Quater-, quinque-, and sexithiophene organogelators: Unique thermochromism and heating-free sol-gel phase transition

Article abstract of DOI:10.1002/chem.200500274

A series of quater-, quinque-, and sexithiophene derivatives bearing two cholesteryl groups at the α-position, which are abbreviated as 4T-(chol)₂, 5T-(chol)₂, and 6T-(chol)₂, respectively, have been synthesized. It has been found that these oligothiophene derivatives act as excellent organogelators for various organic fluids and show the unique thermochromic behaviors through the sol-gel phase transition. It was shown on the basis of extensive investigations, performed with UV-visible spectroscopy, circular dichroism (CD), transmission electron" microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM), that these gelators self-assemble into the one-dimensional structures in the organogels, in which the π-block moieties of the oligothiophenes are stacked in an H-aggregation mode. Surprisingly, an AFM image shows that 4T-(chol) ₂ forms unimolecular fibers in a left-handed helical sense, whereby one pitch of the helical fiber is constructed by 400-540 4T-(chol)₂, molecules. Very interestingly, the conformational change in the oligothiophene moieties can be visually detected: for example, 6T-(chol)₂ shows a specific absorption maximum in the gel (λ_max = 389 nm) and in the solution (λ_max = 439 nm). In addition, a sol-gel phase transition of the 6T-(chol)₂ gel was implemented by addition of oxidizing and reducing reagents such as FeCl₃ and ascorbic acid, respectively. The stimuli-responsive functionality of the oligothiophene-based organogels makes them promising candidates for switchable opto- and electronic soft materials.

Full text of DOI:10.1002/chem.200500274

FULL PAPER
Quater-, Quinque-, and Sexithiophene Organogelators: Unique
Thermochromism and Heating-Free Sol–Gel Phase Transition
Shin-ichiro Kawano, Norifumi Fujita, and Seiji Shinkai*[a]
Abstract: A series of quater-, quinque-,
and sexithiophene derivatives bearing
two cholesteryl groups at the a-posi-
tion, which are abbreviated as 4T-
(chol)2, 5T-(chol)2, and 6T-(chol)2, re-
spectively, have been synthesized. It
has been found that these oligothio-
phene derivatives act as excellent or-
ganogelators for various organic fluids
and show the unique thermochromic
behaviors through the sol–gel phase
transition. It was shown on the basis of
extensive investigations, performed
with UV-visible spectroscopy, circular
dichroism (CD), transmission electron
microscopy (TEM), scanning electron
microscopy (SEM), and atomic force
microscopy (AFM), that these gelators
self-assemble into the one-dimensional
structures in the organogels, in which
the p-block moieties of the oligothio-
phenes are stacked in an H-aggregation
mode. Surprisingly, an AFM image
shows that 4T-(chol)2 forms unimolec-
the conformational change in the oligo-
thiophene moieties can be visually de-
tected: for example, 6T-(chol)2 shows a
specific absorption maximum in the gel
(lmax =389 nm) and in the solution
(lmax =439 nm). In addition, a sol–gel
phase transition of the 6T-(chol)2 gel
was implemented by addition of oxidiz-
ing and reducing reagents such as
FeCl3 and ascorbic acid, respectively.
The stimuli-responsive functionality of
the oligothiophene-based organogels
makes them promising candidates for
switchable opto- and electronic soft
materials.
