Emissive osmium(II) complexes with tetradentate bis(pyridylpyrazolate) chelates

Article abstract of DOI:10.1021/ic302829e

A tetradentate bis(pyridylpyrazolate) chelate, L, is assembled by connecting two bidentate 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole chelates at the 6 position of the pyridyl fragment with a phenylamido appendage. This chelate was then utilized in the synthesis of three osmium(II) complexes, namely, [Os(L)(CO)₂] (4), [Os(L)(PPh₂Me)₂] (5), and [Os(L)(PPhMe₂)₂] (6). Single-crystal X-ray structural analyses were executed on 4 and 5 to reveal the bonding arrangement of the L chelate. Phosphine-substituted derivatives 5 and 6 are highly emissive in both solution and the solid state, and their photophysical properties were measured and discussed on the basis of computational approaches. For application, fabrication and analysis of organic light-emitting diodes (OLEDs) were also carried out. The OLEDs using 5 and 6 as dopants exhibit saturated red emission with maximum external quantum efficiencies of 9.8% and 9.4%, respectively, which are higher than that of the device using [Ir(piq)₃] as a red-emitting reference sample. Moreover, for documentation, 5 and 6 also achieve a maximum brightness of 19540 cd·m^-2 at 800 mA·cm ^-2 (11.6 V) and 12900 cd·m^-2 at 500 mA·cm^-2 (10.5 V), respectively.

Full text of DOI:10.1021/ic302829e

Article
pubs.acs.org/IC
Emissive Osmium(II) Complexes with Tetradentate
Bis(pyridylpyrazolate) Chelates
Shih-Han Chang,† Chun-Fu Chang,† Jia-Ling Liao,† Yun Chi,*,† Dong-Ying Zhou,‡ Liang-Sheng Liao,*,‡
Tzung-Ying Jiang,§ Tsao-Pei Chou,§ Elise Y. Li,*,§ Gene-Hsiang Lee,⊥ Ting-Yi Kuo,⊥ and Pi-Tai Chou⊥
†Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
‡Institute of Functional Nano & Soft Materials, SooChow University, Suzhou 215123, China
§Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
⊥Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
S
* Supporting Information
ABSTRACT: A tetradentate bis(pyridylpyrazolate) chelate, L, is assembled by connecting two bidentate 3-(trifluoromethyl)-5-
(2-pyridyl)pyrazole chelates at the 6 position of the pyridyl fragment with a phenylamido appendage. This chelate was then
utilized in the synthesis of three osmium(II) complexes, namely, [Os(L)(CO)2] (4), [Os(L)(PPh2Me)2] (5), and
[Os(L)(PPhMe2)2] (6). Single-crystal X-ray structural analyses were executed on 4 and 5 to reveal the bonding arrangement
of the L chelate. Phosphine-substituted derivatives 5 and 6 are highly emissive in both solution and the solid state, and their
photophysical properties were measured and discussed on the basis of computational approaches. For application, fabrication and
analysis of organic light-emitting diodes (OLEDs) were also carried out. The OLEDs using 5 and 6 as dopants exhibit saturated
red emission with maximum external quantum efficiencies of 9.8% and 9.4%, respectively, which are higher than that of the
device using [Ir(piq)3] as a red-emitting reference sample. Moreover, for documentation, 5 and 6 also achieve a maximum
brightness of 19540 cd·m−2 at 800 mA·cm−2 (11.6 V) and 12900 cd·m−2 at 500 mA·cm−2 (10.5 V), respectively.
properties, particularly the emission peak wavelengths, were
able to be fine-tuned, the strategy of which has been supported
by a computational approach and experimentally achieved in
numerous reports.2 In studies for osmium(II) complexes, one
example is the trisubstituted [Os(N^N)3]2+ (N^N = 2,2′-
bipyridine, phenanthroline, and analogues), which showed
emission at λmax = 650−680 nm originating from the triplet
metal-to-ligand charge-transfer (MLCT) excited states.3
Relevant ionic osmium(II) metal complexes have emerged as
promising candidates for applications in both OLED and solid-
state electroluminescent (EL) devices.4 In one seminal case, the
successful replacement of phenanthroline in [Os(phen)3]2+
with chelating dppe [i.e., 1,2-bis(diphenylphosphino)ethane]
gave [Os(phen)2(dppe)]2+ and [Os(phen)(dppe)2]2+, for
INTRODUCTION
■
Recently, there is a growing interest in the emissive third-row
transition-metal complexes because they are crucial for the
fabrication of highly efficient organic light-emitting diodes
(OLEDs).1 The strong spin−orbit coupling induced by their
central heavy metal atoms, such as osmium(II), iridium(III),
and platinum(II), promotes an efficient singlet/triplet inter-
system crossing and facilitates strong luminescence in the as-
fabricated OLEDs by harvesting both the singlet and triplet
excitons. As a result, more emphasis has been put forth on
designing such late-transition-metal phosphors, characterizing
their photophysical properties, and fabricating practical OLEDs
with the newly explored materials.
For the emissive transition-metal chelates, the typical
architecture comprises at least one bidentate chelate to serve
as the chromophore. Thus, upon variation of the skeletal
designs or even the attached substituents, their luminescent
Received: December 24, 2012
Published: April 26, 2013
© 2013 American Chemical Society
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which the electron-accepting phosphine stabilized the metal dπ
orbitals and resulted in a substantial blue shift of the emission
wavelengths.5 Recently, relevant examination was switched to
the charge-neutral osmium(II) complexes with chelating
bipyridine,6 cyclometalate,7 or pyridylazolate8 in attempts to
explore their photophysical behaviors and/or as emitters for
latent OLED applications. Furthermore, these osmium(II)
complexes had showed prominent emission in solution or the
solid state, covering the whole visible region and even beyond,
i.e., from true-blue,9 green,10 red, and even near-IR11 depending
on the design of the chromophoric chelate and ligated ancillary;
the latter is mainly for tuning of the oxidation potential of the
central OsII atom.
