Spiro[fluorene-9,9′-xanthene]-based universal hosts for understanding structure-property relationships in RGB and white PhOLEDs

Article abstract of DOI:10.1039/c5ra00694e

Ingenious construction of state-of-the-art models of electrophosphorescent hosts for uncovering the deep role of different molecular modifications in PhOLEDs performances is crucial for rational cumulative improvements of device efficiency and for acceler

Full text of DOI:10.1039/c5ra00694e

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Spiro[fluorene-9,90-xanthene]-based universal
hosts for understanding structure–property
relationships in RGB and white PhOLEDs†
Cite this: RSC Adv., 2015, 5, 29828
Yan Qian,‡a Yeren Ni,‡a Shouzhen Yue,‡b Wenwen Li,a Shufen Chen,a
b
a
a
b
ac
*
*
*
Zhensong Zhang, Linghai Xie, Mingli Sun, Yi Zhao and Wei Huang
Ingenious construction of state-of-the-art models of electrophosphorescent hosts for uncovering the
deep role of different molecular modifications in PhOLEDs performances is crucial for rational
cumulative improvements of device efficiency and for accelerating their commercialization. A series of
conjugation-interrupted spiro[fluorene-9,90-xanthene](SFX)/9-arylfluorenes (AFs) hybrid host materials
have been designed and synthesized by concise two-step reactions, and have been demonstrated to
serve as universal hosts for low voltage blue, green, red and single-emitting layer white PhOLEDs. By
varying the electronegativity and position of the bulky AF substitutes, two independent, selective
methods for fine tuning frontier molecular orbital energy levels without affecting the high triplet energy
level (T1) have been realized. This offers either facilitated hole- or electron-injection/blocking without
influencing the optical properties, which can explain the performance of the corresponding RGB
PhOLEDs. This investigation provides good guidance for the future rational design of high-performance
PhOLED hosts by accumulative structural modifications.
Received 13th January 2015
Accepted 9th March 2015
DOI: 10.1039/c5ra00694e
www.rsc.org/advances
organic light-emitting diodes (WOLEDs) for full-color, at panel
displays and solid-state lighting.4 Universal hosts with blue
1. Introduction
PhOLED EQEs of over 20% have been reported by several
groups.5–7
Phosphorescent organic light-emitting diodes (PhOLEDs) have
attracted intense interest because they can approach 100%
internal quantum efficiency by utilizing both singlet and triplet
excitons,1,2 and are competing with uorescent tube and inor-
ganic LEDs as the next-generation illumination technology. To
achieve high efficiency of PhOLEDs, phosphorescent emitters of
heavy-metal complexes are usually doped into a suitable host
matrix to suppress detrimental effects, such as aggregation
quenching and triplet–triplet annihilation.3 In recent years,
universal RGB hosts that can host different-colored blue, green
and red phosphors have attracted much attention due to their
great potential for the simple and cost-effective fabrication of
highly efficient, all-phosphor, single-emitting-layer, white
However, to date, reports on universal RGB hosts, especially
those with low driving voltages and high device efficiencies, are
still limited.5–15 This is because the molecular design of desir-
able universal host materials is difficult. In one aspect, a T1
higher than blue phosphor (>2.7 eV) generally signies a large
energy bandgap, i.e., a low-lying highest occupied molecular
orbital (HOMO) level and/or a high-lying lowest occupied
molecular orbital (LUMO) level, which may cause large energy
barriers between adjacent layers in PhOLEDs and thus make
charge carrier injection difficult and necessitate high driving
voltages, thus greatly constraining their use for portable appli-
cations, such as mobile communications and consumer elec-
tronics. In another aspect, excessive tuning of FMO energy
levels of universal hosts could result in the loss of the hosting
abilities of one or more low-energy colored phosphors, due to
the possible energy level mismatch, and thus impede energy
transfer between the host and the phosphors.
Therefore, to obtain an efficient universal host for RGB
phosphors, a compromise must be reached between high T1
and ne tuning of frontier molecular orbitals (FMOs). However,
the tuning of HOMO/LUMO energy levels (determining the
electrical properties, such as carrier injection ability) caused by
molecular modications usually simultaneously alters the
HOMO/LUMO bandgaps (Eg), and thus changes both the S1 and
aKey Laboratory for Organic Electronics and Information Displays & Institute of
Advanced Materials (IAM), Jiangsu National Synergistic Innovation Center for
Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications,
Nanjing, 210046, China. E-mail: iamlhxie@njupt.edu.cn; iamwhuang@njupt.edu.cn
bState Key Laboratory on Integrated Optoelectronics, College of Electronic Science and
Engineering, Jilin University, 2699 Qianjin Street, Changchun 130012, China. E-mail:
zhao_yi@jlu.edu.cn
cKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials
(IAM), Jiangsu National Synergistic Innovation Center for Advanced Materials
(SICAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing 211816, China
† Electronic supplementary information (ESI) available: The CIE coordinates
versus the luminance of the RGB devices. See DOI: 10.1039/c5ra00694e
‡ These authors, Y. Qian, Y. R. Ni and S. Z. Yue, contributed equally.
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T1 (determining the optical properties of whether the energy been developed via unconjugated linkage of SFX and various
transfer from the host to phosphor can effectively occur). This AFs, by utilizing our previously reported mild room temperature
intrinsic contradiction between optical and electrical properties Friedel–Cras reaction.18 By varying the number, electronega-
oen induces an uncontrollable adjustment of multiple prop- tivity and position of the bulky AF substitutes, the T1 for all the
erties. It remains a big challenge to simultaneously realize a materials remains almost unchanged, but the latter two modi-
high T1 for exothermic energy transfer and nely tunable FMO cations have been demonstrated to realize respective tuning of
energy levels for balanced carrier injection and low operating the HOMO/LUMO energy levels (Scheme 1). This multiple
voltages.
selective tuning of electrical properties without sacricing the
Ingenious construction of state-of-the-art models of electro- high T1 can explain the performance of the corresponding RGB
phosphorescent hosts to uncover the deep role of each molec- PhOLEDs. This work provides highly valuable design principles
ular modication in PhOLEDs performance is crucial for for developing more efficient universal RGB hosts through
purposely cumulative improvement of device efficiency and for accumulative structural modications.
