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Aug 19, 2016 - The C–H/π interactions as the key driving force for the construction of supramolecular gels remain a great challenge because of thei...
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From Weakness to Strength: C−H/π-Interaction-Guided SelfAssembly and Gelation of Poly(benzyl ether) Dendrimers Yi Peng,†,‡ Yu Feng,*,† Guo-Jun Deng,‡ Yan-Mei He,† and Qing-Hua Fan† †

Beijing National Laboratory for Molecule Sciences (BNLMS), CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China S Supporting Information *

ABSTRACT: The C−H/π interactions as the key driving force for the construction of supramolecular gels remain a great challenge because of their weak nature. We hereby employed for the first time weak C−H/π interactions for the construction of supramolecular dendritic gels based on peripherally methyl-functionalized poly(benzyl ether) dendrimers. Their gelation property is highly dependent on the nature of the peripheral methyl groups. Furthermore, singlecrystal X-ray analysis and NMR spectroscopy revealed that multiple C−H/π interactions between the proton of the methyl group and the electron-rich peripheral methylsubstituted aryl ring played significant roles in the formation of supramolecular nanofibers and organogels. This study uncovers the critical role of weak noncovalent interactions and provides new insights into the further design of self-assembled nanomaterials.

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INTRODUCTION

The rational design of these dendritic gelators was inspired by the fact that C−H/π interactions, involving the methyl group in thymidine base, played significant roles in the stabilization of conformations and structures of DNA helix.23−25 More importantly, recent work from our group has demonstrated that peripherally electron-withdrawinggroup-functionalized poly(benzyl ether) dendrons showed a highly efficient gelation ability.26−33 Multiple unconventional π−π stacking and weak hydrogen-bonding interactions were found to be the key contributor in the formation of the selfassembled gel. A significant highlight of these dendritic organogel systems is that various unconventional gelation units and weak noncovalent interactions can be used to construct effective organogels because of the beneficial effect of dendritic architecture on multivalent cooperative interactions. Thus, it is reasonable to assume that if we introduced methyl substituents at the periphery of poly(benzyl ether) dendrimers (Scheme 1), we should obtain an effective dendritic supramolecular gel that is driven by cooperative multiple C−H/π interactions.34 In this work, we therefore designed and synthesized a series of peripherally methyl-functionalized poly(benzyl ether) dendrimers (Scheme 1). A detailed study on the gelation properties of self-assembled organogels was

C−H/π interactions are a kind of attractive weak hydrogen bond between the protons of an alkyl or aryl group and the π face of an aromatic ring (a combination of soft acids and soft bases).1−4 Despite their weak nature (1.5−2.5 kcal/mol), C− H/π interactions have attracted considerable interest because they are largely recognized as important forces in various fields of chemistry and biological systems, for example, in crystal packing,5,6 host−guest chemistry,7,8 chiral discrimination,9,10 asymmetric catalysis,11,12 and conformation stabilization of proteins and DNA.13−15 On the other hand, supramolecular gels, assembled from low-molecular-weight gelators through weak noncovalent interactions such as hydrogen bonds, π−π stacking, van der Waals interactions, hydrophobic interactions, and metal coordination, are ideal candidates for soft, stimuliresponsive materials with diverse applications.16−18 Although C−H/π interactions have recently been demonstrated to be important noncovalent forces in the stabilization of selfassembled supramolecular structures in solution,19,20 supramolecular gels driven by C−H/π interactions have rarely been reported because of their weaker nature and remain a major challenge.21,22 Indeed, for gelators, strong and highly directional intermolecular noncovalent interactions are needed. Herein, we report for the first time that weaker C−H/π interactions are strong enough to induce the gelation of peripherally methylfunctionalized poly(benzyl ether) dendrimers. © 2016 American Chemical Society

Received: July 19, 2016 Revised: August 16, 2016 Published: August 19, 2016 9313

DOI: 10.1021/acs.langmuir.6b02672 Langmuir 2016, 32, 9313−9320

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Langmuir Scheme 1. Chemical Structures of Dendritic Organogelators

