Bluish-green BMes2-functionalized PtII complexes for high efficiency PhOLEDs: Impact of the BMes2 location on emission color

Article abstract of DOI:10.1002/chem.201201255

New phosphorescent Pt^II compounds based on dimesitylboron (BMes₂)-functionalized 2-phenylpyridyl (ppy) N,C-chelate ligands and an acetylacetonato ancillary ligand have been achieved. We have found that BMes₂ substitution at the 4'-position of the phenyl ring can blue-shift the phosphorescent emission energy of the Pt^II compound by approximately 50 nm, compared to the 5'-BMes₂ substituted analogue, without substantial loss of luminescent quantum efficiencies. The emission color of the 4'-BMes₂ substituted Pt^II compound, Pt(Bppy)(acac) (1) can be further tuned by the introduction of a substituent group at the 3'-position of the phenyl ring. A methyl substituent red-shifts the emission energy of 1 by approximately 10 nm whereas a fluoro substituent blue-shifts the emission energy by about 6 nm. Using this strategy, three bright blue-green phosphorescent Pt^II compounds 1, 2 and 3 with emission energy at 481, 492, and 475 nm and φ_PL=0.43, 0.26 and 0.25, respectively, have been achieved. In addition, we have examined the impact of BMes₂ substitution on 3,5-dipyridylbenzene (dpb) N,C,N-chelate Pt^II compounds by synthesizing compound 4, Pt(Bdpb)Cl, which has a BMes₂ group at the 4'-position of the benzene ring. Compound 4 has a phosphorescent emission band at 485 nm and φ_PL=0.70. Highly efficient blue-green electroluminescent (EL) devices with a double-layer structure and compounds 1, 3 or 4 as the phosphorescent dopant have been fabricated. At 100 cd m ^-2 luminance, EL devices based on 1, 3 and 4 with an external quantum efficiency of 4.7, 6.5 and 13.4 %, respectively, have been achieved. Easily PhOLED: Blue-green phosphorescent organic light-emitting diodes (PhOLEDs) based on BMes₂-functionalized N,C-chelate and N,C,N-chelate Pt^II compounds with external quantum efficiencies of 5 to 22 % have been achieved (see figure). BMes₂ substitution and its location on the N,C- or N,C,N-chelate ligand have been found to be critical in achieving efficient blue-green phosphorescent Pt^II compounds. Copyright

Full text of DOI:10.1002/chem.201201255

FULL PAPER
DOI: 10.1002/chem.201201255
Bluish-Green BMes2-Functionalized PtII Complexes for High Efficiency
PhOLEDs: Impact of the BMes2 Location on Emission Color
Ying-Li Rao,[a] Dylan Schoenmakers,[a] Yi-Lu Chang,[b] Jia-Sheng Lu,[a]
Zheng-Hong Lu,[b] Youngjin Kang,*[c] and Suning Wang*[a]
Abstract: New phosphorescent PtII
compounds based on dimesitylboron
(BMes2)-functionalized 2-phenylpyridyl
(ppy) N,C-chelate ligands and an
position of the phenyl ring. A methyl
substituent red-shifts the emission
energy of 1 by approximately 10 nm
whereas a fluoro substituent blue-shifts
the emission energy by about 6 nm.
Using this strategy, three bright blue-
green phosphorescent PtII compounds
1, 2 and 3 with emission energy at 481,
492, and 475 nm and FPL =0.43, 0.26
and 0.25, respectively, have been ach-
ieved. In addition, we have examined
the impact of BMes2 substitution on
3,5-dipyridylbenzene (dpb) N,C,N-che-
late PtII compounds by synthesizing
compound 4, Pt(Bdpb)Cl, which has
AHCTUNGTRENNUNG
a BMes2 group at the 4’-position of the
benzene ring. Compound 4 has a phos-
phorescent emission band at 485 nm
and FPL =0.70. Highly efficient blue-
green electroluminescent (EL) devices
with a double-layer structure and com-
pounds 1, 3 or 4 as the phosphorescent
dopant have been fabricated. At
100 cdmꢀ2 luminance, EL devices
based on 1, 3 and 4 with an external
quantum efficiency of 4.7, 6.5 and
13.4%, respectively, have been ach-
ieved.
acetylACHTUNGTRENNUNGacetonato ancillary ligand have
been achieved. We have found that
BMes2 substitution at the 4’-position of
the phenyl ring can blue-shift the phos-
phorescent emission energy of the PtII
compound by approximately 50 nm,
compared to the 5’-BMes2 substituted
analogue, without substantial loss of lu-
minescent quantum efficiencies. The
emission color of the 4’-BMes2 substi-
tuted PtII compound, Pt
ACTHNUTRGENN(GU Bppy)CAHTUNGTRENN(UGN acac)
(1) can be further tuned by the intro-
duction of a substituent group at the 3’-
Keywords: OLEDs · organoborons ·
platinum · synthesis · structure
Introduction
emitters have been achieved, stable blue PhOLEDs based
on IrIII compounds remain elusive.[3] Compared with IrIII
complexes, PtII compounds that have a high phosphorescent
quantum efficiency are scarce due to often strong intermo-
lecular p–p stacking interactions caused by the square-
planar geometry, leading to excimer formation and decrease
of emission quantum efficiency and color purity in the solid
state.[4] The square-planar geometry of PtII may have one ad-
vantage over IrIII, namely the access to a higher triplet state,
due to the greater ligand field splitting that greatly increases
the energy of the d–d state, for a given set of chelate lig-
Phosphorescent organic light-emitting diodes (PhOLEDs)
have recently received much attention because of their ap-
plications in next-generation flat-panel displays and solid-
state lighting devices.[1] The key challenge in PhOLEDs re-
search is the development of phosphorescent metal com-
plexes with high quantum efficiency, especially blue phos-
phorescent compounds. Much earlier and current research
efforts on phosphorescent materials for OLEDs focus on 2-
phenylpyridineACHTUNGTRENNUNG
(Hppy)-based IrIII complexes and their deriv-
atives because of their high photoluminescent quantum effi-
ACHTUNGTRENNUNG
ciencies.[2] Although highly efficient PhOLEDs based on IrIII
A
ACHTUNGTRENNUNG
analogue (Scheme 1). Thus, PtII compounds are good candi-
dates for the development of blue phosphorescent emitters,
if low emission efficiency and intermolecular interaction
issues can be addressed. Several examples of green, orange
or red phosphorescent PtII compounds have been demon-
strated recently for successful use in PhOLEDs,[4,6,7] blue
PhOLEDs based on PtII compounds are very rare and only
a few examples are known in the literature.[8,9]
[a] Y.-L. Rao, D. Schoenmakers, J.-S. Lu, Prof. S. Wang
Department of Chemistry, Queenꢀs University
Kingston, Ontario, K7L 3N6 (Canada)
Fax : (+1)6135336669
E-mail: wangs@chem.queensu.ca
[b] Y.-L. Chang, Prof. Z.-H. Lu
Department of Materials Science and Engineering
University of Toronto, Ontario, M5S 3E4 (Canada)
We have recently shown that high efficiency green and
orange PhOLEDs with external quantum efficiency as high
as 20% can be achieved by using dimesitylboron (BMes2)
[c] Prof. Y. Kang
Division of Science Education, Kangwon National University
Chuncheon 200-701 (Republic of Korea)
E-mail: kang@kangwon.ac.kr
(acac) and derivative compounds.[7]
functionalized PtACTHNUTRGENNU(G ppy)ACHUTNGTRENNUGN
We have shown that the introduction of the BMes2 group to
the N,C-chelate backbone plays several important roles in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201255.
