Synthesis and photo- and electroluminescence properties of 3,6-disubstituted phenanthrenes: Alternative host material for blue fluorophores

Article abstract of DOI:10.1039/c1cc12875b

Three deep-blue fluorescent 9,10-bis(4-tert-butylphenyl)phenanthrenes with diphenyl, -naphthyl, and -pyrenyl moieties at C3 and C6 positions were synthesized and used as the host for doped blue fluorescent devices; one of these devices reveals excellent external quantum efficiency of 7.7% and current efficiency of 9.8 cd A^-1 with low efficiency roll-off, deep-blue color coordinates (0.14, 0.14) and long operational lifetime.

Full text of DOI:10.1039/c1cc12875b

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COMMUNICATION
Synthesis and photo- and electroluminescence properties
of 3,6-disubstituted phenanthrenes: alternative host material
for blue fluorophoresw
Yu-Han Chen, Ho-Hsiu Chou, Tsu-Hui Su, Pei-Yu Chou, Fang-Iy Wu and
Chien-Hong Cheng*
Received 16th May 2011, Accepted 27th June 2011
DOI: 10.1039/c1cc12875b
Three deep-blue fluorescent 9,10-bis(4-tert-butylphenyl)phenan-
threnes with diphenyl, -naphthyl, and -pyrenyl moieties at C3
and C6 positions were synthesized and used as the host for doped
blue fluorescent devices; one of these devices reveals excellent
external quantum efficiency of 7.7% and current efficiency of
9.8 cd Aꢀ1 with low efficiency roll-off, deep-blue color coordinates
(0.14, 0.14) and long operational lifetime.
have been reported.8–10 A deeper blue fluorescent device with
high external quantum efficiency (EQE) and current efficiency
(Zc) of 5.1% and 5.4 cd Aꢀ1 at CIE (0.14, 0.13) was demon-
strated by Chen et al..1c In addition, Yoon et al. and Liu et al.
reported pure blue devices (CIE (y) = 0.14) with EQEs (Zc) of
3.47% (3.7 cd Aꢀ1) and 4.24% (5.0 cd Aꢀ1), respectively.11,12a
The search for efficient and stable blue fluorescent materials
with excellent CIE coordinates (y-coordinate value o 0.15) is
important but examples are still relatively rare.11,12
Organic light-emitting diodes (OLEDs) have attracted great
attention due to their application in high-resolution, full-color,
flat-panel displays and lighting.1–3 The search for stable and
efficient three primary color (red, green, and blue) emitters and
devices is particularly important for OLED products to be
commercialized.4 Recently, red and green electroluminescent
devices with high efficiencies, long lifetimes, and proper CIE
(Commission International de l’Eclairage) coordinates have
been developed. For the known blue phosphorescent devices,
high external quantum efficiencies (EQEs) can be achieved, but
there are still some drawbacks such as high CIE coordinates
(y-coordinate value 40.30) and short device lifetimes that
require further improvement. Therefore, to achieve the market-
able OLEDs, the hunt for blue fluorescent materials and devices
is still a subject of current interest.
We have been interested in the application of polyaromatic
compounds for blue fluorescent materials in organic light-emitting
devices.13 Previously, we and Liu’s group employed pyrene- or
triphenylene-containing materials as blue host emitters and as
the host for doped blue devices.4,12,14 As previous reports
demonstrated, the anthracene moiety is the most common
core structure for blue fluorescent materials.8–10,12,15 However,
only few blue fluorescent materials employing phenanthrene, a
structural isomer of anthracene, as the core structure have
been known to date.16–19 It is important to note that phenan-
threne is one of the most stable fused aromatics.16,20 Thus, we
wish to report in this communication the synthesis of phenan-
threne derivatives and their application as stable and very
efficient host for organic blue fluorescent devices.
Three phenanthrene derivatives, 3,6-diphenyl-9,10-bis-
(4-tert-butylphenyl)phenanthrene (TPhP), 3,6-di(naphthalen-
2-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene (TNaP) and
3,6-di(pyrene-1-yl)-9,10-bis(4-tert-butylphenyl)phenanthrene
(TPyP), were synthesized from Suzuki coupling of 3,6-dibromo-
9,10-bis(4-tert-butylphenyl)phenanthrene (see ESIw)16 with
phenyl-, 2-naphthyl- and 1-pyrenylboronic acid, respectively,
as shown in Scheme 1. The core phenanthrene structure
incorporates two tert-butylphenyl groups at C-9 and C-10
positions to prevent extensive intermolecular p,p-interaction
of the compounds in the solid state. In addition, the introduction
of end-capping substituents provides effective tuning of the
emission wavelength, quantum yield and thermal property of
the materials.
It is well known that the device efficiency, color coordinates,
and operational stability of a blue fluorescent device can be
significantly improved by using a doped emitter.5 Nevertheless, in
the dopant/host system, the phase separation upon heating has
become an issue leading to the degradation of efficiency.6,7 The
host material has shown great influence on the efficiency and life
time of the blue-doped fluorescent devices. Thus, it is crucial to
find blue fluorescent host materials with outstanding properties.
Several sky blue-blue doped fluorescent devices using anthra-
cene derivatives as the host materials with high performance
Department of Chemistry, National Tsing-Hua University,
Hsinchu 30013, Taiwan. E-mail: chcheng@mx.nthu.edu.tw;
Fax: +886-3-5724698; Tel: +886-3-5721454
w Electronic supplementary information (ESI) available: Experimental
section, cyclic voltammograms, calculated HOMO/LUMO density
maps, TGA and DSC thermograms, AFM topographic images, 1H and
13C NMR spectra, electroluminescence properties. See DOI: 10.1039/
c1cc12875b
The absorption and photoluminescence spectra (PL) of
TPhP, TNaP, and TPyP thin films are shown in Fig. 1, while
the peak maxima of these spectra are summarized in Table 1.
As revealed in Fig. 1, the absorption spectra are mainly in the
c
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Chem. Commun., 2011, 47, 8865–8867 8865

