Locking the phenyl rings of tetraphenylethene step by step: Understanding the mechanism of aggregation-induced emission

Article abstract of DOI:10.1039/c2cc35641d

Stepwise locking of phenyl rings of tetraphenylethene increases the emission efficiency of luminogen solutions gradually, thus verifying the restriction of intramolecular rotation (RIR) mechanism of the aggregation induced emission phenomenon. The emissio

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Locking the phenyl rings of tetraphenylethene step by step:
Understanding the mechanism of aggregation-induced emission
a
a
a
b
c
a
a
Junqing Shi, Ning Chang, Cuihong Li, , Ju Mei, Chunmei Deng, Xiaoliang Luo, Zhengping Liu,
a
a
b,d
Zhishan Bo, Yong Qiang Dong,* and Ben Zhong Tang*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Stepwise locking of phenyl rings of tetraphenylethene
increases the emission efficiency of luminogen solutions
gradually, thus verifying the restriction of intramolecular
rotation (RIR) mechanism of aggregation induced emission
phenomenon. The emission of the luminogen with one “O”
bridge could be tuned reversibly in solid state through
repeated heating and grinding.
NC
CN
1
0
Si
1
2
3
NC
CN
Many efforts have been devoted to the development of efficient
solid-state luminogens due to their practical application.
Si
1
1
2
2
3
3
4
4
5
0
5
0
5
0
5
However, emissions of many panel-shaped luminogens are
weakened or even quenched in the aggregates due to the
1
a
2a
3a
5
0
Chart 1 Examples of AIE luminophors and their cousins with restricted
formation of detrimental species such as excimers and
2
phenyl rings
exciplexes. To mitigate the aggregation-caused quenching (ACQ)
effect, various approaches have been taken to hamper the
chromophore aggregation, but only limited successes have been
achieved because luminophoric molecules tend to form
while the detailed relation between the number of restricted
phenyl rings and photophysical properties of AIE molecules has
seldom been reported, which may further disclose the RIR
mechanism of AIE effect and may also help to explore the
elaborate application of AIE luminogens. We also wonder
whether it is possible to obtain a luminogen which exhibit similar
PL spectrum and fluorescence quantum yield (Φ ) in both
solution and crystalline states through locking the phenyl rings of
AIE molecule but keeping its twisted conformation.
Here we locked phenyl rings of a popular AIE luminogen,
tetraphenylethene (TPE), with “O” bridge step by step, and Φ of
molecules increased with the number of locked phenyl rings. The
emission spectrum and Φ of 2OTPE solution fitted well with
F
those of its crystal due to the fully locked phenyl rings and
twisted conformation. The emission of 1OTPE can be tuned
reversibly between yellow-greenish and blue in solid state
through grinding-heating process.
The three compounds were prepared conveniently by
McMurry coupling and the detailed synthetic route and
characterization are described in the Supporting Information.
3
aggregates in concentrated solutions or solid state.
5
6
6
7
5
0
5
0
Instead of working against the natural process of chromophore
aggregation, we have recently discovered that a group of
propeller-shaped luminogens which are nonemissive in dilute
solutions emit efficiently upon aggregation. This phenomenon is
coined as aggregation-induced emission (AIE), which is exactly
F
4
opposite to the ACQ effect.
The emission intensities of AIE luminogens increased upon
restriction of intramolecular rotation (RIR) through increasing the
viscosity of solvent, lowering the temperature, or applying high
F
4
b
pressure and so on. Thus RIR in the aggregates is identified as a
5
main cause for the AIE effect. Many new AIE luminogens have
been developed based on the RIR mechanism, and applied as
functional materials, especially as chemosensors, bioprobes and
4
b
solid-state emitters.
In addition to those external control experiments, we prepared
model compounds and exercised an internal control on their
emissions. When the steric hindering groups were introduced to
the phenyl rings at 3 and 4 positions of hexaphenylsilole (1,
6
HPS), the emission of the new luminogen (1a) was turned on due
to the greatly enhanced steric hindrance. Locking of some phenyl
rings in the AIE molecules 2 and 3 afforded two highly emissive
7
luminogens (2a, 3a) in solution, thus further verified the RIR
mechanism of AIE phenomenon. While both 2a and 3a exhibited
ACQ behaviour due to the panel part in their molecules.