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and blue-shifts the emission energy. Thus, our first strategy
toward achieving blue phosphorescent PtII compounds is to
change the location of the BMes2 group from the meta or 5’-
position of the phenyl ring to the para or 4’-position of the
phenyl in the ppy ligand. In addition, we want to further
tune the emission energy by varying the R group
(Scheme 1). Our second strategy in achieving blue phos-
phorescent PtII compounds is to study BMes2-functionalized
3,5-dipyridylbenzene (dpb) N,C,N-chelate PtII compounds
since several members of N,C,N-chelate PtII compounds
have been shown previously to be very promising blue-
green phosphorescent emitters for OLEDs.[8,9] Based on this
consideration, a new N,C,N-PtII compound 4 (Scheme 1) has
been synthesized and investigated. In this report, the details
of synthesis and characterization of compounds 1–4, the
impact of BMes2 substitution on emission color of ppy-
based N,C-chelate PtII compounds and the performance of
these new compounds in PhOLEDs are presented.
Scheme 1. Emission energies of previously synthesized phosphorescent
PtII compounds and their IrIII analogues, and the newly synthesized com-
pounds 1–4.
Results and Discussion
the high performance of the resulting PtII compounds in
PhOLEDs. First of all, it facilitates the mixing of the LC
Synthesis and crystal structure: The synthesis route and de-
tails for boron-functionalized phenylpyridine ligands and
their corresponding PtII complexes are shown in Scheme 1.
In the initial study, we have attempted syntheses of 1–3
3
and the MLCT state, thus enhancing the intrinsic phosphor-
escent efficiency of the molecule. Secondly, it minimizes in-
termolecular interactions, thus enhancing emission efficiency
in the solid state. Thirdly, it facilitates electron injection into
the emissive layer/dopant, thus improving the device effi-
ciency. The excellent perfor-
using PtClACTHNUTRGENN(UG DMSO)ACHTUNTGNER(NUGN acac) as a typical starting material in
the presence of a base according to a procedure established
by our group earlier.[7a] This reaction condition, however,
mance by our first generation
BMes2-functionalized N,C-che-
late PtII compounds inspired us
to investigate blue phosphores-
cent PtII compounds using the
same
boron-functionalization
strategy.
In our earlier study of
BMes2-functionalized Pt(ppy)-
ACHTUNGTRENNUNG(acac) and derivative com-
AHCTUNGTRENNUNG
pounds, the BMes2 group was
placed either at the pyridyl ring
or the phenyl ring, meta to the
Pt atom (e.g., Pt
N
ACHTUNGTRENNUNG
and Pt(BppyB)
A
ACHTUNGTRENNUNG
Scheme 1).[7] By examining two
BMes2 functionalized Ir-ppy
compounds (Scheme 1) report-
ed by Park,[10] Wong and
Marder et al.,[11] it is apparent
that the location of the BMes2
group has a great impact on
emission energy. As shown in
Scheme 1, placing the BMes2
group on the phenyl ring, trans
to the IrIII atom, significantly
Scheme 2. Reagents and conditions: a) nBuLi/Et2O, B
b) 2-Br-pyridine, nBuLi, ZnCl2A(tmeda)2, Pd(PPh3)4, THF, reflux, overnight; c) 2-Br-pyridine, K2CO3, THF, Pd-
(PPh3)4, 708C, 7 h; d) nBuLi/Et2O, BMes2F, ꢀ788C, then room temperature, overnight; e) [PtMe2A(SMe2)]2,
CF3SO3H, Na(acac)·H2O, room temperature, 1 to 2 h; f) 2-(Bu3Sn)pyridine, LiCl, Pd(PPh3)2Cl2, toluene, reflux,
G
U
ACHTUNGTRENNUNG
A
A
widens the HOMO–LUMO gap 3 days; g) K2PtCl4, CH3CN/H2O (3:1), 908C, overnight.
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PtII-Based Blue-Green PhOLEDs
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Table 1. Selected bond lengths [ꢂ] and angles [8] for compounds 1–4.
Compound 1
did not give the desired complexes except 3 and black pre-
cipitate slowly formed as the reaction progressed. Using this
method, compound 3 was obtained only in low yield. To im-
prove the synthesis of 3 and obtain compounds 1 and 2, a re-
cently developed alternative method[12] was used, which in-
volves one-pot reaction of the appropriate ligand with
ꢀ
ꢀ
Pt(1) N(1)
1.992 (9)
2.061 (8)
175.8 (4)
Pt(1) C(1)
1.939 (12)
2.012 (8)
173.9 (4)
ꢀ
ꢀ
Pt(1) O(1)
Pt(1) O(2)
ꢀ
ꢀ
ꢀ
ꢀ
C(1) Pt(1) O(1)
N(1) Pt(1) O(2)
Compound 2
ꢀ
ꢀ
Pt(1) N(1)
1.990 (9)
2.015 (8)
174.8 (4)
Pt(1) C(1)
1.970 (10)
2.102 (7)
174.4 (3)
[PtMe2ACHTUNGTRENNUNG(SMe2)]2, without isolation of any intermediates.
ꢀ
ꢀ
Pt(1) O(1)
Pt(1) O(2)
Using this approach, the new Pt complexes, 1–3 were ob-
tained in high yields (80~90%). The new method is much
better than the old method since it greatly shortens reaction
time, uses mild conditions and produces the desired com-
pounds in high yields. The synthesis of the BMes2-function-
alized N,C,N-chelate ligand 1-dimesitylboryl-3,5-dipyridyl-
benzene (Bdpb) was accomplished by Stille coupling of
excess 2-(tributylstannyl)pyridine with 3,5-dibromophenyldi-
ꢀ
ꢀ
ꢀ
ꢀ
C(1) Pt(1) O(2)
N(1) Pt(1) O(1)
Compound 3
ꢀ
ꢀ
Pt(1) N(1)
1.986 (4)
1.998 (3)
175.08 (16)
Pt(1) C(1)
1.951 (4)
2.089 (3)
174.53 (13)
ꢀ
ꢀ
Pt(1) O(1)
Pt(1) O(2)
ꢀ
ꢀ
ꢀ
ꢀ
C(1) Pt(1) O(2)
N(1) Pt(1) O(1)
Compound 4
ꢀ
ꢀ
Pt(1) N(1)
2.024 (3)
1.905 (4)
161.10 (13)
Pt(1) N(2)
2.037 (4)
2.4018 (11)
177.45 (12)
ꢀ
ꢀ
Pt(1) C(11)
Pt(1) Cl(1)
ꢀ
ꢀ
ꢀ
ꢀ
N(1) Pt(1) N(2)
C(11) Pt1 Cl(1)
mesitylborane by using PdACHTUNGTRENUNG(PPh3)2Cl2 as the catalyst. Com-
pound 4 was synthesized and
isolated in 94% yield from the
treatment of Bdpb with K2PtCl4
in a mixed solvent of CH3CN
and water at 908C.