Scheme 1. Synthetic Procedures to the Tetradentate Chelate
L
a
Along this scientific content, amid assembly of the
phosphorescent metal complexes, there is also a growing
interest of using multidentate chromophores (cf. the traditional
bidentate chromophores) for their extended π conjugation and
enhanced metal chelate stabilization energy. This maneuver
seems to be quite successful for the platinum(II) systems, for
which both tridentate and tetradentate chelates are routinely
employed in the application of luminescent, chemosensing, and
various optoelectronic materials,12 by taking advantage of their
square-planar coordination geometry. In comparison, as to the
osmium(II) system, for which the most stable coordination
geometry is now changed to an octahedron, studies are more
focused on the chelates such as terpyridine13 and functionalized
tridentate analogues,14 leaving only a handful of documents
dealing with the tetradentate chelates such as salen or
porphyrin.15 In contrast to the cyclic geometry of porphyrin,
it is expected that the open framework of salen would allow the
stepwise formation of metal−ligand bonding around the OsII
center, which is essential for gaining insight into the mechanism
en route to the designated reaction products.
a
Reagents and conditions: (i) NaOBut, Pd2(dba)3, DPPF, toluene, 100
°C, 24 h; (ii) 2 N HCl(aq), 80 °C, 12 h; (iii) CF3CO2Et, THF, 70 °C,
12 h; (iv) N2H4, ethanol, reflux, 36 h.
with ethyl trifluoroacetate, followed by cyclization employing
hydrazine hydrate.
Preparation of Osmium(II) Complexes. It has been
reported that Os3(CO)12 can react with 6 equiv of 3-
(trifluoromethyl)-5-(2-pyridyl)pyrazole, the bidentate counter-
part of the tetradentate chelate L, in refluxing diethylene glycol
monomethyl ether (DGME) to afford the dicarbonyl complex
[Os(fppz)2(CO)2] (1),18 for which the carbonyl ligands reside
at the mutual cis orientation, while the pyrazolate and pyridyl
substituents of fppz chelates are located at the trans and cis
disposition, respectively. A structural drawing of 1 is depicted in
Scheme 2. Furthermore, the treatment of 1 with freshly
sublimed Me3NO should kick off the coordinated carbonyls,
followed by the addition of either PPh2Me or PPhMe2 to give
[Os(fppz)2(PPh2Me)2] (2) and [Os(fppz)2(PPh2Me2)2] (3) in
high yields,19 for which the fppz chelates turn to the planar and
head-to-tail cyclic arrangements, with phosphines occupying
the axial sites (Scheme 2).
With this precedence in mind, we then switched to the
reaction of Os3(CO)12 with tetradentate L in an attempt to
synthesize the respective [Os(L)(CO)2] (4). After routine
chromophoric purification, it was then characterized by mass
analysis, NMR spectroscopy, UV/vis, and emission spectral
analyses. To our surprise, complex 4 showed no obvious
emission in both solid and degassed solution at room
temperature, which is in contrast to that of blue-emitting 1,
revealing the first indication of a difference in their bonding
mode. A single-crystal X-ray diffraction study was next
executed, for which the perspective view and selected bond
lengths and angles are shown in Figure 1.
To avoid the inherent constraint imposed by the cyclic
arrangement of the tetradentate chelate, we herein describe a
new kind of flexible chelate, L, which is assembled from two 2-
pyridylpyrazole units by linking them with a single phenyl-
amido appendage. We choose this design because it can be
easily obtained using procedures similar to those previously
published for its bidentate counterpart.16 Moreover, the
noncyclic, relatively flexible skeleton would allow it to form
several stable coordination modes such that the intermediate
species may become stable under the designated synthetic
conditions.
As can be seen, the tetradentate chelate L wraps around the
OsII metal center in a highly distorted manner, while the
carbonyl groups take the mutual cis position with an angle of
89.55(16)°. Apparently, the competition for stronger dative
bonding, i.e., the so-called trans effect, prevents these carbonyl
groups from taking the mutual trans position.20 Otherwise, the
tetradentate chelate L would have adopted a planar geometry,
which could have gained more stabilization than the present
configuration. The Os−N distances of the L chelate vary
considerably, for which the longest [Os−N(2) = 2.128(3) Å]
and shortest [Os−N(6) = 2.044(3) Å] are due to the
RESULTS AND DISCUSSION
■
Preparation of the Tetradentate Chelate. The direct
reaction of aniline with 2 equiv of protected 2-bromopyridine
yields the appropriate dipyridylaniline, i.e., L1; see Scheme 1.
This reaction is in contrast to certain C−N double-bond
formation on functionalized aniline, for which isolation of the
monopyridyl intermediate is recommended.17 After that,
hydrolysis of 1,3-dioxolane groups in L1 afforded the acetyl
compound L2, which could be converted to the bis(2-
pyridylpyrazolate) chelate L by first Claisen condensation
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Scheme 2. Structural Drawing of Osmium(II) Complexes with the Bidentate fppz Chelate
analogous to that of neither its parent complex 1 nor the
isomeric bis(pyridyltriazolate) complex [Os(bptz)2(CO)2],20
for which the anionic triazolate and neutral pyridyl fragments
are located at the mutual cis and trans dispositions, respectively.
The phosphine-substituted complexes [Os(L)(PPh2Me)2]
(5) and [Os(L)(PPhMe2)2] (6) were next prepared by the
treatment of 4 with Me3NO, followed by addition of the
respective phosphines (Scheme 3). In an aim to simplify the
synthetic procedure, we also made attempts to perform a one-
pot method; i.e., after the reaction of Os3(CO)12 with L, we
went directly to the Me3NO and phosphine treatment and
skipped isolation of the carbonyl intermediate 4. This operation
gave the anticipated complexes 5 and 6 in 22 and 29% yield,
comparable to the total yields using the very complicated two-
step process.
