Bolaamphiphiles Bearing Bipyridine as Mesogenic Core: Rational

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Bolaamphiphiles Bearing Bipyridine as Mesogenic Core: Rational Exploitation of Molecular Architectures for Controlled Self-Assembly Guanglu Wu,† Peter Verwilst,‡ Jun Xu,† Huaping Xu,† Ruji Wang,† Mario Smet,‡ Wim Dehaen,‡ Charl F. J. Faul,§ Zhiqiang Wang,† and Xi Zhang*,† †

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Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China ‡ Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium § School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. S Supporting Information *

ABSTRACT: A bolaamphiphile (5,5-B2NBr8) bearing a functional bipyridine moiety as the mesogenic core is reported for the first time. 5,5-B2NBr8 was found to self-assemble into uniform fibrous structure in aqueous solution, when the concentration was higher than cmc. Analogues of 5,5-B2NBr8 with structural differences in chain length, headgroup, mesogenic core, and substituted position were synthesized, elucidating that small variances of the molecular structure could lead to dramatic changes of the resulting assemblies. For example, compound 4,4-B2NBr8 showed only spherical colloidal aggregates rather than fibers as 5,5-B2NBr8 did, while the only difference between them was the position at which the alkyl chains were attached onto bipyridine. A probable model for the fibrous structure of 5,5-B2NBr8 was proposed. Moreover, exploiting the coordination capacity of bipyridine, assembly and disassembly of 5,5-B2NBr8 could be reversibly controlled through the addition of EDTA and Cu(II), respectively.

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INTRODUCTION Self-assembly of amphiphiles is a facile strategy to approach the construction of hierarchical functional nanostructures.1−8 Research into the relationship between the structures of synthetic amphiphiles and resulting self-assembled morphologies has continuously aimed at the ultimate goal of precise regulation and full control of the assembling process.9−13 Mimicking the robust lipid membrane of archaebacteria,14 bolaamphiphiles, as one branch of amphiphiles with two hydrophilic heads connected by hydrophobic spacers,15−23 have been demonstrated to form diverse assemblies24−30 with distinct features.31−36 Moreover, controlled self-assembly of bolaamphiphiles has been realized based on the concepts of supraamphiphiles.37−40 However, more insight into the structure−property relationships of bolaamphiphiles is still required, so that tectons with defined size and shape can be rationally designed and regulated for the goal of fabricating highly ordered assemblies through the bottom-up strategy.41−44 Previously, we have demonstrated that the self-assembled structures of bolaamphiphiles could remain at the solid−air interface through the introduction of a mesogenic core into the hydrophobic skeleton, regardless of the evaporation of solvent. These results implied that the mesogenic core enhanced the interaction among amphiphiles, leading to more stable selfassembled structures.45−48 Herein, we designed and synthesized a new series of bolaamphiphiles using, for the first time, bipyridine as the mesogenic core. It was found that rational © 2012 American Chemical Society

structural changes of bolaamphiphile 5,5-B2NBr8 (Scheme 1a, n = 8) lead to totally different assemblies as fibers or colloidal aggregates. These results suggest that besides enhanced stability, careful choice of the mesogenic core can direct the self-assembly of bolaamphiphiles. Furthermore, exploiting the coordination capacity of bipyridine moiety can reversibly control the assembly properties of 5,5-B2NBr8.

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EXPERIMENTAL SECTION

General Methods. 1H NMR spectra were recorded on JEOL JNM-ECA300 (300 MHz), JNM-ECA400 (400 MHz), and JNMECA600 (600 MHz) apparatuses at 25 °C. ESI−mass spectroscopy measurement was carried out on a PE Sciex API 3000 spectrometer. The room temperature (294 ± 1 K) single-crystal X-ray experiments were performed on a Bruker P4 diffractometer equipped with graphite monochromatized Mo Kα radiation. UV−vis spectra were obtained using a Hitachi U-3010 spectrophotometer at 25 °C using a cuvette with 1 cm path length. Fluorescence spectra were performed on a Hitachi F-7000 spectrofluorometer at 25 °C using a cuvette with 1 cm path length. The conductivities were measured by DDS-307 conductivity meter produced by Shanghai Precision & Scientific Instrument Co. Ltd. A cell containing 2500 μL of concentrated sample solution was titrated by small aliquots (typically 50−200 μL) of pure water. Concentrations were corrected for volume changes. Received: January 25, 2012 Revised: February 27, 2012 Published: March 9, 2012 5023