accelerating their commercialization. To overcome the intrinsic
contradiction between optical and electrical properties, the
selective modulation of a single optoelectronic characteristic,
without changing other properties, has become an urgent issue
for the rational design of high-performance organic hosts. It is
believed that the modulation of a single property is more
2. Experimental section
2.1 Materials
SFX was synthesized by our previously reported method,25 and
the tertiary AFOHs were synthesized by an addition reaction of
various phenyl Grignard reagents to uorenone. SFX/AFs hybrid
hosts were synthesized by the MeSO3H-mediated Friedel–Cras
reaction of the tertiary AFOHs with the SFX.18
important than that of multiple properties, because in this way
the uncontrollable and unexpected change of device perfor-
mance caused by simultaneous variations of multiple properties
can effectively be avoided. Xu reported a short-axis substitution
approach of dibenzothiophene-based phosphine oxide (PO) blue
and yellow hosts, which allows for selectively tuning the LUMO
level by ꢀ0.09 eV without affecting the HOMO level and the T1.16
Thus, selective modication of the electron injection (EI) and
electron transportation (ET) without affecting the hole injection
(HI), hole transportation (HT) and T1 level has been realized.
They also developed a series of ternary PO-dibenzofuran-
carbazoly blue hosts by mixed meso and short-axis linkages17
and another series of ternary PO-uorene-carbazoly blue hosts
through indirect linkage,17 to controllably reduce S1 but keep T1
constant by simultaneously enhancing the HOMO levels and
decreasing the LUMO levels (both within 0.1 eV), which facili-
tates both HT/HI and ET/EI. All these works provide effective
methods for selective single tuning of electrical properties
without changing T1 to achieve better device performance.
On this basis, it will be of more importance to realize
multiple selective tuning of various single electrical properties
without sacricing the optical properties (such as T1) in the
same series of hosts, because it helps to determine which
factors among many are benecial (positive effects) for
improving device performance and which ones are not (negative
effects). Thus, device efficiency can further be accumulatively
improved through purposive and feasible modications, one by
one. In addition, it can also help to determine which positive
factors are more effective and which negative factors are more
detrimental to improving device performance, and thus
provides better guidance for the molecular design in that which
structural modications are the top priorities. However, this
multiple selective tuning of various single electrical properties
through various structural modications without affecting the
optical properties is even more difficult than the above
mentioned single selective tuning of electrical properties, and
has seldom been reported.
20-(9-(4-Methoxyphenyl)-uoren-9-yl)spiro[uorene-9,90-xan-
thene] (MOAF-SFX). MeSO3H (1.20 ml, 9.04 mmol) was added to
15 ml of an anhydrous CH2Cl2 solution of SFX (1.0 g, 3.01 mmol)
and 9-(4-methoxyphenyl)-uoren-9-ol (1.74 g, 6.02 mmol), and
the batch was stirred over a period of 5 h at room temperature.
The resulting mixture was washed with water and extracted with
CH2Cl2. The combined organic layers were then subjected to
silica-gel column chromatography to afford 0.45 g of white solid
in a 24.8% yield. 1H NMR (400 MHz, CDCl3, d): 7.69–7.67 (d, J ¼
7.2 Hz, 2H, Ar H), 7.64–7.62 (d, J ¼ 7.6 Hz, 2H, Ar H), 7.34–7.30
(td, J ¼ 7.2 Hz, 1.2 Hz, 2H, Ar H), 7.24–7.21 (m, 2H, Ar H), 7.20–
7.12 (m, 4H, Ar H), 7.14–7.12 (d, J ¼ 7.2 Hz, 2H, Ar H), 7.09–7.05
(td, J ¼ 7.6 Hz, 1.2 Hz, 2H, Ar H), 7.00–6.96 (d, J ¼ 8.8 Hz, 1H, Ar
H), 6.91–6.89 (d, J ¼ 8 Hz, 2H, Ar H), 6.84–6.81 (dd, J ¼ 8.4 Hz,
2.4 Hz, 1H, Ar H), 6.78–6.74 (m, J ¼ 1.2 Hz, 1H, Ar H), 6.70–6.67
(dt, J ¼ 9.2 Hz, 2.8 Hz, 2H, Ar H), 6.49–6.47 (dt, J ¼ 8.8 Hz, 2.6
Hz, 2H, Ar H), 6.44–6.42 (d, J ¼ 8.4 Hz, 1H, Ar H), 6.38–6.37 (d,
J ¼ 2.4 Hz, 1H, Ar H), 3.68 (s, 3H, CH3); 13C NMR (400 MHz,
CDCl3, d): 157.92, 155.18, 151.59, 151.47, 150.03, 140.10, 139.84,
139.60, 137.74, 128.88, 128.74, 128.15, 128.10, 127.63, 127.53,
127.34, 127.18, 125.74, 125.57, 124.63, 124.61, 123.24, 119.97,
119.90, 116.77, 116.19, 113.23, 63.90, 55.16, 54.32; MALDI-TOF
In this present work, a series of spiro[uorene-9,90-xanthe-
Scheme 1 Multiple selective tuning of electrical properties whilst
ne](SFX)/9-aryluorenes (AFs) hybrid universal RGB hosts have keeping the T1 constant via various structural modifications.
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(m/z): 602.75 [M]+; Anal. calcd for C45H30O2 (602.72): C 89.67, H
5.02; found: C 89.88, H 4.96.
20-(9-(4-Fluorophenyl)-uoren-9-yl)spiro[uorene-9,90-xan-
thene] (FAF-SFX). The synthesis was similar to that of
MOAF-SFX. The residue was puried through silica-gel column
chromatography to yield 0.75 g of white solid in a 42.2% yield.
1H NMR (400 MHz, CDCl3, d): 7.70–7.68 (d, J ¼ 7.6 Hz, 2H, Ar H),
7.65–7.63 (d, J ¼ 7.6 Hz, 2H, Ar H), 7.36–7.33 (t, J ¼ 7.2 Hz, 2H,
Ar H), 7.28–7.18 (m, 6H, Ar H), 7.14–7.12 (d, J ¼ 7.6 Hz, 2H, Ar
H), 7.10–7.07 (t, J ¼ 6.8 Hz, 2H, Ar H), 6.99–6.97 (d, J ¼ 8.4 Hz,
1H, Ar H), 6.88–6.86 (d, J ¼ 8 Hz, 2H, Ar H), 6.79–6.71 (m, 4H, Ar
H), 6.64–6.60 (t, J ¼ 8.4 Hz, 2H, Ar H), 6.46–6.44 (d, J ¼ 7.6 Hz,
1H, Ar H), 6.352–6.346 (d, J ¼ 2.4 Hz, 1H, Ar H); 13C NMR
(400 MHz, CDCl3, d): 155.19, 151.52, 150.99, 150.07, 139.84,
139.70, 139.56, 129.19, 129.19, 128.93, 128.20, 128.15, 128.13,
127.70, 127.62, 127.39, 127.06, 125.68, 125.53, 124.82, 124.42,
123.28, 120.07, 119.89, 116.76, 116.23, 114.65, 114.44, 63.92,
54.26; MALDI-TOF (m/z): 590.692 [M]+; Anal. calcd for C44H27FO
(590.68): C 89.47, H 4.61; found: C 89.67, H 4.72.
