A polyboryl-functionalized triazine as an electron transport material for OLEDs

Article abstract of DOI:10.1021/om2007979

A new pinwheel-like triazine molecule functionalized by three BMes ₂(m-Ph) groups (B3T) has been synthesized and fully characterized. This molecule has been found to have a low LUMO (E_a = 3.25 eV) and a deep HOMO (6.73 eV) energy lev

Full text of DOI:10.1021/om2007979

Communication
pubs.acs.org/Organometallics
A Polyboryl-Functionalized Triazine as an Electron Transport
Material for OLEDs
Christina Sun,† Zachary M. Hudson,† Michael G. Helander,‡ Zheng-Hong Lu,‡ and Suning Wang*,†
†Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario K7L 3N6, Canada
‡Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario M5S 3E4, Canada
S
* Supporting Information
ABSTRACT: A new pinwheel-like triazine molecule functionalized by three BMes2(m-Ph)
groups (B3T) has been synthesized and fully characterized. This molecule has been found to
have a low LUMO (Ea = 3.25 eV) and a deep HOMO (6.73 eV) energy level with a high
triplet energy level (3.07 eV) and is thus very promising as an electron transport material for
phosphorescent OLEDs. Preliminary studies show that B3T is indeed a very effective
electron transport material for OLEDs where the green phosphorescent compound
Ir(ppy)2(acac) is used as the emitter.
moiety. In addition, the use of a triazine core is expected to
riarylboron-containing compounds have demonstrated
Ttremendous potential for applications in electronic confer a deep HOMO level, allowing this material to function
also as a hole-blocking layer in organic electronic devices. By
functionalizing the meta position of the pendant phenyl rings of
the core rather than the para position, the π conjugation of the
pendant groups with the central core is partially disrupted due
to steric interactions, ensuring a large HOMO−LUMO gap and
high triplet level3c that is highly desired for use in
phosphorescent OLEDs.
materials such as organic light-emitting diodes (OLEDs), due
to the electron-accepting nature of the empty pπ orbital on
boron.1 Although remarkable progress has been made in the
development of OLEDs for applications in displays and solid-
state lighting, efficient electron transport materials are still in
high demand.2 Among the desired properties of an effective
electron transport material are a high electron affinity, a high
chemical and thermal stability, and the ability to form
amorphous films.2,3 When sufficient steric bulk is introduced,
triarylboron compounds can undergo reversible reduction while
maintaining a high chemical and thermal stability.1 In addition,
the incorporation of a bulky triarylboron functionality such as
BMes2Ph (Mes = mesityl) often facilitates the formation of
amorphous films.1b As such, this class of compounds is very
promising as electron transport materials (ETM) in OLEDs
and has been explored previously in 1,2,3-tris[5-
(dimesitylboryl)thiophen-2-yl]benzene (TMB-TB) as an
ETM with hole-blocking nature.1j On the other hand, 2,4,6-
triaryl-substituted triazine compounds are known to be effective
electron transport materials for OLEDs, due to the high
electron affinity of the triazine and the thermal stability of the π
skeleton.4 Thus, combining a triarylboron group with a triazine
core is an attractive approach to achieve new and perhaps better
electron transport materials.
B3T is synthesized in two steps from commercially available
starting materials, as shown in Scheme 1. Trimerization of 3-
bromobenzonitrile in the presence of acid readily affords 2,4,6-
tris(3′-bromophenyl)triazine4a in good yield. Subsequent
metal−halogen exchange with n-butyllithium and reaction
with FBMes2 gives the final product, which can be isolated in
high purity by column chromatography. B3T is stable in air in
both solution and in the solid state and has been fully
characterized by NMR and elemental analyses (see the
1
Supporting Information). The H NMR spectrum shows that
the three pendant arms of B3T are equivalent due to the free
rotation of the phenyl group at ambient temperature in
solution.
Cyclic and square-wave voltammetry diagrams of B3T show
multiple but not well-resolved reduction peaks that may be
attributed to the boryl groups and the central triazine as well.
On the basis of electrochemical data (see the Supporting
Information), the LUMO level of B3T is approximately at
−2.70 eV, considerably lower than those of triphenyltriazine
With this in mind, we have designed and synthesized the new
molecule B3T, shown in Scheme 1. The results of our
investigation on the use of B3T in OLEDs are reported herein.
B3T incorporates the low-lying LUMO of a triphenyltriazine
core, as well as the electron-accepting ability of the triarylboron
Received: August 25, 2011
Published: October 14, 2011
© 2011 American Chemical Society
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Scheme 1
