Superbenzene-bridged bis(permethyl-β-cyclodextrin) as a convenient and effective probe for trinitrophenol exploder

Article abstract of DOI:10.1039/c6tc05121a

A novel planar C₂ symmetrical superbenzene-bridged bis(permethyl-β-CD) was synthesized by grafting two permethyl-β-CD units onto a coronene core. Its self-assembly and luminescent behaviors could be smartly controlled by adjusting the polarity

Full text of DOI:10.1039/c6tc05121a

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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Superbenzene-Bridged Bis(permethyl-β-cyclodextrin) as
Convenient and Effective Probe for Trinitrophenol Exploder
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
ab
Jie Yu, Yong Chen, Jing-Jing Li, and Yu Liu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel planar C
2
symmetrical superbenzene-bridged bis(perm-
On the other hand, cyclodextrins (CDs), a kind of cyclic
ethyl-β-CD) was synthesized by grafting two permethyl-β-CD units oligosaccharides, possess good water solubility and low
onto a coronene core. Its self-assembly and luminescent behaviors toxicity and are widely applied in many fields of drug delivery,
7
could be smartly controlled by adjusting the polarity of cellular imaging and fluorescence sensing so on. Recently, in
environment. Significantly, the resultant self-assembly showed our group, a series of supramolecular assemblies have been
the specific fluorescence responses to exploders especially 2,4,6- constructed from cyclodextrin modified perylene, which can
trinitrophenol over various common aromatic compounds, which be used as a kind of fluorescence sensing probes for organic
8
could be readily distinguished either in solution or on the test vapor detection.
paper, and the detection limit of self-assembly towards 2,4,6-
trinitrophenol could reach a ppb level. This finding would enable
the self-assembly as a convenient and highly efficient fluorescence
sensor for the detection of exploders.
As a kind of fascinating organic molecules, polycyclic aromatic
hydrocarbons (PAHs) have drawn much research interest to
1
investigate their special properties and possible applications.
Due to the strong π-π stacking interactions, many PAHs are
inclined to self-assembly to form columnar mesophases, and
charge carrier migration will be occurred among the π-π
2
stacked PAHs. Therefore, the supramolecular assembly of
PAHs showed many exciting applications in fields of organic
3
electronics. As an important member of PAHs, hexa-peri-
hexabenzocoronene (HBC), also named superbenzene, is
4
widely regarded as the smallest graphene unit, and attracted Scheme 1. Supramolecular assembly of PHBC.
more and more attentions in organic electronics, photovoltaics,
photonics, chemical sensors, anticancer drugs and cellular
,6
In this work, we synthesized a superbenzene-bridged
5
imaging agents. Kumar et al synthesized two HBC derivatives bis(permethyl-β-CD) (PHBC, Scheme 1) by attaching two
for the detection of picric acid. Zhang et al reported a three- permethyl-β-CD units to a HBC core. A comprehensive
dimensional nanographene/triptycene conjugate for the investigation by means of UV-vis spectroscopy, fluorescence
detection of nitroaromatic explosives. However, most of spectroscopy, transmission electron microscopy(TEM) and
these works were performed in organic solvents, which scanning electron microscopy(SEM) demonstrated that the
6
a
6
d
inevitably limited their practical application.
self-assembly and luminescent behaviors of PHBC could be
smartly controlled by adjusting the polarity of environment. It
is noteworthy that the resultant self-assembly showed the
specific fluorescence responses to exploders including 2,4,6-
trinitrophenol (TNP), 2,4-dinitrophenol, and 1-methyl-2,4-
dinitrobenzene, which could be readily distinguished either in
solution or on the test paper. Significantly, the detection limit
of self-assembly towards 2,4,6-trinitrophenol could reach 306
ppb in solution and 2 ppb on the test paper, which would
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enable the potential application of PHBC as a highly sensitive be conveniently investigated by means of UV-vis and
and highly selective probe for the detection of nitroaromatic fluorescence spectroscopy. As shown in Figure 1a, the UV-vis
exploders.
spectrum of PHBC exhibited three typical absorption bands (α,
β and p bands), which was consistent with the reported by
2
c
Müllen et al. Typically, PHBC displayed an absorption
maximum peak at 360 nm and fine absorption peaks at p band
in dichloromethane, indicating the existence of un-aggregated
1
0,4a
PHBC monomers in dichloromethane.
