Article pubs.acs.org/Macromolecules
Supramolecular Cross-Linking and Gelation of Conjugated Polycarbazoles via Hydrogen Bond Assisted Molecular Tweezer/ Guest Complexation Tengfei Fu, Zijian Li, Zhongxin Zhang, Xiaolong Zhang, 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, People’s Republic of China S Supporting Information *
ABSTRACT: Supramolecular cross-linking and gelation represent a fascinating approach to improve the performance of πconjugated polymers. Up to now, supramolecular π-conjugated polymer networks have been mainly developed by grafting noncovalent recognition motifs onto the side-chain of π-conjugated polymers. In comparison, much less attention has been paid to the construction of main-chain-type supramolecular polymer networks, in which π-conjugated polymers themselves serve as the noncovalent linkages. Herein we have developed a novel and efficient strategy to attain this objective. The design principle is primarily on the basis of noncovalent molecular recognition between bis[alkynylplatinum(II)]terpyridine molecular tweezer receptor and NH-type carbazole guest, which shows enhanced binding affinity due to the cooperative participation of donor− acceptor and intermolecular N−H---N hydrogen-bonding interactions. The “hydrogen-bond enhanced molecular tweezer/guest recognition” strategy can be further applied for multivalent complexation between π-conjugated polycarbazoles and homoditopic molecular tweezer cross-linker, leading to the formation of main-chain-type supramolecular polymer networks and gels with thermal and solvent responsiveness. Hence, π-conjugated polymers can be endowed with excellent processability via the supramolecular engineering approach, which provides a new avenue toward flexible optoelectronic applications.
1. INTRODUCTION Over the past few decades, π-conjugated polymers have aroused significant attention due to their intriguing optical and electronic properties.1−3 Solution-phase processing of πconjugated polymers offers a facile and low-cost route for the fabrication of functional optoelectronic devices.4 In this respect, multicomponent blending is considered as a feasible approach to achieve precise control over the physical conformation of πconjugated polymers. However, the blending systems commonly suffer from poor stability, which hampers their practical optoelectronic applications. To solve the issue, the supramolecular engineering approach, denoting the implementation of noncovalent interactions in the multicomponent blending systems, represents an intriguing protocol to enhance stability and simultaneously realize the manipulation of chain−chain association at the molecular level.5−11 Up to now, a variety of supramolecular assembled systems with the involvement of πconjugated polymeric segments have been successfully constructed, such as block copolymers,12−16 template-assisted assemblies,17,18 and Langmuir−Blodgett films on the water−air interface.19−21 In addition, supramolecular π-conjugated polymer networks (SCPNs),22−26 representing the crosslinkage of π-conjugated polymers via reversible noncovalent © XXXX American Chemical Society
bonds, have been regarded as a fascinating class of soft materials. On account of their three-dimensional character, SCPNs are prone to form supramolecular gels with outstanding optoelectronic and mechanical properties, which display promising prospects for biomedical, sensor, and flexible electronics applications.27−34 For the successful construction of SCPNs, high binding directionality, together with strong complexation strength, is required for the noncovalent linkages. To attain this objective, previous studies have mainly focused on the development of side-chain-type SCPNs, by grafting various supramolecular recognition motifs onto π-conjugated polymers.35−37 As a consequence, π-conjugated polymers can be connected with the complementary cross-linker units via hydrogen-bonding, metal−ligand, or macrocyclic host−guest interactions. However, such an approach suffers from tedious chemical synthetic steps. Besides, the nonconducting supramolecular recognition units occupy a large amount of weight and volume in the final SCPNs, which are disadvantageous for their device performReceived: June 1, 2017 Revised: September 15, 2017
A
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Scheme 1. Schematic Representation for the Formation of Main-Chain-Type Supramolecular π-Conjugated Polymer Networks between Homoditopic Monomer 2 and Polycarbazoles 3a
the pyridine units on 2. Consequently, main-chain-type SCPNs and supramolecular gels are prone to form between 2 and 3a, attributed to the cooperative participation of electron donor− acceptor and hydrogen-bonding interactions for the noncovalent cross-linking points. In addition, the counterpart polycarbazoles 3b with the absence of NH-type carbazole monomers are also designed (Scheme 1). Noncovalent complexation behaviors are compared between 2/3a and 2/ 3b in order to elucidate the impact of intermolecular hydrogen bonds on the macroscopic self-assembly behaviors. On this basis, stimuli-responsiveness of SCPNs is investigated by regulating the noncovalent binding strength of molecular tweezer/guest linkages.
