Synthesis of pyrimidine-cored host materials bearing phenylcarbazole for efficient yellow phosphorescent devices: Effect of linkage position

Article abstract of DOI:10.1039/c4ra16316h

Two isomeric host materials were synthesized and their photophysical properties and device performances were investigated. In particular, para-substituted pPDPC exhibited the most intense efficiency regardless of the low triplet energy compared to that of meta-substituted mPDPC. The EQE of the pPDPC device reached as high as 22.3% with a yellow color coordinate of (0.49, 0.50). This journal is

Full text of DOI:10.1039/c4ra16316h

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Synthesis of Pyrimidine-cored Host Materials Bearing
Phenylcarbazole for Efficient Yellow Phosphorescent
Devices: Effect of Linkage Position
Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012,
Accepted 00th January 2012
HongꢀGu Jang, Wook Song, Jun Yeob Lee and SeokꢀHo Hwang*
DOI: 10.1039/x0xx00000x
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work, we reported the design, syntheses, characterizations, and
yellow PHOLED application of two isomeric bipolar host materials,
9,9′ꢀ(Pyrimidineꢀ2,5ꢀdiylbis(4,1ꢀphenylene))bis(9Hꢀcarbazole)
(pPDPC) and 9,9′ꢀ(Pyrimidineꢀ2,5ꢀdiylbis(3,1ꢀphenylene))bis(9Hꢀ
carbazole) (mPDPC), comprised of pyrimidine moiety and two Nꢀ
phenylcarbazole units and could be quite simply prepared. By
altering the linking positions such as paraꢀ or metaꢀposition, the
effect of their isomeric molecular structures on the device
performances was evaluated.
Two isomeric host materials were synthesized and their
photophysical properties and device performances were
investigated. In particular, para-substituted pPDPC exhibited
the most intense efficiency regardless of the low triplet energy
compared to that of meta-substituted mPDPC. EQE of the
pPDPC device reached as high as 22.3% with a yellow color
coordinate of (0.49, 0.50).
During the past decade, the development of organic light emitting
devices (OLEDs) has drawn tremendous attention due to their
unique features including large area, flexibility, fast response, high
efficiency, lifespan, manufacturing cost, driving voltage, high
brightness, etc. in fullꢀcolor flatꢀpanel displays and solidꢀstate
lighting applications.1ꢀ4 To fabricate a high efficiency OLED,
phosphorescent material based devices have been used because they
can harvest both singlet and triplet excitons for light emission,
providing
the
opportunity
to
accomplish
internal
electroluminescence (EL) quantum efficiency of close to 100%.5
From molecular design perspective in PHOLEDs, a host material
plays a key role in determining the device performance, such as
external quantum efficiency, current efficiency, and lifetime through
charge balance and energy transfer. As we know, the host material
reported in 1998 was the CBP (4,4′ꢀbis(Nꢀcarbazolyl)ꢀ2,2′ꢀbiphenyl)
molecule which consists of two carbazole moieties and a biphenyl
group and its concise synthetic route makes it widely used in
PHOLED research field.2 Although numerous host materials have
been developed to date, the carbazole derivatives are still the most
favorable building units in design of the most effective host
materials for PHOLEDs, because the derivatives can be readily
modified to fulfill the requirements for designing a good host, e.g.
high triplet energy level, good thermal stability, appropriate transport
mobility and matched HOMO and LUMO levels.6 Unfortunately, the
lack of electron transporting property in carbazole derivatives
usually requires the attachment of electron withdrawing group to
balance the carrier transport.7 Therefore, we selected pyrimidine
moiety as the electron acceptor due to the more electronꢀdeficient
nature.8
Scheme 1. Reagents and conditions: (i) dihalobenzene, CuI, K2CO3,
18ꢀCrownꢀ6, DMF, 130oC, 24hrs; (ii) KOAc, Pd(dppf)Cl2, 1,4ꢀ
dioxane, reflux, 12hrs; (iii) K2CO3, Pd(PPh3)4, toluene, reflux, 36hrs.
