Luminescent iridium(III) complexes supported by a tetradentate trianionic ligand scaffold with mixed O, N, and C donor atoms: Synthesis, structures, photophysical properties, and material applications

Article abstract of DOI:10.1021/acs.organomet.7b00038

A panel of tetradentate H₃-O^N^C^O ligands has been synthesized and employed as a trianionic scaffold for preparing [Ir^III(O^N^C^O)(L)₂], with L = a wide variety of neutral ligands, and also [Ir^III(O^N^C^O)(C≡NAr)(NH₂Ar)], [Ir^III(O^N^C^O)(C≡NAr)(X)] (Ar = 2,6-Me₂C₆H₃; X = 1-methylimidazole, PPh₃, pyridine), and [Ir^III(O^N^C^O)- (NHC)]₂ (NHC = N-heterocyclic carbene). X-ray crystal structure analysis and photophysical studies (including variable-temperature emission lifetime measurements and nanosecond time-resolved emission and absorption spectroscopy) were performed. [Ir(O^N^C^O)(L)₂] display a moderately strong phosphorescence at room temperature (emission quantum yields up to 18% in solution, 51% in PMMA film), with the luminescent properties being strongly affected by axial L ligands. The use of [Ir(O^N^C^O)(NHC)₂] as a phosphorescent emitter in a solution-processed organic light-emitting diode device generated a red electrophosphorescence with an EQE of 10.5% and CIE chromaticity coordinates of (0.64, 0.36).

Full text of DOI:10.1021/acs.organomet.7b00038

Article
pubs.acs.org/Organometallics
Luminescent Iridium(III) Complexes Supported by a Tetradentate
Trianionic Ligand Scaffold with Mixed O, N, and C Donor Atoms:
Synthesis, Structures, Photophysical Properties, and Material
Applications
Daqing Chen,†,§ Kai Li,†,§ Xiangguo Guan, Gang Cheng, Chen Yang, and Chi-Ming Che*
†
†
†
,†,‡
†State Key Laboratory of Synthetic Chemistry, Institute of Molecular Functional Materials, HKU-CAS Joint Laboratory on New
Materials, and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
‡HKU Shenzhen Institute of Research and Innovation, Shenzhen 518053, People’s Republic of China
*
S Supporting Information
∧
∧ ∧
ABSTRACT: A panel of tetradentate H -O N C O ligands has
3
been synthesized and employed as a trianionic scaffold for
III
∧
∧ ∧
preparing [Ir (O N C O)(L) ], with L = a wide variety of
2
III
∧
∧ ∧
neutral ligands, and also [Ir (O N C O)(CNAr)(NH Ar)],
2
III
∧
∧ ∧
[
Ir (O N C O)(CNAr)(X)] (Ar = 2,6-Me C H ; X = 1-
2
6
3
III
∧
∧
∧
methylimidazole, PPh , pyridine), and [Ir (O N C O)-
3
(
NHC)]2 (NHC = N-heterocyclic carbene). X-ray crystal
structure analysis and photophysical studies (including varia-
ble-temperature emission lifetime measurements and nano-
second time-resolved emission and absorption spectroscopy)
∧
∧ ∧
were performed. [Ir(O N C O)(L) ] display a moderately
2
strong phosphorescence at room temperature (emission quantum yields up to 18% in solution, 51% in PMMA film), with the
∧
∧ ∧
luminescent properties being strongly affected by axial L ligands. The use of [Ir(O N C O)(NHC) ] as a phosphorescent
2
emitter in a solution-processed organic light-emitting diode device generated a red electrophosphorescence with an EQE of
10.5% and CIE chromaticity coordinates of (0.64, 0.36).
9
10
INTRODUCTION
comprise corroles and bis(phenolate)dipyrrins, which belong
■
∧
∧
∧
∧
∧
∧
to the N N N N and O N N O donor systems, respectively.
As depicted in Chart 1, bidentate and tridentate ligands are
commonly used to prepare neutral luminescent Ir(III)
complexes. In contrast, photophysical and photochemical
investigations of neutral Ir(III) complexes containing planar
tetradentate ligands are relatively sparse.
co-workers first developed a class of neutral Ir(III)-corrole
complexes (I in Chart 1), which display NIR phosphor-
escence arising from corrole ligand-centered transitions.
The design of new ligands is pivotal in the advancement of
transition-metal chemistry. Over the past decades, there have
been numerous reports on multianionic ligands for their
capability to stabilize metal ions in high oxidation states and
their chelating effects imparting thermal and chemical stability
to the resulting metal complexes. Porphyrin and Schiff base
type ligands are the most commonly studied planar tetradentate
ligand scaffolds, with their transition-metal complexes shown to
11
12−14
Gray, Gross, and
12
12c
1
,2
have profound applications in diverse areas such as catalysis,
3
,4
5
However, the emission quantum yields of Ir(III)-corrole
materials science, and medicines. Iridium(III) complexes
supported by porphyrins or salen ligands have shown
remarkable catalytic activities in various organic transforma-
tions, including cyclopropanation, C−H insertion, and Si−H
1
2c,13,14
complexes,
and also iridium monoazaporphyrin deriva-
1
3a
tives, are very low (≤3.09%) in degassed solutions. It is
noted that these aforementioned tetradentate ligands do not
contain strongly σ donating C atoms which could be useful for
achieving both high emission quantum yields and robustness of
the metal complexes. Furuta and co-workers recently developed
a class of neutral Ir(III) complexes supported by a novel type of
trianionic benzonorrole ligand containing one C-donor atom (J
in Chart 1). In comparison to the Ir(III)-corrole analogue,
these organoiridium(III) complexes exhibit significantly red
shifted emission (λ >900 nm, quantum yield ≤0.62%). Lash
6
insertion. In 2011, Koren and co-workers reported a series of
cationic porphyrin-based red to NIR (near-infrared) emitting7
Ir(III) complexes with emission efficiencies of up to ca. 30%.
The porphyrin and salen ligands are dianionic and, upon
coordination to trivalent metal ions, such as iridium(III) and
gold(III), usually afford cationically charged metal complexes.
We have been interested in developing tetradentate trianionic
14
(
TDTA) ligands that are suitable for the synthesis of charge-
neutral complexes of trivalent metal ions and for the8
stabilization of high-valent metal−ligand bonded complexes.
Examples of planar TDTA ligands in the literature mainly
Received: January 18, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00038
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Article
Chart 1. Structures of a Selection of the Reported Ir(III)
Complexes with Different Combinations of Coordination
Ligands
newly designed ligands have extended π conjugation and adopt
a tetradentate coordination mode in a planar coordination
geometry upon coordination to a metal ion. As depicted in
Scheme 1, H L1 was prepared by two consecutive Pd-
3
(
PPh ) Cl /K CO -mediated Suzuki coupling reactions be-
3 2 2 2 3
tween arylboronic acid and aryl bromides in a total yield of
2
8%. This relatively low yield may be attributed to side
reactions in the synthesis of 2-bromo-6-(3-bromophenyl)-
pyridine, during which a considerable amount of the byproduct
2
,6-bis(3-bromophenyl)pyridine was isolated. The synthesis of
H L2 and H L3 was performed by applying the Krohnke
3
3
pyridine synthesis method as a key step. 2′-Hydroxyl-4-
methylchalcone and 1-[2-(3-bromophenyl)-2-oxoethyl]-
pyridinium iodide underwent Michael addition to give 1,5-
diketones, which reacted with ammonium acetate in methanol
(
4
Scheme 1). Subsequent oxidation gave 2-[6-(3-bromophenyl)-
-p-tolylpyridin-2-yl]phenol. Further Suzuki coupling of the
resulting bromides with arylboronic acids afforded H L2 and
3
H L3 in much higher total yields of 85% and 87%, respectively.
