Poly(aryl ether) Dendrons with Monopyrrolotetrathiafulvalene Unit-Based Organogels exhibiting Gel-Induced Enhanced Emission (GIEE)

Article abstract of DOI:10.1002/chem.201502044

A series of poly(aryl ether) dendrons with a monopyrrolo-tetrathiafulvalene unit linked through an acyl hydrazone linkage were designed and synthesized as low molecular mass organogelators (LMOGs). Two of the dendrons could gelate the aromatic solvents an

Full text of DOI:10.1002/chem.201502044

DOI: 10.1002/chem.201502044
Full Paper
&
Dendrons
Poly(aryl ether) Dendrons with Monopyrrolotetrathiafulvalene
Unit-Based Organogels exhibiting Gel-Induced Enhanced Emission
(GIEE)
Yucun Liu, Wenwei Lei, Tie Chen,* Longyi Jin, Guangyan Sun, and Bingzhu Yin*[a]
Abstract: A series of poly(aryl ether) dendrons with a
monopyrrolo-tetrathiafulvalene unit linked through an acyl
hydrazone linkage were designed and synthesized as low
molecular mass organogelators (LMOGs). Two of the den-
drons could gelate the aromatic solvents and some solvent
mixtures, but the others could not gel all solvents tested
except for n-pentanol. A subtle change on the molecular
structure produces a great influence on the gelation behav-
ior. Note that the dendrons could form the stable gel in the
DMSO/water mixture without thermal treatment and could
also form the binary gel with fullerene (C60) in toluene. The
formed gels undergo a reversible gel–sol phase transition
upon exposure to external stimuli, such as temperature and
chemical oxidation/reduction. A number of experiments
(SEM, FTIR spectroscopy, 1H NMR spectroscopy, and UV/Vis
absorption spectroscopy, and XRD) revealed that these
dendritic molecules self-assembled into elastically interpene-
trating one-dimensional fibrillar aggregates and maintain
rectangular molecular-packing mode in organogels. The
hydrogen bonding, p–p, and donor–acceptor interactions
were found to be the main driving forces for formation of
the gels. Moreover, the gel system exhibited gel-induced
enhanced emission (GIEE) property in the visible region in
spite of the absence of a conventional fluorophore unit and
the fluorescence was effectively quenched by introduction
of C60.
Introduction
gels is thermally reversible and can be further tuned by other
physical and chemical stimuli.
The construction of functional materials through the bottom-
up approach associated with molecular self-assembly allows
the translation of information from the molecular scale into
materials with specific functions.[1] Over the last decade, supra-
molecular gels,[2] representing one of the most important soft
materials, have attracted widespread attention because of
their potential applications in drug delivery,[3] tissue engineer-
ing,[4] sensors and actuators,[5] template materials,[6] removal of
pollutants,[7] enzyme immobilization matrices,[8] catalysis,[9] and
crystal growth,[10] using noncovalent interactions (hydrogen
bonding, p–p interactions, metal–ligand coordination, van der
Waals forces, London dispersion forces, donor–acceptor inter-
actions and hydrophobic effects) for forming nanofibrillar
structures able to “freeze” solvents in the rigid gel frame-
work.[11] Among them, low-molecular mass organogelators
(LMOGs) can be cross-linked into different 3D self-assembled
blocks, such as fibers, rods, ribbons, or other morphologies,
through noncovalent interactions. Because of the weak nature
of these forces, the gel–solution (gel–sol) transition for organo-
So far, a number of physically thermoreversible hydrogel
and organogel systems that consist of gelators with different
functionalities have been reported.[12] Self-assembly in den-
drimers has been intensively investigated because of their po-
tential candidacy in soft matter chemistry and numerous struc-
tural patterns have already been recognized in this regard.[13]
As a new kind of LMOGs, organogelators based on dendrimers
and/or dendrons have also attracted much interest because of
the advantages of significant steric impact and their ability to
form multiple noncovalent interactions.[14] In 2000, Aida and
co-workers reported the first example of a dendritic organogel
by using ester-terminated FrØchet-type dendrons with a dipep-
tide at the focal point.[15] Since then, Zhang and Fan described
the self-assembly of tetraphenylethene, perylenediimide, and
spiropyran-functionalized poly(aryl ether) dendron derivatives
to a photoresponsive organogel.[16] Recently, Prasad and co-
workers reported a series of robust gel systems generated
from poly(aryl ether) dendrons with anthracene, pyrene, glu-
cose, naphthalene and pyridine units attached through an acyl
hydrazone linkage, which own “tunable” morphology or light-
emitting properties.[17] Interestingly, a poly(aryl ether) dendron
with a ferrocene unit-based organogelator could self-assemble
to form a redox-responsive gel.[18] From these advances, it is
clear that the combination of FrØchet-type dendron and acyl
hydrazone unit are capable of forming a large number of inter-
molecular interactions through p–p and hydrogen-bonding
networks, leading to gels with relatively low critical gel
[a] Dr. Y. Liu, W. Lei, Prof. T. Chen, Prof. L. Jin, G. Sun, Prof. B. Yin
Key Laboratory of Natural Resources of Changbai Mountain & Functional
Molecules of Ministry of Education
Yanbian University, Yanji, Jilin 133002 (P.R. China)
Fax: (+86)433-2732456
E-mail: zqcong@ybu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502044.
