Self-Organization and Vesicle Formation of Amphiphilic

Langmuir , 2016, 32 (10), pp 2338–2347. DOI: 10.1021/acs.langmuir.6b00321. Publication Date (Web): February 22, 2016. Copyright © 2016 American Che...
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Self-Organization and Vesicle Formation of Amphiphilic Fulleromonodendrons Bearing Oligo(poly(ethylene oxide)) Chains Mengjun Chen,† Hongxia Zhu,† Shengju Zhou,‡ Wenlong Xu,† Shuli Dong,† Hongguang Li,*,‡ and Jingcheng Hao*,† †

Key Laboratory of Colloid and Interface Chemistry & Key Laboratory of Special Aggregated Materials, Shandong University, Ministry of Education, Jinan 250100, Shandong, P. R. China ‡ State Key Laboratory of Solid Lubrication & Laboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, Gansu, P. R. China S Supporting Information *

ABSTRACT: A new series of N-methylfulleropyrrolidines bearing oligo(poly(ethylene oxide))-appended Percec monodendrons (fulleromonodendrons, 4a−f) have been synthesized. The substituted position of the oligo(poly(ethylene oxide)) chain(s) on the phenyl group of the Percec monodendron for 4a−f was varied, which is at the 4-, 2,4-, 3,5-, 3,4,5-, 2,3,4- and 2,4,6- position, respectively. 4a−e are obtained as solids at 25 °C and can self-organize into lamellar phases as revealed by X-ray diffraction (XRD) and small-angle X-ray scattering (SAXS) measurements, while 4f appears as a viscous liquid. The substitution patterns of the oligo(poly(ethylene oxide)) chain(s) also significantly influence the solubility of 4a−f, especially in ethanol and water. Formation of self-organized supramolecular structures of 4d and 4e in water as well as 4d in ethanol is evidenced from UV−vis and dynamic light scattering (DLS) measurements. Further studies in water using various imaging techniques including transmission electron microscopy (TEM), freeze-fracture TEM (FF-TEM), cryo-TEM and atomic force microscopy (AFM) observations revealed the formation of well-defined vesicles for 4d and plate-like aggregates for 4e, indicating that the aggregation behavior of the fulleromonodendrons is highly dependent on their molecular structures. For 4d in ethanol, only irregular aggregates were noticed, indicating the solvent also plays a role on regulating the aggregation behavior. After functionalization with the Percec monodendrons, 4a−f can preserve the intriguing electrochemical properties of pristine C60 as revealed by cyclic voltammetries. The thermotropic properties of 4a−f have also been investigated. It was found that all of them show good thermal stability, but no mesophases were detected within the investigated temperature ranges. excellent applications in many fields, such as fabrication of vesicles, efficient gene vectors, drug delivery, building block of membrane, and construction of supramolecules.8−13 Grafting dendrimer to the C60 sphere has led to the formation of thermotropic liquid crystals,14 vesicles15 and helix structures.16,17 However, the bulky dendrimer usually induced a relatively low C60 content in the target molecule, which is disadvantageous for further applications especially in optoelectronic devices. Monodendron, the primary element of a dendrimer, has been proven to be a powerful tool to mediate the self-organization of complex systems.18 In recent years, there has been an enthusiasm in the study of C60 derivatives functionalized with Percec monodendrons.19−21 Compared to their dendrimerbased counterparts, these fulleromonodendrons have improved C60 contents and are thus more advantageous in optoelectronic devices. Typical examples are fulleromonodendrons with aliphatic chains as shown by Nakanishi and co-workers,

1. INTRODUCTION Self-assembly of functionalized π-units can lead to the formation of supramolecular structures with various morphologies and dimensionalities.1 They are not only of great fundamental interest, but the highly ordered π-units usually observed within these structures are also the key for further optoelectronic applications. As a π-conjugated molecule with an unusual spherical shape and intriguing physicochemical properties, fullerene C60 (refers to C60 hereafter) has long been a research focus in physics, chemistry, biology, and material science with new discoveries continuously coming.2,3 Specifically, for the applications in optoelectronic devices, the longrange order of the C60 spheres within the self-organized supramolecular structures plays a vital role. It has been shown that pristine C60 can form a variety of crystals where the C60 spheres are highly ordered.4,5 However, these hard and fragile crystals bring difficulties for device fabrication. To solve this problem, suitable chemical modifications are needed on C60 to get relatively soft liquid crystalline and/or amorphous materials. Dendrimer is a collective name of molecules synthesized by repeated growth reaction, which normally bear highly branched chains and own precise molecular structures.6,7 They exhibit © XXXX American Chemical Society

