Detection of nitroaromatics based on aggregation induced emission of barbituric acid derivatives

Article abstract of DOI:10.1016/j.saa.2019.117168

Barbituric acid derivatives with typical aggregation induced emission (AIE) are reported. Their emission wavelengths varied with water fraction of their solution. UV–visible absorption spectroscopy and theoretical calculations revealed the intramolecular

Full text of DOI:10.1016/j.saa.2019.117168

Accepted Manuscript
Detection of nitroaromatics based on aggregation induced
emission of barbituric acid derivatives
Han-Jun Zhang, Yan Tian, Fu-rong Tao, William Yu, Kai-Yue
You, Lin-Rui Zhou, Xi Su, Tian-duo Li, Yue-Zhi Cui
PII:
S1386-1425(19)30558-X
DOI:
https://doi.org/10.1016/j.saa.2019.117168
Article
Number:
Reference:
1
17168
SAA 117168
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
14 December 2018
9 May 2019
26 May 2019
Please cite this article as: H.-J. Zhang, Y. Tian, F.-r. Tao, et al., Detection of nitroaromatics
based on aggregation induced emission of barbituric acid derivatives, Spectrochimica
Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/
j.saa.2019.117168
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ACCEPTED MANUSCRIPT
Detection of Nitroaromatics Based on Aggregation Induced
Emission of Barbituric Acid Derivatives
Han-Jun Zhang, Yan Tian, Fu-rong Tao, William Yu, Kai-Yue You, Lin-Rui Zhou, Xi Su, Tian-duo
Li, Yue-Zhi Cui*
School of Chemical and Pharmaceutical Engineering, Qilu University of Technology, Jinan
250353, P. R. China.
Tel: +86 531 89631208; Fax: +86 531 89631760;
E-mail address: yuezhicui@163.com
Abstract
Barbituric acid derivatives with typical aggregation induced emission (AIE) are reported.
Their emission wavelengths varied with water fraction of their solution. UV-visible
absorption spectroscopy and theoretical calculations revealed the intramolecular charge
transfer (ICT) possibility from donor to acceptor and the mechanism was confirmed as a
restriction of intramolecular motion (RIM). The AIE properties were affected by the different
substituents on barbituric acid. When the molecular volume increased, the AIE effect
decreased. Fluorescent quenching mechanism was applied to detect nitroaromatic
explosives. For 2,4,6-trinitrophenol (PA), one of the derivatives 5-(4-diphenylamino
styrene)-1,3-diphenyl-barbituric acid in THF/H O mixture (1:9, v/v), showed amplified
2
4
-1
fluorescence quenching with a maximum Stern–Volmer quenching constant of 4.1×10 M .
The solid phase paper test based on 5-(4-diphenylamino styrene)-1,3-diphenyl-barbituric
acid also showed a superior sensitivity toward PA both in vapor and solution.
Key words: Aggregation induced emission (AIE); Intramolecular charge transfer (ICT);
Restriction of intramolecular motion (RIM); Fluorescent sensor;

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1
. Introduction
Tang's team firstly reported the aggregation-induced emission (AIE) phenomenon in
001[1, 2], which is opposite to the aggregation-caused quenching (ACQ) [3, 4]. The main
2
source of AIE effect is the inhibition of the appearance of π-π stacking between molecules
5].A common feature of these materials is that they do not emit in molecular solution but
[
emit strong fluorescence in the aggregated state. Therefore AIE provides a new avenue for
applications in optoelectronic devices, bio/chemical sensors, and bioimaging [6-8].
However, typical compounds with AIE require cumbersome multi-step synthesis, and
often result in bulky and structurally complex AIE materials. Therefore, exploring
luminescent materials with AIE-active but simple structures is still interesting and valuable.
Herein, we recommend three AIE active and structurally simple luminescent materials
based on barbituric derivatives. The powerful aggregation ability of barbituric acid units
through intermolecular hydrogen bonding interactions (N-H···O) has been well documented
in the literature [9, 10]. Therefore, we focused on the relationship between AIE effect and
molecular structure of three barbituric acid derivatives.
Nitroaromatic explosives are widely used in many fields because they are easy to prepare
and transport, and they are powerful. The detonation of explosives usually causes damages
to the environments, which in turn affects people's health. Studies disclose that the
explosive residues can pollute the air, soil and water systems. Long-term exposure to these
residues can cause various diseases such as skin diseases, respiratory system problems,
abnormal liver function and cancer [11-15]. All of these have aroused the concern about
how to effectively detect them.
Triphenylamine naturally as a three-dimensional freely rotatable structure, and the
internal energy consumption is large that makes the fluorescence quantum yield low, but the
structure of triphenylamine is easily to be modified, and the fluorescence quantum yield is
generally high after modification. Triphenylamine also exhibits distinct donor characteristics,
which should facilitate interaction with electron-deficient nitroaromatic compounds.
However, there are few reports on triphenylamines for photoluminescence (PL)-based
explosives detection.
Most fluorescent sensors for detecting nitroaromatics cannot be operated in aqueous
media, but compounds with AIE can solve this problem nicely. In this project, we designed

