A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as chemosensor for ClO?

Article abstract of DOI:10.1016/j.saa.2019.117794

Red material is widely used in many fields because it has a lot of high performances such as strong penetrability, little trauma to cell and tissue, easy to prepare, and low background interference. However, a lot of organic materials were troubled with t

Full text of DOI:10.1016/j.saa.2019.117794

SAA-117794; No of Pages 7
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
A red fluorogen: AIEE characteristic, photoluminescence mechanism and
its application as chemosensor for ClO−
⁎
Liqiang Yan , Ya Xie, Jianping Li, Wenyuan Zhu
Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin, Guangxi 541004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2019
Received in revised form 10 November 2019
Accepted 11 November 2019
Available online xxxx
Red material is widely used in many fields because it has a lot of high performances such as strong penetrability,
little trauma to cell and tissue, easy to prepare, and low background interference. However, a lot of organic ma-
terials were troubled with the aggregation caused emission quenching (ACQ) effect, which really limits their
practical applications. In contrast, aggregation induced emission (AIE) and aggregation-induced enhanced emis-
sion (AIEE) effects provide an efficient method to break the obstacle of ACQ effect. Herein, a red light molecule
was developed by integrating cyano and alkyl sulfide moieties. Its photoluminescence mechanism was further
revealed by fluorescence spectrum, density functional theory (DFT) and X-ray single crystal diffraction, respec-
tively. It is found that this compound has good planar construction and has no rotatory unit, it showed typical
AIEE performance because of intramolecular D-π-A structure and the formation of J-aggregation. This molecular
design principle may be able to offer an effective strategy to exploit red AIE/AIEE organic materials. Meanwhile,
this fluorogen showed excellent response capability to ClO− including high selectivity and sensitivity, and cell
imaging performance.
Keywords:
Red fluorogen
AIEE
ClO−
Chemosensor
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
and so on [26–30]. These red AIE molecules share a common character
of complex molecular structure consisting of phenyl rotor, electron-
Luminescent materials based on organic molecules have been fo-
cused widely and continuously in organic light-emitting devices, optical
sensing, liquid crystal display, and stimuli-responsive due to rich colors,
simple synthesis, adjustable structure, good light stability, and no heavy
metal pollution [1–10]. However, many organic luminescent molecules
exhibit intense fluorescence in benign solvent solution while show sig-
nificant fluorescence quenching in the aggregation state or in the solid
state for the formation of excimer and exciplex by the strong intermo-
lecular π-π stacking interaction [11,12]. This is known as aggregation
caused emission quenching (ACQ) effect, which really limits their prac-
tical applications. In contrast, aggregation induced emission (AIE) and
aggregation-induced enhanced emission (AIEE) effects provide an effi-
cient method to break the obstacle of ACQ effect [13]. AIE and AIEE lumi-
nescent materials have no fluorescence or weak emission in solution,
but fluoresce strongly.
donating group (N,N-dialkyl group) and electron-withdrawing group
(cyano group). In solution, the energy of the excited state molecule is
exhausted because of the free rotation of phenyl rotors via the single-
bond axes. In aggregates, such intramolecular rotation is restricted
(RIR) due to the steric hindrance [31]. In addition, the Cyano group pro-
vides an appropriate twisted molecular conformation that prevents the
intermolecular π-π stacking interaction [32–35]. The non-radiative
decay thus turns off, while fluorescence turns on.
Herein, we constructed a cleverish fluorogens with AIEE characteris-
tic and red emission by integrating multiple Cyano groups (acceptor,
A) and methylthio group (donor, D). This molecule exhibited pro-
nounced fluorescence in solution due to intramolecular D-π-A structure
and showed fluorescence enhancement in solid state because of the for-
mation of J-aggregate. Additionally, it can response to ClOˉ with high
specificity and sensitivity, and can be used as fluorescence chemosensor
for ClOˉ detection (Scheme 1).