2,20-Bis(9-(4-methoxyphenyl)-uoren-9-yl)spiro[uorene-9,90-
xanthene] (DMOAF-SFX). The synthesis was similar to that of
MOAF-SFX. The residue was puried through silica-gel column
chromatography to yield 0.34 g of white solid in a 12.9% yield.
1H NMR (400 MHz, CDCl3, d): 7.72–7.70 (d, J ¼ 7.6 Hz, 2H, Ar H),
7.59–7.58 (d, J ¼ 7.2 Hz, 2H, Ar H), 7.51–7.49 (d, J ¼ 7.2 Hz, 2H,
Ar H), 7.36–7.32 (td, J ¼ 7.6 Hz, 0.8 Hz, 2H, Ar H), 7.23–7.20 (t,
J ¼ 6.8 Hz, 5H, Ar H), 7.18–7.13 (m, 6H, Ar H), 7.09–7.07 (d, J ¼
7.6 Hz, 3H, Ar H), 6.98–6.94 (m, 3H, Ar H), 6.84–6.78 (m, 3H, Ar
H), 6.79–6.78 (d, J ¼ 7.2 Hz, 3H, Ar H), 6.71–6.67 (m, 3H, Ar H),
6.46–6.42 (m, 3H, Ar H), 6.14–6.14 (d, J ¼ 2 Hz, 1H, Ar H), 3.65 (s,
3H, CH3), 3.24 (s, 3H, CH3); 13C NMR (400 MHz, CDCl3, d):
157.55, 156.81, 154.95, 151.77, 151.51, 150.27, 149.95, 140.99,
139.86, 139.75, 139.59, 137.85, 137.60, 133.15, 132.58, 129.73,
128.35, 128.13, 128.11, 127.98, 127.59, 127.35, 127.29, 127.23,
127.22, 126.91, 126.12, 125.81, 125.66, 124.89, 124.40, 123.20,
121.05, 119.75, 119.67, 119.56, 63.67, 63.17, 55.31, 55.12, 54.37;
MALDI-TOF (m/z): 982.27 [M + Ag]+; Anal. calcd for C65H44O3
(873.04): C 89.42, H 5.08; found: C 89.58, H 5.12.
2.2 Instrumental methods
The NMR spectra were recorded on a Bruker AV 400 MHz NMR
spectrometer. Mass spectra were collected using a Microex
MALDI-TOF mass-spectrometer (Bruker Daltonics, Germany).
UV-vis absorption and photoluminescence spectra were recor-
ded with a Shimadzu UV-3600 and a Shimadzu RF-5301PC
spectrophotometer, respectively. Differential scanning calo-
rimetry (DSC) and thermogravimetric analyses (TGA) were per-
formed on a Shimadzu DSC-60A and a Shimadzu DTG-60H,
respectively. Cyclic voltammetric (CV) studies were conducted
using an Eco Chemie B. V. Autolab potentiostat.
20-(9-p-Tolyl-uoren-9-yl)spiro[uorene-9,90-xanthene] (MAF-
SFX). The synthesis was similar to that of MOAF-SFX. The
residue was puried through silica-gel column chromatography
to yield 0.73 g of white solid in a 41.5% yield. 1H NMR (400 MHz,
CDCl3, d): 7.68–7.66 (d, J ¼ 7.6 Hz, 2H, Ar H), 7.64–7.62 (d, J ¼
7.2 Hz, 2H, Ar H), 7.34–7.30 (td, J ¼ 7.2 Hz, 1.2 Hz, 2H, Ar H),
7.26–7.22 (t, J ¼ 7.6 Hz, 2H, Ar H), 7.21–7.17 (m, 4H, Ar H), 7.15–
7.14 (d, J ¼ 7.2 Hz, 2H, Ar H), 7.09–7.05 (td, J ¼ 7.6 Hz, 1.2 Hz,
2H, Ar H), 7.02–7.00 (d, J ¼ 8.4 Hz, 1H, Ar H), 6.94–6.92 (d, J ¼
7.6 Hz, 2H, Ar H), 6.89–6.87 (dd, J ¼ 8.8 Hz, 2.4 Hz, 1H, Ar H),
6.79–6.75 (m, 3H, Ar H), 6.69–6.67 (d, J ¼ 8 Hz, 2H, Ar H),
6.45–6.44 (d, J ¼ 7.6 Hz, 1H, Ar H), 6.372–6.366 (d, J ¼ 2.4 Hz,
1H, Ar H), 2.20 (s, 3H, CH3); 13C NMR (400 MHz, CDCl3, d):
155.06, 151.58, 151.29, 150.02, 142.67, 139.98, 139.88, 139.56,
135.66, 128.81, 128.54, 128.10, 128.06, 128.04, 127.56, 127.52,
127.47, 127.43, 127.12, 125.76, 125.53, 124.69, 124.57, 123.19,
119.91, 119.82, 116.73, 116.14, 64.20, 54.31, 20.82; MALDI-TOF
(m/z): 586.93 [M]+; Anal. calcd for C45H30O (586.72): C 92.12, H
5.15; found: C 92.22, H 5.08.
2.3 Theoretical computation methods
Geometrical optimization for the ground and excited states was
carried out at the B3LYP/6-31G and CIS/6-31G levels, respec-
tively. The TDDFT/B3LYP/6-31g calculations of the excitation
energies were then performed at the optimized geometries. All
the quantum-chemical calculations were performed using the
Gaussian 09 suite of programs.26
3. Results and discussion
3.1 Synthesis and characterization
20,70-Bis(9-p-tolyl-uoren-9-yl)spiro[uorene-9,90-xanthene]
(DMAF-SFX). The synthesis was similar to that of MOAF-SFX.
