Exceptional aggregation-induced emission from one totally planar molecule

Article abstract of DOI:10.1016/j.dyepig.2019.107556

Planar molecules usually display aggregation-caused quenching (ACQ). Here, it was found that a completely planar organic compound, 5,7,12,14-tetraoxapentacene A-D-A triad with carbon-oxygen bonds, had no fluorescence in high polar solvents but emitted strong light in suspension and in solid state, showed typical AIE effect. One novel AIE mechanism was disclosed by molecular packing in crystal state. In addition, its exceptional AIE effect showed great potential applications in fluorescent thermometer and highly sensitive sensor for selective detection of nitrophenolic compounds. The detection limit for 2,4,6-trinitrophenol was low to 0.168 nM.

Full text of DOI:10.1016/j.dyepig.2019.107556

DyesandPigments170(2019)107556
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Exceptional aggregation-induced emission from one totally planar molecule
Ying-Xue Yuan, Bai-Xing Wu, Jia-Bin Xiong, Hong-Chao Zhang, Ming Hu, Yan-Song Zheng∗
Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and
Technology, Wuhan, 430074, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
AIE
Planar molecule
Intramolecular charge transfer
Slipped π-π stacking
A-D-A triad
Planar molecules usually display aggregation-caused quenching (ACQ). Here, it was found that a completely
planar organic compound, 5,7,12,14-tetraoxapentacene A-D-A triad with carbon-oxygen bonds, had no fluor-
escence in high polar solvents but emitted strong light in suspension and in solid state, showed typical AIE effect.
One novel AIE mechanism was disclosed by molecular packing in crystal state. In addition, its exceptional AIE
effect showed great potential applications in fluorescent thermometer and highly sensitive sensor for selective
detection of nitrophenolic compounds. The detection limit for 2,4,6-trinitrophenol was low to 0.168 nM.
1. Introduction
or from weak to strong, another class of luminogens with twisted in-
tramolecular charge transfer (TICT) emission mechanism can emit a
In 2001, Tang et al. found a new class of organic compounds that
emitted no or weak fluorescence in solution but had strong emission in
aggregation state, and they coined this phenomenon as aggregation-
induced emission (AIE) [1]. Since then, AIE compounds have been paid
enormously extensive study and are attracting more and more interests
because of their colossal application potential in solid emitter and
chemo/biosensors [2–10]. It is general knowledge that multiple big
substituents (rotors) such as phenyl groups need to be introduced into
π-bond plane or aromatic ring (stator) for getting AIE effect. A planar π-
conjugation molecule shows AIE effect only when it bears substituents
that are not in the same plane while it will display aggregation-caused
quenching (ACQ) if it bears no substituent [11–13]. In this way, the
AIEgen molecules possess a twisted conformation in which all sub-
stituents are not in the same plane due to repulsive force each other
[2,8]. The typical examples of AIE compounds include tetra-
phenylethylene (TPE), hexaphenylsilole, 2-phenylcinnamylnitrile and
their derivatives. In solution, the intramolecular motions including
rotation of the substituents lead to non-radiative decay of the lumino-
gens. In aggregation state, the intramolecular motion is restricted and,
simultaneously, the twisted conformation of the luminogens prevents
them from close cofacial π-π stacking that can cause fluorescent
quenching. Therefore, the luminogen emits strong fluorescence in ag-
gregation state. This is well-known mechanism of restriction of in-
tramolecular motion (RIM) that has been extensively recognized for AIE
phenomena. Multiple substituents and twisted or nonplanar con-
formation are structural features of one AIEgen compound [2–13].
While AIE compound makes the fluorescence change from off to on
dual fluorescence and easily adjust the emission colours by change of
the environmental conditions [13–15]. This class of luminogen is
composed of electron donor (D) and electron acceptor (A) units which
easily lead to intramolecular charge transfer (ICT) at the excited state.
