Efficient non-doped near infrared organic light-emitting devices based on fluorophores with aggregation-induced emission enhancement

Article abstract of DOI:10.1021/cm3008733

A family of donor-acceptor-donor (D-A-D) type near-infrared (NIR) fluorophores containing rigid nonplanar conjugated tetraphenylethene (TPE) moieties was designed and synthesized through Stille coupling reactions with electron-deficient [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) or benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBTD) as acceptors. The absorption, fluorescence, and electrochemical properties were studied. These compounds exhibited good aggregation-induced emission enhancement (AIEE) property, as a result of the twisted TPE units, which restrict the intramolecular rotation and reduce the π-π stacking. Photoluminescence of these chromophores ranges from 600 to 1100 nm, and their HOMO-LUMO gaps are between 1.85 and 1.50 eV. Non-doped organic light-emitting diodes (OLEDs) based on these fluorophores were made and exhibited EL emission spectra peaking from 706 to 864 nm. The external quantum efficiency (EQE) of these devices ranged from 0.89% to 0.20% and remained fairly constant over a range of current density of 100-300 mA cm^-2. The device with the highest solid fluorescence efficiency emitter 1a shows the best performance with a maximum radiance of 2917 mW Sr^-1 m^-2 and EQE of 0.89%. A contrast between nondoped and doped OLEDs with these materials confirms that AIEE compounds are suitable for fabricate efficient nondoped NIR OLEDs.

Full text of DOI:10.1021/cm3008733

Article
pubs.acs.org/cm
Efficient Non-doped Near Infrared Organic Light-Emitting Devices
Based on Fluorophores with Aggregation-Induced Emission
Enhancement
Xiaobo Du,†,§ Ji Qi,†,§ Zhiqiang Zhang,† Dongge Ma,† and Zhi Y. Wang*,†,‡
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, P.R. China
‡Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada
§Graduate School of Chinese Academy of Sciences, Beijing 100039, P.R. China
S
* Supporting Information
ABSTRACT: A family of donor−acceptor−donor (D−A−D) type
near-infrared (NIR) fluorophores containing rigid nonplanar
conjugated tetraphenylethene (TPE) moieties was designed and
synthesized through Stille coupling reactions with electron-deficient
[1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) or benzo[1,2-c;4,5-
c′]bis[1,2,5]thiadiazole (BBTD) as acceptors. The absorption,
fluorescence, and electrochemical properties were studied. These
compounds exhibited good aggregation-induced emission enhance-
ment (AIEE) property, as a result of the twisted TPE units, which
restrict the intramolecular rotation and reduce the π−π stacking.
Photoluminescence of these chromophores ranges from 600 to
1100 nm, and their HOMO−LUMO gaps are between 1.85 and
1.50 eV. Non-doped organic light-emitting diodes (OLEDs) based on these fluorophores were made and exhibited EL emission
spectra peaking from 706 to 864 nm. The external quantum efficiency (EQE) of these devices ranged from 0.89% to 0.20% and
remained fairly constant over a range of current density of 100−300 mA cm−2. The device with the highest solid fluorescence
efficiency emitter 1a shows the best performance with a maximum radiance of 2917 mW Sr−1 m−2 and EQE of 0.89%. A contrast
between nondoped and doped OLEDs with these materials confirms that AIEE compounds are suitable for fabricate efficient
nondoped NIR OLEDs.
KEYWORDS: near infrared, AIEE, OLEDs, donor−acceptor
are more prone to molecular aggregation and fluorescence
quenching in the solid state.17−19 In addition, NIR fluorophores
with low band gaps often have more vibronic coupling between
ground and excited state, which promotes the nonradiative
deactivation pathways.20 Therefore, most NIR-absorbing
chromophores show no or low fluorescence in the solid state
and could only be used as a dopant in NIR OLEDs.21−23 An
effective approach is to employ phosphorescent sensitized
fluorescence by codoped with phosphorescent dye,24,25 which
has been proved useful in improving the device efficiencies of
fluorescent NIR OLEDs to the levels of phosphorescent NIR
OLEDs.26−28 However, efficacious doping often requires
precise control of doping concentration, and the bimolecular
interactions emission quenching at high exciton densities cause
a rapid decrease in quantum efficiency for high currents or high
doping concentrations.29 The non-doped OLEDs with the
emission above 700 nm still show a low external quantum
INTRODUCTION
■
Near-infrared (NIR) fluorescent materials are fundamentally
interesting and practically useful as NIR fluorescent tags for
bioimaging1−3 and chemosensing4 or as NIR organic light-
emitting diodes (OLEDs).5,6 For biological applications, NIR
fluorophores with the emission wavelength between 650 and
1200 nm are more highly desirable than visible fluorescence
probes, because of a better spectral separation from
autofluorescence and low light scattering in biological tissues.7
For potential applications in optical communication,8 night-
vision displays9 and information-secured display,10 highly
efficient NIR fluorophores are urgently needed for use in
OLEDs.