3
∧
∧
∧
Synthesis and Characterization of [Ir(O N C O)(L) ]
2
(
1−14). The synthetic method for Ir(III)-corrole complexes
∧
∧ ∧
and co-workers reported an Ir(III) complex of trianionic
was adopted for the synthesis of complexes [Ir(O N C O)-
15
∧
∧ ∧
carbaporphyrin (which also contains one C-donor atom).
(L) ] (1−14) (Scheme 2). We conceived that a [(O N C O)-
2
I
The luminescence properties of this complex have not been
reported. Yam and co-workers recently reported a class of
strongly luminescient tetradenatate cyclometalated alkynylgold-
Ir (Cod)] intermediate was initially formed by the reaction of
∧
∧ ∧
the H
-O N C O ligands with [Ir(Cod)Cl]
and K CO in
2 2 3
3
degassed THF under N
, which was subsequently oxidized to
2
∧
∧ ∧
(
III) complexes generated through a postsynthetic ligand
give O N C O-ligated Ir(III) complexes upon exposure to
ambient atmosphere. Complexes 1−14 were obtained in
moderate to high yields (29−91%) and, except for 2, 8, and
14, are stable both in the solid state and in solution under
ambient conditions. The bis-NHC Ir(III) complexes 8 and 14
are not stable upon prolonged standing in solution. During the
synthesis of 8 and 14, a minor amount of dinuclear
1
6
modification.
Herein we report the design, synthesis, structure, and
spectroscopic characteristics of a new class of charge-neutral
∧
∧
∧
∧
∧ ∧
[
Ir-(O N C O)(L) ] and [Ir(O N C O)(L)(L′)] complexes
2
∧ ∧ ∧
supported by planar O N C O TDTA ligand scaffolds. These
complexes exhibit moderate emission quantum yields of up to
∧
∧
∧
1
8% in degassed CH Cl and 51% in PMMA thin film at room
[Ir(O N C O)(NHC)] (15 and 16) (Scheme 3) was
2
2
2
temperature. The axial ligands (L or L′) have been observed to
obtained. Attempts to obtain diffraction-quality crystals of the
bis-NHC complexes 8 and 14 by slow evaporation of the
have profound effects on the emission properties of the Ir(III)
∧
∧
∧
complexes. One example of these [Ir-(O N C O)(L) ]
corresponding CH Cl2 solutions (7 days) resulted in the
2
2
complexes has been used to fabricate red OLEDs with
satisfactory performance.
formation of 15 and 16 with structures resembling that of
dinuclear (salen)Ru carbene complexes, which have a distorted-
19
octahedral geometry.
∧
∧ ∧
RESULTS AND DISCUSSION
Synthesis and Characterization of [Ir(O N C O)(L)(L′)].
■
Synthesis of Ligands H L1−H L3. We have recently
In attempts to prepare Ir(III)-L2 complexes containing axial
3
3
∧
∧
∧
∧
∧
∧
developed a series of tetradentate dianionic O N C N ligands
isocyanide (CNAr) ligands, [Ir(O N C O)(CNAr)-
1
7
18
for the preparation of strongly emissive Pt(II) and Pd(II)
complexes. In the current study, we designed a series of TDTA
(NH Ar)] (17 and 18) were unexpectedly obtained as the
2
major products (Scheme 4), both of which contain an
isocyanide and an aniline axial ligand. The aniline ligand is
conceived to come from hydrolysis of the isocyanide ligand.
∧
∧ ∧
ligands, H -O N C O (H L1−H L3), by replacing the pyridyl
3
3
3
∧
∧ ∧
ring of H -O N C N with a phenol moiety (Scheme 1). The
2
Scheme 1. Synthesis of Tetradentate Ligands H L1−H L3
3
3
B
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Article
Scheme 2. Synthesis and Structures of Complexes 1−14
Scheme 3. Synthesis and Structures of Complexes 15 and 16
(push) and a π* orbital of the isocyanide CN moiety (pull).
Reactions of 18 with PPh , pyridine, and 1-methylimidazole
3
∧
∧
∧
(
X) generated [Ir(O N C O)(CNAr)(X)] (19−21) in
good yields (77−89%). All complexes were characterized by
various methods, including H NMR, FAB-MS, and elemental
1
analysis.
1
Variable-Temperature H NMR of Complexes 8 and
∧
∧ ∧
1
9. It is worth noting that although the [Ir(O N C O)] motif
is ruffled, as revealed by the crystal structure of 9 (vide infra),
1
the H NMR spectra of 1, 3, and 9 show singlet peaks
attributed to the N−CH groups on the axial 1-methylimidazole
3
(
MeIm) ligands, revealing the fluxional behavior of the
∧
∧ ∧
O N C O ligand and fast rotation of the axial ligands around
the Ir−N(MeIm) bonds in solution. Instead, the H NMR
1
spectra of 8 and 14 show broad singlet peaks at around 3.52
ppm, assigned to the methyl groups on the NHC ligands; the
peak broadening effect is suggestive of a moderate rotation rate
of the axial NHC ligands on the NMR time scale, possibly due
Scheme 4. Synthesis and Structures of Complexes 17−21
to the close proximity of the N−CH groups to the distorted Ir-
3
∧
∧ ∧
O N C O plane. We thus examined the temperature effect on
the H NMR spectrum of 8. As shown in Figure 1, when the
1
temperature was lowered, the N−CH signal at 3.56 ppm for 8
3
first becomes a broader peak. Further lowering of the
20
Stynes and co-workers reported that the isocyanide ligand of
[
Fe(Pc)(CNR) ] (Pc = phthalocyanine) undergoes ligand
2
dissociation as a result of the strong trans effect of the
isocyanide ligand in solution. According to their findings, the
dissociation rate of isocyanide increases by 10 -fold by changing
4
1
the trans axial amine ligand to an isocyanide ligand. The H
NMR signal of a primary amine coordinated to a metal ion
usually appears as a singlet. However, the signals for the two
N−H protons in 17 appear as two doublets at 5.32 and 4.57
ppm. The split of the N−H signals is attributable to a restricted
rotation of the Ir−N(aniline) bond caused by a push−pull
interaction involving hyperconjugation of an NH σ orbital
1
Figure 1. Variable-temperature H NMR spectra of 8 in CDCl . The
3
asterisk denotes the residual solvent peak, and △ denotes the split
signals of the N−CH protons.
3
C
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Article
temperature induces four discernible peaks at 4.60, 3.61, 3.54,
and 2.41 ppm, revealing a slower rotation of the NHC ligands.