Many AIE luminogens have more than four phenyl rings,
Chart 2 Structures of the TPE and its derivatives with “O” bridge
[Chem. Commun., [year], [vol], 00–00 | 1
This journal is © The Royal Society of Chemistry [year]

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Page 2 of 16
TPE
A
TPE
B
TPE
1OTPE
2OTPE
Table 1. The excited state of TPE in solution deactivated too fast
to be measured with our equipment (the limit is 0.1 ns) which
coincides with some reported TPE derivatives.
1
2
OTPE
OTPE
1
OTPE
OTPE
4
5
The introduction of the "O" bridge significantly alters the
View Online
2
dynamics of the singlet excited states of TPE derivatives. The
excited species of 1OTPE and 2OTPE in solution relaxes much
more slowly with PL lifetime lengthened to 3.76 and 5.14 ns
respectively. The lifetime increases promptly with the number of
locked phenyl rings in molecules from TPE via 1OTPE to 2OTPE,
solutioncrystal380
450
520
590390
Wavelength (nm)
460
530
600
5
0
Fig. 1 (left panel) Photos of THF solutions and crystals of luminogens. (A,
B) PL spectra of (A) THF solutions and (B) crystals of luminogens.
Photos are taken under UV illumination. Excitation wavelength: 350 nm.
Concentration: 10 μM.
F
which coincided well with the change of PL intensity and Φ ,
further verifying the RIR mechanism of AIE phenomenon.
5
The crystals of TPE are highly emissive with the Φ increase
F
TPE is nearly nonemissive when molecularly dissolved in 55 from zero (in solution) to 24.6% and the average lifetime are
solution. However, the solution of 1OTPE emits deep blue light,
and solution of 2OTPE emits strong sky blue light (Fig. 1). The
PL intensity of the luminogens increased promptly with increase
of the number of locked phenyl rings. The ΦF of the TPE is
lengthened to 3.9 ns, which is due to the block of nonradiative
pathway through restriction of rotation of phenyl rings in crystals.
The Φ of 1OTPE also increased from 4.6 to 8.0% when forming
F
1
1
2
2
3
3
4
0
5
0
5
0
5
0
crystals. Thus TPE exhibits AIE activity and 1OTPE shows
nearly zero and cannot be detected by the integrated sphere. 60 aggragation induced emission enhancement (AIEE) activity.
While the Φ
F
of 1OTPE and 2OTPE are 4.6% and 30.1%
However, ΦF of 2OTPE (30.1 to 30.9%) remained nearly
unchanged when transforming from solution to crystals,
exhibiting no AIE activity. The rotation of phenyl rings in
2OTPE is already locked by two “O” bridges, thus when forming
respectively, indicating that the ΦF increases greatly with the
number of locked phenyl rings in the TPE molecules, fitting well
with their photos and PL spectra.
When TPE molecules are dissolved as isolated species in 65 crystals, the phenyl rings of 2OTPE cannot be restricted more
solution less restriction is imposed on their intramolecular
rotation. The intramolecular rotation of phenyl rings in TPE
molecules may induce the efficient nonradiative annihilation
process, thus TPE solution is nearly nonemissive. While if the
rotation of phenyl rings in TPE molecule is restricted, the
emission of the molecule may be turned on due to blocking of the
nonradiative annihilation process. Two phenyl rings of 1OTPE
are locked by one “O” bridge, thus the nonradiative process
caused by intramolecular rotation of phenyl rings may be blocked.
tightly by intermolecule interaction, hence exhibiting no AIE
activity. It is worth noting that PL spectrum and Φ of 2OTPE
F
solution fit well with those of 2OTPE crystals, which is seldom
reported (Fig. S3, ESI†).