Received: January 27, 2016 Revised: February 20, 2016

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Langmuir Scheme 1. Synthetic Route for N-Methylfulleropyrrolidines Bearing Oligo(poly(ethylene oxide))-Appended Percec Monodendrons (4a−f)

Aladdin. Phosphorus tribromide (PBr3, 99.0%) was purchased from J&K Chemical Company. C60 (>99.5%) was obtained from Suzhou Dade Carbon Nanotechnology Co., Ltd. Other chemicals and solvents are from local suppliers with the quality of analytical grade. All the chemicals were used without further purification unless other stated. High-purity water was obtained from a water purification system (Ulupure Instrument Co. Ltd.). 2.2. Synthesis. The synthetic route for oligo(poly(ethylene oxide))-appended fulleromonodendrons (4a−f) is depicted in Scheme 1. The structures of materials synthesized, and the effective evidence (1H NMR, 13C NMR, ESI-MS, MALDI-TOF MS, and elemental analysis) for successful synthesis of materials in each step are listed in the Supporting Information. 2.2.1. Synthesis of 1-Bromo-2-(2-(2-methoxyethoxy)ethoxy)ethane (2). To a two-neck round-bottom flask, 30 mL of diethyl ether containing 2.4 mL (15.0 mmol) of 2-(2-(2-methoxyethoxy)ethoxy)ethanol (1) was added under stirring. The solution was cooled to 0 °C and held for ∼5 min. After that, 0.72 mL (7.5 mmol) PBr3 was added dropwise, and the mixture was allowed to react for 2 h. After that, the reaction mixture was warmed to room temperature and 2.1 mL CH3OH was added to react with the excess PBr3. After 30 min, 3.0 mL H2O was added to stop the rection. The reation mixture was transferred to a separatory funnel and successively washed with 5 wt % NaHCO3 aqueous solution (×1) and brine (×1). The aqueous phase was combined and extracted with ethyl acetate (×3). The organic phase was combined and dried over anhydrous Na2SO4. After removing the organic solvents under reduced pressure, the crude product was subjected to silica gel column chromatography (eluent: 30−50% ethyl acetate in petroleum ether) to afford 2 as a yellowish liquid. 2.2.2. Synthesis of Oligo(poly(ethylene oxide))-Substituted Benzaldehyde (3a−f). (a). Synthesis of 3a. To a 50 mL two-neck flask, 0.174 g (1.43 mmol) of 4-hydroxybenzaldehyde and 0.432 g (3.1 mmol) of K2CO3 were added. After removing the air by repeated vacuum-argon cycles, 5 mL of anhydrous DMF was injected, and the mixture was stirred at room temperature to make a homogeneous suspension. After that, 5 mL of anhydrous DMF with predissolved 0.324 g (1.43 mmol) of 2 was injected, and the mixture was stirred at 65 °C overnight. After the reaction was stopped by the addition of 10 mL of water, the reaction mixture was transferred to a separatory funnel and extracted with CH2Cl2 (×3). The organic phase was combined, washed with brine (×1), and dried over anhydrous MgSO4. The solvent was removed under reduced pressure, and the resulting crude product was purified by silica gel column chromatography using

which can self-organize into a variety of supramolecular structures such as thermotropic liquid crystals22,23 and room temperature liquids in solvent-free states,23−25 perfectly straight nanowires on two-dimensional substrate26 and doughnutshaped objects with rough surfaces produced from bulk solutions.27 Percec monodendrons containing oligo(poly(ethylene oxide)) chains are frequently used to get amphiphilic π-conjugated molecules.28 Positively charged C60 surfactants bearing such structural subunits have been successfully applied as the interfacial layer to enhance the efficiencies of polymer solar cells.29,30 However, compared to fulleromonodendrons containing aliphatic chains, investigations on those containing oligo(poly(ethylene oxide)) chains are highly hysteretic. To the best of our knowledge, only one substituted position (i.e., 2,3,4substitution) has been tried for the charged C60 surfactants,29,30 and the properties of the corresponding neutral molecules are still unknown. Herein, inspired by the interesting properties of both C60 and monodendrons, a new series of N-methylfulleropyrrolidines bearing oligo(poly(ethylene oxide))-appended Percec monodendrons (Scheme 1, 4a−f) have been synthesized and their properties have been investigated in detail. It was found that the substituted positions of the oligo(poly(ethylene oxide)) chains on the Percec monodendron have a significant influence on their solubility, self-organization, and electrochemical properties. Finally, focus has been put on the aggregation behavior of the two water-soluble fulleromonodendrons with a 3,4,5- (4d) and 2,3,4- (4e) substitution pattern. While formation of vesicles in water was confirmed within a wide concentration range for 4d, only irregular plate-like aggregates were noticed for 4e. The successful preparation of these amphiphilic fulleromonodendrons provides new opportunities for potential applications in optoelectronic devices and biomedical fields.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Triethylene glycol monomethyl ether (≥97.0%), 2,4-dihydroxybenzaldehyde (98%), 3,5-dihydroxybenzaldehyde (98%), 3,4,5-trihydroxybenzal-dehyde monohydrate (98%), and 2,4,6-trihydroxybenzaldehyde (≥97.0%) were purchased from SIGMA-Aldrich. 4-hydroxy-benzaldehyde (98%), 2,3,4-trihydroxybenzaldehyde (98%), and sarcosine (99%) were purchased from B