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and synthesized barbituric derivative fluorescence sensors, which can generate aggregates in
high water content and can thus be used for the detection of nitroaromatics in aqueous
media.
2
2
. Experimental section
.1 Materials
Triphenylamine (Aladdin, 99%), dimethylformamide (DMF) (Aladdin, 99%), barbituric acid
(Aladdin, 98%), diphenylurea (Jiuding Chemical, 99%), malonyl chloride (Aladdin, 99%), 1,
3
-dimethyl-barbituric acid (Aladdin, 99%) were used without further purification. DMF is
usually dried sequentially by molecular sieves (4 Å), sodium sand and CaH successively, and
2
distilled prior to use.
2
.2. Characterizations
Absorption spectra were recorded by a UV-2500 spectrometer. The fluorescence spectra
were measured with an F-4600 fluorescence spectrophotometer. The fluorescence quantum
1
13
yield of the solid is commissioned by the compass test platform. The H and C NMR spectra
were recorded on AVANCE II 400 spectrometer using deuterated dimethyl sulfoxide
(
DMSO-d ). FT-IR spectra were obtained recorded by a Nicolet 380 spectrometer using the
6
KBr pellet method. The scanning electron microscope images were obtained with Quanta
2
00 environmental scanning electron microscope (SEM, FEI Company).
.3 Calculation
2
The geometric and electronic structures of the three compounds were calculated at the
B3LYP level with Density Functional Theory (DFT).
2
.4 Synthesis of compounds
The target luminescent materials were synthesized by (1,3-dimethyl, 1,3-diphenyl)
barbituric acid and N, N-diphenylamine benzaldehyde condensation reaction (scheme 1).

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Scheme 1. Synthetic routes to PB-1, -2, and -3
2
.4.1. Synthesis of 4-(diphenylamino)-benzaldehyde (1)
2
ml of dry DMF (25.7 mmol) was added into 5 ml of phosphorus oxychloride (POCl ) (54
3
o
mmol), at 0 C, and remained for 15 min, then triphenylamine (1 g, 4 mmol) was added with
o
stirring. The mixture was heated to 45 C for 2.5 h. After being cooled down, the clear red
solution was dropped into ice water. The resulting mixture was filtered, washed and
redissolved in dichloromethane. This solution was washed with water (150 ml) and dried
with magnesium sulfate, after solvent evaporation, the oily liquid recrystallized into

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1
acetaldehyde in ethyl acetate. Yield: 0.78g (80%). H NMR (400 MHz, DMSO-d ): δ 9.82 (s,
6
1
H), 7.74 - 7.61 (m, 2H), 7.35 (t, J = 7.7 Hz, 4H), 7.22 - 7.13 (m, 6H), 7.07 - 6.96 (m, 2H) (Fig.
S1).
.4.2. Synthesis of 1, 3-diphenyl barbituronic acid (2)
2
Malonyl chloride (33.3 mg, 2 mmol) was added to N, N-diphenyl urea (424.5 mg, 2 mmol)
o
with 6 ml CHCl , and the mixture was heated to 80 C for 4 h. Then the reaction mixture was
3
extracted with dichloromethane and dried over Na SO . After filtration, the filtrate was
2
4
concentrated under reduced pressure and further purified by column chromatography. Yield:
1
3
1
2
43.1 mg (75%). H NMR (400 MHz, DMSO-d ): δ 7.45 (d, J = 7.5 Hz, 3H), 7.41 (d, J = 7.5 Hz,
6
H), 7.28 (d, J = 7.4 Hz, 3H), 4.01 (s, 1H) (Fig. S2).
.4.3. Synthesis of 5-(4-diphenylamino styrene)-barbituric acid (PB-1), 5-(4-diphenylamino
styrene)-1,3-dimethyl-barbituric acid (PB-2), 5-(4-diphenylamino styrene)-1,3-diphenyl
-
barbituric acid (PB-3)
The synthetic steps of PB-1 and PB-2 have been published in the previous papers of our
group [4], so the synthesis and characterization of PB-3 are discussed here.
1
PB-1: H NMR (400 MHz, DMSO-d ) δ (ppm): 8.42 (s, 1H), 8.31 (d, J = 8.9 Hz, 2H), 7.97 (s, 1H),
6
7
.80 (s, 1H), 7.40 (t, J = 7.7 Hz, 4H), 7.24 (t, J = 6.3 Hz, 6H), 6.94 (d, J = 8.9 Hz, 2H) (Fig. S3).
1
3
C NMR (126 MHz, DMSO-d ) δ 155.13, 152.67, 151.2, 150.69, 145.51, 138.13, 130.58,
6
-
1
1
27.21, 126.48, 124.37, 117.30, 113.99 (Fig. S4). FT-IR (KBr, cm ): 1672 (C=O) (Fig. S5).
(The FT-IR of the C=O bond present in the molecule of the barbituric acid derivative is
-1
+
around 1670 cm ). HRMS (ESI) m/z: [M + H] calcd for C H N O , 383.41; found,
2
3 17 3 3
3
84.1278(Fig. S6).
1
PB-2: H NMR (400 MHz, DMSO-d ) δ (ppm): 8.44 (s, 1H), 8.24(d, 2H), 7.38 (t, 4H), 7.23 (m,
6
1
3
6
1
H), 6.95 (d, 2H), 3.40 (d, 6H) (Fig. S7). C NMR (126 MHz, DMSO-d ) δ 161.37, 155.98,
6
52.71, 151.62, 145.49, 138.05, 130.59, 127.20, 126.50, 124.42, 117.30, 113.83, 28.77 (Fig.
-
1
S8). FT-IR (KBr, cm ): 1667(C=O) (Fig. S9). (The FT-IR of the C=O bond present in the
-
1
+
molecule of the barbituric acid derivative is around 1670 cm ). HRMS (ESI) m/z: [M + H]
calcd for C H N O , 411.462; found,412.1557 (Fig. S10).
2
5 21 3 3
PB-3: A mixture of 1 (0.447 g, 3 mmol) and 1, 3-diphenyl barbituronic acid (0.840 g, 3 mmol)
in (10ml) ethanol was refluxed for 4 h. After filtration, the filtrate was concentrated under
reduced pressure and further purified by column chromatography (ethyl acetate:
1
petroleum ether = 2:1). Yield: 1.158 g (90%). H NMR (400 MHz, DMSO-d ) δ (ppm): 8.30
6

