Efficient blue-emitting molecules by incorporating sulfur-containing moieties into triarylcyclopentadiene: Synthesis, crystal structures and photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2015.09.018

Three highly fluorescent blue-emitting molecules, namely 1,2-diphenyl-4-thiophenyl-1,3-cyclopentadiene (DPCP 1), 1,2-diphenyl-4-(4-thiophenyl)phenyl)-1,3-cyclopentadiene (DPCP 2) and 1,2-diphenyl-4-(4-dibenzothiphenyl)phenyl)-1,3-cyclopentadiene (DPCP 3),

Full text of DOI:10.1016/j.dyepig.2015.09.018

Dyes and Pigments 124 (2016) 145e155
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Efficient blue-emitting molecules by incorporating sulfur-containing
moieties into triarylcyclopentadiene: Synthesis, crystal structures and
photophysical properties
,
Junwei Ye a, Yuan Gao a, Lei He a, Tingting Tan a, Wei Chen a, Yu Liu b **, Yue Wang b,
,
*
Guiling Ning a
a State Key Laboratory of Fine Chemicals and School of Chemical Engineering, Dalian University of Technology, 2 Linggong Road, Dalian 116012, PR China
b State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three highly fluorescent blue-emitting molecules, namely 1,2-diphenyl-4-thiophenyl-1,3-
cyclopentadiene (DPCP 1), 1,2-diphenyl-4-(4-thiophenyl)phenyl)-1,3-cyclopentadiene (DPCP 2) and
1,2-diphenyl-4-(4-dibenzothiphenyl)phenyl)-1,3-cyclopentadiene (DPCP 3), have been synthesized by
using aryl-substituted cyclopentadiene and thiophene or dibenzothiphene as ingredients. The single
crystal structure analysis reveals that DPCP 1e3 are non-coplanar structures and bulky substituents on
cyclopentadiene core imposed a significant reduction on intermolecular interactions, hence leading to
their intense blue emission in both solution and solid state. DPCP 1 and DPCP 3 also showed a typical
aggregation-induced emission enhancement in mixed water/acetone solution. These compounds
exhibited good thermal stability with decomposition temperatures between 239 and 383 ꢀC. The non-
doped organic light-emitting diodes using DPCP 3 as the emitting layer displayed a very low turn-on
Received 27 April 2015
Received in revised form
7 August 2015
Accepted 7 September 2015
Available online 30 September 2015
Keywords:
Blue-emitting molecules
Triarylcyclopentadiene
Thiophene
Aggregation-induced emission
enhancement
ꢀ
voltage at 3.2 V and pure blue emission with the Commission Internationale de I'Eclairage (CIEx,y) co-
ordinates of (0.16, 0.16) and a maximum luminance of 2277 cd mꢁ2
.
Crystal structures
Organic light emitting diode
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
pentaphenylsilole and 1-cyano-trans-1,2-bis-(40-methylbiphenyl)
ethylene having strong emission in the aggregated state than that
in dilute solution were reported by Tang et al. [9] and Park et al.
[10], respectively, different type of materials with aggregation-
induced emission (AIE) or aggregation induced emission
enhancement (AIEE) characteristics have been discovered [11]. The
restriction of intermolecular rotations (RIR), molecular planariza-
tion, prevention of exciton diffusion and J-aggregates formation
effects are accepted to understand enhanced emission phenomena
[12]. However, the efficient AIE or AIEE materials systems for blue-
emitting molecules are still quite limited.
The development of high efficient organic optoelectronic ma-
terials has attracted tremendous attention from both the academic
and industrial research communities because of their promising
applications in the fields of organic light-emitting diodes (OLEDs)
[1], illumination [2], photovoltaic cells [3], molecular probes [4],
bio-labeling [5] and photonic devices [6]. Most of organic fluo-
rophores are highly emissive in their dilute solutions, but it be-
comes weakly luminescent when fabricated into thin films. In the
solid state, the molecules aggregate to form less emissive species
such as excimers, leading to a reduction in the luminescence effi-
ciency [7]. Such aggregation-caused quenching (ACQ) effect has
become a drawback to the development of high efficient and stable
OLEDs [8]. Since the anti-ACQ compounds 1-methyl-1,2,3,4,5-
In order to decrease the aggregation quenching of aromatic
hydrocarbons possessing extended
p-conjugation and planar
skeleton, some efforts have been spent to modify the fluorophore
backbone by attaching bulky substituents to the molecular core
[13]. For examples, a series of aryl-functionalized blue emitting
pyrene derivatives were synthesized by steric hindrance intro-
duced through bulky substituents in its 1-, 3-, 5-, 6-, 8- and 9H
position of pyrene core [14], respectively. Several quinacridone
derivatives with high-emission property in both solution and solid
* Corresponding author. Fax: þ86 411 84986065.
** Corresponding author.
E-mail addresses: yuliu@jlu.edu.cn (Y. Liu), ninggl@dlut.edu.cn (G. Ning).
http://dx.doi.org/10.1016/j.dyepig.2015.09.018
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

146
J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
state were synthesized based on the introduction of two propeller-
like pentaphenyl groups to a quinacridone core [15]. Furthermore,
cyclopentadiene derivatives (CPs) are known for versatile organic
molecules due to their good electrical and optical properties [16,17],
as well as multicolor emission in nano-aggregation [18]. Huang
et al. [19] reported two CPs named 1,2,3,4-tetraphenyl-1,3-
2.2. Synthesis
2.2.1. 1,2-Diphenyl-4-thiophenyl-1,3-cyclopentadiene (DPCP 1)
A solution of thiophene-2-carboxaldehyde (2.50 g, 22.29 mmol)
and acetophenone (6.70 g, 55.83 mmol) in ethanol was heated to
65 ꢀC, and aqueous sodium hydroxide (2.23 g, 55.83 mmol) was
added to the solution. The mixture was heated to reflux for 1 h and
then cooled to room temperature. The intermediate product was
cyclized in Zn/CH3COOH system for 5 h, in which zinc dusts were
added per hour. The solution layer was poured into 600 mL water to
get white precipitate. The solid was collected by filtration and then
dried under vacuum, and it was dehydrated in the presence of
concentrated hydrochloric acid for 3 h and then cooled to room
temperature [19]. Light yellow solid precipitate was obtained by
filtration. The obtained solid was purified by column chromatog-
raphy on silica gel using dichloromethane/petroleumether as
eluent to obtain the product in yield 78%. 1H NMR (400 MHz, CDCl3)
cyclopentadiene
(TPCP)
and
1,2,3,4,5-pentaphenyl-1,3-
cyclopentadiene (PPCP) which were used in blue light-emitting
OLEDs. Yoshino et al. [20] reported multicolor OLEDs based on
PPCP or PPCP-doped poly(3-alkylthiophene) as emissive layer.
