Creation of bifunctional materials: Improve electron-transporting ability of light emitters based on AIE-active 2,3,4,5-tetraphenylsiloles

Article abstract of DOI:10.1002/adfm.201303867

2,3,4,5-Tetraphenylsiloles are excellent solid-state light emitters featured aggregation-induced emission (AIE) characteristics, but those that can efficiently function as both light-emitting and electron-transporting layers in one organic light-emitting diode (OLED) are much rare. To address this issue, herein, three tailored n-type light emitters comprised of 2,3,4,5- tetraphenylsilole and dimesitylboryl functional groups are designed and synthesized. The new siloles are fully characterized by standard spectroscopic and crystallographic methods with satisfactory results. Their thermal stabilities, electronic structures, photophysical properties, electrochemical behaviors and applications in OLEDs are investigated. These new siloles exhibit AIE characteristics with high emission efficiencies in solid films, and possess lower LUMO energy levels than their parents, 2,3,4,5-tetraphenylsiloles. The double-layer OLEDs [ITO/NPB (60 nm)/silole (60 nm)/LiF (1 nm)/Al (100 nm)] fabricated by adopting the new siloles as both light emitter and electron transporter afford excellent performances, with high electroluminescence efficiencies up to 13.9 cd A^-1, 4.35% and 11.6 lm W^-1, which are increased greatly relative to those attained from the triple-layer devices with an additional electron-transporting layer. These results demonstrate effective access to n-type solid-state emissive materials with practical utility. Grafting dimesitylboryl groups onto 2,3,4,5- tetraphenylsiloles generates efficient bifunctional materials that can simultaneously serve as light emitters and electron transporters in OLEDs. Remarkably high electroluminescence efficiencies up to 13.9 cd A^-1, 4.35% and 11.6 lm W^-1 are attained from the double-layer OLEDs based on them.

Full text of DOI:10.1002/adfm.201303867

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Creation of Bifunctional Materials: Improve Electron-
Transporting Ability of Light Emitters Based on AIE-Active
2,3,4,5-Tetraphenylsiloles
Long Chen, Yibin Jiang, Han Nie, Ping Lu, Herman H. Y. Sung, Ian D. Williams,
Hoi Sing Kwok, Fei Huang, Anjun Qin, Zujin Zhao,* and Ben Zhong Tang*
1. Introduction
2,3,4,5-Tetraphenylsiloles are excellent solid-state light emitters featured
aggregation-induced emission (AIE) characteristics, but those that can
efficiently function as both light-emitting and electron-transporting layers in
Organic luminescent materials have been
the subject of intense academic and com-
one organic light-emitting diode (OLED) are much rare. To address this issue,
herein, three tailored n-type light emitters comprised of 2,3,4,5-tetraphenyl-
silole and dimesitylboryl functional groups are designed and synthesized. The
new siloles are fully characterized by standard spectroscopic and crystallo-
graphic methods with satisfactory results. Their thermal stabilities, electronic
structures, photophysical properties, electrochemical behaviors and applica-
tions in OLEDs are investigated. These new siloles exhibit AIE characteristics
with high emission efficiencies in solid films, and possess lower LUMO
energy levels than their parents, 2,3,4,5-tetraphenylsiloles. The double-layer
OLEDs [ITO/NPB (60 nm)/silole (60 nm)/LiF (1 nm)/Al (100 nm)] fabricated
by adopting the new siloles as both light emitter and electron transporter
afford excellent performances, with high electroluminescence efficiencies up
to 13.9 cd A–1, 4.35% and 11.6 lm W–1, which are increased greatly relative to
those attained from the triple-layer devices with an additional electron-trans-
porting layer. These results demonstrate effective access to n-type solid-state
emissive materials with practical utility.
mercial interest over the past two decades
because of their various practical applica-
tions such as light emitters for organic
light-emitting diodes (OLEDs).[1] Although
considerable progresses have been made
in the development of organic emitters,
major challenges still remain. One stub-
born problem is that many “conventional”
luminophors that show good emissions in
solutions suffer from aggregation-caused
quenching (ACQ) in the condensed phase.
This becomes one of the thorny obsta-
cles to the evolution of OLEDs, for most
light emitters have to be fabricated into
solid films when used in devices.[2] Var-
ious chemical, physical and engineering
approaches have been proposed to miti-
gate the ACQ effect.[3] But these methods
often lead to some side effects, rendering
efficient solid-state emitters infrequent. It
will be nice, if a system can be developed,
L. Chen, H. Nie, Prof. F. Huang, Prof. A. Qin,
in which light emission is enhanced rather than quenched
by aggregation. In 2001, Tang’s group found that 1-methyl-
1,2,3,4,5-pentaphenylsilole (MPPS) was almost non-fluorescent
in solution but it could emit strong light when aggregated in
poor solvents or fabricated into solid films, demonstrating
an intriguing phenomenon of aggregation-induced emission
(AIE).[4] Based on this finding, many AIE-active luminogens
are developed, making pure solid films with high emission effi-
ciencies readily achievable. This paves a new avenue to realize
efficient non-doped OLEDs,[2,5] which avoid the complicated
and hard-to-control processes of the doping techniques used to
alleviate the ACQ effect of light emitters.
