Self-Assembly of Supra-amphiphiles Based on Dual Charge-Transfer

Article pubs.acs.org/Langmuir

Self-Assembly of Supra-amphiphiles Based on Dual Charge-Transfer Interactions: From Nanosheets to Nanofibers Kai Liu,† Yuxing Yao,† Yu Liu,‡ Chao Wang,† Zhibo Li,‡ and Xi Zhang*,† †

Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: With the elaborate engineering of supraamphiphiles based on dual charge-transfer interactions, the rational design and programmable transformation of welldefined 1D and 2D nanostructures have been demonstrated. First, H-shaped supra-amphiphiles are successfully obtained on the basis of the directional charge-transfer interactions of naphthalene diimide and naphthalene, which self-assemble in water to form 2D nanosheets. Second, by complexation of the H-shaped supra-amphiphiles with pyrene derivatives, the 2D nanosheets transform into ultralong 1D nanofibers. Therefore, this line of research represents a successful example of supramolecular engineering and has enriched its realm.

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INTRODUCTION The rational fabrication of dimensionally controlled organic nanostructures is very important to the further development of supramolecular science and nanoscience.1−5 Toward this goal, supra-amphiphiles are very attractive. The term supraamphiphiles refers to amphiphiles that are formed on the basis of noncovalent interactions.6−17 Various noncovalent interactions can be used in the construction of supraamphiphiles, including host−guest interactions, charge-transfer interactions, and hydrogen bonding. Supra-amphiphiles are advantageous for their noncovalently synthesized nature: mixing different building blocks in solution can lead to large and predicable changes in assembly. As a result, tedious multiple-step covalent chemical synthesis in conventional amphiphiles can be avoided to some extent. Moreover, functional moieties can be easily introduced into supraamphiphiles, thus facilitating the fabrication of functional assemblies.10−12 In addition, with elaborate tuning of the chemical structures of the building blocks, supra-amphiphiles can be engineered to fabricate well-defined nanostructures, thus leading to the further development of supramolecular engineering.12 Herein, with the elaborate engineering of supra-amphiphiles based on dual charge-transfer interactions,18−21 the rational fabrication and programmable transformation of well-defined nanostructures are demonstrated in aqueous media. As shown in Scheme 1, different building blocks have been designed and synthesized. These include a negatively charged, water-soluble dye, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (PYR), and two bola-amphiphiles containing naphthalene (BNAPHV) and naphthalene diimide (BNDIV), in both of © XXXX American Chemical Society

which positively charged viologen derivatives were used as hydrophilic heads. The supra-amphiphiles based on dual charge-transfer interactions are fabricated in two steps. First, the charge-transfer interaction between naphthalene diimide and naphthalene was used as the driving force for the formation of supra-amphiphiles. It is reported that naphthalene diimide and naphthalene preferred a face-centered packing arrangement in which the long axes of the two aromatic rings were nearly parallel. 12,21−23 We envisaged that when BNDIV was complexed with BNAPHV, H-shaped supra-amphiphiles12 would be formed because of the face-centered packing of the naphthalene diimide−naphthalene charge-transfer complex and the 2,6 substitution of the two alkyl chains on the naphthalene ring (Scheme 1). The self-assembly of this supra-amphiphile would lead to the formation of 2D nanostructures. Second, the addition of PYR to the solution of H-shaped supra-amphiphiles could lead to the strong and highly directional charge-transfer interaction between PYR and viologen derivatives,24−27 thus leading to the transformation from 2D to 1D nanostructures.

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EXPERIMENTAL SECTION

Materials. BNDIV: Compounds 1 and 2 were synthesized according to the literature.12 Compound 2 (0.16 g, 0.23 mmol) and 4,4′-bipyridyl (0.43 g, 2.8 mmol) were dissolved in dry N,Ndimethylformamide (8 mL). The mixture was stirred at 100 °C for 24 h in a nitrogen environment. After that, the solution was added dropwise to 300 mL of diethyl ether and filtered. Reprecipitation from N,N-dimethylformamide-diethyl ether several times afforded comReceived: May 5, 2012 Revised: May 22, 2012

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Scheme 1. Schematic Representation of the Rational Fabrication and Programmable Evolution of Well-Defined Nanostructures by Supramolecular Engineering of the Supra-amphiphiles