Recently, our researches are mainly focused on the molecule design
and understanding relationship between molecular structures and
optoelectronic properties of the polyphenyl substituted CPs, and a
new class of triarylsubstituted CPs with good thermal stability and
strong fluorescence in solid state were synthesized [21]. It has been
demonstrated that the introduction of rich electron-donor moieties
into the molecular backbone of CPs would improve the emission
efficiency and color purity of the target molecules.
d
(ppm): 7.39e7.38 (m, 2H), 7.36e7.34 (m, 1H), 7.33e7.30 (m, 4H),
In this work, we would like to introduce thiophene and diben-
zothiphene segments with the good amorphous film forming
ability and the electron-transporting ability [22] into the 4-position
of the 1,2-diphenyl or 1, 2, 4-triphenyl cyclopentadienyl core to
obtain a series of novel blue emission materials, namely 1,2-
7.28e7.25 (m, 2H), 7.24e7.22 (m, 2H), 7.10e7.17 (m, 1H), 7.16e7.02
(m, 1H), 7.01e6.86 (m, 1H), 3.94 (s, 2H). 13C NMR (100 MHz, CDCl3)
d
(ppm): 144.4, 144.3, 142.2, 139.5, 137.1, 136.8, 134.9, 132.9, 132.1,
128.5, 128.4, 128.3, 128.1, 127.8, 127.2, 126.6, 126.1, 125.3, 124.7,
122.9, 44.87. MS (API-ES): calcd for C21H16S, M: 300.1. Elem. Anal.:
C, 83.98%; H, 5.42%; S, 10.71%. Found: C, 83.96%; H, 5.37%; S, 10.67%.
diphenyl-4-thiophenyl-1,3-cyclopentadiene
diphenyl-4-(4-thiophenyl)phenyl-1,3-cyclopentadiene (DPCP 2)
and 1,2-diphenyl-4-(4-dibenzothiphenyl)phenyl-1,3-cyclopen
(DPCP
1),
1,2-
2.2.2. 1,2-Diphenyl-4-(4-thiophenyl)phenyl-1,3-cyclopentadiene
(DPCP 2)
tadiene (DPCP 3). We present a comprehensive investigation on
these three CPs compounds, which not only encompasses their
thermal, photophysical and electrochemical properties, but also the
emphatically studies on their single-crystal structures and DFT
calculation have been investigated to understand the relationship
between chemical structures and luminescent properties. The
resulting non-coplanar structures due to the steric hindrance of
bulky phenyl ring and sulfur-containing moieties could suppress
1,2-Diphenyl-4-(4-bromophenyl)-1,3-diene was synthesized
according to our previously work [21] in essentially similar pro-
cedures. 1,2-diphenyl-4-(4-bromophenyl)-1,3-diene (0.78 g,
2.10 mmol) and the 2-thiopheneboronic acid (0.31 g, 2.40 mmol) in
toluene (15 mL), 2 M aqueous K2CO3 solution (10 mL) and ethanol
(5 mL) were added. The mixture was stirred for 30 min under an
argon atmosphere at room temperature. Then the Pd(PPh3)4 cata-
lyst was added and the reaction mixture was stirred at 80 ꢀC for 24 h
allowing the temperature to decrease gradually to room tempera-
ture [14a,24]. The crude product was concentrated and purified by
silica gel column chromatography using petroleum ether/
dichloromethane (v:v, 10:1, yield 76%). 1H NMR (400 MHz, CDCl3)
the intermolecular
pep interactions and reduce the aggregation
formation. It is worth mentioning that an efficient pure blue OLEDs
based on DPCP 3 as the neat emitting layer was achieved.
2. Experimental section
d
(ppm): 7.61e7.55 (m, 4H), 7.41e7.39 (m, 2H), 7.35e7.28 (m, 6H),
7.26e7.21 (m, 3H), 7.19e7.17 (m,1H), 7.09e7.06 (m, 2H), 3.94 (s, 2H).
13C NMR (100 MHz, CDCl3)
(ppm): 142.0, 140.4, 139.1, 138.1, 136.9,
2.1. Materials and methods
d
136.6,131.5,128.5,128.4,128.3,128.2,127.7,127.2,126.6,123.7,122.8,
45.84. MS (API-ES): calcd for C27H20S, M: 376.1. Elem. Anal.: C,
86.21%; H, 5.39%; S, 8.63%. Found: C, 86.13%; H, 5.35%; S, 8.52%.
All reagents were analytical reagent grade and used as
received. The solvents were purified with standard methods and
dried as needed. All 1H and 13C NMR spectra were referenced to a
Bruker AVANCE-400 MHz magnetic resonance spectrometer.
HRMS were acquired on Micromass-GTC spectrometer. Differen-
tial scanning calorimetry (DSC) curves were obtained with a TA
2.2.3. 1,2-Diphenyl-4-(4-dibenzothiphenyl)phenyl-1,3-
cyclopentadiene (DPCP 3)
The synthesis of DPCP 3 was similar to the above description for
DPCP 2 except that 2-thiopheneboronic acid was replaced by 4-
dibenzothiopheneboronic acid. The light yellow powder was ob-
Instruments thermal analyzer (910S) at
a heating rate of
10 ꢀC minꢁ1 under nitrogen atmosphere. Thermogravimetric
analyses (TGA) were performed with a METTLER TOLEDO (TGA/
SDTA 851e) under nitrogen atmosphere with a heating rate of
10 ꢀC minꢁ1. UV absorption measurements were conducted on
HITACHI U-4100 UVevis Spectrophotometer. The photo-
luminescence (PL) studies were confirmed with a JASCO FP-6300
spectrofluorometer with a 150 W Xe lamp. The relative fluores-
cence quantum yields were estimated relative to solutions of
5 ꢂ 10ꢁ5 M quinine sulfate with FF ¼ 0.55 in 0.1 M sulfuric acid
solution as a standard sample [23]. Cyclic voltammetry (CV)
measurement was carried out on a Shanghai Chenhua electro-
chemical workstation CHI660C in a three-electrode cell with a Pt
disk working electrode, an Ag/AgCl reference electrode, and a
glassy carbon counter electrode. All spectra were carried out at
room temperature.
