Two-phase activated colorimetric and ratiometric fluorescent sensor for visual detection of phosgene via AIE coupled TICT processes

Article abstract of DOI:10.1016/j.saa.2021.119589

In this paper, we specifically designed and synthesized an excellent colorimetric and ratiometric fluorescent sensor DPA-CI for rapid and convenient detection of the highly toxic phosgene. DPA-CI was developed by incorporated a diphenylamine (DPA) and a 2

Full text of DOI:10.1016/j.saa.2021.119589

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Two-phase activated colorimetric and ratiometric fluorescent sensor for
visual detection of phosgene via AIE coupled TICT processes
Qinghua Hu ,1, Tao Gong , Yu Mao, Qiang Yin, Yuyuan Wang, Hongqing Wang
⇑
⇑
1
School of Chemistry and Chemical Engineering, Hunan Key Laboratory for the Design and Application of Actinide Complexes, University of South China, 28 Changsheng West
Road, Hengyang, Hunan 421001, PR China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
A colorimetric and ratiometric
fluorescent sensor DPA-CI for
phosgene sensing.
ꢀ
ꢀ
ꢀ
DPA-CI can be activated in two-phase
for visual detection of phosgene.
DPA-CI showed excellent selectivity,
rapid response, and fair sensitivity.
DPA-CI loaded test strip for phosgene
can be clearly observed by the naked
eyes.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we specifically designed and synthesized an excellent colorimetric and ratiometric fluores-
cent sensor DPA-CI for rapid and convenient detection of the highly toxic phosgene. DPA-CI was devel-
oped by incorporated a diphenylamine (DPA) and a 2-imine-3-benzo[d]imidazole as the enhanced push–
pull electronic structure into the coumarin fluorophore matrix. The sensor DPA-CI towards phosgene
sensing exhibited both visible colorimetric and ratiometric fluorescent color change in solution and in
gaseous conditions with TICT and AIE mechanism respectively, which can be easily distinguished by
using the naked eye. Also, the sensor DPA-CI showed splendid sensing performance such as excellent
selectivity, rapid response (less than 8 s in THF and 2 min in gaseous condition), and fair sensitivity (limit
Received 20 September 2020
Received in revised form 31 January 2021
Accepted 2 February 2021
Available online 16 February 2021
Keywords:
Fluorescent sensors
Phosgene
Ratiometric
AIE and TICT
Colorimetric
of detection less than 0.11 ppm in gaseous condition and 0.27 lM in solution). The design strategy based
on enhanced push–pull electronic structure with AIE and TICT properties will be helpful to construct a
solid optical sensor with excellent potential application prospects for portable and visual sensing of gas-
eous phosgene through distinct color and ratiometric fluorescence change by the naked eyes.
Ó 2021 Elsevier B.V. All rights reserved.
1
. Introduction
Phosgene, as a colorless, highly reactive poisonous gas, was the
synthesis [1–3]. As an insidious toxic gas, phosgene mainly dam-
ages the human respiratory system and has an incubation period
of several hours before the toxic reaction occurs. Specifically,
short-term exposure to more than 3 ppm of phosgene can irritate
to the respiratory system, and exposure to 90 ppm for 0.5 h was
fatal for humans [4–5]. Usually, the use of phosgene is tightly con-
trolled. However, inadvertent leaks or intentional releases (such as
biochemical weapons or terrorism) will seriously threaten the
safety of human life [6–7]. Therefore, it’s a pressing need to con-
struct a fast, convenient, and sensitive monitoring method for
widespread use in the chemical industry, such as pharmaceuticals,
synthetic polymers, chemical intermediates, pesticides, and dyes
⇑
Corresponding author.
E-mail addresses: huqinghua2007@163.com (Q. Hu), hqwang2015@126.com (H.
Wang).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.saa.2021.119589
386-1425/Ó 2021 Elsevier B.V. All rights reserved.
1

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
detecting small amounts of phosgene to stopping the damage in
the early stages.
Compared to other large-scale instrument analysis methods for
phosgene detection (such as GC, electrochemical analysis, and
HPLC) [8–10], colorimetric and/or fluorescence sensors stand out
with some unique advantages, for instance, simple implementa-
tion, low price, high sensitivity, and capable of on-site monitoring
[
11–29]. Up to now, reaction-based phosgene sensors are still the
mainstream design idea, that the sensors reacted with phosgene
can cause color and/or fluorescence change [30–55]. However, they
mostly use the aggregation-caused quenching (ACQ) property
dyes. Unfortunately, the ACQ dyes are not suitable for constructing
solid-state sensors (test-strip or membrane sensors), their photolu-
minescence could be reduced in solid-state or their preparation
was complicated, which will greatly affect the sensing
performance.
