Rigid-flexible block molecules based on a laterally extended aromatic segment: Hierarchical assembly into single fibers, flat ribbons, and twisted ribbons

Article abstract of DOI:10.1002/chem.200800664

Self-assembling rigid-flexible block molecules consisting of a laterally extended aromatic segment and different lengths of hydrophilic coils were synthesized and characterized. The block molecule based on a long polyethylene oxide) coil (1), in the melt state, shows an unidentified columnar structure, whereas the molecule with a shorter poly(ethylene oxide) coil (2) self-organizes into an oblique columnar structure. Further decrease in the poly(ethylene oxide) coil length as in the case of 3, on heating, induces a rectangular columnar structure in addition to an oblique columnar mesophase. In diethyl ether, 1 and 2 were observed to self-assemble into uniform nanofibers with bilayer packing. Remarkably, these elementary fibers were observed to further aggregate in a lateral way to form well-defined flat ribbons (1) and twisted ribbons (2) with solvent exchange of diethyl ether into methanol. Furthermore, the ribbons formed in methanol dissociated into elementary fibers in response to the addition of aromatic guest molecules. This transformation between ribbons and single fibers in response to the addition of guest molecules is attributed to the intercalation of aromatic substrates within the rigid segments and subsequent loosening of the aromatic stacking interactions. These results demonstrate that the introduction of a laterally extended aromatic segment into an amphiphilic molecular architecture can lead to the hierarchical formation from elementary fibers of nanoribbons with a tunable twist through controlled lateral interactions between aromatic segments.

Full text of DOI:10.1002/chem.200800664

FULL PAPER
DOI: 10.1002/chem.200800664
Rigid–Flexible Block Molecules Based on a Laterally Extended Aromatic
Segment: Hierarchical Assembly into Single Fibers, Flat Ribbons, and
Twisted Ribbons
Eunji Lee, Zhegang Huang, Ja-Hyoung Ryu, and Myongsoo Lee*[a]
Abstract: Self-assembling rigid–flexible
block molecules consisting of a lateral-
ly extended aromatic segment and dif-
ferent lengths of hydrophilic coils were
synthesized and characterized. The
block molecule based on a long poly-
diethyl ether, 1 and 2 were observed to
self-assemble into uniform nanofibers
with bilayer packing. Remarkably,
these elementary fibers were observed
to further aggregate in a lateral way to
form well-defined flat ribbons (1) and
twisted ribbons (2) with solvent ex-
change of diethyl ether into methanol.
Furthermore,the ribbons formed in
methanol dissociated into elementary
fibers in response to the addition of ar-
omatic guest molecules. This transfor-
mation between ribbons and single
fibers in response to the addition of
guest molecules is attributed to the in-
tercalation of aromatic substrates
within the rigid segments and subse-
quent loosening of the aromatic stack-
ing interactions. These results demon-
strate that the introduction of a lateral-
ly extended aromatic segment into an
amphiphilic molecular architecture can
lead to the hierarchical formation from
elementary fibers of nanoribbons with
a tunable twist through controlled lat-
eral interactions between aromatic
segments.
A
state,shows an unidentified columnar
structure,whereas the molecule with a
shorter poly(ethylene oxide) coil (2)
self-organizes into an oblique columnar
structure. Further decrease in the
poly(ethylene oxide) coil length as in
the case of 3,on heating,induces a rec-
tangular columnar structure in addition
to an oblique columnar mesophase. In
Keywords: amphiphiles
·
nano-
structures · ribbons · self-assembly ·
solvent effects
Introduction
they are structurally comparable to peptide systems with the
biological b-sheet motif. Peptides with a propensity towards
b-sheet formation self-assemble into a hierarchy of twisted
structures,from tapes to twisted ribbons with increasing so-
lution concentration.[6] In addition,a peptidomimetic self-as-
sembles into various one-dimensional structures including
filaments,helical fibrils,ribbons,and twisted ribbons,de-
pending on solution pH.[7] Inspired by such peptide assem-
blies,extensive efforts have been made to introduce helicity
into artificial aromatic systems to construct twisted nano-
structures. For example,dendron rod–coil molecules give
rise to the transformation between high aspect ratio ribbon-
like nanostructures and helical nanostructures triggered by
the introduction of a chiral moiety.[8] In addition,symmetri-
cal and asymmetrical incorporation of flexible chiral coils
into both ends of a p-conjugated aromatic segment leads to
Precise control of a supramolecular nanostructure with a
well-defined shape and size in self-organizing materials is of
critical importance in acquiring a desired function and spe-
cific properties in the fields of molecular and supramolec-
ular materials.[1] Rigid aromatic rod segments are versatile
building blocks for constructing such controlled nanoarchi-
tectures.[2,3] For example,incorporation of a rigid aromatic
segment into amphiphilic molecular architectures with en-
hanced aggregation stability leads to a variety of nanostruc-
tures including barrels,toroids,helices,and tubes,depending
on the molecular structure.[4,5] Among them,one-dimension-
al helical structures have attracted much attention because
[a] E. Lee,Z. Huang,Dr. J.-H. Ryu,Prof. Dr. M. Lee
Center for Supramolecular Nano-Assembly
and Department of Chemistry
[9]
the formation of twisted and coiled helices,respectively.
Recently,we have shown that a rigid–flexible macrocyclic
molecule consisting of a hexa-p-phenylene aromatic and a
chiral poly(ethylene oxide) coil self-assembles into well-de-
fined ribbonlike aggregates at an initial state which,in turn,
roll up to form a helical tubular structure with a preferred
Yonsei University,Shinchon 134,Seoul 120-749 (Republic of Korea)
Fax : (+82)2-393-6096
E-mail: mslee@yonsei.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200800664.