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Figure 1. (A) Molecular structures of 4a−f optimized at the B3LYP/6-31G(d,p) level. Blue: nitrogen; red: oxygen; dark gray: carbon; light gray: hydrogen. (B) Photos of 4a−f at 25 °C. (C) SEM images of the self-organized structures of 4a−e at 25 °C (scale bar: 2 μm), which were obtained from recrystallization from CH3OH/toluene mixture (for 4a−c) or directly drying a toluene solution (for 4d, 4e). XRD patterns in the wide (D) and small (E) angle region of the self-organized structures of 4a−e at 25 °C. (F) A schematic illustration of the lamellar structure. ethyl ether as an eluent. The final product 3a was obtained as a yellowish oil. (b). Synthesis of 3b. To a 50 mL two-neck flask, 0.213 g (1.54 mmol) of 2,4-dihydroxy-benzaldehyde and 0.886 g (6.41 mmol) of K2CO3 were added. After removing air by repeated vacuum-argon cycles, 10 mL of anhydrous DMF was injected, and the mixture was stirred at room temperature to make a homogeneous suspension. After that, 10 mL of anhydrous DMF with predissolved 1.032 g (3.24 mmol) of 2 was injected, and the mixture was stirred at 100 °C for 4 days. After the reaction was stopped by 20 mL 2 M H2SO4 aqueous solution, the reation mixture was transferred to a separatory funnel and extracted with CH2Cl2 (×3). The organic phase was combined, washed with brine (×1), and dried over anhydrous MgSO4. The solvent was removed under reduced pressure, and the resulting crude product was purified by silica gel column chromatography using 20% actone/ethyl ether as an eluent. The final product 3b was obtained as a yellowish oil. (c). Synthesis of 3c. To a 50 mL two-neck flask, 0.15 g (1.09 mmol) of 3,5-dihydroxybenzaldehyde and 0.356 g (2.5 mmol) of K2CO3 were added. After removing air by repeated vacuum-argon cycles, 7 mL of anhydrous DMF was injected, and the mixture was stirred at room temperature to make a homogeneous suspension. After that, 8 mL of anhydrous DMF with predissolved 0.5 g (2.2 mmol) of 2 was injected, and the mixture was stirred at 100 °C for 4 days. After the reaction was

stopped by 20 mL of 2 M H2SO4 aqueous solution, the reaction mixture was transferred to a separatory funnel and extracted with CH2Cl2 ( × 3). The organic phase was combined, washed with brine (×1), and dried over anhydrous MgSO4. The solvent was removed under reduced pressure, and the resulting crude product was purified by silica gel column chromatography using a quarternary solvent mixture of petroleum ether, CH2Cl2, CH3OH and ethyl acetate (volume ratio of 40%: 30%: 15%: 15%) as the eluent. The final product 3c was obtained as a yellowish oil. (d). General Synthetic Procedures for 3d−f. To synthesize 3d−f, each of which bears three oligo(poly(ethylene oxide)) chains, 0.3 mmol of trihydroxyl-substituted benzaldehyde and 1.94 mmol of K2CO3 were added into a 50 mL two-neck flask. The air in the flask was removed by repeated vacuum-argon cycles. After that, 5 mL of DMF was injected to make the mixture a homogeneous dispersion. To induce the reaction, 5 mL of DMF with predissolved 0.97 mmol of 2 was injected, and the mixture was stirred at 100 °C for 4 days. To stop the reaction, 20 mL of 2 M H2SO4 was added. The mixture was transferred to a separatory funnel and extracted with CH2Cl2 (×3). The organic phase was combined, washed with brine (×1), and dried over anhydrous MgSO4. The solvent was removed under reduced pressure, and the resulting crude product was purified by silica gel column chromatography using a quarternary solvent mixture of petroleum ether, CH2Cl2, CH3OH and ethyl acetate (volume ratio of C