All compounds are stable in
air and soluble in common or-
ganic solvents but not in pen-
tane, hexane and methanol.
Compounds 1–4 have been fully
characterized by NMR spec-
troscopy, elemental analyses,
and X-ray diffraction analyses.
Crystallographic data for all
complexes are provided in the
Supporting Information. Impor-
tant bond distances and angles
are shown in Table 1. The struc-
tures of compounds 1–4 are
shown in Figure 1. For com-
pound 1, three independent
molecules are located in one
asymmetric unit with similar
bond distances and angles. For
4, two independent molecules
are located in one asymmetric
unit with similar bond distances
and angles. Despite the varia-
tion of the R group in struc-
tures of 1–3, the bond lengths
and bond angles around the PtII
atoms are similar. The dihedral
angle between the BC2 (mesi-
Figure 1. Structure diagrams of: a) 1, b) 2, c) 3, and d) 4 with 50% thermal ellipsoids and labels of key atoms.
Hydrogen atoms are omitted for clarity.
tyl) plane and the phenyl ring of the ppy chelate for these
three compounds are also similar (21.48, 22.08 and 24.68 in 1;
23.68 in 2; and 23.78 in 3); this indicates that the change of
the R group from H atom to a methyl group does not have
a significant impact on the conjugation of the BMes2 group
with the phenyl ring of the ppy chelate. The dihedral angles
between the phenyl and the pyridyl ring of the ppy ligand
are 5.9(5)8, 4.4(5)8 and 3.0(5)8 for 1, 12.5(7)8 for 2 and
3.0(3)8 for 3, respectively. The increase of this dihedral
angle from 1 to 2 is clearly caused by the steric effect of the
methyl substitution in 2. Interestingly, however, this dihedral
angle for compound 3 is close to those of 1 and much less
than that of 2, which appears to be the result of strong intra-
molecular hydrogen bonding between fluorine and hydrogen
atoms of both mesityl and pyridine ring (see the Supporting
Information). These small dihedral angles prove the pres-
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Y. Kang, S. Wang et al.
ence of effective p conjugation of the ppy chelate in all
The N,C,N-chelate compound 4 also forms a stacked
dimer in the crystal lattice with the pyridyl ring being locat-
ed directly above the Pt atom. A short interdimer Pt···Cpy
separation distance of 3.64 ꢂ and a short C···C separation
distance of 3.55 ꢂ between the py ring and the central
phenyl ring are observed (see the Supporting Information).
The Pt···Pt separation distance in the dimer of 4 is very long
(5.70 ꢂ) and no further extended stacking interactions are
evident. The lack of direct Pt···Pt interactions in these new
BMes2-functionalized PtII compounds can be attributed to
the presence of the bulky boron group at the 5’-position,
which may inhibit the formation of excimer in either solid
state or thin film.
ꢀ
three compounds. For compounds 1–3, the Pt O bond trans
ꢀ
ꢀ
to the Pt C bond is slightly longer than that trans to the Pt
N bond, attributable to the greater trans effect exerted by
the carbon atom.
Compound 1 forms a stacked dimer in the solid state with
the pyridyl ring being directly above the Pt-acac chelate.
The shortest Pt···Cpy separation distances in the dimer of
1 are found to be 3.64–3.67 ꢂ and the Pt···Pt separation dis-
tances to be 4.64–4.67 ꢂ (see the Supporting Information).
Compound 2 also forms a similar stacked dimer with Pt···Cpy
separation distance of 3.64 ꢂ and a Pt···Pt separation of
4.64 ꢂ. For 1 and 2 the stacking intermolecular interactions
are limited to the discrete dimer and no extended stacking
was observed in the crystal lattice. Compound 3 also forms
a stacked dimer but with a much shorter Pt···Cpy separation
distance (3.38 ꢂ) and a Pt···Pt separation of 4.70 ꢂ that is
similar to those of 1 and 2. Furthermore, the molecules of 3
display extended stacking with short interdimer Cpy···Cpy
separation distance of approximately 3.26 ꢂ and a long
Pt···Pt separation of 6.87 ꢂ (Figure 2). Thus, among N,C-
Thermal property: Because small molecule-based OLEDs
are fabricated by vacuum deposition at elevated tempera-
ture, it is important that the materials used in OLEDs have
a high thermal stability. To investigate the impact of substi-
tution at 4’-position of the phenyl ring on thermal properties
of the new PtII compounds, differential scanning calorimetry
(DSC) diagrams were recorded for compounds 1–3. Al-
though none of the compounds shows glass transitions in
both the first and second heating/cooling cycles, they do dis-
play a high thermal stability with a crystallization tempera-
ture at approximately 2508C (with the order of 1>2>3).
These compounds are thermally stable up to their melting
points (>3008C, see the Supporting Information).
Photophysical and electrochemical properties: Compounds
1–3 display a distinct but weak absorption band at 350–
450 nm region (Figure 3), which may be attributed MLCT
transitions. Compound 4 has a well resolved absorption
band with a moderate intensity at 387 nm, which may be as-
signed to intraACTHNUTRGNENGUliACHUTGTNRENgUNG and charge transfer transition. The main
absorption band at approximately 330 nm for 1–3, which is
attributed to p–p* transitions of the N,C-chelate ligands, is
similar to that observed in Pt
ence between the new compounds 1–3 and Pt
is the luminescence spectra. Like Pt(BppyB)CAHTUNGTRENNNUG
ACHUTGTNRNEUG(N BppyB)ACHTGNUTRENNUNG
N
ACHTUNGTRENNUNG
U
pounds 1–3 are brightly luminescent with quantum efficien-
cies ranging from 0.43–0.26 in solution and 0.35 to 0.21 in
the solid state. In contrast, the quantum efficiency of the
Figure 2. Diagrams showing intermolecular stacking of compound 3 (top)
and 4 (bottom).
non-borylated Pt
phosphorescent decay lifetimes of 1–3 are also similar to
that of Pt(BppyB)(acac). However, compared to Pt(BppyB)-
ACHTUNGTREN(NUGN acac), which emits at 538 nm, the emission energy of 1–3 is
(acac) compound is only 0.15.[13] The
ACHUTGTNRENNUG(ppyACHTUNGTRENNUGN
A
R
ACHTUNGTRENNUNG
chelate compounds 1–3, the fluoro-substituted compound is
most prone to intermolecular stacking interactions. This is
consistent with our earlier observation for the fluoro deriva-
significantly blue-shifted with the emission maximum at 478,
485 and 470 nm, respectively (Figure 3 and Table 2). The
impact of the BMes2 position on the emission energy of the
PtII compounds follows the same trend as that observed for
the IrIII compounds.[10,11] Furthermore, the emission colors of
1–3 do not change significantly from solution to the solid
state (Table 2 and the spectra in Figure 3); this indicates
that the intermolecular interactions observed in the crystal
structures do not have any significant impact on lumines-
cence of these molecules. Among compounds 1–3 the
fluoro-substituted compound 3 has the shortest emission
tive of Pt
separation distance of 3.404(1) ꢂ.[7a] However, compared to
Pt(BppyA)(acac) and Pt(BppyB)(acac) and their deriva-
ACHTUNGTRENNUNG(BppyA)ACHTUNGTRENNUNG(acac) that dimerizes with a short Pt···Pt
A
E
N
ACHTUNGTRENNUNG
tives, which have the tendency to form dimers that stack di-
rectly along the Pt···Pt vector, leading to very short Pt···Pt
separation distances and strong Pt···Pt interactions,[7a] none
of compounds 1–3 shows similar Pt···Pt interactions. In fact,
all intermolecular interactions in 1–3 are between the pyrid-
yl ring and the Pt center.
