Construction of efficient solid emitters with conventional and AIE luminogens for blue organic light-emitting diodes

Article abstract of DOI:10.1039/c1jm10221d

9,9-Bis(9-heptyl-3-carbazolyl)fluorenes (BPyBCF, BAnBCF, BTPABCF, and BTPEBCF, where B = Bis, Py = pyrene, C = carbazole, F = fluorene, An = anthracene, TPA = triphenylamine, and TPE = tetraphenylethene) with different chromophoric units at the 2,7-positi

Full text of DOI:10.1039/c1jm10221d

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PAPER
Construction of efficient solid emitters with conventional and AIE luminogens
for blue organic light-emitting diodes†
a
Zujin Zhao, Shuming Chen,b Chunmei Deng,b Jacky W. Y. Lam,b Carrie Y. K. Chan,b Ping Lu,c
*
Zhiming Wang,c Bingbing Hu,d Xiaopeng Chen,d Ping Lu,d Hoi Sing Kwok,b Yuguang Ma,c Huayu Qiua
b
*
and Ben Zhong Tang
Received 15th January 2011, Accepted 5th May 2011
DOI: 10.1039/c1jm10221d
9,9-Bis(9-heptyl-3-carbazolyl)fluorenes (BPyBCF, BAnBCF, BTPABCF, and BTPEBCF, where
B ¼ Bis, Py ¼ pyrene, C ¼ carbazole, F ¼ fluorene, An ¼ anthracene, TPA ¼ triphenylamine, and TPE
¼ tetraphenylethene) with different chromophoric units at the 2,7-positions are synthesized in
moderate to high yields (52–89%) by Suzuki coupling reactions of 9,9-bis(9-heptyl-3-carbazolyl)-2,7-
dibromofluorene with the corresponding arylboronic acid and utilized as active layers for the
construction of blue organic light-emitting diodes (OLEDs). BPyBCF, BAnBCF and BTPABCF emit
intense blue light with high fluorescence quantum yields (FF ¼ 75–94%) in solution. However, they
exhibit much lower FF values (30–61%) in the film state, revealing that aggregate formation has
quenched their light emission. On the contrary, BTPEBCF is weakly emissive in solution (FF ¼ 0.3%)
but becomes a strong emitter (FF ¼ 100%) when fabricated into solid film, demonstrating
a phenomenon of aggregation-induced emission (AIE). Restriction of intramolecular rotation and
suppression of intermolecular interactions due to the propeller-like tetraphenylethene unit are
responsible for the AIE phenomenon. All theꢀluminogens are thermally and morphologically stable,
showing high glass-transition (Tg ¼ 109–147 C) and thermal-degradation temperatures
ꢀ
(Td ¼ 396–478 C). Non-doped OLEDs using BPyBCF, BAnBCF, and BTPABCF as light-emitting
layers are constructed, which give blue electroluminescence with maximum current (hC,max) and
external quantum (hext,max) efficiencies of 4.8 cd Aꢁ1 and 2.3%. With the same device configuration,
BTPEBCF shows higher hC,max and hext,max values of 7.9 cd Aꢁ1 and 2.9%, respectively, thanks to its
AIE feature.
and pure chromaticity are needed. Thanks to enthusiastic efforts
Introduction
of the scientists, many green and red emitters have been
successfully commercialized. However, high-efficiency blue EL
materials are still rare1 because their intrinsic wide energy band
gaps generally make their electronic levels mismatched with
those of hole- and electron-transporting layers. The balance
between the holes and electrons in the EL device is thus difficult
to achieve, leading to inferior device performance. Another
problem is the aggregation-caused quenching (ACQ) effect: the
emission of conventional luminophors is often weakened in the
solid state in comparison to in solution, due to aggregate
formation in the condensed phase.2 The ACQ problem must be
properly tackled because the luminophors are commonly used as
solid films in their practical applications. Various approaches
have been taken to disrupt the luminophor aggregation.3 The
attachment of bulky alicyclics, encapsulation by amphiphilic
surfactants, and blending with transparent polymers are widely
used methods to impede aggregate formation. All the attempts
have, however, met with only limited success and in most cases,
bring severe side effects.
