Efficient tuning of electroluminescence from sky-blue to deep-blue by changing the constitution of spirobenzofluorene derivatives

Article abstract of DOI:10.1016/j.dyepig.2014.04.019

Two novel benzimidazole-attached spiro[benzofluorene] derivatives, 2,2′-(spiro[benzo[c]fluorine-7,9′-fluorene]-5,9-diylbis(4, 1-phenylene))bis(1-phenyl-1H-benzo[d]imidazole) and 2,2′-(spiro[benzo-[de] anthracene-7,9′-fluorene]-2′,3-diylbis(4,1-phenylene)) bis(1-phenyl-1H-benzo[d]imidazole), were prepared by a Suzuki coupling reaction. Their photophysical and photochemical properties were studied systemically. The fluorescent organic light-emitting diodes were fabricated by using them as the emitters, all of them showed strong blue emission. Interestingly, from the benzoanthracene derived compound a high color purity was found with Commission de L'Eclairage 1931 chromaticity coordinates of (0.15, 0.10) and an efficiency of 1.96 cd/A. To the best of our knowledge, this is the first time to obtain a deep-blue emission with spiro[benzofluorene] derivative in a nondoped device.

Full text of DOI:10.1016/j.dyepig.2014.04.019

Dyes and Pigments 108 (2014) 57e63
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Efficient tuning of electroluminescence from sky-blue to deep-blue by
changing the constitution of spirobenzofluorene derivatives
,
,
,
a,
Houjie Liang a c, Xinxin Wang b, Xingye Zhang a c, Ziyi Ge a , Xinhua Ouyang
,
*
*
Suidong Wang b
,
*
a Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, PR China
b Institute of Functional Nano & Soft Materials, Soochow University, Suzhou 215123, PR China
c University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Two novel benzimidazole-attached spiro[benzofluorene] derivatives, 2,20-(spiro[benzo[c]fluorine-7,90-
fluorene]-5,9-diylbis(4,1-phenylene))bis(1-phenyl-1H-benzo[d]imidazole) and 2,20-(spiro[benzo-[de]
anthracene-7,90-fluorene]-20,3-diylbis(4,1-phenylene))bis(1-phenyl-1H-benzo[d]imidazole), were pre-
pared by a Suzuki coupling reaction. Their photophysical and photochemical properties were studied
systemically. The fluorescent organic light-emitting diodes were fabricated by using them as the emit-
ters, all of them showed strong blue emission. Interestingly, from the benzoanthracene derived com-
pound a high color purity was found with Commission de L’Eclairage 1931 chromaticity coordinates of
(0.15, 0.10) and an efficiency of 1.96 cd/A. To the best of our knowledge, this is the first time to obtain a
deep-blue emission with spiro[benzofluorene] derivative in a nondoped device.
Article history:
Received 9 December 2013
Received in revised form
8 April 2014
Accepted 15 April 2014
Available online 24 April 2014
Keywords:
Spiro[benzofluorene-fluorene]
Spiro[benzoanthracene-fluorene]
Constitution
Ó 2014 Elsevier Ltd. All rights reserved.
Blue OLED
Color purity
Nondoped system
1. Introduction
spirobenzofluorenes containing naphthalene groups into OLEDs,
the results indicated good stabilities, high carrier mobilities and
Blue organic light-emitting diodes (OLEDs) have been attracting
great scientific and commercial attention for their potential appli-
cations in both full-color display and solid-state lighting [1e4].
Much significant progress has been demonstrated and the external
quantum efficiencies (EQE) over 20% have been accessible for
phosphorescent OLEDs (PhOLEDs) and more than 10% for fluores-
cence. However, it is still particularly difficult to generate a deep-
blue emission with Commission Internationale de I’Eclairage (CIE)
coordinates in the range of (CIEx < 0.15, CIEy < 0.15) due to the
intrinsic wide band gap nature of deep-blue emitters [5e7].
Although great effort has been made to develop deep-blue fluo-
rescent emitters in the past decade, there is still a clear need to
improve the color purity and stability of deep-blue OLEDs.
high fluorescence quantum yields [8]. Subsequently, Huang et al.
reported series of novel spirobenzofluorene/pyrene derivatives as
dopant materials and obtained deep-blue emission [9]. Recently,
Gong et al. demonstrated one of spirobenzofluorene dimers as host
materials and the maximum efficiency was up to 7.44 cd Aꢀ1 with
deep-blue emission [10]. However, the reported results based on
spirofluorene derivatives showed that most examples were used as
host or dopant materials. It is well-known that the electrolumi-
nescent properties are extremely sensitive to the dopant concen-
tration and very difficult to control by using co-deposition methods
[11e13]. Furthermore, the energy transformation may be ineffec-
tive because of the potential phase separation in host-dopant sys-
tem [14]. Therefore, to develop the nondoped, highly efficient,
deep-blue emitters based on spirobenzofluorene derivatives is
attractive.
