New carbazole-substituted siloles for the fabrication of efficient non-doped OLEDs

Article abstract of DOI:10.1016/j.cclet.2018.12.020

Luminogenic molecules with aggregation-induced emission (AIE) property are free of aggregation-caused quenching and thus have great potential in the fabrication of efficient non-doped OLEDs. Herein, a series of new carbazole-substituted siloles have been

Full text of DOI:10.1016/j.cclet.2018.12.020

Accepted Manuscript
Title: New carbazole-substituted siloles for the fabrication of
efficient non-doped OLEDs
Authors: Yi Xiong, Jiajie Zeng, Bin Chen, Jacky W.Y. Lam,
Zujin Zhao, Shuming Chen, Ben Zhong Tang
PII:
DOI:
Reference:
S1001-8417(18)30488-1
https://doi.org/10.1016/j.cclet.2018.12.020
CCLET 4757
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
12 November 2018
2 December 2018
18 December 2018
Please cite this article as: Xiong Y, Zeng J, Chen B, Lam JWY, Zhao Z, Chen S, Tang
BZ, New carbazole-substituted siloles for the fabrication of efficient non-doped OLEDs,
Chinese Chemical Letters (2018), https://doi.org/10.1016/j.cclet.2018.12.020
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Chinese Chemical Letters
journal homepage: www.elsevier.com
Communication
New carbazole-substituted siloles for the fabrication of efficient non-doped OLEDs
a
a
a
c
a
b

Yi Xiong , Jiajie Zeng , Bin Chen , Jacky W.Y. Lam , Zujin Zhao , Shuming Chen* , Ben Zhong
Tang*ac
a State Key Laboratory of Luminescent Materials and Devices, Center for Aggregation-Induced Emission, South China University of Technology, Guangzhou
510640, China
b Department of Electrical and Electronic Engineering, South University of Science and Technology of China, Shenzhen 518055, China
c Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration and Reconstruction, The Hong Kong
University of Science & Technology, Hong Kong, China
Corresponding authors.
E-mail addresses: mszjzhao@scut.edu.cn (Z. Zhao), chen.sm@sustc.edu.cn (S. Chen), tangbenz@ust.hk (B.Z. Tang)
Graphical abstract
New aggregation-induced emission molecules of carbazole-substituted siloles are prepared, based on which efficient non-doped
OLEDs are fabricated, offering high external quantum efficiencies of up to 5.63%.
———