By combination of AIE and ICT procedure, luminogens can exhibit more
advantages both in enhancement of emission intensity and variety of
emission colours, which are beneficial for them to be utilized in both
solid emitter and sensors [16,17]. In fact, many D-A rotor luminogens
are born AIEgens according to RIM mechanism [16–23]. On the other
hand, by introducing strong electron-accepting or electron-donating
groups into AIE compounds, the AIEgens can display TICT lumines-
cence [24–29]. Due to easy preparation, a large number of D-A AIEgens
have been reported recently [16–32]. However, from all the D-A AIE-
gens, the donor unit and acceptor moiety are connected by single
covalent bond and the D-A rotor molecules are in twisted or non-planar
conformation just like classical AIE compounds. Here we report that an
A-D-A triad, in which donor and acceptor moieties are linked together
with carbon-oxygen bonds in fused six-member rings, bears all atoms
and substituents in the same plane but displays strong AIE effect. In
solution, non-radiation solvent relaxation from hydrogen bonds be-
tween solvent and the triad arouses no fluorescence. In aggregation
state, due to the repulsive force between multiple oxygen atoms on
different molecular planes, highly slipped pi-pi stacking instead of co-
planar pi-pi stacking was formed, which leads to the AIE effect. This is a
new procedure to AIE phenomenon.
∗ Corresponding author.
E-mail address: zyansong@hotmail.com (Y.-S. Zheng).
https://doi.org/10.1016/j.dyepig.2019.107556
Received 13 April 2019; Received in revised form 11 May 2019; Accepted 11 May 2019
Availableonline15May2019
0143-7208/©2019PublishedbyElsevierLtd.

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
2. Experimental
2.1. Materials and method
Materials: All reagents and solvents were chemical pure (CP) grade
or analytical reagent (AR) grade and were used as received unless
otherwise indicated.
Scheme 1. Synthesis of A-D-A triad 3.
Table 1
Measurements: 1H NMR and 13C NMR spectra were measured on a
Bruker AV 400 spectrometer at 298 K. Infrared spectra were recorded
on BRUKER EQUINAX55 spectrometer. Absorption spectra were re-
corded on a Hewlett Packard 8453 UV–Vis spectrophotometer. Mass
spectrum was measured on an Ion Spec 4.7 Tesla FTMS instrument. The
single crystal data were collected on Rigaku Saturn diffractometer with
CCD area detector. All calculations were performed using the
SHELXL97 and crystal structure crystallographic software packages.
Powder X-ray diffraction (XRD) pattern were measured on a Shimadzu
XRD-6000 diffractometer. Fluorescent emission spectra were collected
on a Shimadzu RF-5301 fluorophotometer at 298 K. Fluorescence life-
times were recorded on Edinburgh instruments (FLS 920 spectro-
meters). Absolute fluorescence quantum yields were recorded on
Absolute PL Quantum Yield Spectrometer C11347 with a calibrated
integrating sphere system. The fluorescence quantum yield was de-
termined using quinine sulfate (Φf = 0.546) in 1 N H2SO4 as standard.
Calculation. The geometrical and electronic properties of the triad
3 were performed with the Amsterdam Density Functional (ADF)
2009.01 program package. The calculation was optimized by means of
the B3LYP (Becke three parameters hybrid functional with Lee-Yang-
Perdew correlation functional) with the 6-31G (d) atomic basis set.
Then the electronic structures were calculated at τ-HCTHhyb/6–311+
+G(d,p) level. Molecular orbitals were visualized using ADFview.
The solubility (mg/mL) and absolute quantum yield (Φf) of 3 in different sol-
vent systems.
Solvent System
Φf%
mg/mL
solid
toluene
benzene
chloroform
dichloromethane
1,2-dichloroethane
1,4-dioxane
CH3CN
DMSO
DMF
THF
Acetone
24.34
31.61
25.24
16.79
6.26
2.65
1.50
0.00
0.00
0.00
0.13
0.00
0.12
0.090
0.050
0.00
5.52
14.70
29.13
/
0.44
0.31
0.24
0.78
0.26
0.15
0.50
0.28
1.97
0.22
0.83
0.40
0.11
0.012
0.0024
/
EA
CH3CH2OH
CH3OH
Hexane
H2O/DMF 90:10
H2O/THF 90:10
H2O/Acetone 50:50
/
/
in DMF, A-D-A triad 3, a derivative of 5,7,12,14-tetraoxapentacene, was
obtained in high yield just by recrystallization from DMF (Scheme 1). In
this D-A system, the two moieties of donor and acceptor were fused
together by double single bonds instead of one single bond.
