Blue‐Emitting Cationic Iridium Complexes with 2‐(1H‐Pyrazol‐1‐yl)pyridine as the Ancillary Ligand for Efficient Light‐Emitting Electrochemical Cells

Two blue‐emitting cationic iridium complexes with 2‐(1H‐pyrazol‐1‐yl)pyridine (pzpy) as the ancillary ligands, namely, [Ir(ppy)₂(pzpy)]PF₆ and [Ir(dfppy)₂(pzpy)]PF₆ (ppy is 2‐phenylpyridine, dfppy is 2‐(2,4‐difluorophenyl) pyridine, and PF₆^? is hexafluorophosphate), have been prepared, and their photophysical and electrochemical properties have been investigated. In CH₃CN solutions, [Ir(ppy)₂(pzpy)]PF₆ emits blue‐green light (475 nm), which is blue‐shifted by more than 100 nm with respect to the typical cationic iridium complex [Ir(ppy)₂(dtb‐bpy)]PF₆ (dtb‐bpy is 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine); [Ir(dfppy)₂(pzpy)]PF₆ with fluorine‐substituted cyclometalated ligands shows further blue‐shifted light emission (451 nm). Quantum chemical calculations reveal that the emissions are mainly from the ligand‐centered ³π–π* states of the cyclometalated ligands (ppy or dfppy). Light‐emitting electrochemical cells (LECs) based on [Ir(ppy)₂(pzpy)]PF₆ gave green‐blue electroluminescence (486 nm) and had a relatively high efficiency of 4.3 cd A^?1 when an ionic liquid 1‐butyl‐3‐methylimidazolium hexafluorophosphate was added into the light‐emitting layer. LECs based on [Ir(dfppy)₂(pzpy)]PF₆ gave blue electroluminescence (460 nm) with CIE (Commission Internationale de L'Eclairage) coordinates of (0.20, 0.28), which is the bluest light emission for iTMCs‐based LECs reported so far. Our work suggests that using diimine ancillary ligands involving electron‐donating nitrogen atoms (like pzpy) is an efficient strategy to turn the light emission of cationic iridium complexes to the blue region.     

Toward Highly Efficient Solid‐State White Light‐Emitting Electrochemical Cells: Blue‐Green to Red Emitting Cationic Iridium Complexes with Imidazole‐Type Ancillary Ligands

Using imidazole‐type ancillary ligands, a new class of cationic iridium complexes ( 1 – 6 ) is prepared, and photophysical and electrochemical studies and theoretical calculations are performed. Compared with the widely used bpy (2,2′‐bipyridine)‐type ancillary ligands, imidazole‐type ancillary ligands can be prepared and modified with ease, and are capable of blueshifting the emission spectra of cationic iridium complexes. By tuning the conjugation length of the ancillary ligands, blue‐green to red emitting cationic iridium complexes are obtained. Single‐layer light‐emitting electrochemical cells (LECs) based on cationic iridium complexes show blue‐green to red electroluminescence. High efficiencies of 8.4, 18.6, and 13.2 cd A^?1 are achieved for the blue‐green‐emitting, yellow‐emitting, and orange‐emitting devices, respectively. By doping the red‐emitting complex into the blue‐green LEC, white LECs are realized, which give warm‐white light with Commission Internationale de L'Eclairage (CIE) coordinates of (0.42, 0.44) and color‐rendering indexes (CRI) of up to 81. The peak external quantum efficiency, current efficiency, and power efficiency of the white LECs reach 5.2%, 11.2 cd A^?1, and 10 lm W^?1, respectively, which are the highest for white LECs reported so far, and indicate the great potential for the use of these cationic iridium complexes in white LECs.     

