Aggregation-induced emission
Yongqiang Donga,b, Jacky Wing Yip Lama, Anjun Qina, Zhen Lia, Jiaxin Sunc, Hoi Sing Kwokc, Ben
Zhong Tang*,a,b,c
a
Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay,
Kowloon, Hong Kong, China
b
Department of Polymer Science & Engineering, Zhejiang University, Hangzhou, Zhejiang 310027,
China
c
Center for Display Research, The Hong Kong University of Science & Technology, Clear Water
Bay, Kowloon, Hong Kong, China
ABSTRACT
New chromophoric molecules of 1,1-di(thiophen-2-yl)-2,3,4,5-tetraphenyl-silole (T2TPS), 9-(diphenylmethylene)-9Hfluorene (DPMF), and tetraphenyletheneare (TPE) are designed and synthesized. When molecularly dissolved in
common organic solvents, the molecules are practically nonemissive. Addition of poor solvents induces the molecules to
aggregate, which turns the emission “on” and boosts their luminescence efficiencies dramatically (“aggregation-induced
emission” or AIE). The photoluminescence (PL) of T2TPS and TPE layers adsorbed on the TLC plates can be turned
“off” and “on” continuously and reversibly by solvent exposure and evaporation. Transformation from amorphous phase
to crystalline structure blue-shifts the PL spectrum of T2TPS and enhances its intensity. A light-emitting devices (LEDs)
device based on TPE is fabricated, which emits a blue light of 447 nm with a low turn-on voltage of 2.9 V.
Keywords: aggregation-induced emission, silole, tetraphenylethylene, fulvene, light-emitting diode, photoluminescence,
electroluminescence, chromism, sensor
1. INTRODUCTION
Design and synthesis of organic luminescent molecules have attracted considerable attention due to their potential
applications in light-emitting diodes (LEDs) full-color flat-panel displays.1,2 However, chromophoric aggregation often
quenches the light emission, which constitutes a formidable obstacle to the development of efficient LEDs because
aggregation is inherently involved in the film-forming processes of luminophoric molecules.3-5 This quenching effect is
believed to be the result of aggregate or excimer formation, which leads to a reduction in the luminescence efficiency.5,6
Many groups have tried to prevent aggregate formation in the solid states by separating the dye chromophores through
the introduction of sterically hindered structures such as dendron substituents.7-9 In fact, it would be more interesting to
explore novel organic molecules or polymers, which not only do not suffer fluorescence quenching in the aggregated
state, but display enhanced light emission in the solid state or in thin films. We have recently observed an exact this
phenomenon, aggregation-induced emission (AIE) in a group of silole molecules. For example, 1-methyl-1,2,3,4,5pentaphenylsilole (PMS) is nonemissive in solutions but is induced to emit efficiently when it aggregates in poor
solvents or in solid films.10-17 A LED using PMS as emitting layer exhibits excellent EL performance, with maximum
current (CE) and power efficiencies (PE) of 20 cd/A and 14 lm/W, respectively.18 Afterwards, sterically biphenyl
substituted ethylenes19 and diaminodicyanoquinodimethanes20 have also been found to show stronger light emission in
solid states than their dilute solutions.
In this work, we describe the synthesis of three new AIE molecules, namely 1,1-di(thiophen-2-yl)-2,3,4,5-tetraphenylsilole (T2TPS), 9-(diphenylmethylene)-9H-fluorene (DPMF), and tetraphenylethylene (TPE) and present their novel
optical properties.
*
Corresponding author (Department of Chemistry of HKUST). Email: tangbenz@ust.hk. Home page: http://home.ust.hk/~tangbenz.
Tel. +852-2358-7375. Fax. +852-2358-1594.
