Octadehydrodibenzo[12]annulene-based organogels: Two methyl ester groups prevent crystallization and promote gelation

Full text of DOI:10.1002/anie.200900501

Angewandte
Chemie
DOI: 10.1002/anie.200900501
Supramolecular Chemistry
Octadehydrodibenzo[12]annulene-Based Organogels: Two Methyl
Ester Groups Prevent Crystallization and Promote Gelation**
Ichiro Hisaki,* Hajime Shigemitsu, Yuu Sakamoto, Yasuchika Hasegawa, Yasuo Okajima,
Kazunori Nakano, Norimitsu Tohnai, and Mikiji Miyata*
Recently, p-conjugated cyclic systems involving benzene rings
and acetylene units, the dehydrobenzoannulenes (DBAs),[1,2]
have attracted substantial interest from the viewpoint of
acting as building blocks of supramolecular assemblies, along
with their optoelectronic functionality, rigid, well-defined
molecular structures, and extensively delocalized p elec-
trons.[3–6] To date, a handful of superstructures based on
triangular DBAs have been reported. They include one-
dimensional (1D) fibrous superstructures in films from Die-
derich, Nielsen et al.[3] and Iyoda et al.,[4] micelles and liquid
crystals from Tew et al.,[5] and two-dimensionally ordered
networks at the liquid–solid interface from Tobe, De Feyter
et al.[6] More recently, we also demonstrated that a single
crystal composed of one-dimensionally p-stacked DBA
exhibited highly anisotropic charge-carrier mobility.[7] How-
ever, superstructures of DBAs have not been extensively
reported except for the case of crystals,[8] and there has been
no report on organogels based on DBAs.[9] One of the reasons
for the lack of DBA superstructures is that the affinity
interactions between DBA rings are weaker than those of
other systems, such as discotic graphene-like molecules, owing
to weaker p–p interactions of DBA rings containing sp-
hybridized carbon atoms.[3] Indeed, for gelators, strong and
highly directional intermolecular interactions are required.
Herein, we describe the construction and structural
characterization of an organogel derived from boomerang-
shaped DBA 1 with methyl ester groups, which was designed
according to the strategy detailed below.
For construction of organogels, it is generally recognized
that gelators have to satisfy the following three requirements:
1) formation of 1D fibrous aggregates through self-comple-
mentary and unidirectional intermolecular interactions,
2) favorable cross-linking of 1D fibers to form 3D networked
structures, and 3) some factors to prevent crystallization and
to preserve metastable gel states. Based on these factors, the
design of low-molecular-weight organic gelators (LMOGs)
has been rationalized by introducing substituents that induce
gelation, such as steroid skeletons and long alkyl chains.[10]
These groups provide favorable van der Waals interactions
and prevent crystallization (Figure 1a). In contrast, in our
[*] Dr. I. Hisaki, H. Shigemitsu, Y. Sakamoto, Dr. N. Tohnai,
Prof. Dr. M. Miyata
Department of Material and Life Science
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita, Osaka, 565-0871 (Japan)
E-mail: hisaki@mls.eng.osaka-u.ac.jp
miyata@mls.eng.osaka-u.ac.jp
Figure 1. A comparison of the one-dimensional assembly of p-conju-
gated systems. a) Conventional typical system, with long alkyl chains.
b) Present system, with an asymmetric boomerang shape and having
polar substituents.
Prof. Dr. Y. Hasegawa, Y. Okajima
Graduate School of Materials Science
Nara Institute of Science and Technology (Japan)
strategy we focused on a dipole–dipole interaction and
molecular symmetry (Figure 1b), though a strategy based
on dipole–dipole interactions was considered by Ajayaghosh
et al.[11] We applied the following two strategies to design
DBA-based gelators: 1) to increase intermolecular affinity of
DBAs, highly-polarized substituents should be introduced in
the periphery;[3] and 2) to align the molecules anisotropically
and exclusively in a certain direction, the molecule should be
asymmetrized into a curved shape, such as the banana-shape
structure applied in liquid crystals.[12] We therefore designed
the boomerang-shaped DBA 1 described herein, which has
two methyl ester moieties in syn positions.
Dr. K. Nakano
Nagoya Municipal Industrial Research Institute (Japan)
[**] This work was financially supported by a Grant-in-Aid for Scientific
Research and the Cooperation for Innovative Technology and
Advanced Research in Evolutional Area (CITYAREA) program by the
Ministry of Education, Culture, Sports, Science, and Technology
(Japan). We thank Prof. Shu Seki and Prof. Mitsuru Akashi at the
Graduate School of Engineering, Osaka University for AFM and
PXRD measurements, respectively.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900501.