tained with the yield of 85%. 1H NMR (400 MHz, CDCl3)
d (ppm):
8.23e8.21 (m, 1H), 8.19e8.17 (m, 1H), 7.88e7.86 (m, 1H), 7.79e7.74
(m, 4H), 7.59e7.57 (d, 1H), 7.56e7.54 (m, 1H), 7.51e7.49 (m, 2H),
7.46e7.44 (m, 2H), 7.39e7.34 (m, 5H), 7.32e7.29 (d, 1H), 7.27e7.25
(d, 1H), 7.22e7.21 (d, 1H), 7.16 (s, 1H), 4.04 (s, 2H). 13C NMR
(100 MHz, CDCl3)
d (ppm): 144.5, 142.2, 139.7, 139.6, 139.1, 138.5,
137.2,136.8,136.7,136.3,135.8,135.4,132.4,128.7,128.6,128.5,128.3,
127.8,127.3,126.9,126.8,126.7,125.3,125.2,124.4,122.7,121.8,120.5,
45.0. MS (API-ES): calcd for C35H24S, M: 476.2. Elem. Anal.: C,
88.29%; H, 5.16%; S, 6.78%. Found: C, 88.20%; H, 5.08%; S, 6.73%.
2.2.4. Single-crystal X-ray crystallography
The single crystals of three compounds were grown from a
mixture of CH2Cl2 and CH3OH. X-ray crystal structure

J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
147
Scheme 1. Synthetic procedures and structures of DPCP 1e3.
determinations were performed on a Bruker SMART APEX CCD
diffractometer with graphite monochromated Mo-Ka radiation
devices were prepared in vacuum at a pressure of 5 ꢂ 10ꢁ6 Torr. All
organics were thermally evaporated at a rate of 1.0 Å Sꢁ1 at a base
pressure of around 3.5 ꢂ 10ꢁ4 Pa. A LiF layer (0.5 nm) was deposited
at a rate of 0.2 Å Sꢁ1. The finishing Al electrode (cathode) was
(
l
¼ 0.71073 Å) using the SMART and SAINT programs. The struc-
tures were solved by direct methods and refined on F2 by full-
matrix least-squares methods with SHELXTL version 5.1. All non-
hydrogen atoms were refined anisotropically. In DPCP 2, one S
and one C atoms are positional disorder and occupy the same po-
sition. Crystallographic data for DPCP 1e3 have been deposited in
the Cambridge Crystallographic Data Centre with reference
numbers CCDC 1052061e1052063. A summary of the crystallo-
graphic data and structure refinements of DPCP 1e3 are tabulated
in Table S1. The selected bond lengths and angles for DPCP 1e3 are
listed in Table S2.
2.2.5. OLED fabrication and measurements
Before device fabrication, the emitting complexes of DPCP 3,
[1,4-bis[(1-naphthylphenyl)amino]-biphenyl] (NPB) [25] and
[bis(2-(2-hydroxylphenyl)-pyridine)beryllium] (Bepp2) [26] were
prepared and purified by sublimation, and ITO glass substrates
were pre-cleaned carefully and treated by UV/O3 for 2 min. The
Fig. 2. UVevis absorption (a) and photoluminescence (b) spectra of DPCP 1e3 in
CH2Cl2 solution (5 ꢂ 10ꢁ5 M). Inset: fluorescent images of DPCP 1e3 in CH2Cl2 solu-
tions under 365 nm UV irradiation.
Fig. 1. The TGA curves of DPCP 1e3.

148
J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
Table 1
Photophysical properties and energy levels of DPCP 1e3.
labsa/nm
lema/nm
lemb/nm
FFa (%)
FFc (%)
FFd (%)
FF (%)
D
Eg/eVf
ELUMO/eV
EHOMO/eV
e
DPCP 1
DPCP 2
DPCP 3
368
370
366
471
465
457
483
491
475
8.96
31.82
37.28
1.28
15.73
16.2
1.52
2.24
24.61
1.89
2.18
26.12
2.92
2.90
2.94
ꢁ2.14
ꢁ2.21
ꢁ2.38
ꢁ5.06
ꢁ5.10
ꢁ5.32
a
Measured in dichloromethane solution (5 ꢂ 10ꢁ5 M).
Measured in powder.
b
c
d
e
f
Measured in acetone (5 ꢂ 10ꢁ5 M).
Measured in water/acetone mixtures (5 ꢂ 10ꢁ5 M).
Measured in film.
Data obtained from CV.
deposited at a rate of 10 Å Sꢁ1 in another chamber. The thicknesses
of the organic materials and the cathode layers were controlled
using a quartz crystal thickness monitor. The electrical character-
istics of the devices were measured with a Keithley 2400 source
meter. The EL spectra and luminance of the devices were obtained
on a PR650 spectrometer. All the devices fabrication and device
characterization steps were carried out at room temperature under
ambient laboratory conditions. Current-Voltage characteristics of
single-carrier devices were measured using the same semi-
conductor parameter analyzer as for OLEDs.
ether as eluent. The structures were characterized by elemental
analyses, 1H NMR, 13C NMR, MS and single-crystal X-ray diffraction.
All of three compounds have good solubility in most organic sol-
vents at room temperature, such as n-hexane, dichloromethane,
tetrahydrofuran, acetonitrile and methanol.
3.2. Thermal and optical properties
The thermal properties of the compounds were carried out by
thermal gravimetric analysis (TGA) (Fig. 1) and by differential
scanning calorimetry (DSC) (Fig. S1). The decomposition tempera-
tures (Td) were around 239, 317 and 383 ꢀC for DPCP 1, DPCP 2 and
DPCP 3, respectively. It is very clear that Td increase in the order of
molecular weight. DSC measurements were examined under the
nitrogen atmosphere. As shown in Table S3 and Fig. S1, all three
compounds have high melting points (94 ꢀC, 208 ꢀC and 130 ꢀC for
DPCP 1, DPCP 2 and DPCP 3, respectively) upon the first heating
process. Only the glass transition temperatures were observed
upon the second heating process which indicated an extreme
tendency toward amorphous state of synthesized compounds [27].
