Mercaptomethylphenyl-modified tetraphenylethene as a multifunctional luminophor: Stimuli-responsive luminescence color switching and AIE-active chemdosimeter for sulfur mustard simulants

Article abstract of DOI:10.1039/c7tc03524a

As a commonly known chemical warfare agent, sulfur mustard (SM) is a severe vesicant that can induce grievous blisters on the skin and mucous membranes. Because the onset of SM toxicity is fast, a rapid, sensitive, and accurate method for the detection of SM is necessary. However, designing an effective detection method for SM is challenging because SM does not have any traditional molecular recognition sites. In this study, a fluorescence probe for the detection of SM and an SM simulant (2-chloroethyl ethyl sulfide, CEES) was developed based on the reaction of benzenemethanethiol with the analytes. A non-emissive mercaptomethylphenyl-modified tetraphenylethene (TPE) probe was designed and synthesized. This probe can emit strong fluorescence upon reacting with CEES for 5 minutes in an aqueous solution in the presence of metal ions due to the formation of a sandwich-type complex, which can effectively restrict the rotation of the phenyl rings of TPE. Furthermore, this procedure exhibited high sensitivity for analytes and can be applied to the detection of CEES in soil. As a TPE derivative, this probe also exhibited a reversible solid-state luminescence color switching between blue emission (467 nm) in the crystal state and green emission (516 nm) in the amorphous state upon fuming, grinding and heating. These results indicate that S4 has potential as a multifunctional material of a smart luminophor and an AIE-active fluorescent probe for SM.

Full text of DOI:10.1039/c7tc03524a

Journal of
Materials Chemistry C
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Mercaptomethylphenyl-modified tetraphenylethene
as a multifunctional luminophor: stimuli-responsive
luminescence color switching and AIE-active
chemdosimeter for sulfur mustard simulants†
Cite this:DOI: 10.1039/c7tc03524a
a
a
a
a
b
Hui Wang,‡ Dong-En Wang,‡ Jianping Guan, Xiang Han, Pengchong Xue,
a
a
a
Wenming Liu,
Mao-Sen Yuan * and Jinyi Wang
*
As a commonly known chemical warfare agent, sulfur mustard (SM) is a severe vesicant that can induce
grievous blisters on the skin and mucous membranes. Because the onset of SM toxicity is fast, a rapid,
sensitive, and accurate method for the detection of SM is necessary. However, designing an effective
detection method for SM is challenging because SM does not have any traditional molecular recognition
sites. In this study, a fluorescence probe for the detection of SM and an SM simulant (2-chloroethyl
ethyl sulfide, CEES) was developed based on the reaction of benzenemethanethiol with the analytes.
A non-emissive mercaptomethylphenyl-modified tetraphenylethene (TPE) probe was designed and
synthesized. This probe can emit strong fluorescence upon reacting with CEES for 5 minutes in an
aqueous solution in the presence of metal ions due to the formation of a sandwich-type complex,
which can effectively restrict the rotation of the phenyl rings of TPE. Furthermore, this procedure
exhibited high sensitivity for analytes and can be applied to the detection of CEES in soil. As a TPE
derivative, this probe also exhibited a reversible solid-state luminescence color switching between blue
emission (467 nm) in the crystal state and green emission (516 nm) in the amorphous state upon
fuming, grinding and heating. These results indicate that S4 has potential as a multifunctional material of
a ‘‘smart’’ luminophor and an AIE-active fluorescent probe for SM.
Received 5th August 2017,
Accepted 22nd October 2017
DOI: 10.1039/c7tc03524a
rsc.li/materials-c
1
3–15
time.
More importantly, due to the lack of effective treat-
Introduction
ments for SM-induced injury and its easy preparation and low
Sulfur mustard (SM, or mustard gas) is a severe vesicant cost, SM is increasingly considered a terrorist threat to human-
that can induce grievous blisters on the skin and mucous kind and homeland security. Therefore, it is of great impor-
1
–5
membranes and, in extreme circumstances, lead to death.