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Langmuir 40%: 30%: 15%: 15%) as the eluent. The final products were obtained as yellowish oils. The yields given below are based on hydroxysubstituted benzaldehyde conversion. 2.2.3. General Synthetic Procedures for Fulleromonodendrons (4a−f). To a 250 mL two-neck round-bottom flask, 0.54 g of C60 (0.75 mmol) and 0.134 g of sarcosine (1.5 mmol) were added. The air in the flask was removed by repeated vacuum-argon cycles. After that, 150 mL of chlorobenzene was injected to make a homegenous solution. To trigger the reaction, 50 mL of chlorobenzene containing 0.5 mmol of 3a−f was injected under stirring. After reacting at ∼140 °C overnight, the mixture was cooled to room temperature, and excess sarcosine was filtered off. Chlorobenzene was removed under reduced pressure. The residue was redissolved in toluene/CH3OH mixture and passed through a short silica gel column to remove most of the unreacted C60. The ratio of toluene and CH3OH was adjusted for different reaction mixtures to get the best separation result. The organic solvent was removed under reduced pressure, and the crude product was redissolved in toluene. After separation by gel permeation chromatography (GPC, Bio-Beads S-X1, 200−400 mesh) using toluene as an eluent, the final products were obtained as brown or yellow solids. In FTIR spectra, a variety of peaks that are characteristic of the oligo(poly(ethylene oxide)) chains have been observed (Figure S1). In addition, the characteristic peak of C60, which locates at 524 cm−1, was clearly detected. Other signals of C60 are not obvious due to their relatively weak intensities. 2.3. Characterizations. NMR spectra were recorded on a Bruker Avance 400 spectrometer (Bruker, Germany) operating at a deuterium frequency of 400 MHz (for 1H NMR) and 100 MHz (for 13C NMR). ESI-MS spectra were performed on a Q-TOF6510 spectrograph (Agilent, USA). MALDI-TOF MS spectra were performed on an AXIMA Confidence TM (Shimadzu, Japan), with gentisic acid as the matrix. MALDI-TOF MS spectra were performed on a matrix-assisted laser desorption time-of-flight mass spectrometer. Small angle X-ray scattering (SAXS) measurements were performed at SAXSess mc2 (Anton Paar, Austria). The incident X-ray wavelength was 0.154 nm. The scattering vector range was chosen from 0.02 to 7.7 nm−1 (q = 4π sin θ/λ, where θ and λ are the scattering angle and wavelength, respectively). The distance from the sample to the detector is 1120 mm, and the data accumulation time is 120 s for each sample. X-ray Diffraction (XRD) patterns were recorded on a PANalytical X’Pert Powder diffractometer (PANalytical, Holland) operating in reflection mode with Cu Kα radiation (λ = 1.54178 Å). The samples were measured at room temperature between 1 and 90° in the 2θ scan mode. Thermogravimetric analysis (TGA) measurements were carried out with DSC 822e (Piscataway, NJ) under nitrogen with the scanning speed of 10 °C/min. Differential scanning calorimetry (DSC) were performed on DSC8500 (PerkinElmer, USA). Samples were measured in aluminum pans under nitrogen flow. An empty aluminum pan was used as the reference. The samples were heated at 10 °C/min. FTIR spectra were obtained on a VERTEX-70/70v FT-IR spectrometer (Bruker Optics, Germany). UV−vis measurements were performed using a HITACHIU-4100 spectrophotometer (Hitachi, Japan) with a scan rate of 600 nm/min. Cyclic voltammogram were obtained on a CHI 600E electrochemical analyzer. Dichlorobenzene was selected as the solvent. A Pt plate was used as the quasi-reference electrode, and the ferrocene/ferrocenium (Fc/Fc+) redox couple was used as internal reference for the potential calibration. Dynamic light scattering (DLS) was performed on a Brookhaven BI-200SM instrument (USA). A 200 mW green laser (λ = 532 nm) with variable intensity was used, and measurements were carried out at room temperature with a scattering angle of 90°. The average radius and polydispersity index were calculated from the intensity autocorrelation data with the cumulants method. The intensity−intensity time correlation functions were analyzed by the CONTIN method. For transmission electron microscopy (TEM) observations, about 5 μL of the specimen was placed on a TEM grid, and the excess solution was wicked away with filter paper. The copper grids were dried with an infrared lamp and observed on a JEOL JEM-100 CXII (Japan) at an accelerating voltage of 100 kV. FE-SEM observations were carried out on a JEOL JSM6700F. For atomic force microscopy (AFM) observations, a drop of

specimen solution was placed on a silica wafer, which was dried off with an infrared lamp for 1−2 h and then performed with a Nanoscope IIIA operating in tapping mode. All images were collected at a scan frequency of 1.5 Hz and a resolution of 512 × 512 pixels.