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concentration (CGC) values and the gel systems have many
potential applications. In spite of the numerous applications of
poly(aryl ether) dendron compounds in material chemistry, it
remains a great challenge to design effective dendron organo-
gelators, especially those with an electron-donating group.
Functional organogels containing electroactive units have
drawn significant attention as these supramolecular structures
have been the focus of immense interest in optoelectronic
applications.[19] Tetrathiafulvalene (TTF) is a redox-active com-
0
pound that can exist in three redox states (TTF , TTF +, TTF2+).
C
Because of this property, TTF and its p-extended derivatives
are particularly interesting as redox-active units in molecular
electronics devices, with the possibility to switch between con-
ducting ON–OFF states by applying, for example, a high bias
potential or an external gating potential.[20] On the basis of this
view, researchers have established that the ability of the TTF
core to act as a redox-active subunit is because of its ability to
self-assemble in solution and in the solid state.[21] Recently,
TTFs have been employed as building blocks for molecular de-
vices such as organic field-effect transistors,[22] liquid crystalline
materials,[23] sensors,[24] molecular switches,[25] conductive mate-
rial,[26] nonlinear optical devices,[27] rectifiers,[20b] and organic
photovoltaic cells.[28] Based on TTF’s good electron-donating
ability and reversible one-electron oxidation action, LMOGs
with TTF units have received considerable attention. In 1994,
the self-assembled molecular wires of a TTF gel were first intro-
duced by Jørgensen and co-workers.[29] Since then, a number
Scheme 1. Molecular structures and synthetic route of the dendrons 1a–d.
region, despite the absence of a conventional fluorophore
unit,[17b] and the fluorescence intensity was quenched by
fullerene (C60) in the gel system.
of physically thermoreversible organogel systems based on Results and Discussion
TTF unit with different chemical structures and functional
Synthesis and gelation
groups have been reported. For example, Becher et al. con-
structed oriented nanowires and organogels based on amphi-
philic TTF-substituted macrocycles.[30] Zhu et al. utilized the
redox properties of the TTF moiety to tune the gel–sol transi-
tion behaviors.[31] After then, this group reported a new TTF-
The starting and intermediate compounds 2, 3b, and 3d were
synthesized according to the previous reported procedur-
es[17e,34] (see the Supporting Information). Target dendrons
were synthesized by an aldimine condensation of 3,4,5-tris-
(benzyloxy) benzohydrazide (2) with corresponding MPTTF
carbaldehydes (3a, 3b, 3c, and 3d) in very good yields. For
comparison purposes, we tuned the length of the alkyl chain
of the attached TTF skeleton and the substituent on the nitro-
gen of pyrrole ring. The chemical structures and purities of all
based molecular gelator with a dendron substituent, in which
+
C
the TTF unit could be oxidized into TTF by chloranil in the
presence of either Sc3+ or Pb2+ [32]
Recently, our group have
.
explored the multiple stimulus responsive organogels based
on monopyrrolotetrathiafulvalene (MPTTF) with different
number of amide units and its CT complex.[33] Incredibly, al-
though a large number of organogelators based on TTF units
have emerged, to our knowledge, the TTFs-linked dendron
organogels have not been reported yet.
1
newly synthesized compounds were characterized by H NMR,
13C NMR spectroscopy and MALDI-TOF mass spectrometry (the
detailed synthetic procedure and spectral data are given in the
Supporting Information).
