Iridium(III) complexes of a dicyclometalated phosphite tripod ligand: Strategy to achieve blue phosphorescence without fluorine substituents and fabrication of OLEDs

Article abstract of DOI:10.1002/anie.201005624

Toss the F: A new series of Ir^III complexes functionalized with pyridyltriazolate chromophores and a tripodal dicyclometalated phosphite achieved highly efficient blue phosphorescence without employing any fluorine substituents. OLEDs with dopa

Full text of DOI:10.1002/anie.201005624

Communications
DOI: 10.1002/anie.201005624
Luminescence
Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod
Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine
Substituents and Fabrication of OLEDs**
Cheng-Huei Lin, Yao-Yuan Chang, Jui-Yi Hung, Chih-Yuan Lin, Yun Chi,* Min-Wen Chung,
Chia-Li Lin, Pi-Tai Chou,* Gene-Hsiang Lee, Chih-Hao Chang,* and Wei-Chieh Lin
Organic light-emitting diodes (OLEDs) based on heavy
transition-metal complexes are playing a pivotal role in next
generation of, for example, flat panel displays and solid-state
lighting.[1] The readily available, OsII-, PtII-, and in particular
IrIII-based phosphorescence complexes grant superior advant-
age over fluorescent materials.[2] This is mainly due to heavy-
atom-induced spin–orbit coupling, giving effective harvesting
of both singlet and triplet excitons. However, tuning of
phosphorescence over the entire visible spectrum still remains
a challenge. Particularly, designing new materials to show
higher energy, such as deep-blue emission—with an ideal
CIEx,y coordinate (CIE = Commission Internationale de
LꢀEclairage) of (0.14, 0.09)—encounters more obstacle than
the progress made for obtaining green and red colors.
Representative blue phosphors are a class of IrIII complexes
possessing at least one cyclometalated 4,6-difluorophenyl
pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and
others.[3] The majority of blue phosphors showed inferior
color chromaticity with a sum of CIEx+y values being much
greater than 0.3 or with single CIEy coordinate higher than
0.25.[4] Such inferior chromaticity, in part, has been improved
upon adoption of carbene-,[5] triazolyl-,[6] and fluorine-sub-
stituted bipyridine (dfpypy) based chelates.[7]
prise a benzyl substituted pyrazole,[10] an N-heterocyclic
carbene,[11] phosphines,[12] and other ingenious molecular
designs.[13]
Herein, we report the preparation of a novel class of
heteroleptic IrIII complexes by incorporation of tripodal,
facially coordinated phosphite (or phosphonite),[14] denoted
as the P^C2 chelate, for serving as the ancillary, together with
the employment of 2-pyridyltriazolate acting as blue chro-
mophore.[15] The reaction intermediate, which possesses an
acetate chelate, was isolated and characterized to establish
the synthetic pathway. The tridentate P^C2 ancillary chelate
offers several advantages: 1) Good stabilization of complex
and necessary long-term stability in application of for
example, emitting devices. 2) The strong bonding of phos-
phorous donors is expected to destabilize the ligand field d–d
excited state, thus minimizing its interference to the radiative
process from the lower lying excited state. 3) P^C2 inherits
profound and versatile functionality (see below) capable of
fine-tuning the electronic character. As a result, highly
efficient blue phosphorescence is attained with good OLED
performance.
