Strong aggregation and directional assembly of aromatic oligoamide macrocycles

Article abstract of DOI:10.1021/ja208548b

Aromatic oligoamide macrocycles exhibit strong preference for highly directional association. Aggregation happens in both nonpolar and polar solvents but is weakened as solvent polarity increases. The strong, directional assembly is rationalized by the co

Full text of DOI:10.1021/ja208548b

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Strong Aggregation and Directional Assembly of Aromatic
Oligoamide Macrocycles
Yongan Yang,† Wen Feng,† Jinchuan Hu,† Shuliang Zou,† Rongzhao Gao,† Kazuhiro Yamato,‡
Mark Kline,‡ Zhonghou Cai,§ Yi Gao,z Yibing Wang,|| Yibao Li,||,r Yanlian Yang,|| Lihua Yuan,*,†
Xiao Cheng Zeng,*,z and Bing Gong*,‡,#
†College of Chemistry, Key Laboratory for Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and
Technology, Sichuan University, Chengdu 610064, Sichuan, China
‡Department of Chemistry, The State University of New York, Buffalo, New York 14260, United States
#College of Chemistry, Beijing Normal University, Beijing 100875, China
§X-ray Operations and Research, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne,
Illinois 60439, United States
zDepartment of Chemistry, University of Nebraska—Lincoln, Lincoln, Nebraska 68588, United States
National Center for Nanoscience and Technology, Beijing 100190, China
S Supporting Information
b
ABSTRACT: Aromatic oligoamide macrocycles exhibit
strong preference for highly directional association.
Aggregation happens in both nonpolar and polar sol-
vents but is weakened as solvent polarity increases. The
strong, directional assembly is rationalized by the co-
operative action of dipoleꢀdipole and πꢀπ stacking
interactions, leading to long nanotubular assemblies that
are confirmed by SEM, TEM, AFM, and XRD. The
persistent nanotubular assemblies contain non-collapsi-
ble hydrophilic internal pores that mediate highly efficient ion
transport observed with these macrocycles and serve as
cylindrical sites for accommodating guests such as
motion due to aggregation.9 At 50 °C, the signals of 1a remain
metal ions.
broadened.3a The 1H NMR spectra of 1b and 1c recorded in CDCl3
also show noticeable broadening of signals, especially in the regions of
aromatic and amide proton resonances.10 These early observations
any unnatural oligomers that fold into defined shapes are
M
known.1 We created foldamers with persistent confor-
prompted us to investigate the intermolecular association of these
macrocyclic molecules.
mations containing lumens from sub-nanometer to >3 nm
across.2 Preparing folding polymers based on the same
strategy led to the one-pot formation of macrocycle 1a in
high yields.3a This one-pot procedure was then extended to
the synthesis of macrocycles such as 2 with much larger
sizes,3b or those with different backbones.3c The availability
of cavity-containing, shape-persistent macrocycles4,5 offers
the possibility of creating higher-order structures6,7 with
novel functions.7d,g For example, we reported highly con-
ducting transmembrane single-ion channels formed by
macrocycles 1 bearing proper side chains.8 It was proposed
that the observed ion transport was mediated by the non-
collapsible internal pores of tubular structures formed from
aligned macrocyces. Herein we report the strong aggregation
of macrocycles 1 and 2, the nanotubular stacks formed from
the directional assembly of these molecules, and the inclu-
sion of metal ions into the internal pores of the nanotubes.
Upon their discovery, macrocycles 1a were noticed for their strong
1
Compounds 1b and 1c were examined using H NMR in
CDCl3 by varying concentration and temperature. The 1H NMR
signals of these macrocycles became better resolved at elevated
temperatures or decreased concentrations.10 It was also observed
that increasing solvent polarity led to improved resolution of the
1H NMR signals of 1b and 1c.10
The aggregation of 1b was confirmed by dynamic light
scattering (DLS).10 In CHCl3, the average size of aggregates
grew from 550 to 1909 nm as the concentrations of 1b changed
from 0.25 to 5 mM, reached 2045 nm at 9 mM, and leveled off at
1
higher concentrations. Consistent with results from H NMR
studies, enhancing solvent polarity decreased the size of the
aggregates. In acetone, the average size of the aggregates formed
by 1b (5 mM) was significantly reduced (102 nm) in comparison
to that in CHCl3. Macrocycle 1c, being less soluble, showed the
same aggregation behavior.10
1
aggregation. The H NMR spectrum of 1a recorded in CDCl3 at
Received: September 10, 2011
Published: October 24, 2011
room temperature contains no signals, indicating decreased molecular
r
2011 American Chemical Society
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The above results demonstrate that the aggregation of macro-
cycles 1a, 1b, and 1c was weakened at elevated temperatures and
lowered concentrations and, interestingly, in polar solvents. The
intermolecular aggregation initially indicated for 1a seems to be
an inherent property of these macrocycles which share the same
oligoamide backbone.