On the contrary, in 2001, Tang and his coworkers discovered a
new photoluminescence phenomenon was known as the
aggregation-induced emission (AIE) [56]. AIE-based fluorophores
showed very weak emission in diluted solutions but give off strong
emission while forming aggregates or in solid-state conditions
Scheme 1. The proposed sensing mechanism of DPA-CI for phosgene in solution
and in solid-state.
state with TICT and AIE mechanism respectively. Also, DPA-CI
exhibits a fast response to phosgene (less than 8 s in THF and
2
min in a gaseous state), great selectivity and sensitivity (detec-
[
57–68]. Therefore, AIE-based fluorophores could be more fit for
tion limit less than 0.11 ppm in gaseous condition and 0 0.27
l
M
the preparation of solid sensors, such as membrane sensors or
test-strip. Nevertheless, AIE-based fluorophores with single-
wavelength emitting can be affected by many factors, such as emit-
ting light collection efficiency, exciting light intensity, and fluores-
cence probe concentration. Ratiometric fluorescent probe with
dual-wavelength emission can effectively avoid these limitations,
which provides built-in self-calibration for sensing process with
higher sensitivity and accuracy [69–73]. Up to now, only two
AIE-activated fluorescent solid sensors have been reported for
phosgene sensing. In 2017, Wu’s group reported the first AIE-
activated test-strip sensing system for gaseous phosgene sensing
in THF). The combined effect of AIE and TICT properties endows
DPA-CI loaded test strip with excellent potential application pro-
spects for on-site portable and visual phosgene sensing through
distinct color and ratiometric fluorescence change in gaseous
conditions.
2
. Experimental sections
2.1. Material and general measurements
[
32]. Although the sensing system shows a ratiometric way, there
The reagents and solvents used in experiments were convenient
are some obvious drawbacks, such as just a 21 nm wavelength shift
after the reaction with phosgene, which will reduce the detection
sensitivity caused by the spectral overlap. Meanwhile, almost no
color change after the reaction was completed. Hence, colorimetric
detection cannot be realized. Another AIE-based sensor for phos-
gene recognition is also not sensitive enough to satisfy the need
for colorimetric detection due to the color just changes from
creamy white to yellow [52]. As a result, it’s of great significance
and remains challenging to design AIE-based colorimetric and
ratiometric fluorescent sensors with significant wavelength
variations.
To improve the performance to satisfy the distinct color and
ratio fluorescent sensing requirements for portable, sensitive, and
visual sensing of phosgene. We herein designed a hopeful colori-
metric and ratiometric fluorescent sensor (DPA-CI) for phosgene
recognition by combined a strong electron-donor diphenylamine
to buy from some common commercial suppliers, such as Sigma-
Aldrich, J&K Chemicals, Aladdin, or TCI, which were used directly
1
does not require any further refinement. H NMRs data were mea-
sured by using the Bruker 400 MHz and JEOL 400 MHz NMR Spec-
trometer by using CDCl
absorption spectra of the samples were tested on a U-3900 model
Hitachi) absorption spectrometer. Fluorescence spectra of the
3 6
or DMSO d as the solvents. UV–Vis
(
solutions and the test strips were tested on an F-7000 model (Hita-
chi) Fluorescence Spectrometer. ESI-HRMS data were collected
through a Waters LCT Premier XE mass spectrometer. The fluores-
cence quantum yield and fluorescence lifetime data were recorded
by using an Edinburgh FLS920 Full-featured fluorescence spec-
trometer. The size distribution was determined on a NanoBrook
0Plus Zeta analyzer at a fixed angle of 90° at 25 °C. Transmission
electronic microscopy (TEM) images of the sample were taken by a
Hitachi HT7700 microscope.
9
(
DPA) and an electron acceptor (also used as the reactive group
including two –NH active site) 2-imine-3-benzo[d]imidazole,
which incorporated the enhanced push–pull electronic structure
into the coumarin fluorophore matrix. DPA moiety is considered
2.2. Synthesis and characterization of the sensor
as a strong electron-donor, which is beneficial to extend the
p-
conjugated system of the sensor molecule simultaneously to delo-
calize the electron distribution. Meanwhile, the conjugate of DPA
moiety and the 3-benzo[d]imidazole group connected coumarin
fluorophore matrix by the single bonds, both the DPA moiety and
the 3-benzo[d]imidazole group have intramolecular rotation char-
acteristics. Therefore, in our designed fluorescent sensor both AIE
and TICT (twisted intramolecular charge transfer) effects could
be observed. At the same time, the redshift of absorption and emis-
sion bands can be achieved by enhancing the push–pull electron
effect. As illustrated in Scheme 1, the fluorescent sensor DPA-CI
towards phosgene sensing can be realized in solution and solid-
As described in Scheme 2, the probe DPA-CI can be easily syn-
thesized in three steps.