On the other hand, in the examined temperature range, the
chemical shifts for aromatic protons only show minor changes,
planes of 23.45°. The iridium atom resides essentially on the
plane defined by the four coordinating O, N, C, and O atoms
with a very small deviation from the plane of 0.02 Å. The
Ir(1)−O(1) distance of 2.092(4) Å is slightly longer than that
of Ir(1)−O(2) (2.030(3) Å), due to the trans effect of the
deprotonated C-donor atom. The chelate angles around the
∧
∧ ∧
suggesting that O N C O is minimally affected by temperature
1
changes. The H NMR spectra of 19 at a wide range of
∧
∧
∧
iridium atom within the O N C O plane are 83.3−94.30°. The
temperatures (220−363 K) were also found to show similar
changes (Figure S1 in the Supporting Information).
Ir−N(ax-py) distances of 2.041(4) and 2.047(4) Å are similar
1
2c
to those found in [Ir(corrole)(Py) ] (2.05−2.06 Å). The two
Thermal Stability. Thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) experiments of 4 and 8
were performed under N2 (Figure S2 in the Supporting
Information). Complex 4 started to decompose at ca. 275 °C;
the weight loss of ca. 20% reached a plateau at 302 °C. This
decomposition is attributable to the loss of two axial pyridine
ligands (calculated weight loss of 20.3%). The second and third
stages of decomposition started at 330 and 468 °C, respectively.
The second weight loss is likely due to the loss of the tolyl
moiety at the peripheral position. The last decomposition stage
2
axial pyridine ligands bind to the Ir(III) ion with their planes
∧
∧ ∧
perpendicular to the Ir−O N C O plane, while they are
mutually almost orthogonal with a torsion angle of 88.40°,
which is in stark contrast to the case for the [Ir(corrole)(Py)2]
complex, in which the two pyridine planes are aligned almost in
12c
parallel. The twisting angles of P1 with respect to P2 in 9
and 13 are 13.24 and 20.93°, respectively. The Ir−P distances
of 2.3614(7) and 2.3543(7) Å in 13 are comparable to those of
the reported Ir(III) complexes containing axial triphenylphos-
21
phine ligands. Notably, the torsion angle of 12.22° between
with T > 468 °C is assigned to decomposition of the
d
∧
∧
∧
P2 and P5 in 9 is much smaller than those in 4 (38.18°) and 13
O N C O ligand framework. Complex 8 shows a decom-
position profile different from that of 4, with decomposition
starting at 320 °C, followed by continuous weight loss at
temperatures higher than 440 °C. A glass transition was not
observed prior to the decomposition for each complex.
X-ray Crystal Structures. Crystals of 4, 9, 13, 16, 17, and
(
29.64°). As shown in Figure S3 in the Supporting Information,
the molecules in the crystal structure of 9 are arranged in pairs
and, within each pair, the two molecules interact with each
other with the P1, P2, and P5 rings in a head-to-tail manner.
This intermolecular interaction, with a π−π distance of around
3
.6 Å, probably forces the P5 ring to adopt a relatively coplanar
19−21 suitable for X-ray diffraction analysis were obtained by
configuration with respect to the P2 ring.
slow diffusion of hexane into the respective chloroform
solutions or by slow evaporation of a mixed dichloro-
methane/hexane solution of the corresponding complexes.
The crystallographic data and selected bond distances and
angles are compiled in Tables S1 and S2 in the Supporting
Information, respectively. All of the complexes adopt a
Complex 16 has a μ-O-bridged structure with an inversion
∧
∧
∧
center between the two Ir-O N C O-NHC units (Figure 3).
∧
∧
∧
distorted-octahedral geometry with the O N C O ligand
scaffold in the equatorial plane. To facilitate comparison,
complexes 4, 9 and 13, with the aromatic rings labeled P1−P5,
are presented in Figure 2. Taking 4 as an example, within the
∧
∧
∧
O N C O ligand scaffold, the twists among the P2, P3, and P4
∧
∧
rings are minor, with P2 P3 and P3 P4 torsion angles of 6.46
and 3.82°, respectively. The twist between P1 and P2 is
relatively more distinct, with a torsion angle between the two
Figure 3. ORTEP plot (50% probability ellipsoids) of 16 with
hydrogen atoms omitted for clarity.
The Ir(III)···Ir(III) distance in 16 is 3.336 Å, indicating the
absence of a metallophilic interaction. The two Ir(III)-L3-NHC
fragments are arranged in a slipped manner in which an Ir(III)
center is coordinated by an O atom of the other half motif. The
Ir(1)−O(1′) distance of 2.136(5) Å is shorter than that of
Ir(1)−O(1) at 2.180(4) Å. The Ir−C(NHC) distance of
1
.962(7) Å is shorter than those of other reported Ir(III)-NHC
22
complexes (1.989−2.137 Å). The plane of the NHC ligand
bisects the N(1)−Ir(1)−C(17) angle, likely to avoid close
contact of the methyl groups of the NHC ligand with P1 and
P4.
Figure 2. ORTEP plots (50% probability ellipsoids) of 4, 9, and 13
with hydrogen atoms omitted for clarity.
Complex 17 crystallized in a triclinic space group. In each
unit cell there are four molecules falling into two pairs (see
D
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Figure S4 in the Supporting Information). As depicted in
Figure 4, a notable feature of 17 is the remarkably distorted
the case of 8, three oxidation peaks (E ) appear at 0.10, 0.70,
pa
+/0
and 1.10 V vs Cp Fe (Figure 5). When the oxidative scan
2
Figure 5. Cyclic voltammograms of 8 (1 mM) in CH Cl with
2
2
n
+/0
Bu NPF (0.1 M) as the supporting electrolyte (for Cp Fe , E1/2
=
4
6
2
0
.15 V).
was reversed immediately after the appearance of the first
oxidation peak, a cathodic wave was observed, revealing the
quasi-reversible nature of this oxidation process (E = 0.05 V
1
/2
+/0
vs Cp Fe ). A similar behavior was observed for the other
2
complexes 3−7 (Figure S6), with E for the first oxidation
pa
+/0
couples at 0.05−0.23 V vs Cp Fe . The dependence of the
2
first E value on the electronic properties of the axial ligands
pa
supports the involvement of Ir d orbitals in the HOMOs. The
less σ donating and more strongly π accepting property of
pyridine, 4-tert-butylpyridine, and pyridazine cause the first
oxidation peaks for 4−6 to occur at a more positive potential,
due to their stabilized dπ orbitals. DFT calculations (vide infra)
reveal that HOMOs of 3, 4, and 8 are mainly localized on Ir d
∧
∧ ∧
orbitals and π(P4) orbitals of the O N C O ligand.
Figure 4. ORTEP plots (50% probability ellipsoids) of 17 and 19 with
hydrogen atoms omitted for clarity.
Absorption Spectra. The complexes are classified into four
∧
∧
∧
groups to elucidate the effects of the O N C O ligand and axial
ligands on the photophysical properties of the Ir(III)
complexes; :(1) complexes 3−8 supported by ligand L2 and
with different axial ligands, (2) complexes 1, 3, and 9 bearing
the same axial MeIm ligands with different O N C O ligands,
(3) complexes 2, 8, and 14 bearing the same axial NHC ligand
∧
∧ ∧
saddlelike O N C O skeleton arching toward the isocyanide
ligand. Within each pair, the mutual orientations of the OMe
groups on the isocyanide and the aniline ligands are different
for the two molecules. However, the structural parameters of
the two isomers, including the two Ir−N(aniline) distances
∧
∧ ∧
∧
∧
∧
with different O N C O ligands and 15 as the dimeric
derivative of 14, and (4) heteroleptic complexes 19−21 bearing
ligand L2. The photophysical data are summarized in Table 1.