Prof. Z. Zhao, Prof. B. Z. Tang
State Key Laboratory of Luminescent
Materials and Devices
South China University of Technology
Guangzhou 510640, China
E-mail: zujinzhao@gmail.com; tangbenz@ust.hk
Y. Jiang, Prof. H. S. Kwok
Center for Display Research
The Hong Kong University of Science & Technology (HKUST)
Kowloon, Hong Kong, China
Dr. P. Lu
State Key Laboratory of Supramolecular Structure and Materials
Jilin University
Changchun 130012, China
Another challenge for next generation of high-performance
OLEDs is to simplify device configuration and cut down the
cost, without sacrificing device efficiency.[6] In this regard,
design and synthesis of new luminescent materials that can
simultaneously serve as light emitter and hole-/electron-
transporting material in one device is a promising alterna-
tive. Compared with organic hole-transporting (p-type) lumi-
nophors,[7] electron-transporting (n-type) materials with low
Dr. H. H. Y. Sung, Prof. I. D. Williams, Prof. B. Z. Tang
Division of Biomedical Engineering
Department of Chemistry
Institute for Advanced Study and Institute
of Molecular Functional Materials
HKUST, Clear Water Bay
Kowloon, Hong Kong
DOI: 10.1002/adfm.201303867
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electron injection barrier and fast electron mobility in addition
to excellent solid-state emission are scarce.[8]
electron affinity and electron-transporting ability.[14] The
thermal, photophysical, electronic, electrochemical, and electro-
luminescent properties of the new siloles are investigated. The
results reveal that these new siloles are efficient bifunctional
materials for OLEDs.
Among numerous AIE luminogens, siloles have attracted
considerable attention because of not only bright emissions in
solid films but also unique electronic structures.[9] The effec-
tive σ*–π* conjugation between the σ* orbitals of two exocy-
clic silicon-carbon bonds and π* orbital of the butadiene moiety
imparts the low-lying lowest unoccupied molecular orbital
(LUMO) and high electron affinity to siloles.[10] So far, efficient
solid-state emitters and electron transporter have been devel-
oped based on siloles.[11] Multiple OLEDs utilizing two different
kinds of silole derivatives as light-emitting and electron-trans-
porting layers, respectively, had been tried by Kafafi et al., which
afforded high efficiencies up to 4.1%.[12] However, the combina-
tion of both merits of siloles to generate efficient n-type light
emitters has been barely realized. In our previous studies, we
found that OLEDs fabricated by using 2,3,4,5-tetraphenylsiloles
(Scheme 1) as light-emitting layers afforded remarkably high
electroluminescence (EL) efficiency.[13] But these devices need
an electron-transporting layer to balance the holes and elec-
trons, implying that the 2,3,4,5-tetraphenylsiloles are excellent
light emitters but their electron-transporting ability needs to be
further enhanced. In view of this, three tailored silole deriva-
tives consisting of 2,3,4,5-tetraphenylsiloles and bimesitylboryl
groups are designed and synthesized in this work (Scheme 1).
It is envisioned that the incorporation of inherently electron
deficient group, dimesitylboryl, is beneficial to advances in
2. Results and Discussion
2.1. Synthesis
The target 2,3,4,5-tetraphenylsiloles carrying dimesitylboryl
groups, (MesB)2DMTPS, (MesB)2MPPS and (MesB)2HPS, were
facilely prepared according to the synthetic routes outlined in
Scheme 1. The detailed procedures and characterization data
are given in the Experimental section. Briefly, compounds 2[15]
and 3[16] were prepared according to the methods described in
the literature. The treatments of 3 with lithium 1-naphthalenide
(LiNaph), and subsequent ZnCl2-TMEDA generated 2,5-meta-
lated silole intermediates (4). Without purification, 4 underwent
coupling reaction with 2 in the presence of a palladium catalyst,
affording final compounds (MesB)2DMTPS, (MesB)2MPPS and
(MesB)2HPS in 80, 50 and 40% yields, respectively. The new
siloles were fully characterized by NMR and mass spectros-
copies, which confirmed their molecular structures. They are
soluble in common organic solvents including THF, dichlo-
romethane, chloroform, toluene, etc., but insoluble in water.