Treatment of a mixture of [IrCl3(tht)3] (tht = tetrahydro-
thiophene) with an equimolar amount of triphenylphosphine
(PPh3), triphenylphosphite {P(OPh)3}, and an excess of
sodium acetate resulted in a high yield conversion (> 80%)
into [Ir(P^C2)(PPh3)(OAc)] (1a); P^C2 = tripodal dicyclo-
metalated phosphite (Scheme 1). Subsequent replacement of
acetate in 1a with chelating 3-tert-butyl-5-(2-pyridyl)triazo-
The above urgency prompted us to search for better and
new blue phosphors. We produced a class of 2-pyridylazolate
chelates possessing very large ligand-centered p–p* energy
gap, as evidenced by the blue-emitting OsII complexes.[8]
Subsequently, room-temperature blue phosphorescence was
also visualized for the respective heteroleptic IrIII com-
plexes,[9] particularly for those dubbed “nonconjugated”
ancillary chelate(s). The nonconjugated ligands so far com-
[*] C.-H. Lin, Y.-Y. Chang, J.-Y. Hung, C.-Y. Lin, Prof. Y. Chi
Department of Chemistry, National Tsing Hua University
Hsinchu, Taiwan 30013 (R.O.C.)
E-mail: ychi@mx.nthu.edu.tw
M.-W. Chung, C.-L. Lin, Prof. P.-T. Chou, Dr. G.-H. Lee
Department of Chemistry, National Taiwan University
Taipei, Taiwan 10617 (R.O.C.)
E-mail: chop@ntu.edu.tw
Prof. C.-H. Chang, W.-C. Lin
Department of Photonics Engineering, Yuan Ze University
Chungli, Taiwan 32003 (R.O.C.)
E-mail: chc@saturn.yzu.edu.tw
[**] This research was supported by the National Science Council (NSC-
98-3114-E-007-005). OLED=organic light-emitting diode.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201005624.
Scheme 1. Synthesis of Ir complexes 1–3. Reaction conditions:
a) 1908C, 6 h; b) 1908C, 12 h.
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Angew. Chem. Int. Ed. 2011, 50, 3182 –3186

late (bptz) gave the blue-emitting phosphor [Ir(P^C2)(PPh3)-
(bptz)] (2a) in high yield (ca. 60%). After establishing the
reaction protocol, for convenience and cost reduction, we
prepared the analogous compounds 2b–d and 3 by employing
a one-pot procedure and skipping isolation of the intermedi-
ate 1a–d. Furthermore, addition of three equivalents of
triphenyl phosphite is necessary for optimizing the synthesis
of 2d and 3; both complexes possess dual monodentate and
dicyclometalated phosphite, showing the intrinsic difference
between phosphine and the stronger p-accepting phosphite.
Of particular interest are those IrIII complexes 2a–d being
free from fluorine substituent; the latter is known to hamper
longevity of OLEDs because of the loss of fluorine atoms
through unavoidable chemical degradation.[16]
Single-crystal X-ray structural determination on 1a and 3
confirms the key feature of dicyclometalated phosphite tripod
(Figures S1 and S2 in the Supporting Information). The
increase of stability with dimetalation is evidenced by the
Figure 1. Absorption and luminescence spectra of 2a–2d and 3–5 in
degassed CH2Cl2 at room temperature (solid lines). Emission spectra
of 2c and 3 in the solid state (dashed lines). The emission scale is
arbitrary, and spectra are plotted so that possible overlap between
peak profiles is avoided.
ꢀ
significantly reduced Ir P distance (1a: 2.142 ꢁ; 3: 2.171 ꢁ)
versus that of monodentate phosphorous donor (1a: 2.378 ꢁ;
3: 2.270 ꢁ) as well as the slightly less idealistic P-Ir-C bite
angles of approximately 798 associated with the P^C2 motif.
Moreover, upon forming 2a–d and 3, the facial configuration
was observed for all anionic and neutral fragments, which are
represented by the cyclometalated phenyl and triazolate
fragments, and the nitrogen and phosphorous donors, respec-
tively. This configuration is evidently driven by the thermo-
dynamic factors.