SM is also an alkylating agent that could react with the guanine systems for this chemical.
nucleotide in DNA, and thus results in carcinogenic, mutagenic Arising from the serious threat of SM, many efforts have
tance and interest to develop simple yet effective detection
6
,7
and teratogenic effects with long-term exposure. As a noto- been devoted to the development of detection techniques
rious class of chemical warfare agent (CWA), SM has caused against SM over the past few years, including gas chromato-
millions of casualties since its first military use in World War I, graphy (GC), gas chromatography-mass spectrometry (GC-MS),
Although SM and GC-MS combined with some other enrichment
has been banned by international organizations, large amounts technologies.
8
–12
as well as in the Iran–Iraq War in the 1980s.
1
6–23
However, the disadvantages of these methods
of SM are still stockpiled or dumped into the sea, which are very distinct, and they are usually costly, have poor selectivity,
may cause serious health and environmental problems over and are susceptible to the environment. In comparison with
these instrumental analysis systems, methods based on colori-
a
metry or fluorometry are much simpler, cost-effective, and easy
College of Chemistry & Pharmacy, Northwest A&F University, Yangling, Shaanxi
12100, P. R. China. E-mail: yuanms@nwsuaf.edu.cn, jywang@nwsuaf.edu.cn
2
4–26
7
to operate.
However, due to the relatively low reactivity and
b
College of Chemistry, Jilin University, Changchun, Jilin 130012, P. R. China
the absence of any traditional molecular recognition sites of SM,
†
Electronic supplementary information (ESI) available: Details of the synthesis,
the design and development of chromogenic and fluorogenic
AIE properties, calculation of the detection limit, fluorescence decay, and DSC
determination. CCDC 1566706. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c7tc03524a
2
7
detection systems are quite challenging. To date, there are only
a few elegant optical probes for successful detection of SM
and its simulants. For example, Kumar and Anslyn reported a
‡
These two authors contributed equally to this work.
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turn-on fluorescent sensor for SM simulants by using a metal-ion
2
8
indicator displacement assay in 2013. This chemosensor can
selectively detect SM simulants at low millimolar levels but
requires an intermediary step of capping the reporter before
detection. Pardasani et al. developed a rhodamine-based fluores-
cence turn-on probe for SM utilizing a tandem S-alkylation
followed by desulfurization reaction of rhodamine–thioamide
2
9,30
with SM.
Wolfbeis et al. developed a test strip for colori-
metric detection of gaseous SM based on the degradation of SM
by the enzyme haloalkane dehalogenase, which could change
the local pH and was thus detected by pH indicators incorpo-
3
1
rated in the strips. In spite of the sensitivity and rapidity of
these methods, some drawbacks still exist, such as the requirement
of capping the indicator and the complex synthetic procedures. The Scheme 1 The synthetic routes of compound S4.
development of new colorimetry or fluorometry-based detection
systems that meet the criteria of simplicity, rapidity, and on-site
analysis of SM is still highly desired.
(516 nm) in the amorphous state upon fuming, grinding and
In 2001, Tang and co-workers reported an abnormal heating. This paper also presents the study of stimuli-
emission phenomenon called ‘‘aggregation-induced emission’’ responsive luminescence color switching of S4. The results
(
AIE), in which a series of silole molecules were found to be indicate that S4 has multi-potential as a ‘‘smart’’ material or
3
2
non-luminescent in solution but emissive in the solid state.
an AIE-active fluorescent probe for SM.
Since this pioneering work, luminescent materials with AIE
attributes have received extensive interest due to their promis-
ing applications in the fields of organic light-emitting Experimental sections
diodes, luminescent sensors, biomedical imaging and
Synthesis and characterization
3
3–43
stimuli-responsive ‘‘smart’’ materials.
