Tailoring the Molecular Properties with Isomerism Effect of AIEgens

Article abstract of DOI:10.1002/adfm.201903834

It is challenging to achieve precise control on the properties of organic π-functional materials to widen their practical applications. On the other hand, the study of aggregation-induced emission luminogens (AIEgens) helps achieve such goals because of i

Full text of DOI:10.1002/adfm.201903834

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Isomeric AIEgens
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Tailoring the Molecular Properties with Isomerism
Effect of AIEgens
Ming Chen, Junkai Liu, Feng Liu, Han Nie, Jiajie Zeng, Gengwei Lin, Anjun Qin,*
Mei Tu, Zikai He, Herman H. Y. Sung, Ian D. Williams, Jacky W. Y. Lam,
and Ben Zhong Tang*
and multidimensional materials for their
It is challenging to achieve precise control on the properties of organic
π-functional materials to widen their practical applications. On the other
hand, the study of aggregation-induced emission luminogens (AIEgens) helps
achieve such goals because of inherent relationships between their lumines-
cence behaviors and conformational variations that allow for the visual moni-
toring of the changes in the material properties. Inspired by this, in this work,
three AIE isomers are fabricated in structures consisting of tetraphenylpyra-
zine and triphenylethene units with para-, meta-, and ortho-position link-
ages, respectively. The isomerism effect brings about significantly decreased
luminescence efficiency, subtly blueshifted emission, basically reduced AIE
effect but boosted porosity in the aggregate state as the conformation of
AIEgens evolves from an extended to a folded one. Based on the distinct
properties, their respective use in blue organic light-emitting diodes, nano-
fluorescent probes, and molecule-capturing porous crystals are investigated.
This work not only achieves precise property control by using the isomerism
effect of AIEgens but also provides useful information on the future design of
π-conjugated materials with advanced functionalities.
intrinsic π-conjugation and rigid struc-
ture.[1] Generally, the functions of organic
conjugated molecules are controlled by
molecular structure, conformation, mole-
cular packing in the aggregate state.[2] Due
to the involvement of multiple factors,
more properties other than desired ones
are usually obtained to lead to unexpected
performance of the designed materials.
For example, the semiconducting layers of
organic field effect transistors often con-
sist of organic conjugated molecules with
planar structures. The close packing of
the planar molecules benefits the overlap
of π-electron cloud between the adjacent
molecules, which dramatically improves
the mobility of the materials. However,
the recombination of excitons in device
operation contributing to luminescence
or heat is still inevitable.[3] On the con-
trary, π-π stacking in organic emitters
should be strongly prohibited because
it is a notorious cause for emission quenching in the aggre-
gate state.[4] To solve such problem, various methods such as
designing molecules with twisted or branched architectures
and covalently hanging them on rigid cubes have been adopted
1. Introduction
Organic conjugated molecules play important roles in conduct-
ance, photoelectronic conversion, luminescence, self-assembly,
Dr. M. Chen, F. Liu, Prof. M. Tu
College of Chemistry and Materials Science
Jinan University
Dr. M. Chen, J. Liu, Dr. J. W. Y. Lam, Prof. B. Z. Tang
HKUST-Shenzhen Research Institute
No.9 Yuexing 1st RD, South Area, Hi-Tech Park, Nanshan 518057, China
Guangzhou 510632, China
Dr. H. Nie, J. Zeng, G. Lin, Prof. A. Qin, Prof. B. Z. Tang
Center for Aggregation-Induced Emission
SCUT-HKUST Joint Research Institute
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640, China
Dr. M. Chen, J. Liu, Dr. H. H. Y. Sung, Prof. I. D. Williams,
Dr. J. W. Y. Lam, Prof. B. Z. Tang
Department of Chemistry
Hong Kong Branch of Chinese National Engineering Research Center
for Tissue Restoration and Reconstruction
Institute for Advanced Study
E-mail: msqinaj@scut.edu.cn
Department of Chemical and Biological Engineering
Division of Life Science
State Key Laboratory of Molecular Neuroscience and Institute
of Molecular Functional Materials
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong, China
E-mail: tangbenz@ust.hk
Prof. Z. He
School of Science
Harbin Institute of Technology Shenzhen
HIT Campus of University Town of Shenzhen
Nanshan, Shenzhen 518055, China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201903834.
DOI: 10.1002/adfm.201903834
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but have achieved with only limited success.[5] Most often, the
π–π stacking cannot be overcome radically by suppressing the
molecular aggregation to exempt influence on the emission
efficiency. Thus, it is of great significance, yet a challenge, to
regulate the molecular properties precisely by reading the
molecular information comprehensively and rationally.
related to plentiful conformational changes that influence the
solid-state properties. Thus, it is interesting to tailor the mole-
cular properties by studying AIEgens as the guideline, which
will assist further design of materials with desired properties.
With these in mind, in this work, we prepared three con-
jugated isomers by connecting tetraphenylpyrazine (TPP) and
triphenylethene (TPE) at the para-, meta-, and ortho-position,
respectively (Figure 1). These isomers show AIE features inher-
ited from TPP and TPE.[11] When the conformation of these iso-
mers evolves from a linear to a folded one, the emission was
weakened gradually because of reduced molecular conjugation
and decreased restriction of intramolecular motion due to the
loose intermolecular packing. On the other hand, the mole-
cular porosity increases, as revealed by the formation of porous
crystals of ortho-isomer with defined nanocavities and nano-
channels easily that can capture guest molecules like dichlo-
romethane (DCM) and tetrahydrofuran (THF). The potential
applications of these AIEgens as luminescent materials (e.g.,
OLED and nanoprobes) were also presented.
Conjugated luminogens with aggregation-induced emis-
sion (AIE) characteristics have attracted considerable attention
for their wide applications in organic light-emitting diodes
(OLEDs), fluorescent sensors, bioimaging, etc.[6] The intrinsic
characteristic of AIE luminogens (AIEgens) is their twisted and
flexible molecular conformation, which makes them to emit
faintly in solution due to the active intramolecular motions
but show remarkably enhanced emission in the aggregate
state due to the restriction of such motions.[7] The nonplanar
structures of AIEgens also endow them with tunable structural
transformability because of the loose molecular packing in the
aggregate state which provides more free space for conforma-
tional change. For example, our group prepared hyperbranched
polymers constructed from AIE unit of tetraphenylethene.