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When freshly purified
1
(synthesized according to
acetone) were subjected to scanning electron microscopy
(SEM) measurements. The gel in 1,2-dichloroethane has
intricately intertwined retiform structures, which are provided
from bundle of tape-like fibrils of about 60 nm of width
(Figure 2b). The gel in benzene has a similar morphology
(Figure 2c). The jelly precipitate from acetone also has a
fibrous structure, although the solvent could not be retained
(Figure 2d). Atomic force microscopy measurements also
support the results (Supporting Information, Figure S1).
These results indicate that 1 prefers to form fibrous super-
structures.
To obtain information pertaining to the gelation behavior
of 1 and the molecular arrangement in the fibrous super-
structures, temperature-dependent UV/Vis spectra of 1 were
measured in 1,2-dichloroethane solutions having concentra-
tions of 5.0 ꢀ 10À3 and 5.0 ꢀ 10À4 m. The former concentration
exceeds the CGC of 3.9 ꢀ 10À3 m, whereas the latter has a
much lower value. The gel was subjected to UV/Vis spectros-
copy at temperatures ranging from À5 to 408C (Figure 3a).
Upon degeneration of the gel by heating, the intensity of the
absorption bands ranging from 345 to 440 nm increased,
indicating that an aggregate forms by p–p interactions
between the annulene rings. It should be noted that the
weak shoulder at about 510 nm gradually decays, and the
band at 484 nm becomes less discernable with increasing
temperature. The decreasing band is attributed to the
energetically favored supramolecular structure in the gel,
while the increasing band is ascribable to the monomeric
species. Absorbance intensities at 355, 372, 440, and 510 nm
are plotted versus temperature (Figure 3a, inset). The tem-
perature at the end of the change corresponds to the Tg value
Scheme S1 in the Supporting Information) was dissolved in
chloroform, gelation successfully occurred, in contrast to
many cases of methyl ester substituted p-conjugated mole-
cules,[13] including DBA,[8d] which usually gave crystalline
precipitates. The detailed gelation ability of 1 was then
investigated in various organic solvents (Table 1). Chloroal-
kanes as solvent tend to result in transparent gels (Figure 2a)
with excellent CGC values (0.11% for 1,2-dichloroethane,
and Tg values of about 308C). Aromatic solvents yield opaque
gels with excellent CGCs (0.36–0.42% and Tg of 43–448C).
DBA 1 was insoluble or provided jelly precipitates in polar
solvents.
To investigate the morphology of the gels, air-dried
representative samples (a transparent gel in 1,2-dichloro-
ethane, an opaque gel in benzene, and a jelly precipitate in
Table 1: Gelation properties of 1 in selected organic solvents.
Solvent
State
CGC[a] [wt%]
Tg[b] [8C]
dichloromethane
chloroform
1,2-dichloroethane
benzene
chlorobenzene
toluene
methyl benzoate
THF
1,4-dioxane
acetone
G(T)[c]
G(T)
G(T)
G(O)[d]
G(O)
G(O)
G(O)
G(O)
G(O)
P[e]
0.24
0.30
0.11
0.36
0.42
0.31
0.38
0.51
0.36
28
25
32
43
44
43
44
44
39
ethyl acetate
triethylamine
ethanol
P
I[f]
I
diethyl ether
I
[a] CGC: critical gelation concentration. [b] Tg: gel–sol dissociation
temperature under CGC conditions. [c] G(T)=transparent gel.
[d] G(O)=opaque gel. [e] P=jelly precipitate. [f] I=insoluble.
Figure 3. Changes in spectra of 1 in 1,2-dichloroethane. a) UV/Vis
spectra recorded at various temperatures (À5 to 408C) at a concen-
tration of 5.0ꢀ10À3 m. b) Fluorescence spectra (lex =345 nm) for
various concentrations (5.0ꢀ10À6 to 5.0ꢀ10À3 m) at 258C. Inset shows
Figure 2. Optical photographs of the gels and precipitate of 1 (a), and
SEM images obtained from the air-dried gels and precipitate (b–d).
b) Transparent gel in 1,2-dichloroethane, c) opaque gel in benzene,
and d) jelly precipitate in acetone. Scale bars: 1 mm.
~
!
*
absorbance changes at 355 ( ), 372 ( ), 440 (&), and 510 ( ) nm.
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determined above (328C). The spectral changes in 5.0 ꢀ 10À3
m
solvent were not observed in the 5.0 ꢀ 10À4 m dilute solution
(Supporting Information, Figure S2).
