Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted N-Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow OLED Application

Article abstract of DOI:10.1021/acs.inorgchem.9b01678

The synthesis, crystal structure, and photophysics of a series of neutral cyclometalated iridium(III) complexes bearing substituted N-heterocyclic carbene (NHC) ancillary ligands ((CN)₂Ir(R-NHC), where CN and NHC refer to the cyclometalating ligand benzo[h]quinoline and 1-phenylbenzimidazole, respectively) are reported. The NHC ligands were substituted with electron-withdrawing or -donating groups on C4′ of the phenyl ring (R = NO₂ (Ir1), CN (Ir2), H (Ir3), OCH₃ (Ir4), N(CH₃)₂ (Ir5)) or C5 of the benzimidazole ring (R = NO₂ (Ir6), N(CH₃)₂ (Ir7)). The configuration of Ir1 was confirmed by a single-crystal X-ray diffraction analysis. The ground- and excited-state properties of Ir1-Ir7 were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the CN ligand based ¹π,π*/¹CT (charge transfer)/¹d,d transitions and very weak spin-forbidden ³MLCT (metal-to-ligand charge transfer)/³LLCT (ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the ¹π,π*/¹CT/¹d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 that had much shorter lifetimes (≤160 ns), the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540-550 nm at room temperature, with the emitting states being predominantly assigned to ³π,π*/³MLCT states. Although the effect of the substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All complexes also exhibited broad triplet excited-state absorption at 460-700 nm with similar spectral features, indicating the similar parentage of the lowest triplet excited states. The highly emissive Ir2 was used as a dopant for organic light-emitting diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (η_c) of 71.29 cd A^-1, a maximum luminance (L_max) of 32747 cd m^-2, and a maximum external quantum efficiency (EQE) of 20.6%. These results suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.

Full text of DOI:10.1021/acs.inorgchem.9b01678

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Neutral Cyclometalated Iridium(III) Complexes Bearing Substituted
N‑Heterocyclic Carbene (NHC) Ligands for High-Performance Yellow
OLED Application
Bingqing Liu,† Mohammed A. Jabed,† Jiali Guo,‡ Wan Xu,†,§ Samuel L. Brown,∥ Angel Ugrinov,†
Erik K. Hobbie,∥ Svetlana Kilina,† Anjun Qin,‡ and Wenfang Sun*,†
†Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108-6050, United States
‡School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of
China
§Engineering Research Center for Nanomaterials, Henan University, Kaifeng 475004, People’s Republic of China
∥Materials and Nanotechnology Program, North Dakota State University, Fargo, North Dakota 58108-6050, United States
S
* Supporting Information
ABSTRACT: The synthesis, crystal structure, and photophysics of a
series of neutral cyclometalated iridium(III) complexes bearing
substituted N-heterocyclic carbene (NHC) ancillary ligands
((C∧N)2Ir(R-NHC), where C∧N and NHC refer to the cyclo-
metalating ligand benzo[h]quinoline and 1-phenylbenzimidazole,
respectively) are reported. The NHC ligands were substituted with
electron-withdrawing or -donating groups on C4′ of the phenyl ring
(R = NO2 (Ir1), CN (Ir2), H (Ir3), OCH3 (Ir4), N(CH3)2 (Ir5))
or C5 of the benzimidazole ring (R = NO2 (Ir6), N(CH3)2 (Ir7)).
The configuration of Ir1 was confirmed by a single-crystal X-ray
diffraction analysis. The ground- and excited-state properties of Ir1−
Ir7 were investigated by both spectroscopic methods and time-dependent density functional theory (TDDFT) calculations. All
complexes possessed moderately strong structureless absorption bands at ca. 440 nm that originated from the C∧N ligand based
3
1π,π*/1CT (charge transfer)/1d,d transitions and very weak spin−forbidden MLCT (metal-to-ligand charge transfer)/3LLCT
(ligand-to-ligand charge transfer) transitions beyond 500 nm. Electron-withdrawing substituents caused a slight blue shift of the
1π,π*/1CT/1d,d band, while electron-donating substituents induced a red shift of this band in comparison to the unsubstituted
complex Ir3. Except for the weakly emissive nitro-substituted complexes Ir1 and Ir6 that had much shorter lifetimes (≤160 ns),
the other complexes are highly emissive in organic solutions with microsecond lifetimes at ca. 540−550 nm at room
3
temperature, with the emitting states being predominantly assigned to π,π*/3MLCT states. Although the effect of the
substituents on the emission energy was insignificant, the effects on the emission quantum yields and lifetimes were drastic. All
complexes also exhibited broad triplet excited-state absorption at 460−700 nm with similar spectral features, indicating the
similar parentage of the lowest triplet excited states. The highly emissive Ir2 was used as a dopant for organic light-emitting
diode (OLED) fabrication. The device displayed a yellow emission with a maximum current efficiency (ηc) of 71.29 cd A−1, a
maximum luminance (Lmax) of 32747 cd m−2, and a maximum external quantum efficiency (EQE) of 20.6%. These results
suggest the potential of utilizing this type of neutral Ir(III) complex as an efficient yellow phosphorescent emitter.
cases. Consequently, phosphorescent Ir(III) complexes appear
promising as robust emissive dopants for OLED fabrication. In
addition, the emission properties of the Ir(III) complexes can
be readily tuned by modification of the cyclometalating and/or
ancillary ligands because the characteristics of the emitting
triplet excited state depends strongly on the nature of the
ligands.
INTRODUCTION
■
Cyclometalated iridium(III) complexes have drawn tremen-
dous attention in the past three decades due to their facile
synthesis and intriguing photophysical characteristics.1,2 To
date, cyclometalated Ir(III) complexes have been explored for
various applications, such as organic light-emitting diodes
(OLEDs),3,4 light-emitting electrochemical cells (LEECs),5−7
photodynamic therapy (PDT),8−10 nonlinear optics
(NLO),11−16 and luminescent biological labeling.17,18 Due to
the strong spin−orbit coupling effect induced by the Ir(III)
ion, nearly quantitative triplet excited-state formation quantum
yield and phosphorescence efficiency can be obtained in some
N-heterocyclic carbenes (NHCs), a type of strong σ-
donating species, have been widely employed as ligands for
Received: June 5, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b01678
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organometallic complexes.19−29 Because of the strong ligand
field strength of NHCs, the metal−carbene antibonding
orbitals lead to a high-lying lowest unoccupied molecular
orbital (LUMO) in the formed complexes and consequently
the emission energy is blue-shifted and the emission quantum
yield improved.20−24 To date, investigations on organoiridium
complexes containing NHC ligands have been primarily aimed
toward OLED applications.20−23 In the pioneering work
reported by Thompson and co-workers,20 two homoleptic
neutral Ir(III) complexes with NHC ligands, i.e. fac-Ir(pmi)3
(pmi = 1-phenyl-3-methylimidazolin-2-ylidene-C,C2′) and fac-
Ir(pmb)3 (pmb = 1-phenyl-3-methylbenzimidazolin-2-ylidene-
C,C2′), showed higher emission energies but decreased
photoluminescence (PL) efficiencies in comparison to the
greenish emissive fac-Ir(ppy)3 (ppy = 2-phenylpyridyl) and the
sky blue emissive fac-Ir(F2ppy)3 (F2ppy = 2-(2,4-
difluorophenyl)pyridine) complexes. By replacing one NHC
ligand with a diimine ligand, Kessler’s group reported several
ium iodide (L2), 3-methyl-1-phenylbenzimidazol-3-ium iodide
(L3), 1-(4-methoxyphenyl)-3-methylbenzimidazol-3-ium io-
dide (L4), 1-(4-dimethylaminophenyl)-3-methylbenzimida-
zol-3-ium iodide (L5), 5-nitro-3-methyl-1-phenylbenzimida-
zol-3-ium iodide (L6), and 5-dimethylamino-3-methyl-1-
phenylbenzimidazol-3-ium iodide (L7)) and benzo[h]-
quinoline as the C∧N ligand. Selection of benzo[h]quinoline
as the C∧N ligand was due to its rigidity that may potentially
reduce the nonradiative decay rates. Different electron-
donating or -withdrawing substituents were introduced to the
NHC ligand in order to understand the effect of the
substituents at the NHC ligand on the photophysics of these
(C∧N)2Ir(NHC)-type complexes. The structure of Ir1 was
determined by X-ray crystallography to confirm the config-
uration of these types of Ir(III) complexes.