However, many known fluorophores exhibit high fluores-
cence in dilute solutions but weak or no emission in the solid
state because of aggregation-caused emission quenching
(ACEQ).11−14 The ACEQ is generally attributed to a
nonradiative deactivation process, such as excitonic coupling,
excimer formation, and excitation energy migration to the
impurity traps.15,16 Most of the NIR organic fluorophores are
donor−acceptor type or flat π-conjugated molecules and, thus,
Received: March 20, 2012
Revised: May 21, 2012
Published: May 22, 2012
© 2012 American Chemical Society
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Scheme 1. Synthetic Route to Compounds 1a,b and 2a,b
efficiency (EQE) and are not suitable for practical applica-
tions.30−33
EXPERIMENTAL SECTION
■
General Information. Synthetic routes of these fluorescent
compounds are described in Scheme 1. All chemicals and reagents
were used as received from commercial sources. 4,7-Dibromo-5,6-
dinitro-2,1,3-benzothiadiazole44 and 1-(4-bromophenyl)-1,2,2-triphe-
nylethylene (TPE-Br)45 were prepared according to literature
methods.
A molecular approach to overcoming the ACEQ in organic
fluorophores was initiated by Tang in 2001, which involves the
introduction of a propeller-like group to reduce or eliminate the
π−π stacking and molecular rotational motions in the solid
state.34 Consequently, a variety of compounds such as
distyrylbenzene, fluorene, pentacene, and pyrene derivatives
are enabled to display the aggregation-induced emission
enhancement (AIEE) and show a higher fluorescence efficiency
in the solid state than in solution.35−38 However, realization of
the AIEE in NIR fluorophores has yet to be demonstrated.
A twisted tetraphenylethene (TPE) has the excellent AIEE
property and has also been shown to be a unique AIEE
enabling unit for many organic chromophores.39,40 Incorpo-
ration of TPE units into the chemical structures of poor
fluorophores could significantly improve their fluorescence
efficiency in the solid state.41 Accordingly, we envision that by
introduction of the TPE units in a low band gap chromophore
such as a π-conjugated donor−acceptor (D−A) system,42,43 the
AIEE in the NIR spectral region can be realized. By design, the
TPE group should be a part of the conjugated moiety to
prevent the π−π stacking and enable the AIEE property of the
designed compounds and also act as an electron donor with an
option of further substitution with additional electron-donating
groups. Guided by this design principle, we report the synthesis
of the D−A−D type NIR fluorescent compounds with the
AIEE property and non-doped NIR OLEDs with a single
emission peak centered above 700 nm and up to 1000 nm.
These devices are stable and show very little efficiency roll-off
at high current densities. The device performance is considered
to be the best for non-doped NIR OLEDs reported to date.
1H NMR (300 MHz) spectra of all the samples in CDCl3 were
recorded on a Bruker Avance NMR spectrometer. The high-
temperature 13C NMR data were obtained on a Varian Unity-400
MHz spectrometer at 393 K. Elemental analysis was performed on a
Vario EL elemental analysis instrument. Matrix-assisted laser
desorption ionization time-of-flight mass spectrometry (MALDI-
TOF-MS) was performed on a Bruker Daltonics Autoflex III TOF/
TOF. High-resolution mass spectrometry (HRMS) was obtained using
a 7.0 T actively shielded Fourier transform ion cyclotron resonance
mass spectrometer. The UV−vis−NIR absorption spectra were
recorded on a Shimadzu UV-3600 spectrophotometer. Cyclic
voltammetry was performed on a CHI660b electrochemical work-
station, in dry dichloromethane containing n-Bu4NPF6 (0.1 M) with a
scan rate of 50 mV/s at room temperature under argon, using a Pt disk
(2 mm diameter) as the working electrode, a Pt wire as the counter
electrode, and a Ag/AgCl electrode as a reference. The redox
potentials were calibrated by using ferrocene as an internal standard.
Thermogravimetric analysis (TGA) was performed on a PerkinElmer
Thermal Analysis Instruments Pyris Diamond TG instrument from 50
to 600 °C at a heating rate of 10 °C/min under a continuous nitrogen
flow.