The structure of 5 is shown in Figure 2, with selected bond
lengths and angles provided in the figure caption. The
geometry of the OsII center is a distorted octahedral, in
which the diaxial positions are occupied by phosphines and the
tetradentate chelate L takes the rest of the equatorial sites. In
comparison, on the one hand, both of the Os−P distances
[Os−P(1) = 2.3606(12) Å and Os−P(2) = 2.3648(12) Å] are
comparable to those observed in parent complex 2 [2.3616(5)
Å]19 and other osmium(II) complexes configured with trans-
disposed phosphines.11a On the other hand, the Os−N
distances seem to be notably different, i.e., the Os−N distances
to ligated pyridyl groups [Os−N(3) = 2.044(4) Å and Os−
N(5) = 2.037(4) Å], which turn out to be much shorter than
the Os−N distances to the pyrazolates [Os−N(2) = 2.073(4) Å
and Os−N(6) = 2.070(4) Å] at the peripheral, with the latter
showing no variance to those in 2 [2.073(2) Å]. This reversal of
the Os−N distances must be related to the formation of a
central OsN(3)C(2) metallacycle, for which the phenylamido
group forces the adjacent pyridyl groups to interact much
tighter to the central OsII atom. Moreover, the tetradentate
chelate L adopts a slightly concave conformation, which is
Figure 1. Perspective view of 4 with ellipsoids shown at the 40%
probability level. Selected bond distances (Å) and angles (deg): Os−
N(2) = 2.128(3), Os−N(3) = 2.090(3), Os−N(5) = 2.099(3), Os−
N(6) = 2.044(3), Os−C(1) = 1.887(4), Os−C(2) = 1.888(4); C(1)−
Os−C(2) = 89.55(16), C(1)−Os−N(5) = 172.76(15), N(3)−Os−
N(5) = 81.57(13), C(2)−Os−N(2) = 170.73(14).
pyrazolates being trans to the carbonyl and pyridyl groups,
respectively, and likewise can be attributed to the trans effect.
Moreover, higher internal strain energy seems to be imposed at
the central OsN(3)C(2) metallacycle, evidence of which are
shown by the acute N(3)−Os−N(5) bond angle of 81.57(13)°,
elongated Os−N(3) and Os−N(5) distances of 2.090(3) and
2.099(3) Å, respectively, and the adoption of a boatlike
conformation rather than the planar geometry. Third, the
arrangement of the four N-donor atoms of 4, i.e., all cis
arrangement for both the pyrazolate and pyridine groups, are
a
Scheme 3. Transformation of Osmium(II) Complexes with the Tetradentate Chelate L
a
Reagents and conditions: (i) Me3NO, DGME, 120 °C, 1 h; PPhMe2 (or PPh2Me), 180 °C, 12 h.
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thus be attributed to a ligand-centered (L; see Scheme 1) ππ*
transition. In comparison, these two ππ* transition bands are
obviously red-shifted with respect to the ∼300 and 405 nm
bands observed in 2 (or 3), the result of which indicates the
involvement of the bridging phenylamino group in the
tetradentate chelate L, giving an elongation of π conjugation
and, hence, a reduced ligand-centered ππ* energy gap in the
higher-lying electronic states for both complexes 5 and 6.
The broad-band absorption at longer wavelength (>450 nm)
for complexes 5 and 6 with relatively lower extinction
coefficient (<3 × 103 M−1·cm−1) is assigned to the metal dπ
→ pyridine (π*) charge-transfer (MLCT), mixed to a certain
extent with pyrazolate (π) → pyridine (π*) intraligand charge
transfer (ILCT) in the singlet manifold. This MLCT/ILCT
band is about 22 nm (∼760 cm−1) blue-shifted with respect to
that of the dual bidentate complexes 2 and 3. From a chemistry
point of view, the bridging phenylamido substituent in 5 and 6
acts as an electron-donating group, which is expected to raise
the energy of the pyridine π* orbital contributing to the lowest
unoccupied molecular orbitals (LUMOs) of ILCT, resulting in
an increase of the MLCT/ILCT band gap and hence a blue
shift of the corresponding absorption peak wavelength.
Complexes 5 and 6 are highly emissive in both solution and
the solid state, with the peak wavelengths appearing at 628 and
634 nm in degassed CH2Cl2 (see Figure 3). All emissions are
confirmed as the phosphorescence because of their drastic
quenching of the emission by O2 upon aeration and relatively
small radiative decay rate constant of kr < 1 × 106 s−1.
Compared with 6, the small blue shift of the emission in 5 may
be rationalized by the slightly stronger electron-withdrawing
PPh2Me than that in 6, resulting in a nonnegligible increase of
the MLCT gap. For this case, the contribution of MLCT in the
triplet manifold may accordingly be reduced in 5 (cf. 6), as
supported by the computational approach elaborated on in the
following section. In comparison to the phosphorescence for 2
and 3, shown in Figure 3, the hypsochromic shift of the
emission for 5 and 6, respectively, correlates well with the result
of the blue shift of absorption for 5 and 6 (vide supra).
We then performed time-dependent density functional
theory (TDDFT) calculations with the aim of gainig more
insight into the fundamentals of the associated photophysical
properties elaborated on above. Figure 4 presents the highest
occupied molecular orbital (HOMO) and LUMO that are
mostly involved in the lowest-lying transitions of 2 and 5 (see
Figure S1 in the Supporting Information (SI) for more frontier
orbitals of 2, 3, 5, and 6). It can be clearly seen from Figures 4
and S1 in the SI that, for complex 2 (also for 3), the HOMO is
predominantly located at the pyrazolate fragments and the
central metal dπ orbitals, whereas the LUMO (and LUMO+1)
shifts toward the pyridyl fragments. For complex 5 (also for 6),
because the two pyrazolate fragments are now trans to the two
pyridyl fragments, the HOMO is contracted even more to the
central metal dπ orbitals. As a result, the lowest-lying singlet and
triplet transitions, which consist primarily of HOMO to LUMO
or HOMO to LUMO+1 transitions, show a strong MLCT
character for as high as 50−60% (see Table 2). Moreover,
although of a very qualitative manner, the contribution of
MLCT of 52% in the triplet states for 5 is slightly smaller than
that of 6 (58%). This observation, in part, supports the
aforementioned stronger electron-withdrawing properties of
the ancillary PPh2Me in 5 than in 6.
