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Table 1 EL performances of methyl-BTPE, isopro-BTPE, Ph-BTPE and
bathochromic shift. Thus, to control the conjugation between
the carbazolyl and TPE moieties, Cz-BTPE is constructed from
BTPE and carbazole units through carbon–nitrogen bonds with
a less conjugation effect. However, the dihedral angle of the
biphenyl core in Cz-BTPE is smaller than those of the other
Cz-BTPEa
lEL Von Lmax
ZP,max
ZC,max Zext,max
(nm) (V) (cd mꢀ2) (lm Wꢀ1) (cd Aꢀ1) (%)
CIE (x, y)
Methyl-BTPE 451 5.3 1976
Isopro-BTPE 451 5.3 1127
0.98
1.43
2.13
2.55
1.53
1.86
3.10
3.74
1.3
1.7
1.9
1.9
0.15, 0.12
0.15, 0.11 derivatives caused by the much stronger p–p interactions between
0.16, 0.19
0.17, 0.26
Ph-BTPE
Cz-BTPE
467 5.3 6497
479 4.9 9911
the planar carbazolyl moieties and the BTPE skeleton. Hence, the
device based on Cz-BTPE shows longer EL emission with greatly
a
Abbreviations: Von = turn-on voltage at 1 cd mꢀ2, Lmax = maximum
luminance, ZP,max, ZC,max and Zext,max = maximum power, current and
external efficiencies, respectively. CIE = Commission International de
enhanced efficiencies, but still in the region of blue emission.
In summary, four new BTPE derivatives with tunable
dihedral angels of the biphenyl cores have been successfully
constructed. The ingenious introduction of methyl, isopropyl,
phenyl and carbazyl groups with different size and conjugation
l’Eclairage coordinates at 100 mA cmꢀ2
.
carbazole units, which is consistent with its electron clouds distribu- degree has resulted in their blue or deep blue EL emissions,
tion and the diminished energy gap between the emitter and the compared to BTPE with sky-blue emission (488 nm) and CIE
hole-transporting layer (NPB).
Generally, devices based on the aromatic-substituted BTPE deri- idea and has shed some light on the further molecular design.
vatives show better EL performance than the alkyl-substituted ones. This work was supported by the National Fundamental Key
coordinates of (0.20, 0.36). This is in accordance with our design
In detail, devices based on methyl-BTPE and isopro-BTPE exhibit Research Program (2013CB834701), the National Natural
maximum luminances (Lmax) of 1976 and 1127 cd mꢀ2, maximum Science Foundation of China (No. 21161160556, 51333007,
current efficiencies (ZC,max) of 1.53 and 1.86 cd Aꢀ1, and and 50973104), and the Foundation of Jilin Research Council
maximum power efficiencies (ZP,max) of 0.98 and 1.43 lm Wꢀ1
,
(2012ZDGG001), and Open Project of State Key Laboratory of
respectively. And both of the devices show deep-blue emissions Supramolecular Structure and Materials (sklssm201302).
(451 nm) with Commission International de l’Eclairage (CIE)
coordinates of (0.15, 0.12) and (0.15, 0.11), due to the introduc-
tion of the non-conjugated alkyl groups and ortho-interaction of
2-methyl or 2-isopropyl substituents. Interestingly, although the
molecular structures of these two emitters are slightly different,
Notes and references
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Much better EL performance were obtained for Ph-BTPE and
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Cz-BTPE with Lmax, ZC,max, and ZP,max of 6497 and 9911 cd mꢀ2
,
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3.10 and 3.74 cd Aꢀ1, and 2.13 and 2.55 lm Wꢀ1, respectively,
showing a more efficient exciton combination in the emissive
layers as expected. Blue emissions were observed from both of the
devices based on Ph-BTPE and Cz-BTPE, with CIE coordinates of
(0.16, 0.19) and (0.17, 0.26). As discussed above, Ph-BTPE bears
two more freely rotatable phenyl rings, leading to its higher
quantum yield in the aggregated state. Therefore, when fabricated
as an emissive layer in the device, it exhibits higher luminance
and efficiencies than methyl-BTPE and isopro-BTPE, since the
radiative channel was activated in the solid state. On the other
hand, the aromatic phenyl ring with larger size is obviously more
conjugated than methyl and isopropyl groups, thus the conjuga-
tion length of Ph-BTPE should be longer than those of methyl-
BTPE and isopro-BTPE, as partially confirmed by their UV-vis and
EL spectra. As we know, carbazole is a vesatile material used in 7 (a) J. Huang, X. Yang, J. Wang, C. Zhong, L. Wang, J. Qin and Z. Li,
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ability, and generally, the direct linkage of carbazole to other
chromophores through 3,6-positions will actually cause a distinct
J. Qin and Z. Li, Adv. Funct. Mater., 2013, 23, 2329.
2138 | Chem. Commun., 2014, 50, 2136--2138
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