Development of chemical stimuli-responsive organogel using boronate ester-substituted cyelotrieatechylene

Article abstract of DOI:10.1246/cl.2008.1238

This study describes the synthesis of a new compound: boronate ester-substituted cyelotrieateehylene 1. The structure-directing property of the boronic acid-diol interactions is responsible for the shuttlecock-shaped structure of 1, which led to the gelation of several solvents. The distribution control of trigonal and tetrahedral boronate esters allows phase transition via chemical stimulus. Copyright

Full text of DOI:10.1246/cl.2008.1238

1238
Chemistry Letters Vol.37, No.12 (2008)
Development of Chemical Stimuli-responsive Organogel
Using Boronate Ester-substituted Cyclotricatechylene
Yuji Kubo,ꢀ1 Wataru Yoshizumi,2 and Tsuyoshi Minami1
1Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University,
1-1 Minami-ohsawa, Hachioji, Tokyo 192-0397
2Department of Applied Chemistry, Graduate School of Science and Engineering, Saitama University,
255 Shimo-ohkubo, Sakura-ku, Saitama 338-8570
(Received September 8, 2008; CL-080849; E-mail: yujik@tmu.ac.jp)
This study describes the synthesis of a new compound: bo-
observed that 1 was insoluble in nonpolar and protic solvents,
while it was soluble in several less polar solvents such as tol-
uene, THF, CHCl3, and CH2Cl2. Next, it was found that 1 was
responsible for the gelation of the solvents. On estimating the
critical gelation concentration (CGC) of the solvents, the lowest
CGC value was obtained: 0.46 wt % when toluene was used.17
The morphological analysis of toluene xerogel was performed
by field-emission scanning electron microscopy (FE-SEM); a
ribbon-phase with a ca. 330 nm width was obtained (Figure 1a).
Further, fine striations of 10–20 nm width were clearly observed
on the ribbons; this is attributable to the molecular packing of
toluene xerogel. The self-assembly of toluene xerogel was ana-
lyzed using X-ray diffraction (XRD) (Figure 1b). A sharp d10
peak and low-intensity d11, d20, and d21 peaks were observed
in the XRD spectra. These peaks could be assigned on the basis
ronate ester-substituted cyclotricatechylene 1. The structure-di-
recting property of the boronic acid–diol interactions is respon-
sible for the shuttlecock-shaped structure of 1, which led to the
gelation of several solvents. The distribution control of trigonal
and tetrahedral boronate esters allows phase transition via chem-
ical stimulus.
Dynamic covalent functionality has been recognized as a
powerful mechanism for the construction of organized architec-
tures in supramolecular chemistry.1 Reversible bond formation
enables the thermodynamic control of the reaction under equilib-
rium so that only the thermodynamically favored entity is
formed as the major product. It is also interesting to note that
an external chemical stimulus could have a significant effect
on the equilibrium of the reaction, which could induce a change
in the product distribution; this has motivated researchers to de-
sign soft materials. Boronic acids exhibit dynamic covalent
functionality, which enables them to rapidly and reversibly form
cyclic boronate esters with diols.2 The formation of these esters
has been advantageous in not only the binding of saccharides3
and polyphenols4 but also the development of unique architec-
tures such as macrocycles,5 porous covalent organic frame-
works,6 and polymers.7 On combining the properties of boronic
acids and Lewis acidic boron,8 self-organized anion chemosen-
sor systems containing diol dye9 and a dynamic molecular cap-
sule are formed.10 The latter can be controlled by a pH switch.
Although much attention has been currently focused on the prep-
aration of boron-based self-assembly, organogels formed from
boronate ester2 remain to be explored from the standpoint of de-
veloping chemical stimuli-responsive smart gels.11 Here we re-
port the synthesis of a new compound: boronate ester-substituted
cyclotricatechylene 1; it is composed of cyclotricatechylene and
4-(4-octyloxybenzeneoxymethyl)phenylborane. We chose cy-
clotricatechylene as one of the components of the newly devel-
oped compound because it has a rigid and cone-shaped molecu-
lar structure that is capable of serving not only as a receptor12 but
also as a skeleton of liquid crystals13 and organogelators.14 We
reasoned that the C3-symmetrical catechol segments of cyclotri-
catechylene could easily participate in boronate esterification to
produce our desired system.