The residue was puried through silica-gel column chroma- SFX/AFs hybrid compounds have been synthesized via two
tography to yield 0.50 g of white solid in a 19.6% yield. 1H NMR simple steps by the very mild room temperature
(400 MHz, CDCl3, d): 7.63–7.61 (d, J ¼ 7.6 Hz, 4H, Ar H), MeSO3H-mediated Friedel–Cras reaction18 of the tertiary
7.55–7.53 (d, J ¼ 7.6 Hz, 2H, Ar H), 7.28–7.21 (m, 6H, Ar H), alcohols of 9-aryl-uoren-9-ols (AFOHs) with the SFX followed
7.18–7.15 (t, J ¼ 6.8 Hz, 2H, Ar H), 7.11–7.10 (d, J ¼ 7.2 Hz, 2H, by the addition reaction of various phenyl Grignard reagents to
Ar H), 7.07–7.04 (d, J ¼ 7.6 Hz, 4H, Ar H), 6.97–6.95 (d, J ¼ 8.4 uorenone (Scheme 2), from which three kinds of products –
Hz, 2H, Ar H), 6.93–6.91 (d, J ¼ 7.6 Hz, 4H, Ar H), 6.85–6.82 (dd, single AF-substituted, symmetrically double AF-substituted,
J ¼ 8.8 Hz, 2.4 Hz, 2H, Ar H), 6.76–6.74 (d, J ¼ 8 Hz, 4H, Ar H), and asymmetrically double AF-substituted SFX derivatives –
6.67–6.65 (d, J ¼ 8 Hz, 4H, Ar H), 6.40–6.39 (d, J ¼ 2.4 Hz, 2H, Ar can be obtained.
H), 2.18 (s, 6H, CH3); 13C NMR (400 MHz, CDCl3, d): 154.96,
The former addition reaction of the various phenyl Grignard
151.35, 150.28, 142.71, 139.89, 139.50, 135.66, 128.91, 128.53, reagents with uorenone allows for easy tuning of the electro-
127.82, 127.52, 127.47, 127.39, 127.32, 127.11, 125.79, 125.33, negativity of the substitutions (CH3O, CH3, F) on the phenyl
124.51, 119.90, 119.74, 116.14, 64.23, 54.47, 20.82; MALDI-TOF rings in the AF moieties. The latter Friedel–Cras reaction
(m/z): 841.24 [M + H]+; Anal. calcd for C65H44O (840.34): C allows for one-step unconjugated connection between SFX and
92.82, H 5.27; found: C 92.71, H 5.34.
various AFs by the saturated sp3-C atom through single or
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Fig. 1 TGA (a) and DSC (b) curves of SFX/AꢂFs1 hybrid compounds
recorded at a heating/cooling rate of 5 ꢁC min
.
(366 ꢁC), MAF-SFX (383 ꢁC) and FAF-SFX (410 ꢁC). For the mono
AF-substituted compounds, with increasing electronegativity of
the substituents on the phenyl ring in the AF moieties (from
CH3O for MOAF-SFX, CH3 for MAF-SFX, to F for FAF-SFX), the Td
increased. This good thermal performance was probably due to
the steric hindrance caused by the spiro-conguration, which
effectively reduces the intermolecular aggregation. Thus, it
gives these compounds advantageous lm morphology stability
and suppresses the phase separation during device operation.
3.3 Photophysical properties
Scheme 2 Synthesis and molecular structures of SFX/AFs hybrid
compounds.
Fig. 2 shows the absorption and uorescence spectra of the SFX/
AFs hybrid compounds in dilute acetonitrile solutions and as
thin lms, respectively. In the solutions, the maximum
absorption peaks at around 261–265 nm are mainly attributed
to the p–p* transition of the SFX and AFs moieties. The small
absorptions in the longer-wavelength region at 293–295 nm and
305–307 nm are possibly due to some extension of intra-
molecular charge transfer, consistent with the FMO charge
distributions, which will be discussed below.
The two emission peaks in the solutions are in the range of
312–320 nm and 322–328 nm, which possibly originate from the
p–p* transitions of AFs and SFX. In the thin lms, probably due
to the some degree of intermolecular aggregation, the
long-wavelength absorptions at around 290–310 nm increase
and become comparable with the absorptions at around
261–265 nm. Consistently, the emissions in lms are red-
shied by 20–30 nm as compared with their solutions. Their
phosphorescence spectra were measured in CH2Cl2 matrix at
77 K (Fig. 3), in which T1 was estimated from the highest energy
vibronic sub-bands. All of the compounds exhibit high T1 with
almost identical values of 2.87–2.88 eV, which are sufficiently
double substitution, which is benecial for maintaining both a
large energy band (Eg) and a high T1. Thus, by varying the elec-
tronegativity of the AF substitutes, the substitution numbers or
positions, the ne tuning of the FMO energy levels (mainly con-
cerning electrical properties) without signicant change of the Eg
and T1 (mainly concerning optical properties) can be expected. In
this work, ve puried compounds have been obtained: 20-(9-(4-
methoxyphenyl)-uoren-9-yl)spiro[uorene-9,90-xanthene] (MOAF-
SFX), 2,20-bis(9-(4-methoxyphenyl)-uoren-9-yl)spiro[uorene-9,90-
xanthene] (DMOAF-SFX), 20-(9-p-tolyl-uoren-9-yl)spiro[uorene-
9,90-xanthene] (MAF-SFX), 20,70-bis(9-p-tolyl-uoren-9-yl)spiro[u-
orene-9,90-xanthene] (DMAF-SFX), and 20-(9-(4-uorophenyl)-uo-
ren-9-yl)spiro[uorene-9,90-xanthene] (FAF-SFX). All these
compounds have been fully characterized by 1H NMR, 13C NMR,
MALDI-TOF MS spectroscopy and elemental analysis (Fig. S1–5†).
3.2 Thermal properties
The thermal properties of the SFX/AFs compounds were inves-
tigated by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) (Fig. 1). All the ve compounds
exhibit high thermal decomposition temperatures (Td) in the
range of 356–436 ꢁC. As indicated in the DSC curves, no
signicant signals of glass transition temperature (Tg) were
observed, except that the Tg of FAF-SFX was detected at about
171 ꢁC. The melting points (Tm) of DMAF-SFX and FAF-SFX can
ꢁ
be derived from the DSC curves to be 360 and 268 C, respec-
tively. In general, the double AF-substituted compounds of
DMOAF (423 ꢁC) and DMAF-SFX (436 ꢁC) exhibit higher Td
Fig.