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handedness.[10] These helical strands are attributed to the
energy balance between repulsive interactions among the
adjacent flexible coils and p-stacking interactions. Although
one-dimensional elementary structures have been extensive-
ly studied,the precise control of the hierarchical assembly
of such elementary structures remains to be explored. One
can envision that introduction of a laterally extended aro-
matic segment into amphiphilic molecules can give rise to
hierarchical formation from elementary fibers to flat ribbons
through enhanced lateral interactions between elementary
structures. With this in mind,we have synthesized rigid–flex-
ible block molecules consisting of a laterally extended aro-
matic segment and poly(ethylene oxide) (PEO) hydrophilic
coils. We present herein the thermotropic liquid-crystalline
phase behavior and hierarchical self-assembly behavior in
solutions of the resulting amphiphilic rigid–flexible block
molecules.
Results and Discussion
Synthesis: The rigid–flexible block molecules are based on a
K-shaped rigid phenazine derivative containing decyl groups
at lateral positions and hydrophilic poly(ethylene oxide)
coils at the termini (Scheme 1). To synthesize the phenan-
threnequinone aromatic scaffolds containing hydrophilic
coils (7),precursor 4 was prepared according to procedures
described previously.[11a] Compound 5 was prepared from
the Suzuki coupling reaction with 4 and a boronic acid de-
rivative in the presence of Pd0 catalyst,and then the subse-
quent etherification with tosylated PEO coils afforded 6.
For the coupling reaction with the aromatic scaffolds con-
taining hydrophobic coils,compound 7 was prepared in
good yield by the cerium(IV) ammonium nitrate (CAN)-
mediated reaction with 6. The aromatic scaffold containing
decyl groups (10) was synthesized by means of Sonogashira
coupling with a benzothiadiazole derivative (8) and subse-
quent reduction with LiAlH4. The final rigid–flexible block
molecules were prepared by the condensation of the appro-
priate dione with 10.[11b,c] The resulting amphiphilic block
Scheme 1. Synthesis of the aromatic amphiphiles 1, 2,and 3. a) Na2CO3,[Pd (PPh3)4],THF,reflux; b) K 2CO3,acetonitrile,reflux; c) cerium(IV) ammoni-
A
um nitrate (CAN),acetonitrile; d) [Pd (PPh3)4],CuI,Et 3N,THF,60 8C; e) LiAlH4,THF,reflux; f) acetic acid,ethanol.
A
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Rigid–Flexible Block Molecules
FULL PAPER
molecules (1, 2,and 3) were characterized by 1H and
13C NMR spectroscopies,elemental analysis,and MALDI-
TOF mass spectrometry,and shown to be in full agreement
with the structures presented. As confirmed by 1H NMR
spectroscopy,the ratio of the protons of the aromatic block
to the alkyl protons is consistent with the ratio calculated.
The experimental mass based on peak positions in the spec-
trum is well matched with the theoretical molecular weight
of each molecule (see the Supporting Information).
thermotropic liquid-crystalline behavior after crystalline
melting,and the transition temperatures and the corre-
sponding enthalpy changes are summarized in Table 1. Mol-
ecule 1,based on long PEO chains,melts into a birefringent
liquid-crystalline phase that transforms into isotropic liquid
at 89.38C. On slow cooling from the optically isotropic
phase,the formation of a fernlike texture that corresponds
to a typical columnar mesophase takes place (see the Sup-
porting Information). To confirm the two-dimensional sym-
metry of the columnar mesophase,X-ray scatterings have
been performed at 808C. However,the diffraction patterns
showed only a strong single peak at the small-angle range
together with a diffuse halo in the wide-angle region,which
did not allow us to identify the columnar structure without
ambiguity (Figure 2a).
Liquid-crystalline behavior: The thermotropic liquid-crystal-
line phase behavior of the block molecules was investigated
by means of differential scanning calorimetry (DSC),ther-
mal optical polarized microscopy,and X-ray scatterings. Fig-
ure 1a reveals the DSC heating and cooling traces of the
rigid–flexible block molecules. All of the molecules exhibit
Figure 2. Small-angle X-ray diffraction patterns of 1–3 plotted against q.
a) The pattern of the mesophase exhibited by 1 at 808C,b) the oblique
columnar lattice exhibited by 2 at 908C,c) the oblique columnar lattice
exhibited by 3 at 708C,and d) the rectangular columnar lattice exhibited
by 3 at 958C.
Figure 1. a) DSC traces recorded during the heating scan and cooling
scan of aromatic molecules (k: crystalline phase; col0: oblique columnar
phase; colr: rectangular columnar phase; i: isotropic phase). b) Represen-
tative optical polarized micrograph of the texture exhibited by a colum-
nar structure of 2 at the transition from the isotropic liquid state.
Table 1. Thermal transitions of aromatic amphiphiles.[a]
Phase transition [8C] and corresponding enthalpy changes [kJmolÀ1
]
[c]
Molecules 1[b] [gcmÀ3
]
fPEO
Calcd molecular length [nm] heating
7.3
4.9
3.7
cooling
1
2
3
1.24
1.12
1.08
0.62
0.45
0.31
cr 77.6 (6.6) col 89.3 (1.5)i
cr 59.4 (6.1) col0 114.5 (3.6)i
I 83.9 (1.2) col 48.8 (4.7) cr
I 105.3 (3.1) col0 24.4 (5.9) cr
cr 41.4 (3.6) col0 91.5 (1.7) colr 126.0 (0.1)i I 103.1 (2.4) colr 74.0 (1.9) col0 22.5 (3.4) cr
[a] Data are from heating and cooling scans. cr: crystalline phase; col0: oblique columnar; colr: rectangular columnar; i: isotropic phase. [b] 1=molecular
density. [c] fPEO =volume fraction of PEO group to hydrophilic part.
