Donor–Acceptor-Type Supramolecular Polymers Derived from Robust

Article pubs.acs.org/Macromolecules

Donor−Acceptor-Type Supramolecular Polymers Derived from Robust yet Responsive Heterodimeric Tweezers Yu-Kui Tian, Yi-Fei Han, Zhi-Shuai Yang, and Feng Wang* CAS Key Laboratory of Soft Matter Chemistry, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, P. R. China S Supporting Information *

ABSTRACT: Molecular tweezer/guest recognition has emerged as a novel motif for the construction of supramolecular polymers. However, the overwhelming majority of ADA- or DAD-type (D = donor, A = acceptor) molecular tweezer/guest recognition systems suffer from relatively low binding affinities and inconspicuous variations toward external stimuli. To address this issue, herein a novel heterodimeric DADA-type complex has been designed and constructed. By engineering of donor− acceptor and hydrogen-bonding interactions, it demonstrates 1000 times enhancement for the complexation strength (Ka = 2.23 × 106 M−1) than the ADA-type counterpart. Moreover, by modulating the intermolecular hydrogen bonds involved in the system, its binding affinity exhibits significantly large variations toward external stimuli (∼102−103-fold change for Ka). The robust yet adaptive heterodimeric complex is employed as a tecton for the fabrication of high-molecular-weight donor−acceptortype supramolecular polymers, demonstrating the efficiency and versatility to develop self-assembled materials via rational engineering of fundamental noncovalent recognition motifs.

1. INTRODUCTION Supramolecular polymers assembled from π-conjugated small molecules, due to the combination of aromatic functionality with ease of processability, have received enormous interest in recent years.1−5 They are conventionally constructed with the assistance of hydrogen-bonding,6−10 metal−ligand,11,12 and host−guest interactions,13−18 by anchoring the corresponding noncovalent recognition motifs on the peripheries of πconjugated chromophores. In comparison, utilizing electron donor−acceptor interactions, which are the intrinsic noncovalent forces between π-conjugated electron-rich and -deficient units, to drive supramolecular polymerization has emerged as a novel and intriguing approach.19−21 Such a protocol ensures the positioning of various aromatic moieties with molecular-level precision. More importantly, the interplay between neighboring π-conjugated donor and acceptor units endows the resulting assemblies with fascinating optical, electronic, and magnetic properties, which are promising for separation, sensing, and optoelectronic applications.22−27 In this respect, π-aromatic molecular tweezer/guest recognition motif has demonstrated its promising prospects to construct well-ordered donor−acceptor-type supramolecular architectures.28,29 For example, Yam et al. and we have reported © XXXX American Chemical Society

a novel molecular tweezer 1 with two electron-deficient alkynylplatinum(II) terpyridine pincers (Scheme 1), which is capable of sandwiching a variety of electron-rich guests such as pyrene 228−32 and thereby imparting highly directional complexation between the electron-rich and -deficient moieties. However, the relatively low binding affinity of the resulting ADA-type (D = donor, A = acceptor) molecular tweezer/guest complex 1/2 (Scheme 1) (association constant Ka = 2.27 × 103 M−1 in chloroform) is detrimental to achieving an appreciable DP (degree of polymerization) value for the resulting supramolecular polymers.33−35 It should be emphasized that the overwhelmingly majority of the ADA- or DAD-type complexes reported in the literature suffer from the similar problems (Ka values ranging from 10−1 to 104 M−1).36−45 Although their binding strength could be increased in principle by enlarging the π-surface of donor/acceptor moieties, tedious synthesis and limited solubility hamper its experimental feasibility. In addition, too strong donor−acceptor communiReceived: May 17, 2016 Revised: August 18, 2016

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Scheme 1. Schematic Representation for the Formation of Donor−Acceptor-Type Supramolecular Polymers 6 and the Counterpart Oligomers 9 on the Basis of the Heterodimeric DADA-Type Molecular Tweezer/Guest Complex 1/3

supramolecular homopolymers, while mixing stoichiometric ratios of the AA- and BB-type monomers yields regularly alternating supramolecular heteropolymers. In this article, the homoditopic AA- and BB-type monomers 4 and 5 are designed (Scheme 1) due to the following reasons.54 First, the symmetrically substituted monomers are easier to prepare and characterize. Second, the control of the extent of polymerization can be achieved through slight stoichiometric imbalances between 4 and 5. As a consequence, it represents an additional level of control when comparing with the uncontrolled AB-type polymerization, which requires additional chain-stopper molecules. Notably, for both monomers 4 and 5, the semirigid spacers decorated with two octadecyl chains not only increase their solubility in chlorinated solvents but also diminish the propensity to form cyclic oligomers.55,56 By taking advantage of the robust yet responsive complexation characters for 1/3, mixing equimolar amounts of 4 and 5 provides convenient access to high-molecular-weight supramolecular polymer 6 with highly adaptive character (Scheme 1).

