Self-organized spiral columns in laterally grafted rods

Article abstract of DOI:10.1039/c0cc00132e

Attachment of a flexible coil on the mid-part of a rigid rod block generates T-shaped rod-coil block molecules that self-assemble into a stepped column. These layers, in turn, self-curve into a spiral column with tunable core structure in the solid state.

Full text of DOI:10.1039/c0cc00132e

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Self-organized spiral columns in laterally grafted rodsw
Dong-Je Hong,ab Eunji Lee,a Moon-Gun Choib and Myongsoo Lee*a
Received 26th February 2010, Accepted 25th May 2010
First published as an Advance Article on the web 11th June 2010
DOI: 10.1039/c0cc00132e
Attachment of a flexible coil on the mid-part of a rigid rod block
generates T-shaped rod–coil block molecules that self-assemble
into a stepped column. These layers, in turn, self-curve into a
spiral column with tunable core structure in the solid state.
laterally attached molecular rods described herein consist of a
penta(p-phenylene) rod and an oligo(ethylene oxide) branched
chain that are laterally grafted together at the mid part of the
rod.10,11 The synthesis of the laterally grafted rod–coil
molecules was performed with preparation of intermediate
rod scaffolds by coupling reaction of 3,6-dibromobenzene-1,2-
diamine and 4-hydroxybenzaldehyde. The final block molecules
were synthesized by an etherification reaction of appropriate
dendritic chain segments and the phenolic intermediates, and
then a Suzuki coupling reaction with biphenylboronic acid. All
of the resulting rod–coil molecules based on a laterally grafted
branched chain were purified by silica gel column chromato-
graphy and then prep-HPLC to yield waxy solids. All molecules
were characterized by 1H and 13C-NMR spectroscopy,
elemental analysis and MALDI-TOF mass spectroscopy,
and shown to be in full agreement with the expected chemical
structures. All of the laterally grafted rods have an ordered
bulk-state structure that melts into isotropic liquid at 134 1C,
105 1C, and 98 1C for 1, 2, and 3, respectively.
The small-angle X-ray scattering (SAXS) pattern of 1 in the
solid state shows several sharp reflections that correspond to a
2D primitive rectangular structure with lattice constants
a = 2.94 nm and b = 1.54 nm (Fig. 2a). 2 and 3 also show
similar SAXS patterns to those of 1, indicative of a 2D
rectangular ordering. The lattice constants are a = 3.45 nm
and b = 1.54 nm for 2, and a = 3.78 nm and b = 1.56 nm for
3, respectively. These results indicate that the lattice parameter
The organization of nanoscale self-assembled structures has
attracted considerable attention in the fields of materials
chemistry, supramolecular chemistry, and biological science.1
To date, many types of molecular components have been
designed for construction of complex structures, most of
which have amphiphilic characteristics.2 Block molecules
based on a rigid rod segment, in this context, are useful
building blocks, especially for confined aromatic nanostructures
when both nanophase separation and anisotropic interactions
are employed as driving forces for molecular assembly.3 On
increasing the volume fraction of flexible chains that are
linearly attached to a rod segment, flat layers are separated
into elongated columns that self-assemble into a 2D organized
structure.4 Within the columns, the rod blocks are aligned
perpendicular to the column long axis. On the other hand, the
rod segments, when they are grafted by flexible chains in a
lateral way, are arranged approximately parallel to the column
long axis.5 However, most of the elongated columnar structures
are based on linear domains without bending. Curved supra-
molecular structures based on rod building blocks can be
constructed by frustration of the growth of rod segments in
a two-dimensional way. This is achieved by macrocyclic
attachment of a flexible chain into both ends of rods or lateral
attachment of a linear chain into a mid-part of rod segments.6
Recently, we have shown that lateral attachment of a
flexible chain into an oligo(p-phenylene) rod enforces planar
layers to roll-up into scrolls that are highly curved structures.7
This result stimulated us to envision whether the scrolls break
up along the rod direction to form spiral columns with
increasing cross-sectional area of a flexible chain (Fig. 1).