Figure 2. Perspective view of 5 with ellipsoids shown at the 40%
probability level. Selected bond distances (Å) and angles (deg): Os−
N(2) = 2.073(4), Os−N(3) = 2.044(4), Os−N(5) = 2.037(4), Os−
N(6) = 2.070(4), Os−P(1) = 2.3606(12), Os−P(2) = 2.3648(12);
P(1)−Os−P(2) = 169.61(4), N(2)−Os−N(6) = 109.49(15), N(3)−
Os−N(5) = 93.68(15).
probably due to the packing effect within the crystal lattice. The
uncoordinated N atom on both pyrazolates hydrogen-bonds
with an adjacent solvated water, which is in sharp contrast to
those in 2; the latter showed the formation of an interligand
hydrogen bond with the adjacent pyridyl group.19
Photophysics and Theoretical Work. The absorption
and emission spectra of complexes 5 and 6 recorded in
degassed CH2Cl2 at room temperature are displayed in Figure
3. For comparison, spectra of 2 and 3 are also depicted in
Figure 3. All pertinent photophysical data of these complexes
are summarized in Table 1. In general, complexes 5 and 6
exhibit allowed absorption bands in the UV region (<350 nm)
and blue visible region (400−450 nm), for which ε values at the
peak wavelengths around 335 and 420 nm are calculated to be
∼3.5 × 104 and ∼6.6 × 103 M−1·cm−1, respectively, and can
The vertically excited energy states and their orbital
decompositions based on the S0-optimized geometries of 2, 3,
Figure 3. UV/vis absorption and emission spectra of osmium(II)
complexes 2, 3, 5, and 6 in a CH2Cl2 solution at room temperature.
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Table 1. Photophysical Data of Osmium(II) Complexes Measured in a Degassed CH2Cl2 Solution at Room Temperature and
the Associated Electrochemical Properties
a
a
abs λmax/nm (ε × 10−3/M−1·cm−1
)
em λmax/nm
Φ
τobs/ns kr × 10−5/s−1 knr × 10−5/s−1 E1/2ox/V [ΔEp/mV ] E1/2red/V [ΔEp/mV ]
2
3
5
6
406 (16), 448 (2.4), 550 (1.3)
411 (21), 465 (2.4), 550 (1.5)
336 (31), 420 (6.4), 528 (1.7)
334 (33), 422 (6.8), 528 (1.7)
632
649
628
634
0.50
0.19
0.40
0.27
855
725
5.85
2.62
1.94
1.62
5.85
11.2
−0.10 [120]
−0.17 [130]
−0.08 [90]
−0.11 [90]
−2.84 [100]
−2.85 [130]
−2.82 [irr]
−2.80 [irr]
2070
1690
2.89
4.32
a
E1/2 (mV) refers to [(Epa + Epc)/2], where Epa and Epc are the anodic and cathodic peak potentials referenced to the Fc+/Fc couple. ΔEp = |Epa
Epc| was reported in millivolts, and the oxidation and reduction experiments were conducted in CH2Cl2 and THF solutions.
−
virtually the state mixing between singlet and triplet manifolds
induced by the spin−orbit coupling. Such a coupling matrix
should be substantial but is not considered in the current
computational approach. The strong MLCT character of the
S0−S1 transition corresponds well to the lower extinction
coefficient observed in Figure 3. The increase of the energy gap
due to the electron-donating phenylamido substituent in 5 and
6 is also reflected by the blue shift of the S0−S1 MLCT
transition from 2 to 5 (or from 3 to 6). For the emission
properties, the calculated energy gaps of S0−T1 states using S0
geometry for 2 (523 nm), 3 (543 nm), 5 (514 nm), and 6 (528
nm) are also in good agreement with the onset of the
phosphorescence spectra.
To more accurately account for the emission spectra of these
complexes, we made further attempts to locate the T1 state by
performing the geometry optimization along the triplet-state
potential energy surface and then carefully examining the
relative energy gaps between the 3MLCT/ππ* and triplet
metal-centered (3MC) dd states for the osmium(II) complexes
2, 3, 5, and 6. The 3MC dd state involves dσ* antibonding and is
considered to be the major nonradiative deactivation channel
and the key factor in accounting for the instability.1c In this
approach, on the one hand, the vertical energy gaps between
3
the lowest triplet states, i.e., the MLCT/ππ* states, and S0
Figure 4. Electron density contours of the HOMO and LUMO for 2
and 5 based on the optimized ground state (S0) geometries.
match very well with the observed phosphorescence peaks for
these complexes. On the other hand, the energy of the 3MC dd
state is located by a triplet geometry optimization starting from
a distorted initial structure with highly lengthened metal−
ligand bonds, as detailed in previous works.20 For 2 and 3, the
3MC dd states are found to be 7.02 and 7.85 kcal·mol−1,
Table 2. Computational Energy Levels, Oscillator Strengths,
and Orbital Transition Analyses of S1 and T1 States for
Osmium(II) Complexes 2, 3, 5, and 6 Based on the
Optimized S0 Ground-State Geometries
3
respectively, above the MLCT/ππ* states (see Figure S3 in
λcal/
the SI). As for the tetradentate complexes 5 and 6, the energy
of MC dd states is even higher and cannot be stabilized by
3
compd state
nm
f
assignment
MLCT/%
2
3
5
T1
S1
523
498
543
517
514
0
HOMO → LUMO (93%)
HOMO → LUMO (98%)
HOMO → LUMO (94%)
60
61
60
61
52
freely lengthening the metal−ligand bond due to the linkage of
0.005
0
two pyridylpyrazolate moieties via the amido bridge. We thus
3
T1
S1
conclude that the thermal population of the MC dd state is
even more unlikely for 5 and 6; thus, the 3MC dd state should
not be an active channel for both nonradiative deactivation and
photoinstability. This, together with the greater rigidity of
terdentate moieties, accounts for the good phosphorescence
yield for both 5 (0.40) and 6 (0.27) in a CH2Cl2 solution at
room temperature and their suitability in the fabrication of
OLEDs elaborated on below.