3. RESULTS AND DISCUSSION 3.1. Self-Organization of 4a−f at 25 °C in Solvent-Free States. The basic molecular structure of 4a−f is quite similar where the oligo(poly(ethylene oxide))-appended Percec monodendrons were connected to the C60 sphere through a pyryolidine ring. The only difference is the number and substitution position of the oligo(poly(ethylene oxide)) chains on the phenyl ring. Figure 1A shows the optimized conformations of 4a−f, which give a clear view on their molecular architectures. 4a−e were obtained as yellow or brown solids at room temperature, while 4f with a 2,4,6substitution pattern appears as a viscous liquid (Figure 1B). Compared to previously reported liquid fullerenes functionalized with Percec monodendrons that bears aliphatic chains,23−25 here 4f has a much higher C60 content (cal 53.8% compared to 41.3−51.2% reported previously), indicating that oligo(poly(ethylene oxide)) chains are more effective to disturb the π−π interaction between C60 units. Figure 1C shows the morphologies of the self-organized structures of 4a−e at 25 °C. Irregular plates and highly stacked layered structures were observed for 4a and 4b, respectively. For 4c, blocks with rough surfaces were noticed while for 4d and 4e, microstructures with winkled but smooth surfaces have been detected. To get further details, XRD analyses were performed both at wide and small angle ranges. From wideangle XRD (WAXRD) patterns (Figure 1D), it can be seen that the oligo(poly(ethylene oxide)) chains still have some crystallinity in 4a-e characterized by the appearance of relatively sharp peaks around 20°. For 4f, however, only a broad halo was noticed, indicating the oligo(poly(ethylene oxide)) chains are totally disordered. In small-angle region (SAXRD, Figure 1E), two diffraction peaks were observed for 4a and up to three ones were detected for 4b−d, all of which can be indexed to a lamellar structure. For 4e, only one diffraction peak was noticed. However, in small-angle X-ray scattering (SAXS, Figure S2) measurements, patterns characteristic of a lamellar structure was also revealed. A schematic illustration of the lamellar structure is given in Figure 1F where formation of periodic layers of C60 and oligo(poly(ethylene oxide)) chains can be envisioned. The spacing of the lamellae (d) formed by 4a−e have been calculated (d = λ/2 sin θ for SAXRD and d = 2π/qmax for SAXS) and summarized in Table 1. It can be seen that d is mainly determined by the number of substituted oligo(poly(ethylene oxide)) chains. For example, d for 4d Table 1. Spacing of the Lamellae (d) Derived from SAXRD, Decomposition Temperature (Td) from TGA, and E1/2 Values from Cyclic Voltammetry for 4a−f d (nm) Td (°C) E1/2 (V)c

I II III

4a

4b

4c

4d

4e

4f

2.23a 302.5 −1.283 −1.691 −2.261

2.70a 314.6 −1.241 −1.659 −2.232

2.61a 334.5 −1.221 −1.639 −2.206

3.16a 130.0 −1.225 −1.633 −2.196

3.08b 334.5 −1.208 −1.623 −2.203

/ 132.0 −1.179 −1.584 −2.148

a

Values obtained from SAXRD. bValue from SAXS. cPotential vs Fc/ Fc+.

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Figure 2. DSC traces of (A) 4b, (B) 4e, (C) 4c, and (D) 4f. Data shown in C and D are from the fourth heating−cooling circle.

During the second heating−cooling circle, both peaks become broad with slight shifts (200 °C for the endothermic peak (ΔH = 4.70 J·g−1) and 137 °C for the exothermic one (ΔH = 4.51 J·g−1)). In the third heating−cooling circle, only a quite broad endothermic peak at 194 °C (ΔH = 2.31 J·g−1) was noticed, and no exothermic one can be observed. Further circling results no detectable endothermic and exothermic peaks. Since the highest temperature in the DSC measurements (250 °C) is far below the Td of 4b (315 °C), thermal decomposition is unlikely. Thus, we suppose that the gradual disappearance of the endothermic and exothermic peaks upon continuous heating−cooling circling is due to the slow structural reorganization of the oligo(poly(ethylene oxide)) chains. Similar phenomenon has been noticed for 4e where a sharp endothermic peak at 161 °C (ΔH = 8.20 J·g−1) together with a shoulder one around 100 °C were observed during the first heating, which totally disappeared in the subsequent circling (Figure 2B). For 4c, a glass transition temperature (Tg) of 28.7 °C (ΔH = 2.33 J·g−1) was detected, which is stable up to four heating−cooling circles (Figure 2C), while for the liquid fullerene 4f, a much smaller Tg of −31 °C was determined (ΔH = 1.22 J·g−1). For 4d, measurements were carried out from −70 to 100 °C and no phase transitions were detected (Figure S4). For all the six molecules, we did not observe the existence of mesophases, which is different with the fulleromonodendrons bearing linear or branched aliphatic chains.22,23 3.3. Optical and Electrochemical Properties. It is wellknown that pristine C60 has abundant absorptions in the UV and visible range.31 Upon the formation of monoadducts, most of the absorptions can be preserved as only one CC bond on the C60 sphere is broken. Specifically, the peak around 430 nm is quite sensitive to the existing form of C60 monoadducts.32 In