The polymers seem rigid but actually can be compressed like
“spring” under external stimuli.[8] Moreover, we and other
groups reported AIEgen-based soft porous crystals, which can
encapsulate organic small molecules by virtue of their porous
framework determined by stereo and flexible conformation
of AIEgens, their intermolecular interactions, etc.[9] Recently,
we also designed AIEgens with near-infrared absorption that
exhibited controllable intramolecular motions in the aggregate
state. Such property equilibrates luminescence and heat forma-
tion due to the excited-state energy dissipation, making these
AIEgens as promising multifunctional biomedical agents in
combination of fluorescence imaging, photoacoustic imaging,
and photothermal therapy.[10] Notably, AIEgens are not only
competent in strong solid-state luminescence but also closely
2. Results and Discussion
2.1. Synthesis, Characterization, and Thermal Properties
of AIE Isomers
The AIE isomers, namely TPP-p-TPE, TPP-m-TPE, and TPP-
o-TPE, were synthesized by Suzuki coupling of Br- or boric acid
ester-substituted TPP or TPE intermediates using Pd(PPh3)4
as catalyst (Figure 1 and Scheme S1, Supporting Informa-
tion). The finial products were carefully purified using column
chromatography and their structures were fully character-
1
ized by H and 13C NMR and high resolution mass spectros-
copies with satisfactory data corresponding to their structures
Figure 1. Schematic illustration of functionalities tuned by the isomerism effect of AIEgens.
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(Figures S1−S23, Supporting Information). Thermogravimetric
analysis (TGA) shows that AIE isomers show 5% weight loss at
temperatures of 338, 309, and 295 °C, respectively, suggestive of
a good thermal stability (Figure S24, Supporting Information).
Analysis by differential scanning calorimetry (DSC) reveals
that TPP-m-TPE and TPP-o-TPE show a similar glass transi-
tion temperature (Tg) of 110 °C, while no Tg is found for TPP-
p-TPE (Figure S25, Supporting Information). Such difference is
probably attributed to the varied molecular conformation and
molecular packing model of the isomers in the aggregate state.
The photoluminescence (PL) of the isomers was then studied
in THF/water mixtures with different water fractions (fw)
(Figure 3). All the molecules exhibit only noisy signals with no
discernible peak maximum in diluted THF solution (10 × 10−6 m).
The emission remains weak when up to 60 vol % of water was
added to the THF solutions. Afterward, intense emission signals
are recorded. This is because the luminogens are molecularly
dissolved in THF and the vigorous intramolecular motions in
solution dissipate the excited-state energy nonradiatively. How-
ever, nanoaggregates form in the presence of a large amount of
water, which restricts such motion and allow the excitons to relax
radiatively. The absolute quantum yields (QYs) of TPP-p-TPE,
TPP-m-TPE, and TPP-o-TPE in films measured by an integrating
sphere are 54.2%, 34.2%, and 5.0%, respectively while no reli-
able values can be collected in the solution state due to the weak
intensity (Table 1). The much higher QYs in films than in THF
and the results from PL spectra both confirm that these isomers
are AIE-active. Both TPP and TPE are well-known AIEgens,
melding them together certainly will produce new luminogens
with property inherited from both the AIE parent units.
The TPP-p-TPE, TPP-m-TPE, and TPP-o-TPE films emit at
477, 468, and 454 nm, and show emission colors from sky-blue
to deep blue. These results are consistent with those of nano-
aggregates in THF/water mixtures (Figure S26, Supporting
Information). The bluer emission in the film state may
attribute to the decreased molecular conjugation due to the
conformation change. The emission properties of AIE isomers
are basically the same as that of tetraphenylethene.[13] How-
ever, they show some difference in AIE effect. For example,
the αAIE values (defined as the ratio between the PL intensity
of the molecules in THF/water mixtures with fw = 95% and
THF) of TPP-p-TPE, TPP-m-TPE, and TPP-o-TPE are calcu-
lated to be 249, 275, and 47, respectively. Since the isomers are
nonemissive in solution, the AIE behavior is often determined
by the luminescent behavior in the aggregate state. Thus, the
excited-state decay of the films was further studied by the for-
mula of kr = ΦF/τ and knr = (1 − Φ)/τ, where kr and knr are the
radiative and nonradiative decay rates, and ΦF and τ are the
absolute quantum yield and excited-state lifetime, respec-
tively. The calculated kr of the isomers in the films state are
2.27 × 108, 0.85 × 108, and 0.19 × 108 S−1, respectively, indi-
cating that the radiative transition is influenced remarkably by
the molecular conjugation. On the other hand, the knr of TPP-
p-TPE (1.92 × 108 S−1) and TPP-m-TPE (1.64 × 108 S−1) are rela-
tively close and are lower than that of TPP-o-TPE (3.57 × 108 S−1)
(Figure S27 in the Supporting Information and Table 1). This
is due to the much folded conformation of TPP-o-TPE, which
makes its molecules to pack loosely in the aggregate state to
introduce free volume to promote molecular motions. Never-
theless, the isomerism effect exerts stronger influence on the
radiative transition than the nonradiative transition of the AIE
isomers.
2.2. Optical Properties
Since the isomers are composed of typical AIE units of TPP and
TPE with para-, meta-, and ortho-linkages, they are anticipated to
show different photophysical properties. We first studied the UV
absorption spectra of the isomers in THF (Figure 2). TPP-p-TPE,
TPP-m-TPE, and TPP-o-TPE exhibit an absorption maximum at
350, 336, and 332 nm, respectively, due to the π–π* transition
of the chromophores. These values are similar to that of TPP
(338 nm), demonstrating that the TPP moiety play a crucial role
in the absorption of the molecules.[11a] Because the molecular
conjugation is weakened from the conformation of the molecules
changed from extended in para-isomer to folded in ortho-isomer,
their absorption maximum blue-shifts accordingly. Interestingly,
TPP-o-TPE shows a molar absorptivity of 4.28 × 104 L mol−1 cm−1,
which is close to TPP-p-TPE (4.26 × 104 L mol−1 cm−1) but larger
than that of TPP-m-TPE (3.22 × 104 L mol−1 cm−1) in spite of its
much folded conformation. It is probably due to the another
intramolecular through-space charge transfer transition, which
takes places from TPE to TPP units as they locate adjacently
in space in the folded conformation, therefore increasing the
probability of transition.[12]
The S0 geometry of the AIE isomers optimized by B3LPY/6-
31(d) level reveals that all the molecules adopt a twisted structure
(Figure 4). TPP-p-TPE shows a relatively more extended confor-
mation, whereas TPP-m-TPE and TPP-o-TPE are much folded
because their TPP and TPE units are located much adjacently
in space. The highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) of TPP-p-TPE
Figure 2. UV–vis spectra of TPP-p-TPE, TPP-m-TPE, and TPP-o-TPE in
THF [dye] = 10 × 10−6 m.