ARTICLE INFO
ABSTRACT
Article history:
Luminogenic molecules with aggregation-induced emission (AIE) property are free of
aggregation-caused quenching and thus have great potential in the fabrication of efficient non-
doped OLEDs. Herein, a series of new carbazole-substituted siloles have been synthesized and
characterized. Their crystal and electronic structures, thermal stabilities, electrochemical
behaviors, and photophysical properties are thoroughly investigated. These silole derivatives
exhibit prominent AIE characteristics with high emission efficiencies in solid films. They can
function as light-emitting layers in non-doped OLEDs, affording eminent electroluminescence
efficiencies of 17.59 cd/A, 12.55 lm/W and 5.63%, amongst the most efficient non-doped
OLEDs based on fluorescent emitters, indicating their promising applications in OLEDs.
Received 12 November 2018
Received in revised form 3 December 2018
Accepted 6 December 2018
Available online
Keywords:
Aggregation-induced emission
Silole
Photoluminescence
Electroluminescence
Organic light-emitting diode
In recent decades, tremendous progresses have been achieved in the development of optoelectronic devices [1]. One of the most
promising optoelectronic devices is organic light-emitting diode (OLED), whose response time and contrast ratio can be even better
than liquid crystal display [2]. The key active materials for OLEDs are the organic luminescent materials, one important kind of organic
semiconductors. The studies on the preparation and application for them have got considerable attentions around the world [3]. Since
organic luminescent materials are applied as solid films in OLEDs, their solid-state emission efficiencies are of significant importance
to the performances of OLEDs. However, there is still a thorny issue of aggregation-caused quenching (ACQ) of conventional
chromophores [4], that is the emissions of these chromophores are weakened or totally quenched in the aggregated state, even if they
can emit strongly in the solution state. In general, a vast majority of the conventional organic luminescent materials have a large and
extended conjugated plane, which makes them prone to experience serious intermolecular interactions, and thus quenches the emissions
in the aggregated state. Although multifarious chemical and engineering methods have been utilized for surmounting the ACQ problem,
the attempts meet with only a limited success [5]. Thus, the exploration of organic luminescent materials with strong emissions in the
aggregated state for the application in OLEDs remains as an important research topic.
Aggregation-induced emission (AIE), a unique phenomenon opposite to ACQ, has evoked intense academic and industrial interest.
The luminogens with AIE property (AIEgens) are almost non-fluorescent in dilute solutions but can emit strongly in aggregates or solid
films, furnishing an effective approach to solve the ACQ problem [6]. In order to figure out the inner connections between the
molecular structures and properties of AIEgens, several of meaningful experiments and calculations have been conducted. And
exploiting more practical applications and deciphering the working mechanism for AIEgens are also of high importance. Currently, the
mostly accepted theory about the AIE mechanism is restriction of intramolecular motions (RIM), including rotation, vibration, twisting,
stretching, etc. [7]. The active intramolecular motions in solutions can nonradiatively deactivate the excited state, but in the aggregated
state, these motions are greatly suppressed by physical constraint as well as weak intermolecular interactions, such as C‒H···π hydron
bonds. In consequence, the nonradiative decay channel is blocked, and thus the excited state energy can dissipate via radiative transition,
rendering greatly enhanced emissions [8].
Siloles are created with high electron affinity and fast electron mobility because of their unique σ*‒π* conjugated electronic
structures, while the exceptional propeller-like molecular structures can bring about distinguished photophysical property of AIE [9].
Consequently, they are widely adopted as effective building units to construct various functional materials for optoelectronic devices
with excellent performances [10,11]. In this work, we have prepared three new silole derivatives by combining 1-methyl-1,3,4-
triphenyl-1H-silole and electron-donating 9-phenyl-9H-carbazole via different manners (Fig. 1). These silole derivatives possess
distinct AIE characteristic and can fluoresce strongly in solid films. The non-doped OLEDs with excellent electroluminescence (EL)
efficiencies have been fabricated based on these silole derivatives.
The target compounds 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP were synthesized according to the synthetic routes
illustrated in Scheme S1 in Supporting inforamtion. These new silole derivatives were yellow solids, and for them, THF,
dichloromethane and toluene are good solvents, but water and ethanol are poor solvents. For the study and application in OLEDs, their
thermal stability was evaluated. Thermogravimetric analysis (TGA) was carried out under nitrogen atmosphere at a heating rate of 10
o
oC/min, and high decomposition temperatures (Td), corresponding to 5% initial weight loss, of 433.8, 452.4 and 440.8 C for 2,2'-
MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP, respectively, were recorded (Fig. S1A in Supporting information), indicating they are
thermally stable enough for film preparation by vacuum deposition. In addition, high glass-transition temperatures (Tg) of 133.4, 147.1
o
and 136.2 C were also detected (Fig. S1B), revealing these compounds are morphological stable, which are conducive to the stability
of the devices.
Crystal structure can provide useful information about the spatial conformation and stacking mode of the compounds in the
aggregated state. Single crystals of 3,3'-MTPS-CaP were grown from CH2Cl2-C2H5OH mixture, and analyzed by X-ray diffraction
crystallography. According to the crystal structure depicted in Fig. 2, 3,3'-MTPS-CaP shows a highly twisted conformation, and the

