Self-assembly of a dendron-attached tetrathiafulvalene: Gel formation and modulation in the presence of chloranil and metal ions

Article abstract of DOI:10.1002/smll.201101917

The self-assembly of a tetrathiafulvalene (TTF)-based molecular gelator with a dendron substituent into a gel is described. Scanning electron microscope and atomic force microscope studies show that the xerogels exhibit rope-like frameworks or interwoven

Full text of DOI:10.1002/smll.201101917

full papers
Molecular Responsive Gels
Self-Assembly of a Dendron-Attached Tetrathiafulvalene:
Gel Formation and Modulation in the Presence of
Chloranil and Metal Ions
Xingyuan Yang, Guanxin Zhang,* Liqiang Li, Deqing Zhang,* Lifeng Chi,
and Daoben Zhu
The self-assembly of a tetrathiafulvalene (TTF)-based molecular gelator with a
dendron substituent into a gel is described. Scanning electron microscope and atomic
force microscope studies show that the xerogels exhibit rope-like frameworks or
interwoven nanofibers, depending on the polarity of solvent from which the gel is
formed. Gels containing chloranil can also be successfully prepared. Interestingly,
this two-component gel can be easily transformed into solution after the addition of
either Sc3+ or Pb2+. Both absorption and electron spin resonance (ESR) spectroscopic
.+
investigations reveal that the TTF unit is oxidized into TTF by chloranil in the
presence of either Sc3+ or Pb2+. Moreover, the gel phase can be restored by reduction
.+
of TTF into the neutral form using magnesium.
or modification agents for paints, inks, cleaning agents, drugs,
1. Introduction
etc.[5] As a result, they have received increased attention in
Self-assembly of organic functional molecules into nano-
recent years. Apart from the studies about the gel formation
structures has been intensively addressed for more than ten
and structures, extensive efforts have been made to investi-
years.[1,2] A number of molecules, referred to as low-molec-
gate functional and stimuli responsive gels. These stimuli-
ular-weight gelators (LMWGs), can be self-assembled into
responsive molecular gels offer us promising opportunities
entangled 3D networks which entrap and immobilize solvent
for designing and constructing new functional materials, such
as sensors, actuators, etc.[6]
molecules by capillary forces leading to solvent gelation and
formation of molecular gels.[3,4] Recent studies have demon-
strated that molecular gels may have potential applications
in a number of areas including nanomaterials and delivery
By incorporation of photoresponsive-/electroactive-/
chemically active segments into the respective LMWGs,
a number of stimuli-responsive gels are prepared. For
instance, molecular gels which respond to metal ions and
anions were obtained by using LMWGs containing spe-
cific moieties that can selectively bind the corresponding
metal ions and anions.[5c,5d] Similarly, LMWGs with photo-
responsive (e.g., azobenzene and stilbene)[7,8] and elec-
troactive moieties[9–12] can lead to gels responding to light
and redox reactions, respectively. We and others have
reported redox reaction-responsive gels with LMWGs
featuring tetrathiafulvalene (TTF) groups which can
be reversibly transformed into radical cations and dica-
tions.[10–13] Moreover, we have also described gels for
which the gel–sol transitions can be triggered by chemical
reactions by designing LMWGs with chemically active
segments.[14]
Dr. X. Yang, Dr. G. Zhang, Prof. D. Zhang, Prof. D. Zhu
Beijing National Laboratory for Molecular Sciences
Organic Solids Laboratory
Institute of Chemistry
Chinese Academy of Sciences
Beijing 100190, China
E-mail: gxzhang@iccas.ac.cn; dqzhang@iccas.ac.cn
Dr. L. Li, Prof. L. Chi
Institute of Physics
Westfälische Wilhelms-Universität Münster
Wilhelm-Klemm-Str.10, 48149 Münster, Germany
DOI: 10.1002/smll.201101917
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Self-Assembly of a Dendron-Attached Tetrathiafulvalene
COOMe
COOMe
O
MeOOC
MeOOC
O
COOMe
COOMe
COOMe
COOMe
S
S
S
S
O
O
S
S
S
S
O
O
O
O
S
S
S
S
K2CO3
HO
S
O
Dimethylformamide
S
COOMe
COOMe
COOMe
O
Br
O
O
COOMe
O
MeOOC
MeOOC
2
1
3
COOMe
COOMe
Scheme 1. The chemical structure of compound 1 and the synthetic approach.
