A hexaazatriphenylene-based organogel that responds to silver(I) with high selectivity under aqueous condition

Article abstract of DOI:10.1016/j.tetlet.2012.01.137

A hexaazatriphenylene (HAT)-based gelator was developed. It exhibits excellent gelation ability. The resulting organogel shows very high selectivity toward Ag⁺ ion under aqueous condition in the form of gel-sol transformation. The mechanism of such transformation was also investigated through UV-vis spectroscopy and powder X-ray diffraction.

Full text of DOI:10.1016/j.tetlet.2012.01.137

Tetrahedron Letters 53 (2012) 1840–1842
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Tetrahedron Letters
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A hexaazatriphenylene-based organogel that responds to silver(I)
with high selectivity under aqueous condition
⇑
⇑
Zhi-Gang Tao, Xin Zhao , Xi-Kui Jiang, Zhan-Ting Li
State Key Lab of Bioorganic and Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A hexaazatriphenylene (HAT)-based gelator was developed. It exhibits excellent gelation ability. The
resulting organogel shows very high selectivity toward Ag ion under aqueous condition in the form of
gel–sol transformation. The mechanism of such transformation was also investigated through UV–vis
spectroscopy and powder X-ray diffraction.
Received 3 November 2011
Revised 18 January 2012
Accepted 31 January 2012
Available online 8 February 2012
+
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Organogel
Responsive
Hexaazatriphenylene
Silver ion
Gel–sol transformation
Organogels, a family of jelly-like assembled structures at the
macroscale in which specific solvents are immobilized in three-
dimensional entangled networks created by the self-assembly of
gelator molecules, have drawn considerable attention in the past
decades for their potential applications in many aspects of materi-
HAT is a disc-like aromatic heterocycle, which was extensively
exploited as n-type semiconductors, magnetic materials, nonlinear
optic chromophores, liquid crystals, micro/nano architectures, and
8
ligands for coordination complexes. For the present study, hexa-
octylhexaazatri-phenylene (HOHAT) was designed and prepared.
We envisioned that the strong stacking between the large planar
HAT core and the van der Waals forces of the six aliphatic chains
1
als science from drug delivery, sensors to nanodevices. The driving
forces that direct the self-assembly of gelator molecules are weak
intermolecular interactions such as hydrogen bonding,
p
-stacking,
3
of this C -symmetric molecule would greatly facilitate the forma-
9
van der Waals force, host–guest interaction, or metal coordination.
Organogels are ideal in the fabrication of smart materials, because
they can be responsive to external stimuli in the form of gel–sol
transformation (or reverse), as a result of the change of intermolec-
ular interactions between the gelator molecules when a stimulus is
tion of organogels. HOHAT was prepared via the condensation of
hexaaminobenzene and 9,10-octadecanedione (see ESI for details).
At room temperature, it exhibited excellent solubility in aliphatic
solvents or halohydrocarbons, but gelated many other organic
solvents of medium to high polarity, which suggested that its
aggregating tendency was promoted by solvophobic interaction
between the peripheral aliphatic chains and solvent molecules. Fif-
teen solvents were tested using the ‘inverse flow’ method,10 and
the results are summarized in Table 1. It can be seen that HOHAT
gelated eleven of the tested solvents, among which ethanol exhib-
ited the lowest critical gelation concentration. The morphology of
the resulting organogels was investigated by the scanning electron
microscopy (SEM) and atomic force microscopy (AFM). The images
clearly showed the entangled fibers of micrometers in length and
hundreds of nanometers in width, which is characteristic of a
typical gel state (Fig. 1a–b and ESI).
2
3
imposed. Recently, organogels capable of responding to light, pH,
4
5
6
redox, ions, and chiral molecules have been reported. Herein we
report a hexaazatriphenylene (HAT)-based organogel that selec-
+
tively responds to Ag ion, a cation that has been actively exploited
as antibacterial reagent.
7
C H
8
17
C H
8
17
N
N
N
N
N
C H
8
17
17
C H
8
N
Since HAT is a good ligand for metal-organic coordination inter-
action, we anticipated that a metal ion responsive organolgel
might be fabricated from HOHAT because the introduction of metal
ions might result in the formation of discrete metal–HOHAT com-
plexes which might lead to the deconstruction of the organogels
due to the collapse of the well-ordered molecular arrangement of
C H
8
17
8f
HOHAT
C H
8
17
⇑
Corresponding authors. Tel.: +86 21 54925023; fax: +86 21 64166128.
E-mail addresses: xzhao@mail.sioc.ac.cn (X. Zhao), ztli@mail.sioc.ac.cn (Z.-T. Li).
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.137