For sensing applica-
Tetrabromo-tetraphenylethene (1) was synthesized according to
tions, AIE fluorogens were typically engineered to dissolve in
aqueous media, in which the molecules would show extremely
5
0
the literature method. Suzuki coupling reaction was used to
prepare the intermediate product tetra(o-tolyl)tetraphenyl-
ethene (2) via reacting 1 with o-tolylboronic acid. The reaction
of 2 with NBS in the presence of AIBN gave tetra(2-(bromo-
methyl)phenyl)-tetraphenylethene (3). The title compound
tetra(mercaptomethyl)phenyl-tetraphenylethene (S4) was ultimately
obtained through the reaction of 3 with thiourea followed by base
hydrolysis and treatment with acid. Details of the synthesis,
purification and single-crystal preparation processes, along with
4
4
weak fluorescence. When target analytes were introduced,
the specific interactions between the AIE fluorogens and the
analytes could light up the fluorescence. Compared with con-
ventional fluorophore based probes, AIE-based probes afford
much higher photo-stability and signal reliability, as AIE
signals are produced only when the intramolecular rotations
are restricted. The last few years have witnessed the continual
emergence of various AIE-based chemo- and bio-sensors for the
assays of pH, amines, ions, peptides, proteins, nucleic acids,
1
13
H and C nuclear magnetic resonance (NMR) spectroscopy
4
5–49
data, mass spectrometry results, elemental analysis characteriza-
tion and X-ray single crystal structure, can be found in the ESI.†
etc.
However, to the best of our knowledge, a fluorescence
probe based on AIE molecules for SM has not been reported.
Tetraphenylethene (TPE), as a star AIE molecule, has been
widely studied for the construction of new AIE materials as well
as AIE-based probes due to a number of advantages, such as its
Spectroscopic characterization
1
13
H and C NMR spectra were recorded using a Bruker Avance
1
13
high efficiency of solid-state emission, simple synthesis, and spectrometer (500 MHz for H and 125 MHz for C) in CDCl3.
5
0–52
easy functionalization.
In this study, the TPE system is Element analyses (C, H) were performed using a German Vario
adopted to discover a novel metal ion coordination assisted EL III elemental analyzer. Mass analyses were performed using
AIE-active probe for selective fluorescence turn-on detection of an Agilent 5973N MSD Spectrometer. UV-visible absorption
SM and its simulant. In our detection system, TPE was function- spectra for the solutions were recorded using a Shimadzu
alized with benzenemethanethiol groups to obtain the TPE-derived UV-2550 spectrometer. Photoluminescence (PL) spectra were
0
0
molecule (Scheme 1), 1,1,2,2-tetrakis(2 -(ethylthio)propyl-[1,1 - recorded using a Shimadzu RF-5301PC fluorescence spectro-
biphenyl]-4-yl)-ethene (S4). This compound could react with SM meter. The fluorescence quantum yield (F) in solution was
and its stimulant, 2-chloroethyl ethyl sulfide (CEES), to gener- determined using rhodamine B in ethanol as a reference
ate strong green AIE signals. The detection process was proven according to a previously reported method. Quantum yields
to be simple, rapid and selective for SM and the SM simulant. of the solid-state powder were determined with a PTI C-701
In addition, as a TPE derivative, compound S4 exhibited calibrated integrating sphere system. Steady-state fluorescence
reversible solid-state luminescence color switching between spectra and decay curves were obtained using an Edinburgh
blue emission (467 nm) in the crystal state and green emission FLS920 fluorescence spectrometer and a time-correlated single
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photon counting (TCSPC) card. Reconvolution fits of the decay
profiles were performed with F900 analysis software to obtain
the lifetime values.