Organic Light Emitting Materials and Devices X, edited by Zakya H. Kafafi, Franky So,
Proc. of SPIE Vol. 6333, 63331D, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.679373
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2. EXPERIMENTAL
2.1 Instrumentations
1
H and 13C NMR spectra were measured on a Varian-300 spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as the
internal standard. Mass spectra were recorded on a Finnigan TSQ 7000 triple quadrupole spectrometer operating in a
chemical ionization (CI) mode using methane as the carrier gas. UV-vis spectra were measured on a Milton Roy
Spectronic 3000 Array spectrophotometer. X-ray diffraction intensity data were collected at 100 K on a Bruker-Nonius
Smart Apex CCD diffractometer with graphite-monochromated Mo Kα radiation. Single crystals of T2TPS were grown
from tetrahydrofuran/ethanol mixture. Processing of the intensity data was carried out using the SAINT and SADABS
routines and the structure solution and refinement were carried out by the SHELXTL suite of X-ray programs (Version
6.10). PL spectra were recorded on Perkin-Elmer LS 55 spectrofluorometer. The particle sizes of T2TPSthe
nanoaggregates in the water/acetone mixtures were measured on a Beckman Coulter Delsa 440SX Zeta potential
analyzer.
2.2 Synthesis
T2TPS, DPMF and TPE were prepared by the synthetic routes shown in Scheme 1. The synthetic procedures are shown
below:
Syntheisis of T2TPS: Lithium shaving (140 mg, 20 mmol) was added into a solution of 3.56 g diphenyacetylene (20
mmol) in THF (20 mL) under dry nitrogen. The mixture was stirred for 12 h at room temperature and the resultant blue
colored solution was added dropwise to a solution of tetrachlorosilane (2.29 mL, 20 mmol) in 80 mL THF. The reaction
mixture was stirred for 2 h at room temperature and then refluxed for 5 h. Hence a slution of Cl2TPS was obtained. Into
another flask were added 2.24 mL (28 mmol) thiophene and 80 mL THF. The mixture was cooled to –40 °C with
acetonitrile/dry ice, into which 10 mL n-BuLi (2.5 M in hexane) was added. After stirring for 1.5 h, the mixture was
transferred dropwise at –40 °C to the Cl2TPS solution prepared before. The reaction mixture was warmed to room
temperature and then stirred overnight. The crude product was obtained by extracting the reaction mixture with diethyl
ether three times, followed by washing with 1 M HCl three times and brine one time. The product was purified by a
silica gel column using hexane/chloroform mixture (2:1 by volume) as eluent. T2TPS was obtained in ~30% yield. Mp:
212−213 °C. 1H NMR (300 MHz, CDCl3): δ (TMS, ppm): 7.68 (d, 2H), 7.47 (d, 2H), 7.20 (m, 2H), 7.06−6.80 (m, br,
20H). 13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 156.9, 139.3, 139.0, 133.7, 130.5, 130.2, 130.0, 129.1, 128.4, 128.1,
127.1, 126.5. UV (THF, 4.0 × 10-5 M), λmax (nm): 370. MS (CI): calcd for M+, 550.1; found 550.0.
Syntheisis of DPMF: To a 250 mL two-necked flask were added 3.33 g (20 mmol) of fluorene and 100 mL THF under
nitrogen. After the solution was cooled down to –78 °C, n-BuLi in hexane (16 mL, 40 mmol) was injected into the flask
and the mixture was stirred at –78 °C for 2 h. Benzophenone (3.64 g, 20 mmol) dissolved in 30 mL THF was then added
into the flask. The mixture was slowly warmed to room temperature. After stirring for 12 h, the mixture was
concentrated by a rotary evaporator. The crude product was purified by a silica-gel column using hexane as eluent.
DPMF was obtained in 43% yield. Mp: 228−229 °C. 1H NMR (300 MHz, CDCl3): δ (TMS, ppm): 7.68 (d, 2H), 7.37 (s,
10H), 7.21 (m, 2H), 6.90 (m, 2H), 6.21 (d, 2H). 13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 145.6, 143.1, 140.7, 138.9,
134.4, 129.9, 129.0, 128.4, 127.8, 126.6, 125.1, 119.5.
TPE was synthesized according to Scheme 1.21 The molecule was purified by silica gel column chromatography using
hexane as eluent and isolated in 70% yield.