Herein, we report a series of poly(aryl ether) dendrons with
an electroactive MPTTFs unit linked through an acyl hydrazone
linkage as organogelators (Scheme 1). As always, the MPTTF
core acts as a redox and electron-donating center in the gel
system and provides S···S and p–p interactions. The combina-
tion of a MPTTF unit and a poly(aryl ether) dendron through
an acyl hydrazone linkage will result in the gelation through
intermolecular p–p and S···S interactions. In addition, the acyl
hydrazone linkage provides hydrogen-bonding interactions to
promote the gelation of the dendrons. The present dendrons
could efficiently gelate aromatic solvents such as benzene, tol-
uene, chlorobenzene, and xylene, and some solvent mixtures.
The gelation leads to fine fibrillar type aggregates, which inter-
estingly exhibits a gelation induced emission in the visible
The gelation ability of dendrons 1a–1d was studied in a vari-
ety of organic solvents as well as solvent mixtures by “stable
to inversion of glass vial”. The critical gel concentration (CGC)
values of the dendrons were determined in each solvent and
the results are given in Table S1 (the Supporting Information).
As seen in Table S1 (the Supporting Information), the dendrons
are easily soluble in the polar aprotic solvents (such as THF,
DMF, DMSO, CHCl3, and so on) but insoluble in saturated hy-
drocarbon and protic solvents (such as n-hexane, cyclohexane,
methanol, and ethanol). The gelation behaviors of dendrons
1a–1b and 1c–1d are different: dendrons 1a and 1b could
gel aromatic solvents (such as benzene, toluene, xylene, and
chlorobenzene) to form the thermo-reversible opaque yellow
organogels and possess low critical gelation concentrations in
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Figure 1. Photographs of sol–gel transition of 1b in toluene (a) and the
mixture of DMSO/water (2:1) (b).
Figure 2. Variation of the Tgel of organogels of 1a and 1b with increasing
gelator concentration in benzene.
the measured solvents. For instance, the gelation of 1 mL of
toluene requires 1.6 mg of 1a and 1.5 mg of 1b, respectively
(Figure 1a and Table S1, the Supporting Information). More-
over, dendrons 1a and 1b could also form a gel in solvent
mixtures. For example, addition of n-hexane to a chloroform
solution of the dendrons leads to the formation of an opaque
organogels (Figure S1, the Supporting Information)). Interest-
ingly, compounds 1a and 1b form translucent gels when
added water to their DMSO solution without heating and cool-
ing treatment. Furthermore, these translucent gels could
change to opaque organogels upon heating and cooling pro-
cess, and are thermo-reversible in nature (Movie 1 (the Sup-
porting Information) and Figure 1b). By comparison with 1a
and 1b, dendrons 1c and 1d, each of which has a methyl
group on nitrogen of pyrrole ring in dendron molecule, could
not gel all solvents tested except for n-pentanol. A subtle
change on the molecular structure produces a great influence
on the gelation ability. The larger solubility and low gelation
ability of 1c and 1d than 1a and 1b in the solvents suggest
that the intermolecular interaction between 1c or 1d is
weaker than that between 1a or 1b. It is clear that the intra-
or intermolecular hydrogen bonding from the hydrogen atom
on the nitrogen atom of the pyrrole ring play a key role.
Figure 3. Optical microscopic images of the xerogels (a) 1a and (b) 1b with
or (c) 1a and (d) 1b without thermal treatment from DMSO/water (2:1, v/v)
(100”magnification).
To investigate the thermal stability of the resulting gels, the
gel–sol phase transition temperatures (Tgel) were measured at
different concentrations of the gelators by a convenient ball-
drop method. As shown in Figure 2, the Tgel value increased
with increasing gelator concentration due to the increasing
participation of intermolecular noncovalent interactions. The
Tgel values of 1a and 1b in benzene varied from 56 to 808C for
1a and 51 to 808C for 1b at their CGC, indicating that the gel
systems had stable supramolecular structures at this concen-
tration and good thermal stability.
crystallites with a well-defined uniform fiber-type self-assembly.