Much research effort has been devoted to the design and
synthesis of luminogenic materials due to their potential appli-
cations in optics and electronics such as organic light-emitting
diodes (OLEDs). To realize full color displays or white lighting,
blue, green, and red emitters with high photoluminescence (PL)
and electroluminescence (EL) efficiencies, good thermal stability
aCollege of Materials, Chemistry and Chemical Engineering, Hangzhou
Normal University, Hangzhou, 310036, China. E-mail: zujinzhao@gmail.
com
bDepartment of Chemistry and Center for Display Research, The Hong
Kong University of Science & Technology, Clear Water Bay, Kowloon,
Hong Kong, China. E-mail: tangbenz@ust.hk
cState Key Laboratory of Supramolecular Structure and Materials, Jilin
University, Changchun, 130012, China
dDepartment of Chemistry, Zhejiang University, Hangzhou, 310027, China
† Electronic supplementary information (ESI) available: Optimized
molecular structure, molecular orbital amplitude plots of HOMO and
LUMO levels and absorption and PL spectra of compound 3, and
NMR spectra of the BArBCF luminogens. See DOI: 10.1039/c1jm10221d
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We have observed a phenomenon of aggregation-induced
emission (AIE): a series of propeller-like molecules such as
silole4,5 and tetraphenylethene (TPE)6 are non-emissive in solu-
tion but emit intensely in the aggregate state. Through a series of
designed experiments and theoretical calculations, we rational-
ized the restriction of intramolecular rotation (IMR) in the
aggregate state as the main cause for the AIE effect. The AIE
luminogens have found an array of high-technological applica-
tions such as fluorescent sensors, biological probes, active layers
for OLEDs, etc.7–9 Recently, a new approach to mitigate the
ACQ effect was developed. By decorating typical ACQ lumino-
phors, such as triphenylamine (TPA) and pyrene, with AIE-
active TPE units, molecules with efficient solid-state emission are
generated.10
To further demonstrate the versatility of such a strategy and
create efficient blue luminophors, in this work, we designed
a series of fluorene derivatives (BArBCFs) with molecular
structures as illustrated in Scheme 1. Two carbazole rings are
attached to the fluorene core as substituents at the 9,9-positions
to endow the resultant molecule with not only good hole-trans-
porting capability11 but also high thermal stability.12 Its three-
dimensional (3D) conformation also helps to hamper the close
packing between adjacent molecules, preventing the formation of
species that are detrimental to the light emission.13 Different
kinds of ACQ and AIE chromophoric units including pyrene,
anthracene, TPA, and TPE are introduced at the 2,7-positions to
further suppress the intermolecular interactions and meanwhile
increase the molecular conjugation. The optical properties of the
BArBCF luminogens and their application as active layers for
OLEDs are investigated, the results of which demonstrate that
molecularly combining ACQ and AIE units is a general and
effective approach for solving the thorny ACQ problem and
generating efficient solid-state emitters.
Scheme 2 Synthetic routes to the BArBCF luminogens.
method.14 Suzuki coupling reactions of 3 with the corresponding
arylboronic acids10 catalyzed by Pd(PPh3)4 in basic medium gave
the target products in moderate to high yields (52%–89%). The
structure and purity of the BArBCF luminogens were charac-
terized by standard spectroscopic methods and elemental anal-
ysis with satisfactory results (see Experimental section and
Fig. S1–S8, ESI†). They are soluble in common organic solvents,
such as THF, toluene, chloroform, and dichloromethane, but
insoluble in methanol and water.
Optical properties
Fig. 1A shows the absorption spectra of BArBCF luminogens in
the dilute THF solutions. Whereas intermediate 3 absorbs at
352 nm (Fig. S9, ESI†), the BArBCF luminogens exhibit an
absorption maximum in the range from 366 to 389 nm, indicative
of their higher conjugation. The spectral pattern and the peak
absorptivity vary with the aromatic substituents at the 2,7-
positions. This is due to their different electronic communica-
tions with the central fluorene core, which change the electronic
structures and hence the conjugation of the luminophors.
BPyBCF, BAnBCF, and BTPABCF are highly emissive when
molecularly dissolved in solution. They emit intense deep blue PL
at 420, 418, and 423 nm, respectively, in diluted THF solutions
(10 mM), showing a large red-shift compared with 3 (361 nm)
(Fig. S9, ESI†). The fluorescence quantum yields (FF’s) esti-
mated using 9,10-diphenylanthracene (FF ¼ 90% in cyclohexane)
as standard are in the range of 75–94% (Table 1), indicating that
they are efficient blue emitters in the solution state. However, the
emission of 3 is quenched when its bromine atoms at the 2,7-
positions are substituted by AIE-active TPE units. As shown in
Fig. 1B, the PL spectrum of BTPEBCF in dilute THF solution is
virtually a flat line parallel to the abscissa. The FF value is as low
as 0.3%, revealing that BTPEBCF is a weak emitter when
existing as an isolated species in good solvents. The structural
difference between BTPEBCF and other luminogens is that
BTPEBCF possesses rotatable AIE units. Evidently, the active
IMR process of the multiple phenyl rings of the TPE units has
drastically deactivated the excited state of BTPEBCF, thus
rendering it weakly luminescent in the solution state.10,11,15
While almost non-emissive in solution, BTPEBCF become
a strong emitter in the aggregate state. Fig. 2A shows the PL
Results and discussion
Synthesis
We designed the molecular structures of a series of fluorene
derivatives and elaborated a multi-step reaction route for their
synthesis (Scheme 2). Compound 3, named 9,9-bis(9-heptyl-3-
carbazolyl)fluorene, was prepared from 2,7-dibromo-9-fluo-
renone (1) and 9-heptylcarbazole (2) according to the literature
Scheme 1 Molecular structures of the BArBCF luminogens.