2.2. Synthesis of the A-D-A triad 3
Synthesis of tetrahydroxybenzene 2. To a flask was added 2,5-di-
hydroxy- [1,4] benzoquinone (1.0 g, 7.15 mmol), concentrated HCl
(20 mL). The mixture was constantly stirred with slowly adding to tin
powder (1.02 g, 8.58 mmol), refluxed for 1 h. The reaction mixture at
50 °C was filtered off, and the filtrate was cooled at 0 °C to yield white
solid. The crude product was recrystallized from THF to yield tetra-
hydroxybenzene (0.65 g, 65%).
3.2. Photophysical properties of triad 3
Compound 3 was insoluble in hexane and cyclohexane while it had
a higher solubility in other conventional solvents such as DMF, acet-
onitrile, THF, dichloromethane and so on (Table 1). As a yellow-green
solid, it showed a main absorption band at about 390 nm in UV–Vis
spectrum (Fig. 1A and Fig. S5). But in DMF and DMSO, the absorption
maximum wavelength (λmax) for this band is at 395 nm and 398 nm,
respectively, which were longer than that in other solvents. In po-
tassium bromide disk, the powder solid had a longer λmax at 415 nm
(Fig. 1A).
Compared with absorption spectra, the emission spectra of 3
showed more obvious change not only in solid state but also in solution.
While the wet crystals isolated from DMF did not show any fluorescence
(Fig. S6), the dry solid powder emitted strong green-yellow light under
irradiation of 365 nm light (Fig. 1B–C and Fig. S7). In solution, as the
solvent polarity increased, the emission was generally weakened and
accompanied by a bathochromic shift. The solution of 3 in benzene
(BZ), toluene (TL) and chloroform (TCM) emitted strong blue light, in
DCM it had green fluorescence, in 1,2-dichloroethane (DCE) and 1,4-
dioxane (DO) a yellow-green fluorescence was observed, and in THF
and ethyl acetate (EA) solution it emitted very faint green-yellow light.
In contrast, the solution in high polar solvents including acetone (AT),
acetonitrile (AN), DMF, DMSO, ethanol, and methanol emitted no any
visual fluorescence (Fig. 1D). The emission spectra also confirmed the
fluorescence change with the solvents. Besides the fluorescence in-
tensity was generally attenuated with the polarity of the solvent, the
emission maximum wavelength λmax was simultaneously increased
from 460 nm to 528 nm (Fig. 1E). However, the wavelength increase
was not linearly relating to the solvent polarity function (Δf) (Fig. S8)
[19,31].
Synthesis of 3. To a flask was added 2,3,5,6-Tetrafluoro-1,4-ben-
zenedicarbonitrile1(800 mg, 4.0 mmol), tetrahydroxybenzene
2
(280 mg, 2.0 mmol) and potassium carbonate (2.21 g, 16.0 mmol), and
DMF (30 mL). The mixture was stirred at room temperature for 2 h. The
mixture was filtered and the solvent was removed on a rotary eva-
porator. The residue was poured into water and was extracted with
dichloromethane. After the combined organic phase was dried over
anhydrous sodium sulfate and filtered, the filtrate was evaporated to
give the crude product, which was recrystallized from DMF to furnish 3
as yellow acicular crystal. After the crystal was stirred to 100 mL die-
thyl ether solvent to remove DMF and filtered, the filter cake was dried
to give pure 3 as yellow solid (813 mg, 88%). Mp > 280 °C; 1H NMR
(400 MHz, DMF-d7) δ 7.42 (s, 2H) ppm. 13C NMR (100 MHz, Acetone-
d6) δ 149.2, 146.4, 142.4, 138.4, 109.4, 107.8; IR (KBr) ν 3060, 2922,
2243, 1649, 1529, 1481, 1412, 1389, 1315, 1266, 1176, 1015, 942,
892, 714, 651 cm−1; HR-MS (ESI+) m/z calculated for C22H2F4N4O4
[M]+ 462.0012, found 462.0017 [M]+
.
