A cyanostilbene-based molecule with aggregation-induced emission properties: amplified spontaneous emission, protonation effect and electroluminescence

Article abstract of DOI:10.1007/s11426-018-9366-7

A terpyridine-substituted cyanostilbene derivative (Z)-2-(4′-([2,2′:6′,2″-terpyridin]-4′-yl)-[1,1′-biphenyl]-4-yl)-3-phenylacrylonitrile (CNSTPy) was synthesized and characterized. The compound exhibits remarkable aggregation-induced emission phenomenon a

Full text of DOI:10.1007/s11426-018-9366-7

SCIENCE CHINA
Chemistry
•ARTICLES•
https://doi.org/10.1007/s11426-018-9366-7
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A cyanostilbene-based molecule with aggregation-induced emission
properties: amplified spontaneous emission, protonation effect and
electroluminescence
Yujie Dong1,4, Suqian Ma2, Xiaoyu Zhang2, Jingyu Qian2, Nianyong Zhu1, Bin Xu2*,
Cheuk-Lam Ho3*, Wenjing Tian2* & Wai-Yeung Wong1,3*
1Institute of Molecular Functional Materials and Department of Chemistry, Hong Kong Baptist University, Hong Kong, China;
2State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China;
3Department of Applied Biology & Chemical Technology, The Hong Kong Polytechnic University, Hong Kong, China;
4State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, College of Chemical Engineering, Zhejiang University of
Technology, Hangzhou 310014, China
Received August 29, 2018; accepted September 29, 2018; published online December 26, 2018
A terpyridine-substituted cyanostilbene derivative (Z)-2-(4′-([2,2′:6′,2″-terpyridin]-4′-yl)-[1,1′-biphenyl]-4-yl)-3-phenylacry-
lonitrile (CNSTPy) was synthesized and characterized. The compound exhibits remarkable aggregation-induced emission
phenomenon and its single crystal shows a blue emission with fluorescent efficiency as high as 45%. Based on its aggregation
state behavior, multiple applications towards exploring its lasing, fluorescence switching and electroluminescence properties
were realized. Amplified spontaneous emission (ASE) was observed from the crystal and verified by the variable pump strip
method, with a threshold value of ~1.5 mJ cm−2. The protonation/deprotonation processes accompanied by the formation of
different molecular aggregation structure result in the distinct blue and yellow emissions. Additionally, the molecule also shows a
moderate electroluminescence performance.
aggregation-induced emission, cyanostilbene, amplified spontaneous emission, protonation effect
Citation:
Dong Y, Ma S, Zhang X, Qian J, Zhu N, Xu B, Ho CL, Tian W, Wong WY. A cyanostilbene-based molecule with aggregation-induced emission
properties: amplified spontaneous emission, protonation effect and electroluminescence. Sci China Chem, 2018, 61, https://doi.org/10.1007/s11426-
018-9366-7
1 Introduction
precisely opposite to the ACQ effect. Since then, the concept
of AIE has been employed to design efficient fluorescent
materials in the solid state and a series of AIE molecules
have been developed [8–12]. Now, many laboratories have
worked on the high-tech applications of the AIE molecules
and much of the effort has been devoted to their uses in
optoelectronic and sensory systems [13–16]. Based on their
high emission intensity in the aggregate state, promising AIE
materials have been used for the fabrication of efficient or-
ganic light-emitting diodes (OLEDs) [17–19]. Some AIE
materials can turn on organic solid-state fluorescence by
environmental stimuli (such as light, temperature, pressure
Fluorescent organic molecules have continued to attract the
attention of scientists owing to their favorable properties and
a broad range of applications [1–3]. Due to aggregation-
caused quenching (ACQ) effect, the applications of numer-
ous organic luminogens are limited in the solid state [4,5]. In
2001, the unusual aggregation-induced emission (AIE)
phenomenon was discovered by Tang et al. [6,7], which is
*Corresponding authors (email: xubin@jlu.edu.cn; cheuk-lam.ho@polyu.edu.hk;
wjtian@jlu.edu.cn; wai-yeung.wong@polyu.edu.hk)
© Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018
chem.scichina.com link.springer.com

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Dong et al. Sci China Chem
and pH), which can be potential candidates for developing
fluorescent sensors [20–24]. Recently, the studies of organic
lasers based on AIE crystals were also reported, which
broadened their application potential [25–27]. However, it is
still a challenging task to develop organic molecules ex-
hibiting highly efficient solid-state luminescence towards
multi-purpose applications.
