Synthesis and application of pyridine-based ambipolar hosts: Control of charge balance in organic light-emitting devices by chemical structure modification

Article abstract of DOI:10.1021/ol102444j

We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host. These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from c

Full text of DOI:10.1021/ol102444j

ORGANIC
LETTERS
2010
Vol. 12, No. 23
5534-5537
Synthesis and Application of
Pyridine-Based Ambipolar Hosts:
Control of Charge Balance in Organic
Light-Emitting Devices by Chemical
Structure Modification
Phillip K. Koech, Evgueni Polikarpov, James E. Rainbolt, Lelia Cosimbescu,
James S. Swensen, Amber L. Von Ruden, and Asanga B. Padmaperuma*
Energy and EnVironment Directorate, Pacific Northwest National Laboratory,
Richland, Washington 99352, United States
asanga.padmaperuma@pnl.goV
Received October 8, 2010
ABSTRACT
We studied the influence of a pyridine moiety versus a phenyl moiety when introduced in the molecular design of an ambipolar host.
These pyridine-based host materials for organic light-emitting diodes (OLEDs) were synthesized in three to five steps from commercially
available starting materials. The isomeric hosts have similar HOMO/LUMO energies; however, data from OLEDs fabricated using the
above host materials demonstrate that small structural modification of the host results in significant changes in its carrier-transporting
characteristics.
The continued development of materials for efficient and
stable blue devices is of interest for both displays1 and
energy-saving solid-state lighting2 based on organic light-
emitting device (OLED) technology. In blue phosphorescent
OLEDs, energy is transferred from both singlet and triplet
states of the host to the triplet states of the phosphorescent
emitter/dopant3 which results in nearly 100% internal
quantum efficiency (IQE).4 This process mandates that the
triplet excited state of the host and the exciton-confining
properties of the surrounding transport materials be higher
than that of the phosphorescent emitter to prevent quenching
(3) Tokito, S.; Lijima, T.; Suzuri, Y.; Kita, H.; Tsuzuki, T.; Sato, F.
Appl. Phys. Lett. 2003, 83, 569.
(1) (a) Forrest, S. R.; Thompson, M. E. Chem. ReV. 2007, 107, 923. (b)
Tang, C. W.; van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913.
(4) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Nature 41998, 395, 151. (b) Baldo,
M. A.; Lamansky, S.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett.
1999, 75, 4. (c) Yersin, H. Top. Curr. Chem. 2004, 241, 1. (d) Kawamura,
Y.; Goushi, K.; Brooks, J.; Brown, J. J.; Sasabe, H.; Adachi, C. Appl. Phys.
Lett. 2005, 86, 071104.
(2) (a) Kido, J.; Kimura, M.; Nagai, K. Science 1995, 267, 1332. (b)
D’Andrade, B. W.; Forrest, S. R. AdV. Mater. 2004, 16, 1585. (c) Service,
R. F. Science 2005, 310, 1762. (d) Sun, Y.; Giebink, N. C.; Ma, B.;
Thompson, M. E.; Forrest, S. R. Nature 2006, 440, 908. (e) Walzer, K.;
Maenning, B.; Pfeiffer, M.; Leo, K. Chem. ReV. 2007, 107, 1233.
10.1021/ol102444j  2010 American Chemical Society
Published on Web 11/05/2010

