichloroethane. The solid-state structure of L1b is illustrated in Figure 7. The unit cell of the

Figure 8. Packing plot of L1b in the solid state. Hydrogens and solvent molecules are omitted for clarity. Face-to-face parameters: centroid−centroid distance (Å) a, 3.69; b, 3.58; c, 5.07.

Figure 7. Top view (a) and side view (b) of the crystal structure of L1b showing the intramolecular quadruple hydrogen-bonded array. Solvent molecules and H atoms not involved in hydrogen bonding have been omitted for clarity. Hydrogen bond parameters are as follows: N···O (N) distance (Å), H···O (N) distance (Å), N−H···O (N) angles (deg): a, 2.72, 1.91, 156.5; b, 2.90, 2.04, 174.0; c, 2.99, 2.16, 161.7; d, 2.71, 1.96, 165.5; e, 2.65, 2.04, 127.8; f, 2.47, 1.81, 132.1. Face-to-face π-stacking parameters: centroid−centroid distance (Å) g, 3.33; ring plane−ring plane inclination (deg): 1.67.

proximity to the other cyclic monomer with an interplanar distance (distance between two dimerized UPy planes) of 3.26 Å. The intermolecular centroid to centroid distance (C−C distance) of one pyrimidinone ring involved in a UPy plane to the other corresponding one of the adjacent monomer is 3.58 Å (Figure 8, b). However, the naphthalene ring is located far from the naphthalene ring of the adjacent monomer (C−C distance 5.07 Å, Figure 8, c), indicating that there is no π−π stacking interaction between them. This is consistent with the results obtained from concentration-dependent 1H NMR spectroscopy of L1a in solution, in which proton signals of UPy showed obvious movements while proton signals of DNP exhibited no change upon increasing its concentration (1.6−16 mM). Viscometry. To further test the ring−chain equilibrium, viscosity measurements of L1a, L2, and L3 were carried out in CHCl3 using a micro-Ubbelohde viscometer. A doublelogarithmic plot of specific viscosity versus monomer concentration was obtained as shown in Figure 9. We found that the slope of L1a in the whole concentration range tended to be 1, which is characteristic for cyclic oligomers with constant size, suggesting the formation of extremely stable cyclic monomer in solution. In contrast to L1a, L2 and L3 showed a ring-opening polymerization process with a sharp rise in the viscosity upon the increasing of the concentration. Their stronger concentration dependence (slopes of 3.14 and 2.81 were found for compound L2 and L3, respectively) indicates the formation of larger entangled polymers of increasing size. CPCs of these two compounds, L2 and L3, were determined based on this plot, yielding values of ca. 69 and 23 mM, respectively, which are in accordance with the results obtained

crystal is triclinic and belongs to the P-1 space group. The crystal structure clearly shows that the two UPy units of L1a adopted an intramolecularly dimerization through a DDAA type of hydrogen-bonded array. Two UPy units were preorganized to bind each other by an intramolecular hydrogen bond from the pyrimidine N−H to the urea carbonyl group. The DNP unit is located on the top of the dimerized UPy motif in an almost parallel fashion, leading to a gap with the plane-toplane distance of 3.33 Å, which is characteristic of effective π−π stacking interaction. This relatively strong π−π stacking interaction was remarkably important for the stability of the cyclic monomer structure of L1b. The crystal structure of L1a’s analogue L1b supported the cyclic monomer form of L1a, and the monomeric cyclization of L1a, L2, and L3 was directly determined by the strength of intramolecular π−π stacking interaction. It would be weakened by the bigger distance between DNP unit plane and dimerized UPy plane, which was determined by the length of the oligoEO chain. This is consistent with the results deduced from 1H NMR investigation. As discussed above, monomer L1a in solution could be associated into supramolecular dimer pairs through weak 9590

