Cyclometalated platinum complexes with luminescent quantum yields approaching 100%

Article abstract of DOI:10.1021/ic302490c

A new class of cyclometalated tetradentate platinum complexes of the type Pt[N^/C-O-LL′] was synthesized and characterized. N ^/C is a cyclometalating ligand such as phenyl-pyrazole (ppz), phenyl-methylimidazole (pmi), or phenyl-pyridine (ppy), and LL′ is an ancillary ligand such as phenoxyl-pyridine (popy). The complexes in this series are highly luminescent, emitting blue to green light in solution with quantum efficiencies ranging from 0.39 to 0.64 and luminescent lifetimes from 2 to 9 μs. When doped in a poly(methyl methacrylate) (PMMA) thin film, measured quantum efficiencies increase to 0.81-0.97 with lifetimes ranging from 4.5-10.4 μs. One notable example, the metal complex PtOO3, emits green light with a luminescent quantum efficiency approaching 100% and achieves approximately 100% electron-to-photon conversion efficiency in device settings.

Full text of DOI:10.1021/ic302490c

Article
pubs.acs.org/IC
Cyclometalated Platinum Complexes with Luminescent Quantum
Yields Approaching 100%
Eric Turner, Nathan Bakken, and Jian Li*
Materials Science and Engineering, Arizona State University, Tempe, Arizona 85287, United States
S
* Supporting Information
ABSTRACT: A new class of cyclometalated tetradentate platinum
complexes of the type Pt[N/\C−O−LL′] was synthesized and
characterized. N/\C is a cyclometalating ligand such as phenyl-pyrazole
(ppz), phenyl-methylimidazole (pmi), or phenyl-pyridine (ppy), and
LL′ is an ancillary ligand such as phenoxyl-pyridine (popy). The
complexes in this series are highly luminescent, emitting blue to green
light in solution with quantum efficiencies ranging from 0.39 to 0.64
and luminescent lifetimes from 2 to 9 μs. When doped in a poly(methyl
methacrylate) (PMMA) thin film, measured quantum efficiencies
increase to 0.81−0.97 with lifetimes ranging from 4.5−10.4 μs. One
notable example, the metal complex PtOO3, emits green light with a
luminescent quantum efficiency approaching 100% and achieves
approximately 100% electron-to-photon conversion efficiency in device settings.
A series of complexes have been developed by Che and co-
INTRODUCTION
■
workers that incorporate O/\N/\C/\N type ligands such as 5,5-
dibutyl-2-(3-(pyridine-2-yl)-phenyl)-5H-indeno[1,2-b]pyridin-
9-olate31 and O/\N/\N/\O type ligands from Schiff bases such
as N,N-bis(5-bromosalicylidene)-1,2-ethylenediamine32 and
bis(phenoxy)diimine based ligands such as 2,9-bis(2′-hydrox-
yphenyl)-4,7-diphenyl-1,10-phenanthroline.33 Other notable
examples include bis-cyclometalated complexes of the type
N/\C*C/\N using ligands such as N,N-di(6-phenylpyridin-2-
yl)aniline,34 those of the type C/\N*N/\C using ligands such as
N,N-di(3-(pyridine-2-yl)-phenyl)aniline34 and 1,1-bis(6-(2,4-
difluorophenyl)-2-pyridyl)-1-methoxyethane,35 and those of
the type C/\C*N/\N using ligands such as N,N-di((1,1′:3′,1″-
terphenyl)-50-yl)-(2,20-bipyridin)-6-amine.36 Several of these
complexes have been incorporated into OLEDs, and while
performance has been promising, it is still well below that of
comparable iridium based emitters. To close the gap, judicious
ligand design is needed to increase radiative decay rates while
simultaneously restricting nonradiative decay mechanisms.
Herein, we report the design, synthesis, and characterization
of a series of highly emissive tetradentate, bis-cyclometalated
platinum complexes that emit light in the blue to green region.
Of note, a complex in this series based on phenyl-pyridine
(ppy) has a luminescent quantum yield approaching 100%, a
fast radiative decay process on a par with analogous iridium
complexes, and approximately a 100% electron-to-photon
conversion efficiency in an OLED.