which has three oligo(poly(ethylene oxide)) chains is calculated to be 3.16 nm. This value only changes slightly (3.08 nm) for 4e which has the same number of oligo(poly(ethylene oxide)) chains. When the number of oligo(poly(ethylene oxide)) chains decreases to two (for 4b and 4c), d also decreases to be 2.70 and 2.61 nm, respectively. For 4a which has only one oligo(poly(ethylene oxide)) chain, a smallest d of 2.23 nm was obtained. As the fully extended length of the oligo(poly(ethylene oxide)) chain is fixed, the decrease of d with decreasing number of oligo(poly(ethylene oxide)) chains indicates that chain interdigitation must occur for 4a−c. For 4f, no well-resolved diffraction peaks were detected in both SAXRD and SAXS measurements (Figure 1E and Figure S2). This indicates that 4f lacks a long-range order, which is consistent with its macroscopic appearance, i.e., a room temperature liquid. 3.2. Thermotropic Properties. From TGA results, it can be seen that 4a−c and 4e have very good thermal stability with the decomposition temperatures (Td) over 300 °C (Figure S3 and Table 1). For 4d and 4f, slight weight loss was noticed below 200 °C before the significant weight loss, and corresponding temperatures for 0.5% weight loss were determined to be ∼153 °C for 4d and ∼189 °C for 4f, respectively. This might be due to the relatively high steric hindrance of 4d and 4f compared to other molecules. Next, DSC measurements were carried out to check the phase transition temperatures and possible existence of mesophases. No phase transitions were detected for 4a from −50 to 300 °C (Figure S4). For 4b, a sharp endothermic peak (ΔH = 10.94 J· g−1) and a broad exothermic one (ΔH = 6.51 J·g−1) were observed during the first heating−cooling circle, which locate at 205 and 127 °C, respectively (Figure 2A). E

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Figure 3. (A) Solubility tests of 4a−f (1.0 mg) in three representative solvents (5 mL) as indicated. (B−D) Corresponding UV−vis absorptions of the samples shown in A. The solvents are CH2Cl2 (B), ethanol (C), and H2O (D), respectively. Inset in D shows the Tyndall phenomenon upon illumination in a solution of 4d in water (100 μM) by a green laser.

aggregation might occur for 4d and 4e in water as well as 4d in ethanol, which can be also seen from the Tyndall phenomenon of the solution (inset of Figure 3D). To see the influence of concentration, solutions of 4d with successively decreased concentrations were examined and the peak at 430 nm did not appear even at a concentration of 10 μM (Figure S5), indicating that 4d forms aggregates in water even at very low concentrations. The electrochemical properties of 4a-f have been probed by cyclic voltammetry measurements in dichlorobenzene. It is known that C60 monoadducts such as N-methylfulleropyrrolidines23,33 can retain the interesting electrochemical properties of pristine C60, which can successively accept up to six electrons.34 Here, up to three couples of reversible redox potentials have been clearly detected (Figure 4). Interestingly, from 4a to 4f it was found that the redox potential (E1/2) gradually moves to more negative values (Table 1), indicating the fulleromonodendron becomes more and more difficult to accept electrons. The differences between E1/2 for the first, second and third redox potentials are 104, 107, and 113 mV, respectively, which are slightly larger compared to the fulleromonodendrons bearing branched aliphatic chains (∼40 mV).23 3.4. Aggregation Behavior of 4d and 4e in Bulk Solutions. The C60 unit has an unusual spherical shape and is rigid. It is known to be both hydrophobic and lipophobic.35 In addition, strong π−π interaction exists among adjacent C60 spheres. These unique properties make C60 quite different from the flexible aliphatic chains in traditional surfactants. Investigations of the aggregation behavior of C60-based amphiphiles are thus of great fundamental interest and have received much