Independent of types of monodentate phosphine, com-
plexes 2a–d exhibit blue emission with similar vibronic peak
profiles at 450, 473, and 498 nm, while CF3-substituted 3
shows significantly blue-shifted photoluminescence, with
vibronic progression resolved at 416, 442, and 458 nm,
corresponding to a true-blue emission with CIEx,y coordinate
of 0.157, 0.130. For another representative example, intense
phosphorescence of 2d was observed, as indicated by its
quantum yield (F) of as high as 0.87 in degassed CH2Cl2
solution at room temperature. From this result, in combina-
tion with an observed lifetime tobs = 44.5 ms, we can deduce a
radiative lifetime of tr = 51 ms. Conversely, the quantum yield
of 3 is exceedingly lower than that of 2d. With an observed
lifetime of 244 ns, the radiative lifetimes was deduced to be
20.3 ms. Both long radiative lifetime (@ 10 ms) and vibronic
Figure 1 and Table 1 show the absorption and emission
spectral data of complexes 2a–d and 3 in degassed CH2Cl2. As
supported by the frontier orbital analyses (see Figure S3 and
Table S2), taking 2a, 2d, and 3, for example, it is reasonable to
assign the lower-lying absorption band around 325 nm (e ꢁ 6–
8 ꢂ 103 mꢀ1 cmꢀ1) for 2a–d and 310 nm for 3 to mainly the p!
p* transition from the cyclometalated phenyl (P^C2) to
pyridyl fragment, that is, ligand to ligand charge transfer
(LLCT), overlapping in part with the IrIII dp!pyridyl (p*)
metal to ligand charge transfer (MLCT) in the singlet
manifold. Attempts to assign higher-lying electronic states
were discouraged because of their complicated shapes and
hence any corresponding assignments may be meaningless.
Owing to the strong spin–orbit coupling, induced by the IrIII
metal center, a nonnegligible absorption cross section for
lower-lying triplet transition is expectable on the red edge, as
evidenced by the small but discernible absorption shoulder
around 350–380 nm.
3
feature of the phosphorescence confirms their dominant p–
p* nature. The major difference in quantum yields among
these IrIII complexes results from the different nonradiative
decay rate constant knr, which is deduced to be as small as
3.00 ꢂ 103 sꢀ1 for 2d but as large as 4.09 ꢂ 106 sꢀ1 for 3. The
difference in knr values among the titled complexes is of
fundamental interest and may be rationalized by the system-
atic variation of bond strength of monodentate phosphine.
Replacing PPh3 with PPh2Me and PPhMe2 (better s donor)
Table 1: Selected photophysical properties of IrIII complexes 2a–d and 3–5 in degassed CH2Cl2 solution.
Abs. lmax [nm] (eꢀ10ꢀ3
)
Em. lmax [nm]
F[a]
tobs
kr [sꢀ1
]
knr [sꢀ1
]
[a]
2a
2b
2c
2d
3
250 (20.4), 271 (15.3), 326 (6.0)
250 (18.3), 273 (13.7), 325 (6.1)
250 (21.2), 273 (15.6), 327 (7.6)
272 (17.6), 279 (17.0), 322 (8.3)
265 (13.6), 271 (13.1), 310 (7.3)
272 (14.5), 329 (6.2)
450, 475, 492
450, 473, 498
451, 473, 498
447, 472, 492
416, 442, 458
454, 477, 495
448, 472, 494
0.07
0.34
0.45 (0.97)
0.87 (0.95)
0.012
2.5 ms
11.1 ms
2.80ꢀ104
3.06ꢀ104
3.15ꢀ104
1.95ꢀ104
4.89ꢀ104
8.36ꢀ105
3.02ꢀ104
3.72ꢀ105
5.95ꢀ104
3.85ꢀ104
3.00ꢀ103
4.09ꢀ106
2.68ꢀ105
1.40ꢀ105
14.3 ms (27.3 ms)
44.5 ms (51.2 ms)
244 ns
2.84 ms (22.3 ms)
5.88 ms (9.6 ms)
4
5
0.24 (ꢁ1.0)
250 (24.7), 297 (22.7), 353 (5.3)
0.18 (0.41)
[a] Data measured in the solid state are given in parentheses.