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PtII-Based Blue-Green PhOLEDs
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wavelength whereas the methyl-substituted compound 1 has
the longest emission wavelength, differing by approximately
15 nm.
The emission spectrum of 4 closely resembles that of the
non-borylated PtACTHNUTRGNEUNG
(dpb)Cl parent molecule[9,14] (Figure 4);
Figure 4. Absorption and emission spectra of compound 4 and Pt
(Pt(NCN)) in CH2Cl2, recorded under the same conditions.
ACHTUNGTRENNUNG(dpb)Cl
AHCTUNGTRENNUNG
this indicates that the phosphorescence of these two mole-
cules share a common origin, which is most likely a mixed
ligand-centered p–p*, MLCT and LC transitions, as estab-
lished previously for PtACTHNUTRGNENGU ACHTUNGTRENNUNG(dpb)Cl,
(dpb)Cl.[14] Compared to Pt
the emission maximum of 4 is blue-shifted by a few nanome-
ters with a significantly enhanced emission efficiency (0.70
in solution vs. 0.60 of Pt
ACHTUNGTRNE(NUNG dpb)Cl). Furthermore, unlike Pt-
AHCTUNGTREG(NNNU dpb)Cl, which has a very poor solubility in common organic
solvents and a strong tendency to produce excimer emission
in concentrated solution and the solid state due to strong in-
termolecular interactions,[14] the emission of 4 does not
change significantly with concentration in solution or from
the solution to the solid state. The bulky BMes2 clearly
plays a role in minimizing intermolecular interactions of 4.
Using Equations (1) and (2), the values for the radiative
(kr) and non-radiative (knr) decay rates for all four new PtII
compounds were determined and compared (Figure 5). Al-
though the kr values are comparable to one another for 1, 2
and 3, the knr value of the highly emissive 4 is considerably
smaller than those of 1, 2 and 3. Thus, the high FPL value
Figure 3. Top: absorption and emission spectra of 1–4 in CH2Cl2 at 298 K.
Middle: emission spectra of 1–4 in the solid state at 298 K (10%wt
PMMA film). Bottom: photos showing the emission color of 1–4 in
CH2Cl2 (top) and in PMMA film (10 wt%, bottom) under 365 nm irradi-
ation.
Table 2. Photophysical and electrochemical data of 1, 2, 3 and 4.
red
Complex
Absorption[a]
Emission, 298 K, solution[a]/solid[b]
Emission, 77 K[d]
E1/2 [V][e]
[c]
lmax [nm], (e [ꢃ104 mꢀ1 cmꢀ1])
lmax [nm]
tp [ms]
FPL
lmax [nm]
tp [ms]
1
2
3
4
271 (1.85), 327 (3.01), 344 (2.13)
282 (2.40), 328 (3.33), 349 (2.56)
274 (2.10), 331 (3.56), 343 (2.71)
283 (3.10), 334 (3.20), 387 (0.87)
481/476
492/487
475/472
485/489
8.7 (2)/9.5 (3)
8.7 (2)/8.7 (1)
9.6 (2)/7.2 (2)
8.9 (2)/6.6 (2)
0.43/0.35
0.26/0.30
0.25/0.21
0.70/0.76
478
485
470
490
9.4 (1)
12.2 (2)
10.2 (7)
9.5 (1)
ꢀ2.34
ꢀ2.36
ꢀ2.27
ꢀ2.09
[a] The absorption spectra were measured in degassed CH2Cl2 solution; [M]=8.0ꢃ10ꢀ5. [b] Doped into PMMA at 10 wt%. [c] Phosphorescence quantum
efficiency measured in CH2Cl2, relative to Ir
(ppy)3 (F=1.0). Solid state quantum efficiency was measured by using an integration sphere (error rangeꢁ
10% of the reported value). [d] In frozen CH2Cl2 glass. [e] The reduction potentials of all compounds were measured in DMF with NBu4PF6 (0.10m) at
a scan rate of either 50 or 100 mVsꢀ1 (vs. FeCp20/+).
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Y. Kang, S. Wang et al.
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Figure 5. Radiative (kr, ) and non-radiative (knr, ) rate constants for 1,
2, 3 and 4.
Figure 6. Comparison of experimental HOMO and LUMO energy levels
of 1–3 with PtCAHTUNGTERN(NNUG ppy)ACHUTNTGERNNUG(acac) and PtACHTNUGERTG(NNUN BppyB)ACHTNUGTNER(NUGN acac).
for 4 can be attributed to the suppression of the non-radia-
tive decay processes. Interestingly, the kr values of 1–3 are
similar with that of Pt
knr values of 1–3 are almost five-times smaller than that of
Pt(ppy)
(acac) (3.2ꢃ105 sꢀ1).[13,15] These results strongly indi-
A
U
that BMes2 substitution at the 4’-position of the phenyl ring
is much better for achieving high phosphorescence energy.
Among compounds 1–3, the electron-withdrawing fluoro
group significantly stabilizes the HOMO level and the
LUMO level as well in 3, albeit to less a degree, leading to
the highest emission energy. In contrast, the electron-donat-
ing methyl group in 2 destabilizes the HOMO level but does
not change the LUMO level significantly, resulting in the
lowest emission energy among the three compounds. The in-
fluence of the fluoro and the methyl group on HOMO and
LUMO energies may be attributed to the electronic induc-
tion effect. These data support that the introduction of
a BMes2 group at the 4’-position of the ppy ligand is an ef-
fective approach in shifting the emission energy of phos-
phorescent PtII compounds toward blue.
A
ACHTUNGTRENNUNG
cate that the BMes2 unit introduced into the ppy ligand sig-
nificantly retards the non-radiative decay process, leading to
improved PL efficiency.
FPL ¼ kr=ðkr þ knrÞ
1=t ¼ kr þ knr
ð1Þ
ð2Þ
To understand the origin of the phosphorescence blue-
shift and the substitution effect observed in compounds 1–4,
electrochemical properties of these compounds were exam-
ined by cyclic voltammetry. All four compounds display a re-
versible reduction peak at E1/2red =ꢀ2.34, ꢀ2.36, ꢀ2.27 for
1–3, respectively, relative to FeCp2+/0. Using the reduction
potentials and the energy of the absorption edge, HOMO
and LUMO energies for compounds 1–4 were estimated
(Table 3). The impact of the BMes2 substitution and location
For compound 4, the reduction potential is at ꢀ2.09 V,
slightly more positive than that[14] of the non-borylated Pt-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
to the N,C,N-chelate must decrease both the LUMO and
the HOMO level, thus maintaining the same HOMO–
LUMO gap.