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Scheme 1. Molecular Structures of Bolaamphiphiles with Structural Differences in Chain Length (a), Head Group (b), Mesogenic Core (c), and Substituted Position (d) Compared to 5,5-B2NBr8

AFM Measurements and Sample Preparation. The atomic force microscopy (AFM) images were collected on a commercial MultiMode Nanoscope IV AFM with tapping-mode and MultiMode 8 with ScanAsyst mode, using silicon cantilevers (RTESP, TESP, TESPA, ScanAsyst-Air, and ScanAsyst-Fluid from Bruker). All the original images, sampled at a resolution of 512 × 512 pixels, were edited and analyzed by free software “Gwyddion”. For ex-situ AFM, 20 μL of the sample solution was placed on freshly cleaved mica (smaller than 1 cm × 1 cm). After adsorption for 15 min, excess solution was removed by absorption onto filter paper. The resultant substrates were air-dried. Sometimes, the substrates were rinsed with small amount of solvent to remove the loosely bound monomers. For very dilute solution, the adsorbed solution was left in the air until dry. For in-situ AFM, 50 μL of the sample solution was placed on the center of freshly cleaved mica. Then the cantilever wetting with a drop of water was set up for the scanning in liquid mode. TEM, Cryo-TEM Measurements, and Sample Preparation. Transmission electron microscopy (TEM) was performed on a Hitashi H-7650B operating at an accelerating voltage of 80 kV. 10 μL of the sample solution (normally 2 mM) was applied to a carbon-coated copper grid for 5 min. After removal of excess solution with filter paper, the grid was negatively stained with 10 μL of 1.5% (w/w) uranyl acetate aqueous solution for 1 min. The excess solution was removed by filter paper. The resultant grid was dried in the air for at least 1 h. Cryo-TEM samples were prepared in a controlled environment vitrification system (CEVS) at 28 °C. The vitrified samples were then stored in liquid nitrogen until they were transferred to a cryogenic sample holder (Gatan 626) and examined with a JEM 2200FS TEM (200 keV) at about −174 °C. The phase contrast was enhanced by underfocus. The images were recorded on a Gatan multiscan CCD and processed with DigitalMicrograph. Synthesis of 5,5-B2NBr8 and Its Analogues. The synthesis routes of 5,5-B2NBr8 and model molecule, dimethyl 2,2′-bipyridine5,5′-dicarboxylate, are shown in Scheme 2. Other analogues of 5,5B2NBr8 were synthesized following similar procedures. More details of the synthetic procedures are included in the Supporting Information. 2,2′-Bipyridine-5,5′-dicarboxylic Acid. White powder; yield: 82%. 1H NMR (300 MHz, CF3COOD, ppm): δ 8.78 (d, 2.1 Hz, 2H), 8.11 (dd, 8.3 Hz, 2.1 Hz, 2H), 7.67 (d, 8.3 Hz, 2H).

Scheme 2. Synthesis Route of 5,5-B2NBr8 and Dimethyl 2,2′-Bipyridine-5,5′-dicarboxylatea

