Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes

Article

Cite This: Inorg. Chem. XXXX, XXX, XXX −XXX

pubs.acs.org/IC

Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with

Easily Tuned Phosphorescent Color and Features for Highly Eﬃcient

Organic Light-Emitting Diodes

†

†,§

†

,‡,∥

†

Zhao Feng, Yue Yu,

Xiaolong Yang, Daokun Zhong, Dongdong Song, Hua Yang, Xi Chen,

,

†

Guijiang Zhou,* and Zhaoxin Wu*

†

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed Matter, Department of Chemistry, School of

‡

Science, State Key Laboratory for Mechanical Behavior of Materials and Key Laboratory of Photonics Technology for Information,

School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China

§

School of Physics and Optoelectronic Engineering, Xidian University, Xi’an 710071, PR China

∥

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, PR China

*

S Supporting Information

∧

ABSTRACT: Three Ir(C N) (acac)-type and one Ir(C N)(C N)(acac)-

2

1

2

type coumarin-based cyclometalated Ir(III) complex isomers (IrC5, IrC7,

IrC7-A, and IrC8) have been obtained using three coumarin-based isomers of

2

-phenylpyridine (ppy)-type cyclometalating ligands (L-C5, L-C7, and L-C8).

Two coordination isomers emerging as principal products (IrC7 and IrC7-A)

are obtained in the synthesis of corresponding coumarin-based cyclometalated

Ir(III) complexes because of two diﬀerent coordination sites in ligand L-C7 to

form a C−Ir bond. To the best of our knowledge, there are no such isomers

reported to date. Interestingly, a broad range of phosphorescent color tuning

from green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been realized

through variation of the pyridyl substitution positions on the fused phenyl ring

of the coumarin skeleton. In addition, based on natural transition orbital

(

NTO) analyses, features of the lowest triplet excited states (T ) from these

1

coumarin-based cyclometalated Ir(III) complex isomers can be tuned easily by

these ligand isomers as well. IrC5, IrC7, and IrC7-A show prevailing MLCT character associated with their T states which

3

1

emit the phosphorescent signals, while the T state of IrC8 exhibits the dominant ligand-centered π−π* transition feature.

1

Importantly, owing to the strong rigidity of the coumarin skeleton, all the coumarin-based cyclometalated Ir(III) complex

isomers can show high phosphorescent quantum yields Φ (ca. 0.4−1). Together with the improved electron-injection/

p

electron-transport (EI/ET) ability, all the phosphorescent emitters display impressive electroluminescence (EL) performance.

The device based on IrC8 gives the highest EL eﬃciencies of external quantum eﬃciency (η ) 22.7%, current eﬃciency (η )

ext

L

−

1

−1

7

9.7 cd A , and power eﬃciency (η ) 58.2 lm W , representing the most state-of-the-art EL ability ever achieved by coumarin-

P

based phosphorescent emitters. All these encouraging data deﬁnitely suggest the great potential of the coumarin skeleton in

both easy tuning of the photophysical properties of ppy-type Ir(III) phosphorescent complexes and developing high-

performance phosphorescent emitters.

INTRODUCTION

complexes. In past decades, numerous coumarin-based cyclo-

metalated Ir(III) complexes have been developed and found

■

Coumarin possesses a rigid structure of a fused α-pyron and a

benzene ring and usually shows no ﬂuorescence emission, while

coumarin derivatives generally exhibit high ﬂuorescence

quantum yields of nearly up to unity.

coumarin derivatives have been extensively exploited in the

14,15

applications in such as luminescent molecular probes,

16,17

photoinduced charge separation,

triplet−triplet annihila-

and visible-light absorbing

However, the potential of the coumarin-

1

8,19

1

−4

tion based upconversion,

To date, a variety of

20,21

photosensitizer.

based cyclometalated Ir(III) complexes in developing electro-

luminescence (EL) materials is still underexplored. Only a

handful of works have been involved in the EL performance

concerning coumarin-based cyclometalated Ir(III) complexes

research ﬁelds of organic optoelectric materials, such as laser

5

,6

7,8

dyes,

^a^g^g^r^e^g^a^tⁱ^oⁿ^-ⁱⁿ^d^u^c^e^d^e^mⁱ^s^sⁱ^oⁿ⁽^A^I^E⁾^e^mⁱ^t^t^e^r^s^,9

thermally activated delayed ﬂuorescence (TADF) emitters,

and dye sensitizers,

10,11

because of the relative ease of their

22−27

so far.

Ren and co-workers reported a series of facial

synthetic modiﬁcation and good photophysical properties

including high quantum yields and superior photostability.

12,13

Thereby, coumarin derivatives can provide great possibility of

molecular design for the corresponding cyclometalated Ir(III)

Received: February 23, 2019

©

XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.9b00534

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

coumarin-based cyclometalated Ir(III) complexes with high Φ_p

for EL research. Although the highest eﬃciencies had been

achieved with maximum eﬃciency of external quantum

isomers. All these coumarin-based cyclometalated Ir(III)

complex isomers have been employed as emitters to construct

OLEDs showing impressive EL performance. This work not

only can represent a new approach to develop high eﬃcient EL

phosphors with ease of color-tuning but also provide some

important information about isomerization in the synthesis of

ppy-type Ir(III) phosphorescent complexes.

−

1

eﬃciencies (η ) 20.5%, current eﬃciency (η ) 74.1 cd A ,

ext

L

−

1

and power eﬃciency (η ) 52.4 lm W , the synthesis of the

P

concerned coumarin-based cyclometalating ligands seems to be

2

very complicated. Yu and co-workers successively investigated

the eﬀect of ancillary ligands on the EL performance of the

coumarin-based cyclometalated momonuclear and dinuclear

Ir(III) complexes. However, OLEDs based on these complexes

showed relatively low EL performance with maximum

EXPERIMENTAL SECTION

■

General Information. All commercially available reagents and

chemicals were not subjected to further puriﬁcation. Solvents used for

the reactions were dried or distilled via standard methods. The

reactions were checked with thin-layer chromatography (TLC)

materials purchased from Merck & Co., Inc. Silica gel (300−400

mesh) for ﬂash column chromatography and preparative TLC plates

−

1

23−25

eﬃciencies of 9.77% (η ), 35.24 cd A (η_L).

Meerholz

ext

and co-workers developed a novel coumarin-based Ir(III)

complexes capable of cross-linking by ancillary ligands, whereas

the ﬁnal OLEDs could just give a maximum eﬃciencies of 18.4

1

13

26,27

were purchased from Shenghai Qingdao. H and C NMR spectra

−

1

−1

cd A (η ) and 15.4 lm W (η_P).

Therefore, coumarin-

L

were obtained in CDCl on a Bruker Avance 400 MHz spectrometer.

3

based cyclometalated Ir(III) complexes with the features of

Chemical shifts were referenced to the signals of the residual solvent

synthetic easiness and high Φ should represent a new class of

1

13

p

(7.26 ppm for H NMR and 77.0 ppm for C NMR). UV−vis

absorption spectra were measured on a PerkinElmer Lambda 950

spectrophotometer. Transient lifetimes and emission spectra for the

obtained coumarin-based cyclometalated Ir(III) complex isomers were

conducted on an Edinburgh Instruments, Ltd., (FLSP 920)

ﬂuorescence spectrophotometer. Photoluminescence quantum yields

(Φ ) were acquired in degassed CH Cl solution and calibrated with

phosphors to expand their potential in development of high-

performance EL materials.

Cyclometalated Ir(III) complexes bearing 2-phenylpyridine

ppy)-type ligands have aroused extensive research interest in

(

the organic EL research ﬁeld because of their outstanding

2

8−31

p

2

performances.

Typically, photophysical characters of

36

fac-[Ir(ppy) ] (Φ = 0.97) at 293 K. The thermal gravimetric

3

p

cyclometalated Ir(III) complexes are dominated by the

analysis data were acquired on a NETZSCH STA 409C instrument.

Fast atom bombardment (FAB) mass spectra were measured with a

Finnigan MAT SSQ710 system. Cyclic voltammetry (CV) measure-

ments were carried out on a Princeton Applied Research model 273A

potentiostat with a glassy carbon working electrode, a Pt-sheet counter

electrode, and an Ag/AgCl reference electrode at a scan rate of 100 mV

32−35

electronic feature of cyclometalating ligands,

so alternating

the structures of the ppy-type ligands has represented the most

important and eﬀective way to modulate the photophysical and

EL properties for the ppy-type cyclometalated Ir(III)

complexes. Unfortunately, this strategy generally involves

tedious synthesis of the ppy-type ligands. Additionally,

nonemissive acetylacetone (acac) is widely employed as an

ancillary ligand with higher T₁than that of ppy-type

cyclometalating ligands of cyclometalated Ir(III) complexes.

Hence, the emission features for the ppy-type coumarin-based

cyclometalated Ir(III) complexes can be governed by the

−

1

s . The supporting electrolyte was a 0.1 M [nBu N]BF in acetonitrile

4

solution calibrated with ferrocene. Elemental analyses for newly

synthetic compounds were performed on a Flash EA 1112 elemental

analyzer. High-performance liquid chromatography (HPLC) spectra

for IrC7 and IrC7-A as well as their mixture were acquired on a

Hitachi L-2000 using ultraviolet (254 nm) detection with methanol/

H O (85/15, v/v) as the mobile phase at a ﬂow rate of 1 mL/min.

2

34

coumarin-based cyclometalating ligands.