To further understand the photophysical properties of
these materials, we have examined their electronic struc-
tures in detail using TD-DFT calculations.[16] Molecular or-
bital and molecular geometry calculations were performed
by using the Gaussian 03 program suite with crystal struc-
can be illustrated by comparing compound 1, Pt
(acac) [HOMO=ꢀ5.45 eV, LUMO=ꢀ2.78 eV][7a] and Pt-
(ppy)
ACHTUNGTRENN(UNG BppyB)-
ACHTUNGTRENNUNG
A
U
The attachment of BMes2 to the ppy chelate leads to the de-
crease of both LUMO and HOMO levels (Figure 6). How-
ever, the decrease of the LUMO level of PtACTHUNTGRENNUG(BppyB)AHCTUNGTRNEN(UGN acac)
is much more pronounced than that of 1, thus supporting
Table 3. Experimental data of HOMO/LUMO energy and DFT calculation results.
Experimental
DFT results
Complex HOMO
LUMO
Eg
[eV]
HOMO
LUMO
HOMO–LUMO gap
[eV]
%HOMO!LUMO
Oscillator strength
(S0!S1)
[eV][a]
[eV][b]
[eV][c]
[eV][c]
(S0!S1)
1
2
3
4
ꢀ5.49
ꢀ5.36
ꢀ5.61
ꢀ5.60
ꢀ2.57
ꢀ2.54
ꢀ2.64
ꢀ2.82
2.92
2.82
2.97
2.78
ꢀ5.54
ꢀ5.42
ꢀ5.62
ꢀ5.56
ꢀ1.67
ꢀ1.64
ꢀ1.73
ꢀ1.98
3.87
3.78
3.89
3.58
78
79
85
86
0.0414
0.0314
0.0239
0.0283
[a] Calculated by using the reduction potential and the optical energy gap (Eg); Eg was determined from absorption edge. [b] Estimated by using the re-
duction potential. [c] HOMO and LUMO energies were determined by DFT calculation.
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PtII-Based Blue-Green PhOLEDs
FULL PAPER
tures as the starting point for geometry optimizations where
possible. Calculations were performed at the B3LYP level of
theory by using LAN2LDZ as the basis set for Pt, and 6-
31G* for all other atoms. The computational results show
that the S0!S1 transitions arise predominantly from the
HOMO and LUMO with reasonable oscillator strengths
(Table 3). Hence, our discussion will focus on these orbitals
(Table 4). The trends in the calculated and observed energy
gaps are in reasonably good agreement. The HOMO levels
of 1–3 have significant contributions from the d orbital of
the Pt atom, some minor contributions from the acac ligand
and major contributions from the phenyl ring of the ppy
chelate. It should be noted that there is a large contribution
by the carbon atom at the 4’ (para to Pt) position of the
phenyl ring at the HOMO level. In contrast, at the LUMO
level, the contribution from the 4’-carbon atom is very
small. Thus, substitution by an electron-withdrawing BMes2
group at this position should be very effective in stabilizing
the HOMO level through both induction effect and p conju-
gation while minimizing the impact on the LUMO level.
This explains the observed increase of the HOMO–LUMO
gap and emission energy of compounds 1–3, compared to
plains the observed decrease of the HOMO–LUMO gap
and the emission energy of Pt
Pt(ppy)(acac) (Figure 6).
For 4, the LUMO level is dominated by the empty p orbi-
ACHUTNGTRENNUG(BppyB)ACHTUGNTREN(NUGN acac), compared to
A
ACHTUNGTRENNUNG
tal of the boron atom and the p* orbital of the N,C,N-che-
late, whereas the HOMO level has major contributions from
the central benzene ring, the Pt atom and the chloro ligand.
Thus, the electronic transition of the first excited state of
compound 4 may be considered as a mixture of intraACHTUNGTRENNUNGliACHTUNGTRENNUNGgand
charge transfer, metal-to-ligand charge transfer and p-to-p*
transition of the chelate ligand. The carbon atom at the 4’-
position of the benzene ring in 4 has a very large contribu-
tion to the HOMO level but has no contributions at all to
the LUMO level. The influence of the BMes2 group to the
LUMO level is therefore limited to induction effect. It can,
however, effectively stabilize the HOMO level through both
induction effect and p-orbital overlap with the carbon atom
at 4’-position of the benzene ring. Based on this analysis, the
slight widening of the HOMO–LUMO gap in 4 relative to
that of PtACTHNUTRGENUG(N dpb)Cl is caused mainly by the stabilization of the
HOMO level through the boron atom.
Pt
(ppy)
E
Electroluminescence: Complexes 1, 3 and 4 were selected
for evaluation of performance in electroluminescent devices
due to their relatively high emission energy and phosphores-
cence efficiency. The devices we fabricated have a double-
layer structure with CBP (4,4’-N,N’-dicarbazolebiphenyl) as
hole-transport layer (HTL) and host and 1,3,5-tris(N-phe-
at the 5’-position of the phenyl ring to the HOMO level is
much less due to the small orbital/electron density contribu-
tion at the 5’-position whereas the LUMO level can be sig-
nificantly stabilized by the Boron center due to the large
contribution at the 5’-position in the LUMO level. This ex-
Table 4. The surfaces and energies of frontier orbitals for 1–4 plotted with an isocontour value of 0.02.
1
2
3
4
LUMO
ꢀ1.67 eV
ꢀ1.64 eV
ꢀ1.73 eV
ꢀ1.98 eV
HOMO
ꢀ5.54 eV
ꢀ5.42 eV
ꢀ5.62 eV
ꢀ5.56 eV
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Y. Kang, S. Wang et al.
Figure 7. EL device structure (top) and energy level diagrams (bottom).
nylbenzimidazole-2-yl)benzene (TPBI) as the electron-trans-
port layer (ETL) as shown in Figure 7. This device structure
has been used successfully recently for green PtII phosphor-
escent emitters by our group.[7c] Previously we have shown
that a transition metal oxide hole injection layer, such as
MoO3, raises the work function of the ITO anode sufficient-
ly so as to allow direct charge injection into the host materi-
al, eliminating the need for a discrete HTL entirely.[17] Using
the double-layer device structure design, we made a series
of EL devices by varying the doping level of the PtII com-
pound in the CBP layer at 2, 4, 8 and 12%. No excimer
emission was observed for any of the devices. For com-
pounds 3 and 4 the best performance was achieved at
a doping level of 12% whereas for 1 the best performance
was observed at a doping level of 8%. For comparison pur-
pose, the performance of EL devices at the 12% doping
level for all three emitters are summarized in Table 5. The
data for devices at 2, 4 and 8% doping levels for all three
compounds can be found in the Supporting Information.
The EL spectra of these devices (lmax =483 nm for device A,
Figure 8. Left: EL spectra of devices A, B and C. Right: The J-L-V char-
acteristics of devices A, B and C by using Pt compounds 1, 3, and 4 as
the phosphorescent dopant, respectively.
478 nm for B and 493 nm for C) are shown in Figure 8,
which match very well with the PL spectra of compounds 1,
3 and 4.