Scheme 1 showed the synthetic route toward isomeric host materials,
pPDPC and mPDPC. Synthesis of the host materials began with
commercially available 9Hꢀcarbazole, which was Nꢀarylated with
1,4ꢀdibromobenzene or 3ꢀbromoiodobenzene using Ullmann
coupling reaction to give 1 (47%) and 2 (36%) that were
subsequently treated with bis(pinacolato)diboron under Pd catalyzed
condition (Miyaura borylation reaction) to form the desired
Since the yellow PHOLED can be used as a lighting source for
lithography labs, traffic lights, and signal lights, it can also be a
major color component for displays such as RGBYꢀTV. In this
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intermediates, 3 (68%) and 4 (62%). These transformations were 350 nm, respectively. The bandgap discrepancy of the isomeric host
supported by the new singlet peaks for 3 and 4 at 1.44 and 1.37 ppm materials should be induced by more extended conjugation length of
(ꢀCH3) in the 1H NMR spectra, respectively. The intermediates were paraꢀsubstituted pPDPC compared to metaꢀsubstituted mPDPC. The
coupled with 5ꢀbromoꢀ2ꢀiodopyrimidine by SuzukiꢀMiyaura solution PL emissions of pPDPC and mPDPC were observed at 427
coupling reaction under basic condition to form the desired pPDPC and 441 nm, respectively. The narrow bandgap of pPDPC
(37%) and mPDPC (39%). The desired host materials were fully contributed to the small redꢀshift (14 nm) of the PL emission peak
1
characterized by H NMR, 13C NMR, and mass spectrometry. The compared to that of mPDPC. The first phosphorescent emission peak
detailed synthetic procedures and structural characterizations were of low temperature (77K) PL emission spectra for the pPDPC and
presented in the electronic supplementary information.
mPDPC were observed at 428 and 441 nm, which corresponded to
The thermal stabilities of the pPDPC and mPDPC were their triplet energies of 2.40 and 2.50 eV, respectively. The triplet
characterized by thermogravimetric analysis (TGA) and differential energy of mPDPC was higher than that of pPDPC due to effective
scanning calorimetry (DSC) under a nitrogen atmosphere. The glass breaking of conjugation through metaꢀsubstitution of the mPDPC.
transition temperatures of pPDPC and mPDPC were clearly Because the triplet energies of the host materials were higher than
observed at 128 oC and 119 oC, respectively. In addition, the the estimated value for iridium(III) bis(4ꢀphenylthieno[3,2ꢀ
decomposition temperatures at which initial 5 wt.ꢀ% loss of mass c]pyridinatoꢀN,C2’)acetylacetonate (POꢀ01) (ET = 2.20 eV),10 they
were 437 oC (pPDPC) and 440 oC (mPDPC). The molecular were sufficient to be used as host materials in POꢀ01 based yellow
configurations of two isomeric host materials should provide them PHOLEDs. As reported previously, the higher triplet energy of the
with high thermal stability. As results, these host materials should host material would facilitate energy transfer from the host to the
construct homogeneous and stable amorphous films upon thermal dopant in hostꢀguest system.11 Accordingly, the two isomeric host
evaporation. Also, they should be suitable to use as PHOLED materials can be effective in the phosphorescent devices because
component for long lifetime. (See Fig. S3 and S4 in ESI†)
energy transfer from the dopant to the host is prevented, and thus,
In order to gain insight into the electronic structures of the pPDPC triplet excitons in the guest are effectively confined to the emission
and mPDPC hosts, their theoretical molecular orbital distributions zone.
were calculated with the Gaussian program at the B3LYP/6ꢀ31G(d)
level, using the density function theory (DFT) for the geometry
optimizations. As shown in the Fig. 1, the highest occupied
molecular orbitals (HOMOs) of both host materials were mostly
dispersed on the electronꢀrich unit of carbazole moiety. In contrast,
their lowest unoccupied molecular orbitals (LUMOs) were localized
on the electronꢀdeficient unit of pyrimidine together with two phenyl
moieties. The clear separation of the HOMO and LUMO suggested
that the HOMOꢀLUMO excitation would shift the electron density
distribution from the donor carbazole moiety to the acceptor
pyrimidine moiety, leading to a polarized excited state. It is more
important, such separation of HOMO and LUMO can provides holeꢀ
and electronꢀtransporting channel respectively.