B
B
Si
Si
R
R'
R
R'
DMTPS
MPPS
HPS
R = Me R' = Me
R = Me R' = Ph
R = Ph R' = Ph
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
R = Me R' = Me
R = Me R' = Ph
R = Ph R' = Ph
Br
1) n-BuLi
B
Br
F
2)
B
Br
Pd(PPh3)2Cl2
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
2
1
1) LiNaph
ClZn
ZnCl
Si
2) ZnCl2-TMEDA
Si
R
R'
R
R'
4a
4b
R = Me R' = Me
R = Me R' = Ph
3a
3b
R = Me R' = Me
R = Me R' = Ph
4c R = Ph R' = Ph
3c R = Ph R' = Ph
Scheme 1. Synthetic routes to dimesitylboryl-functionalized 2,3,4,5-tetraphenylsiloles. LiNaph = lithium 1-naphthalenide; TMEDA =N,N,N′,
N′-tetramethylethylenediamine.
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Table 1. Optical, electronic and thermal properties of (MesB)2DMTPS, (MesB)2MPPS and (MesB)2HPS.
a)
c)
λemb)(ΦF
)
Compounds
Tg/Td
[°C]
HOMO/LUMO
[eV]
Eg
[eV]
λab
[nm]
[nm]([%])
Soln(ΦF)
512(0.87)
516(1.35)
523(1.38)
Film(ΦF)
516(56)
524(58)
526(62)
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
392
395
396
121/240
131/269
123/289
2.6
2.6
2.6
−5.6/−3.0
−5.7/−3.1
−5.7/−3.1
a)Absorption maximum in dilute THF solution (10 µM); b)Emission maximum in THF solution; c)Fluorescence quantum yield in THF solution (soln) determined using qui-
nine sulfate (ΦF = 54.6% in 0.1 N sulfuric acid) as standard or in the film state by a calibrated integrating sphere.
The thermal properties of (MesB)2DMTPS, (MesB)2MPPS and
(MesB)2HPS were evaluated by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). The results
show that they possess good thermal stability with decompo-
sition temperatures (Td) of 240–289 °C, according to 5% loss
of initial weight. High glass-transition temperatures (Tg) of
121–131 °C (Table 1) are recorded from the new siloles, indi-
cating that they are morphologically stable. The good thermal
and morphological stabilities enable them to serve as active
materials for OLEDs.
the solution state. Only faint PL signals peaked at 512, 516 and
523 nm are recorded for (MesB)2DMTPS, (MesB)2MPPS and
(MesB)2HPS, respectively. The fluorescence quantum yields
(ΦF) of (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS
in dilute THF solutions are as low as 0.87, 1.35, and 1.38%,
respectively, estimated using quinine sulfate (ΦF = 54.6% in
0.1 N sulfuric acid) as standard, which indicates that they are
practically very weak emitters when dissolved in good sol-
vents. These siloles become highly emissive in the solid state.
The films of (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS
2.2. Crystal Structure
Single crystals of (MesB)2DMTPS and
(MesB)2HPS were grown from THF/eth-
anol mixture, and analyzed by X-ray dif-
fraction crystallography. Figure 1 displays
the crystal structures of (MesB)2DMTPS
and (MesB)2HPS. It can be seen that in the
crystalline state, both siloles show twisted
conformations and no π–π stacking inter-
actions are found between aromatic rings.
Figure 2 illustrates the molecular packing
patterns of (MesB)2DMTPS and (MesB)2HPS
in the crystalline state. The (MesB)2HPS
molecules are arranged more regularly and
tightly than (MesB)2DMTPS molecules. The
close packing of (MesB)2HPS suggest that it
should have better carrier-transporting ability
than (MesB)2DMTPS.
2.3. Optical Property
The absorption spectra of the new siloles
in dilute THF solutions (10 µM) are
shown in Figure 3A. The spectral profiles
of (MesB)2DMTPS, (MesB)2MPPS and
(MesB)2HPS are similar, with maxima at
392, 395, and 396 nm, respectively, associ-
ated to π−π* transitions. The photolumi-
nescence (PL) spectra of three siloles in
dilute THF solutions (10 µM) are shown
in Figure 3B. Like most silole derivatives,
these new siloles show weak emissions in
Figure 1. ORTEP drawings of (MesB)2DMTPS (CCDC 948629) and (MesB)2HPS (CCDC
948630).
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(Figure 4B). Similar emission behaviors are also recorded
from (MesB)2DMTPS and (MesB)2HPS in THF/water mixtures
(Figure S2, Supporting Information). Since (MesB)2DMTPS,
(MesB)2MPPS, and (MesB)2HPS are insoluble in water, their
molecules must have aggregated in aqueous medium. The
intramolecular rotation that is active in the solution state is
restricted due to steric hindrance in the condensed phase. The
nonradiative relaxation channel is thus blocked and radiative
decay of the excited state is promoted, rendering molecules
highly emissive. These results manifest that the new siloles are
AIE-active indeed.
