Tuning the optoelectronic properties of 4,4′-N, N′-dicarbazole- biphenyl through heteroatom linkage: New host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1021/ol1008872

(Equation Presented). Five carbazole end-capped heterofluorenes (CzHFs) designed by structurally mimicking 4,4′-N,N′-dicarbazole-biphenyl (CBP) via connecting the biphenyl core of CBP with the linking atom of C, P, N, O, and S, respectively, were synthesized successfully, and their optoelectronic properties were investigated. The theoretical calculations and experimental results demonstrate that CzHFs are potential green, red, and even blue hosts for phosphorescent light-emitting diodes (PHOLEDs) with more desirable localization and energy levels of HOMO and LUMO and also higher triplet energy than CBP.

Full text of DOI:10.1021/ol1008872

ORGANIC
LETTERS
2010
Vol. 12, No. 15
3438-3441
Tuning the Optoelectronic Properties of
4,4′-N,N′-Dicarbazole-biphenyl through
Heteroatom Linkage: New Host Materials
for Phosphorescent Organic
Light-Emitting Diodes
Shenglan Zhang, Runfeng Chen,* Jun Yin, Feng Liu, Hongji Jiang, Naien Shi,
Zhongfu An, Cong Ma, Bin Liu, and Wei Huang*
Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute
of AdVanced Materials (IAM), Nanjing UniVersity of Posts and Telecommunications
(NUPT), Nanjing 210046, China
wei-huang@njupt.edu.cn, iamrfchen@njupt.edu.cn
Received June 1, 2010
ABSTRACT
Five carbazole end-capped heterofluorenes (CzHFs) designed by structurally mimicking 4,4′-N,N′-dicarbazole-biphenyl (CBP) via connecting the biphenyl
core of CBP with the linking atom of C, P, N, O, and S, respectively, were synthesized successfully, and their optoelectronic properties were investigated.
The theoretical calculations and experimental results demonstrate that CzHFs are potential green, red, and even blue hosts for phosphorescent
light-emitting diodes (PHOLEDs) with more desirable localization and energy levels of HOMO and LUMO and also higher triplet energy than CBP.
PHOLEDs have attracted much attention, mainly because it
is possible to increase the internal quantum efficiency of the
devices up to 100% as a result of their harvesting of both
single and triplet excitons.1 However, a high-performance
host material is essential for the fabrication of such kind of
devices to prevent the undesired concentration self-quenching
or T1-T1 annihilation. Also, the development of the suitable
host materials especially for blue emitters has turned out to
be a rather challenging task. There are some physical
requirements1 for the host materials: (1) a triplet energy (ET)
higher than the triplet emitter to prevent reverse energy
transfer from the guest back to the host, (2) good and
balanced charge transporting properties with appropriate
highest occupied/lowest unoccupied molecular orbital (HOMO/
LUMO) energy levels, and (3) decent thermal and morpho-
logical stabilities to increase the device’s lifetime.
Carbazole derivatives have been widely used as host
materials in PHOLEDs because of their sufficiently large
triplet energy (up to 3.05 eV) and good hole-transporting
ability.2 CBP is a convenient host for green and red emitters
because of its high triplet energy (2.56 eV); however, for
the general blue triplet guests (2.7 eV), CBP is not applicable
because of inefficient energy transfer from the host to the
guest.2
(2) (a) Hsu, F. M.; Chien, C. H.; Shih, P. I.; Shu, C. F. Chem. Mater.
2009, 21, 1017. (b) Tao, Y. T.; Wang, Q.; Yang, C. L.; Zhong, C.; Zhang,
K.; Qin, J. G.; Ma, D. G. AdV. Funct. Mater. 2010, 20, 304. (c) He, J.; Liu,
H. M.; Dai, Y. F.; Ou, X. M.; Wang, J.; Tao, S.; Zhang, X. H.; Wang,
P. F.; Ma, D. G. J. Phys. Chem. C 2009, 113, 6761. (d) Park, T. J.; Jeon,
W. S.; Park, J. J.; Kim, S. Y.; Lee, Y. K.; Jang, J.; Kwon, J. H. Thin Solid
Films 2008, 517, 896.
(1) Baldo, M. A.; O, B. D.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151.
