Bolaamphiphiles bearing bipyridine as mesogenic core: Rational exploitation of molecular architectures for controlled self-assembly

Article abstract of DOI:10.1021/la300369w

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.

Full text of DOI:10.1021/la300369w

Article
pubs.acs.org/Langmuir
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*
†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 synthe-
sized, 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.
INTRODUCTION
Self-assembly of amphiphiles is a facile strategy to approach the
construction of hierarchical functional nanostructures.
Research into the relationship between the structures of
synthetic amphiphiles and resulting self-assembled morpholo-
gies has continuously aimed at the ultimate goal of precise
regulation and full control of the assembling process.
Mimicking the robust lipid membrane of archaebacteria,
bolaamphiphiles, as one branch of amphiphiles with two
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.
■
1
−8
9
−13
14
EXPERIMENTAL SECTION
■
1
5−23
General Methods. 1H NMR spectra were recorded on JEOL
JNM-ECA300 (300 MHz), JNM-ECA400 (400 MHz), and JNM-
ECA600 (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.
hydrophilic heads connected by hydrophobic spacers,
have
with
2
4−30
been demonstrated to form diverse assemblies
3
1−36
distinct features.
bolaamphiphiles has been realized based on the concepts of
Moreover, controlled self-assembly of
3
7−40
supraamphiphiles.
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
4
1−44
highly ordered assemblies through the bottom-up strategy.
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 self-
45−48
assembled structures.
Herein, we designed and synthesized
Received: January 25, 2012
Revised: February 27, 2012
Published: March 9, 2012
a new series of bolaamphiphiles using, for the first time,
bipyridine as the mesogenic core. It was found that rational
©
2012 American Chemical Society
5023
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Article
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”.
Scheme 2. Synthesis Route of 5,5-B2NBr8 and Dimethyl
a
2,2′-Bipyridine-5,5′-dicarboxylate
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
a
Reagents and conditions: (1) KMnO , reflux, overnight; (2) I -
4
SOCl , II - 8-bromooctan-1-ol, NEt , DCM, reflux, overnight; (3)
pyridine, CH CN, 50 °C, 2 days; (4) MeOH, H SO , reflux, 3 days.
2
3
3
2
4
2
,2′-Bipyridine-4,4′-dicarboxylic Acid. White powder; yield:
1
3
9
1%. H NMR (300 MHz, CF COOD, ppm): δ 9.19 (d, 6.1 Hz, 2H),
3
.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
1
solid; yield: 42%. H NMR (300 MHz, CDCl , 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
1
3
.9 Hz, 4H), 3.41 (t, 6.9 Hz, 4H), 1.93−1.70 (m, 8H), 1.61−1.27 (m,
6H).
(
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.
1
,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxyoctane-8,1-
diyl)]dipyridinium Dibromide (5,5-B2NBr8). Off-white solid;
yield: 99%. H NMR (400 MHz, DMSO-d , ppm): δ 9.20 (dd, 2.1,
0
2
7
Synthesis of 5,5-B2NBr8 and Its Analogues. The synthesis
routes of 5,5-B2NBr8 and model molecule, dimethyl 2,2′-bipyridine-
1
6
.8 Hz, 2H), 9.15 (dd, 6.8, 1.4 Hz, 4H), 8.62 (ddd, 7.8, 4.6, 1.2 Hz,
H), 8.58 (dd, 8.3, 0.7 Hz, 2H), 8.47 (dd, 8.3, 2.2 Hz, 2H), 8.17 (dd,
.6, 6.7 Hz, 4H), 4.63 (t, 7.4 Hz, 4H), 4.33 (t, 6.6 Hz, 4H), 1.98−1.86
5,5′-dicarboxylate, are shown in Scheme 2. Other analogues of 5,5-
B2NBr8 were synthesized following similar procedures. More details
of the synthetic procedures are included in the Supporting
Information.
1
3
(
m, 4H), 1.78−1.67 (m, 4H), 1.47−1.21 (m, 16H). C NMR (101
MHz, DMSO-d , 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,
2
,2′-Bipyridine-5,5′-dicarboxylic Acid. White powder; yield:
6
1
8
8
2%. H NMR (300 MHz, CF COOD, ppm): δ 8.78 (d, 2.1 Hz, 2H),
3
2+
.11 (dd, 8.3 Hz, 2.1 Hz, 2H), 7.67 (d, 8.3 Hz, 2H).