DPCP 2 and DPCP 3 exhibited glass transition temperature (Tg) at
52 and 95 ꢀC, respectively. Compared with DPCP 1 and DPCP 2,
DPCP 3 showed a higher Tg and Td values. This result proved to be
the advantage of the series of DPCP derivatives in terms of thermal
stabilities, especially for DPCP 3, which can prevent them from
decomposing during the process of vacuum deposition as well as
device working.
3. Results and discussion
3.1. Synthesis
The synthetic route for DPCP 1e3 and their chemical structures
are shown in Scheme 1. The intermediates 1,3,5-triaryl-1,5-
pentanediones obtained from the aldol condensation reaction of
appropriate aryl aldehydes and acetyl benzene were cyclized in Zn/
CH3COOH system and dehydrated in the presence of concentrated
hydrochloric acid to give 1,2-diphenyl-4-thiophenyl-1,3-
cyclopentadiene (DPCP 1) as yellow solid in good yields of 78%.
DPCP 2 and DPCP 3 were readily prepared via Suzuki coupling
reactions
between
1,2-diphenyl-4-(4-bromophenyl)-1,3-
cyclopentadiene and corresponding boric acid in the presence of
K2CO3 as base source and Pd(PPh3)4 as catalyst with yields of 76%
and 85%, respectively. The products were purified by column
chromatography on silica gel using dichloromethaneepetroleum
The UVevis absorption and the photoluminescent (PL) spectra
of DPCP 1, DPCP 2 and DPCP 3 in dilute solutions (5 ꢂ 10ꢁ5 M) are
shown in Fig. 2 and the corresponding data are summarized in
Table 1. Intense and similar absorption bands were observed in the
ultraviolet part of the spectrum between 320 and 400 nm,
assigning to the spin-allowed
pe
p* transition of the
Fig. 3. Photoluminescence (PL) spectra of DPCP 1e3 in powders. Inset: photographic
images of powders DPCP 1 (a and b), DPCP 2 (c and d) and DPCP 3 (e and f) under
daylight (a, c, e) and 365 nm UV irradiation (b, d, f), respectively.
Fig. 4. Energy level alignment and pictorial drawings of HOMO and LUMO of DPCP
1e3.

J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
149
cyclopentadiene moiety and conjugated backbone structure. The
other absorption band at about 270 nm was attributed to the
cyclopentadiene unit. The UVevis absorption of 1,2,4-
triphenylcyclopenta-1,3-diene [21b] also exhibited two distinct
bonds at 266 and 352 nm (Fig. S2). The first band of the absorption
spectra of DPCP 2 was red-shifted compared with that of DPCP 1
because of the addition of another phenyl group. Their UVevis
absorption in solvents with different polarities such as n-hexane,
dichloromethane, THF, acetonitrile and methanol had also been
investigated (Fig. S3). Apparently, the maximal absorption intensity
increased and the fluorescence quantum yields mainly decreased
with the increase of the solvent polarity.
As shown in Fig. 2b, three compounds showed blue PL emission
with the maximum wavelength at 471, 465, and 457 nm in the
dilute solution, respectively. Fluorescent peaks of DPCP 1e3
exhibited slight red-shifts and the fluorescent intensity decreased
Fig. 5. UVevis absorption and photoluminescence (PL) spectra of DPCP 1 (a, b), DPCP 2 (c, d) and DPCP 3 (e, f) in water/acetone mixtures with different volume fractions of water.
Concentration: 5 ꢂ 10ꢁ5 M.

150
J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
in polar solvents compared with those in low polar ones (Fig. S3).
This indicated that the polarity of the excited state is lower than
that of the ground state of the compounds. All the compounds in
the solid state also exhibit strong blue PL emission with the
different degree red-shifted compared with those in solutions
(Fig. 3). Furthermore, the fluorescence intensity and fluorescence
quantum yields strengthened gradually from DPCP 1 to DPCP 3.
The detailed data are summarized in Table 1. According to the above
analysis, DPCP 3 shows bright blue emission in both solution and
solid state with the highest fluorescence quantum yields.
is favorable for hole injection and transportation in OLEDs. Among
the three compounds, the oxidation peak of DPCP 3 exhibited the
highest potential. This indicated that DPCP 3 need the highest
energy to remove an electron out of its HOMO. In agreement with
the case in the absorption spectra, the peak blue-shift is the result
of the decreased HOMO level of DPCP 3.
To further investigate the origin of the highest occupied mo-
lecular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels, quantum mechanical computations were carried
out with the optimized geometry using the Gaussian 09 program at
the B3LYP/6-31G level. The plots and data of HOMOs and LUMOs
are shown in Fig. 4. The LUMOs and the electron clouds of the
HOMOs of DPCP 1 and DPCP 2 dispersed on the whole molecules,
showing the weak intramolecular charge transfer of these com-
pounds. On the HOMO orbital of DPCP 3, the electron cloud is
mainly located at the central cyclopentadiene core and surrounding
aromatic rings as well as sulfur atom in dibenzothiophene. The
3.3. Electrochemical properties and theoretical calculation
The electrochemical properties of DPCP 1e3 were investigated
by cyclic voltammetry (CV) which employed a conventional three-
electrode cell containing a Pt working electrode, a glassy carbon
counter electrode and an Ag/AgCl reference electrode. The CV
curves of compounds are shown in Fig. S4. All of the compounds
DPCP 1e3 displayed oxidation peaks in the CV diagrams. The
HOMO energy levels were obtained from the onset oxidation po-
electron cloud in LUMO level was mainly composed of
p
and p*
orbitals of the cyclopentadiene and aromatic rings, showing
electron-donating character of dibenzothiophene.
tentials. The energy band gaps (DEg) of the compounds were esti-
mated from the onset wavelength of the UV absorptions. The LUMO
energy levels were determined from the HOMO energy together
with energy band gap. The resulting data are also summarized in
Table 1 and the HOMO levels are in the region of ꢁ5.06 to 5.32 eV.
