Investigating the effects of side chain length on the AIE properties of water-soluble TPE derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.01.062

The emissive properties of fluorophores in aggregated state are important for the development of bio-sensors or bio-imaging reagents. So three water-soluble TPE derivatives with different lengths of side chains have been synthesized and we investigated the effects of side chains on aggregation-induced emission (AIE) properties in the aggregated states. The results indicate that side chains on the fluorophores play a pivotal role in their emission in aggregated state mediated by heparin or solid state, because the coplanarity of these TPE derivatives was affected by side chains. The rates of radiative decay k_f and non-radiative decay k_nr have been obtained through the quantum yields and lifetime, and a larger k _f and smaller k_nr were present for compound TPE-C4N, suggesting that the aggregated TPE-C4N should posses the most remarkable fluorescent property.

Full text of DOI:10.1016/j.tetlet.2014.01.062

Tetrahedron Letters 55 (2014) 1496–1500
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Investigating the effects of side chain length on the AIE properties
of water-soluble TPE derivatives
a
b
b
Yifan Dong a, Weilun Wang a, Cewen Zhong a, Jianbing Shi a, , Bin Tong , Xiao Feng , Junge Zhi ,
⇑
Yuping Dong a,
⇑
a School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
b School of Chemistry, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The emissive properties of fluorophores in aggregated state are important for the development of bio-
sensors or bio-imaging reagents. So three water-soluble TPE derivatives with different lengths of side
chains have been synthesized and we investigated the effects of side chains on aggregation-induced
emission (AIE) properties in the aggregated states. The results indicate that side chains on the fluoro-
phores play a pivotal role in their emission in aggregated state mediated by heparin or solid state,
because the coplanarity of these TPE derivatives was affected by side chains. The rates of radiative decay
kf and non-radiative decay knr have been obtained through the quantum yields and lifetime, and a larger
kf and smaller knr were present for compound TPE-C4N, suggesting that the aggregated TPE-C4N should
posses the most remarkable fluorescent property.
Received 17 October 2013
Revised 6 January 2014
Accepted 16 January 2014
Available online 23 January 2014
Keywords:
Tetraphenylethylene (TPE)
Aggregation-induced emission (AIE)
Fluorescence enhancement
Heparin
Ó 2014 Elsevier Ltd. All rights reserved.
Introduction
Among all the novel AIE molecules reported, tetraphenylethylene
(TPE) derivatives, which can be easily prepared and derivatized, ap-
The development of fluorescent probes for the detection of bio-
macromolecules has been a fervid research field, presumably be-
cause of their possibility to be revolutionarily applied in
bioscience and biotechnology engineering.1 Generally, fluorescent
bio-probes possess remarkable advantages such as low background
noises, high sensitivity, ease of operation and broad working
scope.2,3 Among the fluorescent bio-probes reported so far, the most
attractive ones are the ‘turn-on’ or ‘light-up’ sensors due to their
excellent availability and reliability. Particularly, signal produced
by those fluorescent sensors is dramatically enhanced, even under
the circumstance of decreasing fluorescence, and it can be detected
by naked eyes.4–6 Traditional fluorescent biosensors usually suffer
from the aggregation quenching effect (ACQ) present in solution
phase, which adequately limited their applications in various bio-
molecular assays. However, molecules capable of aggregation in-
duced emission (AIE), appear to be non-emissive in solution and
can be induced to produce bright emission as molecular aggregates.
Thus, those compounds have been deemed as promising candidates
for the development of effective bio-probes.7–10
pear to be the most eye-catching fluorophore.16–20 By introducing
appropriate functionalities into the TPE scaffold, a diverse set of
TPE derivatives have been developed and proven to be exceedingly
applicable in areas like metal ion detection,21–24 cell imaging,25,26
and biomacromolecular probing.27–32 As evident in most of the
reported examples, both core structures and substituents could
substantially affect the optical properties of TPE-derived molecules,
which are considered crucial factors for developing effective bio-
sensors. However, reports on the effects of side chains are fairly
rare.33 Herein we report the design and syntheses of three water-
soluble TPE derivatives possessing different lengths of side chains.