The absorption spectrum of 3 in CH Cl is shown in Figure 6,
(
(
2.170(5) and 2.171(6) Å) and CN (isocyanide) distances
21
around 1.16 Å) are close to each other. The structure of 19
is also shown in Figure 4, and those of 20 and 21 are shown in
2
2
Figure S5 in the Supporting Information. All three complexes
have saddle-shaped O N C O ligands with the P1 and P4 rings
bent toward the isocyanide ligand, as in 17. The slightly longer
and the absorption spectra of each group of complexes are
shown in Figure S7 in the Supporting Information. In general,
all of the complexes exhibit two intense absorption bands at λ
<300 nm and ∼330−380 nm. The molar absorptivities for
these two bands at their peak wavelengths are in the range of
∧
∧ ∧
Ir−N
, Ir−N , and Ir−P(PPh ) distances of 2.081(4),
MeIm
Py
3
2
.111(3), and 2.383(3) Å in 19−21 are comparable to those in
4
−1
−1
the corresponding complexes 9, 4, and 13 (average distance of
two axial Ir−L bonds: 2.041, 2.044, 2.358 Å), respectively,
which is ascribed to the strong trans effect of isocyanide ligands.
The larger distortion in 20 is evidenced by the 0.112 Å
(1.34−6.60) × 10 M cm . The absorption of ligand H L2
3
in CH Cl is included in Figure 6 for comparison. On the basis
2
2
of the good spectral overlap between the absorption bands of
H L2 and 3 at wavelengths shorter than 300 nm, the strong
3
∧
∧
∧
∧
∧ ∧
displacement of the iridium atom from the O N C O plane,
band at λ <300 nm is assigned to the O N C O ligand-
1
while for 21 the displacement is 0.06 Å and the four chelate
centered (LC) π−π* transition. The band at 366 nm for 3 is
∧
∧
∧
1
angles within the [Ir(O N C O)] core amount to 360°.
Electrochemistry. The electrochemical properties of 3−8
were examined by cyclic voltammetry in CH Cl (0.1 M
also assigned mainly to a LC π−π* transition probably mixed
1
with intraligand charge-transfer ( ILCT) character; the red shift
from that of the free ligand (λmax 323 nm) is due to
deprotonation of the ligand upon coordination.
2
2
n
[
Bu N]PF6 as the supporting electrolyte). All of these
4
complexes show multiple irreversible oxidation waves and no
In comparison to the free ligand, 3 shows an additional
absorption shoulder at around 400 nm (ε ≈ 1 × 10 M cm )
+/0
4
−1
−1
reduction waves at potentials down to −2.5 V vs Cp Fe . In
2
E
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Table 1. Photophysical Data of the Ir(III) Complexes
emission λmax (nm) (τ (μs)); ϕ (%)
a
solution
(298 K)
solid
(298 K)
PMMAc
complex
absorption λmax (nm) (ε (10−3 M−1 cm−1))
a
solid (77 K) glass (77 K)
b
(298 K)
1
283 (16.1), 311 (10.8), 354 (13.4), 393 (6.42), 429 (3.07), 500 (br, 597 (1.6); 18
.42)
596 (0.1)
588 (0.4)
592 (1.2)
607 (0.3)
543 (5.9)
578 (1.4); 16
572 (2.5); 41
601 (1.4); 34
1
2
3
283 (24.2), 295 (sh, 21.6), 339 (br, 14.7), 390 (sh, 6.41), 480 (br,
.15)
586 (0.5); 12
599 (0.2)
531 (5.6),
569
1
261 (40.8), 283 (29.7), 366 (20.3), 394 (sh, 9.98), 438 (3.33), 500 (br, 626 (0.9); 11
.15)
d
577 (4.5)
1
4
5
6
7
263 (44.5), 281 (sh, 36.6), 357 (24.5), 395 (sh, 12.8), 500 (br, 1.06) 611 (0.3); 4
d
572 (0.6)
603 (0.1)
d
552 (3.50)
563 (4.4)
577 (40.5)
591 (0.6); 4
588 (0.5); 6
601 (0.5); 1
584 (1.7); 6
265 (43.2), 279 (38.3), 359 (24.0), 400 (12.3), 500 (br, 1.17)
349 (21.8), 384 (sh, 10.4), 504 (br, 4.2)
603 (0.4); 6
609 (0.1)
d
622 (−); 0.15
303 (sh, 21.8), 338 (sh, 16.0), 364 (18.0), 410 (sh, 7.73), 490 (br,
1.00)
573 (−); <
574 (0.28)
572 (3.26) 561 (5.2)
0.1%
8
9
260 (sh, 43.6), 284 (38.8), 356 (br, 18.1), 392 (sh, 10.7), 480 (br,
.54)
613 (0.4); 12
625 (0.14)
581 (0.1)
634 (0.34)
571 (0.15)
621 (0.77) 559 (4.1)
596 (1.7); 51
608 (1.5); 38
601 (1.8); 48
565 (1.2) 19
1
266 (45.1), 286 (30.4), 368 (22.8), 404 (9.74), 436 (4.55), 504 (br, 631 (1.0); 10
.77)
569 (1.5)
628 (1.2)
571 (1.9)
573 (4.6)
1
1
1
4
5
266 (sh, 42.7), 280 (sh, 36.9), 307 (sh, 18.8), 362 (23.3), 396 (sh,
3.8), 480 (br, 1.47)
626 (0.4); 6
570 (4.03)
1
279 (66.0), 330 (sh, 38.7), 358 (34.7), 383 (sh, 26.7), 411 (sh, 13.2), 581 (0.1); 6
80 (br, 2.33)
557 (3.58),
600
4
1
2
2
9
0
1
260 (58.9), 294 (sh, 34.7), 348 (br, 25.1), 415 (sh, br, 6.26)
259 (46.0), 291 (sh, 30.5), 346 (18.9), 421 (sh, br, 3.83)
262 (sh, 59.2), 295 (sh, 32.2), 329 (19.4), 350 (19.2), 427 (sh, br,
564 (0.3); 4
563 (0.06); 1
564 (−); <0.1
556 (0.21)
558 (0.32)
d
537 (3.6)
556 (2.5)
498 (5.95)
517 (5.09)
545 (1.6); 21
544 (1.2); 11
554 (0.2); 1
554 (28.7) 515 (26.6)
4.78)
a
−5
Determined in degassed CH Cl (∼4 × 10 M). Abbreviations: sh, shoulder; br, broad. The solution emission quantum yields were estimated
2
2
b
c
using [Ru(bpy) ](PF ) in degassed CH CN as a standard (ϕ = 0.062). Determined in 2-methyltetrahydrofuran at 77 K. Doped at 5 wt % into
PMMA film. Not determined due to very weak emission.