a

Reagents and conditions: (1) KMnO4, reflux, overnight; (2) I SOCl2, II - 8-bromooctan-1-ol, NEt3, DCM, reflux, overnight; (3) pyridine, CH3CN, 50 °C, 2 days; (4) MeOH, H2SO4, reflux, 3 days. 2,2′-Bipyridine-4,4′-dicarboxylic Acid. White powder; yield: 31%. 1H NMR (300 MHz, CF3COOD, ppm): δ 9.19 (d, 6.1 Hz, 2H), 9.10 (d, 2.1 Hz, 2H), 8.52 (dd, 6.3 Hz, 2.0 Hz, 2H). Bis(8-bromooctyl) 2,2′-Bipyridine-5,5′-dicarboxylate. White solid; yield: 42%. 1H NMR (300 MHz, CDCl3, ppm): δ 9.29 (d, 1.8 Hz, 2H), 8.56 (d, 8.3 Hz, 2H), 8.43 (dd, 8.3 Hz, 1.8 Hz, 2H), 4.38 (t, 6.9 Hz, 4H), 3.41 (t, 6.9 Hz, 4H), 1.93−1.70 (m, 8H), 1.61−1.27 (m, 16H). 1,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxyoctane-8,1diyl)]dipyridinium Dibromide (5,5-B2NBr8). Off-white solid; yield: 99%. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.20 (dd, 2.1, 0.8 Hz, 2H), 9.15 (dd, 6.8, 1.4 Hz, 4H), 8.62 (ddd, 7.8, 4.6, 1.2 Hz, 2H), 8.58 (dd, 8.3, 0.7 Hz, 2H), 8.47 (dd, 8.3, 2.2 Hz, 2H), 8.17 (dd, 7.6, 6.7 Hz, 4H), 4.63 (t, 7.4 Hz, 4H), 4.33 (t, 6.6 Hz, 4H), 1.98−1.86 (m, 4H), 1.78−1.67 (m, 4H), 1.47−1.21 (m, 16H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 164.45, 157.43, 150.06, 145.49, 144.78, 138.39, 128.09, 126.34, 121.30, 65.23, 60.66, 30.71, 28.42, 28.27, 28.04, 25.33, 25.29. ESI-mass: 312.59 ([M − 2Br]2+, calcd: 312.19); 5024