Single-crystal X-ray diffraction
The single crystal of S4 was obtained through slow diffusion of
its tetrahydrofuran (THF) solution for several days at room
temperature. The data collection was performed without any
inert gas protection at room temperature on a Bruker SMART
APEX-II CCD area detector using graphite-monochromated Mo
Ka radiation (l = 0.71073 Å). The data reduction and integration
and global unit cell refinements were performed using the
INTEGRATE program of the APEX2 software package. Semi-
empirical absorption corrections were applied using the SCALE
program for the area detector. The structure was solved by
direct methods and refined using the full-matrix least-squares
Scheme 2 (a) The reaction of probe S4 with SM or CEES and (b) schematic
illustration of the fluorescent enhancement induced by the metal ion-
coordination.
2
methods on F using SHELX.
phenyl rings of TPE and result in an AIE-active fluorescence
Powder X-ray diffraction and thermal analysis
enhancement in comparison with the non-emissive probe S4.
For phase identification, powder X-ray diffraction (XRD) patterns
were recorded at room temperature with a Bruker AXS X-ray
powder diffractometer using Cu Ka radiation. The data were
recorded in the 2y mode with a step size of 0.026261. Differential
scanning calorimetry (DSC) measurements were performed to
enable a better understanding of the phase transitions, and
the curves were obtained with a NETZSCH thermal analyzer
Detection of mustard simulant
In the current study, we chose 2-chloroethyl ethyl sulfide
(CEES), an SM simulant, which is known as ‘‘half mustard’’,
as the analyte. CEES has the relevant chemical properties of the
real agent without its associated toxicological properties. CEES
ꢀ1
ꢀ1
has a relatively high LD50 of 252 mg kg (rat oral) in compar-
(DSC 204 F1) at heating and cooling rates of 10 K min under a
ꢀ
1
ison with that of SM (2.4 mg kg ), and is an appropriate
N
2
atmosphere.
5
4
substitute of SM for studies with an academic setting.
As expected, probe S4 displayed almost no emission in
dilute solution. When an excess of CEES (15 equivalents) was
added to the probe solution of S4 in a mixture of DMSO and
sodium bicarbonate/sodium hydroxide buffer (1 : 1 vol, pH = 9)
Results and discussion
Design criteria
Designing a fluorescence detection method for SM is challenging and reacted at 80 1C for 5 minutes, the fluorescence intensity
because SM does not have any traditional molecular recognition showed medium enhancement. This phenomenon is due
sites. Furthermore, SM is not a particularly good electrophile largely to the resulting long open chains which decelerate
compared to a simple primary alkyl halide and is difficult to react (or partially restrict) the rotation of the phenyl rings of TPE.
with any sensor molecules in common solvents. Recently, studies The formation of the product L (X = H) was confirmed by mass
have found that the chlorine atoms in SM are reactive with good spectrometry (see Fig. S7 in the ESI†). The next aim was to
nucleophiles in basic medium and can form three-membered choose the metal ion and determine the required concentration
cationic sulfonium heterocycles, which can greatly enhance the of metal ion that saturates the fluorescent enhancement. We
28
electrophilicity of SM. On the basis of this finding, the Anslyn firstly determined the fluorescence change of L in response to
2
+
+
3+
2+
2+
2+
group designed a dithiol derivative with a benzenemethanethiol different metal ions, including Hg , Ag , Al , Ba , Ca , Cd ,
2
+
3+
2+
2+
3+
+
2+
2+
+
2+
structure, which can react with SM in hot water at pH 9 to form a Co , Cr , Cu , Fe , Fe , K , Mg , Mn , Na and Zn . The
2
+
2+
podand and ultimately reached for the detection of SM through results manifested that Hg and Cd can cause obvious
28,53
an optical technique.