cation between electron-rich and -deficient pairs tends to damage their dynamic behaviors. To address this issue, we have sought to pursue an alternative protocol to modulate molecular tweezer/guest complexation, while keeping the pyrene donor and alkynylplatinum(II) terpyridine acceptor structures intact. Herein, compound 3 (Scheme 1), in which two pyrene units are connected via an isophthalic acid diamide backbone, is designed as the guest for 1, by taking the following two factors into account. First, the preorganization effect, rendered by the rigid scaffolds on both 1 and 3, pushes the alkynylplatinum(II) terpyridine and pyrene units in close proximity. Attributing to its structural and binding site complementarities, 1/3 prefers to form a DADA-type heterodimeric complex (Scheme 1) with enhanced donor− acceptor interactions.46−50 Second, due to the existence of two amide groups on 3, intermolecular hydrogen bonds could be potentially embedded, further strengthening its binding affinity. More importantly, by manipulating the strength of the hydrogen-bonding interactions, dynamic features could be simultaneously imparted to the resulting complex 1/3. Hence, we anticipate that elaborate supramolecular control over noncovalent interactions offers a more efficient and versatile approach to improve binding strength and reversibility for molecular tweezer/guest recognition system. The novel DADA-type recognition system could further serve as the tecton51 for the fabrication of donor−acceptor-type supramolecular polymer in a hierarchical manner.52,53 For the successful construction of a linear supramolecular polymer, two types of ditopic monomeric species can be devised with the utilization of the heterocomplementary recognition units. Specifically, one is the self-associating heteroditopic AB-type monomer; the other is the two-component mixtures of homoditopic AA- and BB-type monomers. Polyassociation of the former type monomer leads to the formation of

2. RESULTS AND DISCUSSION Self-Complexation Studies for 1 and 3. Self-association for the individual compounds 1 and 3 was first investigated by means of concentration-dependent 1H NMR experiments. Only slight chemical shift changes are observed for 1 upon varying its concentrations (Figure S1), suggesting that self-complexation of 1 is negligible due to the presence of bulky tert-butyl groups on the pincer units. On the other hand, upon increasing the concentration of 3 from 0.50 to 12.0 mM, the amide NH protons shift downfield from 6.48 to 6.58 ppm, while both pyrene and phenyl signals exhibit moderate upfield shifts (Figure S2). Based on the nonlinear curve fitting of the corresponding chemical shift changes (eq S1), Ka is determined B

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Figure 1. (a) UV−vis absorbance changes upon gradual addition of 3 into 1 (1.00 × 10−4 M). Inset: the intensity changes at λ = 460 nm. (b) ITC data for the consecutive injecting of 3 (0.80 mM in chloroform) into the chloroform solution of 1 (0.05 mM). (c) Optimized geometry of 1/3 obtained from the ONIOM model with semiempirical PM6 and B3LYP/6-31G(d) as the theoretical calculation basis sets. The black dashed line refers to the NH−N hydrogen bond, and the red dashed lines denote NH−π interactions.

to be 28.8 ± 4.0 M−1, which illustrates a rather weak tendency for 3 to form self-associated structures (Figure S3). Molecular Recognition for Complex 1/3. Noncovalent complexation between 1 and 3 was then examined. For UV/vis measurements, the MLCT (metal-to-ligand charge transfer) and LLCT (ligand-to-ligand charge transfer) bands of 1, located in the region of 400−500 nm, undergo gradual decrease in absorption intensity upon progressive addition of 3 (Figure 1a). The changes in the absorbance at 460 nm are saturated upon adding equimolar 3 into 1, supporting 1:1 binding stoichiometry for complex 1/3 (Figure 1a, inset). Moreover, by fitting the exothermic isothermal titration calorimetry (ITC) data with the one-site model, Ka for 1/3 is determined to be (2.23 ± 0.28) × 106 M−1 (Figure 1b and Table 1). Such results, together with the above self-associating studies, demonstrate the exceptional stability and fidelity for the complementary complexation between 1 and 3. It is noteworthy that 3 exhibits 1000-fold enhancement than 2 for the binding affinity toward the tweezer receptor 1. To elucidate the dramatically increased Ka value for 1/3, compound 7 (Figure 2), representing half of structure of 3, was chosen as the model guest. Ka for the resulting complex 1/