Cyclometalated iridium and platinum complexes have been the
focus of considerable research, driven in large part by their
potential use as sensitizers,1−3 photocatalysts,4,5 and chemo-
sensors.6−8 These metal complexes have also attracted strong
interest as luminescent materials for use in organic light
emitting diode (OLED) based display and lighting applica-
tions9−15 due to their ability to harvest both electro-generated
singlet and triplet excitons, resulting in a theoretical 100%
electron-to-photon conversion efficiency.16,17 While both
octahedral iridium complexes and square planar platinum
complexes have rich histories in inorganic photophysics, iridium
complexes have gained more prominence in OLED applications
as their reported quantum yields have approached 100% and
luminescent lifetimes have fallen into the microsecond range.18
Despite the dominance of iridium at present, platinum remains
an exciting field of study. If the intrinsic electroluminescent
properties of phosphorescent platinum complexes can be
unmasked and optimized, these complexes can provide a viable
alternative to existing iridium emitters and spur further growth
in this emerging field.
Square planar platinum complexes possess excellent
structural flexibility, with the ability to employ various
cyclometalating ligands. In doing so, the ground state and
excited state properties of platinum complexes can be altered
significantly.19−21 Traditionally, those based on bidentate22,23
and tridentate24−29 ligands have been the most studied, but
tetradentate complexes have seen expanded interest in recent
years, with many highly efficient examples being reported.
Tetradentate platinum complexes are attractive candidates for
emitters due to their rigid framework which can aid in thermal
stability, as well as reduce nonradiative decay pathways.30
Received: November 14, 2012
© XXXX American Chemical Society
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a
Scheme 1. Synthesis of Tetradentate Platinum Complexes of the Type N/\C−O−LL′
a
Reagents and conditions: (a) 1-methylimidazole (0.5 equiv), K2CO3 (2 equiv), CuI (10%), pyridine/toluene (1:1), 120 °C. (b) Cu2O (10%), syn-
2-pyridinealdoxime (20%), Cs2CO3 (2.5 equiv), acetonitrile, reflux. (c) CuI (2 equiv), Pd(OAc)2 (10%), 1-methylimidazole (1.5 equiv), DMF,
microwave, 150 W, 160 °C. (d) tetrakis(triphenylphosphine)palladium(0) (5%), KF (1.2 equiv), toluene, reflux. (e) K2PtCl4 (1 equiv), AcOH,
reflux.
This design strategy results in a set of asymmetric
tetradentate complexes of the type Pt[N/\C−O−LL′]. To
explore this class, three N/\C motifs were selected: phenyl-
pyrazole (ppz), phenyl-methylimidazole (pmi), and phenyl-
pyridine (ppy). The N/\C−O−popy ligands were prepared by
two successive Williamson ether couplings40 which were
common to all three ligands, followed by a ligand specific
final coupling. The first Williamson reaction produced the
hydroxyl terminated phenoxypridine in modest yield. Special
care was needed during the workup procedure to prevent
deprotonation and decomposition of the intermediate product.
The relatively poor solubility of HO−popy made recrystalliza-
tion preferable to column chromatography for purification. The
product of the second ether coupling was more robust, and the
iodo-terminated intermediate was separated in good yield after
column chromatography. The ligand ppz-O−popy (L-1) was
prepared through an Ullmann-type reaction41 with dimethyl-
pyrazole, pmi-O−popy (L-2) through a palladium catalyzed
C−C cross coupling microwave reaction42 with 1-methylimi-
dazole, and ppy-O−popy (L-3) through a Stille coupling43 with
2-(tributylstannyl)pyridine. All ligands were purified through
column chromatography. Detailed reaction and workup
conditions are available in the Supporting Information. The
Pt[N/\C−O−LL′] complexes were prepared by direct metal-
ation with K2PtCl4 in acetic acid. To achieve purity levels
suitable for OLED devices, the crude products were purified by
column chromatography, recrystallization, and train sublima-
RESULTS AND DISCUSSION
■
Design and Synthesis of Ligands and Complexes. The
bulk of tetradentate complexes that have been reported recently
fall into two main classes. Those reported by the Huo34,36 and
Marder35 groups utilize symmetric ligands that incorporate a
single six membered chelate ring containing a linking group
such as nitrogen or carbon and an identical pair of five
membered chelating rings built from units such as phenyl-
pyridine or phenyl-pyrazole. In contrast, Che and co-workers
have recently reported a diverse set of complexes that use aryl-
phenolate groups, in both symmetric33 and asymmetric31
configurations, which incorporate varying numbers of five and
six membered chelate rings. Given the design flexibility inherent
in square planar systems, it is clear that the complexes that have
been reported thus far represent only a small fraction of the set
of possible designs.