Spirofluorene and their derivatives as important intermediates
have attracted great interest due to their special structures and
properties. In 2008, Gong et al. introduced asymmetric
In this study, we designed and synthesized two novel blue
emitting materials, 2,20-(spiro[benzo[c]fluorine-7,90-fluorene]-5,9-
diylbis(4,1-phenylene))bis(1-phenyl-1-H-benzo[d]imidazole)
(SBFBI) and 2,20-(spiro[benzo[de]anthracene-7,90-fluorene]-20,3-
diylbis(4,1-phenylene))bis(1-phenyl-1H-benzo[d]imidazole)
(SAFBI), integrating spirobenzofluorene and benzoimidazole
* Corresponding authors.
E-mail addresses: geziyi@nimte.ac.cn (Z. Ge), ouyangxh@nimte.ac.cn
(X. Ouyang), wangsd@suda.edu.cn (S. Wang).
http://dx.doi.org/10.1016/j.dyepig.2014.04.019
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

58
H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
moieties. It is particularly intriguing to compare the electrolumi-
nescent properties of SBFBI and SAFBI, both have the same mo-
lecular constructing units. We only changed the constitution of
spirobenzofluorene core between two benzoimidazole units, the
emission was found to be changed from sky-blue to deep-blue. Our
results provide a way to design and synthesize deep-blue materials
based on spirofluorene derivatives. The thermal properties and
energy levels were fully investigated. The OLEDs based on SAFBI as
the emitting layer achieved a current efficiency of 1.96 cd Aꢀ1 and a
deep-blue CIE(x,y) of (0.15, 0.10), which was the first time to obtain
a deep-blue emitter with spirobenzofluorene derivative in a non-
doped system to our knowledge.
product was purified by column chromatography to give a yellow
solid. Yield, 93%. 1H NMR (400 MHz, Acetone-d6)
9.01 (d, 1H,
J ¼ 8.32 Hz), 8.59 (d, 1H, J ¼ 8.32 Hz), 8.35 (d, 1H, J ¼ 8.32 Hz), 8.08
(d, 2H, J ¼ 7.39 Hz), 7.91 (t, 1H, J ¼ 7.39 Hz), 7.81 (t, 1H, J ¼ 7.86 Hz),
7.72 (d, 1H, J ¼ 8.32 Hz), 7.49 (t, 2H, J ¼ 7.39 Hz), 7.20 (t, 2H,
J ¼ 7.86 Hz), 7.10 (s, 1H), 6.91 (s, 1H), 6.78 (d, 2H, J ¼ 7.39 Hz).
d
2.3. Preparation of 20,3-dibromospiro[benzo[de]anthracene-7,90-
fluorene] (20,3-dBr-SBAF)
Using a similar approach for 5,9-dBr-SBFF, yellow powder was
finally obtained. Yield, 98%. Mp 231 ꢁC. FTIR (KBr, cmꢀ1) 3053, 3023,
739, 765 (aromatic CeH), 1587, 1489, 1463, 1440 (aromatic C]C),
2. Experimental
753 (aromatic CeBr). 1H NMR (400 MHz, CDCl3)
d 8.19 (d, 1H,
J ¼ 7.86 Hz), 8.16e8.10 (m, 2H), 7.95 (d, 1H, J ¼ 8.06 Hz), 7.81 (d, 1H,
J ¼ 7.67 Hz), 7.70 (d, 1H, J ¼ 8.06 Hz), 7.49 (dd, 1H, J1 ¼ 8.15 Hz,
J2 ¼ 1.75 Hz), 7.38 (td, 1H, J1 ¼ 7.57 Hz, J2 ¼ 0.78 Hz), 7.35e7.30 (m,
2H), 7.17 (td, 1H, J1 ¼ 7.57 Hz, J2 ¼ 0.97 Hz), 7.05e7.01 (m, 2H), 6.94
(d, 1H, J ¼ 7.67 Hz), 6.68 (d, 1H, J ¼ 7.28 Hz), 6.51 (dd, 1H,
J1 ¼ 8.01 Hz, J2 ¼ 0.58 Hz). HRMS (m/z): cacld for (MþþH) C29H17Br2
(524.9598), found 524.9582.
2.1. Materials and measurements
Manipulations involving air-sensitive reagents were performed
under an inert atmosphere of dry nitrogen. Commercially available
reagents were used without further purification unless otherwise
stated. Naphthalene-1,8-diamine, phenylboronic acid, 2-bromo-
9H-fluoren-9-one, 9H-fluoren-9-one, naphthalen-1-ylboronic acid,
1-bromo-2-iodobenzene, 2-(4-bromophenyl)-1-phenyl-1H-benzo
[d]imidazole were purchased from TCI co. and used as received. 1,8-
diiodonaphthalene [15], 1-iodo-8-phenylnaphthalene [16], 3-
2.4. Synthesis of 2,20-(spiro[benzo[c]fluorene-7,90-fluorene]-5,9-
diylbis(4,1-phenylene))bis (1-phenyl-1H-benzo[d]imidazole)
(SBFBI)
bromospiro[benzo[de]-anthracene-7,90-fluorene]
[17],
1-(2-
bromophenyl)naphthalene, spiro[fluorene-7,90-benzofluorene], 5-
bromospiro-[benzo[c]fluorene-7,90-fluorene] [8], and 1-phenyl-2-
(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-benzo
[d]imidazole [18] were synthesized according to the methods of the
reported literature. Melting points were obtained with Shanghai
YiCe WRX-4 melting-point apparatus. Fourier transform-infrared
(FTIR) spectra were performed using Thermo Nicolet 6700 spec-
trophotometer. Nuclear magnetic resonance (NMR) spectra were
measured on a Bruker DRX-400 spectrometer with chemical shifts
reported as ppm (in CDCl3 or Acetone-d6, TMS as internal standard).
