Rational Design and Characterization of ... - ACS Publications

Subscriber access provided by University of Newcastle, Australia

Article

Rational Design and Characterization of Heteroleptic Phosphorescent Complexes for Highly Efficient Deep-Red Organic Light-Emitting Devices Guomeng Li, Ping Li, Xuming Zhuang, Kaiqi Ye, Yu Liu, and Yue Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00348 • Publication Date (Web): 15 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Rational Design and Characterization of Heteroleptic Phosphorescent Complexes for Highly Efficient DeepRed Organic Light-Emitting Devices Guomeng Li, Ping Li, Xuming Zhuang, Kaiqi Ye, Yu Liu,* Yue Wang State Key Laboratory of Supramolecular Structure and Materials, Institute of Theoretical Chemistry, Jilin University, Changchun, 130012, P. R. China KEYWORDS. Electroluminescence (EL), Phosphorescence, Deep-red, Iridium complex, Fourmembered heterocycles.

ABSTRACT: Two new deep-red iridium(III) complexes, (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2), comprising two cyclometaling ligands of fluorophenyl-isoquinoline derivatives (fpiq and f2piq) and a N-heterocyclic carbene (NHC)-based ancillary ligand of N,N’-diisopropylbenzamidinate (dipba) are designed, synthesized and characterized. Given the unique four-membered Ir-N-C-N backbone built by the metal center and the ancillary ligand, both phosphors achieve significant improvement for their comprehensive optoelectronic characteristics. Density function theory (DFT) calculations and electrochemical measurements support the genuine pure red phosphorescent emission of fIr1 and dfIr2 based on their clearly distinct electron density distributions of the HOMO/LUMO orbitals compared with other red-emitting Ir(III) derivatives. Both new phosphors show deep-red emission with λmax values in the region of 650-660 nm with high PLQYs and short excited-state lifetimes. The phosphorescent

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 29

organic light emitting diodes (PhOLEDs) based on fIr1 and dfIr2 realize deep-red EL with the stable CIEx,y coordinates of (0.70, 0.30) and (0.69, 0.31), the peak EQE/PE values of 15.4%/9.3 lm W−1 and 16.7%/10.4 lm W−1 respectively, which maintain such high levels as 10.6%/3.5 lm W−1 and 10.8%/3.6 lm W−1 at the practical luminance of 1000 cd m−2. They are the highest EL values reported for the OLEDs with such deep-red CIE coordinates.

1. INTRODUCTION Phosphorescent organic light-emitting diodes (PhOLEDs) have attracted great attention and significantly improved electroluminescence (EL) efficiency due to their utilizing both singlet and triplet excitons for light emission,1-4 and the iridium (III) complexes are the most promising emitters for PhOLEDs due to their short phosphorescence lifetimes, easily sublimable feature and excellent color tunability.5-8 Currently, great success has been obtained in green, yellow and orange emitting complexes. However, the development of efficient deep red phosphors remains an unaddressed issue. Suffered from the energy gap law together with the drop in luminous flux in the saturated-red region,9-14 the high-efficiency deep-red PhOLEDs with the EL maximum at 640-680 nm and the Commission Internationale de L’Eclairage (CIEx,y) coordinates of x≥0.70 and y≤0.30 are rare so far.15-18 Although several more efficient deep-red devices have been achieved recently,19-22 but their EL efficiencies at the practical luminance of 100 and/or 1000 cd m−2, especially for the power efficiencies, still needed to be further improved. Therefore, further developing new high-performance red phosphorescent emitters by finding an effective molecule-design strategy that can improve both the OLED efficiency and the color purity, rather than merely reach a trade-off between them, thus achieving the balanced three primary colors in the upcoming commercial OLEDs for flat-panel displays and solid-state lighting, is still the research emphasis at present, of course, is also a great challenge.


Page 3 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For a deep red emitter its emission maximum and spectral profile are two important elements. A deep red color light required that emission maximum locate at the region of ≥640 nm and the emission wavelengths shorter than 600 nm must be avoided, because human eyes are very sensitive to this spectral fragment, which can significantly decrease the CIEx value and thus lead to the poor color purity of the corresponding red light.14 On the other hand, relatively short phosphorescent lifetime of the phosphors can effectively reduce the chance of the triplet-triplet annihilation (TTA) happened,12,23,24 and thus significantly improve the EQEs of the corresponding OLEDs, especially when they are working at high current density and/or high luminance. For the hereroleptic bis-cyclometalated Ir(III) complexes [(C^N)2Ir(LX), C^N= cyclometalated ligand and LX= ancillary or third ligand], it has been demonstrated that the combination of well-chosen LX ligand provided a simple and effective alternative approach to optimize the comprehensive optoelectronic characteristics of phosphors as opposed to adjusting the conjugation of the C^N ligands and/or introducing the electron-withdrawing or -donating groups to their different substituted position through complex synthesis procedure.25 In our previous efforts to develop highly efficient iridium complexes possessing the bipolar charge transporting ability, we have long noticed that employing the amidinate or guanidinate groups as the LX ligands,26,27 can lead to the significant red-shift of the resulting phosphorescent complexes and also induce their considerably shorter lifetimes, compared to their counterpart phosphorescent molecules with other LX ligands such as acac (acetylacetonate) group.28,29 Obviously, these facts prompt us to design and obtain some new red phosphors with simultaneously enhanced properties of color purity and EL efficiency. In this study, we design and synthesize two heteroleptic iridium complexes (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2) (as shown in Scheme 1), which employed 1-(4-fluorophenyl)isoquinoline (fpiq) and 1-(2,4-difluorophenyl)isoquinoline (f2piq) as the C^N ligands respectively and N,N’diisopropylbenzamidinate (dipba) possessing the classical N-heterocyclic carbene (NHC) structure as the LX ligand.30 Given the unique four-membered Ir-N-C-N backbone built by the metal center and the NHC-based ancillary ligand, both new phosphors not only showed remarkable red-shift color, shorter ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 29

lifetimes relative to their parent complexes (fpiq)3Ir and (fpiq)2Ir(acac),12,13 but exhibited the desirable balanced and sufficient charge-transporting properties. The combined effects of these multiple property enhancements led to the high-performance EL emission in the PhOLEDs based on fIr1 and dfIr2, which realize the most efficient deep-red OLEDs with similar CIE coordinates up to date, in terms of their low driving voltages, high maximum efficiencies and the values at the practical luminance based on a rather wide doping-concentration range of 5-20 wt%.20,23,25 It is also worth mentioned that both EL spectra are very high quality possessing almost 100% red emission bands beyond 600 nm with steeply risen curves those extending into the near-infrared region, thus either one of them lie at the right bottom corner of the CIE chromaticity coordinates, which will effectively expand the color gamut realized by OLEDs and thus lead to more saturated visual effect for their potential display application. More importantly, this result indicated that our molecular design strategy for new high-performance red emitters is feasible, which indeed realizes a win-win improvement for both EL efficiency and color purity. 2. EXPERIMENTAL SECTION General Information.