In this paper, we report a new TTF-based gelator, 1 acetonitrile (1:1, v/v), and the gel was transformed into the
(Scheme 1), with a dendron substitutent. The molecular respective solution after further heating. Interestingly, gels
design is based on the following considerations: 1) the den- of 1 from either benzene or p-xylene were opaque, but they
dron moiety in 1 has been found to be favorable for the gela- were thermally reversible. The corresponding critical gela-
tion of solvents because of the formation of 3D self-assembly tion concentrations (CGC) at 25 °C were also measured and
structures due to the interactions of benzene rings.[8b,8c,15] As
a result, guest molecules may be easily captured into the 3D
listed in Table 1.
The self-assembly structures of 1 after formation of gels
were examined in the following way: the gels were firstly
self-assembly structures of the dendron; 2) the oxidation of 1
will lead to the transformation of TTF into TTF•+, which may
cooled and the solvents in gels were carefully removed under
affect the interactions of dendron moieties and, as a result,
reduced pressure to yield the respective xerogels; the xero-
the gel–sol transition can be triggered. We have recently
gels were further dispersed in poor solvents and the suspen-
reported the oxidation of TTF by quinones such as chloranil
sions were subjected to scanning electron microscope (SEM)
and atomic force microscope (AFM) analysis. Figure 2a–d
show the SEM images of xerogels of 1 from benzene and
in the presence of metal ions, if the TTF and quinone units
are spatially close.[16] It is anticipated that the self-assembly
structures of 1 can be modulated, and consequently the gel–
p-xylene, respectively. Interconnected rope-like nanostruc-
sol transition can occur after the addition of chloranil and
certain metal ions. As to be discussed below, molecules of 1
can self-assemble into interconnected nanofibers in solution
leading to gel formation; moreover, the gel–sol transition
occurs easily after the addition of chloranil and Pb2+/Sc3+.
tures, being typical of the organogels, were observed. These
rope-like structures were ca. 500 nm in width and several
micrometers in length. Moreover, magnified images revealed
Table 1. The gelation experimental results with compound 1 in dif-
ferent solvents.
2. Results and Discussion
Solvents tested
Gelation resultsa)
Benzene
Go (26 mg mL−1
Go (40 mg mL−1
)
)
2.1. Self-Assembly and Solution/Gel Formation
p -Xylene
Compound 1 was synthesized by the reaction of compounds
2
Toluene
Gt (44 mg mL−1
)
[17] and 3[15] in the presence of K2CO3, as shown in Scheme 1.
Benzene/Acetonitrile = 1: 1
Gt (27 mg mL−1
)
Its chemical structure and purity were established and con-
firmed by spectroscopic and elemental analysis data (see
Experimental Section). The gelation ability of 1 was exam-
ined in different solvents, and the results are listed in Table 1.
Among the solvents tested, compound 1 can gel aromatic sol-
vents including benzene, toluene, p-xylene, and a mixture
of benzene and acetonitrile (1:1, v/v). As demonstrated in
Figure 1, a transparent yellow gel was formed by cooling a
hot solution of 1 (28 mg mL−1) in a mixture of benzene and
Ethyl acetate; 1,2-Dichloroethane; Tetrahydrofuran
CCl4; Acetone
S
P
I
Methanol; Ethanol; Acetonitrile; Cyclohexane; n-Hexane;
Methylcyclohexane
a)Gt: transparent gel; Go: opaque gel; I: insoluble; S: solution; P: precipitate; each CGCs was
measured at 25 °C; all gels listed in this table were found to be stable in closed tubes at room
temperature for at least three months; the gel–sol transition temperatures of all gels listed in
this table (Tg) were ca. 25 °C.
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Figure 1. Left part: illustration of the gel formation with compound 1 (28 mg mL−1) in the absence of p -chloranil in benzene/CH3CN (1:1, v/v); Right
part: illustration of the gel formation with compound 1 (28 mg mL−1) in the presence of 1.0 eq. of p -chloranil in benzene/CH3CN (1:1, v/v).
that the surfaces of these ropes were hierarchically struc-
The self-assembly structures of 1 in solutions were also
tured with nanofibers of 50–100 nm in diameter. However, it investigated with AFM. As an example, Figure 3 shows the
is interesting to note that xerogels of 1 from the mixture of topography and phase images (recorded in taping mode) of
benzene and acetonitrile (1:1, v/v), contain bundles of long the xerogel from the mixture of benzene and acetonitrile.
nanofibers which are randomly interconnected as depicted Again, molecules of 1 are assembled into nanofibers of ca.
in Figure 2e,f. It can be seen from the magnified image that 50 nm in diameter which are further interconnected to form
the nanofibers are smooth, being a few micrometers in length 3D nanostructures. This is consistent with the SEM observa-
and ca. 50 nm in width. Thus, these results also manifest that tions as discussed above.