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Figure 3. PL spectra of A) TPP-p-TPE, B) TPP-m-TPE, and C) TPP-o-TPE in THF/water mixtures with different water fractions (fw). Dye concentration =
10 × 10−6 m; excitation wavelengths = 350, 336, and 332 nm, respectively. D) Plot of relative PL intensity (I/I0 − 1) versus the composition of THF/water
mixtures of AIE isomers, where I0 = PL intensity in pure THF solution. Inset: fluorescent photographs of TPP-p-TPE, TPP-m-TPE, and TPP-o-TPE (from
top to bottom) in THF (left) and THF/water mixtures (right) with fw = 90%.
are contributed by orbitals of all units, while in TPP-m-TPE and
TPP-o-TPE, the orbitals of the HOMO and LUMO are mainly con-
tributed by TPE and TPP, respectively. Such theoretical analysis
further confirms that TPP- p-TPE shows a better conjugation
than TPP-m-TPE and TPP-o-TPE to contribute greater radiative
transition.
2.3. Organic Light-Emitting Diodes
Thanks to the high PL efficiency of TPP-p-TPE and TPP-m-TPE
in the film state, they are promising to serve as light-emitting
layers of blue OLEDs. Before device fabrication, their energy
levels are first evaluated. The HOMO and LUMO energy
Table 1. Photophysical properties of the AIE isomers.
λ
abs [nm]
350
λ
em [nm]
477
Φ
F [%]
τ [ns]
2.39
4.01
2.66
k [108 S−1] knr [108 S−1
]
TPP-p-TPE
TPP-m-TPE
TPP-o-TPE
54.2
34.2
5.0
2.27
0.85
0.19
1.92
1.64
3.57
336
468
332
454
Abbreviation: λabs: absorption maximum in THF; λem: emission maximum in films;
ΦF: fluorescence quantum yields of films measured by an integrating sphere; τ:
fluorescence lifetime of films; kr: rate of radiative decay of films = ΦF/τ; knr: rate
of nonradiative decay of films = (1 − ΦF)/τ. The films are obtained by coating the
THF solution of molecules onto quartz sheet followed by removing the solvent by
evaporation.
Figure 4. Electron cloud distributions of the HOMO and LUMO based on
S0 geometry of AIE isomers optimized by B3LPY/6-31(d) level.
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Figure 5. A) The schematic energy level diagram of the materials used in the OLED devices. B) EL spectra of the devices. C) Plot of luminance and
current density with the applied voltage of devices. D) CIE coordinates of the devices, where the triangle and the star represent TPP-p-TPE- and TPP-
m-TPE-based devices, respectively.
levels of TPP-p-TPE and TPP-m-TPE are deduced as −5.58 and
−5.62 eV, and −2.46 and −2.30 eV, respectively, by cyclic voltam-
metry and UV spectroscopy (Figure S28, Supporting Informa-
tion). Considering the importance of energy level matching in
device design, commercial N,N′-di(1-naphthyl)-N,N′-diphenyl-
benzidine (NPB) and 1,3,5-tri(1-phenyl-1H-benzo[d]imi-
dazol-2-yl)phenyl (TPBi) are chosen as hole-transporting and
electron-transporting layers, respectively. Besides, ITO, Al, and
LiF are separately used as anode, cathode and electron-injecting
layer. This forms typical triple-layer electroluminescence (EL)
devices with a configuration of ITO/NPB (60 nm)/AIE isomer
(20 nm)/ TPBi (40 nm)/LiF (1 nm)/Al (Figure 5A).
The TPP-p-TPE-based device (device I) turns on at 5.1 V
and shows maximum luminance (Lmax), current efficiency (ηc),
powder efficiency (ηp) and external quantum efficiency (ηext) of
18 431 cd m−2, 6.12 cd A−1, 3.07 lm W−1, and 2.74%, respectively.
The device displays sky-blue emission at 488 nm with a Commis-
sion Internationale de L’Eclairage (CIE) coordinate of (0.19, 0.34)
(Figure 5B–D and Table 2). Such result is comparable to many
AIEgen-based OLEDs with sky-blue emission. However, its
emission color is much redder and still conflicts with blue light
for practical application. In contrast, device II fabricated using
TPP-m-TPE as luminescent layer shows a deep-blue emission at
454 nm. The CIE coordinate is (0.16, 0.15). Thus, the emission
color is much purer than the former.[14] Nevertheless, its perfor-
mance is poorer. For example, its Lmax, ηc, and ηp are measured
to be 4571 cd m−2, 1.65 cd A−1, and 0.72 lm W−1, respectively,
which are much lower than those of device I. Besides, the ηext
is almost half of that of device I. It is understandable as TPP-
m-TPE possess a lower quantum yield than TPP-p-TPE in the
film state because of its shorter conjugation length caused by
the more crooked molecular conformation. On the other hand,
such a molecule cannot pack well in the aggregate state due to
the steric effect of its V-shaped structure. Because carrier’s trans-
portation is through hopping between its adjacent molecules in
the emitting layer,[15] the loose packing of TPP-m-TPE will lessen
its charge transport capacity to collectively deteriorate the device
performance. These results imply that the emission wavelength
of a compound can be tuned to meet desirable OLED applica-
tion by isomerism strategy, but the best device performance is
achieved in molecules with good conjugation.
2.4. Nanofluorescent Probes
The AIE isomers exhibit varied luminescent behaviors and
are likely to form nanoaggregates in THF/water mixtures with
different morphologies caused by the isomerism effect. Since
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Table 2. EL performances of the devices.