2.2.1. Preparation and characterization of Compound 3
Compound 3 was prepared on the basis of previous report [74]
1
in two steps as off-white solid with 64% yield. H NMR (400 MHz,
6
DMSO d , Figure. S1): d 10.74 (s, 1H), 9.93 (s, 1H), 7.54–7.39 (m,
5H), 7.31–7.20 (m, 6H), 6.30 (dd, J = 8.7, 2.2 Hz, 1H), 6.20 (d,
+
J = 2.2 Hz, 1H). HR-MS (ESI) [M + H] : m/z = 290.1183; Calculated:
19 2
C H15NO = 290.1136.
2

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
Scheme 2. The synthetic procedure of DPA-CI.
2
.2.2. Preparation of sensor DPA-CI
Compound 3 (0.4 mmol, 116 mg) and (2-benzimidazolyl) ace-
and THF solutions at 25 °C, all the data were performed in standard
fluorescent quartz cuvettes in size of 10.0 Â 10.0 mm. In our study,
phosgene is produced by dropped a nonvolatile and less toxic
triphosgene to the sensing system DPA-CI (10 lM) in THF through
the decomposition of triphosgene in the presence of 0.1% triethy-
tonitrile (0.4 mmol, 63 mg) was dissolved and transferred to a
0 mL pressure-resistant reaction tube with 15 mL ethanol, con-
tinue stirring for 30 min at room temperature, then 70 L piperi-
5
l
dine was dropped in the reaction liquid. The mixture was
allowed to heated and reflux for 2 h at 82 °C, a yellow product
was precipitated out. After cooled, the sediment was filtered under
reduced pressure and washed several times with absolute ethanol,
a final light yellow solid power DPA-CI was obtained by drying
lamine (TEA).
2.4. Preparation of test strips with DPA-CI
The probe DPA-CI loaded test strips were prepared by immers-
ing the filter papers (2 Â 5 cm) in the stock solution of DPA-CI
under vacuum (154 mg, 90% yield). 1H NMR (400 MHz, DMSO d
6
,
-3
Figure. S3): d 12.94 (s, 1H), 10.49 (s, 1H), 8.46 (s, 1H), 8.11 (d,
J = 8.9 Hz, 1H), 7.63 (d, J = 6.9 Hz, 1H), 7.45–7.41 (m, 5H), 7.24–
(1 Â 10 M) and taken out drying under air. The test strips can
be stored in a dryer after preparation.
7
1
.20 (m, 7H), 6.68 (dd, J = 8.5, 2.2 Hz, 1H), 6.46 (d, J = 2.2 Hz,
H), 6.41 (dd, J = 8.9, 2.3 Hz, 1H). 13C NMR (400 MHz, CDCl
, Figure.
3
2.5. Detection of phosgene in solvents
S4): d 157.90, 154.61, 151.64, 148.41, 146.18, 136.01, 129.84,
29.39, 126.28, 125.19, 122.71, 116.57, 112.81, 105.87. HR-MS
1
In this part, triphosgene has been used as a precursor, which
can be decomposed by trimethylamine(TEA) to generate phosgene
in-situ. Spectrum response of phosgene in solution was conducted
by adding different amounts of triphosgene stock solution into the
+
(
20 4
ESI) [M + H] : m/z = 397.1197; Calculated: C28H N O = 376.1463.
2
.2.3. Preparation of DPA-CI-PS
The probe DPA-CI (0.3 mmol, 128 mg) and 606 mg Et
6 mmol) was added into a 250 mL two-neck flask contained
N
THF sensor solution DPA-CI (10 lM) containing 0.1% TEA at 25 °C.
3
(
After a 1 min reaction, spectral changes in the emission spectra
(kex = 480 nm) and absorption (340 nm-580 nm) of the sensing sys-
tem were then recorded in a fluorescence spectrophotometer and a
UV–Vis spectrophotometer, respectively.
1
0
00 mL dichloromethane, then the mixture was immersed into
°C ice bath. After stirred for 5 min under a N atmosphere, then
2
triphosgene (0.5 mmol, 149 mg) was completely dissolved in ace-
tonitrile (15 mL) and slowly dripping into the reaction fluid with
an injector. The reaction mixture was further stirred under room
temperature over 10 h. Next, concentrated with reduced pressure
distillation, the crude solid product was further purified by silica
2.6. Phosgene sensing with test strips
Relative to other available materials, the commonly neutral fil-
ter paper is more suitable for preparing the sensing test strips.
These paper strips were prepared by the method described previ-
ously and use directly. The different concentrations of gaseous
phosgene were generated by mixing different amounts of triphos-
gene solutions in dichloromethane and 0.1% TEA in dichloro-
methane at the bottom of a sealed conical flask (200 mL). The
test strips were hanged and exposed to these phosgene vapors
and other analytes for 2 min. Then, taken out the test strip and
recorded it on a fluorescence spectrophotometer (kex = 365 nm).