As a classical molecular system, some cyanostilbene mo-
lecules were also reported for their AIE property [28,29].
This kind of materials owns twisted molecular conformation
and multiple supramolecular interactions, which can lead to
high solid-state emission quantum efficiency and special
molecular aggregation state [29–32]. By modifying the
molecular structure, some interesting phenomena have been
observed, such as self-assembly behavior [32] and amplified
spontaneous emission (ASE) [33].
Here, a terpyridine group was introduced into this mole-
cular system to build up a new AIE compound (CNSTPy),
and its photophysical properties and applications in different
aspects were studied. Based on the obvious AIE effect, the
strong blue emission of the solid has been considered to be
applied in ASE and OLED. In addition, due to protonation of
the terpyridine unit, its fluorescence switching has also been
studied, and different acid-base atmosphere can induce its
emission to be switched from blue to yellow.
2.1.2 Synthesis of 4'-(4-pinacolatoboronphenyl)-
2,2':6',2''-terpyridine (2)
A Schlenk tube was charged with Pd(dppf)Cl2 (32 mg, 0.03
equiv.), potassium acetate (379 mg, 3.29 mmol) and bis(pi-
nacolato)diboron (344 mg, 1.35 mmol) and flushed with ni-
trogen. Dry tetrahydrofuran (THF) (5 mL) and compound 1
(500 mg, 1.29 mmol) were then added. The mixture was
stirred at 80 °C for 12 h under nitrogen. After being cooled to
room temperature, the product was extracted with ethyl
acetate. The ethyl acetate layer was dried over anhydrous
MgSO4 and the solvent was removed by rotary evaporation.
The crude product was purified by chromatography on a
silica gel column using n-hexane/CH2Cl2 as eluent to give a
white solid (0.24 g, yield: 42%). 1H NMR (400 MHz,
CDCl3), δ (ppm): 8.76 (s, 2H), 8.74 (d, J=8.0 Hz, 2H), 8.67
(d, J=7.9 Hz, 2H), 7.96–7.91 (m, 4H), 7.89 (td, J=7.8,
1.8 Hz, 2H), 7.36 (ddd, J=7.4, 4.8, 1.1 Hz, 2H), 1.39 (s,
12H); 13C NMR (100 MHz, CDCl3), δ (ppm): 156.3, 156.0,
150.1, 149.2, 141.0, 136.9, 135.4, 126.6, 123.9, 121.4, 118.9,
84.0, 24.9; HRMS (m/z), 435.21 [M]+.
2.1.3 Synthesis of (Z)-2-(4-bromophenyl)-3-phenylacrylo-
nitrile (3)
To a solution of benzaldehyde (1.0 g, 3.40 mmol) and 2-(4-
bromophenyl)-acetonitrile (1.0 g, 2.4 mmol) in 125 mL of
ethanol, NaOH (1.09 g, 1.0 mmol) was added. The mixture
was refluxed for 2 h. After being cooled to room tempera-
ture, the mixture was filtered and washed with ethanol. The
crude product was purified by chromatography on a silica gel
column using n-hexane/CH2Cl2 (4:1, v/v) as eluent. Then the
product was obtained as a white solid (0.79 g, yield: 77%).
1H NMR (400 MHz, CDCl3), δ (ppm): 7.88 (d, J=8.0 Hz, 2H,
Ar), 7.56 (q, J=8.0 Hz, 4H, Ar), 7.53 (s, 1H, –CH=CCN–),
7.47 (m, 3H, Ar); 13C NMR (100 MHz, CDCl3), δ (ppm):
142.64, 133.45, 132.26, 132.13, 130.88, 129.37, 129.07,
127.51, 123.46, 117.66, 110.57; HRMS (m/z): 285.00 [M]+.
2 Experimental
2.1 Materials and synthesis
4-Bromobenzaldehyde, 2-acetylpyridine, bis(pinacolato)
diboron and 2-(4-bromophenyl)acetonitrile were purchased
from Aldrich Chemical Co. (USA). Pd(dppf)Cl2 was pur-
chased from Acros Organics (Thermo Fisher Scientific, USA).
All starting materials were used without further purification.
