host materials.4 A typical example is 4,40-(bis(9-carbazoyl))-
biphenyl (CBP) that has a low HOMO and large band
gap. In addition, the ultrahigh singlet (∼4.5 eV) and triplet
(∼3.5 eV) energies of tetraarylsilanes (UGHx) make them
potentially useful host materials for deep blue PHOLEDs.5
On the other hand, examples of electron transporting
(ET) host materials for PHOLED are relatively limited.6
1,3,4-Oxadiazoles (OXDs) form a family of electron trans-
port molecules that have been widely used in OLED.7 We
recently reported that OXDs are appropriate ET hosts for
fac-tris(phenylpyridine) iridium (IrPPy3) and iridium(III)
bis(4,6-(difluorophenyl)pyridine-N,C2) picolinate (FIrpic)
in PHOLEDs.8
Scheme 1. Synthetic Pathways for 1ꢀ5
Herein we report the study of OXD-silanes 1ꢀ5 that
have high triplet energy as well as good ET properties.
By using these OXDs as the host for FIrpic,9 high efficiency
blue PHOLEDs could be fabricated. When 1 and 5 were
employed as the host for FIrpic in a PHOLED, current
efficienciesof 36.6and39.9cd/A, withanexternalquantum
efficiency of 12.9% and 13.1%, were achieved.
The syntheses of 1ꢀ5 are depicted in Scheme 1. OXDs
1ꢀ4 were prepared from the corresponding phenylsilyl-
benzoic acid.10 By adopting the Huisgen reaction,11 the
benzoic acids was converted to acylchlorides, followed
by reaction with tetrazole12 to give 1ꢀ4. OXD 5 was
synthesized from 4-(triphenylsilyl)benzoic acid through a
reaction sequence of hydrazide formation and dehydrative
ring formation.13
Crystallographic analysis is a particularly important
tool for understanding material properties.14 Figure 1
shows the ORTEPs of 1, 2 and 5. One can easily perceive
that the diphenyloxadiazole moiety favors to have copla-
nar arrangementin all cases. Forexample, the OXD ring in
the single crystal of 1 is nearly coplanar with the neighbor-
˚
ing phenyl groups. The estimated distances of 2.544 A
˚
˚
for H4ꢀO1, 2.536 A for H14ꢀO1, 2.656 A for H8ꢀN2, and
˚
2.657 A for H10-N1 suggest the presence of CꢀHꢀO and
CꢀHꢀN intramolecular hydrogen bond interactions.15
On the other hand, the orientation of the Ph3Siꢀ groups
is relatively flexible, dependent on the molecular packing
in the lattice. A dihedral angle of 81.41° for C11ꢀC12ꢀ
Si1ꢀC27 in 1 clearly suggested that one PhꢀSi bond is
nearly perpendicular to the diphenyloxadiazole moiety.
Hyperconjugation interactions are therefore expected.
Similar to that of 1, a dihedral angle of 89.13° for C21ꢀ
C20ꢀSi1ꢀC29 in 2 was observed. Different from the pre-
vious two cases, the Ph3Siꢀ groups of 5 show another
conformational preference, with one PhꢀSi bond almost
eclipsed with the OXD moiety.
(4) (a) Shih, P.-I.; Chiang, C.-L.; Dixit, A. K.; Chen, C.-K.; Yuan,
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Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.; Thompson, M. E. Chem.
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Figure 1d shows the molecular stacking of 5 in the single
crystal viewed along the b-axis. Alternating layer struc-
tures were observed, in which the stacking of oxadiazole is
formed. Electron conduction might therefore be enhanced
through the OXD layer.
Differential scanning calorimetry (DSC) analysis of 1
(Table 1) shows aglass-transition temperature(Tg) of 57 °C
which is higher than that of UGH1 (26 °C).5 We attribute
this to the lollipop shape structure of 5, with the rigid
oxadiazole group sticking out from the spherical Ph4Si-
surface, which restricts the molecular motion in the solid
matrix. Other OXDs 2ꢀ4 also have high Tg’s of 87, 104,
and135°C respectively, along with anincrease in molecular
diameter. It is noteworthy to point out that the dumbbell
shape of 5 gives rise to a high Tg of 98 °C. In addition, 1ꢀ5
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