dihedral angles between phenyls and the silole core at the 1,3,4-positions are 67.99°, 82.55° and 80.01°, respectively. In addition, the
two substituents at 2,5-positions are unsymmetrical, where the dihedral angles between carbazoles and silole core at these two positions
are 47.83° and 9.39°, respectively. Further analysis for packing pattern of 3,3'-MTPS-CaP indicates that there are multiple
intramolecular and intermolecular C–H···π hydrogen bonds within the range of 2.648–3.039 Å (Fig. S2 in Supporting informaiton).
These weak interactions will rigidify molecular structure and restrict the intramolecular motions in the solid state. However, no π–π
stacking interactions in crystalline state of 3,3'-MTPS-CaP are found. A highly twisted conformation is helpful to prevent strong
intermolecular π‒π interaction, and suppress the emission quenching in the condensed phase.
The absorption maxima of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP in dilute THF solution (10–5 mol/L) are located at
398, 404 and 385 nm, owing to the π–π* electron transition (Fig. 3A). The optical band gaps of 2,2'-MTPS-CaP and 3,3'-MTPS-CaP are
relatively narrower than that of 9,9'-MTPS-CaP, estimated from the onset of absorption spectra, which indicate that 2,2'-MTPS-CaP and
3,3'-MTPS-CaP have better effective conjugation than 9,9'-MTPS-CaP. The photoluminescence (PL) emissions of 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP in dilute THF solutions are very weak, with PL peaks at 525, 526 and 520 nm, and low ΦFs of
5.75%, 4.00% and 3.20%, respectively. After fabricated into solid films, they present slightly red-shifted and greatly enhanced PL peaks
within the range of 520–539 nm (Fig. 3B). Their ΦFs are significantly increased to 69.93%, 60.33% and 80.00% (Table 1), indicating
their AIE characteristics. To further assess the PL properties of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP, the fluorescence
lifetimes (τ), important parameters used to describe the excited-state decay processes, were measured and fitted. The intersystem
crossing (ISC) process had been ignored in the excited-state decay process of silole derivatives because of their fluorescent nature. The
ΦF and τ can be determined by radiative decay rate (kr) and nonradiative decay rate (knr) [12]. Thus, the decay rates of 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP had been calculated, including in solution and solid states (Table S1 in Supporting information).
For these silole derivatives, once fabricated into solid films, there are great increases in their ΦF and τ, but small increases in kr.
However, the knr is decreased significantly. These results manifest that the RIM is triggered in the aggregated state, which blocks the
nonradiative decay channel, and thus boosts the PL emissions.
In order to gain an in-depth insight into their AIE property of these silole-based luminogens, their PL spectra in THF/water
mixtures were measured and the results are shown in Fig. 4. Since they are insoluble in water, their PL intensity was increased along
with the increase of water fraction (fw) in the mixture. Strong PL emissions were observed in the aggregated state with a high water
fraction (fw = 90%), further validating the AIE property of these silole derivatives.
To investigate the electronic structures of these new silole derivatives, their highest occupied molecular orbitals (HOMOs) and
lowest unoccupied molecular orbitals (LUMOs) were calculated by density function theory (DFT) calculation using a B3LYP/6-31G (d,
p) basis set on the Gaussian 09 program. The optimized structures and spatial distributions of HOMOs and LUMOs for 2,2'-MTPS-CaP,
3,3'-MTPS-CaP and 9,9'-MTPS-CaP are illustrated in Fig. 5. The HOMOs are located on the entire molecular backbones consisting of
central silole ring and carbazole substituents. Their LUMOs, however, are mainly concentrated on the silole ring. The exocyclic single
bonds at the 1,1-positions also contribute to the LUMOs, indicative of the unique σ*‒π* conjugation.
In order to get the experimental HOMO and LUMO energy levels of these novel silole-based luminogens, the electrochemical
properties were investigated by cyclic voltammetry (CV) in dichloromethane solution containing 0.1 mol/L tetra-n-butylammonium
hexafluorophosphate at a scan rate of 100 mV/s. The working electrode was platinum and the reference electrode was Ag/AgNO3
electrode. They showed a good electrochemical stability with reversible oxidation processes (Fig. 6). The values of Eox
and Ere
onset
onset
were represented by the onset oxidation and reduction potentials relative to Fc/Fc+. The onset potentials of oxidation (Eoxonset) of 2,2'-
MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP occurred at 0.68, 0.51 and 0.82 V, respectively. Thus, the HOMO energy levels can
be determined as –5.20, –5.04 and –5.34 eV, respectively, according to the equation [HOMO = – (Eox
+ 4.8) eV]. While the onset
onset
potentials of reduction (Ereonset) of these silole derivatives occurred at –1.92, –2.21 and –2.09 V, respectively, and their LUMO energy
levels were calculated to be –2.70, –2.49 and –2.62 eV, respectively, from the equation [LUMO = – (Ere
+ 4.8) eV]. The variation
onset
tendency of energy band gaps between HOMOs and LUMOs are consistent with optical band gaps (Eg) (Table 1).
Given the high solid-state emission efficiency and favorable thermal stability of these new silole derivatives, their potentials as
light-emitting layers in non-doped OLEDs were evaluated. The non-doped OLEDs with a configuration of ITO/NPB (60 nm)/emitter
(20 nm)/TPBi (40 nm)/LiF (1 nm)/Al were fabricated, in which new silole derivatives worked as light-emitting layers, NPB (N,N'-di(1-
naphthyl)-N,N'-diphenyl-benzidine) functioned as the hole-transporting layer, and TPBi (1,3,5-tri(1-phenylbenzimidazol-2-yl)-benzene)
served as the electron-transporting layer. The EL performance data for the non-doped OLEDs based on these silole derivatives are
shown in Fig. 7 and Table 2. These non-doped OLEDs could be turned on at voltages of 2.9–5.3 V, and radiated green lights. The
maxima EL peaks of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP are located at 555 (CIEx,y = 0.244, 0.435), 552 (CIEx,y
=
0.399, 0.559) and 542 nm (CIEx,y = 0.376, 0.549), respectively, which are red-shifted by 16–22 nm in comparison with the PL emissions
in films. Actually, this is a common phenomenon for the luminescent materials because of the microcavity effect [13]. The devices of
2,2'-MTPS-CaP and 3,3'-MTPS-CaP showed comparable EL performances, with maxima luminance (ηL,max) of 83870 and 78780 cd/m2,
maxima current efficiencies (ηC,max) of are 12.72 and 12.44 cd/A, maxima power efficiencies (ηP,max) of 9.87 and 10.64 lm/W and
maxima external quantum efficiencies (ηext,max) of 4.01 and 3.57%, respectively. The device of 9,9’-MTPS-CaP exhibited the best EL
performance, affording high ηL,max, ηC,max, ηP,max and ηext,max of 91920 cd/m2, 17.59 cd/A, 12.55 lm/W, and 5.63%, respectively. The
excellent ηext,max is actually approaching the theoretical efficiency limit of fluorescent OLEDs.