ular fibers in
a left-handed helical
sense, whereby one pitch of the helical
fiber is constructed by 400–540 4T-
(chol)2 molecules. Very interestingly,
Keywords: nanostructures · oligo-
thiophenes
· sol–gel processes ·
supramolecular chemistry · thermo-
chromism
Introduction
syn–anti mixing in thiophene sequences often causes poly-
morphism leading to the serious lack of the physical proper-
ties characteristic of these p-conjugation systems.[2]
Effective conjugation length is an index expressing the p-
electronic nature of p-conjugated organic molecules that
now play a central role of materials chemistry. In case of
thiophene sequences in oligo- and polythiophenes, the effec-
tive conjugation length is mainly governed by anti versus
syn conformations of thiophene rings.[1] However, although
the anti conformation of the thiophene sequences is thermo-
dynamically more favorable than that of the syn conforma-
tion, effective conjugation length becomes rather short in
solution because oligothiophenes enjoy a dynamic rotation
of thiophene rings. Even in the solid state such as thin films,
Recently, p-block-based low-molecular-weight gels have
occupied the attention of supramolecular chemists, because
these one-dimensional objects inspire construction of novel
energy-transfer systems and nanowire-based devices.[3] In
such integrated systems, p blocks should interact with each
other through p–p interactions in the self-assembled archi-
tectures; a requirement for this is that the p-conjugated se-
quence needs to be as planar as possible and the anti confor-
mation should be favored. In this context, we have demon-
strated an effective molecular design strategy for organoge-
lators, in which p blocks are covalently linked together and
contain a cholesteryl group at each end.[4] The p–p stacking
between the p blocks and van der Waals forces between the
cholesteryl groups cooperatively work, leading to formation
of stable gels in various organic fluids. In addition, these ge-
lators often offer high transparency that enables us to readi-
ly investigate optical properties of these gels. Herein, we
report oligothiophene-based gelators bearing cholesteryl
groups at both ends of the thiophene sequences (4T-(chol)2,
[a]S.-i. Kawano, Dr. N. Fujita, Prof. Dr. S. Shinkai
Department of Chemistry & Biochemisry
Graduate School of Engineering, Kyushu University
6-10-1 Hakozaki, Higashi-ku, Fukuoka 812–8581 (Japan)
Fax : (+81)92-642-3611
E-mail: seijitcm@mbox.nc.kyushu-u.ac.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
Chem. Eur. J. 2005, 11, 4735 – 4742
DOI: 10.1002/chem.200500274
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5T-(chol)2, and 6T-(chol)2), expecting that anti oligothio-
phene cores should stack through strong p–p interactions in
the gel phase and the resultant gel systems should have new
physical and chemical properties.[5]
tors in the view of the chromophore, which can be regulated
by the conformational change in the p-conjugation system.
For the present system, as well as extensive spectrometric
analyses, we have investigated the unique chromatic proper-
ties of the longer oligothiophene-based gelators, which are
controlled with the sol–gel phase-transition.
Results and Discussion
Syntheses of oligothiophene-based gelators: Each step of
the synthetic procedures of 4T-(chol)2–6T-(chol)2 was based
on literature methods previously reported.[9–11] This entails a
straightforward coupling reaction of cholesterol-attached
mono- or bithiphene derivatives with oligothiophene cores
to give the desired molecules (Schemes 1–3). 3b-Cholest-5-
en-3yl-N-(2-aminoethyl) carbamate (1),[9] 5-bromo-2-thio-
phenecarboxylic acid (2),[10] 5,5’-bis[tris(n-butyl)stannyl]-2,2’-
bithiophene (5),[10] 2,2’-bithiphene-5-carboxaldehyde (7),[11]
and 5-bromo-2,2’-bithiphene-5’-carboxaldehyde (8),[11] and
the acid analogue (9)[10] were prepared according to the lit-
eratures reported previously. The acid chlorides 3 and 10
were prepared according to a standard reaction with SOCl2.
These compounds were characterized by IR and 1H NMR
spectroscopy and used for the following key steps. Differing
from the most cases in the final step of the syntheses of ge-
lator molecules, appropriate selection of the solvent and the
proper concentration for the Stille coupling reaction enabled
us to avoid the reaction system from becoming viscous. As a
result, 4T-(chol)2–6T-(chol)2 were successfully obtained in
practical yields from 4+5, 4+6, and 5+11 simply by repre-
To design these oligothiophene-based gelators, we took
the following points into consideration:
1) Their reversible sol–gel phase-transition behavior be-
tween the solution and the gel should induce significant
transformation of effective conjugation length (Figure 1).
Figure 1. Schematic representation of a chromatic change accompanied
with self-assembly.
2) Oligothiophene derivatives
consisting of more than
quaterthiophene sequence
possess strong absorption
bands in visible region
leading to a dramatic color
change when they are cou-
pled with the sol–gel phase
transition
3) Oligothiophene
moieties
can be easily oxidized to
form radical cation species,
leading to electrostatically
repulsive disintegration of
the gelators and resulting
in the reversible sol–gel
phase control.[4b,6,7]
Although Feringa et al. al-
ready reported the investiga-
tion on thiophene- or bithio-
phene-based organogelators,[8]
the authors mainly focused on
the conductivities of the gels
and did not evaluate the gela-
Scheme 1. Synthesis of oligothiophene-based gelator 4T-(chol)2. a) Ethylenediamine, CH2Cl2, 0–208C, 4 h;
b) NaClO2, H2O2, CH3CN, and H2O, 10 8C, 1 h; c) SOCl2, 808C, 4 h; d) 1, triethylamine, CH2Cl2, 0–208C, 3.5 h;
e) nBuLi, nBu3SnCl, diethyl ether, 208C, 12 h; f) [Pd(PPh3)2Cl2], DMF, 80 8C, 12 h.
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Sol–Gel Processes
FULL PAPER
tures with transmission and
scanning electron microscopes
(TEM and SEM; Figure 2).