3
6
2
3
r
d
follow the trend: 2 (λmax 339 nm) > 8 (λmax 351 nm) > 14 (λmax
67 nm). The progressive bathochromic shift is likely due to
3
∧
∧ ∧
the incorporation of the phenyl ring into the [O N C N]
ligand, thus extending the π conjugation and electron-donating
t
effects of the Bu substituent. Complexes 8 and 15 exhibit
similar absorption profiles while the molar absorptivity of 15 is
essentially twice that of 8, which is in agreement with the
dinuclear structure of 15. Similarly, the heteroleptic complexes
∧
∧ ∧
1
9−21 show intense [O N C O] π−π* absorptions with λ
max
in the region of 346−350 nm and moderately intense ILCT/
MLCT absorptions at around 400 nm (Figure S7D). It is
∧
∧ ∧
noteworthy that the [O N C O] π−π* absorptions of 19−21
are slightly blue shifted from those of their analogues 3, 4, and
7
(357−366 nm), which is likely attributable to the distortion
∧ ∧ ∧
−5
Figure 6. Absorption spectra of H L2 and 3 in CH Cl (4 × 10 M).
of the [O N C O] ligand plane.
3
2
2
Emission. The luminescent properties of selected com-
3
and a weak absorption band at around 500 nm (ε ≈ 1 × 10
plexes in degassed CH Cl2 (298 K), PMMA film, glassy
2
−
1
−1
1
M
cm ). The former is assigned to mixed ILCT and
MLCT (metal-to-ligand charge-transfer) transitions, while the
latter, extending to 600 nm, is tentatively assigned to spin-
forbidden MLCT/ ILCT transitions in view of the closeness
solution (2-methyltetrahydrofuran, 77 K), and the solid state
1
(298 and 77 K) were examined. The emission data of Ir(III)-
∧
∧ ∧
O N C O complexes are summarized in Table 1. Complexes
2, 8, and 14, bearing axial NHC ligands, exhibit orange to red
(λmax 586−626 nm) emission with quantum yields of 6−12%
and lifetimes of 0.1−0.5 μs, in CH Cl . The long emission
3
3
of its energy to its emission. The absorption spectra of 3−8
∧
∧ ∧
exhibit a [O N C O]-centered π−π* absorption with λ in
max
2
2
the range of 349−366 nm and ILCT absorptions extending to
lifetimes reveal their phosphorescent nature. As shown in
Figure 7, the emission energies of 2, 8, and 14 follow the trend
observed for their absorption energies in CH Cl with
5
00 nm (Figure S7A in the Supporting Information). Of note,
6
shows an additional absorption of moderate intensity at 504
nm (ε = 4.2 × 10 M cm ), which is tentatively assigned to
LLCT/MLCT transitions from O N C O/Ir to the highly
electron deficient pyridazine motif.
2
2
3
−1
−1
bathochromic shifts of λmax from 586 nm for 2 to 626 nm for
∧
∧
∧
14. Taking together the structureless emission profile, the
3
3
emissions of 2, 8, and 14 are assigned to be MLCT/ ILCT in
nature. The emission of 15 at λmax 581 nm is hypsochromically
shifted from that of its mononuclear counterpart 8 (λmax 613
nm); this shift is presumably due to replacing one of the two
strong-field NHC ligands that are trans to each other with an
oxygen atom donor. In addition, the severely distorted ligand
As depicted in Figure S7B in the Supporting Information,
complexes 1, 3 and 9, bearing axial MeIm ligands, exhibit the
lowest LC π−π* absorption, with energies in the order of 1
(λ
354 nm) > 3 (λ 366 nm) > 9 (λ 368 nm). The
max max max
lowest LC π−π* absorptions for 2, 8, and 14 (Figure S7C) also
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Article
The emissions of the axially heteroleptic complexes 19−21
are highly similar to λ 564 nm (Figure 9). In the cases of 19
max
−5
Figure 7. Emission spectra of 2, 8, 14, and 15 in CH Cl (4 × 10 M;
2
2
λex 365 nm).
−
5
Figure 9. Emission spectra of 19−21 in CH Cl (4 × 10 M; λ 365
2
2
ex
scaffold in 15, as depicted in Figure 3 for its analogue, 16, is
also expected to contribute to the blue-shifted emission.
The emission spectra of complexes 3, 4, and 6−8 in CH Cl
nm).
2
2
and 20, their emission λmax values are remarkably blue shifted
48−62 nm) in comparison to those of 3 and 4. This is
attributable to the stabilization of the Ir(III) d orbitals through
are shown in Figure 8. These complexes, bearing the same
(
π
π back-bonding interactions with the axial isocyanide ligands. In
spite of having the same emission energy, complex 21 with axial
phosphine ligands displays an emission quantum yield 2 orders
of magnitude lower as well as a very short emission lifetime.
Solvent Effect on Photophysical Properties. The
absorption and emission spectra of complexes 3, 4, and 8 in
a panel of solvents were examined. As depicted in Figure 10,
−5
Figure 8. Emission spectra of 3−4 and 6−8 in CH Cl (4 × 10 M;
ex
2
2
λ
365 nm).
equatorial ligand L2, exhibit emissions with λmax values in the
range of 573−626 nm and quantum yields and lifetimes varying
with the axial ligands (Table 1). Among these complexes, 3 and
8
display the highest emission quantum yields of 11% and 12%,
respectively. The radiative (k = ϕ/τ) and nonradiative (k = (1
r
nr
−
ϕ)/τ) decay rate constants for 3 and 4 were estimated on the
basis of emission efficiency and lifetime. The k values for 3 and
Figure 10. Absorption and emission spectra of 8 in various solvents (2
× 10 M).
r
5
5
−1
−5
4
are comparable, being 1.7 × 10 and 1.3 × 10 s ,
5
−1
respectively. The knr value is 8.5 × 10 s for 3, which is
6
−1
considerably smaller than that of 3.2 × 10 s for 4, suggesting
a profound effect of the axial ligand on the nonradiative decay
process. Complex 6 exhibits a relatively low emission energy
when the solvent was changed from toluene to methanol the
strong absorption of 8 at 400 nm blue-shifts by ∼16 nm. This
negative solvatochromic effect is suggestive of charge-transfer
character for this transition. Similarly, the emission maximum
of 8 displays a blue shift upon increasing the solvent polarity.
The solvent-dependent absorption and emission spectra of 3
and 4 exhibit similar solvatochromic behavior (Figure S8 in the
Supporting Information).
(λmax 622 nm) with a low emission efficiency. Complex 7,
bearing axial PPh ligands, affords the highest emission energy
3
(
0
λmax 573 nm), but with the lowest emission efficiency (ϕ <
.1%). The π-accepting phosphine is envisioned to stabilize the
Ir d orbitals, thereby resulting in a blue shift of the emission
π
energy. Significant decreases in emission quantum yields upon
Emission Properties in PMMA Film. All of the complexes,
except 4 and 5, exhibit significantly enhanced emission
quantum yields in PMMA thin film, in comparison to those
using PPh ligand(s) has also been reported in luminescent
3
23
Ir(III) and Pt(II) complexes.