These are very closed to the work function of ITO anode [28], which
3.4. AIEE properties
To confirm the possible AIEE properties of all compounds, their
fluorescent behaviors with water added into acetone were studied
[29]. The UVevis absorption and PL spectra for DPCP 1e3 in the
Fig. 6. UVevis absorption and photoluminescence (PL) spectra of DPCP 1 (a, b) and DPCP 3 (c, d) in glycerolemethanol mixtures with different volume fractions of glycerol.
Concentration: 5 ꢂ 10ꢁ5 M.

J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
151
Table 2
spectra in a mixture of methanol and glycerol, with different vol-
ume proportion was tested (Fig. 6) [30]. The emission intensities
drastically increased on the continued addition of glycerol. It
indicated that glycerol restricted the rotation of single bond of the
molecules, thus generating AIEE phenomenon.
The dihedral angles of the selected planes of the compounds.
Dihedral angle/ꢀ
P1eP2
P1eP3
P1eP4
P4eP5
3.5. Crystal structures
DPCP-1-I
DPCP-1-II
DPCP-2-I
DPCP-2-II
DPCP-3
55.80
61.90
39.41
40.51
61.58
27.39
31.46
39.41
40.51
31.76
25.59
16.14
29.48
5.88
e
e
In order to further understand the relationship between pho-
tophysical properties and structures, the single crystals of com-
pounds DPCP 1e3 were successfully obtained from
dichloromethane/methanol mixtures. Single crystal X-ray analysis
revealed that DPCP 1e3 crystallize in orthorhombic crystal system
with space group Pna21, Fdd2 and Pca21, respectively (Table S1). All
molecules adopted non-coplanar conformations and possessed
different packing patterns because of strict repulsion which can
influence some of their electronic and physical properties such as a
suppression of the conjugation [31]. Table 2 gives the selected
dihedral angles between the substituent moieties and central
cyclopentadiene ring of the three compounds. As shown in Fig. 7a,
there are two constituent molecules (I and II) in the asymmetric
unit of DPCP 1. Two phenyl rings and one thiophenylring are forced
to twist out of the cyclopentadiene plane with dihedral angles in
the range 55.80ꢀ, 27.39ꢀ and 25.59ꢀ, respectively. Interestingly, the
3.59
21.13
37.35
7.00
water/acetone mixtures with different volume proportion are
shown in Fig. 5. From the absorption spectra we can see that a level-
off tail caused by the formation of aggregates was seen in the visible
spectral region when the water fraction was over 50%. In addition,
both DPCP 1 and DPCP 3 displayed an obvious red-shift with
increasing water contents, which indicated the capable formation of
J-aggregation [28]. On the contrary, DPCP 2 displayed the opposite
effect with blue-shift in the absorption spectra. Such phenomenon
is the characteristic of the formation of H-aggregation [10]. In the PL
spectra, the emission intensities kept unchanged when the water
fraction was in the range 0e60% for DPCP 1 and 0e30% for DPCP 3,
then dramatically increased with the continued addition of water
and the maximum wavelength was red-shifted to a certain extent.
However, with the increase of water fraction, PL intensity for DPCP 2
decreased and the maximal emission peak moved to longer wave
which can be interpreted as fluorescence caused quenching.
To explore whether or not restriction of intramolecular rotation
is responsible for AIEE of DPCP 1 and DPCP 3, their fluorescence
spiral chain structure (Fig. 7b) derives from the weak CeH…
teractions between adjacent II-type single molecules. These chains
and I-type molecules are further packed without strong aromatic
p…p interactions (Fig. 7c). This packing structure could improve
the fluorescence intensity in aggregation state.
p in-
In DPCP 2, the molecule has two different spatial conformations,
DPCP 2-I and DPCP 2-II (Fig. 8a). Two phenyl rings connected with
Fig. 7. (a) Crystal structure of DPCP 1, (b) spiral chain structure and (c) packing arrangements in the crystal of DPCP 1. The dotted line represented CeH…
p interactions.

152
J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
Fig. 8. Molecular structures (a) and packing arrangements (b and c) in the crystal structure of DPCP 2.
Fig. 9. Molecular structures (a), dimer structure (b) and packing arrangements (c) in the crystal structure of DPCP 3.

J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
153
cyclopentadiene core were twisted out with dihedral angles in the
range of 31.46e39.41ꢀ. The dihedral angle (3.59ꢀ) between thio-
phenylring and neighboring phenyl rings in DPCP DPCP 2-I is
smaller than that (21.13ꢀ) in DPCP 2-II. Compared with DPCP 1, the
effective conjugation length in the molecule was increased by using
4-thiophenyl-phenyl to replace thiophenyl moieties. As shown in
Fig. 8b, both DPCP 2-I and DPCP 2-II columns were formed. There is
(30 nm)/Bepp2 (50 nm)/LiF (10 nm)/Al (200 nm)] was fabricated.
The device configuration and the energy diagram of the materials
used in the EL device above are shown in Fig. 10a and b. NPB and
Bepp2 were employed as the hole transport layer (HTL) and elec-
tron transport layer (ETL), respectively, and the neat film DPCP 3
was adopted as the emitting layer (EML). Fig. 10c shows the EL
spectrum of this device at the brightness of 100 cd mꢁ2. It shows
the emission peak at around 462 nm with the Commission Inter-
no noteworthy intermolecular
p…p interaction within each mo-
ꢀ
lecular column, but alike H-aggregate enables the compound to
have AIEE-inactive characteristics.
nationale de I'Eclairage (CIEx,y) coordinates of (0.16, 0.16) and
remain almost unchanged in the driving voltage range from 4 V to
8 V as shown in Fig. 10d, which is consistent well with the PL
spectral characteristics of this compound. The current densi-
tyevoltageeluminance (JeVeL) characteristics and the luminance
efficiency (LE) and power efficiency (PE) plotted with respect to the
voltage of the device are shown in Fig. S5. All the devices displayed
low driving voltages, and the corresponding current density and
luminance exhibited sustained increase upon increasing driving
voltage. The quite low turn-on voltage about 3.2 V was realized due
to the easily charge injection from the HTL and/or ETL into the EML.
The driving voltage for the luminance of 100 cd mꢁ2 is 5.1 V and the
maximum luminance of 2277 cd mꢁ2 is obtained at 8.5 V. More
importantly, the LE and PE values of 0.42 cd Aꢁ1 and 0.26 lm Wꢁ1
were achieved at the practical luminance of 100 cd mꢁ2. Such EL
performance is comparable to the non-doped blue OLEDs based on
The molecular structure and packing structure of DPCP 3 are
shown in Fig. 9. The dihedral angle between the central cyclo-
pentadiene plane and the connector benzene plane is only 7.00ꢀ,
and terminal 4-dibenzothiphene plane has certain distortions from
the bulky conjugation skeletons with dihedral angles of 37.35ꢀ.
