Supramolecular Copolymer Constructed by Hierarchical Self- Assembly of Orthogonal Host−Guest, H‐Bonding, and Coordination Interactions

Supramolecular copolymers with complex architectures and emergent functions constitute a class of challenging but enticing synthetic targets in polymer science. Individual building blocks can be tailored to endow a resulting supramolecular copolymer with increased structural and functional complexity. Herein, we describe the construction of a linear supramolecular copolymer comprising mechanically interlocked segments with hydrogen-bonding metallorhomboidal units. Specifically, a hierarchical supramolecular polymerization of a crown ether-based [2]rotaxane and a discrete organoplatinum(II) metallacycle driven by 2-ureido-4pyrimidinone (UPy) quadruple hydrogen bonding provides the impetus for its formation. This system demonstrates enhanced structural complexity accessed by the unification of orthogonal noncovalent interactions: metal coordination, host− guest chemistry, and multiple hydrogen bonding interfaces. N juxtaposes modular building blocks in a precise manner to create vast libraries of functional polymeric scaffolds in a simultaneous multistep self-assembly process based on the interplay of covalent and noncovalent bonds. Supramolecular copolymers (SCs), in which noncovalent interactions replace the covalent linkages found in traditional copolymers, bridge the gap between synthetic and natural systems. The interface of supramolecular chemistry and polymer science enables unique dynamic and reversible properties that are not achievable in covalently bonded copolymers. Whereas many supramolecular polymers contain only a single repeating unit, a copolymer formulation allows for the incorporation of additional functionalities to grow the complexity of the resultant material. This efficient and modular assembly strategy motivates the design of novel building blocks to ultimately govern and deliver densely multifunctionalized polymeric architectures. The motifs available for the construction of SCs include mechanically interlocked structures and metallo-supramolecular architectures. The high degree of freedom of the mechanical interfaces and the possibility to control the motions and relative positioning of the subcomponents enable the formation of materials with unusual physical and mechanical properties. In particular, rheological, mechanical, dynamic, and self-assembly behaviors, which critically depend on molecule-level flexibility and mobility, should be affected by the introduction of the defined mechanical bonds. In artificial systems, coordinationdriven self-assembly has proven to be another versatile and valuable design methodology to control predesigned and welldefined topological modules. The programmed organization of functional molecular components into unified entities using directional metal−ligand interactions may impart considerable stability to a given architecture and also introduces the possibility of hierarchical assembly. Such SCs may be applied to the fields of heterocatalysis, light harvesting, and gas storage, etc. It can be difficult to access SCs that contain complex, functional modules when utilizing only single or dual noncovalent interactions. Orthogonal self-assembly, an emerging principle for the combination of multiple noncovalent interactions into one architecture, provides an alternative route for the construction of SCs. Herein, we unify the modules of mechanically interlocked monomers and discrete metallacycle units, both decorated with self-complementary UPy quadruple hydrogen bonding motifs, into a SC. The orthogonal assembly of crown-ether-based [2]rotaxane host−guest interReceived: April 13, 2016 Accepted: May 11, 2016 Published: May 16, 2016 Letter pubs.acs.org/macroletters © 2016 American Chemical Society 671 DOI: 10.1021/acsmacrolett.6b00286 ACS Macro Lett. 2016, 5, 671−675 actions, coordination-driven self-assembly of organoplatinum(II) metallacycles, and multiple hydrogen bonding delivers a SC that possesses a random distribution of mechanically interlocked moieties with metallorhomboidal cores (Scheme 1). The reaction of UPy-functionalized 3 (Scheme S1) and 4, in a Huisgen-type copper(I)-catalyzed 1,3-dipolar azide−alkyne cycloaddition method, gave [2]rotaxane 1 with UPy units on the “axle” and the “wheel” in 50% isolated yield (Scheme S2). Electrospray ionization mass spectrometry (ESI-MS) of the [2]rotaxane provided evidence for its formation. In the mass spectrum of the [2]rotaxane (Figure S11), peaks at m/z = 889.9 (100%) corresponding to [M − PF6 + H] and m/z = 1778.6 (40%) corresponding to [M − PF6] were observed, and almost no peaks of other compounds were found, which gave strong evidence for the formation of the [2]rotaxane. By employing correlation spectroscopy (COSY), partial signals from the “axle” and the “wheel” could be identified clearly (Figure S12). The assignment and correlation of the protons of [2]rotaxane 1 were also validated by its NOESY NMR spectrum (Figure S13). For example, NOE correlation signals were found for H1c,e and protons of the glycol chain on the “wheel” and H1b,i,j,k on the “axle”, indicating that the formation of the threaded structure brought these two sets of protons very close to each other in space. With the UPy-functionalized [2]rotaxane 1 in hand, we investigated the formation of a linear supramolecular rotaxane polymer (SRP-1 in Scheme 1). Though head-to-tail interactions of the UPy units may exist in the polymer chain since the same H-bonding motif is utilized on both ends of 1, we only display head-to-head and tail-to-tail interactions in the cartoon illustration of SRP-1 for clarity. H NMR was used as a diagnostic tool with [2]rotaxane concentrations in the range of 1.25−110 mM (Figure 1). A mixed-solvent condition (CDCl3/ CD3CN (8:1, ν/ν)) was chosen to make the [2]rotaxane more soluble. The concentration-dependent H NMR spectra indicated the involvement of fast-exchanging noncovalent interactions in solution. The large downfield chemical shifts (between 10.0 and 13.5 ppm) and lower intensities observed on UPy N−H indicated the dimerization of the UPy motifs. At a concentration of 10.0 mM, the signal of H1a on 1 split into two peaks. With the increase of the initial concentration of [2]rotaxane to 20.0 mM, H1b, PhH on the “axle” eventually merged together with H1c on the “wheel” (Figure S14). Further increases of concentration made both of the signals shift downfield along with that of H1d on rotaxane 1. These chemical shift changes demonstrated that the percentage of intermolecular UPy dimerization was concentration-dependent, and the supramolecular polymer was favored at high concentrations. Moreover, the splitting of peaks was gradually attenuated, along with broadening of all proton signals at high concentrations, further indicating the formation of high-molecular-weight polymeric structures. Considering the existence of host−guest interactions in this supramolecular polymer, we also studied the dynamics of the polymer by H NMR (Figure S21). However, no obvious chemical shift changes were observed by the sequential addition of triethylamine (TEA) and trifluoroacetic acid (TFA) into the solution of the [2]rotaxane, indicating that it was difficult to realize the reversible shuttling of the [2]rotaxane upon the addition of base/acid. This phenomenon is possibly a result of the combination of the following two factors: first, the distance between the sec-ammonium site and the counterion (PF6 −) may be very close in this mechanically interlocked structure, so the Scheme 1. Cartoon Representations of (a) [2]Rotaxane 1 and Metallacycle 2 and (b) Formation of Supramolecular Polymers SRP-1 and SP-2 and Supramolecular Copolymer SC-3 Figure 1. Partial H NMR spectra of [2]rotaxane 1 (CDCl3/CD3CN (8:1, ν/ν), 298 K, 500 MHz) at different [2]rotaxane concentrations: (a) 110 mM; (b) 75.0 mM; (c) 50.0 mM; (d) 30.0 mM; (e) 20.0 mM; (f) 10.0 mM; (g) 5.00 mM; (h) 2.50 mM; (i) 1.25 mM. ACS Macro Letters Letter DOI: 10.1021/acsmacrolett.6b00286 ACS Macro Lett. 2016, 5, 671−675 672 strong ion−ion interactions between them are difficult to be broken under the basic condition; second, there is no other strong binding site on the “axle” even if the sec-ammonium site can be basified. To substantiate the formation of SRP-1, two-dimensional diffusion-ordered H NMR spectroscopy (2D DOSY) was used to evaluate the dimensions of the supramolecular aggregates. As the concentration of [2]rotaxane 1 increased from 1.25 to 30.0 mM, the measured weight-average diffusion coefficient D decreased from 4.56 × 10−10 to 0.55 × 10−10 m s−1 (D30.0 mM/ D1.25 mM ≈ 8.3) (Figure 2a and Figures S15−S20). These results are consistent with the aforementioned concentrationdependent H NMR results, indicating that concentration plays a significant role in the supramolecular polymerization process. DOSY NMR spectra confirmed that the presence of the rotaxane structure improved the efficiency of long-chain polymerization over cyclic oligomers. This phenomenon can be attributed to the introduction of relatively rigid mechanical interfaces along the polymer backbone that provide