Z.-G. Tao et al. / Tetrahedron Letters 53 (2012) 1840–1842
1841
Table 1
Gelation Property of HOHAT
The formation of the organogels was proposed to be driven by
the aromatic stacking of the HAT units and the van der Waals force
among the octyl groups, which induced the formation of one-
dimensional fibrous structures which further aggregated into
three-dimensional networks to immobilize the solvent molecules.
UV–vis experiments were further performed to investigate the
arrangement of HOHAT in the gel state. Compared to that in hex-
ane in which both the aromatic stacking and the van der Waals
force are considered relatively weak, the maximum absorption
(276 nm) of the native HOHAT gel of ethanol displayed a red-shift
of 4 nm (Fig. 3), which might be attributed to a J-type arrange-
Entry
Solvent
Statea
CGCb
1
2
3
4
5
6
7
8
9
Decaline
S
S
S
n
Ethyl acetate
Tetrahydrofuran
1,4-Dioxane
Nitromethane
Acetonitrile
n
n
G
PG
PG
G
G
G
G
G
G
G
G
P
23.0
n
n
Acetone
14.3
21.7
14.3
13.2
20.6
25.6
6.61
5.51
n
Dimethyl sulfoxide
Dimethylformamide
Dimethylacetamide
Butanone
n-Butanol
Methanol
10
11
12
13
14
15
12
+
ment. After the addition of the Ag salt, the resulting solution
exhibited a maximum absorption at 287 nm (Fig. 3), which could
+
be attributed to the binding between HOHAT and Ag ions. In con-
Ethanol
Metanol/water (4:1)
trast, upon the addition of alkali and alkaline-earth metal ions such
+
2+
2+
as Na , Ca and Mg , the wavelength of the maximum absorption
of the samples, which were still in the gel state, displayed almost
no change (see ESI), reflecting the fact that alkali and alkaline-earth
metal ions hardly display coordinating ability to HAT. Upon the
addition of the other transition metal ions, red-shifting of the max-
imum absorption of the gel was observed, albeit in a smaller extent
a
G, P, PG, and S denote gelation, precipitation, part gelation and solution,
respectively.
b
The critical gelation concentration (mM).
(
see ESI), indicating that weaker metal-organic coordination
a
b
existed in the systems.
The samples obtained before and after the addition of aqueous
3
AgNO were also characterized with the powder X-ray diffraction
(
XRD). The XRD of the xerogel of HOHAT fabricated with ethanol
exhibited several strong peaks (Fig. 4). The peak with a distance
of 2.26 nm could be assigned to the molecular width of HOHAT,
which has a size of 2.3 nm in extended conformation as shown
by CPK modeling. The peak at 0.45 nm should be attributed to
the ordered aggregation of the aliphatic side-chains of the discot-
8
a
ics. A broad peak at 0.35 nm was also observed, which is typically
produced by the aromatic stacking, and here by that of the HAT
Figure 1. (a) SEM and (b) AFM images of the xerogel fabricated with ethanol.
13
3
core. After the addition of 3 equiv of aqueous AgNO , the peaks
the gelators in the native organogels. Aqueous solutions of various
metal nitrates were chosen because nitrates have very good solu-
bility in water. The experiments were carried out by putting a layer
of aqueous solution of each metal nitrate (1.0 M, 10-fold) onto the
top of the preformed ethanol organogel of HOHAT (11 mM) at
room temperature. Fifteen nitrates were tested, with the metals
ranging from alkali, alkaline-earth to transition metals, and at least
at 0.45 and 0.35 nm disappeared (see ESI), indicating that the
ordered stacking of the HOHAT molecules was destroyed by the
coordination of Ag ions. In contrast, the XRDs of the gel samples
+
3 3 2 3 2
after the addition of aqueous NaNO , Zn(NO ) , and Co(NO ) ,
showed almost the identical diffraction patterns as the native gel
(Fig. 4 and ESI), indicating that the interactions between HOHAT
and these cations, if any, is too weak to destroy the molecular
packing of HOHAT in the gel state.
The nitrogen atoms of the HAT unit might be protonized by
strong acids, which might also lead to deconstruction of the organ-
ogel, as a result of the electrostatic repulsion between protonized
HAT units. Therefore, the influence of acid on the HOHAT gel was