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M. Lee et al.
Similarly,compound 2 based on short PEO chains also
displays a columnar mesophase. After a crystal melting tran-
sition at 59.48C,compound 2 exhibits a birefringent liquid-
crystalline phase,followed by an isotropic phase at 114.5 8C.
Upon cooling from the isotropic liquid,a spherulitic fan tex-
ture can be observed by using an optical polarized micro-
scope,characteristic of a columnar mesophase (Figure 1b).
To corroborate the detailed liquid-crystalline structure of 2,
small- and wide-angle X-ray scattering experiments were
performed at 908C. The small-angle X-ray scattering
(SAXS) of this mesophase displays three sharp reflections
that can be indexed as a two-dimensional oblique columnar
structure (a P1 space group symmetry) with lattice constants
the lower temperature liquid-crystalline phase,the corre-
sponding small-angle X-ray diffraction patterns show a
number of well-resolved reflections,which can be indexed
as a two dimensional oblique columnar lattice with lattice
parameters a=6.9 nm, b=6.1 nm,and g=698. On further
heating to the second liquid-crystalline phase,the oblique
columnar structure transforms into a rectangular columnar
structure with lattice parameters a=7.7 nm, b=6.0 nm,and
g=908. Calculation based on the lattice constants and mea-
sured densities of both 2 and 3 reveals that the number of
molecules in a single slice of the column is about four,indi-
cating the same number of molecules per columnar cross
section irrespective of the length of the PEO chain.[13] Based
on these results,the column can be considered to consist of
three distinct molecular regions that result from the micro-
segregation of the flexible chains from the rigid aromatic
segments.[14] The alkyl chains in the core are surrounded by
aromatic segments arranged in a square shape,which are
held together by means of the hydrophobic interaction of
alkyl chains,whereas hydrophilic PEO chains in the periph-
ery are filled with the intercolumnar matrix (Figure 4).
a=b=7.2 nm and
a characteristic angle of 608 (Fig-
ure 2b).[12] The wide-angle X-ray scattering (WAXS) shows
only a broad halo centered at approximately 4.5 Š,which is
indicative of the liquid-crystalline order of aromatic seg-
ments within domains (see the Supporting Information).
To further confirm the formation of an oblique columnar
structure,the sample was quickly quenched in liquid nitro-
gen after annealing at 908C and then was cryomicrotomed
to a thickness of approximately 50 to 70 nm. The microfilms
of 2 were stained with RuO4 vapor and observed by trans-
mission electron microscopy (TEM). As shown in Figure 3,
Figure 4. Schematic representation of the self-assembly of 2 (a=7.2 nm,
b=7.2 nm,and g=608) and 3 (a=6.9 nm, b=6.1 nm,and g=698) into a
two-dimensional columnar structure with an oblique lattice (alkyl chains
in green,aromatic segments in blue,PEO coil matrix in black).
Figure 3. TEM images of ultramicrotomed films of 2 stained with RuO4,
revealing a columnar array of alternating light-colored flexible coils and
dark aromatic layers. The inset image at perpendicular beam incidence
shows an oblique columnar array of aromatic core.
Aggregation behavior in solution: The rigid–flexible block
molecules,when dissolved in a solvent suitable for the oli-
go(ethylene oxide) segments,can self-assemble into an ag-
gregation structure due to their amphiphilic characteris-
tics.[15] The aggregation behavior was subsequently investi-
gated with 1 and 2,which show good solubility in polar sol-
vents. Dynamic light scattering (DLS) experiments per-
formed in diethyl ether,a selective solvent of the PEO
chains,showed that both molecules aggregate into nano-
structures with a monomodal size distribution,which indi-
cates well-equilibrated structures (Figure 5a).[16] The average
hydrodynamic radii (RH) of 1 and 2 were observed to be ap-
proximately 160 and 110 nm,respectively. TEM micrographs
of both molecules showed fibrous aggregates with uniform
diameters of 11 and 7 nm for 1 and 2,respectively (Fig-
ure 5b and c). The extended molecular lengths are 7.3 and
the TEM image of 2 shows organized,darkly stained hydro-
phobic domains consisting of aromatic and alkyl segments
with a thickness of around 6 nm. The inset TEM image
shows in-plane order of an oblique symmetry in which dark
hydrophobic domains are regularly arrayed in a matrix of
light-colored coil segments. The interdomain distance was
measured to be approximately 7.5 nm,which is consistent
with that obtained from X-ray scattering. Interestingly, 3 ex-
hibits an additional liquid-crystalline phase above a lower
temperature mesophase (Figure 1a). On heating,compound
3 melts into a liquid-crystalline phase at 41.48C,and then
converts into a second liquid-crystalline phase,which in turn
undergoes isotropization at 126.08C. Similar to that of 2,in
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Figure 5. a) Size distribution graphs of 1 (c) and 2 (c) in diethyl ether at a scattering angle of 908C (from CONTIN analysis of the autocorrelation
function; 0.1 wt%). TEM images of b) 1 and c) 2 in diethyl ether (scale bars: 100 nm; 0.01 wt%). d) FTIR spectra (2700–3025 cmÀ1) of solutions of self-
assembled 1 in methanol (c) and diethyl ether (c).
