Highly efficient solid-state emission of diphenylfumaronitriles with full-color AIE, and application in explosive sensing, data storage and WLEDs

Article abstract of DOI:10.1016/j.dyepig.2019.107829

Luminescent materials with aggregation-induced emission (AIE) have attracted extensive attentions for their strong emission in aggregated states. Although all kinds of AIE luminogens have been recently developed, it is difficult to realize full-color AIE which is popular for the various applications in optoelectronic fields. Herein, a class of diphenylfumaronitrile core-based dyes emitting in a wide region from near ultraviolet (382 nm) to near-infrared (682 nm) have been synthesized and well characterized. It is found that the emission wavelength of the dyes severely depends on the peripheral substituent, including electron-withdrawing trifluoromethyl (1), methyl (2), electron-pushing methoxyl (3), and bulky aromatic amine (7–9), and that the substituting location imposes great effect on the emission intensity of multiple-substituted dyes (4–6). Compound 4 with two methyl substituting at 3,4-position gives a ultrahigh quantum yield of 93percent for unique packing structure. 1–3 and 7 can be used in explosive detecting and data storage for their emission sensitive to picric acid and external stimuli, respectively. When 4 and 8 applied in white-light-emitting diodes (WLEDs), a pure white emission can be obtained with 90 of CRI and (0.32, 0.32) of CIE.

Full text of DOI:10.1016/j.dyepig.2019.107829

DyesandPigments172(2020)107829
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Highly efficient solid-state emission of diphenylfumaronitriles with full-
color AIE, and application in explosive sensing, data storage and WLEDs
Duobin Wu, Wenjing Gong, Huimei Yao, Limei Huang, Zhenghuan Lin∗, Qidan Ling
Fujian Key Laboratory of Polymer Materials, College of Chemistry and Materials Science, Fujian Normal University, Fuzhou, 350007, Fujian, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fumaronitrile
AIE
Luminescent materials with aggregation-induced emission (AIE) have attracted extensive attentions for their
strong emission in aggregated states. Although all kinds of AIE luminogens have been recently developed, it is
difficult to realize full-color AIE which is popular for the various applications in optoelectronic fields. Herein, a
class of diphenylfumaronitrile core-based dyes emitting in a wide region from near ultraviolet (382 nm) to near-
infrared (682 nm) have been synthesized and well characterized. It is found that the emission wavelength of the
dyes severely depends on the peripheral substituent, including electron-withdrawing trifluoromethyl (1), methyl
(2), electron-pushing methoxyl (3), and bulky aromatic amine (7–9), and that the substituting location imposes
great effect on the emission intensity of multiple-substituted dyes (4–6). Compound 4 with two methyl sub-
stituting at 3,4-position gives a ultrahigh quantum yield of 93% for unique packing structure. 1–3 and 7 can be
used in explosive detecting and data storage for their emission sensitive to picric acid and external stimuli,
respectively. When 4 and 8 applied in white-light-emitting diodes (WLEDs), a pure white emission can be ob-
tained with 90 of CRI and (0.32, 0.32) of CIE.
Emission
Data storage
WLED
Explosive
1. Introduction
state emissive materials based on the AIE effect, it is still challenging to
design AIE-active fluorophores with tunable solid-state emission that
Organic materials with efficient luminescence in the aggregate
states are very important for the practical advanced applications of
biomedicine [1,2] and optoelectronic devices, such as organic laser [3],
luminescent displays and illumination [4–9]. One of the main chal-
lenges in this field is the fact that the emission of conventional lumi-
nophores is often weakened or even totally quenched in the solid or
aggregate states, as a result of the notorious photophysical effect of
aggregation-caused quenching (ACQ) [10]. The reason causing ACQ is
that most luminophores with a panel-like structure exhibit undesirable
intermolecular interactions (like π-π stacking) in the aggregate states,
or form excimers or exciplexes in excited states, resulting in non-ra-
diation decay of excitons. To reduce ACQ, several strategies, such as
introducing bulky substituent, adopting spiro and branched molecular
structure, have been proposed [11–15]. Tang, Park, and their respective
co-workers observed the aggregation-induced emission (AIE) phenom-
enon which overcomes fluorescence quenching in the aggregated state
[16,17]. For AIE luminophores, no elaborate work needs to be done to
interrupt the aggregation process. Instead, the aggregate formation can
be taken advantage of, to generate efficient solid emission. To date, a
large number of AIE-active dyes based on silole or arylethene structure
have been developed [18–25]. Despite successful examples of solid-
covers the whole visible (especially near IR) region.