2 UV-vis absorption and PL spectra of SFX/AFs hybrid
values than the mono AF-substituted compounds of MOAF-SFX compounds in solutions (a) and as thin films (b).
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electronegativity of the phenyl substitutes in the AF moieties,
the HOMO and LUMO energy levels simultaneously increased
with both the Eg and T1 remaining almost constant. For the
monosubstituted FAF-SFX, MAF-SFX and MOAF-SFX, with the
sequence in electronegativity of uorophenyluoren > tolyl-
uorene > methoxyphenyluorene, the HOMO and LUMO
energy levels simultaneously increased. The HOMO energy
levels of FAF-SFX, MAF-SFX and MOAF-SFX are ꢂ6.10, ꢂ6.08
and ꢂ6.03 eV, respectively, while the LUMO energy levels are
ꢂ2.15, ꢂ2.14 and ꢂ2.09 eV, respectively. Although the tuning
from tolyluorene substituted MAF-SFX to methoxyphenyl-
uorene substituted MOAF-SFX is small, the relatively large
tuning from uorophenyluorene substituted FAF-SFX to
methoxyphenyluorene substituted MOAF-SFX proves that this
tuning of HOMO/LUMO levels by changing electronegativity
does work. (3) From the mono methoxyphenyluorene
substituted MOAF-SFX to the double asymmetrically substituted
DMOAF-SFX, the HOMO and LUMO energy levels are simulta-
neously elevated by 0.09 eV and 0.07 eV, respectively, with T1
remaining constant. The HOMO energy levels of MOAF-SFX and
DMOAF-SFX are ꢂ6.03 and ꢂ5.94 eV, respectively, while the
LUMO energy levels are ꢂ2.09 and ꢂ2.02 eV, respectively. (4)
From the mono tolyluorene substituted MAF-SFX to the double
symmetrically substituted DMAF-SFX, both the HOMO and
LUMO energy levels remain almost constant.
Therefore, either by decreasing the electronegativity of the
AF substituents (Route I) or introducing asymmetrical substi-
tution (Route II), the HOMO and/or LUMO energy levels are
enhanced and thus the hole injecting and/or electron blocking
will be facilitated without affecting the T1. Thus, FAF-SFX
exhibits the lowest HOMO/LUMO energy levels, while
DMOAF-SFX, with accumulative modications of Route I and
Route II (see Scheme 1), exhibits the highest HOMO/LUMO
energy levels. This multiple selective ne tuning of electrical
properties, without sacrice of optical properties, is very helpful
for understanding how each modication can affect the
subsequent device performance and which factors can cause
the most prominent impact.
Fig. 3 The phosphorescence spectra of SFX/AFs hybrid compounds in
CH2Cl2 matrix at 77 K.
high to conne the triplet excitons of the blue phosphor of
iridium(III) bis(4,6-(diuorophenyl)pyridine-N,C20) picolinate
(FIrpic, T1 ¼ 2.65 eV).
3.4 Tunable frontier molecular orbitals and electrical
properties
Cyclic voltammetry (CV) was performed to investigate the elec-
trochemical properties of the SFX/AFs hybrid compounds
(Fig. 4). During the anodic scan in CH2Cl2, all these compounds
exhibit one quasi-reversible, one-electron oxidation process,
from which the HOMO energy levels can be derived. The LUMO
energy levels can be calculated from the values of HOMO energy
levels and the optical energy band gaps, which are determined
from the absorption onset. As anticipated, both the HOMO and
LUMO energy levels fall in the relatively small ranges of
ꢂ6.10–5.94 eV and ꢂ2.15–2.02 eV, respectively. The Eg values of
the SFX/AFs hybrid compounds are in proximity (3.92–3.95 eV),
which is in good agreement with their similar photophysical
parameters and T1 values.
The physical data, including Tg, Td, absorption and emission
wavelengths, HOMO/LUMO values, Eg and T1, are all summa-
rized in Table 1. From these data, several conclusions can be
drawn: (1) whatever the modication to the electronegativity,
the substitution numbers or the substitution position of the AF
substituents, T1 exhibits almost no change for all these
compounds at 2.87–2.88 eV. (2) By decreasing the
Nevertheless, whether changing the electronegativity of the
AF substitutes or changing the substitution positions, these
structural modications are insulated by the spiro sp3-C atoms
in both the SFX moiety and on the 9-position of the AF moieties.
How these structural modications effectively affect, across the
insulated segments, the FMO energy levels needs further
investigation. Thus, the DFT calculations were performed for
these compounds. It is interesting to nd that, unlike most
previously published hosts in which the charge distributions on
the HOMO and the LUMO are mainly located on the donor/HT
and acceptor/ET moieties, respectively, this series of SFX/AFs
hybrid compounds exhibits an unusual shi of FMO charge
distributions with the same parent segments of SFX and AFs
(Fig. 5).
Several comparisons are conducted as follows: (1) for the
mono AF-substituted MOAF-SFX, MAF-SFX, and FAF-SFX,
although the charge distributions on the HOMO orbitals are
similarly located on the xanthene ring and the uorene ring in
the AFs, the charge distributions on the LUMO orbitals are quite
Fig. 4 Cyclic voltammograms of the oxidation for SFX/AFs hybrid
compounds in CH2Cl2.
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Table 1 The physical parameters of SFX/AFs hybrid compounds
HOMO/LUMOc (eV)
ꢂ6.026/ꢂ2.090
ꢂ5.937/ꢂ2.017
ꢂ6.084/ꢂ2.144
ꢂ6.071/ꢂ2.145
ꢂ6.100/ꢂ2.149
Egd (eV)
3.936
3.920
3.940
3.925
3.950
T1e (eV)
2.87
sf (ns)
1.1
Qfg
a
b
Compound
MOAF-SFX
DMOAF-SFX
MAF-SFX
Tg (ꢁC)
Td (ꢁC)
labs (nm)
lem (nm)
—
366
423
383
436
410
262, 293s, 305h
265, 297, 308i
264, 293s, 306h
263, 293, 309i
261, 294s, 306h
265, 302, 309i
266, 295s, 307h
265, 288, 306i
261, 293s, 306h
264, 289, 307i
317s, 325h
337, 352si
320s, 328h
338, 352i
313, 324h
337, 350i
315, 326h
332, 348si
312, 322h
333, 352si
0.5%
0.6%
0.8%
1.2%
0.8%
—
2.87
1.1
—
2.87
2.1
DMAF-SFX
FAF-SFX
—
2.88
3.6
171
2.87
3.5
Obtained from DSC measurements. b Obtained from TGA measurements. c HOMO levels were measured from the onset of oxidation potentials in
a
CH2Cl2, LUMO levels was deduced from HOMO and Eg. d Estimated from the absorption edges. e Determined from the phosphorescence spectra in
CH2Cl2 at 77 K. Lifetime (see Fig. S6). Quantum yields in powder. Measured in acetonitrile solutions. Measured in thin lms; “s” means
shoulder peaks; “–” means no observation of obvious Tg signals.