of a hexagonal columnar aggregate.18 The diameter a (¼2 ꢁ
pffiffi
˚
d10= 3) of the gel estimated to be 28.5 A is smaller than the
lateral dimension of the shuttlecock-shaped structure of 1. This
difference in size can be explained by intercolumnar interdigita-
R
O
O
(a)
ca. 330 nm
O
R
O
R
fine striations
O
B
O
R = B
O
1
H17
OC8
O
O
O
OC8H17
2
(b)
10000
8000
6000
5.3 Å
d(10)
4000
2000
0
d(11)
d(20)
d(21)
0
5
10
2
15
20
25
The synthesis of 1 was straightforward (Scheme S115) and
was observed by NMR and MALDI-TOFMS; m=z ¼ 1349:632
[1 + Na]þ, Calcd. for C84H93B3O12Na: 1349.687. Compound
2 was also synthesized as a control.15 Gelation tests16 were
carried out using conventional solvents (Table S115). It was
/ deg
Figure 1. (a) FE-SEM image of xerogel of 1. The scale bar
corresponds to 1.0 mm. (b) XRD pattern of i) xerogel of 1 and
ii) blank at room temperature. The xerogels were prepared from
toluene (2.5 wt %).
Copyright ꢀ 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.12 (2008)
1239
(a)
(c)
ca. 28 Å
(b)
5.3 Å
Figure 3. Images of the toluene gel of 1, which responded to
Et2NH. (a) Toluene gel of 1 (1.5 wt %). (b) Toluene gel of 1 con-
taining Et2NH (3 equiv for 1); the gel was then sonicated for
3 min. (c) Solution (b) was evaporated, and its residue was dried
at 90 ꢃC in vacuo, cooled, and dissolved in toluene.
Figure 2. Possible molecular packing structure of toluene gel.
This research has been partly supported by a Grant-in-Aid
for Scientific Research (C) (No. 18550116) from the Ministry
of Education, Culture, Sports, Science and Technology of Japan.
We thank Prof. H. Yoshida, Tokyo Metropolitan University, for
the assistance of XRD measurement.
tion of the alkyl chains through van der Waals interactions. A
˚
broad peak around 5.3 A in the wide-angle region of the XRD
pattern can be assigned to molecular stacking within the col-
umns. Figure 2 shows a possible molecular packing structure
of toluene gel.
References and Notes
As the next stage, we were interested in investigating wheth-
er the obtained gel would exhibit a phase transition in response
to chemical stimulus such as anions. We reasoned that the chem-
ical stimulus capable of binding trigonal boronate ester in the
system would induce a change in its coordination mode, thereby
bringing about a change in the morphology of the gel. As a
standard examination method, CHCl3 solution (7.5 mL) contain-
ing AcOꢂ (4:5 ꢁ 10ꢂ3 mmol; 3 equiv for 1) as tetrabutylammo-
nium salt as putative anions9a was added to the toluene gel of 1
(10 mM, 1.5 wt %); the resulting mixture was sonicated for
3 min. Subsequently, the use of AcOꢂ resulted in a gel-to-sol
transition at room temperature (Figure S115), which can be ex-
plained on the basis of an anion-triggered sp2 ! sp3 conversion
of the boron element in 1. The reaction was also confirmed by
the 1H NMR spectrum of the sol phase. Because of the presence
of AcOꢂ, proton resonances arising from phenyl boronate in 1
were clearly distinguishable signals; one set of the signals is ob-
served at 7.88 (d, J ¼ 7:5 Hz) and 7.36 (d, J ¼ 7:3 Hz), while a
set of broad signals is observed between 7.67–7.46 and 7.23–
7.04 ppm (Figure S115). The sol phase may contain disintegrated
1 and acetate-triggered sp3-boronate derivatives, indicating that
the boronate ester segments play a significant role in the gel-to-
sol transition. Further, the chemical stimulus-responsive transi-
tion is not limited by the addition of anions; when Et2NH (3
equiv) was used as chemical stimuli under similar conditions,
gel-to-sol transition occurred. It is noteworthy that regelation oc-
curred owing to a treatment involving heat under reduced pres-
sure (Figure 3). The removal of Et2NH enables the reaction equi-
librium to shift toward the formation of trigonal boronate ester 1.
Thus, reversible sol–gel transition in the system was achieved.
In conclusion, we present for the first time a boronate-ester-
type organogelator which shows a phase transition in response to
a chemical-stimulus such as anion and amine. The phase transi-
tion is due to a change in the coordination mode of boron in 1.
The results obtained in this study could provide significant infor-
mation on how boron-based dynamic covalent functionality can
be employed to prepare smart gels. The development of an en-
hanced system having optical functions is in progress according
to this strategy.
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Published on the web (Advance View) November 8, 2008; doi:10.1246/cl.2008.1238