28.04, 25.33, 25.29. ESI-mass: 312.59 ([M − 2Br] , calcd: 312.19);
5
024
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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).
+
7
03.31, 705.31 ([M − Br] , calcd: 703.29, 705.28); 865.34 ([M +
138.33, 128.09, 126.31, 121.25, 65.26, 60.67, 30.75, 28.93, 28.91,
−
2+
Br] , calcd: 865.12).
28.80, 28.68, 28.40, 28.07, 25.40. ESI-mass: 368.56 ([M − 2Br] ,
+
1
,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxyoctane-8,1-
calcd: 368.25); 815.44, 817.44 ([M − Br] , calcd: 815.41, 817.41).
diyl)]ditriethylammonium Bromide Dibromide (5,5-B2NBr8-
Bis(8-bromooctyl) 2,2′-Bipyridine-4,4′-dicarboxylate. Off-
1
1
TEA). Light brown solid; yield: 74%. H NMR (400 MHz, DMSO-d ,
6
white solid; yield: 52%. H NMR (600 MHz, CDCl , ppm): δ 8.95
3
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,
(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).
4
7
1
2
H), 1.83−1.68 (m, 4H), 1.57 (s, 4H), 1.48−1.24 (m, 16H), 1.15 (t,
.0 Hz, 18H). 13C NMR (101 MHz, DMSO-d , ppm): δ 164.49,
6
1,1′-[2,2′-Bipyridine-4,4′-diylbis(carbonyloxyoctane-8,1-
57.47, 150.10, 138.42, 126.39, 121.32, 65.27, 51.92, 51.89, 28.47,
diyl)]dipyridinium Dibromide (4,4-B2NBr8). Pale brown solid;
2+
1
8.42, 28.06, 25.76, 25.31, 20.91, 7.17. ESI-mass: 334.56 ([M − 2Br] ,
yield: 97%. H NMR (400 MHz, DMSO-d , ppm): δ 9.12 (d, 5.7 Hz,
6
+
calcd: 334.26); 747.42, 749.42 ([M − Br] , calcd: 747.44, 749.44);
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,
−
9
09.48 ([M + Br] , calcd: 909.27).
Bis(4-bromobutyl) 2,2′-Bipyridine-5,5′-dicarboxylate. White
6.7 Hz, 4H), 1.99−1.85 (m, 4H), 1.80−1.68 (m, 4H), 1.48−1.18 (m,
1
13
solid; yield: 87%. H NMR (300 MHz, CDCl , ppm): δ 9.30 (d, 2.0
16H). C NMR (101 MHz, DMSO-d
6
, ppm): δ 164.52, 155.43,
3
Hz, 2H), 8.60 (d, 8.3 Hz, 2H), 8.44 (dd, 8.3 Hz, 2.0 Hz, 2H), 4.44 (t,
150.92, 145.49, 144.76, 138.52, 128.09, 123.40, 119.22, 65.69, 60.70,
6
.7 Hz, 4H), 3.50 (t, 6.8 Hz, 4H), 1.93−1.78 (m, 8H).
30.70, 28.40, 28.27, 27.99, 25.34, 25.26. ESI-mass: 312.42 ([M −
2
+
+
1
,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxybutane-4,1-
2Br] , calcd: 312.19); 703.29, 705.29 ([M − Br] , calcd: 703.29,
−
diyl)]dipyridinium Dibromide (5,5-B2NBr4). Pale brown solid;
705.28); 865.34 ([M + Br] , calcd: 865.12).