High-resolution mass spectra were obtained from Bruker Esquire
LC/Ion Trap Mass Spectrometer and JEOL/HX-110. Elemental ana-
lyses were performed with a PerkineElmer 2400 II elemental
analyzer. UVevis absorption spectra (UV) were recorded on a Per-
kineElmer Lambda 950 spectrophotometer. Fluorescence (PL)
measurements were carried out with a FLSP920 spectrophotometer
in a solution of 10ꢀ6 mol/L and solid state, respectively. Differential
scanning calorimetry (DSC) curves were obtained with Metler
Toledo DSC822 instrument at 20 ꢁC minꢀ1 under nitrogen flushing.
Thermogravimetric analyses (TGA) were carried out using a Per-
kineElmer Pyris thermogravimeter under a dry nitrogen gas flow at
a heating rate of 10 ꢁC minꢀ1. The electrochemical properties of
derivatives was studied through cyclic voltammetry (CV) on a CHI
660D analyzer with a three electrode configuration with a Pt disk as
the working electrode of 0.01 cm2, a Pt wire as the counter elec-
trode, and an Ag/AgCl as the reference electrode, and in a
dichloromethane solution containing 0.1 M of tetrabutylammo-
nium hexafluorophosphate as supporting electrolyte.
1-phenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)-1H-benzo[d]imidazole (1.75 g, 4.4 mmol), 5,9-dBr-SBFF
(1.0 g, 2.0 mmol), bis(triphenyl phosphine)palladium(II)chloride
(0.20 g, 0.28 mmol) and toluene (100 mL) were stirred in a three-
necked flask for 30 min. To the above solution was added potas-
sium carbonate (2 M, 20 mL) and anhydrous ethanol (30 mL). The
resulting solution was refluxed 2 days at 105 ꢁC. The reaction
mixture was extracted with dichloromethane and water. After the
organic layer was evaporated with a rotary evaporator, the resulting
powdery product was purified by column chromatography to give a
white solid. Yield, 45%. Mp 385 ꢁC. FTIR (KBr, cmꢀ1) 3064, 3053,
3038, 818, 756, 742 (aromatic CeH), 1597, 1499, 1450, 1406 (aro-
matic C]C), 1476 (C]N), 1260 (CeN). 1H NMR (400 MHz, CDCl3)
d
8.95 (d, 1H, J ¼ 8.51 Hz), 8.53 (d, 1H, J ¼ 8.19 Hz), 8.00e7.94 (m,
3H), 7.89 (d, 2H, J ¼ 7.53 Hz), 7.79e7.73 (m, 2H), 7.66e7.23 (m, 27H),
7.13 (td, 2H, J1 ¼7.61 Hz, J2 ¼ 0.74 Hz), 7.00 (d, 1H, J ¼ 1.64 Hz), 6.79
(d, 2H, J ¼ 7.53 Hz), 6.75 (s, 1H). 13C NMR (100 MHz, CDCl3):
d 151.4,
151.3, 151.2, 150.7, 147.7, 147.3, 142.4, 142.3, 142.3, 142.2, 142.1, 140.0,
138.6, 136.9, 136.7, 136.4, 136.3, 135.9, 132.0, 130.3, 130.2, 130.1,
129.9,129.8,129.2,129.1, 128.0,127.4,127.4, 127.3, 127.1,127.0, 126.9,
125.9, 124.2, 124.1, 124.0, 123.9, 123.8, 123.7, 123.4, 122.8, 122.3,
120.2, 119.3, 119.2, 110.7. ESI-MS (m/z): 904.5 (MþþH). HRMS (m/z):
cacld for (MþþH) C67H43N4 (903.3409), found 903.3417. Anal. Calcd
for C67H42N4: C, 89.11; H, 4.69; N, 6.20; Found: C, 89.03; H, 4.63; N,
6.21.
2.5. Synthesis of 2,20-(spiro[benzo[de]anthracene-7,90-fluorene]-
20,3-diylbis(4,1-phenylene))bis(1-phenyl-1H-benzo[d]imidazole)
(SAFBI)
2.2. Preparation of 5,9-dibromospiro[benzo[c]fluorene-7,90-
fluorene] (5,9-dBr-SBFF)
The synthesis of SAFBI is similar to that of SBFBI by a Suzuki
coupling reaction, white powder was finally obtained. Yield, 87%.