When considering design criteria for new luminescent
materials, the resulting complexes should remain rigid for the
purposes of thermal stability and reduced nonradiative decay
rates, be easily tunable to various emission energies, and have
sufficiently fast radiative decay rates. A class of complexes was
envisioned in which a portion of the ligand could be freely
modified with widely reported cyclometalating motifs (N/\C)37
and coupled to a shared ancillary portion (LL′)38 that would
not directly contribute to the radiative decay process. This
ancillary portion should be relatively easy to reduce and form
an additional platinum−carbon bond to destabilize the well-
known metal centered quenching states found in platinum
complexes.39 Phenoxyl pyridine (popy) was chosen to fill this
role. The addition of the bridging oxygen between the (N/\C)
and popy portions enables facile metal coordination, whereas
(N/\C)Pt(popy) still remains as a synthetic challenge.
1
tion. These new platinum complexes were characterized by H
and 13C NMR and CHN elemental analysis.
X-Ray Crystal Structure. Single crystals of PtOO1 were
prepared by slow sublimation under high vacuum (10−6 Torr)
conditions. The molecular structure deviates from square planar
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geometry due to the boat-like conformation of two six
membered rings containing the oxygen linking atom, i.e.,
phenyl rings, and the two phenyl rings themselves, which is
comparable to the distribution found in Pt[N/\C−N−C/\N]
complexes using the ligands N,N-di(3-(3-methyl-1H-pyrazol-1-
yl)phenyl)aniline and N,N-di(3-(pyridin-2-yl)-phenyl)aniline.34
Of note, the oxygen bridge of the Pt[N/\C−O−popy]
complexes provides significantly less contribution to the
HOMO than the triaryl amine in the comparable Pt[N/\C−
N−C/\N] complexes. In the case of PtOO1 and PtOO2, the
LUMO density is localized almost entirely on the pyridyl ring
of the LL′ ligand, with minor contributions from the metal and
the oxygen attached to the pyridyl ring. The LUMO density of
PtOO3 has additional contribution from the (N/\C) pyridyl
ring.
Pt1−C1−C−O1−C−C2 and Pt1−C2−C−O2−C−N3 (Figure
1), where the latter shows a larger degree of distortion. The
Electrochemistry. The electrochemical properties of Pt-
[N/\C−O−popy] complexes and analogs were examined using
cyclic voltammetry and differential pulsed voltammetry. The
electrochemical data (Table 1) reported here were measured
relative to Fc/Fc+. PtOO1, PtOO2, and PtOO3 all show an
irreversible oxidation process typical of platinum complexes45
between 0.33 and 0.62 V. Reduction occurs between −2.62 and
−2.50 V. PtOO1 and PtOO2 both have levels of reversibility
that are detectable only at scan speeds on the order of volts per
second and are thus considered largely irreversible processes. In
contrast, PtOO3 shows two well-defined quasi-reversible peaks
at a scan speed of 100 mV/s (Figure S2). The reduction range
of the tridentate complexes is significantly larger (−2.73 to
−2.18 V) than their tetradentate analogues. This difference can
be attributed to the fact that the pyrazolyl and imidazoyl groups
in the tridentate complexes are more difficult to reduce than the
pyridyl groups in PtOO1 and PtOO2, indicating that the first
reduction process of the latter occurs primarily on the pyridyl
group from the popy ligand. This is supported by the LUMO
assignment calculated with density functional theory (Figure 2),
which is composed mainly of this pyridyl ring.
Figure 1. Perspective views of (a) PtOO1 with thermal ellipsoids
representing the 25% probability limit, (b) the ring joining the two
oxygen bridged phenyl rings, and (c) the ring joining the oxygen
bridged phenyl and pyridyl rings. Hydrogen atoms were omitted for
clarity.
existence of a bulky tetradentate ligand in the complex system
also results in an increase of corresponding bond length.
Compared with Pt(pzpyt)Cl,44 where pzpyt is the ligand of 1-
(1-pyrazolyl)-3-(2-pyridinyl)-toluene, the metal−ligand bond
lengths in PtOO1 (Pt−N1pz = 2.097(3) Å, Pt−C1 = 1.970(4)
Å, Pt−C2 =1.980(4) Å, and Pt−N3py = 2.093(3) Å) are
significantly longer than their Pt(pzpyt)Cl counterparts (Pt−
Npz = 2.023(6) Å, Pt−C = 1.917(7) Å, and Pt−Npy = 2.040(6)
Å).