To gain a detailed insight into the gelation behavior and
nature of the self-assembly, the morphology of xerogels of ge-
lators 1a and 1b was observed through scanning electron mi-
croscopy (SEM). As seen in Figure 4a and b, the xerogels of 1a
and 1b from toluene aggregated into a fibrillar self-assembly
with fiber diameters of 100–150 nm and lengths of several
dozens of micrometers. Similarly, the entangled fibrillar net-
work morphologies of thicker fibers with 200–300 nm in diam-
eter in which a few of fibers linked with each other were ob-
served for the xerogels of 1a and 1b obtained from CHCl3/n-
hexane (6:5 for 1a and 2:1 for 1b, v/v) (Figure 4c and d). By
comparison, 1a and 1b in the solvent mixture of DMSO/water
(2:1, v/v) self-aggregated into a rod-type self-assembly with
0.1–0.5 mm in width and more than 100 mm in length, which is
a few of slim rods as seen in Figure 4e and f. Notably, the rod-
like translucent gel without thermal treatment transformed
into an opaque gel when subjected to heating and cooling
Morphologies and architectures
The dendron in the gel phase were initially examined by using
optical microscopic techniques.[18,35] The optical microscope
images of the gels of 1a and 1b obtained from the mixture of
DMSO/water with and without thermal treatment were depict-
ed in Figure 3. The aggregation in all gel phases shows fibrous
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Figure 4. The SEM images of the xerogels obtained from 1a (a) and 1b (b)
from toluene; 1a (c) and 1b (d) from CHCl3/n-hexane; 1a (e) and 1b (f) with
or 1a (g), and 1b (h) without thermal treatment from DMSO/water. The
scale bars correspond to1mm (a, b, c, d, e, f, g) and 10 mm (h), respectively.
Figure 5. SAXS patterns of xerogels of 1b from toluene (a) and DMSO/water
(2:1, v/v) without thermal treatment (b).
process but no microscopic structures of the gel system was
found (Figure 4 g and h).
SAXS study
To understand the molecular packing pattern of the dendrons
in gel phase, the X-ray diffraction patterns of the xerogels of
gelator 1b from toluene and DMSO/water (without thermal
treatment) were analyzed. In both cases, the diffraction pat-
terns are characterized by two reflection peaks in the small-
angle region. In the case of xerogel obtained from toluene,
two reflection peaks of scattering pattern was observed at 2.6
and 1.86 nm in the small-angle region with a scattering vector
p
ratio of 1: 2, corresponding to a 2D rectangular structure with
a=2.6 nm and b=1.31 nm, as shown in Figure 5a. Similarly,
the xerogel obtained from a mixture of DMSO/water also self-
aggregated to a 2D rectangular structure (Figure 5b). In both
cases, the average number of dendrons in a disk was calculat-
ed to be approximately 1 based on calculations using the
Equation in ref. [23]. In addition, considering that the molecu-
lar length was approximately 2.55 nm (based on the CPK
model), the molecules have a monomolecular arrangement in
the matrix. On the basis on these results, we assumed that the
Figure 6. Cartoon representation of the self-assembly of 1b in the both
cases.
molecule might favor packing in a parallel fashion to maximize
p-stacking, hydrogen-bonding, and S···S interactions.[36] The
packing model of the rectangular columnar phase is shown in
Figure 6.
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started to appear and became gradually sharper and stronger
and the original broad peaks also became gradually sharper
and stronger with increasing temperature. A broad peak for
amide NH and pyrrole NH could be found at d=8.38 ppm
over 323 K, which was split and shifted upfield gradually to
d=8.40 and 8.12 ppm, respectively. Another peak for the
ÀCH=NÀ unit appeared at d=7.57 ppm and shifted downfield
to d=7.62 ppm. In the same way, the weak and broad signals
for pyrrole CH and aryl ether CH2O became gradually sharper
and stronger with increasing temperature and shifted down-
field from d=5.81 and 4.85 ppm to 5.83 and 4.91 ppm, respec-
tively. Furthermore, the signals from the protons on the alkyl
segment were shifted downfield by about 0.047–0.083 ppm
and the signals of the benzene rings in the range of d=7.47–
7.00 ppm showed slight upfield shifts (<0.01 ppm). The NMR
result revealed that supramolecular aggregates were formed
through intermolecular hydrogen bonding, p–p and van der
Waals interactions of gelator molecules.
Figure 7. FTIR spectra of 1a in DMSO solution (a), xerogel from toluene (b)
and CHCl3/n-hexane (6:5, v/v, c).