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the HOMO/LUMO energy gap of TPyP is substantially lower
than those of TPhP and TNaP owing to the longer p conjugation
of the pyrenyl group. To gain insight into the electronic states
of these phenanthrene compounds, density functionalized
theory (DFT) calculation was performed. Interestingly, the
results revealed that the HOMO/LUMO electron distributions
of TPhP and TNaP are similar mainly on the 9,10-bis(4-tert-
butylphenyl)phenanthrene structure and to a less degree on
the 3,6-disubstituted groups. In contrast, the HOMO/LUMO
electron distributions of TPyP are mostly over the two pyrene
groups and to a less extent over the phenanthrene structure
(see ESIw). This difference in the electron distributions provides
the basis for the difference in the HOMO/LUMO level, the
photophysical properties and the electrochemical properties of
these phenanthrene derivatives.
Scheme 1 Synthesis of phenanthrene derivatives.
The thermal-property data of these compounds were measured
by differential scanning calorimetry (DSC) and thermo-
gravimetric analysis (TGA) as shown in Table 1. In the DSC
measurement of TPhP, the Tm and Tg were not detected. On
the other hand, the Tg and Tm of both TNaP and TPyP were
observed clearly and TPyP showed a very high Tg of 211 1C
and Tm of 378 1C (see ESIw). To look into the impact of
thermal stability toward the surface morphology, we used
atomic force microscopy (AFM) to measure the topography
of a TPyP thin film doped with 5% 4,40-bis(9-ethyl-3-carbazo-
vinylene)-1,10-biphenyl (BCzVBi) at room temperature and
heated at 110 1C for 18 h; we found that the root-mean-square
roughness (Rrms) were 0.237 and 0.220, respectively. These
observations demonstrated that the doped TPyP film has no
phase separation problem upon heating and formed a high-
quality amorphous film. We think that the phenanthrene
structure attached by the tert-butylphenyl and the large rigid
pyrene groups is the major reason for the high thermal and
morphological stability.
Fig. 1 Absorption and photoluminescence emission spectra of TPhP,
TNaP and TPyP in the thin film and electroluminescence spectra of
devices A–D.
UV range and are due to p–p* transition, but the PL spectra span
the UV and deep blue regions. Comparison of the PL spectra of
these three phenanthrene derivatives indicates that as the conjuga-
tion increases, the emission maximum is red-shifted significantly.
In addition, the thin film PL spectra of TPhP and TNaP
reveal mild stacking in the thin film resulting in ca. 14 nm red-
shifted compared with those in dichloromethane solution. On
the other hand, the TPyP thin film shows a broadened PL
spectrum red-shifted by 33 nm relative to that in the solution,
suggesting an increase of p–p stacking of the two end-capping
pyrene moieties (see Fig. 1). Such a red-shift phenomenon of
pyrene-based material has been previously reported.4 The
fluorescence quantum yields of these thin films (30 nm) are
measured using an integrating sphere and are listed in Table 1.
The results show that all these three phenanthrenes give high
quantum yields and TPyP is significantly higher than TNaP
and TPhP.
To find the suitability of these phenanthrene derivatives as
EL materials, we fabricated devices A–C by utilizing these
derivatives as the host and BCzVBi as the dopant. The devices
consist of the layers: NPB (20 nm)/TCTA (30 nm)/phenanthrene