The optical properties of AIE molecules have been closely examined
with several aggregation methods, including the one using heparin
as aggregation template. Furthermore, we have measured the quan-
tum yields and fluorescence lifetime of those TPE analogues, which
were subsequently employed to render the radiative rate and non-
radiative rate. Plus, the influence of side chain length on the AIE
properties of three fluorescent compounds has been intensively
investigated, embracing their latent applications as bio-sensors or
bio-imaging reagents.
In 2001, Tang and co-workers first reported a novel AIE system,11
which has attracted much attention since then. Consequently, a
wide variety of analogous molecules have been designed and
synthesized, aiming to produce a robust AIE fluorophore.12–15
Results and discussion
As shown in Scheme 1, several TPE derivatives possessing dif-
ferent alkyl chains have been readily synthesized by a three-step
sequence. First, 4,40-dihydroxy benzophenone (1) was allowed to
⇑
Corresponding authors. Tel./fax: +86 10 6891 7390.
E-mail addresses: bing@bit.edu.cn (J. Shi), chdongyp@bit.edu.cn (Y. Dong).
0040-4039/$ - see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2014.01.062

Y. Dong et al. / Tetrahedron Letters 55 (2014) 1496–1500
1497
Br
N
d Br
N
O
O
O
O
e
c
e
NaOH
b
+
Br
Br
O
a
*
n
CH2Cl2
HO
OH
Br
O
O
Br
n
n
2
3
4
5
6
7
1
n=2
n=4
n=6
n=2
n=4
n=6
Br
N
Br
N
*
O
O
a b
Br
N
Br
N
c
d
TPE-C2N
TPE-C4N
Br
Br
Br
O
O
O
n
n
Br
g'
n
d' Br
N
e'
n
n
O
O
c'
b'
N
f'
Zn/TiCl4
THF
N(CH3)3
Methanol/THF
e'
a'
*
*
Br
N
Br
Br
N
Br
N
Br
O
O
N
O
O
n
n
O
O
n
TPE-Cn
: n=2, 4, 6
TPE-CnN
: n=2, 4, 6
a'
b'
g'
c'
f'
*
*
d'
g''
Br
N
i''
d'' Br
N
e''
O
O
O
Scheme 1. Synthetic routes to compounds TPE-CnN.
c''
h''
f''
e''
b''
a''
O
Br
N
Br
N
react with different dibromides to afford three alkylation products
5–7, which subsequently underwent a McMurry coupling reaction
to generate the desired tetrabromides TPE-Cn (n = 2, 4, 6). Finally,
these tetrabromide intermediates were treated with trimethyla-
mine in THF/methanol to furnish three water-soluble TPE deriva-
tives TPE-C2N, TPE-C4N and TPE-C6N in 74–80% yields. The
chemical structures of TPE-C2, TPE-C4 and TPE-C6 have been con-
firmed by MALDI-TOF-MS, 1H NMR and 13C NMR analyses. Figure 1
showed the 1H NMR spectra of the three TPE derivatives prepared
in our laboratory. Specifically, it appears that the resonance peaks
of methylene protons adjacent to the phenoxy group (Ph–OCH2–)
are closely associated with the length of alkyl chains, and the
chemical shifts for compounds TPE-C2, TPE-C4 and TPE-C6 are
4.24, 3.94 and 3.90 ppm, respectively. Generally, protons that are
weakly shielded should absorb in the lower applied field, and the
chemical shift of a proton adjacent to substituents often depends
h''
b''
a''
d''
c''
g''
+
f''
TPE-C6N
i''
7
6
5
4
3
2
Chemical shifts (ppm)
Figure 2. 1H NMR spectra of TPE-CnN in CD3OD (solvent peaks are marked with
asterisks).
of fact, the study of water-soluble TPE derivatives has attracted
much of our attention, simply because of their fascinating potential
in the development of bio-sensors or bio-imaging reagents.
Figure 3 showed the normalized UV–vis absorption spectra of
compounds TPE-C2N, TPE-C4N and TPE-C6N in aqueous solution.