examine the thermally reversible gel−sol transition temperatures (Tgel). Briefly, a sealed vial containing a gel was immersed upside down in a thermostated water bath. The temperature of the bath was raised at the rate of approximately 2 °C/min. Tgel was defined as the temperature at which the gel moved on tilting of the vial. The experimental error of Tgel in repeated independent measurements was less than 1 °C. Single-Crystal X-ray Diffraction. Diffraction-quality single crystals were obtained by dissolving the compounds in a suitable solvent followed by slow evaporation of the solvent at room temperature. The crystals of dendrimers C3v-G1-2Me and C3v-G13Me were obtained from CH3CN−CH3OH (v/v, 2:1) and CHCl3, respectively. All data were collected on a Rigaku Saturn X-ray diffractometer with graphite-monochromator Mo-Kα radiation (λ = 0.71073 Å) at 113 K. Intensities were measured for absorption effects using the multiscan technique SADABS. The structures were solved by direction methods and refined using a full-matrix least-squares technique based on F2 using the SHELXL-97 program (Sheldrick, 1997). The extended packing plots and data from crystal packing were obtained using the software Mercury 3.8. Rheological Measurements. Rheological measurements were carried out with a stress-controlled rheometer (TA Instruments, ARG2) equipped with steel-coated parallel-plate geometry (40 mm diameter). The gap distance was fixed at 750 μm. A solvent-trapping device was placed above the plate to avoid evaporation. When gel samples were mounted on the plate, they were allowed to stand for at least 5 min to facilitate the recovery of the structure. All measurements were made at 10 °C. Strain sweep at a constant frequency (6.28 rad/s) was performed in the 0.01%−200% range to determine the linear viscoelastic region (LVER) of the gel sample. The frequency sweep was obtained from 0.1 to 100 rad/s at a constant strain of 0.05%, well within the linear regime determined by the strain sweep. Scanning Electron Microscopy (SEM). The morphologies and sizes of the dendritic xerogels were characterized by using a fieldemission scanning electron microscope (FE-SEM, Hitachi SU8020) at an accelerating voltage of 15 kV. Samples were prepared by dropcasting the suspension of diluted gels on a silicon substrate. To minimize sample charging, a thin layer of Au was deposited onto the samples before SEM examination. Transmission Electron Microscopy (TEM). TEM was performed on a Hitachi HT7700 microscope. Samples were prepared by dropcasting the suspension of diluted gels on carbon-coated copper grids, and the TEM images were obtained without staining.

pursued to elucidate the methyl-substituted effects on the selfassembly and organogelation propensity. Furthermore, the mechanistic driving forces of this spontaneous self-assembly process were also investigated. To the best of our knowledge, this work represents the first example that employs only weaker CH/π interactions for the construction of supramolecular dendritic gels.

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EXPERIMENTAL SECTION

General Information. All starting materials were obtained from commercial suppliers and used as received. All solvents were distilled using suitable drying agents. Moisture-sensitive reactions were performed under an atmosphere of dry argon. 1H NMR and 13C NMR spectra were recorded on a Bruker AMX-400 spectrometer (1H: 400 MHz; 13C: 100 MHz) or a Bruker AMX-600 spectrometer (1H: 600 MHz; 13C: 150 MHz) at 298 K. Chemical shifts were reported in parts per million (ppm) relative to the internal standards, partially deuterated solvents or tetramethylsilane (TMS). HRMS-ESI mass spectra were obtained on a Bruker APEX IV instrument. Matrixassisted laser desorption−ionization (time of flight) mass spectrometry (MALDI-TOF) was performed on a Bruker Biflex III MALDI-TOF spectrometer with α-cyano-4-hydroxylcinnamic acid (CCA) as the matrix. Elemental analyses were performed on a Carlo-Erba-1106 instrument. Synthesis. Dendrimers were synthesized by using an ether reaction between the first-generation dendritic bromide or 1,3,5-tris(bromomethyl)benzene and commercially available 3,5-dimethylphenol (or 3,4,5-trimethylphenol). Details of synthetic information can be found in the Supporting Information. Gelation Test. A weighed sample of the dendritic organogelator was dissolved in an appropriate solvent in sealed test tubes and heated in an oil bath (150 °C) until the solid was dissolved. A stable turbid organogel was obtained when the prepared sample was treated with ultrasound (0.40 W/cm2, 40 kHz) for 1−5 min at the beginning of the cooling process in a certain solvent under ambient conditions (25 °C). Gel formation was evaluated by its stability to the inversion of the test tube. Repeated heating and cooling under ultrasound treatment (0.40 W/cm2, 40 kHz, 1−5 min) confirmed the thermoreversibility of the gelation process. The critical gelation concentration (CGC) of the organogelator was determined by measuring the minimum amount of the gelator required for the formation of a stable gel at room temperature. Measurement of the Thermally Reversible Gel−Sol Transition Temperature (Tgel). The “tube-inversion method” was used to 9314