2.3 Fabrication of EL Devices
Electroluminescence (EL) device using TPE as emitting layer was fabricated in the usual manner with sequential
vacuum evaporation of various layers on 30Ω/cm2 indium-tin oxide (ITO)-coated glass substrate. The structure of the
device is shown in Chart 1. The ITO glass was pretreated in ultrasonic assisted detergent followed by rinsing with
deionized water before being dried in oven at 100 °C. After 25 min of UV-ozone treatment the substrate was transferred
into vacuum chamber with a base pressure 2 × 10–4 Pa for device preparation. N,N’-bis(1-naphthyl)-N,N’diphenylbenzidine (NPB), TPE, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), LiF, and Al were deposited
sequentially.
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Scheme 1
SiCl4
Li
THF, rt
Si
THF
Li Li
Cl
Cl
Cl2 TPS
n-BuLi
S
THF
S
Li
Cl2 TPS
THF
Si
S
S
T2 TPS
1. n-BuLi/THF
2. Ph2 CO
DPMF
TiCl3 /AlCl3/Zn, THF
reflux, 24 h
O
TPE
Chart 1
Al (1200 Å)
LiF (10 Å)
BCP (200 Å)
TPE (300 Å)
NPB (500 Å)
ITO Glass
λ max = 447 nm
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3. RESULTS AND DISCUSSION
3.1 Aggregation-induced emission of T2TPS
Figure 1 shows the photoluminescence (PL) spectra of T2TPS. Almost no PL signals are detected when its dilute acetone
solution is excited at 376 nm, suggesting that it is a weak emitter when it is molecularly dissolved in a good solvent.
Upon addition of large amounts of water (70%) to its acetone solution, while keeping the final concentration unchanged,
the PL intensity, astonishingly, increases dramatically under identical measurement conditions. Water is a nonsolvent for
T2TPS and the T2TPS molecules will certainly aggregate in such high water fraction. The resultant suspensions are,
however, macroscopically homogenous with no visible precipitates, suggesting that the silole aggregates are of
nanodimension.
H2O content (%)
PL intensity (au)
90
80
70
0
x 50
400
490
580
670
Wavelength (nm)
Figure 1. Photoluminescence spectra of T2TPS in water/acetone mixtures. Concentration of T2TPS: 10 µM; excitation
wavelength: 376 nm.
When the water content further increases to 90%, the PL becomes weaker but the peak maximum shifts from 470 to 505
nm, which has not been observed in its silole congeners.10-14 This phenomenon may be caused by the kinetics of their
molecular packing arrangement. Solvent mixtures with water contents less than 70% lead to a slow formation of
nanoaggregates, allowing sufficient time for the silole molecules to pack in a proper way (thermodynamically
controlled). The packing may resemble to its crystal form as suggested by their pure blue emission similar to those of
crystals at ~470 nm. As the water content is higher than 70%, the T2TPS molecules will cluster together quickly and
form aggregates in an uncontrolled manner (kinetically controlled). This assumption is supported by the fact that the
emission spectrum of T2TPS solution with 80% water content blue shifts after standing at room temperature for 12 h
(Figure 2.).
iLJLTAL
Figure 2. Photos of T2TPS taken under UV illumination in water/acetone mixtures where water fractions (%) are labeled on
the vials. Concentration: 10 µM. The two samples on the rightest side are T2TPS in solutions with 80% and 90% water
fractions that stand at room temperature for 12 h.
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To further confirm our speculation, we studied the PL spectra of T2TPS solution with 70 and 90% water at different time
intervals (Figure 3). At 70% water content, the PL is peaked at 505 nm. However, the emission progressively shifts from
green (505 nm) to blue (470 nm) with time, meanwhile, the PL intensity becomes stronger. On the other hand, the color
and intensity of the mixture with 90% water fraction hardly change with time. (Figure 4B). These results verify our
former assumption. When the water content is less than 70%, not all the T2TPS molecules are precipitated and form
nanoparticles. The molecularly dispersed T2TPS molecules may thus have enough time to deposit on the formerly
formed nanoparticles in a proper way similar to recrystalization. This supported by the fact that the average particles size
also increases from 160 to 330 nm in three hours. However, for a mixture with 90% water fraction, nearly all the T2TPS
molecules are precipitated and aggregated in a random way. Therefore, the particle size keeps at 110 nm even the
mixture is standing for 12 h.