a steric hindrance for the formation of cyclic oligomers and, therefore, improve the efficiency of polymerization. Viscometry is a convenient and reliable method to test the propensity of monomers to self-assemble into large aggregates. Therefore, viscosity measurements were carried out in CHCl3/CH3CN (8:1, ν/ν) using a Cannon−Ubbelohde semimicro dilution viscometer to address the self-assembly behavior of the linear supramolecular rotaxane polymer (Figure 2b). Considering the strong hydrogen bonding interaction of UPy units, the [2]rotaxane was expected to form an extremely viscous soft material at relatively high concentrations, obviating viscosity measurements. As such, the UPy concentration was varied from 3.40 to 133 mM. For most reported linear supramolecular polymers, a sharp transition from oligomers to polymers is evidenced by a change in slope in the double logarithmic plot of specific viscosity versus concentration. For our system, a linear relationship between specific viscosity and concentration with only one slope (slope = 2.04) was observed, which indicated the absence of cyclic oligomers in the concentration range of study as well as the formation of a linear supramolecular polymer. The relatively low slope compared with that of the similar ureidopyrimidone assembly system with slope of ∼3.5 reported by Meijer can be attributed to the introduction of the large rotaxane units. This conclusion is in good agreement with the results of the H NMR measurement and the DOSY NMR investigation. After the successful preparation of the mechanically interlocked subunit, we constructed a second building block based on a metallacycle for the synthesis of the desired SC. Multinuclear (H and P NMR) analyses (Figures S22−S24) and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) of the product supported the formation of a highly symmetric [2 + 2] assembly, 2 (Figure S25). A related linear supramolecular polymer constructed from a similar metallacycle has been reported by our group. DLS revealed that the solutions of the [2]rotaxane and metallacycle have sizes of about 79 and 164 nm at 1.00 mM, respectively (Figure S26). This result indicated that the organoplatinum(II) metallacycle monomer had a higher tendency to polymerize than the [2]rotaxane monomer. The morphologies of the gel-like solutions of both SRP-1 and SP-2 (Scheme 1) prepared by a freeze-drying methodology were examined by SEM, revealing porous structures in which the high UPy dimerization efficiency and the entanglement of long-chain polymers were responsible for the observed gel-like behaviors (Figure S27). We expected that a copolymer held together by both host/ guest interactions and hydrogen bonding would form when mixing these two building blocks together. As such, [2]rotaxane 1 and rhomboidal metallacycle 2 were mixed in a 1:1 molar ratio in CH2Cl2, triggering the formation of a supramolecular copolymer (Scheme 1a, route (ii)). NMR was employed to track the formation of SC-3. Although the H NMR spectrum of SC-3 appears like a simple overlay of the NMR spectrum of SRP-1 and the NMR spectrum of SP-2, this result is reasonable considering that the mixing process involves only an exchange of the hydrogen bonding between the two building blocks (Figure 3). For the P{H} NMR spectra, both SP-2 and SC-3 possessed a sharp singlet at ∼12.37 ppm with concomitant Pt satellites (Figure S33). In the P{H} NMR spectrum of SC-3, five more peaks between −137 and −152 ppm appeared due to the PF6 − counterions of the SRP-1 building blocks. The metallacycle cores thus remain intact within the SC, due to the orthogonality of the noncovalent interactions employed. In addition, the SC could also be directly obtained via orthogonal self-assembly by mixing [2]rotaxane 1, 120° UPy-functionalized dipyridyl ligand 5, and 60° organoplatinum(II) acceptor 6 in a 1:2:2 molar ratio (Figures S28 and S29). Figure 2. (a) Concentration dependence of diffusion coefficient D [CDCl3/CD3CN (8:1, ν/ν), 298 K, 500 MHz)] of SRP-1. (b) A double logarithmic plot of specific viscosity of solution of [2]rotaxane 1 in CHCl3/CH3CN (8:1, ν/ν) at 298 K versus UPy unit concentrations. Figure 3. Partial H NMR spectra (CD2Cl2, 293 K, 500 MHz): (a) SP-2; (b) SC-3; (c) SRP-1. c (UPy unit concentration) = 30.0 mM. ACS Macro Letters Letter DOI: 10.1021/acsmacrolett.6b00286 ACS Macro Lett. 2016, 5, 671−675 673 The polymeric nature of SC-3 was investigated by differential scanning calorimetry (DSC). Figure 4a shows the DSC traces of SRP-1, SP-2, and their blend SC-3 in a 1:1 molar ratio recorded during a second heating