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Fig. 1 (A) Absorption and (B) PL spectra of THF solutions (10 mM) of BArBCF luminogens. (C) PL spectra of thin films of BArBCF luminogens. Inset
in (B): photographs of BTPABCF and BTPEBCF in THF solutions (10 mM) taken under 365 nm UV illumination.
spectra of BTPEBCF in THF/water mixtures with different
water fractions (fw). The PL intensity remains unchanged at low
fw values (#50%) but starts to increase afterwards (Fig. 2B). At
a fw value of 99.5%, intense blue light is recorded at 484 nm, the
intensity of which is ꢂ300-fold higher than that in pure THF.
Since BTPEBCF is insoluble in water, addition of a large amount
of water into its THF solution has caused its molecules to
aggregate. Clearly, the PL of BTPEBCF is induced by aggregate
formation and it, like TPE, is AIE-active.
The PL properties of the BArBCF luminogens are further
studied in the solid state because luminogenic materials are
fabricated as thin films in optical and electronic devices. The thin
films of BPyBCF, BAnBCF, and BTPABCF show strong blue
light with PL peaks at 449, 443, and 446 nm, respectively
(Fig. 1C). The associated FF values measured by a calibrated
integrating sphere are 61, 30, and 32%, respectively, which are
much lower than those in solution (Table 1). The redder emission
and lower FF values of BPyBCF, BAnBCF, and BTPABCF in
the solid state suggest that they still suffer the ACQ problem even
when bulky aromatic rings are attached to their fluorene core.
On the contrary, the thin film of BTPEBCF exhibits intense
sky blue light at 487 nm, which is slightly redder than the PL of
the aggregates in a poor solvent. The FF value measured under
the same conditions is 100%, which is 1.6–3.3-fold higher than
those of BPyBCF, BAnBCF, and BTPABCF. The film emission
of BTPEBCF is also more efficient than TPE (FF ¼ 49.2%).9d
Apparently, the attachment of TPE units to 3 has endowed
BTPEBCF with not only AIE features but also efficient
Fig. 2 (A) PL spectra of BTPEBCF in THF/water mixtures with
different water fraction (fw). (B) Plots of (I/I0 ꢁ 1) values versus water
fractions in THF/water mixtures, where I0 is the PL intensity in pure THF
solution. Inset: photos of BTPEBCF in THF/water mixtures (fw ¼ 0 and
99.5%) taken under the illumination of a UV lamp.
solid-state emission. In the solid state, the IMR process is
restricted, which blocks the nonradiative pathway and populates
the radiative decay. On the other hand, the propeller-like TPE
units have hampered the close packing of the BTPEBCF mole-
cules that tends to induce nonradiative energy decay and red-
shift in the PL spectrum as observed in conventional lumino-
phors with strong p–p interactions. Coupled with the electronic
interactions between the TPE units and the central fluorene core,
all these collectively boost the FF value of BTPEBCF to unity in
the condensed phase.
Table 1 Optical, electronic, and thermal properties of the BArBCF luminogensa
lem (nm)
FF (%)
lab (nm)
Solnb
Solnb
Film
Soln
Film
Eg (eV)
HOMO (eV)
LUMO (eV)
Tg/Td (ꢀC)
BPyBCF
368
389
372
366
420
418
423
—
449
443
446
487
80
75
94
0.3
61
30
32
100
3.05
3.02
3.03
3.06
ꢁ5.21
ꢁ5.12
ꢁ5.18
ꢁ5.27
ꢁ2.16
ꢁ2.09
ꢁ2.15
ꢁ2.21
137/449
138/478
109/396
147/474
BAnBCF
BTPABCF
BTPEBCF
a
Abbreviation: lab ¼ absorption maximum, lem ¼ emission maximum, FF ¼ fluorescence quantum yield in THF solution (soln) determined using 9,10-
diphenylanthracene (FF ¼ 90% in cyclohexane) as standard or in the film state measured by a calibrated integrating sphere, Eg ¼ energy gap calculated
from the onset absorption wavelength, HOMO ¼ highest occupied molecular orbital determined from cyclic voltammetry, LUMO ¼ lowest unoccupied
molecular orbital, Tg ¼ glass-transition temperature, Td ¼ temperature for 5% weight loss. b 10 mM.