Scheme 2. Synthesis Route of BNDIV

pound 3 as a gray solid. The yield was 55%. Compound 3 (0.12 g, 0.12 mmol) was dissolved in dry N,N-dimethylformamide (5 mL) and stirred and then methyl iodide (0.5 mL, 8.0 mmol) was added dropwise to the solution, after which the solution was heated to 100 °C under a nitrogen environment and stirred for 48 h. The solution was added dropwise to 100 mL of acetonitrile. The solid precipitate was filtered. Reprecipitation from dimethyl sulfoxide−acetonitrile several times afforded BNDIV as a red powder. The yield was 78%. 1 H NMR (600 MHz, DMSO, 25 °C, TMS): δ 9.39 (d, J = 6.44 Hz, 4H), 9.29 (d, J = 6.80 Hz, 4H), 8.79 (d, J = 6.80 Hz, 4H), 8.76 (d, J = 6.80 Hz, 4H), 8.68 (s, 4H), 4.68 (t, J = 7.44 Hz, 4H), 4.44 (s, 6H), 4.04 (t, J = 8.18 Hz, 4H), 1.1−2.1 (m, 32H). ESI-MS: m/z = 221.89 (calculated: 221.79). BNAPHV: Compound 4 was synthesized according to the literature.12 BNAPHV was synthesized in the same way as BNDIV. The yield was 40%. 1 H NMR (400 MHz, DMSO, 25 °C, TMS): δ 9.38 (d, J = 6.92 Hz, 4H), 9.28 (d, J = 6.96 Hz, 4H), 8.78 (t, J = 6.76 Hz, 4H), 8.75 (d, J = 6.88 Hz, 4H), 7.69 (d, J = 8.96 Hz, 2H), 7.24 (d, J = 2.32 Hz, 2H), 7.09 (dd, J1 = 2.28 Hz, J2 = 6.40 Hz, 2H), 4.68 (t, J = 7.36 Hz, 4H),4.44 (s, 6H), 4.02 (t, J = 6.48 Hz, 4H), 1.1−2.1 (m, 32H). ESI-MS: m/z = 195.31 (expected: 195.28). Characterization. 1H NMR spectra were obtained using a JEOL JNM-ECA400 apparatus. ESI−MS spectra were recorded using a PE Sciex API 3000 apparatus. UV/vis spectra were measured using a Hitachi U-3010 spectrophotometer (path length is 10.0 mm for Figure 4a but 1.0 mm in other experiments). Fluorescence spectra were recorded using a Hitachi F-7000 apparatus. Zeta potential measurements were made on a Malvern ZS 90 Zetasizer instrument. Transmission electron microscope (TEM) measurements were carried out on a JEMO 2010 electron microscope operating at an

Scheme 3. Synthesis Route of BNAPHV

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Figure 1. (a) UV/vis and (b, c) fluorescence spectra of BNDIV (red curve), BNAPHV (black curve), and BNDIV−BNAPHV supra-amphiphiles (blue curve). Inset of (a) Difference UV/vis spectra obtained by subtracting the spectrum of BNDIV and BNAPH from that of the BNDIV− BNAPHV complex. The concentrations of BNDIV and BNAPHV are 1.0 × 10−3 M.

Figure 2. (a) Concentration-dependent UV/vis spectroscopy of the BNDIV−BNAPHV supra-amphiphiles. Arrows indicate changes with increasing concentration. (b) 1H NMR spectra of BNDIV, BNAPHV, and BNDIV−BNAPHV in pH 9 buffer solution (D2O). The concentrations of BNDIV and BNAPHV are 1.0 × 10−3 M. The chart shows the chemical shifts of protons on the naphthalene diimide and naphthalene aromatic rings before and after complexation. (c) Proposed face-centered packing geometry of naphthalene diimide and naphthalene in the charge-transfer complex. acceleration voltage of 120 kV. The samples were prepared by dropcasting the aqueous solution on the carbon-coated copper grid and were then negatively stained with a phosphotungstic acid solution. Cryogenic transmission electron microscope (cryo-TEM) samples were prepared in a controlled-environment vitrification system (CEVS) at 28 °C. The vitrified samples were stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined by a JEM2200FS TEM (200 kV) at about −174 °C. Atomic force microscopy (AFM) was performed using tapping mode in air on a commercial multimode nanoscope IVAFM. For sample preparation, a few drops of the solution were placed on a mica surface and were incubated for 10 min under moist conditions; excess solution was removed by absorption onto filter paper, and then the substrate was washed slightly with water and air-dried. X-ray diffraction (XRD) measurements: For sample preparation, a few drops of the solution were placed on a glass surface, and then the solvent was evaporated at room temperature. The sample was then directly used for XRD measurements. The Bragg peak was extracted from the XRD data, and the d-spacing could be obtained according to the Bragg equation d = λ/(2 sin θ), where λ = 0.15405 nm.