Specifically, similar spectral patterns are evident for each com-
pound, and two typical absorption peaks have been exhibited at
ꢀ250 and 310 nm, corresponding to the
p–
p⁄ transition of phenyl
on the effect of
p electrons, especially in conjugated systems. Con-
rings and TPE segments, respectively. Surprisingly, the absorption
bands of both phenyl and TPE moieties in compounds TPE-C4N
and TPE-C6N have been shifted to the red, when compared with
compound TPE-C2N. Those results indicated that the coplanarity
and/or conjugation degree of the former two compounds were
higher, which was consistent with the previous 1H NMR analyses.
When the solutions of TPE-C2N and TPE-C4N were excited at
330 nm, almost no photoluminescence (PL) signals were evident
in their emission spectra, as shown in Figure 4a and b. However,
when compound TPE-C6N was excited at the same wavelength, a
very weak emission was observed (Fig. 4c). As a matter of fact,
the discrepancy exhibited in the AIE associated properties of TPE
compounds, presumably due to the possession of different side
chains, are of particular interest for us and others. Therefore, we
next investigated the PL behaviours of TPE derivative aggregates,
which have been readily prepared by adding THF into the aqueous
solution, because those TPE compounds are not suitable solutes for
THF. In an effort to study the relationship between fluorescent
sequently, the side-chain protons of TPE derivatives absorb on the
right side of NMR spectra, because a higher applied field for the
resonance is produced by the coplanar TPE scaffolds. This effect
is more distinctive for the cationic TPE species, as illustrated in Fig-
ure 2. The chemical shifts of methylene protons adjacent to phen-
oxy groups (Ph–OCH2–) are 4.41, 3.99 and 3.91 ppm for
compounds TPE-C2N, TPE-C4N and TPE-C6N, respectively.
Presumably, the positive charge of compound TPE-C2N could give
rise to a repulsive force, which certainly would lead to a highly
twisted conformation, eventually affording a less planar structure.
In addition, the structural distortion should be more severe for TPE
analogues possessing shorter alky chains. Those results clearly
indicated that the length and electronic nature of side chains, adja-
cent to the TPE core, could significantly influence the overall
coplanarity, and subsequently alter optical properties. As a matter
d
d
c
O
O
a
b
Br
Br
c
b
a
1.0
TPE-C2N
Br
Br
c'
O
O
*
*
*
TPE-C4N
TPE-C2
TPE-C4
TPE-C6
TPE-C6N
d'
f'
d'
0.8
O
O
Br
Br
Br
b'
a'
c'
e'
b'
a'
e'
f'
0.6
0.4
0.2
0.0
Br
O
O
O
f''
d''
d''
h''
O
Br
Br
g''
c''
e''
b''
b''
a''
c''
g''
a''
O
h''
+
e''
f''
Br
Br
O
7
6
5
4
3
2
250
300
350
400
450
500
Chemical shifts (ppm)
Wavelength (nm)
Figure 1. 1H NMR spectra of TPE-Cn in CDCl3 (solvent peaks are marked with
asterisks).
Figure 3. Normalized UV–vis absorption spectra of TPE-CnN in aqueous solution.

1498
Y. Dong et al. / Tetrahedron Letters 55 (2014) 1496–1500
enhancement was obtained when 99% of the solvent was THF. In
120
90
60
30
0
(a)
1500
1000
500
contrast, the fluorescent emission of compounds TPE-C2N and
TPE-C6N in the mixed solvent has not been enhanced until the per-
centage of THF reached 99%, and the magnitudes were 128-folds
and 41-folds for TPE-C2N and TPE-C6N, respectively. These results
demonstrated that the AIE properties of TPE derivatives could be
readily modulated by altering their side chain lengths.