70
Some AIE luminogens transform to ACQ molecules when the
phenyl rings are locked due to the close interaction of the panel
7
parts of the molecules (Chart 1, 2a, 3a). However, 1OTPE and
2OTPE does not exhibit such ACQ activity. Closer inspection of
molecular conformations and packing patterns of 1OTPE and
And the increase of Φ
validated our hypothesis. As we expected, the Φ
phenyl rings of which are locked with two “O” bridges, boosts to
0.1% due to further blocking of the nonradiative way caused by
intramolecular rotation. Then we can see that the ΦF can be
F
from 0% (TPE) to 4.6% (1OTPE) 75 2OTPE unveiled that molecules of both 1OTPE and 2OTPE take
F
of 2OTPE, all
twisted conformation, thus avoid close packing which may
quench the excited states (Fig. S4, ESI†).
Several AIE active luminogens have been reported to exhibit
piezochromic luminescence. Grinding normally amorphizes
3
gradually increased by restriction of intramolecule rotation step 80 organic crystals. Crystals and amorphous solid of some AIE
by step, hence verifying that RIR in the aggregates is a main
cause for the AIE effect.
The emission colors of luminogens depend on the chemical
structures and conformation of their molecules. The energy band
gaps of TPE, 1OTPE and 2OTPE are 4.116, 4.165, 3.736 eV, 85 TPE derivatives could be modulated by mechanical stimulus.
respectively, nicely correlating with their different emission
colors and PL spectra in solutions. (Fig. S2, ESI†)
In addition to the steady-state spectra measurements, time-
resolved PL spectra can provide information on the excited states
active luminogens exhibit different emissions due to different
conformations of molecules in various aggregates. Therefore
emissions of those AIE active luminogens will change upon
grinding. Thus we checked whether the emissions of the three
8
Emission of 1OTPE crystals changed from bright blue to
yellow-greenish (Φ = 6.0 %) upon grinding in a mortar (Fig. 2).
F
And the emission reverted to blue (Φ = 7.8 %) upon heating.
F
Thus the emission of 1OTPE could be tuned reversibly between
of fluorogens. We measured the time-resolved PL spectra of the 90 blue and yellow-greenish through repeating heating and grinding.
three luminogens and dynamic parameters were summarized in
Table 1 Optical Properties of TPE, 1OTPE and 2OTPE
a
b
λ
em [nm](Φ
F
[%])
fluorescence decay of solution
fluorescence decay of crystal
(ns) (ns)
sample
TPE
c
d
e
Solution
nd (nd)
401 ( 4.6)
466 (30.1)
cyrstal
A
1
/A
2
τ
1
(ns)
τ
2
(ns)
A
1
/A
2
τ
1
τ
2
<τ> (ns)
3.988
4.596
7.57
453 (24.6)
458 ( 8.0)
467 (30.8)
nd
1/0
1/0
nd
3.76
5.14
nd
/
/
48/52
24/76
100/ 0
1.7
1.1
7.57
6.1
5.7
/
1
2
OTPE
OTPE
a
b
c
d
λ
em = emission maximum, Φ = fluorescence quantum yield determined using a calibrated integrating sphere, Measured in THF, Fraction (A, %)
F
e
1 1
and lifetime (τ, ns) of shorter (1) or longer lived species (2); The weighed mean lifetime <τ>was calculated according to equation <τ> = (A τ +
A
2
τ
2
)/( A
1
+ A
2
),
2
|
Chem. Commun., [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 16
ChemComm
such material through combination of highly rigid and twisted
annealed ground
conformation.
5
0
This work was partially supported by the National Science
Foundation of China (51173018), the 973 Programs
View Online
(
2011CB935702 and 2009CB623605), and the Fundamental
Research Funds for the Central Universities. The work was also
partially supported by RPC Grants of HKUST (RPC11SC09), the
Research Grants Council of Hong Kong (HKUST2/CRF/10).