There is aromatic C-H…p interactions among adopt parallel
arrangement model with two kinds of molecular two single mol-
ecules with an interaction distance of 2.97 Å to form a dimer
(Fig. 9b) as X-aggregates which could increase the resistance be-
tween molecules. There are no strong aromatic
pep interactions
because the centerecenter distance between two molecules is
8.473 Å along b-axis. This structure feature further avoided the
maximum face-to-face stacking (Fig. 9c) to improve the fluores-
cence intensity in aggregation state. According to crystal structures
and the spectroscopy, the AIEE properties of DPCP 1, DPCP 3 should
be attributed to combined effects of restricted intramolecular
rotation (RIR) and the formation of J- and X-aggregations.
cyclopentadiene derivatives with
a
maximum luminance
950 cd mꢁ2 [19]. Although there is no optimization of device con-
figurations, but DPCP 3 shows good performance.
3.6. Electroluminescent properties
4. Conclusions
DPCP 3 with the highest fluorescence quantum yield as well as
high thermal stability indeed has the potential as blue-emitting
material. To evaluate the EL performance of DPCP 3, a non-doped
OLED with a simple configuration of [ITO/NPB (20 nm)/DPCP 3
In conclusion, we have successfully designed and simply syn-
thesized three cyclopentadiene derivatives by introducing thio-
phene ring and dibenzothiphene group. DPCP 1 and DPCP 3
exhibited typical AIEE character as a result of the combined effects
Fig. 10. (a) The EL device structure and the chemical structure of NPB and Bepp2; (b) the energy level diagram of the materials in EL device; (c) EL, PL spectra and CIE coordinates of
device (inset); (d) EL spectra of devices at different applied voltages.

154
J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
[6] Li C, Yan H, Zhao LX, Zhang GF, Hu Z, Huang ZL, et al.
A
trident
of restricted intramolecular rotation (RIR) and the formation of J-
and X-aggregations. These compounds emit strong blue fluores-
cence both in solutions and in powders. In addition, they showed
high thermal stability with the decomposition temperature of
239e363 ꢀC and appropriate energy levels with the region of
5.06e5.32 eV which is favorable for hole injection and trans-
portation in OLEDs. Non-doped three-layer blue OLEDs based on
DPCP 3 has a high performance with a turn-on voltage of 3.2 V and
the maximum luminance of 2277 cd mꢁ2 as well as the CIE co-
ordinates of (0.16, 0.16). This work presents a promising approach
for designing new luminescent materials with AIEE character and
gives some efficient evidence for understanding the relationship
between molecular structures and photophysical properties.
dithienylethene-perylenemonoimide dyad with super fluorescence switching
speed and ratio. Nat Commun 2014;5:5709e19.
[7] (a) Chen CT. Evolution of red organic light-emitting diodes: materials and
devices. Chem Mater 2004;16:4389e400.
(b) Yang MD, Zhang Y, Zhu WJ, Wang HZ, Huang J, Cheng LH, et al. Difunc-
tional chemosensor for Cu(II) and Zn(II) based on Schiff base modified anthryl
derivative with aggregation-induced emission enhancement and piezochro-
mic characteristics. J Mater Chem C 2015;3:1994e2002.
(c) Zhang Y, Haske W, Cai D, Barlow S, Kippelen B, Marder S. Efficient blue-
emitting electrophosphorescent organic light-emitting diodes using 2-(3,5-
di (carbazol-9-yl) phenyl)-5-phenyl-1, 3, 4-oxadiazole as an ambipolar host.
RSC Adv 2013;3:23514e20.
[8] (a) Chan LH, Lee RH, Hsieh CF, Yeh HC, Chen CT. Optimization of high-
performance blue organic light-emitting diodes containing tetraphenylsilane
molecular glass materials. J Am Chem Soc 2002;124:6469e79.
(b) Zeng Q, Li Z, Dong YQ, Di CA, Qin AJ, Hong YN, et al. Fluorescence en-
hancements of benzene-cored luminophors by restricted intramolecular ro-
tations: AIE and AIEE effects. Chem Commun 2007:70e2.
Acknowledgment
(c) Yuan WZ, Lu P, Chen S, Lam JWY, Wang ZM, Liu Y, et al. Changing the
behavior of chromophores from aggregation-caused quenching to
aggregation-induced emission: development of highly efficient light emitters
in the solid state. Adv Mater 2010;22:2159e63.
(d) Tang X, Yao L, Liu H, Shen FZ, Zhang ST, Zhang HH, et al. An efficient AIE-
active blue-emitting molecule by incorporating multifunctional groups into
tetraphenylsilane. Chem Eur J 2014;20:7589e92.
This work was supported by the National Natural Science
Foundation of China (51003009), the Fundamental Research Funds
for the Central Universities of China (DUT14LK32) and the Science
and Technology Research Foundation of Education Department of
Liaoning Province (L2014033).
[9] Luo JD, Xie ZL, Lam JWY, Cheng L, Chen HY, Qiu CF, et al. Aggregation-induced
emission of 1-methyl-1, 2, 3, 4, 5-pentaphenylsilole. Chem Commun 2001;18:
1740e1.
[10] An BK, Kwon SK, Jung SD, Park SY. Enhanced emission and its switching in
fluorescent organic nanoparticles. J Am Chem Soc 2002;124:14410e5.
[11] (a) Barr JW, Bell TW, Catalano VJ, Cline JI, Phillips DJ, Procupez R. Syntheses,
structures, and photoisomerization of (E)-and (Z)-2-tert-butyl-9-(2, 2, 2-
triphenylethylidene) fluorene. J Phys Chem A 2005;109:11650e4.
(b) Huang J, Sun N, Yang J, Tang R, Li QQ, Ma DG, et al. Blue aggregation-
induced emission luminogens: high external quantum efficiencies up to
3.99% in LED device, and restriction of the conjugation length through rational
molecular design. Adv Funct Mater 2014;24:7645e54.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.09.018.