Von[V]
5.1
EQE [%]
2.74
CIE [x, y]
(0.19, 0.34)
(0.16, 0.15)
λ
EL [nm]
488
L
max [cd m−2
]
ηc [cd A−1
6.12
]
η
p [lm W−1
]
TPP-p-TPE
TPP-m-TPE
18341
3.07
454
5.5
4571
1.65
0.72
1.41
Abbreviation: λEL: emission maximum of the EL spectra; Von: turn-on voltage; Lmax, ηc, ηp, EQE: maximum values of luminescence, current efficiency, power efficiency, and
external quantum efficiency of the devices, respectively; CIE (x, y): CIE coordinate at 1000 cd m−2
.
AIE nanoaggregates are highly emissive and have found poten-
tial applications as fluorescent sensors, how does the molecular
isomerism affect the detection results? To answer this ques-
tion, we first evaluated their detection behavior for aromatic
explosives because these analytes are electron-deficient and
interact readily with most fluorescent molecules to alter their
PL. Herein, we used 2,4,6-trinitrophenol (picric acid, PA) as
model explosive for study due to its commercial availability.
Figure 6A−C shows the PL spectra of the nanoaggregates of
concentration of as low as 5 × 10−6 m and the PL signals are
almost lost at a PA concentration of 300 × 10−6 m.
Remarkably, the Stern-Volmer plots of I/I0 − 1 versus
[PA] show overall upward bent curves instead of linear ones
(Figure 6D). This means the PL quenching becomes efficient
as the analyte concentration increases, indicative of a super-
amplification effect for PA detection. It is probably because
each PA molecule can have more probability to interact with
AIEgens in the surroundings once PA is encapsulated into
the probe. It is worth noting that TPP-o-TPA shows a stronger
emission quenching than TPP-m-TPE at high quencher con-
centration, in spite of its much weaker PL in nanoaggregates.
It is mostly due to the porous structure of the nanoparticle
the AIE isomers formed in THF/water mixtures with fw
=
90% in the presence of PA. With an increase of the amount
of PA added, the PL of the molecules decreases progressively.
The fluorescence quenching can be clearly discerned at a PA
Figure 6. PL spectra of A) TPP-p-TPE, B) TPP-m-TPE, and C) TPP-o-TPE nanoaggregates formed in THF/water mixtures with fw = 90% in the presence
of PA with different concentrations. Dye concentration = 10 × 10−6 m; excitation wavelengths = 350, 336, and 332 nm, respectively. D) Plots of I0/I − 1
versus PA concentration, where I = peak intensity and I0 = peak intensity at [PA] = 0 × 10−6 m. Inset: chemical structure of PA.
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suspension generated by the loose packing of TPP-o-TPE
molecules upon aggregation. The porosity of TPP-o-TPE
aggregates facilitates the interaction of the probe with ana-
lyte to improve its sensitivity at high quencher concentration.
However, its quenching efficiency is somewhat smaller than
that of TPP-p-TPE. This is because the stronger Lewis acid-
base interactions are inclined to take place between PA and
AIE isomers with better conjugation, which helps to quench
the emission. The two effects are competitive in determining
the probe behavior.[16]
On the other hand, careful examination of the Stern-Volmer
plots depicts that linear relationships exist at PA concentra-
tion from 0 to 75 × 10−6 m with similar quenching constants of
25 457, 25 459, and 19 351 m−1, respectively (Figure S29, Sup-
porting Information). This indicates that the morphologies of
the nanoaggregates and Lewis acid-base interactions caused
by isomerism effect exert less influence on the sensitivity of
PA sensors at Low PA concentration than at high quencher
concentration.
More specifically, the PL decay curves of nanoparticle sus-
pensions treated with PA of 0, 50 × 10−6, and 150 × 10−6 m are
investigated. Before PA addition, lifetimes of 2.81 ns, 3.79 ns
and 2.29 ns, respectively are deduced, whose values remain
almost unchanged during the PA titration process (Figure S30,
Supporting Information). The independence of the lifetime
of nanoaggregates on the PA concentration indicates that the
quenching mechanism is ascribed to a static quenching model,
in which the electron transition takes place from the ground
state of AIEgen to the excited-state of PA to form nonlumines-
cent complexes in the ground state. The shift in UV absorption
spectra of probe in the presence of PA further confirms the
formation of such complexes (Figure S31, Supporting Informa-
tion).[17] In static quenching, the lifetime of the probe will not
be affected because those luminogens which are not complexed
will have normal excited-state properties. Thus, the fluores-
cence quenching is caused neither by photoinduced electron
transfer nor energy transfer because no excited-state behavior
of AIEgen is participated in the process. In other words, the PL
efficiency of the AIE nanoaggregates determined by the isom-
erism effect will subtly influence the quenching process for PA
detection.
Next, we investigated whether the fluorescent probes
based on AIE isomers could respond to the metal ion of
ruthenium(III) (Ru3+) because it interact readily with fluo-
rescent molecules with heteroatoms as reported.[18] Similarly,
the nanoaggregates of AIEgens formed in THF/water mix-
ture with fw = 90% are used as probes. The emission of TPP-
p-TPE, TPP-m-TPE, and TPP-o-TPE quenches progressively
with gradual addition of Ru3+ (Figure 7A−C). The responses
can be clearly recorded at a Ru3+ concentration of as low as
1 × 10−6 m. The Stern-Volmer plots of I/I0 − 1 versus [Ru3+] are
nearly linear until the Ru3+ concentration reaches 12 × 10−6,
8× 10−6, and 16 × 10−6 m for TPP-p-TPE, TPP-m-TPE, and TPP-
o-TPE, respectively. Afterward, plateaus are reached. At these
concentrations, the PL intensity of the isomers decreases by
about 79%, 67%, and 65%, respectively (Figure 7D). These
results are different from previous AIEgen-based Ru3+ sen-
sors, which show remarkable quenching behavior but no
observable plateau.[19]
The quenching constants of these probes at low Ru3+ con-
centration are 343 535, 28 5062, and 137 893 m−1, respectively,
determined from the slopes of the linear curves (Figure S32,
Supporting Information). The gradual decline in quenching
constant seems to indicate that the quenching efficiency is
closely related to the PL efficiencies of the nanoaggregates.