Meanwhile, a digital camera was used to collect the photos under
visible light and a 365 nm hand-held lamp.
gel column using CH
der DPA-CI-PS (125 mg, 92%). H NMR (400 MHz, CDCl
S6): d 8.86 (s, 1H), 8.51 (dd, J = 6.4, 2.4 Hz, 1H), 7.83 (dd, J = 6.4,
.2 Hz, 1H), 7.53 (d, J = 8.9 Hz, 1H), 7.43–7.48 (m, 5H), 7.34–7.26
2 2
Cl /methanol (v/v, 40/1) to yield a red pow-
1
3
, Figure.
2
1
3
(
m, 7H), 7.02 (dd, J = 8.8, 2.2 Hz, 1H), 6.98 (d, J = 1.9 Hz, 1H).
C
3
NMR (400 MHz, CDCl , Figure. S7): d 165.71.90, 155.63, 154.99,
1
1
1
C
52.02, 145.64, 144.56, 138.75, 130.70, 130.33, 127.08, 126.98,
25.55, 124.86, 119.26, 117.53, 115.79, 113.19, 106.03, 104.43,
+
00.00. HR-MS (ESI) [M + H] : m/z = 455.1505; Calculated:
29
H
18
N
4
O
2
= 454.1430.
2.3. Experimental Methods.
3
. Results and discussion
-
3
The stock solution (1 Â 10 M) of DPA-CI and DPA-CI-PS were
prepared by dissolving the solid in HPLC grade THF, while other
analytes were dissolved in dichloromethane respectively. Optical
studies (including UV–vis and fluorescent spectrum) were per-
formed in HPLC grade THF, or the different proportions of water
3.1. Probe DPA-CI preparation and characterization
Compound 3 was obtained by using 3-Methoxytriphenylamine
3 3
BBr and POCl as the reactant in two steps and finally produced an
3

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
off-white solid in 64% yield. For the synthesis of the probe DPA-CI,
showed a red-shift from 532 nm to 558 nm, which could con-
tribute to the TICT effect due to the suppressing rotation of
diphenylamine moiety and 3-benzo[d]imidazole moiety in viscous
media. Also, the TICT effect of DPA-CI can be explained in terms of
DFT calculations under Gaussian 16 at the B3LYP/6-31G(d) basis
set. As illustrated in Figure S12, the photoexcitation (electron
clouds) from the HOMO to LUMO of DPA-CI involves an obvious
ICT process from the diphenylamine unit to the 3-benzo[d]imida
zole-chromen-2-imine unit. Since the phenylamine unit in the
diphenylamine unit and the single bond between 3-
benzimidazole and the chromen-2-imine unit are freely rotatable.
So the previously mentioned ICT process is an actual TICT process
formed by a significant molecular geometry change.
(
2-benzimidazolyl)acetonitrile, and compound 3 were refluxed 1 h
in ethanol with a catalytic amount of piperidine to obtain a yellow
solid power with 90% yield. The probe DPA-CI can react efficiently
with phosgene (generated by triphosgene with TEA) in DCM, giving
an expected reference sensing product DPA-CI-PS in very good
yield (92%). The structures of these three compounds were fully
1
13
verified by H NMR, C NMR, and HR-MS(ESI) (as shown in Sup-
plementary Data Figure S1-S8).
3.2. Optical properties measurement
In our design idea, we expect to obtain a sensor with AIE and
TICT properties. To verify our thoughts, we tested the optical prop-
erties (including emission spectrum, fluorescence quantum yield,
Uv–vis absorption, and fluorescence lifetime) of the compounds
DPA-CI and DPA-CI-PS in solution and solid condition. The sensor
DPA-CI consists of a diphenylamine moiety which acts as a strong
electron donor and 3-benzo[d]imidazole moiety is considered to be
the electron-acceptor, which is connected by a rotatable single
bond resulting in a twist propeller-like structure and incorporated
Besides the TICT activity, we also investigated the AIE behavior
of DPA-CI and the sensing product DPA-CI-PS. The experiments
were carried out by monitoring its emission spectra in mixture sol-
vent systems (THF/water) with different fractions of water (f ),
w
which is a classic method applied to evaluate the AIE effect. As
revealed in Fig. 2(A, C, E), DPA-CI shows an emission peak at
500 nm in THF with a quantum yield (U ) of 24.4%. With the f
f w
increases from 0 to 99%, the peak intensity of DPA-CI decreased
gradually, along with the maximum emission wavelength red-
shift from 500 nm to 554 nm. This phenomenon could be caused
by the TICT effect arising from the fraction of water increase. In
comparison, TPE-CI-PS exhibits completely different photolumi-
nescence properties. As shown in Fig. 2(B, D, F), DPA-CI-PS exhibit
an orange light at around 570 nm in dilute THF solution with the
the D-p-A push–pull electronic structure.