ances. To avoid these drawbacks, it is appealing to develop main-chain-type SCPNs, in which π-conjugated polymers themselves act as the noncovalent complexation sites.38−40 Based on this consideration, preorganized molecular tweezer receptor represents a suitable choice, which is capable of encapsulating π-conjugated polymers via noncovalent donor− acceptor interactions. Shinkai et al. have previously adopted this concept by cross-linking polyphenylene-acetylenes or polyanilines via homoditopic zinc porphyrin molecular tweezers.39,40 Nevertheless, their studies paid more emphasis on aligning πconjugated polymers in the short-range scale. In contrast, the macroscopic self-assembly behaviors have yet to be reported, primarily ascribed to the relatively weak molecular tweezer/ guest binding affinity. In this context, we sought to realize macroscopic supramolecular cross-linking and gelation of π-conjugated polymers by taking advantage of robust molecular tweezer/guest complexation as the noncovalent linkages. Our research group is especially interested in implementing d8 transition metal (such as Pt2+ and Pd2+) units into molecular tweezer receptor, in light of their square-planar geometry and fascinating optical behaviors.41−43 We have previously demonstrated that molecular tweezer 1 (Scheme 1), consisting of two alkynylplatinum(II) terpyridine pincers, is capable of sandwiching electron-rich arene guests via electron donor−acceptor interactions.44,45 More interestingly, the pyridine moiety on 1 could serve as the hydrogen bond accepting site. When hydrogen bond donating units are anchored on the complementary guests (such as 1- and 2-naphthol), intermolecular O−H---N hydrogen bonds tend to form, leading to the dramatic increase of noncovalent binding strength.46,47 Herein, the “hydrogen-bond assisted molecular tweezer/ guest complexation” strategy is employed for the construction of polycarbazole-based main-chain-type SCPNs. Specifically, polycarbazoles 3a and molecular tweezer cross-linker 2 have been designed (Scheme 1). The presence of bulky N-alkylated carbazole monomers guarantees excellent solubility for 3a. Meanwhile, NH-type carbazole monomers on 3a are expected to act as the hydrogen-bond-donating sites, facilitating the formation of intermolecular N−H---N hydrogen bonds with
2. EXPERIMENTAL SECTION Materials and Methods. Diphenylammonium triflate (DPAT), 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine ( t Bu 3 tpy), 3-bromoacetephenone, CuI, 4-hydroxybenzaldehyde, NH-type carbazole 4, 9methylcarbazole 5, fluorine 6, 3,6-dibromo-9-H-carbazole 7, 1,4benzenediboronic acid bis(pinacol) ester, and polycaprolactone (PCL) diol (average Mn ∼ 2000) were reagent grade and used as received. PCL-OTs 11,48 [Pt(tBu3tpy)Cl](BF4),44 and molecular tweezer 144 were synthesized according to the previously reported procedures. Other reagents and solvents were employed as purchased. 1 H NMR spectra were collected on a Varian Unity INOVA-300 spectrometer with TMS as the internal standard. UV/vis spectra were recorded on a UV-1800 Shimadzu spectrometer. Fluorescent spectra were recorded on a Fluoromax-4 spectrofluorometer. Titration calorimetry (ITC) experiments were carried out with a Microcal VPITC apparatus at 298 K. Molecular weights and molecular weight distributions were determined by a gel permeation chromatograph (GPC) (three linear Styragel columns (HR2 and HR4), DMF as the eluent at a flow rate of 1.0 mL/min), equipped with a Waters 1515 pump and a Waters 2414 differential refractive index detector. A series of low-polydispersity polystyrene standards were employed for GPC calibration. Sample Synthesis. General Synthetic Route toward Polycarbazoles 3a,b. π-Conjugated polycarbazoles 3a,b were synthesized via the Suzuki coupling reactions between dibromic functionalized aromatic units and 1,4-benzenediboronic acid bis(pinacol) ester. Specifically, a mixture of 3,6-dibromo-9-H-carbazole 7, 3,6-dibromo-9-(2-hexyldecyl)-carbazole 9, 1,4-benzenediboronic acid bis(pinacol) ester, Pd(PPh3)4, NaHCO3, THF, toluene, and H2O were carefully degassed. B