Mp > 300 ꢁC. FTIR (KBr, cmꢀ1) 3060, 3034, 831, 762, 748 (aromatic
CeH), 1598, 1499, 1449, 1402 (aromatic C]C), 1478 (C]N), 1262
5-Bromospiro[benzo[c]fluorene-7,90-fluorene]
(4.01
g,
9.0 mmol) was dissolved in trichloromethane (50 mL) in a two-
necked flask; bromine (2.16 g, 13.5 mmol) was then added slowly
in a dropwise fashion over a period of 20 min. The mixture was
stirred at room temperature, then the reaction mixture was
extracted with dichloromethane and water. After the organic layer
was evaporated with a rotary evaporator, the resulting powdery
(CeN). 1H NMR (400 MHz, CDCl3)
d
8.33 (d, 1H, J ¼ 7.85 Hz), 8.26 (d,
1H, J ¼ 7.91 Hz), 8.02 (d, 1H, J ¼ 8.01 Hz), 7.97 (d, 1H, J ¼ 7.91 Hz),
7.92 (d, 1H, J ¼ 8.01 Hz), 7.86 (d, 1H, J ¼ 7.56 Hz), 7.77 (d, 2H,
J ¼ 8.11 Hz), 7.74 (dd, 1H, J1 ¼ 8.42 Hz, J2 ¼ 0.82 Hz), 7.65 (dd, 1H,
J1 ¼ 7.91 Hz, J2 ¼ 1.54 Hz), 7.62e7.54 (m, 8H), 7.52e7.39 (m, 9H),

H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
59
Scheme 1. The synthetic routes to SBFBI and SAFBI.
7.37e7.29 (m, 7H), 7.23 (d, 2H, J ¼ 8.21 Hz), 7.18e7.11 (m, 2H), 7.01
(t,1H, J ¼ 7.70 Hz), 6.96 (d,1H, J ¼ 7.49 Hz), 6.68 (dd,1H, J1 ¼7.29 Hz,
J2 ¼ 0.82 Hz), 6.57 (dd, 1H, J1 ¼ 8.00 Hz, J2 ¼ 1.03 Hz). 13C NMR
by vacuum deposition of the materials at 10ꢀ6 Torr onto ITO glass
with a sheet resistance of 6
cmꢀ2. All of the organic layers and
U
inorganic layers were prepared in sequence by vacuum deposition.
The electroluminescence spectra and Commission Internationale
De L’Eclairage (CIE) coordination of these devices were measured
by a PR655 spectra scan spectrometer. The luminance-current and
density-voltage characteristics were recorded simultaneously with
the measurement of the EL spectra by combining the spectrometer
with a programmable voltageecurrent source. All measurements
were carried out at room temperature under ambient conditions.
(100 MHz, CDCl3):
d 157.6, 157.5, 151.7, 151.4, 142.5, 142.2, 140.2,
140.1, 139.3, 138.7, 138.5, 137.3, 137.0, 136.8, 136.7, 136.5, 131.8, 131.6,
130.2, 130.1, 130.0, 129.8, 129.6, 129.5, 129.0, 128.9, 128.6, 128.6,
128.3, 127.8, 127.6, 127.5, 127.4, 127.4, 126.9, 126.8, 126.6, 125.5,
125.3, 124.6, 124.1, 123.8, 123.6, 123.5, 123.3, 120.5, 120.3, 119.5,
119.3, 119.1, 110.6, 110.6. HRMS (m/z): cacld for (MþþH) C67H43N4
(903.3409), found 903.3349. Anal. Calcd for C67H42N4: C, 89.11; H,
4.69; N, 6.20; Found: C, 89.04; H, 4.65; N, 6.21.
3. Results and discussion
2.6. Device fabrication and testing
3.1. Synthesis and characterization
SBAFI and SBFBI were purified by vacuum sublimed and the
purity of the materials is above 99% before the OLEDs fabrication.
The ITO-coated glass substrates were routinely cleaned and sub-
sequently treated with UV-ozone. The EL devices were fabricated
The synthetic routes and molecular structures of SAFBI and
SBFBI are illustrated in Scheme 1. Some starting materials were
synthesized according to the reported methods of literature [8,15e
Table 1
Thermal properties, optional properties and energy levels of SBFBI and SAFBI.
Compounds
Tm/Td [ꢁ C]
lmax Absa [nm]
lmax PLa,b [nm]
HOMO/LUMOexp(Egopt
)
c [eV]
HOMO/LUMOcal(Ecgal d [eV]
)
SBFBI
SAFBI
385/524
e/426
359
325
440/461
440/456
ꢀ5.62/ꢀ2.62 (3.00)
ꢀ5.62/ꢀ2.51 (3.11)
ꢀ5.22/ꢀ1.68 (3.54)
ꢀ5.28/ꢀ1.55 (3.73)
a
Measured in CHCl3.
b
Measured in solid film on quartz plates.
c
Estimated based on absorption onset and cyclic-voltammetry. EHOMO ¼ ꢀ(qEox þ 4.4) eV, ELUMO ¼ Eogpt þ EHOMO
.
d
DFT calculation with B3LYP/6-31G.