EXPERIMENTAL SECTION
■
Synthesis and Characterizations. All chemicals and solvents
were obtained from commercial sources and used as received without
further purification. Silica gels (60 Å, 230−400 mesh) for column
chromatography were purchased from Sorbent Technology. The
precursor compounds 1 (1-(4-nitrophenyl)benzimidazole),30 2 (1-(4-
cyanophenyl)benzimidazole),31 3 (1-phenylbenzimidazole),30 and 4
(1-(4-methoxyphenyl)benzimidazole)30 were prepared according to
the reported procedures (see Scheme 1 for the synthetic route). The
C∧N ligand benzo[h]quinoline was purchased from Alfa-Aesar, and its
Ir dimer [Ir(benzo[h]quinolone)2(μ-Cl)]2 was prepared by following
the reported method.32 The synthetic schemes for the Ir(III)
complexes Ir1−Ir7 are provided in Scheme 1. The synthetic details
for the precursor compounds 5−7, the ligands L1−L7, and the
complexes Ir1−Ir7, as well as the characterization data, are reported
below.
cationic Ir(N∧N)(NHC)2 complexes with yellow emission in
+
neat films with PL efficiencies of up to 0.83.25
The seminal work on the development of heteroleptic
(C∧N)2Ir(NHC) type of complexes was reported by Bielawski
and co-workers, in which one NHC ligand and two
cyclometalating 2-phenylpyridyl (C∧N) ligands were em-
ployed.27 This complex emitted at 497 nm in MeTHF with
an emission yield of 19%. More recently, Zuo and co-workers22
reported a series of heteroleptic neutral (C∧N)2Ir(NHC)
complexes with various cyclometalating C∧N and NHCs as the
ligands. Among them, in the complexes with 2-(2,4-
difluorophenyl)pyridine (dfppy) as the cyclometalating C∧N
ligand and NHCs bearing electron-withdrawing −F or −CF3
substituents on the phenyl ring or NHCs with the phenyl ring
being replaced with pyridine or pyrimidine rings, the emission
was shifted to a bluer region. The blue-emitting OLED device
utilizing (dfppy)2Ir(CF3-pmi) (CF3-pmi = 1-(3-trifluorome-
thylphenyl)-3-methylimidazole) as the dopant gave a max-
imum current efficiency of 37.83 cd A−1, an external quantum
efficiency of 10.3%, and a maximum luminance of 8709 cd
m−2.
Although the obstacles of synthesis have been solved, further
exploration of the (C∧N)2Ir(NHC)-type Ir(III) complexes
remains dormant. Because of the promising applications of
these types of complexes in OLEDs, we designed and
synthesized seven (C∧N)2Ir(NHC)-type Ir(III) complexes
(Ir1−Ir7, Chart 1) with different electron-donating (−OCH3
or −N(CH3)2) or -withdrawing (−NO2 or −CN) substituents
on C5 of the benzimidazole or C4′ of the phenyl rings at the
NHC ligand (i.e., 3-methyl-1-(4-nitrophenyl)benzimidazol-3-
ium iodide (L1), 1-(4-cyanophenyl)-3-methylbenzimidazol-3-
1
The structures of the ligands L1−L7 were characterized by H
NMR spectroscopy, and the complexes Ir1−Ir7 were characterized by
1H NMR, electrospray ionization mass spectrometry (ESI−HRMS),
and elemental analyses. In addition, the configuration of complex Ir1
was determined by single-crystal X-ray crystallography. The 1H NMR
spectra were recorded on a Varian Oxford-400/Bruker-400
spectrometer in deuterated solvents (i.e. CDCl3 or DMSO-d6) using
tetramethylsilane (TMS) as the internal standard. A Waters Synapt
G2-Si mass spectrometer was used for the mass analyses. Elemental
analyses were carried out by NuMega Resonance Laboratories, Inc. in
San Diego, CA, USA. A Kappa Apex II Duo X-ray diffractometer was
used to collect the X-ray diffraction (XRD) pattern of complex Ir1.
1-(4-Dimethylaminophenyl)benzimidazole (Compound 5).
In a two-neck flask, 4-bromo-N,N-dimethylbenzenamine (2.00 g, 10
mmol), benzimidazole (1.18 g, 10 mmol), CuI (318 mg, 1.67 mmol),
L-proline (383 mg, 3.33 mmol), and K2CO3 (2.76 g, 20 mmol) were
mixed in 15 mL of DMSO and degassed with nitrogen for 30 min.
The resulting mixture was heated to 130 °C for 24 h before it was
partitioned between water and ethyl acetate. The organic layer was
washed with brine, dried over Na2SO4, and concentrated in vacuo.
The crude product was then loaded on a silica gel column and eluted
with ethyl acetate/hexane (v/v 1/1) to afford the product as a yellow
Chart 1. Chemical Structures of (C∧N)2Ir(NHC)
Complexes Ir1−Ir7
1
solid (960 mg, 40%). H NMR (400 MHz, CDCl3): δ 8.03 (s, 1H),
7.86 (dd, J = 6.0, 3.1 Hz, 1H), 7.45 (dd, J = 5.9, 3.3 Hz, 1H), 7.38−
7.27 (m, 4H), 6.85 (d, J = 3.4 Hz, 1H), 6.82 (d, J = 3.3 Hz, 1H), 3.04
(s, 6H).
2,4-Dinitro-N-phenylbenzenamine (Compound a). A mixture
of 2,4-dinitrochlorobenzene (4.05 g, 20 mmol), aniline (2.05 g, 22
mmol), and sodium acetate (1.64 g, 20 mmol) was dissolved in
ethanol (10 mL) and heated to refluxed for 1 h. After the mixture was
cooled to room temperature, the precipitated red-orange crystals were
filtered out, washed with ethanol twice (10 mL each), and dried in
1
vacuo to afford compound a as a red solid (4.70 g, 91%). H NMR
(500 MHz, CDCl3): δ 9.98 (br, 1H), 9.19 (d, J = 2.6 Hz, 1H), 8.17
(dd, J = 9.5, 2.6 Hz, 1H), 7.51 (t, J = 7.8 Hz, 2H), 7.39 (t, J = 7.5 Hz,
1H), 7.31 (d, J = 7.8 Hz, 2H), 7.17 (d, J = 9.5 Hz, 1H).
B
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Scheme 1. Synthetic Routes for Complexes Ir1−Ir7
4-Nitro-N1-phenyl-1,2-benzenediamine (Compound b). A
suspension of compound a (4.70 g, 18.1 mmol) in alcohol (30 mL)
was added to a solution of Na2S (2.82 g, 36.2 mmol) and sulfur
powder (1.15 g, 36.2 mmol) in water (30 mL). The mixture was
heated to reflux for 2 h, diluted with water (30 mL), and cooled to
room temperature. The obtained precipitate was filtered out and
washed with warm water to give dark brown crystals of the title
compound (4.10 g, 99%). 1H NMR (400 MHz, CDCl3): δ 7.77−7.64
(m, 2H), 7.35 (t, J = 7.8 Hz, 2H), 7.15 (d, J = 8.9 Hz, 1H), 7.12−6.97
(m, 3H), 5.76 (br, 1H), 3.70 (br, 2H).