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Thermal stability
The thermal properties of BArBCF luminogens are examined by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) measurements. As depicted in Fig. 3, BPyBCF,
BAnBCF, BTPABCF and BTPEBCF are thermally stable, with
5% weight loss being observed at 449, 478, 396, and 474 ꢀC,
respectively. They also enjoy high morphological stability, as
revealed by their high glass-transition temperatures
(109–147 ꢀC). The high thermal and morphological stabilities of
BArBCF luminogens are helpful to increase the stability and
lifetime of their optoelectronic devices.
Electrochemical properties
The electrochemical properties of the BArBCF luminogens are
investigated by cyclic voltammetry (CV). All the molecules
display similar CV curves (Fig. 4). Their oxidation onset poten-
tials (Eonset) occur at 0.72–0.87 V, from which the HOMO energy
levels are calculated to fall in the range from ꢁ5.12 to ꢁ5.27 eV
by the equation: HOMO ¼ ꢁ(4.4 + Eonset). The HOMO of 4,4-
bis(1-naphthylphenylamino)biphenyl (NPB) is reported to be
located at ꢁ5.3 eV. The higher HOMO energy levels of the
present molecules suggest that they possess good hole-trans-
porting properties, presumably due to the fine contribution from
the carbazole units.16 Their LUMOs can be obtained by
subtraction of the optical band gap energies (estimated from the
onset absorption wavelengths) from the HOMO values and are
located from ꢁ2.09 to ꢁ2.21 eV.
Fig. 4 Cyclic voltammograms of the BArBCF luminogens measured in
dichloromethane containing 0.1
M tetra-n-butylammonium hexa-
fluorophosphate. Scan rate: 100 mV sꢁ1
.
carbazole substituents at the 9,9-positions. In BPyBCF and
BAnBCF, the pyrene and anthracene rings are twisted out of the
fluorene plane due to the severe steric hindrance between the two
chromophoric units, which lowers their electronic communica-
tion. The conjugation between the TPA and the TPE moieties
with the fluorene core in BTPABCF and BTPEBCF is much
better due to their comparatively smaller sizes. Whereas the
HOMO and LUMO of intermediate 3 are dominated by the
orbitals from the carbazole and fluorene rings, respectively
(Fig. S10, ESI†), the orbitals from the fluorene core and the
aromatic substituents at the 2,7-positions contribute mainly to
both energy levels of the BArBCF luminogens. These results
Electronic structure
Density functional theory calculations of the BArBCF lumi-
nogens are carried out using a suite of Gaussian 03 program in
order to understand their properties at the molecular level. The
nonlocal density functional of B3LYP with 6-31G(d) basis sets
was used for the calculation. The optimized structures and the
orbital distributions of HOMO and LUMO energy levels of
BPyBCF, BAnBCF, BTPABCF, and BTPABCF are shown in
Fig. 5. All the luminogens possess a 3D conformation due to the
Fig. 3 TGA thermograms of BArBCF luminogens recorded under
nitrogen at a heating rate of 10 ꢀC minꢁ1. Inset: DSC curves recorded
during the second heating scan.
Fig. 5 Optimized molecular structures and molecular orbital amplitude
plots of HOMO and LUMO levels of the BArBCF luminogens calculated
using the B3LYP/6-31G(d) basis set.
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Table 2 EL properties of the BArBCF luminogensa
device
lEL (CIE) (nm)
Von (V)
Lmax (cd mꢁ2
)
hP, max (lm Wꢁ1
)
hC,max (cd Aꢁ1
)
hext,max (%)
BPyBCF
BAnBCF
BTPABCF
I
I
I
II
I
468 (0.17, 0.19)
472 (0.19, 0.24)
444 (0.16, 0.13)
440 (0.15, 0.10)
508 (0.24, 0.42)
502 (0.21, 0.37)
4.2
4.4
4.4
3.6
5.5
4.5
10 160
8170
4780
7600
13 760
6400
2.5
1.1
0.8
2.4
2.8
3.7
4.8
2.2
1.5
2.8
7.2
7.9
2.3
1.0
1.2
2.2
2.6
2.9
BTPEBCF
II
a
Device configuration: ITO/NPB (60 nm)/BArBCF (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) (device I) and ITO/NPB (60 nm)/BPyBCF or
BAnBCF (20 nm)/TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) (device II). Abbreviation: lEL ¼ electroluminescence maximum,
Von ¼ turn-on voltage at 1 cd mꢁ2, Lmax ¼ maximum luminance, hP,max ¼ maximum power efficiency, hC,max ¼ maximum current efficiency, and
hext,max ¼ maximum external quantum efficiency.