Br-C5 and Br-C7. Concentrated H SO (98%, 13 mL) was slowly

2

4

Herein, three coumarin-based isomers of ppy-type cyclo-

metalating ligands (L-C5, L-C7, and L-C8) have been readily

obtained by attaching the pyridine ring to diﬀerent positions of

the fused phenyl ring in the coumarin skeleton through Stille

cross-coupling reactions. Moreover, owing to the strong rigidity

of the coumarin skeleton, cyclometalated Ir(III) complexes

with the coumarin-based cyclometalating ligands should be

added to a mixture of 3-bromophenol (10 g, 57.8 mmol) and malic

acid (5.2 g, 38.8 mmol) at 0 °C in a nitrogen atmosphere. The

resultant solution was magnetically stirred at 120 °C for 6 h. Once the

reaction mixture was cooled to room temperature, we poured it into

ice water and collected the precipitated solid by ﬁltration. Both Br-C5

and Br-C7 were obtained as white solid through silica-gel column

chromatography using petroleum ether/CH Cl (3/2, v/v) as eluent.

2

1

Br-C5. Yield: 1.36 g, 16%. H NMR (400 MHz, CDCl , δ): 8.07 (d,

3

highly rigid, which is crucial to furnish high Φ by suppress the

J = 10.0 Hz, 1H), 7.51 (dd, J = 8.0, 1.2 Hz, 1H), 7.38 (t, J = 8.2 Hz,

p

1

3

nonradiative decay pathways. With these three ligands, three

1H), 7.29 (d, J = 8.4 Hz, 1H), 6.49 (d, J = 10.0 Hz, 1H). C NMR

(100 MHz, CDCl , δ): 159.95, 154.72, 142.19, 132.19, 128.46, 122.33,

18.81, 117.73, 116.47. FAB-MS (m/z): 224, 226 [M] . Anal. Calcd

for C H BrO : C, 48.04; H, 2.24. Found: C, 47.95; H, 2.29.

∧

Ir(C N) (acac)-type and one Ir(C N)(C N)(acac)-type

3

2

1

2

+

1

coumarin-based cyclometalated Ir(III) complex isomers

IrC5, IrC7, IrC7-A, and IrC8) have been obtained. All the

9

5

2

(

1

Br-C7. Yield: 5.66 g, 65%. H NMR (400 MHz, CDCl , δ): 7.67 (d,

3

obtained coumarin-based cyclometalated Ir(III) complex

J = 9.6 Hz, 1H), 7.52 (d, J = 1.6 Hz, 1H), 7.42 (dd, J = 8.2, 2.0 Hz,

isomers can show high Φ (ca. 0.4−1). Two coordination

13

p

1

H), 7.35 (d, J = 8.0 Hz, 1H), 6.44 (d, J = 9.6 Hz, 1H). C NMR (100

isomers emerging as principal products (IrC7 and IrC7-A)

were obtained in the synthesis of corresponding coumarin-

based cyclometalated Ir(III) complexes because of two diﬀerent

coordination sites in ligand L-C7 to form a C−Ir bond. To the

best of our knowledge, there are no such isomers reported to

date. Clearly, it is meaningful to explore their diﬀerence

including crystal structures, photophysical and electrochemical

properties, and EL ability. In addition, a broad range of

phosphorescent color tuning from green (IrC8, λ = 516 nm) to

red (IrC5, λ = 608 nm) can be realized by modulating the

electron density on the chelating carbon atom in the ligands for

these ppy-type coumarin-based cyclometalated Ir(III) complex

MHz, CDCl , δ): 159.87, 154.28, 142.69, 128.74, 127.87, 125.79,

3

+

1

20.22, 117.73, 116.90. FAB-MS (m/z): 224, 226 [M] . Anal. Calcd

for C H BrO : C, 48.04; H, 2.24. Found: C, 47.93; H, 2.21.

9

5

2

3-Bromo-2-hydroxybenzaldehyde. Under ambient conditions,

paraformaldehyde (7.4 g), Et N (6.4 mL), and magnesium chloride

3

(2.62 g, 27.53 mmol) were added to an acetonitrile solution of 2-

bromobenphol (3.17 g, 18.32 mmol) in sequence. The reaction

mixture was allowed to be stirred at reﬂux for 3 h. The reaction mixture

was recovered to room temperature, diluted with 1 M HCl (100 mL),

and then extracted with dichloromethane (3 × 60 mL). All the organic

phase was combined, dried with anhydrous Na SO , and concentrated

2

4

under reduced pressure. The residue was chromatographed with a

silica gel column with petroleum ether/CH Cl (1/5, v/v) as eluent to

2

B

DOI: 10.1021/acs.inorgchem.9b00534

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Inorganic Chemistry

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1

give a white solid product (3.24 g, 88%). H NMR (400 MHz, CDCl ,

added to an dry CH Cl solution of colored Ir(III) μ-chloro-bridged

3

2

dimer (0.32 g, 0.24 mmol)/(0.55 g, 0.41 mmol)/(0.28 g, 0.21 mmol).

The reaction solution was magnetically stirred at room temperature for

6 h. The inorganic salt was removed by centrifugation, and the solvent

was evaporated to dryness under vacuum. The designed coumarin-

based Ir(III) complex isomers were isolated with silica gel preparative

thin-layer chromatography (TLC) using proper developing solvents.

Caution: Thallium(I) acetylacetonate (Tl(acac)) must be dealt with

1

3

carefully because of its extreme toxicity.

1

IrC5. Yield: 0.13 g, 36%. H NMR (400 MHz, CDCl , δ): 8.67 (dd, J

3

= 5.6, 0.8 Hz, 2H), 8.53 (d, J = 10.0 Hz, 2H), 8.06 (d, J = 8.4 Hz, 2H),

7.88 (td, J = 8.0, 1.6 Hz, 2H), 7.28 (m, 2H), 6.72 (d, J = 8.4 Hz, 2H),

6.43 (d, J = 10.0 Hz, 2H), 6.23 (d, J = 8.4 Hz, 2H), 5.27 (s, 1H), 1.81

1

3

(s, 6H). C NMR (100 MHz, CDCl , δ): 185.15, 167.13, 160.50,

3

151.75, 149.14, 145.45, 140.48, 139.25, 137.46, 136.11, 122.80,

122.42, 117.37, 116.62, 115.93, 100.76, 28.68. FAB-MS (m/z): 736

+

[M] . Anal. Calcd for C H IrN O : C, 53.87; H, 3.15; N, 3.81.

3

23

2

6

Found: C, 53.76; H, 3.10; N, 3.72%.

1

IrC7. Yield: 0.11 g, 17%. H NMR (400 MHz, CDCl , δ): 8.55 (d, J

3

= 5.2 Hz, 2H), 7.96 (d, J = 8.0 Hz, 2H), 7.90 (td, J = 7.6, 1.2 Hz, 2H),

7.55 (s, 2H), 7.31 (m, 2H), 7.30 (d, J = 9.6 Hz, 2H), 6.23 (s, 2H), 6.20

1

3

(d, J = 9.6 Hz, 2H), 5.27 (s, 1H), 1.81 (s, 6H). C NMR (100 MHz,

CDCl , δ): 185.09, 166.70, 161.30, 150.65, 148.67, 148.53, 143.57,

3

139.52, 137.63, 130.72, 123.28, 119.97, 119.78, 116.25, 111.55,

+

100.81, 28.74. FAB-MS (m/z): 736 [M] . Anal. Calcd for

C H IrN O : C, 53.87; H, 3.15; N, 3.81. Found: C, 53.79; H,

33 23

2

6

0

.33 mmol)/(0.51 g, 0.44 mmol)/(0.46 g, 0.40 mmol) was

3.09; N, 3.71%.

1

correspondingly added for the Stille cross-coupling reaction. The

reaction was allowed to proceed at 110 °C for 16 h. After the reaction

solution was recovered to room temperature, the solvent was

evaporated to dryness under reduced pressure. The target ligands

were isolated by silica gel column chromatography using CH Cl as

IrC7-A. Yield: 0.10 g, 15%. H NMR (400 MHz, CDCl , δ): 8.53 (d,

3

J = 5.6 Hz, 1H), 8.51 (d, J = 6.0 Hz, 1H), 8.00 (d, J = 8.4 Hz, 1H), 7.91

(dd, J = 6.4, 1.6 Hz, 2H), 7.86 (td, J = 7.8, 1.6 Hz, 1H), 7.61 (d, J = 8.0

Hz, 1H), 7.54 (s, 1H), 7.39 (d, J = 9.6 Hz, 1H), 7.31−7.23 (m, 3H),

6.99 (d, J = 8.0 Hz, 1H), 6.16 (d, J = 9.6 Hz, 1H), 6.11 (s, 1H), 6.03 (d,

J = 9.6 Hz, 1H), 5.29 (s, 1H), 1.81 (s, 3H), 1.78 (s, 3H). C NMR

(100 MHz, CDCl , δ):185.08, 184.55, 167.74, 165.79, 161.52, 161.42,

2

1

3

eluent.

1

L-C5. Yield: 1.23 g, 83%. H NMR (400 MHz, CDCl , δ): 8.76 (dd,

3

160.89, 150.95, 150.38, 149.66, 149.21, 148.42, 143.63, 143.30,

137.70, 137.34, 136.85, 133.54, 130.49, 123.10, 122.54, 121.56,

120.11, 119.74, 119.15, 119.02, 116.56, 115.61, 110.95, 100.78, 28.68.