References
(c) Huang J, Jiang Y, Yang J, Tang R, Xie N, Li QQ, et al. Construction of efficient
blue AIE emitters with triphenylamine and TPE moieties for non-doped
OLEDs. J Mater Chem C 2014;2:2028e36.
[1] (a) D'Andrade BW, Forrest SR. White organic light-emitting devices for solid-
state lighting. Adv Mater 2004;16:1585e95.
(b) Ye SH, Chen JM, Di C, Liu YQ, Lu K, Wu WP, et al. Phenyl-substituted
fluorene-dimer cored anthracene derivatives: highly fluorescent and stable
materials for high performance organic blue-and white-light-emitting diodes.
J Mater Chem 2010;20:3186e94.
(c) Jung BJ, Yoon CB, Shim HK, Do LM, Zyung T, et al. Pure-red dye for organic
electroluminescent devices: bis-condensed DCM derivatives. Adv Mater
2001;11:430e4.
[12] (a) Chen J, Law CCW, Lam JWY, Dong YP, Lo SMF, Williams LD, et al. Synthesis,
light emission, nanoaggregation, and restricted intramolecular rotation of 1,
1-substituted 2, 3, 4, 5-tetraphenylsiloles. Chem Mater 2003;15:1535e46.
(b) Zhang P, Wang HT, Liu HM, Li M. Fluorescence-enhanced organogels and
mesomorphic superstructure based on hydrazine derivatives. Langmuir
2010;26:10183e90.
[13] (a) Qian G, Dai B, Luo M, Yu DB, Zhan J, Zhang ZQ, et al. Band gap tunable,
donoreacceptoredonor charge-transfer heteroquinoid-based chromophores:
near infrared photoluminescence and electroluminescence. Chem Mater
2008;20:6208e16.
(d) Zhu MR, Yang CL. Blue fluorescent emitters: design tactics and applications
in organic light-emitting diodes. Chem Soc Rev 2013;42:4963e76.
(e) Gong SL, Sun N, Luo JJ, Zhong C, Ma DG, et al. Highly efficient simple-
structure blue and all-phosphor warm-white phosphorescent organic light-
emitting diodes enabled by wide-band gap tetraarylsilane-based functional
materials. Adv Funct Mater 2014;24:5710e8.
(b) Qian Y, Li SY, Zhang GQ, Wang Q, Wang SQ, Xu HJ, et al. Aggregation-
induced emission enhancement of 2-(20-hydroxyphenyl) benzothiazole-based
excited-state intramolecular proton-transfer compounds.
J Phys Chem B
[2] (a) Huang ZY, Chiu SW, Chen CW, Chen YH, Lin LY, Wong KT, et al. Sponta-
neous formation of light-trapping nano-structures for top-illumination
organic solar cells. Nanoscale 2014;6:2316e20.
2007;111:5861e8.
(c) Huang C, Barlow S, Marder SR. Perylene-3, 4, 9, 10-tetracarboxylic acid
diimides: synthesis, physical properties, and use in organic electronics. J Org
Chem 2011;8:2386e407.
(b) Harafuji K, Sato H, Matsuura T, Omoto Y, Kaji T, Hiramoto M, et al.
Degradation in organic solar cells under illumination and electrical stresses in
air. Jpn J Appl Phys 2014;53:122303.
[14] (a) Feng X, Hu JY, Iwanaga F, Seto N, Redshaw C, Elsegood MRJ, et al. Blue-
emitting butterfly-shaped 1, 3, 5, 9-tetraarylpyrenes: synthesis, crystal
structures, and photophysical properties. Org Lett 2013;15:1318e21.
(b) Zhang Y, Ng TW, Lu F, Tong QX, Lai SL, Chan MY, et al. A pyrene-phe-
nanthroimidazole derivative for non-doped blue organic light-emitting de-
vices. Dyes Pigments 2013;98:190e4.
[15] (a) Sato T, Jiang DL, Aida T. A blue-luminescent dendritic rod: poly (phenyl-
eneethynylene) within a light-harvesting dendritic envelope. J Am Chem Soc
1999;121:10658e9.
[3] (a) Chen CP, Huang CY, Chuang SC. Organic photovoltaics: highly thermal
stable and efficient organic photovoltaic cells with crosslinked networks
appending open-cage fullerenes as additives. Adv Funct Mater 2015;25:206.
(b) Atkin P, Farid MM. Improving the efficiency of photovoltaic cells using
PCM infused graphite and aluminium fins. Sol Energy 2015;114:217e28.
ꢀ
(c) Ottone C, Berrouard P, Louarn G, Beaupre S, Gendron D, Zaqorska M, et al.
Donoreacceptor alternating copolymers containing thienopyrroledione elec-
tron accepting units: preparation, redox behaviour, and application to
photovoltaic cells. Polym Chem 2012;3:2355e65.
(b) Zhao QL, Zhang S, Liu Y, Mei J, Chen SJ, Lu P, et al. Tetraphenylethenyl-
modified perylenebisimide: aggregation-induced red emission, electro-
chemical properties and ordered microstructures. J Mater Chem 2012;22:
7387e94.
[4] (a) Xu JP, Fang Y, Song ZG, Mei J, Jia L, Qin AJ, et al. BSAetetraphenylethene
derivative conjugates with aggregation-induced emission properties: fluo-
rescent probes for label-free and homogeneous detection of protease and a1-
antitrypsin. Analyst 2011;136:2315e21.
(b) Fan Z, Zhou H, Li PY, Speer JE, Cheng H, et al. Structural elucidation of cell
membrane-derived nanoparticles using molecular probes. J Mater Chem B
2014;2:8231e8.
(c) Zhang QC, Divayana Y, Xiao JC, Wang ZJ, Tiekink ERT, Doung HM, et al.
Synthesis, characterization, and bipolar transporting behavior of
a new
twisted polycyclic aromatic hydrocarbon: 10,40-diphenyl-naphtho-(20.30:1.2)-
pyrene-60-nitro-70-methyl carboxylate. Chem Eur J 2010;25:7422e6.
[16] Adachi C, Tsutsui T, Saito S. Blue light-emitting organic electroluminescent
devices. Appl Phys Lett 1990;56:799e801.
(c) Zhao J, Ji S, Chen Y, Guo HM, Yang P, et al. Excited state intramolecular
proton transfer (ESIPT): from principal photophysics to the development of
new chromophores and applications in fluorescent molecular probes and
luminescent materials. Phys Chem Chem Phys 2012;14:8803e17.