Although curved 1D objects have been observed in block
copolymer solutions and Langmuir–Blodgett films of an
amphiphilic molecule,8,9 they have not yet been realized in
the bulk state without any template or solvents.
a
increases with increasing the coil volume fraction,
while b remains nearly unaltered irrespective of the coil
volume fraction. The wide angle X-ray scatterings (WAXS)
of all the molecules show a reflection centered at a q-spacing
of 12.1 nmÀ1, which is due to crystal packing of the rod
Here we report the formation of unusual spiral columns
from the self-assembly of molecular rods with a laterally
attached branched chain in the bulk. The spirally curved
columns are self-organized into a 2D rectangular lattice. The
a Center for Supramolecular Nano-Assembly and Department of
Chemistry, Seoul National University, Gwanak-ro 599,
Seoul 151-747, Korea. E-mail: myongslee@snu.ac.kr;
Fax: +822 3936096; Tel: +822 880 4340
b Department of Chemistry, Yonsei University, Seoul, Korea
w Electronic supplementary information (ESI) available: Details of
synthetic procedures, characterization. See DOI: 10.1039/c0cc00132e
Fig. 1 Schematic representation of spiral columns and molecular
structure of 1–3.
ꢀc
This journal is The Royal Society of Chemistry 2010
4896 | Chem. Commun., 2010, 46, 4896–4898

View Article Online
segments within the aromatic domains with an inter-rod
distance of 5.1 A.12
Considering the calculated rod length of 2.4 nm, the a and b
dimensions are too much deviated to be interpreted as a
perpendicular arrangement of the rods to the column axis.
Therefore, parallel arrangements of the rods along the column
axis would be an appropriate model, similar to other laterally
grafted rods.5 Based on the lattice constants and measured
densities, the average numbers of rod units per cross section of
the column are calculated to be B8 for all the samples
(Table S2w). Taking the lattice parameters into account, this
number of molecules suggests that four rods are faced parallel to
each other to produce a brick-like rectangular solid which
subsequently stacks together with an overlap of an approxi-
mately half of the rod length to form stepped columns
(Fig. 2b, 3b). The bricks have a thickness of 1.54 nm and widths
of 2.94 nm, 3.45 nm, and 3.78 nm for 1, 2, and 3, respectively.
These dimensions indicate that the lateral chains are located in
the side faces of the rectangular solid. This packing arrangement
of rod segments is similar to how stepped column structure is
formed from the laterally grafted elongated aromatic rods.5a
At room temperature, the sample could be mechanically
aligned by shearing on a polyimide film, as observed in the 2D
X-ray diffraction pattern (Fig. 3a). When the X-ray beam is
Fig. 3 (a) 2D-SAXS pattern obtained from the sheared sample of 1 at
25 1C. The X-ray pattern was recorded on the sample along a direction
perpendicular to the shear. The diffraction pattern shows {100}, {010}
reflections. (b) Schematic representation of the packing model of 1.
Fig. 4 (a) The high magnification image of the cross-section shows
spiral columns from 1. (b) TEM image of 2. The insets in (a) and (b)
are high-magnification images of the cross-section. Scale bar (a) 10 nm
and (b) 20 nm. (c) TEM micrographs of intermediate structures
kinetically trapped by quick quenching from the melt of 2. TEM
analysis was performed on thin films cryo-microtomed at À70 1C.
TEM images of the cross sections from the experiment show the
intermediate structures between straight and spiral column.
oriented perpendicular to the shear direction, a pair of (100)
reflections is diffracted along the equator, while the (010)
reflections are diffracted in the cross direction at an angle of
901. This result indicates that 2D ordered columns adopt an
orientation perpendicular to the shear direction, as opposed to
conventional columnar structures in which the columns are
aligned parallel to the shear direction.13
To further investigate the unusual 2D supramolecular
structure, transmission electron microscopy experiments were
performed with the cryo-microtomed films of 1 and 2.
Remarkably, the columns of both samples are curved to form
spirals, as revealed by TEM images in Fig. 4. The images of 1
and 2 stained with RuO4 show curved columns of a regular
thickness of B3 nm with a spiral arrangement throughout the
samples. These dimensions are consistent with those obtained
from SAXS. The periodic dimensions of B3 nm in the spiral
packing suggest that the columns bend toward rod–coil
junctions, that is, the side face of the rectangular solid. The
Fig. 2 (a) SAXS patterns of 1–3 plotted against q (= 4p sin ylÀ1
)
(Inset: magnified SAXS reflections in the range of q = 3.5–5.5 nmÀ1).