Inspired by this promising reaction, we fluorescent enhancement of L (see Fig. S1 in the ESI†), and
realized that, when connecting benzenemethanethiol with the Hg was ultimately selected as the assistant ion. Toward this
TPE moiety, the tetrathiol compound would be a multi-site end, fluorescence titration was carried out for the quantitative
2
+
2
reactant for SM. The reaction of the tetrathiol compound S4 with determination of HgCl in the achieved system containing L. As
the target analyte SM will give a sulfur-based podand molecule L shown in Fig. 1a, the fluorescence emission of the mixed
with four open chains, which are excellent receptors for some solution was gradually increased with an increase in the
2
+
2+
2+
metal ions, e.g. Hg and Cd . As illustrated in Scheme 2, in the concentration of Hg , and became saturated with the addition
2
+
presence of metal ions, it is expected that the metal ions can be of five equivalents of Hg (5 mM). After confirming the required
chelated by two stagger sulfur-based chains and then sandwiched equivalents of Hg , we went on to explore the detection of
2
+
by two podand molecules (L). The face-to-face molecular stacking CEES. In this process, S4 in the mixture of DMSO and buffer
will ultimately lock the TPE moieties to restrict the rotation of the (1 : 1 vol) containing five equivalents of HgCl
2
was allowed to
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2+
Fig. 1 (a) Fluorescence titration of a reaction solution of S4 (1.0 mM) and 15 equivalents of CEES with increasing concentrations of Hg . (b) Fluorescence
2+
titration of a mixed solution of S4 and five equivalents of Hg with increasing concentrations of CEES, inset: colorimetric response of the solution with
sequential addition of CEES from left to right. (c) The relationship of the fluorescence intensity of S4 at 482 nm and CEES concentration in the presence
2+
and absence of Hg respectively. (d) Reaction-time profiles: the fluorescence intensity of the reaction solution of S4 (5.0 mM) with 10 equivalents of
2+
CEES in the presence of five equivalents of Hg at 482 nm versus time (within 7 minutes).
react with CEES with varied equivalents from 1 to 15 at 80 1C for CEES. The reaction in this sensing process needed to be heated
5
minutes and then the fluorescence spectrum of each solution at 80 1C, which is one downside of this detection method. It is
was subsequently recorded. As shown in Fig. 1b, the solutions one of our aims to improve the reaction conditions and
containing excess of nine equivalents of CEES exhibited max- decrease the reaction temperature in our follow-on work.
imum fluorescence intensity, which was approximately 30-fold
Next, we investigated the selectivity of S4 for CEES by testing
higher than that of the solution without CEES. The fluores- the potential interfering agents like alkyl halides (butyl iodide,
cence intensity of the probe solution increased gradually and butyl bromide) and the oxy derivative of mustard (bis-(2-
showed good linearity with the concentration of CEES. The chloroethyl) ether, 2-chloroethyl ethyl ether) using the same
detection limit of the probe for CEES is about 1.10 mM based on protocol. No evident fluorescence enhancement was observed
a signal-to-noise ratio (S/N) of 3 under such test conditions, (Fig. 2), which indicated that the halogens in simple alkyl
which is much lower than the lethal dose (Fig. S2 in the ESI†). halides and the oxy derivative of mustard are not as reactive
Moreover, a visual fluorescent boost of the S4 solution can be as that in CEES. In addition, when CEES was added to the
readily observed by the naked eye after reacting with CEES, solution of S4 containing all the above potential interfering
which suggests that S4 can be utilized as a convenient probe for agents, fluorescence enhancement appeared, indicating that
naked eye detection of CEES (inset in Fig. 1b). As a control, we these species do not interfere with the detection of CEES.
also inspected the fluorescence response of S4 to CEES in the
absence of metal ions (Hg ), showing that the fluorescence stability and can remain active in neutral soil for as long as
enhancement was rather limited under such conditions several weeks following deployment. With an aim to mimic
It is known that SM behaves with high environmental
2
+
(Fig. 1c). The results indicate that the ion-coordination- real-life scenarios for on-site and off-site detection, the sensing
induced sandwich-type structure plays a crucial role in the protocol was further implemented in spiked soil samples. To
fluorescence enhancement during the process of CEES detec- determine the presence of CEES in soil samples, soil was spiked
tion. In addition, we further studied the response time. The using a solution of CEES in THF. The solvent was evaporated to
fluorescence intensity of S4 treated with nine equivalents of obtain dry soil spiked with CEES. The sample was then divided
CEES reached a maximal value within about 5 minutes into two equal portions, one of them was treated with S4 in
2
+
(
Fig. 1d), which indicated that probe S4 could rapidly monitor DMSO/buffer (1 : 1 vol) containing five equivalents of Hg at
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Scheme 3 Schematic presentation of the proposed reaction between
real-life agent SM and compound S4.
rings of TPE to cause fluorescence enhancement. In this model,
probe S4 can directly monitor SM sensitively with no need of
the assistance of metal ions.