Donor-acceptor-donor (D-A-D) type compounds based on fumar-
onitrile core (A unit) have recently attracted significant attention for
their AIE or aggregation induced emission enhancement (AIEE) activity
[26–30]. For example, The DPF derivatives containing aromatic amine
could display strong red AIE, and be employed as nanoprobe for tar-
geted cellular and computed tomography (CT) imaging with good
photostability and low cytotoxicity [27,29]. N-Phenylcarbazole-based
fumaronitrile dye could switch its emission color between green and
orange and be used to fabricate two-color emission in organic light-
emitting diodes (OLEDs) [26]. Tetraphenylethene and hex-
aphenylbenzene was employed as the donor unit to construct D-A-D
diphenylfumaronitrile (DPF) derivatives exhibiting strong emission in
aggregate state and pronounced and sensitive response on the stimuli of
mechanic force and amines [28,30].
Motivated by the good performance of DPF dyes in optoelectronic
fields, we chose DPF as core to design and synthesize AIE-active
fluorophores (1–9) with highly efficient and tunable solid-state emis-
sion from two aspects (Fig. 1): (i) introducing the different peripheral
substituted groups with push-pull electronic effect (1–3 and 7–9); (ii)
changing the substituted location of groups at benzene ring of DPF
∗ Corresponding author.
E-mail address: zhlin@fjnu.edu.cn (Z. Lin).
https://doi.org/10.1016/j.dyepig.2019.107829
Received 12 July 2019; Received in revised form 20 August 2019; Accepted 20 August 2019
Availableonline21August2019
0143-7208/©2019ElsevierLtd.Allrightsreserved.

D. Wu, et al.
DyesandPigments172(2020)107829
bath before the temperature rose to above 0 °C. The reaction solution
was stirred for another 3–4 h at room temperature, and then the reac-
tion was quenched with 3–6% hydrochloric acid at less than 10 °C, and
filtered to isolate the solid which was rinsed with cold methanol-water
solution to wash away ionic substances. The reaction mixture was ex-
tracted with ethyl acetate, washed by saturated brine and then dried
over anhydrous MgSO4. After the solvent was removed under reduced
pressure, the crude product was purified by column chromatography or
recrystallization to give the pure product.
2.2.1.1. 2,3-Bis(4-(trifluoromethyl)phenyl)fumaronitrile
(1). White
powder; Yield 74%; mp 228.3 °C; 1H NMR (CDCl3, 400 MHz) δ 7.98
(d, J = 8.34 Hz, 4H), 7.84 (d, J = 8.4 Hz, 4H); 13C NMR (CDCl3,
100 MHz) δ 134.70, 133.99, 133.66, 129.27, 126.50, 125.86, 115.68;
HRMS (ESI) m/z [M+H]+ calcd 367.0670, found 367.0673.
2.2.1.2. 2,3-Di-p-tolylfumaronitrile (2). White powder; Yield 80%; mp
203.5 °C; 1H NMR (CDCl3, 400 MHz) δ 7.74 (d, J = 8.3 Hz, 4H), 7.33 (d,
J = 8.1 Hz, 4H), 2.44 (s, 6H); 13C NMR (CDCl3, 100 MHz): δ 142.33,
129.94, 129.40, 128.64, 124.60, 117.03, 21.58; HRMS (ESI) m/z [M
+Na]+ calcd 281.1055, found 281.1055.
Fig. 1. Structure of DPF derivatives.
(4–6). All of these DPF derivatives display AIE with high quantum
yields in the solid state. The maximum improved times of fluorescent
quantum yield (Φf) in solid than solution reach 4650 for compound 6. A
systematic fine-tuning of the band gap can be realized with emissive
peaks cover a very wide range from near ultraviolet (382 nm, for 1) to
near-infrared (682 nm, for 9) regions. To the best of our knowledge,
such wide-range AIE from the same fluorophore is rarely reported. In-
terestingly, the substituted location of methoxy has big influence on the
emissive intensity. Compound 4 shows ultrahigh luminescence effi-
ciency with 93% of quantum yield for unique packing structure. These
dyes with strong AIE display huge potential in the application of ex-
plosive sensing, data storage and white LEDs (WLEDs).
2.2.1.3. 2,3-Bis(4-methoxyphenyl)fumaronitrile (3). Green powder;
Yield 73%; mp 201.5 °C; 1H NMR (CDCl3, 400 MHz) δ 7.79 (d,
J = 8.9 Hz, 4H), 7.01 (d, J = 8.9 Hz, 4H), 3.89 (s, 6H); 13C NMR
(CDCl3, 100 MHz) δ 161.89, 130.46, 124.65, 122.73, 117.33, 114.60,
55.57; HRMS (ESI) m/z [M+Na]+ calcd 313.0953, found 313.0957.