+

FAB-MS (m/z): 736 [M] . Anal. Calcd for C H IrN O : C, 53.87;

33 23

2

6

H, 3.15; N, 3.81. Found: C, 53.80; H, 3.10; N, 3.74%.

1

IrC8. Yield: 0.12 g, 37%. H NMR (400 MHz, CDCl , δ): 9.12 (d, J

3

Anal. Calcd for C H NO : C, 75.33; H, 4.06; N, 6.27. Found: C,

= 8.4 Hz, 2H), 8.53 (dd, J = 5.6 Hz, 2H), 7.94 (td, J = 8.0, 1.6 Hz, 2H),

7.54 (d, J = 9.6 Hz, 2H), 7.28 (m, 2H), 6.75 (d, J = 8.0 Hz, 2H), 6.23

(d, J = 1.6 Hz, 2H), 6.21 (s, 2H), 5.27 (s, 1H), 1.81 (s, 6H). C NMR

14

9

2

7

5.22; H, 4.13; N, 6.16%.

L-C7. Yield: 1.65 g, 83%. H NMR (400 MHz, CDCl , δ): 8.74 (dd,

1

13

3

J = 4.8, 0.8 Hz, 1H), 7.98 (dd, J = 8.0, 1.6 Hz, 1H), 7.95 (s, 1H), 7.81

(100 MHz, CDCl , δ): 185.17, 165.33, 160.88, 159.60, 151.79, 148.10,

3

(

m, 2H), 7.75 (d, J = 9.2 Hz, 1H), 7.58 (d, J = 8.0 Hz, 1H), 7.31 (m,

145.38, 138.27, 132.20, 129.54, 127.97, 124.82, 122.32, 113.74,

1

3

+

1

H), 6.46 (dd, J = 9.4, 1.4 Hz, 1H). C NMR (100 MHz, CDCl , δ):

111.34, 100.71, 28.67. FAB-MS (m/z): 736 [M] . Anal. Calcd for

3

60.80, 155.28, 154.46, 150.00, 142.99, 142.94, 137.04, 128.14,

C H IrN O : C, 53.87; H, 3.15; N, 3.81. Found: C, 53.77; H, 3.16;

3

23

2

6

23.19, 122.96, 120.92, 119.10, 116.94, 115.00. FAB-MS (m/z): 223

N, 3.75%.

+

[

M] . Anal. Calcd for C H NO : C, 75.33; H, 4.06; N, 6.27. Found:

X-ray Crystallography. High-quality single crystals of IrC5, IrC7,

and IrC7-A for X-ray diﬀraction investigation were obtained by slowly

diﬀusing methanol into their chloroform solutions. The selected single

crystals were measured with a Φ and ω scan modes on a Bruker

SMART CCD diﬀractometer (Mo Kα radiation and λ = 0.71073 Å).

Direct methods and diﬀerence Fourier syntheses were applied to solve

the molecular structures. Structural reﬁnements were achieved by full-

1

4

9

2

C, 75.19; H, 4.09; N, 6.18%.

1

L-C8. Yield: 1.45 g, 81%. H NMR (400 MHz, CDCl , δ): 8.75 (dd,

3

J = 4.8, 0.8 Hz, 1H), 8.11 (dd, J = 8.0, 1.6 Hz, 1H), 8.03 (d, J = 8.0 Hz,

1

=

H), 7.83 (td, J = 7.8, 1.6 Hz, 1H), 7.79 (d, J = 9.6 Hz, 1H), 7.54 (dd, J

7.6, 1.6 Hz, 1H), 7.41 (t, J = 7.6 Hz, 1H), 7.31 (ddd, J = 7.6, 4.8, 0.8

1

3

Hz, 1H), 6.47 (d, J = 9.6 Hz, 1H). C NMR (100 MHz, CDCl , δ):

3

2

37

1

60.27, 157.04, 153.06, 151.24, 149.81, 143.97, 136.45, 133.66,

matrix least-squares techniques against F in SHELXL-97 program. A

28.57, 125.39, 124.56, 122.78, 119.21, 116.43. FAB-MS (m/z): 223

riding model was used to calculate and reﬁne the positions of hydrogen

isotropically. Likewise, all other non-hydrogen atoms were also reﬁned

+

[

M] . Anal. Calcd for C H NO : C, 75.33; H, 4.06; N, 6.27. Found:

1

4

9

2

38

C, 75.22; H, 4.11; N, 6.19%.

isotropically. SADABS was applied for absorption corrections.

General Synthetic Procedure for IrC5/IrC7/IrC7-A/IrC8. To a

Computational Details. DFT calculations for all of the obtained

ppy-type coumarin-based cyclometalated Ir(III) complex isomers were

conducted using the B3LYP. C, H, O, and N atoms adopted the 6-31G

(d, p) basis set for calculations, while a LanL2DZ basis set was

solvent mixture of 2-ethoxyethanol and H O (3/1, v/v) in a nitrogen

2

atmosphere were added L-C5 (0.25 g, 1.12 mmol)/L-C7(0.44 g, 1.97

mmol)/L-C8(0.21 g, 0.94 mmol) and IrCl ·nH O (60 wt % Ir

3

2

39,40

content) (0.16 g, 0.51 mmol)/(0.29 g, 0.90 mmol)/(0.14 g, 0.43

mmol). The reaction mixture was magnetically stirred at 110 °C for 16

h. Once the resultant mixture was cooled to room temperature, we

poured it into a saturated solution of NaCl. The precipitated colored

Ir(III) μ-chloro-bridged dimer was ﬁltrated out and dried in vacuum.

Afterward, thallium(I) acetylacetonate [Tl(acac)] (0.16 g, 0.53

mmol)/(0.27 g, 0.90 mmol)/(0.14 g, 0.46 mmol) was correspondingly

employed for eﬀective core potentials of Ir atoms.

TD-DFT

calculations were conducted to obtain the excitation behaviors based

on the optimized ground state (S ) geometries. In addition, the

0

geometries of the lowest triplet excited state (T ) were optimized with

1

the UB3LYP method and analyses of the natural transition orbital

(NTO) was conducted for S → T excitation. All the theoretical

0

1

41

calculations were performed in the Gaussian 09 program.

C

DOI: 10.1021/acs.inorgchem.9b00534

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

Scheme 1. Speciﬁc Synthetic Scheme of the Coumarin-Based Isomers of ppy-Type Cyclometalating Ligands

Scheme 2. Speciﬁc Synthetic Scheme of ppy-Type Coumarin-Based Cyclometalated Ir(III) Complex Isomers

OLED Fabrication and Measurements. The precleaned ITO

glass substrates were treated with ozone for 20 min. Subsequently, a 3

form a ca. 20 nm thick doped CBP region. Finally, successive

deposition of 1,3,5-tris[N-(phenyl)-benzimidazole]-benzene (TPBi)

(40 nm), LiF (1 nm), and Al cathode (100 nm) was performed at a

base pressure of less than 10 Torr. The CIE coordinates and EL

spectra were acquired with a PR650 spectra colorimeter. The

luminance was recorded on a PR650 SpectraScan spectrometer. The

J−V−L data of all the devices were obtained on a Keithley 2400/2000

nm thick hole injection layer of MoO was deposited onto the surface

3

−

6

of ITO glass. Then, deposition of 4,4′-bis[N-(1-naphthyl)-N-phenyl-

amino]biphenyl (NPB) was carried out to form ca. 40 nm thick hole-

transporting and electron-blocking layer. Coevaporation of the Ir(III)

complex and 4,4′-N,N′-dicarbazole-biphenyl (CBP) was conducted to

D

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Article

source meter. All of the relevant measurements and experiments were

conducted under ambient conditions.

RESULTS AND DISCUSSION

■

Synthesis and Characterization. The detailed synthetic

protocols of the coumarin-based isomers of ppy-type cyclo-

metalating ligands and the ppy-type coumarin-based cyclo-

metalated Ir(III) complex isomers are illustrated in Schemes 1

and 2, respectively. Br-C5 and Br-C7 can be prepared

concurrently with a reaction yield ratio of ca. 1:4 by heating

the concentrated H SO solution of 3-bromophenol and malic

2

4

2

acid at 120 °C, whereas Br-C8 has to be synthesized by a two-

step reaction. First, 3-bromo-2-hydroxybenzaldehyde has

been prepared by introduction of aldehyde group into 2-

bromobenphol in the presence of paraformaldehyde according

43

to the reported literature. Second, Br-C8 can be obtained in a

high yield by simply heating a acetic anhydride solution of 3-

bromo-2-hydroxybenzaldehyde in the presence of acetate

4

potassium. Target ligands L-C5/L-C7/L-C8 were readily

synthesized through Stille cross-coupling reaction between the

corresponding brominated coumarins (Br-C5/Br-C7/Br-C8)

and 2-(tributylstannyl)pyridine.