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3183

Communications
and also P(OPh)3 (better p acceptor) reduces the respective
state, accounting for the highest emission quantum yield for
ꢀ
ꢀ
Ir P distance, as supported by both X-ray structural analysis
2d (F = 0.87) in solution. Upon decreasing this Ir P bond
and theoretical calculations (see Table S2). Accordingly,
stronger phosphines are able to further destabilize the
metal-centered d–d excited states, thereby suppressing non-
radiative decay pathways. In contrast, their radiative rate
constant kr increases with increase of electron-donating
character of phosphine: PMe2Ph (2c) > PMePh2 (2b) > PPh3
(2a) > P(OPh)3 (2d), reflecting the increased MLCT partic-
ipation in the lowest-lying excited states.
strength from 2d to 2c, the energy gap between 3MLCT/p–p*
and 3MC d–d energy accordingly decreases. The order of
energy level starts to reverse in 2b, and the MC d–d level
3
turns out to be the lowest-lying excited state in 2a. In other
3
3
words, upon excitation of 2a, the MLCT/p–p*! MC!S0
radiationless pathway is expected to be efficient,[18] account-
ing for its highest knr value, that is, the lowest emission
quantum yield (F = 0.07) and short observed lifetime (2.5 ms,
see Table 1). Furthermore, as the substituent on the triazolate
moiety in 2d is changed from tert-butyl to trifluoromethyl to
form 3, the dp orbital energy of IrIII metal ion is decreased and
The proof of concept was then confirmed by a computa-
tional study, in which tuning the relatively energy gap
between MLCT/p–p* and MC d–d states was examined.
3
3
3
3
In this approach, the energy of higher-lying MC d–d states
hence MLCT/p–p* energy increased (cf. 2d, see Figure 2).
ꢀ
was calculated by following the methodology illustrated by
Persson and co-workers (see the Supporting Information for
details).[17] Although being qualitative, the results shown in
Figure 2 show a correlation between the bond strength of
phosphorus donors and 3MLCT/p–p* versus 3MC d–d energy
Thus, despite the possession of the strongest Ir P bonding,
3
3
the net results also reverse the order of MC and MLCT/p–
p* from 2d to 3. Again, the 3MC d–d state becomes the
lowest-energy excited state in 3, resulting in dominant
quenching process and hence much lower solution quantum
yield (F = 0.012) and rather short observed lifetime (244 ns).
Because of the poor emission quantum yield of 3 and
insufficient volatility of 2d, we decided to skip these
phosphors, but to utilize more suitable dopant 2c (Fp ꢁ 0.97
in solid state, see Table 1) for fabrication of blue OLEDs. The
as-fabricated device consist of a multilayer architecture of
ITO/TAPC (30 nm)/TCTA (10 nm)/CzSi (3 nm)/CzSi doped
with 4 wt% of 2c (25 nm)/UGH2 doped with 4 wt% of 2c
(3 nm)/UGH2 (2 nm)/TmPyPB (50 nm)/LiF (0.8 nm)/Al
(150 nm) (ITO = indium tin oxide, TAPC = di[4-(N,N-ditoly-
lamino)phenyl]cyclohexane, TCTA = 4,4’,4’’-tris(carbazol-9-
yl)triphenylamine, CzSi = 9-(4-tert-butylphenyl)-3,6-bis(tri-
phenylsilyl)-9H-carbazole, UGH2 = p-bis(triphenylsilyl)ben-
zene, and TmPyPB = 1,3,5-tri[(3-pyridyl)phen-3-yl]benzene),
for which configurations and structure of materials are shown
in Figure S4. Notably, the wide-gap hosts CzSi and UGH2,
which possess a triplet energy gap of 3.02 eV and 3.18 eV,
were employed for optimal efficiency. In addition, good
confinement of excitons and carriers was realized by using
double emitting layers (4 wt% of dopant in both hole-
transporting CzSi and electron-transporting UGH2 layers)
and double buffer layers to balance the charge transport and