10.1021/ol1008872  2010 American Chemical Society
Published on Web 07/07/2010

Table 1. Absorption and Emission Peaks (in nm) in THF and Thin Solid Film and DFT Calculations of the Absorption Peaks (in nm),
Dipole Moment (µ), Torsion Angle (σ), and Singlet Band Gap (calEg, eV) of CzHFs
THF
thin film
λem
calculated
a
compound
λabs
λem
369
362 (377)
427
386, 406
378
378
λabs
opEg
λabs
µ
σ
calEg
CBP2d
292, 318
291, 339
289, 339
332
378
367, 406
364, 469
(365, 384) 414, 437
375, 410, 435
361, 391 (412, 434)
303, 348
308, 361
305, 397
300, 352
297, 361
305, 368
0.0003
0.0972
2.8079
4.8042
1.6235
1.6481
53.8
55.4
60.7
61.0
59.0
57.6
4.08
3.95
3.64
3.97
3.92
3.93
p-CzCF
p-CzPF
m-CzNF
m-CzOF
m-CzSF
293, 342
289, 338
291, 319, 345
284, 332
293, 330
3.37
2.96
3.34
3.41
3.39
262, 289, 339
280, 325, 336
289, 326, 338
a Optical band gaps obtained from the absorption edge on spin-coated films (Eg ) hc/λonset).
One strategy to design new functional materials is to mimic
the structure of the famous and well-studied compounds,
although their synthesis may be a great challenge. Structur-
ally, CBP can be considered as a carbazole end-capped
biphenyl. Then, what will the material be if the diphenyl of
CBP were connected by other bridging atoms? The nonplanar
biphenyl core becomes a planar heterofluorene, and a new
series of compounds (CzHFs) can be produced. Heterofluo-
renes3 such as nitrogafluorene (carbazole, NF),4 sulfafluorene
(SF),5 silafluorene (SiF),6 germafluorene (GeF),7 and phos-
phafluorene (PF)8 have received interest due to their par-
ticular electronic and optical properties. The marriage of
carbazole with heterofluorene in a way as that in CBP may
produce interesting materials, especially of high-performance
host materials for blue emitters.
The heteroatom linkage of the biphenyl of CBP was
realized in two steps with high yields. The first step is to
prepare the dibromoheterofluorenes.8 The second step is to
attach the 9-position of the carbazole moiety to the terminal
ends of the heterofluorene cores using an optimized Ullmann
C-N cross-coupling reaction between the carbazole and
dibromoheterofluorene as shown in Scheme 1. The optimized
Ullmann reaction11 proceeded in refluxing nitrobenzene in
the presence of copper bronze as catalyst and K2CO3 as base
and produced the desired compounds in high yields. The
target compounds of CzHFs were fully characterized by 1H
NMR, 13C NMR, mass spectrometry, and elemental analysis.
The detailed synthetic procedures and structural characteriza-
tions are presented in Supporting Information. CzHFs show
good solubility in common solvents and good thermal
stabilities (see Table S1 in Supporting Information) revealed
by differential scanning calorimetry (DSC).
The optical properties of CzHFs in dilute THF and in solid
film were investigated and are summarized in Table 1. Both
in solution and in solid film, two absorption bands can be
observed (see Figure 1). The first absorption band around
290 nm that is also observed in CBP can be assigned to the
carbazole-centered n-π* transition,12 and the other long
wavelength absorption around 320-340 nm is attributed to
the π-π* transition of the entire conjugated backbone. This
two absorption bands are quite different for p- and m-CzHFs,
in that they show the strongest absorption at the second and
Scheme 1. Synthesis of the CzHFs
(3) Chen, R. F.; Zheng, C.; Fan, Q. L.; Huang, W. J. Comput. Chem.
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43, 952.
In this paper, we successfully synthesized a series of
carbazole end-capped fluorene (CzCF),9 phosphafluorene
(CzPF), nitrogafluorene (CzNF),10 oxygafluorene (CzOF),
and sulfafluorene (CzSF) by connecting the biphenyl core
of CBP with the linking atom of C, P, N, O, and S,
respectively. The optoelectronic properties of the CzHFs have
been measured and discussed by a combined research of
experimental and theoretical study. This study suggests that
CzHFs are good host materials with more desirable localiza-
tion and energy level of HOMO and LUMO and also higher
triplet energy than CBP, and p-CzPF is also an excellent
blue light emitting materials.
(6) Chan, K. L.; McKiernan, M. J.; Towns, C. R.; Holmes, A. B. J. Am.
Chem. Soc 2005, 127, 7662.
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Sci.China. Ser. B 2009, 52, 212.
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2913. (b) Chen, R. F.; Fan, Q. L.; Zheng, C.; Huang, W. Org. Lett. 2006,
8, 203.
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Keawin, T. Tetrahedron. Lett. 2007, 48, 89.