Besides, the quenching extent of TPP-m-TPE nanoaggregates is
nearly the same as that of TPP-o-TPE, though the response of
the later is much slower. This demonstrates that the porosity of
AIE isomers may play another role in affecting the quenching
behavior.
Different from PL lifetime study of PA detection, the
lifetime of the AIE nanoparticles decreases obviously when
titrated with Ru3+. The lifetimes of TPP-p-TPE, TPP-m-TPE,
and TPP-o-TPE were recorded as 0.62, 1.80, and 1.39 ns,
respectively, in the presence of 10 × 10−6 m of Ru3+, which
were much lower than their initial values without quenchers
(2.81, 3.79, and 2.29 ns, respectively) (Figure S33, Sup-
porting Information). The changes of lifetime manifests
that a dynamic (collisional) quenching model dominates
the mechanism of Ru3+ detection.[20] Since this mechanism
requires a close contact of the excited fluorophores with
the analytes, the quenching effect is thus related to Dexter
energy transfer because such a short distance may help
electron exchange between the donor and the acceptor. The
mechanism is different from the previous Ru3+ probe with
quenching behavior based on AIEgens because Ru3+ only act
as acceptor here for energy transfer.[19] However, the fluo-
rescence resonance energy transfer (FRET) is almost impos-
sible to happen, though the quenching behavior is seemingly
related to the PL intensities of probes. It is because FRET
is a long-distance interaction between donor and acceptor.
If FRET exists, it will help to quench all the emissions by
long-distance action. Since the plateau occurs, there must
have some AIEgens which are not influenced by Dexter
energy transfer due to distance limit to give the emissions.
The spectral overlap between the absorption of Ru3+ and the
emissions of probes is beneficial for Dexter energy transfer
process. (Figure S34, Supporting Information).
For probes fabricated with TPP-p-TPE, the Ru3+ should be
encapsulated more compactly and homogeneously into the
nanoparticles, which can facilitate the interaction between
probe and analytes. Whereas, in TPP-m-TPE and TPP-o-
TPE-based probes, more Ru3+ are gathering in the cavities
of probes, thus possessing fewer opportunities to participate
in Dexter energy transfer. Nevertheless, because TPP-o-TPE
shows the best porosity, the Ru3+ is very likely to diffuse
along the 3D voids in the probe, which can explain why TPP-
o-TPE shows similar maximum quenching extent with TPP-
m-TPE as probes but reaches the plateau at a higher quencher
concentration.
Besides, the selectivity of Ru3+ detection is also evaluated in
the presence of other metal ions. As shown in Figures S35−37
(Supporting Information), addition of other metal ions, such
as Ag+, Cd2+, Cu2+, Fe2+, Fe3+, Ir3+, Mg2+, Ni2+, Pb2+, Rh3+, and
Zn2+, exerts negligible changes in the PL spectra of TPP-p-TPE,
TPP-m-TPE, and TPP-o-TPE-based probes, indicative of a good
selectivity toward Ru3+, which is independent on the isomerism
effect.
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Figure 7. PL spectra of A) TPP-p-TPE, B) TPP-m-TPE, and C) TPP-o-TPE nanoaggregates formed in THF/water mixtures with fw = 90% in the presence
of Ru3+ with different concentrations. Dye concentration = 10 × 10−6 m; excitation wavelengths = 350, 336, and 332 nm, respectively. D) Plots of I0/I − 1
versus Ru3+ concentration, where I = peak intensity and I0 = peak intensity at [Ru3+] = 0 × 10−6 m.

2.5. Porous Crystals
distance of 2.80 and 3.35 Å are found between C H bonds
and adjacent phenyl rings, which assists in locking the mole-

Studies on the X-ray crystal structures of the AIE isomers
provide further insight into the molecular information in the
aggregate state. Fortunately, three crystals of TPP-p-TPE and
TPP-o-TPE suitable for X-ray analysis were obtained in mixed
solvent of CHCl3/hexane, CH2Cl2/methanol and THF/meth-
anol, respectively (Figure 8A−C). We also tried to grow single
crystals of TPP-m-TPE but only obtained cotton-like powders
even various methods had been adopted, which is possibly due
to its V-shaped conformation that is not favorable for ordered
molecular packing. The crystal structure of TPP-p-TPE shows
an extended conformation as a whole while the phenyl rings at
the periphery of molecules still twist against the central pyra-
zine ring and the double bond. By comparison, the conforma-
tion of TPP-o-TPE is highly folded as the TPE unit is linked to
the TPP unit at ortho-position. Such geometrical structures of
AIE isomers are well in accordance with the results obtained by
the theoretical simulation.
cular conformation. Besides, strong C H…π interaction with

a distance of 2.77 Å and C H…N interaction with a distance
of 2.89 Å exist between molecules, which facilitate their close
packing in the crystal lattice. Both factors induce the formation
of compact aggregate-state structure without obvious cavities

(Figure 8G). In contrast, except the intramolecular C H…N
hydrogen bond formed between the C-H bond of the TPE units
and the N atom of the pyrazine ring, no other interactions are
found in TPP-o-TPE crystals (Figure 8E,F). The bonding dis-
tances (2.63 and 2.68 Å) are shorter than those in TPP-p-TPE,
indicative of strong interactions that help to self-lock the folded
conformation of TPP-o-TPE. Interestingly, the fold conforma-
tion of TPP-o-TPE also provides effective free volume for mole-
cule capture. For example, during the single crystal culture,
the solvent molecules of DCM and THF are easy to enter the
free space and generate complexes with host in a molar ratio
of 1:1. Notably, the DCM molecule captured by TPP-o-TPE
shows a blurred structure, especially for the two chlorine
atoms, which exhibit additional trace beyond the original struc-
ture (Figure S38, Supporting Information). This is because the
The stretching conformation of TPP-p-TPE makes them easy
to pack well in the aggregate state. As shown in Figure 8D,

multiple intramolecular C H…π interactions with short
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(Supporting Information), the nanopores
arrange and connect almost in a straight line,
indicative of the formation of long-range nan-
ochannels. As shown above, the TPP-o-TPE
crystals obtained in THF/methanol mixture
adopt a molecular conformation similar to
that obtained in DCM/methanol mixture.
Does it imply that they possess a similar
ability to capture THF molecules? The
answer is yes. Indeed, the analogous nano-
pores with cylinder shape are generated by
incorporating two THF molecules (Figure 8I).