0.0032 HOMO → LUMO (98%)
T1
0
HOMO → LUMO+1
(79%)
T2
S1
512
467
528
516
0
HOMO → LUMO (82%)
0.0009 HOMO → LUMO (97%)
63
58
6
T1
T2
0
0
HOMO → LUMO (89%)
HOMO → LUMO+1
(80%)
Electroluminescence of OLEDs. The as-prepared
osmium(II) complexes are used as a red dopant in OLEDs.
The devices are fabricated with the structure of ITO/NPB (60
nm)/TCTA (10 nm)/TPBi: metal complex (20 nm, 4 vol
%)/TPBi (40 nm)/LiF(1 nm)/Al (150 nm). Device A contains
the common red phosphorescent dopant tris(1-phenylisoqui-
noline)-C2,N-iridium(III) [Ir(piq)3], while devices B and C
employ the newly prepared osmium(II) dopants 5 and 6,
respectively. In addition, 4,4′-bis[N-(1-naphthyl)-N-
S1
477
0.0004 HOMO → LUMO (97%)
61
5, and 6 are listed in Table 2. The calculated energy gaps of the
S0−S1 transition for 2 (498 nm), 3 (517 nm), 5 (467 nm), and
6 (477 nm) reveal a trend similar to that observed
experimentally, showing a lower-lying, broad absorption band
peaked at 550 nm for 2 and 3 and 528 nm for 5 and 6 (see
Table 1 and Figure 3). The discrepancy could be attributed to
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phenylamino]biphenyl (NPB), 4,4′,4″-tris(N-carbazolyl)-
triphenylamine (TCTA), and 1,3,5-tris(N-phenylbenzimida-
zol-2-yl)benzene (TPBi) function as a hole-transporting layer,
an electron-blocking layer, and a host material or electron-
transporting layer, respectively.
Figure 5 displays the EL spectra driven at the same current
density of 5 mA·cm−2. In principle, all three phosphors show
Figure 5. Respective EL spectra of devices A−C fabricated with
emitter [Ir(piq)3], 5, and 6 at the same current density of 5 mA·cm−2.
satisfactory deep-red emission with the CIE coordinates located
at (0.67, 0.33), (0.63, 0.36), and (0.65, 0.35) for devices A−C,
respectively. Concomitantly, osmium(II) phosphors 5 and 6
show structureless emission with centers located at 616 and 620
nm, which are in contrast with those of [Ir(piq)3], showing an
intense peak at 620 nm, together with a shoulder at 674 nm.
Besides, complex 6 exhibits a much more red-shifted emission
versus that of 5, the result of which is consistent with the
photoluminescence data shown in Figure 3 as well as the higher
electron-donating nature of PPhMe2 ancillaries.
Moreover, all of these devices exhibit good performance
because of the effective exciton confinement and recombination
in the emitting layer. As shown in Figure 6b,c, the devices with
dopants 5 and 6 both exhibit EL properties superior to those of
the reference device with [Ir(piq)3]. The device with 5 shows
the best performance characteristics with a maximum current
efficiency of 14.0 cd·A−1 at 5 mA·cm−2 and an external
quantum efficiency (EQE) of 9.8%; both are better than those
for the reference device A with [Ir(piq)3] showing a current
efficiency of 7.6 cd·A−1 and an EQE of 8.5% at 10 mA·cm−2.
However, the osmium(II)-based devices B and C exhibit higher
operation voltages than that of the [Ir(piq)3] device, as shown
in Figure 6a. This is caused by the shallow HOMO level of
osmium(II) phosphors, which may induce a large carrier
injection barrier and charge trap.21 Devices B and C achieve
maximum power efficiencies of 9.5 and 7.3 lm·W−1 (at 2.5
mA·cm−2), respectively, while the reference device with
[Ir(piq)3] shows a slightly lower maximum power efficiency
of 5.3 lm·W−1 (at 2.5 mA·cm−2) under identical conditions.
The combined device performance data are depicted in Table 3.
It is believed that the power efficiency of devices B and C could
be further improved if their drive voltage could be comparable
to that of the reference device.
Figure 6. (a) Current density−voltage characteristics, (b) current
efficiency−current density and power efficiency−current density
relationships, and (c) brightness−current density and EQE−current
density relationships for devices with different red dopants.
CONCLUSION
■
In summary, we have prepared the tetradentate bis-
(pyridylpyrazolate) chelate L via the assembly of two bidentate
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Table 3. EL Performances of Devices Fabricated with [Ir(piq)3], 5, and 6 as Phosphors
b
c
dopant
Von /V
Lmax/cd·m−2
Imax/mA·cm−2
ηext,max/%
ηc, max/cd·A−1
ηp, max/lm·W−1
ηext/% at 1000 cd·m−2
CIE [x, y]
a
A
[Ir(piq)3]
3.0
3.3
3.3
17800 (12.6 V)
19540 (11.6 V)
12900 (10.5 V)
800
800
500
8.5
9.8
9.4
7.6
14.0
10.6
5.3
9.5
7.3
5.1 (8.3 V)
4.8 (9.5 V)
0.67, 0.33
0.63, 0.36
0.65, 0.35
a
B
5
6
a
C
5.6 (8.7 V)
a
b
The notations A−C indicate the devices fabricated with employment of dopants [Ir(piq)3], 5, and 6, respectively. Turn-on voltage at which
c
emission became detectable. The driving voltage at 1000 cd·m−2 is depicted in parentheses.
extracted with CH2Cl2 (30 mL × 3). The solution was concentrated,
3-(trifluoromethyl)-5-(2-pyridyl)pyrazole chelates at the 6
position of the pyridyl fragment with a phenylamido
appendage. Subsequently, this chelate was utilized to prepare
osmium(II) complexes 4−6, among which the structures of 4
and 5 have been determined by single-crystal X-ray structural
analysis. Phosphine-substituted derivatives 5 and 6 are highly
emissive in both solution and the solid state. Thus, the OLED
using dopant 5 exhibits a saturated red emission with a
maximum brightness of 19540 cd·m−2 at 800 mA·cm−2 (11.6
V). The tetradentate coordination property is expected to offer
an advantage in enhanced stability, and its extension in
chemistry should be of great perspective because of the
versatile derivation for the pyridylazolate moieties. This new
progress should open up another dimension for the multi-
dentate transition-metal complexes exploited in OLEDs and
photovoltaics.
and the residue was separated by silica gel column chromatography,
eluting with a 1:1 mixture of ethyl acetate and hexane. Recrystallization
from mixed CH2Cl2 and hexane gave 2.0 g of yellow crystals (6.1
mmol, 94%).