2.4. Theoretical Calculation
To gain a deep insight into the electronic structures of the
new siloles, theoretical calculations are performed by den-
sity function theory (DFT) method with a B3LYP/6–31G(d)
basis set using Gaussian 09 package. The optimized struc-
tures and orbital distributions of the HOMOs and LUMOs of
(MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS are shown in
Figure 5. All the molecules adopt twisted conformations, which
would hamper the strong intermolecular interactions between
the molecules. In LUMOs, significant contributions from the
boron centers are observed, owing to the pπ– π* conjugation
between empty pπ orbital of boron atom and π* orbitals of the
phenyl rings. Meanwhile, the electronic cloud of LUMOs can
also delocalize to exocyclic Si-C bonds through σ*–π* conjuga-
tion. Such distinctive electronic structures result in low LUMO
energy levels of −1.97, −2.05, and −2.04 eV for (MesB)2DMTPS,
(MesB)2MPPS, and (MesB)2HPS, respectively. The values
are much lower than those of DMTPS (−1.59 eV) and other
2,3,4,5-tetraphenylsiloles,[17] demonstrating that the incorpora-
tion of dimesitylboryl groups has effectively lowered the LUMO
energy levels. The electron injection and transport become
more favorable in the new siloles than their parents.
2.5. Electrochemical Property
Figure 2. Molecular packing patterns of (MesB)2DMTPS and (MesB)2HPS
in the crystalline state.
The
(MesB)2MPPS, and (MesB)2HPS were investigated by cyclic
voltammetry (CV) in dichloromethane solution with 0.1
electrochemical
properties
of
(MesB)2DMTPS,
M
fluoresce intensely with emission maxima at 516, 524, and 526
nm, respectively (Figure S1, Supporting Information). Only
slight bathochromic shifts are observed in the PL spectra of
the films compared to those of the solutions, which is attribu-
tive to the propeller-like molecular conformations as well as
the branched dimestylboryl groups that have impeded close
π–stacking interactions. The ΦF values of the films, deter-
mined by integrating sphere, are as high as 56, 58, and 62%
for (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS, revealing
that they are AIE-active and are excellent solid-state emitters.
The AIE attributes of the new siloles are further confirmed
by their emission behaviors in THF/water mixtures. Figure 4A
illustrates the PL spectra of (MesB)2MPPS in THF/water mix-
tures as an example. It can be seen that when the water con-
tent is low, the emission is weak. But when the water con-
tent becomes high, the emission intensity enhances swiftly
tetrabutylammonium hexafluorophosphate as the supporting
electrolyte at a scan rate of 50 mV s−1 using platinum as the
working electrode and saturated calomel electrode (SCE) as
the reference electrode. The new siloles exhibit similar CV
curves (Figure 6). The oxidation onset potentials (Eonset) of
(MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS occur at
1.2, 1.3, and 1.3 V, respectively. The energy levels of HOMO
[HOMO = −(4.4 + Eonset)] and LUMO [LUMO = –(HOMO + Eg)]
were determined by Eonset and optical band gaps. The HOMO
energy levels are calculated to be −5.6, −5.7, and −5.7 eV,
and the LUMO energy levels are −3.0, −3.1, and −3.1 eV for
(MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS, respectively.
The LUMO energy levels of the present siloles are equal to or
even lower than those of widely used electron-transporting mate-
rials, for example, tris(8-hydroxyquinoline) aluminum (Alq3,
−3.0 eV) and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
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2.5
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
2.0
1.5
1.0
0.5
0.0
B
A
340
400
460
520
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 3. A) Absorption and B) PL spectra of (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS in THF solutions (10 µM). Excitation wavelength:
380 nm.
(TPBi, −2.7 eV), indicating their great potential as electron-trans-
is located at 552 nm, which is red-shifted by 28 nm in com-
parison with its PL in film (Figure 7A). Such kind of red shifts
could be ascribed to space charge accumulation caused by the
imbalance of charge transport.[18] The performances of devices
I summarized in Table 2 are moderate and the turn-on volt-
ages are relatively high. These results indicate that the standard
device configuration with three active layers may not suitable
for the present siloles.
Considering the low-lying LUMO energy levels of the new
siloles, it is envisioned that they may function as electron trans-
porter in addition to light emitter in device. To confirm this,
double-layer OLEDs with a configuration of ITO/NPB (60 nm)/
silole (60 nm)/LiF (1 nm)/Al (100 nm) (Device II) are fabricated,
in which (MesB)2DMTPS, (MesB)2MPPS, or (MesB)2HPS serve
as both EML and ETL, and NPB acts as HTL. Devices II of
(MesB)2MPPS and (MesB)2HPS exhibit similar EL emissions
to the PL emissions of films. For instance, the EL spectrum of
(MesB)2MPPS with a peak at 520 nm is almost identical to the
porting materials for OLEDs.
2.6. Electroluminescence
The efficient solid-state PL emissions of (MesB)2DMTPS,
(MesB)2MPPS, and (MesB)2HPS inspire us to evaluate their
electroluminescence (EL) properties in non-doped OLEDs.