Because the size of THF molecules is much
larger than DCM, the pore thus has longer r
and h of ≈3.5 and 11 Å and a larger volume
of ≈0.42 nm3. Besides, nanochannels are also
formed along a certain direction (Figure S40,
Supporting Information). All these indicate
that TPP-o-TPE is apt to form porous crystals
by virtue of its folded conformation caused
by the isomerism effect. The intramolecular

C H…N hydrogen bond existed in the crys-
tals helps to fix the solid geometry of TPP-
o-TPE, thus providing a definitive manner
to construct pore structure. However, the
volume of nanopores in the porous crystals
is still tunable in consideration of different
sizes of the guest molecules.
3. Conclusions
In this contribution, we synthesized three
AIE isomers, named TPP-p-TPE, TPP-
m-TPE, and TPP-o-TPE, by connecting TPE
to TPP at the para-, meta-, and ortho-posi-
tions, respectively. The isomerism effect of
AIEgens leads to some obvious difference
Figure 8. Crystal structures of A) TPP-p-TPE and TPP-o-TPE grown from B) CH2Cl2/methanol in the luminescence behavior and solid-state
and C) THF/methanol. D) Multiple intermolecular and intramolecular interactions in crystals
property during conformational changes to
direct to various applications. For example,
the stretching structure of AIEgen enables
it to possess the best conjugation and tight
packing in the aggregation state, making the
molecules to emit efficiently in the film state

of TPP-p-TPE with indicated distances (Å). Intramolecular C H…N interaction of TPP-o-TPE
crystals grown from E) CH2Cl2/methanol and F) THF/methanol with indicated distances (Å)
and their complexes with CH2Cl2 and THF. G) Molecular packing of TPP-p-TPE crystals. Porous
crystals of TPP-o-TPE for capturing H) CH2Cl2 and I) THF inside.
volume of DCM molecules is smaller than the space of cavity.
Thus, the DCM molecules can undergo uncontrollable mole-
cular motions during the single crystal X-ray diffraction analysis.
However, no such behavior was observed in THF-encapsulated
crystal mostly due to the volume matching between the host
and the guest.
The packing information of TPP-o-TPE in both crystals is fur-
ther investigated. In case of DCM-containing crystals, every four
molecules coordinately form a cylinder-like pore with radius
(r) and height (h) of ≈3.1 and 9 Å, respectively, with the aid of
their folded structures (Figure 8H). The pore volume is further
calculated as ≈0.27 nm3 and competent in accommodating
two DCM molecules. Besides, the nanopores are uniformly
distributed but highly oriented. For example, in Figure S39
and possess enhanced interactions with analytes suitable for
OLED and nanoprobe applications. However, slightly bending
the structure of AIEgen will shorten its conjugation length
and induce deep-blue emission. The fabricated device emits a
much pure blue light but at the expense of performances due
to its decreased luminescence efficiency and loose packing in
the aggregate state. The folded structure of AIEgens enables
them to form porous nanoaggregates, which show low emis-
sion efficiency but improved sensitivity as fluorescent probes.
The folded conformation also led them easy to assemble into
porous crystals with similar structure but produce different
pore volume to capture different guest molecules. All these
results indicate that the study of isomerism effect of AIEgens
is instructive for precise control of material properties, which
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Synthesis of 2-(3-bromophenyl)-3,5,6-Triphenylpyrazine (3): Into
a 250 mL round bottom flask was added 15.5 g (53.6 mmol) of 2,
13.7 g (64.4 mmol) of 1,2-diphenylethane-1,2-diamine, and 80 mL of
acetic acid. The mixture was stirred under reflux for 4 h. Afterward, the
mixture was cooled to room temperature and the powder was collected
by filtration. The crude product was further recrystallized in acetic acid.
A pale-yellow powder of 17.5 g (37.8 mmol) was obtained in a yield of
70%. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.95−7.94 (t, 1H), 7.65−7.62
(m, 6H), 7.47−7.45 (m, 1H), 7.42−7.40 (m, 1H), 7.37−7.32 (m, 9H),
7.14−7.10 (t, 1H). 13C NMR (100 MHz, CDCl3), δ (ppm): 148.4, 148.0,
147.9, 146.2, 139.9, 137.6, 137.4, 132.1, 131.0, 129.3, 128.9, 128.3, 128.2,
127.9, 127.8, 127.7, 121.9.
provides useful information for developing functional materials
with high performance.
4. Experimental Section
Materials and Instrumentation: All the commercially available chemicals
were purchased from Sigma-Aldrich or Energy Chemical and used directly
without further purification. 2-(4-Bromophenyl)-3,5,6-triphenylpyrazine
was synthesized according to the previous report.[11d] THF was distilled
from sodium benzopheone ketyl under dry nitrogen immediately prior
1
to use. H and 13C spectra were recorded on a Bruker AVIII 400 MHz
Synthesis of 2-(2-bromophenyl)-1-Phenylethanone (4): The synthetic
procedure was similar to that of 1. A white powder of 9.4 g (34.2 mmol)
NMR spectrometer using CDCl3 or CD2Cl2 as solvent. High-resolution
mass spectra (HRMS) were measured with a GCT premier CAB048 mass
spectrometer operated in MALDI-TOF mode. UV–visible absorption
spectra were performed on a Cary 50 Conc spectrophotometer. PL
spectra were recorded on a HORIBA spectrofluorometer. The absolute
ΦF values were measured with a Hamamatsu Quantaurus-QY C11347
spectrometer. The PL decay curves were recorded using an Edinburgh
FLSP920 fluorescence spectrophotometer equipped with a xenon laser
arc lamp (Xe900), a microsecond flash lamp (uF900), a picosecond
pulsed diode laser (EPL-375), and a closed-cycle cryostate (CS202*I-
DMX-1SS, Advanced Research Systems). TGA was carried out on a
TA TGA Q5000 under nitrogen at a heating rate of 10 °C min−1. DSC
was performed on a TA DSC Q1000 under nitrogen at a heating rate of
10 °C min−1. Cyclic voltammetry was performed at room temperature
in a three-electrode cell using CHI610E electrochemical workstation.