to move the exciton-formation zone away from the adjacent
carrier-transport layers. The current–voltage–luminance (I–
V–L) characteristics and other electroluminescence perfor-
mance data are depicted in Figure S5. These data are also
summarized in Table 2, showing a turn-on voltage of 4.1 V, a
peak external quantum efficiency (hext) of 11.0% photons per
electron, a peak luminance efficiency (hl) of 22.3 cdAꢀ1, and a
peak power efficiency (hp) of 16.7 lmWꢀ1, respectively. Upon
ꢀ
gap. Note that the P Ir bond strength follows the order
P(OPh)3 > PPhMe2 > PPh2Me > PPh3. The monodentate P-
ꢀ
(OPh)3 ligand in 2d, which renders the strongest Ir P
bonding, significantly increases the MC d–d state, such that
3
its relative energy is higher than that of the lowest-lying
3MLCT/p–p* excited state. The large separation in energy
also inhibits thermal population to the higher-lying 3MC d–d
3
Figure 2. Energy level diagram of complexes 2a–d and 3–5. MLCT/p–
p* excited states were obtained from unrestricted optimization,
employing X-ray structural data derived from complex 3. The 3MC d–d
state was also refined by the same method, but with an initial
structure derived from a distorted molecular geometry; see the
Supporting Information for a detailed description. Note that the S0
levels for all complexes are normalized.
Table 2: Electroluminescent characteristics of blue-emitting OLEDs with different phosphors.
LE [cdAꢀ1
]
PE [lmWꢀ1
]
Von
Max. L [cdmꢀ2] (V [V])
CIEx,y
CIEx,y
[c]
[b]
[d]
Device
EQE [%]
[a]
[b]
[a]
[b]
[a]
[b]
2c
4
5
11.0
8.4
8.7
8.4
8.0
8.5
22.3
17.3
14.3
16.9
16.5
13.9
16.7
13.2
10.8
8.1
8.1
10.5
4.1
4.1
4.2
1431 (14.0)
6027 (13.0)
4084 (13.6)
0.179, 0.286
0.173, 0.303
0.169, 0.247
0.220, 0.336
0.185, 0.302
0.171, 0.248
[a] Maximum efficiencies. [b] Data measured at a brightness of 100 cdmꢀ2. [c] Turn-on voltage measured at 1 cdmꢀ2. [d] Data recorded at 1000 cdmꢀ2
.
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increasing to the practical brightness of 100 cdmꢀ2, the hext, hl,
and hp values remain above 8.4%, 16.9 cdAꢀ1, and 8.1 lmWꢀ1,
respectively, which are consistent with less significant effi-
ciencies roll-off, and with CIEx,y coordinates of (0.179, 0.286).
The saturated nature of P^C2 makes fine-tuning the
energy gap feasible by replacing substituents on the triazolate
chromophore. To demonstrate such a possibility, we synthe-
sized the dicyclometalated diphenyl phenylphosphonite
derivatives 4 and 5, which are analogues of 2c, to be the
next generation phosphors (Scheme 2). Pertinent photophys-
Keywords: iridium · luminescence · N ligands · P ligands
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all have good quantum yields in solution and the solid state
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the theoretical results (see Figure 2). Table 2 lists the device
properties for 4 and 5 as well. Obviously, OLEDs made of
either 4 or 5 achieved a three times better peak luminescence
of 6027 and 4084 cdAꢀ1, and particularly for 5, a blue CIEx,y
color chromaticity of (0.169, 0.247). However, shifting of the
CIE coordinates upon increasing the driving voltage was
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energy levels between dopant and electron/hole transporting
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substituents. The molecules were assembled using dicyclome-
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pyridyl triazolate chromophore and a monodentate phospho-
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performance includes: peak efficiencies of 11.0%,
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