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(s, 1H), 8.23 (d, J = 8.4 Hz, 2H), 7.49 - 7.38 (m, 12H), 7.34 (d, J = 7.5 Hz, 6H), 7.24 (d, J = 8.4
13
Hz, 8H), 6.74 (d, J = 8.5 Hz, 2H) (Fig. S11). C NMR (126 MHz, DMSO-d ) δ 163.54, 161.57,
6
1
1
56.45, 152.84, 145.37, 138.22, 130.58, 129.60, 129.47, 129.24, 128.72, 127.25, 126.59,
-1
25.10, 117.15, 114.39 (Fig. S12). FT-IR (KBr, cm ): 1667 (C=O) (Fig. S13). (The FT-IR of the
-
1
C=O bond present in the molecule of the barbituric acid derivative is around 1670 cm ).
+
HRMS (ESI) m/z: [M + H] calcd for C H N O ,535.686; found,536.1873 (Fig. S14).
3
5 35 3 3
3
3
Results and discussion
.1 Optical properties
Fig. 1 shows absorption spectra of PB-1, -2, and -3 at different THF/H O percentages
2
(
water fraction f = 0, 30, 50, 70%). The results show that the absorbance decreases with the
w
increase of water fractions and the reason may be that the degree of torsion of the
molecules increases with the increase of water fractions, leading to a weak intramolecular
charge transfer. The absorption band in the range of 270 to 300 nm is assigned to the π-π*
electronic transition of the corresponding benzene ring unit. The absorption peaks at 454,
4
55 and 464 nm (for PB-1, -2, and -3, respectively) can be attributed to the ICT from
diphenylamine units to part of barbiturates [16].
At the same time, it was also founded that absorption spectra of the PB-1 had a wide and
relatively weak peak at around 350 nm, which may be consistent with the characteristics of
n-π* transition. This mechanism can be depicted in Fig. 2. As water is added, PB-1 undergoes
tautomerization from the keto to the enol structure.
The IR spectra of the three compounds were measured to verify the tautomerization of
-
1
PB-1, as shown in Fig.3. A wide peak at 3413~3418 cm appeared in all the three spectra,
which may be attributed to the trace of water in the samples. It is noteworthy that a unique
-
1
peak around 3220 cm appeared in PB-1, while it is absent in PB-2 and PB-3. Reasonably, the
-
1
peak at 3220 cm is attributed to the hydroxyl groups in the enol structure of PB-1. However,
for PB-2 and PB-3, with the hydrogen atom in NH group is replaced by a methyl group and a
phenyl group, respectively, cannot undergo tautomerization from the keto to the enol
-
1
structure, thus there is no hydroxyl group to produce the IR peak at 3200 cm .
In the enol structure of PB-1, the lone pairs of electrons on the N atom can generate n-π*
transitions with N=C double bonds, thus produces the absorption peak at 350 nm.

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Accordingly, without enol structure for PB-2 and -3, there is no n-π* transitions, thus the
peak at 350 nm is absent in the absorption spectra of PB-2 and -3.
Fig. 1. UV-visible absorption spectra of PB-1, -2, and -3 in different THF/H O percentage
2
-4
solutions (solution concentration: 10 M).
Fig. 2. Mechanism of PB-1 tautomerism.

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Fig. 3. FT-IR spectra of PB-1, PB-2 and PB-3.
3
.2 AIE effect
It can be seen from the photograph that the emissivity of compounds PB-1, -2, and -3 is
weak in the low concentration of pure THF (Fig. 4), because the intermolecular interaction in
solution is relatively weak, so they rotate freely in solution state and energy is expended in a
non-radiative transition showing a weak fluorescence[17].

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Fig.4. Fluorescence photographs of PB-1, -2, and -3 under ultraviolet light irradiation in
different states (365 nm).
To test whether PB-1, -2, and -3 have AIE property, their PL behavior in THF/H O of
2
-
4
-1
different water fractions (f ) was investigated with the concentration of PB series of 10 M
w
(
Fig.5). PB-1, -2, and -3 exhibited weak emissions at 619.0, 628.0 and 634.0 nm in pure THF
Table 1).
(
Table 1 Optical properties of PB-1, -2, and -3.
Solution / THF
Aggregation solution
fw = 90%)
Solid
I /I
A
P
(
powder
λabs / nm
454.5
λ / nm
Stokes shift / nm λ / nm
λ / nm
Φ / %
em
em
em
F
PB-1
PB-2
PB-3
619.0
628.0
634.0
164.5
172.5
170
625.0
620.0
638.0
629.0
589.0
672.0
41.50%
23.27%
32.34%
7.2
3.6
2.1
455.5
464.0
Note: I is the fluorescence intensity in the aggregation solution, I is the fluorescence
A
P
intensity in pure THF.
Significantly, PB-1 exhibits enhanced AIE effect compared to PB-2 and PB-3: in aggregation
solution, the luminescence intensity of PB-1 is 7.2-fold higher than that in THF. In addition,
the solid fluorescence quantum yields of PB-1, PB-2, and PB-3 were 41.50%, 23.27%, and
3
2.34%, respectively [18]. As mentioned previously, the better AIE performance of PB-1 is
attributable to H-bonding, which helps to further solidify the molecular conformation of the
aggregated state and block the non-radiative transition.
We found that the AIE effect is related to the molecular structure of barbituric derivatives,
which as the volume of barbituric acid derivatives increased, the AIE index (I /I ) decreased
A
P
(Table 1). The aggregation induced fluorescence indexes of PB-1, PB-2 and PB-3 are 7.2, 3.6
and 2.1, respectively. Fluorescence intensity of PB-3 is weaker than those of PB-1 and PB-2 in
aggregation state, but stronger than those of PB-1 and PB-2 in pure solution, fluorescence
intensity of PB-2 is weaker than that of PB-1 in aggregation, but stronger than that of PB-1 in