reveal that the optical properties of the luminogens are mainly
controlled by the fluorene core and the 2,7-peripheries and vary
by changing the aromatic substituents.
7.2 cd Aꢁ1, and a hext,max of 2.6%. The good EL properties of
BTPEBCF are clearly related to its AIE feature, which enables it
to show efficient PL in the solid state and hence higher device
performance.
We further investigated the EL properties of the BArBCF
luminogens by fabricating two more OLEDs with a configura-
tion of ITO/NPB (60 nm)/BTPABCF or BTPEBCF (20 nm)/
TPBi (10 nm)/Alq3 (30 nm)/LiF (1 nm)/Al (100 nm) (device II).
The introduction of an additional electron-transporting layer
of tris(8-hydroxyquinolinolato)aluminum (Alq3) aims to
balance the electrons and holes in the EL devices because the
carbazole unit in BTPABCF and BTPEBCF is a well-known
hole-transporting material. As shown in Fig. 7 and summarized
in Table 2, device II exhibits a higher performance than device
I, which is in some sense expected. The EL device of
BTPABCF is turned on at a lower voltage of 3.6 V, emitting
a deep blue light at 440 nm (CIE ¼ 0.15, 0.10) with a higher
Lmax of 7600 cd mꢁ2. The hP,max,hC,max, and hext,max values
(2.4 lm Wꢁ1, 2.8 cd Aꢁ1, and 2.2%, respectively) also become
much higher because better recombination of electrons and
holes has been achieved due to the improved balance between
the carriers. Similarly, the device performance of BTPEBCF is
enhanced by further engineering endeavor. Comparison of the
EL properties of BTPABCF with BTPEBCF reveals that the
TPA moieties in BTPABCF possess the advantage of higher
hole-transporting capability than the TPE units in BTPEBCF,
as evidenced by the much higher current density and lower
turn-on voltage in the EL device of BTPABCF. However,
BTPABCF shows a much lower EL efficiency owing to its
ACQ feature.
Electroluminescence
The high solid PL and good thermal and morphological stabili-
ties of the present molecules encouraged us to investigate their
EL properties. We fabricated multilayer EL devices with
a configuration of ITO/NPB (60 nm)/BArBCF (20 nm)/TPBi
(40 nm)/LiF (1 nm)/Al (100 nm) (device I) by vapor deposition
processes, in which BArBCF serves as emitter, NPB functions as
hole-transporting layer, 2,20,200-(1,3,5-benzinetriyl)tris(1-phenyl-
1-H-benzimidazole) (TPBi) works as both a hole-blocking and
electron-transporting material, respectively. Fig. 6 shows their
EL spectra and device performances. Both BPyBCF and
BAnBCF exhibit blue EL at 468 and 472 nm with Commission
International d’Eclairage (CIE) chromaticity coordinates of
(0.17, 0.19), and (0.19, 0.24), respectively (Table 2). They start to
emit at a similar voltage (4.2 V for BPyBCF and 4.4 V for
BAnBCF). However, BPyBCF performs better as an EL emitter
than BAnBCF. Its EL device radiates more brillantly with
maximum luminance (Lmax) of 10 160 cd mꢁ2. The maximum
power (hP,max), current (hC,max), and external quantum (hext,max
)
efficiencies are 2.5 lm Wꢁ1, 4.8 cd Aꢁ1, and 2.3%, respectively,
which are all more than 2-fold higher than the values achieved by
BAnBCF. BTPABCF exhibits deep blue EL at 444 nm
(CIE ¼ 0.16, 0.13) with lower efficiency (Table 2). On the
contrary, BTPEBCF emits brightly at 508 nm (CIE ¼ 0.24, 0.42)
with a Lmax of 13 760 cd mꢁ2, a hP,max of 2.8 lm Wꢁ1, a hC,max of
Fig. 6 (A) EL spectra and (B) change in luminance and current density with the applied bias. (C) Plot of current efficiency versus current density in
multilayer EL devices of the BArBCF luminogens with a configuration of ITO/NPB/emitter/TPBi/LiF/Al.