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in solution (Table 1). Along with the emission quantum yields,
the emission lifetimes also increase noticeably. Such findings
are attributable to decreased nonradiative decay rate as a result
of restricted molecular motion in the rigid solid matrix. In the
present cases, the molecular motions that affect the emission
properties comprise mainly the excited state O N C O ligand
distortion and axial ligand rotation. Suppression of the former
will result in blue-shifted emission as well as decreased
nonradiative decay by eliminating vibronic crossing between
blue-shifted λmax values and lifetimes significantly prolonged, in
comparison to those values measured in solutions at 298 K
(Table 1). For example, in the case of 8, the emission λmax blue
shifts from 613 nm (CH Cl , 298 K) to 559 nm (2-MeTHF, 77
K) and the emission lifetime increases from 0.4 to 3.7 μs. A
similar blue shift of 66 nm, along with a 20-fold increase in
emission lifetime, has also been found for 19. The remarkable
2
2
∧
∧ ∧
blue shifts in emission λ can be attributed to the flexible
max
∧
∧ ∧
O N C O ligand scaffold, which undergoes a severe excited-
state geometry distortion in solution. Thus, the enhanced
emission intensity and increased emission lifetimes in 77 K
glassy solutions for all of these Ir-O N C O complexes are due
to suppression of excited state geometry distortion. In the solid
state at 298 K, all four groups of complexes exhibit weak
phosphorescence with lifetimes in the submicrosecond regime.
The emission λmax values are close to those recorded in solution
the T and S states. Indeed, thin film emission λ values of all
1
0
max
complexes except 7 are blue-shifted (14−25 nm) in comparison
to those in solution (Figure S9 in the Supporting Information).
The rigidochromic shift behavior is exemplified by 8 and 19, as
is evident in the data shown in Figure 11. On the other hand,
∧
∧ ∧
(
Table 1). Upon cooling to 77 K, these complexes in the solid
state display intense emissions accompanied by prolonged
emission lifetimes and slight blue shifts in emission λmax (Table
1
).
Temperature-Dependent Emission Lifetimes. To shed
light on the nonradiative decay pathways of the Ir(III)-
∧
∧ ∧
O N C O complexes, the emission lifetimes of 3 and 8 in
degassed CH Cl at variable temperatures (233−293 K) were
examined. In this temperature range, identical normalized
emission spectra were recorded (Figure S14 in the Supporting
2
2
Information). The typical pathways for deactivating the T state
1
in this temperature range consist of temperature-independent
radiative (k ) and nonradiative (k ) pathways and a temper-
r
nr
ature-dependent nonradiative (NR) channel (k (T); Figure
nr
2
4
1
2A). The temperature-independent nonradiative decay
Figure 11. Emission spectra of (A) 8 and (B) 19 in various states.
rotation of the axial ligands provides an efficient channel for
depopulating the emitting states. The effect of axial ligands on
the emission properties of the present complexes in thin film is
also evident. The highest values of 41−51% were recorded for
complexes 2, 8, and 14, all of which contain axial NHC ligands.
Complexes 7 and 21, having PPh axial ligands, are also weakly
3
emissive in thin film samples with emission quantum yields of
6
% and 1%, respectively.
Solid-State and Glassy-State Emission. The emission
spectra of 8 and 19 in glassy 2-methyltetrahydrofuran (2-
MeTHF) and in the solid state (298 and 77 K) are presented in
Figure 11, and those of the other complexes are given in
Figures S10−S13 in the Supporting Information. In 2-MeTHF
at 77 K, all complexes show intense phosphorescence with
Figure 12. (A) Schematic diagram showing the decay pathways for the
T states of the metal complexes. (B) Temperature-dependent k
1
obs
values of 3 and 8 in degassed CH Cl .
2
2
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Article
pathway is mainly due to the vibronic coupling process between
the T and S states. The temperature-dependent nonradiative
1
0
decay processes are due to thermal population of d−d states
and other possible molecular motions which may affect the
excited-state dynamics. The emission lifetime is affected by the
interplay of these three parameters, as described in eq 1.
1
/τ = k + k + k exp(−E /RT)
(1)
r
nr
1
a1
As depicted in Figure 12B, the emission lifetimes for 3 and 8
decrease with increasing temperature and the temperature
dependence agrees well with an Arrhenius relationship. From
eq 1, the E values for 3 and 8 were found to be 489 and 1578
a1
−1
cm , respectively. Given the low emission energies of 3 and 8,
the NR states are unlikely the ligand-field d−d states. The d--d
energy levels of a series of cyclometalated Ir(III) complexes
−1
have been estimated to lie higher than 20000 cm above their
2
5
ground states. On the other hand, the values of the pre-
exponential term k for the dissociative d−d state of Ir(III)
1
1
1
14 −1 25
complexes are usually in the range of 10 −10 s , which are
7
strikingly larger than the observed values for 3 (k = 1.0 × 10
1
−1
9
−1
s ) and 8 (k = 3.1 × 10 s ). In the present cases, the NR
1
states are probably higher-lying states involving free rotation of
the axial ligands. This attribute is supported by the temper-
1
ature-dependent H NMR experiments showing that rotation
of the axial NHC ligands of 8 is fast at room temperature but
slows down at lower temperatures.
Nanosecond Transient Absorption (ns-TA) and Time-
Resolved Emission (ns-TRE) Spectra. The ns-TA and ns-
TRE spectra of complexes 3 and 8 in degassed CH Cl2
2
Figure 13. Nanosecond (A) time-resolved absorption (ns-TA) and
following 355 nm laser pulse excitation were recorded. As
shown in Figure 13A, the ns-TA of 3 exhibits a broad
absorption at 380−800 nm. In addition, a bleaching band
corresponding to ground-state absorption at 368 nm was
observed. The absorption decay monitored at 413 and 693 nm
showed a dominant single-exponential decay behavior with
lifetimes of ca. 0.7 μs, which are close to the emission lifetime
−5
(
M).
B) emission spectra (ns-TRE) of 3 in degassed CH Cl (2 × 10
2 2
the emission of [Ir(corrole)Py ] in 77 K glassy solution exhibits
2
no blue shift in energy and has a lifetime increased by less than
2-fold in comparison to the values recorded in solutions at
room temperature. The lack of a rigidochromic shift in the
(
0.9 μs; Figure 13B and Figure S15 in the Supporting
Information). This suggests that the ns-TA spectrum can be
emission λmax of [Ir(corrole)Py ] is due to the rigidity of the
2
attributed to the T absorption. The ns-TA spectral profile of 8
corrole ligand that undergoes minimal geometry distortion in
the emissive excited state. This accounts for its slower
nonradiative decay rate in comparison to that of the Ir(III)-
1
at 380−800 nm is similar to that of 3; both feature intense,
broad excited state absorption in the visible to near-infrared
region (Figure S16 in the Supporting Information).
∧
∧ ∧
O N C O complexes.
