Synthesis of dimesitylborane-substituted phenylcarbazoles as bipolar host materials and the variation of the green PHOLED performance with the substituent position of the boron atom

Article abstract of DOI:10.1039/c4dt00101j

Two structural isomers of the bipolar host material containing the dimesitylborane moiety were synthesized and their device performance was investigated. The quantum efficiency of the devices depends on the substituent position of the dimesitylborane moiety. The maximum external quantum efficiency of the device reached as high as 23.8% with a green color coordinate of (0.30, 0.63). This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00101j

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Synthesis of dimesitylborane-substituted
phenylcarbazoles as bipolar host materials and the
variation of the green PHOLED performance with
the substituent position of the boron atom†
Cite this: Dalton Trans., 2014, 43,
7712
Received 12th January 2014,
Accepted 26th March 2014
DOI: 10.1039/c4dt00101j
Hong-Gu Jang, Bo Seong Kim, Jun Yeob Lee and Seok-Ho Hwang*
www.rsc.org/dalton
Two structural isomers of the bipolar host material containing the in the molecular designs based on carbazole, various moieties
dimesitylborane moiety were synthesized and their device per- capable of electron-accepting, such as pyridine, triazole,
formance was investigated. The quantum efficiency of the devices triazine, phenanthroline, oxadiazole, benzimidazole, phos-
depends on the substituent position of the dimesitylborane phine oxide, and phosphine sulfide, were incorporated to give
moiety. The maximum external quantum efficiency of the device novel bipolar hosts.3
reached as high as 23.8% with a green color coordinate of (0.30,
As another electron acceptor, boron-containing derivatives
have attracted great deal of attention due to their
intriguing electronic and photophysical properties as
0.63).
a
a
For more than ten years, much progress has been made in the
field of phosphorescent organic light-emitting diodes
(PHOLEDs) because they effectively harvest electro-generated
singlet and triplet excitons to accomplish an internal quantum
efficiency close to 100%, which is much superior to the 25%
upper limit imposed by the formation of singlet excitons in
fluorescence.1 To realize highly efficient PHOLEDs, a suitable
host material is usually employed to suppress the intrinsic
detrimental effects, such as aggregation quenching and/or
triplet–triplet annihilation, of transition metal-centered phos-
phors.2 However, designing host materials having higher
triplet energy levels than that of the phosphorescent dopant
still remains a challenge for materials researchers. Recently,
PHOLED-related research has significantly drifted to the develop-
ment of host materials possessing bipolar properties. With
bipolar host materials consisting of both hole- and electron-
injecting/transporting functional groups, the wise selection
and proper linking of hole- and electron-transporting moieties
can promote balancing of the charge fluxes in the devices and
alignment of energy levels with the neighboring functional
layers, and meanwhile still provide large enough triplet ener-
gies compatible with emitters.
result of the overlap between the vacant p-orbitals of the boron
atom.4 Although these derivatives had been demonstrated
to be highly efficient fluorophores,5 electron-transporting
materials,6 emissive materials,7 and even bipolar materials in
some cases,5,7b,d thus far bipolar phosphorescent host
materials based on the combination of phenylcarbazole and
dimesitylborane have not been much exploited for green
PHOLED devices. In this communication, we reported the syn-
thesis and characterization of novel bipolar and high triplet-
energy phosphorescent host materials, 3-(dimesitylboryl)-9-
phenyl-9H-carbazole (1) and 9-(4-(dimesitylboryl)phenyl)-9H-
carbazole (3), that coupled between dimesitylborane and phe-
nylcarbazole to correlate the position of the boron atom of
dimesitylborane with the device performance (Scheme 1).
The bipolar host material 1 (33%) was synthesized by the
reaction of 3-bromo-N-phenylcarbazole first with n-butyllithum
to give the lithiate intermediate which was subsequently
quenched with fluorodimesityl borane. Synthesis of 3 began
with the commercially available 9H-carbazole, which was
N-arylated with 1,4-dibromobenzene using the Ullmann coup-
ling reaction to give 2 (36%) that was subsequently treated
with n-butyllithium and fluorodimesityl borane to form the
desired host material 3 (28%). The synthesis of the host
materials was confirmed from 1H and 13C NMR, and mass
data. The detailed synthetic procedure and analytical methods
are described in the ESI.†
One of the most important core structures of bipolar host
molecules is the carbazole moiety due to its good hole-trans-
port properties and large triplet energy. To achieve bipolarity
The thermal behavior was studied by TGA and DSC under a
nitrogen atmosphere. Their glass transition temperatures
clearly appeared to be 105 °C (1) and 89 °C (3), respectively.