210
40
Time (min)
B
32
70%
140
λ−λ0 (nm)
PL intensity (au)
16
14
12
10
8
6
4
2
0
24
16
70
8
0
90%
0
0
400
480
560
I/I0−1 (%)
A
16
640
0
5
10
15
20
25
Time (min)
Wavelength (nm)
Figure 3. (A) Photoluminescence spectra of T2TPS in water /acetone mixtures (70:30 by volume) at different time interval
and (B) change in the emission position and intensity of T2TPS solutions with 70 and 90% water fractions with time.
Concentration: 10 µ M; excitation wavelength: 376 nm.
3.2 Vapochromism of T2TPS
B
chloroform
acetone
Figure 4. Photos of the T2TPS spots on the TLC plates in the petri-dish sets in the (A and D) absence and (B and E)
presence of vapors of organic solvents. Photos in C and F are taken after the solvents are evaporated. All the photos
are obtained under UV illumination.
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Recently we have found that hexaphenylsilole (HPS) show interesting vapochromism: its emission is quenched by
solvent vapors.22 Thus, it is of interest to check whether T2TPS is also vapor sensitive. We dropped T2TPS solutions onto
TLC plates and put them in the petri dishes saturated with different solvents. To avoid direct contact of the plates with
the solvents, we put some plastic “spacers” in between the samples and the solvents. As shown in Figure 4, the emission
from the silole spot is turned “off” when they are exposed to acetone or chloroform vapors. The emission, however,
becomes visible and switched on again when the solvent is removed (column C). This off/on process is fully reversible
and can be repeated for many times because the involved process is a nondestructive physical cycle of dissolution–
aggregation. This fluorescence switching behavior makes the T2TPS a promising candidate for fluorescence devices
sensing volatile organic compounds.
In addition to visual observations, we studied the solvent effect on the silole emission spectroscopically. We coated a
thin film of T2TPS on the inner wall of a quartz cell and added several drops of acetone in a small container placed at the
bottom of the cell. The change in the PL of the T2TPS film was then investigated. Unlike the situation on the TLC plate,
the PL of the T2TPS film on the quartz cell becomes stronger progressively with time (Figure 5). Meanwhile, the
emission color progressively shifts gradually from green (492 nm) to blue (470 nm).
PL intensity (au)
5.5
Time (min)
5.5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
400
450
500
550
600
650
Wavelength (nm)
Figure 5. Effect of acetone vapor on the photoluminescence spectrum of a T2TPS film coated on a quartz cell at different
time intervals. Excitation wavelength: 370 nm.
HPS film have been found to undergo crystallization upon solvent fumigation, leading to stronger emission at lower
wavelength region. As a result, we checked the morphology of T2TPS films before and after solvent exposure. As shown
in Figure 6, the untreated silole film is amorphous. Silole crystals are, however, formed after fumigated by the solvent
vapor. Similar results are also obtained from the TEM analysis. The ED pattern of untreated film shows only a diffuse
halo (Figure 6C). On the contrary, diffraction spots are clearly seen in the fumed film, suggestive of its crystalline
nature. These results also confirm our speculation that T2TPS in water/acetone mixture (70:30 by volume) may form
crystal nanoparticles, which blue-shifts PL spectrum accompanied with higher intensity.
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Figure 6. (A and B) SEM and (C and D) TEM images of T2TPS films (A and C) before and (B and D) after exposure to
acetone vapor for 10 h. The insets in panels C and D are the ED patterns.
3.3 Aggregation-induced emission of DPMF
Similar to T2TPS, upon addition of large amounts of poor solvent (water) into the solutions of DPMF, the emission of
the dye is boosted dramatically. Unlike T2TPS, the emission peak shifts little even at high water content and remains at
460 nm.
H2O content (%)
PL intensity (au)
90
80
70
60
0
380
440
500
560
620
Wavelength (nm)
Figure 7. Photoluminescence spectra of DPMF in acetonitrile/water mixtures. Concentration: 10 µM; excitation
wavelength: 330 nm. Photos of DPMF taken under UV illumination in water/acetone mixtures with the water fractions
(%) labeled on the vials are shown on the right panel.