After the coumarin-based isomers of ppy-type cyclometalat-

ing ligands were obtained, reaction of IrCl ·nH O and the

3

2

corresponding cyclometalating ligands (L-C5/L-C7/L-C8)

aﬀorded the colored cyclometalated Ir(III) μ-chloro-bridged

dimers. Subsequently, the obtained μ-chloro-bridged dimers

were transformed into the designed ppy-type coumarin-based

cyclometalated Ir(III) complex isomers by mixing the relevant

cyclometalated Ir(III) μ-chloro-bridged dimer with Tl(acac) in

dry CH Cl at room temperature (Scheme 2). These air-stable

2

Ir(III) complex isomers were isolated with acceptable yields as

colored solids by preparative silica gel TLC plates with the

proper developing solvents. Owing to the two diﬀerent

coordination sites in ligand L-C7 forming a C−Ir bond, there

should theoretically be three coordination isomers to be

obtained in the synthesis of the ppy-type coumarin-based

cyclometalated Ir(III) phosphorescent complexes using ligand

L-C7. Unfortunately, one of the three coordination isomers,

namely, IrC7-B, appeared with rather low yield at approx-

imately <2%. Hence, two main products of coordination

isomers IrC7 and IrC7-A were obtained (Figure 1). HPLC

spectra for IrC7 and IrC7-A as well as their mixture indicates

that the retention times of IrC7 and IrC7-A are ca. 5.70 and

6

.35 min with methanol/H O (85/15, v/v) as the mobile phase

2

at a ﬂow rate of 1 mL/min, respectively, and their purity

exceeds 99.3% (Figure S1). According to the comparable yields

of IrC7 (17%) and IrC7-A(15%), the two coordinating sites

should show similar coordinating activity, so it can be

concluded that the very low yield for the third coordination

isomer, IrC7-B, might be ascribed to the steric hindrance eﬀect

Figure 1. Perspective drawings of (a) IrC5, (b) IrC7, and (c) IrC7-A

with thermal ellipsoids at a probability level of 25%. Most of the labels

and all the hydrogen atoms are omitted just for clarity.

signals at 5.27 and 1.81 ppm are safely attributed to the ancillary

ligand acetylacetonate. These results agree well with the

(

Figure S2 ). However, this isomerization behavior has not been

∧

49,50

1

reported in the cyclometalated Ir(III) phosphorescent com-

plexes bearing ppy-type ligands with two possible coordinating

sites.

All the obtained ppy-type coumarin-based cyclometalated

Ir(III) complex isomers have been characterized by H and C

NMR spectra (Figure S3 and S4 ). In the H NMR spectrum of

reported [Ir(N C) (acac)]-type complexes.

In the H

2

NMR spectra of IrC7-A, two doublet resonance signals located

at 8.53 and 8.51 ppm are ﬁrmly ascribed to the aromatic

protons attached to the C atoms linked to the N atoms in the

pyridyl units of the two ligands, L-C7. The singlet resonance

signal at 5.29 ppm can be ﬁrmly assigned to the proton linked to

the C atom between the two carbonyl groups in the

acetylacetonate after coordinating to the Ir atom, while two

sets of singlet resonance signals at 1.81 and 1.78 ppm can be

ascribed to the protons of the two methyl groups in the ancillary

ligand acetylacetonate. The results are in accordance with our

4

5−48

1

13

1

each IrC5, IrC7, and IrC8, only one doublet resonance signal

located in the range of 8.55−9.12 ppm has been observed,

which can be safely assigned to two aromatic protons linked to

the C atoms adjacent to the N atoms in the two pyridine rings

of the two relevant ligands, while two sets of singlet resonance

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Table 1. Crystal and Data Parameters for the Structures of IrC5, IrC7, and IrC7-A

IrC5

IrC7

IrC7-A

1866016

CCDC no.

formula

1866014

C H IrN O

1866015

C H IrN O ·3CHCl ·C₀_.₅Cl₁_.₅

C H IrN O ·CHCl

33 23 2 6 3

3

23

2

6

33 23

2

6

3

formula weight

crystal system

space group

a (Å)

b (Å)

c (Å)

α (deg)

β (deg)

γ (deg)

V (Å³)

735.73

orthorhombic

fdd2

32.682 (10)

32.196 (10)

10.239 (3)

90

1153.02

triclinic

855.10

triclinic

P1

̅

P1

̅

12.379 (3)

12.848 (4)

15.351 (4)

92.052 (4)

105.609 (3)

112.228 (3)

2167.7 (10)

2

8.753 (3)

12.557 (4)

14.606 (4)

81.485 (4)

85.990 (4)

81.404 (4)

1568.0 (8)

2

90

10774 (6)

16

Z

D_c_a_l_c_d(g cm⁻³)

crystal size (mm³)

F (000)

1.814

1.765

1.811

0.380 × 0.120 × 0.060

5760

5.009

0.250 × 0.120 × 0.050

1123

3.772

0.350 × 0.140 × 0.080

836

4.563

μ (mm⁻¹)

θ range (deg)

diﬀrn reﬂns number

reﬂns number total

no. of parameters

R , wR [I > 2.0σ(I)]

2.17−25.73

20472

5080

1.72−25.62

15939

7960

571

0.0424, 0.1063

0.0519, 0.1117

2.32−25.55

11853

5832

382

417

a

0.0231, 0.0527

0.0254, 0.0537

1.001

0.0261, 0.0579

0.0321, 0.0609

0.872

1

2

R , wR (all data)

1

2

GOF on F²

b

1.048

a

R = Σ||F | − |F ||/Σ|F |. wR = {Σ[w(F − F ) ]/Σ[w(F ) ]} . GOF = [(Σw|F | − |F |) /(N − N_p_a_r_a_m₎_]1/2

2

1/2

b

2

.

1

0

c

0

2

0

c

0

c

obs

Table 2. Photophysical and Thermal Data of the ppy-Type Coumarin-Based Cyclometalated Ir(III) Complex Isomers

emission λ_e_m(nm)

Φ_p

τ_P(μs)

T

d

a

b

c

d

e

f

g

absorption (293 K) λ_a_b_s(nm)

293 K

608

77 K

ﬁlm

solution

ﬁlm

solution

ﬁlm

(°C)

IrC5

IrC7

253 (4.67), 283 (4.64), 319 (4.72), 416 (3.81), 495 (3.70),

592,

608

600

584

520

0.39

0.42

0.45

0.99

0.47

0.58

0.66

0.99

1.51 (608)

1.57 (604)

2.00 (586)

1.73 (608)

1.85 (600)

2.01 (584)

1.48 (520)

358

5

16 (3.70)

244 (4.50), 287 (4.57), 322 (4.65), 417 (3.67), 476 (3.52),

00 (3.51)

IrC7-A 287 (4.48), 322 (4.60), 406 (3.62), 466 (3.49), 496 (3.39)

636

586,

642

572,

18

508,

546

604

586

516

324

335

358

5

6

IrC8

259 (4.50), 304 (4.39), 322 (4.37), 348 (4.39), 368 (4.36),

08 (3.97), 460 (3.80), 499 (2.89)

1.36 (516)

4

a

−5

b

Recorded in CH Cl solution at a concentration of ca. 10 M and log ε values are presented in parentheses. sh: shoulder. Recorded in degassed

2

−

5

c

d

CH Cl solution at a concentration of ca. 10 M, λ ex = 380 nm. Measured in an 8 wt % doped CBP ﬁlm. Calibrated with fac-[Ir(ppy) ] (Φ =

0

CH Cl solution at a concentration of ca. 10 M at 293 K. Measured in an 8 wt % doped CBP ﬁlm and the monitoring wavelength in parentheses

2

3

p

e

f

.97) in degassed CH Cl solution, λ ex = 380 nm. Performed with an integrating sphere in an 8 wt % doped CBP ﬁlm. Recorded in degassed

2 2

g

−

5

2

is presented (in nm).

∧

previously reported asymmetric heteroleptic [Ir(N C )-

̅

IrC7-A adopt the same packing mode of space group P1 to

1

∧

51,52

(

N C )(acac)]-type complexes,

demonstrating that the

aﬀord a triclinic crystal system. From the structures of IrC7 and

IrC7-A (Figure 1), as expected, L-C7 employs two

coordinating sites to form complexes. The isomerization

phenomenon aforementioned has been conﬁrmed properly.

As indicated in Figure 1, two same ppy-type ligands and one

acetylacetone anion centering on Ir(III) atom adopt a distorted

octahedral chelating skeleton for IrC5, IrC7, and IrC7-A,

which show the typical cis-C,C, cis-O,O, and trans-N,N chelating

disposition. For IrC5, the bond angles of N(24)−Ir(1)−N(26),

C(43)−Ir(1)−O(9), and C(70)−Ir(1)−O(1) are 175.30(2),

2

same two ligands, L-C7, oﬀer diﬀerent carbon atoms

coordinating to Ir atom, and the obtained single-crystal

structure of IrC7-A further supports this (vide infra).

Combined with the results of their ¹³C NMR spectra, the

structures of all the obtained ppy-type coumarin-based

cyclometalated Ir(III) phosphorescent complex isomers can

be properly conﬁrmed.

Single-Crystal X-ray Crystallography. High-quality

single crystals of IrC5, IrC7, and IrC7-A have been successfully

cultivated by slowly diﬀusing methanol to their chloroform

solution. Their structures are investigated by single-crystal X-

ray crystallography. Selected detailed data of each structure are

presented in Tables 1, S1, S2, and S3. As for their single crystals,

the molecules of IrC5 pile in the space group fdd2 to give an

orthorhombic crystal system, while those of both IrC7 and

1

75.70(2), and 171.80(2)°, while those are 176.03(19),

76.60(2), and 175.92(19)° (for IrC7) and 175.84(12),

173.86(13), and 175.03(13)° (for IrC7-A), respectively. The

bond lengths of Ir−N, Ir−C, and Ir−O are similar to those of

50,51

the reported Ir(ppy) (acac)-type complexes.

2

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Figure 2. UV−vis absorption spectra (a) and photoluminescence (PL) spectra (b) of the obtained ppy-type coumarin-based cyclometalated Ir(III)

complex isomers in CH Cl solution measured at 293 K.