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RESULTS AND DISCUSSION Synthesis of Dendritic Gelators. According to our previous method,26 four peripherally methyl-functionalized poly(benzyl ether) dendrimers were divergently synthesized using an ether reaction between the first-generation dendritic bromide or 1,3,5-tris(bromomethyl)benzene and commercially available 3,5-dimethylphenol (or 3,4,5-trimethylphenol). To evaluate the role of the peripheral methyl group in the gelation process, dendrimer C3v-G2-Ph without surface methyl groups was also synthesized for comparison. Their chemical structures and purities were confirmed by 1H NMR, 13C NMR, HRMSESI, and MALDI-TOF mass spectroscopy as well as by elemental analysis. Gelation Properties. Gelation ability of these dendrimers was investigated in detail in various organic solvents and in mixed solvents (Tables 1 and S1). It is interesting to note that

G2-3Me could form gels in 2-methoxyethanol with CGCs of 8.0 and 3.6 mg/mL, respectively, indicating that one dendritic molecule could entrap about 1.9 × 103 and 4.6 × 103 solvent molecules, respectively. However, dendrimer C3v-G2-Ph turned out to be a poor gelator: no gelation was observed in all tested solvents (Table S1), which therefore suggests that the peripheral methyl group plays a critically important role in gel formation. Interestingly, the gelation efficiency properties are highly dependent on the nature of gelation solvents.36,37 For dendrimers C3v-G2-2Me and C3v-G2-3Me, no gelation is observed in ketone solvents such as cyclohexanone and acetone, halogenated solvents, or in aromatic solvents such as benzene, toluene, pyridine, and anisole. This is probably due to the excellent solubility in these solvents; the gelation cannot happen unless the solvophobic interaction is strong enough. In addition, dendritic gelators C3v-G2-2Me and C3v-G2-3Me also exhibited different gelation abilities in different gelation solvents. For example, dendrimer C3v-G2-2Me exhibited better excellent gelation abilities than those of C3v-G2-3Me in acetonitrile, benzylalcohol, and nitromethane. However, dendrimer C3v-G2-3Me is a more effective gelator than C3vG2-2Me in 2-methoxyethanol and 2-ethoxyethanol. These experimental results demonstrate the importance of the various factors that govern the gelation abilities, and we propose that the significant methyl effect on gelation properties of these dendritic gelators may be the consequence of subtly balancing the various noncovalent interactions and solubility of dendrimers. Thermotropic and Rheological Behaviors of Dendritic Gels. The gel−sol phase transition temperatures (Tgel) of the dendritic organogels formed in 2-methoxyethanol were measured at different concentrations by the tube-inversion method (Figure 1A). The thermal stability of the gels increased with increasing concentration, indicating a higher stability of the gel networks in a higher concentration of gelators. In addition, the mechanical properties of these dendritic organogels were investigated in detail by rheological techniques (Figures 1B and S2−S4). As shown in Figure 1B, the storage modulus (G′) exceeded the loss modulus (G″) by more than one order of magnitude, indicating that they are viscoelastic in nature and behave as typical organogels. Furthermore, both moduli of the gel exhibited slight dependence as the frequency decreased in the range of 0.1−100 rad/s, demonstrating that the gel has a good tolerance to external forces. The organogel of C3v-G2-3Me formed in 2-methoxyethanol was mechanically stiffer than that formed from the corresponding dendrimer C3vG2-2Me, with G′ values of 24 and 8.6 kPa, respectively. Morphologies of Organogels. To investigate the morphology of the organogels, air-dried xerogels from respective solvents were subjected to SEM and TEM measurements. Fiberlike or ribbonlike morphologies of C3vG2-2Me and C3v-G2-3Me in different solvents were observed (Figures 2 and S5−S8). It was worth noting that the morphologies of xerogels strongly depend on the properties of the gelling solvents. For example, the xerogel of C3v-G2-2Me from acetonitrile exhibited an entangled three-dimensional (3D) network composed of thin solid fibers, of widths 30−50 nm and more than several micrometers long (Figure 2A). These nanofibers intertwined together to assemble into a dense 3D fibrillar network, which is responsible for the gel formation. The xerogel of C3v-G2-3Me in 2-methoxyethanol has a similar morphology (Figure 2D). Notably, the xerogel of C3v-G2-2Me in 2-methoxyethanol showed a shorter and straight ribbonlike