The OLED was fabricated by thermal vacuum deposition on
prepatterned ITO-coated glass substrates with sheet resistances of 10
Ω/square. The substrate was ultrasonically cleaned with acetone,
detergent, deionized water, and ethanol in sequence, followed by
oxygen-plasma cleaning. The thermal evaporation of organic materials
was carried out at a chamber pressure of 10−4 Pa. The thickness of
each layer was determined by a quartz thickness monitor. The effective
size of the light-emitting diode was 16 mm2. Current−voltage−power
measurements were performed simultaneously using a Keithley 2400
source meter and a Newport 1830-C optical meter equipped with a
Newport 818-UV silicon photodiode, respectively. The NIR photo-
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Chemistry of Materials
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luminescence (PL) and electroluminescence (EL) spectra were
measured using a PTI fluorescence spectrophotometer with InGaAs
detector. The visible EL was measured using a PR650 spectroscan
photometer. The fluorescence quantum yield in THF solutions was
estimated using IR-125 (ΦF = 0.13 in DMSO)50 as standards. All the
measurements were carried out under ambient atmosphere at room
temperature. The EQE of the NIR EL was determined according to
the literature method, by measuring the light intensity in the forward
direction and assuming the external emission profile to be
Lambertian.32,47
Synthesis. 1-(4-Bromophenyl)-2,2-bis(4-methoxyphenyl)-1-phe-
nylethene (MTPE-Br). Bis(4-methoxyphenyl)methanone (9.73 g, 40.0
mmol), 4-bromo-benzophenone (12.54 g, 48.0 mmol), and Zn powder
(13 g, 200 mmol) in dried THF (500 mL) were stirred at 0 °C under
an argon atmosphere; then, TiCl4 (17.6 mL, 160 mmol) was dropped
into the reaction mixture with rapid stirring over 30 min. The mixture
was heated to 80 °C for 24 h. After being cooled to room temperature,
the reaction was quenched by the addition of 250 mL 10% aq K2CO3;
the mixture was filtered to remove insoluble materials and washed with
CH2Cl2. The organic layer was dried with anhydrous MgSO4 and
filtered. The solution was evaporated. The crude product was purified
by column chromatography (PE/DCM = 1:1) to give 9.62 g (20.4
mmol) of MTPE-Br as a light yellow solid in 51% yield. 1H NMR (300
MHz, CDCl3) δ (ppm): 7.22 (d, 2H, J = 8.4 Hz), 7.12−7.08 (m, 3H),
7.01−6.98 (m, 2H), 6.95−6.87 (m, 6H), 6.68−6.61 (m, 4H), 3.75 (d,
6H, J = 8.4 Hz).
4H, J = 8.1 Hz), 7.14 (s, 10H), 7.03−6.96 (m, 8H), 6.69−6.64 (m,
8H), 3.75 (s, 16H).
4,9-Bis[4-(1,2,2-triphenylvinyl)phenyl][1,2,5]thiadiazolo-[3,4-g]-
quinoxaline (1a). TPE-BTNH (0.414 g, 0.50 mmol) and 1,4-dioxane-
2,3-diol (0.120 g, 1.00 mmol) in acetic acid (30 mL) and CHCl3 (30
mL) were stirred at 30 °C for 6 h under an argon atmosphere. After
being cooled to the room temperature, the mixture was poured into
water and the resulting solids were collected by filtration. The crude
product was purified by column chromatography (PE/DCM = 1:3) to
give compound 1a as a red solid (0.297 g, 70%). 1H NMR (300 MHz,
CDCl3) δ (ppm): 8.82 (s, 2H), 7.64(d, 4H, J = 8.4 Hz), 7.27−7.24 (m,
4H), 7.19−7.07 (m, 30H). 13C NMR (100 MHz, C2D2Cl4, 393K) δ
(ppm): 154.07, 146.34, 144.84, 144.70, 144.36, 142.93, 142.10, 139.31,
134.13, 133.11, 132.32, 132.24, 132.17, 131.18, 128.68, 128.55, 127.37,
127.28. HRMS (ESI, m/z): (M+H)+ calcd for C60H40N4S, 849.30518;
found, 849.30487.