4.9 nm for 1 and 2 (by CPK: Corey–Pauling–Koltun),re-
spectively; these widths are consistent with an interdigitated
bilayer packing of the hydrophobic segments. The bilayer
feature is also reflected in FTIR spectroscopic experiments
(Figure 5d). The films of 1 and 2 cast from solutions in di-
ethyl ether showed two bands at 2918 (nanti) and 2850 cmÀ1
(nsym),which contribute to modes corresponding to CH
2
stretching vibrations in the crystalline packing of alkyl
chains.[17] These results suggest that the fibrous aggregates
consist of hydrophobic aromatic segments and decyl alkyl
groups in the core surrounded by poly(ethylene oxide) coils
that are exposed to the diethyl ether environment. Within
the core,the extended aromatic segments are stacked in a
bilayer packing arrangement (Figure 6).
We expected that replacing diethyl ether with methanol,
which is a poorer solvent for ethylene oxide coils,would
strengthen hydrophobic and p–p stacking interactions be-
tween aromatic segments.[18] The fluorescence spectrum of 1
in diethyl ether (4”10À4 m) exhibits a strong emission maxi-
mum at 525 nm without any noticeable fluorescence quench-
ing (Figure 7b),which suggests that the aromatic segments
Figure 6. Schematic representation of the transformation from nanofibers
to flat ribbon or twisted ribbon in response to solvent exchange.
within aggregates in diethyl ether are loosely packed. How-
ever,the emission maximum in methanol (4”10 À4 m) is red-
shifted with respect to that observed in chloroform,and
fluorescence is significantly quenched,which indicates that
strong p–p stacking interactions between the aromatic seg-
ments are induced upon exchange of the solvent into metha-
nol.[18,19] The fluorescence experiments with a solution of 2
in methanol exhibit a similar optical behavior,which indi-
cates that the aromatic segments of 2 also pack closely to
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M. Lee et al.
Figure 7. a) Absorption and b) emission spectra of molecule 1 in methanol (c) and diethyl ether (c) compared with chloroform (c) (c=4”
10À4 m; I=10 mm; lex =295 nm). c) TEM image of the flat ribbonlike aggregates of 1 in methanol (0.01 wt%; scale bar: 200 nm; inset: thickness of
ribbon,scale bar: 100 nm). d) TEM image of the twisted ribbons of 2 in methanol with negative staining (0.01 wt%; scale bar: 200 nm).
each other through enhanced p–p interactions in methanol
(see the Supporting Information).
the fully extended molecular length of 4.9 nm,the ribbon
thickness of 7 nm indicates interdigitated bimolecular pack-
ing of the hydrophobic segments. This was further confirmed
by FTIR spectroscopy that shows characteristic bands asso-
ciated with bilayer packing of hydrophobic segments (see
the Supporting Information).[17] Considering that the fibers
formed in diethyl ether have a width of 7 nm,the ribbons
with a width of 20 nm consist of three laterally assembled el-
ementary fibers. These results suggest that aromatic amphi-
phile 2 self-assembles at the initial stage into elementary
fibers with a bilayer thickness; these further self-assemble in
a hierarchical way through side-by-side hydrophobic interac-
tions between the fibers into twisted ribbons.
Considering that 1 self-assembles into nontwisted,flat rib-
bons,the formation of twisted ribbons in 2 seems to be at-
tributed to enhanced steric constraints imposed by closer
packing of K-shaped aromatic segments. The molecule with
longer poly(ethylene oxide) coils would be arranged into
flat ribbons with bilayer packing as in the case of 1. Howev-
er,a decrease in the PEO length drives the K-shaped aro-
matic segments to be packed more closely due to reduced
steric crowding of the PEO coils that are located at the ends
of the ribbons. To fit more closely together,the aromatic
segments are aligned with each other in a slight tilting ar-
rangement,and consequently,the tilted stacks of the aro-
matic segments lead to the formation of twisted ribbon
(Figure 6).[20] This packing consideration of the aromatic
segments is reflected in the increased extent of fluorescence
TEM investigations of 1 cast onto a TEM grid from a so-
lution in methanol showed one-dimensional strands with an
average width of approximately 30 nm (Figure 7c). Close ex-
amination of the objects revealed the folded edges of indi-
vidual strands with a thickness of 11 nm (Figure 7c,inset).
This dimension is consistent with the expected thickness of
interdigitated bimolecular packing (11 nm),clearly indicat-
ing a ribbonlike shape. Based on these results,aromatic am-
phiphile 1 can be concluded to self-assemble into a flat
ribbon structure with a width of 30 nm and a thickness of
11 nm. These dimensions suggest that the elementary fibers
based on bimolecular packing are laterally associated into
stacks of three fibers to produce flat ribbons with solvent
exchange from diethyl ether to methanol. This result clearly
demonstrates that strong p–p interactions enhance the ag-
gregation characteristics of the laterally extended aromatic
amphiphiles,which results in the hierarchical formation of
the ribbons.
In contrast,the elementary fibers of 2 transform into
twisted ribbons with solvent exchange of diethyl ether into
methanol. As shown in Figure 7d,the TEM micrograph re-
veals similar flat,ribbonlike structures to those of 1 in meth-
anol,but with a regular twist about their long axis. The di-
mensions of the twisted ribbons were shown to be approxi-
mately 20 nm in width,around 7 nm in cross-sectional thick-
ness,and with a pitch length of about 125 nm. Considering
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Rigid–Flexible Block Molecules
FULL PAPER
quenching in 2 compared with that of 1,which implies that
the p–p stacking interaction increases as the volume fraction
of the PEO coil decreases (Figure 8).