One can consistently observe
well-developed network struc-
tures composed of fibrous ag-
gregates. Moreover, these gels
have a high aspect ratio of the
fibers, in which the length is
more than several micrometers
and the diameters are 7–20 nm.
In the TEM image of the 4T-
(chol)2 gel, the shortest diame-
ter of the fiber is about 7 nm,
which is comparable with the
molecular long axis of 4T-
(chol)2 (6.6 nm). In the SEM
image of the 4T-(chol)2 gel,
one can clearly recognize a
left-handed helical motif (Fig-
ure 2b, inset). This helical
motif is consistent with the
model expected in the Figure 1
and also supported by the neg-
ative exciton coupling (S chir-
ality) obtained from the CD
measurements (vide infra). It
was also found that 5T-(chol)2
and 6T-(chol)2 gels form the
three-dimensional fibrous net-
works in organic fluids (Fig-
Scheme 2. Synthesis of oligothiophene-based gelator 5T-(chol)2. a) nBuLi, nBu3SnCl, diethyl ether, 208C, 12 h;
b) [Pd(PPh3)2Cl2], DMF, 80 8C, 12 h.
Scheme 3. Synthesis of oligothiophene-based gelator 6T-(chol)2. a) POCl3, DMF, 1,2-dichloroethane, 608C, 4 h;
b) Br2, NaHCO3, CHCl3, 708C, 4 h; c) NaClO2, H2O2, CH3CN, and H2O, 10 8C, 1 h; d) SOCl2, 808C, 4 h; e) 1,
triethylamine, CH2Cl2, 0–208C, 3.5 h; f) [Pd(PPh3)2Cl2], DMF, 80 8C, 12 h.
Table 1. Gelation properties of 4T-(chol)2, 5T-(chol)2, and 6T-(chol)2 at
258C.[a–c]
cipitation. The final products were characterized by
1H NMR spectroscopy, MALDI-TOF-MS, and elemental
analyses.
Solvent
4T-(chol)2
5T-(chol)2
6T-(chol)2
benzene
toluene
p-xylene
diphenyl ether
anisole
benzonitrile
TCE
chloroform
decalin
cyclohexane
hexane
acetonitrile
1-butyronitrile
acetone
ethyl acetate
THF
1-octanol
1-butanol
ethanol
G (2.0)
G (2.0)
G (2.0)
G (2.0)
G (5.0)
G (7.5)
G (2.0)
G (2.1)
G (3.3)
I
G (2.5)
G (2.5)
G (2.2)
G (1.7)
G (1.7)
G (3.3)
G (3.3)
S+
IÀÀÀ
IÀÀÀ
IÀÀÀ
G (2.0)
G (2.9)
G (3.3)
G (5.0)
IÀÀÀ
IÀÀ
Gelation properties of 4T-(chol)2–6T-(chol)2: The gelation
properties of oligothiophene derivatives 4T-(chol)2–6T-
(chol)2 were evaluated in various solvents (Table 1). Deriva-
tives 4T-(chol)2 and 5T-(chol)2 gelated various kinds of non-
polar solvents such as benzene, toluene, benzonitrile, 1,1,2,2-
tetrachloroethane (TCE), and Decalin as well as a few kinds
of polar solvents such as THF, DMSO, and so forth. In
many cases, the gels showed high transparency and stability
even at concentrations as low as 1.5 mm. In contrast, 6T-
(chol)2 was insoluble in most common solvents tested and
gelated only five solvents, that is, anisole, benzonitrile, di-
phenyl ether, TCE, and pyridine. The insolubility of 6T-
(chol)2 is ascribed to the strong p–p interaction between the
sexithiophene moieties. Derivative 6T-(chol)2 had a high
propensity to aggregate by itself in these concentration
region. This problem is discussed more in detail in a later
section.
G (5.0)
G (10)
IÀÀÀ
IÀÀ
I
I
I
I
IÀÀ
IÀÀÀ
IÀÀ
IÀÀÀ
IÀÀ
IÀÀÀ
IÀÀ
I
IÀÀÀ
IÀÀ
G (7.5)
G (2.0)
IÀÀ
G (5.0)
G (2.9)
IÀÀÀ
IÀÀ
SÀÀ
IÀÀÀ
I
IÀÀÀ
IÀÀ
pyridine
DMSO
DMF
S+
G (10)
G (6.7)
IÀÀÀ
PÀ
G (15)
G (15)
P++
P++
[a]X ++, X+, X, XÀ, XÀÀ, and XÀÀÀ denote the concentrations of 10, 5.0,
4.0, 3.3, 2.5, and 2.0 gdmÀ3, respectively. [b]G: gel, P: precipitation, S:
solution, and I: insoluble when heated. [c]The critical gelation concen-
trations [gdmÀ3]of gelators are shown in the parentheses.