5-Nitro-1-phenylbenzimidazole (Compound 6). A mixture of
compound b (687 mg, 3 mmol) and formic acid (50 mL) was stirred
at 100 °C for 12 h. Then, the reaction mixture was cooled to room
temperature and concentrated in vacuo to afford a solid. The crude
solid was partitioned between ethyl acetate (250 mL) and 30%
ammonium hydroxide (25 mL). The organic layer was collected and
concentrated in vacuo to afford compound 6 as an orange solid (710
mg, 99%). 1H NMR (400 MHz, CDCl3): δ 8.83 (d, J = 2.1 Hz, 1H),
8.33−8.26 (m, 2H), 7.67 (t, J = 7.7 Hz, 2H), 7.63−7.57 (m, 2H),
7.57−7.51 (m, 2H).
1-Phenylbenzimidazol-5-amine (Compound 7). Compound 6
(239 mg, 1 mmol) was dissolved in 10 mL of ethanol/water (v/v 4/
1). Then, sodium dithionite (1.74 g, 10 mmol) was added to the
solution and the resulting mixture was heated to reflux for 4 h. The
reaction mixture was diluted with ethyl acetate and washed with
water. The water layer was extracted several times with ethyl acetate.
The organic layers were combined, dried over sodium sulfate, and
concentrated to yield compound 7 as a white solid (200 mg, 96%). 1H
NMR (400 MHz, CDCl3): δ 8.02 (s, 1H), 7.61−7.52 (m, 2H), 7.50
(t, J = 4.3 Hz, 2H), 7.44 (d, J = 7.3 Hz, 1H), 7.34 (d, J = 8.6 Hz, 1H),
7.16 (d, J = 2.0 Hz, 1H), 6.76 (dd, J = 8.6, 2.1 Hz, 1H), 3.35 (br, 2H).
General Synthetic Procedure for Ligands L1−L7. A mixture
of 1 mmol of the corresponding precursor (compounds 1−7), 1.5
mmol of iodomethane (3 mmol for L7), and 10 mL of
tetrahydrofuran was placed in a pressure tube with a Teflon cap
and heated to 100 °C for 24 h. After the mixture was cooled to room
temperature, the resulting solid was filtered out, washed with
tetrahydrofuran, and dried in vacuo.
L1. A white solid was obtained as the target compound (358 mg,
94%). 1H NMR (400 MHz, DMSO-d6): δ 10.25 (s, 1H), 8.60 (d, J =
9.2 Hz, 2H), 8.18 (d, J = 7.6 Hz, 1H), 8.13 (d, J = 9.1 Hz, 2H), 7.97−
7.90 (m, 1H), 7.85−7.68 (m, 2H), 4.20 (s, 3H).
L2. A white solid was obtained as the target compound (336 mg,
93%). 1H NMR (400 MHz, DMSO-d6): δ 10.19 (d, J = 0.6 Hz, 1H),
8.33−8.23 (m, 2H), 8.21−8.12 (m, 1H), 8.05 (d, J = 8.5 Hz, 2H),
7.92 (dd, J = 8.2, 0.8 Hz, 1H), 7.84−7.67 (m, 2H), 4.18 (s, 3H).
L3. A white solid was obtained as the target compound (218 mg,
65%). 1H NMR (400 MHz, DMSO-d6): δ 10.12 (s, 1H), 8.16 (d, J =
8.2 Hz, 1H), 7.87−7.68 (m, 8H), 4.17 (s, 3H).
L4. A white solid was obtained as the target compound (245 mg,
67%). 1H NMR (400 MHz, DMSO-d6): δ 10.02 (s, 1H), 8.13 (d, J =
8.1 Hz, 1H), 7.80−7.66 (m, 5H), 7.28 (d, J = 8.9 Hz, 2H), 4.16 (s,
3H), 3.89 (s, 3H).
L5. A white solid was obtained as the target compound (349 mg,
1
92%). H NMR (400 MHz, DMSO-d6): δ 9.95 (s, 1H), 8.12 (d, J =
8.1 Hz, 1H), 7.82−7.64 (m, 3H), 7.56 (d, J = 9.0 Hz, 2H), 6.96 (d, J
= 9.0 Hz, 2H), 4.14 (s, 3H), 3.03 (s, 6H).
L6. A yellow solid was obtained as the target compound (127 mg,
33%). 1H NMR (400 MHz, DMSO-d6): δ 10.40 (s, 1H), 9.21 (d, J =
2.1 Hz, 1H), 8.52 (dd, J = 9.2, 2.1 Hz, 1H), 8.06 (d, J = 9.2 Hz, 1H),
7.86−7.73 (m, 5H), 4.28 (s, 3H).
L7. A white solid was obtained as the target compound (281 mg,
1
71%). H NMR (400 MHz, DMSO-d6): δ 9.83 (s, 1H), 7.80−7.68
(m, 5H), 7.62 (d, J = 9.2 Hz, 1H), 7.16 (dd, J = 9.3, 2.2 Hz, 1H), 7.12
(d, J = 2.1 Hz, 1H), 4.07 (s, 3H), 3.06 (s, 6H).
General Synthetic Procedure for Complexes Ir1−Ir7. The
corresponding ligand L1−L7 (0.06 mmol), [Ir(benzo[h]-
quinoline)2(μ−Cl)]2 (35 mg, 0.03 mmol), and Ag2O (28 mg, 0.12
mmol) were dissolved in 10 mL of 1,2-dichloroethane, and the
reaction mixture was heated to reflux in the dark for 12 h (1 h for
Ir6). The solution was then cooled to room temperature. After
removal of the solvent under reduced pressure, purification by silica
C
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gel column chromatography with dichloromethane and hexane (v/v
1/1) as eluent afforded a yellow solid as the target complex.
Ir1: 40 mg (yield: 83%). 1H NMR (400 MHz, CDCl3): δ 8.29 (d, J
= 5.4 Hz, 1H), 8.24 (d, J = 5.4 Hz, 1H), 8.19 (d, J = 8.1 Hz, 1H), 8.09
(d, J = 8.0 Hz, 1H), 8.05 (d, J = 7.9 Hz, 1H), 7.96−7.87 (m, 2H),
7.80 (d, J = 6.4 Hz, 1H), 7.78 (d, J = 6.4 Hz, 1H), 7.62−7.48 (m,
3H), 7.46−7.39 (m, 2H), 7.38−7.31 (m, 2H), 7.28 (s, 1H), 7.24−
7.04 (m, 4H), 6.71 (d, J = 6.8 Hz, 1H), 6.35 (d, J = 7.1 Hz, 1H), 3.28
(s, 3H). ESI-HRMS (m/z): calcd for [C40H26N5O2Ir + H]+,
802.1796; found, 802.1772. Anal. Calcd for C40H26N5O2Ir: C,
59.99; H, 3.27; N, 8.74. Found: C, 59.96; H, 3.62; N, 8.73.
(m/z): calcd for [C42H32N5Ir + H]+, 800.2367; found, 800.2357.