[5] (a) van de Linde S, Sauer M. How to switch a fluorophore: from undesired
blinking to controlled photoswitching. Chem Soc Rev 2014;43:1076e87.
(b) Gaillard C, Adumeau P, Canet JL, Gautier A, Boyer D, Beaudoin C, et al.
Monodisperse silica nanoparticles doped with dipicolinic acid-based lumi-
nescent lanthanide (III) complexes for bio-labelling. J Mater Chem B 2013;1:
4306e12.
[17] Yoshino K, Tada K, Hirohata M, Kajii H, Hironaka Y, Tada N, et al. Novel
properties of molecularly doped conducting polymers and junction devices.
Synth Met 1997;84:477e82.
[18] Zhao YS, Fu H, Hu F, Peng AD, Yao J. Multicolor emission from ordered as-
semblies of organic 1D nanomaterials. Adv Mater 2007;19:3554e8.
[19] Gao XC, Cao H, Huang L, Huang YY, Zhang BW, Huang CH. Comparison of the
electroluminescence and its related properties of two cyclopentadiene de-
rivatives. Appl Sur Sci 2003;210:183e9.

J. Ye et al. / Dyes and Pigments 124 (2016) 145e155
155
[20] Ohmori Y, Hironaka Y, Yoshida M, Fujii A, Yoshino K. Enhancement of elec-
troluminescence intensity from dye-doped poly(3-alkylthiophene) light
[25] Hirai Y, Uozumi Y. Clean synthesis of triarylamines: BuchwaldeHartwig re-
action in water with amphiphilic resin-supported palladium complexes.
Chem Commun 2010;46:1103e5.
emitting diode with different alkyl-side-chain length. Jpn
J Appl Phys
1996;35:4105e9.
[26] (a) Li YQ, Liu Y, Bu WM, Lu D, Wu Y, Wang Y. Hydroxyphenyl-pyridine
[21] (a) Yang LJ, Ye JW, Xu LF, Yang XY, Gong WT, Lin Y, et al. Synthesis and
properties of aggregation-induced emission enhancement compounds
derived from triarylcyclopentadiene. RSC Adv 2012;2:11529e35.
(b) Zhang XD, Ye JW, Xu LF, Yang LJ, Deng D, Ning GL. Synthesis, crystal
structures and aggregation-induced emission enhancement of aryl-
substituted cyclopentadiene derivatives. J Lumin 2013;139:28e34.
(c) Ning GL, Li XC, Munakata M, Gong WT, Maekawa M, Kamikawa T. Con-
version of phenyl-substituted cyclopentadienes to pyrylium cations. J Org
Chem 2004;69:1432e4.
beryllium complex (Bepp2) as a blue electroluminescent material. Chem
Mater 2000;12:2672e5.
(b) Liu Y, Guo JH, Feng J, Zhang HD, Li YQ, Wang Y. High-performance blue
electroluminescent devices based on hydroxyphenyl-pyridine beryllium
complex. Appl Phys Lett 2001;78:2300e2.
[27] Shih PI, Chuang CY, Chien CH, Diau EWG, Shu CF. Highly efficient non-doped
blue-light-emitting diodes based on an anthrancene derivative end-capped
with tetraphenylethylene groups. Adv Funct Mater 2007;17:3141e6.
[28] (a) Li HY, Chi ZG, Xu BJ, Zhang XQ, Li XF, Liu SW, et al. Aggregation-induced
(d) Zhang XD, Ye JW, Wang SN, Gong WT, Lin Y, Ning GL. Heterocyclic radical
mediated synthesis and fluorescence properties of conjugated polyene ke-
tones. Org Lett 2011;13:3608e11.
emission
enhancement
compounds
containing
triphenylamine-
anthrylenevinylene and tetraphenylethene moieties. J Mater Chem 2011;21:
3760e7.
(e) Yang LJ, Ye JW, Gao Y, Deng D, Lin Y, Ning GL. One-step stereoselective
synthesis of (2Z, 4Z, 6Z, 8Z)-decatetraene diketone from pyrylium salts. Eur J
Org Chem 2014;3:515e22.
(b) Hirata S, Kubota K, Jung HH, Hirata O, Goushi K, Yahiro M, et al.
Improvement of electroluminescence performance of organic light-emitting
diodes with a liquid-emitting layer by introduction of electrolyte and a
hole-blocking layer. Adv Mater 2011;23:889e93.
[22] (a) Jeong SH, Lee JY. Dibenzothiophene derivatives as host materials for high
efficiency in deep blue phosphorescent organic light emitting diodes. J Mater
Chem 2011;21:14604e9.
[29] Zhang XQ, Chi ZG, Li HY, Xu BJ, Li XF, Liu SW, et al. Synthesis and properties of
novel aggregation-induced emission compounds with combined tetraphe-
(b) Huang TH, Whang WT, Shen JY, Lin JT, Zheng HG. Organic electrolumi-
nescent derivatives containing dibenzothiophene and diarylamine segments.
J Mater Chem 2005;15:3233e40.
nylethylene and dicarbazolyltriphenylethylene moieties. J Mater Chem
2011;21:1788e96.
[30] Kim KH, Bae SY, Kim YS, Hur JA, Hoang MH, Lee TW, et al. Highly photosen-
sitive J-aggregated single-crystalline organic transistors. Adv Mater 2011;23:
3095e9.
[23] (a) Dawson WR, Windsor MW. Fluorescence yields of aromatic compounds.
J Phys Chem 1968;72:3251e60.
(b) Crosby GA, Demas JN. Measurement of photoluminescence quantum
yields. J Phys Chem 1971;8:991e1024.
[24] Huang C, Zhen CG, Su SP, Loh KP, Chen ZK. Solution-processable poly-
phenylphenyl dendron bearing molecules for highly efficient blue light-
emitting diodes. Org Lett 2005;7:391e4.
[31] Prachumrak N, Namuangruk S, Keawin T, Jungsuttingwong S, Sudyoadsuk T,
Promarak V. Synthesis and characterization of 9-(fluoren-2-yl) anthracene
derivatives as efficient non-doped blue emitters for organic light-emitting
diodes. Eur J Org Chem 2013;2013:3825e34.