TEM and SEM investigations: To obtain visual images of
the 4T-(chol)2–6T-(chol)2 aggregates in the gels we took pic-
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S. Shinkai et al.
adopts a left-handed helical motif along its one-dimensional
aggregate. In the height profile of the fibers (Figure 3b), the
periodic top-to-top length of the helical structure is about
76–100 nm. This length corresponds to half of the helical
pitch (Figure 3c). As the distance between the p-stacked oli-
gothiophene moieties can be estimated to be about
0.38 nm,[12] one can consider that this half of one pitch is
constructed by 200–270 4T-(chol)2 molecules.
Variable-temperature 1H NMR studies: Variable-tempera-
1
ture H NMR studies showed that the peak signals assign-
able to the urethane and amide NH protons of 4T-(chol)2
are more downfield shifted at room temperature in 1,1,2,2-
C2D2Cl4 (1.0 mm) than those at 1008C (Dd=0.40 and
0.15 ppm, respectively). The finding clearly indicates that
formation of the intermolecular hydrogen bonds also con-
tributes to the aggregation of 4T-(chol)2. These results are
well compatible with the aggregation model proposed for
4T-(chol)2 (Figure 1). However, the detailed analyses of the
1H NMR spectra were difficult because of the significant
line broadening, particularly in the gel phase.
UV-visible and CD spectroscopic analyses: Interestingly, re-
versible thermochromic behavior was observed for 4T-
(chol)2–6T-(chol)2 through the sol–gel phase transition
(Figure 4). For example, when the orange 4T-(chol)2 gel was
Figure 2. TEM (left) and SEM (right) images of xerogels prepared from
4T-(chol)2 (top), 5T-(chol)2 (middle), and 6T-(chol)2 (bottom) gels. Con-
ditions: a) [4T-(chol)2]=1.5mm in TCE, b) [4T-(chol)2]=1.0mm in tolu-
ene, c) and d) [5T-(chol)2]=2.9mm in TCE, e) and f) [6T-(chol)2]=
3.8mm in TCE.
ure 2c–f). The high aspect ratio of the fibers composed of
oligothiophene derivatives is certainly reminiscent of a func-
tional nanowire.
AFM investigations: More surprisingly, AFM observation
offered important evidence of unimolecular helical aggre-
gates of 4T-(chol)2 (Figure 3a). Detailed examination of this
image revealed that the height of the fibrous aggregate is
approximately 6.8–7.3 nm (Figure 3b), which is consistent
with the molecular long axis of 4T-(chol)2. Furthermore, it
was again confirmed that the fibrous aggregate of 4T-(chol)2
Figure 4. Phase transition and thermochromic behavior of the sol (right)
and gel (left) phases prepared from 4T-(chol)2, 5T-(chol)2, and 6T-(chol)2
in TCE.
heated, a yellow solution was produced. To the best of our
knowledge, there is no preceding example that such a clear
color change can be induced by a conformational change of
the p-conjugation system. Hence, we measured the absorp-
tion spectra of this gel at various temperatures to elucidate
the origin of this color change. As shown in Figure 5a, a
spectrum of the 4T-(chol)2 gel with TCE (208C) showed a
Figure 3. a) AFM image of 4T-(chol)2 (1.0mm in TCE), b) the height pro-
file along the solid line in Figure 3a, and c) schematic illustration of the
helical fiber of 4T-(chol)2.
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Sol–Gel Processes
FULL PAPER
phene moieties are p-stacked and adopt an H-aggregation
mode in the gel phase, similar to rigid p-systems (e.g., pery-
lene) in the organogel phase.[4a]
In addition, the 4T-(chol)2 gel showed a temperature-de-
pendent, strong negative Cotton effect in the p–p* transi-
tion band region with the q=0 crossing wavelength near the
absorption maximum (367 nm) in the CD spectra (Fig-
ure 5b). It was confirmed that the contribution of linear di-
chroism to these spectra is negligible. These results indicate
that the chromophores of the quaterthiophene moieties of
4T-(chol)2 self-assemble in a helical sense and the dipole
moments are oriented in an anticlockwise direction.[17] The
results are in accord with the morphological observation by
SEM and AFM. Derivatives 5T-(chol)2 and 6T-(chol)2 show
the similar behavior to that of 4T-(chol)2 in the UV-visible
absorption and CD experiments (see Table 2 and Supporting
Information).
Table 2. Absorption maxima of the aggregated species (lagg), the absorp-
tion maxima of the nonaggregated species (lfree), and binding constants K
of 4T-(chol)2, 5T-(chol)2, and 6T-(chol)2 in TCE.