Anal. Calcd for C42H32N5Ir·0.9C6H14·4H2O: C, 60.02; H, 5.59; N,
7.38. Found: C, 59.89; H, 5.98; N, 7.76.
Photophysical Studies. The solvents used for the photophysical
studies are spectrophotometric grade and were purchased from Alfa
Aesar Co., Inc. The ultraviolet−visible (UV−vis) absorption spectra
of Ir1−Ir7 were recorded on a Varian Cary 50 spectrophotometer.
The steady-state emission spectra of Ir1−Ir7 were collected on a
HORIBA FluoroMax 4 fluorometer/phosphorometer. The absolute
emission quantum yields (Φem) were determined via a fiber coupled
Ocean Optics integrating sphere. A broad white light LED was used as
the excitation light source, which was connected to a set of Delta
linear-variable filters (LVFs) and then to the integrating sphere via a
fiber. The excitation wavelength was narrowed and positioned at 450
nm using the LVFs. Ocean Optics UV−vis QE65000 and Ocean
Optics NIRQ512 spectrometers were coupled through a bifurcated
fiber to the integrating sphere, which provides an effective detection
range from 350 to 1700 nm. Each sample solution was degassed and
placed in an anaerobic environment prior to the measurement.
The nanosecond transient absorption (TA) spectra and decays,
Ir2: 26 mg (yield: 56%). 1H NMR (400 MHz, CDCl3): δ 8.28 (d, J
= 5.4 Hz, 1H), 8.26 (d, J = 5.5 Hz, 1H), 8.19 (d, J = 7.7 Hz, 1H), 8.11
(d, J = 8.0 Hz, 1H), 8.08 (d, J = 8.1 Hz, 1H), 7.92 (d, J = 8.3 Hz, 1H),
7.83 (d, J = 8.8 Hz, 1H), 7.80 (d, J = 8.7 Hz, 1H), 7.59 (d, J = 5.3 Hz,
1H), 7.56 (d, J = 5.2 Hz, 1H), 7.49−7.31 (m, 5H), 7.29 (s, 1H),
7.24−7.03 (m, 5H), 6.72 (d, J = 6.9 Hz, 1H), 6.32 (d, J = 7.1 Hz,
1H), 3.30 (s, 3H). ESI-HRMS (m/z): calcd for [C41H26N5Ir + H]+,
782.1897; found, 782.1872. Anal. Calcd for C41H26N5Ir: C, 63.06; H,
3.36; N, 8.97. Found: C, 63.32; H, 3.75; N, 8.89.
Ir3: 9 mg (yield: 20%). 1H NMR (400 MHz, CDCl3): δ 8.36 (d, J
= 4.5 Hz, 1H), 8.30 (d, J = 4.7 Hz, 1H), 8.22 (d, J = 8.2 Hz, 1H), 8.04
(d, J = 7.8 Hz, 1H), 8.00 (d, J = 7.9 Hz, 1H), 7.89 (d, J = 7.8 Hz, 1H),
7.76 (d, J = 8.7 Hz, 2H), 7.54 (d, J = 8.8 Hz, 1H), 7.51 (d, J = 8.7 Hz,
1H), 7.40−7.31 (m, 3H), 7.28 (d, J = 8.1 Hz, 1H), 7.23 (d, J = 8.0
Hz, 1H), 7.17−7.00 (m, 5H), 6.81 (d, J = 5.7 Hz, 1H), 6.74 (t, J = 6.2
Hz, 2H), 6.37 (d, J = 7.0 Hz, 1H), 3.27 (s, 3H). ESI-HRMS (m/z):
calcd for [C40H27N4Ir + H]+, 757.1945; found, 757.1938. Anal. Calcd
for C40H27N4Ir·0.1CH2Cl2: C, 63.01; H, 3.59; N, 7.33. Found: C,
62.77; H, 3.30; N, 7.26.
Ir4: 10 mg (yield: 4%) (by scaling up all of the reagents and solvent
to 5-fold of the scale described in the above general synthetic
procedure). 1H NMR (400 MHz, CDCl3): δ 8.39 (d, J = 4.9 Hz, 1H),
8.30 (d, J = 5.7 Hz, 1H), 8.16 (d, J = 8.3 Hz, 1H), 8.03 (d, J = 7.1 Hz,
1H), 8.00 (d, J = 7.4 Hz, 1H), 7.80 (d, J = 8.7 Hz, 1H), 7.76 (d, J =
3.0 Hz, 1H), 7.74 (d, J = 3.2 Hz, 1H), 7.53 (d, J = 8.9 Hz, 1H), 7.50
(d, J = 8.7 Hz, 1H), 7.40−7.30 (m, 3H), 7.25−6.95 (m, 6H), 6.75 (d,
J = 6.8 Hz, 1H), 6.55 (dd, J = 8.6, 2.9 Hz, 1H), 6.40 (d, J = 3.0 Hz,
1H), 6.37 (d, J = 7.0 Hz, 1H), 3.48 (s, 3H), 3.25 (s, 3H). ESI-HRMS
(m/z): calcd for [C41H29N4OIr + H]+, 787.2051; found, 787.2041.
triplet excited-state quantum yields (ΦT), and triplet lifetimes (τTA
)
were measured on the laser flash photolysis spectrometer (Edinburgh
LP920) in the nitrogen-purged toluene solutions. The third harmonic
output (355 nm) of a Quantel Brilliant Nd:YAG laser with 4.1 ns
pulse duration and 1 Hz repetition rate was utilized as the excitation
source. Each sample solution was purged with nitrogen for 40 min
before the measurement. The triplet excited-state molar extinction
coefficients (εT) at the TA maxima were deduced using the singlet
depletion method.33 To obtain the triplet quantum yields, the relative
actinometry method34 was applied using a silicon naphthalocyanine
(SiNc) in benzene solution (ε590 nm = 70000 L mol−1 cm−1, ΦT
=
0.2)35 as the reference.
DFT Calculations. All calculations were performed using the
Gaussian16 software package.36 The ground-state geometries of the
complexes were optimized using density functional theory (DFT).37
The long-range corrected functional CAM-B3LYP38 was used with a
mixed basis set of LANL2DZ39 for Ir(III) and 6-31G*40 for all other
atoms. Solvent effects were incorporated in the calculations by using
the conductor like polarizable continuum model (CPCM) reaction
field method.41 Toluene was chosen as the solvent for consistency
with the experimental studies.
Anal. Calcd. for C41H29N4OIr·0.2CH2Cl2·0.1C6H14 (C6H14
=
hexane): C, 61.87; H, 3.83; N, 6.90. Found: C, 62.10; H, 3.67; N,
6.51.
A linear response time dependent DFT (TDDFT)42 methodology
was adopted to calculate 70 excited states in toluene using the same
functional and basis set as those used for the ground-state
calculations. Although our previous joint experimental and computa-
tional studies demonstrated that the hybrid functional PBE043 was
capable of accurately representing the excited properties of many
Ir(III) complexes,12,44 PBE0 failed in reproducing the correct trends
for the complexes studied in this work, especially for complexes
containing the nitro substituent. It has been discussed in the literature
that long-range corrected functionals, such as CAM-B3LYP, more
accurately describe the localization/delocalization properties of charge
transfer states in the donor−acceptor molecules than hybrid
functionals, despite the overall overestimation of the excited
energies.45,46 As such, we chose CAM-B3LYP for our calculations,
while we applied a constant shift of −0.82 eV to correct the blue shift
and align the calculated absorption spectra with the experimental
spectra. To construct the absorption spectra, each optical transition
obtained from TDDFT calculations was broadened by the Gaussian
function with a width of σ = 0.08 eV, which well reproduces the
thermal broadening of the experimental spectra.