3.4 Aggregation-induced emission of TPE
TPE emits almost no light when molecularly dissolved in acetonitrile. When a nonsolvent of water is added, the TPE
molecules are clustered together and intense PL is observed. The PL intensity increases progressively when the water
fraction becomes higher. Using 9,10-diphenylanthracene in cyclohexane [quantum efficiency ΦF = 90%] as reference,
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the ΦF of TPE at 99% water fraction is determined to be 12.5%, which is 52-fold higher than that in pure acetonitrile.
Clearly, similar to T2TPS and DPMF, TPE is AIE-active. This is further supported by the photos in the right panel of
Figure 8. While the acetonitrile solution is nonemissive, intense blue PL is observed in acetonitrile with 99% water
fraction. TPE also exhibits vapochromism similar to T2TPS.
PL intensity (au)
H2O content (%)
99
90
80
70
60
0
370
430
490
550
610
Wavelength (nm)
Figure 8. (Left) Photoluminescence spectra of TPE in acetonitrile and acetonitrile/water mixtures. Concentration: 10 µM;
excitation wavelength: 330 nm. (Right) Photos of TPE solutions taken under UV illumination in acetonitrile (0% H2O)
and acetonitrile/water mixture (99% H2O).
3.5 Electroluminescence of TPE
The novel AIE behavior of TPE promotes us to check its EL performance. We fabricated an LED with a configuration of
ITO/NPB (50 nm)/TPE (30 nm)/BCP (20 nm)/LiF (1 nm)/Al (120 nm). As shown in Figure 5A, the EL spectrum is
nearly identical to the PL spectrum peaked at 447 nm, indicating that the emission is truly from TPE and the excimer
emission are unlikely involved in the EL process. The turn-on voltage is 2.9 V and the luminance reaches 877 cd/m2 at 5
V. We believe further optimization of the device configuration will lead to better results.
4800
1500
B
current density
Intensity (au)
PL
2
2
Current density ( A/m )
EL
3200
1000
luminance
1600
500
0
380
450
520
Wavelength (nm)
590
Luminance ( cd/m )
A
0
0
2
4
6
Voltage (V)
Figure 9. (A) Photoluminescence and electroluminescence spectra of TPE films and (B) current density and luminance
against voltage curve of the light-emitting diode of TPE
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Till now, we have developed several types of AIE-active molecules. Most of these molecules are nonplanar due to steric
hindrance between the peripheral phenyl rings. For example, the interplane molecular distance of TPE is larger than 6 Å
(Figure 10), suggestive of the lack of strong molecular interactions that tends to induce nonradiative recombination or
red-shifts as seen in the “normal” crystals with strong π-π interactions. However, we are still uncertain why the
crystalline form emits PL with shorter wavelength or bluer color.
Figure 10. Packing diagrams of TPE crystals, where the interplane distance is 9.172 Å and the intermolecular distance
within the unit cell is 6.088 Å.
4. CONCLUSION
In summary, new AIE-active molecules, T2TPS, DPMF, TPE are synthesized. The molecules are almost nonluminescent
when molecularly dispersed in solution but become highly emissive when aggregated into nanoclusters or fabricated into
thin films. The emission of T2TPS and TPE on the TLC plates can be turned “off” and “on” by solvent exposure and
evaporation. Transformation from an amorphous phase to a crystalline structure blue-shifts the PL spectrum of T2TPS
and boosts its emission intensity. An LED device based on TPE is constructed, which emits a blue light of 447 nm with a
low turn-on voltage of 2.9 V.
ACKNOWLEDGEMENTS
This work was partly supported by the Research Grants Council of Hong Kong (602706, HKU2/05C, 603505, 603304
and 664903), the 973 program of the Ministry of Science and Technology (2002CB613401), and the National Science
Foundation of China (N_HKUST606_03). B.Z.T acknowledges the support of Cao Guangbiao Foundation of Zhejiang
University.
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