Figure 2. Partial 1H NMR spectra (300 MHz, CDCl3, room temperature) of (a) 7, (b) a 1:1 mixture of 1 and 7, (c) 1, (d) a 1:1 mixture of 1 and 3, and (e) 3 (total concentration = 3.00 mM).

7 is determined to be (1.80 ± 0.11) × 105 M−1 on the basis of the ITC measurement (Table 1, Figures S6 and S7). In view of the fact that both 2 and 7 are encapsulated into the cavity of molecular tweezer 1 to form ADA-type molecular tweezer/ guest complexes, the approximately 80 times increase for Ka of 1/7 than that of 1/2 is ascribed to the presence of amide group on 7. We failed to grow single crystals of 1/7 suitable for X-ray crystallography measurements, but DFT theoretical calculations depict that an intermolecular N−H---N hydrogen bond prefers to form between the amide unit on 7 and the pyridine moiety on 1 (Figure S22b). Such a result is validated by means of 1H NMR, which displays a remarkable downfield shift from 6.42 to 9.92 ppm for the amide proton resonance on 7 upon complexation with equimolar 1 (Figure 2a,b).57 Noncovalent recognition was compared between complexes 1/3 and 1/7. Considering that the Ka for 1/3 is 12-fold higher than that for 1/7, while both complexes show 1:1 binding

Table 1. Binding Constants between Molecular Tweezer 1 and the Complementary Guests guest 2 3 7 10 18c

Ka (M−1) in CHCl3 (2.27 (2.23 (1.80 (1.31 (2.00 (6.67

± ± ± ± ± ±

0.05) 0.28) 0.11) 0.35) 0.57) 0.41)

× × × × × ×

103 a 106 b 105 b 105 a 104 a 102 a

a

Ka value determined on the basis of UV−vis titration measurements. Ka value determined on the basis of ITC measurement. cChemical structure of 18 is shown in Figure S12.

b

C

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Macromolecules stoichiometries, 1/3 is thereby confirmed to form a DADAtype heterodimeric complex (Scheme 1). In 1H NMR of equimolar amounts of 1 and 3 in CDCl3, the terpyridine protons exhibit significant chemical shift changes (Δδ = 1.00, 0.39, 0.27, and 0.35 ppm for H1−4, respectively, Figure 2c,d), which are more upfield than those of 1/7 under the same circumstances (Δδ = 0.88, 0.35, 0.21, and 0.27 ppm for H1−4, respectively, Figure 2b,c), explicitly supporting the enhancement of donor−acceptor interactions in 1/3 due to the multivalency effect.58 Meanwhile, the two amide protons Ha and Hb on 3 appear significantly downfield (10.0 and 9.45 ppm, respectively, Figure 2d). The tendency for Ha is almost in line with that of the NH signal in 1/7 (Figure 2b), demonstrating that intermolecular NH−N hydrogen bonds are favored to form between Ha and the pyridine unit on 1 due to the accommodation of the affiliated amide in the cavity of 1 (Figure 2d). With respect to the latter amide which lies on the outer face of 1, DFT theoretical calculation illustrates that it tends to form intermolecular NH−π interactions with the phenylethyne scaffold on 1,59 as manifested by the short distances of 2.87 and 2.65 Å for NH−ethynyl (144.3°) and NH−phenyl (161.4°) bonds, respectively (Figure 1c). Noteworthy, after Nmethylation of both amides on 3, its binding affinity toward 1 is reduced considerably (Ka = (6.67 ± 0.41) × 102 M−1, Table 1 and Figure S12), highlighting the importance of hydrogen bonds to maintain strong complexation between 1 and 3. Hence, the remarkably enhanced binding affinity for 1/3 is attributed to the integration of electron donor−acceptor and hydrogen-bonding interactions in a synergistic manner. Formation of Donor−Acceptor-Type Supramolecular Polymers 6. After establishing the robust DADA-type heterodimeric recognition motif 1/3, we focused on the formation of supramolecular polymers 6 via equimolar mixing of the homoditopic monomers 4 and 5. In addition, the counterpart supramolecular assemblies 9 deriving from the equimolar mixture of 4 and 8 were also constructed to elucidate the impact of noncovalent binding strength on the size of the resulting supramolecular assemblies. The double-logarithmic plot of specific viscosity versus concentration evidences a distinct slope change for 6, accompanied by a critical point at 7.00 mM, thus demonstrating ring−chain transition mechanism for the supramolecular polymerization process (Figure 3a). Notably, the slope value (2.87) above the critical polymerization concentration is significantly higher than that of 9 (slope = 1.70, Figure S21), suggesting the pronounced tendency for supramolecular polymerization at high monomer concentrations for 6. Twodimensional diffusion-ordered (DOSY) NMR measurements were then employed to quantify the size variations for the resulting supramolecular polymers. As the monomer concentration of 4 increases from 2.00 to 21.0 mM, the diffusion coefficient values for 6 reveal 195 times decrease from 2.24 × 10−9 to 1.15 × 10−11 m2 s−1 (Figure 3b), thus implying dramatic size expansion at high monomer concentrations. By contrast, for the counterpart supramolecular assemblies 9, the diffusion coefficient values exhibit only 1.7 times change under the same conditions (Figure S20). Meanwhile, the specific viscosity for 9 exhibits relatively shallow curves, while it varies exponentially as a function of monomer concentration in terms of 6 (Figure 3c). Such phenomena definitely reflect the enhanced binding affinity for the noncovalent repeating units (Ka: (2.23 ± 0.28) × 106 M−1 for 6 vs (6.67 ± 0.41) × 102 M−1