Ir5: 10 mg (yield: 4%) (by scaling up all of the reagents and solvent
to 5-fold of the scale described in the above general synthetic
procedure). 1H NMR (400 MHz, CDCl3): δ 8.44 (d, J = 4.4 Hz, 1H),
8.32 (d, J = 5.5 Hz, 1H), 8.15 (d, J = 8.1 Hz, 1H), 8.02 (d, J = 8.0 Hz,
1H), 7.98 (d, J = 7.3 Hz, 1H), 7.82−7.70 (m, 3H), 7.53 (d, J = 8.8
Hz, 1H), 7.49 (d, J = 8.7 Hz, 1H), 7.34 (d, J = 7.9 Hz, 3H), 7.22 (d, J
= 6.6 Hz, 2H), 7.17−7.09 (m, 3H), 7.06 (dd, J = 8.0, 5.4 Hz, 1H),
6.81 (d, J = 7.0 Hz, 1H), 6.49−6.36 (m, 2H), 6.32 (d, J = 2.9 Hz,
1H), 3.25 (s, 3H), 2.55 (s, 6H). ESI-HRMS (m/z): calcd for
[C42H32N5Ir + H]+, 800.2367; found, 800.2333. Anal. Calcd for
C42H32N5Ir·0.1CH2Cl2: C, 62.62; H, 4.02; N, 8.67. Found: C, 62.59;
H, 3.85; N, 9.05.
1
Ir6: 23 mg (yield: 48%). H NMR (400 MHz, CDCl3): δ 8.48−
8.22 (m, 4H), 8.19 (s, 1H), 8.10 (d, J = 7.8 Hz, 1H), 8.07 (d, J = 7.9
Hz, 1H), 7.89 (d, J = 7.9 Hz, 1H), 7.81 (d, J = 8.7 Hz, 2H), 7.58 (d, J
= 8.8 Hz, 1H), 7.55 (d, J = 8.8 Hz, 1H), 7.41 (t, J = 8.2 Hz, 2H),
7.25−7.02 (m, 5H), 6.91−6.79 (m, 2H), 6.76 (d, J = 6.9 Hz, 1H),
6.33 (d, J = 6.8 Hz, 1H), 3.39 (s, 3H). ESI-HRMS (m/z): calcd for
[C40H26N5O2Ir + H]+, 802.1796; found, 802.1785. Anal. Calcd. for
C40H26N5O2Ir·0.1C6H14·3H2O: C, 56.47; H, 3.90; N, 8.11. Found: C,
56.38; H, 4.20; N, 8.06.
Emission energies were calculated by the ΔSCF method.47−49 The
triplet ground-state geometry was optimized (using the CAM-B3LYP
functional and LAN2DS/6-31G* basis set) and then used for
TDDFT calculations of the vertical triplet excitations.
To better understand the nature of the excited states, natural
transition orbitals (NTOs)50 were calculated. NTOs are compact
representations of the density of the excited electron−hole pair using
diagonalized transition density matrices obtained from the TDDFT
Ir7: 19 mg (yield: 40%). 1H NMR (400 MHz, CDCl3): δ 8.39 (d, J
= 5.4 Hz, 1H), 8.36 (d, J = 5.3 Hz, 1H), 8.07−8.00 (m, 2H), 7.83 (d,
J = 7.7 Hz, 1H), 7.78 (d, J = 8.7 Hz, 2H), 7.55 (d, J = 8.7 Hz, 1H),
7.53 (d, J = 8.8 Hz, 1H), 7.39 (d, J = 7.7 Hz, 1H), 7.36 (d, J = 7.8 Hz,
1H), 7.23−6.99 (m, 6H), 6.89−6.72 (m, 4H), 6.47 (d, J = 2.1 Hz,
1H), 6.41 (d, J = 7.0 Hz, 1H), 3.22 (s, 3H), 3.00 (s, 6H). ESI-HRMS
D
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Figure 1. ORTEP (Oak Ridge Thermal Ellipsoid Plot)54 diagram displaying thermal ellipsoids at the 50% probability level and atom labels for Ir1.
Hydrogen atoms are omitted for clarity. (a) and (b) are viewed from different angles.
calculations. All NTOs were visualized by VMD51 code using 0.02
isosurface resolution.
X-ray Crystallography. The single crystal of Ir1 was grown by
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Ir1
Ir1−N3
Ir1−N4
Ir1−C2
2.059(5)
2.061(5)
2.054(5)
Ir1−C3
Ir1−C4
Ir1−C6
2.059(5)
2.073(6)
2.086(6)
slow diffusion of hexane into a dilute dichloromethane solution of Ir1.
The crystallographic data were collected on a Bruker Kappa Apex II
Duo X-ray diffractometer with an Apex 2 CCD area detector at
200.01 K. The structure was solved with the ShelXT52 structure
solution program embedded in Olex2,53 using Intrinsic Phasing, and
refined with the ShelXL refinement package with least-squares
minimization.
N4−Ir1−C4
N4−Ir1−C6
N3−Ir1−N4
N3−Ir1−C3
N3−Ir1−C4
N3−Ir1−C6
C2−Ir1−N4
C2−Ir1−N3
80.6(2)
97.3(2)
169.6(19)
80.1(2)
92.2(2)
89.9(2)
91.5(2)
97.3(2)
C2−Ir1−C3
C2−Ir1−C4
C2−Ir1−C6
C3−Ir1−N4
C3−Ir1−C4
C3−Ir1−C6
C4−Ir1−C6
171.9(2)
101.6(2)
77.7(2)
92.0(2)
86.2(2)
94.6(2)
177.8(2)
Fabrication and Characterization of the OLEDs. The
multilayer OLEDs were fabricated by the vacuum-deposition method.
The 95 nm indium tin oxide (ITO) coated glass substrates with a
sheet resistance of 15−20 Ω sq−1 were subjected to a routine cleaning
process of acetone, isopropyl alcohol, detergent, deionized water, and
isopropyl alcohol in an ultrasonic bath and treated with O2 plasma for
10 min. Organic layers and the cathode were sequentially deposited
on the ITO-coated glass substrates by thermal evaporation under high
vacuum (<5 × 10−4 Pa). The deposition rates were 1.0 Å s−1 for
organic layers, 0.1 Å s−1 for the LiF layer, and 3−5 Å s−1 for the Al
cathode, respectively. The active area of each device was 9 mm2. The
electroluminescence spectra (EL), the current density−voltage
characteristics (J−V), and the current density−voltage−luminance
curves (J−V−L) of the OLEDs were detected on a Photo Research
SpectraScan PR-745 spectroradiometer and a Keithley 2450 source
meter, and the data were recorded simultaneously. All device
characterization was carried out at room temperature under ambient
laboratory conditions. The device configuration is ITO/HATCN (5
nm)/TAPC (25 nm)/TCTA (5 nm)/CBP:10 wt % Ir2 (20 nm)/
TmPyPB (55 nm)/LiF (1 nm)/Al, where 1,4,5,8,9,11-hexaazatriphe-
nylenehexacarbonitrile (HATCN) is the hole injection layer (HIL), 1-
bis[4-[N,N-di-4-tolylamino]phenyl]cyclohexane (TAPC) works as
the hole-transporting layer (HTL), tris(4-carbazoyl-9-ylphenyl)amine
(TCTA) serves as the electron-blocking layer (EBL), 4,4′-bis(9H-
carbazol-9-yl)-1,1′-biphenyl (CBP) functions as the host, 1,3,5-tris(m-
pyrid-3-ylphenyl)benzene (TmPyPB) acts as the electron-trans-
porting layer (ETL) and the hole-blocking layer (HBL), and LiF is
the electron injection layer (EIL). The molecular structures of
HATCN, TAPC, TCTA, CBP, and TmPyPB are shown in Figure S1
in the Supporting Information.