also examined. Unexpectedly, no disruption of the gel was
one chosen from each subgroup B of the periodic table of elements
+
(
except the VB group).11 Remarkably, only Ag ion was found capa-
3
ble of breaking the organogel. Once aqueous AgNO was added,
part corrosion of the gel was observed immediately. Within 0.5 h,
the organogel turned into a clear solution. The organogel could also
3
be destroyed slowly when just 1 equiv of AgNO was empolyed.
However, for all other fourteen nitrates, the gel state with ethanol
was found to be preserved even 48 h after the addition of their
aqueous solutions, although the diffusion of the solution of the
salts into the organogel was observed (see ESI). SEM investigation
revealed that no fibrous structures were observed for the sample
observed upon the addition of an excess amount of HNO
CF CO H, or even stronger CF SO H, again reflecting the high selec-
tivity of the HOHAT gel in response to the outside stimuli.
The high selectivity of HOHAT gel toward Ag ion might be
ascribed to the strong complexation between HOHAT and Ag+ in
aqueous medium. The similar complexation for the other metal
cations examined, if any, should be too weak in water to inhibit
the stacking of HOHAT. To obtain more evidence for this assump-
tion, control experiments were carried out. The solution of
3
,
3
2
3
3
+
3
after the addition of aqueous AgNO (Fig. 2a), also suggests total
destruction of the well-ordered molecular aggregates. For the cases
of all other salts, however, the fibrous structures of the organogels
showed almost no change (see ESI), after the samples were stood
for 48 h after the addition of aqueous solutions of the nitrates. Fur-
3 2 3 2 3 2
Co(NO ) , Cu(NO ) , or Ni(NO ) in ethanol, instead of water, was
+
thermore, in the case of the Ag salt, the transformation between
added on the top of the native HOHAT gels, respectively. The gel-
to-sol transformation was observed for all the three samples in
2 h, although the rate of the deaggregation of the organogels was
the gel and solution states was reversible. After the deaggregation
of the organogel by the introduction of aqueous AgNO , adding
3
+
aqueous NaI solution to the resulting solution led to the formation
of yellow AgI precipitate. The mixture was heated to reflux and
then cooled down to room temperature slowly. An organogel with
AgI being entrapped was generated again (Fig. 2b–d). This process
can be repeated several times without observing the failure of the
formation of the organogel.
slower than that in the presence of Ag ion. SEM investigation also
revealed total deconstruction of the fibrous structures, as observed
+
upon the addition of Ag ion.
In conclusion, a novel HAT-based organogel, which selectively
+
responds to Ag ion in aqueous medium, has been developed. Such
organogel might be further exploited as a gel-based sensor to

1
842
Z.-G. Tao et al. / Tetrahedron Letters 53 (2012) 1840–1842
a
c
d
b
Ag+
I-
Figure 2. (a) SEM images of the sample after the addition of a solution of aqueous AgNO
solution phase, mediated by Ag and I , respectively.
3
, and (b–d) pictures illustrating the reversible transformation between gel phase and
+
ꢀ
organogels that can selectively (or even exclusively) respond to a
certain metal cation can be expected, providing gelators integrated
with highly selective recognition sites to given metal ions are
developed. In this way, new gel-based sensing systems or smart
soft materials can be fabricated.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
Acknowledgments
We thank NSFC (Nos. 21172249, 20972180, 20921091 and
2
0974118) and the Science and Technology Commission of Shang-
hai Municipality (10PJ1412200) for financial support.
Supplementary data
250
300
350
400
Wavelength (nm)
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2012.01.137.
Figure 3. Normalized UV–vis spectra of HOHAT in hexane (black), in gel of ethanol
red), and in the solution of ethanol after the addition of AgNO (blue).
(
3
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1