60
H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
100
SBFBI (sol.)
SBFBI (film)
SAFBI (sol.)
SAFBI (film)
1.0
1.0
0.8
0.6
0.4
0.2
0.0
80
0.8
60
0.6
0.4
0.2
0.0
40
20
0
SBFBI
SAFBI
0
100 200 300 400
Temperature
500 600
700 800
300
400
500
600
o
C)
(
Wavelength (nm)
Fig. 1. TGA thermograms of SBFBI and SAFBI recorded at a heating rate of 10 ꢁC minꢀ1
and DSC traces of SBFBI and SAFBI recorded at a heating rate of 20 ꢁC minꢀ1
Fig. 3. UV absorption and PL spectra of SBFBI and SAFBI.
.
18]. The 20,3-dBr-SBAF or 5,9-dBr-SBFF were obtained by reaction
with bromine and 20,3-dBr-SBAF or 5,9-dBr-SBFF were obtained in
good yield. Bromination reactions were employed in the synthesis
of the intermediates with a yield of over 90%. The target com-
pounds were obtained by the bromides reacting with boronate
compounds through Suzuki coupling reaction in aqueous potas-
sium carbonate solution. The compounds were purified by column
chromatography using common solvent systems. The chemical
structures of SAFBI and SBFBI were confirmed by 1H NMR and 13C
NMR. The results of mass spectrometry also supported the forma-
tion of the two target materials and matched well with the calcu-
lated data. Compounds SAFBI and SBFBI had good solubility in
common organic solvents so that they could be easily purified by
column chromatography to high purity for spectroscopic charac-
terization and OLEDs application.
analysis (TGA) under a nitrogen atmosphere, and their thermal data
are shown in Table 1 and Fig. 1. In the DSC, no obvious glass tran-
sition temperatures (Tg) were observed for them and they did not
show any melting transition even when SAFBI was heated up to
420 ꢁC, while endothermic melting transition temperature (Tm)
appeared obviously at 385 ꢁC for SBFBI. These results indicate that
highly twisted structures, due to the different constitution of SAFBI
compared to SBFBI, might mitigate the intermolecular interactions
of SAFBI in the solid state [19]. From the TGA results, SBFBI and
SAFBI exhibited decomposition (Td, corresponding to 5% weight
loss) as high as 524 ꢁC and 426 ꢁC, respectively. Such high Td values
indicate that these molecules are stable and have the potential to be
fabricated into devices by vacuum thermal evaporation technology
[20e22].
3.3. Theoretical calculations
3.2. Thermal properties
To understand the electronic structures of these compounds, the
molecular geometry and frontier molecular orbitals were calcu-
lated at B3LYP/6-31G (d,p) level by using DFT in Gaussian 03
The thermal properties of SBFBI and SAFBI were measured by
differential scanning calorimetry (DSC) and thermogravimetric
Fig. 2. Molecular orbital distribution and optimized geometries of SBFBI and SAFBI.

H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
61
100
2500
2000
1500
1000
500
SBFBI
SAFBI
SBFBI
SAFBI
10
1
0
4
5
6
7
8
9
10
Voltage (V)
Fig. 6. Current densityevoltage-luminance characteristics of SBFBI and SAFBI devices.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
Voltage (V)
shown in Table 1 and Fig. 4. HOMO values are calculated from as
Fig. 4. Cyclic voltammograms of compounds SBFBI and SAFBI in CH2Cl2.
HOMO ¼ ꢀ([Eonset ox þ 4.4) [24]. Both HOMO values of SBFBI and
]
SAFBI are ꢀ5.62 eV. Such a low HOMO energy level greatly reduced
the energy barrier for hole injection from indium tin oxide (ITO) to
the emissive derivatives. The energy band gaps of the compounds
are estimated by analyzing absorption edge with a plot of UVevis
curve. The energy band gaps of SAFBI and SBFBI are found to be
3.11 eV and 3.00 eV, respectively. The LUMO levels of SAFBI and
SBFBI calculated by HOMO þ Eogpt are ꢀ2.51 eV and ꢀ2.62 eV. It was
found that SAFBI showed a relatively higher LUMO energy with a
program. As shown in Fig. 2, the HOMO and LUMO of SBFBI were
delocalized in the SBFF and two benzimidazole units, while the
HOMO and LUMO were delocalized in the SBAF and one benz-
imidazole moiety. Compared with SBFBI, the titled angle of the two
benzimidazoles for SAFBI is almost 90ꢁ because of the steric hin-
drance. Consequently, the non-planar geometry of SAFBI efficiently
prevents the recrystallization due to the large torsional stresses,
which can induce the formation of either an exciplex or excimer
[23].
larger band gap due to the distorted p-conjugation of SAFBI [25].