Moreover, the
similar absorption features of PHBC were observed in acetone
and methylbenzene, but the intensity of absorption decreased.
These results might be attributed to the π-π stacking
interaction leading to partial aggregation in acetone and
methylbenzene. Furthermore, with the increasing of solvent
polarity, an obvious decrease of absorption intensity was
observed, accompanied by the disappearance of fine
absorption peaks (from 300 nm to 450 nm), probably owing to
the aggregation of PHBC in polar solvents such as acetonitrile,
1
0
methanol and methanol/H O.
2
Scheme 2. Synthetic route to PHBC.
The synthetic route of superbenzene-bridged bis(permethyl-
β-CD) was shown in Scheme 2. Therein, hexaphenylbenzene
derivative
3
and 6-deoxy-6-azido-permethyl-β-CD were
synthesized in good yields according to the literature reported
9
methods. C
2
symmetrical compound
4
was obtained by Suzuki
with 4-
4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phenol. However,
coupling reaction of hexaphenylbenzene derivative
3
(
due to the rigid planar and C
2
symmetrical properties of
-5
Figure 1. a) UV-vis spectra of PHBC (1.0×10 M) in different solvents at 25°C, and b)
compound
solvents. Furthermore, owning to the attached phenol group in
compound , making it is easy to suffer from oxidation. Then,
the crude compound , which was insoluble in many common
4
, which was insoluble in many common organic
-6
emission spectra of PHBC (1.0×10 M) in different solvents at 25 °C (λex=365 nm).
Similarly, the fluorescence spectra of PHBC also showed the
fine structures in the wavelength region of 450-540 nm in
dichloromethane, acetone and methylbenzene (Figure 1b and
Figure S10), where the emission peaks at 470 nm and 493 nm
4
4
organic solvents and easy to be oxidized, was directly used
without further purification for the reaction with 3-bromo-1-
11
maybe assigned to the fluorescence of PHBC monomers.
propyne in the presence of K
Subsequently, by means of “click chemistry” method, the
intermediate reacted with an excess amount of 6-deoxy-6-
azido-permethyl-β-CD in the presence of CuI to give
permethyl-β-CD-modified hexaphenylbenzene in 78% yield.
Finally, through a Scholl oxidation reaction catalyzed by
anhydrous FeCl PHBC was successfully synthesized from in
3% yield. The composition and structure of and PHBC were
fully characterized by H NMR, C NMR and MALDI-TOF
2 3
CO to give the intermediate 5.
However, when the solvent became more polar, these
emission peaks red-shifted to 525 nm assigned to the excimer
emission, accompanied by the disappearance of fine
5
11
structures. In addition, the polarity-induced change of
6
aggregation state could readily be distinguished by naked eyes.
As seen in Figure 2, the color of PHBC solution varied from
green to yellow with the increase of solvent polarity.
Furthermore, the fluorescence life-time of PHBC was also
3
,
6
6
6
1
13
investigated in different solvents including PhCH
3
, DCM,
(Figures S1–S8).
acetone, CH CN, CH OH and CH OH/H O (1/9). The results
2
3
3
3
Owning to the good solubility of permethyl-β-CD, the
showed that PHBC displayed the significantly longer lifetime
photophysical properties of PHBC in different solvents could
and a non-monoexponential fluorescence decay especially in
2
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methanol/water (1/9), indicating the higher interchain
interactions (e.g. with formation of weakly-coupled aggregates
or excimers) and reduced rigidity (Figure S29).
-
6
Figure 4. a) Quenching efficiency of PHBC (1.0×10 M, λex=365 nm) with TNP at
-
6
different solvents, b) Emission spectra of PHBC (1.0×10 M in CH
3 2
OH/H O=1/9, λex=365
nm) with TNP at various concentrations.
Subsequently, the molecular recognition of PHBC to
exploders was investigated by means of fluorescence
spectroscopy using 2,4,6-trinitrophenol (TNP) as a model
substrate. Typically, with the addition of TNP (0-120 eq), the
fluorescence of PHBC gradually quenched, which could be
easily observed by either the fluorescence spectroscopy or the
Figure 2. The color of PHBC in different solvents under (a)visible light and (b)UV light
(
365 nm): A) toluene, B) dichloromethane, C) acetone, D) acetonitrile, E) methanol, F)
methanol/H O=1/9.