To gain further insight into the hydrogen bonding is one of
the main driving forces for gelation, we subsequently exam-
ined the stimuli-responsive property of gel of 1a in toluene to-
wards the various anions (as the tetrabutylammonium salt). As
seen in Figure 8a and Figure S4 (the Supporting Information),
upon addition of 3.0 equiv of FÀ, ClÀ or AcOÀ, as time passed,
the gel gradually transformed into a yellow solution but the
color of the sol formed had no a drastic change in the whole
process (the Supporting Information, Figure S5), suggesting
that the collapse of gel phase arise from the anion-binding in-
teractions rather than the deprotonation. The gel could be ad-
versely preserved upon addition of the same amount of other
anions (BrÀ, IÀ, H2PO4À, HSO4À). In addition, the gel could not
be restored by adding the protic solvents (such as methanol or
H2O), which provides another evidence for the aforementioned
Driving-force study
Fourier transform infrared spectroscopy (FTIR) is a powerful
tool to study noncovalent interactions, especially hydrogen
bonding in the gelation.[37] To find out the involvement of acyl
hydrazone moieties in dendron molecule in the self-assembly,
FTIR spectra of xerogels (from toluene and CHCl3/n-hexane)
and the non-gelated DMSO solution (about 5%) of gelator 1a
were recorded. As shown in Figure 7, the FTIR spectrum of 1a
in DMSO solution shows a broad transmission band at
3443 cmÀ1 which was assigned to n NÀH on nitrogen of the
pyrrole ring and a narrow band at 3282 cmÀ1 due to non-hy-
drogen-bonded n NÀH (amide A) as well as other two bands at
1664 and 1582 cmÀ1, the latter bands are characteristic of n C= mechanism. However, the gel obtained from DMSO/H2O could
O (amide I) and d NÀH (amide II) frequencies, respectively.
However, in the FTIR spectra of the xerogels both from toluene
and CHCl3/n-hexane, the correspondent amide A and amide I
were redshifted by 52 and 33 cmÀ1, respectively, whereas ami-
de II was slightly blueshifted by 2 cmÀ1. A noticeable change in
FTIR spectrum is that the transmission band of NÀH on nitro-
gen of pyrrole ring in gels was also redshifted about 46 cmÀ1
with respect to the DMSO solution. These change of transmis-
sion bands indicate the involvement of hydrogen bonding
from amide group and, in particular, the NÀH bond of pyrrole
ring in the gelation process. Similar FTIR spectral changes were
also observed for gelator 1b under same experimental
conditions (the Supporting Information, Figure S2).
not be destroyed upon addition of anions. Additional evidence
To clarify the existence of noncovalent interaction in the
self-assembled network, taking 1b (5.0 mgmLÀ1 in C6D6) as an
example, temperature-dependent 1H NMR experiment was
performed (the Supporting Information, Figure S3). Because of
the strong self-assembly and the opaque gel state at room
temperature, no obvious signals of amide NH, pyrrole NH and
ÀCH=NÀ were detected, and other proton signals presented
broad peaks, such as pyrrole CH and the alkyl residue. Upon
heating from 293 K (gel state) to 353 K (solution state), the
peaks ascribed to the amide NH, pyrrole NH and ÀCH=NÀ
Figure 8. (a) Photographs of the gel–sol and color transitions of 1a in
toluene in the presence of 3.0 equiv FÀ or ClÀ or AcOÀ and (b) 1H NMR
([D6]DMSO) spectra of 1a in the presence of FÀ in [D6]DMSO.
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for the stimuli-responsive mechanism was obtained from
1H NMR experiments in [D6]DMSO. The peaks of the amide NÀ
H, pyrrole NÀH, pyrrole ÀCHÀ and ÀCH=NÀ protons in gelator
1a appeared at d=11.87, 11.68 and 7.04 and 8.40 ppm, re-
spectively. After adding 0.5 equiv of FÀ, the peaks at d=11.87
and 11.68 ppm for the proton of amide unit and the proton on
nitrogen of pyrrole, respectively, disappeared, but no peak was
À
observed for the HF2 anion around d=16 ppm, which further
supported the anion-binding mechanism. Upon treatment
with the increasing amounts of FÀ, the peak at d=7.04 for the
proton of pyrrole ring was shifted upfield to d=6.80 ppm,
probably because of the enhanced self-assembly through p–p
interactions (Figure 8b). Moreover, addition of anions to DMSO
solution of 1a caused hardly any significant spectral change
(the Supporting Information, Figure S6), which also refuted the
deprotonation mechanism.
Next, to prove the p–p interactions between aromatic moi-
eties during gelation, the UV/Vis spectra of 1a both in the
dilute toluene solution (5”10À5 m) and xerogel state were re-
corded. In dilute solution, gelator 1a showed a strong absorp-
tion band at 346 nm, which was attributed to the p–p* transi-
tions of the molecular backbone. As for the xerogel state, this
band took an obvious redshift to 350 nm and accompanied
with the decrease of the intensity. This result suggested that J-
aggregates were formed in the toluene gel. The UV/Vis absorp-
tion spectra for gelator 1a in DMSO were also recorded with
addition of increasing amounts of H2O at room temperature.