of the dopant emission.5 Previously, we reported phospho-
rescent blue OLEDs with low drive voltages using phosphine
oxide (PO)-based wide bandgap host/charge transporting
materials and the sky blue emitter iridium(III)bis(4,6-
(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic).6,7 For
these materials, the PdO moiety acts to break the conjugation
between adjacent aryl groups resulting in a triplet energy
greater than 2.7 eV. Here, we report the design, synthesis,
electrochemical characterization, and OLED properties of
four new ambipolar phosphine oxide based hosts (4-(9H-
carbazol-9-yl)phenyl)(phenyl)(pyridin-3-yl)phosphine oxide
(HM-A4), (5-(9H-carbazol-9-yl)pyridin-2-yl)diphenylphos-
phine oxide (HM-A5), and (5-(diphenylamino)pyridin-2-
yl)diphenylphosphine oxide (HM-A6), (4-(diphenylami-
no)phenyl)(phenyl)(pyridin-3-yl)phosphine oxide (HM-A8)
(Schemes 1 and 2). The device properties of these hosts were
compared to those of two previously reported hosts (4-
(diphenylamino)phenyl)diphenylphosphine oxide (HM-A1)8
and (4-(9H-carbazol-9-yl)phenyl)diphenylphosphine oxide
(PO12) (Figure 1).8b,9
Figure 1. Chemical structures for hosts HM-A1 and PO12.
most host materials preferentially transport holes rather than
electrons.10 Pyridine rings have low-lying lowest unoccupied
molecular orbital (LUMO) energies. This, coupled with the
ELUMO lowering properties of the appendant diphenylphos-
phine oxide group, results in enhanced electron injection and
transport. Hole transport and injection are accomplished by
using either of the well-known and commonly used hole
transport moieties carbazole or triphenylamine. Compounds
HM-A4, HM-A5, HM-A6, and HM-A8 are combinations
of the functional groups above and are designed to produce
ambipolar host materials for blue phosphorescent OLEDs.
The synthesis of the pyridine bridge hosts HM-A5 and HM-
A6 starts with Grignard metathesis of the commercially avail-
able 2,5-dibromopyridine 1 using i-PrMgCl followed by
treatment with chlorodiphenylphosphine to afford the common
intermediate 2-bromo-5-diphenylphosphanylpyridine 2 (Scheme
1). Copper-catalyzed N-amination of this intermediate 2 in the
presence of carbazole followed by H2O2 oxidation of the
resulting product furnishes HM-A5 in excellent yields. This
product was characterized by 1H, 13C, 31P, FTIR, and HRMS.
The regiochemistry of the nitrogen of the central pyridyl ring
was assigned on the basis of 13C NMR analysis. The 13C NMR
spectrum of HM-A5 contains two diagnostic doublets at 153.0
and 142.3 ppm, a product of 3J 13C-31P coupling, corresponding
to two of the three pyridyl protons. Only the HM-A5 regioi-
somer shown in Scheme 1 is consistent with this pattern. This
observation is consistent with the literature report indicating that
the Grignard methathesis takes place preferentially at the
5-position of the 2,5-dibromopyridine.11 HM-A6, a derivative
of HM-A5 containing a diphenylamine group in place of the
carbazole moiety, was synthesized via a copper-catalyzed
N-amination of compound 2 in the presence of diphenylamine
to afford the intermediate 4 in good yield. Subsequent oxidation
using H2O2 affords HM-A6 in excellent yield. Both HM-A5
and HM-A6 were purified by column chromatography, fol-
lowed by multiple gradient sublimations to obtain high purity
materials.
Scheme 1. Syntheses of HM-A5 and HM-A6
Scheme 2. Syntheses of HM-A4 and HM-A8
Synthesis of the more challenging p-chiral phosphine oxide
based hosts HM-A4 and HM-A8 was achieved via Grignard
metathesis of 3-bromopyridine 6 using i-PrMgCl followed by
reaction with an excess (2 equiv) of dichorophenylphosphine
(Scheme 2). This was performed so as to obtain 1 equiv of the
monosubstituted chlorophenyl(3-pyridyl)phosphine. Subsequent
reaction of this product with 4-bromophenyllithium affords the
Our design strategy takes advantage of the pyridine moiety
which has been shown to enhance electron transport because
(5) Adachi, C.; Kwong, R.; Djurovich, P.; Adamovich, V.; Baldo, M. A.;
Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 2001, 79, 2082.
(6) Padmaperuma, A. B.; Sapochak, L. S.; Burrows, P. E. Chem. Mater.
2006, 18, 2389
.
(7) (a) Vecchi, P. A.; Padmaperuma, A. B.; Qiao, H.; Sapochak, L. S.;
Burrows, P. E. Org. Lett. 2006, 8, 4211. (b) Sapochak, L. S.; Padmaperuma,
A. B.; Cai, X.; Male, J. L.; Burrows, P. E. J. Phys. Chem. C. 2008, 112,
(10) (a) Su, S.-J.; Chiba, T.; Takeda, T.; Kido, J. AdV. Mater. 2008, 20,
2125. (b) Sasabe, H.; Gonmori, E.; Chiba, T.; Li, Y.-J.; Tanaka, D.; Su,
S.-J.; Takeda, T.; Pu, Y.-J.; Nakayama, K.-I.; Kido, J. Chem. Mater. 2008,
20, 5951. (c) Su, S.-J.; Sasabe, H.; Takeda, T.; Kido, J. Chem. Mater. 2008,
20, 1691.
7989
.
(8) (a) Polikarpov, E.; Swensen, J. S.; Chopra, N.; So, F.; Padmaperuma,
A. B. Appl. Phys. Lett. 2009, 94, 223304. (b) Xu, H.; Yin, K.; Huang, W.
Chem.sEur. J. 2007, 13, 10281.
(9) Cai, X.; Padmaperuma, A. B.; Sapochak, L. S.; Vecchi, P. A.;
(11) Trecourt, F.; Breton, G.; Bonet, V.; Mongin, F.; Marsais, F.;
Queguiner, G. Tetrahedron 2000, 56, 1349.
Burrows, P. E. Appl. Phys. Lett. 2008, 92, 083308
.
Org. Lett., Vol. 12, No. 23, 2010
5535