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sizes of hydrogen-bonded aggregates exhibit different diffusion coefficients.6,27 So, we also measured the DOSY NMR spectra of L1a, L2, and L3 to confirm their self-assembly property in solution (Figure 10). On the basis of 1H NMR diffusion measurements, the aggregate for L1a was determined to be monomeric in size which was in line with other evidence mentioned above. The diffusion coefficient of the aggregate of L1a (5.25 × 10−10 m2 s−1, 16 mM, Mn = 805 g/mol, 2Mn = 1610 g/mol) showed a larger value than heptakis(2,3,6-tri-Omethyl)-β-cyclodextrin (4.03 × 10−10 m2 s−1, Mn = 1429 g/ mol), implying that L1a exhibited as cyclic monomers in solution although it existed as a dimer pair associated by weak intermolecular π−π stacking interaction (Figure 10a). As shown in Figure 10, the signals for L2 and L3 in concentrated solution displayed a broad distribution, reflecting the broad molecular weight distribution of different aggregates. On the contrary, the peaks for L1a in concentrated solution showed a very well-defined distribution, corresponding to a single type of aggregate in solution. Moreover, the diffusion coefficient of the predominant species of L2 and L3 in concentrated solution is smaller than L1a, corresponding to the formation of sizable supramolecular polymers. It should be noted that the signals of L2 are more disordered than L3, which might be due to the bigger CPC of L2 than L3. Host−Guest Studies. Since each monomer of L1−L3 employed DNP as a π-donor unit, which is also an electron-rich group, it provided us a convenient way to study the complexation of L1−L3 with the well-known electron-deficient macrocycle CBPQT4+. Our previous study23 showed that L2 could perform selective cyclization over a wide range of

Figure 9. Specific viscosity of chloroform solutions of L1a (▲), L2 (●), and L3 (■) versus the concentration (298 K). Values on the curves indicate the slope.

from 1H NMR experiments. There are two points here needed to be noted. First, the slope of L3 (2.81) in high concentration is smaller than L2 (3.14). Second, both slopes of L2 and L3 are relatively small compared to previous reported bifunctional UPy molecules. These results should be attributed to the competitive hydrogen bonding produced from the oligoEO chain.26 DOSY Study. Diffusion-ordered 1H NMR spectroscopy (DOSY) is a sensitive technique to investigate the quadruple hydrogen-bonding supramolecular polymers, since the different

Figure 10. DOSY spectra (400 MHz, CDCl3, 298 K) of (a) L1a in 16 mM with the addition of peralkylated β-CD as internal standard, (b) L1a in 200 mM, (c) L2 in 200 mM, and (d) L3 in 200 mM. 9591

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concentration after threading into CBPQT4+, resulting in the formation of a novel dynamic [2]catenane instead of noncovalent main-chain polypseudorotaxanes (Figure 11).

threading procedure in the mixed solvent CDCl3/CD3CN. In contrast to monomer L2 or L3, no chemical shift change was observed for C2 or C3 over the entire concentration range, reflecting that C2 or C3 was in the form of the stable [2]catenane in solution (Figures S2 and S3).

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EXPERIMENTAL SECTION

General Methods. All solvents and reagents were purchased from commercial suppliers and used as received unless otherwise stated. Details of the synthesis and characterization of all new compounds are given in the Supporting Information. Dry chloroform for the synthesis was obtained by purification of analytical grade chloroform by extractions with water for 8 times, followed by drying over anhydrous MgSO4 for 24 h and distillation. NMR spectra were recorded on a Bruker DPX 300 MHz or a Bruker AVANCE III 400 MHz spectrometer with internal standard tetramethylsilane (TMS) and solvent signals as internal references, where CDCl3 and CD3CN were dried using neutral aluminum oxide. Low-resolution electrospray ionization mass spectra (LR-ESI-MS) were obtained on Finnigan Mat TSQ 7000 instruments. High-resolution electrospray ionization mass spectra (HR-ESI-MS) were recorded on an Agilent 6540Q-TOF LCMS equipped with an electrospray ionization (ESI) probe operating in positive-ion mode with direct infusion. Single-Crystal X-ray Analysis. Single crystal X-ray data were measured on a Bruker SMART APEX II CCD diffractometer (Mo Kα radiation, λ = 0.710 73 Å). Structure solutions and refinements were carried out using the SHELXTL-PC software package.28 Crystallographic data of L1b: colorless, C30H36N8O9, Mr = 654.68, triclinic, space group P1̅, a = 11.3045(12), b = 12.4390(14), c = 14.2130(15) Å, α = 72.0610(10)°, β = 74.1310(10)°, γ = 73.0230(10)°, V = 1781.2(3) Å3, z = 2, Dc = 1.221 g cm−3, T = 296(2) K, μ = 0.092 mm−1, 7017 measured reflections, 4438 independent reflections, F(000) = 692.0, R1 = 0.0948, wR2 = 0.1291(all data), R1 = 0.0642, wR2 = 0.1243 [I > 2sigma(I)], largest difference peak and hole 0.179 and −0.705 e·Å−3, goodness-of-fit on F2 = 1.191, CCDC − 889596. DOSY NMR. 1H NMR diffusion measurements were carried out at 298 K on a Bruker AVANCE III 400 spectrometer. The ledbpgp2s pulse sequence from Bruker Biospin was selected for the DOSY NMR by using gradients varied linearly from 5% up to 95% in 32 steps, with 16 scans per step. The diffusion time (Δ) was set at 20 ms, and the gradient length (δ) was set at 2 ms. Heptakis(2,3,6-tri-O-methyl)-βcyclodextrin (peralkylated β-CD) was used as internal standard if necessary. Viscometry. Solution viscosities were measured using Ubbelohde microviscometers (Shanghai Liangjing Glass Instrument Factory, 0.40 mm and 0.71 mm inner diameter) at 298 K in chloroform. The microviscometers were thermostated in a water bath. Each sample was filtered over a short cotton plug before measurement.