2.1.1 Synthesis of 4'-(4-bromophenyl)-2,2':6',2''-
terpyridine (1)
2.1.4 Synthesis of (Z)-2-(4'-([2,2':6',2''-terpyridin]-4'-yl)-
[1,1'-biphenyl]-4-yl)-3-phenylacrylonitrile (CNSTPy)
4-Bromobenzaldehyde (3 g, 16.2 mmol) and 2-acetylpyr-
idine (3.9 g, 32.4 mmol) were stirred in methanol (360 mL)
followed by addition of sodium hydroxide (NaOH) (0.66 g,
16.2 mmol) and ammonium hydroxide (NH4OH) (90 mL).
The mixture was refluxed for 24 h, and then cooled down to
room temperature. The precipitate was filtered and washed
using methanol and water to obtain a white powder (2.5 g,
40%). 1H NMR (400 MHz, CDCl3), δ (ppm): 8.76–8.72 (m,
2H), 8.71 (s, 2H), 8.68 (d, J=8.0 Hz, 2H), 7.89 (td, J=7.7,
1.8 Hz, 2H), 7.82–7.83 (m, 2H), 7.69–7.59 (m, 2H), 7.37
(ddd, J=7.5, 4.8, 1.2 Hz, 2H); 13C NMR (100 MHz, CDCl3),
δ (ppm): 156.0, 155.9, 149.1, 148.9, 137.3, 136.9, 132.0,
128.8, 123.9, 123.4, 121.3, 118.5; HRMS (m/z), 388.03
[M−H]+.
To a solution of 2 (1.0 g, 3.52 mmol) in 30 mL of THF, 3
(2.30 g, 5.28 mmol), Pd(PPh3)4 (0.20 g, 0.17 mmol, 0.05
equiv.) and 2 M K2CO3 aqueous solution (12 mL) were ad-
ded. The mixture was heated at the reflux temperature for
2 d. After being cooled to room temperature, water was ad-
ded and the solution mixture was extracted with ethyl acet-
ate. The combined organic layer was dried by anhydrous
Na2SO4 and filtered. Afterwards, the mixture was con-
centrated under reduced pressure. The residue was purified
by chromatography on a silica gel column using CH2Cl2 as
eluent. CNSTPy was obtained as a white crystalline product
(yield: 46%). 1H NMR (400 MHz, CDCl3), δ (ppm): 8.80 (s,
2H), 8.77–8.73 (m, 2H, Ar), 8.69 (d, J=8.0 Hz, 2H, Ar), 8.02

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Dong et al. Sci China Chem
(d, J=8.0 Hz, 2H, Ar), 7.92–7.87 (m, 4H, Ar), 7.78–7.74 (m,
6H, Ar), 7.61 (s, 1H, –CH=C(CN)–), 7.49–7.47 (m, 3H, Ar),
7.37 (ddd, J=7.5, 4.8, 1.1 Hz, 2H, Ar); 13C NMR (100 MHz,
CDCl3), δ (ppm): 156.24, 156.05, 149.57, 149.17, 142.01,
141.21, 140.52, 137.90, 136.90, 133.75, 133.71, 130.61,
129.35, 129.00, 127.89, 127.67, 127.50, 126.50, 123.88,
121.40, 118.69, 117.94, 111.27; HRMS (m/z): 513.21
[M−H]+.
and irradiated by the third harmonic (355 nm) of a Nd:YAG
(yttrium-aluminum-garnet) laser at a repetition rate of 10 Hz
and pulse duration of about 10 ns. By using a cylindrical lens
and a slit, the beam was adjusted to a strip with the area of
3.6 mm×0.5 mm. The edge emission of the crystal was de-
tected by an optical fiber connected with Maya2000pro op-
tical fiber spectrophotometer. All the measurements were
carried out at room temperature under ambient conditions.
The luminance intensity enhances exponentially with the
increasing pump stripe length according to the following
equation: I(λ)=A(λ)P0(eg(λ)l−1)/g(λ), where A is a parameter
proportional to the photoluminescence quantum yield, P0 is
the pump intensity, g is the net gain coefficient (the differ-
ence between optical gain and loss) and l is the pump stripe
length. The loss coefficient was calculated according to the
following equation: I=I0e−αx, where I0 is the emission from
the end of the pump stripe and is a constant, α is the loss
coefficient and x is the distance between the crystal tip and
the pump stripe.