Comparing the 1H NMR spectrum of 1b with those of 3, 4,
and 5 provided additional details. In CDCl3, Ha of 1b shows a
significant (0.29 ppm) upfield shift compared to Ha of 3. The
amide protons of 4 and 5 also show upfield shifts (0.10 ppm for
Ha of 4; 0.16 ppm for Ha and 0.21 ppm for Hb of 5) relative to
Ha of 3. Given that the H-bonding capacities of these amide
protons are “saturated” upon forming three-center H-bonds,2c,11
the observed upfield shifts accompanying the larger oligoamides
cannot be rationalized by intermolecular H-bonding.
Figure 1. Chemical shifts of the three-center H-bonded amide protons
of 1b, 3, 4, and 5 (2.0 mM) versus percent of CD3OD in CDCl3. The 1H
NMR spectra were recorded at 300 K.
The shifts of the 1H NMR resonances of 4 and 5 relative to
those of 3, being proportional to the surface area of the
corresponding molecules, reflect the strength of the stacking
interaction of these relatively flat molecules.12 Besides amide
protons, the aromatic protons d and c of 1b also exhibit
pronounced shifts (from 0.3 to 0.4 ppm) relative to the signals
of the corresponding aromatic protons of 3, 4, and 5.10 The large
shifts observed for the amide and aromatic protons of 1b suggest
that the cyclic oligoamide backbone has a particularly strong
propensity for stacking interaction, which results in especially
strong self-aggregation.
Figure 2. (a) SEM (on mica) and (b) TEM (on carbon film) images of
the solid samples of 1b. (c) SEM (on mica) and (d) TEM (on carbon film)
images of the solid samples of 2. All solid samples were formed by drop-
casting solutions in CHCl3 (for 1b) or in CHCl3/p-xylene (2/1, for 2).
The 1H NMR spectra of 1b, 3, 4, and 5 recorded in mixtures of
CDCl3 and CD3OD revealed that the three-center H-bonded amide
protons, although unavailable for additional H-bonding,2c,11 all
exhibited noticeable downfield shifts with increasing CD3OD
content (Figure 1). At 20% CD3OD, Ha of 4 and Ha and Hb of
5 shifted to positions nearing Ha of 3, which undergoes the weakest
stacking interaction because it has the smallest surface area among
these oligoamides.12 This result suggests that the stacking of 4 and 5
was weakened with increasing solvent polarity. The amide 1H signal
of 1b, appearing at much more upfield positions relative to those of
3, 4, and 5, also underwent downfield shifts with increasing solvent
polarity, indicating weakened stacking interactions in polar media.
CDCl3 solvent containing different percentages of other polar
solvents including DMSO-d6 or CD3CN also led to the same trend
of line-sharpening and downfield shifting, indicating that the
observed disruption of aggregation in polar solvents is caused by
hydrogen-bonding between solvent molecules and the macrocycles.
These observations are consistent with results from earlier 1H NMR
and DLS studies discussed above.
stacking, which is usually promoted in solvents of high dielectric
constants, such as water and methanol, and discouraged in
solvents of low dielectric constants, such as CHCl3.13 To explain
the observed aggregation and solvent effect, ab initio computa-
tion was performed. A large binding energy of ꢀ57.9 (DFT-D)
or ꢀ55.3 kcal/mol (DFT-D3) for the association of two
molecules of 1d was revealed.10 With the shape of a shallow bowl,
the macrocycle has a large (15.5D) overall dipole moment passing
through the center of the macrocyclic backbone (Figure S82).10
Being at least 10 times stronger than that of regular πꢀπstacking, the
association of these macrocycles is best rationalized by the coopera-
tive action of dipoleꢀdipole and πꢀπ interactions.10 The contribu-
tion of a strong dipoleꢀdipole interaction also explains the effect of
polar solvents on the aggregation of macrocycles 1.