Figure 11. Representation of the selective cyclization of L2 or L3 after threading into CBPQT4+.

Therefore, we are curious to investigate the assembly behavior of L1a and L3 with CBPQT4+, which would provide valuable information for ring−chain mechanism research. To our surprise, L1a is too stable to thread into the cavity of CBPQT4+ even subjected to heat at 50 °C for 36 h, which agrees well with the results above (Figure S5). The highly stable property of L1a as cyclic monomers prolonged preexchange lifetime of UPy dimer, which gave no chance for L1a to break its cyclic form to penetrate into the CBPQT4+ (Figure S5). Without CBPQT4+, similar to L2, L3 self-assembled to form linear supramolecular polymers in concentrated solution, which followed a ring−chain mechanism causing a significant increase in the viscosity. Moreover, the final results showed that L3 could also form dynamic [2]catenane (Figure 11) although its stability is not so good, which might be due to the size mismatch between L3 having longer oligoEO chain and CBPQT4+. As shown from the 1H NMR spectra (Figure S8), partial dissociation of C3 upon time extension was observed. In contrast to monomer L2 or L3 alone, the resulting [2]catenane of L2 or L3 with CBPQT4+ was highly stable in solution over a wide concentration range, leading to relatively low viscosity in concentrated solution. Their process of “threading followed by selective cyclization” was studied by 1H NMR spectra taken at regular time intervals (Figures S6 and S7). The spectra showed that the threading rate of L3 with CBPQT4+ was much faster than L2, indicating that L3 is more flexible than L2. The original signals of DNP protons both in L2 and L3 gradually decreased, accompanied by the appearance of a new set of signals upfield, indicating a slow-exchange process of the

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CONCLUSIONS In summary, we have discovered that there exists π−π stacking interaction between the electron-rich DNP unit and dimerized UPy motif in the cyclic monomer form of DNP bridged bifunctional UPy derivatives. By taking advantage of the π−π stacking interaction, we have designed and synthesized several model compounds with different lengths of spacer which can determine the intensity of the intramolecular π−π stacking interaction to investigate their ring−chain equilibrium. As a result, the CPC could be tuned over a broad concentration range based on this strategy. The ring−chain equilibrium of our model compounds have been studied by employing a combination of techniques, such as 1H NMR, DOSY NMR, and viscosity measurements. During the research process, the bifunctional UPy derivative L1a, which possesses the shortest flexible oligoEO chain, was found to form highly stable cyclic monomer in solution over a wide concentration range. And for the first time, we have observed the solid-state structure of 9592

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bifunctional UPy derivative in its monomeric cyclic form. Furthermore, the selective cyclization of bifunctional UPybased supramolecular polymers could be attained by introducing a macrocycle molecule based on the host−guest interaction. Either the intramolecular π−π stacking interaction or the intercomponent host−guest interaction involved in this study to guide the self-assembly of bifunctional UPys represents a new useful strategy for controlling the ring−chain equilibrium of supramolecular polymers or engineering specific supramolecular architectures. The detailed parameters involved in the ring−chain equilibrium of the present study may have great significance in devising supramolecular “smart materials”, such as well-defined molecular machines, foldamers, and tunable materials.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures and characterization data for new compounds; analytical data including various 1H NMR, NOESY; details of the X-ray analyses including CIF file. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+86)25-83317761; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2013CB922101, 2011CB808600), the National Natural Science Foundation of China (No. 20932004, 21072093, 91227106), and Natural Science Foundation of Jiangsu (BK2011551). We are grateful to Prof. Bert Meijer from Eindhoven University of Technology and Dr. Clever Guido from Göttingen University for fruitful discussions and suggestions on the NMR measurement and analysis.

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