To confirm the TICT activity of the DPA-CI, the solvent polarity
effect on the UV–vis absorption spectrum and fluorescence spec-
troscopy of DPA-CI was investigated. The good solubility of sensor
DPA-CI in some familiar organic solvents such as EtOAc, chloro-
form, THF, DCM, DMF, DMSO, MeCN, and MeOH allows us to inves-
tigate the TICT activity in the solvents with different polarities, but
it hardly soluble in water. The UV–Vis spectrum can be seen in Fig-
ure S9, the sensor DPA-CI has a peak at around 430 nm in THF,
which changes little and exhibit quite a similar absorption peak
in other polarity solvents. On the contrary, the fluorescent emis-
sion spectra of DPA-CI changed more obviously with the solvent
polarity changed. As illustrated in Fig. 1A, the emission peaks var-
ied gradually from 496 nm to 533 nm with the solvent polarity
increased. Simultaneously, the emission intensity in chloroform
and DCM were much higher than other solvents. These spectral
data confirmed that the fluorescent properties of DPA-CI are signif-
icantly affected by the change of solvent polarity, which results
f
U of 5.6%. However, when 10% of water added, the fluorescence
intensity at 570 nm decreased dramatically. This fluorescence
quenching effect could be explained as the TICT effect in the
DPA-CI-PS solution due to the addition of water and increased sol-
vent polarity. When the water fractions ranging from 10% to 70%,
the fluorescence intensity of DPA-CI-PS continues to decrease
slowly. Upon further increasing the f
w
from 70% to 80%, a rapid
enhance in the peak intensity at 592 nm together with a redshift
in the peak emission wavelength from 570 nm to 592 nm was
observed. Due to the poor solubility in the high water fractions
THF solution, DPA-CI-PS will be aggregated and activates the AIE
process. Surprisingly, when the f
emission decreased rapidly. This phenomenon may be attributed
to two possible factors [67]: (1) When the f above 80%, the exces-
w
increased from 80% to 95%, the
from the TICT effect in the DPA-CI molecule with D-p-A structures.
Moreover, a large Stokes shift over 100 nm was realized.
To further explored the impact of the polarity of the solvents on
the photoluminescence spectra of DPA-CI, the emission maximum
w
sive aggregation DPA-CI-PS molecules leads to the reduction of
internal radiation, which results in a less emissive; (2) In a high
shift of DPA-CI in different solvents versus the E
T
(30) (solvent
w
f solution (above 80%) the sensor molecules may form more
polarity empirical parameter) is plotted in Fig. 1B and the other
related data were listed in Table S1. A linear relationship fitting
was found with a good correlation coefficient (R2 = 0.986) and a
amorphous particles, which could decrease the emission intensity.
To prove that assertion, the aggregation of 80% and 95% water frac-
tion of TP-CI-PS was characterized by dynamic light scattering
(DLS) and transmission electron microscopy (TEM). As shown in
Figure S13A and S13B, the average hydrodynamic diameter is
3
.72 slope was obtained between the emission maximum and
T
E (30), which showed a weak solvatochromism of DPA-CI. To fur-
ther confirmed the TICT activity of the sensor DPA-CI. The solva-
tochromic properties of DPA-CI were further evaluated by the
112.56 ± 0.58 nm (f
tively. The TEM images revealed good dispersion nanoparticles in
the f of 80% (Figure S13C). While in the f of 95%, the dispersed
nanoparticles produced significant aggregation (Figure S13D).
Besides, we also measured and fitted the average fluorescence life-
w w
80%) and 381.33 ± 0.78 nm (f 95%), respec-
Lippert-Mataga equation of the
polarity parameter) for TICT molecules (Table S1 and Fig. S10). As
shown in Fig. S10, the of each DPA-CI solvent increases with
D
m
(Stokes shift) versus
D
f (solvent
w
w
D
m
the increase of solvent polarity. A high slope of 8993.70 was
obtained by the linear line, which shows the striking difference
times (
solid-state. As shown in Figure S14 and S15, the average fluores-
cence lifetimes of DPA-CI in THF, 80% f solution, and solid-state
w
s) of DPA-CI and DPA-CI-PS in THF, 80% f solution, and
(D
l) between the
l
g
(ground state dipole moment) and the
l
e
(ex-
w
cited dipole moment). The results indicated that the charge separa-
tion exists in the DPA-CI structure and further confirmed the TICT
assumption. Subsequently, the emission responding toward viscos-
ity of DPA-CI in methanol and glycerol mixture was investigated.
By adding the high viscosity glycerol into methanol solution, while
were fitted for 2.87 ns, 3.08 ns, and 1.79 ns at 554 nm, respectively.