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Figure 1. Self-assembly of bolaamphiphile, 5,5-B2NBr8, into fibers in water (2 mM). (a) TEM image (negatively stained by 1.5% uranyl acetate aqueous solution) of micrometer-long nanofibers with (b) uniform width of 9 nm. (c) Tapping-mode AFM image (phase) of condensed stacked fibers on freshly cleaved mica with a thickness of 3.3 nm from the section analysis of AFM image (height) (d). 703.31, 705.31 ([M − Br]+, calcd: 703.29, 705.28); 865.34 ([M + Br]−, calcd: 865.12). 1,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxyoctane-8,1diyl)]ditriethylammonium Bromide Dibromide (5,5-B2NBr8TEA). Light brown solid; yield: 74%. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.22 (d, 1.5 Hz, 2H), 8.60 (d, 8.3 Hz, 2H), 8.49 (dd, 8.3, 2.2 Hz, 2H), 4.35 (t, 6.6 Hz, 4H), 3.22 (q, 7.2 Hz, 12H), 3.15−3.05 (m, 4H), 1.83−1.68 (m, 4H), 1.57 (s, 4H), 1.48−1.24 (m, 16H), 1.15 (t, 7.0 Hz, 18H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 164.49, 157.47, 150.10, 138.42, 126.39, 121.32, 65.27, 51.92, 51.89, 28.47, 28.42, 28.06, 25.76, 25.31, 20.91, 7.17. ESI-mass: 334.56 ([M − 2Br]2+, calcd: 334.26); 747.42, 749.42 ([M − Br]+, calcd: 747.44, 749.44); 909.48 ([M + Br]−, calcd: 909.27). Bis(4-bromobutyl) 2,2′-Bipyridine-5,5′-dicarboxylate. White solid; yield: 87%. 1H NMR (300 MHz, CDCl3, ppm): δ 9.30 (d, 2.0 Hz, 2H), 8.60 (d, 8.3 Hz, 2H), 8.44 (dd, 8.3 Hz, 2.0 Hz, 2H), 4.44 (t, 6.7 Hz, 4H), 3.50 (t, 6.8 Hz, 4H), 1.93−1.78 (m, 8H). 1,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxybutane-4,1diyl)]dipyridinium Dibromide (5,5-B2NBr4). Pale brown solid; yield: Quantitative. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.25 (s, 2H), 9.18 (d, 5.7 Hz, 4H), 8.71−8.56 (m, 4H), 8.51 (d, 8.2 Hz, 2H), 8.19 (t, 6.8 Hz, 4H), 4.73 (t, 7.3 Hz, 4H), 4.39 (t, 6.0 Hz, 4H), 2.22− 2.05 (m, 4H), 1.86−1.72 (m, 4H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 164.46, 157.46, 150.22, 145.57, 144.87, 138.54, 128.14, 126.30, 121.25, 64.53, 60.30, 27.57, 24.82. ESI-mass: 256.32 ([M − 2Br]2+, calcd: 256.12); 591.19, 593.19 ([M − Br]+, calcd: 591.16, 593.16); 751.17, 753.16 ([M + Br]−, calcd: 751.00, 752.99). Bis(12-bromododecyl) 2,2′-Bipyridine-5,5′-dicarboxylate. White solid; yield: 43%. 1H NMR (300 MHz, CDCl3, ppm): δ 9.30 (d, 2.0 Hz, 2H), 8.58 (d, 8.3 Hz, 2H), 8.44 (dd, 8.3 Hz, 1.9 Hz, 2H), 4.38 (t, 6.6 Hz, 4H), 3.40 (t, 6.9 Hz, 4H), 1.93−1.67 (m, 8H), 1.57− 1.01 (m, 32H). 1,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxydodecane12,1-diyl)]dipyridinium Dibromide (5,5-B2NBr12). Pale brown solid; yield: 77%. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.33−9.08 (m, 6H), 8.62 (t, 7.7 Hz, 2H), 8.55 (d, 8.3 Hz, 2H), 8.45 (d, 8.2 Hz, 2H), 8.17 (t, 6.9 Hz, 4H), 4.62 (t, 7.3 Hz, 4H), 4.31 (t, 6.3 Hz, 4H), 1.89 (s, 4H), 1.81−1.62 (m, 4H), 1.50−1.01 (m, 32H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 164.42, 157.39, 150.03, 145.49, 144.77,