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pure solution. The reason is that large steric hindrance of the larger group make it difficult to
rotate freely in pure solution. On the other hand, in the state of aggregation of groups with
larger molecular spacing, the intermolecular interaction is weakened.
We also investigated the effect of the emission spectra of PB-1, -2 and -3 by varying the
amount of glycerol in the THF-glycerol mixture. As seen from Fig. S15, the fluorescence
intensity increases with increased glycerol contents in the THF-glycerol mixtures, because
the intramolecular rotation is limited by more glycerol, which attenuates the non-radiative
transition process and enhances the emission.
We carefully analyzed the AIE behavior of the three compounds. Compound of PB-1
exhibits a more complicated tendency with the increase of f . The emission peak of PB-1
w
red-shifts from 619.4 nm to 648.4 nm when f increases from 0% to 70%. Furtherly, it
w
blue-shifts to 624.4 nm as f increase to 90%. It is known from section 3.1 that PB-1
w
produces the tautomerization from keto to enol structure, so the intermolecular hydrogen
bond (N-H···O) interaction exists, which increases the solubility of PB-1 in aqueous medium.
Therefore, when f ≤ 70%, the PB-1 molecule is completely dissolved in the mixed solution
w
-
4
-1
(
CPB-1=10 M ), and the increased f enhances the polarity of mixture, generating the
w
red-shifted emission due to the ICT effect. When 70%＜f ≤90%, the tightly aggregated
w
particles formed, in which the free rotation of the molecules was restricted, resulting in the
so-called RIR mechanism. By this mechanism, the emission intensity increased abruptly with
an AIE effect of 7.2.
As show in Fig. 5, PB-2 and PB-3 have similar behaviors with the f changes. When f ≤
w
w
4
0% for PB-2 and f ≤ 20% for PB-3, the molecules are still completely soluble in the mixture
w
solution. With hydrophobic methyl and phenyl groups on the N atom of barbituric acid,
respectively, they exhibits lower solubility than PB-1. In the resolvable range, the increase in
fw enhances the polarity of the mixture, the fluorescence intensity decreases due to ICT [19],
while the emission wavelength is accompanied by a red shift. However, as the water content
increases (40%＜f ≤80% for PB-2 and 20%＜f ≤60% for PB-3), the molecules pack into
w
w
loose aggregates firstly, and the polarity of the micro-environment of the luminescent
molecule is reduced due to self-wrapping, thereby generates blue-shift of the emission
wavelength. Meanwhile, the loose packing does not restrict the free rotation of the
molecules, which leads to the continuing decrease in fluorescence intensity with f increases.
w

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When 80%＜f ≤90% for PB-2 and 60%＜f ≤90% for PB-3, the molecules begin to pack
w
w
tightly into nano-aggregates due to the strong intermolecular forces, and the RIR mechanism
takes effect. As a result, the emission wavelength red-shifts while the fluorescence intensity
increases.
To verify that the gradual decrease in fluorescence at low f values is due to ICT, emission
w
spectra were recorded in different solvents of different polarities. As can be seen from Fig.
S16, The fluorescence intensity of the barbituric acid derivatives PB-1, PB-2 and PB-3
decreased with the increase of the polarity of different solutions and the emission
wavelength was red-shifted. It was verified that the fluorescence decreased gradually at low
fw values due to ICT.

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Fig. 5. Left: Fluorescence spectra of PB-1, -2 and -3 in different water content (f ) of THF/H O
w
2
mixture; Right: emission peak (square) and emission intensity (triangle) relative to the water
fraction in the mixture. The excitation wavelengths were 450, 450 and 430 nm, respectively.
The fluorescence photos of PB-1, -2 and -3 under UV irradiation (365 nm) in different
water contents are shown in Fig. 6. The fluorescence intensity of the solution first decreases,

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then increases significantly, and the color of luminescence changes significantly, from
yellow-green emission to orange or red emission.
Fig. 6. Fluorescent photographs of PB-1, -2 and -3 in different f THF/H O mixed solvents
w
2
-4
irradiated by a 365 nm ultraviolet lamp (solution concentration: 10 M)
Scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to
detect the morphology and size of nano-aggregates in different f solvents (Fig. 7) which
w
show the presence of spherical and blocky aggregates. As shown in Fig. 7, the solution was
uniformly stable at a water content of 90%, and the average diameters (d) of the PB-1, -2
and -3 aggregates were 645.2, 346.0 and 426.0 nm, respectively. These data indicate that the
enhanced emission of the compounds is related to the formation of nano-aggregates.
The intermolecular hydrogen bond (N-H···O) interaction exists in the barbituric acid unit
on PB-1 due to the tautomerization from the keto to the enol structure, resulting in the
larger size of aggregates. On the contrary, without intermolecular hydrogen bond, both the
aggregate sizes of PB-2 and -3 are much smaller than that of PB-1.

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Fig. 7. SEM images and particle size distribution histograms of PB-1, -2, -3 in a THF/H O
2
-4
mixture (f = 90%) ( solution concentration: 10 M).
w
3
.2 Theoretical calculation
In order to better understand their optical properties, we calculated the frontier
molecular orbitals using the DFT of the B3LYP/6-31G (d) basis set [22, 23]. Optimized
geometry and HOMO, LUMO energy levels of PB-1, PB-2 and PB-3 are given in Fig. 8 and Fig.
9
. They exhibit highly distorted conformations that facilitate active intramolecular rotation of
the phenyl group in solution, thereby effectively dissipating exciton energy for weak
emission in the solvent. Its HOMO is located on the electron-donor triphenylamine, and
LUMO is mainly distributed on the electron-acceptor barbituric acid. The above results
further illustrate the occurrence of an ICT process from the donor to the acceptor moiety,
which is consistent with the observed optical properties [24].