0
20
40
60
80
100
THF Percentage (vol%)
Certain factors, such as aggregation-caused quenching of light
emission, could disturbingly interfere with the AIE behaviour of a
fluorophore, thus heparin is employed as an aggregation template
to minimize those effects.34 Figure 5 showed the PL spectra of
TPE-C2N, TPE-C4N and TPE-C6N in water when heparin was grad-
ually added. The concentration of heparin can be calculated using
sugar dimer as the repeating unit, which theoretically contains
four negative charges. In addition, every TPE derivative possesses
exactly four positive charges. As shown in Figure 5a–c, a linear
relationship between the emission of three TPE derivatives and
heparin titration has been observed. However, the saturation point
for each compound is distinctively different, even though all three
of the TPE derivatives contain the same amount of positive
charges. On the other hand, the binding constant can be obtained
by the correlation between the PL increase (I/I0) and the concentra-
tion of heparin, as demonstrated by the well-known Stern–Volmer
equation.35 Specifically, the saturation point for compound
TPE-C2N was 1.7 equiv of heparin, and the corresponding PL signal
has been increased to 62 folds (Fig. 5d). Consequently, it was found
that the binding constant of TPE-C2N and heparin should be
468 nm
×50
0
400
450
500
550
600
Wavelength (nm)
180
150
120
90
(b)
2700
60
30
0
1800
900
0
0
20
40
60
80
100
THF Percentage (vol%)
469 nm
3.76 ꢁ 106 Mꢂ1
. At the same time, the saturation points for
TPE-C4N and TPE-C6N were 1.4 and 1.0 equivalent of heparin,
with a 134- and 37-folds of signal increase; therefore, their binding
constant has been determined to be 1.00 ꢁ 107 Mꢂ1 and
3.63 ꢁ 106 Mꢂ1, which appear to be substantially different. Theo-
retically, a molecule possessing shorter side chain should be less
flexible, thus it may be more difficult to adjust the conformation
to maximize its interaction with heparin. Consequently, some neg-
ative charges of the heparin molecules may not be paired and re-
mained accessible in the solution. Thus, more heparin is often
needed to achieve maximal emission for the TPE derivatives with
shorter side chains. Presumably, the emission enhancement of
TPE derivatives should be originated from the restriction of intra-
molecular rotations, because they could electrostatically interact
with heparin molecules, effectively blocking the non-radiative
decay channel. Based on those results, we concluded that the most
significant intensity enhancement was observed in the spectra of
compound TPE-C4N, and the best sensitivity for heparin was
obtained in plain aqueous solution. In contrast, compounds
TPE-C2N and TPE-C6N have afforded inferior signal enhancement.
Generally, only TPE derivatives with suitable length of side chains
could pair well with heparin molecules to afford a strong electro-
static interaction, which consequently should restrict intramolecu-
lar motions and lead to enhanced signals. As a matter of fact,
compound TPE-C4N appears to possess the most suitable side
chain and its intramolecular movement has been severely
hampered, affording a favourable conformation for fluorescence
emission. Hence, the signal enhancement of compound TPE-C4N
turned out to be more visible. Moreover, we also investigated the
pH effect on the fluorescent responses of compounds TPE-CnN,
as shown in Figure 6. To our delight, these TPE derivatives have
exhibited remarkable independence on pH, and no obvious PL
change was evident within the pH range explored (pH 3–11).
In order to understand the intrinsic mechanism involved in the
better emission enhancement of TPE-C4N aggregates, we have sys-
tematically conducted a number of experiments and deliberately
×
50
400
450
500
550
600
Wavelength (nm)
40
30
20
10
0
1200
900
600
300
0
(c)
0
20
40
60
80
100
THF Percentage (vol%)
470 nm
10
×
400
450
500
550
600
Wavelength (nm)
Figure 4. PL spectra of (a) TPE-C2N, (b) TPE-C4N and (c) TPE-C6N in water-THF
mixtures; Insert: Correlation between the PL increase (I/I0) of TPE-C2N, TPE-C4N
and TPE-C6N and THF percentage. Compounds concentration is 10ꢂ5 M; excitation
wavelength is 330 nm.
emission and aggregation, the percentage of THF has been gradually
increased from 0 to 100 percent, as shown in the inset of Figure 4.
Interestingly, the fluorescence intensity of compounds TPE-C2N,
TPE-C4N and TPE-C6N did not exhibit significant change when
the percentage of THF was relatively low. However,
when the percentage of THF reached 80 percent, a 74-fold intensity
enhancement was observed in the spectra of compound TPE-C4N.