2

Table 3. Theoretical Calculation Results for These ppy-Type Coumarin-Based Cyclometalated Ir(III) Complex Isomers Based

on Their Optimized S Geometries

0

contribution percentages of metal d

π

orbitals and π orbitals of ligand to

main conﬁguration of S → S excitation/

main conﬁguration of S → T excitation/E

0

1

0

1

cal

a

b

complex

MO

MOs (%)

λ

(nm)/E /f

/λ (nm)

cal

Ir

L-C5

acac

1.29

0.67

4.34

49.97

acac

0.88

0.56

4.95

49.99

acac

0.59

1.09

4.96

47.60

acac

2.01

5.35

50.94

L+1

L

H

3.68

3.16

44.30

36.42

Ir

2.28

2.33

46.11

39.17

Ir

1.50

3.73

45.91

40.42

Ir

7.68

4.92

45.03

37.24

95.03

96.17

51.36

13.61

H → L (95.03%)

2.475 eV

501.0

H → L (82.45%)

2.085 eV

594.6

IrC5

H−1

0.0673

L-C7

L+1

L

H

96.84

97.11

48.94

10.84

H → L (95.93%)

2.437 eV

508.8

H → L (81.39%)

2.076 eV

597.3

IrC7

H−1

0.0415

L-C7_C₆L-C7_C₈

L+1

L

H

96.88

0.91

34.41

5.31

1.03

94.27

14.72

6.67

H → L (96.50%)

2.420 eV

512.5

H → L (85.94%)

2.162 eV

573.4

IrC7-A

IrC8

H−1

0.0129

L-C8

L+1

L

H

90.31

93.07

49.62

11.82

H → L (90.32%)

2.874 eV

431.4

H → L (71.53%)

2.438 eV

508.6

H−1

0.1147

a

L-C7_C₆denotes ligand L-C7 with C6-position in the coumarin skeleton coordinated to Ir atom, while L-C7_C₈represents ligand L-C7 with C8-

position in the coumarin skeleton coordinated to Ir atom. H → L means transition from HOMO to LUMO. E , λ , and f represent calculated

b

cal cal

excitation energy, calculated absorption wavelength, and calculated oscillator strength, respectively. The spin-forbidden character of singlet−triplet

transition determines no oscillator strength for S → T because TD-DFT calculations in the Gaussian program take no consideration of spin−

0

1

orbital coupling.

Thermal and Photophysical Properties. Thermal

stability of these ppy-type coumarin-based cyclometalated

Ir(III) complex isomers have been investigated with thermog-

ravimetric analysis (TGA) under a nitrogen ﬂow. The TGA

results demonstrated their good thermal stability with their 5%

weight loss temperature (ΔT ) in the range from 324 to 358

absorption spectra can be safely attributed to the spin-allowed

π−π* transition in the two same ppy-type ligands because of

their large extinction coeﬃcients (log ε > 4). According to the

calculation results of time-dependent density functional theory

(TD-DFT), their HOMO → LUMO (H → L) transitions can

embody the feature of their S → S transitions involved with

the major absorption bands because of their overwhelming

contribution of over 90% (Table 3). On the basis of their H →

L transition features, the major absorption bands of IrC5, IrC7,

and IrC8 should mainly display intraligand charge transfer

(ILCT) characters from the phenyl π orbitals to the pyridyl π*

orbitals and the π* orbitals of the heterocycle in the coumarin

skeleton of ligand L-C5/L-C7/L-C8 (Figure 3 and Tables 3

and S4 ), while the major absorption band of IrC7-A should

primarily display ligand-to-ligand charge transfer (LLCT)

feature from the π orbitals of ligand L-C7_C₆(L-C7_C₆denotes

ligand L-C7 with the C6-position in the coumarin skeleton

5

%

0

1

°

C (Table 2 and Figure S5 )). The ΔT above 320 °C should

5%

make them suitable as phosphorescent emitters for application

in OLEDs in terms of stability.

UV−vis absorption spectra of all the obtained ppy-type

coumarin-based cyclometalated Ir(III) complex isomers are

−

5

recorded in CH Cl solution at a concentration of ca. 10 mol

L

2

−

1

at 293 K, and the relevant data are tabulated in Table 2.

There are two absorption bands in their UV−vis absorption

spectra (Figure 2a), similar to the reported results of

∧

32,50,52

[

Ir(N C) (acac)]-type complexes.

Clearly, their high-

2

energy absorption bands before ca. 400 nm in the UV−vis

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Figure 3. Molecular orbital (MO) patterns (isocontour value = 0.030) of these ppy-type coumarin-based cyclometalated Ir(III) complex isomers

based on their optimized S geometries. (a) IrC5, (b) IrC7, (c) IrC7-A, and (d) IrC8.

0

Figure 4. Experimental UV−vis absorption spectra (red proﬁle) and simulated absorption spectra (black proﬁle) with discrete vertical vibronic

transition for these ppy-type coumarin-based Ir(III) complex isomers.

coordinated to Ir atom) to the π* orbitals of ligand L-C7_C₈(L-

C7_C₈represents ligand L-C7 with the C8-position in the

coumarin skeleton coordinated to Ir atom) and intraligand

charge transfer (ILCT) feature from the phenyl π orbitals to the

pyridyl π* orbitals and the π* orbitals of the heterocycle in the

coumarin skeleton of ligand L-C7_C₈(Figure 3 and Table 3 and

S4 ). In order to clearly interpret the low-energy absorption

behaviors of these ppy-type coumarin-based cyclometalated

Ir(III) complex isomers, simulation of UV−vis spectra with

discrete vertical vibronic transitions have been conducted. All of

the simulated UV−vis spectra are nearly identical to the

experimental ones of the corresponding ppy-type coumarin-

based cyclometalated (III) complex isomers in terms of both

wavelengths and line-shapes, demonstrating the validity of the

calculation method (Figure 4). For IrC5 and IrC7, the low-

energy absorption bands centered on ca. 420 nm with vertical

vibronic transitions are from HOMO−1 to LUMO+1

transitions (H−1 → L+1) according to the simulation results

H

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Figure 5. NTO patterns (isocontour value = 0.030) for S → T excitation for these ppy-type coumarin-based cyclometalated Ir(III) complex

0

1

isomers based on their optimized T geometries. (a) IrC5, (b) IrC7, (c) IrC7-A, and (d) IrC8.

1

of the UV−vis spectrum; based on the H−1 → L+1 transition

features, the low-energy absorption bands between 400 and 450

nm should exhibit metal-to-ligand charge transfer (MLCT)

characters from the Ir(III) center to the pyridyl π* orbitals and

the π* orbitals of the heterocycle in the coumarin skeleton of

ligand L-C5/L-C7 and LLCT transition features from the π

orbitals of acac to the π* orbitals of ligand L-C5/L-C7 (Figures

transition feature, the weak low-energy absorption bands after

400 nm should exhibit MLCT character from the Ir(III) center

to the π* orbitals of ligand L-C8 together with ILCT feature

from the phenyl π orbitals to the pyridyl π* orbitals and the π*

orbitals of the heterocycle in the coumarin skeleton of ligand L-

C8 (Figures 3 and 4 andTable 3 and S4). Interestingly, the

MLCT absorption band of IrC8 shows a marked blue-shift

relative to those of IrC5, IrC7, and IrC7-A. As shown from the

molecular orbital (MO) pattern of IrC8 (Figure S6), the

alkenyl groups in the coumarin skeletons are linked with the

electron-rich sites at para-position of Ir-coordinated carbon

atom. The electron-withdrawing conjugation eﬀect of the

carbonyl group in the coumarin skeleton can reduce the

electron density on the coordinated phenyl (Table S4), hence

inducing a lowered HOMO level. Meanwhile, electron-

withdrawing conjugation eﬀect of the carbonyl group will

enrich electron density on the coumarin heterocycle and

weaken its electron-accepting ability (Table S4), thus leading to

an elevated LUMO level. Therefore, an enlarged band gap

between the HOMO and LUMO levels and a hypsochromic

MLCT absorption band can be observed for IrC8 relative to

IrC5, IrC7, and IrC7-A.

3

and 4 and Table 3). The low-energy absorption bands

centered around 501 nm for IrC5 and 508 nm for IrC7 with

vertical vibronic transitions originate from H → L transitions

according to the simulation results of UV−vis spectra; based on

the H → L transition features, the weak lowest-energy

absorption bands after 450 nm should mainly include MLCT

transition features from the Ir(III) center to the π* orbitals of

ligand L-C5/L-C7 together with ILCT transition features from

the phenyl π orbitals to the pyridyl π* orbitals and the π*

orbitals of the heterocycle in the coumarin skeleton of ligand L-

C5/L-C7 (Figures 3 and 4 and Table 3 and S4 ). As for IrC7-A,

on the basis of the simulation results of UV−vis spectra, the

low-energy absorption band centered on 460 nm with vertical

vibronic transition comes from HOMO to LUMO+1 (H → L

+

1) transition; on the basis of H → L+1 transition character,

the low-energy absorption band between 400 and 480 nm

should exhibit MLCT transition feature from Ir(III) center to

the π* orbital of ligand L-C7_C₆and LLCT transition feature

from the π orbital of ligand L-C7_C₈to the π* orbital of ligand L-

C7_C₆as well as ILCT transition character from the phenyl π

orbital to the pyridyl π* orbital and the π* orbital of the

heterocycle in the coumarin skeleton of ligand L-C7_C₆(Figures

Photoluminescent spectra of all the ﬁnal ppy-type coumarin-

based cyclometalated Ir(III) complex isomers are recorded in

CH Cl solution at 293 and 77 K as well as CBP ﬁlms at 293 K.