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Fig. 8. Optimized molecular ground state geometries of PB-1, -2, and -3.
Fig. 9. Electron density distributions of LUMO and HOMO molecular orbitals of PB-1, -2, and
-
3.

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3
.3 Compound fluorescence enhancement mechanism
Single crystal X-ray diffraction (SXRD) analysis can provide direct evidence of the
molecular conformation and packing structure of the compound. The single crystal of PB-2
was successfully cultured before, but it was unsuccessful for PB-1 and PB-3 crystals. Since
our group already has the single crystal data of PB-2, we cite them directly here. The specific
analysis of Fig. 10 has been presented in the previous work [4] of our group.
According to our previous work, the reason why PB-1, -2, and -3 weakly emit when the
particles dissolved in a solvent is due to the free rotations and vibrations inside molecules.
Intramolecular interaction inhibits the process of radiation decay and promotes enhanced
emission, which is primarily due to the limited intramolecular motion and some specific
molecular packing models, such as herringbone stacking [25, 26].
In the dilute solution, the steric hindrance in the molecule reduces the π-conjugation,
PB-1, -2, and -3 are almost isolated and adopted a twisted conformation. Movements such
as rotation, vibration, and stretching within the molecule consume the energy of the excited
state and suppress the decay process of the radiation. While in the aggregate state,
intramolecular vibration or rotation is locked by the interaction forces between molecules,
such as C-H···O, C=O···C and C=O···N and many CH··π interactions. Therefore, intramolecular
motion is significantly suppressed, hindering the non-radiative process.
In general, it can be said that the mechanism for enhancing PB-1, PB-3 emission is the
restriction of intramolecular motion (RIM) caused by molecular aggregation. Additionally,
PB-2 also shows a herringbone stacked conformation in the crystal, Herringbone stacking
and RIM are considered the reasons for the enhancement of PB-2 emission. Therefore,
according to the results of SEM and DLS, the formation of nano-aggregates of PB-1, -2 and -3
at high f can be attributed to the phenomenon of fluorescence enhancement.
w

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Fig. 10. ORTEP (thermal ellipsoids at 50% probability level) diagram (a), one-dimensional
structure (b), herringbone stack structure (c), and π-π interaction (d) of PB-2. (Adopted from
Royal Society of Chemistry with permission for reference 4).
3
.4 Detection of nitroaromatic explosives
As shown in Fig. 11, fluorescence of PB-3 (f = 90%) did not change within 30 minutes
w
under continuous UV irradiation, while the fluorescence of PB-1 and PB-2 decreased
remarkably in f = 90%. These results means that PB-3 (f = 90%) has excellent
w
w
photo-stability and could be used for explosive detection.
1
The H NMR spectra of PB-1, PB-2 and PB-3 were measured before and after being
irradiated for 24 hours under the UV lamp (365 nm). The results can be found in S17-S19 in
the supporting information. It is obvious from Fig. S17 that N-H peak at 11.0 ppm on the
barbituric acid of PB-1 disappears after being irradiated by UV, accompanied by a new peak
1
near 3.5 ppm which could be ascribe to the peak of O-H. This change of H NMR spectra is in
agreement with the fact that PB-1 undergoes tautomerization from keto to enol structure,
which is responsible for great decrease of fluorescence intensity (by 95% in 30 minutes). For
PB-2, the chemical shifts of the hydrogen atom in the triphenylamine moiety remained
unchanged after being irradiated, while those in both the methyls and vinyls around the
barbituric acid shifted to the high field (Fig. S18). This spectral changes meant the photo
reaction occurred around barbituric acid group, which leading to the severe photobleaching
of PB--2 that the fluorescence intensity decreased by 95% in 30 minutes. Unfortunately, the
reason for the photo reaction has not yet been clarified. For PB-3, there was almost no
1
change in H NMR spectra after being irradiated for 24 hours under the UV lamp (365 nm)

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(Fig. S19), indicating that its chemical structure did not change, this phenomenon well
explained the great photo-stability of PB-3.
Fig. 11. Light stability of PB-1, PB-2, PB-3 in f = 90%.
w
The fluorescence quenching of PB-3 (f = 90%) nano-aggregates was tested in the
w
presence of 2,4-dinitrotoluene (DNT), 2,4,6-Trinitrotoluene (TNT) and 2,4,6-trinitrophenol
-
2
(
PA) with equivalents of nitroaromatics (0.8×10 g/L). As shown in Fig. 12, PA, TNT or DNT
were gradually added to PB-3 (f = 90%) solution, and the fluorescence gradually weakened,
w
but we could see that the dripping of PA led to the fastest fluorescence reduction. The
-
2
fluorescence intensity of quenching at different concentrations (01.6×10 g/L) was
recorded, and the Stern-Volmer curve was obtained. It was found that the quenching curve
-
2
of nitroaromatic compounds (00.8×10 g/L) at low concentration was linear. As shown in
Fig. 13, the quenching constants (K ) of various nitroaromatic compounds were obtained by
SV
fitting the data [27]. It can be seen from the figure that the values of K of PA, TNT and DNT
SV
4
4
3
-1
are 4.1×10 , 1.4×10 and 2.0×10 M , respectively, when the concentration range of