Gelator
lagg [nm]
lfree [nm]
K [LmolÀ1
]
4T-(chol)2
5T-(chol)2
6T-(chol)2
367
370
388
419
424
439
0.5”103
2.2”104
2.7”105
Figure 5. Temperature dependence of a) absorption and b) CD spectra of
4T-(chol)2 gel in TCE: i) [4T-(chol)2]=1.5mm, 20 to 808C (red to blue)
and ii) a dotted line in Figure 5a shows a spectrum of the solution ([4T-
(chol)2]=20mm, 208C).
These findings consistently support the view that the con-
formation of the oligothiophene moieties is drastically
changed to facilitate the p–p stacking in the gel phase,
which is sensitively reflected by the color change.
p–p transition band at 367 nm, whereas a spectrum of the
dilute solution 4T-(chol)2 in TCE (208C) had the absorption
maximum at 418 nm. Moreover, the p–p transition band of
the 4T-(chol)2 gel significantly decreased and the absorb-
ance at 411 nm, which was assigned to the nonaggregated
4T-(chol)2, gradually appeared when the temperature was
raised to 808C.[13] These results can be explained as forma-
tion of an H-aggregate from 4T-(chol)2 by means of a mo-
lecular exciton model, assuming a parallel orientation of oli-
gothiophene moieties.[14] In addition, appearance of vibra-
tional fine structures (shoulder bands at l=408, 434, and
467 nm) was also confirmed in the gel phase. These results
indicate an increase in the effective conjugation length as
well as an increase in the conformational order in the aggre-
gates.[15] Raman spectral measurements of the 4T-(chol)2 sol
and the gel in TCE also supported this proposal. Thienyl
groups have characteristic Raman bands in 1550–1000 cmÀ1
spectral region. The sol phase of 4T-(chol)2 at 0.15mm
showed four bands at 1550–1500 cmÀ1 attributed to the in-
phase C=C antisymmetric vibration. On the other hand, the
gel phase at 2.0mm had only one band in this region. In ad-
dition, the intensity of this band was weakened relative to
the strongest n(C=C) band at 1461 cmÀ1, more than that in
the sol phase (see Supporting Information). The results indi-
cate that the thienyl moieties in the gel phase adopt an anti-
rich conformation and have a simpler distribution among
several possible conformations than that in the sol phase.[16]
It is likely from these results, therefore, that the quaterthio-
Sol–gel phase transition of the 6T-(chol)2 gel by redox stim-
uli: One of the most promising properties that organogels
can offer is a stimulated response leading to the reversible
sol–gel phase transition.[18] Although various types of the
stimuli have been applied so far, a redox stimulus is impor-
tant for the construction of the electromechanical soft mate-
rials, such as artificial muscles, electrorheoloigical fluids, and
so forth.[6] When a 1.0 molar equivalent of FeCl3 was added
to a 6T-(chol)2 gel in TCE (4.0mm) as an oxidizing reagent
and the mixture was vigorously stirred for two minutes, the
red gel turned into the dark-brown solution.[19] Then a 1.2
molar equivalent of ascorbic acid (AsA) was added to this
solution as a reducing reagent and the mixture was left with-
out heating. After one hour, the red gel was reproduced
(Figure 6). It should be emphasized that this sol–gel phase
transition can be attained simply by adding the chemicals
Figure 6. Sol–gel phase transition of the 6T-(chol)2 gel in TCE triggered
by chemical oxidation and reduction.
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S. Shinkai et al.
without any heating–cooling process: in general, the gel is
regenerated only by heating the sol above the sol–gel phase
transition temperature. The oxidized sample shows the
ESR-active species attributed to radical cation generation in
the sexithiophene derivative (see Supporting Information).
We believe, therefore, that this is a unique example of a
heating-free sol–gel phase transition attained by combina-
tion with the redox reaction of 6T-(chol)2. We consider that
this property is related to the nature of the unimolecular
fiber structure: even in the sol phase, short fibrous pieces do
not precipitate and easily reconstruct the long fibers capable
of suppressing solvent fluidity.
(chol)2 (K=2.7”105 mÀ1) tends to self-assemble five hun-
dreds times more effectively than 4T-(chol)2 (K=0.5”
103 mÀ1). These results show, therefore, that most of 6T-
(chol)2 should be integrated in the aggregate at the concen-
tration lower than the sol–gel phase-transition concentra-
tion. As mentioned in the gelation test section, 6T-(chol)2
has poor gelation properties relative to those of 4T-(chol)2
and 5T-(chol)2. Therefore, the aggregation tendency of
6T-(chol)2 might be too strong to form an organogel
(K ꢀ105 mÀ1). In other words, good gelators should have an
adequate aggregation tendency (Kꢀ103–104 mÀ1), as long as
the aggregation obeys the equal-K model. We presume that
a much stronger growth toward the one-dimensional direc-
tion of 6T-(chol)2 might lose the curvature of the fibers,
leading to crystallization or precipitation by means of rapid
bundling of fibrous aggregates.