Figure 3. (a) Double-logarithmic plot of specific viscosity of 6 versus the monomer concentration. (b) DOSY spectra (400 MHz, CDCl3, 298 K) of supramolecular polymers 6 at different monomer concentrations. (c) Specific viscosities of 6 (red ●), 9 (blue ◀) and 4 (black ■) as a function of monomer concentrations (chloroform, 298 K). (d) Estimated DP values of 6 (red line) and 9 (black line) as a function of monomer concentration.

for 9) is crucial for the formation of supramolecular polymeric assemblies with larger sizes. The above experimental results are further correlated with mathematical calculations. Based on the isodesmic model,55 when the monomer concentration of 4 approaches 17.5 mmol/ L−1, the calculated DP value for 6 is determined to be approximately 395, whereas the value is 8 for 9 under the same conditions (Figure 3d). Although the real DP for 6 could be lower than the calculated value due to the unavoidable existence of cyclic oligomers,34 it is still apparent that thanks to the sufficiently strong binding capability for the heterodimeric DADA-type complex 1/3, 6 forms a high-molecularweight donor−acceptor-type supramolecular polymer at high monomer concentrations. Stimuli Responsiveness for Supramolecular Polymers 6. Next, we turned to the adaptive properties for 6, via modulating the complexation strength of the noncovalent repeating units. As an initial step, the binding affinity for 1/3 was regulated by manipulating hydrogen bonds. Hexafluoroisopropanol (HFIP), a well-known denaturant capable of interfering with intermolecular hydrogen bonds,60 is titrated into the chloroform solution of 1/3. When 0%, 2%, 4%, and 8% (v/v) amounts of HFIP are added, Ka values are determined to be (2.23 ± 0.28) × 106, (1.00 ± 0.67) × 105, (3.33 ± 0.59) × 104, and (3.33 ± 0.21) × 103 M−1, respectively (Figure 4a). Clearly small amounts of HFIP give rise to thousand times decrease for the binding affinity between 1 and 3. The enormous decrease of Ka values is also observed for the ADAtype tweezering complex 1/7, declining from (1.80 ± 0.11) × 105 M−1 to (3.03 ± 0.17) × 103, (4.44 ± 0.75) × 102, and (1.69 ± 0.65) × 102 M−1 under the same conditions (Figure 4a). To exclude the solvent polarity effects brought by HFIP, the model complex 1/10 (structure of 10: see Figure 4a), which is solely driven by electron donor−acceptor interactions without the participation of hydrogen bonds,30 was also evaluated for its binding strength toward HFIP. As shown in Figure 4a, Ka values for 1/10 are only slightly influenced by varying the amount of HFIP in chloroform ((2.00 ± 0.57) × 104 and (1.43 D

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Figure 4. (a) Ka values of 1/3 (red ◀), 1/7 (blue ■), and 1/10 (black ●) upon varying the amount of HFIP (v/v) in chloroform. (b) Ka values of 1/3 upon varying the amount of TFA (v/v) in chloroform.