As shown in Figure 9b, gelator 1a in DMSO solution (5”
10À5 m) showed two absorption bands centered at 267 and
346 nm, which could be ascribed to the p–p* transitions of the
MPTTF and conjugated molecular backbone, respectively.
When increasing the water content to 33.3% (2:1, v/v), these
two absorption bands redshifted to 303 and 358 nm, respec-
tively. By increasing the water content further to 66.7% (1:2,
v/v), the band at 346 nm continued to redshift to 369 nm and
resulted in an obvious Tyndall phenomenon (the Supporting
Information, Figure S7), indicative of the formation of J-aggre-
gates within self-assembly process. There is no doubt that the
p–p interactions plays a key role in the gelation process.[33b,38]
On the other hand, as seen from the optimized structure (the
Supporting Information, Figure S8), the MPTTF and dendron
unit in the respective plane provide a favorable environment
for p–p stacking between molecules.
Figure 9. UV/Vis spectra of 1a (a) of the dilute toluene solution (5”10À5 m)
and xerogel from toluene and (b) in DMSO (5”10À5 m) with addition of
increasing amounts of H2O at room temperature.
The electrochemical properties of the gelators and the corre-
sponding gels were evaluated by cyclic voltammetry in tolu-
ene/acetonitrile (v/v=4:1, 0.5 mm, vs. Ag/AgCl). Taking gelator
1a as example, as shown in Figure 10a, the solution of 1a
showed two reversible single-electron oxidation waves at ap-
proximately E= +0.587 V (I), +0.938 V(II), which were assigned
to the formation of radical cation and dications of TTF, respec-
tively.[39] By comparison, the xerogel of 1a from toluene exhib-
ited two quasi-reversible oxidation waves at 0.562 and0.913 V,
(Figure 10b), respectively, and both were negatively shifted by
0.025 V compared with those in solution under the same con-
ditions. The lower oxidation potentials of the TTF gel phase
should be attributed to the stabilization effect arising from the
strong p–p and S···S interactions.[31a]
Figure 10. Cyclic voltammograms of (a) 1a solution (0.5 mm); (b) native
xerogel of 1a from toluene; (c) binary xerogel of 1a with C60 from
toluene; and (d) C60 (0.25 mm) in toluene/CH3CN (4:1, v/v). Containing 0.1m
Bu4NPF6 and the scan rate was 100 mVsÀ1
.
Chemical redox
As we know, the TTF moiety could be oxidized in an oxidative
environment, leading to the gel’s response to certain external
chemical stimuli. When an equivalent amount of Fe (ClO4)3 was
carefully placed above the gel of 1a in toluene, the gel gradu-
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which is become a thorny obstacle to the development of
their light-emitting properties.[41] Recently, aggregation-in-
duced emission (AIE) or aggregation-induced enhanced emis-
sion (AIEE) have been reported by many scientists in organic
molecules with a structural feature of one conjugated stator
carrying multiple aromatic peripheral rotors because of the re-
striction of the intramolecular rotation.[42] Therefore, the close
packing of MPTTF and aromatic units in the dendron will re-
sults in the aggregation induced fluorescence emission.[43] To
verify the hypothesis, the fluorescence spectroscopic investiga-
tion of the gelator was carried out taking 1a as example. Gela-
tor 1a is almost non-emissive in neat DMSO solution when ex-
cited at 346 nm. Addition of 33% water (2:1, v/v) to the DMSO
solution of 1a results in the gelation and elicited a big fluores-
cence enhancement (ꢀ240 fold) at 549 nm. The remarkable
fluorescence enhancement can be explained by gel-induced
enhanced emission (GIEE). The gelation of 1a in toluene simi-
larly induces 100 fold fluorescence enhancement as compared
with the DMSO solution (Figure 12a). It is interesting that the
change of the fluorescence emission of the system is easily to
survey due to that the gelation in DMSO/water mixture does
not need the heating and cooling process. To further under-
stand the role of p–p stacking for the emission nature of gel
state, therefore, the fluorescence emission change of 1a in the
sol–gel transition was tracked by gradual addition of water to
the DMSO solution (0 to 33.3%, v/v). As shown in Figure 12b,
with gradual addition of water to the DMSO solution, the emis-
sion intensity of 1a gradually increased. When addition about
11 % of water, the system started to elicit fluorescence emis-
sion at about 500 nm. With an increase of water content, the
emission wave gradually shifts to 549 nm, showing the GIEE
phenomenon occurred. Note that when the water content
reached 25%, a sudden change of the emission intensity was
observed, and a maximal fluorescent signal is achieved until
the water content reached 29.4%. However, the emission in-
tensity at 549 nm was slightly decreased along with increase
the water content (33.3%) likely due to the lower transparency
of the gel (the Supporting Information, Figure S10).