2.2 Instrumentation
The NMR spectra were recorded on a Bruker Ultrashield
400 MHz FT-NMR spectrometer (Germany) with tetra-
methylsilane as the internal standard. The time of flight mass
spectra was recorded using a Kratos MALDI-TOF mass
system (USA). Ultraviolet-visible spectrum (UV-Vis) was
recorded on a Hewlett Packard 8453 spectrophotometer
(USA). Photoluminescence (PL) spectra were collected on a
Perkin Elmer LS 55B spectrophotometer (USA) and Maya
2000Pro. optical fiber spectrophotometer (USA). Crystalline
state PL efficiencies were measured with an integrating
sphere (C-701, Labsphere Inc., USA) with a 405 nm Ocean
Optics LLS-LED as the excitation source, and the laser was
introduced into the sphere through the optical fiber.
2.5 OLED device fabrication
Before device fabrication, the indium tin oxide (ITO) glass
substrate was precleaned carefully and treated in UV-ozone
for 15 min. Then ZnO nanocrystals solution was spin-coated
onto the substrate which was transferred into a glove box
later. Polyetherimide (PEI) solution was subsequently spin-
coated onto the ZnO film and then the substrate was annealed
at 110 °C for 10 min. Then the substrate was transferred to
vacuum deposition system and CNSTPy, tris(4-carbazoyl-9-
ylphenyl)amine (TCTA), MoO3 and Au were thermally
evaporated in sequence under vacuum at a pressure of
5×10−7 Torr. The Keithley 2400 source meter (USA) was
used to measure the current-voltage characteristics of the
device. The light output was detected by a calibrated New-
port power meter (model 1936-R, Newport Corporation,
USA) with a Newport silicon photodetector (918D-SL-
0D3R) placed at a fixed distance and directed toward the ITO
glass side of the device, while the brightness was determined
from the light reaching photodetector. The electro-
luminescence (EL) spectrum was recorded by a Maya
spectrometer (Ocean Optics) connected with an optical fiber.
2.3 X-ray crystallography
The diffraction experiments were carried out on a single-
crystal X-ray diffractometer (Bruker APEX DUO CCD Area
Detector, Germany) equipped with a IµS Cu Kα copper
microfocus tube (λ=1.54184 Å) at 173 K. The absorption
corrections were applied (SADABS) to the collected re-
flections. The structure was solved by the direct methods
using the SHELXTL programs and refined with a full-matrix
least-squares technique on F2. Anisotropic thermal para-
meters were refined for all the non-hydrogen atoms. The
hydrogen atoms were generated in their idealized position
and refined with isotropic displacement parameters. The
powder X-ray diffraction patterns were recorded by the
Bruker AXS D8 Advance X-ray diffractometer with Cu Kα
radiation. Crystal data for CNSTPy (CCDC-1862220) (de-
tailed see the Supporting Information online): C36H24N4, M
512.59, T=173(2) K, monoclinic, space group P21/n, a=
11.5480(10) Å, b=9.7030(10) Å, c=24.206(2) Å, α=90°,
β=101.97(2)°, γ =90°, U=2653.3(5) Å3, Z=4, Dc=
1.283 Mg m−3, μ=0.076 mm−1, F(000)=1072, 2θ=1.829°–
25.687°, reflections collected 10625, independent reflections
5003 (Rint=0.0511), refined parameters 361, GOF on
F2=0.975, final R1=0.0474, wR2=0.1049 [I>2σ(I)], R1=
0.1140, wR2=0.1356 (all data).
3 Results and discussion
3.1 Synthesis and crystal structure
The general synthetic route for the preparation of the com-
pound CNSTPy is illustrated in Scheme 1. All the com-
1
pounds involved were fully characterized by H NMR, 13C
NMR and MALDI-TOF.
By slow volatilization of a dichloromethane solution of
CNSTPy, single crystals suitable for X-ray structural ana-
2.4 ASE measurement
The crystal was fixed on a quartz substrate by vacuum grease

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Dong et al. Sci China Chem
Scheme 1 Synthesis route of CNSTPy.
lysis were obtained. The unit cell of CNSTPy is monoclinic
with space group P21/n. As shown in Figure 1, the molecular
configuration in the crystal of CNSTPy is very twisty. The
terpyridine moiety is almost coplanar, while the cyanos-
tilbene group is highly distorted with a large twist angle of
57° between the phenyl rings (1 and 2) linked by the cyano-
vinyl unit. Connected by phenyl ring 3, the phenyl ring 2 is
almost perpendicular to the terpyridine plane 4 with the large
torsional angle of 75°. The largely twisted molecular con-
figuration makes CNSTPy molecules stack crossly with
large distance and nearly no effective overlap between each
other, owing to the large molecular steric hindrance. The
major intermolecular interactions are the CH/π interaction
and N–H hydrogen bond, which are all relatively weak, and
there is no close π-π interaction between the terpyridine
planes for the neighboring units. Such special stacking mode
can protect the molecules from the strong quenching effect,
and this indicates that the aggregation state of CNSTPy
should result in highly efficient fluorescence emission
[34,35].