The aggregation tendency of the oligothiophene-based
organogels: Although a lot of organogelators have already
been reported thus far, most of the organogels have been
accidentally produced. That is the reason why numerical
principles for designing effective organogelators have been
long-awaited. In this context, the aggregation tendency and
binding constants of these p-block molecules, the concept of
which was introduced by Martin,[20] have been evaluated by
applying the equal-K model to this one-dimensional self-as-
sembled system.[21] The model assumes equal binding con-
stants for all binding events in a one-dimensional columnar
aggregate of a homo species, which is driven by the p stack-
ing of aromatic moieties. The present oligothiophene-based
organogels well-suit this model because 1) these organogels
offer the one-dimensional unimolecular aggregates, directly
supported by TEM and AFM investigations, and 2) one can
distinguish the aggregated species from the nonaggregated
in the concentration-dependent UV-visible absorption spec-
tra (Table 2 and Supporting Information), which are similar
to those in the temperature-dependent UV-visible absorp-
tion spectra (Figure 5a). Thus, as the present oligothio-
phene-based gelators exactly meet the requirements, this
system is suitable for testing Martinꢁs theory through sys-
tematic correlation between the gelation property and the
aggregation tendency, focusing on the difference of the oli-
gothiophene moieties. From the concentration-dependent
UV-visible analyses, it has become clear that the longer the
oligothiophene moieties are, the larger the binding constants
are by one order of magnitude (Figure 7). For example, 6T-
Conclusion
In conclusion, we have proposed that the p-conjugation
length of thiophene sequences can be controlled under the
self-assembled circumstances through the sol–gel phase tran-
sition; this results in the construction of the novel thermo-
chromic organogels. According to this concept, we have suc-
cessfully demonstrated that chromatic changes can influence
a broad range of the spectra, in which the color of the sam-
ples can be changed from yellow to red. In addition, the
redox active oligothiophene system has made the design of
heating-free sol–gel phase control possible. Bearing this con-
cept and results in our minds, this methodology can be ap-
plicable, more in general, to construction of other stimuli-re-
sponsive soft materials triggered by electro- or photochemi-
cal signals.
Experimental Section
General: All chemical reagents were obtained from Aldrich, TCI, KISH-
IDA CHEMICAL or Wako were used without further purification. Di-
chloromethane was distilled over CaH2. 1,2-Dichloroethane purchased
from KISHIDA CHEMICAL was used. Distilled N,N-dimethylform-
amide was used. Diethyl ether and acetonitrile purchased from Wako
were used as received unless otherwise noted. 1H NMR spectra were
measured on Brucker AC-250PC and DMX 600 spectrometers. ATR-IR
spectra were obtained using a Perkin–Elmer instrument Spectrum One
FT-IR spectrometer. Mass spectral data were obtained using a Perseptive
Voyager RP MALDI TOF mass spectrometer. UV/Vis, CD, ESR, and
Raman spectra were measured on a Shimadzu UV-2500PC spectrometer,
a Jasco J-720WI spectrometer, a JOEL JES-FE1XG ESR spectrometer,
and a Perkin–Elmer System 2000 FT-Raman spectrometer, respectively.
Gelation test of organic fluids: The gelators and the solvents were put in
a septum-capped test tube and heated until the solid was dissolved. The
sample vial was cooled to 258C and left for 2 h at the ambient conditions.
The state was evaluated by the “stable to inversion of a test tube”
method.
TEM and SEM measurements: The gel was put on a carbon-coated
copper grid and dried in vacuo for 12 h at room temperature. Then, the
xerogel sample was subjected to TEM observations with a JEOL JEM-
2010, operating at 120 kV. For SEM observations, the xerogels were
Figure 7. Concentration dependence of the molar ratio of the aggregate
~
&
*
(aagg) on 4T-(chol)2
( ), 5T-(chol)2 ( ), and 6T-(chol)2 ( ) in TCE: for
the method to determine aagg, see reference [21a].
4740
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www.chemeurj.org
Chem. Eur. J. 2005, 11, 4735 – 4742

Sol–Gel Processes
FULL PAPER
shielded by Pt and examined with a Hitachi S-5000 scanning electron mi-
croscope. The accelerating voltage of SEM was 15 kV.