desired chiral phosphine and 2 equiv of bis(4-bromophe-
nyl)phenylphosphine as a byproduct. The chiral phosphine was
oxidized using H2O2 to afford compound 7. HM-A4 was
obtained from compound 7 via copper-catalyzed N-arylation
using conditions similar to those for HM-A5. Synthesis of HM-
A8 is analogous to that of HM-A4, starting with Grignard
metathesis followed by reaction with chlorodiphenylphosphine
to provide an intermediate which is then reacted with 2 equiv
of (4-(diphenylamino)phenyl)lithium to give the intermediate
N,N-diphenyl-4-(phenyl(pyridin-3-yl)phosphino)aniline. This
intermediate was partially purified by column chromatography
to remove the byproduct 4,4′-(phenylphosphinediyl)bis(N,N-
diphenylaniline). Oxidation of this crude intermediate using
hydrogen peroxide followed by chromatographic purification
affords HM-A8 in modest yields.
Having successfully synthesized the host materials, elec-
trochemical characterization was performed in order to
estimate the energies of highest occupied molecular orbital
(HOMO) and LUMO levels. Knowledge of HOMO/LUMO
energy levels of OLED materials is important as they may
affect the charge injection from adjacent layers. It is
established that there exists a correlation between the
HOMO/LUMO energy levels derived from direct methods
such as ultraviolet photoelectron spectroscopy (UPS) and
inverse photoemission spectroscopy (IPES) and much simpler
solution-based electrochemical methods.12 In this work, we
measured the oxidation and reduction potentials of all of the
materials in solution and used these electrochemical values
to estimate the orbital energies based on the published
protocols.12a,13 The oxidation and reduction potentials, EHOMO
and ELUMO values derived from cyclic voltammetry, and the
experimentally obtained triplet energy (ET) values for the
hosts are shown in Table 1.
responsible for the hole transport. The LUMO is localized on
the lowest energy aryl group around the PdO moiety (see the
Supporting Information). For nonpyridine-containing hosts
PO12 and HM-A1, the LUMO state is localized on the bridging
phenyl group, whereas for pyridine-containing hosts the LUMO
is localized on the pyridine. The ELUMO’s of the pyridine-
containing hosts are deeper than those of the aryl-bridged hosts
as a consequence of the electronegative nature of the pyridine
moiety (Table 1). The HOMO levels of the hosts are determined
by the hole transport moiety (HTm). As a result, changes in
the electron-transporting moiety have a minor effect on the
HOMO levels of the molecules.
Charge balance in an OLED, particularly in the light-emitting
region of the organic stack, strongly influences the quantum
efficiency of the emitting device.14 Our chemistry approach
based on functionalization of host materials with charge-
transporting groups allows for the control of charge transport
and charge balance in the emissive zone of an OLED. A method
to determine whether the host material provides ambipolar
charge transport or is predominantly hole- or electron-conduc-
tive is to monitor the emission zone in an operating device.
The location of the emission zone during device operation
correlates with the charge-transport character of the host
materials.15 For a host with ambipolar character, the emission
zone is expected to be broad with the maximum emission
coming from the bulk of the emissive layer (EML). On the other
hand, if a given host preferentially transports one sign of carriers,
then the emission zone is narrowed and moved to the interface
of the EML with the opposite sign charge transporting layer,
as depicted in Figure 2. On the basis of the location of the
Table 1. Solution Oxidation and Reduction Potentials,
Electrochemistry-Based Frontier Orbital Energies, and Triplet
Energies Obtained from Low Temperature Delayed
Luminescence Spectra
a
b
Eox, V Ered, V EHOMO
,
eV ELUMO
,
eV ET,c eV
HM-A1
PO12
0.71
0.96
0.89
0.96
0.80
0.65
n/ad
-2.91
-2.85
-2.69
-2.56
-2.80
-2.78
-2.61
-5.59
-5.94
-5.85
-5.94
-5.72
-5.51
-6.55e
-2.56
-2.52
-2.78
-2.91
-2.68
-2.69
-2.86
2.80
2.84
2.99
2.99
2.80
2.82
3.10
HM-A4
HM-A5
HM-A6
HM-A8
PO15
Figure 2. Schematic representation of the organic layers of OLEDs
in which a host material possesses different degrees of charge
balance.
a Calculated from the oxidation potential using the equation EHOMO
)
-1.4Eox - 4.6 eV according to ref 12a unless stated otherwise. b Calculated
from reduction potentials using a bicarbazole biphenyl (CBP) LUMO as a
reference, as described in ref 13. c Estimated from the photoluminescence
of frozen dichloromethane solutions at 77 K. d Outside the solvent window.
e Estimated from the optical gap using the onset of the absorption spectrum.
maximum emission region, conclusions can be drawn regarding
the charge transport in a given host material. We examined the
(12) (a) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P. I.;
Polikarpov, E.; Thompson, M. E. Org. Electr. 2005, 6, 11. (b) Djurovich,
P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Org. Electr. 2009, 10,
515.
Geometry optimization was carried out using PNNL’s
NWChem package at the B3LYP/6-31G* level to extract
theoretical electronic properties and correlate with experimental
trends. The HOMO of the host molecules are predominantly
localized on the TPA or carbazole fragments which are
(13) Kolosov, D.; Adamovich, V.; Djurovich, P. I.; Thompson, M. E.;
Adachi, C. J. Am. Chem. Soc. 2002, 124, 9945.
(14) Giebink, N. C.; Forrest, S. R. Phys. ReV. B 2008, 77, 235215.
(15) Polikarpov, E.; Swensen, J. S.; Cosimbescu, L.; Koech, P. K.;
Rainbolt, J. E.; Padmaperuma, A. B. Appl. Phys. Lett. 2010, 96, 053306.
5536
Org. Lett., Vol. 12, No. 23, 2010