At present, more and more AIE and AIEE organic molecules have
been prepared [14,15]. Nevertheless, the vast majority of them are
blue and green light materials [16–25], the number of red light mate-
rials is still very limited including quinoline nitrile derives, diphenyl
fumaronitrile substituted by carbazole derives, TPE-BODIPY derives
2. Experimental
2.1. Apparatus and materials
1H and 13C NMR spectra were measured on nuclear magnetic reso-
nance spectrometer (AVANCE III HD 600 MHz, Bruker, Switzerland)
using TMS as the internal standard. Mass spectrum was recorded with
⁎
Corresponding author.
E-mail addresses: liqiangyan@glut.edu.cn (L. Yan), likianping@263.net (J. Li),
wyzhu@glut.edu.cn (W. Zhu).
https://doi.org/10.1016/j.saa.2019.117794
1386-1425/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
chemosensor..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117794

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L. Yan et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy xxx (xxxx) xxx
Scheme 1. Synthetic route of compound 1.
a Bruker solan X70 FT-MS. Fluorescence spectra were recorded by Cary
Eclipse fluorescence spectrophotometer (Varian, America). Fluores-
cence quantum efficiency was measured with a Fluorolog 3-TSCPC
and integrating sphere accessory (Horiba Jobin Yvon Inc. France). Ultra-
violet visible absorption spectrum was tested using UV–vis spectrome-
ter (Lambda 950, PerkinElmer, USA). Cell images were got under laser
scanning confocal microscopy (LSM700, Zeiss, Germany). Diffraction
data was collected on Agilent G8910A CCD diffractometer. Fluorescence
microscopy images were taken under fluorescence microscope (Olym-
pus CX33). FTIR spectra were acquired by a Bruker TENSOR27 infrared
spectrophotometer with KBr pellet in the range of 4000 cm−1 to
400 cm−1. All of the reagents and organic solvents are analytically
pure. They were purchased from Sahn chemical technology (Shanghai)
corporation and were used directly. Deionized water was used through-
out the test process.
3. Results and discussion
3.1. Photophysical and AIEE characteristics
Ultraviolet absorption spectra (Fig. S4) and fluorescence spectra
(Fig. S5) of compound 1 in different organic solution including
dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), acetoni-
trile (CH3CN), ethyl acetate (EA), absolute ethyl alcohol (EtOH) and di-
chloromethane (DCM). The absorption peaks are located at 449 nm,
448 nm, 444 nm, 450 nm, 442 nm and 450 nm, respectively. The maxi-
mum emission peaks are 610 nm, 604 nm, 596 nm, 581 nm, 593 nm and
586 nm, respectively.
AIEE characteristic of compound 1 was investigated in DMSO/H2O
mixture because it is soluble in organic solvents but insoluble in
water, and compound 1 molecules must aggregate in DMSO/H2O mix-
tures with a high water fraction (ƒw, v:v). As shown in Fig. 1, compound
1 displayed purple fluorescence in pure DMSO solution, and its fluores-
cence quantum yield was 4.69%. Along with the increasing of ƒw, the
fluorescence enhanced gradually. When ƒw reached 90%, the fluores-
cence quantum yield of compound 1 increased to 16.44%, which im-
proved by 3.5-fold. In addition, the emission wavelength red shifted to
640 nm from 609 nm. The colour of compound 1 solution changed to
red from pink (Fig. 1 inset). However, the fluorescence intensity de-
creased slightly when ƒw was 95%, which can be caused by the precipi-
tation of molecular aggregation. With the increase of water fraction, the
fluorescence intensity enhancement (black square line) and the wave-
length redshift (red dot line) of compound were revealed visually as
shown in Fig. 2. These results proved that compound 1 has remarkable
AIEE performance.
2.2. Syntheses
2-(3-Cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile
(2) was synthesized by the reaction between 3-hydroxy-3-
methylbutan-2-one (3) and malononitrile according to the published
literature [36].