3H; CH3), 0.86–2.29 (m, 34H; CH, CH2, CH3), 3.42–3.43 (m, 2H; CH2),
3.52–3.56 (m, 2H; CH2), 4.47–4.51 (m, 1H; CH), 5.08–5.09 (m, 1H; NH),
5.31–5.32 (m, 1H; CH), 6.98–6.99 (m, 2H; Ar), 7.04 (d, J=3.8 Hz, 1H;
Ar), 7.10 (brs, 1H; NHCOO), 7.39(d, J=3.8 Hz, 1H; Ar); ATR-IR: n=
3335, 1710, 1615, 1539, 750 cmÀ1; MS (MALDI-TOF, matrix; dithranol):
766.89 [M+Na]+ (calcd: 767.41); elemental analysis calcd (%) for
C39H55BrN2O3S2: C 62.97, H 7.45, N 3.77; found: C 62.94, H 7.46, N 3.77.
AFM measurements: The 4T-(chol)2 gel was cast on a freshly cleaved
highly oriented pyrolytic graphite (HOPG) with spin-coating (2000 rpm,
60 s). The sample was examined by Topometrix TMX-1010 (non-contact
mode).
Preparation of 3: Compound 2 (2.6 g, 13 mmol) was added to SOCl2
(46 mL, 0.63 mol) and the mixture was refluxed under nitrogen for 3 h.
The excess SOCl2 was evaporated under reduced pressure, and the resul-
tant red oil was subjected to the next reaction.
Preparation of 6T-(chol)2: This compound was prepared by using a simi-
lar procedure to that described for compound 4T-(chol)2 from 5 (1.5 g,
2.0 mmol) and 11 (3.0 g, 4.1 mmol) and obtained as a red solid (2.6 g,
1.7 mmol, 85%). M.p. 2478C (decomp); 1H NMR (600 MHz, 1,1,2,2-
C2D2Cl4, 373 K): d=0.73 (s, 6H; CH3), 0.92–2.37 (m, 68H; CH, CH2,
CH3), 3.44–3.46 (m, 4H; CH2), 3.57–3.58 (m, 4H; CH2), 4.52–4.53 (m,
2H; CH), 4.93 (brs, 2H; NH), 5.38–5.39 (m, 2H; CH), 6.55 (brs, 2H;
NH), 7.07–7.44 ppm (m, 12H; Ar); ATR-IR: n=3354, 1692, 1627, 1537,
Preparation of 4: A solution of 3 (2.8 g, 13 mmol) in CH2Cl2 (50 mL) was
added dropwise over a period of 2 h to a solution of 1 (6.0 g, 13 mmol)
and triethyl amine (2.1 mL, 15 mmol) in CH2Cl2 (50 mL) at 08C, and the
reaction mixture was stirred at room temperature for 3.5 h. The solvent
was removed by evaporation and the residue was purified by column
chromatography (SiO2; CHCl3/AcOEt=10:1) to give 4 (5.1 g, 7.7 mmol,
60%) as a yellow solid. M.p. 176–1778C; 1H NMR (250 MHz, CDCl3,
298 K): d=0.68 (s, 3H; CH3), 0.86–2.30 (m, 34H; CH, CH2, CH3), 3.42–
3.43 (m, 2H; CH2), 3.51–3.52 (m, 2H; CH2), 4.47–4.48 (m, 1H; CH),
5.08–5.09 (m, 1H; NH), 5.35–5.36 (m, 1H; CH), 7.03 (d, J=4.1 Hz, 1H;
Ar), 7.15–7.16 (br, 1H; NH), 7.26 ppm (d, J=4.1 Hz, 1H; Ar); ATR-IR:
n=3314, 1723, 1697, 1625, 1572 cmÀ1; MS (MALDI-TOF, matrix; dithra-
nol): 685.28 [M+Na]+ (calcd: 684.77); elemental analysis calcd (%) for
C35H53BrN2O3S: C 63.52, H 8.07, N 4.23; found: C 63.46, H 8.07, N 4.31.
790 cmÀ1
;
UV/Vis
(1,1,2,2-C2H2Cl4):
lmax
(e)=439 nm
(26770 molÀ1 LcmÀ1); MS (MALDI-TOF, matrix; dithranol): 1493.78
[M+H]+ (calcd: 1493.24); elemental analysis calcd (%) for
C88H114N4O6S6·4H2O: C 66.54, H 7.74, N 3.53; found: C 66.75, H 7.67, N
3.56.