Selected spectral data for L2. 1H NMR (400 MHz, CDCl3, 298 K):
δ 7.70−7.64 (m, 4H), 7.44 (t, JHH = 8.0 Hz, 2H), 7.33−7.26 (m, 5H),
2.38 (s, 6H).
Preparation of L. A mixture of L2 (1.2 g, 3.5 mmol) and sodium
ethoxide (0.68 g, 10 mmol) in 30 mL of THF was stirred at 0 °C for 1
h. After that, to the mixture was added ethyl trifluoroacetate (0.83 mL,
8.5 mmol), and it was heated at 70 °C for 12 h. After cooling to room
temperature, the solvent was removed under reduced pressure and
neutralized with 2 N HCl(aq) until pH = 4−5. Extraction with CH2Cl2
(50 mL × 3) followed by evaporation of solvent produced the crude
diketone. Without further purification, this product was allowed to
react with 98% hydrazine hydrate (3.4 mL, 70 mmol) in a refluxing
ethanol solution for 36 h. After that, the solvent was removed under
vacuum and the residue was washed with deionized water to remove
the unreacted hydrazine. Further purification was carried out by silica
gel column chromatography, eluting with a 1:1 mixture of ethyl acetate
and hexane. Recrystallization from mixed ethyl acetate and hexane
gave 1.2 g of a pale-yellow solid (2.3 mmol, 65%).
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed under
nitrogen, and solvents were distilled from appropriate drying agents
prior to use. Commercially available reagents were used without
further purification unless otherwise stated. All reactions were
monitored using precoated thin-layer chromatography plates (0.20
mm with fluorescent indicator UV254). 2-Bromo-6-(2-methyl-1,3-
dioxolan-2-yl)pyridine was obtained by the treatment of 2-acetyl-6-
bromopyridine with ethylene glycol in the presence of a catalytic
amount of p-toluenesulfuric acid.22 Mass spectra were obtained on a
JEOL SX-102A instrument operating in electron impact or fast atom
1
Selected spectral data for L. H NMR (400 MHz, acetone-d6, 298
K): δ 7.82 (t, JHH = 8 Hz, 2H), 7.61 (d, JHH = 7.6 Hz, 2H), 7.46 (t, JHH
= 8 Hz, 2H), 7.33−7.30 (m, 3H), 7.16 (s, 2H), 7.06 (d, JHH = 8.4 Hz,
2H). 13C NMR (150 MHz, CDCl3, 298 K): δ 157.6 (s, 2C), 144.9 (s,
2C), 144.6 (s, 1C), 143.7 (q, JCF = 38.3 Hz, 2C), 142.3 (s, 2C) 138.8
(s, 2C), 130.2 (s, 2C), 128.7 (s, 2C), 127.3 (s, 1C), 121.2 (q, JCF
=
268.7 Hz, 2C), 117.3 (s, 2C), 115.8 (s, 2C), 102.0 (s, 2C). 19F NMR
(376 MHz, CDCl3, 298 K): δ −62.5 (s, 6F). Anal. Calcd for
C24H15F6N7: C, 55.93; H, 2.93; N, 19.2. Found: C, 55.84; H, 3.03; N,
18.9.
1
bombardment (FAB) mode. H and 13C NMR spectra were recorded
on a Varian Mercury-400, an INOVA-500, or a Bruker AVANCE-600
instrument. Elemental analysis was carried out with a Heraeus CHN-O
rapid elemental analyzer. Cyclic voltammetry measurements were
performed using a BAS 100 B/W electrochemical analyzer. The
oxidation and reduction measurements were recorded using a
platinum wire and a gold disk coated with mercury as working
electrodes, respectively, in anhydrous CH2Cl2 and anhydrous
tetrahydrofuran (THF) containing 0.1 M TBAPF6 as the supporting
electrolyte, at a scan rate of 50 mV·s−1. The potentials were measured
against an Ag/Ag+ (0.01 M AgNO3) reference electrode with
ferrocene as the internal standard.
Preparation of L1. A stirred solution of 2-bromo-6-(2-methyl-1,3-
dioxolan-2-yl)pyridine (4.0 g, 16 mmol), aniline (0.70 g, 7.5 mmol),
(1,1′-diphenylphosphino)ferrocene (DPPF; 0.33 g, 0.60 mmol),
sodium tert-butoxide (2.0 g, 21 mmol), and Pd2(dba)3 (0.23 g, 0.25
mmol) in dried toluene (30 mL) was heated at 100 °C for 24 h. The
reaction mixture was cooled to room temperature and neutralized with
2 N HCl(aq) until pH = 6−7. Extraction with CH2Cl2 (50 mL × 3)
followed by evaporation of solvent produced the crude product, which
was further purified by silica gel column chromatography, eluting with
a 1:1 mixture of ethyl acetate and hexane. Yield: 2.7 g, 6.4 mmol, 87%.