Fig. 2 UV/vis absorption and PL spectra of pPDPC and mPDPC.
The cyclic voltammetry (CV) of the host materials was measured in
CH3CN to study the energy levels of the host materials. The HOMO
levels of pPDPC and mPDPC were calculated from the
electrochemical oxidation onset potential peaks in cyclic
voltammogram. During the first anodic scan, all the materials
exhibited an irreversible oxidation peaks at around 1.75 eV. (See Fig.
5 in ESI†) These peak irreversibility indicated the short lifetime of
the radical ions formed during the electrochemical processes. Their
HOMO levels were calculated as ꢀ6.01 and ꢀ6.08 eV, respectively.
The LUMO levels of them were ꢀ2.88 and ꢀ2.54 eV, calculated from
the HOMO level and energy bandgaps determined from the UV/vis
absorption threshold.
Fig. 1 DFT calculations of HOMO and LUMO distribution for
pPDPC and mPDPC.
To understand the hole and electron density of host materials, holeꢀ
Fig. 2 presented the UV/vis absorption, photoluminescence (PL),
and electronꢀonly devices composed of pPDPC and mPDPC were
and lowꢀtemperature PL emission spectra of the synthesized host
fabricated as follow structures: ITO/PEDOT:PSS (60nm)/TAPC
materials in dilute solution. The relatively weak peaks at around 340
(20nm)/mCP (10nm)/ Host (25nm)/TAPC (5nm)/Al (hole);
nm which appeared in absorption spectra of all these materials can
ITO/PEDOT:PSS (60nm)/TSPO1 (10nm)/Host (25nm)/TSPO1
be attributed to the nꢀπ* transitions of extended conjugation of the
(5nm)/TPBi (30nm)/LiF (0.15nm)/Al (electron). Fig. 3 showed the
carbazole moiety. And the absorption peaks below 290 nm could be
assigned to πꢀπ* transitions of carbazole moiety.9 The optical
current densityꢀvoltage curves of holeꢀ and electronꢀonly devices
bearing pPDPC and mPDPC. Considering the molecular structures
bandgaps of pPDPC and mPDPC were calculated from the
of isomeric host materials, the hole current density of pPDPC and
absorption edges of the UV/vis spectra. The calculated optical
mPDPC was higher than their electron current density due to the
bandgaps from the data acquired for pPDPC and mPDPC were 3.14
carbazole unit. Furthermore, hole and electron current densities of
and 3.54 eV, which corresponded to absorption edges of 396 and
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pPDPC were higher than those of mPDPC, which indicates that those of the mPDPCꢀ and CBPꢀbased yellow devices. The maximum
pPDPC shows better performance than mPDPC, in terms of charge quantum efficiency of the pPDPC, mPDPC and CBP devices were
transportation. Generally, the hole and electron current densities 22.3, 19.7 and 18.8%, while the quantum efficiencies at 1000 cd/m2
reflect charge injection and charge transport properties. From the of pPDPC, mPDPC and CBP devices were 19.2, 15.8 and 16.1%,
holeꢀ and electronꢀonly devices, carbazole unit should play a role of respectively. These results imply that the design concept for
strong hole transporting unit and pyrimidine unit should work as a constructing new bipolar hosts is adequate to improve the efficiency
strong electron transporting unit, indicating the balance of hole and of PHOLEDs. The much higher quantum efficiency of pPDPC
electron in the emitting layer.
device could be explained by the efficient energy transfer, balanced
charge density and exciton confinement in the emitting layer. The
higher quantum efficiency of the pPDPC device than that of the
mPDPC device is due to better charge balance as can be proved by
the hole and electron current density of the hole and electron only
devices. In addition to that, the maximum current efficiency of
pPDPC yellow was 68.3 cd/A, and the maximum power efficiency
was 47.5 lm/W as best value most of all. (See Fig. S6 and S7 in ESI†)
Fig. 3 Current densityꢀvoltage curves of holeꢀ and electronꢀonly
devices for pPDPC and mPDPC.