Acknowledgements
Preparation of 5: A solution of 2,2’-bithiophene (0.38 g, 2.3 mmol) in di-
ethyl ether (50 mL) was added dropwise over a period of 2 h to a solu-
tion of 15% nBuLi (15% in hexane; 3.4 mL) in diethyl ether (20 mL) at
08C under N2. Then the reaction mixture was stirred at room tempera-
ture for 1 h. The reaction mixture was then cooled to 08C. Tri-n-butyltin
chloride (1.3 mL, 4.8 mmol) was added to the solution and the reaction
mixture was stirred at room temperature for 12 h. The solvent was re-
moved under reduced pressure and then the residue was subjected to the
next step without further purification.
We thank Ms. M. Fujita of Kyushu University for AFM measurements
and Prof. Dr. H. Wariishi, Dr. T. Kitaoka, and A. Mayumi of Kyushu
University for Raman measurements. This work was partially supported
by Grant-in-Aid for Young Scientists (B) (No. 16750122) and the 21st
Century COE Program, “Functional Innovation of Molecular Informat-
ics” from the Ministry of Education, Culture, Science, Sports and Tech-
nology of Japan and JSPS fellowships (for S.-i.K.).
Preparation of 4T-(chol)2: A three-necked round bottom flask was charg-
ed with 4 (3.0 g, 4.5 mmol) and 5 (1.7 g, 2.3 mmol). The reagents were
dissolved in distilled DMF (100 mL), and the solution was deaerated
under vacuum and backfilled with argon ten times prior to the addition
of the [Pd(PPh3)2Cl2](0.08 g, 0.11 mmol). The reaction mixture was stir-
red for 12 h at 808C. The reaction was quenched with methanol, and the
orange precipitate was filtered and washed with methanol (500 mL). The
residue was suspended in THF and the mixture was refluxed for 12 h and
filtered to give 4T-(chol)2 as a yellow solid (2.9 g, 2.1 mmol, 94%). M.p.
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1
2548C (decomp); H NMR (600 MHz, 1,1,2,2-C2D2Cl4, 373 K): d=0.73 (s,
6H; CH3), 0.92–2.37 (m, 68H; CH, CH2, CH3), 3.43–3.44 (m, 4H; CH2),
3.56–3.57 (m, 4H; CH2), 4.51–4.52 (m, 2H; CH), 4.94 (brs, 2H; NH),
5.37–5.38 (m, 2H; CH), 6.58 (brs, 2H; NH), 7.16 (s, 4H; Ar), 7.20 (s,
2H; Ar), 7.44 ppm (s, 2H; Ar); ATR-IR: n=3347, 1696, 1630, 1533,
791 cmÀ1; MS (MALDI-TOF, matrix; dithranol): 1328.92 [M+H]+ (calcd:
1328.99);
UV/Vis
(1,1,2,2-C2H2Cl4):
lmax
(e)=418 nm
(38820 molÀ1 LcmÀ1); elemental analysis calcd (%) for C78H110N4O6-
S4·1.9CH3OH: C 69.10, H 8.53, N 4.03; found: C 68.96, H 8.34, N 4.16.
Preparation of 5T-(chol)2: This compound was prepared by using a simi-
lar procedure to that described for compound 4T-(chol)2 from 4 (2.0 g,
3.0 mmol) and 6 (1.2 g, 1.5 mmol) and obtained as an orange solid
(0.81 g, 0.57 mmol, 38%). M.p. 238–2398C; 1H NMR (600 MHz, 1,1,2,2-
C2D2Cl4, 373 K): d=0.73 (s, 6H; CH3), 0.92–2.37 (m, 68H; CH, CH2,
CH3), 3.43–3.44 (m, 4H; CH2), 3.57–3.58 (m, 4H; CH2), 4.53–4.54 (m,
2H; CH), 4.93 (brs, 2H; NH), 5.38–5.39 (m, 2H; CH), 6.56 (brs, 2H;
NH), 7.01–7.44 ppm (m, 10H; Ar); ATR-IR: n=3330, 1691, 1628, 1539,
791 cmÀ1; MS (MALDI-TOF, matrix; dithranol): 1409.51 [M+H]+ (calcd:
1411.11);
UV/Vis
(1,1,2,2-C2H2Cl4):
elemental analysis
lmax
(e)=424 nm
(%) for
(43430 molÀ1 LcmÀ1);
calcd
C82H112N4O6S5·0.55CHCl3: C 67.18, H 7.69, N 3.80; found: C 67.23, H
7.69, N 3.80.
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86%). Mp 189–1908C; 1H NMR (600 MHz, CDCl3, 298 K): d=0.67 (s,
Chem. Eur. J. 2005, 11, 4735 – 4742
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4741

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Received: March 10, 2005
Published online: May 24, 2005
4742
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2005, 11, 4735 – 4742