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Fig. 7 (A) EL spectra and (B) change in luminance and current density with the applied bias. (C) Plot of current efficiency versus current density in
multilayer EL devices of BTPABCF and BTPEBCF with a configuration of ITO/NPB/emitter/TPBi/Alq3/LiF/Al.
Internal quantum efficiency (hint) for a fluorescent OLED can
injection efficiences and probabilities for carrier recombination.
Consequently, the PL efficiency of their thin films would play
a key role in determining their EL properties. The order of their
EL efficiency is BTPEBCF > BPyBCF > BAnBCF and
BTPABCF, which is consistent with the ranking of their PL
efficiency. The introduction of an additional electron-trans-
porting layer in device II has improved the balance between the
holes and the electrons. The generation of excitons becomes
more efficient, thus resulting in higher EL efficiency and
enhanced device performance.
be determined by eqn (1)
1
2n2
hext ¼ bhint
¼
FFgELhr
(1)
where FF ¼ fluorescent quantum yield of the emitter, gEL
¼
electro-excitation efficiency defined by the ratio of the number of
exciton formation to the number of electrons flowing in the
external circuit or the probability for carrier recombination to
form excitons, and hr ¼ efficiency or probability for the forma-
tion of singlet excitons defined by the ratio of the number of
singlet excitons to the total number of excitons. hr is sometimes
also known as the singlet to triplet branching ratio. On the other
hand, the hext value is the product of hint multiplied by the light
output coupling factor (b) as shown by eqn (2)
Conclusions
In summary, a group of fluorene derivatives with different 2,7-
substituents and carbazole units at the 9,9-positions are synthe-
sized and utilized as emitters for the construction of blue OLEDs.
The attachment of different types of chromophoric units at the
2,7-positions of the fluorene core results in luminogens with
opposite emission behaviors. BPyBCF, BAnBCF, and
BTPABCF containing conventional chromophors show strong
blue PL in solution with high FF values but much weaker
emission when fabricated into thin film, revealing that they suffer
the ACQ problem in the aggregate state. On the contrary,
BTPEBCF is weakly emissive in solution, but emit intensely with
a FF value of 100% in the aggregate state, thanks to the AIE
feature of the TPE unit. Multilayer OLEDs using the present
molecules as emitters are constructed, which give deep blue to
sky blue EL (Fig. 8) with hP,max, hC,max, and hext,max of
3.7 lm Wꢁ1, 7.9 cd Aꢁ1, and 2.9%, respectively.
hint ¼ FFgELhr
(2)
Since n (refractive index) is a constant for a given material and hr
is also a constant (25%) according to the spin statistics, eqn (2)
can be rewritten to eqn (3)
hext ¼ cFFgEL
(3)
where c is a constant and is equal to hr/2n2. In other words, hext
depends on the PL efficiency and the probability for carrier
recombination that leads to exciton formation.
The BArBCF luminogens possess similar molecular structures
and energy levels, and thus should exhibit analogical carrier
Experimental
General information
THF was distilled from sodium benzophenone ketyl under dry
nitrogen immediately prior to use. All other chemicals and
reagents were purchased from Aldrich and used as received
without further purification. 1H and 13C NMR spectra were
measured on a Bruker AV 300 spectrometer in deuterated chlo-
roform using tetramethylsilane (TMS; d ¼ 0) as internal refer-
ence. UV spectra were measured on a Milton Roy Spectronic
3000 Array spectrophotometer. PL spectra were recorded on
a Perkin-Elmer LS 55 spectrofluorometer. The high resolution
mass spectra (HRMS) were recorded on a GCT premier CAB048
mass spectrometer operated in MALDI-TOF mode.
Fig. 8 CIE diagram for the EL emission of the BArBCF luminogens.
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Thermogravimetric analysis (TGA) was carried on a TA TGA
Q5000 under dry nitrogen at a heating rate of 10 ꢀC minꢁ1
2,7-Bis(1-pyrenyl)-9,9-bis(9-heptyl-3-carbazolyl)fluorene
(BPyBCF). A mixture of 314 (0.85 g, 1 mmol), pyrenylboronic
acid (0.59 g, 2.4 mmol), Pd(PPh3)4 (0.11g, 0.1 mmol), and
potassium carbonate (1.1 g, 8 mmol) in 150 mL of toluene/
ethanol/water (8/1/1 v/v/v) was heated to reflux for 12 h under
nitrogen. After filtration and solvent evaporation, the residue
was purified by silica-gel column chromatography using hexane/
dichloromethane as eluent. Pale yellow solid of BPyBCF was
.