1

H NMR spectra were measured by a Bruker AVANCE 500 MHz

spectrometer, where CDCl3 and tetramethylsilane were used as the solvent and internal standard respectively. Mass spectra were performed on a GC/MS mass spectrometer. Elemental analyses were evaluated using a flash EA 1112 spectrometer. A Shimadzu UV-2550 spectrophotometer was used to obtain Ultraviolet−visible (UV−vis) absorption spectra, and Photoluminescent (PL) spectra in CH2Cl2 was undertaken by Shimadzu 5301PC fluorescence spectrophotometer. Photoluminescent (PL) spectra of films were performed by a Perkin-Elmer LS-55 fluorescence spectrometer, where a Xe arc lamp was used as the excitation source and their absolute phosphorescence quantum yields were quantified by an Edinburgh FLS920 spectrometer combined with a calibrated integrating sphere. TGA characteristics were demarcated by a TA Q500 thermogravimeter while the heating rate was 10 °C min-1 and the


Page 5 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


measuring range was 25 to 900 °C under nitrogen to obtain compound weight loss. DSC measurements were quantified by a NETZSCH DSC204 instrument and the heating rate was 10 °C min-1 in the nitrogen atmosphere. The time-correlated single-photon counting (TCSPC) system was adopted to determine compound emission lifetime data and a Edinburgh Instruments EPL375 379 nm picosecond diode laser was applied to excite the complexes. All solvents were degassed three times before using, Cyclic voltammetry (CV) curves were obtained by a BAS 100W Bioanalytical electrochemical work station, with a platinum working electrode. A platinum wire was used as auxiliary electrode with Ag/AgCl wire as reference electrode. The ferrocene/ferrocenium (Fc/Fc+) couple was performed to reference the voltammograms. A 0.1 M solution of n-Bu4NPF6 in dry CH2Cl2 was used as the supporting electrolyte for oxidation. Due to the limitation27 in detecting reduction potentials in the range of -2.7 V to -3.5 V in CH2Cl2, only the oxidation potential for (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2obtained) were detected. The HOMO levels of fIr1 and dfIr2 were calculated from the onset of the oxidation potential in the cyclic voltammetry (CV) waves. The bandgaps (Eg) of fIr1 and dfIr2 were determined from the absorption edge of their UV-vis spectra. The LUMO levels of fIr1 and dfIr2 were evaluated by considering the difference between their HOMO levels and the corresponding Eg. AFM pictures and RMS values were measured by an S II Nanonaviprobe station 300Hz (Seiko, Japan). Device Fabrication and Measurement. Patterned indium tin oxide (ITO)-coated glass substrates (20 Ω/square) were cleaned in turn by using detergent solution in an ultrasonic bath, then deionized water, ethanol, acetone, and isopropanol. All organics were purified by sublimation before using in vacuum deposition process, and the depositing rate was 0.2 Å s-1 at a base pressure of 5 x 10-4 Pa. the depositing rate of LiF layer (1 nm) was 0.1 Å s-1 and the depositing rate of Al layer (200 nm) was 3 Å s-1. The overlapping active area of devices was 2 × 2.5 mm2. EL spectra and L-J-V values were obtained by combining a computer-controlled Keithley model 2400 voltage-current source and Spectrascan PR-650 spectrophotometer under room temperature in ambient conditions. The samples for TOF (time-of-flight) ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 29

measurements were prepared on the glass substrate covered with indium tin oxide (ITO) layer, the corresponding devices structures were [ITO/fIr1 (2 µm)/Al], [ITO/dfIr2 (2 µm)/Al]. A frequencytripled (355 nm) Nd:YAG laser was used to emit an intense short light pulse to generate photocarriers at one side of the devices. They were measured at the electric field of 5.0 × 105 V cm−1 at 293 K under ambient conditions. Hole-only and electron-only single-carrier devices based on the active layers of the neat fIr1 and dfIr2 films and the corresponding doped thin films of Bebq2: fIr1 and dfIr2 (5, 10 and 20 wt%) with the structures of [ITO/NPB (10 nm)/active layer (50 nm)/NPB (10 nm) Al (100 nm)] for hole-only device and [ITO/TPBi (10 nm)/active layer (50 nm)/TPBi (10 nm) /LiF (1 nm)/Al (100 nm)] for electron-only device were fabricated. Materials. All the chemical reagents/solvents purchased from commercial sources were used without further purification. Anhydrous tetrahydrofuran was distilled by sodium together with benzophenone ketyl under inert atmosphere and further degassed by the freeze-pump-thaw method before being used. All experimental tools including glassware, magnetic stirring bars were dried in a convection oven for more than 4 hours. Thin layer chromatography (TLC) was used to monitor reaction progress under UV light of 254 and 365 nm (Silica gel 60 F254, Merck Co.), and then silica gel 60 G (particle size 5~40 µm, Merck Co.) was used for silica column chromatography. Scheme 1. The synthetic routes of complexes fIr1 and dfIr2.