Apparently, increasing the percentage of THF can substantially im-
pact the PL spectra after certain stage. As expected, the fluorescence
collected data generated. Theoretically, quantum yield (
fluorescence lifetime ( ) are the most important parameters for a
fluorophore, and the emission efficiency can be quantitatively
U) and
s
intensity of TPE-C4N maintained mounting and
a 186-folds

Y. Dong et al. / Tetrahedron Letters 55 (2014) 1496–1500
1499
(b)
(a)
1500
600
450
300
150
0
1200
900
600
300
0
400
450
500
550
600
650
400
(c)
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
1200
900
600
300
0
150
120
90
60
30
0
TPE-C2N
TPE-C4N
TPE-C6N
(d)
0.0
0.5
1.0
1.5
2.0
400
450
500
550
600
650
[Heparin] / [TPE-CnN]
Wavelength (nm)
Figure 5. PL spectra of (a) TPE-C2N, (b) TPE-C4N and (c) TPE-C6N in water with heparin; (d) correlation between the PL increase (I/I0) of TPE-CnN (n = 2, 4, 6) and the ratio of
[heparin] to [TPE-CnN]. [TPE-CnN] = 1.0 ꢁ 10ꢂ5 M; excitation wavelength is 330 nm.
Table 1
TPEC2N
TPEC4N
TPEC6N
1.4
1.2
1.0
0.8
0.6
The quantum yields and lifetime of compounds TPE-C2N, TPE-C4N and TPE-C6N in
different states
Samples
U
(%)
s
(ns)
kf/ꢁ108 sꢂ1
—
knr/ꢁ108 sꢂ1
—
TPE-C2N in solutiona
TPE-C2N in aggregationb
TPE-C2N in filmc
0.21d
8.52d
7.86e
0.22d
29.70d
10.62e
0.47d
9.74d
4.99e
—
3.428
3.834
—
4.935
4.120
—
4.269
3.662
f
f
f
0.2480
0.2050
2.6692
2.4032
TPE-C4N in solutiona
TPE-C4N in aggregationb
TPE-C4N in filmc
—
—
f
f
f
0.6018
0.2578
1.4245
2.1694
TPE-C6N in solutiona
TPE-C6N in aggregationb
TPE-C6N in filmc
—
—
f
f
f
0.2272
0.1363
2.1152
2.5945
a
b
c
[TPE-CnN] = 1.0 ꢁ 10ꢂ5 M in water.
Aggregated by interacting with heparin in aqueous solution.
Spin-coated from 5.0 mg/mL solution in methanol.
2
4
6
8
10
12
d
Quantum yield is estimated using quinine sulfate (
U = 55% in 0.1 M H2SO4) as
pH
the standard.
e
Quantum yield is the absolute value obtained from an integrating sphere.
‘—’ means no data available.
Figure 6. Fluorescent responses (I/I0) of TPE-CnN (n = 2, 4, 6) at different pH values.
[TPE-CnN] = 1.0 ꢁ 10ꢂ5 M; excitation wavelength is 330 nm; I0 is the PL intensity at
pH = 7.0.
f
when aggregated by heparin. Interestingly, compound TPE-C4N
exhibited the highest quantum yield and the largest signal increase
(135 folds), which is consistent with the results obtained from the
PL titration. On the other hand, the fluorescence lifetime for all
three compounds in aqueous solution was less than 0.05 ns, be-
yond the detection limit of our instrument. To our delight, the life-
time of TPE-C2N, TPE-C4N and TPE-C6N aggregates appears to be
3.428, 4.935 and 4.269 ns, respectively, as demonstrated in Table 1.
evaluated accordingly. As a matter of fact, the quantum yields of
compounds TPE-C2N, TPE-C4N and TPE-C6N in both solution
and aggregate states have been determined, using quinine sulfate
in 0.1 M H2SO4 (U = 0.55) as a standard. Table 1 summarizes the
quantum yields of the three fluorescent compounds in different
states. Specifically, their quantum yields in aqueous solutions were
all below 0.5%, but they unanimously exhibited a sharp increase

1500
Y. Dong et al. / Tetrahedron Letters 55 (2014) 1496–1500
Notably, compound TPE-C4N exhibited the longest lifetime
whereas compounds TPE-C2N and TPE-C6N showed relatively
shorter lifetime. Generally, the quantum yield of a fluorophore is
defined as the ratio of emitted photons to absorbed photons. In
addition, it has been proved that the emissive rate of a fluorophore
(kf) and the rate of non-radiative decay to S0 (knr) both can depop-
ulate the excited state. For the fractions of a fluorophore decaying
through emission, the quantum yield can be provided by Eq. 1.