Fig. 2 Fluorescence spectra of the solutions of S4 (5 mM) and five
equivalents of HgCl
chloroethyl)ether (BCEE), 2-chloroethyl ethyl ether (CEEE), butyl iodide
BI) and butyl bromide (BB), respectively, inset: colorimetric response of
2
treated with 10 equivalents of CEES, HCl, bis(2-
(
Stimuli-responsive luminescence switching
2+
Hg -containing S4 solution to different analytes.
As a TPE derivative, the AIE properties of compound S4 were
inevitably explored. We measured the fluorescence change of S4
in the THF/water mixed system with varied water fractions and
found that not only was the fluorescence intensity of the solution
enhanced with increasing water fractions, but also the fluorescence
maximum of the mixture with 90 vol% water showed a 25 nm blue-
shift relative to its pure THF solution (from 503 to 478 nm, Fig. S3 in
the ESI†). The aggregation-induced luminous blue-shift phenom-
enon indicates that S4 has the potential to be a smart luminescence
switching molecule. Based on the previous studies of our group,
the stimuli-responsive luminescence switching properties of S4 were
therefore systematically investigated.
We first cultured the crystals of S4 through slow diffusion of
its THF solution to obtain the colorless crystals B-S4, which
emit intense blue fluorescence in 365 nm UV light with a high
8
0 1C. The other was treated with a solution of S4 in a similar
2
+
manner without Hg . Meanwhile, the soil sample without
spiked CEES was also treated with the probe solution contain-
2
+
ing Hg as a control. As shown in Fig. 3, the solution of the
unspiked soil sample exhibited almost no emission, suggesting
the absence of CEES in the soil. A medium emissive intensity
was observed for the probe solution without Hg when treated
with the spiked soil sample. While a strong fluorescence
emission was observed when the spiked soil sample was treated
with S4 and Hg . Moreover, the measured concentration of
CEES in the spiked soil sample was consistent with the calibra-
tion curve with a 7% error, which indicates that this method
can be applied in practice.
2
+
55,56
2
+
In a real-life SM assay, besides the sandwich-type construc-
tion containing metal ions (Scheme 2), our strategy would likely
create compound S6, as presented in Scheme 3, by reacting SM
with two adjacent sulfydryl groups of S4. The two formed
macrocycles can likewise restrict the rotation of the phenyl
fluorescence quantum yield F
smoking the lamellar blue crystals with CH
observed that the crystals gradually turned pale green and their
emissive wavelength also changed from 467 nm (blue) to
F
of 47%. When fuming or
Cl vapor, we
2
2
516 nm (green) with a relatively low F of 23% (Fig. 4a and b).
F
Grinding the blue crystalline powder of B-S4 in an agate mortar gave
a pale green powder, which exhibited a green emission (508 nm)
similar to the CH
responsive conversion from blue to green can be reversed by
thermal treatment. Upon heating the CH Cl -fumed B-S4 samples
green fluorescence) to 97 1C using a microscopic melting-point
2 2
Cl -fumed B-S4. Furthermore, the stimuli-
2
2
(
ꢀ
1
apparatus with a heating rate of B5 K min under ambient
conditions (or directly heating the green sample at 97 1C for
approximately 5 s), we observed the solid-state fluorescence switch-
ing from green to blue.