Also, the decomposition temperatures for the initial 5 wt%
loss of mass were 290 °C (1) and 296 °C (3). These results
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. See DOI: 10.1039/
c4dt00101j
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hand, the signals in the photoluminescence spectra shifted
significantly to longer wavelengths upon increasing the solvent
polarity. We attribute this phenomenon to a mechanism invol-
ving rapid photoinduced electron transfer between the carba-
zole donor and the dimesitylborane acceptor units, resulting
in a large change in the dipole moment in the excited state; a
subsequent solvent relaxation process leads to the solvent
polarity-dependent emission.8 When we plotted the emission
peak frequencies of 1 and 3 in various organic solvents as a
function of solvent polarities, we obtained an almost linear
relationship together with a slope as steep as −4936 (1) and
−7990 (3) cm−1, consistent with our assignment of charge-
transfer emission (see Fig. S6 in ESI†).8 The absorption spectra
of 1 and 3 show strong absorption peaks corresponding to
π–π* absorption of the carbazole linked dimesitylborane back-
bone appearing below 300 nm. Also, two compounds exhibit a
strong absorption band around 340 nm assigned to an intra-
molecular charge transfer (ICT) band from the HOMO deloca-
lized over the carbazole moiety to the LUMO mainly localized
on the boron center. In contrast to this similarity in the
absorption spectra, significant differences are observed in the
fluorescence properties. The solution PL emissions of 1 and 3
were observed at 400 nm and 430 nm, respectively. The wave-
length of the emission maximum (λem) of 3 is more than
30 nm longer than that of 1. The Stokes shift of 3 exceeds
86 nm (5810 cm−1), whereas that of 1 is only 56 nm
(3980 cm−1). This large Stokes shift is one notable character-
istic of the present 4-boryl-phenylcarbazole skeleton; presum-
ably, it may be attributed to the change from the twisted
structure to the planar structure in the excited state.7f The
triplet energies of 1 and 3 were 2.88 and 2.72 eV, which could
be calculated from the first phosphorescent emission peak of
the low temperature (77 K) PL spectrum at 430 and 455 nm,
respectively. Their triplet energy was high enough to use the
host material as a green phosphorescent dopant. The triplet
energy of the tris[2-phenylpyridinato-C2,N]iridium (Ir(ppy)3)
dopant is 2.41 eV and effective energy transfer from the
synthesized hosts to the Ir(ppy)3 dopant is expected (Fig. 2).
On the basis of the roughly evaluated electrochemical oxi-
dation onset potential, the HOMO levels of 1 and 3 were esti-
Scheme 1 Reagents and conditions: (i) n-BuLi, (Mes)2BF, THF, −78 °C to
20 °C; (ii) K2CO3, CuI, 18-crown-6, DMF, 130 °C; (iii) n-BuLi, (Mes)2BF,
THF, −78 °C to 20 °C.
Fig. 1 HOMO and LUMO distribution of 1 and 3.
mean that they should be suitable to use as a PHOLED
component.
Spatial distributions of the HOMO and LUMO orbitals of 1 mated as −6.21 and −6.17 eV, respectively. Their LUMO levels
and 3 were calculated with the Gaussian program at the are −2.86 and −2.95 eV, calculated from the HOMO level and
B3LYP/6-31G(d) level, using the density functional theory energy bandgap determined from the UV/vis absorption
(DFT) for the geometry optimizations. As shown in Fig. 1, the threshold. The LUMO level of 3 was much lower than that of 1
HOMO of 1 and 3 is mainly localized at the phenylcarbazole because there may be a different electron accepting ability of
because of the strong electron donating character of nitrogen the boron center depending on the substituent position onto
of carbazole. In contrast, those LUMO showed a different be- phenylcarbazole.
havior. The LUMO of 3 was localized to the central phenyl and
dimesitylborane unit; however, in the case of 1, a quite exten- fabricated to compare the hole and electron density in the
sion of the LUMO to the carbazole unit was observed.
host materials. Fig. 3 shows the current density–voltage curves
Hole- and electron-only devices composed of 1 and 3 were
Fig. S5† depicts the room temperature absorption and PL of hole- and electron-only devices bearing 1 and 3. The hole
spectra of 1 and 3 in various solutions. The absorption spectra and electron current density of 3 was much higher than those
in 1,3-dioxane, CH2Cl2, acetone and DMSO are almost identi- of 1, which indicates that 3 shows better performance than 1,
cal, i.e., independent of the solvent polarity, implying that the in terms of charge transportation. The HOMO and LUMO
Franck–Condon excited-state is subject to a rather small levels of 1 and 3 were determined to be suitable for hole and
dipolar change with respect to the ground state. On the other electron injection, which accounted for the high hole and elec-
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Fig. 2 UV/vis, solution PL and low temp. PL spectra of 1 and 3.
Fig. 4 Current
density–voltage–luminance
(A)
and
quantum
efficiency–luminance (B) curves of 1 and 3 green PHOLEDs.
Fig. 3 Current density–voltage of hole- and electron-only devices
made of 1 and 3.
electron current densities of the 3 device were higher than
those of the 1 device, high current density and luminance were
observed for the 3 device. The maximum luminance of the 1
device could not reach 1000 cd m−2 compared with over 10 000
cd m−2 of the 3 device. Poor charge transport properties of 1
limited the maximum current density and luminance of the 1
device.
tron current densities with better charge transport properties.