the self-assembled structures of 1 in solution through gela-
1
On the basis of previous studies[8b,8c,15] and the H-NMR
tion can be affected by the solvent properties: by increasing investigation, the intermolecular π–π and the hydrophobic
the solvent polarity, the rope-like self-assembly structures of interactions due to the dendron segment in 1 may be respon-
1 is changed into nanofibers.
sible for the self-assembly and gel formation. Figure 4 dis-
1
plays the H-NMR spectra for the gel of
1 from the mixture of d6-benzene and
d3-acetonitrile (1:1, v/v) at different tem-
peratures and the corresponding solution
at 343 K. The signals around 8.1 ppm,
7.6 ppm, and 6.9 ppm due to the aro-
matic protons (marked as a, b, c, and d)
of the benzene units in 1 were gradually
shifted downfield when the temperature
increased from 298 to 343 K. In addition,
down-field shifts were also observed for
the signals around 4.8 ppm due to the
benzyl-CH2 protons (marked as e). This
result implies that π–π stacking owing to
the dendron units in 1 exists in the gel
phase and it becomes weak after heating
as anticipated.
2.2. Tuning the Gel–Sol Transition
By taking advantage of the electroactive
feature of TTF, molecular gels responding
toredoxreactionswerereportedearly.[10–12]
Gels of 1 can be easily converted into the
respective solution after the addition of
Fe(ClO4)3. Alternatively, TTF can also be
oxidized into the radical cation (TTF•+) by
quinone in the presence of certain metal
ions.[16] Thus, it is interesting to study the
responsiveness of gels of 1 toward the
ensemble of quinone and metal ions.
Figure 2. SEM image of the xerogel of 1 from benzene (a) and the corresponding high-
magnification image (b); that from p -xylene (c) and the corresponding high-magnification
image (d); that from the mixture of benzene and acetonitrile (e) and the corresponding high-
magnification image (f).
Figure 5 shows the absorption spec-
trum of 1 and those of the ensemble of 1
and chloranil at different concentrations.
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Self-Assembly of a Dendron-Attached Tetrathiafulvalene
Figure 3. The topography (left) and phase (right) AFM images of the xerogel of 1 from the mixture of benzene and acetonitrile.
Obviously, no new absorptions were detected when the 8.0 mm (12.5 mg mL−1). This new absorption should be
concentrations of 1 and chloranil were lower than 1.0 × owing to the intermolecular charge-transfer interaction
10−4 m. But, weak new absorption around 900 nm was between TTF unit in 1 and chloranil. It is interesting to note
observed when the concentrations of 1 and chloranil reached that such weak intermolecular charge-transfer interaction
COOMe
H(a)
MeOOC
H(b)
O
COOMe
COOMe
S
S
O
S
S
H(e)
O
S
S
S
O
COOMe
COOMe
O
H(d)
O
O
H(c)
MeOOC
COOMe
1
Figure 4. Partial H-NMR spectra for the gel formed with 1 (28 mg mL−1) in the mixture of d6-benzene and d3-acetonitrile (1:1, v/v) at different
temperatures and that for the corresponding solution at 343 K.
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0.6
1 ( 8.0 mM)
1 ( 8.0 mM) and chloranil
1 ( 0.1 mM)
1 ( 0.1 mM) and chloranil
0.4
0.2
Figure 7. The transition of the gel formed with 1 (30 mg mL−1) and 1.0 eq.
of chloranil from the mixture of benzene and acetonitrile (1:1, v/v) into
the solution after addition of Sc(CF3SO3)3; the recovery of the gel phase
after further stirring with magnesium strips, followed by heating and
cooling.
0.0
860 nm indicates the generation of TTF•+. In fact, direct
oxidation of 1 by Fe3+ led to the same absorptions at around
460 and 860 nm. Moreover, a strong ESR signal due to TTF•+
(g = 2.0097, aH = 1.13 G) was detected.The absorption spectra
of 1 and chloranil in the gel phase before and after the addi-
tion of Sc3+ were also recorded (see SI, Figure S1).Again, new
absorptions around 460 nm and in the range of 600–1200 nm
were observed. Other metal ions were also allowed to react
with the ensemble of 1 and chloranil. Among the ions tested,
Pb2+ can induce similar absorption spectral changes for the
ensemble of 1 and chloranil (see SI, Figure S2). Therefore, it
can be concluded that TTF•+ is generated for the ensemble
of 1 and chloranil in the presence of Sc3+/Pb2+. The formation
of TTF•+ is assumed to be due to the intermolecular electron
transfer between the TTF unit in 1 and chloranil, facilitated
by Sc3+/Pb2+. Our recent studies indicated that metal ions
promote smooth electron transfer between TTF and quinone
when they are spatially close.[16] Briefly, the reduction poten-
tial of quinone (e.g., chloranil) is positively shifted in the
presence of certain metal ions (see SI, Figure S3,S4) and, as
a result, the electron transfer is thermodynamically more fea-
sible; in addition, the TTF•+ and the radical anion of quinone
(e.g., chloranil),[19] which are generated through electron
transfer, can be stabilized by the electrostatic interaction.
