Organogels formed by substituent-free pyrene-appended oligo(m-phenylene ethynylene)s

Article abstract of DOI:10.1039/c5cc04006j

A new class of pyrene-appended m-phenylene ethynylene oligomers, which bear no alkyl chains or heteroatoms, have been demonstrated to gelate organic solvents, and one of them forms chiral twisted gels in cyclohexane.

Full text of DOI:10.1039/c5cc04006j

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Organogels formed by substituent-free
pyrene-appended oligo(m-phenylene ethynylene)s†
Cite this:DOI: 10.1039/c5cc04006j
Yuan-Yuan Chen, Hui Wang,* Dan-Wei Zhang,* Jun-Li Hou and Zhan-Ting Li*
Received 14th May 2015,
Accepted 17th June 2015
DOI: 10.1039/c5cc04006j
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A new class of pyrene-appended m-phenylene ethynylene oligomers, subunit was introduced to all the compounds as a steering
which bear no alkyl chains or heteroatoms, have been demonstrated stacking subunit,10 while the oligo(m-phenylene ethynylene)
to gelate organic solvents, and one of them forms chiral twisted gels chains were explored to see if they could serve as tunable flexible
in cyclohexane.
chains for the formation of 3D fibrous networks, as observed
for alkyl chains in many reported conventional gelators for the
p-Conjugated low-molecular-mass organic gelators (LMOGs) have gelation of organic solvents.
received great attention due to their potential applications in
biomedical and optoelectronic materials.1,2 Typically, the presence
of long alkyl chain(s) or steroidal or other non-conjugated unit(s) in
the gelator molecules is necessary to facilitate the formation of three-
dimensional (3D) networks and to promote the aggregation of the
whole molecular framework.2 However, for the development of
LMOGs with optoelectronic properties, which are usually designed
on the basis of large aromatic systems, these inactive and isolating
groups are undesirable and their introduction also reduces the atom
The gelation properties of these pyrene derivatives were first
economy of the resulting LMOGs. In the past decade, several kinds investigated by the ‘‘stable-to-inversion of a test tube’’ method
of conjugated backbones which bear no aliphatic chains or steroidal in discrete organic solvents. The results are summarized in Table 1.
units have been reported to gelate organic solvents.3–9 However, In a typical experiment, one compound was dissolved in an organic
discrete auxiliary polar groups, including cyano and trifluoromethyl,3 solvent by heating at 75 1C. The solution was then allowed to cool
amide,4 acid,5a amino,6a,b heterocycle,4,5a,6b,7,8a b-diketone-boron down to room temperature under ultrasound. All the compounds
difluoride,6a imine,3b ether,8a,b ester9 and imide8b units, have been could not gelate dichloromethane, chloroform, tetrahydrofuran,
introduced to enhance the stacking of the designed conjugated 1,4-dioxane or toluene, and most of them, except 2b which partially
cores through electrostatic interaction and/or hydrogen bonding. In gelated n-hexane, were either insoluble or precipitated in apolar
one case, the gelation of cyano-bearing stilbene derivatives requires n-hexane or polar acetonitrile or methanol. However, the longest 1d
the assistance of additional pyridine–Ag+ coordination.3f Herein, we and 2c gelated cyclohexane, decahydronaphthalene, ethyl acetate
report that, without bearing any polar groups or heteroatoms, and acetone as well as the mixtures of dichloromethane with
pyrene-appended oligo(m-phenylene ethynylene)s are able to gelate n-hexane and methanol. In all the cases, the critical gelation
organic solvents of low and modest polarity. We further describe concentration (CGC) of 2c was lower than that of 1d for the
that fibres formed by one oligomer exhibit uneven symmetry break- identical solvent, implying that the ethynyl group at the one end
ing, leading to the formation of chiral assemblies.
of 1d was unfavourable to the gelating process. Compounds 1b and
Compounds 1a–1d and 2a–2c were designed and synthesized 2b could gelate cyclohexane, whereas 2b could also gelate the 1 : 1
and the synthetic details are presented in the ESI.† The pyrene mixture of dichloromethane and methanol. However, the CGC was
pronouncedly higher than that of the respective analogues 1d and
Department of Chemistry, Collaborative Innovation Center of Chemistry for Energy
2c, and it took a longer time for the solution of 1b and 2b to realize
the solution-to-gel transition. These facts indicate that the longer
3-mer m-phenylene ethynylene chain was stronger than the shorter
2-mer chain in providing the gelating ability for both series
Materials (iChEM), Fudan University, 220 Handan Road, Shanghai 200433, China.