Since the isomeric host materials showed the suitable
HOMO/LUMO levels for good hole and electron injection and
enough triplet energy for energy transfer to yellow emitting POꢀ01,
they were examined as the host materials for the yellow PHOLEDs.
Fig. 4 showed the current densityꢀvoltageꢀluminance curves of the
yellow PHOLEDs with two different structural isomers as host
materials. The yellowꢀemitting POꢀ01 dopant was optimized at a
doping concentration of 5 wt.ꢀ% and the device performances were
compared. The current density in the pPDPC device was higher than
that in the mPDPC device, as predicted by holeꢀ and electronꢀonly
device data. The luminance showed the same tendency as the current
density.
Fig. 5 Quantum efficiencyꢀluminance curves of yellow PHOLEDs
based on pPDPC, mPDPC and CBP.
The electroluminescence (EL) spectra of the yellow PHOLEDs were
shown in Fig. 6. As shown in Fig. 6, the yellow emission spectra
were observed with a maximum emission peak around 558 nm and
vibrational shoulder peak around 590 nm of typical POꢀ01 based
devices. Only these two emission peaks implied that complete
energy transfer from the hosts (pPDPC, mPDPC and CBP) to the
POꢀ01 dopant and charge confinement were occurred inside the
emitting layer. Consequently, high quantum efficiencies were
achieved in the yellow PHOLED with pPDPC and mPDPC host
materials. The color coordinate of the yellow PHOLEDs were
(CIEx,y= 0.49, 0.50) for pPDPC device, (CIEx,y= 0.50, 0.50) for
mPDPC device and (CIEx,y= 0.50, 0.50) for CBP device, respectively.
Fig. 4 Current densityꢀvoltageꢀluminance curves of pPDPC and
mPDPC yellow PHOLEDs.
We investigated the device performances based on two structural
isomeric hosts and also compared them with commercial available
host, 4,4′ꢀbis(Nꢀcarbazolyl)ꢀ1,1′ꢀbiphenyl (CBP), based yellow
PHOLED. Quantum efficiencyꢀluminance curves of the yellow
PHOLEDs based on pPDPC, mPDPC and CBP were depicted in Fig.
5. The quantum efficiency of pPDPC yellow device was better than
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C4RA16316H
Fig. 6 EL spectra of yellow PHOLEDs based on pPDPC, mPDPC
and CBP.
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In conclusion, two structural isomers comprised of pyrimidine and
phenylcarbazole moieties were synthesized as new bipolar host
materials for yellow PHOLED, successfully. All the compounds
could be afforded by a facile synthetic route. Although relatively low
triplet energy was obtained in pPDPC incorporating a pyrimidine on
phenylcarbazole with paraꢀposition, it exhibited better charge
transport properties compared to those of mPDPC. These
phenomena led to increasing the maximum quantum efficiency,
current efficiency and power efficiency. Using the employed POꢀ01
based yellow device structures, the maximum quantum efficiency,
current efficiency and power efficiency of the pPDPC device were as
high as 22.3%, 68.3 cd/A and 47.5 lm/W, respectively. This newly
designed host materials should be useful for the future development
of host materials for yellow PHOLEDs as well as white PHOLEDs.
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Acknowledgements
The present research was conducted by the research fund of
Dankook University in 2014 (Grant no. 122531).
Notes and references
Department of Polymer Science and Engineering, Dankook University,
Yongin, Gyeonggi 448-701, Republic of Korea.
E-mail: bach@dankook.ac.kr, Fax: +82 31 8005 3585; Tel: +82 31 8005
3588
.
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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