Comparison with Ir(III)-Corrole Complexes. In 2010,
DFT and TDDFT Calculations. To understand the excited
state properties, density functional theory (DFT) and time-
dependent density functional theory (TDDFT) calculations
Gray and co-workers reported neutral phosphorescent Ir(III)
12c
complexes containing tetradentate trianionic corrole ligands,
which exhibit NIR phosphorescence. However, the emission
quantum yields of Ir(III)-corroles are relatively low, with the
highest value being only 1.2% in degassed toluene. It is
therefore of interest to compare the emission properties of
were performed on 3, 4, and 8. The optimized S structures of
0
3, 4, and 8 are depicted in Figure S17 in the Supporting
Information. The optimized structure of 4 agrees well with its
crystal structure (Table S3 in the Supporting Information). The
calculated Ir−O, Ir−N, and Ir−C bond lengths and the bite
angles are within 0.03 Å and 0.5° deviations from the
experimental data. The key structural parameters of the
optimized structure of 3 are in good agreement with the
crystal structure data of 9, which should have a structure similar
to that of 3. The plots of selected frontier orbitals of 3, 4, and 8
∧
∧ ∧
Ir(III)-corroles and Ir(III)-O N C O complexes. On the basis
of the emission lifetime data, the k values of the Ir(III)-corrole
r
3
−1
complexes were estimated to be on the order of 10 s , 2
∧
∧ ∧
orders of magnitude lower than those of the Ir(III)-O N C O
complexes described in this work as well as most of the other
1
1
cyclometalated Ir(III) complexes. This is in line with the
emissive excited states of Ir(III)-corroles being localized to the
corrole ligand. In contrast, the much higher radiative decay
in their respective S states are shown in Figure 14. In all cases
0
the electron density in the HOMO is mainly localized on the Ir
dπ and π orbitals of the P4 ring with small contributions from
the π(P3) orbitals. The LUMO orbital mainly consists of π*
orbitals of the P2 and P5 rings. The significant involvement of
the peripheral P5 ring in the LUMO is in line with the observed
red shift of absorption and emission λmax values of 3/9 and 8/
14 from those of 1 and 2, respectively. The simulated
∧
∧
∧
rates for the Ir(III)-O N C O complexes are suggestive of
decent MLCT character in their emissive excited states, which
promotes a spin-forbidden T → S transition. In addition, the
1
0
5
−1
k value of 2.0 × 10 s for [Ir(corrole)Py ], which was the
nr
2
best emitter reported in Gray’s work, is smaller than those of
∧
∧
∧
6
−1
Ir(III)-O N C O complexes (∼10 s ). It is noteworthy that
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Figure 14. Plots of calculated frontier orbitals of 3, 4, and 8 at the optimized S geometry.
0
absorption profiles for 3 and 8 are in good agreement with the
experimentally observed absorption spectra (Figures S18 and
S19 in the Supporting Information). As given in Table S4 in the
Supporting Information, the lowest energy S → S absorptions
efficiency (EQE) values of the devices are shown in Figure 15C,
and the performance data are given in Table 2. The best device
performance was obtained using 8 at a doping concentration of
12 wt % with a maximum current efficiency of 12.9 cd A− and
an EQE of 10.5%. Such an EQE is among the high values for
1
0
1
are mainly composed of HOMO → LUMO transitions (94−
26
9
6% contribution). The calculated S → S transition energies
0
1
solution-processed red phosphorescent OLEDs. However,
of 475.6, 466.6, and 466.5 nm for 3, 4, and 8, with relatively
small oscillator strength, correspond well to the relatively weak
absorption at λ >450 nm observed in the absorption spectra of
these two complexes. Therefore, the S → S transitions are
−2
the maximum brightness was only thousands of cd m and the
devices exhibited severe efficiency rolloff at high brightness. For
example, the EQE of the device doped with 8 dropped from
−2
0
1
1
0.5% to 5.3% at a brightness of 1000 cd m . These
intraligand charge transfer (ILCT) combined with a small
amount of MLCT character. For 3 and 8, the higher energy
absorptions with strong oscillator strengths mainly involve
transitions from the HOMO-1, HOMO-2, or HOMO-3 to
LUMO or LUMO+1 orbitals (Table S4). As depicted in Figure
S20 in the Supporting Information, all of the orbitals involved
characteristics are attributable to emission self-quenching effect.
CONCLUSION
■
∧
∧ ∧
A new type of tetradentate trianionic O N C O ligand was
developed and used for the preparation of Ir(III) complexes. A
∧
∧
∧
∧
∧ ∧
are localized to the O N C O ligand, without appreciable
involvement of the metal ion or the axial ligands. Therefore, the
library of new Ir-O N C O complexes bearing different neutral
axial ligands was synthesized and examined with a suite of
spectroscopic techniques. The complexes containing axial NHC
ligands undergo slow dimerization in CH Cl , resulting in μ-O-
∧
∧ ∧
higher energy absorptions are assigned to O N C O ligand-
centered transitions. The assignment is slightly different in the
2
2
case of 4; the calculated intense S → S , S → S , S → S , and
0
2
0
4
0
5
bridged dinuclear complexes. The dimerization is likely caused
S → S transitions at 417, 403, 398, and ∼371 nm for 4 are
0
6
by the strong mutual trans influence of the NHC ligands.
mainly due to transitions involving the HOMO-2, HOMO-1,
LUMO, LUMO+1, LUMO+2, and LUMO+3 orbitals. Since
the HOMO-1, HOMO, and LUMO of 4 are mainly localized
∧
∧
∧
Although the emissive excited states of these Ir-O N C O
3
3
complexes are mainly ILCT/ MLCT in nature with L =
∧
∧
∧
O N C O, the axial ligands have been shown to exert
profound effects on the luminescent properties of the
complexes, presumably through stabilizing the Ir dπ orbitals
and affecting nonradiative decay rates. The complexes
containing axial NHC ligands show superior emission proper-
ties in both degassed solutions and PMMA films at room
temperature. Temperature-dependent emission lifetime experi-
ments suggest that, in the cases of 3 and 8, there are closely
∧
∧ ∧
on the O N C O ligand while LUMO+1 is mainly localized on
the two axial pyridine ligands, the S (n ≥ 2) states are assigned
n
to have mixed IL and LLCT character.
OLED Performances. Complex 8, which has the highest
emission quantum yield in thin film, was used as a
phosphorescent emitter for OLED fabrication. The device
structure was ITO/PEDOT:PSS (30 nm)/PYD-2:Ir(III)
complex (30 nm)/DEPEO (10 nm)/TPBI (40 nm)/LiF (1.2
nm)/Al (150 nm). The molecular structures of PYD-2,
DEPEO, and TPBI are provided in Figure 15A. With increasing
Ir(III)-dopant concentration, the electroluminescence (EL)
λmax slightly red shifted, presumably as the result of
intermolecular π−π interactions (Figure 15B). The EL
spectrum was stable in the operational voltage range of 7−12
V. The CIE chromaticity coordinates of (0.64, 0.36) for the
devices with 8 at 12 and 20 wt % doping levels revealed the
complex to be a promising red emitter. The external quantum
high-lying nonradiative states above the emitting T states,
1
which could be attributed to the rotation of the axial ligands.
Efficient red OLEDs have been fabricated with a high EQE of
1
0.5% by using 8 as the emitter. This novel tetradentate
∧
∧ ∧
trianionic O N C O-type ligand system, complementary to the
well-documented systems featuring combinations of bidentate
and/or tridentate ligands, is conceived to provide access to a
new class of phosphorescent Ir(III) complexes with tunable
photophysical and luminescent properties.
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EXPERIMENTAL SECTION
■
Materials and Reagents. All materials were purchased from
commercial sources and were used without further purification, unless
otherwise indicated. [Ir(Cod)Cl] was prepared from IrCl ·xH O and
2
3
2
cyclooctadiene. Palladium-catalyzed coupling reactions were used for
preparing ligands H L1−H L3. Dry THF was freshly distilled from
3
3
sodium−benzophenone.