4,9-Bis{4-[2,2-bis(4-methoxyphenyl)-1-phenylvinyl]phenyl}
[1,2,5]thiadiazolo-[3,4-g]quinoxaline (1b). MTPE-BTNH (0.474 g,
0.50 mmol) and 1,4-dioxane-2,3-diol (0.120 g, 1.00 mmol) in acetic
acid (30 mL) and CHCl3 (30 mL) were stirred at 80 °C for 24 h under
an argon atmosphere. After being cooled to room temperature, the
mixture was poured into water, and the resulting solids were collected
by filtration. The crude product was purified by column chromatog-
raphy (PE/DCM = 1:3) to give compound 1b as a purple solid (0.372
g, 77%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.83 (s, 2H), 7.64(d,
4H, J = 8.4 Hz), 7.23 (s, 2H), 7.17−7.08 (m, 14H), 6.97(d, 4H, J = 8.7
Hz), 6.70(d, 4H, J = 8.7 Hz), 6.65(d, 4H, J = 8.7 Hz), 3.75(d, 12H, J =
0.9 Hz). 13C NMR (100 MHz, C2D2Cl4, 393K) δ (ppm): 160.04,
154.49, 146.71, 145.61, 145.36, 142.53, 140.96, 139.72, 138.05, 134.22,
133.89, 133.82, 133.51, 133.06, 132.78, 132.65, 131.63, 131.02, 129.09,
127.48, 115.09, 114.95, 56.76. HRMS (MALDI, m/z): (M)+ calcd for
C64H48N4O4S, 968.33959; found, 968.33942.
4,7-Bis[4-(1,2,2-triphenylvinyl)phenyl]-5,6-dinitrobenzo-2,1,3-
thiadiazole (TPE-BTNO). TPE-Br (8.23 g, 20.0 mmol) in dried THF
(150 mL) was stirred at −78 °C under an argon atmosphere, then n-
BuLi (2.5 M in hexane, 12.0 mL, 30.0 mmol) was dropped into it over
20 min. After the mixture was stirred for 2 h, tributylstannyl chloride
(10.1 mL, 36.0 mmol) was added to it and reacted for 12 h at 25 °C.
The mixture was poured into water and extracted with ethyl acetate.
Upon evaporation of the solvent, the crude product, 4,7-dibromo-5,6-
dinitro-2,1,3-benzothiadiazole (3.07 g, 8.00 mmol), and PdCl2(PPh3)2
(100 mg) in dried THF (150 mL) were stirred at 80 °C for 24 h under
an argon atmosphere. After being cooled to the room temperature, the
mixture was diluted with dichloromethane and washed with 100 mL
saturated aqueous potassium fluoride solution and brine. The crude
product was purified by column chromatography (PE/DCM = 1:2) to
4,8-Bis[4-(1,2,2-triphenylvinyl)phenyl]benzo[1,2-c:4,5-c′]bis-
[1,2,5]thiadiazole (2a). TPE-BTNH (0.414 g, 0.50 mmol), N-
sulfinylaniline (0.143 g, 1.00 mmol) and chlorotrimethylsilane
(0.100 g, 0.95 mmol) in dried pyridine (30 mL) were stirred at 30
°C for 6 h under an argon atmosphere. After being cooled to the room
temperature, the mixture was poured into water and the resulting
solids were collected by filtration. The crude product was purified by
column chromatography (DCM) to give compound 2a as a blue solid
(0.247 g, 58%). 1H NMR (300 MHz, CDCl3) δ (ppm): 8.05 (d, 4H, J
= 8.1 Hz), 7.30−7.27 (m, 4H), 7.17−7.05 (m, 30H). 13C NMR (100
MHz, C2D2Cl4, 393K) δ (ppm): 154.42, 145.47, 145.13, 145.01,
143.51, 142.26, 134.75, 132.84, 132.72, 132.62, 132.57, 132.24, 129.17,
129.12, 128.96, 127.95, 127.84, 127.77, 122.46. HRMS (MALDI, m/
z): (M)+ calcd for C58H38N4S2, 854.25377; found, 854.25342.
4,8-Bis{4-[2,2-bis(4-methoxyphenyl)-1-phenylvinyl]phenyl}benzo-
[1,2-c:4,5-c′]bis[1,2,5]thiadiazole (2b). MTPE-BTNH (0.474 g, 0.50
mmol), N-sulfinylaniline (0.143 g, 1.00 mmol) and chlorotrimethylsi-
lane (0.100 g, 0.95 mmol) in dried pyridine (30 mL) were stirred at 80
°C for 24 h under an argon atmosphere. After being cooled to the
room temperature, the mixture was poured into water, and the
resulting solids were collected by filtration. The crude product was
purified by column chromatography (DCM) to give compound 2b as
a green solid (0.358 g, 73%). Because of the limited solubility of 2b in
the NMR solvent, taking the 13C NMR spectrum of 2b was not
1
give compound TPE-BTNO as an orange solid (6.72 g, 95%). H
NMR (300 MHz, CDCl3) δ (ppm): 7.30−7.27 (m, 4H), 7.21−7.02
(m, 34H). Anal. calcd for C58H38N4O4S: C, 78.54; H, 4.32; N, 6.32.