To gain further insight into the role of the packing ar-
rangement of the aromatic segments on the twisted ribbon,
intercalation experiments of 1 and 2 have been performed
with hydrophobic dye,Nile Red. The intercalation of the
dye molecules between the aromatic segments in the ribbons
would disrupt packing constraints imposed by close packing
of K-shaped aromatic segments. The intercalation of Nile
Red was confirmed by using fluorescence microscopy (Fig-
ure 9a). When the solution of 1 in methanol containing Nile
Red was excited at 266 nm,the fluorescence intensity of
1
was suppressed while exhibiting strong emission at 624 nm
corresponding to Nile Red due to energy transfer,clearly in-
dicating that the guests are efficiently intercalated between
the aromatic segments to interrupt p–p stacking interac-
tions.[9c,21] This packing frustration between the aromatic
segments of the molecules would drive the ribbons to be dis-
sociated into smaller aggregates. Upon addition of Nile Red,
indeed,the flat ribbons were shown to transform into single
fibers with a uniform diameter of approximately 11 nm (Fig-
ure 9b). Considering that the diameter of the ribbons
formed in methanol is about 30 nm,the ribbons are dissoci-
ated into three elementary fibers upon addition of aromatic
guest molecules. This result indicates that the flat ribbons of
1 are composed of three lateral stacks of elementary fibers,
suggesting that the block molecules self-assemble,in a hier-
archical fashion,into flat ribbons from single fibers. This
transformation between ribbons and single fibers in re-
sponse to the addition of guest molecules is attributed to the
Figure 8. Emission spectra of molecule 1 (c) and 2 (c) in methanol
compared with chloroform (c) (c=4”10À4 m; I=10 mm; lex =325 nm).
Figure 9. a) Emission spectra of a solution of 1 (b; c=4”10À4 m) and of a solution of 1 and Nile Red (c; c=4”10À4 m; 20 mol% relative to 1; lex
=
266 nm). b) TEM image of 1 (0.01 wt%) containing 0.2 equiv of Nile Red (scale bar: 200 nm). c) Emission spectra of a solution of 2 (b; c=4”10À4 m)
and of a solution of 2 and Nile Red (c; c=4”10À4 m; 20 mol% relative to 2; lex =266 nm). d) TEM image of 2 (0.01 wt%) containing 0.2 equiv of
Nile Red (scale bar: 200 nm).
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M. Lee et al.
tography was carried out with silica gel 60 (230–400 mesh) from EM sci-
ence.
intercalation of aromatic substrates within the rigid seg-
ments and subsequent loosening of aromatic stacking inter-
actions. Upon addition of Nile Red,compound 2 also
showed similar optical behavior to 1 (Figure 9c). TEM
showed that 2 containing Nile Red forms flat ribbons to-
gether with a few irregular twists (Figure 9d). Instead,the
ribbons are split into ꢀthree elementary fibers at irregular
intervals. This result indicates that packing constraints of the
K-shaped aromatic segments imposed by strong p–p stack-
ing interactions are primarily responsible for the regular
twisting of the flat ribbons.
Techniques: 1H NMR spectra were recorded from samples in CDCl3 or
DMSO by using a Bruker AM 250 spectrometer. The purity of the prod-
ucts was checked by thin-layer chromatography (TLC; Merck,silica gel
60). A Nikon Optiphot 2-pol optical polarized microscope (magnifica-
tion: 100”) equipped with a Mettler FP 82 hot-stage and a Mettler FP
90 central processor was used to observe the thermal transitions and to
analyze the anisotropic texture. A Perkin–Elmer DSC-7 differential scan-
ning calorimeter equipped with a 1020 thermal analysis controller was
used to determine the thermal transitions,which were reported as the
maxima and minima of their endothermic or exothermic peaks. In all
cases,the heating and cooling rates were 5 8CminÀ1. X-ray scattering
measurements were performed in transmission mode using synchrotron
radiation at the 10C1 X-ray beam line at Pohang Accelerator Laboratory,
Korea. Microanalyses were performed using a Perkin–Elmer 240 elemen-
tal analyzer at the Organic Chemistry Research Center,Sogang Universi-
ty,Korea. MALDI-TOF mass spectrometry was performed by using a
Perseptive Biosystems Voyager-DE STR with 2,5-dihydroxybenzoic acid
as matrix. DLS measurements were performed by using an ALV/CGS-3
Compact Goniometer System. UV/Vis absorption spectra were obtained
from a Shimadzu 1601 UV spectrometer. The fluorescence spectra were
obtained from a Hitachi F-4500 fluorescence spectrometer. FTIR spectra
were recorded by using an Equinox 55 FTIR spectrophotometer with a
ZnSe pellet. TEM was performed at 120 kV by using a JEOL-JEM 2010
instrument. Compounds were synthesized according to the procedure de-
scribed in Scheme 1 and then purified by silica gel column chromatogra-
phy and preparative HPLC. Recycling preparative HPLC was performed
at room temperature by using a 20 mm”600 mm polystyrene column on
a Japan Analytical Industry Model LC-908 recycling preparative HPLC
system,equipped with UV detector 310 and RI detector RI-5. Chloro-
form (HPLC grade) was used as eluent.
Conclusion
Self-assembling rigid–flexible block molecules consisting of
a laterally extended aromatic segment and different lengths
of PEO coils were synthesized,and their self-assembling be-
havior in both bulk and solution was investigated. In the
melt state,the rigid–flexible block molecule based on a long
PEO coil (1) was observed to self-assemble into an unidenti-
fied columnar structure,whereas the molecule with shorter
PEO coils (2) self-organizes into an oblique columnar struc-
ture. Further decrease in the PEO coil length as in the case
of 3,on heating,gives rise to transformation from an obli-
que columnar to a rectangular columnar structure. In diethyl
ether,both 1 and 2 self-assemble into a fibrous structure
with bilayer packing. Notably,these elementary fibers were
observed to further aggregate in a lateral way to form flat
ribbons with solvent exchange of diethyl ether into metha-
nol. Decreasing the PEO length as in the case of 2 forces
the flat ribbons to be curved to form twisted ribbons with a
regular pitch along their axis due to steric constraints caused
by enhanced p–p stacking interactions. Another interesting
point to be noted is that the ribbons are dissociated into ele-
mentary fibers in response to addition of aromatic guest
molecules due to interruption of p–p stacking interactions.