Electrochemical investigations were carried out in anhydrous CH2Cl2
with Pt disk, Pt wire, and saturated calomel electrode as working
electrode, auxiliary electrode, and reference electrode, respectively. 0.1 m
n-tetrabutylammonium hexafluorophosphate and ferrocene were used as
supporting electrolyte and standard, respectively. The electrolyte solution
was purged with nitrogen before measurements and the scanning rates
were 50 mV s−1.
1
was obtained in a yield of 68.3%. H NMR (400 MHz, CDCl3), δ (ppm):
8.07-8.05 (m, 2H), 7.61−7.58 (m, 2H), 7.51−7.48 (m, 2H), 7.31−7.24
(m, 2H), 7.17−7.13 (m, 1H), 4.46 (s, 2H). 13C NMR (100 MHz, CDCl3),
δ (ppm): 195.7, 136.0, 134.4, 132.7, 132.2, 131.1, 128.1, 127.7, 126.9,
124.5, 45.2.
Synthesis of 1-(2-bromophenyl)-2-Phenylethane-1,2-Dione (5): The
synthetic procedure was similar to that of 2. A yellow viscous liquid of
9.1 g (31.5 mmol) was obtained in a yield of 94.6%. 1H NMR (400 MHz,
CDCl3), δ (ppm): 8.10−8.07 (m. 2H), 7.84−7.82 (m, 1H), 7.70−7.63 (m,
2H), 7.57−7.43 (m, 4H). 13C NMR (100 MHz, CDCl3), δ (ppm): 193.6,
190.9, 135.4, 133.9, 133.8, 133.1, 132.1, 131.9, 129.7, 128.3, 127.2, 121.2.
Synthesis of 2-(2-bromophenyl)-3,5,6-Triphenylpyrazine (6): The
synthetic procedure was similar to that of 3. A pale-yellow powder of
1
9.5 g (20.5 mmol) was obtained in a yield of 68%. H NMR (400 MHz,
CDCl3), δ (ppm): 7.66−7.63 (m, 2H), 7.61−7.56 (m, 5H), 7.51−7.48 (m,
1H), 7.36−7.21 (m, 11H). 13C NMR (100 MHz, CDCl3), δ (ppm): 149.6,
149.1, 148.5, 148.4, 139.3, 137.8, 137.7, 137.2, 132.5, 131.5, 129.4, 129.3,
128.9, 128.1, 128.0, 127.7, 127.6, 127.4, 126.9, 122.4.
Synthesis
of
2,3,5-Triphenyl-6-(2-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl) Pyrazine (7): Into a 100 mL round bottom flask
was added 2 g (4.3 mmol) of 6 and 50 mL of dry tetrahydrofuran under
nitrogen. The system was cooled to −78 °C and then 4.3 mL (8.6 mmol)
of n-BuLi solution (2 m) was injected dropwise. After stirring at
−78 °C for 1 h, 2.6 mL (12.9 mmol) of 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane was added. The reaction was warmed to room
temperature and stirred overnight. Then, aqueous solution of NH4Cl
was added to quench the reaction and the mixture was extracted with
dichloromethane. The organic phase was washed with water several
times and condensed under reduced pressure. The crude product
was purified on a silica-gel column using hexane/dichloromethane
mixture (v/v = 2:1) as eluent. A white powder of 1.26 g (2.47 mmol)
was obtained in a yield of 57.4%. 1H NMR (400 MHz, CDCl3), δ (ppm):
7.82−7.80 (d, 1H), 7.69−7.65 (m, 4H), 7.60−7.58 (m, 2H), 7.36−7.32
(m, 4H), 7.27−7.24 (m, 7H), 7.21−7.19 (d, 1H), 0.99 (s, 12H). 13C NMR
(100 MHz, CDCl3), δ (ppm): 150.3, 148.0, 147.9, 144.3, 138.3, 138.1,
137.7, 134.3, 129.5, 129.3, 129.1, 127.8, 127.7, 127.6, 127.5, 127.4,
126.9, 82.9, 24.1.
Synthesis of TPP-p-TPE: Into a 250 mL round bottom flask was added
500 mg (1.08 mmol) of 2-(4-bromophenyl)-3,5,6-triphenylpyrazine,
495 mg (1.3 mmol) of 4,4,5,5-tetramethyl-2-(1,2,2-triphenylvinyl)-
1,3,2-dioxaborolane, 62 mg (0.05 mmol) of Pd(PPh3)4, and 60 mL of
dry tetrahydrofuran under nitrogen. After the mixture was dissolved,
30 mL of 2 m aqueous solution of K2CO3 was injected. The mixture was
stirred at 80 °C overnight. Afterward, the mixture was extracted with
dichloromethane. The organic phase was washed by water several times
and condensed under reduced pressure. The crude product was purified
on a silica-gel column using hexane/dichloromethane mixture (v/v =
10:1) as eluent. A white powder of 440 mg (0.69 mmol) was obtained
in a yield of 63.9%. 1H NMR (400 MHz, CDCl3), δ (ppm): 7.62−7.60
(m, 6H), 7.39−7.37 (d, 2H), 7.33−7.31 (m, 9H), 7.16−7.04 (m, 15H),
7.00−6.98 (d, 2H). 13C NMR (100 MHz, CDCl3), δ (ppm): 148.4, 148.3,
144.3, 143.6, 143.4, 141.2, 140.3, 138.2, 136.3, 131.4, 131.3, 131.2, 129.9,
129.3, 128.6, 128.2, 127.7, 126.7, 126.6. HRMS (MALDI-TOF): m/z
638.2754 ([M]+), calcd for C48H34N2 638.2722).
Synthesis of 2-(3-bromophenyl)-1-Phenylethanone (1): The product was
prepared according to the literature.[21] Into a 500 mL round bottom
flask was added 21.5 g (100 mmol) of 3-bromophenylacetic acid and
14.6 mL (200 mmol) of thionyl chloride. The mixture was stirred under
reflux for 3 h. Afterward, the residual thionyl chloride was removed
by reduced pressure. The crude product was dissolved in 200 mL of
dichloromethane, followed by adding 11.52 mL (130 mmol) of benzene.