Table 1. Gelation Properties and Critical Gelator Concentrations (CGCs) of Dendrimers in Various Organic Solvents and Mixed Solvents at 25 °Ca entry

solvents

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

toluene benzene anisole pyridine benzaldehyde cyanobenzene tetrachloromethane ethyl acetate methyl pyruvate cyclohexanone acetone acetonitrile 2-methoxyethanol 2-ethoxyethanol benzylalcohol nitromethane nitroethane pyridine/H2O = 4:1 anisole/CH3OH = 1:3 THF/CH3OH = 1:3 ethyl acetate/CH3OH = 1:3

C3v-G2-2Me S S S S S S S S S S S G G G G G S G G G G

(7.4) (8.0) (7.0) (23.5) (19.4) (45.0) (19.2) (21.3) (40.8)

C3v-G2-3Me S S S S S S S S S S S G (7.9) G (3.6) G (6.6) G (60.0) PG S G (55.0) G (19.2) G (15.0) G (36.6)

a

G = stable gel; S = soluble (>60 mg/mL); and CGC is the critical gelator concentration at which gelation was observed to restrict the flow of the medium at 25 °C (g/L).

sonication played an intriguing effect on gelation.30,35 For example, if directly cooling the hot solution of C3v-G2-2Me in acetonitrile led to precipitation, no gel was formed. However, when ultrasound was applied to the hot solution for a few minutes before cooling (0.4 W/cm2, 40 kHz), a translucent gel was formed rapidly. To understand the effect of sonication on the gelation process, the aggregate morphologies before and after sonication were studied using SEM. As shown in Figure S1, it is shown that the prominent morphological changes from microspheres to nanofibers were responsible for the gelation process triggered by sonication. Notably, dendrimers C3v-G2-2Me and C3v-G2-3Me showed excellent gelation abilities in polar solvents, such as acetonitrile, 2-methoxyethanol, 2-ethoxyethanol, benzylalcohol, nitromethane, and their critical gelation concentrations are almost below 10 g/L. For example, dendrimers C3v-G2-2Me and C3v9315

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from acetonitrile was composed of poorly connected unbranched bundles of straight solid fibers with diameters of 50−80 nm and lengths of 1−2 μm (Figure 2C). Furthermore, the morphologies were further confirmed by TEM images (Figures S7 and S8). Crystal Structure. To obtain deeper understanding of the intermolecular interactions taking place between these dendrimers in gel-phase aggregation, single crystals of lowgeneration dendrimers C3v-G1-2Me and C3v-G1-3Me suitable for X-ray diffraction were obtained by slow evaporation of dendritic solutions at room temperature (see the Experimental Section), and the molecular structures were determined unambiguously by X-ray crystallography analysis. The crystal structure and packing diagram of C3v-G1-2Me and C3v-G1-3Me are shown in Figure 3. In the crystal structure of C3v-G1-2Me, the three peripheral methyl-substituted aromatic rings were in the nearly same plane as that of the core benzyl ring. There were two molecules in the asymmetric unit, and the two molecules were held together by multitype intermolecular weak C−H/π interactions (the protons of methyl or benzyl with the peripheral methyl-substituted aryl rings or the core aryl ring of other dendrimers) with distances of 2.77−2.82 Å (Figure S11), which are shorter than 3.05 Å.38 Interestingly, the asymmetric unit was further stabilized by weak C−H/π interactions (2.83 Å) between the protons of the methyl group with the electronrich peripheral methyl-substituted aryl rings of other dendrimers that aid the formation of an infinite twodimensional (2D) array of dendrimers (Figure 3A). For C3vG1-3Me, it was noted that the asymmetric unit formed from the two dendrimers was stabilized by the cooperation of the intermolecular weak C−H/π interactions between the proton of benzyl with the core aryl of dendrimer (2.85 Å) and multiple CH···O hydrogen bonding interactions (2.55−2.68 Å) (Figure S13). Furthermore, each asymmetric unit was involved in up to eight C−H/π interactions between the proton of the methyl group and the electron-rich peripheral methyl-substituted aryl rings with four other asymmetric units (2.80 and 2.87 Å), facilitating the growth of the structure in a 2D array. Although the C−H/π interaction is the weakest hydrogen bond,

Figure 1. (A) Plots of Tgel against the concentrations of C3v-G2-2Me and C3v-G2-3Me (solvent: 2-methoxyethanol). (B) Storage modulus G′ and loss modulus G″ for dendritic organogels as a function of frequency with a strain of 0.05% (2-methoxyethanol, 10 °C, 10 mg/ mL).

structure with large diameters of 80−120 nm and length less than 3 μm (Figure 2B). However, the xerogel of C3v-G2-3Me