Consequently,the incorporation of a laterally extended aro-
matic segment into an amphiphilic system provides a unique
strategy to construct elementary fibers that further self-as-
semble into well-defined nanoribbons with a tunable twist.
This result,which provides the precise control of hierarchi-
cal assembly,may have significant impact on the design of
controlled one-dimensional supramolecular structures with
electro-optical functions.
TEM: For the study of the self-assembled structure of K-shaped aromatic
molecules in solution,a drop of solution of amphiphilic molecules was
placed on a carbon-coated copper grid and the solution was allowed to
evaporate under ambient conditions. The samples were stained by depos-
iting uranyl acetate onto the surface of the sample-loaded grid. The dried
specimen was observed by using a JEOL-JEM 2010 instrument operating
at 120 kV. The data were analyzed using Digital Micrograph software.
Fluorescence spectroscopy: To investigate the co-assembled system of
the K-shaped molecule and Nile Red,the prepared Nile Red solution
(1.0”10À3 m) was added to 1 (4.0”10À4 m in methanol) with a 0.2 ratio,
[Nile Red]/[1],and fluorescence spectroscopy measurements were per-
formed. Molecule 2 was also characterized using the same procedure.
DLS spectroscopy: DLS measurements were performed by using a UNI-
PHASE He–Ne laser operating at 632.8 nm. The scattering was kept at
908 during the whole experiment. The maximum operating power of the
laser was 30 mW. The detector optics employed optical fibers coupled to
an ALV/SOSIPD/DUAL detection unit,which employed an EMI PM-
28B power supply and ALV/PM-PD preamplifier/discriminator. The
signal analyzer was an ALV5000/E/WIN multiple-tau digital correlator
with 288 exponentially spaced channels. The hydrodynamic radius (RH)
was determined from the DLS autocorrelation functions by the cumu-
lants and the CONTIN methods by using the software provided by the
manufacturer.
FTIR spectroscopy: Samples were prepared by dissolving an appropriate
amount of aromatic amphiphiles in methanol (or diethyl ether),and then
several drops of solution were coated on a ZnSe tablet.
Experimental Section
Synthesis: The synthetic procedure used in the preparation of the K-
shaped molecules is described in Scheme 1. Compounds 4 and 8 were
synthesized according to the procedures described previously.[11]
Materials: p-Toluenesulfonyl chloride (98%) from Tokyo Kasei was used
as received. Poly(ethylene glycol) methyl ether (MW =350,750),12, -phe-
nylenediamine (99.5%),phenanthrenequinone (95%),bromine, n-butyl-
lithium (1.6m solution in n-hexane),triisopropyl borate (98%),4-iodo-
phenol (99%),ammonium cerium(IV) nitrate (98.5%),and 1-bromode-
cane (98%) from Aldrich were used as received. Tetrakis(triphenylphos-
phine)palladium(0) and tetraethylene glycol monomethyl ether (98%)
from TCI,and the conventional reagents were used as received. Visuali-
zation was accomplished with UV light and iodine vapor. Flash chroma-
Synthesis of compound 5: Compound 4 (1.6 g,4.04 mmol) and 4-(tetrabu-
tyldimethylsilyoxy)phenylboronic acid (2.55 g,10.1 mmol) were dissolved
in degassed THF (25 mL). Degassed 2m aqueous Na2CO3 (25 mL) was
added to the solution and then tetrakis (triphenylphosphine)palladium(0)
(5 mg,0.004 mmol) was added. The mixture was heated at reflux for 24 h
with vigorous stirring under nitrogen,cooled to room temperature,the
layers were separated,and the aqueous layer was then washed twice with
6964
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Rigid–Flexible Block Molecules
FULL PAPER
dichloromethane. The combined organic layer was dried over anhydrous
magnesium sulfate and filtered. The solvent was removed with a rotary
evaporator,and the crude product was purified by column chromatogra-
phy (silica gel) using ethyl acetate to yield a yellow solid (1.7 g,99%).
1H NMR (250 MHz,CDCl 3): d=8.74 (s,2H; ArH),8.19 (d, J=8.5 Hz;
2Ar-H),7.75 (d, J=8.5 Hz; 2Ar-H) 7.62 (d, J=8.3 Hz,4H; m to OH),
7.01 (d, J=8.3 Hz,4H; o to OH),4.10 ppm (s,6H; OCH 3).
in a rotary evaporator and the resulting mixture was poured into water
and extracted with dichloromethane. The dichloromethane solution was
washed with water,dried over anhydrous magnesium sulfate,and filtered.
The crude products were purified by column chromatography (silica gel)
using dichloromethane/hexane (1:1 v/v) to yield a yellow solid (0.35 g,
80%). 1H NMR (250 MHz,CDCl 3): d=7.74 (s; 2Ar-H),7.45 (d, J=
8.8 Hz,4H; m to OCH2),6.89 (s; 2Ar-H),6.86 (d, J=8.8 Hz,4H; o to
OCH2),3.97 (t,4H; CH2Ophenyl),1.79 (t,4H; OCH 2CH2),1.46–1.27
(m,28H; CH2CH2),0.88 ppm (t,6H; CH 2CH3).