3.5. Electroluminescent properties
3.4. Photophysical and electrochemical properties
Nondoped OLED devices were fabricated to investigate the EL
performances of SBFBI and SAFBI, the structure of devices is ITO/
MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/SBFBI or SAFBI (20 nm)/
TPBi (40 nm)/LiF (1 nm)/Al (100 nm). Indium tin oxide (ITO) and Al
were utilized as anode and cathode, MoO3 was as a hole injecting
The UVevis absorption and PL spectra of SBFBI and SAFBI in
diluted CHCl3 solutions and in the solid film spin-coated on quartz
plates were explored as shown in Fig. 3. The photophysical data of the
compounds are summarized in Table 1. SBFBI showed a UVevis ab-
sorption maximum at 359 nm, whereas SAFBI showed a UV absorp-
layer,
N,N0-bis(naphthalene-1-yl)-N,N0-bis(phenyl)-benzidine
tion maximum at 325 nm originated from the
pe
p* transition of the
(NPB) was as a hole transporting layer, 4,40,400-tri(N-carbazolyl)
benzene (TCTA) was as an electron blocking layer, 1,3,5-tri(phenyl-
2-benzimidazolyl)benzene (TPBi) was as an electron transporting
and hole blocking layer, and LiF was used as an electron injecting
layer. The EL spectra of SBFBI or SAFBI devices at different current
density are shown in Fig. 5. The maximum emission lmax of both
SBFBI and SAFBI are located at 436 nm, and the FWHM values are
85 nm and 54 nm. There is a 25 nm blue-shift for SBFBI and a 20 nm
blue-shift for SAFBI. It is common that the w20 nm shift is observed
in OLEDs [26,27]. For the EL spectra of SBFBI device at different
current density, there is a slight difference in the intensity of the
shoulder at w500 nm. This phenomenon should be ascribed to the
spiro[benzoanthracene-fluorene] core in CHCl3 solution [17]. For the
fluorescence spectra of SBFBI and SAFBI, they both emit blue fluo-
rescence peaking at 440 nm in dilute CHCl3 solution. In the film state,
the emissions from these two molecules show a red shift but still
localized in the blue spectral region 461 nm for SBFBI, and 456 nm for
SAFBI, respectively. Additionally, it is worth noting that the PL spec-
trum of SBFBI is more red-shifted compared with SAFBI. It can be
attributed to the constitution of SAFBI with a large steric hindrance,
which prevents close molecular packing in the solid film [19].
Cyclic voltammetry analysis were carried out to measure the
HOMO values of the synthesized compounds, and the results are
2.5 mA/cm2
2.5 mA/cm2
(a)
(b)
1.0
1.0
5 mA/cm2
5 mA/cm2
10 mA/cm2
10 mA/cm2
0.8
0.8
20 mA/cm2
20 mA/cm2
40 mA/cm2
40 mA/cm2
100 mA/cm2
100 mA/cm2
0.6
0.6
0.4
0.2
0.0
0.4
0.2
0.0
400
500
600
700
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Fig. 5. Normalized EL spectra for the blue FLOLEDs with SBFBI (a) and SAFBI (b).

62
H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.24
SBFBI
SAFBI
3
2
1
0
SBFBI CIE x
SBFBI CIE y
SAFBI CIE x
SAFBI CIE y
0.20
0.16
0.12
0.08
0
20
40
60
80
100
Current Density (mA cm-2)
0
20
40
60
80
100
Current density(mA cm-2)
Fig. 7. Luminous efficiency curves and power efficiency curves versus current density
for SBFBI and SAFBI devices.
Fig. 9. CIE coordinates deviation of EL spectra in response to the current change of
SBFBI and SAFBI EL devices.
3
SBFBI
SAFBI
shoulder at w500 nm in the EL spectra of SBFBI device is almost
disappeared at high current density. However, the CIE coordinates
(x, y) of SBFBI- and SAFBI-based on devices are found to be (0.18,
2
1
0.18) and (0.15, 0.10) when the current density is at 100 mA cmꢀ2
.
We can confirm the CIE coordinates difference between these two
devices results from different FWHM of two EL spectra, which
contributes to the much more twisted molecular structure of the
SAFBI [23]. The EL properties of the devices for SBFBI and SAFBI are
summarized in Table 2.
Notably, the CIE values deviation of SAFBI device is negligible
(x ¼ 0.001, y ¼ 0.008) with the current density increasing from
5 mA cmꢀ2 to 100 mA cmꢀ2 (see Fig. 9). According to the color
stability of SAFBI-based device, we can confirm that the energy
levels of SAFBI match well with neighboring layers, and the charge
carrier recombination at the emitting layer is efficient at all current
density [23].
0
20
40
60
80
100
Current density (mA cm-2)
Fig. 8. The external quantum efficiency of SBFBI and SAFBI based FLOLEDs.
4. Conclusion
electromer formation of SBFBI, since there is no evidence of exci-
mer formation from the film and solution PL spectra [21]. Fig. 6
shows the current density-voltage-luminance characteristics of
the devices.