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Macromolecules Scheme 2. Synthetic Routes to Polycarbozoles 3a,b and the Homoditopic Molecular Tweezer 2
7.6 Hz, 4H), 7.06 (dd, J = 14.3, 8.5 Hz, 4H), 4.24 (t, J = 4.6 Hz, 4H), 4.06 (t, J = 6.7 Hz, 34H), 3.70 (t, J = 4.8 Hz, 4H), 3.16 (s, 4H), 2.48− 2.26 (m, 36H), 1.66 (m, 78H), 1.39 (q, J = 8.0 Hz, 36H). Synthesis of 2. Compound 10 (900 mg, 0.33 mmol), [Pt(tBu3tpy)Cl](BF4) (1.30 g, 1.66 mmol), and CuI (80.0 mg, 0.40 mmol) were dissolved in CH2Cl2 (30 mL) and triethylamine (8 mL). After stirring at room temperature for 36 h, the solvent was removed with a rotary evaporator. The crude product was purified by flash column chromatography (silica gel, CH2Cl2/CH3OH, 100:1 v/v as the eluent) to afford 2 as a reddish-brown solid (1.30 g, 68%). 1H NMR (300 MHz, C2D2Cl4, room temperature) δ (ppm): 9.12 (s, 8H), 8.26 (s, 4H), 8.06 (d, J = 24.8 Hz, 24H), 7.84 (s, 4H), 7.67 (d, J = 7.9 Hz, 4H), 7.57 (s, 12H), 7.43 (s, 4H), 6.96 (d, J = 8.6 Hz, 4H), 4.10 (s, 4H), 3.93 (t, J = 6.4 Hz, 34H), 3.57 (s, 4H), 2.21 (q, J = 9.4, 7.4 Hz, 34H), 1.53 (t, J = 7.6 Hz, 61H), 1.47 (s, 43H), 1.36 (s, 84H), 1.31−1.22 (m, 56H). Determination of Molecular Tweezer/Guest Association Constant via UV−Vis Measurements. Progressive addition of the aromatic guests to molecular tweezer 1 leads to the intensity change of MLCT/LLCT absorption bands. Treatment of the collected absorbance data at 460 nm versus the concentration of guest added with a nonlinear least-squares curve-fitting equation affords the corresponding association constants. Specifically, for 1:1 molecular tweezer/guest complexation, the binding constant is determined according to the equation
The mixture was heated to reflux and stirred under nitrogen for 96 h. After the reaction was complete, the resulting mixture was extracted with H2O/CH2Cl2 (3 × 50 mL), and the combined organic layers were dried over anhydrous Na2SO4. After the removal of solvent under reduced pressure, the residue was precipitated into methanol, and the resulting precipitate was collected by filtration and dried under vacuum to afford polycarbazoles 3a,b. Synthesis of 3a. Compounds 7 (100 mg, 0.30 mmol), 9 (650 mg, 1.00 mmol), 1,4-benzenediboronic acid bis(pinacol) ester (430 mg, 1.30 mmol), NaHCO3 (2.00 g, 23.8 mmol), Pd(PPh3)4 (100 mg, 0.09 mmol), THF (15 mL), toluene (3 mL), and H2O (3 mL) were used. Polymer 3a was obtained as an off-white powder (0.45 g, 75%). The molecular weight and molecular weight distribution were determined by GPC experiment, revealing the number-average molecular weight of 50.2 kDa and polydispersity value of 2.01. The content of NH-type carbazole monomers was determined to be ∼20.9% by 1H NMR analysis in CDCl3. 1H NMR (300 MHz, CDCl3, room temperature) δ (ppm): 8.50−8.04 (m, 3H), 7.78 (dd, J = 19.2, 8.1 Hz, 6H), 7.56−7.26 (m, 3H), 4.11 (s, 2H), 2.15 (s, 1H), 1.40−0.95 (m, 24H), 0.80 (s, 6H). Synthesis of 3b. Compound 9 (486 mg, 0.91 mmol), 1,4benzenediboronic acid bis(pinacol) ester (300 mg, 0.91 mmol), NaHCO3 (1.00 g, 11.9 mmol), Pd(PPh3)4 (60.0 mg, 0.05 mmol), THF (10 mL), toluene (2 mL), and H2O (2 mL) were used. Polymer 3b was obtained as an off-white powder (0.32 g, 76%). The molecular weight and molecular weight distribution were determined by GPC experiment, revealing the number-average molecular weight of 61.1 kDa and polydispersity value of 1.89. 