The TTF•+ can be reduced to a neutral TTF unit. For
instance, the absorptions around 460 and 860 nm due to TTF•+
for the ensemble of 1 and chloranil, to which an equivalent
amount of either Sc3+ or Pb2+ was added, became gradually
weaker after the solution was stirred with magnesium strips
(see SI, Figure S5,S6). This is simply due to reduction of the
TTF•+ into the neutral form by magnesium. Simultaneously,
the solution was transformed into a gel after the excess mag-
nesium strips were removed, followed by heating and cooling
the solution, as indicated in Figure 7. These results clearly
indicate that the self-assembled structures and the gel phase
of 1 are influenced by the oxidation state of TTF; the trans-
formation of TTF into TTF•+, which can be realized by either
direct oxidation with Fe3+ or with the ensemble of chloranil
and Sc3+/Pb2+, accompanies the gel–sol transition, and the gel
phase can be restored after the reduction of TTF•+ into the
neutral form by magnesium.
400
600
800
/nm
1000
1200
Figure 5. The absorption spectra of 1 in solution with concentrations
of 0.1 mm (green) and 8.0 mm (black); that of the equivalent mixture
of 1 (0.1 mm) and chloranil (blue); that of the equivalent mixture of 1
(8.0 mm) and chloranil (red); in each case the mixture of benzene and
acetonitrile (1:1, v/v) was used as the solvent.
cannot trigger the gel–sol transition as depicted in Figure 1.
The gels of 1 containing an equiv amount of chloranil can
be formed by cooling the corresponding hot solutions and
the gels can be tuned by alternating heating and cooling. As
shown in Figure 6, the xerogel of 1 and chloranil also con-
tains interwoven nanofibers of ca. 60 nm in width. Clearly, the
self-assembly structure of 1 was not largely modulated by
the presence of chloranil. This is indeed in agreement with
the weak charge-transfer interaction between TTF unit in 1
and chloranil.
Interestingly, gels of 1 and chloranil can be transformed
into the solutions after introducing metal ions such as Pb2+
and Sc3+ as shown in Figure 7. But, other ions including Na+,
K+, Cd2+, Mg2+, Ca2+ and Ba2+ cannot induce the gel–sol tran-
sition. Both absorption and ESR spectroscopic studies indi-
cate such gel–sol transition is induced by the oxidation of
TTF unit in 1.
Figure 8 shows the absorption spectra of 1 (0.1 mm)
and chloranil (0.1 mm) in the absence and presence of Sc3+.
Before the addition of Sc3+ there is no strong absorption
above 400 nm. However, new absorptions around 460 and
860 nm emerge after the addition of Sc3+. The color of the
solution changed from light-yellow to red-brown after intro-
ducing Sc3+, as shown in Figure 8. According to previous
reports,[12,13,18] the appearance of absorptions around 460 and
3. Conclusion
Figure 6. SEM images of the xerogel of 1 containing 1.0 eq. of chloranil
from the mixtue of benzene and acetonitrile (1:1, v/v) (left) and the
corresponding high-magnification image (right).
A new TTF-based molecular gelator (1) with a dendron
substituent is reported. Compound 1 can gel several solvent
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Self-Assembly of a Dendron-Attached Tetrathiafulvalene
(a)
3.5
(b)
(c)
g = 2.0097
aH= 1.13 G
a
b
c
3.0
2.5
2.0
1.5
1.0
0.5
0.0
d
3460 3470 3480 3490
Magnetic field / G
300
500
900
1100
λ/7n00m
Figure 8. Left: the absorption spectra of 1 in solution (0.1 mm, black), chloranil (0.1 mm, red), the equivalent mixture of 1 (0.1 mm) and chloranil
(green), and the equivalent mixture of 1 (0.1 mm) and chloranil in the presence of 1.0 eq. of Sc3+ (blue); Top Right: the ESR spectrum of 1 (0.1 mm)
and 1.0 eq. of chloranil in the presence of Sc3+ (1.0 eq. vs. 1) recorded at room temperature; Lower Right: illustration of the color change for the
solution of 1 (0.1 mm) and 1.0 eq. of chloranil before and after addition of 1.0 eq. of Sc3+ (red-brown). For each measurement, a mixture of benzene
and acetonitrile (1:1, v/v) was employed as the solvent.