Therefore, hole and electron transport properties of 3 were
enhanced, resulting in high hole and electron current den-
sities in the hole- and electron-only devices. The better hole
transport properties of 3 than that of 1 are due to the localiz-
ation of the HOMO on carbazole which shows good hole trans-
port properties. The dimesitylborane moiety degraded the hole
transport character of carbazole in 1, which resulted in low
hole current density in 1. Similarly, poor electron transport
character of 1 is due to extensive dispersion of the LUMO
on the carbazole unit which has poor electron transport pro-
perties. In contrast, the LUMO of 3 was localized on the elec-
tron deficient phenyl unit modified with dimesitylborane,
which improved the electron transport properties of 3.
The triplet energy of 1 and 3 was suitable for evaluation as
the host material of green PHOLEDs. The green PHOLEDs
were fabricated using the host materials with 10% of Ir(ppy)3
doping concentration in all devices. Fig. 4(A) shows current
density–voltage–luminance curves of the green PHOLEDs. The
current density of the green PHOLED with 3 as the host was
much higher than that of the green PHOLED with 1 as the
host, which is in agreement with the current density–voltage
curve of the single carrier devices in Fig. 3. As the hole and
External quantum efficiency–luminance curves of the green
PHOLEDs are shown in Fig. 4(B). The 3 device exhibited much
higher quantum efficiency than the 1 device at the same lumi-
nance. The maximum quantum efficiency and the quantum
efficiency at 1000 cd m−2 of the 3 device were 23.8% and
21.7% compared with the maximum quantum efficiency of
6.5% of the 1 device. There was more than three times
improvement of the quantum efficiency of the 3 device. The
high quantum efficiency of the 3 device is related to balanced
charge density, exciton confinement and efficient energy trans-
fer. As shown in the single carrier device data in Fig. 3, the
hole current density of 3 was similar to the electron current
density of 3, which indicates bipolar charge transport pro-
perties. The bipolar charge transport properties of 3 balanced
holes and electrons in the emitting layer, which contributed to
the high quantum efficiency of the 3 device. However, the hole
current density was much higher than the electron current
density in the 1 device, which leads to poor charge balance
and low quantum efficiency. Exciton confinement by the high
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Acknowledgements
The present research was supported by the research fund of
Dankook University in 2012 (grant no. 108863).
Notes and references
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triplet energy host, hole transport material and electron trans-
port material also played an important role in enhancing the
quantum efficiency of the 3 device. As the triplet energy of the
host and charge transport materials was higher than that of
Ir(ppy)3, triplet excitons of Ir(ppy)3 could be effectively con-
fined in the emitting layer with little quenching. Finally,
efficient energy transfer as can be observed in the electro-
luminescence (EL) spectrum of the 3 device in Fig. 5 was
another key factor for the high quantum efficiency of the 3
device. Only green emission of Ir(ppy)3 without any emission
from the 3 host material was observed in the EL spectra, imply-
ing good energy transfer from the 3 host to Ir(ppy)3. The low
quantum efficiency of the 1 device is due to poor charge
balance in the emitting layer because excitons can be effec-
tively confined in the 1 device and no emission from the 1
host was observed in the EL spectrum of the 1 device.
EL spectra of the 1 and 3 devices are shown in Fig. 5.
Strong green emission by Ir(ppy)3 appeared at 509 nm with a
shoulder at 539 nm. No other emission peaks from host or
charge transport materials were observed, confirming effective
charge confinement in the emitting layer. Color coordinates of
the 1 and 3 devices were (0.31, 0.62) and (0.30, 0.63),
respectively.
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Conclusions
In summary, two structural isomers, 3-(dimesitylboryl)-9-
phenyl-9H-carbazole and 9-(4-(dimesitylboryl)phenyl)-9H-car-
bazole, with the dimesitylboryl group at different positions of
the phenylcarbazole moiety, were synthesized. Thereof, the
photophysical properties and device performances were corre-
lated with the molecular structure. The maximum quantum
efficiency and the quantum efficiency at 1000 cd m−2 of the
device with 3 were 23.8% and 21.7% compared with the
maximum quantum efficiency of 6.5% of the device with 1. 8 (a) Y. Y. Chien, K. T. Wong, P. T. Chou and Y. M. Cheng,
Therefore, the dimesitylboryl group should be selectively intro-
duced into the molecular structure of host materials to
manage the charge transport properties of host materials.
Chem. Commun., 2002, 2874; (b) K. T. Wong, S. Y. Ku,
Y. M. Cheng, X. Y. Lin, Y. Y. Hung, S. C. Pu, P. T. Chou,
G. H. Lee and S. M. Peng, J. Org. Chem., 2006, 71, 456.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 7712–7715 | 7715