Figure 1 Molecular configuration and stacking in CNSTPy crystal (color
online).
3.2 Photophysical properties
The UV-Vis and PL spectra of CNSTPy in THF solution are
shown in Figure 2, as well as the PL spectrum of its single
crystal. In THF solution, the compound shows a main ab-
sorption peak at 335 nm and a deep-blue emission with the
maximum PL peak (λmax) at around 423 nm. In contrast to its
solution, the PL spectrum of the crystal displayed a red-shift
of wavelength by 11 nm, with λmax at around 436 nm. Re-
ferring to its crystal structure, this small red-shift may be
mainly ascribed to the molecular interaction in the ag-
gregation state. Moreover, through the integrating sphere
measurement, a quite high fluorescence quantum yield
(~0.45) was obtained for the crystal of CNSTPy, compared
to that of the solution (~0.11), which indicated that an ef-
fective AIE effect should exist in the solid state of CNSTPy.
In order to further investigate the AIE effect of CNSTPy in
the solid state, we carried out the experiments of measuring
its PL spectra by adding water into its THF solution, as
Figure 2 Normalized absorption and PL spectra of CNSTPy (color on-
line).
shown in Figure 3(a). The emission of CNSTPy is very weak
in THF solution, and with increasing the water’s ratio in
solution, the PL intensities remain at low values in the
aqueous solution at the water fraction less than 80%. When
the water fraction was beyond 80%, the PL intensity tended
to increase obviously, and significantly reached the nearly
15-fold level at the water fraction of 90% compared to the

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Dong et al. Sci China Chem
this possibility, one isolated single crystal of CNSTPy was
excited with a pulsed laser and the PL spectra were subse-
quently detected from its edge area.
The ASE spectra of the sample were collected, as shown in
Figure 4(a). At a low pump energy of 1.5 mJ cm−2, the
emission spectrum was initially featured as a broad band
with a full width at half-maximum (FWHM) of ~57 nm.
When the pump energy was gradually increased, the emis-
sion showed obviously narrowed bands with the FWHM
decreased to ~20 nm in this system. Figure 4(b) shows the
dependence of the emission intensity and FWHM on the
pump energy. The peak intensity displays a sharp increase
accompanied with a decrease of FWHM, and the nonlinear
curves demonstrate the ASE behavior of CNSTPy crystal.
The threshold value was determined to be about 1.5 mJ cm−2,
which was calculated by the slope change of FWHM or PL
intensity versus pump energy curves. Greater than this
threshold value, spontaneous emission can effectively sti-
mulate emission while it is propagating in the crystal sample,
and then the amplification occurs in the stimulated-emission
region [36].
In addition, the gain coefficient of this crystal was also
determined by a successively variable pump stripe method,
in which the lengths of pump stripe were adjusted by a slit.
As shown in Figure 5(a, b), exponentially grown peak in-
tensity and gradually narrowed emission band were observed
with the increase of pump stripe length. Also, the higher
pump energy can accelerate the emission band narrowing,
which is consistent with the ASE theory. Figure 5(c) displays
that the maximum of the gain coefficient is about 42 cm−1 at
2.741 mJ cm−2 pump intensity. The loss coefficient was also
calculated by measuring the emission at the tip of the crystal.
As shown in Figure 5(d), the loss is around 17 cm−1 at the
peak of 436 nm. Generally, the optical loss is caused by both
scattering and self-absorption, and a lower loss is beneficial
to the high net gain.
Figure 3 PL spectra of CNSTPy in THF solution and THF/water mixture
(a) and in THF solution at room temperature and 77 K (b) at a con-
centration of ~10−5 M (color online).
initial PL intensity in THF. We also measured its PL spec-
trum in THF at low temperature and found that the PL in-
tensity enhanced enormously compared to that at room
temperature (Figure 3(b)). These results indicate that the
inhibition of intramolecular vibrational and rotational motion
should be the origin for the high quantum yield in solid,
relating to its AIE property. The largely twisty molecular
configuration, which could be seen from the crystal struc-
ture, may induce the strong intramolecular vibrational and
rotational motion in the solution of CNSTPy and thus result
in a fast nonradiative relaxation to quench the fluorescence at
room temperature. At low temperature or in the aggregation
state of crystal or THF/water (1:9) solution in which mole-
cular interactions exist, the strong intramolecular vibrational
and rotational motion should be effectively inhibited or
weakened which gives the AIE effect [6,7].