1
yield: Quantitative. H NMR (400 MHz, DMSO-d , ppm): δ 9.25 (s,
Bis(8-bromooctyl) Biphenyl-4,4′-dicarboxylate. White solid;
6
1
2
8
2
H), 9.18 (d, 5.7 Hz, 4H), 8.71−8.56 (m, 4H), 8.51 (d, 8.2 Hz, 2H),
yield: 49%. H NMR (300 MHz, CDCl , ppm): δ 8.13 (d, 8.3 Hz,
3
.19 (t, 6.8 Hz, 4H), 4.73 (t, 7.3 Hz, 4H), 4.39 (t, 6.0 Hz, 4H), 2.22−
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
3
.05 (m, 4H), 1.86−1.72 (m, 4H). C NMR (101 MHz, DMSO-d ,
6
ppm): δ 164.46, 157.46, 150.22, 145.57, 144.87, 138.54, 128.14,
1,1′-[Biphenyl-4,4′-diylbis(carbonyloxyoctane-8,1-diyl)]-
dipyridinium Dibromide (5,5-B0NBr8). Pale brown solid; yield:
1
2
5
26.30, 121.25, 64.53, 60.30, 27.57, 24.82. ESI-mass: 256.32 ([M −
1
2
+
+
8
8
4
1
0%. H NMR (400 MHz, DMSO-d , ppm): δ 9.13 (d, 5.6 Hz, 4H),
Br] , calcd: 256.12); 591.19, 593.19 ([M − Br] , calcd: 591.16,
6
−
.61 (tt, 7.9, 1.2 Hz, 2H), 8.17 (dd, 7.6, 6.7 Hz, 4H), 8.06 (d, 8.4 Hz,
H), 7.91 (d, 8.5 Hz, 4H), 4.62 (t, 7.4 Hz, 4H), 4.29 (t, 6.5 Hz, 4H),
.99−1.85 (m, 4H), 1.77−1.66 (m, 4H), 1.48−1.20 (m, 16H).
NMR (101 MHz, DMSO-d , ppm): δ 165.46, 145.49, 144.76, 143.29,
93.16); 751.17, 753.16 ([M + Br] , calcd: 751.00, 752.99).
Bis(12-bromododecyl) 2,2′-Bipyridine-5,5′-dicarboxylate.
1
3
1
C
White solid; yield: 43%. H NMR (300 MHz, CDCl , ppm): δ 9.30
3
(
4
1
d, 2.0 Hz, 2H), 8.58 (d, 8.3 Hz, 2H), 8.44 (dd, 8.3 Hz, 1.9 Hz, 2H),
.38 (t, 6.6 Hz, 4H), 3.40 (t, 6.9 Hz, 4H), 1.93−1.67 (m, 8H), 1.57−
.01 (m, 32H).
6
1
2
7
29.85, 129.50, 128.09, 127.40, 64.78, 60.69, 30.70, 28.42, 28.28,
2+
8.13, 25.34. ESI-mass: 311.47 ([M − 2Br] , calcd: 311.19); 701.35,
+
03.35 ([M − Br] , calcd: 701.30, 703.29); 861.43, 863.41 ([M +
1
,1′-[2,2′-Bipyridine-5,5′-diylbis(carbonyloxydodecane-
−
1
2,1-diyl)]dipyridinium Dibromide (5,5-B2NBr12). Pale brown
Br] , calcd: 861.13, 863.13).
1
solid; yield: 77%. H NMR (400 MHz, DMSO-d , 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,
6
(
2
1
(
RESULTS AND DISCUSSION
■
H), 8.17 (t, 6.9 Hz, 4H), 4.62 (t, 7.3 Hz, 4H), 4.31 (t, 6.3 Hz, 4H),
13
.89 (s, 4H), 1.81−1.62 (m, 4H), 1.50−1.01 (m, 32H). C NMR
For bolaamphiphile 5,5-B2NBr8, 2,2′-bipyridine is employed as
the central mesogenic core, connected to two hydrophobic
101 MHz, DMSO-d , ppm): δ 164.42, 157.39, 150.03, 145.49, 144.77,
6
5
025
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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.
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 triethylammo-
nium (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.
Self-Assembly of 5,5-B2NBr8 into Fibrous Structure.
TEM of 5,5-B2NBr8 revealed the formation of micrometer-
long 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
5
,5-B2NBr12, with a longer 12-carbon chain, presented helical
fibers with a width of 12 ± 2 nm as observed by TEM (Figure
b) and AFM (Figure S5b). It is well-known that hydrophobic
(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, concentration-
dependent fluorescence of 5,5-B2NBr8 was also observed
2
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.
(Figure S3). In very dilute solutions, the emission of 5,5-
B2NBr8 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 self-
assembled 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
Further confirmation of the significance of π−π interactions
49
was obtained from the single crystal structure of dimethyl
2,2′-bipyridine-5,5′-dicarboxylate, a model for the mesogenic
5
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49
Figure 3. Crystal structure 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 c-
axis. L of 3.49 Å and α of ca. 65° are respectively the vertical interval and the slip angle between two adjacent bipyridine planes.