Figure 2. Experimental UV−vis absorption spectra of complexes Ir1−
Ir7 at room temperature in toluene. The inset is the expansion of the
spectra in the region of 415−620 nm.
crystallographic data are compiled in Table S1 in the
Supporting Information.
Figure 1 shows the top-down and side views of the crystal
structure for Ir1. The selected bond lengths and bond angles
around the Ir(III) center are presented in Table 1. The crystal
structure clearly indicates that the N atoms on the C∧N ligands
coordinate to the metal center in a trans configuration with a
RESULTS AND DISCUSSION
■
Single-Crystal Structure of Ir1. A suitable single crystal
of Ir1 for X-ray crystallographic analysis was obtained by slow
diffusion of hexane into a dilute dichloromethane solution of
Ir1 at room temperature. The crystal structure of Ir1 was
determined by X-ray diffraction at 200.01 K, and the obtained
E
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Table 2. Photophysical Properties of Complexes Ir1−Ir7
d
λ
/nm (τem/ns);
λem/nm
(77 K)
λT1‑Tn/nm
a
emb
c
c
e
λabs/nm (log ε)
Φem (room temp)
kr/s−1
knr/s−1
8.86 × 107
(τTA/ns, log εT1−Tn); ΦT
f
g
Ir1 290 (4.60), 340 (4.41), 386 (4.11), 433 (3.82)
Ir2 290 (4.59), 330 (4.44), 387 (4.12), 436 (3.82)
544 (45); 0.0027
540 (5300); 0.16
2.40 × 105
1.21 × 105
−
532 (50, − ); −
g
6.34 × 105 522, 560, 608
502 (5700, − ); −
(sh.)
Ir3 290 (4.57), 349 (4.35), 398 (4.11), 442 (3.81)
Ir4 290 (4.59), 349 (4.33), 397 (4.04), 444 (3.79)
554 (3520); 0.15
549 (2640); 0.064
1.85 × 105
1.28 × 105
1.05 × 106 531, 568, 617
547 (4050, 4.45); 0.23
547 (2430, 4.53); 0.19
(sh.)
1.87 × 106 519, 551, 597
(sh.)
Ir5 290 (4.65), 326 (4.35), 391 (3.94), 445 (3.64)
Ir6 290 (4.63), 354 (4.41), 390 (4.27), 433 (3.94)
552 (2490); 0.046
545 (160); 0.016
552 (4030); 0.088
1.15 × 105
3.33 × 105
7.04 × 104
2.39 × 106 524, 554
2.05 × 107 528, 566
7.30 × 105 534, 569
550 (2550, 4.57); 0.16
523 (160, 4.37); 0.30
540 (4000, 4.20); 0.31
Ir7 290 (4.63), 315 (4.51), 355 (4.37), 400 (4.05),
443 (3.72)
a
b
Electronic absorption band maxima (λabs) and molar extinction coefficients (log ε) in toluene at room temperature. Room-temperature emission
band maxima (λem) and lifetimes (τem) for Ir1−Ir7 in toluene at a concentration of 1 × 10−5 mol L−1. The absolute quantum yields were measured
c
via the Ocean Optics integrating sphere (λex = 450 nm). The emission signals were integrated in the range of 350−820 nm. Radiative (kr) and
nonradiative decay rates (knr) in toluene calculated by kr = Φem/(ΦTτem) and knr = (1 − Φem)/(ΦTτem), respectively. For Ir3−Ir7, the estimated
d
triplet quantum yields (ΦT) from the transient absorption measurement were used. For Ir1 and Ir2, ΦT was assumed to be 0.25. Emission band
e
maxima in MTHF glassy matrix at 77 K. Nanosecond TA band maxima (λT1‑Tn), triplet excited-state lifetimes (τTA), molar extinction coefficients
f
g
(εT1−Tn), and quantum yields (ΦT) measured in toluene at room temperature. The emission signal was too weak to be recorded. Due to the lack
of a bleaching band, the εT1−Tn values were unable to be estimated by the singlet depletion method. Consequently, the ΦT values could not be
calculated.
bond angle of 169.6°, which is slightly distorted from the
octahedral geometry. The other bond angles around the Ir(III)
center also deviate from the angles for an ideal octahedral
geometry. In comparison with the Ir(III) complex bearing 1-
methyl-3-phenylbenzimidazolylidene and 2-phenylpyridine
ligands,22 similar bond angles and bond lengths were found.
The Ir−C2 and Ir−C6 bonds in Ir1 were 0.010 and 0.025 Å
shorter than the corresponding bonds in the reported complex,
while slightly longer Ir−N3 and Ir−N4 bonds of 0.005 and
0.008 Å, respectively, were found in Ir1 in comparison to those
in the reported complex. These minor differences are likely due
to the rigidity of the cyclometalating benzo[h]quinoline
ligands. Complexation of the NHC ligand with the metal
center prohibited the rotation of the N-phenyl substituent,
resulting in a nearly coplanar configuration (7.80°) between
the phenyl ring and the benzimidazole moiety (Figure S2 in
the Supporting Information).
effect of these complexes in solvents with varied polarities,
which is a characteristic of π,π* transitions (see Figure S3 in
1
the Supporting Information). These assignments are also
confirmed by the TDDFT calculations that will be discussed in
the following paragraph. In addition, all complexes exhibited
very weak absorption tails beyond 500 nm. With reference to
the other reported Ir(III) complexes with NHC ligands,22 we
attribute this tail to spin-forbidden charge transfer transitions
(3MLCT/3LLCT).
To better understand the natures of the electronic
transitions, TDDFT calculations were carried out for
complexes Ir1−Ir7 in toluene. The spectral features and the
trend of the calculated spectra matched the experimental
spectra very well (Figures S4 and S5 in the Supporting
Information). The natural transition orbitals (NTOs) for the
electrons and holes corresponding to the lowest-energy
electronic transitions are provided in Table 3, and the NTOs
corresponding to the transitions contributing to the high-
energy major absorption bands are provided in Tables S2 and
S3 in the Supporting Information. As the NTOs in Table 3
show, the predominant contributors to the low-energy
absorption band at ca. 440 nm are the C∧N ligand-localized
1π,π* transition, admixing with a 1MLCT (metal-to-ligand
charge transfer) transition. In addition, 1LLCT (ligand-to-
ligand charge transfer), 1LMCT (ligand-to-metal charge
Electronic Absorption. The electronic absorption spectra
of complexes Ir1−Ir7 were measured in toluene, dichloro-
methane, tetrahydrofuran, and acetonitrile. The spectra in
toluene are displayed in Figure 2, and the absorption band
maxima and molar extinction coefficients are compiled in
Table 2. The normalized absorption spectra in the other
solvents are presented in Figure S3 in the Supporting
Information. The concentration-dependence study from 5 ×
10−6 to 1 × 10−4 mol L−1 showed that the absorption of these
complexes in toluene obeyed Beer’s law without forming
ground-state aggregates. All complexes possessed intense
absorption bands (ε ≈ 104 L mol−1 cm−1) in the wavelength
range of 290−420 nm and a broader and weaker band (ε ≈ 103
L mol−1 cm−1) at 420−500 nm. Taking into account the
spectral features and the molar extinction coefficients, we
tentatively assign the 290−420 nm bands predominantly to
1
transfer), and d,d transitions also made minor contributions.