Scrutinizing Design Principles toward Efficient,Long‐Term Stable Green Light‐Emitting Electrochemical Cells

Enhancing the efficiency and lifetime of light emitting electrochemical cells (LEC) is the most important challenge on the way to energy efficient lighting devices of the future. To avail this, emissive Ir(III) complexes with fluoro‐substituted cyclometallated ligands and electron donating groups (methyl and tert ‐butyl)‐substituted diimine ancillary (N^{^}N) ligands and their associated LEC devices are studied. Four different complexes of general composition [Ir(4ppy)₂(N^{^}N)][PF₆] (4Fppy = 2‐(4‐fluorophenyl)pyridine) with the N^{^}N ligand being either 2,2′‐bipyridine ( 1 ), 4.4′‐dimethyl‐2,2′‐bipyridine ( 2 ), 5.5′‐dimethyl‐2,2′‐bipyridine ( 3 ), or 4.4′‐di‐tert ‐butyl‐2,2′‐bipyridine ( 4 ) are synthesized and characterized. All complexes emit in the green region of light with emission maxima of 529–547 nm and photoluminescence quantum yields in the range of 50.6%–59.9%. LECs for electroluminescence studies are fabricated based on these complexes. The LEC based on ( 1 ) driven under pulsed current mode demonstrated the best performance, reaching a maximum luminance of 1605 cd m^?2 resulting in 16 cd A^?1 and 8.6 lm W^?1 for current and power efficiency, respectively, and device lifetime of 668 h. Compared to this, LECs based on ( 3 ) and ( 4 ) perform lower, with luminance and lifetime of 1314 cd m^?2, 45.7 h and 1193 cd m^?2, 54.9 h, respectively. Interestingly, in contrast to common belief, the fluorine content of the Ir‐iTMCs does not adversely affect the LEC performance, but rather electron donating substituents on the N^{^}N ligands are found to dramatically reduce both performance and stability of the green LECs. In light of this, design concepts for green light emitting electrochemical devices have to be reconsidered.     

Green and blue–green light-emitting electrochemical cells based on cationic iridium complexes with 2-(4-ethyl-2-pyridyl)-1H-imidazole ancillary ligand

The ionic iridium complexes, [Ir(ppy)₂(EP-Imid)]PF₆ (Complex 1) and [Ir(dfppy)₂(EP-Imid)]PF₆ (Complex 2) are used as the light-emitting material for the fabrication of light-emitting electrochemical cells (LECs). These complexes have been synthesized, employing 2-(4-ethyl-2-pyridyl)-1H-imidazole (EP-Imid) as the ancillary ligand, 2-phenylpyridine (ppy) and 2-(2,4-difluorophenyl)pyridine (dfppy) as the cyclometalated ligands, which were characterized by various spectroscopic, photophysical and electrochemical methods. The photoluminescence (PL) emission spectra in acetonitrile solution show blue–green and blue light emission for Complexes 1 and 2 respectively. However, LECs incorporating these complexes resulted in green (522 nm) light emission for Complex 1 with the Commission Internationale de L’Eclairage (CIE) coordinates of (0.33, 0.56) and blue–green (500 nm) light emission for Complex 2 with the CIE coordinates of (0.24, 0.44). Using Complex 1, a maximum luminance of 1191 cd m⁻² and current efficiency of 1.0 cd A⁻¹ are obtained while that of Complex 2 are 741 cd m⁻² and 0.88 cd A⁻¹ respectively.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene Blue Host and Side‐Chain‐Located Orange Dopant

Four single polymers with two kinds of attachment of orange chromophore to blue polymer host for white electroluminescence (EL) were designed. The effect of the side‐chain attachment and main‐chain attachment on the EL efficiencies of the resulting polymers was compared. The side‐chain‐type single polymers are found to exhibit more efficient white EL than that of the main‐chain‐type single polymers. Based on the side‐chain‐type white single polymer with 4‐(4‐alkyloxy‐phenyl)‐7‐(4‐diphenylamino‐phenyl)‐2,1,3‐benzothiadiazoles as the orange‐dopant unit and polyfluorene as the blue polymer host, white EL with simultaneous orange (λ_max = 545 nm) and blue emission (λ_max = 432 nm/460 nm) is realised. A single‐layer device (indium tin oxide/poly(3,4‐ethylenedioxythiophene)/polymer/Ca/Al) made of these polymers emits white light with the Commission Internationale de l'Éclairage coordinates of (0.30,0.40), possesses a turn‐on voltage of 3.5 V, luminous efficiency of 10.66 cd A^–1, power efficiency of 6.68 lm W^–1, and a maximum brightness of 21 240 cd m^–2.     