Concentration-dependent changes in fluorescence spectra
were also determined. Figure 3b shows the normalized
spectra in 1,2-dichloroethane solution at 258C. Upon increas-
ing the concentration, bands at 411, 426, and 439 nm decay
rapidly, and in particular for concentrations higher than 1.0 ꢀ
10À4 m, which is probably due to self-absorption. At 5.0 ꢀ
10À3 m, above the CGC, a very weak band with lmax
=
551 nm remained (Fem < 0.01). The fluorescence band is
probably due to the aggregated species, in which excitation
energy migration efficiently occurs within and/or between the
fibrous assemblies, as reported in the case of the oligo(p-
phenylene vinylidene) (OPV) gels.[14] Time-resolved experi-
ments for 1 in 1,2-dichloroethane (5.0 ꢀ 10À3 m) at À58C
(Supporting Information, Figure S3) support this fact: the
lifetime t at 540 nm (1.5 ns) is shorter than that at 450 nm
(2.4 ns).
Figure 4. Crystal structures of 3. Molecular conformations and packing
diagrams for the J-type (a,b) and H-type (c,d) structures. Atoms in (a)
and (c) are drawn as thermal ellipsoids with 50% probability, except
for hydrogen atoms (sticks). Oxygen atoms are shown in dark gray.
To verify our strategy and characterize the fibrous super-
structure in more detail, DBA 1 was compared with
compounds 2 and 3. DBA 2 is the anti-isomer of 1
(synthesized according to Scheme S2 in the Supporting
Information). Naphthalene derivative 3 is formally derived
by removing two diyne groups from 1.
Noncentrosymmetric 1 has a dipole moment of 2.22–
3.58 Debye along its short axis as it has C2v symmetry,
whereas 2 in the C2h symmetric conformation has no dipole
moment (Supporting Information, Figure S4). The difference
in the dipole moment must affect the aggregation behavior.
For 1, molecular aggregation along the p-stacked direction
should be significantly accelerated by dipole–dipole interac-
tions. Such highly anisotropic growth of the aggregate should
yield no crystals, but instead yield 1D fibrous aggregates,
providing gelation. In contrast, the aggregation growth of 2 is
expected to be less anisotropic than that of 1, and results in a
3D crystalline superstructure. In fact, isomer 2 did not yield
any gel-like materials under the same conditions as 1, but did
yield crystalline precipitates by cooling of a chloroform
solution (Supporting Information, Figure S5).
Figure 5. Comparison of the PXRD patterns between a) a cryodried gel
of 1, and b) the J-type and c) the H-type crystals of 3. The patterns in
(b) and (c) are simulated on the basis of the single-crystal diffraction
data.
According to our strategy, boomerang-shaped ester 3
should also yield 1D fibrous aggregates as in the case of 1. In
contrary to the gelation found for 1, however, slow evapo-
ration of a solution of 3 dissolved in chloroform/methanol
yielded crystalline precipitates that contain two polymorphic
crystals: a majority being columnar crystals, and the remain-
der platelet crystals. Crystallographic analysis revealed that
molecule in the former, which has a down–down carbonyl
conformation (Figure 4a), arranges in a slipped p-stacked
fashion (i.e., J aggregation),[15] whereas those in the latter,
with an up–up conformation (Figure 4c), arrange in an
alternately p-stacked fashion (i.e., H aggregation)[16] such as
to cancel out their dipole moments. Thus, the up–up
conformation is a key structure for the J-type stacking.
Comparison of the powder X-ray diffraction (PXRD) pat-
terns of the cryodried gel of 1 with that of the crystals of 3
revealed that the gel has a similar pattern to that of the J-type
crystal, rather than the H-type (Figure 5). The peaks from the
gel at 2q = 14.8, 15.5, 23.5, and 27.48 (d spacing: 6.1, 5.7, 3.8,
and 3.3 ꢁ, respectively) in Figure 5a correspond to the peaks
of (010), (011), (10 Æ 1), and (11 Æ 1) planes of the J-type
crystal in Figure 5b: the former two peaks are attributed to
the periodic distances related to the molecular length along
the shorter axis, and the latter two to the periodic distance in
the p-stacking direction.[17] The peaks in the small angle
region at 2q = 4.3, 6.6, and 9.28 (d spacing: 21.0, 13.5, and
10.8 ꢁ) are related to the molecular length along the longer
axis. Therefore, 1 is expected to arrange mainly in the J-
aggregated fashion to form fibrous superstructures. The
reason that 3 did not give a gel despite its desired 1D
molecular arrangement is that exclusive 1D elongation of the
superstructure did not occur because its aromatic core is
smaller than that of 1.