Page 7 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Synthesis of [(fpiq)2Ir(µ-Cl)] 2 (1). According to a modified Nonoyama procedure,28 IrCl3·3H2O (2.5g, 7 mmol), 2.5 equiv of the ligand of 1-(4-fluorophenyl) isoquinoline (3.9 g, 17.5 mmol), a mixture solvent of 60 mL 2-methoxyethanol/water (3:1, v/v) were added to a 250 mL two neck flask, and heated to reflux ( ～ 110 oC) for 6-7 h under nitrogen. After the reaction solution was cooled to room temperature, then the deionized water (60 mL) was slowly added and the resulting mixture was filtered. The precipitate was washed with hexane and ethyl ether several times to provide the desired complex as a red solid (4.1 g, 86% yield). Synthesis of [(f2piq)2Ir(µ-Cl)] 2 (2). According to the similar procedure for the preparation of [(fpiq)2Ir(µ-Cl)]2 (1), complex [(f2piq)2Ir(µ-Cl)]2 (2) was prepared from 1-(2,4-fluorophenyl) isoquinoline (4.2 g, 17.5 mmol) and IrCl3·3H2O (2.5g, 7mmol) as a red solid (4.4 g, 89% yield). Synthesis of (fpiq)2Ir(dipba) (fIr1). A hexane solution with BuLi (0.20 mL x 2.5 M) was dropped to bromobenzene (78 mg, 0.5 mmol) in 10 mL hexane. After being stirred about 45 min at room temperature for half an hour, N,N-diisopropylcarbodiimide (63 mg, 0.5 mmol) was dropped into and then stirred it for another 30 min. The resulting mixture solution was added dropwise to 1 (335 mg, 0.25 mmol) in 25 mL hexane solution. This reaction solution was stirred at 80 oC for 8 h and gradually cooled to room temperature. The solvent was removed by evaporated under reduced pressure and the resultant precipitate was washed by Et2O (20 mL) twice to give a red powder (fpiq)2Ir(dipba) (fIr1) (306mg, 73% yield). ESI-MS: m/z 839.91 (M+) (calcd: 840.27). Anal. Calcd for C43H37F2IrN4: C, 61.48; H, 4.44; N, 6.67. Found: C, 61.87; H, 4.26; N, 6.68. 1H NMR (500 MHz, CDCl3) δ 9.34 (d, J = 6.4 Hz, 2H), 8.88 (d, J = 8.1 Hz, 2H), 8.18 (dd, J = 8.6, 5.8 Hz, 2H), 7.98 (d, J = 7.4 Hz, 2H), 7.80 – 7.71 (m, 4H), 7.60 (d, J = 6.3 Hz, 2H), 7.43 (t, J = 7.3 Hz, 2H), 7.40 – 7.28 (m, 3H), 6.58 (t, J = 7.7Hz, 2H), 5.99 (d, J =7.6Hz, 2H), 3.23 - 3.27 (m, 2H), 0.67 (d, J = 6.2 Hz, 6H), -0.14 (d, J = 6.2 Hz, 6H). Synthesis of (f2piq)2Ir(dipba) (dfIr2). According to the same procedure for fIr1, (f2piq)2Ir(dipba) (dfIr2) was synthesized as a red powder (346mg, 79% yield). ESI-MS: m/z 876.13 (M+) (calcd: ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 29

876.24). Anal. Calcd for C43H35F4IrN4: C, 58.96; H, 4.03; N, 6.40. Found: C, 59.28; H, 3.93; N, 6.40. 1H NMR (500 MHz, CDCl3) δ 9.29 (d, J = 6.4 Hz, 2H), 8.35 (t, J = 9.4 Hz, 2H), 7.97 (d, J = 8.1 Hz, 2H), 7.78 – 7.71 (m, 4H), 7.66 (t, J = 7.5 Hz, 2H), 7.45 (d, J = 7.5 Hz, 2H), 7.40 (t, J = 7.3 Hz, 1H),7.30 7.34 (m, 2H), 6.35 (t, J = 7.7Hz, 2H), 5.54 (d, J = 7.6 Hz, 2H), 3.28 - 3.23 (m, 2H), 0.65 (d, J = 6.3 Hz, 6H), -0.07 (d, J = 6.3 Hz, 6H). Table 1. Thermal, photophysical and electrochemical data for fIr1 and dfIr2. Tm

Td

λabs,maxa

λPL,maxa

(oC)

(oC)

(nm)

(nm)

fIr1

330

332

300, 342, 455

658

0.10

dfIr2

331

348

295, 340, 448

650

0.15

Compound

a

Ф

τobsa

Egb

HOMOc LUMOd

(µs) (eV)

(eV)

(eV)

0.08

2.01

-4.95

-2.94

0.11

2.06

-5.05

-2.99

a

Measured in neat film; b Calculated from the onset of absorption spectra; c Estimated from the E1/2 values of oxidation potentials in deaerated CH2Cl2; d Deduced from the energy gap (Eg) and HOMO levels. 3. RESULTS AND DISCUSSION Molecular Structure and Photophysical Properties. The synthetic routes of the IrIII complexes (fpiq)2Ir(dipba) (fIr1) and (f2piq)2Ir(dipba) (dfIr2) are illustrated in Scheme 1. Both new phosphors exhibited high thermal decomposition temperatures (Td, corresponding to 5% weight loss) of more than 350 °C (see Figure S3 in S.I and the data summarized in Table 1), indicating their high morphologic stability of amorphous phase in the deposited films. As shown in Figure 1a, the four-membered heterocycles containing the NHC analogue in both fIr1 and dfIr2 were accessed by employing a more N-electron-rich ligand with a bulkier backbone substituent to favor N,N-chelation of the electrondeficient metal center.30,33 Thus, the isopropyl on the nitrogen and the phenyl ring on the carbon atoms tend to push the lone pairs of electrons on the nitrogen atoms toward the metal center, which could greatly favor the chelating bonding mode.34-36 On the other hand, the four-membered Ir-NCN backbones could lead to significantly more acute N-Ir-N bite angles than seen for the larger sixmembered heterocycles based on such as acac group and its derivatives.37 It is well understood, this ACS Paragon Plus Environment

Page 9 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


compact and tensile bonding mode is favorable to the short phosphorescent lifetime, and the relatively short distances between Ir atom and NCN ligand is also crucial for improving the MLCT (metal-ligand charge transfer) of the resulting phosphorescent complexes,38 thus leading to the increased radiative transfer and the enhanced photoluminescence (PL) quantum efficiency. The UV/Vis absorption and PL spectra of fIr1 and dfIr2 in the neat films are shown in Figure 1b. The intense absorption bands below 380 nm arise from the spin-allowed intraligand 1π-π* transitions, and the relatively weak and broad bands in the visible region (400-650 nm) should be assigned to the 1MLCT (metal-ligand charge transfer) and 3MLCT. Upon light irradiation at 450 nm, the neat films of fIr1 and dfIr2 show deep-red emission peaks of 658 and 650 nm with the quantum yield values of 0.10±0.01 and 0.15±0.01, and the lifetimes as short as 0.08 and 0.11 µs respectively (Figure 1b, inset), indicating that efficient spin−orbit coupling results in intersystem crossing from the singlet to triplet states in both phosphors. Moreover, the short lifetimes are very useful for increasing spin-state mixing and decreasing the triplet exciton aggregation, and thus reduce the EL efficiency roll-off.39,40 All the photophysical data are summarized in Table 1. Theoretical Calculations, Electrochemical and Charge-Transporting Properties. Density functional theory (DFT) was used to gain insight into the fundamental properties and rationalization of fIr1 and dfIr2,41 which could hopefully serve as an important supplement to experiments in guiding the design of more efficient heteroleptic phosphorescent Ir(III) complexes.42-44 The geometrical configurations were optimized by the Becke three-parameter hybrid exchange and the Lee-Yang-Parr correlation functional (B3LYP) and 6-31G** basis set using the Gaussian 03 software package.45,46 Figure 2a and 2b depict the electron density distributions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the two complexes. In order to deeply understand that these complexes' optoelectronic properties are affected by tuning dipba group instead of other LX ligands such as the classical acac group, we also calculated two counterpart