Physical Measurements and Instrumentation. The solvents
used for photophysical measurements were of HPLC grade. Elemental
analyses were performed by the Institute of Chemistry at the Chinese
Academy of Sciences, Beijing, People’s Republic of China. Fast atom
bombardment (FAB) mass spectra were obtained on a Finnigan Mat
9
5 mass spectrometer. Electrospray ionization (ESI) mass spectra were
collected on a Finnigan LCQ quadrupole ion trap mass spectrometer
1
19
31
(
samples were dissolved in HPLC grade solvent). H, F, and
P
NMR spectra were recorded on DPX300 and Avance400 Bruker FT-
NMR spectrometers. UV−vis absorption spectra were recorded on a
PerkinElmer Lambda 19 UV/vis spectrophotometer. Steady-state
emission spectra at 298 K were obtained on a Spex 1681 Fluorolog-2
Model F111 spectrophotometer equipped with a Hamamatsu R928
PMT detector. All solutions for photophysical measurements, except
as stated otherwise, were degassed in a high-vacuum line with at least
four freeze−pump−thaw cycles. Emission lifetimes were measured
with a Quanta-Ray Q-switch DCR-3 Nd:YAG pulsed laser system.
Emission quantum yields of solutions were measured using a degassed
acetonitrile solution of [Ru(bpy) ](PF ) (bpy = 2,2′-bipyridine) (ϕ
3
6
2
r
=
0.062) as the standard. Errors for ϕ (10%) are estimated. The
nanosecond time-resolved absorption difference spectra (ns-TA) and
nanosecond time-resolved emission spectra (ns-TRE) were performed
with a LP920-KS Laser Flash Photolysis Spectrometer (Edinburgh
Instruments Ltd., Livingston, U.K.). The excitation source was the 355
nm output (third harmonic) of an Nd:YAG laser (Spectra-Physics
Quanta-Ray Lab-130 pulsed Nd:YAG laser). The signals were
processed by a PC plugin controlled with L900 software. Cyclic
voltammetric measurements were performed with a Princeton Applied
Research electrochemical analyzer (potentiostat/galvanostat Model
2
73A). nBu NPF6 (0.1 M) in CH Cl2 was used as a supporting
4
2
electrolyte for the electrochemical measurements at room temperature.
All solutions used in electrochemical measurements were deaerated
with argon gas. Ag/AgNO3 (0.1 M in MeCN), a glassy-carbon
electrode, and a platinum wire were used as reference electrode,
working electrode, and counter electrode, respectively. The
+
/0
ferrocenium/ferrocene (Cp Fe ) redox couple was used as an
2
internal reference.
X-ray Diffraction Measurements. The X-ray diffraction data
were collected on a Bruker X8 PROTEUM single-crystal X-ray
diffractometer. Raw frame data were integrated using the SAINT
program. Multiscan SADABS was applied for absorption correction.
Figure 15. (A) Structures of PYD-2, TPBI, and DPEPO used in
2
7
OLED fabrication. (B) Normalized EL spectra of the devices doped
The structures were solved by using Olex2 software with employing
−2
28
with 8 at 100 cd m . (C) EQE of the devices doped with 8 vs
the ShelXS structure solution program by direct methods and refined
luminance.
29
with the ShelXL refinement package using least-squares minimiza-
tion. The positions of the hydrogen atoms were calculated on the basis
of the riding mode with thermal parameters equal to 1.2 and 1.5 times
that of the associated C atoms and terminal methyl groups, and these
positions participated in the calculation of the final R indices. In the
final stage of least-squares refinement, all non-hydrogen atoms were
refined anisotropically.
Table 2. Electroluminescence Performances of OLEDs
Doped with 8
current efficiency
−1
(
cd A
)
EQE (%)
@ 1000
concentration max luminance
@ 1000
−2
(cd m−
2
−2
Computational Methods. The density functional theory (DFT)
and time-dependent density functional theory (TDDFT) calculations
(wt %)
)
max
cd m
max
cd m
4
8
2650
2910
3390
5410
4.07
8.34
4.07
4.59
6.46
7.20
2.92
6.78
2.92
3.73
5.25
6.86
30
for complexes 3, 4, and 8 were carried out using the PBE0 functional
31
with a basis set of 6-31+G* for the C, H, and N atoms and a
pseudopotential Stuttgart/Dresden (SDD) basis set for the Ir atom.
1
2
2
0
12.92
10.12
10.49
9.64
32
Solvent effects were examined using the self-consistent reaction field
3
3
(
SCRF) method based on PCM models. The Gaussian 09 package
3
4
was used for the calculations.
K
DOI: 10.1021/acs.organomet.7b00038
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
007, 107, 2388−2410. (l) Gupta, K. C.; Sutar, A. K. Coord. Chem.
ASSOCIATED CONTENT
Supporting Information
this material is available free of charge via the Internet at The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.7b00038.
■
Rev. 2008, 252, 1420−1450. (m) Kleij, A. W. Eur. J. Inorg. Chem. 2009,
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Synthesis and characterization data of the ligands and
complexes, crystal data and selected bond distances and
bond angles, crystal structures, absorption and emission
spectra of selected complexes, and DFT optimized
structures and TDDFT calculated transition energies
for 3, 4, and 8 (PDF)
(
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AUTHOR INFORMATION
Corresponding Author
E-mail for C.-M.C.: cmche@hku.hk.
ORCID
Kai Li: 0000-0002-5869-1006
Author Contributions
D.C. and K.L. contributed equally.
■
*
2
54, 2297−2310. (h) Panda, M. K.; Ladomenou, K.; Coutsolelos, A.
G. Coord. Chem. Rev. 2012, 256, 2601−2627. (i) Carvalho, C. M. B.;
Brocksom, T. J.; de Oliveira, K. T. Chem. Soc. Rev. 2013, 42, 3302−
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§
(k) Gao, W.-Y.; Chrzanowski, M.; Ma, S. Chem. Soc. Rev. 2014, 43,
Notes
5
841−5866. (l) Urbani, M.; Gratzel, M.; Nazeeruddin, M. K.; Torres,
̈
The authors declare no competing financial interest.
́
̈
T. Chem. Rev. 2014, 114, 12330−12396. (m) Auwarter, W.; Ecija, D.;
Klappenberger, F.; Barth, J. V. Nat. Chem. 2015, 7, 105−120.
(n) Tanaka, T.; Osuka, A. Chem. Soc. Rev. 2015, 44, 943−969.
ACKNOWLEDGMENTS
■
(o) Chakraborty, J.; Nath, I.; Verpoort, F. Coord. Chem. Rev. 2016,
This work was supported by the National Key Basic Research
Program of China (No. 2013CB834802), the University Grants
Committee (Area of Excellence Scheme AoE/P-03/08),
SRFDP & RGC ERG Joint Research Scheme (M-HKU702/
3
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26, 135−163.
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planar tetradentate Schiff base ligands in materials science:
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HKSAR Government (ITS/084/14). This work was also
supported by the Guangdong Special Project of the
Introduction of Innovative R&D Teams.
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DOI: 10.1021/acs.organomet.7b00038
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Organometallics
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DOI: 10.1021/acs.organomet.7b00038
Organometallics XXXX, XXX, XXX−XXX