desirable host materials in PhOLEDs. However, their
practical applications are usually limited by relatively
poor-electron transporting mobility.4 In addition, most
of the carbazole based small molecules and polymers are
constructed by modification at 3,6, 2,7, 1,8 and N linkage
positions of carbazole.5 For example, Professor Wang
synthesized a dendrimer host, by conjugating a moiety to
the 3,6 and N positions of carbazole; a fully white
PhOLED with a maximum power efficiency of 80.9 lm/
W was harvested.6 By the modification at the 3,6 and 2,7
positions of the carbazole, some novel compounds were
developed and the solution-processed blue PhOLEDs with
FIrpic as a dopant showed a low turn-on voltage of 4.0 V,
a maximum current efficiency of 27.2 cd/A, a maximum
efficiency of 11.8 lm/W, and a maximum external quantum
efficiency 14.0%.7 Recently, a variety of 1,8-disubstituted
carbazole derivatives were reported as fluorescent probes8
or to demonstrate the relationship between the rigidity
structure and the linking positions.9 Generally, the simple
structure of a carbazole unit actually offers many choices
for introducing building blocks into the system via an easy
chemistry approach. However, the tetrasubstituted carba-
zole derivatives used as the host materials for PhOLEDs
have been rarely reported.
In this paper, spirofluorene and benzene units are con-
nected to the 1,3,6,8-positions of the carbazole to form
the butterfly-shaped compounds (illustrated in Scheme 1).
Compared to the disubstituted carbazole derivatives,10
the unique butterfly-shaped structure of the molecules
enhances the thermal stability of these materials, and the
nonplanar linkage modes keep the triplet energy gap at
a high level.11 In addition, the long alkyl groups at the N
position of the carbazole are beneficial to the solubility
of the host materials in the most common solvents. The
solution-processed green PhOLEDs with the novel hosts
showed excellent performance, with a luminous efficiency
(ηp,max) of 41.0 cd/A and an external quantum efficiency
(ηEQE,max) of 11.8%. This result is comparative with
the recently reported small-molecule host for solution-
processed green PhOLEDs.12
The second step is to attach the N-9-position of the car-
bazole with a long alkyl group by a simple CꢀN coupling
reaction. The last step involves linking the electron-
deficient moiety to the 1,3,6,8-positions of carbazole using
a typical Suzuki CꢀC cross-coupling reaction between the
different functional boronic esters and the 1,3,6,8-tetrabromo-
N-octylcarbazole (2). Tetrasubstituted derivatives of car-
bazole have been rarely reported because it is difficult to
obtain tetrabromocarbazole. Only Smith et al.13 reported
a method of bromination of carbazole by treatment with
NBS in the presence of silica gel at ambient temperature.
However, the reaction is less selective and gives mixtures
of different bromo-substituted derivatives and purifica-
tion of the tetrabromocarbazole is difficult. Here, we used
bromine instead of NBS and used FeCl3 as the initiator,
with the temperature enhanced to 100 °C to promote full
substitution in the carbazole. The intermediate 1,3,6,8-
tetrabromocarbazole (1) can be recrystallized and purified
with the solvent ethanol/toluene. Afterwards, 1,3,6,8-
tetrabromocarbazole (1) was treated with the base NaH,
followed by 1-bromooctane, to afford the desired com-
pound 2 in a high yield of 98%. The butterfly-shaped
molecules were synthesized by a Suzuki reaction of 2 with
4 equiv of 2-(9,90-spirobi[fluoren]-7-yl)-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane or 2-([1,10:30,100-terphenyl]-50-yl)-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane with high yields
in the range of 75ꢀ80% (Scheme 1). All the compounds
1
have been fully characterized by H NMR, 13C NMR,
mass spectrometry, and elemental analysis (see Supporting
Information).
Scheme 1. Synthetic Route for TSPFCz and TTPhCz
The butterfly-shapedhostmaterials wererealized in three
steps with high yields. As shown in Scheme 1, the first
step is to prepare the tetrabromocarbazole intermediates.
(3) Tsai, M. H.; Lin, H. W.; Su, H. C.; Ke, T. H.; Wu, C. c.; Fang,
F. C.; Liao, Y. L.; Wong, K. T.; Wu, C. I. Adv. Mater. 2006, 18, 1216.
(4) Zhang, S. L.; Chen, R. F.; Yin, J.; Liu, F.; Jiang, H. J.; Shi, N. E.;
An, Z. F.; Ma, C.; Liu, B.; Huang, W. Org. Lett. 2010, 12, 3438.
(5) (a) Sasabe, H.; Kido, J. Chem. Mater. 2011, 23, 621. (b) Park,
M. S.; Lee, J. Y. Chem. Mater. 2011, 23, 4338. (c) Su, S.-J.; Cai, C.; Kido,
J. Chem. Mater. 2011, 23, 274.
(6) Zhang, B.; Tan, G.; Lam, C. S.; Yao, B.; Ho, C. L.; Liu, L.; Xie,
Z.; Wong, W. Y.; Ding, J.; Wang, L. Adv. Mater. 2012, 24, 1873.
(7) Jiang, W.; Duan, L.; Qiao, J.; Dong, G.; Zhang, D.; Wang, L.;
Qiu, Y. J. Mater. Chem. 2011, 21, 4918.
(8) Gee, H. C.; Lee, C. H.; Jeong, Y. H.; Jang, W. D. Chem. Commun.
2011, 47, 11963.
(9) Wang, H.-Y.; Liu, F.; Xie, L.-H.; Tang, C.; Peng, B.; Huang, W.;
Wei, W. J. Phys. Chem. C 2011, 115, 6961.
(10) Sasabe, H.; Pu, Y. J.; Nakayama, K.; Kido, J. Chem. Commun.
The compounds TSPFCz and TTPhCz exhibited excel-
lent thermal stability with 5% weight-loss decomposition
temperaturesof483 and 487 °C, withglasstransition temp-
eratures of 236 and 146 °C, respectively, which suggests
that they could form morphologically stable and uniform
amorphous films. The surface morphologies of thin films
of TSPFCz and TTPhCz were also investigated. The
thin film was prepared by spin-coating and then annealed
under N2 gas conditions at 100 °C for 2 h. The annealed
film had a fairly smooth surface morphology with a root-
mean-square (rms) roughness of 0.45 and 0.38 nm for
2009, 6655.
(11) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953.
(12) Zhu, M.; Ye, T.; He, X.; Cao, X.; Zhong, C.; Ma, D.; Qin, J.;
Yang, C. J. Mater. Chem. 2011, 21, 9326.
(13) Smith, K.; James, D. M.; Mistry, A. G.; Bye, M. R.; Faulkner,
D. J. Tetrahedron. 1992, 48, 7479.
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