2
High-resolution transmission electronic microscopy (HR-TEM) naked eyes (Figure 2 and Figure 4). Moreover, the color of the
and scanning electron microscopic (SEM) were applied to solution turned from colorless to yellow (Figure 2). The
0 0
investigate the morphology of PHBC assembly in methanol and quenching efficiency ((I -I)/I )×100%) in different solvents were
methanol/water (1:9). The TEM images (Figure 3a) in methanol also calculated under the same condition, and the highest
showed a number of needle-like nanostructures with average
quenching efficiency was up to 94.3% in methanol/water (1:9).
A possible reason for the fluorescence quenching may be
attributed to the penetration of TNP into the self-aggregated
PHBC via the π-π stacking interactions, leading to the charge
transfer from PHBC (electron donor) to the adjacent TNP
6
a,6d
(electron acceptor).
Through a quantitative study, the self-
4
−1
assembly of PHBC gave a KSV constant as 1.7×10 M and a
detection limits up to 306 ppb (Figure S22 and S23) for TNP in
methanol/water(1:9), which was lower than the corresponding
1
2
value of reduced graphene oxide. It was reported that the
exciton migration would be enhanced by the longer-range
molecular aggregation (due to the stronger π-π interaction),
then fewer analyte molecules could effectively disturb the
exciton migration and further result in the fluorescence
Figure 3. TEM (a, c) and SEM (b, d) images of PHBC in methanol (a, b) and
13
quenching. In addition, the intermolecular distance might
2
methol/H O=1/9 (c, d).
also be influenced by the inclusion of analytes into the
permethyl-β-CD cavity and then caused the change of
13a
length of 112 nm and an average width of 17 nm. When
altering the solvent from methanol to methanol/water (Figure
fluorescence. As a result, the highly self-aggregated PHBC
3
c), the original needle-like nanostructures changed to rod-like
exhibited the good quenching behavior, great Stern-Volmer
constants and low detection limit in methanol/water.
ones, and their averaged length and width increased to 200
nm and 67 nm, respectively. The similar morphological change
of PHBC was also observed in SEM images (Figure 3b, 3d). A
possible reason may be the solvent-dependent self-assembly
behaviors of PHBC, that PHBC tended to exist as monomers in
the weakly polar solvents but tended to form large self-
assemblies through the π-π interactions among planar
superbenzene molecules in the polar solvents, leading to the
different morphology in the different environments.
It is also important to examine the detection universality
and selectivity towards various exploders. Figure 5 illustrated
the fluorescence quenching efficiency of PHBC self-assembly at
5
25 nm in the presence of various aromatic compounds,
including exploders (such as TNP, 2,4-dinitrophenol and 1-
methyl-2,4-dinitrobenzene) and possible competing aromatic
compounds (such as aniline and phenol) when the PHBC self-
assembly was used as a exploder sensor. As shown in Figure 5,
the quenching efficiencies of PHBC self-assembly towards the
common aromatic compounds (such as aniline and phenol)
were less than 40%, and this quenching efficiency in no case
approached the corresponding values observed for exploders
(
more than 70%), especially the fairly high value for TNP
94.3%). Among the examined exploders, the PHBC self-
(
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assembly gave the highest quenching efficiency towards TNP
Journal Name
,
Notes and references
which was the most electron-deficient. In the control
experiment, either the absorbance intensity change or the
fluorescence intensity change of TNP with the gradual addition
of free PM-β-CD was too little to calculate the association
constant (page S22, Figure S30), indicating that the association
of PM-β-CD cavity with TNP was very weak and could be
negligible. There we deduced that the charge transfer between
PHBC (donor) and TNP (acceptor) may play an important role
in the fluorescence quenching.
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7
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6
2
In conclusion, a novel planar C symmetrical hexa-peri-
hexabenzocoronene bridged bis(permethyl-β-CD)s was
successfully synthesized, and its self-assembly exhibited the
obvious fluorescence responses, which could be readily
monitored by both eyes and fluorescence spectroscopy, to
various exploders either in solution or on test paper, and this
fluorescence sensing was not obviously affected by common
aromatic compounds. Significantly, the detection limit of self-
assembly towards TNP could reach 306 ppb in solution and 2
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We thank NNSFC (21432004, 21672113 and 91527301) for
financial support.
,
1
4
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Table of Contents
Superbenzene-bridged bis(permethyl-β-cyclodextrin) as convenient and effective probe for
trinitrophenol exploder
.