Two benzimidazole-attached spiro[benzofluorene] derivatives
(SBFBI and SAFBI) were successfully designed and synthesized for
blue emitters in nondoped OLEDs. The nondoped, blue fluorescent
organic light-emitting diodes were fabricated with the structure of
ITO/MoO3 (10 nm)/NPB (80 nm)/TCTA (5 nm)/SBFBI or SAFBI
(20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm). The devices fabri-
cated with SAFBI exhibited efficiencies of 1.96 Cd/A, 1.34 lm/W as
well as a deep-blue emission with CIE coordinates of (0.15, 0.10) and
little efficiency roll-off and color change at high brightness. It is
particularly intriguing to compare the electroluminescent proper-
ties of SBFBI and SAFBI, both have the same molecular constructing
units. We only changed the constitution of spirobenzofluorene core
between two benzoimidazole units, the emission was found to be
changed from sky-blue to deep-blue. Our results provide a way to
design and synthesize deep-blue materials based on spirofluorene
derivatives.
As shown in Fig. 7, the maximum luminance efficiencies for
SBFBI and SAFBI are 2.63 cd Aꢀ1 and 1.96 cd Aꢀ1, and the maximum
power efficiencies are 1.56 lm Wꢀ1 and 1.34 lm Wꢀ1, respectively.
The external quantum efficiencies (EQE) maximum of these non-
doped FLOLEDs are 1.81% for SBFBI and 2.23% for SAFBI (see Fig. 8.).
When the current density is at 20 mA cmꢀ2, the CIE coordinates (x,
y) of SBFBI- and SAFBI-based on devices are found to be (0.18, 0.20)
and (0.15,0.10), which is close to the blue standards (0.14, 0.08) of
NTSC. As expected, SAFBI showed deep-blue emission, while SBFBI
showed sky-blue emission. The effect on the change in CIE co-
ordinates of SBFBI device from the shoulder at w500 nm can be
neglected because the intensity of the shoulder at w500 nm only
changes a little as increasing the current density. Especially, the
Table 2
Electroluminescent characteristics of SBFBI and SAFBI.
Devices
CIEa [x, y]
lmax EL [nm]
LEmax [cd Aꢀ1
]
PEmax [lm Wꢀ1
]
EQEmax [%]
Devices performance at 20 mA cmꢀ2
LE [cd Aꢀ1
]
PE [lm Wꢀ1
]
EQE [%]
SBFBI
SAFBI
0.18, 0.20
0.15, 0.10
436
436
2.63
1.96
1.56
1.34
1.81
2.23
2.62
1.88
1.12
0.80
1.79
2.23
a
Values collected at 20 mA cmꢀ2
.

H. Liang et al. / Dyes and Pigments 108 (2014) 57e63
63
[12] Linton KE, Fisher AL, Pearson C, Fox MA, Pålsson LO, Bryce MR, et al. Colour
tuning of blue electroluminescence using bipolar carbazole-oxadiazole mol-
ecules in single-active-layer organic light emitting devices (OLEDs). J Mater
Chem 2012;22:11816e25.
[13] Lee YH, Wu TC, Liaw CW, Wen TC, Feng SW, Lee JJ, et al. Nondoped active
layer, benzo[k]fluoranthene-based linear acenes, for deep blue- to green-
emissive organic light-emitting diodes. Org Electron 2013;14:1064e72.
[14] Zhang Y, Lai SL, Tong QX, Lo MF, Ng TW, Chan MY, et al. High efficiency
nondoped deep-blue organic light emitting devices based on imidazole-p-
triphenylamine derivatives. Chem Mater 2012;24:61e70.
[15] Weimar M, Dürner G, Bats JW, Göbel MW. Enantioselective synthesis of
(þ)-estrone exploiting a hydrogen bond-promoted Diels-Alder reaction. J Org
Chem 2010;75:2718e21.
Acknowledgments
This work was financial supported from National Natural Sci-
ence Foundation of China (21102156, 51273209), Ningbo Interna-
tional Cooperation Foundation (2012D10009, 2013D10013), the
Open Fund of the State Key Laboratory of Luminescent Materials
and Devices (South China University of Technology) (2013-
SKLLMD-02) and the External Cooperation Program of the Chinese
Academy of Sciences (No. GJHZ1219).
[16] Lejeune M, Grosshans P, Berclaz T, Sidorenkova H, Besnard C, Pattisoncd P,
et al. Role of the aromatic bridge on radical ions formation during reduction of
diphosphaalkenes. New J Chem 2011;35:2510e20.
[17] Kim JY, Lee CW, Jang JG, Gong MS. Orange phosphorescent organic light-
emitting diodes using new spiro[benzoanthracene-fluorene]-type host ma-
terials. Dyes Pigments 2012;94:304e13.
[18] Yang XH, Zheng SJ, Bottger R, Chae HS, Tanaka T, Li S, et al. Efficient fluo-
rescent deep-blue and hybrid white emitting devices based on carbazole/
benzimidazole compound. J Phys Chem C 2011;115:14347e52.
[19] Park H, Lee J, Kang I, Chu HY, Lee JI, Kwon SK, et al. Highly rigid and twisted
anthracene derivatives: a strategy for deep blue OLED materials with theo-
retical limit efficiency. J Mater Chem 2012;22:2695e700.