Found: C, 77.91; H, 4.58; N, 6.12. MALDI-TOF-MS: m/z 886.3
(M+).
4,7-Bis{4-[2,2-bis(4-methoxyphenyl)-1-phenylvinyl]phenyl}-5,6-
dinitrobenzo-2,1,3-thiadiazole (MTPE-BTNO). Following the proce-
dure for the preparation of TPE-BTNO, MTPE-BTNO was obtained
as a purple solid (7.58 g, 94%). 1H NMR (300 MHz, CDCl3) δ
(ppm): 7.30−7.25 (m, 4H), 7.19−7.12 (m, 14H), 7.00−6.93 (m, 8H),
6.71−6.64 (m, 8H), 3.76 (d, 12H, J = 5.7 Hz). Anal. calcd for
C62H46N4O8S: C, 73.94; H, 4.60; N, 5.56. Found: C, 73.40; H, 4.63;
N, 5.32. MALDI-TOF-MS: m/z 1006.3 (M+).
4,7-Bis[4-(1,2,2-triphenylvinyl)phenyl]-5,6-diaminobenzo-2,1,3-
thiadiazole (TPE-BTNH). TPE-BTNO (4.44 g, 5.00 mmol) and iron
dust (3.35 g, 60.0 mmol) in acetic acid (180 mL) and CHCl3 (180
mL) was stirred at 80 °C for 36 h under an argon atmosphere. After
being cooled to the room temperature, the mixture was poured into
water, and the resulting solids were collected by filtration. The mixture
was dissolved in chloroform and was filtered to remove insoluble
materials. Upon evaporation of the filtrate, the crude product was
recrystallized by chloroform to afford the product as light yellow solid
(2.65 g, 64%). 1H NMR (300 MHz, CDCl3) δ (ppm): 7.32 (d, 4H, J =
8.4 Hz), 7.22 (d, 4H, J = 8.4 Hz), 7.14−7.04 (m, 30H), 3.87 (s, 4H).
4,7-Bis{4-[2,2-bis(4-methoxyphenyl)-1-phenylvinyl]phenyl}-5,6-
diaminobenzo-2,1,3-thiadiazole (MTPE-BTNH). MTPE-BTNH was
synthesized according to the same procedure as that used in the
1
possible, even in C2D2Cl4 at 393 K. H NMR (300 MHz, CDCl3) δ
(ppm): 8.06 (d, 2H, J = 8.7 Hz), 7.33−7.29 (m, 4H), 7.19−7.07 (m,
14H), 7.00−6.93 (m, 6H), 6.71−6.64 (m, 8H), 3.76 (t, 12H, J = 6.0
Hz). HRMS (MALDI, m/z): (M)+ calcd for C62H46N4O4S2,
974.29601; found, 974.29694.
RESULTS AND DISSCUSSION
■
Design and Synthesis. To design the AIEE NIR
fluorophores suitable for use in OLED by vapor deposition,
we selected [1,2,5]thiadiazolo[3,4-g]quinoxaline (QTD) and
benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBTD) as electron
acceptors and tetraphenylethene (TPE) and 2,2-bis(4-methox-
yphenyl)-1-phenylethene (MTPE) as both electron donors and
1
preparation of TPE-BTNH to obtain a yellow solid at 59% yield. H
NMR (300 MHz, CDCl3) δ (ppm): 7.33 (d, 4H, J = 8.4 Hz), 7.21 (d,
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Table 1. Characterizations of Compounds 1a,b and 2a,b
a
ab
,
c
max
abs
max
PL
λ
(nm)
λ
(nm)
Φf (%)
d
e
e
f
cmpd
soln
film
soln
film
soln
agg.