3. Results and discussions
3.1. Synthesis of A-D-A planar triad 3
Tetrafluoroterephthalonitrile 1 is an ideal precursor of acceptor,
which can be easily substituted by nitrogen or oxygen atoms to give
excellent D-A rotor luminogens [32–34]. When it reacted with 1,2,4,5-
tetrahydroxybenzene 2 [35,36] in the presence of potassium carbonate
2

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
Fig. 1. (A) The absorption spectra of 3 in po-
tassium bromide disk and in partial solvents. (B)
Photo of solid powder under daylight. (C, B)
Photos of solid powder (C) and solution of 3 in
different solvents (D) under a portable 365 nm
lamp. (E) The emission spectra of 3 in different
solvents.
Em/ex
slit
widths = 3/5,
[3] = 2 × 10−5 M except methanol and ethanol
in which the saturation supernatant of 3 was
used.
3.3. AIE phenomenon of the triad 3
water, the solution became turbid and started to emit fluorescence. As
more water was added and the solution became more turbid, the so-
lution emitted stronger light. At 90% water fraction, the solution
emitted brilliant green-yellow light, which was 793 fold stronger than
that in pure DMF (Fig. 2B). In addition, besides the major emission
band at 520 nm, one minor emission band at 460 nm was also observed.
Moreover, while the emission at long wavelength increased, the emis-
sion at short wavelength decreased with water, showing a continuous
increase of the fluorescence intensity ratio at 520 nm and 460 nm with
water fraction. This result indicated that the emission band at long
wavelength was ascribed to the intermolecular interaction instead of
conformation change of single molecule at the excited state.
Exceptionally, when non-solvent water was added into the solution
of 3 in high polar solvents, the solution became strong emissive from no
fluorescence. As shown in Fig. 2A, when less than 20% water (volume
ratio, the same below) was added into the solution of 3 in DMF, the
solution was still clear and non-emissive. By adding more than 20%
In general, the AIE effect was measured in H2O/THF solvent system.
When water was added into the solution of 3 in THF, the similar phe-
nomenon to the DMF solution was observed (Fig. 3A). At 80% water,
the fluorescence intensity got to the largest and increased 103 times
more than that in pure THF. When water was increased to 90%, the
intensity decreased a little probably due to higher polarity of the mixed
solvent and smaller aggregates [19,31]. But it still emitted 71 fold
stronger green fluorescence than that in pure THF (Fig. 3B). This
change in fluorescence intensity with water fraction was the same as
that of some typical AIE molecules [37–39]. Therefore, luminogen 3
could display AIE effect. Meanwhile, a shoulder peak could be made out
at short wavelength, showing dual fluorescence just like H2O/DMF
system. Using H2O/acetone solvent system as test, the fluorescence
intensity also had a big increase after water was added, which could get
to 100 fold increase at 90% water fraction compared with that in pure
acetone (Fig. S9). Simultaneously, obvious dual fluorescence and
growth of intensity ratio I518/I465 with water were also observed, just
like H2O/DMF and H2O/THF system.
The measurement of the fluorescence quantum yield further corro-
borated the conclusion of the AIE effect (Table 1 and Table S1). Mea-
sured by integrate sphere, the dry solid powder had 24.3% fluorescence
quantum yield. Using quinine sulfate (Φf = 0.546) in 1 N H2SO4 as
standard, the relative fluorescence quantum yield in the three aqueous
solvent systems was estimated. In DMF and 90% H2O/DMF, it was 0%
and 4.96%, respectively; In THF and 80% H2O/THF (intensity
Fig. 2. (A) Change in the emission spectra of 3 in DMF with water fraction.
Inset: curve of intensity ratio at 520 nm and 460 nm vs water fraction. (B) Curve
of fluorescence intensity at 520 nm vs water fraction. Inset, photos of solution of
3 in pure DMF and in 90:10 H2O/DMF under irradiation of a portable 365 nm
lamp. λex = 409 nm, em/ex slit widths = 3/5, [3] = 1 × 10−5 M.
3

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
Fig. 3. (A) Change in the emission spectra of 3 with water fraction in THF. (B) Curve of fluorescence intensity vs water fraction in acetone. Inset, photos of solution of
3 in pure THF and in 90:10 H2O/THF under irradiation of a portable 365 nm lamp. λex = 416 nm, em/ex slit widths = 5/5, [3] = 1 × 10−5 M.
maximum, the same below), it was 0.36% and 16.4%, respectively; In
acetone, 50% H2O/acetone, and 90% H2O/acetone, it was 0%, 22.0%,
and 4.9%, respectively. After aggregation, the fluorescence quantum
yield increased very significantly.