The NTOs indicate that the NHC ligand essentially is not
involved in the low-energy transitions. Therefore, introducing
electron-donating or -withdrawing substituents to the NHC
ligand has a minor effect on the low-energy absorption band at
ca. 440 nm. Nevertheless, electron-donating substituents, such
as OCH3 and N(CH3)2 groups, induced a slight red shift of the
low-energy absorption band in Ir4, Ir5, and Ir7 regardless of
whether the substituent was introduced at the phenyl ring or at
the benzimidazole motif. In contrast, electron-withdrawing
substituents, such as CN and NO2 groups, caused a slight blue
shift of the low-energy absorption band in Ir1, Ir2, and Ir6.
The NTOs in Table S2 in the Supporting Information show
that the transitions in the region of 380−425 nm have
predominant 1π,π*/1LLCT/1MLCT characters, with the
1
spin-allowed ligand-localized π,π* transitions. For the 420−
500 nm band, the structureless feature suggests charge transfer
transitions. However, the moderate molar extinction coefficient
implies significant contributions of the 1π,π* transitions.
Attribution of the major absorption bands, including the
absorption band centered at 433−445 nm, to predominant
1π,π* transitions is supported by the minor solvatochromic
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Table 3. NTOs of the Holes and Electrons for the Low-Energy Transitions in Ir1−Ir7, Calculated by the TDDFT Method with
the CAM-B3LYP Functional and LANL2DZ/6-31G* Basis Set in Toluene
contribution from the 1LLCT (π(NHC) → π*(C∧N))
transition becoming more significant. Similar to the case for
the lowest-energy absorption band at ca. 440 nm, electron-
donating substituents caused a red shift of the 380−425 nm
band while electron-withdrawing substituents induced a blue
shift. For the high-energy absorption bands at <380 nm, they
emission. The normalized emission spectra and the emission
characteristics in other solvents are provided in Figure S6 and
Table S4 in the Supporting Information, respectively.
Comparison of the experimental and calculated emission
energies of Ir1−Ir7 in toluene is provided in Figure S7 in the
Supporting Information. The emission spectra for all
complexes are broad and featureless, indicative of charge-
transfer characters. On the other hand, the emission energies in
1
are also composed of mixed π,π*/1CT/1d,d transitions, with
the holes or electrons becoming more delocalized and thus the
oscillator strengths being increased.
3
all solvents are quite similar, which is a characteristic of π,π*
Photoluminescence. The emission of complexes Ir1−Ir7
was studied in different solvents at room temperature. The
emission spectra in deaerated toluene are displayed in Figure 3,
and the emission parameters, i.e. the emission quantum yields
(τem) and lifetimes (Φem), are given in Table 2. All complexes
exhibited long-lived (several microseconds except for Ir1 and
Ir6) and oxygen-sensitive bright yellow luminescence with
large red shifts in comparison to their corresponding excitation
wavelength, suggesting the phosphorescent nature of the
emission. Moreover, the emission energies of all complexes
resemble each other, suggesting the similar nature of the
emitting states. However, the longer lifetimes of Ir2 in
comparison to those of the other complexes in each
corresponding solvent along with the clearer vibronic structure
3
in toluene imply a somewhat larger contribution of the π,π*
character in the emitting state of Ir2.
The NTOs in Table 4 indicate that the holes of the T1 states
in all complexes are delocalized on one of the C∧N ligands and
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Figure 4. Nanosecond transient absorption spectra of complexes Ir1−
Ir7 in deaerated toluene immediately after 355 nm excitation (A355 nm
= 0.4 in a 1 cm cuvette).
Figure 3. Experimental emission spectra of complexes Ir1 (λex = 434
nm), Ir2 (λex = 437 nm), Ir3 (λex = 442 nm), Ir4 (λex = 444 nm), Ir5
(λex = 445 nm), Ir6 (λex = 433 nm), and Ir7 (λex = 444 nm) in
deoxygenated toluene at room temperature.
Table 5. Summary of the Ir2-Based OLED Performance
Table 4. NTOs Contributing to the T1 States of Complexes
Ir1−Ir7 in Toluene, Calculated by the ΔSCF-TDDFT
Method Using the CAM-B3LYP Functional and LANL2DZ/
6-31G* Basis Set
a
b
b
b
b
Von (V)
L (cd/m2)
ηC (cd/A)
ηP (lm/W)
EQE (%)
20.6
2.9
32747
71.29
68.90
a
b
Von is the turn-on voltage at 1 cd/m2. The luminance (L), current
efficiency (ηc), power efficiency (ηP), and EQE are the maximum
values of the device.
Although the emission energies and natures for these
complexes are similar and the NHC ligand is not involved in
the emitting state, the substituents on the NHC ligand still
exerted minor effects on the emission energies. Electron-
withdrawing substituents caused a slight blue shift of the
emission bands in Ir1, Ir2, and Ir6 in comparison to that in
Ir3, while electron-donating substituents had a negligible effect
on the emission energies of Ir4, Ir5, and Ir7. In contrast, the
effects of the substituents at the NHC ligand on the emission
lifetimes and quantum yields are drastic. An examination of the
emission parameters given in Table 2 and Table S4 in the
Supporting Information found that complexes Ir1 and Ir6,
which contain the strongly electron-withdrawing nitro
substituent, exhibited much shorter emission lifetimes and
much weaker emission in comparison to those of the other
complexes regardless of the solvent used. While the electron-
donating substituents OCH3 and N(CH3)2 also reduced the
emission lifetimes and quantum yields in Ir4, Ir5, and Ir7 with
respect to those in Ir3, the reductions were generally more
pronounced in Ir5 and Ir7, which contain the more strongly
electron-donating substituent N(CH3)2. Interestingly, the
longest emission lifetime and strongest emission were found
in Ir2, which bears an electron-withdrawing CN substituent.
The effects of the substituents on the emission lifetimes and
quantum yields can be rationalized by the different influences
of the substituent on the radiative decay rate constant (kr) and
nonradiative decay rate constants (knr) for these complexes.
The kr and knr data given in Table 2 indicate that the different
substituents at the NHC ligand do not change the kr values
significantly (∼4.7-fold), while they alter the knr values by
approximately 140-fold. The CN-substituted complex Ir2
possesses the lowest knr value but a comparable kr value with
a
The emission energies were calculated by ΔSCF method.
the d orbital of the Ir(III) ions, while the electrons are mainly
distributed on the same C∧N ligand. Therefore, the emission of
these complexes predominantly emanates from the
3π,π*/3MLCT (d(Ir) →π*(C∧N)) states.
To further confirm the nature of the emitting states in Ir1−
Ir7, emission of these complexes in a glassy matrix of 2-
methyltetrahydrofuran (MTHF) at 77 K was studied and the
emission band maxima are given in Table 2. A comparison of
the emission spectrum of each complex (except for Ir1, in
which the emission at 77 K was too weak to be monitored) in
fluid MTHF solution at room temperature and in an MTHF
glassy matrix at 77 K is provided in Figure S8 in the Supporting
Information. The emission spectra of these complexes at 77 K
were all blue-shifted and became structured. The vibronic
progressions were in the range of 1033−1410 cm−1, which are
in line with the ring stretching modes of the aromatic rings. In
addition, the thermally induced Stokes shifts for these
complexes were in the range of 489−836 cm−1. These features
are consistent with the predominant π,π*/3MLCT nature of
the emitting states in these complexes.
3
H
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Figure 5. Electroluminescence device data: (a) EL spectrum; (b) EQE−L curve; (c) J−V−L curves. Device architecture: ITO/HATCN (5 nm)/
TAPC (25 nm)/TCTA (5 nm)/CBP:10 wt % Ir2 (20 nm)/TmPyPB (55 nm)/LiF (1 nm)/Al.
respect to the other complexes, resulting in the strongest
emission and longest lifetime among this series of complexes.