(b) Schematic representation of a 2D rectangular structure.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4896–4898 | 4897

View Article Online
side view image shows a dimensional array of dark aromatic
spots in a matrix of light oligo(ethylene oxide) chains
(Fig. 4a,b, inset).
This work was supported by the National Creative Research
Initiative Program of the National Research Foundation and
the Pohang Accelerator Laboratory (Beamline 10C1 and 4C1),
Korea. E.L. acknowledges a fellowship from the BK21
program of the Ministry of Education and Human Resources
Development.
The transmission electron microscopy observations together
with the X-ray results lead to the structural model of the spiral
structure as shown in Fig. 3. The rod segments self-assemble
into a rectangular brick with lateral chains in the side faces
which stacks together with a half overlap of the rod length to
form stepped columns. Subsequently, the resulting stepped
column bends toward the rod–coil junctions through staggering
between the bricks at a certain angle to form a spiral columnar
structure with a 2D rectangular lattice. Considering that the
laterally grafted rods based on ether linkages in rod–coil
junctions form linear columns without curvature,5a hydrogen
bonding interactions between the imidazole units in this
system seem to play an important role in the curved structure.
This was confirmed by FT-IR spectroscopy that shows
characteristic bands associated with hydrogen bonding of
imidazole units (Fig. S5w).
To elucidate the formation mechanism of the curved
columns from the melt, we have investigated the cryomicrotomed
films of 2 with different annealing times (Fig. 4c). When
quickly quenched from the melt by blowing cold argon, the
TEM images show the loosely bent columns together with
non-curved columns. However, when the films were cooled
slowly from the melt to ambient temperature, we observed
closely packed spirals throughout the image area. This result
implies that the molecule first assembles into a linear columnar
structure which subsequently bends into spirals.
Notes and references
1 (a) J. A. A. W. Elemans, R. V. Hameren, R. J. M. Nolte and
A. E. Rowan, Adv. Mater., 2006, 18, 1251; (b) T. Kato, Science,
2002, 295, 2414; (c) H. Dietz, S. M. Douglas and W. M. Shih,
Science, 2009, 325, 725; (d) T. Douglas and M. Young, Nature,
1998, 393, 152; (e) C. Du, G. Falini, S. Fermani, C. Abbott and
J. Moradian-Oldak, Science, 2005, 307, 1450.
2 (a) H. Cui, Z. Chen, S. Zhong, K. L. Wooley and D. J. Pochan,
´
Science, 2007, 317, 647; (b) M. Dubois, B. Deme, T. Gulik-
Krzywicki, J.-C. Dedieu, C. Vautrin, S. Desert, E. Perez and
´
T. Zemb, Nature, 2001, 411, 672; (c) M. Schappacher and
A. Deffieux, Science, 2008, 319, 1512; (d) V. Percec,
M. R. Imam, T. K. Bera and V. S. K. Balagurusam, Angew.
Chem., Int. Ed., 2005, 44, 4739; (e) M. Lee, J.-W. Kim, Y.-S. Yoo,
S. Peleshanko, K. Larson, D. Vaknin, S. Markutsya and
V. V. Tsukruk, J. Am. Chem. Soc., 2002, 124, 9121; (f) L. Sun,
L. Zhu, L. Rong and B. S. Hsiao, Angew. Chem., Int. Ed., 2006, 45,
7373; (g) J.-K. Kim, E. Lee, Z. Huang and M. Lee, J. Am. Chem.
Soc., 2006, 128, 14022.
3 (a) M. Lee, B.-K. Cho and W.-C. Zin, Chem. Rev., 2001, 101, 3869;
(b) C. Tschierske, Chem. Soc. Rev., 2007, 36, 1930.
4 M. Lee, B.-K. Cho, H. Kim, J.-Y. Yoon and W.-C. Zin, J. Am.
Chem. Soc., 1998, 120, 9168.
5 (a) D.-J. Hong, E. Lee, J.-K. Lee, W.-C. Zin, M. Han, E. Sim and
M. Lee, J. Am. Chem. Soc., 2008, 130, 14448; (b) M. Prehm, F. Liu,
X. Zeng, G. Ungar and C. Tschierske, J. Am. Chem. Soc., 2008,
130, 14922.