2

Upon excited with 360 nm UV light, IrC5 and IrC7 emit red

phosphorescent bands at 608 and 604 nm, respectively, while

IrC7-A shows an orange phosphorescent band at 586 nm. In

addition, a green phosphorescent band is observed at 516 nm

for IrC8 (Figure 2b and Table 2). All of these ppy-type

coumarin-based cyclometalated Ir(III) complex isomers show

high Φ values in degassed CH Cl solution: 0.39 for IrC5, 0.42

3

and 4 andTable 3). The low-energy absorption band centered

on 512 nm with vertical vibronic transition are from H → L

transition, according to H → L transition feature, the low-

energy absorption band after 480 nm should be primarily

induced by MLCT transition feature from Ir(III) center to the

π* orbital of ligand L-C7_C₈and LLCT transition feature from

the π orbital of ligand L-C7_C₆to the π* orbital of ligand L-C7_C₈

with as well as ILCT transition character from the phenyl π

orbital to the pyridyl π* orbital and the π* orbital of the

p

2

for IrC7, 0.45 for IrC7-A, and 0.99 for IrC8 (Table 2). These

results indicate the advantage of introduction of the coumarin

scaﬀold to Ir(III) complexes in furnishing high Φ .

p

Furthermore, the Φ values of these Ir(III) complex isomers

p

in CBP ﬁlms are relatively higher than those in degassed

CH Cl solution, which may originate from some suppressed

2

heterocycle in the coumarin skeleton of ligand L-C7 (Figure 3

nonradiative pathways in the rigid matrix (Table 2). The room-

temperature luminescent lifetimes in both degassed CH Cl

C8

and 4, and Table 3 and S4 ). For IrC8, the low-energy

absorption band centered on 431 nm with strong vertical

vibronic transition is from H → L transition according to the

simulation results of the UV−vis spectrum; based on the H → L

2

solution and CBP ﬁ lms fall in between 1.36 and 2.01 μs (Table

2 and Figures S7 and S8), indicating emission from triplet

excited states. The emission peak wavelengths of these ppy-type

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coumarin-based cyclometalated Ir(III) complex isomers show a

blueshift in CH Cl solution at 77 K relative to those in CH Cl

2

atoms in these ligands is noticeably diﬀerent (Figure 6 and

Table S5 ). According to our previous report, the chelating

53

2

solution at 293 K, which may result from the restraint of

molecular vibrations in the rigid matrix (Table 2 and Figures 2b

and S9). While their emission peak wavelengths in CBP ﬁlms

are almost identical to those in CH Cl solution at 293 K

2

(

Table 2, Figures 2b and S10 ). In order to gain insight into the

phosphorescent character, i.e., the lowest triplet excited state

T ) characters of these ppy-type coumarin-based cyclo-

(

1

metalated Ir(III) complex isomers, natural transition orbital

(

NTO) analyses were carried out for their S → T excitations

0 1

based on the optimized T geometries.

1

As shown from the distribution patterns of the NTO hole

and particle orbitals for IrC5, IrC7 and IrC7-A (Figure 5 and

Table 4. NTO Results for these ppy-Type Coumarin-Based

Cyclometalated Ir(III) Complex Isomers Based on

Figure 6. Molecular orbital (MO) patterns for these coumarin-based

isomers of the ppy-type cyclometalating ligands. (a) L-C5, (b) L-C7,

and (c) L-C8. The electron density on the chelating carbon atoms is

highlighted by the green circles.

Optimized T Geometries

1

contribution percentages of metal d orbitals and π

π

a

b

complex

NTO

orbitals of ligand to NTOs (%)

Ir

L-C5

65.44

94.50

L-C76

67.67

96.53

L-C7_C₆

13.08

1.01

acac

2.86

1.06

acac

2.76

0.72

acac

4.38

1.52

acac

2.02

H

P

31.70

4.44

Ir

29.57

2.75

Ir

43.55

5.91

Ir

20.69

6.49

IrC5

carbon atom with higher electron density obviously shows

stronger electron-donating ability which will deﬁnitely increase

the electron density on the Ir(III) center of the concerned

cyclometalated Ir(III) complexes. Thereby, it will facilitate the

MLCT process and hence stabilize the MLCT state, inducing a

H

P

IrC7

L-C7_C₈

38.99

91.56

H

P

bathochromic eﬀect in the emission bands with MLCT features

IrC7-A

IrC8

30,53

as well as the corresponding absorption bands.

Thereby,

L-C8

77.29

91.80

IrC5 (λ = 608 nm) gives a longer emission wavelength relative

to that of IrC7 (λ = 604 nm) due to the fact that the electron

density on the chelating carbon atom of ligand L-C5 (17.63%)

in HOMO is higher than that in L-C7_C₆(11.97%) (Figures 2b

and 6 and Table S5). Both IrC7 (λ = 604 nm) and IrC7-A (λ =

586 nm) exhibit long-wavelength emission (Figure 2b),

implying that their emission behaviors are mainly dominated

H

P

1.71

a

b

H and P represent NTO hole and particle orbitals, respectively. L-

C7_C₆denotes the ligand L-C7 with C6-position in the coumarin

skeleton coordinated to Ir atom, while L-C7_C₈represents the ligand

L-C7 with C8-position in the coumarin skeleton coordinated to Ir

atom.

by ligand L-C7_C₆rather than L-C7 . This can be ascribed to

C8

the following facts. Both IrC7 and IrC7-A show prevailing

3

Table 4), the character of their T states is dominated by the

MLCT, which can be further conﬁrmed by their structureless

line proﬁles (Figure 2b). As for IrC8, the hole orbitals are

mainly located on the ligands L-C8 (77.29%). The Ir(III)

center also makes some contribution to their hole orbitals

MLCT character of their T states, so as aforementioned,

1

3

electron density on the Ir(III) center should mainly aﬀect the

energy-level of the MLCT states and hence the emission

wavelength. The electron density on the Ir(III) center shows

positive correlation with that on the chelating carbon atom. For

ligand L-C7_C₆(11.97%), there is much higher electron density

on the chelating carbon atom, while the situation is totally

diﬀerent in ligand L-C7_C₈(0.36%) (Figure 6 and Table S5), so

the emission wavelengths for both IrC7 and IrC7-A are mainly

(

20.69%), while its particle orbitals show a prevailing

occupation on ligand L-C8 with a percentage of 91.80%

(

Figure 5 and Table 4). Hence, the ligand-centered π−π*

transition dominates the character of the T state which is also

1

identiﬁed by the relatively low contribution of MLCT in its PL

spectrum (Figure 2b). As shown from the PL spectra of these

ppy-type coumarin-based cyclometalated Ir(III) complex

isomers, a broad range of emission color tuning from green

up to ligand L-C7 . In addition, the emission wavelength of

C6

IrC7-A (λ = 586 nm) is hypsochromic by 18 nm in comparison

with that of IrC7 (λ = 604 nm). This result can be explained as

follows: the reduced electron density on the Ir(III) center

induced by ligand L-C7_C₈with much lower electron density on

the chelating carbon atom will restrain the MLCT process to

(

IrC8 λ = 516 nm) to red (IrC5 λ = 608 nm) can be realized

through variation of the pyridyl substitution positions on the

fused phenyl ring of the coumarin skeleton.

3

furnish higher energy-level of MLCT states in IrC7-A (Figure

It is well-known that the photophysical features of the

cyclometalated Ir(III) complexes are generally dominated by

the electronic structures of the cyclometalating ligands.

Therefore, in order to interpret this unique color-tuning

behavior, density functional theory (DFT) calculations have

been conducted for ligands L-C5, L-C7, and L-C8 to obtain

their molecular orbital (MO) patterns. The DFT results

demonstrate that the electron density on the chelating carbon

6 and Table S5 ). Accordingly, IrC7-A exhibits a shorter

phosphorescent wavelength than that of IrC7. Despite the

noticeable electron density on the chelating carbon atom of

ligand L-C8 (Figure 6 and Table S5), IrC8(λ = 516 nm) still

shows the shortest emission wavelength among these

complexes. This should be due to IrC8 showing a ligand-

32−34

centered π−π* feature of the T states. Hence, the electron

1

density on the chelating carbon atom in L-C8 (2.85%) and that

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Table 5. Redox Properties of These ppy-Type Coumarin-Based Cyclometalated Ir(III) Complex Isomers

E_g(eV)

E_g^o^p^t/E_g^c^v

c

,

d

E_H_O_M_O(eV)

e

E_L_U_M_O(eV)

f

complex

E_p_a(V)

E_p_c(V)

a

b

IrC5

IrC7

IrC7-A

IrC8

0.54

0.52

0.53

0.64

−1.99, −2.12, −2.42

−2.05, −2.19, −2.49

−2.18, −2.32, −2.56

−2.29, −2.42, −2.51

2.06/2.53

2.08/2.57

2.12/2.71

2.40/2.93

−5.34

−5.32

−5.33

−5.44

−2.81

−2.75

−2.62

−2.51

a

b

Reversible; the value was set as E . All three values were irreversible or quasi-reversible; values were derived from the cathodic peak potential.