While the average fluorescence lifetimes of DPA-CI-PS in THF, 80%
w
f solution, and solid-state at 592 nm were fitted as 7.19 ns,
9.03 ns, and 2.46 ns, respectively. The fluorescence quantum yields
of DPA-CI and DPA-CI-PS in solid states were measured at 3.4%
and 17.7% respectively, these results are consistent with the previ-
ous AIE behavior test results in THF/water mixture systems.
To have a better insight into the novel optical properties change
of the sensing strategy, the trends in the frontier orbital distribu-
the glycerol fraction (f
DPA-CI was reduced gradually. In sharp contrast, as the f
G
) increase to 50%, the emission at 532 nm of
contin-
G
ues to rise to 100%, the emission of DPA-CI was enhanced (Fig-
ure S11 A and B). Meanwhile, the emission peak of DPA-CI
4

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
Fig. 1. (A) Fluorescent spectra of DPA-CI in different solvents. (B) The plot of the maximum emission peaks of DPA-CI in some common solvents versus each solvent polarity
empirical parameter E (30). Final concentration: 10 M; Excitation at 475 nm.
T
l
Fig. 2. Emission spectra of (A) DPA-CI and (B) DPA-CI-PS in THF/water mixture solvent systems with different fractions of water (f
different water fraction (f ) of (C) DPA-CI (at 500 nm) and (D) DPA-CI-PS (at 592 nm); Fluorescent pictures of (E) DPA-CI and (F) DPA-CI-PS in THF/water mixture systems
with the water fractions (f ) increased. Final concentration: 10 M; excited at 475 nm.
w
); The plot of maximum intensity versus
w
w
l
tions and the electronic transition energies for DPA-CI and the
expected sensing product DPA-CI-PS were optimized and calcu-
lated with the density functional theory (DFT) method. As illus-
trated in Fig. 3, the HOMO electron clouds of two optimized
molecules were nearly uniformly distributed vertically over the
diphenylamine unit and 3-benzimidazole unit to the inocoumarin
cyclic urea part. That because the inocoumarin cyclic urea struc-
ture in DPA-CI-PS was considered as a rigid planar, which inhibits
single bond rotation between 3-benzimidazole and the iminocou-
marin unit, and further enhanced the ICT effect and activate the
AIE properties in DPA-CI-PS. In general, the enhanced ICT effect
will cause a redshift in absorption wavelength and enhanced
fluorescence-emission. Also, the supposed optical properties
change was in qualitative support the HOMO-LUMO energy gap
difference between DPA-CI (0.120 eV) and DPA-CI-PS (0.101 eV),
p-p conjugation. While excited, on the LUMO of DPA-CI, just the
electron clouds on the diphenylamine unit transferred to the imi-
nocoumarin part, forming the TICT process. However, by compar-
ison, we find that the TICT process on the LUMO of DPA-CI-PS
was realized by transferring the most electron clouds from the
5

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
8
s upon the triphosgene added (Fig. 4D). The above results showed
that DPA-CI can be used as a colorimetric and ratiometric fluores-
cent senor for phosgene sensing in THF solvent.
3.4. Sensing properties of DPA-CI in solid State.
Inspired by the promising optical properties in solution, since
the phosgene is existing as a poisonous air pollutant, there is little
point in identifying phosgene gas in solvents. So we need to estab-
lished a method for the monitoring of gaseous phosgene. Com-
pared with other types of solid-state based optical sensors, the
paper-based test strip is simpler to prepare and convenient to
use. Therefore, a new test strip was prepared by immersed a qual-
itative filter paper in the DPA-CI solution and take out to dry nat-
urally. The gaseous phosgene was produced by mixing various
concentrations of triphosgene and a proper amount of TEA in a
cork-sealed conical flask. The test strips were suspended in the
flask and exposed to phosgene vapors for 2 min, then recorded
on a fluorimeter (kex = 365 nm). As shown in Fig. 5A, without the
phosgene, the DPA-CI loaded test strip is initially emitted at
around 554 nm. Upon exposure to phosgene, the peak intensity
at 554 nm gradually dropped off, a new emission band at around
5
92 nm was generated and its intensity enhanced along with the
phosgene concentration increasing. Therefore, the test strips in
response to the gaseous phosgene also exhibited a ratiometric
way. The redshift of the wavelength and the increasing emission
intensity at 592 nm is due to the reaction between DPA-CI and
phosgene. The formation of hexatomic cyclic urea inhibited the
intramolecular rotation in the 3-benzimidazole part and further
enhanced the AIE effect of the test strip. Moreover, as shown in
Fig. 5B, in the range of 2–45 ppm, the ratio of I592 nm/I554 nm versus
Fig. 3. Comparison of the frontier molecular orbitals of DPA-CI and DPA-CI-PS and
the orbital energies.
which were following the red-shift of absorption peaks from
4
5
30 nm to 506 nm and the maximum emission peaks from
02 nm to 578 nm in solution.