138.33, 128.09, 126.31, 121.25, 65.26, 60.67, 30.75, 28.93, 28.91, 28.80, 28.68, 28.40, 28.07, 25.40. ESI-mass: 368.56 ([M − 2Br]2+, calcd: 368.25); 815.44, 817.44 ([M − Br]+, calcd: 815.41, 817.41). Bis(8-bromooctyl) 2,2′-Bipyridine-4,4′-dicarboxylate. Offwhite solid; yield: 52%. 1H NMR (600 MHz, CDCl3, ppm): δ 8.95 (d, 1.5 Hz, 2H), 8.87 (d, 8.9 Hz, 2H), 7.91 (dd, 8.6 Hz, 1.5 Hz, 2H), 4.40 (t, 6.7 Hz, 4H), 3.41 (t, 6.8 Hz, 4H), 1.93−1.76 (m, 8H), 1.52− 1.30 (m, 16H). 1,1′-[2,2′-Bipyridine-4,4′-diylbis(carbonyloxyoctane-8,1diyl)]dipyridinium Dibromide (4,4-B2NBr8). Pale brown solid; yield: 97%. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.12 (d, 5.7 Hz, 4H), 8.96 (d, 5.0 Hz, 2H), 8.83 (s, 2H), 8.61 (t, 7.8 Hz, 2H), 8.21− 8.12 (m, 4H), 7.94 (dd, 4.9, 1.5 Hz, 2H), 4.61 (t, 7.4 Hz, 4H), 4.36 (t, 6.7 Hz, 4H), 1.99−1.85 (m, 4H), 1.80−1.68 (m, 4H), 1.48−1.18 (m, 16H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 164.52, 155.43, 150.92, 145.49, 144.76, 138.52, 128.09, 123.40, 119.22, 65.69, 60.70, 30.70, 28.40, 28.27, 27.99, 25.34, 25.26. ESI-mass: 312.42 ([M − 2Br]2+, calcd: 312.19); 703.29, 705.29 ([M − Br]+, calcd: 703.29, 705.28); 865.34 ([M + Br]−, calcd: 865.12). Bis(8-bromooctyl) Biphenyl-4,4′-dicarboxylate. White solid; yield: 49%. 1H NMR (300 MHz, CDCl3, ppm): δ 8.13 (d, 8.3 Hz, 4H), 7.69 (d, 8.3 Hz, 4H), 4.35 (t, 6.6 Hz, 4H), 3.41 (t, 6.8 Hz, 4H), 1.94−1.72 (m, 8H), 1.52−1.20 (m, 16H). 1,1′-[Biphenyl-4,4′-diylbis(carbonyloxyoctane-8,1-diyl)]dipyridinium Dibromide (5,5-B0NBr8). Pale brown solid; yield: 80%. 1H NMR (400 MHz, DMSO-d6, ppm): δ 9.13 (d, 5.6 Hz, 4H), 8.61 (tt, 7.9, 1.2 Hz, 2H), 8.17 (dd, 7.6, 6.7 Hz, 4H), 8.06 (d, 8.4 Hz, 4H), 7.91 (d, 8.5 Hz, 4H), 4.62 (t, 7.4 Hz, 4H), 4.29 (t, 6.5 Hz, 4H), 1.99−1.85 (m, 4H), 1.77−1.66 (m, 4H), 1.48−1.20 (m, 16H). 13C NMR (101 MHz, DMSO-d6, ppm): δ 165.46, 145.49, 144.76, 143.29, 129.85, 129.50, 128.09, 127.40, 64.78, 60.69, 30.70, 28.42, 28.28, 28.13, 25.34. ESI-mass: 311.47 ([M − 2Br]2+, calcd: 311.19); 701.35, 703.35 ([M − Br]+, calcd: 701.30, 703.29); 861.43, 863.41 ([M + Br]−, calcd: 861.13, 863.13).

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RESULTS AND DISCUSSION For bolaamphiphile 5,5-B2NBr8, 2,2′-bipyridine is employed as the central mesogenic core, connected to two hydrophobic 5025

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Figure 2. TEM images showed fibrous structures for 5,5-B2NBr8-TEA and 5,5-B2NBr12. (a) Bundles of fibers of 5,5-B2NBr8-TEA with a width of ca. 7 nm (inset). (b) Helical fibers of 5,5-B2NBr12 with a width of ca. 12 nm (inset).

alkyl chains of eight carbons, ending with pyridinium bromide as the hydrophilic head groups. A transparent aqueous solution of 5,5-B2NBr8 was found to be stable at room temperature for at least 3 months, with a critical micelle concentration (cmc) of ca. 0.4 mM at 298 K confirmed by conductivity and pyrene fluorescence (Figure S1). Unless stated, otherwise, 2.0 mM aqueous solutions at room temperature were used for most of the measurements to ensure the formation of ordered assemblies. Self-Assembly of 5,5-B2NBr8 into Fibrous Structure. TEM of 5,5-B2NBr8 revealed the formation of micrometerlong nanofibers with a uniform width of 9 ± 1 nm in pure water (Figure 1a,b). The length of the fibers was dependent upon the concentration. At a concentration of 0.5 mM, which was just above the cmc, nanorods with ca. 150 nm length were observed (Figure S2). At a concentration of 1.0 mM, micrometer-long fibers were formed. And at a concentration of 2.0 mM, fibers longer than 20 μm, i.e., an aspect ratio of more than 2000, were distributed densely on the grid. When the 5,5-B2NBr8 concentration was increased to 4 mM or higher, most of the fibers assembled into bundles. Accordingly, concentrationdependent fluorescence of 5,5-B2NBr8 was also observed (Figure S3). In very dilute solutions, the emission of 5,5B2NBr8 at 405 nm increased linearly with concentration. At the onset of aggregation, the emission decreased quickly, with the fluorescence totally quenched when concentrations higher than the cmc were reached. AFM images of 5,5-B2NBr8 also revealed homogeneous one-dimensional (1D) assemblies (Figure 1c). Deposition on freshly cleaved mica revealed rows and layers of fibers stacked closely in a condensed and orderly fashion. Such stacking facilitated the statistical analysis of the dimensional data of the fibers and confirmed a width of 9.4 ± 0.3 nm (Figure S4), consistent with the width measured by TEM. The layer height was determined to be 3.3 ± 0.2 nm (Figure 1d). Compared to the extended length of 3.9 nm from the CPK model, it suggests that 5,5-B2NBr8 should align in a tilted fashion to form fibrous structures. To further understand how 5,5-B2NBr8 self-assembled into a fibrous structure and what factors determined the selfassembled morphology, five analogues of 5,5-B2NBr8 were synthesized. Compared to 5,5-B2NBr8, only one structural change was made in alkyl chain length, headgroup, mesogenic core, or substituted position, respectively (Scheme 1). To ensure a consistent and coherent comparison, bromide was