Triple-layer OLEDs with a configuration of ITO/NPB (60 nm)/
silole (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (Device
I) are fabricated, in which (MesB)2DMTPS, (MesB)2MPPS or
(MesB)2HPS serve as light-emitting layer (EML), N,N′-di(1-
naphthyl)-N,N′-diphenyl-benzidine (NPB) acts as hole-trans-
porting layer (HTL) and TPBi functions as electron-transporting
layer (ETL). The devices I of the new siloles radiate yellow
lights, whereas the PL emissions of the films are in the green
region. Taking (MesB)2MPPS as an example, its EL maximum
30
(MesB) DMTPS
2
fw (vol%)
A
B
(MesB) MPPS
2
90
25
20
15
10
5
(MesB) HPS
2
80
70
60
50
40
30
20
10
0
0
90
0
0
20
40
60
80
100
420
470
520
570
620
670
Wavelength (nm)
f
w (vol %)
Figure 4. A) PL spectra of (MesB)2MPPS in THF/water mixtures with different water fractions (fw). B) Plot of (I/I0 − 1) values versus water fractions in
THF/water mixtures of (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS. I0 is the PL intensity in pure THF solution. Inset: photos of (MesB)2MPPS
in THF/water mixtures (fw = 0 and 90%), taken under the illumination of a UV lamp.
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Figure 5. B3LYP/6–31G(d) calculated molecular orbital amplitude plots and energy levels of HOMOs and LUMOs of (MesB)2DMTPS, (MesB)2MPPS,
and (MesB)2HPS.
PL spectrum of film (524 nm) (Figure 7A), indicating that the
holes and electrons get more balanced, and the exciton recom-
bination zone has been confined inside the silole layer.
are advanced noticeably. Excellent maximum current (ηC), and
external quantum (ηext) efficiencies of 13.0 cd A−1, and 4.12%,
respectively, are attained by device II, which are almost twice
as much as those of device I (6.6 cd A−1, and 2.13%). The max-
imum power efficiency (ηP) of device II reaches 10.5 lm W−1,
being about four-fold higher than that of device I (2.4 lm W−1).
Significantly, device II shows a small roll-off in EL efficiencies
when luminance increases, and a high current efficiency of
7.0 cd A−1 is reserved at a luminance of 1000 cd m−2. This is
very important because the lack of a hole-blocking layer often
causes a large efficiency roll-off at high luminance range. Simi-
larly, device II of (MesB)2HPS also shows better performances
than device I. The turn-on voltage of device II is decreased
(4.3 vs. 5.4 V), and the peak EL efficiencies are enhanced obvi-
ously (ηC, 13.9 vs 8.4 cd A−1; ηext, 4.35 vs 2.62%; ηP, 11.6 vs
4.1 lm W−1) (Table 2 and Supporting Information Figure S3).
These values are much better than those obtained from
double-layer OLEDs of the reported n-type light emitters with
close emission colors, such as copolymers of silafluorene and
2,1,3-benzothiadiazole (10.6 cd A−1; 3.81%),[19] folded tetraphe-
nylethene derivatives (7.9 cd A−1; 3.1%),[5b] dimesitylboryl-func-
tionalized tetraphenylethene (7.13 cd A−1; 2.7%),[20] cyano-sub-
stituted pyridine derivatives (5.4 cd A−1),[21] Alq3 (3.3 cd A−1),[22]
spiro-bisilole derivatives (1.98 cd A−1),[23] and so forth.
The device II adopting (MesB)2MPPS as both EML and ETL is
turned on at a much lower voltage (3.9 V) than its device I with
TPBi as ETL (7.5 V). It also shows higher current densities, and
better luminance at the same voltages than device I (Figure 7B).
The maximum luminance (Lmax) of device II is 13 900 cd m−2,
which is higher than that of device I (9610 cd m−2). Figure
7C,D depict the current efficiency and external quantum effi-
ciency as a function of luminance for both devices I and II of
(MesB)2MPPS. It can be seen that both efficiencies of device II
90
(MesB)2DMTPS
(MesB)2MPPS
(MesB)2HPS
60
30
0
Since both (MesB)2MPPS and (MesB)2HPS have closer
LUMO energy levels to that of LiF/Al, the electron-injection is
more favorable from LiF/Al to (MesB)2MPPS and (MesB)2HPS
than to TPBi (Figure 8). The TPBi layer is virtually not neces-
sary. It may even cause adverse effect to device performances
because of the contact resistance at the interfaces. In addi-
tion, the excellent electron-transporting ability stemmed from
the synergistic effect of silole ring and boron atoms leads to
the shift of the current density-voltage characteristics to lower
-30
0.0
0.5
1.0
1.5
2.0
2.5
Voltage (V)
Figure 6. Cyclic voltammograms of (MesB)2DMTPS, (MesB)2MPPS, and
(MesB)2HPS, measured in dichloromethane containing 0.1 M tetra-n-but-
ylammonium hexafluorophosphate. Scant rate: 50 mV s−1
.