E-mail: wanghui@fudan.edu.cn, zhangdw@fudan.edu.cn, ztli@fudan.edu.cn
† Electronic supplementary information (ESI) available: Synthesis and character-
ization of new compounds, 1H and 13C NMR spectra and additional images and
spectra. See DOI: 10.1039/c5cc04006j
of compounds. The alcohol precursor for the preparation of 2c,
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Table 1 Gelation properties of compounds 1b–1d, 2b and 2c in organic
solventsa
Solvents
1a 1b
1c 1d
2a 2b
PG
2c
HE
Cyclohexane
DHN
AcOEt
Acetone
CH2Cl2/HE (2 : 5, v/v)
CH2Cl2/MeOH (1 : 1, v/v) S
PhMe
CH2Cl2
CHCl3
THF
1,4-Dioxane
MeCN
P
P
S
S
S
S
P
I
I
P
I
G (4.72) PG G (2.56) P G (8.50) G (1.60)
P
S
S
P
P
S
S
S
S
S
P
P
P
S
P
P
P
S
S
S
S
S
P
I
G (4.65) S
G (21.2) S
G (11.7) S
G (8.64) S
S
S
S
S
G (2.46)
G (12.5)
G (6.67)
G (7.64)
G (5.86) PG G (22.5) G (3.83)
S
S
S
S
S
S
P
S
S
S
S
S
I
S
S
S
S
S
P
P
S
S
S
S
S
S
I
S
S
S
S
S
I
MeOH
I
I
a
The values in parentheses are the critical gelation concentrations
(CGCs) in mg mLꢀ1. HE: n-hexane; DHN: decahydronaphthalene; G: gel;
I: insoluble; PG: partial gel; P: precipitation; S: solution.
i.e., compound 5 in the ESI,† did not gelate any organic solvents,
suggesting that intermolecular hydrogen bonding formed by the
hydroxyl group weakened the gelating ability of the conjugated
backbone. For comparison with the most efficient gelator 2c,
compound 3 was also prepared. It was found that this compound
did not gelate any organic solvent, reflecting that the pyrene
subunit in 2c played a crucially important role in endowing the
backbone with the gelating capacity.
Fig. 1 SEM (a and c) and TEM (b and d) images of air-dried organogels
formed in cyclohexane by 2b (a and b), and 2c (c and d).
structures were observed from both the SEM and TEM images
(Fig. 5, vide infra).
The rheological properties of the gels formed by 1d and 2c
were determined by a frequency sweep test at 20 1C. Both
organogels exhibited higher storage modulus (G0) values than
loss modulus (G00) values over the whole scanning frequency
range, which is typical for viscoelastic soft solids (Fig. S51, ESI†).
The ratio of the storage modulus value to the loss modulus value
The thermal stability of gels 1b, 1d, 2b and 2c was also (G0/G00) is in the range of 9–16 for 1d and 5–9 for 2c. It suggests
determined by the ‘‘stable-to-inversion of a test tube’’ method. The that the dissipation of energy is less for the gels of 1d, thus
gel–sol transition temperatures (Tgel) ranged from 24 to 43 1C at their indicating a stronger network. This result is consistent with
respective critical gelation concentrations (Fig. S49a, ESI†). It was those obtained for the Tgel
noted that the expected higher Tgel values of 2c than those of 1d were To get insight into the role of the pyrene subunit in the formation
.
not observed, although 2c had lower CGCs compared to 1d. The of the gels, we also investigated the spectroscopic properties of
results indicated that the 1d gels are more stable than the 2c gels. compounds 1d and 2c in homogeneous solutions. As shown in
Moreover, the plot of Tgel as a function of the 2c gelator concen- Fig. 2, 1d and 2c gave rise to almost the same UV-visible absorption
tration showed a nonlinear increase in thermal stability with spectra in cyclohexane. The absorption maxima around 295 nm,
increasing concentration (Fig. S49b, ESI†).
364 nm and 385 nm correspond to the absorptions of the pyrene
To investigate the morphologies of the new organogels, air- subunit. The ratio of the intensities I385/I295 increases nonlinearly
dried samples were subjected to both scanning electron micro- with the increase of the concentration from 1.0 mM to 30 mM at
scopy (SEM) and transmission electron microscopy (TEM) analyses 25 1C. The hyperchromic effect of the absorption centred around
(Fig. 1 and Fig. S50, ESI†). It was found that the morphologies of 385 nm supports J-type aggregation of the pyrene subunit.11 For
all the xerogels were dependent on the length of the backbone. For comparison, the UV-visible spectrum of controls 3 and pyrene were
example, the SEM images of both 1d and 2c in cyclohexane also recorded in cyclohexane. Within the identical concentration
showed that these gelators generated multi-layered entangled 3D range, the absorption intensity of both compounds was linearly
networks consisting of bundles of long fibrils, which must be related to the concentration (Fig. S52 and S53, ESI†), suggesting that
responsible for the observed gelation. Different from those of 1d no aggregation took place.