The large dipole moment of macrocycles 1 should lead to the
alignment or directional assembly of these structurally symme-
trical molecules. This expectation was confirmed by examining
the assembly of 1b in the solid state. The scanning electron
microscopy (SEM) image of a sample of 1b, prepared by
dropping a solution in CHCl3 onto mica followed by evaporating
solvent, reveals fibers of over 400 μm in length and 2ꢀ3 μm in
The effect of solvent polarity on the aggregation of these
oligoamides is in contrast to what is known about aromatic
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Figure 4. Diffractogram of the solid samples of cesium picrate alone, the
complex of 1b with Cs+Picꢀ, and macrocycle 1b alone. The stacking
distance between adjacent macrocyclic cores is 3.8 Å for the 1b Cs+Picꢀ
3
complex.
The confirmed nanotubular assembly of 1b (and similarly 1c
and 2) has provided structural evidence confirming the validity of
a previously proposed model for explaining the highly conduct-
ing ion channels formed by analogues of 1aꢀc.8 Besides serving
as ion channels, the hydrophilic sub-nanometer pores formed by
1b should also serve as sites for accommodating polar species,
including various metal ions.
This possibility was probed by mixing an excess amount of
cesium picrate (Cs+Picꢀ) with 1b in CHCl3, in which the salt is
nearly insoluble.10 After filtering off excess Cs+Picꢀ, the other-
wise colorless solution of 1b became yellow. Removing CHCl3
Figure 3. (a) Diffractogram of the solid sample of 1b. (b) Modeling of
the columnar packing of 1b and the hexagonal lattice. The hexagonal
lattice parameter a is 27.6 Å. The orientation of the macrocycles in a
column is currently unknown.
1
left a solid residue with a H NMR spectrum (in CDCl3)
containing the signals of 1b along with that (at 8.78 ppm) of
diameter (Figure 2a). The transmission electron microscopy (TEM)
image of a sample of 1b, prepared by drop-casting the same solution
onto carbon film (Figure 2b) or mash grid,10 reveals straight, rod-like
assemblies. SEM and TEM of 1c also reveal similar fibers.10 Direc-
tional assembly is not limited to 1. SEM and TEM (Figure 2c,d) of 2
show long, rod-like fibers. Additionally, the atomic force microscopy
(AFM) images of solid samples of 1b and 2 reveal long fibers on the
surface.10 These microscopic images point to a highly directional
assembly of these largely flat macrocyclic molecules.
the picrate ion.10 The presence of Cs+Picꢀ led to dramatic
1
shifts of the H NMR signals of 1b. The observed changes
suggest complxation of Cs+ ion by 1b. By integrating the 1H
NMR signals of the picrate ion and the olefin moiety of the
side chains, the stoichiometry of the 1b Cs+Picꢀ complex was
3
found to be 1:0.83. DLS measurements revealed that mixing
1b (2 mM, CHCl3) with Cs+Picꢀ formed aggregates with an
average size of 320 nm. These results indicate that Cs+Picꢀ
underwent strong interaction with the aggregates of 1b, which
suggests that the Cs+ ions are enclosed within the hydrophilic
pores of the nanotubular assemblies. A similar change in the
1H NMR signals of 1b was observed upon addition of K+Picꢀ
(Figure S77), suggesting that the hydrophilic pores could
accommodate metal ions of different sizes.
Details on the assembly of 1b were gained from X-ray
diffraction (XRD). An intense peak at 23.9 Å and three others
at 13.9, 11.8, and 9.6 Å, with ratios of d-spacings being 1:1/
√
√
3:1/2:1/ 7, are detected (Figure 3a). The XRD pattern
points to cylindrical stacks that further packed on a typical
hexagonal (colh) lattice (Figure 3b).14 The lattice parameter a
(27.6 Å), i.e., the intercolumnar spacing, agrees with the
diameter of 1b given partially (∼55%) collapsed or interdigi-
tated side chains, which confirms that these molecules aligned
into a tubular assembly with its outer and inner diameters being
defined by the macrocycles.