Selected spectral data for L1. 1H NMR (400 MHz, CDCl3, 298 K):
Preparation of 4. To a 50 mL flask was added Os3(CO)12 (100
mg, 0.11 mmol), L (180 mg, 0.35 mmol), and DGME (20 mL). The
mixture was heated at 190 °C for 24 h. After cooling to room
temperature, the mixture was concentrated under reduced pressure
and the resulting solid was washed with deionized water and diethyl
ether. Further purification was carried out by silica gel column
chromatography, eluting with a 3:1 mixture of ethyl acetate and
hexane. Recrystallization from mixed ethyl acetate and hexane gave 95
mg of yellow crystals (4; 0.13 mmol, 38%). Single crystals suitable for
X-ray diffraction were obtained from the slow diffusion of hexane into
a CH2Cl2 solution saturated with the complex at room temperature.
1
Selected spectral data for 4. H NMR (400 MHz, acetone-d6, 298
K): δ 8.07−8.00 (m, 2H), 7.92−7.89 (m, 3H), 7.83 (t, JHH = 7.6 Hz,
2H), 7.74 (t, JHH = 7.6 Hz, 1H), 7.67 (d, JHH = 7.6 Hz, 1H), 7.27 (s,
1H), 7.18−7.09 (m, 3H). 19F NMR (376 MHz, CDCl3, 298K): δ
−60.7 (s, 3F), −60.8 (s, 3F). Anal. Calcd for C26H13F6N7O2Os·H2O:
C, 40.16; H, 1.94; N, 12.61. Found: C, 39.97; H, 2.09; N, 12.31.
Selected structural data of 4: C28.5H19F6N7O2Os, M = 795.71,
monoclinic, space group P21/n, a = 13.0288(12) Å, b = 13.2670(13)
Å, c = 16.9620(16) Å, β = 109.168(2)°, V = 2769.4(5) Å3, Z = 4, ρcalcd
= 1.908 Mg·m−3, F(000) = 1540, crystal size = 0.40 × 0.15 × 0.06
mm3, λ(Mo Kα) = 0.71073 Å, T = 150(2) K, μ = 4.687 mm−1, 21102
reflections collected, 6339 independent reflections (Rint = 0.0442),
GOF = 1.042, final R1 [I > 2σ(I)] = 0.0311, and wR2 (all data) =
0.0741.
δ 7.51 (t, JHH = 8.0 Hz, 2H), 7.35 (t, JHH = 8.0 Hz, 2H), 7.21 (d, JHH
=
6.8 Hz, 3H), 7.10 (d, JHH = 7.6 Hz, 2H), 6.95 (d, JHH = 8.4 Hz, 2H),
4.00 (t, JHH = 6.8 Hz, 4H), 3.87 (t, JHH = 6.8 Hz, 4H), 1.59 (s, 6H).
Preparation of L2. To a 100 mL flask was added L1 (2.7 g, 6.5
mmol) and 2 N HCl(aq) (30 mL). The solution was heated at 80 °C
for 12 h. The mixture was cooled to room temperature and neutralized
with 0.5 M NaOH(aq) until pH = 6−7. The product was then
Preparation of 5. To a 50 mL flask was added Os3(CO)12 (100
mg, 0.11 mmol), L (180 mg, 0.35 mmol), and DGME (20 mL). The
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mixture was heated at 190 °C for 24 h. The solution was cooled to
room temperature, trimethylamine N-oxide (52 mg, 0.66 mmol) was
added, and the resulting mixture was briefly heated at 120 °C for 1 h.
To this mixture was added PPh2Me (0.14 mL, 0.73 mmol) at room
temperature, and the mixture was then heated at 190 °C for 24 h.
Finally, the solution was concentrated, and the residue was dissolved in
ethyl acetate and washed with deionized water. Further purification
was carried out by silica gel column chromatography, eluting with a 3:1
mixture of ethyl acetate and hexane. Recrystallization from mixed ethyl
acetate and hexane gave red crystals of 5 (80 mg, 0.072 mmol, 22%).
Single crystals of 5 were obtained by the slow diffusion of hexane into
a CH2Cl2 solution saturated with the complex at room temperature.
The derivative 6 was prepared using PPhMe2 and under the similar
procedure. Yield: 29%.
sum of the exponential functions with a temporal resolution of ∼300
ps by using a nonlinear least-squares procedure in combination with an
iterative convolution method.
Computational Methodology. Calculations on the electronic
singlet and triplet states of all titled complexes were carried out using
the DFT with B3LYP hybrid functional.25 Restricted and unrestricted
formalisms were adopted in the singlet and triplet geometry
optimization, respectively. A “double-ζ” quality basis set consisting
of Hay and Wadt’s effective core potentials (ECPs; LANL2DZ)26 was
employed for the IrIII metal atom, and a 6-31G* basis set,27 for the rest
of the atoms. The relativistic ECPs replaced the inner-core electrons of
the OsII metal atom, leaving only the outer-core valence electrons
(5s25p65d6) to be concerned with. TDDFT calculations using the
B3LYP functional were then performed based on the optimized
structures at ground states.28 Moreover, considering the solvation
effect, the calculations were then combined with an integral equation
formalism polarizable continuum model (IEF-PCM in dichloro-
methane), implemented in Gaussian 09.29 Typically, 10 lower triplet
and singlet roots of the nonhermitian eigenvalue equations were
obtained to determine the vertical excitation energies. The oscillator
strengths were then deduced from dipole transition matrix elements
(for singlet states only). All calculations were carried out using
Gaussian 09.30
Fabrication and Characterization of OLEDs. The OLEDs are
fabricated on prepatterned indium−tin oxide (ITO) substrates with a
sheet resistance of 10 Ω·square−1 and an optical transmittance over
85% in the visible spectral range. Before being used, the ITO
substrates are cleaned in an ultrasonic bath of acetone, alcohol, and
deionized water, respectively, and treated by UV ozone for 15 min to
enhance the work function of the ITO anode. All materials are
deposited on the ITO substrates in a vacuum thermal evaporator
without breaking the vacuum. All of the devices are encapsulated in a
nitrogen-filled glovebox after being removed from the evaporator. The
fabricated devices are then characterized at room temperature in air.
The electrical and EL properties of the devices are measured with a
constant-current source meter (Keithley 2400) and photometer
(Photoresearch PR-650).