a good solvent where the C60 monoadducts exist as individuals, this peak is sharp. However, once molecular self-assemblies form either induced by increasing concentration or by the addition of a pure solvent, this peak will become broad or even disappear totally. Thus, from the shape of this peak, the aggregation state of a given C60 monoadduct can be inferred.32 To fully reveal the optical features of 4a−f, we first checked their solubilities. It was found that 4a−f are soluble in a variety of common organic solvents including dichloromethane (CH2Cl2), toluene, carbon disulfide, ethyl acetate, 1,4-dioxane, tetrahydrofuran, acetone, ethyl ether, N,N-dimethylformamide, acetonitrile and dimethyl sulfoxide. In ethanol and water, the solubility shows a strong dependence on the substitution pattern of the oligo(poly(ethylene oxide)) chains, as shown in Figure 3A. For comparison, results of solubility tests in CH2Cl2 are also given. Typically, to each glass bottle containing 1.0 mg of each molecule, 5 mL different solvents were added (Figure 3A). As already demonstrated, 4a−f dissolved totally in CH2Cl2. In ethanol, 4d−f dissolved totally, while 4b is only partially soluble. The other two molecules (4a and 4c) cannot dissolve as seen from the almost colorless supernatants. In water, 4d is the most soluble with only tiny amount of solid at the bottom. 4e can be also dissolved but the solubility is smaller compared to 4d based on visual inspections. The UV−vis absorptions of 4a−f are given in Figure 3B−D. In CH2Cl2, all the six fulleromonodendrons exhibit sharp peaks at 430 nm (Figure 3B). Similar phenomenon was observed for 4b, 4e, and 4f in ethanol (Figure 3C). However, it is not the case for 4d in ethanol (Figure 3C) and for 4d and 4e in water (Figure 3D), where the peak at 430 nm has disappeared. Based on the discussions mentioned above, these results show that F

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(PDI = 0.181). Meanwhile, a tiny peak is also evidenced between 20 and 30 nm. The concentration effects have also been examined by successively diluting the stock solution of 4d or 4e (100 μM), and the results are given in Figure S6. In both cases, aggregates with similar size distributions have been observed, although slight variations of Rh also occurred in certain concentrations. We have also prepared colloidal suspensions of 4d by injecting different amount of a stock solution of 4d in THF (1.0 mM) into water (5.0 mL) under stirring. THF was then removed by slow evaporation. By this means, aqueous solutions of 4d with a concentration range of 1−20 μM have been successfully prepared. DLS measurements revealed the presence of aggregates with a mean diameter (⟨Rh⟩) of 60 ± 20 nm (Figure S7). These results indicate that 4d and 4e can self-assemble into aggregates in water within a wide concentration range, which is consistent with the results of UV−vis measurements. The high propensity of 4d and 4e to self-assemble into aggregates in water can be ascribed to the strong hydrophobicity of the C60 units. It also implies that the attached monodendrons are not bulky enough to fully cover the surface of the C60 spheres. To reveal the morphologies of the aggregates, various imaging techniques have been applied including TEM, freeze-fracture TEM (FF-TEM) and cryoTEM. Typical results of 4d are summarized in Figure 6. From TEM images, in both saturated (images a−c) and 100 μM (image d) aqueous solutions, 4d formed globular aggregates with the average diameter of ∼40 nm. These globular aggregates tend to further organize into big clusters, which should be induced by the drying process during sample preparation as similar structures are absent in FF-TEM and cryo-TEM observations. The drying process also causes the shrinkage of the aggregates, accounting for the slightly smaller sizes of the aggregates in TEM observations compared to those in DLS measurements. To eliminate the influence of the drying process, FF-TEM and cryo-TEM observations were further conducted on the 100 μM solution. In both cases, globular aggregates with diameters of 100−200 nm were observed (images e, f, h, i). In addition, aggregates with much smaller sizes (∼8 nm) have also been detected (image g) by FF-TEM observations. Indeed, in DLS measurements peak splitting was frequently seen for certain samples (Figure 5, Figures S6 and S7). It turns out that the single peak appeared in DLS at this concentration (Figure 5) also reflects the existence of aggregates with two different sizes, just as the case for the saturated aqueous solution. From the magnifications of single globular aggregates (Figure 6c,i, core−shell structures can be distinguished. This indicates that the globular aggregates have hollow interiors and can be classified as vesicles (Scheme 2). To get further details, AFM observations were carried out on the saturated and 100 μM solutions of 4d in water. In both cases, globular aggregates with quite similar sizes as detected in DLS measurements and various imaging techniques have been observed (Figure 7 and Figure S8). Analyses on selected aggregates revealed a much smaller height, which is only 10−20 nm, indicating that the globular aggregates are empty, which collapsed on the substrates during AFM observations. As the fully extended length of 4d is calculated to be ∼2.0 nm, the vesicles probably contain multilayers. From Figure 7, it can be also seen that besides the large globular aggregates, the presence of smaller ones is also evidenced especially for the saturated aqueous solution of 4d,