3.4 Protonation effect
The protonation/deprotonation process of CNSTPy was also
investigated. As shown in Figure 6(a), the as-synthesized
sample exhibited a strong blue emission peaking at 436 nm
with a fluorescence quantum yield of (45±3)%, while after
being fumed by hydrochloride (HCl) vapor, the emission
turned yellow and shifted to 559 nm with a fluorescence
quantum yield of ~3% under 365 nm UV light illumination.
Interestingly, after being fumed by triethylamine (TEA), the
refumed sample recovered to the blue emissive state and the
emission peak was close to that obtained from the as-syn-
thesized sample. The switch between blue and yellow
emission can be repeated by alternately fuming with HCl and
TEAvapors (Figure 6(b)). The wavelength shift of 123 nm in
fluorescence and the recovery towards the initial state de-
3.3 Amplified spontaneous emission
From the crystal photo under UV light, crystal of CNSTPy
exhibits a stronger blue emission at the edge than its body,
indicating that its emission is highly confined inside the
crystal. This phenomenon reflects that self-waveguided
emission may occur in the crystal and thus this crystal is a
possible candidate for use in organic blue laser media. To test

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Dong et al. Sci China Chem
Figure 4 (a) Normalized PL spectra of a single crystal at various pulse excitation energies (Inset: crystal photo under 365 nm UV light); (b) a plot of PL
intensity versus pump energy and the calculation of threshold value (Inset: dependence of PL intensity and FWHM on pump laser energy) (color online).
Figure 5 (a) Peak intensity of the emission as a function of pump stripe length under different pump energies; (b) the dependence of the FWHM of PL
spectra on pump stripe length; (c) net gain coefficients as a function of wavelength; (d) loss coefficient as a function of wavelength (color online).
Figure 6 (a) PL spectra of the initial, HCl fumed and TEA refumed samples; (b) emission wavelength of CNSTPy samples by repeated fuming with HCl
and TEA vapors (Inset: images of the fumed samples under 365 nm UV light) (color online).

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Dong et al. Sci China Chem
monstrate the significant fluorescence switching properties
of the as-synthesized CNSTPy sample under protonation/
deprotonation stimuli.
For the solution sample of CNSTPy, the same red-shift
can also be observed. As shown in Figure 7(a), when excess
hydrochloride (HCl) was added into the THF solution, red-
shifts were observed in both absorption and emission (from
425 to 548 nm). To investigate the protonation process, we
compared the 1H NMR spectra of CNSTPy solution samples
before and after protonation. As shown in Figure 7(b), when
trifluoroacetic acid (TFA) was added, the shifts towards the
1
low field in the H NMR spectrum revealed the decreased
electron cloud density on hydrogen atoms, which confirmed
the protonation of the pendant pyridine moieties. Therefore,
the increased electron-withdrawing ability of pyridine moi-
eties is beneficial to the electron delocalization, which leads
to the formation of the intramolecular charge transfer (ICT)
state and results in the shift of absorption and fluorescence
[37].
To gain more insights into the origin of the switched
emission between blue and yellow, powder X-ray diffraction
(PXRD) experiments were conducted (Figure 8). The dif-
fraction pattern of the as-synthesized sample accords well
with the simulated XRD pattern from the single crystal data,
suggesting that the initial sample should adopt the same
molecular arrangement as that of the single crystal. After
Figure 8 PXRD patterns of CNSTPy: fumed samples, initial sample and
the simulated pattern obtained from the crystal data (color online).
being fumed by HCl, the weak and altered diffraction pattern
indicates the significant decrease of crystallinity and for-
mation of different molecular aggregation structure of the
CNSTPy sample, possibly due to the destruction of planarity
of the terpyridyl group after the interaction of HCl with
CNSTPy molecule. When treated with TEA, the refumed
sample shows sharp reflections, which are partly consistent
with the initial sample. The PXRD patterns indicate that
protonation/deprotonation processes result in a change of the
degree of crystallinity and different stacking modes of the
CNSTPy sample, which may contribute to the switched
emission.