In summary, a series of new silole derivatives functionalized by carbazole groups had been synthesized and characterized
successfully. All of these new silole derivatives showed good thermal and electrochemical stabilities. They exhibited prominent AIE
properties with high ΦFs in the solid state. Different connection pattern between silole and carbazole groups resulted in slightly varied
optical properties. Efficient non-doped OLEDs based on these silole derivatives were fabricated. The device of 9,9'-MTPS-CaP
afforded the best EL performance, with excellent ηL,max, ηC,max, ηP,max and ηext,max of 91920 cd/m2, 17.59 cd/A, 12.55 lm/W, and 5.63%,
respectively, indicating the good potential for the application in OLEDs.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China (Nos. 21788102 and 21673082), the
National Basic Research Program of China (973 Program, No. 2015CB655004) founded by MOST, the Guangdong Natural Science
Funds for Distinguished Young Scholar (No. 2014A030306035), the Science and Technology Program of Guangzhou (No.
201804020027), International Science and Technology Cooperation Program of Guangzhou (No. 201704030069) and the Innovation
and Technology Commission of Hong Kong (No. ITC-CNERC14SC01).

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Fig. 1. Molecular structures of these new silole derivatives.

Fig. 2 ORTEP drawing of the crystal structure of 3,3'-MTPS-CaP.