Efficient and Long Lived Green Light‐Emitting Electrochemical Cells

The combination of high efficiencies and long lifetime in a single light‐emitting electrochemical cell (LEC) device remain a major problem in LEC technology, preventing its application in commercial lighting devices. Three green light‐emitting cationic iridium‐based complexes of the general composition [Ir(C^{^}N)₂(N^{^}N)][PF₆] with 4‐Fppy (2‐(4‐fluorophenyl)pyridinato) as the cyclometalating C^{^}N ligand and 1,10‐phenanthroline ( 1 ), 4,7‐diphenyl‐1,10‐phenanthroline (bathophenanthroline, bphen, 2 ), and 2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (bathocuprione, dmbphen, 3 ) as ancillary N^{^}N ligands are synthesized and characterized. Computational studies are carried out in order to compare the electronic structure of the three ionic transition metal complexes (iTMCs) and provide insights into their potential as LEC emitter materials. LECs are then fabricated with complexes 1 – 3 . Driven under a pulsed current, they display a high luminance and current and power efficiencies. As the LEC based on complex 2 displays the overall best device performance, including the longest lifetime of 474 h, it is selected for subsequent driving conditions optimization. An extraordinary power efficiency of 25 lm W^?1 and current efficiency of 30 cd A^?1 are achieved under optimized operation conditions with reduced current density, resulting in a long device lifetime of 720 h. Altogether, ligand design in iTMCs and optimization of the device driving conditions leads to a significant improvement in LEC performance.     

Efficient Organic Light‐Emitting Diodes based on Sublimable Charged Iridium Phosphorescent Emitters

The synthesis and characterization of two new phosphorescent cationic iridium(III) cyclometalated diimine complexes with formula [Ir( L )₂(N‐N)]⁺(PF₆^–) ( HL = (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)diphenylamine); N‐N = 4,4′‐dimethyl‐2,2′‐bipyridine ( 1 ), 4,7‐dimethyl‐1,10‐phenanthroline ( 2 )) are reported. Both complexes are coordinated by cyclometalated ligands consisting of hole‐transporting diphenylamino (DPA)‐ and fluorene‐based 2‐phenylpyridine moieties. Structural information on these heteroleptic complexes has been obtained by using an X‐ray diffraction study of complex 2 . Complexes 1 and 2 are morphologically and thermally stable ionic solids and are good yellow phosphors at room temperature with relatively short lifetimes in both solution and solid phases. These robust iridium complexes can be thermally vacuum‐sublimed and used as phosphorescent dyes for the fabrication of high‐efficiency organic light‐emitting diodes (OLEDs). These devices doped with 5 wt % 1 can produce efficient electrophosphorescence with a maximum brightness of up to 15 610 cd m^–2 and a peak external quantum efficiency of ca. 7 % photons per electron that corresponds to a luminance efficiency of ca. 20 cd A^–1 and a power efficiency of ca. 19 lm W^–1. These results show that charged iridium(III) materials are useful alternative electrophosphors for use in evaporated devices in order to realize highly efficient doped OLEDs.     

Synthesis and Characterization of Red‐Emitting Iridium(III) Complexes for Solution‐Processable Phosphorescent Organic Light‐Emitting Diodes

A new series of highly efficient red‐emitting phosphorescent Ir(III) complexes, (Et‐CVz‐PhQ)₂Ir(pic‐N‐O), (Et‐CVz‐PhQ)₂Ir(pic), (Et‐CVz‐PhQ)₂Ir(acac), (EO‐CVz‐PhQ)₂Ir(pic‐N‐O), (EO‐CVz‐PhQ)₂Ir(pic), and (EO‐CVz‐PhQ)₂Ir(acac), based on carbazole (CVz)‐phenylquinoline (PhQ) main ligands and picolinic acid N‐oxide (pic‐N‐O), picolinic acid (pic), and acetylacetone (acac) ancillary ligands, are synthesized for phosphorescent organic light‐emitting diodes (PhOLEDs), and their photophysical, electrochemical, and electroluminescent (EL) properties are investigated. All of the Ir(III) complexes have high thermal stability and emit an intense red light with an excellent color purity at CIE coordinates of (0.65,0.34). Remarkably, high‐performance solution‐processable PhOLEDs were fabricated using Ir(III) complexes with a pic‐N‐O ancillary ligand with a maximum external quantum efficiency (5.53%) and luminance efficiency (8.89 cd A^?1). The novel use of pic‐N‐O ancillary ligand in the synthesis of phosphorescent materials is reported. The performance of PhOLEDs using these Ir(III) complexes correlates well with the results of density functional theory calculations.     