3.5 Electroluminescence
The efficient blue-light emission of CNSTPy in the solid
state prompts us to investigate their electroluminescence
(EL) properties. The device was fabricated with the struc-
ture: ITO/ZnO (40 nm)/PEI (5 nm)/emitting layer (30 nm)/
TCTA (20 nm)/MoO3/Au, where MoO3 was used as the hole-
injection material, TCTA (TCTA=tris[4-(carbazol-9-yl)phe-
nyl]amine) as the hole-transport material, CNSTPy as the
emitting layer and ZnO/PEI (PEI=polyethylenimine) as the
electron-transport material. Figure 9 shows the current-
voltage-luminance characteristic curve of the device. The
device exhibited a blue-green emission with a maximum
peak at 497 nm in the EL spectrum, accompanied with a
turn-on voltage of 3.7 V. The maximum current and power
efficiencies of 0.84 cd A−1 and 0.42 lm W−1, respectively,
were obtained, and the maximum brightness could reach
1281 cd m−2. As a blue emission material with ASE property,
we believe it can show good photoelectric performance in the
future research study.
Figure 7 (a) Normalized UV-Vis absorption and PL spectra of CNSTPy
in THF and in THF solution with excess HCl; (b) 1H NMR spectra of
CNSTPy in CDCl3 (down) and CDCl3 with TFA (up) expanded in the
7–9 ppm region (color online).
4 Conclusions
We present a blue-emitting molecule CNSTPy with re-

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Dong et al. Sci China Chem
2
3
Wang N, Evans JS, Mei J, Zhang J, Khoo IC, He S. Opt Express, 2015,
23: 33938
Han T, Feng X, Tong B, Shi J, Chen L, Zhi J, Dong Y. Chem Com-
mun, 2012, 48: 416–418
4
5
6
Thomas SW, Joly GD, Swager TM. Chem Rev, 2007, 107: 1339–1386
Chen CT. Chem Mater, 2004, 16: 4389–4400
Luo J, Xie Z, Lam JWY, Cheng L, Tang BZ, Chen H, Qiu C, Kwok
HS, Zhan X, Liu Y, Zhu D. Chem Commun, 2001, 381: 1740–1741
Tang BZ, Zhan X, Yu G, Sze Lee PP, Liu Y, Zhu D. J Mater Chem,
2001, 11: 2974–2978
Mei J, Leung NLC, Kwok RTK, Lam JWY, Tang BZ. Chem Rev,
2015, 115: 11718–11940
Hong Y, Lam JWY, Tang BZ. Chem Soc Rev, 2011, 40: 5361–5388
7
8
9
10 Xie Z, Chen C, Xu S, Li J, Zhang Y, Liu S, Xu J, Chi Z. Angew Chem
Int Ed, 2015, 54: 7181–7184
11 Goswami N, Yao Q, Luo Z, Li J, Chen T, Xie J. J Phys Chem Lett,
2016, 7: 962–975
12 Tsujimoto H, Ha DG, Markopoulos G, Chae HS, Baldo MA, Swager
TM. J Am Chem Soc, 2017, 139: 4894–4900
13 Zhou Y, Liu H, Zhao N, Wang Z, Michael MZ, Xie N, Tang BZ, Tang
Y. Sci China Chem, 2018, 61: 892–897
14 Ding S, Liu M, Hong Y. Sci China Chem, 2018, 61: 882–891
15 Hang Y, Cai X, Wang J, Jiang T, Hua J, Liu B. Sci China Chem, 2018,
61: 898–908
16 Qiu Z, Hao B, Gu X, Wang Z, Xie N, Lam JWY, Hao H, Tang BZ. Sci
China Chem, 2018, 61: 966–970
17 Qin W, Yang Z, Jiang Y, Lam JWY, Liang G, Kwok HS, Tang BZ.
Chem Mater, 2015, 27: 3892–3901
18 Huang J, Yang M, Yang J, Tang R, Ye S, Li Q, Li Z. Org Chem Front,
2015, 2: 1608–1615
19 Liu Y, Chen S, Lam JWY, Lu P, Kwok RTK, Mahtab F, Kwok HS,
Tang BZ. Chem Mater, 2011, 23: 2536–2544
20 Dong Y, Xu B, Zhang J, Tan X, Wang L, Chen J, Lv H, Wen S, Li B,
Ye L, Zou B, Tian W. Angew Chem Int Ed, 2012, 51: 10782–10785
21 Li H, Zhang X, Chi Z, Xu B, Zhou W, Liu S, Zhang Y, Xu J. Org Lett,
2011, 13: 556–559
Figure 9 (a) The electroluminescence (EL) spectra and image of the
fabricated OLED device; (b) the current efficiency-current density-power
efficiency curves for the device (Inset: the current-voltage-luminance
characteristic curves) (color online).
markable AIE effect, which exhibits multiple applications.