2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
B
A
2,2'-MTPS-CaP, Eg = 2.62 eV
3,3'-MTPS-CaP, Eg = 2.66 eV
9,9'-MTPS-CaP, Eg = 2.74 eV
450
500
550
600
650
700
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 3. (A) Absorption spectra in THF solutions, (B) photoluminescence (PL) spectra in solid films of these new silole derivatives. Eg = optical
band gap.

f
w
(vol%)
90
80
70
60
50
40
30
20
2, 2'-MTPS-CaP
3, 3'-MTPS-CaP
9, 9'-MTPS-CaP
f
w
(vol%)
90
80
70
60
50
40
30
20
A
f
w
(vol%)
D
C
B
20
15
10
5
90
80
70
60
50
40
30
20
10
0
10
0
10
0
0
90
0
0
20
40
60
80
100
400 450 500 550 600 650 700
Wavelength (nm)
400 450 500 550 600 650 700
Wavelength (nm)
400 450 500 550 600 650 700
Wavelength (nm)
fw (vol%)
Fig. 4 Photoluminescence (PL) spectra of (A) 2,2'-MTPS-CaP, (B) 3,3'-MTPS-CaP and (C) 9,9'-MTPS-CaP in THF/water mixtures with
different water fractions (fw), and (D) plots of (I/I0 ‒ 1) correspond to different water fractions (fw), where I0 is the PL intensity when fw = 0%, I
is the PL intensity in each THF/water mixture (fw = 0 to 90%).
Fig. 5. The molecular orbital amplitude plots and energy levels of HOMOs and LUMOs of these new silole derivatives, calculated by
B3LYP/6-31G(d, p).

2,2'-MTPS-CaP
HOMO = -5.20 eV
LUMO = -2.70 eV
3,3'-MTPS-CaP
9,9'-MTPS-CaP
LUMO = -2.49 eV
HOMO = -5.04 eV
HOMO = -5.34 eV
LUMO = -2.62 eV
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Potential (V vs. Ag/Ag+)
Fig. 6. Cyclic voltammograms of the films of 2,2'-MTPS-CaP, 3,3'-MTPS-CaP, 9,9'-MTPS-CaP, measured in acetonitrile containing 0.1 mol/L
tetra-n-butylammonium hexafluorophosphate. Scan rate: 100 mV/s.

2000
1500
1000
500
0
20
15
10
5
105
104
103
102
101
100
B
C
A
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
D
6
4
2
0
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
0
1000 2000 3000 4000 5000
Luminance (cd/m2)
0
1000 2000 3000 4000 5000
Luminance (cd/m2)
400
500
600
700
800
2
7
12
17
Voltage (V)
Wavelength (nm)
Fig. 7 (A) EL spectra, (B) current density-voltage-luminance, (C) current and (D) external quantum efficiencies with the luminance of the non-
doped OLEDs based on 2,2'-MTPS-CaP, 3,3'-MTPS-CaP and 9,9'-MTPS-CaP.

Table 1
Optical properties, thermal stabilities and energy levels of these silole-based luminogens.
a
d
Compounds
λabs
λem (nm)
ФFc (%)
filma
αAIE
Tg/Td (oC)
HOMO/LUMOe (eV)
Egf (eV)
solna
filmb
539
531
520
soln
5.75
4.00
3.20
(nm)
398
404
385
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
525
526
520
69.93
60.33
80.00
12.16
15.08
25.00
133.4/433.8
147.1/452.4
136.2/440.8
–5.20/–2.70
–5.04/–2.49
–5.34/–2.62
2.50
2.55
2.72
a In THF solution (10−5 mol/L).
b Drop-casted film on quartz plate.
c Fluorescence quantum yield, determined by a calibrated integrating sphere.
d Values of AIE effect, calculated by F (film)/F (soln).
e Determined by CV measurement in solutions. f Energy band gaps, Eg = [LUMO – HOMO] eV.

Table 2
EL performances of OLEDs based on these silole derivatives.a
Emitters
λEL (nm)
555
552
542
Von (V)
2.9
3.1
5.3
Lmax (cd/m2)
83870
78780
ηc,max (cd/A)
12.72
12.44
ηp,max (lm/W)
9.87
ηext,max (%)
4.01
3.57
CIE (x, y)
2,2'-MTPS-CaP
3,3'-MTPS-CaP
9,9'-MTPS-CaP
(0.244, 0.435)
(0.399, 0.559)
(0.376, 0.549)
10.64
12.55
91920
17.59
5.63
a The maximum luminescence (Lmax), maximum current efficiency (ηc,max), maximum power efficiency (ηp,max) and the maximum external
quantum efficiency (ηext,max) of the devices; Von is the turn-on voltage at 1 cd/m2 and CIE coordinates at 100 mA/cm2.