band appears between 400−600 nm, indicating the chargetransfer interaction (Figure 1a). Second, the fluorescence of both naphthalene diimide and naphthalene are strongly quenched after complexation, confirming the charge-transfer interaction between naphthalene diimide and naphthalene chromophores (Figure 1b,c). All of these data suggest the formation of BNDIV−BNAPHV supra-amphiphiles based on the charge-transfer interaction of naphthalene diimide and naphthalene. We wondered whether the BNDIV−BNAPHV supraamphiphiles were H-shaped as expected. To answer this question, we have employed UV/vis and 1H NMR spectroscopy to gain further structural information. As shown in Figure 2a, the intensity of the charge-transfer absorption band arises as the concentration of the supra-amphiphiles increases, suggesting the formation of the supra-amphiphiles. However, significant hypochromism has been observed in the main absorption bands upon self-assembly. Hypochromism is dependent on the distance between chromophores and on the orientation of chromophores with respect to the angle formed by the ring planes.31 The observed hypochromism indicates the face-centered stacking of the charge-transfer complex, in which the long axes of the naphthalene diimide and naphthalene aromatic rings are nearly parallel.21,31 The face-

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RESULTS AND DISCUSSION To construct the desired H-shaped supra-amphiphiles, BNDIV and BNAPHV were mixed in water with a molar ratio of 1:1. Different methods were employed to confirm the formation of the supra-amphiphiles.12,28−30 First, a broad new absorption C

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centered stacking arrangement is further confirmed by the 1H NMR spectroscopy. As shown in Figure 2b, all the resonances of the protons on naphthalene and naphthalene diimide aromatic rings are shifted upfield after complexation. It should arise from the ring-current shielding effects after the formation of the charge-transfer complex.10,12,17,32 The variation of the chemical shifts of each proton before and after complexation is then estimated. As shown in Figure 2b, the protons of n1, n2, and n3 underwent much larger chemical resonance shifts when compared with that of m1, therefore suggesting that the protons of naphthalene are located at the center of the chargetransfer complex, whereas those of naphthalene diimide are located at the edge.10,12 Considering that the surface area of the naphthalene diimide aromatic ring is larger than that of naphthalene, the face-centered stacking geometry is confirmed, as shown in Figure 2c. Thus, the intended H-shaped supraamphiphiles are indeed formed in aqueous solution. The H-shaped supra-amphiphiles self-assemble in water to form 2D nanosheets as indicated by TEM, cryo-TEM, XRD, and AFM. As shown by the TEM images in Figure 3a,

this purpose, H-shaped supra-amphiphiles were further complexed with PYR in water. The formation of a chargetransfer complex between viologen derivatives and PYR was confirmed by zeta potential measurement and UV/vis and fluorescence spectroscopy.10,24−27 In the zeta potential measurement, the assemblies change from positively charged (15.8 mV) to negatively charged (−47.4 mV) before and after complexation with PYR, indicating the formation of the viologen derivatives−PYR charge-transfer complex. In addition, a broad new charge-transfer absorption band appears between 500 and 700 nm (Figure 4a). Moreover, the fluorescence of

Figure 4. (a) UV/vis absorption and (b) fluorescence emission spectra of the BNDIV−BNAPHV (black curve), PYR (blue curve), and BNDIV−BNAPHV/(PYR)4 (red curve) complex solutions. (c) TEM and (d) cryo-TEM images of the BNDIV−BNAPHV/(PYR) 4 assemblies in pH 9 buffer solution. The concentrations of BNDIV and BNAPHV are 2.5 × 10−4 M, respectively, and that of PYR is 1.0 × 10−3 M.