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-
2
nitroaromatics are matched (00.8×10 g/L). It can be seen that the K of PA is much higher
SV
than those of TNT and DNT.
Fig. 12. PB-3 vs. PA, DNT, and TNT quenching fluorescence spectra.
Fig. 13. Fluorescence quenching Stern-Volmer curves of PB-3 with PA, TNT and DNT.
Selectivity is a key criterion for the practicality of sensing materials. In order to test the
selectivity of PB-2, PA, TNT, DNT, 1,4-benzoquinone (BQ), chlorobenzene, benzenesulfonic
acid, phenol and methanol are used as organic analytes, and meanwhile HCl and KOH as
inorganic interferences were studied (Fig. 14). As show in Fig. 14, PA displayed the highest
fluorescence quenching efficiency ((1-I/I ) ×100%) of 90%, and the quenching efficiencies of
0
TNT and DNT with nitro groups were 60% and 50%, respectively. However, there almost no
reaction of other compounds. Obviously, PB-3 has better selectivity for PA, TNT and DNT.

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The limit of detection (LOD) of PB-3 in THF-H O mixture (1: 9 v/v) for PA is 2.4 μM, which
2
can be obtained by the equation LOD = 3S/K , where S is the standard deviation of 15
sv
measurements of blank samples and K is known from above. [28, 29].
sv
By analyzing the literatures on fluorescence sensors for NACs, it was found that the K of
sv
-
4
-1
most of the sensing materials concentrates on the order of magnitude of 10 M [30-35].
This means that PB-3 exhibited comparable K with most of the reported sensing materials.
sv
Additionally, PB-3 exhibits outstanding photo-stability among the reported ones, which is an
important property for a fluorescent sensor.
Water from different resources (Yellow River water, tap water, and pond water spiked) was
tested to see any effect on fluorescence quenching. The fluorescence quenching of PB-3 (f =
w
9
0%) nano-aggregates was tested in the presence of PA with equivalents of nitroaromatics
-2
(
0.8×10 g/L). As shown in Fig. 15, it is obvious that the quenching effect is the same in
different water environments, so it can be concluded that water from different sources does
not affect the quenching behavior.
-
2
Fig. 14. Quenching efficiencies of PB-3 for (0.8×10 g/L) NACs and various interferents in

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THF-H O mixture (1:9 v/v) at room temperature.
2
Fig. 15. Fluorescence response of PB-3 (THF:H O mixture (1:9 v/v)) toward PA in different
2
water resources.

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Fig. 16. Column diagrams of fluorescence detection of PA performed in the presence of
-
5
different metal ions. Green bars: metal ion solution (c = 10 M). Purple bars: metal ion
-
4
solution with PA (c = 10 M) (PB-3, f = 90%) + metal ion + PA).
w
Simultaneously, there may be a lower concentration of metal ions in different waters,
which may affect the detection of PA. Therefore, fluorescence detection of PA was
performed in the presence of different metal ions. As shown in Fig. 16, the presence of
metal ions does not affect the detection of PA.
The availability of solid-state sensor equipment is critical for practical explosive detection.
To achieve this goal, we prepared the test paper by soaking Whatman filter paper in PB-3 (fw
-
4
=
90%) (10 M) solution and then dried it in air stream. The first is the vapor mode test. We
placed the fluorescent test paper on a glass vial containing solid PA (0.2 g) for 5 minutes. The
circular area of the test strip was exposed to PA vapor. It can be seen that the PL of the
aggregate is obviously quenched in the exposed area as shown in Fig. 17A. Then, the

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solution state was tested. The test paper were immersed into pure THF (as reference) and a
-
4
solution of PA in THF (10 M). As shown in Fig. 17B, the fluorescence of the test paper
quenched completely when it was immersed in PA/THF (Fig. 17B.e). On the contrary, there is
no obvious PL quenching phenomenon in the reference paper immersed in pure THF (Fig.
17B.d). These experimental results indicate that PB-3 can be used to prepare the sensitive
sensor for detecting nitroaromatic explosive.
Fig. 17. Paper test. (A) Gas phase mode detection of PA: before (a) and after (b) the test strip
is placed on top of a glass bottle containing solid PA for 5 minutes; (B) Solution mode.
3
.4.1 Sensing mechanism
The two mechanisms of PB-3 quenching may be Förster resonance energy transfer (FRET)
and photoinduced electron transfer (PET) [36-38]. In order to verify whether the quenching
of PB-3 is due to the presence of FRET, different nitroaromatics (NACs) absorption spectra
and PB-3 emission spectra were tested. As shown in Fig. 18, the absorption spectra of NACs
(PA, TNT and DNT) do not overlap with the emission spectra of PB-3 (as a prerequisite for
Forster-type energy transfer), and the results suggest that FRET mechanism is negligible.

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Fig. 18. Absorption and emission spectra of TNT, DNT, PA and PB-3.
Theoretical calculations were made on the basis of B3LYP/6-31g (d) to determine the
LUMO and HOMO energies of PB-3 and NACs. As shown in Fig. 8, the optimized geometry of
PB-3 exhibits a distorted conformation which can be free to rotate in a pure solution, thus
energy consumption due to non-radiative transitions, so its emission intensity is weak in
pure solution. The calculated results show that the electron densities of HOMO is located in
the triphenylamine, whereas the electron densities of LUMO is confined to barbituric acid
group (Fig. 19). These indicate that charge transfer from the triphenylamine group to the
barbituric acid group occurs in the excited state of PB-3. When the excited PB-3 is exposed to
NACs, the excited electrons are transferred from the LUMO of PB-3 to the LUMO of NACs.
The main driving force of PET is the difference between the LUMO value of the tested
compound and the NACs. The driving forces of PA, TNT and DNT were 1.194, 0.627 and 0.176
eV, respectively (Fig. 19). Compared with other NACs, PA has the most significant driving
force (1.194 eV) in PET mechanism, so PB-3 shows special selectivity for PA in three NACs
[
39].

