lower than that of Ir(ppy)3 (2.42 eV),8 implying CZBP was
not suitable as a host material for Ir(ppy)3.
has more significant localization of the HOMO and LUMO
than CZBP, which is desirable for charge separation and
prevention of back excitation transfer. Furthermore, the triplet
energy gap of m-CZBP is higher than the guest Ir(ppy)3 by
2.61 to 2.44 eV, whereas that of CZBP is lower than Ir-
(ppy)3 by 2.41-2.44 eV, which allows back energy transfer
from the guest to the host. Although phase separation or
aggregation of dopants was observed from the EL spectra
of m-CZBP, the charge localization may play a more
important role in giving rise to the improved device
performance. Consequently, m-TPAP with both obvious
charge localization and a high triplet energy gap also exhibits
excellent device performance with the maximum luminance
and current efficiency of 19 000 cd m-2 and 10.69 cd A-1,
respectively. BPABP device indicates a relatively low
performance (7000 cd m-2 and 1.34 cd A-1) attributed to
the incomplete localization of HOMO and LUMO and low
triplet energy gap (2.44 eV). Clearly, the experimental results
validate our theoretical modeling that the localization of
HOMO and LUMO at their respective hole- and electron-
transporting moieties and modification of triplet energy gap
are two important factors that should be considered when
designing bipolar molecules for PHOLEDs.
The thermal properties of the bathophenanthroline deriva-
tives were investigated by differential scanning calorimetry
(DSC) and thermal gravimetric analysis (TGA). As shown
in Table 1, the compounds possess high Tg of 133-174 °C
and high melting point (Tm ) 219-360 °C). TGA measure-
ments show the compounds are thermally stable up to
480 °C with the decomposition temperatures (Td) in the range
of 487-541 °C in nitrogen. The excellent thermal stability
enables preparation of homogeneous and stable amorphous
thin films by vacuum deposition.
OLED devices based on CZBP, m-CZBP, m-TPAP, and
BPABP were fabricated by multilayer vacuum deposition
with the structure of indium tin oxide (ITO)/ PEDOT (20
nm)/host + 6 mol % of Ir(ppy)3 (70 nm)/TPBI (30 nm)/Cs:
BCP (1:1) (20 nm)/Al (100 nm). Poly(3,4-ethylenediox-
ythiophene) (PEDOT)9 is employed to promote the hole
injection, and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
(TPBI) serves as the hole and exciton blocker. Cs-doped BCP
acts as electron injection layer. Bathophenanthroline deriva-
tives doped with 6 mol % of Ir(ppy)3 are used as the emitting
layer.
In summary, we have modeled a series of novel bipolar
compounds containing bathophenanthroline and carbazole/
triphenylamine and investigated their energy levels by DFT
calculations. Four representative compounds were success-
fully synthesized and employed as host materials to afford
highly efficient OLEDs with the luminance and current
efficiency in the range of 1149-19 000 cd m-2 and 0.15-
16.20 cd A-1, respectively. The theoretical and experimental
results demonstrate that the effective disruption of π-con-
jugation is an efficient way to obtain desirable localization
of HOMO and LUMO and increase the triplet energy gap
in bipolar hosts, which provides a promising avenue in
molecular design for PHOLEDs.
In the EL spectra (Figure 4b), the devices based on CZBP
and m-TPAP exhibit green luminescence around 510 nm
from Ir(ppy)3. It is surprising to note that the luminescence
from the m-CZBP and BPABP device was blue-shifted by
60 and 20 nm, respectively, compared to the typical green
emission (510 nm), which might be attributed to the phase
separation or aggregation of the dopants.
The current-luminance (I-L) characteristics of the de-
vices are outlined in Figure 5b. The EL performance of
device m-CZBP (maximum luminance ) 18 000 cd m-2;
current efficiency ) 16.20 cd A-1) is much better than that
for CZBP (1149 cd m-2; 0.15 cd A-1), which can be
attributed to the combined effects of enhancement in charge
localization and the increase of triplet energy gap of the host
material. The DFT calculation results indicate that m-CZBP
Supporting Information Available: Calculation details
1
and results, measurements, experimental, and H and 13C
NMR data. This material is available free of charge via the
(8) Wong, K. T.; Chen, Y. M.; Lin, Y. T.; Su, H. C.; Wu, C. C. Org.
Lett. 2005, 7, 5361.
(9) Elschner, A.; Bruder, F.; Heuer, H. W.; Jonas, F.; Karbach, A.;
Kirchmmeyer, S.; Thurm, S.; Wehrmann, R. Synth. Met. 2000, 111, 139.
OL702773D
424
Org. Lett., Vol. 10, No. 3, 2008