2.2.1.4. 2,3-Bis(3,4-dimethoxy-5-methylphenyl)fumaronitrile (4). Yellow
powder; Yield 45%; mp 201.3 °C; 1H NMR (400 MHz, CDCl3) δ 7.49 (m,
2H), 7.38 (d, J = 2.2 Hz, 2H), 6.98 (d, J = 8.5 Hz, 2H), 3.97 (s, 12H);
13C NMR (100 MHz, CDCl3) δ 151.81, 149.15, 124.78, 122.74, 122.71,
117.44, 111.17, 111.07, 56.18, 56.13; MALDI MASS m/z
[M + H + Na]2+ calcd 374.1243, found 374.1260.
2. Experimental section
2.2.1.5. 2,3-Bis(3,5-dimethoxyphenyl)fumaronitrile (5). Yellow powder;
Yield 63%; mp 227.4 °C; 1H NMR (400 MHz, CDCl3) δ 6.94 (d,
J = 2.2 Hz, 4H), 6.61 (t, J = 2.2 Hz, 2H), 3.86 (s, 12H); 13C NMR
(100 MHz, CDCl3) δ 161.17, 133.48, 125.66, 116.54, 106.64, 104.14,
55.70; MALDI MASS m/z [M+H]+ calcd 351.1345, found 351.1310.
2.1. General information
Electronic spray ion (ESI) mass spectra were recorded on an Agilent
G6520B spectrometer. MALDI-TOF mass spectra were recorded on
Bruker microflex LRF spectrometer. NMR spectra were measured in
CDCl3 on a Bruker Ascend 400 FT-NMR spectrometer; 1H and 13C
chemical shifts were quoted relative to the internal standard tetra-
methysilane. UV–vis spectra were obtained on a Shimadzu UV-2600
spectrophotometer. The emission spectra were probed on Shimadzu RF-
5301PC fluorescence spectrophotometer. Differential scanning calori-
metry was done on a Mettler DSC822e instrument under flowing N2 gas
at a heating rate of 10 °C min−1. The fluorescence lifetime and absolute
ФF values of solution and solid were measured using an Edinburgh
Instruments FLS920 Fluorescence Spectrometer with a 6 inch in-
tegrating sphere.
2.2.1.6. 2,3-Bis(3,4,5-trimethoxyphenyl)fumaronitrile
(6). Yellow
powder; Yield 80%; mp 201.1 °C; 1H NMR (CDCl3, 400 MHz) δ 7.07
(s, 4H), 3.94 (d, J = 2.6 Hz, 18H); 13C NMR (CDCl3, 100 MHz) δ
153.44, 141.07, 127.03, 124.10, 117.09, 106.23, 61.11, 56.44;
MALDI MASS m/z [M+H]+ calcd 411.1556, found 411.1557.
2.2.2. General procedure for the synthesis of 7–9
Compound 10 (0.12 g, 0.3 mmol) and aromatic amine (0.8 mmol)
were mixed with 5 mL of dry toluene in a round flask containing a stir
bar. Then, Pd(OAc)2 (14.4 mg, 0.64 mmol), P(t-Bu)3 (0.2 mL, 0.8 mmol)
and K2CO3 (0.32 g, 2.4 mmol) were also added and stirred under ni-
trogen at 110 °C for about 12 h. Reaction mixture was cooled to room
temperature, extracted with ethyl acetate, and washed by saturated
brine and then dried over anhydrous MgSO4. After the solvent was
removed under reduced pressure, the crude product was and purified
by column chromatography using ethyl acetate and petroleum ether as
the eluent, affording a pure dark-red solid.
All of the reagents and solvents used, were obtained from commercial
suppliers and were used without further purification unless otherwise
noted. Thin layer chromatography was performed on G254 silica gel plates
of Qingdao Haiyang Chemical. Column chromatography was conducted
on Yantai Huanghai brand silica gel (200–300 mesh).
2.2. Synthesis
2.2.1. General procedure for the synthesis of 1–6
2.2.2.1. 2,3-Bis(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)fumaronitrile
(7). Aurantius powder; Yield 52%; mp 317.5 °C; 1H NMR (400 MHz,
CDCl3) δ 8.14 (m, 4H), 7.55 (m, 8H), 7.02 (m, 8H), 6.40 (m, 4H), 1.70
(s, 12H); 13C NMR (100 MHz, CDCl3) δ 145.12, 140.39, 131.44, 131.38,
131.14, 130.99, 126.55, 125.39, 124.62, 121.57, 116.71, 114.93,
36.23, 30.88; HRMS (ESI) m/z [M+H]+ calcd 645.3018, found
645.3020.