Density Functional Theory. The calculated geometry of
PtOO1 compares reasonably well with the one determined by
X-ray crystallography, with both showing significant distortion
from planarity of the popy ligand. The DFT generated metal−
ligand bond lengths for Pt−N1pz (2.17 Å), Pt−C1 (2.00 Å),
Pt−C2 (1.98 Å), and Pt−N3py (2.16 Å) are longer than those
found in the crystal (Figure 1). The highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) of the three tetradentate complexes are shown in
Figure 2. In all reported compounds, the HOMO density is
comprised of platinum metal, the oxygen bridging the two
Photophysical Properties. The emission spectra in
solution at room and cryogenic (77 K) temperatures and in a
doped PMMA film were recorded for all three Pt[N/\C−O−
popy] complexes along with their Pt(N/\C/\N)Cl and ppy
analogs (Table 1). The absorption features of PtOO1, PtOO2,
1
and PtOO3 are largely similar (Figure 3) with π−π* ligand
centered (LC) transitions occurring below 300 nm, metal-to-
ligand charge transfer (MLCT) transitions in the 300−420 nm
range, and weaker, broad triplet transitions occurring near the
energy of maximum emission at 77 K (Figure 4).
At 77 K, PtOO1, PtOO2, and PtOO3 show sharp vibronic
progressions of 1380 cm−1, 1440 cm−1, and 1380 cm−1,
respectively, which is typical of complexes emitting from a
predominantly ligand centered triplet state. The emission
energy of PtOO1, PtOO2, and PtOO3 is strongly dependent
on the reducibility of the N contributing aryl ring of the N/\C
ligand, with emission maxima ranging from 420 to 487 nm.
At room temperature, the synthesized Pt[N/\C−O−popy]
compounds are highly luminescent in degassed dichloro-
methane solution, emitting light in the blue to green region
of the spectrum (Figure 5). The vibronic progressions of
PtOO1 and PtOO3 become poorly resolved at this temper-
ature, while those of PtOO2 are comparatively intact. Quantum
efficiencies in solution range from 0.39 to 0.64 and in thin film
from 0.81 to 0.97, which is among the highest reported for
platinum based emitters. In the case of PtOO1, it far exceeds
the comparable iridium compound fac-Ir(ppz)3 (Φ < 0.01),18 as
well as the analogous tridentate Pt(dpzb)Cl (Φ<0.01, Table 1).
Furthermore, it emits at significantly higher energy than
Figure 2. Highest occupied molecular orbital (HOMO) (bottom) and
lowest unoccupied molecular orbital (LUMO) (top) of Pt[N/\C−O−
popy] complexes determined through density functional theory
(DFT) calculations.
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Table 1. Photophysical and Electrochemical Properties of Pt[N/\C−O−popy] Complexes and Their Analogs
a
b
emission at RT
emission at 77K
c
d
d
e
e
λmax [nm]
Φ
τ [μs]
kr [x105 s−1
]
knr [x105 s−1
]
λmax [nm]
τ [μs]
Eox [V]
Ered [V]
PtOO1
430, 456
432
0.39 (0.83)
<0.01 (0.02)
0.64 (0.81)
0.56
3.0 (7.5)
<0.1 (0.7)
9.0 (10.4)
11
1.3 (1.1)
(∼0.3)
0.71 (0.78)
2.0 (0.23)
>100 (14.8)
0.40 (0.18)
0.40
420
426
462
465
487
487
480
13
14
12
12
4.9
7.2
9.0
0.62
0.57
0.33
0.31
0.38
0.41
0.42
−2.59
−2.72
−2.62
−2.73
−2.50
−2.18
−2.41
Pt(dpzb)Cl29
PtOO2
468
Pt(dmib)Cl29
PtOO3
Pt(dpyb)Cl24
(ppy)Pt(acac)22
470
0.51
512
0.63 (0.97)
0.60 (0.73)
0.15 (0.53)
2.0 (4.5)
3.8 (5.7)
2.6 (6.0)
3.2 (2.2)
1.6 (1.3)
0.58 (0.88)
1.8 (0.07)
1.1 (0.47)
3.3 (0.78)
490
484
a
b
Room temperature emission spectra were measured in a solution of dichloromethane or in a doped PMMA film (in parentheses). The 77 K
emission spectra were measured in a solution of 2-MeTHF. Coumarin 47 was used as a reference for quantum efficiency measurement in a dilute
solution. The radiative decay rate (kr) and the nonradiative decay rate (knr) are calculated based on the quantum efficiency and luminescent lifetime
c
d
e
of samples in a dilute solution or a doped PMMA film (in parentheses). Redox measurements were carried out in anhydrous DMF solution using
DPV. The redox values are reported relative to Fc/Fc+.