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 29

phosphors of (fpiq)2Ir(acac) (RfIr1) and (f2piq)2Ir(acac) (RdfIr2) for comparison (their chemical formulas as shown in Figure S4 in S.I.). Compared with that the HOMO orbitals of RfIr1 and RdfIr2

Figure 1. (a) Schematic structures of four- and six-membered backbones. (b) UV-vis absorption and PL spectra of fIr1 and dfIr2 neat films. Inset: the corresponding transient PL spectra. mostly localized on π* orbitals of both C^N ligands (54.4 and 53.3% respectively) together with the dorbitals of iridium atom (41.3 and 41.9%) and slight dependence on the π orbitals of acac ligands (4.2 and 4.7%), the HOMO of fIr1 and dfIr2 were distributed over the π* orbitals of C^N ligands (13.1 and 11.4%) and d metal orbitals (24.5 and 23.9%), with a large contribution from the π orbitals of ancillary ligand (dipba group, 62.3 and 64.6%) as well (Table 2). Their HOMO localized predominantly on the dipba part should also be ascribed to the distinctive and compact Ir-N-C-N four-membered backbone, which indeed led to this ancillary ligand being largely involved to construct the molecular orbitals of


Page 11 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fIr1 and dfIr2. This clear difference above resulted in that the HOMO levels of fIr1 and dfIr2 (-4.74 and -4.87 eV) were clearly higher than those of RfIr1 and RdfIr2 (-5.01 and -5.21 eV respectively). On the contrary, the electrons in the LUMO orbitals of the four complexes were distributed similarly, which are distributed mostly on the π* orbitals of C^N ligands (~95%) as well as the d orbitals of iridium atom (less than 5%), leading to the similar LUMO levels of -1.81 and -1.86 eV for fIr1 and RfIr1, and -1.95 and -1.92 eV for dfIr2 and RdfIr2 respectively.

Figure 2. Molecular orbital diagrams and HOMO-LUMO (H-L) energy gaps for (a) fIr1 and (b) dfIr2 and two reference complexes (RfIr1/RdfIr2). The structures of RfIr1 and RdfIr2 are shown in Figure S4. Table 2. HOMO, LUMO levels and MOs on the complexes from DFT calculations. Compound

Orbital

Energy (eV)

MO composition (%)

HOMO

-4.74

24.5% d(Ir)+ 13.1% π*(sfpiq)+62.3% π(dipba)

LUMO

-1.81

3.2% d(Ir)+95.7% π*(sfpiq)

HOMO

-4.87

23.9% d(Ir) +11.4% π*(dfpiq)+64.6% π(dipba)

LUMO

-1.95

2.9% d(Ir) + 95.9% π*(dfpiq)

HOMO

-5.01

41.3% d(Ir)+54.4% π*(sfpiq) +4.2% π(acac)

fIr1

dfIr2 RfIr1



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

LUMO

-1.86

3.9% d(Ir) + 95.5% π*(sfpiq)

HOMO

-5.21

41.9% d(Ir)+ 53.3% π*(dfpiq) +4.7% π(acac)

LUMO

-1.92

4.2% d(Ir) + 95.4% π*(dfpiq)

Page 12 of 29

RdfIr2

Figure 3. Cyclic voltammograms of fIr1 and dfIr2 in a freshly distilled CH2Cl2 for oxidation behaviors. The respective HOMO and LUMO energy levels were experimentally determined from redox curves relative to the vacuum level (as shown in Figure 3), and all the electrochemical data are summarized in Table 1. The HOMO and LUMO values are found to be -4.9 and -2.9 eV for fIr1, -5.0 and -2.9 eV for dfIr2, which showed much narrower HOMO-LUMO band gap energies than that of RfIr1 with the HOMO/LUMO of -5.47/-2.89 eV.13 This is consistent well with the result of the computational investigation above. Such good linear correlation between theoretical and electrochemical energy gaps together with the remarkably red-shift phosphorescent emission, which were evaluated in photophysical measurement of fIr1 and dfIr2 compared with the reference complex such as RfIr1 with orange-red emission peak of ~600nm, indeed confirmed the significant effect on the frontier molecular orbitals in fIr1 and dfIr2 through choosing the dipba group as the ancillary ligand, leading to their distinctively different photoelectric properties from those of other Ir(III) molecules using the acac-based ancillary ACS Paragon Plus Environment

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ligands. On the other hand, such effective separation of electron densities of HOMO and LUMO in fIr1 and dfIr2 molecules provided the bidirectional charge-transporting channels, where the holes and electrons can flow smoothly along their respective pathways.27,28,47,48 This is indeed conductive to endow both phosphorescent molecules with the bipolar charge-transporting ability. Correspondingly, the time-of-flight (TOF) measurements (see Figure S5 in S.I) revealed that the comparable hole and electron mobilities of fIr1 are 1.0 × 10−3 and 6.4 × 10−4 cm2 V−1 s−1, and dfIr2 are 7.8 × 10−4 and 5.6 × 10−4 cm2 V−1 s−1, respectively, which are the high levels for a phosphorescent emitter and even higher than that of the typical hole-transporting material of NPB (~10−4 cm2 V−1 s−1) and the electron-transporting material of TPBi (~10−5 cm2 V−1 s−1).49-51 This desirable bipolar character indicates that such multifunctional phosphors could play another important role for conducting both hole and electron in addition to emitting light, and thus serve as new emitters in the quest for highly efficient deep-red PhOLEDs. Electrophosphorescent Devices. A series of deep-red OLEDs were fabricated by employing fIr1/dfIr2 dopant emitters co-evaporated with a well-matched energy gap host molecule bis(10hydroxybenzo(h)quinolinato)beryllium complex (Bebq2) as the emitting layers (EMLs).52-54 All the devices contained 4,4′-bis(N-(1-naphthyl)-N-phenylamino)biphenyl (NPB) served as a hole-transporting layer (HTL) and bis(2-(2-hydroxyphenyl)-pyridine)beryllium) (Bepp2) as an electron-transporting layer (ETL) (Figure 4a). The device structures were [ITO/NPB (40 nm)/emitting layer (EML, 30 nm)/Bepp2 (30 nm)/LiF (0.5 nm)/Al], with the doping EML corresponding to Bebq2:fIr1 of 5 wt%, 10 wt% and 20 wt% for device F5, F10 and F20 respectively, and Bebq2:dfIr2 of 5 wt%, 10 wt% and 20 wt% for device DF5, DF10 and DF20 respectively. Here, there is the slight barrier (~0.1 eV) against hole injection from NPB into these EMLs based on Bebq2 host, and no electron injection barrier from Bepp2 into such EMLs. Complex Bebq2 was employed as the host owing to its high enough triplet energy (~2.5 eV) and sufficient electron-transporting ability based on its rather high and similar high electron