Phenylacetonitrile derivatives (1 mmol) and iodine (0.25 g, 1 mmol)
were dissolved in 5 mL of dry diethyl ether. Sodium methoxide solution
newly prepared from sodium (0.05 g, 2.1 mmol) and 5 mL of methanol
was added slowly (over a period of 30 min) into the reaction solution at
dry ice temperature under a nitrogen atmosphere. The reaction solution
was allowed to warm by replacing the dry ice bath with an ice-water
2

D. Wu, et al.
DyesandPigments172(2020)107829
Scheme 1. Synthesis routes of DPF derivatives.
2.2.2.2. 2,3-Bis(4-(di-p-tolylamino)phenyl)fumaronitrile (8). Red powder;
Yield 46%; mp 223 °C; 1H NMR (400 MHz, CDCl3) δ 7.65 (d, J = 8.9 Hz,
4H), 7.12 (dd, J = 28.2, 8.3 Hz, 16H), 7.00 (d, J = 8.9 Hz, 4H), 2.37 (s,
12H); 13C NMR (100 MHz, CDCl3) δ 150.72, 143.77, 134.56, 130.28,
129.75, 126.04, 123.79, 120.45, 119.24, 117.91, 20.94; MALDI MASS m/z
[M+H]+ calcd 621.3018, found 621.3034.
adhesive (7550-1A/B). They are stirred evenly and clockwise, and put
into vacuum to remove bubbles for 10 min, and then deposited on the
LED chip. The forward current of the LED is 350 mA.
Electroluminescence (EL) measurements of the LEDs were carried out at
room temperature using an Everfine HAAS-2000. In order to collect
light, the LED is placed in an integrating sphere with a diameter of
30 cm, and a spectral radiometer (wavelength accuracy < 0.3 nm) and
a programmable test power LED 300 E are coupled to a high-precision
array.
2.2.2.3. 2,3-Bis(4-(bis(4-methoxyphenyl)amino)phenyl)fumaronitrile
(9). Deep red powder; Yield 46%; 1H NMR (CDCl3, 400 MHz) δ 7.62 (d,
J = 8.9 Hz, 4H), 7.12 (d, J = 9.0 Hz, 8H), 6.88 (d, J = 8.9 Hz, 12H),
3.81 (d, J = 3.3 Hz, 12H); 13C NMR (CDCl3, 100 MHz) δ 157.05,
151.10, 139.21, 129.78, 127.75, 123.14, 120.12, 118.01, 117.76,
115.03, 55.52; HRMS (ESI) m/z [M+H]+ calcd 685.2815, found
685.2819.
3. Results and discussion
3.1. Molecular design and synthesis
To tune the emission properties of solid state of DPF, a series of DPF
derivatives were designed through changing substituted group (com-
pound 1–3 and 7–9) and substituted location (compound 4–6) of ben-
zene rings (Fig. 1). Substituted groups including electron-withdrawing
trifluoromethyl (1), electron-pushing methoxyl (3), and bulky aromatic
amine (7–9) are adopted to adjust the intramolecular charge transfer
(ICT) effect, and consequently luminescent color of DPF derivatives.
Changing substituted location is expected to regulate the emissive in-
tensity of DPF in solid state by controlling intermolecular packing
mode.
2.3. Theory calculation
Molecular Simulations were performed on Gaussian 09 program.
Geometry at ground state was fully optimized by the density functional
theory (DFT) method with the Becke three-parameter hybrid exchange
and the Lee-Yang-Parr correlation functional (B3LYP) and 6-31G* basis
set.
2.4. Single-crystal X-ray diffraction (SXRD) analysis
Compound 1–6 were synthesized by a simple one-step reaction from
corresponding arylacetonitrile in good yield (Scheme 1). Such a di-
merization reaction also gave a intermediate (compound 10) which
could react with different aromatic amine through C–N bond coupling
catalyzed by Pd(OAc)2 to yield compound 7–9. All of the compounds
were purified by recrystallization or silica gel column chromatography.
Their structure was confirmed by 1HNMR, 13CNMR, HRMS (ESI) and
MALDI MASS. The method of solvent evaporation was adopted to cul-
ture single crystals of these dyes. Only the crystals of 6 and 7 have good
quality for the characterization of single-crystal x-ray diffraction (SC-
XRD). Their ellipsoid plot of molecules in the crystals is shown in Fig.
S1 with fine data summarized in Table S1. Two benzene rings in both 6
and 7 are parallel and twisted with fumaronitrile core. The dihedral
angle between benzene and core is 49.4° for 6, and 40.4° for 7 (Fig. S2).