2
the phosgene concentration existed good linearity (R = 0.996). The
limit of detection (LOD) of the test strip for gaseous phosgene sens-
ing was calculated as low as 0.11 ppm according to the equation
3.3. Optical sensing experiments of DPA-CI in solution.
3r/k, which was far below the critical concentration (20 ppm) that
can result in immediate danger to human health [34]. To further
demonstrate the promising application of DPA-CI for phosgene
visual detection, the exposed test strips were photographed by a
digital camera under a daylight lamp and a portable 365 nm lamp.
As illustrated in Fig. 5C, with the phosgene level increasing, under a
daylight lamp, the test strips color changed from yellow to peach,
while under the 365 nm lamp, the emission color changed from
light yellow to rose. The color change can be clearly distinguished
by the naked eyes even in the very low level of phosgene vapor.
These results confirmed that the DPA-CI loaded test strip can be
used for portable, visual, and reliable sensing of phosgene gas
based on AIE processes.
The optical sensing assays of DPA-CI were conducted by titrat-
ing various concentrations of triphosgene stock solution into the
sensing solution of DPA-CI (10 M) in THF at 25 °C containing
.1% TEA. The changes in fluorescent emission (kex = 470 nm)
l
0
and the absorption spectrum were monitored after the triphosgene
was added for one minute. As shown in Fig. 4A, sensor DPA-CI in
THF mainly showed a maximum absorption peak at 425 nm. With
the triphosgene amount increasing, the absorption band at 425 nm
gradually decreased, and a new absorption band at 506 nm gradu-
ally enhanced, which accompanied a noticeable color change from
light yellow to orange (Fig. 4A, inset), showed a colorimetric sens-
ing process. Then, we tested the fluorescence change of the sensor
DPA-CI with phosgene. As expected, we can see in Fig. 4B, the sen-
sor exhibited an excellent ratiometric fluorescence change to phos-
3.5. The selectivity of the test strip for phosgene.
gene. With the phosgene concentrations increasing (0–200
l
M),
A good sensor should have excellent selectivity, to evaluate the
selectivity of the DPA-CI loaded test strip for phosgene over several
related gaseous contaminants, such as toluenesulfonyl chloride
the maximum emission peak at 502 nm decreased dramatically
along with a new emission band increased at 578 nm. A distinct
fluorescence change from green to orange-red is easily identified
under a 365 nm desk lamp (Fig. 4B, inset), indicating a ratiometric
fluorescence detection process. These remarkable optical proper-
ties change could be ascribed to the phosgene induced cyclization
reaction with the –NH groups in 3-benzo[d]imidazole-chromen-2-
imine unit to form hexatomic cyclic urea, which inhibited part of
the TICT behavior and increasing the conjugate system. As illus-
trated in Fig. 4C, the ratio of the emission intensity at 578 and
2 2 2 3 2 2 2 3 3
(TsCl), SO Cl , SOCl , POCl , C Cl O , CH COCl, NH , TEA, HCl, and
diethyl chlorophosphate (DCP). The fluorescence spectra and the
photos were collected. As shown in Fig. 6A and 6C, only in the pres-
ence of phosgene the test strips could induce significant fluores-
cence spectra and color change, while other related gaseous
contaminants could not cause any obvious fluorescence spectra
or color change compared to the control samples. Although some
other acyl chlorides could couple with one –NH group in DPA-CI,
they cannot further combine with another amine group to form
hexatomic cyclic urea. Thus, they cannot influence the fluorescence
and absorption spectrum of DPA-CI due to the TICT behavior
between 3-benzimidazole and the iminocoumarin unit cannot be
inhibited and the conjugate system has not been prolonged. As a
5
0
02 nm (I578/I502) versus the increasing amount of phosgene from
2
to 90
lM showed an excellent linear relationship (R = 0.994),
and the LOD (limit of detection) was calculated as 0.27
l
M (accord-
ing to the formula 3d/k). Meanwhile, the time-dependent studies
showed that the peak intensity at 502 nm reaches a plateau within
6

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
Fig. 4. (A) Absorption spectrum and (B) emission spectrum of DPA-CI (10
l
M) in THF upon the addition of different amounts of phosgene (0–200
M) vs different concentrations of phosgene in THF. (D) Time-dependent intensity
lM) in THF. Excited at: 475 nm, slits: 10/10 nm.
lM) for 1 min at 25 °C. (C)
The plot of the intensity ratio I578 nm/I502 nm (inset) and linear relationships of DPA-CI (10
response at 502 nm of DPA-CI (10 M) with phosgene (200
l
l
Fig. 5. (A) Fluorescent spectra of DPA-CI test strips in the presence of different amounts of gaseous phosgene for 2 min. Insert the fluorescence spectra from phosgene
concentration 20 ppm to 45 ppm of DPA-Cl (B) Linear correlation of the peak intensity ratio (I592 nm/I554 nm) of DPA-CI test strips vs the content of phosgene. Insert the peak
intensity ratio of I592 nm/I554 nm as a function of phosgene concentrations. Excited at: 365 nm. (C) Pictures of the color (a) and fluorescent color change (b) of DPA-CI test strips
in the presence of increasing content of phosgene vapor for 2 min at 25 °C. Under a 365 nm lamp.