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 29

mobility level (~10−4 cm2 V−1 s−1) to Bepp2.55-58 Moreover, the LUMO level of Bebq2 was 0.5 eV lower than that of NPB, leading to effective exciton blocking. Therefore, it was expected that charge carrier and triplet exciton would be confined within the EMLs. All the devices based on respective doping concentrations of 5, 10 and 20 wt% represented almost same emission curves with the EL maximum at the range of 644-652 nm. They are independent of the applied voltage and there is no host emission being observed, which are consistent with their PL emission of the doped thin films corresponding to these EMLs (see Figure S6 and S7 in S.I). Although all six devices emitted pure red light as shown in Figure 4b, the detailed EL data summarized in Table 3 indicated that the slightly different spectroscopic features resulted in more red CIEx,y coordinates of ~(0.70, 0.30) in devices F5, F10 and F20 than those of ~(0.69, 0.30) in devices DF5, DF10 and DF20, which are measured at the high and practical luminance of 1000 cd m−2.


Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) Energy diagram of the materials used in the OLEDs. (b) EL spectra of F5, F10, F20, DF5, DF10 and DF20 at the practical luminance of 1000 cd m−2. Inset: a photograph of F5 illuminating at 1000 cd m−2. Table 3. EL data of all the devices. Vturn-on Device

a

Lmax /cd m−2

PE b /lm W−1

EQE b /%

EL λmaxc/nm, CIE (x,y)c

(V)

(V at Lmax)

F5

2.4

16330 (9.3)

9.3/6.1/3.5

15.4/13.3/10.6

648, (0.70, 0.30)

F10

2.4

13300 (9.0)

7.4/5.0/2.5

13.7/11.7/8.9

652, (0.70, 0.30)

F20

2.3

11090 (8.7)

5.5/3.4/1.6

11.2/9.3/6.9

652, (0.70, 0.29)

DF5

2.6

16050 (8.8)

10.4/6.2/3.6

16.7/13.6/10.8

644, (0.69, 0.31)

DF10

2.6

12200 (8.7)

8.2/4.6/2.6

14.1/10.9/8.0

644, (0.69, 0.30)

DF20

2.5

12380 (8.6)

6.6/3.6/1.8

11.6/9.1/6.5

648, (0.69, 0.30)

a

Recorded at 1 cd m−2. b Order of maximum, then values at 100 and 1000 cd m–2. c Measured at 1000 cd m−2.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 29

Figure 5. (a) Current density-voltage-luminance (J-V-L) curves of F5, F10, F20. (b) Current densityvoltage-luminance (J-V-L) curves of DF5, DF10 and DF20. (c) Power efficiency (PE)-luminance (L)external quantum efficiency (EQE) curves of F5, F10 and F20. (d) Power efficiency (PE)-luminance (L)-external quantum efficiency (EQE) curves of DF5, DF10 and DF20. The current

density-voltage-luminance

(J-V-L) and

EL

efficiency-luminance

(PE/EQE-L)

characteristics of F5, F10, F20, DF5, DF10 and DF20 are shown in Figures 5a, 5c and Figures 5b, 5d, respectively. Obviously, all devices exhibited rapidly increasing J-V and L-V curves based on rather low turn-on voltages of 2.3~2.6 V, which reached the extremely high luminance of ~10000 cd m−2 at the driving voltages as low as 7.8-8.4 V. To the best of our knowledge, these devices are the brightest OLEDs with similar CIE coordinates. Furthermore, all these deep-red devices based on different doping concentrations generally realized the desired peak EQEs of more than 11% with remarkably low rolloff. Among them, F5 and DF5 showed higher EQE/PE than those of the other cases with the relatively higher doping concentrations, in terms of not only their peak values: 15.4%/9.3 lm W−1 and 16.7%/10.4 ACS Paragon Plus Environment

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lm W−1 respectively, but also the rather high levels of 10.6%/3.5 lm W−1 and 10.8%/3.6 lm W−1 maintained at the practical luminance of 1000 cd m−2. Achieving such high efficiencies at the highluminance level of 1000 cd m−2 requiring for displays and/or solid-state lighting, accompanied with such desired CIE coordinates are indeed the highest reported for deep-red devices based on either spincoated or vacuum-deposited fabrication process,19-22 especially those employing such simple device structure as ITO/HTL/EML/ETL/LiF/Al. These devices showed good EL performance durability also, their maintaining highly efficient (the efficiency degradation ≤15%), stable EL spectra and CIE without host emission observed after continuous open-condition operation at a brightness of 100 cd m-2 for 2 hours. In addition, it is worth noting that, the current densities of the devices with the relatively high doping concentrations is generally higher than those of the low-concentration cases, which are consistent with the J-V characteristics of the corresponding hole-only and electron-only single-carrier devices as shown in Figure 6, indicating that these emitting molecules can also serve as the hole- and electron-transporting channels, which facilitate the charge injection and transport across the EMLs. These processes could result in the improved and sufficient charge fluxes, and thereby favor the triplet exciton formation in EMLs. On the other hand, the decrease in EL efficiencies (Figures 5c, 5d) together with the same dependences of luminous intensity on the increasing dopant concentration (see Figure S11 and S12 in S.I), reflect the gradual emerging TTA within EMLs owing to the enhancing exciton density along with the concentration increased from 5 to 20 wt%. Nevertheless, such low dependence of the EL performance on doping concentration in a certain range as these PhOLEDs is indeed the rare advantage of fIr1 and dfIr2 as efficient deep-red phosphorescent emitters and also renders their multifunction in nature, leading to the easily-fabricated highly efficient PhOLEDs being good for the commercial mass production in future.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 29