Thermal transitions were investigated by differential scanning
calorimetry (DSC) using a TA DSC Q1000 under dry nitrogen at
a heating rate of 10 ꢀC minꢁ1. The ground-state geometries were
optimized using density functional theory with B3LYP hybrid
functional at the basis set level of 6-31G(d). All the calculations
were performed using Gaussian 03 package. Cyclic voltammetry
was performed on a CHI660A electrochemical work station at
room temperature with use of a standard three-electrode elec-
trochemical cell in dichloromethane containing 0.1 M tetra-n-
butylammonium hexafluorophosphate. The working and refer-
ence electrodes were platinum and Ag/AgCl. The reference
electrode was checked versus ferrocene as internal standard as
recommended by IUPAC. All the solutions were deaerated by
bubbling nitrogen gas for a few minutes prior to the electro-
chemical measurements.
1
obtained in 85% yield (0.93 g). H NMR (300 MHz, CDCl3),
d (TMS, ppm): 8.32–7.95 (m, 24H), 7.83 (d, 2H, J ¼ 9.3 Hz), 7.74
(d, 2H, J ¼ 8.1 Hz), 7.59 (d, 2H, J ¼ 8.7 Hz), 7.43–7.24 (m, 6H),
7.15–7.10 (m, 2H), 4.22 (t, 4H, J ¼ 7.2 Hz), 1.84–1.79 (m, 4H),
1.31–1.19 (m, 16H), 0.81–0.76 (m, 6H). 13C NMR (75 MHz,
CDCl3), d (TMS, ppm): 153.8, 141.5, 141.2, 139.7, 138.7, 137.8,
132.1, 131.6, 131.2, 129.2, 128.4, 128.2, 128.1, 128.0, 126.6, 126.0,
125.7, 125.4, 125.3, 123.2, 121.1, 120.3, 109.3, 66.6, 43.9, 32.4,
29.7, 28.0, 23.2, 14.7. HRMS: m/z 1092.5374 (M+, calcd
1092.5383). Anal. Calcd for C83H68N2: C, 91.17; H, 6.27; N, 2.56.
Found: C, 91.19; H, 6.22; N, 2.41.
Device fabrication
The devices were fabricated on 80 nm-ITO coated glass with
a sheet resistance of 25U ,ꢁ1. Prior to loading into the
pretreatment chamber, the ITO-coated glasses were 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 ꢃ 10ꢁ7 Torr for the
deposition of NPB, emitter, TPBi, and Alq3. The samples were
then transferred to the metal chamber for cathode deposition
which was 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 EL spectra were
obtained by a PR650 spectrophotometer. All the measurements
were carried out under air at room temperature without device
encapsulation.
2,7-Bis(9-anthryl)-9,9-bis(9-heptyl-3-carbazolyl)fluorene
1
(BAnBCF). Pale yellow solid; yield 52%. H NMR (300 MHz,
CDCl3), d (TMS, ppm): 8.45 (s, 2H), 8.16 (d, 2H, J ¼ 7.8 Hz),
8.10 (d, 2H, J ¼ 1.5 Hz), 8.00 (d, 6H, J ¼ 9.0 Hz), 7.91 (d, 4H, J ¼
9.0 Hz), 7.77 (s, 2H), 7.58–7.52 (m, 4H), 7.43–7.14 (m, 16H), 4.18
(t, 4H, J ¼ 7.2 Hz), 1.81–1.76 (m, 4H), 1.30–1.20 (m, 16H), 0.82–
0.78 (m, 6H). 13C NMR (75 MHz, CDCl3), d (TMS, ppm): 154.0,
141.4, 140.0, 138.7, 138.0, 137.6, 132.1, 131.5, 130.9, 130.5, 129.0,
127.8, 127.6, 127.2, 126.1, 125.7, 123.5, 122.9, 120.9, 119.9, 119.2,
109.3, 66.5, 43.8, 32.4, 29.7, 27.9, 23.3, 14.7. HRMS: m/z
1044.5360 (M+, calcd 1044.5383). Anal. Calcd for C79H68N2: C,
90.76; H, 6.56; N, 2.68. Found: C, 90.79; H, 6.61; N, 2.54.