7

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
Fig. 6. (A) Fluorescent spectrum and (B) The peak intensity ratios (I592 nm/I554 nm) of DPA-CI test strips upon the addition of each analyte for 2 min. (1) air (blank), (2)
phosgene, (3) toluenesulfonyl chloride (TsCl), (4) SO Cl , (5) SOCl , (6) POCl , (7) C Cl , (8) CH COCl, (9) NH , (10) TEA, (11) HCl, (12) diethyl chlorophosphate (DCP). (C) The
picture of the color (top) and fluorescent color change (bottom) of the test strips with the existence of different gaseous analytes.
2
2
2
3
2
2
O
2
3
3
result, the probe has better selectivity to phosgene molecule with
two active reaction sites. In Fig. 6B, the intensity ratio (I592 nm/I554
nm) histogram further illustrates the excellent selectivity of the
DPA-CI loaded test strip. These desirable results confirmed that
the test strips made from DPA-CI can be a valuable portable sensor
for a visual and sensitive monitor of phosgene gas.
tivity and rapid colorimetric and ratiometric fluorescence sensing
process and be finished within 8 s in solution and 2 min in gaseous
states, and the LOD can be calculated less than 0.11 ppm in gaseous
condition and 0.27
lM in solution. Especially, the sensor DPA-CI
loaded test strip features excellent selectivity for phosgene over
other potential interferes. As compared with the other probes in
Table S2, based on the unique advantage of the AIE characteristics
and obvious dual-modes color change, our strategy would present
some helpful insights into the manageable development of AIE
based long-wavelength solid-state colorimetric and ratiometric
fluorescent sensors for convenient and visual recognition of highly
toxic gaseous pollutants.
3.6. Exploration of sensing mechanism
In the synthesis section, the reaction product (DPA-CI-PS) of
DPA-CI with phosgene has been purified by column chromatogra-
phy, and the molecular structure of DPA-CI-PS was fully character-
1
ized by H NMR spectra and HR-MS. To further investigate the
1
sensing mechanism, thin-layer chromatography (TLC) and
H
CRediT authorship contribution statement
NMR titration along with HR-MS titration of TPE-CI with phosgene
in DMSO d were performed. As illustrated in Fig. 7A, the proton
6
Qinghua Hu: Formal analysis, Funding acquisition, Writing -
review & editing. Tao Gong: Investigation, Writing - original draft,
Formal analysis. Yu Mao: Software. Qiang Yin: Software, Formal
analysis. Yuyuan Wang: Resources. Hongqing Wang: Conceptual-
ization, Methodology, Funding acquisition, Supervision, Validation,
Writing - review & editing.
peak at 10.49, and 12.94 ppm were attached to the adjacent NH
protons of Ha and Hb in the DPA-CI molecule, respectively. After
phosgene bubbled into the DPA-CI solution, as seen in Fig. 7B,
the proton peaks of Ha and Hb disappeared due to the two active
–
NH groups reacted with phosgene and generated hexatomic cyc-
1
lic urea. Also, the generation of the product via the H NMR titra-
tion was further confirmed by HR-MS (Figure S16), where two
dominant peaks at m/z value of 455.1507 (M + H ) and 477.1322
+
Declaration of Competing Interest
+
(
M + Na ), which confirmed the expected product m/z value at
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
4
54.1430 (calculated by ChemDraw software). Besides, the TLC
analysis (Figure. S17) intuitively verified the product changes in
the sensing process. Finally, combined with the previous DFT
results for photophysical properties change, the possible sensing
mechanism of the sensor DPA-CI for phosgene was followed pro-
posed in Figure. S18.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (11804146, 11875161 and 51703075), and the Natural
Science Foundation of Hunan Province (2020JJ5462), are gratefully
acknowledged.
4
. Conclusion
In conclusion, we have successfully developed
a
diphenylamine-iminocoumarin-3-benzo[d]imidazole based fluo-
rescent sensor DPA-CI for portable and visual recognition of highly
toxic phosgene. The sensor undergoes TICT and AIE process trans-
forms through twice carbamylation reactions with phosgene in
solution or the solid-state test strips. More interestingly, the sensi-
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.saa.2021.119589.
8

Q. Hu, T. Gong, Y. Mao et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 253 (2021) 119589
Fig. 7. 1H NMR analysis of (A) DPA-CI in DMSO d
(B) DPA-CI after the addition of TEA and triphosgene in DMSO d .
6 6
separated emissions with independent excitations, Biosens. Bioelectron. 81
2016) 341–348.
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