Figure 1. (left) 298 K absorption (solid line) and emission (dashed line) spectra of B3T at 10−5 M in CH2Cl2. (right) 77 K phosphorescence
spectrum of B3T in 2-MeTHF. The arrow indicates the triplet energy level.
Figure 3. Device structure and energy level diagram of OLEDs
fabricated using Ir(ppy)2(acac) as the emitter at a doping
concentration of 8% and B3T as the electron transport material.
Table 1. OLED Device Data
a
b
device
luminance /cd m−2
ηext,max/%
ηL /cd/A
Figure 2. Frontier molecular orbitals of B3T plotted with an
isocontour value of 0.03.
A
B
C
1510
1019
14.8
17.6
21.5
50.5
61.1
68.9
521
and its derivatives,4 supporting the improved electron-accepting
ability provided by the BMes2 groups. UV photoelectron
spectroscopic (UPS) measurements for the B3T solid revealed
that it has an exceptionally low HOMO energy level of −6.73
eV in the solid state, consistent with previous triazine-based
electron transport materials.4a Using the HOMO value
determined by UPS and the absorption edge of the solid-
state absorption spectrum (see the Supporting Information),
the LUMO energy of B3T in the solid state was determined to
be −3.25 eV, significantly lower than those of simpler
a
b
Values obtained at 12.0 V. Values obtained at 100 cd/m2.
for certain triarylboron-derivative compounds.6 The value
obtained from UPS and the solid-state UV absorption spectrum
is clearly more realistic and meaningful. The exceptionally low
LUMO and the deep HOMO level of B3T are reminiscent of
the well-known triarylboron electron transport molecule tris-[3-
(3-pyridyl)mesityl]borane (3TPYMB), reported by Kido and
co-workers.1c,d
B3T shows three strong and broad absorption bands in the
UV−vis spectrum (Figure 1) due to charge-transfer from the
mesityl π orbitals to the empty pπ orbitals on boron and the π*
triphenyltriazines.4a 5a,b The discrepancy between the LUMO
,
value estimated from CV data and that obtained using UPS and
the solid-state absorption spectrum has been noted previously
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OLEDs increased in proportion to the thickness of the ETL;
the most efficient device was obtained using a 60 nm thick layer
of B3T (device C). At a display-relevant luminescence of 100
cd/m2, device C displays a high current efficiency of 68.9 cd/A,
with an impressive external quantum efficiency of 19.4%, as
shown in Figure 4. These devices do, however, show turn-on
voltages notably higher than those demonstrated in previous
reports using triazine-based electron transport materials,4a likely
due to inefficient charge injection from the LiF/Al cathode.
In conclusion, a new triarylboron-based electron transport
material with a triazine core has been achieved. This compound
exhibits a remarkably high triplet energy level with low-lying
HOMO and LUMO energy levels due to the presence of three
electron-accepting boron centers. Preliminary experiments
incorporating this material into phosphorescent OLEDs as an
electron transport layer gave devices with high current and
external quantum efficiencies at brightness levels appropriate
for display applications. Future work will focus on optimizing
the device structure and the use of this material as an electron-
transporting host material for high triplet energy blue
electrophosphorescent devices.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and a table giving synthesis and characterization
details for B3T, TD-DFT calculation data, and details of OLED
fabrication and performance data. This material is available free
of charge via the Internet at http://pubs.acs.org.
Figure 4. Diagrams of current efficiency (top) and L−J−V
characteristics (bottom) for devices A−C.
AUTHOR INFORMATION
■
orbitals of the triazine. These assignments are supported by
TD-DFT calculations7 carried out at the B3LYP level of theory
with 6-31G* as the basis set. The top three occupied orbitals
have contributions almost entirely on the mesityl groups, and
the LUMO and LUMO+1 have contributions mainly from the
empty p orbitals on boron and the electron-deficient core
(Figure 2).
Corresponding Author
*E-mail: suning.wang@chem.queensu.ca.
ACKNOWLEDGMENTS
■
We thank the Natural Science and Engineering Research
Council of Canada for financial support. C.S. and Z.M.H. thank
the NSERC for Canada Graduate Scholarships.
At room temperature, B3T shows a purple fluorescent
emission at 395 nm (Φ = 0.13 in CH2Cl2). The compound has
a remarkably high triplet energy level (ET) of 3.07 eV
determined from its low-temperature phosphorescence spec-
trum at 77 K (Figure 1), which is considerably higher than
those of common green phosphorescent emitters (e.g.,
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NOTE ADDED AFTER ASAP PUBLICATION
■
The version of this paper published on October 14, 2011, had
errors in the table of contents graphic, abstract graphic, and
Scheme 1. The version of this paper that now appears as of
October 20, 2011, has the correct artwork.
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