As shown in Figure 9, the EL devices based on com-
pounds 1, 3 and 4 have an impressive performance. For
1 and 3, the turn-on voltage of the devices is about 3.5 V
whereas for 4 it is 2.8 V with maximum brightness reaching
3025 cdmꢀ2 and 2765 cdmꢀ2, respectively. At the 12%
doping level, the external quantum efficiencies of devices
1 and 3 at 100 cdmꢀ2 are 4.7 and 6.5%, respectively. At the
8% doping level, the EQE of the EL device of 1 is 5.7% at
100 cdmꢀ2 with a maximum brightness of 3340 cdmꢀ2. Al-
though the device efficiencies of 1 and 3 are lower than the
green PhOLEDs based on Pt
ACHUTGTNRENN(GU BppyA)ACHUTNGTREN(NUGN acac) we reported
previously,[6a,c] it is certainly among the most efficient
Table 5. OLEDs characteristics for 1, 3 and 4 at 12 wt% doping level.
[d]
[d]
12 wt% dopant
Von [V][a]
L [cdmꢀ2], [V][b]
hext [%][c]
hc [cdAꢀ1
]
hp [lmWꢀ1
]
CIE (x,y)
10 cdmꢀ2
100 cdmꢀ2
1000 cdmꢀ2
1
3
4
3.6
3.5
2.8
3025, 11.0
2765, 10.5
2417, 9.0
5.1
6.7
15.3
4.7
6.5
13.4
3.5
4.5
3.6
8.9
16.5
30.1
8.7
16.2
33.8
(0.33, 0.34)
(0.28, 0.44)
(0.34, 0.56)
[a] The applied voltage (Von) is defined as brightness of 1 cdmꢀ2. [b] The luminance (L) is maximum value. [c] External quantum efficiency (EQE, hext).
[d] Current efficiency (hc) and power efficiency (hp) are maximum values.
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PtII-Based Blue-Green PhOLEDs
FULL PAPER
pound 4 is certainly much better in terms of device efficien-
cy, thus demonstrating that the BMes2 group is also highly
effective in enhancing the performance of N,C,N-chelate PtII
compounds.
Conclusion
We have demonstrated that substitution by a BMes2 group
in a ppy chelate ligand is a highly effective strategy to en-
hance the phosphorescent efficiency of the corresponding
PtACTHNUTRGNEU(GN acac) compounds. The location of the BMes2 group on
the ppy has a dramatic impact on the phosphorescent
energy. Substitution at the 4’-position of the phenyl ring of
ppy by BMes2 has been found to effectively shift the phos-
phorescent emission energy of the PtII compound to the
blue-green region, which can be further tuned by a substitu-
ent at the 3’-position. BMes2 substitution on the dipyridyl-
benzene N,C,N-chelate ligand does not significantly change
the phosphorescent energy of the PtII compound, it does,
however, increase phosphorescent quantum efficiency, mini-
mize excimer emission and greatly enhance the performance
of the EL devices. High performance bluish-green EL devi-
ces with a simple double-layer structure and compounds 1, 3
and 4 as the dopants have been achieved, demonstrating
once again the important role played by the BMes2 group in
achieving high efficiency PhOLEDs.
Figure 9. The external quantum efficiencies of EL devices versus lumi-
nance (top) and current density (bottom).
Experimental Section
General consideration and measurement: All experiments were carried
out under a dry nitrogen atmosphere by using standard Schlenk tech-
niques. All starting materials were purchased from either Aldrich Chemi-
cal Co. or Strem Chemicals, Inc., and used without further purification.
1H and 13C NMR spectra were recorded on a Bruker Avance 300 or
bluish-green PhOLEDs based on PtACHTUNGTRNEUNG
(acac) compounds.[4c,8]
The relatively low external quantum efficiencies of EL devi-
ces of 1 and 3 can be attributed to the relatively low photo-
luminescent quantum efficiencies of 1 and 3, compared to
400 MHz spectrometer. UV/Vis spectra were obtained on
a Varian
that of Pt
(BppyA)
E
Cary 50 UV/Vis spectrophotometer with all sample concentrations in the
range 10~50 mm. Excitation and emission spectra were recorded on
a Photon Technologies International QuantaMaster Model 2 spectrome-
ter. Photoluminescent lifetimes were measured on a Photon Technology
International Phosphorescent lifetime spectrometer, Timemaster C-631F
equipped with a xenon flash lamp and digital emission photon multiplier
tube for both excitation and emission. All solutions for photophysical ex-
periments were degassed with nitrogen. Phosphorescence quantum yields
the photoluminescent quantum efficiency (0.25 in CH2Cl2)
of 3 into consideration, the EQE of EL device 3 is close to
the commonly accepted theoretical limit for typical
OLEDs.[18] Most significant is that the EL devices based on
1 and 3 maintain the high efficiency at high brightness and
high current density and show little tendency of high current
“roll-off” often observed in OLEDs.
The EL devices based on compound 4 have the most im-
pressive performance (Figure 9 and Table 5). At the 12%
doping level, the EQE of device C is 15.3% at 10 cdmꢀ2 lu-
minance and 13.4 cdmꢀ2 at 100 cdmꢀ2 luminance with
a brightness of 2417 cdmꢀ2 at 9 V. At the 8% doping level,
the device performance remains impressive with EQE of
16.1% at 10 cdmꢀ2 luminance and 10.6 cdmꢀ2 at 100 cdmꢀ2
luminance, respectively, and a brightness of 2242 cdmꢀ2 at
9.2 V. Efficient blue-green and green PhOLEDs based on
of complexes 1 to 4 were obtained relative to IrACTHNURGTNEUNG(ppy)3 in degassed
CH2Cl2 (FPL =1.0)[19] at 298 K, and calculated by using previously report-
ed procedures.[20] Cyclic voltammetry was performed by using a BAS CV-
50W voltammetric analyzer with scan rates of either 50 or 100 mVsꢀ1
.
The electrolytic cell used was a conventional three compartment cell,
with a Pt working electrode, Pt wire auxiliary electrode, and Ag/AgCl
reference electrode. All experiments were performed at room tempera-
ture by using NBu4PF6 (0.10m) in DMF as the supporting electrolyte. The
ferrocenium/ferrocene couple (Eonset =0.55 V) was used as the internal
standard. The starting materials, (3-bromophenyl)dimesitylborane and 2-
(2’-(dimesitylboryl)phenyl)pyridine (Bppy) were synthesized according to
a literature procedure.[10] TD-DFT computations were performed by
using Gaussian 03 software[16] and the High Performance Computing Vir-
tual Laboratory (HPCVL) at Queenꢀs University.
PtACHTUNGTRENNUNG(dpb)Cl and its derivatives have been demonstrated previ-
ously by Williams and colleagues, and others.[9] Compared to
the previously reported Pt-N,C,N compounds-doped EL de-
vices, the performance of the EL devices based on com-
Synthesis of ligands
2-(5’-Dimesitylboryl-6’-fluorophenyl)pyridine (Bfppy): n-Butyllithium
(0.63 mL, 1.60m in hexanes, 1.00 mmol) was slowly added to a solution of
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Y. Kang, S. Wang et al.
2-(2-fluro-3-bromophenyl)pyridine (0.25 g, 1.00 mmol) in THF at ꢀ788C.