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500
400
300
200
100
0
104
103
102
101
100
Device I
Device II
PL in film
B
A
Device I
Device II
2
8
14
20
430
530
630
730
Voltage (V)
Wavelength (nm)
10
100
Device I
Device II
Device I
Device II
D
C
10
1
1
0.1
0.1
0
200
400
600
800
1000
0
200
400
600
800
1000
2
2
Luminance (cd/m )
Luminance (cd/m )
Figure 7. A) PL and EL spectra of solid thin film of (MesB)2MPPS. B) Current density-voltage-luminance characteristics, and C) current and
D)externalquantumefficiencieswiththeluminanceinELdevicesof(MesB)2MPPS. Deviceconfiguration:ITO/NPB(60nm)/(MesB)2MPPS(20nm)/TPBi
(40 nm)/LiF (1 nm)/Al (100 nm) (Device I); ITO/NPB (60 nm)/(MesB)2MPPS (60 nm)/LiF (1 nm)/Al (100 nm) (Device II).
voltages. At the same time, electron accumulation and exciton
quenching at the cathode are reduced. The balance of holes and
electrons is improved, and thus the carrier recombination prob-
ability is increased, leading to advance in EL performances.
The current and external quantum efficiencies of device II of
(MesB)2DMTPS are increased slightly relative to those of device
I (Table 2, Figure S4, Supporting Information). However, the
turn-on voltage is increased and the luminance is decreased.
Since electron-transporting ability is not dependent solely on the
energy levels of the materials, the arrangement of the molecules
in solid film also functions as a crucial factor. Close packing of
molecules is conducive to the carrier transport. From crystalline
Table 2. EL performances of OLEDs based on (MesB)2DMTPS, (MesB)2MPPS, and (MesB)2HPS. Abbreviations: λEL = electroluminescence max-
imum, Von = turn-on voltage at 1 cd m−2, Lmax = maximum luminance, ηc = maximum current efficiency, ηext = maximum external quantum efficiency,
ηp = maximum power efficiency, CIE = Commission Internationale de I’Eclairage coordinates.
Emitter
Devicea)
Von
[V]
Lmax
[cd m−2
CIE
(x, y)
λEL
[nm]
ηc
ηext
[%]
ηp
[cd A−1
]
[lm W−1
]
]
(MesB)2DMTPS
I
II
I
540
536
552
520
548
524
6.9
6.9
7.5
3.9
5.4
4.3
10500
1810
7.4
7.8
2.25
2.35
2.13
4.12
2.62
4.35
3.2
2.3
0.35, 0.55
0.35, 0.57
0.40, 0.54
0.30, 0.56
0.39, 0.55
0.33, 0.56
(MesB)2MPPS
(MesB)2HPS
9610
6.6
2.4
II
I
13900
15200
12200
13.0
8.4
10.5
4.1
II
13.9
11.6
a)Device configuration: ITO/NPB (60 nm)/silole (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (Device I); ITO/NPB (60 nm)/silole (60 nm)/LiF (1 nm)/Al (100 nm)
(Device II).
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other chemicals and reagents were purchased from Aldrich and J&K
Scientific Ltd., and used as received without further purification. 1H
and 13C NMR spectra were measured on a Bruker AV 400 or AV 300
spectrometer in deuterated dichloromethane or chloroform using
tetramethylsilane (TMS; δ = 0) as internal reference. UV spectra were
measured on a Milton Roy Spectronic 3000 Array spectrophotometer.
PL spectra were recorded on a Perkin-Elmer LS 55 spectrofluorometer.
High resolution mass spectra (HRMS) were recorded on a GCT premier
CAB048 mass spectrometer operating in MALDI-TOF mode. Single
crystal X-ray diffraction intensity data were collected on a Bruker–
Nonices Smart Apex CCD diffractometer with graphite monochromated
Mo Kα radiation. Processing of the intensity data was carried out using
the SAINT and SADABS routines, and the structure and refinement were
conducted using the SHELTL suite of X-ray programs (version 6.10).
TGA analysis was carried on a TA TGA Q5000 under dry nitrogen at a
heating rate of 10 °C min–1. The ground-state geometries were optimized
using the density functional with B3LYP hybrid functional at the basis set
level of 6–31G(d). All the calculations were performed using Gaussian
09 package.
-2.3 eV
-2.3 eV
NPB
-2.7 eV
-3.1 eV
-3.7 eV
-3.1 eV
-3.7 eV
LiF/Al
LiF/Al
NPB
TPBi
ITO
ITO
-4.8 eV
-4.8 eV
-5.3 eV
-5.7 eV
-5.3 eV
-5.7 eV
-6.2 eV
Device I
Device II
Figure 8. Energy level diagrams of multilayer EL devices of (MesB)2MPPS
with or without TPBi as electron-transporting layer.