3.4. Crystal structure and AIE mechanism of triad 3
Fortunately, single crystal that was suitable for X-ray diffraction was
obtained by slow evaporation of the solution of 3 in DMF at room
temperature. The crystal structure disclosed that the molecule was
composed of five fused six-membered rings, which gave an A-D-A
system [40]. The angles between all five six-membered rings were 0.00°
and even the fluorine atoms and cyano groups were in the same mo-
lecular plane. Therefore, the fluorophore molecule was completely
planar (Fig. 4A−4B). Due to D-A structure, there was a slipped π-π
stacking between the donor moiety of one molecule and the acceptor
moiety of another one along the long axis of the molecule. However, the
π-π stacking area was very small and there was only an edge to edge
contact (3.334 Å) between the aromatic rings. The measured pitch
angle and roll angle in the slipped π-π stacking were 54.08° and 46.22°,
respectively (Fig. S10). The roll angle that was a little larger than 45°
confirmed almost no overlapping area between two adjacent molecules
[41–43]. However, the short contact was less than the typical π-π
stacking distance (3.4 − 3.6 Å) [39] and there were additional inter-
actions (3.350 Å) between electron deficient carbon of cyano groups
and centroid of electron-rich tetraoxybenzene rings (Fig. 4C), demon-
strating the efficient π-π stacking interaction through edge to edge
contact instead of cofacial overlap. Moreover, the slipped π-π stacking
could be formed at different slipped π-π stacking directions (Fig. 4D),
suggesting the facile availability of the slipped π-π stacking in ag-
gregation state. The different slipped π-π stacking series were bound
together by F–C (CN) short contacts (3.091 Å) and interactions between
cyano nitrogen and electron-deficient centroid of difluorobenzene ring
(3.248 Å). The very small contact area between the molecular planes
should be ascribed to the four oxygen atoms bearing two lone electron
pairs, which would lead to large repulsive force between oxygen atoms
Fig. 4. Crystal structure of 3 viewed (A) perpendicular to the molecular plane
if two molecular planes were cofacially overlapping. Noticeably, more
and (B) along the molecular plane. (C) The packing of the molecules. (D) The
than 1:1 DMF molecules were cocrystallized due to strong hydrogen
bond (2.095 Å) between aromatic hydrogen of 3 and carbonyl oxygen
atom of DMF (Fig. 4C).
slipped π-π stacking at different directions with the solvent molecules being
removed for clarity. The number denotes the intermolecular distance in ang-
strom and the red ball denotes the centroid of a ring. (For interpretation of the
references to colour in this figure legend, the reader is referred to the Web
version of this article.)
The slipped π-π stacking of the molecular plane should be the
reason why the fluorophore emitted dual fluorescence. In nonpolar or
less polar solvent, the excited ICT state could not be stabilized by sol-
vation and gave an excited state at high energy level. Therefore, the
emission that was like local emission (LE) or monomer emission ap-
peared at short wavelength. Meanwhile, the less solvation attenuated
the non-radiative solvent relaxation, which aroused a strong fluores-
cence. In highly polar solvent, especially in those solvents bearing atom
that could form hydrogen bond with the aromatic protons of 3, the
excited ICT state could be stabilized by the solvation. If the
concentration was higher or in aggregation state, the ICT slipped π-π
stacking occurred, which would bring down the energy level of the
excited state due to strong Coulomb interactions between ICT states,
leading to a bathochromic emission. Meanwhile, due to the strong
solvation, the non-radiative solvent relaxation reduced the emission
intensity and even caused no any fluorescence. Especially, it should be
4

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
mentioned that although THF, 1,4-dioxane and ethyl acetate had less
polarity function than that of DCM and DCE, they possessed oxygen
atom that could form strong hydrogen bond with the aromatic protons.
Therefore, 3 displayed more bathochromic and weaker emission in
THF, 1,4-dioxane and EA than in DCM and DCE (Fig. S8). In the case of
weak monomer emission or the slipped π-π stacking one that they did
not completely overlay each other, dual fluorescence was observed.