6 W.-Y. Yang, J.-H. Ahn, Y.-S. Yoo, N.-K. Oh and M. Lee, Nat.
Mater., 2005, 4, 399.
The results described here demonstrate that the aromatic
rods with a lateral branched chain self-assemble into a 2D
spiral columnar structure, where the rod segments are
arranged parallel to the column axis in a stepped fashion.
Although the definitive driving forces responsible for this
curvature are not clear at present, a delicate balance between
the energetic gain obtained through maximized hydrogen
bonding together with p–p stacking interactions and the
penalty associated with large mismatch between the cross
sectional area of rod and coil seems to play a crucial role in
the formation of a curved columnar structure.14 Compared to
the scrolls in laterally grafted rods based on a linear chain,7
this result indicates that the scrolls break up along the rod
direction into spiral columns with increasing cross-sectional
area of the flexible chains. This is similar to how nanostruc-
tures with interfacial curvatures are formed in preference to
lamellar structures in linear rod–coil systems with increasing
volume fraction of flexible chains.3a,4,15 The spiral bending of
elongated columns is in marked contrast to structural behavior
of repeated 2D structures.13,16 In this respect, this system
provides access to a large variety of theoretical and experi-
mental investigations to expand the scope of curved nano-
structures formed in the bulk of self-assembling molecules.
7 D.-J. Hong, E. Lee, H. Jeong, J.-K. Lee, W.-C. Zin, T. D. Nguyen,
S. C. Glotzer and M. Lee, Angew. Chem., Int. Ed., 2009, 48, 1664.
8 L. Zhang, C. Bartels, Y. Yu, H. Shen and A. Eisenberg, Phys. Rev.
Lett., 1997, 79, 5034.
9 X. Huang, C. Li, X. Jiang, X. Wang, B. Zhang and M. Liu, J. Am.
Chem. Soc., 2004, 126, 1322.
10 J. W. Ciszek and J. M. Tour, Tetrahedron Lett., 2004, 45, 2801.
11 Y.-S. Yoo, J.-H. Choi, J.-H. Song, N.-K. Oh, W.-C. Zin, S. Park,
T. Chang and M. Lee, J. Am. Chem. Soc., 2004, 126, 6294.
12 See supporting information.
13 (a) V. Percec, B. C. Won, M. Peterca and P. A. Heiney, J. Am.
Chem. Soc., 2007, 129, 11265; (b) K. Kishikawa, K. Oda, S. Aikyo
and S. Kohmoto, Angew. Chem., Int. Ed., 2007, 46, 764.
14 M. A. Horsch, Z. Zhang and S. C. Glotzer, Nano Lett., 2006, 6,
2406.
15 (a) M. Lee, D.-W. Lee and B.-K. Cho, J. Am. Chem. Soc., 1998,
120, 13258; (b) M. Lee, B.-K. Cho, Y.-G. Jang and W.-C. Zin,
J. Am. Chem. Soc., 2000, 122, 7449.
16 (a) R. J. Bushby and O. R. Lozman, Curr. Opin. Colloid Interface
Sci., 2002, 7, 343; (b) B. M. Rosen, C. J. Wilson, D. A. Wilson,
M. Peterca, M. R. Imam and V. Percec, Chem. Rev., 2009, 109,
6275; (c) K. Venkatesan, P. H. J. Kouwer, S. Yagi, P. Muller and
¨
T. M. Swager, J. Mater. Chem., 2008, 18, 400; (d) J. Wu,
M. D. Watsom, L. Zhang, Z. Wang and K. Mullen, J. Am. Chem.
¨
Soc., 2004, 126, 177; (e) I. Bury, B. Heinrich, C. Bourgogne,
D. Guillon and B. Donnio, Chem.–Eur. J., 2006, 12, 8396;
(f) P. K. Karahaliou, P. H. J. Kouwer, T. Meyer, G. H. Mehl
and D. J. Photinos, Soft Matter, 2007, 3, 857.
ꢀc
This journal is The Royal Society of Chemistry 2010
4898 | Chem. Commun., 2010, 46, 4896–4898