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Fig. 19. Energy levels, energy gaps and electron cloud distributions of the HOMO and LUMO
of PA, TNT, DNT and PB-3 calculated by the B3LYP/6-325 31G (d) program.
4
Conclusions
Three D-π-A type barbituric derivatives were synthesized and the AIE property related to
the molecular structure of barbituric derivatives. In comparison with PB-1 and -2), PB-3
showed strikingly fluorescence stability at high water fractions THF/H O = 1:9 (f = 90%),
2
w
therefore further for great sensitivity detection as a prototype of the nitroaromatic analyte
of PA. Solid phase paper test based on PB-3 was also showed great sensitivity toward PA
both in the vapor and solutions.
Acknowledgements
This work was financially supported by the Natural Science Foundation of Shandong
province (ZR2018MB025), Key R&D Program Projects of Shandong province (2017GGX202001)
and the Funds for Research Leader in Jinan （2018GXRC028）.

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References
[
1] Luo J, Xie Z, Lam J W Y, et al. Aggregation-induced emission of
1
2
-methyl-1,2,3,4,5-pentaphenylsilole[J].
001(18):1740-1741.
Chemical
Communications,
[
[
2] Zhao Z, He B, Tang B Z. ChemInform Abstract: Aggregation‐Induced Emission of Siloles
J]. Cheminform, 2015, 6:5347-5365.
[
3] Chen B, Jiang Y, He B, et al. Synthesis, Structure, Photoluminescence, and
Electroluminescence of Siloles that Contain Planar Fluorescent Chromophores [J]. Chem
Asian J, 2015, 9(10):2937-2945.
[
4] Li K, Zhang Y, Qiao B, et al. Facile fabrication of AIE/AIEE-active fluorescent nanoparticles
based on barbituric for cell imaging applications[J]. Rsc Advances, 2017,
7
(48):30229-30241.
[
[
5] Qian J, Zhu Z, Qin A, et al. High-Order Non-linear Optical Effects in Organic Luminogens
with Aggregation‐Induced Emission [J]. Advanced Materials, 2015, 27(14):2332-2339.
6] Zhang Y, Li D, Li Y, et al. Solvatochromic AIE luminogens as supersensitive water detectors
in organic solvents and highly efficient cyanide chemosensors in water [J]. Chemical
Science, 2014, 5(7):2710-2716.
[
[
[
[
7] Zhang X, Liu M, Yang B, et al. Cross-linkable aggregation induced emission dye based red
fluorescent organic nanoparticles and their cell imaging applications[J]. Polymer
Chemistry, 2013, 4(19):5060-5064.
8] Wang C, Ji H, Li M, et al. A highly sensitive and selective fluorescent probe for
hypochlorite in pure water with aggregation induced emission characteristics [J].
Faraday Discussions, 2017, 196:427-438.
9] Misra R, Jadhav T, Dhokale B, et al. Reversible mechanochromism and enhanced AIE in
tetraphenylethene substituted phenanthroimidazoles [J]. Chemical Communications,
2
014, 50(65):9076-9078.
10] L i R, Xiao S, Li Y, et al. Polymorphism-dependent and piezochromic luminescence based
on molecular packing of a conjugated molecule[J]. Chemical Science, 2014,
5
(10):3922-3928.

ACCEPTED MANUSCRIPT
[
[
11] Germain M E, Knapp M J. Optical explosives detection: from color changes to
fluorescence turn-on. [J]. Cheminform, 2009, 38(9):2543-2555.
12] Gole B, Song W, Lackinger M, et al. Explosives sensing by using electron-rich
supramolecular polymers: role of intermolecular hydrogen bonding in significant
enhancement of sensitivity. [J]. Chemistry - A European Journal, 2015,
2
0(42):13662-13680.
[
13] Pon S N, Venugopalan S, Senthilkumar N, et al. Voltammetric determination of
nitroaromatic and nitramine explosives contamination in soil [J]. Talanta, 2006,
6
9(3):656-662.
[
[
14] Salinas Y, MartãNez-Mã Ã R, Marcos M D, et al. Optical chemosensors and reagents to
detect explosives [J]. Chemical Society Reviews, 2012, 41(3):1261-1296.
15] Wang D.H, Cui Y.Z, Tao F.R. et al. A novel film of conjugated polymer grafted onto gelatin
for detectingnitroaromatics vapor with excellent inhibiting photobleaching.Sensors and
Actuators B, 2016, 225,319-326.
[
[
16] NakamaruKatsumi. Synthesis, Luminescence Quantum Yields, and Lifetimes of
Trischelated Ruthenium (II) Mixed-ligand Complexes Including 3, 3′ -Dimethyl-2,
2
′ -bipyridyl [J]. Bulletin of the Chemical Society of Japan, 1982, 55(9):2697-2705.
17] Santra D C, Bera M K, Sukul P K, et al. Charge-Transfer-Induced Fluorescence Quenching
of Anthracene Derivatives and Selective Detection of Picric Acid [J]. Chemistry - A
European Journal, 2016, 22(6):2012-2019.
[
[
18] J. N. Demasa, G. A. Crosby. Measurement of Photoluminescence Quantum Yields [J].
Chemistry International, 2015, 37(5-6).
19] P.Y. Gu, C.J. Lu, Z.J. Hu, N.J. Li, T.T. Zhao, Q.F. Xu, et al. The AIEE effect and two-photon
absorption (TPA) enhancement induced by polymerization: synthesis of a monomer with
ICT and AIE effects and its homopolymer by ATRP and a study of their photophysical
properties. Journal of Materials Chemistry C: Materials for Optical and Electronic Devices,
2
013, 1(14), 2599-2606.
[
20] Thilagar P, Mukherjee S. Molecular Flexibility Tuned Emission in “V” Shaped
Naphthalimides: Hg (II) Detection and Aggregation-Induced Emission Enhancement
(AIEE) [J]. Chemical Communications, 2013, 49(66):7292-7294.