Figure 11. (a) The gel–sol transitions of the gel of 1a by chemical redox
reaction in toluene, and (b) the corresponding changes of UV/Vis spectra of
1a upon chemical redox in THF/CHCl3 (5”10À5 m, 1:1, v/v).
ally turned into a dark-green solution (Figure 11 a).[31] The
mechanism of phase transition is identical to that observed
during electrochemical oxidation, that is, TTF was oxidized to
TTF +, which results in the repulsion between the MPTTF units
C
to destroy the p–p and S···S interactions between TTF units,
and thereby the gel network collapses.[18] Notably, when an
equivalent ascorbic acid (Vitamin C, Vc) dissolved in small
amount of water was added to the oxidized solution, which
+
C
was able to reduce TTF to the neutral TTF, the organogel re-
formed upon cooling the hot solution. Such a gel–sol transi-
tion could be repeated for several cycles. Further evidence for
redox reaction processes was monitored by a UV/Vis spectros-
copy. The UV/Vis spectrum of 1a in THF/CHCl3 (1:1, v/v) did
not show any absorption bands in the range of 400 to 900 nm.
However, upon addition of increasing amounts of Fe3+, the
peak assignable to dendron molecular backbone absorption
(at 349 nm) was split into two peaks at 317 and 356 nm. Mean-
while, two new absorption peaks centered at 429 and 760 nm
On the other hand, gelator 1a showed concentration-
dependent emission property in toluene. Upon increasing the
concentration of 1a from 0.4 to 2.0 mgmLÀ1, the emission
intensity at 549 nm gradually increased and accompanied by
the gelation (Figure 12c). The time-dependent change of the
fluorescence intensity was also investigated in toluene
(1.6 mgmLÀ1) (Figure 12d). The fluorescence emission of 1a in
the hot toluene solution exhibited a negligibly small emission
intensity. With the aging time at room temperature, the emis-
sion intensity of 549 nm was gradually strengthened. When
the hot toluene solution of 1a was naturally cooled for 12 min,
the process of sol–gel transition was finished and the emission
intensity of the gel was almost reached the maximum. Note
that at early stage of gelation, the initial fluorescence silence
was observed (<1.8 min) and then, fluorescence emission
intensity increased drastically and finally reached a maximum
value.[44] The above results proved further that the p–p
interactions is a the main driving force.
emerged, respectively, which were assigned to the absorption
+ [33b,40]
C
bands of MPTTF
.
In turn, the two characteristic bands
+
C
assignable to the MPTTF cation gradually decreased with the
increase in Vc concentration. Finally, the original spectrum of
1a was totally regenerated upon the addition of 4 equivalents
of Vc (Figure 11 b) and the color of the solution also
experienced reversible transformation in the whole redox
process (the Supporting Information, Figure S9).
Fluorescence properties of the gels
It is well known that the “aggregation-caused quenching”
(ACQ) effect exists in the gel formation process in most cases,
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Figure 12. Fluorescence spectra of compound 1a (A) in DMSO solution (a), the gel in toluene (b, 1.6 mgmLÀ1) and DMSO/water (c, 0.8 mgmLÀ1, 2:1, v/v);
(B) in DMSO (2.25 mgmLÀ1) with addition of different amount of H2O; (C) in toluene with increasing of the concentration and (D) of the time-dependent
changes in toluene (1.6 mgmLÀ1). The inset is the fluorescent time sweep experiment for the gel of 1a from toluene.
Binary organogel formation with C60
TTF and its derivatives have received considerable attention as
donor components in the molecular assembly because they
have strong electron-donating ability.[21a,25,45] It is expected that
the p–p and S···S interactions in the gel phase would be de-
stroyed by introduction of an acceptor molecules, such as C60,
through the intermolecular charge-transition (CT) interactions.