1
Selected spectral data for 5. MS (FAB): m/z 1107 [(M + 1)+]. H
NMR (400 MHz, acetone-d6, 298 K): δ 7.80 (t, JHH = 7.6 Hz, 2H),
7.71 (t, JHH = 6.8 Hz, 1H), 7.35 (t, JHH = 8.0 Hz, 2H), 7.19 (d, JHH
=
8.0 Hz, 2H), 7.13 (t, JHH = 6.8 Hz, 6H), 6.99 (t, JHH = 7.6 Hz, 8H),
6.87−6.86 (m, 8H), 6.73 (s, 2H), 5.88 (d, JHH = 8.8 Hz, 2H), 1.04 (s,
6H). 13C NMR (150 MHz, CDCl3, 298 K): δ 156.8 (s, 1C), 151.5 (s,
2C), 148.3 (s, 2C), 143.3 (s, 2C), 142.8 (q, JCF = 35.7 Hz, 2C), 131.6
(s, 2C), 130.8 (t, JCP = 20.3 Hz, 4C), 130.4 (s, 8C), 130.2 (s, 2C),
129.8 (s, 1C), 129.5 (s, 2C), 128.6 (s, 4C), 127.8 (s, 8C), 122.3 (q, JCF
= 267.1 Hz, 2C), 113.0 (s, 2C), 112.0 (s, 2C), 102.0 (s, 2C), 9.9 (t, JCP
= 15.3 Hz, 2C). 19F NMR (376 MHz, CDCl3, 298 K): δ −59.8 (s, 6F).
31 P NMR (202 MHz, acetone-d6, 298 K): δ −13.4 (s). Anal. Calcd for
C50H39F6N7OsP2·H2O: C, 53.52; H, 3.68; N, 8.74. Found: C, 53.11;
H, 3.90; N, 8.80.
Selected structural data of 5: C50H41F6N7OOsP2, M = 1122.04,
monoclinic, space group P21/n, a = 12.5123(9) Å, b = 20.0068(14) Å,
c = 17.7109(12) Å, β = 93.438(2)°, V = 4425.6(5) Å3, Z = 4, ρcalcd
=
1.684 Mg·m−3, F(000) = 2232, crystal size = 0.38 × 0.12 × 0.03 mm3,
λ(Mo Kα) = 0.71073 Å, T = 150(2) K, μ = 3.028 mm−1, 27722
reflections collected, 10161 independent reflections (Rint = 0.0579),
GOF = 1.081, final R1 [I > 2σ(I)] = 0.0461, and wR2 (all data) =
0.0849.
1
Selected spectral data for 6. MS (FAB): m/z 982 [(M + 1)+]. H
NMR (400 MHz, acetone-d6, 298 K): δ 7.74 (t, JHH = 6.6 Hz, 2H),
7.65 (t, JHH = 7.2 Hz, 1H), 7.45−7.37 (m, 4H), 7.14−7.10 (m, 4H),
6.95 (t, JHH = 8 Hz, 6H), 6.30 (t, JHH = 7.6 Hz, 4H), 5.70 (d, JHH = 9.2
Hz, 2H), 0.76 (s, 12H). 13C NMR (150 MHz, DMSO-d6, 298 K): δ
155.6 (s, 1C), 150.6 (s, 2C), 147.2 (s, 2C), 142.7 (s, 2C), 141.3 (q, JCF
= 35.1 Hz, 2C), 133.2 (t, JCP = 19.4 Hz, 2C), 131.0 (s, 4C), 129.7 (s,
4C), 129.6 (s, 2C), 128.9 (s, 1C), 127.6 (s, 2C), 127.4 (s, 2C), 126.6
(s, 2C), 122.5 (q, JCF = 265.6 Hz, 2C), 112.7 (s, 2C), 111.4 (s, 2C),
101.3 (s, 2C), 7.4 (t, JCP = 14.8 Hz, 2C). 19F NMR (376 MHz, CDCl3,
298 K): δ −59.7 (s, 6F). 31P NMR (202 MHz, acetone-d6, 298 K): δ
−17.0 (s). Anal. Calcd for C40H35F6N7OsP2·H2O: C, 47.29; H, 3.87;
N, 9.65. Found: C, 46.96; H, 3.98; N, 9.45.
Single-Crystal X-ray Diffraction Studies. Single-crystal X-ray
diffraction data were measured on a Bruker SMART Apex CCD
diffractometer using Mo radiation (λ = 0.71073 Å). The data
collection was executed using the SMART program. Cell refinement
and data reduction were performed with the SAINT program. An
empirical absorption was applied based on the symmetry-equivalent
reflections and the SADABS program. The structures were solved
using the SHELXS-97 program and refined using the SHELXL-97
program by full-matrix least squares on F2 values. The structural
analysis and molecular graphics were obtained using the SHELXTL
program on a PC computer.23
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data file (CIF) of 4 and 5 and detailed
results concerning the TDDFT calculations of all studied
osmium(II) complexes. This material is available free of charge
via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*E-mail: ychi@mx.nthu.edu.tw (Y.C.), lsliao@suda.edu.cn (L.-
S.L.), eliseytli@ntnu.edu.tw (E.Y.L.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Cross-Strait Collaboration
Fund sponsored by both the National Science Council (Grant
100-2119-M-001-030-MY3) and the National Natural Science
Foundation of China (Grant 21161160446).
Photophysical Measurement. Measurement of the steady-state
absorption, emission, and phosphorescence lifetimes in both solution
and the solid state was described in our previous reports.24 To
determine the phosphorescence quantum yield (QY) in solution, the
samples were degassed by three freeze−pump−thaw cycles. A
fluorescence dye, 4-(dicyanomethylene)-2-methyl-6-(4-dimethylami-
nostyryl)-4H-pyran (λmax = 615 nm; QY = 0.44 in methanol), was used
as the standard reference for the QY measurement. Lifetime studies
were measured with an Edinburgh FL 900 photon-counting system
and a hydrogen lamp as the excitation source. Data were fitted by the
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