The large dihedral angle of 6 would weaken ICT and cause the blue-
shifted emission in crystal, relative to solution.
The single crystals of 6 and 7 were mounted on a glass fiber for the
X-ray diffraction analysis. Data sets were collected on an Agilent
Technologies SuperNova Single Crystal Diffractometer equipped with
graphite monochromatic Cu-Kα radiation (λ = 1.54184 Å). The single
crystals were kept at 100 K for 6 and 296 K for 7 during data collection.
The structures were solved by SHELXS (direct methods) and refined by
SHELXL (full matrix least-squares techniques) in the Olex2 package. All
non-hydrogen atoms were refined anisotropically displacement para-
meters, and all hydrogen atoms in the ideal positions attached to their
parent atoms.
2.5. Preparation of aggregates
Stock THF solutions of the molecules with a concentration of
0.1 mM were prepared. An aliquot (1 mL) of this stock solution was
transferred to a 10 mL volumetric flask. After adding an appropriate
amount of THF, water was added quickly under vigorous stirring to
furnish 10 μM THF/water mixtures with water fractions of 0–90 vol %.
Emission spectra of the resultant mixtures were measured immediately.
3.2. Regulation on emission wavelength
The optical properties of the synthesized diaryfumaronitrile deri-
vatives were investigated in solids and DCM solution at room tem-
perature. As expected, DPF derivatives 1–3 and 7–9 display distinctly
different luminescent color in solids under 365 nm UV light, and a red-
shifted emission with the increasing electron-pushing ability of the
terminal groups (Fig. 2). Their maximum emission peaks cover a very
wide range from near ultraviolet to near-infrared region. For example,
2.6. LED devices
The light-emitting diodes (LEDs) used for the white electro-
luminescence were supplied by Shenzhen Chundaxin Optoelectronic
Corp. A certain proportion of sieved samples are mixed into curable
3

D. Wu, et al.
DyesandPigments172(2020)107829
between fumaronitrile (as acceptor) and aromatic amine (as donor). It
can be confirmed by simulated calculation using density functional
theory (DFT) with B3LYP/6-31G (d) as basis sets. Theory calculations
give their frontier orbital distribution and excited energy levels (Fig.
S5). The HOMO and LUMO of 1–3 delocalize on whole molecules.
However, the HOMO of 7–9 mainly distributes in the aromatic amine
section, while their LUMO concentrates on fumaronitrile core. As a
result, the electron transfer from HOMO to LUMO is of an ICT char-
acter, which results in the large red-shift of absorption and emission
band of 7–9, relative to 1–3. The time-dependent DFT (TD-DFT) cal-
culated results of the first 3 singlet excited states of compound 1–3 and
7–9 are given in Tables S2–S7. The calculated vertical absorption
transition energy (S1) has much better agreement with the experi-
mental ones with a deviation less than 0.31 eV (Table S8).
3.3. Regulation on emission intensity
Diphenylfumaronitrile derivatives 4–6 have same terminal groups
(methoxyl) as compound 3, but different substituted location and
quantity. Fig. 3a shows the photograph of 4–6 in solids under UV light,
as well as photoluminescence spectra. All of them present yellow-green
light with emission band located at 545 nm, 560 nm and 535 nm, re-
spectively. It indicates that the quantity and locality of the substitutes
have little influences on the luminescent wavelength. However, 4–6
exhibit distinctly different emission intensity with Φf changing from
93% of compound 4–6% of compound 5 (Table 1). High luminescent
efficiency of 4 and 6 in solid is originated from their high kr of 0.066,
while 5 gives a very high knr of 0.357. Their lifetime in solid is in the
range of 7.01–14.2 ns with the emissive decay plot shown in Fig. S3b.
Different solid-state emissive efficiency of 4–6 should be caused by
their different packing structure. Single crystal structure of 6 shows that
the methoxy group at 4-position of benzene ring is perpendicular to the
ring (Fig. S2). The conformation can effectively inhibit molecules from
forming π-π stacking. Besides, every molecule can be fixed by sur-
rounding other six molecules through hydrogen bonding of C–H⋯N
(2.736 Å) and C–H⋯O (2.504 Å and 2.347 Å), and weak interaction of
C–H … π (3.465 Å), as shown in Fig. 3b and c. Consequently, the in-
hibition of molecular motion results in large kr of 6. Unfortunately, the
crystal quality of 4 and 5 is not enough good for SC-XRD analysis.