Consistent with strong, directional dipoleꢀdipole interaction,
the stacked macrocycles exhibit an extraordinary long-range
ordering. The 3.60-Å reflection, typical of πꢀπ stacking, is
attributed to the intra-columnar spacing between the parallel
macrocyclic cores, giving a correlation length of 69.6 nm based
on Scherrer’s equation.15 This correlation length indicates a
nanotube consisting of ∼193 continuously stacked macrocycles.
Similar XRD analysis on the solid samples of 1c and 2 revealed
the same tubular phases in which long columns consisting of
large numbers of macrocycles pack on a hexagonal lattice.10
In the solid state, directional assembly was observed for 1b
with added Cs+Picꢀ. SEM of a sample of Cs+Picꢀ and 1b (1:1),
prepared by drop-casting a CHCl3 solution onto mica, shows
long fibers.10 XRD of 1b (Cs+Picꢀ) revealed a hexagonal lattice
3
similar to that of 1b (Figure 4). Lattice parameter a (28.6 Å),
corresponding to the intercolumnar spacing, is very similar to
that of pure 1b. An intra-columnar spacing of 3.8 Å, larger than
the 3.6 Å observed for 1b alone, indicates that the distance
between the macrocyclic cores increased in the presence of Cs+
ions, due likely to electrostatic repulsion. Given the small
increase of stacking distance, the picrate ions, instead of being
sandwiched between the macrocyclic backbones, are more likely
“dissolved” by the alkenyl side chains and located between the
columnar stacks. The 3.8-Å reflection gives a correlation length
of 77.82 nm,10 corresponding to a tube consisting of ∼205
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Journal of the American Chemical Society
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continuously stacked macrocycles. Thus, the columnar assembly
and hexagonal packing of 1b remain in the presence of Cs+Picꢀ,
suggesting that the metal ions are complexed within the
hydrophilic pore of the nanotubular stacks.
Ferguson, J. S.; Yamato, K.; Liu, R.; He, L.; Zeng, X. C.; Gong, B. Angew.
Chem., Int. Ed. 2009, 48, 3150.
(4) (a) Bunz, U. H. F.; Rubin, Y.; Tobe, Y. Chem. Soc. Rev. 1999,
28, 107. (b) Sessler, J. L.; Tvermoes, N. A.; Davis, J.; Anzenbacher, P.;
Jursikova, K.; Sato, W.; Seidel, D.; Lynch, V.; Black, C. B.; Try, A.;
Andrioletti, B.; Hemmi, G.; Mody, T. D.; Magda, D. J.; Kral, V. Pure
Appl. Chem. 1999, 71, 2009. (c) Grave, C.; Schluter, A. D. Eur. J. Org.
Chem. 2002, 2002, 3075. (d) Zhang, W.; Moore, J. S. Angew. Chem., Int.
Ed. 2006, 45, 4416. (e) MacLachlan, M. J. Pure Appl. Chem. 2006,
78, 873. (f) H€oger, S. Pure Appl. Chem. 2010, 82, 821.
(5) (a) Yamaguchi, Y.; Yoshida, Z.-I. Chem. Eur. J. 2003, 9, 5430. (b)
H€oger, S. Angew. Chem., Int. Ed. 2005, 44, 3806. (c) Xing, L. Y.; Ziener, U.;
Sutherland, T. C.; Cuccia, L. A. Chem. Commun. 2005, 5751. (d) Zhang, W.;
Moore, J. S. J. Am. Chem. Soc. 2005, 127, 11863. (e) Campbell, F.; Wilson,
A. J. Tetrahedron Lett. 2009, 50, 2236. (f) Qin, B.; Ren, C. L.; Ye, R. J.; Sun,
C.; Chiad, K.; Chen, X. Y.; Li, Zhao.; Xue, F.; Su, H. B.; Chass, G. A.; Zeng,
H. Q. J. Am. Chem. Soc. 2010, 132, 9564. (g) Frischmann, P. D.; Facey, G. A.;
Ghi, P. Y.; Gallant, A. J.; Bryce, D. L.; Lelj, F.; MacLachlan, M. J. J. Am. Chem.
Soc. 2010, 132, 3893.
(6) Reviews on columnar/tubular assemblies: (a) Bong, D. T.;
Clark, T. D.; Granja, J. R.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2001,
40, 988. (b) Block, M. A. B.; Kaiser, C.; Khan, A.; Hecht, S. Top. Curr.