Figure 5. Room temperature emission spectra of PtOO1, PtOO2, and
PtOO3 in dichloromethane.
Figure 3. The comparison of the absorption spectra of PtOO1,
PtOO2, and PtOO3 in CH2Cl2 at room temperature. The T1
absorption transitions are shown in the inset.
linking atoms, which inhibits the intermolecular interactions
necessary for excimer formation.
Study of PtOO3 and its Analogues. In order to better
understand the influence of the (popy) ligand on the
photophysical properties of platinum complexes, a series of
ppy-based cyclometalated metal complexes were synthesized
and characterized for comparison (Figure 6). This included the
bidentate complex (ppy)Pt(acac),22 the tridentate complex
Pt(dpyb)Cl,24 where dpyb is dipyridylbenzene, and the tris-
cyclometalated complex fac-Ir(ppy)3.17
All three (ppy)Pt-based complexes have an irreversible
oxidation process but a quasi-reversible reduction process. The
reduction potential of PtOO3 is similar to that of (ppy)Pt-
(acac), which is about 300 mV more negative than that of
Pt(dpyb)Cl. This can be attributed to the influence of
extending conjugation from ppy to the dpyb ligand. A similar
trend was observed when other (C/\N)Pt(acac) complexes
were compared to their Pt(N/\C/\N)Cl analogs.25
Figure 4. The 77 K emission spectra of PtOO1, PtOO2, and PtOO3
in 2-methyltetrahydrofuran.
A comparison of absorption features among PtOO3,
Pt(dpyb)Cl, (ppy)Pt(acac), and fac-Ir(ppy)3 complexes is
shown in Figure 7. The platinum and iridium complexes
exhibit very strong absorption bands below 300 nm due to
1π−π* ligand centered (LC) transitions, together with a set of
intense bands in the region 300−420 nm, which is attributed to
MLCT transitions involving both ligands and metal ions. The
weak absorption bands between 470 and 500 nm can be
identified as a triplet transition (S0→T1) on the basis of the
efficient platinum complexes based on N,N-di(3-(1H-pyrazol-1-
yl)phenyl)aniline ligands.34 Interestingly, unlike several other
reported tetradentate platinum complexes,31,33,34 no excimer
based emission was observed regardless of concentration for
any of the presented compounds. This is likely due to the
considerable distortion from planarity induced by the oxygen
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Figure 6. Structures of the synthesized ppy based comparative
complexes (top) and the structures of literature reported ppz and pmi
based comparative N/\C/\N tridentate complexes (bottom) discussed
in this paper.
Figure 8. The emission spectra of PtOO3, Pt(dpyb)Cl, (ppy)Pt(acac),
and fac-Ir(ppy)3 complexes in CH2Cl2 at room temperature.
reported tetradentate homoleptic bis-cyclometalated platinum
complexes [1,1-bis(6-(4,6-difluorophenyl)-2-pyridyl-N,C2)-1-
methoxyethane]-platinum(II) (Φ = 0.54, τ = 0.38 μs)35 and
the highly efficient [N,N-di(2-phenylpyrid-6-yl)aniline]-
platinum(II) (Φ = 0.74, τ = 7.6 μs)34 while possessing a
shorter lifetime than the latter. All comparisons indicate that
the use of the (popy) ligand favorably alters the ground and
excited state properties of platinum complexes, making them
comparable to the state-of-the-art cyclometalated iridium
complexes.
OLED Application. Devices employing PtOO3 as an
emitter were compared with fac-Ir(ppy)3 in the device
structure: ITO/PEDOT:PSS/20 nm TAPC/25 nm 26mCPy:e-
mitters(8%)/10 nm PO15/30 nm BmPyPB/LiF/Al. As shown
in Figure 9, both devices exhibit external quantum efficiencies
Figure 7. The comparison of the absorption spectra of PtOO3,
Pt(dpyb)Cl, (ppy)Pt(acac), and fac-Ir(ppy)3 complexes in CH2Cl2 at
room temperature. The T1 absorption transitions are shown in the
inset.
small energy shift between absorption and emission at room
temperature. PtOO3 possesses similar absorption features to
Pt(dpyb)Cl and (ppy)Pt(acac) for LC and MLCT transitions
in the range of 350−420 nm. The T1 absorption of PtOO3 has
fewer features and is more intense (based on integration area)
than its platinum analogues, but weaker than fac-Ir(ppy)3.