Td (°C)
HOMO (eV)
LUMO (eV)
Eg (eV)
1a
1b
2a
2b
518
534
612
632
532
544
635
658
700
780
787
857
704
761
803
883
10.1
0.28
13.0
0.20
36.9
8.20
0.26
6.39
480
454
443
418
−5.39
−5.21
−5.38
−5.22
b
−3.54
−3.53
−3.71
−3.72
1.85
1.68
1.67
1.50
a
Solution: measured in THF with a concentration of 1 × 10−5 M. Film: spin-coated on quartz plate. Excitation wavelengths for the emission
c
measurements are equal to their maximum absorption wavelengths. Fluorescence quantum yield (Φf) was measured relative to IR-125 (13% in
DMSO). Corrections due to the change in solvent refractive indices were applied. Aggregation was measured in a water−THF mixture with 95 vol %
d
e
water. The onset temperature for 5% weight loss in nitrogen. Calculated from the formula, E(HOMO) = −(Eox + 4.34) (eV), E(LUMO) = −(Ered
f
+ 4.34) (eV). The HOMO−LUMO energy gap (Eg) is derived from the HOMO subtracting LUMO.
AIEE-enabling units. Accordingly, four compounds 1a,b and
2a,b (Scheme 1) were designed and expected to have a low
HOMO−LUMO gap due to the presence of strong electron-
withdrawing acceptors of QTD and BBTD.48,49 The key step in
the entire synthetic route involves the Stille cross-coupling
reaction of 4,7-dibromo-5,6-dinitrobenzo[c][1,2,5]thiadiazole
with the corresponding tributyltin compounds derived from
TPE and MTPE. Reduction of the nitro groups and subsequent
cyclization with 1,4-dioxane-2,3-diol or PhNSO furnished the
construction of the acceptor moieties of QTD and BBTD,
respectively. All the compounds are relatively more soluble in
dichloromethane and chloroform than in other common
organic solvents. The solubility of 1a,b and 2a in chloroform
is more than 10 mg/mL at room temperature, while 2b has a
relatively lower solubility of about 2 mg/mL at room
temperature. TGA indicates that they are thermally stable
and suitable for vapor deposition in OLED device fabrication,
as the onset temperatures for 5% weight loss (Td) in nitrogen
range from 418 to 480 °C (Figure S1, Support Information).
Optical Properties. Photophysical properties of these four
compounds are summarized in Table 1. The absorption bands
from 300 to 450 nm are attributed to π−π* and n−π*
transitions of the conjugated aromatic segments, and those at
longer wavelengths are due to the intramolecular charge
transfer between the donors and acceptors (Figure 1). As
expected, the use of a stronger donor (e.g., MTPE) or a
stronger acceptor (e.g., BBTD) led to a bathochromic shift of
the long-wavelength peaks in the absorption and emission
Figure 1. (a) Absorption and PL spectra of compounds 1a,b and 2a,b
spectra. Among all the four compounds, compound 2b exhibits
in THF. (b) Absorption and PL spectra of compounds 1a,b and 2a,b
the longest absorption and emission centered at 658 and 883
as thin films.
nm (with tailing above 1000 nm) in the solid state, respectively
(Figure 1 and Table 1). A larger bathochromic shift in
photoluminescence observed for 2a and 2b as thin films rather
than in solution is an indication of more aggregation in the
solid state brought by the rigid and polar BBTD acceptor.
Accordingly, the NIR emission above 900 nm can be expected
by introduction of stronger donors in the BBTD-containing
fluorophores.
The AIEE property of these compounds was revealed by
comparison of the PL intensity in a water−THF mixture with
different water fractions (Table 1 and Figure 2). By a gradual
increase of the amount of a nonsolvent of water in the THF
solution of these fluorophores, the molecular aggregation
occurs, which leads to the changes in PL intensity. In THF
solution, compounds 1a and 2a are more fluorescent than 1b
and 2b, while the emission enhancement of the latter pair is
relatively more significant than the former pair. The additional
bond rotations brought by the four methoxy groups in 1b and
2b seem to be responsible for the nonradiative deactivation of
Figure 2. PL quantum yields of compounds 1a,b and 2a,b in a water−
THF mixture (10 μM) relative to a different fraction of water.