1H NMR (300 MHz, CDCl3, room temperature) δ (ppm): 8.45 (s, 2H), 7.97−7.58 (m, 6H), 7.45 (s, 3H), 4.16 (s, 2H), 2.20 (s, 1H), 1.29 (m, 24H), 0.86 (s, 6H). Synthesis of 10. PCL-OTs 11 (2.00 g, 0.87 mmol), compound 12 (0.80 g, 2.20 mmol), K2CO3 (0.60 g, 4.30 mmol), and acetonitrile (70 mL) were mixed together and refluxed overnight. The whole mixture was then cooled down to room temperature, and the insoluble salts were removed by filtration. The filtrate was concentrated to remove the solvent. The residue was extracted between CH2Cl2 and water, and the combined organic layers were dried over anhydrous Na2SO4 and concentrated. The mixture was purified by flash column chromatography (silica gel, petroleum ether/CH2Cl2, 3:1 v/v as the eluent) to afford 10 as a colorless oil (1.60 g, 67%). 1H NMR (300 MHz, CDCl3, room temperature) δ (ppm): 8.30 (s, 4H), 8.21 (d, J = 7.9 Hz, 4H), 7.87 (s, 4H), 7.80−7.66 (m, 4H), 7.58 (d, J = 7.6 Hz, 4H), 7.50 (q, J =
A = A0 +
Alim − A 0 [C0 + CA + 1/KS 2C0
− [(C0 + CA + 1/KS)2 − 4C0CA ]1/2 ] A0 and A are the absorbance of 1 at 460 nm with and without the presence of the guest, respectively. C0 is the total concentration of the molecular tweezer receptor. CA is the concentration of the guest. Alim is the limiting value of absorbance with the presence of excess guest, and KS is the binding constant. Gel Preparation for SCPNs 2/3a. Polycarbazoles 3a (60.0 mg) and molecular tweezer cross-linker 2 (150 mg) were mixed together in 0.5 mL of chloroform. After heating the sample to 60 °C and subsequent cooling to room temperature, black gels tend to form, with the critical gelation concentration (CGC) of around 2.40 mmol/L for 3a (excessive amount of 2 is used for the complete noncovalent crosslinking of 3a). C
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Figure 1. (a) ITC data for titrating 4 (8.00 mM in CHCl3) into the CHCl3 solution of 1 (0.40 mM). (b−d) UV−Vis intensity changes at λ = 460 nm upon gradual addition of 4, 5, and 9 into 1 (5.00 × 10−5 M). The red lines were obtained from the nonlinear curve-fitting. Inset: arrows show UV−vis absorbance changes upon progressive addition of the corresponding guests into 1.
Table 1. Binding Constants (Ka, M−1) of 1 with Various Guests in CHCl3 via UV−Vis Titration Measurements
3. RESULTS AND DISCUSSION Synthesis of Polycarbazoles 3a,b and Molecular Tweezer-Based Cross-Linker 2. For the synthesis of polycarbazoles 3a,b (Scheme 2), Suzuki coupling reaction was proceeded between 1,4-benzenediboronic acid bis(pinacol) ester and the corresponding carbazole monomers. The mole percentage of NH-type carbazole units in 3a is around 21%, by calculating the 1H NMR integration ratio of benzylic peak (4.16 ppm for protons He, see Scheme 1) versus aromatic peaks (7.41−8.39 ppm) (Figure S22). Such a value is comparable to the monomer feed ratio (23%), suggesting similar polymerization reactivity for the two carbazole monomers 7 and 9. Based on GPC measurements, the number-average molecular weight (Mn) of 3a is around 50.2 kDa, while the polydispersity index (PDI) value is 2.01. It indicates that approximately 31.5 NH-type carbazole monomers exist on per single polymer chain of 3a. For the counterpart polycarbazoles 3b, Mn and PDI values are determined to be 61.1 kDa and 1.89, respectively. Meanwhile, the homoditopic molecular tweezer receptor 2 was synthesized via the copper(I)-catalyzed reaction between the intermediate compound 10 and [Pt(tBu3tpy)Cl](BF4) (Scheme 2). Noncovalent Molecular Tweezer/Carbazole Guest Complexation. Noncovalent recognition was first examined between molecular tweezer 1 and NH-type carbazole guest 4 (Scheme 1). For isothermal titration calorimetry (ITC) measurement (Figure 1a), it shows a negative signal upon