Figure 5. (a) Specific viscosity of supramolecular polymer 6 (monomer concentration = 9.00 mM) upon varying the amount of HFIP (v/v) in chloroform. (b) Diffusion coefficient of 6 (monomer concentration: 15.8 mM) with different percentage of HFIP in chloroform.

± 0.02) × 104 M−1 for 0 and 8% HFIP (v/v), respectively). Hence, it can be concluded that the dramatic decrease of binding affinity for 1/3 triggered by HFIP primarily originates from the breakage of hydrogen bonds. Alternatively, the binding strength of 1/3 could be modulated via protonation of the nitrogen atom on the rigid diphenylpyridine skeleton of 1, which is the hydrogen-bonding acceptor. Specifically, upon adding trifluoroacetic acid (TFA) to 1, the absorption band located at 450−500 nm exhibits an obvious hypsochromic shift, accompanied by an isosbestic point at 447 nm (Figure S16). Blue-shifting of the low-energy MLCT/LLCT band primarily derives from the decreased electron-donating capability of the diphenylpyridine alkynyl ancillary ligand,61 denoting the conversion of 1 to its protonated form. The reverse transition is accomplished via addition of triethylamine (TEA), as manifested by the restoration of the original UV−vis absorption bands. Subsequently, the influence of TFA on the noncovalent complexation of 1/3 was evaluated (Figure 4b). When 0.17% (v/v) TFA is added into the chloroform solution of 1/3, the Ka value drops from (2.23 ± 0.28) × 106 to (6.40 ± 1.30) × 103 M−1, revealing that trace amounts of TFA bring about 350 times decrease for the complexation strength of 1/3. After clarifying the stimuli-responsive complexation for the DADA-type complex 1/3, the dynamic behavior of supramolecular polymer 6 was further examined toward HFIP and TFA. Upon adding a trace amount of TFA into equimolar mixture of 4 and 5 (10 mM for each monomer), the salt precipitates out rapidly, whereas redissolution occurs upon addition of TEA. Such a phenomenon is attributed to the protonation of the pyridine units on 4 and the 1,2,3-triazole moieties on 5. Gratifyingly, HFIP exhibits high efficiency to mediate supramolecular polymerization of 6 in solution. When 2% (v/v) volume of HFIP is introduced as the hydrogen bond competing solvent, the specific viscosity of 6 decreases and gradually levels off (Figure 5a). Moreover, the diffusion coefficient values derived from DOSY NMR measurements vary from 4.79 × 10−11 m2 s−1 to 4.36 × 10−10 and 7.94 × 10−10 m2 s−1, when 0, 2, and 4% amounts of HFIP are added into the chloroform solution of 6 (Figure 5b). Accordingly, it is evident that small amounts of HFIP are sufficient to trigger the depolymerization of 6.

improving molecular tweezer/guest complexation via the supramolecular engineering approach. Moreover, by modulating hydrogen bonds involved in 1/3, its binding strength exhibits noticeable variations (∼102−103-fold change for Ka) toward small amounts of polar solvent and acid. Considering that 1/3 concomitantly features high binding directionality, strong complexation affinity, and stimuli-sensitive responsiveness, it facilitates the formation of high-molecular-weight supramolecular polymers 6 with highly adaptive character, which are promising for the development of smart π-conjugated materials with tailored functionalities.

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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/acs.macromol.6b01032. Synthesis, characterization, spectroscopic titration, theoretical calculation data, and other materials (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Fax (+86) 551 6360 6095; e-mail [email protected] (F.W.). 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 (21274139), CAS Youth Innovation Promotion Association, and the Fundamental Research Funds for the Central Universities (WK3450000001 and WK2060200012).

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REFERENCES

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3. CONCLUSION In summary, herein a novel heterodimeric DADA-type complex 1/3 has been constructed. By virtue of the synergistic donor− acceptor and hydrogen-bonding interactions, 1/3 exhibits 1000 times enhancement for the binding strength than the ADA-type counterpart 1/2, thus demonstrating the efficiency for E

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DOI: 10.1021/acs.macromol.6b01032 Macromolecules XXXX, XXX, XXX−XXX