All three ppy based platinum complexes are strongly
luminescent with quantum yields (Φ) ranging between 0.15
and 0.63 in solution and 0.53 and 0.97 in a doped PMMA film.
All have luminescence lifetimes (τ) at room temperature which
fall between 2 and 9 μs in solution and 4 and 10 μs in a PMMA
film. The emission spectra of PtOO3, Pt(dpyb)Cl, (ppy)Pt-
(acac), and fac-Ir(ppy)3 complexes at room temperature are
shown in Figure 8. While both Pt(dpyb)Cl and (ppy)Pt(acac)
possess structured luminescent spectra with well resolved
vibronic progressions, the spectrum of PtOO3 is significantly
less resolved, similar to that of fac-Ir(ppy)3. Moreover, when
compared to its platinum analogues, PtOO3 demonstrates a
shorter luminescent lifetime (around 2 μs), a higher quantum
yield (0.63), and a larger rigidochromic shift between room
temperature and 77 K.46 Similar to fac-Ir(ppy)3,17 PtOO3 has a
measured emission quantum yield in doped PMMA film that
approaches 100%. Taking this thin film data into account, the
quantum yield of PtOO3 compares favorably with the currently
Figure 9. Quantum efficiency-current density characteristics of PtOO3
and fac-Ir(ppy)3 devices with the structure of ITO/PEDOT:PSS/
TAPC/26mCPy:emitters(8%)/PO15/BmPyPB/LiF/Al. Inset shows
the EL spectra of the PtOO3 and fac-Ir(ppy)3 devices.
(EQE) of over 20%, peaking at 22.3% ph/el and 23.6% ph/el
for PtOO3 and fac-Ir(ppy)3 respectively, which is close to the
theoretical limit of device efficiency for an OLED on a planar
glass substrate.16 Despite their similar current−voltage (I−V)
characteristics (Supporting Information, Figure S3), at high
currents the EQE of the device employing PtOO3 begins to
drop off more rapidly than that of the fac-Ir(ppy)3. This can be
E
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PL quantum efficiency measurements of doped thin film were carried
out on a Hamamatsu C9920 system equipped with a xenon lamp,
integrating sphere, and a model C10027 photonic multichannel
analyzer. PL transient measurements of doped thin films were carried
out using the time correlated single photon counting method using a
Horiba Jobin Yvon Fluorolog-3 integrated with an IBH Datastation
Hub using a 370 nm NanoLED as the excitation source while the
sample compartment was purged thoroughly in nitrogen.
Material Identification. Elemental analysis was performed at the
Shanghai Institute of Organic Chemistry. NMR spectra were recorded
on a Varian Gemini-400 instrument, and chemical shifts were
referenced to residual protiated solvent.
Density Functional Theory. Density functional theory (DFT)
calculations were performed using the Titan software package (Wave
function, Inc.) at the B3LYP/LACVP** level.22,51,52 The HOMO and
LUMO energies were determined using a minimized singlet geometry
to approximate the ground state.
Electrochemistry. Cyclic voltammetry and differential pulsed
voltammetry were performed using a CH Instrument 610B electro-
chemical analyzer. Anhydrous dimethylformamide was used as the
solvent under a nitrogen atmosphere, and 0.1 M tetra-n-butylammo-
nium hexafluorophosphate was used as the supporting electrolyte. A
silver wire was used as the pseudo-reference electrode. A platinum wire
was used as the counter electrode, and glassy carbon was used as the
working electrode. The redox potentials are based on the values
measured from differential pulsed voltammetry and are reported
relative to a ferrocenium/ferrocene (Fc+/Fc) redox couple used as an
internal reference (0.45 V vs SCE).53 The reversibility of reduction or
oxidation was determined using cyclic voltammetry.54 As defined, if
the magnitude of the peak anodic and the peak cathodic currents have
an equal magnitude at scan speeds of 100 mV/s or slower, then the
process is considered reversible; if the magnitude of the peak anodic
and the peak cathodic currents are not equal, but the return sweeps are
nonzero, the process is considered quasi-reversible; otherwise, the
process is considered irreversible.
attributed to charge imbalance within the emissive layer at high
currents.28 Nevertheless, PtOO3 devices were still able to
demonstrate remarkably high quantum efficiencies at practical
emission intensities of 20.6% ph/el at 100 cd/m2 and 17.6%
ph/el at 1000 cd/m2.