titrating 4 into the chloroform solution of 1, denoting the enthalpy-driven complexation process (ΔHassoc < 0). Depending on the abrupt change in the ITC curves, the binding stoichiometry between 1 and 4 is determined to be 1:1. Fitting the exothermic isotherm data with one-site model provides the Ka value of (1.63 ± 0.13) × 105 M−1 for 1/4. For UV−vis measurements, the MLCT (metal-to-ligand charge transfer) and LLCT (ligand-to-ligand charge transfer) absorption bands of 1 predominately locate in the region of 400−500 nm in the UV−Vis spectrum (Figure 1b, inset). Upon progressive addition of 4, the intensity of MLCT/LLCT absorption signal gradually decreases (Figure 1b, inset). Nonlinear curve-fitting of the collected absorbance data at 460 nm (Figure 1b) provides the Ka value of (2.65 ± 0.02) × 105 M−1 for 1/4, which is highly consistent with the ITC results. In contrast, when 1-methylcarbazole 5 serves as the guest instead of 4, Ka value for the resulting complex 1/5 is determined to be (1.73 ± 0.06) × 103 M−1 on the basis of UV− Vis titration measurements (Figure 1c). The noncovalent binding affinity shows a further decrease between 1 and the fluorine guest 6 (Ka = (6.49 ± 0.30) × 102 M−1) (Table 1 and Figure S4). Hence, it is evident that N−H unit on carbazole guest plays a pivotal role for enhanced complexation strength between 1 and 4. Driving Forces for Molecular Tweezer/NH-Type Carbazole Complexation. Noncovalent complexation difference between 1/4 and 1/5 was then elucidated via 1H NMR experiments (Figure 2). In detail, for a 1:1 mixture of 1 and 4 in D
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Figure 2. 1H NMR spectra (300 MHz, CDCl3, room temperature, 2.00 mM for each species) of (a) 9, (b) a 1:1 mixture of 1 and 9, (c) 4, (d) a 1:1 mixture of 1 and 4, (e) 1, (f) a 1:1 mixture of 1 and 5, and (g) 5.
Figure 3. Spectroscopic changes of 1 (5.00 × 10−5 M) upon gradual addition of 3a (●) and 3b (■): (a) UV−vis absorption at 460 nm and (b) emission at 600 nm. The total concentration for the polycarbazole solution is 1.00 mg/mL.
d-chloroform, both the terpyridine protons on 1 and the aromatic protons on 4 undergo remarkable upfield shifts (0.65, 0.39, 0.49, 0.44 ppm for H1, H2, H3, H4 on 1 and 0.47, 0.57, 0.46, 0.10 ppm for Ha, Hb, Hc, Hd on 4, respectively, Figure 2c− e). In the meantime, obvious downfield shifts are observed for proton H5 located in the inner cavity of 1 (−0.43 ppm). Hence, it suggests that donor−acceptor interactions exist between electron-deficient alkynylplatinum(II) terpyridine pincers on 1 and electron-rich carbazole unit on 4 due to the encapsulation of 4 into the cavity of 1. In terms of 1/5, although the chemical shift tendency is similar, the resonance changes are relatively smaller than those of 1/4 under the same conditions (0.24, 0.25, 0.14, 0.13 ppm for H1, H2, H3, H4 on 1 and 0.05, 0.09, 0.12, 0.18 ppm for Ha′, Hb′, Hc′, Hd′ on 5, respectively, Figure 2e−g). Such phenomena imply higher binding affinity of 1/4 than that of 1/5, which is consistent with the aforementioned ITC and UV−vis titration measurements.