systems, leading to transparent or opaque gels. SEM and AFM
Synthesis of Compound 1: A solution of compound 2 (446 mg,
studies show that the self-assembled structures of 1 in the 1.0 mmol), compound 3 (650 mg, 0.55 mmol), and K2CO3 (760 mg,
form of xerogels can be rope-like frameworks or interwoven 5.50 mmol) in dimethyl formamide (DMF, 10 mL) was stirred at room
nanofibers, depending on the solvent polarity. The gels of 1 temperature for 3 days. The resulting reaction mixture was extracted
containing chloranil can also be prepared. Interestingly, fur- with dichloromethane and washed with water. The organic layer was
ther addition of either Sc3+ or Pb2+ led to the gel–sol transition. dried over anhydrous Na2SO4. The solvents were removed under
Both absorption and ESR spectroscopic investigations reveal vacuum, and the residue was purified by column chromatography
that the TTF unit in 1 is oxidized into TTF•+ by chloranil in on silica gel with CH2Cl2/EtOAc (30/1, v/v) to give compound 1 as
the presence of either Sc3+ or Pb2+. Moreover, the gel-phase a yellow solid powder (653 mg, 76%). 1H NMR (600 MHz, CDCl3,
can be restored by reduction of TTF•+ into the neutral form 25 °C, δ): 8.29 (s, 4 H), 7.82 (s, 8 H), 7.13 (s, 2 H), 7.10 (s, 1 H),
with magnesium.This responsive molecular gel is based on the 7.05 (s, 4 H), 6.95 (s, 2 H), 5.13 (s, 8 H), 5.08 (s, 4 H), 4.08-4.10
modulation of the intermolecular interactions between gelator (m, 2 H), 3.93 (s, 24 H), 3.25 (s, 4 H), 2.93-2.95 (m, 2 H), 2.09-
(i.e., compound 1) and guest (i.e., chloranil) molecules by 2.11 (m, 2 H). 13C NMR (150 MHz, CDCl3, 25 °C, δ): 166.03, 159.32,
external stimuli (Sc3+/Pb2+). Therefore, the current studies may 158.64, 138.65, 138.19, 131.87, 123.35, 120.10, 118.79, 118.62,
open a new venue for designing responsive molecular gels.
113.60, 113.08, 70.08, 69.91, 65.54, 51.65, 30.18, 29.24. MALDI-
TOF m/z:[M+] 1560.2. Anal. calcd. for C75H68O23S7·H2O: C, 57.02; H,
4.47; S, 14.21. Found: C, 57.09; H, 4.33; S, 14.50.
4. Experimental Section
Gel Formation: In a typical gelation experiment, a weighed
amount of gelator was mixed with a solvent (0.5 mL) in a test vial,
which was sealed and than heated. If the compound was unable
to dissolve, it was noted as insoluble (I). After cooling down to
room temperature, if a stable and transparent gel was formed
when inverting the vial, it was noted as gelation Gt; if a stable and
opaque gel was formed, it was noted as gelation Go; if the clear
solution (>60 mg mL−1) was retained, it was marked as soluble
(S); if precipitate was formed, it was marked P. Repeated heating
and cooling confirmed the thermoreversibility of the gelation
process. The critical gelator concentration (CGC) was determined
by measuring the minimum amount of gelator required for the for-
mation of a stable gel at 25 °C.
General: All chemicals were from commercial sources and
used as received, and solvents were purified and dried following
standard procedures unless otherwise stated. Compounds 2 and 3
were prepared according to the reported procedures.[15,17] 1H-NMR,
13C-NMR, MS, elemental analysis, UV–vis absorption, cyclic
voltammograms, and ESR spectra were measured with conventional
spectrometers.
Scanning electron micrographs were taken with Hitachi S-4800
microscope equipped with a digital camera. Samples of suspen-
sion were dropped on silicon and the solvent was evaporated
under ambient conditions. All the samples were sputtered with Pt.
AFM measurements were taken on a commercial AFM instrument
(DI Dimension 3000 with Nanoscope IIIa controller) in tapping
mode.
Reduction with Magnesium: A few magnesium strips, which
were treated with HCl (1.0 m), washed sequentially with water and
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Acknowledgements
The present research was financially supported by NSFC, the State
Basic Program and Chinese Academy of Sciences. This work was
also supported by the NSFC-DFG joint project (TRR61).
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Received: September 14, 2011
Published online: January 19, 2012
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