However, from the optimized structure of 4–6 (Fig. S6), it is found that
they have different conformation. In the case of 5, two methoxyl groups
are coplanar with benzene ring, which easily makes the molecules of 5
form two kinds of π-π stacking in aggregated state: head to head, or
head to tail (Fig. S7). The detrimental π-π stacking results in large knr
and low Φf of 5. For compound 4, one of two methoxy groups is also
perpendicular to benzene ring, like the conformation of 6 in single
crystal structure, which can avoid forming π-π stacking of molecules.
Additionally, two benzene rings in 4 are twisted with a 78° of dihedral,
Fig. 2. Photographs (a) under nature light (upper) and UV light (bottom), and
emission spectra (b) of dye 1–3 and 7–9 in solid.
compounds 1 and 9 in solid exhibits 382 nm and 682 nm of emission,
respectively. The compounds present a fast decay with the average
lifetime (τav) ranging from 2.4 ns to 35.7 ns. It indicates that their
emission is originated from the prompt fluorescence of single excited
state. The representative time-resolved transient emissive decay of 1–3
and 7–9 in solids are plotted in Fig. S3a. Six compounds show relatively
high Φf in solid, especially for compounds emitting visible light with Φf
up to 76.8%. All of them are poorly emissive in DCM solution with Φf
less than 2%, consequently exhibiting an obvious AIE activity. The
improved times of Φf in solid than solution ranges from 16 to 710. Their
photophysical data are summarized in Table 1.
Absorption and emission spectra of these dyes in DCM are shown in
Fig. S4. Compared with compounds 1–3, 7–9 show a significant red
shift in absorption and emission band. It should be attributed to the
strong electron-rich properties of the bulky aromatic amines, which
makes 7–9 form D-π-A-π-D structure and cause strong ICT effect
Table 1
Experimental data of photophysical properties of dye 1–9 in DCM solution and solid state.
Dye
Solution
Solid
λab nm
λem nm
Φf
%
λem nm
Φfa
%
τav ns
kr,b ns−1
knr,cns−1
1
2
3
4
5
6
7
8
9
314
348
380
407
346
390
456
503
510
389
401
431
516
585
540
588
671
677
0.33
0.13
0.15
1.75
0.29
0.01
1.07
2.04
0.01
382
431
504
545
560
535
600
660
682
15 (45)
77 (591)
67 (445)
93 (53)
6 (21)
2.4
0.062
0.043
0.019
0.066
0.023
0.066
0.016
0.100
0.009
0.355
0.013
0.009
0.005
0.357
0.076
0.082
0.122
0.122
17.7
35.7
14.2
2.63
7.01
10.2
4.5
47 (4650)
17 (16)
45 (22)
7 (710)
7.6
a
b
c
Value in the bracket is improved times of Φf in solid than solution.
Radiative rate constant kr = Φf/τav
Non-radiative rate constant knr = (1-Φf)/τav
.
.
4

D. Wu, et al.
DyesandPigments172(2020)107829
Fig. 3. (a) Emission spectra of dye 4–6 in solid. Inset: Photographs of dye 4–6 under UV light. (b, c) Stacking structure of dye 6 in crystal.
not like the parallel conformation in 5 and 6. The twisted conformation
of 4 makes molecules form unique head-to-tail packing structure which
is different from that of 5 (Fig. S7). The unique head-to-tail packing of 4
not only prevents molecular π-π stacking, but also induces two adjacent
molecules form weak interaction, such as CH … π, which fixes mole-
cules and suppresses their movement, consequently producing very
high Φf. The twisted structure of two benzene rings reasonably exists in
crystal of a PDF derivative [31]. A weak interaction of CH … π (2.91 Å)
is found in its packing structure (Fig. S7).
nanoaggregates of compound 1–3 and 7–9 in the mixture solvents of
THF-water (Fig. S8), it is found that there is a spectral overlap between
the absorption band of PA and the emissive bands of compound 1–3.
Consequently, the nanoaggregates of compound 1–3 are selected as
probes and expected to recognize PA through fluorescence quenching
resulting from the energy transfer from nanoaggregates to PA. As shown
in Fig. 5, the emissions of the nanoaggregates are really weakened when
PA was gradually added but without changing its emission peak posi-
tions, indicating that no other emissive species are formed. The emis-
sion quenching can be clearly discerned at a PA concentration as low as
1 ppm. At a PA concentration of 0.18 mM for compound 1, 0.22 mM for
compound 2, and 0.26 mM for compound 3, virtually no light can be
detected by the spectrofluorometer. The Stern-Volmer plot of relative
emission intensity (I0/I-1) values versus PA concentrations in nanoag-
gregates in the aqueous mixtures give curve bending upward instead of
linear (Fig. 5 right), indicating that the fluorescence quenching be-
comes more efficient with increasing quencher concentration. The plots
in low content of PA are linear and give quenching constants of
56082 M−1 for 1, 30353 M−1 for 2 and 17760 M−1 for 3. Compound 1
shows higher sensitivity to PA than compound 2 and 3, due to the large
overlap between the absorption band of PA and the emissive band of
compound 1 (Fig. S8).