and 2c, the SEM images of shorter 1b and 2b in cyclohexane
The fluorescent spectra of 1b–1d and 2a–2c were also
showed broad fibrous networks. The fibres of 1b are approximately recorded in cyclohexane at different concentrations (1.0 mM
0.13 mm and that of 2b are 1.3 mm in width. The TEM pictures also to 1.0 mM) at 25 1C. For 2c, two sharp peaks located at 392 nm
confirmed the above results, but provided even clearer network and 413 nm, respectively, and a shoulder at around 432 nm
structures. Interestingly, for the gels of 2c, obvious chiral twisted were detected. Upon increasing the concentration, the sharp
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Fig. 2 UV-vis absorption spectra of (a) 1d and (c) 2c in cyclohexane at
different concentrations and the plots of I385/I295 vs. concentration for (b)
1d and (d) 2c.
peak at 392 nm faded and red-shifted (for 0.1 mM) or disap-
peared (for 1.0 mM), while the peak at 413 nm and the shoulder
became enhanced (Fig. 3a). Similar results were also observed
for other compounds (Fig. S55 and S56, ESI†). When the gel of
2c in cyclohexane was formed, the shoulder at around 432 nm
became enhanced dramatically and a strong band centered at
484 nm emerged (Fig. 3b), which should reflect the enhanced
stacking of the pyrene subunit in the gel state. However, no
such emissions were observed in the fluorescent spectra of
control 3 in cyclohexane. These results indicate that, compared
with 3, all the compounds bearing the pyrene subunit have a higher
tendency of stacking.11b,12 The fact that the shorter analogues did
not gelate organic solvents or just had a low gelating ability
suggests that the longer oligo(m-phenylene ethynylene) chain
of 1d and 2c also played a key role in enabling the gelating
ability by providing additional stacking and holding the 1D
aggregates into bundle fibrils (Fig. 4). Thus, to some extent, the
function of the aromatic chain is similar to that of long alkyl
chains in many reported organogelators.1,2
Fig. 3 (a) Emission spectra of 2c in cyclohexane with increasing concentra-
tion from 1.0 mM to 1.0 mM. (b) Emission spectra of 2c in solution (0.02 mM)
and in the gel state (5.0 mM) in cyclohexane.
On the basis of the above experimental observations, we
proposed a possible mechanism for the hierarchical self-assembly
of the new pyrene-derived conjugated compounds into gels (Fig. 4).
In the first stage, the pyrene core stacked into one-dimensional (1D)
column-styled aggregates with the assistance of the stacking of the
benzene subunits.11b,12 Such 1D aggregates further assembled into
bundle fibrils, which were stabilized by the stacking of the benzene
subunits of the adjacent aggregates. These fibrils gelated organic
solvents by forming 3D entangled networks. The m-substitution
feature of the benzene subunits was expected to enable the stacking
of the benzene subunits from different molecules because it
allowed for the orientation of the conjugated backbones in differ-
Fig. 4 Schematic representation of the hierarchical self-assembly of
pyrene-appended oligo(m-phenylene ethynylene)s into 3D networks for
gelating organic solvents.
The SEM and TEM images of the dried gels of achiral 2c
ent directions. Thus, to some extent, the conjugated backbones in cyclohexane exhibited fibres with a statistically dominant
resemble the aliphatic chains of many reported organogelators in left-handed (M) or right-handed (P) twist. Although we could
forming 3D networks through van der Waals force,1,2 whereas the not control the M or P twist for a specific sample, all the
pyrene subunits allowed for the formation of the 1D aggregates due investigated samples formed an M- or a P-dominant twist in a
to a strong face-to-face stacking tendency.
random manner. The average width of the twists was ca. 21 nm.
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investigate the possibilities of replacing the pyrene subunit
with other large aromatic moieties and replacing the ethynylene
linkers with vinylene subunits.
This work was financially supported by The Ministry of
Education Research Fund for the Doctoral Program and The
Ministry of Science and Technology (2013CB834501) of China.
Notes and references
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Fig. 5 CD spectra of 2c gels (red line, gel suspension) and 2c solutions
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through the p–p stacking of the pyrene subunits. The certain
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conformation, probably due to the steric hindrance arising from
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of these small helical aggregates further produced larger twists of
the P or M conformation.
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Chem. Commun.
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