kept as the counterion for all the compounds included in this work. Influence of Head Group and Alkyl Chain Length on Self-Assembled Fibrous Structure. Using triethylammonium (TEA) instead of pyridinium as the cationic headgroup (Scheme 1b), 5,5-B2NBr8-TEA still showed a fibrous morphology (TEM, Figure 2a; AFM, Figure S5a). However, the average width of the fibers was 7 ± 1 nm, smaller than measured for 5,5-B2NBr8, which can be explained by the steric hindrance exerted by the triethylammonium groups limiting the condensed packing along the width. Changes in the alkyl chain length had the following effects on the morphology of the fibers: for 5,5-B2NBr4, with a shorter four carbon chain, no 1D assembly was observed either by TEM or AFM at any of the concentrations used in this study. 5,5-B2NBr12, with a longer 12-carbon chain, presented helical fibers with a width of 12 ± 2 nm as observed by TEM (Figure 2b) and AFM (Figure S5b). It is well-known that hydrophobic interactions are the essential driving force for the self-assembly of amphiphiles. The longer the alkyl chains are, the greater the hydrophobic interactions are: for 5,5-B2NBr4, the hydrophobic interactions are not sufficient to sustain a fibrous structure. However, with increased hydrophobic interactions from a longer alkyl chain, the fibers of 5,5-B2NBr12 assembled to form broader structures. Furthermore, longer alkyl chains and broader widths rendered ribbonlike fibers the flexibility to twist into helical shapes. No Fibrous Structure Was Formed When Using Biphenyl as Mesogenic Core Rather than Bipyridine. When the mesogenic core was changed from 2,2′-bipyridine to biphenyl (5,5-B0NBr8, Scheme 1c), no fibrous structures were found, even in sample solutions of very high concentrations (up to 20 mM). A careful comparison of these two mesogenic cores showed that while the two aromatic rings of biphenyl could rotate freely, the 2,2′-bipyridine has an energy barrier to prevent free rotation, even in solution. The optimized structures by DFT also showed that 2,2′-bipyridine has a flat π-conjugated plane, while biphenyl adopts a twisted conformation with a lower degree of π-conjugation (Figure S6) and thus weaker π−π interactions. This result indicates that the π−π stacking should be the main driving force to sustain the growth along the length of fibers. Further confirmation of the significance of π−π interactions was obtained from the single crystal structure49 of dimethyl 2,2′-bipyridine-5,5′-dicarboxylate, a model for the mesogenic 5026

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Figure 3. Crystal structure49 of dimethyl 2,2′-bipyridine-5,5′-dicarboxylate as a model for 5,5-B2NBr8, suggesting self-assembly driven by π−π stacking and dipole−dipole interactions. (a) π−π stacking of the bipyridine moiety viewed from a-axis. (b) Lateral zigzag alignment viewed from caxis. L of 3.49 Å and α of ca. 65° are respectively the vertical interval and the slip angle between two adjacent bipyridine planes.