In order to understand the stimuli-responsive fluorescence
change, we studied the phase characteristics of the relevant sam-
ples using powder XRD analysis and DSC measurements. The
results indicated that the stimuli-responsive reversible fluorescence
switching is derived from the stimuli-responsive reversible phase
transformations between the crystal state and the amorphous state
(Fig. 4c). In other words, the samples underwent a phase transfor-
mation between the crystal state and the amorphous state upon
fuming, grinding, or heating, accompanying the fluorescence
Fig. 3 The fluorescent responses of S4 (1 mM) with the CEES-spiked soil
2+
sample in the presence of Hg , the CEES-spiked soil sample in the
2+
absence of Hg and unspiked soil sample, inset: the visual fluorescence
color of the CEES-spiked and unspiked soil sample treated with S4
2
+
(
and Hg ).
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Fig. 4 (a) Photographs of the crystalline phase transformation and luminescence change of S4 between blue and green upon fuming, heating, or grinding.
b) Normalized PL spectra of blue-crystal B-S4, CH Cl fumed B-S4, ground B-S4, and heated fumed-B-S4 (heating the green sample obtained by CH Cl
fumed blue-crystal B-S4 at 98 1C for 1 min). (c) Powder XRD profiles of the heated green S4 sample, CH Cl fumed B-S4, and blue crystal B-S4.
(
2
2
2
2
-
2
2
switching. The DSC results were consistent with the experimental
Next, we explored the single-crystal structure of S4 to under-
phenomenon and we found that the sample of CH Cl -fumed B-S4 stand the solid-state fluorescence switch. The molecular struc-
2
2
displayed an exothermic peak at 97.3 1C (Fig. S5 in the ESI†).
ture and packing of S4 are shown in Fig. 5. The molecules in
Fig. 5 X-ray crystallographic packing of compound S4. (a) The single molecular structure of S4. (b) Side view of the X-ray crystallographic packing of
blue crystal S4, where the layer-by-layer stacking can be clearly observed and illustration of the intermolecular C–Hꢁ ꢁ ꢁp interactions in the crystal.
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crystal B-S4 were highly twisted with a big torsion angle of (2016M592837) and Science and Technology Plan Projects by
the four peripheral biphenyl moieties, respectively (Fig. 5a). Yangling (2016NY-32).
The crystal packing showed a typical layer-by-layer stacking
characteristic, the neighboring layers were linked by weak
C–Hꢁ ꢁ ꢁp interactions (Fig. 5b), and no strong intermolecular
Notes and references
interactions were found between molecules in the same layer.
The highly twisted configuration of S4 in crystals is just
attributed to the interlamellar C–Hꢁ ꢁ ꢁp interactions. The large
torsion angle of biphenyl fragments will block intramolecular
charge transfer upon excitation and enhance the energy gap
between the HOMO and LUMO, ultimately leading to the blue
emission of the crystal B-S4. In addition, the C–Hꢁ ꢁ ꢁp inter-
actions also effectively restrict the rotation of the phenyl rings
of S4 to result in higher fluorescence quantum yield of the
crystals than that of the amorphous powder. Although the
C–Hꢁ ꢁ ꢁp interactions can facilitate molecular crystallization, they
will also result in loose crystal packing due to the formation of a
highly twisted molecular configuration. Such a loose crystalline
structure can be readily destroyed upon external physical per-
turbations, which is a prerequisite for a stimuli-responsive
luminescence switching molecule.
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Conclusions
In this study, we successfully designed and synthesized a
mercaptomethylphenyl-modified TPE compound S4 as a fluores-
cence probe for SM and an SM simulant (CEES). Compound S4
exhibited almost no emission in dilute solution. However,
it emitted strong fluorescence upon reacting with CEES in a
hot aqueous solution in the presence of metal ions due to the
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agent SM because there is no need for metal ions. On the other
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crystal state and green emission (516 nm) in the amorphous
state upon fuming, grinding and heating. The allochroic behav-
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and crystal structure analysis. The results indicate that S4 has
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