[20] Zhang XQ, Chi ZG, Xu BJ, Li HY, Yang ZY, Li XF, et al. Synthesis of blue light
emitting bis(triphenylethylene) derivatives: a case of aggregation-induced
emission enhancement. Dyes Pigments 2011;89:56e62.
[21] Huang JH, Su JH, Li X, Lam MK, Fung KM, Fan HH, et al. Bipolar anthracene
derivatives containing hole- and electron-transporting moieties for highly
efficient blue electroluminescence devices. J Mater Chem 2011;21:2957e64.
[22] Tang S, Li WJ, Shen FZ, Liu DD, Yang B, Ma YG. Highly efficient deep-blue
electroluminescence based on the triphenylamine-cored and peripheral
blue emitters with segregative HOMO-LUMO characteristics. J Mater Chem
2012;22:4401e8.
[23] Cho I, Kim SH, Kim JH, Park S, Park SY. Highly efficient and stable deep-blue
emitting anthracene-derived molecular glass for versatile types of non-
doped OLED applications. J Mater Chem 2012;22:123e9.
[24] Zhang H, Wan X, Xue X, Li Y, Yu A, Chen Y. Selective tuning of the HOMO-
LUMO gap of carbazole-based donor-acceptor-donor compounds toward
different emission colors. Eur J Org Chem 2010;9:1681e7.
[25] Wan W, Du HL, Wang J, Le YP, Jiang HZ, Chen H, et al. Novel blue luminescent
materials for organic light-emitting diodes based on C9-fluorenyl anthra-
cenes. Dyes Pigments 2013;96:642e52.
References
[1] Li WJ, Liu DD, Shen FZ, Ma DG, Wang ZM, Feng T, et al. A twisting donor-
acceptor molecule with an intercrossed excited state for highly efficient,
deep-blue electroluminescence. Adv Funct Mater 2012;22:2797e803.
[2] Shin MG, Kim SO, Park HT, Park SJ, Yu HS, Kim YH, et al. Synthesis and
characterization of ortho-twisted asymmetric anthrancene derivatives for
blue organic light emitting diodes (OLEDs). Dyes Pigments 2012;92:1075e82.
[3] Zhang P, Dou W, Ju ZH, Yang LZ, Tang XL, Liu WS, et al. A 9,90-bianthrancene-
cored molecule enjoying twisted intramolecular charge transfer to enhance
radiative-excitons generation for highly efficient deep-blue OLEDs. Org Elec-
tron 2013;14:915e25.
[4] Hu JY, Pu YJ, Yamashita Y, Satoh F, Kawata S, Katagiri H, et al. Excimer-
emitting single molecules with stacked p-conjugated groups covalently linked
at the 1,8-positions of naphthalene for highly efficient blue and green OLEDs.
J Mater Chem C 2013;1:3871e8.
[5] Kamino BA, Chang YL, Lu ZH, Bender TP. Phthalonitrile based fluorophores as
fluorescent dopant emitters in deep-blue OLEDs: approaching the NTSC
standard for blue. Org Electron 2012;13:1479e85.
[6] Xing X, Xiao LX, Zheng LL, Hu SY, Chen ZJ, Qu B, et al. Spirobifluorene deriv-
ative: a pure blue emitter (CIEy z 0.08) with high efficiency and thermal
stability. J Mater Chem 2012;22:15136e40.
[7] Chang YC, Yeh SC, Chen YH, Chen CT, Lee RH, Jeng RJ. New carbazole-
substituted anthracene derivatives based non-doped blue light-emitting de-
vices with high brightness and efficiency. Dyes Pigments 2013;99:577e87.
[8] Jeon SO, Jeon YM, Kim JW, Lee CW, Gong MS. Blue organic light-emitting
diode with improved color purity using 5-naphthyl-spiro[fluorene-7,90-ben-
zofluorene]. Org Electron 2008;9:522e32.
[9] Liu F, Xie LH, Tang C, Liang J, Chen QQ, Peng B, et al. Facile synthesis of spirocyclic
acromatic hydrocarbon derivatives based on o-halobiaryl route and domino
reaction for deep-blue organic semiconductors. Org Lett 2009;11:3850e3.
[10] Lee CW, Jang JG, Gong MS. Deep blue fluorescent host materials based on
spirobenzofluorene-fluorene dimers and their properties. Dyes Pigments
2013;98:471e8.
[26] Liang ZQ, Zeng ZC, Zou DC, Wang XM, Tao XT. Novel 4-(2,2-diphenyl-vinyl)-
phenyl substituted pyrene derivatives as efficient emitters for organic light-
emitting diodes. Org Electron 2012;13:2898e904.
[27] Feng XJ, Chen SF, Yong Ni, Wong MS, Lam MM, Cheah KW, et al. Fluorene
derivatives for highly efficient non-doped single-layer blue organic light-
emitting diodes. Org Electron 2014;15:57e64.
[11] Jeong SJ, Kim MK, Kim SH, Hong JI. Efficient deep-blue emitters based on
triphenylamine-linked benzimidazole derivatives for nondoped fluorescent
organic light-emitting diodes. Org Electron 2013;14:2497e504.