3.4. Nanoparticles and sensing for explosive
The AIE properties of these derivatives were assessed by another
typical experiment, in which a good solvent (THF) and a poor solvent
(water) are added successively to generate their nanoparticles. The
nanoparticles would be strongly emissive for the restriction of in-
tramolecular motions [32]. As expected, the aggregates of 1–3 and 7–9
in THF/water are more strongly emissive, compared with their solution
(Fig. 4a–f). The compounds are fairly weakly emissive in the dilute THF
and in the mixture solvents of THF-water with low water fraction.
However, the intensity of emission peak increases with the content of
water, reaching maximum at the water fraction of 90% (for 2, 3, 8 and
9), 80% (for 1), and 50% (for 7), respectively. The emissive intensity in
nanoparticles is improved ca. 80–1000 times than that in THF. As
shown in Fig. 4, the formed aggregates of these compounds emit similar
light as their crystal, covering a wide range from near ultraviolet to
near-infrared region. Interestingly, different water fraction causes 7
different luminescent colors, which should be attributed to the different
packing mode of molecules of 7, because dihydroacridine-based deri-
vatives are polymorphic forms in aggregated states [33–35].
3.5. Multi-stimuli-responsive emission
Materials with multi-stimuli-responsive emission have recently
captured significant attention due to the potential application in in-
formation processing [14,15,23–25,36–38]. Compound 7 displays a
morphology-dependent emission, as shown in Fig. 4d. It is possible to
regulate the morphology, and eventually emission by external stimuli,
like heat, mechanical pressure and solvent. To study the effect of heat
on the emission of 7, the pristine orange solid (named as 7-O) of 7 was
first analyzed by DSC. Fig. S9 shows that two endothermic peak at 62 °C
Prompted by the AIE characteristic of these compounds, we utilized
them to detect explosive. Picric acid (PA) was used as a model ex-
plosive, due to its commercial availability. From the emission spectra of
5

D. Wu, et al.
DyesandPigments172(2020)107829
Fig. 4. Emission spectra of dye 1 (a), 2 (b), 3 (c), 7 (d), 8 (e) and 9 (f) in H2O/THF mixture (5 × 10−5 mol/L) at different content of water.
and 173 °C in the DSC plot of 7-O. The former is corresponding to Tg,
while the latter should be caused by phase transition of 7. Conse-
quently, 7-O was first treated by heating at 200 °C for 5 min, and found
to be turned into a yellow-light-emitting solid (7-Y) with 20.2% of Φf,
as shown in Fig. 6a. Then, both of 7-O and 7-Y can be transferred into a
red sold (7-R, Φf = 9.7%) through grinding with a glass rod. Interest-
ingly, 7-R can be changed back into 7-O by fuming in acetone, and into
7-Y by heating at 200 °C. As a result, fluorescence switching of 7 be-
tween three kinds of solid-states has been realized by external stimuli.
Their emission spectra are shown in Fig. 6b.
Based on the above multi-stimuli-responsive emission of compound
7, an erasable data storage process was designed by employing me-
chanical force and solvent as external stimuli, as shown in Fig. 6c. A
glass rod is used to handwrite on the 7-Y powders supported by a piece
6

D. Wu, et al.
DyesandPigments172(2020)107829
Fig. 5. Sensing for explosive PA by 1 (a), 2 (b) and 3 (c) in H2O/THF mixture (5 × 10−5 mol/L). Emission spectra change (left) and Stern-Volmer plots of (I0/I-1)
values (right) versus PA concentration. I = peak intensity and I0 = peak intensity at [PA] = 0 mM.
of filter paper. Two letters ‘FJ’ are written and recognized for their
different fluorescent color from background. Then the red-light-emitted
letters can be erased by grinding the background with the glass pod,
because the 7-Y turned into 7-R. In the next writing, the letters ‘FJ’ with
yellow luminescence are written with a Chinese brush, employing
CH2Cl2 as ink. Last, the yellow letters can be erased by fuming in
CH2Cl2 vapor, due to 7-R returned into initial 7-Y. As a result, a reu-
sable and reversible writing/erasing process is developed.