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.
lateral direction, since each dipole of the pyridine rings in 5,5-
B2NBr8 adopts a head-to-tail arrangement. However, the head-
to-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 5,5-
B2NBr8. 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 5,5-
B2NBr8. 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.
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
Reversibly Controlled Assembly through Coordina-
tion. 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,
CuBr was employed to avoid introducing other counterions.
2
II
Coordination of Cu with 5,5-B2NBr8 leads to a metal-to-
ligand-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
50
5
,5-B2NBr8 and CuBr2 with different compositions.
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 CuBr (monitored at 324 nm), the binding
2
6
−1
51
constant was calculated to be 1.62 × 10 M (Figure S9).
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.
Structural investigations by TEM of a 1:1 mixture of 5,5-
B2NBr8 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
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
II 52
to Cu , which destroys the beneficial conformation for the
II
formation of fibrous structures. In addition, binding to Cu 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
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
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
II
(EDTA), which has a much higher binding constant with Cu
5
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Article
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.
II
Figure 6. The assembly and disassembly of the fibrous structure can be controlled reversibly by adding and removing Cu , respectively. (a) Job plot
II
showed a 1:1 binding profile for 5,5-B2NBr8 and Cu in pure water. (b) An MLCT band appeared at 324 nm, and all peaks were bathochromically
shifted when 1 equiv of CuBr was added (II). The spectrum (III), on further addition of 1 equiv of EDTA, exhibited the same profile as the
2
spectrum (I) of pure 5,5-B2NBr8. The concentration was 0.05 mM for all samples. (c) TEM images of [5,5-B2NBr8]:[CuBr ]:[EDTA] equaled
2
1:0:0 (I), 1:1:0 (II), and 1:1:1 (III).
5
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5
3
than bipyridine, fibers appeared again (Figure 6c-III). The
reversibility was further demonstrated by UV/vis spectroscopy:
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of Supramolecular Materials. Adv. Mater. 2003, 15, 673.
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after addition of CuBr , to a 5,5-B2NBr8 solution, the MLCT
2
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Self-
Assembled Hexa-peri-hexabenzocoronene Graphitic Nanotube. Science
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 5,5-
B2NBr8 can be controlled reversibly.
2
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(
2
(
CONCLUSIONS
Dimensional Sheets from the Self-Assembly of Laterally Grafted
Amphiphilic Rods. Angew. Chem., Int. Ed. 2009, 48, 3657.
■
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
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further exploit the coordination capacity of the bipyridine
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ASSOCIATED CONTENT
■
*
S
Supporting Information
́
(12) Brizard, A.; Aime, C.; Labrot, T.; Huc, I.; Berthier, D.; Artzner,
Cmc measurement, concentration-dependent assembly of 5,5-
B2NBr8, 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.
F.; Desbat, B.; Oda, R. Counterion, Temperature, and Time
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AUTHOR INFORMATION
■
*
(
16) Menger, F. M.; Wrenn, S. Interfacial and Micellar Properties of
Bolaform Electrolytes. J. Phys. Chem. 1974, 78, 1387.
17) Yiv, S.; Kale, K. M.; Lang, J.; Zana, R. Chemical Relaxation and
Corresponding Author
Fax: (+86) 10-6277-1149; e-mail: xi@mail.tsinghua.edu.cn.
(
Notes
Equilibrium Studies of Association in Aqueous Solutions of Bolaform
Detergents. 1. Dodecane-1,12-Bis(trimethylammonium Bromide). J.
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The authors declare no competing financial interest.
(
18) Fuhrhop, J. H.; David, H. H.; Mathieu, J.; Liman, U.; Winter, H.
ACKNOWLEDGMENTS
■
J.; Boekema, E. Bolaamphiphiles and Monolayer Lipid Membranes
Made from 1,6,19,24-Tetraoxa-3,21-Cyclohexatriacontadiene-
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
2
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22) Benvegnu, T.; Brard, M.; Plusquellec, D. Archaeabacteria
(
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.
(
1
(
Bipolar Lipid Analogues: Structure, Synthesis and Lyotropic Proper-
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