National Natural Scientific Foundation of China (Nos. 21004004
and 51061160500), the Specialized Research Fund for the Doctoral
Program of Higher Education of China (Nos. 20101101120029 and
20091101110031), and the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry
(No. 20120932001).
Supplementary data
U
¼ kf =ðkf þ knrÞ
ð1Þ
Supplementary data (experimental details) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2014.01.062.
For convenience, all possible non-radiative decay processes
have been grouped with a single rate constant knr. Thus, the life-
time of an excited state can be expressed in Eq. 2, which is defined
as the average time that molecules can stay in the excited state be-
fore they return to the ground state.
References and notes
1. Goncalves, M. S. T. Chem. Rev. 2009, 109, 190–212.
s
¼ 1=ðkf þ knrÞ
Based on the experimental quantum yields (
ð2Þ
) and lifetime ( ),
2. Silva, A. P. D.; Guaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C.
P.; Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515–1566.
3. Patrick Ambrose, W.; Goodwin, Peter M.; Jett, James H.; Van Orden, Alan;
Werner, James H.; Keller, R. A. Chem. Rev. 1999, 99, 2929–2956.
4. Han, T. Y.; Dong, Y. F.; Li, Z. Y.; Tong, B.; Shi, J. B.; Zhi, J. G.; Dong, Y. P. Acta Chim.
Sinica 2012, 70, 1187–1192.
5. Hong, Y. N.; Haussler, M.; Lam, J. W. Y.; Li, Z.; Sin, K. K.; Dong, Y. Q.; Tong, H.;
Liu, J. Z.; Qin, A. J.; Renneberg, R.; Tang, B. Z. Chem. Eur. J. 2008, 14, 6428–6437.
6. Shi, H. B.; Liu, J. Z.; Geng, J. L.; Tang, B. Z.; Liu, B. J. Am. Chem. Soc. 2012, 134,
9569–9572.
U
s
we have calculated rate constants kf and knr according to Eqs. 1 and
2, as shown in Table 1. Specifically, a larger kf and a smaller knr were
obtained for compound TPE-C4N when it was aggregated with the
assistance of heparin, clearly suggesting that fluorescence emission
was favoured and stronger fluorescence intensity should be ob-
served. Furthermore, a solid state can also be considered as a special
type of aggregation state, even though it is distinctively different
from the ones aggregated with heparin. Thus, the TPE derivatives
have been spin-coated and placed on quartz plates, which were
subsequently measured to afford absolute PL quantum yields, using
an integrating sphere. In addition, the lifetime of these films has
also been determined with a fluorescence lifetime spectrometer.
Three parallel experiments have been conducted to obtain quan-
tum yields and the corresponding fluorescence lifetime, and the re-
sults were averaged to minimize possible errors. These data are
summarized in Table 1 as well. Specifically, the quantum yields of
compounds TPE-C2N, TPE-C4N and TPE-C6N in film state were
7.86%, 10.62% and 4.99%, respectively. On the other hand, their life-
time appeared to be 3.834, 4.120 and 3.662 ns. Accordingly, kf and
knr have been obtained using Eqs. 1 and 2, as demonstrated in Ta-
ble 1. A higher kf and a smaller knr have been observed in compound
TPE-C4N, which should be beneficial for emission enhancement.
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In conclusion, three water-soluble TPE derivatives possessing
different lengths of side chains have been synthesized in our labo-
ratory, and their optical properties in different states were also
investigated. It appears that side chains on the fluorophores play
a pivotal role on their emission, especially when they were exam-
ined in solution, aggregated state mediated by heparin, or solid
state. Based on the quantum yields and lifetime, which were ob-
tained from the experiments employing compounds in different
states, the rates of radiative decay kf and non-radiative decay knr
have been determined, respectively. It appears that a larger kf and
smaller knr were present for compound TPE-C4N, whether it has
been aggregated by heparin or examined in solid state, suggesting
that the aggregated TPE-C4N should posses the most remarkable
fluorescent property. Consequently, this study not only provides a
fundamental principle for the design of water-soluble AIE com-
pounds, but also endows an excellent TPE aggregrate, which could
find broad applications in the field of biosensors or bioimaging.
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This work was financially supported by the National Basic
Research Program of China (973 Program; 2013CB834704), the