Synthesis of compounds 6a, 6b, and 6c: These compounds were synthe-
sized using the same procedure. A representative example is described
for 6a. Compound 5 (0.38 g,0.9 mmol),monotosylated poly(ethylene
glycol) (Mr=750,1.2 g,2.25 mmol),and excess K 2CO3 were dissolved in
anhydrous acetonitrile (30 mL). The mixture was heated to reflux for
24 h. The resulting solution was poured into water and extracted with di-
chloromethane. The dichloromethane solution was washed with water,
dried over anhydrous magnesium sulfate,and filtered. The solvent was
removed with a rotary evaporator,and the crude product was purified by
column chromatography (silica gel) using ethyl acetate/methanol (10:1 v/
Synthesis of compounds 1, 2, and 3: These compounds were synthesized
using the same procedure. A representative example is described for 1. A
mixture of 10 (97 mg,0.196 mmol) and 7a (162 mg,0.142 mmol) in etha-
nol (20 mL) and acetic acid (2 mL) was heated to reflux for 2 h. The mix-
ture was cooled to room temperature,poured into water,and extracted
with dichloromethane. The dichloromethane solution was washed with
water,dried over anhydrous magnesium sulfate,and filtered. The crude
products were purified by column chromatography (silica gel) using di-
chloromethane/methanol (20:1 v/v) as eluent and by preparative HPLC
to yield a yellow solid (0.19 g,53%). 1: Yield 53%; 1H NMR (250 MHz,
CDCl3): d=9.47 (d, J=8.4 Hz,2H),8.71 (s; 2Ar-H),7.95 (s; 2Ar-H),
v) to yield
a
waxy solid (0.77 g,72%). 6a: Yield 72%; 1H NMR
(250 MHz,CDCl 3): d=8.75 (s,2H; ArH),8.19 (d, J=8.4 Hz; 2Ar-H),
7.75 (d, J=8.4 Hz; 2Ar-H),7.64 (d, J=8.5 Hz,4H; m to OH),6.89 (d,
7.93 (d, J=8.4 Hz,2H),7.73 (m,8H;
m to OCH2),7.09 (d, J=8.7 Hz,
J=8.5 Hz,4H; o to OH),4.13 (t,4H;
CH2Ophenyl),3.83 (t,4H;
4H; o to OCH2),6.97 (d, J=8.8 Hz,4H; o to OCH2),4.23 (t,4H;
CH2Ophenyl),4.02 (t,4H; O CH2),3.92 (t,4H; CH2Ophenyl),3.78–3.53
(m,128H),3.36 (s,6H; O CH3),1.83 (t,4H; OCH 2CH2),1.50–1.29 (m,
28H; CH2CH2),0.89 ppm (t,6H; CH
CDCl3): d=159.7,159.0,142.7,132.3,128.8,126.7,123.6,115.6,115.2,
114.9,97.9,86.1,72.0,71.0,69.9,68.3,67.7,59.1,32.0,29.7,29.6,29.4,
26.2,22.8,14.2 ppm; elemental analysis calcd (%) for C 138H208N2O38: C
66.22,H 8.38,N 1.12; found: C 65.86,H 8.62,N 1.00; MALDI-TOF-MS:
m/z calcd for C138H208N2O38: 2503.12 [M+H]+; found: 2504.28. 2: Yield
55%; 1H NMR (250 MHz,CDCl 3): d=9.47 (d, J=8.4 Hz,2H),8.70 (s;
OCH2),3.67–3.38 (m,128H; O CH2),3.29 ppm (s,6H; O CH3). 6b: Yield
69%; 1H NMR (250 MHz,CDCl 3): d=8.70 (s,2H; ArH),8.11 (d, J=
8.3 Hz; 2Ar-H),7.66 (d, J=8.3 Hz; 2Ar-H) 7.55 (d, J=8.4 Hz,4H; m to
OH),6.89 (d, J=8.4 Hz,4H; o to OH),4.01 (t,4H; CH2Ophenyl),3.57
(t,4H; O CH2),3.51–3.36 (m,56H; O CH2),3.21 ppm (s,6H; O CH3). 6c:
Yield 67%; 1H NMR (250 MHz,CDCl 3): d=8.81 (s,2H; ArH),8.26 (d,
J=8.5 Hz; 2Ar-H),7.81 (d, J=8.5 Hz; 2Ar-H),7.69 (d, J=8.7 Hz,4H;
2CH3); 13C NMR (100 MHz,
m
to OCH2),6.89 (d, J=8.7 Hz,4H;
o to OCH2),4.01 (t,4H;
CH2Ophenyl),3.57 (t,4H; O CH2),3.51–3.36 (m,56H; O CH2),3.21 ppm
(s, 6H; OCH3).