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Saengsuwan, S.; Keawin, T. Tetrahedron. Lett. 2006, 47, 8949. (b) Albrecht,
K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244.
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Jungsuttiwong, S.; Keawin, T. Synth. Met. 2007, 157, 17.
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B. X.; Xia, P. F.; Cheah, K. W.; Chen, C. H. AdV. Mater. 2009, 21, 688.
Org. Lett., Vol. 12, No. 15, 2010
3439

CzHFs (362-386 nm). Most CzHFs have only one emission
peak, but p-CzCF and m-CzNF have two. However, in the
thin solid film, more PL peaks appear and the spectra become
quite complicated. The emission in the thin film was
generally red-shifted as a result of the stronger intermolecular
interactions in the solid state, but blue-shifted emission peaks
still can be founded. The short wavelength emission of the
p-CzCF, m-CzNF, m-CzSF, and m-CzOF in the film can be
attributed to the emission of the single molecule as with the
similar emission behaviors in the dilute THF solutions.
However, in the p-CzPF film, an even bluer emission than
that in dilute THF solution was observed. Among all of the
CzHFs, p-CzPF has the highest PL quantum efficiency (Φ
)1.14, in cyclohexane), suggesting its great potential as a
highly efficient blue (469 nm) light-emitting material.
Figure 1. Calculated absorption spectra in vacuum (a, b) and
normalized absorption and photoluminescent spectra in dilute THF
(c, d) and in thin solid film (e, f) of CzHFs
the first bands, respectively. This difference is due to the
different substitution pattern of the carbazole, which can be
verified by DFT calculations. Theoretical transition config-
uration analysis reveals that the first absorption corresponding
to a S0 f S2 electronic transition is mainly related to HOMO
f LUMO+1 and HOMO f LUMO+2, whereas the second
band of S0 f S1 transition is mainly contributed by HOMO
f LUMO. As a result of different frontier orbital electron
distribution overlap between the initial and the final states,
the different oscillator strength that stands for different
absorption intensity results. The theoretical prediction is quite
consistent with the actual spectra of the CzHFs. In compari-
son with the absorption in THF solution, CzHFs exhibit little
spectral shift (within 5 nm) in solid films, suggesting that in
the solid state the molecular interaction of CzHF is weak as
a result of its nonplanar molecular geometry (see the torsion
angle in Table 1). The optical energy band gaps (opEg) of
the CzHFs determined from the onset of the UV-vis spectra
in solid film are are very similar (3.34-3.41 eV), and
m-CzOF has the highest one, except for p-CzPF, which has
a very low opEg (2.96 eV) due to its para-linkage effects of
carbazole and the oxidized phosphorus atom linkage.
For the photoluminescene (PL) of CzHFs, identical emis-
sion spectra can be obtained whether excited at the π-π*
transition (S0 f S1) or at the n-π* transition (S0 f S2)
absorption band, indicating the efficient energy transfer from
the peripheral carbazole dendrons to the heterofluorene core.
In THF solution, p-CzPF has the longest emission peak at
427 nm, which is significantly longer than that of other
Figure 2. Computed isodensity surface of the HOMO and LUMO
obitals of CzHFs
From the HOMO and LUMO orbitals shown in Figure 2,
CzHFs have quite similar frontier orbital electron density
distributions. The HOMOs are mainly determined by car-
bazole, although heterofluorenes also have their influence,
whereas the LUMOs are predominately localized on and are
primarily determined by the central heterofluorenes. A clear
difference between p- and m-CzHFs can be observed;
LUMOs of p-type CzHFs have also contributions from the
N atoms in the carbazole substituent, since the nitrogen
would provide its lone pair to form an extended conjugation,
suggesting the better conjugation of p-CzHFs compared with
that of the m-CzHFs, which explains the facts found in the
optical and electrochemical measurements. The separately
determined HOMO and LUMO by carbazole and heterof-
3440
Org. Lett., Vol. 12, No. 15, 2010

luorene result in the separated electron density distribution
between the HOMO and LUMO of CzHFs. This separation
is preferable for efficient hole- and electron-transporting
properties and the prevention of reverse energy transfer,13
enabling CzHFs the excellent potential host materials.
singlet band gaps (expEg), p-CzPF has the lowest one (2.86
eV)m and the other CzHFs have expEg’s very close (3.12-3.22
eV) but lower than that of CBP (3.7 eV). The CV measured
frontier obitals are well predicted by the DFT calculations
with good coordination as listed in Table 2. The higher
HOMO and lower LUMO of CzHFs compared with that of
CBP suggest that CzHFs have better hole and electron
injection and transfer abilities than CBP that are beneficial
for the device performance improvement.