Notably, the NH proton on guest 4 appears significantly downfield for complex 1/4 (δ = −9.44 ppm, Figure 2d), denoting the involvement of intermolecular hydrogen bond for molecular tweezer/guest complexation. It is rationalized that the N−H---N hydrogen bond forms between the carbazole NH moiety on 4 and the pyridine unit on 1. The presence of intermolecular hydrogen bond was further validated by means of solvent-dependent UV−vis titration experiments. Specifically, when 0%, 3.2%, 6.7%, and 9.1% (v/v) amounts of acetonitrile were titrated into the chloroform solution of 1/4, Ka values were determined to be (2.65 ± 0.02) × 105, (1.41 ± 0.04) × 104, (3.43 ± 0.09) × 103, and (1.33 ± 0.04) × 103 M−1, respectively (Figure S10). The dramatic influence of solvent polarity on noncovalent binding affinity of 1/4 can be ascribed to the disruption of intermolecular N−H---N hydrogen bond, in view of the fact that polar solvent generally leads to the weakening of hydrogen-bonding strength. E
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Macromolecules After validating the cooperative participation of electron donor−acceptor and hydrogen-bonding interactions for molecular tweezer/NH-type carbazole recognition, we turned to investigate guest substitution effect on noncovalent binding affinity. Briefly, when two para-substituted bromo and phenyl groups are anchored on 4, Ka values for the resulting complexes 1/7 and 1/8 are determined to be (1.10 ± 0.20) × 105 M−1 (Figure S5) and (3.03 ± 0.12) × 104 M−1 (Figure S7), respectively. Hence, the periphery substitutes on the carbazole guest exert minor influence on the noncovalent binding affinity. On the other hand, for guest 9 bearing a bulky N-alkylated unit, noncovalent complexation toward molecular tweezer receptor 1 can be hardly detected, as reflected by the negligible changes in both 1H NMR (Figure 2a,b,e) and UV−vis (Figure 1d) experiments. These studies lay the basis for the further investigation of multivalent complexation between molecular tweezer receptors 1, 2 and polycarbazole guests 3a,b. Formation of Main-Chain-Type SCPNs via Hydrogen Bond Enhanced Molecule Tweezer/Guest Complexation. Noncovalent complexation between molecular tweezer receptors and polycarbazoles 3a,b was then clarified. Upon progressive addition of 3a into 1 (Figures S15 and S16), both MLCT/LLCT absorption (λmax = 460 nm, Figure 3a) and emission (λmax = 600 nm, Figure 3b) bands of 1 display steady intensity decreases, which are consistent with the above model systems. In stark contrast, almost no emission signal changes upon adding the counterpart polymer 3b into 1, while UV−vis absorbance exhibits a slight and abnormal increase for the intensity (Figure 3a,b). Furthermore, 1H NMR measurements provide explicit evidence for noncovalent complexation difference between 3a and 3b. In particular, upon addition of 3a (3.50 mg) into the solution of homoditopic cross-linker 2 (9.00 mg in 1 mL of CDCl3), protons H5 shift to the downfield region (δ = 8.28 ppm), whereas terpyridine protons H1−4 on 2 exhibit upfield shifts (δ = 9.16, 7.61, 8.05, 8.11 ppm) (Figure S17). Conversely, no resonance changes occur for complex 2/ 3b under the same circumstance (Figure S18). Accordingly, these results highlight the importance of intermolecular hydrogen bonds for molecular tweezer/polycarbazole guest complexation. Additionally, fluorescent measurements were also performed by the reversal titration of 1 into the CHCl3 solution of 3a. Significant quenching of polycarbazole emission at 410 nm can be observed upon gradual addition of 1, indicating the presence of energy transfer process between donor 3a and acceptor 1 (Figures S19a and S20). In sharp contrast, the polycarbazole emission at 410 nm only shows a slight decrease upon addition of 1 to 3b (Figure S19b). It is primarily ascribed to the negligible noncovalent complexation between 1 and 3b due to the lacking of hydrogen bond donating units on 3b. Hence, it is apparent that distances between polycarbazole donors and molecular tweezer acceptors exert a huge impact on the energy transfer efficiency. Considering that main-chain-type SCPNs tend to form between homoditopic cross-linker 2 and polycarbazoles 3a, capillary viscosity measurements were employed to probe the size expansion of the resulting supramolecular assemblies. In detail, variation of specific viscosity as a function of monomer concentration for complex 2/3a (mixing 125 mg of 2 and 50.0 mg of 3a together in CHCl3) was performed (Figure 4). As a comparison, specific viscosity of the individual species (2 and 3a) was also plotted. As the monomer concentration increases from 1.00 to 15.0 mM, specific viscosity changes almost linearly
Figure 4. Specific viscosity (chloroform, 298 K) of complex 2/3a (■), monomer 2 (▲), polycarbazoles 3a (▼), and complex 2/3b (●).