geometry illustrated in Figure 2, we can see that (MesB)2HPS
molecules adopt
a much tighter packing manner than
Devices Fabrication: The devices were fabricated on 80 nm-ITO coated
glass with a sheet resistance of 25Ω ٗ–1. Prior to loading into the
pretreatment chamber, the ITO-coated glass was soaked in ultrasonic
detergent for 30 min, followed by spraying with de-ionized water for
10 min, soaking in ultrasonic de-ionized water for 30 min, and oven-
baking for 1 h. The cleaned samples were treated by perfluoromethane
plasma with a power of 100 W, gas flow of 50 sccm, and pressure of 0.2
Torr for 10 s in the pretreatment chamber. The samples were transferred
to the organic chamber with a base pressure of 7 × 10−7 Torr for the
deposition of NPB, emitter (and TPBi), which served as hole-transport,
light-emitting (and electron-transport) layers, respectively. The samples
were then transferred to the metal chamber for cathode deposition which
composed of lithium fluoride (LiF) capped with aluminum (Al). The light-
emitting area was 4 mm2. The current density-voltage characteristics of
the devices were measured by a HP4145B semiconductor parameter
analyzer. The forward direction photons emitted from the devices were
detected by a calibrated UDT PIN-25D silicon photodiode. The luminance
and external quantum efficiencies of the devices were inferred from the
photocurrent of the photodiode. The electroluminescence spectra were
obtained by a PR650 spectrophotometer. All measurements were carried
out under air at room temperature without device encapsulation.
Preparation of Nanoaggregates: Stock THF solutions of the siloles with
a concentration of 10−4 M were prepared. Aliquots of the stock solution
were transferred to 10 mL volumetric flasks. After appropriate amounts
of THF were added, water was added dropwise under vigorous stirring
to furnish 10−5 M solutions with different water contents (0–90 vol%).
The PL measurements of the resultant solutions were then performed
immediately.
(MesB)2DMTPS. Thereby, it is highly possible that (MesB)2HPS
molecules could pile up more closely than (MesB)2DMTPS mol-
ecule even in amorphous films. This renders easier electron
injection and better electron transport, and hence, superior EL
property of (MesB)2HPS to (MesB)2DMTPS. It should also be
noted that the devices have not yet been optimized thoroughly.
Many factors, for example, cathode materials, carrier injection
layers, hole-transporting layers, the layer thicknesses, and sub-
strate temperature for film deposition, have subtle impacts on
the balance of carrier injection, carrier mobility, carrier recom-
bination zone and efficiency, and so on. Therefore, given the
excellent optoelectronic property of the new siloles, more effi-
cient OLEDs are expectable via device engineering.
3. Conclusions
In summary, three thermally stable 2,3,4,5-tetraphenylsiloles
modified with dimesitylboryl groups are synthesized, and their
molecular structures and arrangement in the crystalline state
are investigated. The new siloles are weakly fluorescent in solu-
tions but become highly emissive in the aggregate state, pre-
senting AIE characteristics. Thanks to the synergistic effect
between silole ring and dimesitylboryl groups, the LUMO
energy levels of the new siloles are lowered and the electron-
transporting ability is improved. These merits enable them
to serve simultaneously as bifunctional materials of light
emitter and electron transporter in OLEDs. Efficient double-
layer devices are achieved based on them, exhibiting greatly
enhanced performances relative to triple-layer devices with
these siloles as light emitters and TPBi as electron transporter.
To the best of our knowledge, (MesB)2MPPS and (MesB)2HPS
are among the best n-type luminescent materials used in the
non-doped OLEDs. These results demonstrate an effective
molecular design approach to bifunctional candidates (n-type
light emitters) for OLEDs, for the sake of simplifying device
configuration and reducing manufacturing cost.
Synthesis
of
2,5-Bis(4-(dimesitylboryl)phenyl)-1,1-dimethyl-3,4-
diphenylsilole ((MesB)2DMTPS): A solution of lithium 1-naphthalenide
(LiNaph) was prepared by stirring a mixture of naphthalene (2.56 g,
20 mmol) and lithium granular (0.14 g, 20 mmol) in dry THF (30 mL)