5
5
Notes and references
3
90
480
570
660
a
Department of Chemistry, Beijing Key Laboratory of Energy Conversion
Wavelength (nm)
and Storage Materials Beijing Normal University, Beijing, 100875,
China.; Tel: 86-10-5880-6896; E-mail: dongyq@bnu.edu.cn
Fig. 2 (left panel) Normalized PL spectra and (right panel) photos
of 1OTPE in three repeating cycles of (red lines) grinding and
(blue lines) annealing (170 °C, 25 min).
b
6
6
7
7
8
0
5
0
5
0
Institute of Biomedical Macromolecules, MOE Key Laboratory of
5
Macromolecular Synthesis and Functionalization, Department of Polymer
Science and Engineering, Zhejiang University, Hangzhou 310027, China.
To unveil the mechanism of the mechanochromic emission of
OTPE, powder X-ray diffraction (XRD) (Fig. S5, ESI†) and
DSC measurements were carried out on the pristine blue crystals,
the ground and annealed powder of 1OTPE.
c
Institute of Chemistry, Chinese Academy of Sciences, Zhongguancun
1
North First Street 2,100190 Beijing, China
d
Department of Chemistry, The Hong Kong University of Science &
Technology (HKUST), Clear Water Bay, Kowloon, Hong Kong, China.
† Electronic Supplementary Information (ESI) available: [Experimental
procedures, crystal structure of 1OTPE and 2OTPE (CCDC 893966 and
1
1
2
2
3
3
0
5
0
5
0
5
The diffraction curves of both the original blue crystals and the
annealed samples display many sharp and intense reflection peaks
that coincide with each other, indicating their same crystalline
orders and that ground solid can revert to the original crystalline
state by heating. Diffraction of the ground powder also exhibits
some reflections that agree with those of the original crystal and
annealed ground sample but does not have as many or as sharp
peaks, hinting of a not absolutely amorphous phase (Fig. S5,
ESI†), which may be caused by the spontaneous recover during
grinding.
The DSC thermogram of ground 1OTPE shows a broad
exothermic peak (at around 86 °C, Fig. S6, ESI†) before melting,
which is absent in the DSC curves of both crystalline and
annealed sample, indicating that the ground sample is in a
metastable amorphous state and can crystallize promptly in the
solid state upon heating. After crystallization, both the ground
and annealed sample melt at a similar temperature to that of the
pristine sample, suggesting that the ground sample can revert
back to the original crystalline state. Thus the mechanochromic
fluorescence of 1OTPE is ascribed to the transformation from a
crystalline to an amorphous phase upon grinding.
However, the emissions of ground solids of TPE and 2OTPE
are same as their crystals, that is, both TPE and 2OTPE exhibit no
response to grinding. TPE crystallizes very quickly, and we
cannot obtain amorphous solid of TPE through grinding or
quenching of its melt, thus TPE is irresponsive to mechanical
stimulus. While the all phenyl rings of 2OTPE are locked and the
conformation of 2OTPE molecules keep unchanged in different
aggregate states, thus 2OTPE does not exhibit mechanochromic
fluorescence too.
8
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luminogens, DSC thermograms and powder XRD patterns of 1OTPE in
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4
0
Conclusions
In summary, emission intensity and Φ
increased with increasing of number of locked phenyl rings in
TPE molecules, thus the RIR mechanism of AIE phenomenon
was further verified. Emission of 1OTPE can be tuned reversibly
of luminogens are 105
F
(
d)X. Q. Zhang, Z. G. Chi, J. Y. Zhang, H. Y. Li, B. J. Xu, X. F. Li, S.
W. Liu, Y. Zhang and J. R. Xu, J. Phys. Chem. B, 2011, 115, 7606;
e)X. Gu, J. Yao, G. Zhang, Y. Yan, C. Zhang, Q. Peng, Q. Liao, Y.
(
4
5
between blue and yellow-greenish through repeating grinding and 110 Wu, Z. Xu, Y. Zhao, H. Fu and D. Zhang, Adv. Funct. Mater., doi:,
annealing. 2OTPE exhibit nearly same emission spectra and Φ in
F
10.1002/adfm.201201482.
both solution and crystal state, hinting of a design strategy for
This journal is © The Royal Society of Chemistry [year]
Chem. Commun., [year], [vol], 00–00 | 3