Interestingly, when the methyl groups in 1 were replaced
by longer alkyl chains, such as ethyl, propyl, and butyl
groups,[18] the resulting annulenes gave no organogels but
rather crystalline precipitates. Preliminary X-ray crystallo-
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temperature (in the cases of benzene and 1,4-dioxane) or À198C
(for other solvents) and allowed to stand for 10–15 min. The
formation of the gels was evaluated by determining whether they
were stable to inversion in the test tube. The gel-destruction
temperature was determined by the dropping-ball method (rate of
temperature increase: ca. 18CminÀ1). A glass ball weighing 0.18 g was
placed on the gel surface, and the test tube was heated in a water bath.
The temperature (Tg) was noted when the ball fell to the bottom of
the test tube. See the Supporting Information for other detailed
experiments.
graphic analysis on single crystals of the ethyl and n-propyl
ester derivatives revealed that the esters did not have a down–
down conformation, and the DBAs no longer aggregated in
the J-type assemblies owing to steric hindrance of the ester
moieties (Supporting Information, Figure S7). Thus, methyl
ester groups are also a crucial factor for gelation.
The gelation of 1 is assumed to occur in the following way.
One appropriate conformer of 1, that is, the down–down
conformer, and having a boomerang-shaped structure, stacks
along a certain direction, with uniform molecular direction-
ality, by dipole–dipole and p–p interactions (Figure 6a).
Received: January 26, 2009
Revised: March 19, 2009
Published online: May 26, 2009
Keywords: annulenes · dipole–dipole interactions · gels ·
.
self-assembly · supramolecular chemistry
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Figure 6. a) Representation of the formation of the 1D columnar
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Elongation takes place exclusively in the p-stacking direction
because of effective dipole–dipole interactions to yield a 1D
fibrous superstructure (Figure 6b). The fibers then construct
a 3D network to form the gel.
In summary, we have described the formation and
characterization of an organogel provided by boomerang-
shaped DBA 1, which was designed and synthesized based on
the strategy that focuses on dipole–dipole interactions and
asymmetrization of the molecule. Despite a lack of long alkyl
chains but rather equipped with only two methyl ester groups,
1 acts as an excellent LMOG for various organic solvents. To
the best of our knowledge, there has been no examples of
LMOGs that have a p-conjugated core and only methyl ester
groups.[19] As DBA 1 has a reactive distorted butadiyne
moiety, 1 could undergo intermolecular reactions, such as
polymerization, converting the self-assembled superstruc-
tures into covalently-bonded nanostructures. Current efforts
are now focused on surveying the physical properties and
conversion of the gel into polymeric materials.
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Experimental Section
Preparation of gels: Compound 1, along with an appropriate solvent,
was placed in a test tube and sealed and heated until the compound
was dissolved. The solution was then rapidly cooled to room
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[11] For gelation by dipole–dipole interactions, see: A. Ajayaghosh,
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can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[16] Crystal data for the H-type crystal of 3: C14H12O4, Mr = 244.25,
a = 7.2916(5), b = 7.5568(6), c = 12.0759(12) ꢁ, a = 102.755(5),
b = 93.669(4), g = 115.632(5)8, V= 574.40(8) ꢁ3, T= 153 K, tri-
clinic, space group P1 (No. 2), Z = 2, m(CuKa) 0.865 mmÀ1, 1calcd
=
ꢀ
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1.410 gcmÀ3, 6172 collected, 2053 unique (Rint = 0.075) reflec-
tions, Final R1 = 0.058 (I > 2.0s(I)) and wR2 = 0.164 (all data).
CCDC 725720 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[17] When absolute value of l in the (hkl) plane is larger than 1, for
example (013), (10 Æ 2), (103), (11 Æ 2), and (11À3) in pat-
tern (b), the corresponding peak was not or weakly observed in
pattern (a), probably because of irregularly, slippage, and/or the
limited dimension of the aggregate.
[18] Transesterification from the methyl ester to other esters was
carried out concurrently with the Glaser reaction by using the
corresponding alcohols in the cross-coupling reactions (see the
Supporting Information).
[15] Crystal data for the J-type crystal of 3: C14H12O4, Mr = 244.25,
a = 3.8914(5), b = 6.0548(7), c = 24.256(3) ꢁ, b = 90.532(6), V=
571.47(11) ꢁ3, T= 153 K, monoclinic, space group P21 (No. 4),
Z = 2, m(CuKa) 0.871 mmÀ1, 1calcd = 1.419 gcmÀ3, 6540 collected,
2068 unique (Rint = 0.075) reflections. Final R1 = 0.058 (I >
2.0s(I)) and wR2 = 0.145 (all data). CCDC 725719 contains the
supplementary crystallographic data for this paper. These data
[19] For examples of organogelators without typical gelation-induc-
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H.-S. Song, S.-Y. Park, J. Am. Chem. Soc. 2004, 126, 10232 –
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