shifts in the OLED emission region as a function of the host
material chemical structure and categorized the hosts on the
basis of their charge-transport type. The results are summarized
in Table 2.
dominated to balanced ambipolar compared to the electron-
dominant HM-A6. However, HM-A4 and HM-A5 both
exhibit electron-dominated transport characteristics. This
observation demonstrates that the location of the pyridine
ring in an HM-Ax structure has a greater influence on charge
transport when a TPA moiety is present than when a CBz
moiety is present.
Table 2. Charge-Transport Character of the Emissive Layer of
Blue OLEDs Based on Phosphine Oxide Host Materials
Augmented by Different Functional Groupsa
In summary, slight but distinct structural changes were
incorporated into host design to study the impact of structural
modification on carrier-transport properties. We have reported
the synthesis of host materials (HM-A4, HM-A5, HM-A6,
and HM-A8) as well as their corresponding electrochemical
properties. The syntheses utilize known, well established
methods. More importantly, from emission zone location data
we identified differences in transport properties of host
materials described herein. Compounds HM-A4, HM-A5,
and HM-A6 show preference for electron transport, while
HM-A8 shows ambipolar characteristics. The electronic
changes associated with structural modification of the host
materials have a significant effect on the charge transport
properties of the hosts in an OLED and allow for the control
of the charge balance.
charge transport
functionality
TPA, no py
TPA + py bridge
TPA + py in outer position
character
HM-A1
HM-A6
HM-A8
hole-dominant
electron-dominant
ambipolar
ambipolar to
PO12
HM-A5
HM-A4
CBz, no py
CBz + py bridge
CBz + py in outer position
electron-dominant
electron-dominant
electron-dominant
a “py” is pyridine, “TPA” is triphenylamine, and “CBz” is carbazole.
It can be seen from Table 2 how a minor chemical
modification in the host determines the preference for hole
or electron transport in the emissive layer of an operating
OLED. Replacement of a phenyl linkage with a pyridine in
both PO12 and HM-A1 results in an increase in electron
transport, which induces a shift of the emission zone toward
the HTL interface (compare HM-A5 with PO12). When the
charge transport in HM-A1 and PO12-based devices is
compared, it can be seen that HM-A1 is hole-dominant, and
PO12 is mostly ambipolar with a slight preference for
electron transport. When a pyridine bridge is placed between
the phosphine oxide and TPA/CBz moieties instead of a
simple phenyl bridge (compounds HM-A6 and HM-A5), we
see that the pyridine moiety overwhelms hole transport for
either a TPA or CBz group, resulting in materials that,
overall, are electron-transporting. When the pyridine ring is
moved from between the phosphine oxide and TPA/CBz
group and affixed to the phosphine oxide at an outer position
(compounds HM-A8 and HM-A4), the result depends on
the HTm in the host molecule. For the TPA-containing HM-
A8, the charge-transport character shifts from electron-
Acknowledgment. This project was funded by the Solid
State Lighting Program of the U.S. Department of Energy
(US DOE), with the Building Technologies Program (BT)
(Award No. M68004043 managed by the National Energy
Technology Laboratory (NETL). A portion of this research
was performed using EMSL. Computations were carried out
using “NWChem, A Computational Chemistry Package for
Parallel Computers, Version 5.1” (2007). Pacific Northwest
National Laboratory (PNNL) is operated by Battelle Memo-
rial Institute for the US Department of Energy under Contract
DE_AC06-76RLO 1830. We thank Dr. Rui Zhang (PNNL)
for performing electrospray ionization mass spectrometry.
1
Supporting Information Available: H, 13C, and 31P
NMR spectra for all novel compounds, orbital density maps,
and experimental details. This material is available free of
charge via the Internet at http://pubs.acs.org.
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