Table 2. Experimental and Theoretical HOMO, LUMO, Singlet
Band Gap (expEg,), and Triplet State Energy (expET and calET) of
CzHFs (in eV)
Phosphorescent spectra of CzHFs were detected at 77K
with 5 ms delay as shown in Figure S21 in Supporting
Information, and the triplet state energy (expET) was read by
taking the highest energy peak of phosphorescence as the
transition energy of T1 f S0. The CzHFs show higher expET’s
(2.61-2.98 eV) compared with that of CBP (2.56 eV) except
p-CzPF (2.30 eV), indicating that these compounds are
suitable host materials for green, red, and even blue triplet
emitters. From Table 2, the m-CzHFs have expET’s much
higher than those of the p-CzHFs, and with the triplet energy
higher than 2.8 eV, they can host blue emitters. A small
systemic error of about 0.1 eV between DFT calculation and
experimental measurement on evaluating triplet state energy
was observed with good coordination. It is interesting that
CzHFs have ET’s higher than those of CBP, although their
Eg’s are lower, probably due to the great modification abilities
of the heteroatom linkage. The higher HOMOs, lower
LUMOs, but still higher expET’s compard with those of CBP
make CzHFs attractive host materials for triplet emitters.
In summary, by mimicking the molecular structure of CBP,
we have successfully synthesized a new series of host
materials of CzHFs using Ullmann C-N coupling reaction,
and their optoelectronic properties were systematically
studied. The combined theoretical and experimental results
demonstrate that CzHFs are a kind of promising green, red,
and even blue host material for PHOLEDs with their more
desirable localization and energy level of HOMOs and
LUMOs and also higher triplet energies compared with those
of CBP. Also, p-CzPF is also an excellent blue light-emitting
material with strong fluorescent emission.
from CV
B3LYP/6-31G*
compound HOMO LUMO expEg
HOMO LUMO calET expET
CBP14
-5.70 -2.00 3.70 -5.31 -1.23 2.72 2.56
-5.42 -2.20 3.22 -5.24 -1.29 2.69 2.61
-5.44 -2.58 2.86 -5.39 -1.75 2.48 2.30
-5.34 -2.22 3.12 -5.17 -1.20 2.97 2.86
-5.40 -2.26 3.14 -5.37 -1.45 3.08 2.98
-5.42 -2.26 3.16 -5.36 -1.43 3.05 2.87
p-CzCF
p-CzPF
m-CzNF
m-CzOF
m-CzSF
The electrochemical behaviors of CzHFs were investigated
by cyclic voltammetry (CV) as shown in Figures S18 and
S19 in Supporting Information. Except for p-CzCF, which
is chemically reversible as reported in the literature, the other
CzHFs show quasi-reversible or irreversible redox behaviors,
and their irreversibility becomes more apparent when the
CV scans are performed in solution as illustrated in Figure
S20 in Supporting Information. The irreversible redox
behaviors of CzHFs (except for p-CzCF) are due to the active
3- and 6-positions of carbazole that can undergo dimerization
reactions during the CV scan as proposed by Ma et al., when
they were investigating the carbazole-containing host
materials.2c The first oxidation process of CzHFs around
+0.8 V can be assigned to the simultaneous multielectron
oxidation process of the peripheral carbazole units resulting
the formation of radical cations, and the second oxidation
process around +1.0 V corresponds to the removal of
electrons from the interior heterofluorene core generating
radical dications.11 From the onset potential of electrochemi-
cal reduction and oxidation, HOMO and LUMO were
calculated and are listed in Table 2. Because after the first
CV scan irreversible cross-linking through the 3- and
6-positions of carbazole occurs and the material works at
that state, the HOMOs were calculated from the second CV
scan. The highest HOMO is found for m-CzNF (-5.34 eV),
and the other CzHFs have very close HOMOs (-5.40 to
-5.44 eV), which are much higher than CBP (-5.7 eV).
The lowest LUMO is found for p-CzPF (-2.58 eV), and
the other CzHFs all have LUMOs (-2.20 to -2.26 eV) lower
than that of CBP (-2.0 eV). As for the electrochemical
Acknowledgment. This work was financially supported
by the National Basic Research Program of China (973
Program, 2009CB930601), National Natural Science Foun-
dation of China (20804020, 60976019, and 20974046), and
Natural Science Foundation of Jiangsu College Council
(Grant No. 08KJB150012).
Supporting Information Available: Experimental pro-
cedures and spectra for the new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
OL1008872
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