Figure 3. (a) TEM image, (b) cryo-TEM image, and (c) XRD pattern of the BNDIV−BNAPHV assemblies. (d) AFM image of the BNDIV− BNAPHV assemblies. (Inset in d) Height profile along the blue line.

PYR is strongly quenched after complexation with the Hshaped supra-amphiphiles (Figure 4b), further confirming the charge-transfer interaction between viologen derivatives with PYR. Furthermore, the emission of PYR in the BNDIV/ BNAPHV/(PYR)4 dual charge-transfer complex is lower than that in the single charge-transfer complexes (Figure S2, i.e., BNDIV/(PYR)2 or BNAPHV/(PYR)2). In other words, the quenching extent is enlarged in the ternary systems, indicating a stronger intermolecular interaction in the dual charge-transfer complexes. This should arise from the cooperativity between the dual charge-transfer interactions. Therefore, supra-amphiphiles based on the dual charge-transfer interaction (i.e., naphthalene diimide−naphthalene and viologen derivatives− PYR) are successfully fabricated. TEM observations indicate that 2D nanosheets formed by Hshaped supra-amphiphiles have evolved into ultralong 1D nanofibers after complexation with PYR. As shown by TEM (Figure 4c) and cryo-TEM (Figure 4d) images, the ultralong nanofibers are quite uniform with a diameter of ∼4.6 nm (Figure S3). In addition, the characteristic X-ray diffraction peaks for nanosheets in Figure 3c disappear completely, indicating the successful transformation. Therefore, the feasibility of using supra-amphiphiles for the programmable evolution of dimension-controlled nanostructures indicates that

nanosheets can be seen clearly. The formation of nanosheets is further confirmed by cryo-TEM (Figure 3b) in which the influence of the negative staining of phosphotungstic acid in TEM can be excluded. In addition, XRD patterns (Figure 3c) indicate the existence of a layered structure with a d spacing of ∼3.5 nm. Moreover, 2D nanosheets are indicated by AFM images as shown in Figure 3d. The section analysis of AFM images shows that the thickness of a single layer of the nanosheets is about 3.6 nm. Hence, the feasibility of obtaining 2D nanostructures by using the supramolecular engineering of the H-shaped supra-amphiphiles is clearly demonstrated. It is anticipated that the 2D nanosheets can transform into 1D nanostructures by the rational tuning of the supraamphiphiles. Generally, the fabrication of 1D nanostructures relies on highly directional intermolecular interactions.33−42 Thus, the strong, highly directional charge-transfer interactions between the viologen head of the H-shaped supra-amphiphile and PYR (Scheme 1) are expected to transform the 2D nanostructures into 1D nanostructures, thus leading to the programmable evolution of well-defined nanostructures. For D

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supra-amphiphiles can be used as building blocks for the fabrication of higher-order structures.

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CONCLUSIONS By elaborate engineering of the building blocks of the supraamphiphiles, the rational design and programmable evolution of well-defined 1D and 2D nanostructures have been demonstrated. First, H-shaped supra-amphiphiles are successfully obtained on the basis of the directional charge-transfer interactions of naphthalene diimide and naphthalene, leading to the rational fabrication of 2D nanosheets. Second, by complexation of the H-shaped supra-amphiphiles with PYR, the nanosheets transform into ultralong nanofibers. The advantage of supra-amphiphiles is their noncovalently synthesized nature, for which different building block can be simply added to the solution stepwise, thus leading to the programmable evolution of the geometry of supra-amphiphiles as well as their self-assembled nanostructures in the same aqueous medium. In addition, the rational design and programmable evolution of well-defined nanostructures in aqueous solution could be crucial to a better understanding of various cellular events and for the fabrication of supramolecular materials with tailored structures and functions.

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ASSOCIATED CONTENT

S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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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 This work was financially supported by the National Basic Research Program (2007CB808000, 2011CB808200), the NSFC (50973051, 20974059), and an NSFC-DFG joint grant (TRR 61) as well as by the Tsinghua University Initiative Scientific Research Program (2009THZ02230). We thank Dr. Huaping Xu for helpful discussions.

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