Figure 6. Current density-voltage (J-V) characteristics of the hole-only devices for (a) the neat fIr1 film and the doped thin films of Bebp2: fIr1 (5, 10 and 20 wt%), and (b) the neat dfIr2 film and the doped thin films of Bebp2: dfIr2 (5, 10 and 20 wt%). Current density-voltage (J-V) characteristics of the electron-only devices for (c) the neat fIr1 film and the doped thin films of Bebp2: fIr1 (5, 10 and 20 wt%), and (d) the neat dfIr2 film and the doped thin films of Bebp2: dfIr2 (5, 10 and 20 wt%). 4. CONCLUSION In summary, we have designed and synthesized two highly efficient deep-red IrIII phosphors, by employing our original amidinate group (dipba) as the ancillary ligand, which indeed is an effective molecule-design strategy for simultaneously enhancing the charge transporting ability, the color purity as well as the emission property of the phosphorescent emitters. As a result, both phosphors realized the best EL performance for the red OLEDs according with the EL maximum at 640-660 nm and the ACS Paragon Plus Environment

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


excellent pure red CIEx,y of (0.70±0.01, 0.30±0.01) reported up to date. They showed rather low driving voltages with the turn-on voltage of 2.6 V and reached the luminance of more than 10000 cd m−2 at 7-8 V, as well as the very high maximum EQE of 15.4/16.7% and PE of 9.3/10.4 lm W−1 for fIr1/dfIr2based OLEDs respectively. These findings demonstrate that our design strategy for developing highperformance red emitting phosphorescent complexes is indeed feasible, and more importantly, it injects new energy to realize the simple, high-efficiency and high-quality deep-red PhOLEDs. ASSOCIATED CONTENT Supporting Information. 1

H NMR Spectra, typical time of flight transient, differential scanning calorimetric (DSC) and

thermogravimetric analysis (TGA) data of new complexes, PL spectra and the absolute phosphorescence quantum yields of doped films, X-ray crystallographic data files (CIF), crystal data and structure refinement parameters for fIr1 and dfIr2. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (973 Program, 2013CB834805, 2015CB65500), Natural Science Foundation of China (51373062, 91333201, 21221063, 51473028, 21473214), Key Scientific and Technological Project of Jilin Province (20150204011GX), and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

REFERENCES (1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151-154. (2) Chi, Y.; Chou, P. T. Transition-Metal Phosphors with Cyclometalating Ligands: Fundamentals and Applications. Chem. Soc. Rev. 2010, 39, 638-655. (3) Xiao, L.; Chen, Z.; Qu, B.; Luo, J.; Kong, S.; Gong, Q.; Kido, J. Recent Progresses on Materials for Electrophosphorescent Organic Light-Emitting Devices. Adv. Mater. 2011, 23, 926-952. (4) Xu, H.; Chen, R.; Sun, Q.; Lai, W.; Su, Q.; Huang, W.; Liu, X. Recent Progress in MetalOrganic Complexes for Optoelectronic Applications. Chem. Soc. Rev. 2014, 43, 3259-3302. (5) Holder, E.; Langeveld, B. M. W.; Schubert, U. S. New Trends in the Use of Transition Metal–Ligand Complexes for Applications in Electroluminescent Devices. Adv. Mater. 2005, 17, 1109-1121. (6) Bettington, S.; Tavasli, M.; Bryce, M. R.; Beedy, A.; Al-Attar, H.; Monkman, A. P. TrisCyclometalated Iridium(III) Complexes of Carbazole(fluorenyl)pyridine Ligands: Synthesis, Redox and Photophysical Properties, and Electrophosphorescent Light-Emitting Diodes. Chem. Eur. J. 2007, 13, 1423-1431. (7) Tavasli, M.; Moore, T. N.; Zheng, Y. H.; Bryce, M. R.; Fox, M. A.; Griffiths, G. C.; Jankus, V.; Al-Attar, H. A.; Monkman, A. P. Colour Tuning From Green to Red by Substituent Effects in Phosphorescent Tris-Cyclometalated Iridium(III) Complexes of Carbazole-Based Ligands: Synthetic, Photophysical, Computational and High Efficiency OLED Studies. J. Mater. Chem. 2012, 22, 6419-6428.


Page 21 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Fan, C.; Yang, C. Yellow/Orange Emissive Heavy-Metal Complexes as Phosphors in Monochromatic and White Organic Light-Emitting Devices. Chem. Soc. Rev. 2014, 43, 64396469. (9) Caspar, J. V.; Meyer, T. J. Application of the Energy Gap Law to Nonradiative, Excited-State Decay. J. Phys. Chem. 1983, 87, 952-957. (10) Cummings, S. D.; Eisenberg, R. Tuning the Excited-State Properties of Platinum(II) Diimine Dithiolate Complexes. J. Am. Chem. Soc. 1996, 118, 1949-1960. (11) Chen, C. T. Evolution of Red Organic Light-Emitting Diodes: Materials and Devices. Chem. Mater. 2004, 16, 4389-4400. (12) Su, Y. J.; Huang, H. L.; Li, C. L.; Chien, C. H.; Tao, Y. T.; Chou, P. T.; Datta, S.; Liu, R. S. Highly Efficient Red Electrophosphorescent Devices Based on Iridium Isoquinoline Complexes: Remarkable External Quantum Efficiency Over a Wide Range of Current. Adv. Mater. 2003, 15, 884-888. (13) Li, C. L.; Su, Y. J.; Tao, Y. T.; Chou, P. T.; Chien, C. H.; Cheng, C. C.; Liu, R. S. Yellow and Red Electrophosphors Based on Linkage Isomers of Phenylisoquinolinyliridium Complexes: Distinct Differences in Photophysical and Electroluminescence Properties. Adv. Funct. Mater. 2005, 15, 387-395. (14) Fan, C. H.; Sun, P.; Su, T. H; Cheng, C. H. Host and Dopant Materials for Idealized DeepRed Organic Electrophosphorescence Devices. Adv. Mater. 2011, 23, 2981-2985. (15) Song, Y. H.; Yeh, S. J.; Chen, C. T.; Chi, Y.; Liu, C. S.; Yu, J. K.; Hu, Y. H.; Chou, P. T.; Peng, S. M.; Lee, G. H. Bright and Efficient, Non-Doped, Phosphorescent Organic Red-LightEmitting Diodes. Adv. Funct. Mater. 2004, 14, 1221-1226.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