f
g
h
i
different. For MOAF-SFX, the LUMO orbital is mainly distrib- and the uorene from the xanthenes-linked AF. (4) For the
uted on the two uorenes from the SFX and AF moiety. For mono- and double symmetrically tolyluorene substituted
MAF-SFX, the charge on the LUMO is mainly distributed on the MAF-SFX and DMAF-SFX, the charge distribution on the FMOs
uorene from the SFX moiety, while for FAF-SFX it is mainly also varies. For the HOMO, the charge is mainly located on the
distributed on the uorene from the AF moiety. (2) For the xanthene ring in the SFX and the uorene ring in the AF for
double substituted DMOAF-SFX and DMAF-SFX, the charge MAF-SFX, while mainly on the xanthene in the SFX (with very
distributions of both the HOMO and the LUMO are quite little charge distributed on the two uorenes on the AF substi-
different. For the asymmetrically double methoxyphenyl- tutes) for DMAF-SFX. For the LUMO, the charge distributes on
uorene substituted DMOAF-SFX, the HOMO orbital is mainly the uorene in the SFX for MAF-SFX and on the two uorenes
located on the SFX and the two uorenes in the two AFs from the AF substitutes for DMAF-SFX. It is regarded that the
substitutes, while the LUMO orbital is mainly distributed on the above mentioned two methods for selective ne tuning of the
uorene in the SFX and the uorene in the xanthene-linked AF. HOMO and LUMO energy levels with various structural modi-
For the symmetrically double tolyluorene substituted cations are possibly related to this unusual shiing of FMOs
DMAF-SFX, the HOMO orbital is mainly located on the SFX unit charge distributions.
(with little charge distribution on the uorenes of the two AF
substitutes), while the LUMO orbital is mainly distributed on
the two uorenes of the two AF substitutes. (3) For the mono-
and double asymmetrically methoxyuorene substituted
MOAF-SFX and DMOAF-SFX, the charge distribution on the
FMOs is different as well. For the HOMO, the charge is mainly
located on the xanthene ring in the SFX and the uorene in the
AF for MOAF-SFX, while it is located on the whole SFX unit and
the uorene in the spiro-uorene-linked AF substitute for
DMOAF-SFX (the charge on the uorene in the xanthenes-
linked AF is very small). For the LUMO, the charge for
MOAF-SFX is mainly located on the two uorenes from the SFX
and the AF, and for DMOAF-SFX on the uorene from the SFX
3.5 Electroluminescent device performances
In view of the high triplet energies (T1 ¼ 2.87–2.88 eV) and the
photophysical characteristics, the FIrpic-based blue phospho-
rescent devices B1–B5, hosted by MOAF-SFX, DMOAF-SFX, MAF-
SFX, DMAF-SFX and FAF-SFX, respectively, are rstly fabricated
to investigate how this multiple ne tuning of FMO levels can
affect device performance. The device structures are ITO/MoOx
(2 nm)/m-MTDATA : MoOx (3 : 1) (15 nm)/m-MTDATA (25 nm)/
Ir(ppz)3 (10 nm)/host:FIrpic (10 wt%, 10 nm)/3TPYMB (10 nm)/
BPhen (30 nm)/LiF (1 nm)/Al (100 nm), where MoOx and LiF
served as hole- and electron-injecting layers, m-MTDATA and
BPhen as the HTL and ETL, Ir(ppz)3 as hole transporting and
electron-blocking material, 3TPYMB as the electron trans-
porting layer and hole-blocking material, and FIrpic doped in
the SFX/AFs hybrid hosts as the EML. Fig. 6 shows the current
density–voltage–brightness (J–V–L) characteristics and effi-
ciency curves versus luminance. The driving voltages are
remarkably lower than 3.0 V for onset and still lower than 4.0 V
at a luminance of 100 cd mꢂ2. Device B2 is hosted by
DMOAF-SFX, which possesses the highest-lying HOMO
(ꢂ5.94 eV) and LUMO (ꢂ2.02 eV) levels, and thus achieves the
best electroluminescence performance with
a maximum
current efficiency (CE) of 25.3 cd Aꢂ1, PE of 21.6 lm Wꢂ1 and
Fig. 5 The calculated charge spatial distributions on the HOMOs and
the LUMOs of the SFX/AFs hybrid compounds.
EQE of 11.9%. This is because the doping of FIrpic can
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Fig. 6 J–V–L characteristics (a, c, e) and efficiency curves (b, d, f) of the blue, green and red PhOLEDs hosted by the SFX/AFs hybrid compounds.
dramatically enhance the electron transporting characteristic in phosphorescent emitters. The green (G1–G5) and red devices
EML,19,20 and thus a higher HOMO/LUMO energy level facili- (R1–R5) are fabricated with almost the same device structure as
tated hole injection/electron blocking and better balanced the blue devices except for removing the hole blocking layer of
charge injection/transportation. Consistently, device B2 3TPymb, by using MOAF-SFX, DMOAF-SFX, MAF-SFX, DMAF-
exhibits a lower efficiency roll-off of 9.4% at a brightness of SFX and FAF-SFX as the hosts, respectively, and 8 wt% iridiu-
1000 cd mꢂ2 with EQE still as high as 10.6%. Besides, the color m(III) bis(2-phenylpyridinato-N,C20) acetylacetonate [(ppy)2-
stabilities of all the devices are excellent, with almost no change Ir(acac)] and 6 wt% bis(2-methyldibenzo-[f, h]-quinoxaline)
in the Commission International de I'Eclairage (CIE) coordi- acetylacetonate iridium(III) [Ir(MDQ)2(acac)] as the green and red
nates in the full range of the operating voltages (Fig. S7†).