It is also worth noting that although the π-extended rigid
benzo[h]quinoline C∧N ligand lowered the emission energies
of Ir1−Ir7, it prolonged the emission lifetimes for most of the
complexes in all solvents (except for Ir1 and Ir6 in toluene, see
Table S4 in the Supporting Information) by approximately 1
order of magnitude in comparison to that of the most relevant
Ir(III) complex (0.69 μs in MTHF) reported in the literature,
which bears the 3-methyl-1-phenylbenzimidazol-3-ium NHC
ligand and more flexible 2-phenylpyridine C∧N ligands.27 The
prolonged lifetimes should be attributed to the drastically
reduced knr values and/or kr values in Ir1−Ir7. For example,
the knr and kr values for Ir3 (which has the same NHC ligand
as that in complex 6 in ref 27) in THF are 4.17 × 105 and 1.18
× 105 s−1, respectively, while these values for complex 6 in ref
27 in MTHF are 5.10 × 106 and 1.20 × 106 s−1, respectively,
assuming the same triplet quantum yield of 0.23 as that for Ir3.
Apparently, replacing the more flexible 2-phenylpyridine ligand
by the rigid benzo[h]quinoline ligand dramatically reduced the
knr and kr values for Ir3 in comparison to those of complex 6 in
ref 27, resulting in a much longer lifetime (8.14 μs for Ir3 in
THF vs 0.69 μs for complex 6 in ref 27 in MTHF) and slightly
increased the emission quantum yield for Ir3 (0.22 in THF vs
0.19 for complex 6 in ref 27 in MTHF). An increased emission
quantum yield and/or emission lifetime due to the
incorporation of a rigid C∧N or N∧N ligand in Ir(III)
complexes has been previously reported by our group for the
region. The triplet lifetimes deduced from the decay of TA
signals matched well with the respective lifetimes obtained
from the decay of emission signals, implying that the transient
absorbing excited states are the emitting excited states: i.e., the
C∧N ligand-centered 3π,π* state mixed with 3MLCT
configurations.
Phosphorescent OLEDs (PhOLEDs). Among this series
of complexes, Ir2 possessed the highest emission quantum
yields in all of the solvents studied. Thus, we chose this
complex as the emitter for fabrication of a prototype
electroluminescence (EL) device. The device architecture
consisted of ITO/HATCN (5 nm)/TAPC (25 nm)/TCTA (5
nm)/CBP:10 wt % Ir2 (20 nm)/TmPyPB (55 nm)/LiF (1
nm)/Al. The data for the device performances, such as the
luminance (L), current efficiency (ηc), power efficiency (ηP),
and EQE, are summarized in Table 5. The obtained EL
spectrum, EQE−L curve, and J−V−L curves are shown in
Figure 5. The EL spectral feature and the emission band
maximum (548 nm) resemble those of its photoluminescence
in CH2Cl2, THF, and CH3CN solutions (Figure S6 in the
Supporting Information). Thus, the parentage of the EL is
assigned to the triplet excited state of the dopant: namely, the
3MLCT/3π,π* state. The turn-on voltage (Von) of this device
was 2.9 V. In terms of the brightness, the maximum current
efficiency (ηc) and external quantum efficiency (EQE) are
71.29 cd A−1 and 20.6%, respectively. Note that the device did
not suffer from serious efficiency roll-off even at high
brightness (Figure 5b). In comparison to the performance of
a blue OLED using (2-(2,4-difluorophenyl)pyridine)2Ir-
(NHC) as the emitter,22 the OLED based on Ir2 exhibited a
lower turn-on voltage, doubled ηc and EQE, and almost 4-fold
higher maximum luminance. These results demonstrate the
potential of using these types of complexes as yellow emitters
for OLED applications.
55−57
monocationic Ir(N∧N)(C∧N)2 -type complexes.
Similar
+
results are observed for the neutral Ir(NHC)(C∧N)2 type
complexes reported in this work.
Transient Absorption (TA). To further explore the triplet
excited-state characteristics of complexes Ir1−Ir7, a TA study
was carried out in deaerated toluene solutions for all
complexes. The TA spectra of Ir1−Ir7 immediately after 355
nm excitation are shown in Figure 4, and the time-resolved
spectra are provided in Figure S9 in the Supporting
Information. The TA band maxima, the triplet excited state
lifetimes, and quantum yields are summarized in Table 2. The
TA spectral features of all complexes resemble each other, all
exhibiting a broad structureless positive absorption band in the
region of 460−700 nm. However, the TA band maxima differ
slightly, with electron-withdrawing substituents, especially the
CN substituent, causing a noticeable blue shift, while electron-
donating substituents showing a minor effect in comparison to
that of Ir3. The major difference appeared at 380−450 nm,
probably due to the different ground-state absorption in this
CONCLUSIONS
■
Seven neutral cyclometalated Ir(III) complexes bearing the
benzo[h]quinoline C∧N ligand and different 1-phenylbenzimi-
dazole (NHC) ancillary ligands with substituents at the C4′ of
the phenyl ring or C5 of the NHC ligand were synthesized and
characterized. The influence of the electron-donating or
-withdrawing substituents on their photophysical properties
was investigated. All complexes exhibit 1π,π*/1CT/1d,d
transitions at ca. 440 nm and yellow π,π*/3MLCT emission
3
at room temperature in fluid solutions. They all exhibited
broad triplet transient absorption at 460−700 nm with similar
I
DOI: 10.1021/acs.inorgchem.9b01678
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Notes
spectral features. Generally, electron-withdrawing substituents
caused minor blue shifts of the low-energy absorption band
and the emission band, while the electron-donating sub-
stituents induced red shifts somewhat. Although the effects of
the substituents at the NHC ligand on the T1 state energies of
these complexes are quite minor, the effects on the T1 lifetimes
and emission quantum yields are drastic. The most emissive
complex, Ir2, was utilized as the dopant for PhOLED
fabrication. The obtained device displayed a yellow emission
with a maximum current efficiency of 71.29 cd A−1, a
maximum luminance of 32747 cd m−2, a maximum EQE of
20.6%, a low turn-on voltage (2.9 V), and a low efficiency roll-
off. In comparison to the reported blue OLED using (2-(2,4-
difluorophenyl)pyridine)2Ir(NHC) as the emitter, the OLED
based on Ir2 exhibited a lower turn-on voltage, doubled ηc and
EQE, and almost 4-fold higher maximum luminance. These
results demonstrate the potential of using these types of
complexes as yellow emitters for OLED applications.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
W.S. and S.K. acknowledge the financial support from the
National Science Foundation (DMR-1411086 and CHE-
1800476) for complex synthesis, characterization, and
computational simulation of the optical spectra. S.K. is grateful
to the US Department of Energy (DE-SC008446) for financial
support and the Center for Computationally Assisted Science
and Technology (CCAST) at North Dakota State University
and the National Energy Research Scientific Computing
Center (NERSC) allocation Award No. 86678 (supported by
the Office of Science of the Department of Energy under
Contract No. DE-AC02-05CH11231) for computer access and
administrative support.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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AUTHOR INFORMATION
■
Corresponding Author
*W.S.: e-mail, Wenfang.Sun@ndsu.edu; tel, 701-231-6254; fax,
701-231-8831.
ORCID
Bingqing Liu: 0000-0002-1540-2235
Mohammed A. Jabed: 0000-0001-8552-0301
Erik K. Hobbie: 0000-0001-6158-8977
Svetlana Kilina: 0000-0003-1350-2790
Anjun Qin: 0000-0001-7158-1808
Wenfang Sun: 0000-0003-3608-611X
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