(16) You, Y.; An, C. G.; Kim, J. J.; Park, S. Y. A Deep Red Phosphorescent Ir(III) Complex for Use in Polymer Light-Emitting Diodes: Role of the Arylsilyl Substituents. J. Org. Chem. 2007, 72, 6241-6246. (17) Bae, H. J.; Chung, J.; Kim, H.; Park, J.; Lee, K. M.; Koh, T. W.; Lee, Y. S.; Yoo, S.; Do, Y.; Lee, M. H. Deep Red Phosphorescence of Cyclometalated Iridium Complexes by o-Carborane Substitution. Inorg. Chem. 2014, 53, 128-138. (18) Sree, V. G.; Cho, W.; Shin, S.; Lee, T.; Gal, Y. S.; Song, M.; Jin, S. H. Highly Efficient Solution-Processed Deep-Red Emitting Heteroleptic Thiophene-Phenylquinoline Based Ir (III) Complexes for Phosphorescent Organic Light-Emitting Diodes. Dyes and Pigments. 2017, 139, 779-787. (19) Jiang, B.; Gu, Y.; Qin, J.; Ning, X.; Gong, S.; Xie, G.; Yang, C. Deep-Red Iridium (III) Complexes Cyclometalated by Phenanthridine Derivatives for Highly Efficient SolutionProcessed Organic Light-Emitting Diodes. J. Mater. Chem. C, 2016, 4, 3492-3498. (20) Zhou, G.; Wong, W. Y.; Yao, B.; Xie, Z.; Wang, L. Triphenylamine-Dendronized Pure Red Iridium Phosphors with Superior OLED Efficiency/Color Purity Trade-Offs. Angew. Chem. Int. Ed. 2007, 46, 1149-1151. (21) Jou, J. H.; Su, Y. T.; Hsiao, M. T.; Yu, H. H.; He, Z. K.; Fu, S. C.; Chiang, C. H.; Chen, C. T.; Chou, C. H.; Shyue, J. J. Solution-Process-Feasible Deep-Red Phosphorescent Emitter. J. Phys. Chem. C, 2016, 120, 18794-18802. (22) Nagai, Y.; Sasabe, H.; Takahashi, J.; Onuma, N.; Ito, T.; Ohisa, S.; Kido, J. Highly Efficient, Deep-Red Organic Light-Emitting Devices using Energy Transfer from Exciplexes. J. Mater. Chem. C, 2017, 5, 527-530.


22

Page 23 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(23) Fujii, H.; Sakurai, H.; Tani, K.; Wakisaka, K.; Hirao, T. Bright and Ultimately Pure Red Electrophosphorescent Diode Bearing Diphenylquinoxaline. IEICE Electronics express, 2005, 2, 260-266. (24) Wang, H.; Meng, L.; Shen, X.; Wei, X.; Zheng, X.; Lv, X.; Yi, Y.; Wang, Y.; Wang, P. Highly Efficient Orange and Red Phosphorescent Organic Light-Emitting Diodes with Low Roll-Off of Efficiency using a Novel Thermally Activated Delayed Fluorescence Material as Host. Adv. Mater. 2015, 27, 4041-4047. (25) Lu, K. Y.; Chou, H. H.; Hsieh, C. H.; Ou Yang, Y. H.; Tsai, H. R.; Tsai, H. Y.; Hsu, L. C.; Chen, C. Y.; Chen, I. C.; Cheng, C. H. Wide-Range Color Tuning of Iridium Biscarbene Complexes from Blue to Red by Different N^N Ligands: an Alternative Route for Adjusting the Emission Colors. Adv. Mater. 2011, 23, 4933-4937. (26) Liu, Y.; Ye, K.; Fan, Y.; Song, W.; Wang, Y.; Hou, Z. Amidinate-Ligated Iridium(III) Bis(2-pyridyl)phenyl Complex as an Excellent Phosphorescent Material for Electroluminescence Devices. Chem. Commun. 2009, 3699-3701. (27) Peng, T.; Li, G.; Ye, K.; Wang, C.; Zhao, S.; Liu, Y.; Hou, Z.; Wang, Y. Highly Efficient Phosphorescent OLEDs with Host-Independent and Concentration-Insensitive Properties Based on a Bipolar Iridium Complex. J. Mater. Chem. C, 2013, 1, 2920-2926. (28) Li, G.; Zhu, D.; Peng, T.; Liu, Y.; Wang, Y.; Bryce, M. R. Very High Efficiency OrangeRed Light-Emitting Devices with Low Roll-Off at High Luminance Based on an Ideal Host– Guest System Consisting of Two Novel Phosphorescent Iridium Complexes with Bipolar Transport. Adv. Funct. Mater. 2014, 24, 7420-7426.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

(29) Feng, Y.; Li, P.; Zhuang, X.; Ye, K.; Peng, T.; Liu, Y.; Wang, Y. A Novel Bipolar Phosphorescent Host for Highly Efficient Deep-Red OLEDs at a Wide Luminance Range of 1000–10000 cd m−2. Chem. Commun. 2015, 51, 12544-12547. (30) Jones, C.; Junk, P. C.; Platts, J. A.; Stasch, A. Four-Membered Group 13 Metal (I) Nheterocyclic Carbene Analogues: Synthesis, Characterization, and Theoretical Studies. J. Am. Chem. Soc. 2006, 128, 2206-2207. (31) Jung, S. G.; Kang, Y. J.; Kim, H. S.; Kim, Y. H.; Lee, C. L.; Kim, J. J.; Lee, S. K.; Kwon, S. K. Effect of Substitution of Methyl Groups on the Luminescence Performance of IrIII Complexes: Preparation, Structures, Electrochemistry, Photophysical Properties and Their Applications in Organic Light-Emitting Diodes (OLEDs). Eur. J. Inorg. Chem. 2004, 3415-3423. (32) Nonoyama, M. Benzo [h] Quinolin-10-yl-N Iridium (III) Complexes. Bull. Chem. Soc. Jpn. 1974, 47, 767. (33) Zhang, L.; Nishiura, M.; Yuki, M.; Luo, Y.; Hou, Z. Isoprene Polymerization with Yttrium Amidinate Catalysts: Switching the Regio- and Stereoselectivity by Addition of AlMe3. Angew. Chem. Int. Ed. 2008, 47, 2642-2645. (34) Schmidt, J. A. R.; Arnold, Mono-Amidinate Complexes Stabilized by a New StericallyHindered Amidine. J. Chem. Commun. 1999, 2149-2150. (35) Abeysekera, D.; Robertson, K. N.; Cameron, T. S.; Clyburne, J. A. C. M-terphenylSubstituted Amidinates: Useful Ligands in the Preparation of Robust Aluminum Alkyls. Organometallics, 2001, 20, 5532-5536. (36) Bambirra, S.; Leusen, D. V.; Meetsma, A.; Hessen, B.; Teuben, J. H. Yttrium Alkyl Complexes with a Sterically Demanding Benzamidinate Ligand: Synthesis, Structure and Catalytic Ethene Polymerisation. Chem. Commun. 2003, 522-523.