The solution was then stirred for 1 h at ꢀ788C before BMes2F (0.30 g,
1.10 mmol) was added. The resulting solution was maintained at ꢀ788C
for an additional hour, then slowly warmed to room temperature and
stirred, overnight. The concentration of the solution under vacuum and
further purification by chromatography on silica gel afforded the desired
product in 80% yield. 1H NMR (400 MHz, CD2Cl2, 258C, TMS): d=8.76
(d, 3J=4.5 Hz, 1H), 8.19 (t, 3J=8.0 Hz, 1H), 7.79 (d, 3J=8.5 Hz, 1H),
ported recently for N,C-chelate Pt
dure is given below.
The starting material[21] [PtMe
2ACHTUNGTRENUNG(SMe2)]2 and corresponding BMes2-func-
tionalized phenylpyridine were dissolved in THF (~5 mL) and then this
mixture was stirred for 1 h at ambient temperature. Trifluoromethanesul-
fonic acid (TfOH) in THF was added slowly to this solution. After
(acac) compounds.[12] A typical proce-
ACHTUNGTRENNUNG
30 min, Na
ACHTUNGTRNE(NUNG acac)·H2O in methanol was added and stirred for 1 to 2 h.
The pale yellow precipitate formed was filtered and washed with Et2O.
An analytically pure BMes2-functionalized PtACTHNUTRGENNGU(ppy)CAHTUNGTRENN(UGN acac) was isolated in
3
3
3
7.68 (t, J=7.0 Hz, 1H), 7.39 (t, J=7.0 Hz, 1H), 7.32 (t, J=7.5 Hz, 1H),
3
7.23 (t, J=6.0 Hz, 1H), 6.80 (s, 4H), 2.36 (s, 6H), 2.16 ppm (s, 12H).
the range of 80~90% yield.
Synthesis of Pt-Bppy (1): Yield: 92%; 1H NMR (400 MHz, CD2Cl2,
2-Bromo-6-dimesitylboryltoluene: A solution of n-butyllithium (6.00 mL,
1.60m solution in hexane, 9.6 mmol) was added dropwise to a solution of
2,6-dibromotoluene (2.00 g, 8.00 mmol) in Et2O (30 mL) at ꢀ788C. The
reaction mixture was maintained at this temperature for an additional
hour, and then BMes2F (2.57 g, 9.60 mmol) in Et2O (50 mL) was added
slowly. The reaction mixture was allowed to warm to room temperature
and stirred, overnight. The reaction mixture was quenched with water,
and then extracted with CH2Cl2. The organic layers were combined and
washed with water, dried over MgSO4, and then filtered. The filtrate was
concentrated under reduced pressure. The residue was purified by silica
column chromatography by using hexane to give the product in 75%
yield as a white solid. 1H NMR (300 MHz, CDCl3, 258C TMS): d=7.63
3
3
258C, TMS): d=9.24 (d, sat, J=5.6 Hz, JPtꢀH =36 Hz, 1H), 7.81 (td, J=
8.0 Hz, 4J=1.6 Hz, 1H), 7.65–7.53 (m, 3H), 7.28 (dd, 3J=7.6 Hz, 4J=
1.2 Hz, 1H), 7.18 (td, 3J=6.4 Hz, 4J=1.2 Hz, 1H), 6.87 (s, 4H), 5.55 (s,
1H), 2.35 (s, 6H), 2.07 (s, 12H), 2.06 (s, 3H), 2.02 ppm (s, 3H); 13C NMR
(100 MHz, CD2Cl2, 258C, TMS): d=186.2, 184.4, 167.9, 147.9, 147.2,
144.9, 140.7, 138.2, 131.2, 130.3, 128.2, 128.1, 121.5, 118.6, 102.3, 27.9,
26.9, 23.2, 20.9 ppm; elemental analysis calcd (%) for C33H35BNO2Pt: C
57.99, H 5.16, N 2.05; found: C 57.89, H 5.19, N 2.08.
Synthesis of Pt-Bmppy (2): Yield: 89%; 1H NMR (400 MHz, CD2Cl2,
258C, TMS): d=9.10 (dd, sat, 3J=5.6 Hz, 4J=1.2 Hz, JPtꢀH =42 Hz, 1H),
7.79–7.74 (m, 2H), 7.45 (d, sat, 3J=7.7 Hz, JPtꢀH =28 Hz, 1H), 7.05 (td,
3J=7.2 Hz, 4J=1.2 Hz, 1H), 6.84 (d, 3J=7.6 Hz, 1H), 6.70 (s, 4H), 5.41
(s, 1H), 2.40 (s, 3H), 2.20 (s, 6H), 1.93 (s, 12H), 1.87 (s, 3H), 1.45 ppm (s,
3H); 13C NMR (100 MHz, CD2Cl2, 258C, TMS): d=186.3, 184.4, 168.4,
147.8, 146.2, 143.8, 143.4, 140.2, 138.5, 137.9, 134.8, 128.2, 128.0, 123.2,
120.8, 102.2, 28.0, 26.9, 22.8, 20.9 ppm; elemental analysis calcd (%) for
C34H37BNO2Pt: C 58.54, H 5.35, N 2.01; found: C 58.52, H 5.38, N 2.05.
(d, 3J=7.8 Hz, 1H), 7.05–7.15 (m, 2H), 6.83 (s, 4H), 2.32
ACHTNUTRGNEUNG(s, 6H), 2.24 (s,
3H), 1.99 ppm (s, 12H).
2-(5’-Dimesitylboryl-6’-methylphenyl)pyridine (Bmppy): A Schlenk flask
was charged with 2-bromopyridine (0.87 g, 5.50 mmol) and dry THF
(15 mL). This solution was cooled to ꢀ788C, then n-butyllithium (4.1 mL,
1.60m solution in hexane, 6.56 mmol) was added dropwise via syringe.