for 4 h at room temperature under nitrogen. A solution of 3 (1.3 g,
5 mmol) in THF (20 mL) was then added dropwise into the solution
of LiNaph, and the resultant mixture was stirred for 30 min at room
temperature. The solution was cooled to −10 °C, into which ZnCl2-
TMEDA (6.3 g, 25 mmol) and 20 mL of THF were added. After the fine
suspension was stirred for 1h, Pd(PPh3)2Cl2 (100 mg) and a solution of
2 (4.04 g, 10 mmol) in 10 mL THF were added. After reflux for 12 h, the
reaction was cooled to room temperature and terminated by addition of
2 M hydrochloric acid. The mixture was poured into water and extracted
with dichloromethane. The organic layer was washed successively
with aqueous sodium chloride solution and water and then dried over
magnesium sulfate. After filtration, the solvent was evaporated under
reduced pressure and the residue was purified by silica-gel column
chromatography using hexane as eluent. Recrystallization gave a yellow
solid of product in ∼80% yield. 1H NMR (400 MHz, CD2Cl2), δ (ppm):
7.20 (d, J = 8.0 Hz, 4H), 7.04–6.95 (m, 6H), 6.89 (d, J = 8.0 Hz, 4H),
6.81–6.79 (m, 12H), 2.27 (s, 12H), 1.95 (s, 24H), 0.47 (s, 6H). 13C
4. Experimental Section
Materials and Instruments: THF was distilled from sodium
benzophenone ketyl under dry nitrogen immediately prior to use. All
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NMR (75 MHz, CDCl3), δ (ppm): 155.0, 144.0, 143.0, 142.6, 141.8,
140.8, 138.5, 138.3, 136.3, 129.9, 128.3, 128.0, 127.3, 126.4, 23.3, 21.2,
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Synthesis
of
2,5-Bis(4-(dimesitylboryl)phenyl)-1-methyl-1,3,4-
triphenylsilole ((MesB)2MPPS): The procedure was analogous to that
described for (MesB)2DMTPS. Yellow solid; yield 50%. 1H NMR
(400 MHz, CD2Cl2), δ (ppm): 7.65 (d, J = 8.0 Hz, 2H), 7.38–7.30 (m,
3H), 7.10 (d, J = 8.0 Hz, 4H), 7.07–6.98 (m, 6H), 6.87 (d, J = 8.0 Hz,
4H), 6.82 (d, J = 8.0 Hz, 4H), 6.76 (s, 8H), 2.24 (s, 12H), 1.90 (s, 24H),
0.80 (s, 3H). 13C NMR (100 MHz, CDCl3), δ (ppm): 156.5, 143.5, 143.0,
141.8, 141.3, 140.7, 138.6, 138.3, 136.2, 134.6, 133.2, 129.9, 128.5, 128.2,
128.0, 127.4, 126.6, 23.3, 21.2, −6.3. HRMS (MALDI-TOF): m/z 972.5442
[M+, calcd 972.5433].
Synthesis of 2,5-Bis(4-(dimesitylboryl)phenyl)-1,1,3,4-tetraphenylsilole
((MesB)2HPS): The procedure was analogous to that described for
(MesB)2DMTPS. Yellow solid; yield 40%. 1H NMR (400 MHz, CD2Cl2),
δ (ppm): 7.64 (d, J = 8.0 Hz, 4H), 7.45–7.42 (m, 2H), 7.35–7.32 (m, 4H),
7.08 (d, J = 8.0 Hz, 4H), 7.06–7.00 (m, 6H), 6.92 (d, J = 8.0 Hz, 4H), 6.86
(d, J = 8.0 Hz, 4H), 6.75 (s, 8H), 2.25 (s, 12H), 1.89 (s, 24H). 13C NMR
(75 MHz, CDCl3), δ (ppm): 157.6, 143.8, 143.3, 141.8, 140.8, 140.4,
138.6, 138.3, 136.0, 131.5, 130.2, 130.0, 128.8, 128.2, 128.0, 127.4, 126.6,
23.3, 21.2. HRMS (MALDI-TOF): m/z 1034.5597 [M+, calcd 1034.5589].
X-Ray Crystallography: Crystal data for (MesB)2DMTPS (CCDC
948629): C66H68B2Si, MW = 910.91, orthorhombic, P2(1)2(1)2(1), a =
12.0690(3), b = 17.7421(5), c = 26.1832(7) Å, V = 5606.6(3) Å3, Z = 4,
Dc = 1.079 g cm−3, µ = 0.644 mm−1 (MoKα, λ = 1.5418), F(000) = 1952,
T = 173.00(14) K, 2θmax = 66.50° (99.36%), 32045 measured reflections,
9924 independent reflections (Rint = 0.0702), GOF on F2 = 1.002, R1
= 0.0903, wR2 = 0.1230 (all data), Δe 0.206 and −0.185 eÅ−3. Crystal
data for (MesB)2HPS (CCDC 948630): C76H72B2Si, MW = 1035.05,
monoclinic, C2/c, a = 37.0234(15), b = 10.5134(4), c = 15.9686(6) Å, β =
90.774(4)°, V = 6215.1(4) Å3, Z = 4, Dc = 1.106 g cm−3, µ = 0.639 mm−1
(MoKα, λ = 1.5418), F(000) = 2208, T = 173.00(14) K, 2θmax = 66.50°
(95.3%), 17 100 measured reflections, 5275 independent reflections
(Rint = 0.0676), GOF on F2 = 1.005, R1 = 0.0764, wR2 = 0.1531 (all data),
δe 0.454 and −0.262 eÅ−3
.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors acknowledge the financial support from the National Natural
Science Foundation of China (21104012 and 51273053), the National
Basic Research Program of China (973 Program, 2013CB834702), the
Fundamental Research Funds for the Central Universities (2013ZZ0002),
the Guangdong Innovative Research Team Program of China
(20110C0105067115) and the Research Grants Council of Hong Kong
(HKUST2/CRF/10).
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DOI: 10.1002/adfm.201303867