Compound 2 (1.0 g, 5.0 mmol), 4-methylmercaptobenzaldehyde
(0.76 g, 5 mmol) and EtONH4 (0.77 g, 10 mmol) were dissolved in the
mixture solution (THF/EtOH = 4:1, 60 mL). The reaction mixture was
stirred for 24 h at room temperature. The deep red precipitate was ob-
tained after filtration and was further purified by silica gel column chro-
matography with CH2Cl2 as mobile phase and recrystallization from the
mixture solution of CH2Cl2 and ethanol. Compound 1 (yield: 78.4%). 1H
NMR (600 MHz, DMSO‑d6), δ (ppm): 7.93, 7.89 (d, J = 24 Hz, 1H); 7.86,
7.84 (d, J = 12 Hz, 2H); 7.38, 7.36 (d, J = 12 Hz, 2H); 7.20, 7.16 (d, J =
24 Hz, 1H); 2.56 (s, 3H); 1.79 (s, 6H). 13C NMR (151 MHz, DMSO‑d6), δ
(ppm): 177.59, 175.76, 147.55, 145.53, 131.10, 130.36, 126.01, 114.55,
113.21, 112.38, 111.45, 99.75, 98.92, 54.52, 25.66, 14.47. EI-MS: calcu-
lated for C19H15N3OS 333.09, found 334.1 [M + H]+.
The aggregation process of compound 1 can be proved by fluores-
cence microscope (Fig. 3). When ƒw were respectively 0% and 20%, no
aggregation was observed under the fluorescence microscope. A small
number of aggregates were not observed until the ƒw reached 40%.
2.3. X-ray crystallography
X-ray single crystal diffraction data was collected on Agilent G8910A
CCD diffractometer with graphite-monochromated Mo-Ka radiation (λ
= 0.71073 Ǻ). The structure was solved by the SAINT program. The
crystallographic data has been uploaded to the Cambridge Crystallo-
graphic Data Centre (CCDC Number: 1915083), and the crystallographic
data sheet has been shown in Table S1.
2.4. Cell imaging
A549 cells were purchased from Shanghai Chaoyan Biotechnology
Co., Ltd. and were incubated for 24 h in Dulbecco's modified eagle me-
dium at 37 °C. Then they were cultivated for approximately 20 min
with compound 1 (20 μM) and were imaged with a laser scanning con-
focal microscopy. Next, the cells were consequently cultivated with ClOˉ
(30 μM) for another 10 min, and imaged using a laser scanning confocal
microscopy again.
Fig. 1. Fluorescence spectra of compound 1 (10 μM) in DMSO containing different rate of
water (λex = 455 nm).
Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
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beneficial to enable the electron transition and is easy to result in a
long wavelength fluorescence emission.
For AIEE molecule, the molecular packing mode has an important ef-
fect on the luminescence performance. Therefore, X-ray diffraction of
compound 1 single crystal was also performed. The molecular confor-
mation of compound 1 in the crystalline state is in accordance with its
optimized structure, showing good planarity. Tactfully, two methyl
groups and two hydrogen atoms out of the molecular plane can effec-
tively hamper the intermolecular π-π stacking interaction (Fig. 5a),
where the distance between the two molecular planes is 4.661 Ǻ and
the angle between the methyl group out of the plane and the molecular
plane is 123.25 degrees. Furthermore, compound 1 could form rod-
shaped crystal (length L = 3 cm) from the mixture solution of CH2Cl2
and EtOH and show strong red fluorescence (Fig. 5a inset). In addition,
molecules of compound 1 formed effective J-aggregates (Fig. 5b), thus
prevent the fluorescence quenching in aggregate state. This leads to a
conclusion that plane molecule without rotational units and rigid conju-
gate structure can also get AIE/AIEE and red fluorescence properties by
introducing the right group.
Fig. 2. Plot of maximum fluorescence intensity (black squares) and wavelength (red dots)
of compound 1 (10 μM) versus water fraction (λex = 455 nm).
Dramatically, there were a lot of aggregates in solution when ƒw was in
the range of 50% to 80%. When ƒw was 90%, lots of tiny particles were ap-
peared. The fact that the change of aggregate state with increasing con-
centration of water also illustrated the existence of the aggregation
process of compound 1 in DMSO/water mixtures. These results are con-
sistent with the variation trend of fluorescence intensity.