dopants, respectively. All the green and red PhOLEDs exhibit
The higher device efficiency of the blue PhOLEDs prompted good color stabilities (Fig. S8 and 9†). The J–V–L characteristics
us to further test the applicability of these hosts to green and red and efficiency curves versus luminance (Fig. 6c–f) indicate that
Table 2 Electroluminescence performances of the SFX/AF-based PhOLEDs
CEb [cd Aꢂ1
]
PEb [lm Wꢂ1
]
EQEtb [%]
CIE (x, y)c
a
Devices
Von [V]
B1
B2
B3
B4
B5
G1
G2
G3
G4
G5
R1
R2
R3
R4
R5
W
2.8, 3.8
3.0, 3.6
2.8, 3.6
2.8, 3.8
2.8, 4.0
3.5, 3.9
3.0, 3.8
3.2, 3.3
3.0, 3.4
3.5, 3.7
2.8, 4.3
3.0, 4.4
2.8, 3.8
2.6, 4.0
2.6, 3.3
2.8, 4.2
23.1, 22.1, 11.9
25.3, 24.5, 22.4
22.0, 22.0, 18.9
20.5, 17.9, 9.0
17.2, 15.4, 6.6
25.0, 24.4, 18.6
48.0, 35.7, 23.4
30.3, 28.2, 25.2
25.4, 24.1, 19.1
34.7, 33.4, 25.8
9.4, 6.0, 2.9
10.1, 7.7, 3.9
11.8, 5.6, 3.3
8.5, 7.6, 2.8
11.7, 11.6, 10.5
18.9, 15.0, 6.3
22.2, 18.2, 6.9
21.6, 21.2, 14.7
21.0, 19.4, 12.3
20.1, 14.8, 5.0
16.8, 12.0, 3.6
20.2, 19.7, 11.8
33.5, 30.1, 14.7
26.5, 26.5, 18.8
22.8, 21.9, 13.4
31.1, 28.8, 17.8
9.3, 4.3, 1.4
9.0, 5.4, 1.8
12.4, 6.2, 1.7
9.0, 4.4, 1.4
11.6, 10.9, 7.9
16.9, 11.3, 3.2
10.7, 10.2, 5.6
11.9, 11.5, 10.6
10.4, 10.4, 8.9
9.4, 8.2, 4.1
8.1, 7.2, 3.1
6.3, 6.1, 4.7
9.2, 8.8, 5.8
7.0, 6.5, 5.8
5.8, 5.5, 4.4
8.0, 7.7, 5.9
5.1, 3.2, 1.6
6.2, 4.8, 2.4
6.5, 4.2, 1.9
4.8, 3.3, –
(0.15, 0.31)
(0.15, 0.34)
(0.15, 0.32)
(0.15, 0.32)
(0.15, 0.31)
(0.30, 0.64)
(0.30, 0.64)
(0.31, 0.64)
(0.31, 0.64)
(0.31, 0.64)
(0.60, 0.40)
(0.61, 0.39)
(0.60, 0.39)
(0.61, 0.39)
(0.62, 0.38)
(0.27, 0.38)
7.4, 7.3, 6.2
7.6, 6.1, 2.6
a
b
c
Turn-on voltages and driving voltages at 100 cd mꢂ2
.
Maximum values, then values at 100 and 1000 cd mꢂ2
.
At 7 V.
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the turn-on voltages of the green and red devices are lower than Bphen(40 nm)/LiF(1 nm)/Al(100 nm). FIrpic and Po-01 were
3.5 and 3.0 V, respectively, and at a luminance of 100 cd mꢂ2 the doped in the universal host DMOAF-SFX, serving as the single
driving voltages are still lower than 4.0 and 4.5 V, respectively. EML. Fig. 7a displays the normalized EL spectra of the WOLED at
For the green PhOLEDs, device G2 obtained the highest device various voltages. Clearly, the emission from Po-01 slightly
efficiency of CE, as high as 48.0 cd Aꢂ1 (PE of 33.5 lm Wꢂ1 and decreases with increasing driving voltage, which is possibly
EQE of 9.2%). The reason is similar to that in the blue devices. In ascribed to the competition between electron trapping on the
the red PhOLEDs, device R5, with the host FAF-SFX possessing yellow-emitting
chromophore
sites
and
unperturbed
the lowest HOMO/LUMO levels, achieved the highest device charge through the organic layer.23,24 The CIE coordinates are
efficiency with a maximum CE of 11.7 cd Aꢂ1, PE of 11.6 lm/W (0.275 ꢃ 0.015, 0.39 ꢃ 0.01) over a wide range of luminance
and EQE of 7.4%, and exhibited a much reduced efficiency (100–4932 cd mꢂ2), which shows good color stability (Fig. 7). The
roll-off. At luminances of 100 and 1000 cd mꢂ2, the CEs are still J–V–L characteristics and efficiency curves indicate that Device W
as high as 11.6 cd Aꢂ1 (EQE ¼ 7.3%) and 10.5 cd Aꢂ1 (EQE ¼ exhibits a low onset voltage of 2.4 V with a maximum CE of
6.2%), with efficiency roll-offs of 1.4% and 16.2%, respectively. 18.8 cd Aꢂ1, PE of 16.9 lm Wꢂ1 and EQE of 7.6%.
This is due to the hole transport ability being higher than the
electron transport ability in the red phosphor of Ir(MDQ)2(-
acac),21,22 because some of the holes may directly inject into and
4. Conclusions
transport on the HOMO level of Ir(MDQ)2(acac) caused by the In summary, we have developed a new, simple, two-step
large hole injection barrier between the electron blocking layer molecular design strategy to synthesize SFX/AF-based host
and the host (>1.0 eV), and the large concentration of the materials by unconjugated connection of two separate func-
phosphorescent emitters. FAF-SFX possesses the lowest-lying tional segments of SFX and various AFs via very mild room
HOMO (ꢂ6.10 eV) and LUMO levels (ꢂ2.15 eV), which can best temperature MeSO3H-mediated Friedel–Cras reaction.
facilitate the electron injecting and hole blocking, and thus Multiple selective ne tuning of the FMO energy levels with
achieves better balanced carrier injection/transporting and constant high T1 has been achieved by either asymmetric
higher device efficiency (Table 2).
substitution or electronegativity variation, which is regarded to
Overall, among all these hosts, the DMOAF-SFX host ach- be related to the unusual shi of the FMO charge distributions.