2,7-Bis[4-(diphenylamino)phenyl]-9,9-bis(9-heptyl-3-carbazolyl)-
fluorene (BTPABCF). Pale yellow solid; yield 76%. 1H NMR (300
MHz, CDCl3), d (TMS, ppm): 8.06 (d, 2H, J ¼ 2.1 Hz), 7.91–7.85
(m, 4H), 7.75 (d, 2H, J ¼ 1.5 Hz), 7.62–7.59 (m, 2H), 7.49–7.35
(m, 10H), 7.28–7.04 (m, 24H), 7.01–6.96 (m, 4H), 4.23 (t, 4H,
J ¼ 7.2 Hz), 1.86–1.81 (m, 4H), 1.34–1.23 (m, 16H), 0.86–0.81 (m,
6H). 13C NMR (75 MHz, CDCl3), d (TMS, ppm): 154.4, 148.3,
147.7, 141.4, 140.7, 140.0, 139.3, 137.7, 136.2, 129.9, 128.5, 127.3,
126.7, 126.1, 125.3, 125.0, 124.6, 123.5, 123.2, 121.1, 120.5, 119.2,
109.2, 66.5, 43.9, 32.4, 29.7, 28.0, 23.3, 14.7. HRMS: m/z
1178.6299 (M+, calcd 1178.6226). Anal. Calcd for C87H78N4: C,
88.59; H, 6.67; N, 4.75. Found: C, 88.65; H, 6.69; N, 4.57.
Preparation of nanoaggregates
Stock THF solutions of the compounds with a concentration of
10ꢁ4 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 10ꢁ5 M solutions with different water
contents (0–99.5 vol %). The PL measurements of the resultant
solutions were then performed immediately.
2,7-Bis[4-(1,2,2-triphenylvinyl)phenyl]-9,9-bis(9-heptyl-3-car-
1
bazolyl)fluorene (BTPEBCF). White solid; yield 89%. H NMR
(300 MHz, CD2Cl2), d (TMS, ppm): 8.02 (d, 2H, J ¼ 1.2 Hz),
7.89–7.85 (m, 4H), 7.73 (s, 2H), 7.61 (d, 2H, J ¼ 8.1 Hz), 7.46–
7.29 (m, 14H), 7.11–6.98 (m, 34H), 4.24 (t, 4H, J ¼ 7.1 Hz), 1.85–
1.80 (m, 4H), 1.34–1.24 (m, 16H), 0.86–0.81 (m, 6H). 13C NMR
(75 MHz, CD2Cl2), d (TMS, ppm): 154.6, 144.6, 144.5, 143.5,
141.8, 141.5, 141.3, 140.8, 140.1, 139.7, 137.5, 132.4, 132.0, 131.9,
128.5, 128.4, 128.3, 127.2, 127.1, 127.0, 126.9, 126.2, 125.3, 123.4,
123.2, 121.3, 121.0, 120.3, 119.2, 109.4, 66.6, 43.9, 32.5, 29.8,
29.7, 28.0, 23.3, 14.6. HRMS: m/z 1234.5381 (M+, calcd
1234.5307). Anal. Calcd for C103H88N2: C, 91.38; H, 6.55; N,
Synthesis
The fluorene derivatives were prepared according to Scheme 2.
The procedure for the synthesis of BPyBCF is given below as
example.
ꢀ
2.07. Found: C, 91.34; H, 6.62; N, 2.01. m.p.: 225 C.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10949–10956 | 10955

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J. W. Y. Lam, Y. Hong, F. Mahtab, W. Yuan and B. Z. Tang,
€
Acknowledgements
Chem.–Eur. J., 2010, 16, 8433; (c) Y. Hong, M. Haußler,
J. W. Y. Lam, Z. Li, K. K. Sin, Y. Dong, H. Tong, J. Liu,
A. Qin, R. Renneberg and B. Z. Tang, Chem.–Eur. J., 2008, 14, 6428.
8 (a) M. Wang, G. Zhang, D. Zhang, D. Zhu and B. Z. Tang, J. Mater.
Chem., 2010, 20, 1858; (b) M. Wang, X. G. Gu, G. X. Zhang,
D. Q. Zhang and D. Zhu, Anal. Chem., 2009, 81, 4444; (c) T. Sanji,
K. Shiraishi, M. Nakamura and M. Tanaka, Chem.–Asian J., 2010,
5, 817.
We thank the financial support from the Research Grants
Council of Hong Kong (603509, HKUST13/CRF/08, and
HKUST2/CRF/10), the Innovation and Technology Commis-
sion (ITP/008/09NP), the University Grants Committee of Hong
Kong (AoE/P-03/08), the Initial Funding of Hangzhou Normal
University (HSQK0085), and the National Natural Science
Foundation of China (21074028).
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10956 | J. Mater. Chem., 2011, 21, 10949–10956
This journal is ª The Royal Society of Chemistry 2011