CONCLUSION
■
On the basis of a novel molecular engineering method, a series
of efficient platinum complexes of the type Pt[N/\C−O−LL′]
were synthesized, including PtOO3, a cyclometalated platinum
complex with an emission quantum yield close to unity. When
incorporated into an OLED device, it achieved approximately
100% electron-to-photon conversion efficiency. A similar
influence of the popy ligand on the excited state was observed
for the deep-blue emitting complex PtOO1 and the blue-green
emitting complex PtOO2, which both demonstrated emission
quantum efficiencies of over 0.80 (Table 1). This method could
be used to develop more efficient phosphorescent complexes,
including those in the blue region where there is currently a
deficiency. Progress in this area is vital to the continued
development of OLED technology for full-color display and
solid state lighting applications. It is worth mentioning that the
popy ligand will not be the only candidate for LL′ ligands in
such a study. More work needs to be completed in order to
uncover the relationship between the structure and the
photophysical properties of platinum complexes. Moreover,
the method of controlling the excited state properties with
ancillary ligands can be expanded to different metal systems,
continuing to enrich the understanding of the photophysics of
luminescent metal complexes.
EXPERIMENTAL SECTION
Synthesis. Tetradentate ligands and comparative platinum
compounds were prepared using literature methods from commer-
cially available starting materials which were used without further
purification. A summary of the procedures employed for ligand
synthesis is available in the Supporting Information.
■
X-Ray Crystallography. X-ray diffraction data were collected on a
Bruker SMART APEX CCD diffractometer with graphite-monochro-
mated Mo Kα radiation (λ = 0.71073 Å) at 296.0(2) K for PtOO1. A
sphere of diffraction data was collected up to a resolution of 0.84 Å,
and the intensity data were processed using the SAINT program. The
cell parameters for the platinum complexes were obtained from the
least-squares refinement of spots (from 1818 collected frames for each
compound) using the SAINT program. Absorption corrections were
applied by using SADABS.47 All calculations for the structure
determination were carried out using the SHELXTL package (version
6.14).48 Initial platinum atomic positions were located by Patterson
methods using XS, and the remaining structure of PtOO1 was found
using difference maps and refined by least-squares methods using
SHELXL-97 with 2112 independent reflections within the range of θ =
2.61−24.92° (completeness 100.0%). Calculated hydrogen positions
were input and refined in a riding manner along with the attached
carbons. A summary of the refinement details and resulting factors is
given in the Supporting Information (Table S1).
PtOO1. A mixture of 2-(3-(3-(3,5-dimethyl-1H-pyrazol-1-yl)-
phenoxy)phenoxy)pyridine (0.34 g, 0.001 mol), K2PtCl4 (0.41 g,
0.001 mol), and acetic acid (35 mL) was refluxed under nitrogen for 3
days. After the solvent was removed under vacuum conditions, a
dichloromethane solution was added to the solid. Then the solution
was filtered, and the filtrate was rotary evaporated to dryness. The oily
raw product was then flash chromatographed on a silica column using
dichloromethane and isolated with a 45% yield. The raw product was
further purified by recrystallization through slow diffusion of ether into
dichloromethane followed by thermal evaporation. Anal. Calcd. for
C22H17N3O2Pt: C, 48.00%; H, 3.11%; N, 7.63%. Found: C, 48.06%; H,
1
3.21%; N, 7.62%. H NMR (400 MHz, CDCl3): δ 2.17 (s, 3H), 2.66
(s, 3H), 6.03 (s, 1H), 6.88−7.01 (m, 3H), 7.04−7.12 (m, 3H), 7.17
(vt, 1H, J 7.8 Hz), 7.32 (d, 1H, J 8.3 Hz), 7.83 (ddd, 1H, J 8.5, 7.1, 1.7
Hz), 8.78 (dd, 1H, J 5.9, 1.7 Hz). 13C NMR (d6-DMSO): 14.45 (1C),
14.58 (1C), 105.89 (1C), 107.90 (1C), 110.49 (1C), 110.66 (1C),
112.24 (1C), 112.59 (1C), 112.70 (1C), 116.09 (1C), 120.54, (1C),
124.57 (1C), 125.24 (1C), 142.00 (1C), 142.53 (1C), 147.57
(1C),150.33 (1C), 152.31 (1C), 152.43 (1C), 153.45 (1C), 155.81
(1C), 159.80 (1C).