ieves the best device performance in the RGB PhOLEDs and This series of SFX/AFs hybrid compounds has been demon-
serves as the most efficient universal host. The maximum effi- strated to be utilized as universal hosts for low voltage multi-
ciencies of the blue, green, and red PhOLEDs are 25.3 (11.9%), color RGB and white PHOLEDs. The multiple selective ne
48.0 (9.2%), 10.1 (6.2%) cd Aꢂ1, respectively. Based on host tuning of the electrical properties without interference of the T1
DMOAF-SFX, further application in a single-emitting-layer levels, can well explain the corresponding RGB PhOLEDs
white PhOLED has been performed. The blue/yellow comple- performances. This work provides good guidance for further
mentary single-emitting-layer white PhOLED (Device W) was rational design of more efficient SFX-based monochromic and
fabricated, with the conguration of ITO/MoOx (2 nm)/ universal RGB hosts by cumulative structural modications.
m-MTDATA : MoOx (3 : 1) (15 nm)/m-MTDATA (20 nm)/Ir(ppz)3
(10 nm)/DMOAF-SFX : FIrpic:bis(4-phenylthieno[3,2-c]pyridinato-
Acknowledgements
N,C2) acetylacetonate (Po-01) (10 wt%,
1 wt%, 10 nm)/
We express our sincere gratitude to the 973 projects
(2012CB723402 and 2012CB933301), Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD)
(YX03001), the National Natural Science Foundation of China
(Grant nos 21373114, 61275033, and 21274064), National Natural
Science Funds for Excellent Young Scholar (21322402), Doctoral
Fund of Ministry of Education of China (20133223110007),
Excellent science and technology innovation team of Jiangsu
Higher Education Institutions (2013), and the Natural Science
Foundation of Jiangsu Province (Grant nos BK2011761,
10KJB510013, and BM2012010) for their nancial support.
Notes and references
1 M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151–
154.
2 A. Kohler, J. S. Wilson and R. H. Friend, Adv. Mater., 2002, 14,
701–707.
Fig. 7 The EL spectra (a) and CIE coordinates (b) at different operating
voltages/luminance, the J–V–L characteristics (c) and efficiency
curves (d) of the white device.
3 Y. Tao, C. Yang and J. Qin, Chem. Soc. Rev., 2011, 40, 2943–
2970.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 29828–29836 | 29835

View Article Online
RSC Advances
Paper
4 A. Chaskar, H.-F. Chen and K.-T. Wong, Adv. Mater., 2011, 23,
3876–3895.
H.-C. Ting, K.-T. Wong, Y.-H. Liu and Y. Chi, J. Mater.
Chem., 2012, 22, 870–876.
5 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468– 16 C. Han, Z. Zhang, H. Xu, J. Li, G. Xie, R. Chen, Y. Zhao and
2471. W. Huang, Angew. Chem., Int. Ed., 2012, 51, 10104–10108.
6 S. J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274–284. 17 D. Yu, F. Zhao, C. Han, H. Xu, J. Li, Z. Zhang, Z. Deng, D. Ma
7 S.-J. Su, C. Cai and J. Kido, J. Mater. Chem., 2012, 22, 3447–
3456.
8 S. Gong, Y. Chen, J. Luo, C. Yang, C. Zhong, J. Qin and D. Ma,
Adv. Funct. Mater., 2011, 21, 1168–1178.
and P. Yan, Adv. Mater., 2012, 24, 509–514.
18 L.-H. Xie, X.-Y. Hou, Y.-R. Hua, C. Tang, F. Liu, Q.-L. Fan and
W. Huang, Org. Lett., 2006, 8, 3701–3704.
19 T.-L. Chiu and P.-Y. Lee, Int. J. Mol. Sci., 2012, 13, 7575–7585.
9 S. Gong, Y. Chen, C. Yang, C. Zhong, J. Qin and D. Ma, Adv. 20 N. Matsusue, S. Ikame, Y. Suzuki and H. Naito, J. Appl. Phys.,
Mater., 2010, 22, 5370–5373. 2005, 97, 123512.
10 C. Cai, S.-J. Su, T. Chiba, H. Sasabe, Y.-J. Pu, K. Nakayama 21 S. Reineke, F. Lindner, G. Schwartz, N. Seidler, K. Walzer,
and J. Kido, Org. Electron., 2011, 12, 843–850. B. Luessem and K. Leo, Nature, 2009, 459, 234–238.
11 W. Y. Hung, L. C. Chi, W. J. Chen, E. Mondal, S. H. Chou, 22 W. S. Jeon, T. J. Park, S. Y. Kim, R. Pode, J. Jang and
K. T. Wong and Y. Chi, J. Mater. Chem., 2011, 21, 19249–
19256.
12 H.-F. Chen, T.-C. Wang, S.-W. Lin, W.-Y. Hung, H.-C. Dai,
J. H. Kwon, Org. Electron., 2009, 10, 240–246.
23 M. C. Gather, R. Alle, H. Becker and K. Meerholz, Adv. Mater.,
2007, 19, 4460–4465.
H.-C. Chiu, K.-T. Wong, M.-H. Ho, T.-Y. Cho, C.-W. Chen 24 Q. Wang, J. Q. Ding, D. G. Ma, Y. X. Cheng, L. X. Wang,
and C.-C. Lee, J. Mater. Chem., 2012, 22, 15620–15627. X. B. Jing and F. S. Wang, Adv. Funct. Mater., 2009, 19, 84–95.
13 C.-L. Ho, L.-C. Chi, W.-Y. Hung, W.-J. Chen, Y.-C. Lin, H. Wu, 25 L.-H. Xie, F. Liu, C. Tang, X.-Y. Hou, Y.-R. Hua, Q.-L. Fan and
E. Mondal, G.-J. Zhou, K.-T. Wong and W.-Y. Wong, J. Mater.
Chem., 2012, 22, 215–224.
14 W.-Y. Hung, G.-M. Tu, S.-W. Chen and Y. Chi, J. Mater.
Chem., 2012, 22, 5410–5418.
15 M.-S. Lin, L.-C. Chi, H.-W. Chang, Y.-H. Huang, K.-C. Tien,
C.-C. Chen, C.-H. Chang, C.-C. Wu, A. Chaskar, S.-H. Chou,
W. Huang, Org. Lett., 2006, 8, 2787–2790.
26 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci and G. A. Petersson, et al., Gaussian 09,
Revision A.1, Gaussian, Inc., Wallingford CT, 2009.
29836 | RSC Adv., 2015, 5, 29828–29836
This journal is © The Royal Society of Chemistry 2015