Figure 4. Cyclic voltammetries of 4a−f (1.0 mM) in dichlorobenzene at 25 °C using ferrocene (Fc) as a reference and 50 mM TBABF4 as supporting electrolyte. The scan rate is 0.1 V/s.

attention in recent years.36−40 Moreover, the formation of selfassemblies in water with interesting morphologies such as vesicles15,36−38 may also pave the way for the practical applications of C60-based amphiphiles in biomedicine, as C60 is well-known to exhibit a variety of biological activities.41 The aggregation behavior of 4d and 4e, which show relatively good solubility in water, has been explored in detail. First, the three samples used for UV−vis absorptions as shown in Figure 3D were checked by dynamic light scattering (DLS) measurements, and the results are summarized in Figure 5. Two types

Figure 5. Distribution of the diameter (Rh) of the aggregates of 4d and 4e in water derived from DLS measurements.

of aggregates were detected for the saturated aqueous solution of 4d. The larger ones have diameters (Rh) ranging from 80 to 200 nm, while the Rh of the smaller ones ranges from 25 to 50 nm. When the concentration of 4d decreases to 100 μM, the two peaks merge into one, but the polydispersity index (PDI) also slightly increases. For the saturated aqueous solution of 4e, the aggregates have similar sizes with a broader size distribution G

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Figure 6. TEM images at different magnifications of the saturated aqueous solution of 4d (a−c) and TEM (d), FF-TEM (e−g) and cryo-TEM (h, i) images of 100 μM 4d in water.

effects on the properties of fulleromonodendrons including thermal stability, solubility, self-organization in solvent-free state, and the aggregation behavior in water. While 4a−e with a 4-, 2,4-, 3,5-, 3,4,5- and 2,3,4- substituted position, respectively, are solids at 25 °C and can self-organize into lamellar phases, 4f with a 2,4,6- substituted position appears as a viscous liquid. 4a−e show good thermal stability as revealed by TGA especially for 4a−c and 4e whose Td can exceed 300 °C. Cyclic voltammetries revealed that 4a−f have rich electrochemical properties. These properties provide the possibility of utilizing the fulleromonodendrons as n-type organic semiconductors in optoelectronic devices. Investigations on the aggregation behavior reveal the formation of well-defined vesicles for 4d in water. For 4e in water and 4d in ethanol, plate-like aggregates and irregular aggregates were noticed. These studies can not only enrich our knowledge on the selfassembly algorithm of C60-based amphiphiles, but also potentially lead to new applications of these aggregates (especially the vesicles) in biomedicine. In addition, the successful preparation of the oligo(poly(ethylene oxide))bearing fulleromonodendrons also creates new opportunities for the construction of novel supramolecular structures. Efforts toward this direction are currently underway and will be reported in the near future.

Scheme 2. A Schematic Illustration of the Vesicles in Water

which is consistent with DLS measurements and FF-TEM observations. Besides water, UV−vis measurements indicate that 4d may also self-assemble in ethanol (Figure 5). However, TEM observations did not reveal the formation of any regular aggregates (Figure S9). The morphologies of the aggregates of 4e in water have also been checked. Unlike the case in 4d, which self-assembles into well-defined vesicles, formation of irregular plate-like aggregates was noticed for 4e (Figure 8). These results indicate that the interaction between the C60based amphiphiles is very delicate, which shows high sensitivity toward both solvent and the molecular structures of fulleromonodendrons.

4. CONCLUSIONS In summary, we have synthesized a new series of Nmethylfulleropyrrolidines bearing oligo(poly(ethylene oxide))appended Percec monodendrons (fulleromonodendrons, 4a− f). It was found that variation of the number and substituted position of the oligo(poly(ethylene oxide)) chain(s) have great H

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Figure 7. Typical AFM images of 100 μM (A) and saturated (B) solutions of 4d in water. The bottom figures are variations of the height along the marked lines in the upper images.

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Figure 8. A typical TEM image showing the presence of plate-like aggregates in the saturated aqueous solution of 4e.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00321. Synthetic procedures, FTIR spectra, SAXS results, TGA and DSC measuremnts, UV−vis spectra, DLS data, AFM and TEM images (PDF)

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REFERENCES

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Corresponding Authors

*E-mail: [email protected]; Phone: +86-931-4968829; Fax: +86931-4968163. *E-mail: [email protected]; Phone: +86-531-88366074; Fax: +86-531-88364750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Nos. 21420102006 and 21273134, J.H.), National Natural Science Foundation of China (61474124, H.L.) and by the Hundred Talents Program of Chinese Academy of Sciences (Y20245YBR1, H.L.). I

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