scan. For SRP-1, the heat capacity change at 68 °C was attributed to the glass transition temperature (Tg). No clear Tg could be observed for SP-2 in the measured temperature range. For the 1:1 molar mixture of 1 and 2, we observed an increased Tg value (Tg3 = 104 °C) compared with that of SRP-1, confirming the formation of a homogeneous blend. The thermal stabilities of the three polymers were investigated by thermogravimetric analysis (TGA). The decomposition curve for 1·2 is almost completely located between those for SRP-1 and SP-2 (Figure S36), which again is consistent with the formation of a novel supramolecular polymer. We also investigated the DSC thermograms of SC-3 by changing the feed ratio of the [2]rotaxane and metallacycle (Figures S38−S40). The glass transition temperature of the supramolecular copolymer decreased upon the increase of the percentage of the [2]rotaxane, indicating that the copolymerization of the [2]rotaxane monomer and the metallacycle monomer occurred. 2D DOSY NMR spectra were obtained to reveal spatial information about the supramolecular aggregates (Figure 4b). For comparison, the DOSY NMR experiments of SRP-1, SP-2, and the copolymer were all performed at 30.0 mM (UPy unit concentration) in CD2Cl2 to yield average diffusion coefficients as 16.6 × 10−11, 2.62 × 10−11, and 6.31 × 10−11 m s−1, respectively. Here the stiffness of the metallacycle moiety is higher than that of the rotaxane unit, so the degree of polymerization of SRP-1 is smaller than that of SP-2 at the same monomer concentration. Accordingly, we observed that the diffusion coefficient of SP-2 was smaller than that of SRP-1. The copolymer showed a smaller diffusion coefficient compared to that of SRP-1 and a larger diffusion coefficient compared to that of SP-2. The polymerization progress of SC-3 was investigated by H NMR spectroscopy at UPy unit concentrations ranging from 1.25 to 60.0 mM (Figures S34 and S35). Upon increasing the concentration, the signals from 1 or 2 shifted downfield (H5a,5b, H1b,1c,1d), and the signal on acceptor 6 (H6a) shifted downfield and eventually merged together with one set of H5a protons at 10.0 mM. Above 20.0 mM, the peak splitting disappeared gradually, along with broadening of all signals, indicating the formation of a linear supramolecular copolymer. The blend SC-3 was further characterized by measuring the solution viscosity in CH2Cl2. The plot of the specific viscosity of the solution of 1·2 with a 1:1 molar ratio vs the UPy unit concentration was linear over the concentration range 4.00−24.0 mM with only one slope (slope = 1.41) (Figure S37). Meanwhile, scanning electron microscopy (SEM) studies showed that long rod-like fibers could easily be drawn from a concentrated solution of SC-3 in CH2Cl2, reflecting significant chain extension to generate the desired high molecular polymeric aggregates (Figure 4c). In conclusion, we have synthesized a UPy-functionalized [2]rotaxane and investigated the construction of a linear supramolecular rotaxane polymer by supramolecular polymerization of a [2]rotaxane driven by a quadruple hydrogen bonding motif. A linear polyrhomboid supramolecular polymer was prepared with high efficiency by means of the directionalbonding approach. On the basis of the presence of selfcomplementary UPy units, a homogeneous linear SC randomly linked by mechanically interlocked moieties and H-bonding metallacycles was obtained by mixing the UPy-functionalized [2]rotaxane and UPy-decorated metallacycle together in one system. This study found that employing organoplatinum(II) metallacycles and rotaxanes as building blocks of supramolecular polymeric backbones enhanced supramolecular polymerization efficiency by limiting the formation of cyclic oligomers. Moreover, the orthogonal strategy based on noncovalent bonds used here offered advantages over covalent methods for obtaining complicated and advanced structures in a highly modular fashion. As we can control the properties of both precursors in the SC, the resultant material benefits from the unique functionalities and growing complexity introduced by hierarchical self-assembly. Given the improved properties of supramolecular coordination complexes and the favorable properties of mechanical bond-containing materials, we expect that the fundamental results presented here define a simple yet highly efficient way to construct modular multifunctional advanced materials. ■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00286. Experimental details and additional data (PDF) ■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].