Then, 17.4 g (130 mmol) of anhydrous AlCl3 was added in portion at
0 °C and the solution was stirred for 1 h. The mixture was poured onto
ice water and was extracted by dichloromethane. The organic phase
was washed with water several times and condensed under reduced
pressure. The crude product was purified on a silica-gel column using
hexane/ethyl acetate mixture (v/v = 20:1) as eluent. A white powder of
1
21.2 g (77 mmol) was obtained in a yield of 77%. H NMR (400 MHz,
CDCl3), δ (ppm): 7.98−7.96 (m, 2H), 7.56−7.52 (t, 1H), 7.45−7.40
(m, 3H), 7.37−7.34 (m, 1H), 7.16−7.14 (2H), 4.21 (s, 2H). 13C NMR
(100 MHz, CDCl3), δ (ppm): 196.2, 136.2, 135.8, 132.9, 132.0, 129.6,
129.5, 128.2, 128.0, 127.8, 122.0, 44.2.
Synthesis of 1-(3-bromophenyl)-2-Phenylethane-1,2-Dione (2): The
product was prepared according to the literature.[22] Into a 500 mL round
bottom flask was added 16 g (58.2 mmol) of 1, 108 mL of dimethyl
sulfoxide, 54 mL of water and 108 mL of HBr in acetic acid (33 wt%).
The mixture was stirred at 100 °C overnight. Afterward, the mixture was
cooled to room temperature, poured onto ice water and extracted with
the ethyl acetate several times. The organic phase was washed with
water several times and condensed under reduced pressure. The crude
product was purified on a silica-gel column using hexane/ethyl acetate
mixture (v/v = 20:1) as eluent. A yellow solid of 16.7 g (57.8 mmol)
1
was obtained in a yield of 99.3%. H NMR (400 MHz, CDCl3), δ (ppm):
8.15−8.14 (t, 1H), 7.99−7.97 (m, 2H), 7.91−7.88 (m, 1H), 7.81−7.78 (m,
1H), 7.71−7.67 (m, 1H), 7.56−7.52 (t, 2H), 7.42−7.38 (t, 1H). 13C NMR
(100 MHz, CDCl3), δ (ppm): 192.9, 192.2, 137.1, 134.6, 134.1, 132.0,
131.9, 130.0, 129.4, 128.5, 128.0, 122.7.
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Synthesis of TPP-m-TPE: The synthetic procedure was similar to that
of TPP-p-TPE except bromine-substituted derivative was replaced by 3. A
white powder of 510 mg (0.8 mmol) was obtained in a yield of 74.1%.
1H NMR (400 MHz, CDCl3), δ (ppm): 7.60−7.56 (m, 4H), 7.49−7.47 (m,
2H), 7.36−7.29 (m, 11H), 7.18−7.12 (m, 3H), 7.09−6.96 (m, 14H). 13C
NMR (100 MHz, CDCl3), δ (ppm): 148.6, 148.4, 148.2, 144.0, 143.6,
143.5, 143.2, 141.2, 140.6, 138.6, 138.5, 138.3, 132.6, 131.5, 131.3,
131.2, 130.0, 129.9, 128.6, 128.5, 128.2, 128.1, 127.7, 127.6, 126.5,
126.4. HRMS (MALDI-TOF): m/z 638.2710 ([M]+), calcd for C48H34N2
638.2722).
Keywords
aggregation-induced emission, isomerism effect, nanofluorescent
probes, organic light-emitting diodes, porous crystals
Received: May 14, 2019
Revised: June 19, 2019
Published online:
Synthesis of TPP-o-TPE: The synthetic procedure was similar to that of
TPP-p-TPE except bromine and boron ester-substituted derivatives were
replaced by bromotriphenylethylene and 7. A white powder of 80 mg
(0.13 mmol) was obtained in a yield of 19.3%. 1H NMR (400 MHz,
CD2Cl2), δ (ppm): 7.56−7.54 (m, 4H), 7.36−6.93 (m, 23H), 6.84−6.83
(m, 3H), 6.78−6.76 (d, 2H), 6.70 (d, 2H). 13C NMR (100 MHz, CD2Cl2),
δ (ppm): 145.8, 145.7, 145.6, 140.7, 140.2, 135.8, 132.6, 131.0, 130.7,
130.1, 129.8, 128.5, 128.3, 128.0, 127.8, 127.4, 127.0, 126.4, 126.2, 126.0,
125.8. HRMS (MALDI-TOF): m/z 638.2758 ([M]+), calcd for C48H34N2
638.2722).
Fabrication of OLEDs: Multilayer OLEDs were fabricated by the
vacuum-deposition method. Organic layers were deposited by high-
vacuum (5 × 10−4 Pa) thermal evaporation onto a glass substrate pre-
coated with a 90 nm indium tin oxide (ITO) layer, which was thoroughly
cleaned and treated by O2 plasma before conducting experiments. All
organic layers were deposited sequentially. N,N′-di(1-naphthyl)-N,N′-
diphenyl-benzidine (NPB) was used as the hole-transporting layer
(HTL). TPBi was used as the electron-transporting layer (ETL) and
LiF/Al was used as the cathode. The thermal deposition rates for the
organic materials, LiF, and Al were 1.0, 0.1, and 3 Å s−1, respectively.
The active area of each device was 9 mm2. The electroluminescence
spectra, current density–voltage–luminance (J–V–L) characteristics,
and the electroluminescence spectra of the OLEDs were carried out
with a Photo Research SpectraScan PR-745 Spectroradiometer and a
Keithley 2450 Source Meter and they were recorded simultaneously.
All measurements were done at room temperature under ambient
conditions.
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Detection of PA and Ru3+: The detections were carried out by adding
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into the THF solution of AIEgens in volume ratio of 9:1. [CCDC
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Supporting Information
Supporting Information is available from the Wiley Online Library or
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Acknowledgements
This work was partially supported by the National Natural Science
Foundation of China (21788102), the University Grants Committee of
Hong Kong (AoE/P-03/08), the Research Grants Council of Hong Kong
(16305015, A-HKUST 605/16 and C6009-17G), the Innovation and
Technology Commission (ITC-CNERC14SC01 and ITCPD/17-9), and the
Science and Technology Plan of Shenzhen (JCYJ20170818113602462 and
JCYJ20160229205601482).
Conflict of Interest
The authors declare no conflict of interest.
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©
Adv. Funct. Mater. 2019, 1903834
1903834 (12 of 12)
2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim