A dibenzofuran-based host material for blue electrophosphorescence

Article abstract of DOI:10.1021/ol0614121

(Chemical Equation Presented) Dibenzofuran (DBF) is converted to a vacuum-sublimable, electron-transporting host material via 2,8-substitution with diphenylphosphine oxide moieties. Close π-π stacking and the inductive influence of P=O moieties impart favorable electron-transport properties without lowering the triplet energy. A maximum external quantum efficiency of 10.1% and luminance power efficiency of 25.9 lm/W are realized using this material as the host for the blue-green electrophosphorescent molecule, iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C²′)picolinate (Flrpic).

Full text of DOI:10.1021/ol0614121

ORGANIC
LETTERS
2006
Vol. 8, No. 19
4211-4214
A Dibenzofuran-Based Host Material for
Blue Electrophosphorescence
Paul A. Vecchi, Asanga B. Padmaperuma, Hong Qiao, Linda S. Sapochak,* and
Paul E. Burrows
Material Science DiVision, Pacific Northwest National Laboratory,
Richland, Washington 99352
linda.sapochak@pnl.goV
Received June 9, 2006
ABSTRACT
Dibenzofuran (DBF) is converted to a vacuum-sublimable, electron-transporting host material via 2,8-substitution with diphenylphosphine
oxide moieties. Close π−π stacking and the inductive influence of P O moieties impart favorable electron-transport properties without lowering
the triplet energy. A maximum external quantum efficiency of 10.1% and luminance power efficiency of 25.9 lm/W are realized using this
material as the host for the blue-green electrophosphorescent molecule, iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C2
)picolinate (FIrpic).
d
′
Currently, the most efficient organic light-emitting devices
(OLEDs) are based on organic electrophosphorescence,
where an organometallic phosphor is physically doped into
a conductive organic host matrix and electroluminescence
(EL) from the dopant results from energy transfer from the
host and/or direct trapping of charge on the phosphor.1,2
Highly efficient and long-lived red and green phosphorescent
OLEDs have been demonstrated.3 However, efficient and
stable blue electrophosphorescence remains a challenge in
part because of the lack of appropriate organic charge
transporting host materials with a triplet energy (ET) high
enough to prevent quenching of the dopant emission (ideally
∼3 eV). Such a high ET requires either a very small molecule
with poor thermal stability or one with a short conjugation
length defined by saturated centers such as Si.4
We recently reported that both 4,4′-bis(diphenylphosphine
oxide)biphenyl and 2,7-bis(diphenylphosphine oxide)-9,9-
dimethylfluorene function as wide band gap and charge-
transporting host materials for the sky blue phosphorescent
dopant, iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-
N,C2′)picolinate (FIrpic) in OLEDs with peak quantum
efficiencies of ∼8% and low drive voltages.5,6 For both
materials, the phosphine oxide (PdO) moieties act as points
of saturation between the “active” chromophore bridge
(fluorene or biphenyl) and outer phenyl groups resulting in
materials with high triplet energies characteristic of the
bridges, but with physical properties suitable for device
(1) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048.
(2) Holmes, R. J.; D’Andrade, B. W.; Forrest, S. R.; Ren, X.; Li, J.;
Thompson, M. E. Appl. Phys. Lett. 2003, 83, 3818.
(3) Kwong, R. C.; Weaver, M. S.; Lu, M.-H. M.; Tung, Y.-J.; Chwang,
A. B.; Zhou, T. X.; Hack, M.; Brown, J. J. Org. Electron. 2003, 4, 155.
10.1021/ol0614121 CCC: $33.50
© 2006 American Chemical Society
Published on Web 08/18/2006

fabrication by thermal sublimation. However, both these
bridges have triplet energies comparable to the phosphores-
cent dopant FIrpic (ET ) 2.7 eV), which may limit the device
quantum efficiency due to back transfer of excitons from
the dopant to the host molecules.
In this paper, we show that the higher triplet energy (ET
) 3.1 eV) building block, dibenzofuran (DBF), when
substituted at the 2,8-positions with diphenylphosphine oxide
groups serves as a more efficient charge-transporting host
for blue electrophosphorescent OLEDs than similar mol-
ecules built using hydrocarbon bridges.
Synthesis of 2,8-bis(diphenylphosphine oxide)dibenzofuran
(3) was accomplished by a modified literature procedure
outlined in Scheme 1.7 The starting material, 2,8-dibromo-
Scheme 1
Figure 1. (a) Thermal ellipsoid plot (ORTEP) for 3. Ellipsoids
shown at the 50% probability level and hydrogen atoms omitted
for clarity. Crystal packing showing (b) view down the b-axis, with
edge-to-face interactions indicated in yellow and (c) side view of
1-dimensional π-π stacking facilitated by one P(2)-O(2)- - -
H(15)-C(15) (cyan dashed lines) and two P(1)-O(1)- - -H(18 &
12)-C (18 & 12) (red dashed lines) H-bonding interactions.
(∼1.81 Å). The PdO groups are oriented antiparallel in a
transoid conformation with respect to the DBF plane with
O-P-C-C torsion angles of 79.19° and 86.80° for P(1)-
O(1) and P(2)-O(2), respectively. Close π- - -π interactions
(3.385 Å) are observed between the DBF bridges in 3,
forming 1-dimensional arrays along the b-axis (see Figure
1b) facilitated by two close P-O- - -H-C interactions (all
C- - -O distances ∼3.4 Å) with the DBF ring on adjacent
molecules (see Figure 1c). Although 3 exhibits strong π-π
stacking in the crystal, thermal analysis (DSC, 10 K/min,
N2) showed stable glass formation with Tg of 105 °C and no
crystallization exotherm.
Computational modeling (B3LYP/6-31G*) predicts that
Ph2PdO substitution of DBF at the 2,8-positions does not
significantly change the energy gap between the highest
occupied and lowest unoccupied molecular orbitals (HOMO
and LUMO) but lowers the energy of each by 0.39 and 0.44
eV, respectively. These predictions correspond well with the
minimal difference observed between the absorption energy
onsets for DBF (4.06 eV) and 3 (4.02 eV). The experimental
absorption spectra are shown in Figure 2, where the
absorption maximum of 3 (290 nm, log ꢀ ) 4.26) is shown
to be slightly red shifted from the absorption of the
unsubstituted DBF bridge because of the inductive influence
of the PdO moieties. The solid-state absorbance of 3 is also
shown in Figure 2 and is similar to the solution spectrum.
The fluorescence spectrum of 3 was independent of
solution polarity and is approximately the mirror image of
dibenzofuran (1) was obtained by addition of Br2 (2.1 equiv)
to DBF in CHCl3. Compound 1 was rigorously purified to
remove isomeric impurities by repeated solvent washings
followed by multiple sublimations in a tube furnace (∼10-7
1
Torr) until the product was pure by H NMR. A lithium-
halogen exchange reaction between 1 and n-butyllithium (2
equiv) in THF (-77 °C, Ar) followed by reaction with
chlorodiphenylphosphine (2 equiv) gave crude 2,8-bis-
(diphenylphosphine)dibenzofuran (2). The diphosphine prod-
uct was rigorously purified by solvent washings and by
column chromatography. Compound 2 was then oxidized
with an excess of 30% H2O2 to give 3 as a white solid.
Compound 3 was purified by multiple sublimations in a
three-zone tube furnace to give the final purified product
subliming at 250-255 °C (∼10-7 Torr). Complete spectro-
scopic and structural characterization of compounds 1-3 is
reported in the Supporting Information.
The structure of 3 was confirmed by single-crystal X-ray
diffraction from a sample obtained by sublimation (see Figure
1 and Supporting Information). Similar to organic diphos-
phine oxides reported previously,6,8 PdO bond lengths are
identical [1.480(2) Å] and all P-C bond lengths are similar
(4) Ren, X.; Li, J.; Holmes, R. J.; Djurovich, P. I.; Forrest, S. R.;
Thompson, M. E. Chem. Mater. 2004, 16, 4743.
(5) Burrows, P. E.; Padmaperuma, A. B.; Sapochak, L. S.; Djurovich,
P.; Thompson, M. E. Appl. Phys. Lett. 2006, 88, 183503.
(6) Padmaperuma, A. B.; Sapochak, L. S.; Burrows, P. E. Chem. Mater.
2006, 18, 2389.
(8) Fie, Z.; Kocher, N.; Mohrschladt, C. J.; Ihmels, H.; Stalke, D. Angew.
Chem., Int. Ed. 2003, 42, 783.
(7) Baldwin, R. A.; Cheng, M. T. J. Org. Chem. 1967, 32, 1572.
4212
Org. Lett., Vol. 8, No. 19, 2006