Chem. 2005, 245, 89. (c) Keizer, H. M.; Sijbesma, R. P. Chem. Soc. Rev.
2005, 34, 226. (d) Pasini, D.; Ricci, M. Curr. Org. Syn. 2007, 4, 59.
(7) Examples of columnar/tubular assemblies: (a) Ghadiri, M. R.;
Granja, J. R.; Milligan, R. A.; McRee, D. E.; Khazanovich, N. Nature
1993, 366, 324. (b) Seebach, D.; Matthews, J. L.; Meden, A.; Wessels, T.;
Baerlocher, C.; McCusker, L. B. Helv. Chim. Acta 1997, 80, 173. (c)
Gattuso, G.; Menzer, S.; Nepogodiev, S. A.; Stoddart, J. F.; Williams, D. J.
Angew. Chem., Int. Ed. 1997, 36, 1451. (d) Mindyuk, O. Y.; Stetzer, M. R.;
Heiney, P. A.; Nelson, J. C.; Moore, J. S. Adv. Mater. 1998, 10, 1363. (e)
Kim, Y.; Mayer, M. F.; Zimmerman, S. C. Angew. Chem., Int. Ed. 2003,
42, 1121. (f) Fenniri, H.; Deng, B. L.; Ribbe, A. E.; Hallenga, K.; Jacob, J.;
Thiyagarajan, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 6487. (g) Gallant,
A. J.; MacLachlan, M. J. Angew. Chem., Int. Ed. 2003, 42, 5307. (h)
Percec, V.; Dulcey, A. E.; Balagurusamy, V. S. K.; Miura, Y.; Smidrkal, J.;
Peterca, M .; Nummelin, S.; Edlund, U.; Hudson, S. D.; Heiney, P. A.;
Hu, D. A.; Magonov, S. N.; Vinogradov, S. A. Nature 2004, 430, 764. (i)
Pisula, W.; Kastler, M.; Yang, C.; Enkelmann, V.; Mullen, K. Chem. Asian
J. 2007, 2, 51. (j) Yang, J.; Dewal, M. D.; Profeta, S.; Smith, M. D.; Li,
Y. Y.; Shimizu, L. S. J. Am. Chem. Soc. 2008, 130, 612. (k) Fischer, L.;
Decossas, M.; Briand, J. P.; Didierjean, C.; Guichard, G. Angew. Chem., Int.
Ed. 2009, 48, 1625. (l) Frischmann, P. D.; Guieu, S; Tabeshi, R.;
MacLachlan, M. J. J. Am. Chem. Soc. 2010, 132, 7668. (m) Ren, C. L.;
Maurizot, V.; Zhao, H. Q.; Shen, J.; Zhou, F.; Ong, W. Q.; Du, Z. Y.; Zhang,
K.; Su, H. B.; Zeng, H. Q. J. Am. Chem. Soc. 2011, 133, 13930. (n) Sato, K.;
Itoh, Y.; Aida, T. J. Am. Chem. Soc. 2011, 133, 13767. (o) Fritzsche, M.;
Bohle, A.; Dudenko, D.; Baumeister, U.; Sebastiani, D.; Richardt, G.; Spiess,
H. W.; Hansen, M. R.; Hoger, S. Angew. Chem., Int. Ed. 2011, 50, 3030.
(8) Helsel, A. J.; Brown, A. L.; Yamato, K.; Feng, W.; Yuan, L. H.;
Clements, A.; Harding, S. V.; Szabo, G.; Shao, Z. F.; Gong, B. J. Am.
Chem. Soc. 2008, 130, 15784.