PtOO2. The complex was prepared in a synthetic procedure similar
to PtOO1 with a 50% yield. Anal. Calcd. for C21H15N3O2Pt: C,
47.02%; H, 2.82%; N, 7.83%. Found: C, 47.00%; H, 2.94%; N, 7.77%.
1H NMR (400 MHz, CDCl3): δ 4.02 (s, 3H), 6.91 (dd, 1H, J 7.2, 1.7
Hz), 6.93 (d, 1H, J 1.2 Hz), 7.01 (d, 1H, J 1.4 Hz), 7.04−7.13 (m,
4H), 7.17 (vt, 1H, J 8.1 Hz), 7.27 (dd, 1H, J 7.1, 1.0 Hz), 7.32 (d, 1H,
J 8.2 Hz), 7.90 (ddd, 1H, J 8.4, 7.1, 1.8 Hz), 8.81 (dd, 1H, J 5.9, 1.6
Hz). 13C NMR (CDCl3): 35.87 (1C), 104.00 (1C), 109.85(1C),
112.83 (1C), 116.26 (1C), 117.22 (1C), 117.37 (1C), 119.16 (1C),
122.16 (1C), 122.64 (1C), 124.23 (1C), 124.44 (1C), 124.50 (1C),
Photophysical Measurements. The UV−visible spectra were
recorded on a Hewlett-Packard 4853 diode array spectrometer. Steady
state emission experiments at room temperature were performed on a
Horiba Jobin Yvon FluoroLog-3 spectrometer. Solution quantum
efficiency measurements were carried out at room temperature in a
solution of dichloromethane. Before emission spectra were measured,
the solutions were thoroughly bubbled with nitrogen inside of a
glovebox with an oxygen content less than 1 ppm. Solutions of
coumarin 47 (coumarin 1, Φ = 0.73)49 in ethanol were used as a
reference. The equation Φs = Φr((ηs2ArIs)/(ηr2AsIr)) was used to
calculate the quantum yields. The subscript s denotes the sample,
subscript r the coumarin reference, Φ the quantum yield, η the
refractive index, A the absorbance, and I the integrated area of the
emission band.50 Phosphorescence lifetime measurements were
performed on the same spectrometer with a time correlated single
photon counting method using a LED excitation source. The absolute
F
dx.doi.org/10.1021/ic302490c | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
137.85 (1C), 139.82 (1C), 149.00 (1C), 152.84 (1C), 154.54 (1C),
155.07 (1C), 159.55 (1C).
XRD measurements. Financial support was provided by NSF
(CHE-0748867).
PtOO3. The complex was prepared in a synthetic procedure similar
to PtOO1 with a 70% yield. Anal. Calcd. for C22H14N2O2Pt: C,
49.53%; H, 2.65%; N, 5.25%. Found: C, 49.57%; H, 2.78%; N, 5.19%.
1H NMR (400 MHz, CDCl3): δ 6.95 (dd, 1H, J 7.2, 1.8 Hz), 7.12−
7.27 (m, 6H), 7.33 (d, 1H, J 8.3 Hz), 7.50 (dd, 1H, J 7.2, 1.4 Hz), 7.82
(ddd, 1H, J 8.6, 7.2, 1.6 Hz), 7.88 (ddd, 1H, J 8.7, 7.1, 1.9 Hz), 7.92 (d,
1H, J 8.1 Hz), 8.30 (d, 1H, J 5.5 Hz), 8.48 (dd, 1H, J 5.7, 1.9 Hz). 13C
NMR (d6-DMSO): 106.24 (1C), 110.25 (1C), 112.69 (1C), 116.11
(1C), 117.38 (1C), 119.31 (1C), 120.46 (1C), 122.00 (1C), 124.23
(1C), 124.82 (1C), 124.89, (1C), 125.72 (1C), 139.44 (1C), 142.20
(1C), 147.66 (1C), 148.69 (1C), 149.46 (1C), 152.06 (1C), 154.01
(1C), 155.99 (1C), 159.82 (1C), 164.44 (1C).
OLED Fabrication and Testing. Devices employing PtOO3 and
fac-Ir(ppy)3 as emitters were fabricated on glass substrates precoated
with a patterned transparent indium tin oxide (ITO) anode using a
device architecture of ITO/PEDOT:PSS/20 nm TAPC/25 nm 8%
emitter:26mCPy/10 nm PO15/30 nm BmPyPB/LiF/Al, where
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AUTHOR INFORMATION
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