with the concentration for the individual monomer, indicating that no significant physical entanglements take place. In sharp contrast, specific viscosity changes exponentially (from 0.80 to 18.8) for complex 2/3a in the examined concentration regime. Besides, for the counterpart polycarbazoles 3b with the absence of hydrogen-bonding sites, addition of cross-linker 2 leads to a shallow increase of specific viscosity. Hence, it unambiguously supports that fundamental hydrogen bonds between 2 and 3a dictate the formation of large-sized SCPNs at the macroscopic scale. Macroscopic Gelation of Main-Chain-Type SCPNs. When polycarbazoles 3a (60.0 mg, off-white solid) and crosslinker 2 (150 mg, reddish-brown solid) were mixed together in chloroform (0.5 mL), black gels form upon successive heating and cooling processes (Figure 5c). Gelation can be directly reflected by the capability to invert the vial without sample movement. Rheological experiments were further carried out to investigate the viscoelastic properties of the two-component gels (Figure 5a,b). Specifically, the elastic modulus and viscous modulus curves are almost parallel to each other. The elastic modulus values (G′) are higher than viscous modulus values (G″) at all tested frequencies (Figure 5a), indicating that the supramolecular gel is elastically strong. In the meantime, G′ and G″ values are unchanged within a long time (Figure 5b), revealing sufficient stability for the gels derived from mainchain-type SCPNs 2/3a. Dynamic behaviors for the two-component gels were further exploited. As shown in Figure 5c, heating the gels to 60 °C led to the recovery of fluid solution, while subsequent cooling to the room temperature immediately restored the gel state. Such phenomena demonstrate the thermoresponsive sol−gel transition behavior. Moreover, the impact of hexafluoroisopropanol (HFIP) on the macroscopic gelation behaviors was also studied, considering that HFIP displays strong tendency to interrupt intermolecular hydrogen bonds. As an initial step, the influence of HFIP on noncovalent binding affinity of the model complex 1/4 was examined. When 0%, 1%, 1.5%, and 2% (v/v) amounts of HFIP were titrated into the chloroform solution, Ka values were determined to be (2.65 ± 0.02) × 105, (7.99 ± 0.38) × 103, (3.92 ± 0.18) × 103, and (1.88 ± 0.15) × 103 M−1, respectively. Upon further increasing the amount of HFIP to 5% (v/v), it is unable to get the accurate Ka value, revealing rather weak binding strength between 1 and 4 (Figure S12). F
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Figure 5. Elastic modulus G′ (■) and viscous modulus G″ (●) values of the gels derived from main-chain-type SCPNs 2/3a as a function of (a) oscillation frequency at 303 K with a strain of 0.2% and (b) time at 298 K with a frequency of 1 Hz and a strain of 0.2%. (c) Reversible sol−gel transition of main-chain-type SCPNs 2/3a.
ability via supramolecular engineering approach, which provides a new avenue toward flexible optoelectronic applications.
Nevertheless, for gels derived from SCPNs 2/3a, direct addition of 5% of HFIP at room temperature fails to achieve gel-to-sol transition, presumably due to its rather small volume. Alternatively, we tried to add HFIP (5%) into the heated sample of 2/3a and found that no gelation can be visualized upon cooling back to room temperature (Figure 5c). Hence, it can be concluded that supramolecular cross-linking degree is significantly decreased for SCPNs 2/3a due to the breakup of intermolecular N−H---N hydrogen bonds by HFIP.
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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.7b01149. Synthesis, characterization, spectroscopic titration data, and other materials (PDF)
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4. CONCLUSION In summary, herein we have developed an efficient approach for supramolecular cross-linking and gelation of polycarbazoles. Because of the synergistic integration of electron donor− acceptor and hydrogen-bonding interactions, alkynylplatinum(II) terpyridine molecular tweezer 1 displays sufficiently strong binding affinity toward NH-type carbazole guest (Ka: 104−105 M−1). Hydrogen-bond-enhanced recognition49−52 can be further applied to the noncovalent complexation between molecular tweezer receptors 1, 2 and polycarbazole guest 3a. Considering the structural similarly for N-alkylated and NHtype carbazole monomers on 3a, highly specific recognition between NH-type monomers and 1 shows promising prospects for information storage and processing.53 Furthermore, mainchain-type SCPNs and gels tend to form for complex 2/3a at high monomer concentration, which can be reversibly disrupted by varying temperature and solvent composition. Hence, πconjugated polymers can be endowed with excellent process-
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (F.W.). ORCID
Feng Wang: 0000-0002-3826-5579 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 (21674106), the Fundamental Research Funds for the Central Universities (WK3450000001), CAS Youth Innovation Promotion Association (2015365), and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. G
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DOI: 10.1021/acs.macromol.7b01149 Macromolecules XXXX, XXX, XXX−XXX