3.3. Fluorescence response to ClOˉ
As one of reactive oxygen species, ClOˉ has important effect on
human health [37–41]. A moderate amount of ClOˉ benefits people's
health in aspects of bacteria disinfection in vivo and body immuniza-
tion. However, abnormal concentration of ClOˉ will produce negative ef-
fects such as oxidative stress response and tissue lesion. Hence, the
development of convenient detection technology for ClOˉ is of great sig-
nificance in the diagnosis and prevention of related diseases. It was
found that compound 1 showed double optical response through ultra-
violet absorption signal and fluorescence signal simultaneously, which
can detect ClOˉ with colorimetric and fluorescent changes with good se-
lectivity and sensitivity for ClOˉ over other analytes. The absorption
spectra of compound 1 with ClOˉ were measured in aqueous solution
containing 10% DMSO (pH = 7.4) (Fig. S6). Compound 1 has a strong
absorption band around 459 nm and a weak absorbance band around
337 nm. With the addition of a series of concentration of ClOˉ, the
peak at 575 nm reduced gradually, while the peak at 337 nm increased
sharply. Additionally, with increasing concentrations of ClOˉ, the colour
of probe 1 solution changed to colorless from yellow. The results
3.2. Photoluminescence mechanism
The AIEE behavior of compound 1 encouraged us to explore the
mechanism of fluorescence emission. The theoretical calculation was
carried out by means of the Gaussian 09 program with the B3LYP/6-
31G basis set. Optimized structure, energy levels and electron density
distributions of compound 1 are displayed in Fig. 4. The optimized struc-
ture indicates that all the atoms except C7, C8, H25–27 of C7, H28–30 of
C8, H38, H39 are in the same plane, which is very helpful to extend the
π-conjugation system, easily resulting in a red-shifted fluorescence.
Moreover, their HOMO and LUMO are distributed throughout most of
molecule, which induce small energy gap between HOMO and LUMO.
The energy levels of HOMO and LUMO and their energy gap are
−6.14 eV, −3.39 eV and 2.95 eV, respectively. The small energy gap is
Fig. 3. Fluorescence microscopy images of compound 1 (10 μM) in DMSO containing different rate of water (scale bar: 50 μm).
Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
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Fig. 4. Optimized structure, energy levels and electron density distributions of compound 1.
indicate the excellent colorimetric recognition ability of compound 1 for
ClOˉ.
The emission peak at 640 nm reduced quickly and was nearly stable
value in 1 min after addition of 80 μM ClOˉ. From these results it can
be certainly inferred that compound 1 has high sensitivity and selectiv-
ity for quantitative detection of ClOˉ with quick response time.
Similarly, the variation tendency of fluorescence with the increase of
ClOˉ concentration was also analyzed as shown in Fig. 6a. The fluores-
cence of compound 1 showed a reduced trend along with the gradual
increase of ClOˉ from 0 to 80 μM. In addition, It can be easily found
that there is a good linear relation between the maximum values
(640 nm) of fluorescence emission of compound 1 and the concentra-
tions of ClOˉ from 0 to 40.0 μM: y = 895.1–18.8 x (R2 = 0.9959) as
shown in Fig. 6b. The limit of detection (LOD) of compound 1 can be cal-
culated to be about 15.9 nM where the signal-to-noise ratio is 3 [42].
Moreover the fluorescence response of compound 1 is not disturbed
by other analytes. As shown in Fig. 6c, the fluorescence intensity of com-
pound 1 weaken only upon addition of ClOˉ, while didn't change obvi-
ously in the presence of other analytes (black bars). When ClOˉ was
added to the compound 1 solution in the presence of other competitive
analytes, the fluorescence intensity displayed forceful quenching with-
out significant interference (red bars). In addition, the response time
of compound 1 to ClOˉ was further examined as displayed in Fig. 6d.