24

Page 25 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Hay, J. Theoretical Studies of The Ground and Excited Electronic States in Cyclometalated Phenylpyridine Ir(III) Complexes Using Density Functional Theory. J. Phys. Chem. A. 2002, 106, 1634-1641. (38) Zhang, Q; Wang, X.; Wang, X.; Wang, L.; Zhang, J. Computational Studies of Electronic Structures and Photophysical Properties of Luminescent Iridium(III) Complexes based on Amidinate/Bis (pyridylphenyl) Ligands. Org. Electron. 2016, 33, 281-289. (39) Zhu, Y. C.; Zhou, L.; Li, H. Y.; Xu, Q. L.; Teng, M. Y.; Zheng, Y. X.; Zuo, J. L.; Zhang, H. J.; You, X. Z. Highly Efficient Green and Blue-Green Phosphorescent OLEDs Based on Iridium Complexes with the Tetraphenylimidodiphosphinate Ligand. Adv. Mater. 2011, 23, 4041-4046. (40) Murawski, C.; Liehm, P.; Leo, K.; Gather, M. C. Influence of Cavity Thickness and Emitter Orientation on the Efficiency Roll-Off of Phosphorescent Organic Light-Emitting Diodes. Adv. Funct. Mater. 2014, 24, 1117-1124. (41) Runge, E.; Gross, E. K. U. Density-Functional Theory for Time-Dependent Systems. Phys. Rev. Lett. 1984, 52, 997. (42) Srivastava, R.; Rao Joshi, L. The Effect of Substituted 1,2,4-Triazole Moiety on the Emission, Phosphorescent Properties of the Blue Emitting Heteroleptic Iridium(III) Complexes and the OLED Performance: a Theoretical Study. Phys. Chem. Chem. Phys. 2014, 16, 1728417294. (43) Hisamatsu, Y.; Kumar, S.; Aoki, S. Design and Synthesis of Tris-Heteroleptic Cyclometalated Iridium(III) Complexes Consisting of Three Different Nonsymmetric Ligands Based on Ligand-Selective Electrophilic Reactions via Interligand HOMO Hopping Phenomena. Inorg. Chem. 2017, 56, 886-899.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

(44) Skórka, Ł.; Filapek, M.; Zur, L.; Małecki, J. G.; Pisarski, W.; Olejnik, M.; Danikiewicz, W.; Krompiec, S. Highly Phosphorescent Cyclometalated Iridium (III) Complexes for Optoelectronic Applications: Fine Tuning of the Emission Wavelength through Ancillary Ligands. J. Phys. Chem. C. 2016, 120, 7284-7294. (45) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648. (46) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; 2135 Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (47) Ge, Z.; Hayakawa, T.; Ando, S.; Ueda, M.; Akiike, T.; Miyamoto, H.; Kajita, T.; Kakimoto, M. Spin-Coated Highly Efficient Phosphorescent Organic Light-Emitting Diodes Based on Bipolar Triphenylamine-Benzimidazole Derivatives. Adv. Funct. Mater. 2008, 18, 584-590.


26

Page 27 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48) Tao, Y. T.; Wang, Q.; Yang, C. L.; Zhong, C.; Zhang, K.; Qin, J. G.; Ma, D. G. Tuning the Optoelectronic Properties of Carbazole/Oxadiazole Hybrids Through Linkage Modes: Hosts for Highly Efficient Green Electrophosphorescence. Adv. Funct. Mater. 2010, 20, 304-311. (49) Tse, S. C.; Kwok K. C.; So, S. K. Electron Transport in Naphthylamine-Based Organic Compounds. Appl. Phys. Lett. 2006, 89, 262102. (50) Tsung, K. K.; So, S. K. Carrier Trapping and Scattering in Amorphous Organic Hole Transporter. Appl. Phys. Lett. 2008, 92, 103315. (51) Li, Y. Q.; Fung, M. K.; Xie, Z. Y.; Lee, S. T.; Hung, L. S.; Shi, J. M. An Efficient Pure Blue Organic Light-Emitting Device with Low Driving Voltages. Adv. Mater. 2002, 14, 1317-1321. (52) Park, T. J.; Jeon, W. S.; Park, J. J.; Kim, S. Y.; Lee, Y. K.; Jang, J.; Kwon, J. H.; Pode, R. Efficient Simple Structure Red Phosphorescent Organic Light Emitting Devices with Narrow Band-Gap Fluorescent Host. Appl. Phys. Lett. 2008, 92, 113308. (53) Jeon, W. S.; Park, T. J.; Kim, K. H.; Pode, R.; Jang, J.; Kwon, J. H. High Efficiency Red Phosphorescent Organic Light-Emitting Diodes with Single Layer Structure. Org. Electron. 2010, 11, 179-183. (54) Jou, J. H.; Su, Y. T.; Hsiao, M. T.; Yu, H. H.; He, Z. K.; Fu, S. C.; Chiang, C. H.; Chen, C. T.; Chou, C. H.; Shyue, J. J. Solution-Process-Feasible Deep-Red Phosphorescent Emitter. J. Phys. Chem. C 2016, 120, 18794-18802. (55) Li, Y. Q.; Liu, Y.; Bu, W. M.; Lu, D.; Wu, Y.; Wang, Y. Hydroxyphenyl-Pyridine Beryllium Complex (Bepp2) as a Blue Electroluminescent Material. Chem. Mater. 2000, 12, 2672-2675.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

(56) Liu, Y.; Guo, J. H.; Feng, J.; Zhang, H. D.; Li, Y. Q.; Wang, Y. High-Performance Blue Electroluminescent Devices Based on Hydroxyphenyl-Pyridine Beryllium Complex. Appl. Phys. Lett. 2001, 78, 2300-2302. (57) Peng, T.; Yang, Y.; Bi, H.; Liu, Y.; Hou, Z. M.; Wang, Y. Highly Efficient White Organic Electroluminescence Device Based on a Phosphorescent Orange Material Doped in a Blue Host Emitter. J. Mater. Chem. 2011, 21, 3551-3553. (58) Du, M. X.; Feng, Y. S.; Zhu, D. X.; Peng, T.; Liu, Y.; Wang, Y.; Bryce, M. R. Novel Emitting System Based on a Multifunctional Bipolar Phosphor: An Effective Approach for Highly Efficient Warm-White Light-Emitting Devices with High Color-Rendering Index at High Luminance. Adv. Mater. 2016, 28, 5963-5968

Table of Contents


28

Page 29 of 29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment

29

Rational Design and Characterization of ... - ACS Publications

Recommend Documents