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the excitons, resulting in the observed low fluorescence in
solution.46 To quantitatively estimate the aggregation-induced
emission process, the PL quantum yields were calculated for the
compounds in the water−THF mixtures with different water
fractions using IR-125 (Φ = 0.13 in DMSO) as the reference
(Figure 2). The highly aggregated PL quantum yield of 1a is
attributed to the AIEE effect caused by cross-stacking of
propeller-like TPE groups when forming molecular aggregates,
in which the suppression of π−π stacking interaction and the
restriction of intramolecular rotations lead to enhancement of
fluorescence intensity. Surprisingly, compound 2a did not show
the AIEE, probably because the stronger donor−acceptor
system of 2a is easy to aggregate in a more polar solvent such as
water, and the more planar conformation of 2a, without the
methoxy group, leads to more aggregation-caused emission
quenching. The photoluminescence of 1a, 1b, and 2b was
significantly enhanced in solution upon addition of water. At
95% water fraction, the PL quantum yield of 1a jumped to
36.9% and the PL quantum yields of 1b and 2b increased more
than 30 times relative to those in pure THF solution. The PL
enhancement is attributed to the AIEE effect caused by the
formation of molecular aggregates, in which the suppression of
π−π stacking interaction and the restriction of intramolecular
rotations lead to enhancement of fluorescence intensity. Due to
the lack of a required instrument, the solid-state quantum yield
is difficult to determine accurately. Since the fluorescence
quantum yield is proportional to the emission intensity and
inversely proportional to the absorption intensity, from the
absorption and emission intensity of films with similar thickness
(Figure S2, Support Information), it can be estimated that
compound 2a has a relatively higher solid-state quantum yield,
which higher than 1b and 2b but lower than 1a. Thus, the AIEE
effect of 2a is weaker than that of 1a, and it is suppressed when
in a polar environment.
calculated (Table 1). Compounds 1a and 2a have the similar
HOMO levels (−5.39 eV and −5.38 eV), as they have the same
TPE donor, while the HOMO levels of 1b and 2b with the
MTPE group are slightly higher (−5.21 eV and −5.22 eV).
Similarly, the LUMO energy levels of compounds 2a and 2b,
with the stronger acceptor of BBTD (−3.71 eV and −3.72 eV),
are lower than those of 1a and 1b (−3.54 eV and −3.53 eV).
Among all, compound 2b has the lowest HOMO−LUMO gap
of 1.50 eV.
Electroluminescent Property. Given the enhanced PL
property and good processability, all the four compounds were
further evaluated as emitters in OLEDs using the following
device configuration: ITO/MoO3(8 nm)/NPB(40 nm)/
emitter(20 nm)/TPBi(10 nm)/Alq3(30 nm)/LiF(1 nm)/Al
(100 nm). N,N-Bis(1-naphtyl)-N,N′-diphenyl-1,1,1′-biphenyl-
4,4′-diamine (NPB) is chosen for hole-transporting and
electron-blocking, and tris(8-hydroxyquinolinato) aluminum
(Alq3) is a typical electron-transporting material. 1,3,5-Tris(N-
phenylbenzimidazol-2-yl)benzene (TPBi) is used as the hole-
blocking layer, in order to eliminate the visible emission from
Alq3. LiF on the Al cathode and MoO3 on the ITO anode are
used to aid the electron injection and hole injection,
respectively. According to the energy diagram of these devices
(Figure 4a), the energy level of the emission layer falls between
those of NPB and TPBi. Since the carriers are expected to be
completely trapped in the emission layer, there should be no
emission from NPB and Alq3, and the device should give
exclusively NIR emission from the emitter. Indeed, a single EL
peak at above 700 nm was observed in all non-doped devices
(Figure 4b). A good match between the EL and PL spectra of
these compounds indicates that the observed EL comes solely
from the emitter layer due to the blocking layers used in
OLEDs. By comparing the device characteristics (Table 2 and
Figure 4), one can see a correlation or trade-off between the
emission wavelength and device performance. The non-doped
device with compound 1a emits the light with a maximal peak
at the shortest wavelength of 706 nm and shows the best
performance among all, with the highest radiance of 2917 mW
Sr−1 m−2 at 14 V and a maximum EQE of 0.89% at a current
density of 100 mA cm−2. In comparison, the device based on
compound 2b gives the EL peak at the longest wavelength of
864 nm with a lower turn-on voltage (4.4 V) but exhibits a
lower EQE (0.20%) at a current density of 100 mA cm−2. The
non-doped devices using compounds 1b and 2a displayed the
EL spectra with a maximum at 749 and 802 nm and the EQE of
0.29% and 0.43% at a current density of 100 mA cm−2,
respectively. The relatively low EQE for the non-doped OLEDs
based on 1b and 2b is mainly due to the low fluorescence
quantum yield in the solid state, in addition to the longer
emission wavelengths, which lead to the lower photon energy.
It is worth noting that the EQE of these devices remained fairly
constant over a range of current density of 100−300 mA cm−2
(Figure 4d). The result implies that the current-induced
fluorescence quenching is effectively suppressed in these
devices which should due to the nonplanar structure in solid
state of TPE group. The stability and very little efficiency roll-
off of these devices as current density increase are suitable for
applications requiring high NIR light radiance under high
current density.