derivatives: BCzVBi (5%) (40 nm)/BCP (30 nm)/LiF (1 nm)/
Al (100 nm), where NPB = N,N0-di(1-naphthyl)-N,N0-diphenyl-
(1,10-biphenyl)-4,40-diamine, TCTA = 4,40,40 0-tri(9-carbazoyl)
triphenylamine and BCP = 2,9-dimethyl-4,7-diphenyl-1,10-
phenanthroline. Devices A, B and C employing TPhP, TNaP,
and TPyP as the host materials have achieved excellent EQEs
of 5.2, 5.6 and 6.4%, and current efficiencies of 5.0, 6.5 and
7.7 cd Aꢀ1, respectively (see Table 2). The EQEs and current
efficiencies vs. luminance of these devices are displayed in
Fig. 2. At luminance between 10 to 10000 cd mꢀ2, the EQE
The electrochemical properties of TPhP, TNaP and TPyP
were investigated by cyclic voltammetry (CV) (see ESIw).
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) levels and thus the
energy gaps were estimated from their oxidation and reduction
potentials. The results as indicated in Table 1 are in agreement
with the observed absorption and emission data showing that
Table 1 Photophysical and thermal properties of phenanthrene derivatives
Compound
lsola/nm/(e)b
PLsola/nm
PLfilmc/nm
HOMOd/eV
LUMOd/eV
Ege/eV
Tg /Tmf/Tdg/1C
f
Fi
TPhP
TNaP
TPyP
272 (1238), 328 (381)
268 (1361), 338 (433)
242 (1404), 282 (1103), 346 (872)
380, 395
395, 406
427
397
418
460
ꢀ5.8
ꢀ5.7
ꢀ5.6
ꢀ2.1
ꢀ2.2
ꢀ2.3
3.7
3.5
3.3
N.Dh/N.Dh/367
134/291/438
211/378/504
0.57
0.64
0.83
a
b
c
Absorption spectra measured in CH2Cl2 with concentration = 1 ꢁ 10ꢀ5 M. Extinction coefficient (102 Mꢀ1 cmꢀ1). Fluorescence measured in
d
e
f
the thin film. HOMO and LUMO levels were estimated based on the CV data of each compound. Eg = HOMO ꢀ LUMO. Glass transition
g
temperature (Tg) and melting point (Tm). Decomposition temperature (Td). Not detected. The quantum yields of the films (30 nm) were
h
i
determined with an integrating sphere.
c
8866 Chem. Commun., 2011, 47, 8865–8867
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Table 2 EL performance of the host materials TPhP, TNaP and
TPyP
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Fig. 2 EQE and current efficiency versus luminance of devices A–D.
of TPhP- and TNaP- based devices A and B showed relatively
high efficiency roll-off. But surprisingly, the efficiency of the
TPyP-based device C increases gradually as the luminance
increases from 10 to 10000 cd mꢀ2. The performance of the
device C-based structure can be further improved by inserting
a thin layer of MoO3 (2 nm) between ITO and NPB.21 Device
D thus fabricated shows an EQE and current efficiency as high
as 7.7% and 9.8 cd Aꢀ1, respectively. The operational lifetime
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In summary, we have successfully synthesized three phenan-
threne derivatives TPhP, TNaP and TPyP that show excellent
photoluminescent and thermal properties for electrolumines-
cent (EL) applications. In particular, the pyrene-substituted
TPyP-based devices have achieved one of the highest external
quantum efficiencies ever reported and excellent stability with
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We thank the Ministry of Economy (98-EC-17-A-08-S1-042)
and the National Science Council of the Republic of China
(NSC-99-2119-M-007-010) for support of this research and the
National Center for High-Performance Computing (Account
number: u32chc04) of the Republic of China for providing
computing time.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 8865–8867 8867