1

/2

c

Optical energy gap E_g^o^p^twas calculated from the absorption onset of their UV−vis absorption spectra. CV energy gap E = LUMO − HOMO.

d

cv

g

e

f

HOMO levels were calculated according to the equation HOMO = −(4.8 + E ) eV. LUMO levels were derived from the peak potential of the

pa

ﬁrst reduction wave of the CV data, and calculated according to the equation LUMO = −(4.8 + E ) eV.

pc

Figure 7. Device structure of these vacuum-deposited OLEDs as well as molecular structures and energy level diagram of the materials employed in

the OLEDs.

on the Ir(III) center will have a much weaker inﬂuence on the

the Ir(III) complexes in the oxidative scan while the Ir(III)

complexes can also capture electrons in the reverse scan) in the

range of 0.52−0.64 V, which can be attributed to oxidation of

the Ir(III) center (Table 5 and Figure S11). In the cathodic

scan, three irreversible reduction waves have been observed

(Figure S11). Their ﬁrst reduction wave should be ascribed to

the reduction of the heterocycle in the coumarin skeleton

because of the stronger electron-accepting ability, while the

other two reduction waves can be assigned to the reduction of

the pyridyl units in the organic ligands. Typically, the major

contribution to LUMO orbitals of ppy-type Ir(III) complexes

emissive T states of IrC8. In addition, the short conjugation of

1

L-C8 should aﬀord a high-energy level of its T states, which

1

coincides well with the short phosphorescent wavelength of

IrC8. On the basis of these results, we can conclude that

modulation of the electron density on the chelating carbon

atoms of the cyclometalating ligands can show eﬀective color-

tuning for the corresponding ppy-type cyclometalated Ir(III)

3

complexes possessing emissive T states with dominant MLCT

1

character, while it has a much weaker eﬀect on emission

wavelength of those with predominant ligand-centered π−π*

32−34

character for their emissive T states.

originates from the π* orbitals of pyridyl ring,

whereas

1

Electrochemical Properties. Electrochemical properties

of these ppy-type coumarin-based cyclometalated Ir(III)

complex isomers were investigated by cyclic voltammetry

measurements in acetonitrile with reference to ferrocene (Fc)

under a nitrogen atmosphere. The results are compiled in Table

according to the results of TD-DFT calculations for these ppy-

type coumarin-based cyclometalated Ir(III) complex isomers,

the π* orbitals of the pyridyl units are no longer the only major

contributor to the LUMOs, which also include the π* orbitals

of the heterocycles (between 24.33 and 28.30%) and the phenyl

rings (ranging from 24.81 to 33.23%) in the coumarin skeletons

(Table S4 ). In analogy with our previous reported liter-

5

. In the anodic scan, all of the ppy-type coumarin-based

cyclometalated Ir(III) complex isomers can give a reversible

oxidation wave (meaning that electrons can be removed from

30,50,51

atures,

the heterocycles in the coumarin skeletons can

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Inorganic Chemistry

Article

oﬀer a good electron sink for stabilization of the MLCT states,

thus lowering the LUMOs levels. Consequently, the LUMO

levels for these ppy-type coumarin-based cyclometalated Ir(III)

complex isomers range from −2.51 to −2.81 eV, much lower

triplet exited states of these ppy-type coumarin-based cyclo-

metalated Ir(III) complex isomers.

Current density−voltage−luminance (J−V−L) curves and

EL-eﬃciency−luminance characteristics for these vacuum-

deposited OLEDs are depicted in Figures 9 and 10 as well as

49,54

than the typically reported ppy-type Ir(III) complexes.

More importantly, the substantially lower LUMO levels will

deﬁnitely be beneﬁcial to promote EI/ET properties of these

ppy-type coumarin-based cyclometalated Ir(III) complex

isomers, which are crucial to give high EL performance of the

concerned OLEDs. The HOMO levels of these ppy-type

coumarin-based cyclometalated Ir(III) complex isomers are in

the range from −5.32 to −5.44 eV. In addition, the obtained

HOMO and LUMO levels based on CV data for these ppy-type

coumarin-based cyclometalated Ir(III) complex isomers

correlate well with the corresponding ones based on DFT

calculations (Figure 3 and Table 5).

Electroluminescent Properties. To investigate their

electroluminescence performance, vacuum-deposited OLEDs

using these ppy-type coumarin-based cyclometalated Ir(III)

complex isomers as dopants have been constructed with the

Figure 9. Current−density−voltage−luminance (J−V−L) character-

conﬁguration ITO/MoO (3 nm)/NPB (40 nm)/ Ir x wt

3

istic curves of their optimized OLEDs.

%

(

:CBP (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm)

Figure 7). In this device structure, well-known CBP is used as

the host material for these phosphorescent emitters, and the

in Figures S12 and S13. The EL performance of these vacuum-

deposition OLEDs are compiled in Table 6. For all the ppy-type

coumarin-based phosphorescent emitters, the optimal EL

device performance is achieved at a doping level of 8 wt % in

the emission layer. All the devices possess a relatively low turn-

on voltage in the range of 3.0−3.4 V, which might beneﬁt from

the enhanced EI/ET ability in association with coumarin

hole-injection layer (HIL) of MoO is fabricated by vacuum

3

deposition. NPB is used to form a hole-transporting and

electron-blocking layer, while TPBi is employed to fulﬁll both

electron-transporting and hole-blocking function of the

OLEDs. In addition, LiF works as an electron-injection layer.

Only the doping level is optimized to further explore the EL

potential of these ppy-type coumarin-based cyclometalated

Ir(III) complex isomers.

With a proper voltage applied, the OLEDs with IrC5 and

IrC7 as emitters can emit a red electrophosphorescence at 608

and 600 nm, respectively; an orange electrophosphorescence at

skeletons. Owing to high Φ and enhanced EI/ET ability, all the

p

phosphorescent OLEDs can show impressive EL performance

(

Table 6). The optimized red-emitting device A2 with IrC5 as

emitter can give a maximum external quantum eﬃciency (η )

of 10.5%, a maximum current eﬃciency (η ) of 29.0 cd A , and

a maximum power eﬃciency (η ) of 19.2 lm W , while the EL

ext

−1

L

5

84 nm can be observed for the OLEDs with IrC7-A as

−1

P

emitters. The OLEDs with IrC8 as emitter can emit a green

data for red-emitting device B based on IrC7 are 12.4% (η ),

ext

electrophosphorescence at 520 nm (Figure 8). These EL

−1

3

7.0 cd A (η ), and 24.1 lm W (η ), respectively (Table 6,

L P

Figures 10a,b). Additionally, the orange-emitting device C2

with IrC7-A as a dopant can show maximum EL eﬃciencies of

−

1

−1

1

3.9% (η ), 44.4 cd A (η ), and 32.1 lm W (η ) (Table 6

ext L P

and Figure 10c). Eventually, the green-emitting device D2

based on IrC8 achieves the highest EL eﬃciencies with η of

ext

−1

2

2.7%, η of 79.7 cd A , and η of 58.2 lm W , respectively

L P

(

Table 6 and Figure 10d). Besides their impressive EL

eﬃciencies, these vacuum-deposition OLEDs can exhibit very

−

2

high luminance of over 30 000 cd m at a driving voltage of less

than 20 V (Figure 9 and Table 6). Ren and co-workers reported

a series of coumarin-based cyclometalated Ir(III) complexes

with facial conﬁguration for EL research. The highest EL

−

1

−1

eﬃciencies of 20.5% (η ), 74.1 cd A (η ), and 52.4 lm W

(

ext

L

2

η_P) can be achieved. Yu and co-workers successively

Figure 8. EL spectra of the optimized OLEDs at ca. 10 V.

investigated the eﬀect of ancillary ligands on the EL

performance of the coumarin-based cyclometalated momonu-

clear and dinuclear Ir(III) complexes. OLEDs based on these

complexes show EL performance with maximum eﬃciencies of

emission wavelengths are consistent with PL emission wave-

lengths in CBP ﬁlms (Figures 8 and S10 and Table 2). As

depicted in Figure 8, the EL spectral line shapes of each

optimized OLEDs based on these ppy-type coumarin-based

cyclometalated Ir(III) complex isomers are greatly similar to

those of their PL spectra in both CH Cl solution and doped

−

1

23−25

9.77% (η ) and 35.24 cd A (η_L).

Meerholz and co-

ext

workers developed a novel coumarin-based Ir(III) complexes

capable of cross-linking by ancillary ligand. The ﬁnal OLEDs

−

1

can give maximum eﬃciencies of 18.4 cd A (η ) and 15.4 lm

L

−

1

26,27

W

(η_P).

Compared with all these EL results, the great

2

CBP ﬁlms (Figures 2b and S10), demonstrating that the

electrophosphorescence authentically arises from the lowest

potential of these novel ppy-type coumarin-based cyclo-

metalated Ir(III) phosphorescent emitters can been seen. To

L

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Figure 10. Relationships between EL eﬃciencies and luminance of their optimized devices. Devices: (a) A2, (b) B, (c) C2, and (d) D2.