2Ar-H),7.95 (s; 2Ar-H),7.93 (d, J=8.4 Hz,2H),7.74 (m,8H;
OCH2),7.10 (d, J=8.6 Hz,4H; o to OCH2),6.86 (d, J=8.8 Hz,4H; o to
OCH2),4.24 (t,4H; CH2Ophenyl),4.02 (t,4H; O CH2),3.94 (t,4H;
CH2Ophenyl),3.78–3.53 (m,56H),3.36 (s,6H; O CH3),1.83 (t,4H;
m to
Synthesis of compounds 7a, 7b, and 7c: These compounds were synthe-
sized using the same procedure. A representative example is described
for 7a. Ammonium cerium(IV) nitrate (0.188 g,0.343 mmol) in water
(10 mL) was dropped slowly into a solution of compound 6a (0.19 g,
0.156 mmol) in acetonitrile (10 mL). The reaction mixture was stirred at
room temperature for 1 h. The solution was quenched with dichlorome-
thane. Then the resulting solution was washed with water and the di-
chloromethane solution,dried over anhydrous magnesium sulfate,and fil-
tered. The solvent was removed in a rotary evaporator,and the crude
product was purified by column chromatography (silica gel) using ethyl
acetate/methanol (10:1 v/v) to yield a waxy solid (0.17 g,91%). 7a: Yield
91%; 1H NMR (250 MHz,CDCl 3): d=8.17 (m,4H; ArH),7.59 (m,6H;
ArH),7.06 (d, J=8.7 Hz,4H; o to OH),4.13 (t,4H; CH2Ophenyl),3.84
OCH2CH2),1.50–1.29 (m,28H; CH2CH2),0.89 ppm (t,6H; CH 2CH3);
13C NMR (100 MHz,CDCl 3): 159.7,159.0,142.6,132.5,128.7,126.7,
123.6,115.7,115.2,114.9,97.9,86.2,72.0,71.0,70.7,68.3,67.7,59.1,32.0,
29.7,29.6,29.4,26.2,22.8,14.2 ppm; elemental analysis calcd (%) for
C102H136N2O20: C 71.64,H 8.02,N 1.64; found: C 71.26,H 8.12,N 1.49;
MALDI-TOF-MS: m/z calcd for C102H136N2O20: 1710.17 [M+H]+; found:
1710.67. 3: Yield 54%; 1H NMR (250 MHz,CDCl 3): d=9.46 (d, J=
8.4 Hz,2H),8.69 (s; 2Ar-H),7.95 (s; 2Ar-H),7.93 (d,
7.74 (m,8H; m to OCH2),7.10 (d, J=8.6 Hz,4H; o to OCH2),6.98 (d,
J=8.8 Hz,4H; o to OCH2),4.24 (t,4H; CH2Ophenyl),4.03 (t,4H;
OCH2),3.94 (t,4H; CH2Ophenyl),3.78–3.53 (m,24H),3.38 (s,6H;
J=8.4 Hz,2H),
(t,4H; O CH2),3.69–3.46 (m,128H; O CH2),3.31 ppm (s,6H; O CH3).
1
7b: Yield 89%; H NMR (250 MHz,CDCl ): d=8.22 (d, J=8.2 Hz; 2Ar-
3
OCH3),1.84 (t,4H; OCH 2CH2),1.49–1.28 (m,28H; CH2CH2),0.89 ppm
(t,6H; CH CH3); 13C NMR (100 MHz,CDCl ): 158.2,156.6,142.1,132.1,
H),8.20 (s,2H; ArH),7.63 (m; 6Ar-H),7.06 (d,
J=8.7 Hz,4H; o to
2
3
OH),4.21 (t,4H; CH2Ophenyl),3.90 (t,4H; O CH2),3.75–3.53 (m,56H;
OCH2),3.36 ppm (s,6H; O CH3). 7c: Yield 91%; 1H NMR (250 MHz,
CDCl3): d=8.22 (d, J=8.2 Hz; 2Ar-H),8.19 (s,2H; ArH),7.62 (m; 6Ar-
H),7.06 (d, J=8.7 Hz,4H; o to OCH2),4.21 (t,4H; CH2Ophenyl),3.90
(t,4H; O CH2),3.75–3.53 (m,56H; O CH2),3.36 ppm (s,6H; O CH3).
128.5,126.7,123.6,115.7,115.2,114.9,97.9,86.4,72.7,71.0,70.7,68.9,
59.3,31.7,29.4,29.1,28.7,26.2,22.8,14.2 ppm; elemental analysis calcd
(%) for C86H104N2O12: C 76.08,H 7.72,N 2.06; found: C 75.87,H 7.86,N
2.12; MALDI-TOF-MS: m/z calcd for C86H104N2O12: 1358.7 [M+H]+,
1380.7 [M+Na]+,1396.7
[
M+K]+; found: 1358.5 [M+H]+,1379.5
[M+Na]+,1397.5 [ M+K]+.
Synthesis of compound 9: 1-(Decyloxy)-4-iodobenzene (650 mg,
1.8 mmol),compound 8 (125 mg,0.679 mmol),[Pd (PPh3)4] (116 mg),and
G
CuI (19 mg) were added to a mixture of triethylamine (20 mL) and tetra-
hydrofuran (10 mL). The mixture was degassed and then stirred at 608C
for 24 h. Solvent was removed in a rotary evaporator and the resulting
mixture was poured into water and extracted with dichloromethane. The
dichloromethane solution was washed with water,dried over anhydrous
magnesium sulfate,and filtered. The crude products were purified by
column chromatography (silica gel) using dichloromethane/hexane (1:1 v/
Acknowledgements
This work was supported by the National Creative Research Initiative
Program of the Korean Ministry of Science and Technology. E.L. and
Z.H. acknowledge a fellowship of the BK21 program from the Ministry
of Education and Human Resources Development.
v) to yield a yellow solid (0.35 g,80%). 1H NMR (250 MHz,CDCl ): d=
3
7.74 (s; 2Ar-H),7.59 (d, J=8.8 Hz,4H; m to OCH2),6.90 (d, J=8.8 Hz,
4H; o to OCH2),3.99 (t,4H; CH2Ophenyl),1.80 (t,4H; OCH 2CH2),
1.48–1.28 (m,28H; CH2CH2),0.88 ppm (t,6H; CH 2CH3).
Synthesis of compound 10: Compound 9 (0.3 g,0.46 mmol) and lithium
aluminum hydride (61 mg,1.85 mmol) were added to tetrahydrofuran
(20 mL). The mixture was heated to reflux for 4 h. Solvent was removed
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Received: April 8,2008
Published online: July 4,2008
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Chem. Eur. J. 2008, 14,6957 – 6966