The resulting solution was stirred at ꢀ788C for 1 h. ZnCl
(1.60 g, 6.60 mmol) in THF(66 mL) was added slowly at the same temper-
ature via cannula. After 30 min at ꢀ788C, the reaction mixture was al-
lowed to warm to room temperature and stirred for 1 h. Pd(PPh3)4
(0.32 g, 5 mol%) and (3-bromo-2-methylphenyl)dimesitylborane (2.32 g,
5.5 mmol) in THF (20 mL) were added to the reaction mixture. The reac-
tion mixture was refluxed, overnight, and then cooled to room tempera-
ture. All volatiles were removed under reduced pressure, and the residue
was then extracted with CH2Cl2. Organic layers were combined and
washed with water, dried over MgSO4, and then filtered. The filtrate was
concentrated under reduced pressure. The residue was purified by silica
column chromatography by using ethylacetate/hexane (1:3 v/v) to give
the product in 50% yield as a white solid. 1H NMR (300 MHz, CDCl3,
2ACHTUNGTRNE(NUNG tmeda)
Synthesis of Pt-Bfppy (3): The Pt complex was synthesized the same way
as indicated in the literature.[6a] Bright yellow crystals were obtained after
recrystallization from DCM/hexane. 1H NMR (400 MHz, CD2Cl2, 258C,
TMS): d=9.11 (d, 4J=0.5 Hz, 1H), 8.02 (d, 3J=8.5 Hz, 1H), 7.86 (t, 3J=
8.0 Hz, 1H), 7.45 (d, 3J=8.5 Hz, 1H), 7.20 (t, 3J=7.5 Hz, 1H), 7.06 (t,
3J=7.5 Hz, 1H), 6.85 (s, 4H), 5.54 (s, 1H), 2.33 (s, 6H), 2.10 (s, 12H),
1.75 (s, 3H), 1.15 ppm (s, 3H); 13C NMR (100 MHz, CD2Cl2, 258C,
TMS): d=186.4, 184.5, 147.4, 140.2, 138.8, 138.5, 137.6, 137.5, 128.1,
126.4, 123.4, 123.2, 121.6, 102.4, 27.9, 26.8, 22.8, 20.9 ppm; elemental anal-
ysis calcd (%) for C34H35BFNO2Pt: C 57.15, H 4.94, N 1.96; found: C
56.97, H 4.95, N 1.87.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
Synthesis of Pt-(Bdpb)Cl (4): Bdpb (0.029 g, 0.06 mmol) and K2PtCl4
(0.028 g, 0.066 mmol) were stirred in degassed an acetonitrile and water
(3:1; 5 mL) mixture at 908C, overnight. After removal of the solvent and
further recrystallization from hexane/CH2Cl2, orange crystals of the com-
plex 4 were obtained (0.04 g, 94%). 1H NMR (400 MHz, CDCl3, 258C,
TMS): d=9.40 (d, sat, 4J=1.0 Hz, J PtꢀH =60 Hz, 1H), 7.92 (td, 3J=
10 Hz, 4J=1.5 Hz, 2H), 7.65 (d, 3J=10 Hz, 2H), 7.61 (s, 2H), 7.31 (td,
4
3
4
258C TMS): d=8.68 (d, J=3.9 Hz, 1H), 7.76 (td, J=7.8 Hz, J=1.8 Hz,
3
4
3
1H), 7.44 (dd, J=6.0 Hz, J=2.7 Hz, 1H), 7.38 (d, J=7.8 Hz, 1H), 7.25
(m, 3H), 6.84 (s, 4H), 2.32(s, 6H), 2.20 (s, 3H), 1.99 ppm (s, 12H).
1-Dimesitylboryl-3,5-dibromobenzene: solution of n-butyllithium
AHCTUNGTRENNUNG
A
(2.0 mL; 1.60m in hexanes, 3.20 mmol) was slowly added to a solution of
1,3,5-tribromobenzene (1.00 g, 3.20 mmol) in THF at ꢀ788C. The solu-
tion was then stirred for 1 h at ꢀ788C before BMes2F (0.94 g, 3.50 mmol)
was added. The resulting solution was maintained at ꢀ788C for an addi-
tional hour, then slowly warmed to room temperature and stirred, over-
night. The concentration of the solution under vacuum and further purifi-
cation by chromatography on silica gel gave a white solid. Recrystalliza-
tion produced the product in 45% yield. 1H NMR (300 MHz, CDCl3,
4
3J=8.0 Hz, J=1.5 Hz, 2H), 6.87 (s, 4H), 2.36 (s, 6H), 2.07 ppm (s, 12H);
13C NMR (100 MHz, CDCl3, 258C, TMS): d=168.4, 167.2, 152.2, 141.6,
141.0, 140.8, 139.2, 138.7, 132.5, 128.3, 123.2, 119.6, 23.6, 21.3 ppm; ele-
mental analysis calcd (%) for C34H32BClN2Pt: C 57.52, H 4.54, N 3.95;
found: C 57.02, H 4.37, N 3.85.
EL device fabrication: Devices were fabricated in a Kurt J. Lesker LU-
MINOSꢄ cluster tool with a base pressure of approximately 10ꢀ8 Torr
without breaking vacuum. The ITO anode is commercially patterned and
coated on glass substrates 50ꢃ50 mm2 with a sheet resistance less than
15 W. Substrates were ultrasonically cleaned with a standard regiment of
Alconoxꢄ, acetone, and methanol followed by UV ozone treatment for
15 min. The active area for all devices was 2 mm2. The film thicknesses
were monitored by a calibrated quartz crystal microbalance and were fur-
ther verified for single-carrier devices by using capacitance-voltage meas-
urements (Agilent 4294A). I-V characteristics were measured by using
a HP4140B picoammeter in ambient air. Luminance measurements and
EL spectra were taken by using a Minolta LS-110 luminance meter and
an Ocean Optics USB200 spectrometer with bare fiber, respectively. The
external quantum efficiency of EL devices was calculated following the
standard procedure.[22]After deposition, single carrier devices were trans-
ferred to a homebuilt variable temperature cryostat for measurement at
4
4
258C TMS): d=7.82 (t, J=1.5 Hz, 1H), 7.53 (d, J=1.5 Hz, 2H), 6.83 (s,
4H), 2.36 (s, 6H), 2.02 ppm (s, 12H).
1-Dimesitylboryl-3,5-dipyridylbenzene (Bdpb): Excess 2-(tributylstannyl)-
pyridine (1.82 mL, 4.80 mmol), LiCl (0.40 g, 9.50 mmol) and PdACTHNUTRGNE(UNG PPh)3Cl2
(0.055 g, 0.08 mmol) were added to the toluene solution of 1-dimesityl-
boryl-3,5-dibromobenzene (0.95 mmol). The solution was then stirred
and refluxed for 3 days under nitrogen. Purification through column
chromatography by using CH2Cl2/hexanes afforded a white solid of Bdpb
(0.14 g, 30%). 1H NMR (300 MHz, CD2Cl2, 258C, TMS): d=8.91 (s, 4J=
1.5 Hz, 1H), 8.72 (d, 3J=3.5 Hz, 2H), 8.20 (d, 4J=1.5 Hz, 2H), 7.80 (m,
4H), 7.29 (t, 4J=1.5 Hz, 2H), 6.93 (s, 4H), 2.38 (s, 6H), 2.10 ppm (s,
12H).
Synthesis of Pt
ACHTUNGTRENNUNG(acac) compounds: BMes2-functionalized PtACHTUGNTREN(NUNG acac) com-
pounds were synthesized by the general procedure similar to the one re-
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PtII-Based Blue-Green PhOLEDs
FULL PAPER
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Received: April 13, 2012
Published online: && &&, 0000
Chem. Eur. J. 2012, 00, 0 – 0
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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Y. Kang, S. Wang et al.
Platinum
Y.-L. Rao, D. Schoenmakers,
Y.-L. Chang, J.-S. Lu, Z.-H. Lu,
Y. Kang,* S. Wang* .......... &&&& — &&&&
Bluish-Green BMes2-Functionalized
PtII Complexes for High Efficiency
PhOLEDs: Impact of the BMes2 Loca-
tion on Emission Color
Easily PhOLED: Blue-green phos-
phorescent organic light-emitting
diodes (PhOLEDs) based on BMes2-
functionalized N,C-chelate and N,C,N-
chelate PtII compounds with external
quantum efficiencies of 5 to 22% have
been achieved (see figure). BMes2 sub-
stitution and its location on the N,C-
or N,C,N-chelate ligand have been
found to be critical in achieving effi-
cient blue-green phosphorescent PtII
compounds.
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Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!