lateral direction, since each dipole of the pyridine rings in 5,5B2NBr8 adopts a head-to-tail arrangement. However, the headto-tail arrangement of dipoles cannot be achieved when two molecules of 4,4-B2NBr8 approach each other. A Model Proposed for the Fibrous Structure of 5,5B2NBr8. Now, with the acquired data presented above in hand, we are able to propose a probable model for the fibrous structure of 5,5-B2NBr8 as shown in Figure 5. The assembly of the ribbonlike fiber of 3.2 nm thickness indicates a single molecular layer, as indicated by the tilted monolayer of 5,5B2NBr8. Strong π−π interactions direct the alignment along the length. Along the width, similar to that in the crystal structure of dimethyl 2,2′-bipyridine-5,5′-dicarboxylate, a zigzag alignment of the mesogenic planes is favored by dipole−dipole interactions. As expected, hydrophobic interactions also play a very important role in the assembly process: with longer alkyl chains, the ribbonlike fibers will be broad and flexible enough to twist into helical shapes. Reversibly Controlled Assembly through Coordination. As mesogenic cores such as bipyridine are also bidentate binding ligands, this provides the possibility to control the assembly by coordinating transition metals. As a first attempt, CuBr2 was employed to avoid introducing other counterions. Coordination of CuII with 5,5-B2NBr8 leads to a metal-toligand-charge-transfer (MLCT) band to appear at 324 nm in the UV/vis spectrum. A Job plot was then constructed by monitoring the absorbance at 324 nm for aqueous mixtures of 5,5-B2NBr8 and CuBr2 with different compositions.50 A maximum MLCT absorption at the molar ratio of 0.5 was obtained, indicating 1:1 binding (Figure 6a). From a titration of 5,5-B2NBr8 against CuBr2 (monitored at 324 nm), the binding constant was calculated to be 1.62 × 106 M−1 (Figure S9).51 Structural investigations by TEM of a 1:1 mixture of 5,5B2NBr8 and CuBr2 showed no formation of fibers at concentrations of 2 mM or higher (Figure 6c-II). The bipyridine moiety has to change its favorable planar conformation from trans to cis (inset of Figure 6a) to bind to CuII,52 which destroys the beneficial conformation for the formation of fibrous structures. In addition, binding to CuII may also reduce the electron density of the conjugated plane, thus causing weakening of the π−π stacking interactions. These effects, in combination with the high binding constant, lead to the complete destruction of the fibrous structure on addition of 1 equiv of CuBr2. More importantly, reversible control can be exerted over the self-assembly process of 5,5-B2NBr8 by simple competitive binding: upon adding 1 equiv of ethylenediamine tetraacetate (EDTA), which has a much higher binding constant with CuII

core of 5,5-B2NBr8. As shown in Figure 3, the conjugated bipyridine planes stacked orderly with a vertical interval of 3.49 Å, which is the typical force range for π−π stacking. The slip angle α of the stacked molecules is ca. 65°, suggesting a stacking fashion similar to fibrous H-aggregates. Moreover, as shown in Figure 3b, a lateral zigzag alignment was observed, which may result from the dipole−dipole interactions. Compared to biphenyl, bipyridine connected by two polar pyridine rings should have much stronger dipole−dipole interactions between adjacent molecules, thus providing another reason why 5,5-B0NBr8 cannot assemble as effectively as 5,5-B2NBr8 does. Besides π−π interactions, dipole−dipole interactions are therefore also responsible for the assembly process. Significant Changes on Self-Assembled Structure Because of Different Substituted Position. Changing the position at which the alkyl tails are attached onto bipyridine caused a surprising change in the assembly behavior. As shown in Figure 4, only spherical colloidal aggregates rather than 1D

Figure 4. Spherical colloidal aggregates formed by 4,4-B2NBr8 in solution observed through (a) cryo-TEM and (b) ex situ-AFM (peak force error) with consistent diameter of ca. 33 nm.