The fluorescence response mechanism was also explored by mass
spectrum. After reacted with ClOˉ, the molecular peak at 334.10 in
mass spectrum disappeared and a new peak at 350 grown up
(Fig. S7). This can be attributed to the possible that alkyl sulfide of com-
pound 1 was oxidized to carbonyl sulfide as shown in Fig. 7. Further-
more, the FTIR spectra also confirmed this shift of carbonyl sulfide.
The peaks at 2997 cm−1 and 2926 cm−1 in the curve of compound 1
(Fig. S8) are the st(C\\H) vibration of S\\CH3, and the peaks at
1299 cm−1 and 1277 cm−1 are the ν(C\\H) vibration of S\\CH3. How-
ever, the four peaks disappeared and a new peak at 1048 cm−1 ap-
peared in the curve of compound 1 with ClO− (Fig. S9). The peak at
1048 cm−1 is the st(S_O) vibration, showing the transition form alkyl
sulfide group to carbonyl sulfide group. Methylthio moiety is a strong
electron withdrawing group, which can build D-π-A structure of com-
pound 1 through intramolecular charge transfer (ICT) principle. When
Fig. 5. The molecular typical packing modes in the single-crystal structure of compound 1 along a direction (a) and b direction (b), respectively. Inset: Crystal photograph of compound 1
under UV light lamp (365 nm).
Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
chemosensor..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117794

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Fig. 6. (a) Fluorescence changes of compound 1 (10 μM) in the presence of a series of dopamine concentration of NaOCl in a water containing 10% DMSO (v:v). (b) The linear relation
between fluorescence intensities of compound 1 and the concentrations of ClOˉ in the range of 0–40 μM. (c) Fluorescence intensity of compound 1 (10 μM) in the presence of various
analytes (80 μM) (1. probe 1 only, 2. ClOˉ, 3. SO24−, 4. NO23−, 5. H2PO4ˉ, 6. Fˉ, 7. ClO4ˉ, 8. NO2ˉ, 9. O2−, 10. 1O2, 11. ·OH, 12. H2O2, 13. HSO3ˉ, 14. HCO3ˉ, 15. HSˉ). (d) The changes of
fluorescence intensity of compound 1 (10 μM) over time with ClOˉ (80 μM).
alkyl sulfide was oxidized to carbonyl sulfide by ClOˉ, the electron do-
nating property of alkyl sulfide was replaced by the electron-
withdrawing property of alkyl sulfide, breaking ICT process and D-π-A
structure. As a result, the red fluorescence of compound 1 greatly
decreased.
in the fluorescence signal, which is very applicable for the image of ClOˉ
in living cells.
4. Conclusions
The biological application capacity was further investigated in living
A549 cells. As shown in Fig. 8, A549 cells incubated with compound 1
were involved in obvious red fluorescence. However, after incubated
successively with compound 1 and ClOˉ, the red fluorescence of A549
cells was almost completely quenched. The results indicated that com-
pound 1 can penetrate cells very easily and display conspicuous changes
In conclusion, a cleverish red fluorogen was developed by in-
troducing D-π-A molecular structure. Though this compound has
good planar construction and has no rotatory unites, it showed
typical AIEE performance by regulating molecular stacking type
and forming J-aggregation. This molecular design principle may
be able to offer an effective strategy to exploit red AIE/AIEE
Fig. 7. Reaction mechanism of compound 1 to ClOˉ.
Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
chemosensor..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117794

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Fig. 8. Laser confocal images. Bright-field image (a), fluorescence image (b) and merged image (c) of A549 cells incubated with compound 1. (d) Fluorescence image of A549 cells treated
with compound 1 and then loaded with ClOˉ.
organic materials. Meanwhile, this fluorogen showed excellent re-
sponse performances to ClOˉ including high selectivity and sensi-
tivity, and cell imaging performance, which has potential
application for the detection of ClOˉ in vitro and in vivo.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.saa.2019.117794.
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Please cite this article as: L. Yan, Y. Xie, J. Li, et al., A red fluorogen: AIEE characteristic, photoluminescence mechanism and its application as
chemosensor..., Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, https://doi.org/10.1016/j.saa.2019.117794