Based on the high solid-state fluorescence efficiency, fasci-
nating optical properties such as ASE and EL were observed
for CNSTPy, which indicates that it may serve as a pro-
mising candidate for OLEDs and organic lasers. On the other
hand, the study of the protonation process for CNSTPy
suggests that this kind of materials may be a potential can-
didate for applications in sensing, detection and display de-
vices with remarkable fluorescence-switching properties.
22 Aldred MP, Li C, Zhang GF, Gong WL, Li ADQ, Dai Y, Ma D, Zhu
MQ. J Mater Chem, 2012, 22: 7515–7528
23 Chen ZQ, Chen T, Liu JX, Zhang GF, Li C, Gong WL, Xiong ZJ, Xie
NH, Tang BZ, Zhu MQ. Macromolecules, 2015, 48: 7823–7835
24 Chen T, Chen ZQ, Gong WL, Li C, Zhu MQ. Mater Chem Front,
2017, 1: 1841–1846
25 Wang Y, Liu T, Bu L, Li J, Yang C, Li X, Tao Y, Yang W. J Phys
Chem C, 2012, 116: 15576–15583
26 Wang L, Zhang Z, Cheng X, Ye K, Li F, Wang Y, Zhang H. J Mater
Chem C, 2015, 3: 499–505
27 Ma S, Zhang J, Liu Y, Qian J, Xu B, Tian W. J Phys Chem Lett, 2017,
8: 3068–3072
28 Li W, Wang S, Zhang Y, Gao Y, Dong Y, Zhang X, Song Q, Yang B,
Ma Y, Zhang C. J Mater Chem C, 2017, 5: 8097–8104
29 Li Y, Li F, Zhang H, Xie Z, Xie W, Xu H, Li B, Shen F, Ye L, Hanif
M, Ma D, Ma Y. Chem Commun, 2007, 265: 231–233
30 Yeh HC, Wu WC, Wen YS, Dai DC, Wang JK, Chen CT. J Org Chem,
2004, 69: 6455–6462
31 Palayangoda SS, Cai X, Adhikari RM, Neckers DC. Org Lett, 2008,
10: 281–284
32 An BK, Gierschner J, Park SY. Acc Chem Res, 2012, 45: 544–554
33 Fang HH, Chen QD, Yang J, Xia H, Gao BR, Feng J, Ma YG, Sun HB.
J Phys Chem C, 2010, 114: 11958–11961
34 Zhang J, Xu B, Chen J, Ma S, Dong Y, Wang L, Li B, Ye L, Tian W.
Adv Mater, 2014, 26: 739–745
35 Xie Z, Yang B, Li F, Cheng G, Liu L, Yang G, Xu H, Ye L, Hanif M,
Liu S, Ma D, Ma Y. J Am Chem Soc, 2005, 127: 14152–14153
36 Park S, Kwon OH, Kim S, Park S, Choi MG, Cha M, Park SY, Jang
DJ. J Am Chem Soc, 2005, 127: 10070–10074
Acknowledgements This work was supported by the Hong Kong Research
Grants Council (C6009-17G), the Areas of Excellence Scheme of HKSAR
(AoE/P-03/08), the Clarea Au Endowed Professorship in Energy (847S) and
the Hong Kong Polytechnic University (1-ZE1C). Y.J. Dong thanks the
Hong Kong PhD Fellowship Scheme (HKPFS) from the Hong Kong Re-
search Grants Council for the financial support.
Conflict of interest The authors declare that they have no conflict of
interest.
Supporting information The supporting information is available online at
http://chem.scichina.com and http://link.springer.com/journal/11426. The
supporting materials are published as submitted, without typesetting or
editing. The responsibility for scientific accuracy and content remains en-
tirely with the authors.
1
Li W, Liu D, Shen F, Ma D, Wang Z, Feng T, Xu Y, Yang B, Ma Y.
Adv Funct Mater, 2012, 22: 2797–2803
37 Zhang J, Chen J, Xu B, Wang L, Ma S, Dong Y, Li B, Ye L, Tian W.
Chem Commun, 2013, 49: 3878–3880