3.6. White LEDs
Solid-state illumination from WLEDs will become mainstream light
sources in recent 10 years for the significant advantages, such as lower
energy consumption, safer application, and higher efficiency, compared
with traditional ones [9,39–42]. However, most of reported white-light
materials and WLEDs exhibit color rendering index (CRI) values less
than 85 for the mismatching emission at blue, green and red regions,
7

D. Wu, et al.
DyesandPigments172(2020)107829
which does not satisfy the demand of solid-state lighting applications
[43–47]. The yellow emission of compound 4 and 6 with high quantum
yields encourage us to apply them in white LEDs driven by blue chip.
The compounds were first mixed with A/B glue, and then deposited on
a commercially available blue LED consisting of a packaged complete
Epileds InGaN chip exhibiting an emission at about 460 nm under a
350 mA of current (Fig. S10). The loading amount of compound 4 or 6
in glue was controlled in the range of 0.3–5%. The electroluminescent
data are summarized in Table S9. Compared with compound 6, 4 give
higher luminous efficiency (up to 56 lm/W) at low loading amount, due
to higher Φf of 4 than 6. However, the CRI of WLEDs based on 4 is low
and below 60 for the absence of blue or red emission. 6-based WLEDs
show relatively high CRI up to 82, but unsatisfactory CIE in the range
from (0.23, 0.25) to (0.23, 0.31). The correlated colour temperature
(CCT) of these WLEDs can be adjusted from 5760 K to 26540 K. Their
electroluminescence spectra are shown in Figs. S11 and S12.
To improve the CRI and regulate the CIE of WLEDs, A very small
amount of compound 8 with red emission was added into the glue to
fabricate the four WLED devices. Such small amount of 8 can improve
CRI of WLEDs (Device 1–3) containing 4 from 60 to 90, which is close
to true nature white light, accompanied by (0.30, 0.30) of CIE and
8049 K of CCT (Table 2). 8 also make 6-based WLED (Device 4) realize
89 of CRI, 6358 K of CCT, and (0.32, 0.32) of CIE which is very close to
the CIE of pure white light (0.33, 0.33). Electroluminescence spectra of
the WLEDs based on 4, 6 and 8 under a 350 mA of current are displayed
in Fig. 7. All of electroluminescent spectra cover whole visible region
from 380 nm to 750 nm, consequently producing a white emission with
CIE of ca. (0.30
0.02, 0.31
0.02). Photos of the WLED (Device 4)
based on compound 6 and 8 with and without a forward current are
shown in Fig. 7d (inset).
4. Conclusions
In conclusion, we designed and synthesized successfully a series of
AIE-active fluorophores (1–9) base on diphenylfumaronitrile. Their
photoluminescence spectra can be tuned from near ultraviolet to near-
infrared regions, due to the different push-pull electronic effect of
peripheral substituted groups. Single crystal structure and calculation
results indicate the substituted location of peripheral groups can affect
the packing mode of 4–7, consequently adjust their luminescent effi-
ciency. The nanoparticles of dyes with trifluoromethyl (1), methyl (2)
and methoxyl (3), formed in aqueous solution, could effectively sensing
explosive PA for the fluorescence quenching resulted from energy
transfer. Compouond 7 with dihydroacridine shows morphology-de-
pendent and multi-stimuli-responsive emission, which can be used in
data storage technology. Compound 4 with two methyl substituting at
3,4-position displays a highly efficient solid-state emission (Φf = 93%)
and can be employed to fabricated WLED with high CRI of 90 and pure
white-light CIE of (0.30, 0.30). These results are highly valuable for the
further design of smart AIE materials with highly-efficient and tunable
solid-state emissions.
Fig. 6. (a) Photos of 7-O, 7-Y and 7-R, and mutual conversion upon external
stimuli of heating (H), grinding (G), and fuming (F). (b) Emission spectra of 7-
O, 7-Y, 7-R, fumed 7-R, and heated 7-R. (c) Photos of rewritable data recording
processes.
Table 2
Electroluminescent data of WLEDs based on dye 4 and 6 with small amount of 8.
Device
Dye
Loading Amount (%)
CIE
CCT (K)
CRI
Luminous flux (lm)
Luminous efficiency (lm/W)
1
2
3
4
4 : 8
0.33:0.0016
0.33:0.0020
0.3:0.0020
2.5:0.004
(0.29, 0.33)
(0.29, 0.31)
(0.30, 0.30)
(0.32, 0.32)
7652
8106
8049
6358
83
87
90
89
38
40
32
45
35
36
29
40
6 : 8
8

D. Wu, et al.
DyesandPigments172(2020)107829
Fig. 7. Electroluminescent spectra of WLEDs device 1–4 (a–d). Inset: photos of device 4 without and with forward current.
Acknowledgements
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This work was supported by the National Natural Science
Foundation of China (21374017 and 21574021), the Natural Science
Foundation of Fujian Province (2017J01684).
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