Table 6. EL Data of the Electrophosphorescent OLEDs

luminescence L_m_a_x(cd m⁻²)^a

η_e_x_t(%)

η_L(cd A⁻¹)

η_P(lm W⁻¹)

λ_m_a_x(nm)

d

device

dopant (amt (wt %))

V_t_u_r_n_‑_o_n(V)

a

8

7.7

8

1

10.1

1

7

7.0

7

1

12.2

1

10.7

1

13.4

.6 (7.4)

23.8 (7.4)

21.2

23.7

29.0 (6.4)

27.7

28.4

20.6 (6.7)

19.3

20.5

37.0 (5.8)

36.3

35.9

34.7 (5.1)

34.3

33.4

44.4 (5.4)

43.1

43.0

27.8 (6.1)

26.0

27.7

55.4 (6.3)

48.7

12.6 (4.3)

12.4

10.5

19.2 (3.3)

17.4

12.6

12.3 (4.7)

12.1

9.1

24.1 (4.1)

22.3

15.8

25.9 (3.7)

22.7

17.2

32.1 (3.7)

30.7

22.1

17.3 (4.4)

17.2

13.5

30.3 (4.0)

29.9

608 (0.59, 0.40)

600 (0.54, 0.42)

584 (0.56, 0.43)

520 (0.29, 0.65)

b

A1

IrC5 (6.0)

3.3

47214 (16.9)

c

.6

0.5 (6.4)

A2

A3

B

IrC5 (8.0)

3.0

3.3

3.4

3.1

36246 (16.2)

37924 (17.6)

38776 (16.0)

36628 (15.6)

44486 (15.3)

37829 (15.3)

74748 (13.2)

73362 (12.5)

0.3

.5 (6.7)

IrC5 (10.0)

IrC7 (8.0)

.4

2.4 (5.8)

2.1

0.8 (5.1)

C1

C2

C3

D1

D2

IrC7-A (6.0)

IrC7-A (8.0)

IrC7-A (10.0)

IrC8 (6.0)

0.4

3.9 (5.4)

1

8

3.4

.7 (6.1)

8.1

8

1

.6

5.7 (6.3)

13.8

1

2

5.5

2.7 (5.4)

54.6

79.7 (5.4)

73.9

25.5

58.2 (3.7)

57.1

520 (0.29, 0.65)

IrC8 (8.0)

21.0

2.7

2

79.7

49.3

a

b

Maximum values achieved in the devices. The voltages given in the parentheses denoted the corresponding values achieved. Values were

−2

c

−2

d

obtained at 100 cd m . Values were achieved at 1000 cd m . Values were achieved at ca. 10 V, and CIE coordinates (x, y) are presented in

parentheses.

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the best of our knowledge, these phosphors have shown the

highest EL eﬃciencies for the phosphorescent complexes with

coumarin skeletons. In addition, Ma and co-workers recently

reported highly eﬃcient green noncoumarin Ir-based OLEDs

with CIE coordinates similar to those of device D2 showing

states. Due to high Φ and enhanced EI/ET ability, all the

p

phosphorescent OLEDs show impressive EL performance and

low eﬃciency roll-oﬀ. Finally, the vacuum-deposited green-

emitting OLED based on IrC8 achieves the highest EL

eﬃciencies, with external quantum eﬃciency (η ) 22.7%,

ext

−

1

−1

maximum eﬃciencies of 26.7% (η ) and 94.7 lm W (η ) and

current eﬃciency (η ) 79.7 cd A , and power eﬃciency (η )

ext

P

L

P

55

−1

low eﬃciency roll-oﬀ; Zhong and co-workers developed a

high-performance orange noncoumarin Ir-based OLEDs with

the CIE coordinates similar to those of device C2 giving

58.2 lm W , respectively, representing the most state-of-the-art

EL performances ever achieved by coumarin-based phosphor-

escent emitters. This work not only represents a new approach

to develop high eﬃcient EL phosphors with ease of color-

tuning but also provides some important information about

isomerization in the synthesis of ppy-type Ir(III) phosphor-

escent complexes.

−

1

maximum eﬃciencies of 17.4% (η ), 45.8 cd A (η ), and

ext

L

−

1

56

2

4.5 lm W (η_P). In comparison with these impressive

results, the EL data of our OLEDs demonstrates that these

novel ppy-type coumarin-based cyclometalated Ir(III) complex

isomers are promising candidates for high-performance

phosphorescent OLEDs.

ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.9b00534.

■

Importantly, all the devices can show a relatively low

eﬃciency roll-oﬀ less than 12% even at a high luminance of

*

S

−

2

1

000 cd m (Table 6). These results can be interpreted from

the viewpoints of both energy-level layout of the emission layer

in the OLEDs and the electronic characters of these coumarin-

based phosphorescent emitters. These ppy-type coumarin-

based phosphorescent emitters possess much higher HOMO

levels (ca. 5.35 eV, Figure 7) than does CBP (ca. 5.90 eV,

Figure 7). Meanwhile, they also have relatively lower LUMO

levels than does the host material CBP (ca. 2.50 eV, Figure 7).

Hence, these ppy-type coumarin-based phosphorescent emit-

ters can be eﬀectively electronically excited through direct

Selected structural data of X-ray crystallographic results

for IrC5, IrC7, and IrC7-A; HPLC spectra for IrC7 and

IrC7-A; optimized S geometry of the third isomer IrC7-

0

1

B by DFT and molecular orbital pattern of IrC8; H and

C NMR spectra for all the ﬁnal Ir(III) complex isomers;

13

TGA and CV traces of these ppy-type coumarin-based

cyclometalated Ir(III) complex isomers; photophysical

properties at 77 or 293 K in degassed CH Cl solution as

5

7−59

2

charge-trapping.

Furthermore, CV results have indicated

well as in CBP ﬁlms at 293 K for all the ﬁnal Ir(III)

complex isomers; contribution percentages of diﬀerent

segments to MOs for all the obtained Ir(III) complex

isomers; contribution percentages of electron density on

the chelating carbon atom to the HOMOs for all the ﬁnal

ligands; EL characteristics for the devices except the

optimized ones (PDF)

that introduction of the coumarin skeleton can endow these

phosphorescent emitters with enhanced EI/ET ability (Table

22

5

) in accordance with the reported literature. All these results

can imply that these devices should show an excellent balanced

charge transporting properties. As listed in Table 6, EL ability of

these coumarin-based phosphorescent emitters falls in the order

of IrC8 > IrC7-A > IrC7 > IrC5, which shows a trend similar to

that of Φ (Table 2). Hence, the EL eﬃciencies of these

Accession Codes

p

coumarin-based phosphorescent emitters should be mainly

CCDC 1866014−1866016 contain the supplementary crys-

tallographic data for this paper. These data can be obtained free

of charge via www.ccdc.cam.ac.uk/data_request/cif, or by

emailing data_request@ccdc.cam.ac.uk, or by contacting The

Cambridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

governed by their Φ . Thus, the OLED based on IrC8 show the

p

highest EL eﬃciencies among these ppy-type coumarin-based

phosphorescent complex isomers.

CONCLUSION

■

In conclusion, three coumarin-based isomers of ppy-type

cyclometalating ligands (L-C5, L-C7, and L-C8) have been

readily obtained by Stille cross-coupling reactions. With these

AUTHOR INFORMATION

Corresponding Authors

*E-mail: zhougj@mail.xjtu.edu.cn (G.Z.).

*E-mail: zhaoxinwu@mail.xjtu.edu.cn (X.W.).

■

∧

three ligands, three Ir(C N) (acac)-type and one Ir(C N)-

2

1

∧

(

C N)(acac)-type coumarin-based cyclometalated Ir(III)

2

complex isomers (IrC5, IrC7, IrC7-A, and IrC8) have been

obtained. Owing to the strong rigidity of the coumarin skeleton,

all the obtained ppy-type coumarin-based cyclometalated

ORCID

Yue Yu: 0000-0002-4758-8492

Xiaolong Yang: 0000-0002-0243-6698

Guijiang Zhou: 0000-0002-5863-3551

Zhaoxin Wu: 0000-0003-2979-3051

Ir(III) complex isomers show high Φ (ca. 0.4−1) to beneﬁt

p

their EL performances. Two coordination isomers emerging as

principal products (IrC7 and IrC7-A) are obtained in the

synthesis of corresponding coumarin-based cyclometalated

Ir(III) complexes because of two diﬀerent coordination sites

in ligand L-C7 forming a C−Ir bond. Their chemical structures

were further identiﬁed by single-crystal X-ray crystallography.

In addition, a broad range of phosphorescent color tuning from

green (IrC8, λ = 516 nm) to red (IrC5, λ = 608 nm) has been

realized, and a unique color-tuning mechanism is presented as

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This research was ﬁnancially supported by the National Natural

Science Foundation of China (21572176, 21875197, and

21602170), Fundamental Research Funds for the Central

Universities (cxtd2015003 and xjj2016061), the China

Postdoctoral Science Foundation (2015M580831 and

20130201110034), the Shaanxi Province Postdoctoral Science

well. It has been found that complexes IrC5, IrC7, and IrC7-A

3

show a prevailing MLCT character of the T states, while IrC8

1

exhibits a dominant ligand-centered π−π* character of T₁

N

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Inorganic Chemistry

Article

Foundation (Grant No. 2017BSHYDZZ02), the Key Creative

Scientiﬁc Research Team in Yulin City (2015cxy-25), Key

Laboratory Construction Program of Xi'an Municipal Bureau of

Science and Technology (2015cxy-25), Key Laboratory

Construction Program of Xi'an Municipal Bureau of Science

and Technology (201805056ZD7CG40). The characteriza-

tions of materials are supported by Instrument Analysis Center

of Xi’an Jiaotong University.

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∧

Cyclometalated Ir(III) Complexes with Pyreno[4,5-d]imidazole C N

Ligands as Triplet Photosensitizers for Triplet−Triplet Annihilation

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DOI: 10.1021/acs.inorgchem.9b00534

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Isomers of Coumarin-Based Cyclometalated Ir(III) Complexes with Easily Tuned Phosphorescent Color and Features for Highly Efficient Organic Light-Emitting Diodes

DOI: 10.1021/acs.inorgchem.9b00534

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