Electrochemical Properties. The electrochemical proper-
ties of compounds 1a,b and 2a,b were investigated by cyclic
voltammetry in 0.1 M solution of (Bu4N)PF6 in dry
dichloromethane under argon atmosphere. As shown in Figure
3, they are electrochemically active and undergo the reversible
redox reaction. With the methoxy substituents in 1b and 2b, an
additional oxidation peak is shown at a lower potential and is
due to the oxidation of the methoxy group. From the onset of
oxidation/reduction potentials, the HOMO, LUMO, and
HOMO−LUMO gap levels of these compounds were
To demonstrate the benefit of using the AIEE fluorophores
in NIR OLEDs, we fabricated the OLEDs using 1a and 2a as a
dopant in the device configuration of ITO/MoO3(8 nm)/
NPB(40 nm)/Alq3:dopant emitter(wt%)(20 nm)/TPBi(10
Figure 3. Cyclic voltammograms of compounds 1a,b and 2a,b in
dichloromethane containing 0.1 M TBAPF6 (scan rate = 50 mV s−1;
concentration = 1 × 10−3 M).
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Chemistry of Materials
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Figure 4. (a) Energy diagram of the devices based on compounds 1a,b and 2a,b (relative to the vacuum energy level). (b) Normalized NIR
electroluminescence spectra of non-doped devices. (c) Voltage−current density−radiance (V−J−R) characteristics of the non-doped devices. (d)
EQE−J characteristics of the non-doped devices.
the visible and NIR light. The visible emission at 535 nm comes
from the host Alq3 as a result of incomplete energy transfer
from the host to the dopant (1a or 2a). As the doping
concentration increased, the visible EL peak diminished and
completely disappeared when the 8% dopant was used. The
voltage−current density−radiance (V−J−R) characteristics of
these doped devices are shown in Figure S3 (Support
Information). In comparison with the non-doped OLEDs, the
EQE of the doped devices based on 1a was significantly lower
by at least 4 times (Figure 6). The EQE decreases when the 1a
doping concentration increased to 8% because the energy
transfer from Alq3 was more inefficient in high doping
concentration and the more nonradiative deactivation from
1a via intramolecular rotation. The observed higher EQE for
Table 2. Electroluminescent Data of Non-Doped OLED
Devices Using Compounds 1a,b and 2a,b
a
b
max
EL
max
Von (V)
λ
(nm)
Rmax (mW Sr−1 m−2
)
η
(%)
ext
1a
1b
2a
2b
5.5
4.6
4.5
4.4
706
2917
1576
1604
1003
0.89(0.81)
0.29(0.27)
0.43(0.41)
0.20(0.20)
749
802
864
a
b
Von is onset voltage obtained at 1 mW Sr−1 m−2. Values in the
parentheses taken at a current density of 300 mA cm−2n
.
nm)/Alq3(30 nm)/LiF(1 nm)/Al(100 nm). The doping
concentration in Alq3 as a host was set at 2%, 4%, and 8%.
Figure 5 shows the EL spectra in visible region of the devices
with different dopant concentration. At a low doping
concentration (2%), the devices based on 1a and 2a emitted
Figure 5. Normalized electroluminescence spectra of doped devices
based on compounds 1a and 2a.
Figure 6. EQE−J characteristics of the doped devices in comparison
with non-doped devices based on compounds 1a and 2a.
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Chemistry of Materials
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR, 13C NMR, and HRMS spectra and TGA traces of
1a,b and 2a,b, absorption and PL spectra of films with the
actual measured intensity, the VJR characteristics of 1a-doped
and 2a-doped devices. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Email: wwjoy@ciac.jl.cn.
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Ma, D. Adv. Mater. 2009, 21, 111.
Notes
The authors declare no competing financial interest.
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̈
ACKNOWLEDGMENTS
■
(34) Zhao, Z.; Chen, S.; Lam, J. W. Y.; Jim, C. K. W.; Chan, C. Y. K.;
Wang, Z.; Lu, P.; Deng, C.; Kwok, H. S.; Ma, Y.; Tang, B. Z. J. Phys.
Chem. C 2010, 114, 7963.
(35) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Chem. Commun. 2009,
4332.
This work was supported by the Natural Science and
Engineering Research Council of Canada and National Natural
Science Foundation of China (21134005, 21074132,
20920102032, and 20834001).
(36) Qin, A.; Lam, J. W. Y.; Tang, L.; Jim, C. K. W.; Zhao, H.; Sun, J.;
Tang, B. Z. Macromolecules 2009, 42, 1421.
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