The photophysical behaviour of the solid further confirmed this
speculation. In wet crystals isolated from DMF solution, no fluorescence
was observed under 365 nm light because of cocrytallized DMF mole-
cules that were linked to 3 by hydrogen bonds. After the all DMF mo-
lecules were removed, however, the solid emitted strong green-yellow
light. By comparing XRD pattern of wet crystals and dry powder, the
later kept a good crystal state (Fig. S11). The IR spectrum also had
bathochromic shift of about 20 nm compared with the DMF solution,
indicating the existence of the slipped π-π stacking. When water was
added into the solution, the water could form hydrogen bond with
DMF, THF, or acetone etc., and replaced the hydrogen bonds between 3
and the organic solvents (Fig. S6E). The formed aggregates did not
involve solvent molecules that could form efficient hydrogen bonds
with 3. Therefore, the formed aggregates by water addition could avoid
non-radiative solvent relaxation and make fluorescence increase. In
addition, the aggregation could restrict intramolecular motions such as
flapping and vibration, which made the emission further increase ac-
cording to RIM mechanism.
derivatives [44,45]. Therefore, the emission band at long wavelength
should come from the aggregation or the slipped π-π stacking instead of
TICT process. Furthermore, the bathochromic shift aroused by TICT
process was linearly relating to the solvent polarity function, but this
emission of 3 was not so [11,19,31]. For A-D-A triad 3, donor and ac-
ceptor moieties were fused through oxygen atoms, which had a stable
sp2 hybridization that led to difficulty of simultaneous rotation of two
carbon-oxygen single bonds.
Time-resolved emission decay showed that the fluorescence life-
times (τ) had an obvious prolonging change from monomer emission to
the slipped π-π stacking emission (Fig. S12 and Table S1). The solution
in toluene and DCE had 7.05 ns and 1.53 ns fluorescent lifetime, re-
spectively, while the powder solid had 23.8 ns. The lifetime for sus-
pension in 90:10 H2O/DMF and 80:20 H2O/THF was 19.4 ns and 56.9
ns, respectively, further indicating longer lifetime of aggregates than
monomer. In 50:50 H2O/acetone, there appeared distinct dual fluor-
escence, the lifetime was 1.60 ns for the monomer emission at 460 nm
and 6.50 ns for the aggregate emission at 520 nm, which was also in
accordance with the above observation. This result confirmed the ag-
gregates had stronger fluorescence and longer lifetime.
3.5. Application properties of the triad 3
More interestingly, the A-D-A luminogen displayed emission in-
crease with temperature in 1,2-dichloroethane. When temperature was
raised from 0 °C to 80 °C at 10 °C interval each time, the intensity was
linearly increased with temperature (Fig. 6A). Meanwhile, the emission
maximum wavelength showed hypsochromic shift with temperature.
The fatigue resistance for the fluorescence change with temperature
was high. By repeatedly heating to 80 °C and then cooling to 25 °C for
ten times, the fluorescence spectra and intensity had almost no change
(Fig. S13). This result suggested that this fluorophore had a potential
for using as thermometer, especially in the conventional temperature
Density functional theory (DFT) analysis suggested that the HOMO
orbitals of 3 resided on the tetraoxybenzene unit but not on the cyano
groups (Fig. 5). In contrast, the LUMO orbitals localized on the cyano
groups but not on the tetraoxybenzene unit both at the ground state and
at the excited state. The completely separation of the molecules orbital
indicted the ICT process. Meanwhile, at the excited state, the molecular
geometry was still planar (Fig. 5B), demonstrating that no bending or
twisting occur at the excited state, unlike dihydrobenzophenazine
Fig. 5. Frontier molecular orbitals and geometry of 3 calculated by DFT. (A) HOMO and LUMO orbitals at the ground state. (B) Molecular geometry, HOMO and
LUMO orbitals at the excited state.
5

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
Fig. 6. Change in the emission spectra of 3 in 1,2-dichloroethane with temperature (A) and with hexane fraction (B). The arrows indicating the change tendency of
the intensity and emission maximum λmax; inset, linear change of the intensity maximum with temperature. λex = 395 nm, [3] = 2 × 10−5 M.
Fig. 7. (A) Fluorescence spectra of 3 in 90:10
H2O/THF by addition of different NACs.