Contrary to expectations, when different amounts of C60 (from
0~1 equiv) dissolved in toluene were placed above the gels of
1a in toluene, a dark-brown gel was formed upon heating and
cooling process (Figure 13). To determine the optimum stoichi-
ometry for the gel, Tgel was measured with increasing amounts
of C60 at a constant concentration of 1a. A plot of Tgel versus
the molar ratio of C60 showed a maximum value of Tgel at a 2:1
molar ratio (Figure 13). The results suggested that two MPTTF
moieties in 1a bind with one C60 molecule to form a 2:1 1a/
C60 sandwich-type complex (Figure 14).[46] To explore the driv-
Figure 13. Variation of the gel melting temperature of 1a (2.0 mgmLÀ1) with
ing-force in the gel, the electron paramagnetic resonance
different amounts of C60 (from 0 to 1.0 equiv) in the toluene; the inset
shows the color change for the gels based on compound 1a after addition
of different amounts of C60.
spectrum was recorded at 298 K but no electron spin reso-
nance (ESR) signal was observed in the region characteristic of
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Figure 15. (a) The SEM images of the binary gel of 1a after adding C60 (mole
ratio=1:0.5) and (b) Fluorescence quenching of organogel 1a from toluene
upon titration with C60. The scale bars correspond to (left and the inset of
right) 1 mm and (right) 10 mm, respectively.
change of the optical property of the gel. To verify this hypoth-
esis, the emission intensity is monitored as a function of added
C60 in toluene. As shown in Figure 15b, the emission intensity
of the gel system was quenched dramatically by addition of
increasing amounts of C60 (ca., 5 fold). A similar observation
was made for the binary gel of gelator 1b under same experi-
mental condition (the Supporting Information, Figure S13).
Figure 14. Cartoon representation of the possible molecular packing mode
for 1a in the gel with C60.
the TTF radical cation (the Supporting Information, Figure S11),
indicative of that the formed gel is not a charge-transfer (CT)
complex gel but a binary gel, which is derived by intermolecu-
lar interactions. On the other hand, the change of redox poten-
tial of the binary xerogel from toluene supported further the
intermolecular interaction mechanism. The binary gel from tol-
uene/acetonitrile (v/v=4:1) mixture exhibits two quasi-reversi-
ble oxidation waves at 0.656 and1.017 V (Figure 10c), respec-
tively, which were far higher than those of the native gel of
1a, and an irreversible reduction wave at À1.169 V that is
10 mV more negative than that of C60 itself in toluene/acetoni-
trile mixture. The features of optical absorption of the binary
gels containing different amounts of C60 confirmed further the
absence of CT complex (the Supporting Information,
Figure S12). These results are in agreement with our proposed
interaction mechanism.
Conclusion
We have developed a new series of low-molecular mass
organogelators based on poly(aryl ether) dendron with a
monopyrrolotetrathiafulvalene unit. Dendrons 1a and 1b with
a hydrogen atom on the nitrogen in pyrrole ring could gel aro-
matic solvents and the mixed solvents to form the thermo-re-
versible stable gels with low critical gelation concentration. By
comparison, dendrons 1c and 1d with a methyl group on the
nitrogen in pyrrole ring could not gel all solvents tested
except for n-pentanol. It is interesting to note that the den-
drons could form the stable gel in the DMSO/water mixture
without thermal treatment and could form the binary gel with
C60 in toluene. The formation of the binary gel is attributed to
the intermolecular interaction between the MPTTF unit and
C60, which is confirmed by absorption and ESR spectral studies.
The formed gels undergo a reversible gel–sol phase transition
upon exposure to external stimuli, such as temperature and
chemical oxidation/reduction. The cooperation of hydrogen
bonding, p–p and S···S interactions was determined to be the
main driving force for the formation of the gels. Moreover, the
gel system exhibits gel-induced enhanced emission (GIEE)
property in the visible region, despite the absence of a conven-
Interestingly, an evident change took place in the
morphology of the binary gel compared with those of the
corresponding native gel. The fibrous morphology of the
native gel of 1a obtained from toluene transformed into the
elliptical morphology, which consists of many short rod-like
structures (Figure 15a). The coexistence of C60, which is an
electron-acceptor, in binary gel will inevitably cause the
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quenched by introduction of C60. The present gelators with ex-
cellent gelation ability, GIEE property and the inherent elec-
tronic properties of TTF, it may thus provide important insight
into the relevant applications, such as light-harvesting systems
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The authors acknowledge financial support from the National
Natural Science Foundation of China (grant no. 21262038).
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Keywords: dendrons · gels · fluorescence · redox chemistry ·
self-assembly
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Received: May 25, 2015
Published online on September 7, 2015
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