ACCEPTED MANUSCRIPT
[
[
[
[
[
[
[
[
21] Fujii K, Iyi N, Sasai R, et al. Preparation of a Novel Luminous Heterogeneous System:
Rhodamine/Coumarin/Phyllosilicate Hybrid and Blue Shift in Fluorescence Emission
[
J] . Chemistry of Materials, 2008, 20(9):2994-3002.
22] Piton A, K M, Neogra P, Reza J, et al. Benzene Dimer: High-Level Wave Function and
Density Functional Theory Calculations [J]. Journal of Chemical Theory and
Computation, 2008, 4(11):1829-1834.
23] Kai L, Xi S, Yi xuan W, et al. D-π-A type Barbituric Derivatives: Aggregation induced
emission, mechanofluorochromic and solvatochromic properties [J]. Journal of
Luminescence, 2018, 203:50-58.
24] Dong W, Pina J, Pan Y, et al. Polycarbazoles and polytriphenylamines showing
aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) behavior
for the optical detection of nitroaromatic compounds [J]. Polymer, 2015, 76:173-181.
25] S. Vyas, R. Gutzler, J. Nuss, K. Kernab and V. Bettina, Optical gap in herringbone and
p-stacked crystals of [1] benzothieno [3, 2-b] benzothiophene and its brominated
derivative, Cryst Eng Comm, 2014, 16, 7389–7392.
26] C. W. Chang, C. J. Bhongale, C. S. Lee, W. K. Huang, C. S. Hsu, W. G. Diau, et al.,
Relaxation Dynamics and Structural Characterization of Organic Nanobelts with
Aggregation-Induced Emission, J. Phys. Chem. C, 2012, 116,15146–15154.
27] Ma X, Tao F, Zhang Y, et al. Detection of nitroaromatic explosives by a 3D hyperbranched
σ–π conjugated polymer based on a POSS scaffold [J]. Journal of Materials Chemistry A,
2
017, 5(27).
28] H. Zhou, J.S. Li, M. H. Chua, H. Yan, B.Z. Tang and J.W. Xu. Poly (acrylate) with a
tetraphenylethene pendant with aggregation-induced emission (AIE) characteristics:
highly stable AIE-active polymer nanoparticles for effective detection of nitro
compounds. Polym. Chem., 5 (2014) 5628-5637.
[
[
29] S. Chemate, Y. Erande, D. Mohbiya and N. Sekar. Acridine derivative as a “turn on” probe
for selective detection of picric acid via PET deterrence. RSC Adv., 6 (2016),
8
4319-84325.
30] Z.J Luo, B. Liu, S.F. Si, Y.J. Lin, Crystal S.J. Luo, C.J. Pan, C. Zhao and Lei Wang. A
fluorescent chemosensor based on nonplanar donor-acceptor structure for highly
sensitive and selective detection of picric acid in water. Dyes and Pigments, 143(2017),
4
63-469.

ACCEPTED MANUSCRIPT
[
[
31] Ma X, Tao F, Zhang Y, et al.2D hyperbranched conjugated polymer for detecting TNT with
excellent exciton migration[J]. Sensors & Actuators B Chemical, 2017, 238:48-57.
32] C. T. Zhou, X. T. Han, G. F. Liao, et al. A Fluorescent Chemosensor with a Hybridized Local
and Charge Transfer Nature and Aggregation Induced Emission Effect for the Detection
of Picric Acid [J]. Electro, Physical & Theoretical Chemistry, 2019, 4 (10):
2
868-2873.
[
[
33] Qi L, Xiao G, Yan F, Jiao W, You Z, and Tai W, et al. A tripodal supramolecular sensor to
successively detect picric acid and CN through guest competitive controlled AIE [J]. New
J. Chem., 2019, 43, 2030-2036.
34] Laxmi Raman Adil, Peddaboodi Gopikrishna, and Parameswar Krishnan Iyer.
Receptor-Free Detection of Picric Acid: A New Structural Approach for Designing
Aggregation-Induced Emission Probes [J]. ACS Appl. Mater. Interfaces 2018, 10, 32,
27260-27268.
[
[
[
35]Jian T, Xiao Z, Yen W, et al. Ultrafast construction and biological imaging applications of
AIE-active sodium alginate-based fluorescent polymeric nanoparticles through a
one-pot microwave-assisted Döbner reaction [J]. 2018.153: 99-105
36] Mirco Natali, Sebastiano Campagna and Franco Scandola Photoinduced electron
transfer across molecular bridges: Electron - and hole-transfer superexchange pathways,
Chem. Soc. Rev. 2014, 43:4005-4018.
37] Kaur M, Mehta S K, Kansal S K. A fluorescent probe based on nitrogen doped graphene
quantum dots for turn off sensing of explosive and detrimental water pollutant, TNP in
aqueous medium[J]. Spectrochim Acta A Mol Biomol Spectrosc, 2017, 180:37-43.
38] Yuan L, Lin W, Zheng K, et al. FRET-based small-molecule fluorescent probes: rational
design and bioimaging applications [J]. Acc Chem Res, 2013, 46(7):1462-1473.
39] Dong W, Fei T, Palmacando A, et al. Aggregation induced emission and amplified
explosive detection of tetraphenylethylene-substituted polycarbazoles [J]. Polymer
Chemistry, 2014, 5(13):4048-4053.
[
[