[P] = [PNT] = [DNT] = [TNT] = [NB] = [DNB] = [TNB] = [PNP] = [DNP] = [TNP] = 1.0 × 10−4 M. (B) Change in fluorescence spectra of 3 in 90:10 H2O/THF
with concentration of TNP. (C) Curve of fluorescence intensity at 496 nm vs. concentration of TNP from 0 to 5.0 × 10−4 M. (D) Change in fluorescence intensity at
496 nm with concentrations of TNP from 0 to 2.0 × 10−6 M, the red line indicating the fitted one by Origin 7.5 software. [3] = 1.0 × 10−5 M λex = 416 nm, ex/em
slit widths = 5/5 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
range [13,18,19,29].
aromatic compounds (NACs) both bearing electron-attracting unit and
lone-pair electrons of oxygen of nitro groups [46,47]. Moreover, the
planar structure will facilitate these interactions due to less steric hin-
drance. Therefore, with the suspension of 3 in 90:10 H2O/THF as a
sensing system, the effect of NACs on the emission was tested. NACs
included 1,3,5-trinitrobenzene (TNB), 2,4,6-trinitrotoluene (TNT),
2,4,6-trinitrophenol (TNP), 1,3-dinitrobenzene (DNB), 2,4-dini-
trotoluene (DNT), 2,4-dinitrophenol (DNP), nitrobenzene (NB), p-ni-
trotoluene (PNT), and p-nitrophenol (PNP) as well as phenol (P). When
the NACs were added to the suspension, respectively, only nitrophenol
derivatives obviously alleviated the fluorescence (Fig. S15). Among
nitrophenol compounds, TNP showed the largest quenching. This was
contrary to other AIE compounds which were more sensitive to TNT
rather than TNP [46–49]. The detection limit for TNP was low to
1.68 × 10−7 M (Fig. 7 and Fig. S14).
The phenomenon of fluorescence increase with temperature may be
ascribed to thermal activity or polarity decrease of the solvent. Due to
lack of TICT process and the triple state emission, the thermal activity
could be excluded. In order to prove the role of the solvent polarity on
the fluorescence, the polarity of DCE was continuously lowered by
addition of hexane. As expected, when hexane fraction was increased
from 0% to 60%, the monomer fluorescence displayed a gradual in-
tensity increase and hypsochromic shift thanks to the polarity decrease
(Fig. 6B). When the hexane fraction was got to 70%, 3 started to ag-
gregate and turbid appeared in the solution due to insolubility of 3 in
hexane. As the hexane fraction was more than 70% and more ag-
gregation occurred, the monomer emission decreased while the slipped
π-π stacking emission increased. This observation not only demon-
strated the key reason of fluorescence change with temperature but also
further corroborated the AIE effect of 3 even in the pure organic media.
Given that 3 possessed both electron-attracting and electron-do-
nating unit, it should have strong n-π and p-π interactions with nitro
6

Y.-X. Yuan, et al.
DyesandPigments170(2019)107556
4. Conclusion
[18] Sasaki S, Drummen GPC, Konishi G. Recent advances in twisted intramolecular
charge transfer (TICT) fluorescence and related phenomena in materials chemistry.
J Mater Chem C 2016;4(14):2731–43.
In conclusion, it was found for the first time that a totally planar A-
D-A triad could emit strong light in suspension and in solid but no
fluorescence in highly polar solvents, displaying a typical AIE effect,
although all reported AIE phenomena are based on twisting or non-
planar molecules and the planar molecules often usually encounter
aggregation-caused quenching problems. In this triad, the repulsive
force between oxygen atoms on the different molecular planes pre-
vented the planar molecules from cofacial π-π stacking that could cause
quenching. Therefore, the triad showed AIE effect due to RIM process in
aggregation state. Considering a large number of fused heterocyclic
aromatic compounds, this finding will largely extend the content of
AIEgen molecules. More and better AIEgen compounds will be devel-
oped from planar aromatic compounds besides twisting molecules.
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The authors thank National Natural Science Foundation of China
(91856125 and 21673089) and HUST Graduate Innovation Fund for
financial support, and thank the Analytical and Testing Centre at
Huazhong University of Science and Technology for measurement.
Appendix A. Supplementary data
Supplementary data related to this article can be found at https://
doi.org/10.1016/j.dyepig.2019.107556.
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