Figure 2. SEM images of air-dried xerogels from C3v-G2-2Me in (A) acetonitrile and (B) 2-methoxyethanol and from C3v-G2-3Me in (C) acetonitrile and (D) 2-methoxyethanol. Scale bar: 1 μm. 9316

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Figure 3. C−H/π interactions in the crystal of (A) C3v-G1-2Me and (B) C3v-G1-3Me.

cooperation of multiple C−H/π interactions is strong enough to drive self-assembly and gelation of these dendrimers. Hirshfeld Surface Analysis. To understand and visualize the intermolecular interactions of the superstructure of the crystals, Hirshfeld surface analysis was used to gain additional insights into the close-contact interactions.39−41 As shown in Figure 4, the molecular surfaces and the relevant fingerprint plots suggested closer C−H/π contacts in the crystals of C3vG1-2Me and C3v-G1-3Me. In C3v-G1-2Me, the CH···C (C−H/ π) interactions consist of a significant proportion of the Hirshfeld surface, contributing 30.8% of the interactions to the total intermolecular surface−contact interaction, which was undoubtedly responsible for the main structural feature seen in the crystal structure of C3v-G1-2Me. Moreover, the tips of the spikes were located at di = 1.63 Å and de = 1.12 Å or di = 1.12 Å and de = 1.63 Å (di + de representing the shortest distance between the atoms inside and outside of the molecular surface, correspondingly). Thus, the shortest calculated interaction of about 2.75 Å is in perfect agreement with the shortest CH/π interactions observed in the crystal of C3v-G1-2Me (2.79−2.83 Å). Furthermore, with 5.6% contribution of the surface, CH···O hydrogen bonds are slightly less important for the overall stability of the crystal packing. It should be noted that only 0.8% of the interactions are attributable to π−π interactions and

thus should not be as important (Figure S16). Similarly, in the case of C3v-G1-3Me, CH···C (C−H/π) interactions contribute 21.8% to the total interaction (Figure 4F), which may play a vital role in the stabilization of the crystal structure. The minimal de + di distance calculated between the tips of the spikes is about 2.76 Å, which agrees with the observed 2.80 and 2.85 Å values in the crystal structure. In addition, the CH···O hydrogen bonds make up a total of 6.6% of the surface, but only 2.9% of the interactions are attributable to the π−π interactions (Figure S18). Self-Assembly Behavior in Solution. To shed some light on the aggregation behavior of these dendrimers in solution, we have performed concentration-NMR dependent (CD) and temperature-dependent (TD) 1H NMR spectroscopic studies. As shown in Figures S19 and S20, CD-1H spectroscopic experiments in CD3CN/CDCl3 (v/v = 4:1) at ambient temperature show a simultaneous shielding and broadening of all resonance signals upon increasing the concentration from 22.2 μM to 11.1 mM, indicating that both aromatic and methyl units are strongly involved in the formation of self-assembled structures. In particular, the clear upfield shift of the benzyl and methyl proton signals of the dendrimers upon increasing the concentration would agree with the contribution of C−H/π interactions in these aggregation processes.42 Furthermore, 9317

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Figure 4. (A,B) Calculated Hirshfeld surfaces, (C,D) the fingerprint plots of all of the intermolecular interactions, and (E,F) the fingerprint plots of the CH···C (C−H/π) interactions for C3v-G1-2Me and C3v-G1-3Me, respectively.

regarding the TD-1H NMR experiments, the chemical shifts of these protons undergo a pronounced low-field shift, as illustrated in Figures S21 and S22, when the temperature is increased from 25 to 55 °C.

interactions are strong enough to induce supramolecular organogels. Their gelation property is highly dependent on the nature of the peripheral methyl groups. Single-crystal X-ray analysis and NMR spectroscopy revealed that the cooperative multiple C−H/π interactions are the key contributor in the formation of self-assembled gels. This work illustrates the importance of weak C−H/π interactions in the self-assembly process and provides new insights into the design of selfassembled nanomaterials.

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CONCLUSIONS In summary, we have established a new series of dendritic gelators based on peripheral methyl-functionalized poly(benzyl ether) dendrimers and proved that the weaker C−H/π 9318

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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.langmuir.6b02672. Synthetic and experimental details, additional characterization data, and the Hirshfeld surface analysis (PDF) Crystallographic information of C3v-G1-2Me (CIF) Crystallographic information of C3v-G2-3Me (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the financial support from the National Basic Research Program of China (973 program, No. 2013CB932800), the National Natural Science Foundation of China (91027046 & 21472192) and the Youth Innovation Promotion Association, Chinese Academy of Sciences.

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REFERENCES

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