A CH2Cl2 glass containing 3 at 77 K exhibits phospho-
rescence peaking at 431 nm with a υ0,0 peak at 395 nm,
corresponding to a high ET of 3.14 eV, which is almost
identical to the triplet energy of DBF (ET ) 3.12 eV) (see
Figure 2b). The PdO moieties slightly reduce the triplet life-
time of 3 (τphos) 3.05 ( 0.06 s) compared to the unsubsti-
tuted DBF bridge (τphos) 4.1 s),10 which can be attributed
to weak spin-orbital coupling with the phosphorus atom.
To evaluate 3 as a host material for blue electrophospho-
rescence, OLEDs were fabricated with the shortest wave-
length commercially available phosphorescent dopant, FIrpic,
doped at 10% and 20% by mass via coevaporation with 3.
The device structure grown on ITO glass (<15 Ω/square)
by thermal evaporation as described previously5,6 was
composed of 100 Å copper phthalocyanine (CuPc)/200 Å
N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-di-
amine (R-NPD)/50 Å 4,4′,4”-tris(carbazol-9-yl)triphenyl-
amine (TCTA)/200 Å 10 or 20% FIrpic in (3)/200 Å (3)/6
Å LiF/1000 Å Al. Compound 3 served both as electron-
transporting host in the emissive layer and as an exciton
blocking layer in this device configuration.
As shown in Figure 3 (inset), only FIrpic emission is
observed in the both the EL spectrum from the device and
Figure 2. Absorption (a) and emission (λexc ) 290 nm, rt); (b)
spectra of DBF (black line) and 3 (red line) in CH2Cl2 (∼10-5 M)
compared to a 400 Å thick vapor deposited film of 3 (blue line).
Phosphorescence spectra of DBF (black circles) and 3 (red circles)
(77 K, CH2Cl2, λexc ) 280 nm, delay time 300 µs) are also shown
in (b) with υ0,0 peak used to determine triplet energy identified
with arrow.
the absorption spectrum with UV emission at λmax (em) )
322 nm and a small Stokes shift (3427 cm-1), similar to the
unsubstituted DBF bridge (318 nm, 4140 cm-1, respectively)
(see Figure 2b). The fluorescence quantum yield measured
in CH2Cl2 vs quinine sulfate was 0.44 with a singlet lifetime
of 3.2 ( 0.5 ns. Although the solid-state film of 3 exhibited
similar structured emission in the UV, a new broad emission
peak centered at 361 nm was also observed. Excitation
spectra indicated that both the structured UV emission and
broad lower energy emission originated from the same
ground state, suggesting that the latter was excimer emission.
This emission was much weaker in solution and only
observed by subtraction of a lower concentration (2 × 10-6
M) spectrum from a higher concentration (2 × 10-5 M)
spectrum revealing a broad peak at 355 nm, which is similar
to excimer emission at 360 nm reported for DBF at high
solution concentration.9
Figure 3. Comparison of the external quantum efficiencies (black)
and luminance power efficiencies (red) for OLEDs with 10%
(squares) and 20% (circle) Firpic doped into PO14. Inset: device
EL spectrum and film PL spectrum for 20% FIrpic in compound
3.
the PL spectrum from a 20% FIrpic in compound 3 film
prepared by thermal evaporation. Hence, complete energy
transfer to the phosphorescent dopant occurs with no
measurable spectral effects due to excimers of 3. The external
quantum efficiency (ηex) and luminous power efficiency (ηp)
of these devices are also depicted in Figure 3, and all device
results are summarized in Table 1. Using 20% FIrpic in
compound 3, a maximum ηex of 10.1% at 70 µA/cm2 and
(9) Pinion, J. P.; Minn, F. L.; Filipescu, N. J. of Lumin., 1971, 3, 245
and Wang, X.; Tian, J.; Kofron, W. G.; Lim, E. C. J. Phys. Chem. A 1999,
103, 1560.
(10) Davydov, S. N.; Rodionov, A. N.; Shigorin, D. N.; Syutkina, O.
P.; Krasnova, T. L. Russ. J. Phys. Chem. 1981, 55, 444.
Org. Lett., Vol. 8, No. 19, 2006
4213

testing on devices appropriately encapsulated for long
operating lifetimes are currently being investigated.
Table 1. Device Properties
In conclusion, PdO moieties were used to electronically
isolate a high triplet energy DBF bridge allowing the
photophysical properties to be incorporated into a vacuum
sublimable material (3) for OLED applications. While other
schemes for increasing the triplet energy of organic host
materials have been published, the resulting loss of molecular
conjugation length invariably raises the operating voltage of
the blue OLEDs. The ability of 3 and other organic diphos-
phine oxide compounds to combine high ET with low
operating voltage devices is attributed to the inductive effect
of the PdO moieties, and subsequent energy lowering of
the LUMO resulting in enhancement of both electron
injection and transport in the device. The overall result is a
device with EQE > 10% at high brightness but Vop < 5.5 V
compared to literature voltages of ∼10 V. Given the >3 eV
triplet energy of 3, we expect similar high efficiency and
low voltage results from shorter wavelength organometallic
phosphors as they become available.
device propertya
10% FIrpic
20% FIrpic
ηex,max (%) [J, mA/cm2]
ηc,max (cd/A)
7.6 (0.04)
20.2
3.3
20.1 (0.02)
4.7 (6.4)
12.4
10.1 (0.07)
26.9
3.6
25.9 (0.02)
8.0 (3.8)
21.2
Vopt @ max QE
ηp,max (lm/W) [J, mA/cm2]
ηexb (%) [J, mA/cm2]
ηcb (cd/A)
ηpb (lm/W)
Voptb (V)
7.0
5.6
12.5
5.4
a ηex,max, maximum external QE; ηc,max, maximum luminance efficiency;
p,max, maximum luminance power efficiency; and Vopt, operating voltage
η
at a specified current. b Reported at 800 cd/m2.
3.6 V was obtained. As the drive current was increased to a
brightness of 800 cd/m2 the ηex dropped to 8.0% at 5.4 V.
The measured luminous efficiencies were 26.9 cd/A and 21.2
cd/A, respectively. Assuming Lambertian emission into π
steradians, the corresponding derived power efficiencies were
25.9 and 12.5 lm/W.
Acknowledgment. We acknowledge the U.S. Department
of Energy/Building Technologies Solid-State Lighting Pro-
gram for financial support and thank Dr. A. Joly and Dr. S.
Burton from the Environmental Molecular Sciences Labora-
tory (EMSL) and Dr. A. Ellern from the Molecular Structure
Laboratory, Iowa State University. Pacific Northwest Na-
tional Laboratory (PNNL) is operated by Battelle Memorial
Institute for the US Department of Energy (DOE) under
Contract DE-AC06-76RLO 1830.
These device properties of OLEDs incorporating 3 as an
host for FIrpic are superior to those reported previously using
other organic diphosphine oxide host materials (i.e., anala-
gous diphosphine compounds with biphenyl and fluorene
bridges).5,6 Specifically, a larger ηex is realized and main-
tained at higher drive current and similar low operating
voltages (∼5.4 V at 800 cd/m2) are achieved. The higher
device external quantum efficiency is attributed to the higher
triplet energy of the DBF bridged phosphine oxide derivative
(3) and consequent prevention of back energy transfer
between host and dopant molecule. Further, the morphologi-
cal stability of 3 is greater than both the biphenyl or fluorene
bridged counterparts (i.e., higher Tg and no recrystallization
exotherm). However, previous studies of furan-based opto-
electronic materials, including polyfurans11 and benzofuran
oligomers12 have reported poor photooxidative stability of
the furan ring. Preliminary studies of compound 3 films also
showed some photobleaching on long-term exposure to UV
excitation (∼254 nm) in the presence of air (see the
Supporting Information). More detailed stability studies and
Supporting Information Available: Detailed experi-
mental and computational procedures, NMR and X-ray
structural information (CIF), and crystal structure data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0614121
(11) Saadeh, H.; Goodson, T., III; Yu, L. Macromolecules 1997, 30, 4608;
Politis, J. K.; Nemes, J. C.; Curtis, M. D. J. Am. Chem. Soc. 2001, 123,
2537.
(12) Anderson, S.; Taylor, P. N.; Verschoor, G. L. B. Chem.sEur. J.
2004, 10, 518.
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