In summary, oligoamide macrocycles undergo strong, direc-
tional assembly. In contrast to the stacking of typical aromatic
hydrocarbons, aggregation of these macrocycles is weakened in
polar media. The interplay of dipoleꢀdipole and πꢀπ stacking
leads to strong aggregation and the observed effect of solvent
polarity. These macrocycles readily form long fibers in the solid
state, in contrast to the fabrication of 1D assemblies of many disk-
like aromatics, which requires demanding conditions.16 XRD
data reveal well-defined, long nanotubes, which confirms the
highly directional stacking of these macrocycles. With their
persistent shape, non-deformable cavities, and high propensity
of aggregation, tubular assemblies of oligoamide macrocycles
provide a reliable supramolecular motif, based on which a variety
of organic nanotubes containing non-collapsible internal pores
can be created. The ability of 1b to accommodate metal ions
while maintaining the same nanotubular assembling and packing
order bodes well for creating new porous materials.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, ana-
b
lytical data, and microscopic images. This material is available
free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
bgong@buffalo.edu; lhyuan@scu.edu.cn;xzeng@unlserve.unl.edu
Present Address
rKey Laboratory of Organo-pharmaceutical Chemistry, Gannan
Normal University, Ganzhou 341000, P.R. China
’ ACKNOWLEDGMENT
This work is supported by the National Natural Science Founda-
tion of China (20774059, 21172158), the Doctoral Program of the
Ministry of Education of China (20090181110047), and the
U.S. National Science Foundation (CBET-1036171 and CBET-
1066947). Analytical & Testing Center of Sichuan University
(Dr. Pengchi Deng) is acknowledged for NMR experiments.
’ REFERENCES
(1) Reviews: (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173. (b)
Hill, D. J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. Rev.
2001, 101, 3893. (c) Gong, B. Chem. Eur. J. 2001, 7, 4336. (d) Huc, I.
Eur. J .Org. Chem. 2004, 17. (e) Li, Z. T.; Hou, J. L.; Li, C.; Yi, H. P. Chem.
Asian J. 2006, 1, 766. (f) Goodman, C. M.; Choi, S.; Shandler, S.;
DeGrado, W. F. Nature Chem. Biol. 2007, 3, 252.
(2) (a) Gong, B. Acc. Chem. Res. 2008, 41, 1376. (b) Gong, B.; Zeng,
H. Q.; Zhu, J.; Yuan, L. H.; Han, Y. H.; Cheng, S. Z.; Furukawa, M.;
Parra, R. D.; Kovalevsky, A. Y.; Mills, J. L.; Skrzypczak-Jankun, E.;
Martinovic, S.; Smith, R. D.; Zheng, C.; Szyperski, T.; Zeng, X. C. Proc.
Natl. Acad. Sci. U.S.A. 2002, 99, 11583. (c) Yuan, L. H.; Zeng, H. Q.;
Yamato, K.; Sanford, A. R.; Feng, W.; Atreya, H. S.; Sukumaran, D. K.;
Szyperski, T.; Gong, B. J. Am. Chem. Soc. 2004, 126, 16528.
(9) (a) Nelson, J. C.; Saven, J. G.; Moore, J. S.; Wolynes, P. G. Science
1997, 277, 1793. (b) Brunsveld, L.; Zhang, H.; Glasbeek, M.; Vekemans, J.
A. J. M.; Meijer, E. W. J. Am. Chem. Soc. 2000, 122, 6175.
(10) Please see Supporting Information for details.
(11) Parra, R. D.; Zeng, H. Q.; Zhu, J.; Zheng, C.; Zeng, X. C.; Gong,
B. Chem. Eur. J. 2001, 7, 4352.
(12) Zhang, Y. F.; Yamato, K.; Zhong, K.; Zhu, J.; Deng, J. G.; Gong,
B. Org. Lett. 2008, 10, 4339.
(13) Norberg, J.; Nilsson, L. Biophys. J. 1998, 74, 394.
(14) Prasad, S. K.; Rao, D. S. S.; Chandrasekhar, S.; Kumar, S. Mol.
Cryst. Liq. Cryst. 2003, 396, 121.
(15) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction, 3rd ed.;
Prentice-Hall: Englewood Cliffs, NJ, 2001.
(3) (a) Yuan, L. H.; Feng, W.; Yamato, K.; Sanford, A. R.; Xu, D. G.;
Guo, H.; Gong, B. J. Am. Chem. Soc. 2004, 126, 11120. (b) Feng, W.;
Yamato, K.; Yang, L. Q.; Ferguson, J.; Zhong, L. J.; Zou, S. L.; Yuan,
L. H.; Zeng, X. C.; Gong, B. J. Am. Chem. Soc. 2009, 131, 2629. (c)
(16) Zang, L.; Che, Y. K.; Moore, J. S. Acc. Chem. Res. 2008, 41,
1596.
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