fibers were observed for compound 4,4-B2NBr8 (Scheme 1d). As shown in Figure 4a, spherical colloidal aggregates with an average diameter of 34 ± 4 nm were observed by cryo-TEM, which was consistent with the size cross-verified by ex-situ AFM (Figure 4b), in-situ AFM, and normal TEM (Figure S7). Figure S8 is drawn to explore why 4,4-B2NBr8, in contrast to 5,5-B2NBr8, cannot undergo an effective assembly into anisotropic structures. First, the 4,4′-substituted alkyl chain experiences more steric hindrance, which prevents molecules to pack closely. Second, as mentioned above, besides π−π stacking along the normal direction of the aromatic plane, dipole−dipole interactions facilitate the assembly of the mesogenic core in the 5027

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Figure 5. The assembly of 5,5-B2NBr8 was a monolayer ribbonlike fiber. Along the length, molecules stacked closely through strong π−π interaction. Along the width, a lateral alignment was adopted, similar to that in the crystal of dimethyl 2,2′-bipyridine-5,5′-dicarboxylate, to form a tilted monolayer with a thickness of 3.2 nm. With longer alkyl chains, the fibers could twist into helical shape. By changing the position at which the alkyl tails are attached, spherical colloidal aggregates were formed. The dipole moment is shown as a yellow arrow.

Figure 6. The assembly and disassembly of the fibrous structure can be controlled reversibly by adding and removing CuII, respectively. (a) Job plot showed a 1:1 binding profile for 5,5-B2NBr8 and CuII in pure water. (b) An MLCT band appeared at 324 nm, and all peaks were bathochromically shifted when 1 equiv of CuBr2 was added (II). The spectrum (III), on further addition of 1 equiv of EDTA, exhibited the same profile as the spectrum (I) of pure 5,5-B2NBr8. The concentration was 0.05 mM for all samples. (c) TEM images of [5,5-B2NBr8]:[CuBr2]:[EDTA] equaled 1:0:0 (I), 1:1:0 (II), and 1:1:1 (III).

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than bipyridine,53 fibers appeared again (Figure 6c-III). The reversibility was further demonstrated by UV/vis spectroscopy: after addition of CuBr2, to a 5,5-B2NBr8 solution, the MLCT band appeared at 324 nm (Figure 6b-II); upon EDTA addition to this mixture, the spectrum (Figure 6b-III) was virtually identical to the spectrum of the pure 5,5-B2NBr8 solution (Figure 6b-I). Thus, the assembly and disassembly of 5,5B2NBr8 can be controlled reversibly.

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CONCLUSIONS In conclusion, we have designed and synthesized a new class of bolaamphiphiles containing bipyridine as mesogenic core. The rational exploitation of the new bola-form architectures leads to control over the self-assembly process and structure formation, which represents a successful example of supramolecular engineering. Further tuning of the self-assembly properties should also be possible by changing the counterion. In order to further exploit the coordination capacity of the bipyridine moiety, other metal ions may be introduced into these versatile bolaamphiphiles, leading to fabrication of advanced optoelectronic nanomaterials.

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ASSOCIATED CONTENT

* Supporting Information S

Cmc measurement, concentration-dependent assembly of 5,5B2NBr8, section analysis of AFM images, AFM and TEM images of analogues, structure optimization through DFT:B3LYP/6-31g** basis set, elaboration on the assembly of 4,4-B2NBr8, binding constant measurement, details of single-crystal X-ray experiment, and details of synthesis including NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: (+86) 10-6277-1149; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21121004), Tsinghua University Initiative Scientific Research Program (2009THZ02230), the NSFC (50973051, 20974059), NSFC-DFG joint grant (TRR61), and Bilateral Scientific Cooperation between Tsinghua University and K. U. Leuven. We thank Eric Hill from the University of New Mexico for DFT optimization and thank Yu Liu and Prof. Zhibo Li from ICCAS for cryo-TEM measurements.

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REFERENCES

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