Ionic Iridium(III) Complexes with Bulky Side Groups for Use in Light Emitting Cells: Reduction of Concentration Quenching

Here, the photophysics and performance of single‐layer light emitting cells (LECs) based on a series of ionic cyclometalated Ir(III) complexes of formulae and where ppy, bpy, and phen are 2‐phenylpyridine, substituted bipyridine and substituted phenanthroline ligands, respectively, are reported. Substitution at the N?N ligand has little effect on the emitting metal‐ligand to ligand charge‐transfer (MLLCT) states and functionalization at this site of the complex leads to only modest changes in emission color. For the more bulky complexes the increase in intermolecular separation leads to reduced exciton migration, which in turn, by suppressing concentration quenching, significantly increases the lifetime of the excited state. On the other hand, the larger intermolecular separation induced by bulky ligands reduces the charge carrier mobility of the materials, which means that higher bias fields are needed to drive the diodes. A brightness of ca. 1000 cd m^?2 at 3 V is obtained for complex 5, which demonstrates a beneficial effect of bulky substituents.     

A DCM‐Type Red‐Fluorescent Dopant for High‐Performance Organic Electroluminescent Devices

2‐(2‐tert‐Butyl‐6‐((E)‐2‐(2,6,6‐trimethyl‐2,4,5,6‐tetrahydro‐1H‐pyrrolo[3,2,1‐ij]quinolin‐8‐yl)vinyl)‐4H‐pyran‐4‐ylidene)malononitrile (DCQTB) is designed and synthesized in high yield for application as the red‐light‐emitting dopant in organic light‐emitting diodes (OLEDs). Compared with 4‐(dicyanomethylene)‐2‐tert‐butyl‐6‐(1,1,7,7,‐tetramethyljulolidyl‐9‐enyl)‐4H‐pyran (DCJTB), one of the most efficient red‐emitting dopants, DCQTB exhibits red‐shifted fluorescence but blue‐shifted absorption. The unique characteristics of DCQTB with respect to DCJTB are utilized to achieve a red OLED with improved color purity and luminous efficiency. As a result, the device that uses DCQTB as dopant, with the configuration: indium tin oxide (ITO)/N,N′‐bis(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4′‐diamine (NPB; 60 nm)/tris(8‐quinolinolato) aluminum (Alq₃):dopant (2.3 wt %) (7 nm)/2,9‐dimethyl‐4,7‐diphenyl‐1,10‐phenanthroline (BCP; 12 nm)/Alq₃(45 nm)/LiF(0.3 nm):Al (300 nm), shows a larger maximum luminance (L_max = 6021 cd m^–2 at 17 V), higher maximum efficiency (η_max = 4.41 cd A^–1 at 11.5 V (235.5 cd m^–2)), and better chromaticity coordinates (Commission Internationale de l'Eclairage, CIE, (x,y) = (0.65,0.35)) than a DCJTB‐based device with the same structure (L_max = 3453 cd m^–2 at 15.5 V, η_max = 3.01 cd A^–1 at 10 V (17.69 cd m^–2), and CIE (x,y) = (0.62,0.38)). The possible reasons for the red‐shifted emission but blue‐shifted absorption of DCQTB relative to DCJTB are also discussed.     

High Efficiency White Organic Light‐Emitting Devices Incorporating Yellow Phosphorescent Platinum(II) Complex and Composite Blue Host

A new class of charge neutral, strongly luminescent cyclometalated platinum(II) complexes supported by dianionic tetradentate ligand are synthesized. One of these platinum(II) complexes, Y‐Pt , displays a high photoluminescence quantum yield of 86% and electroluminescence efficacy (η_power) of up to 52 lm W^?1, and is utilized as a yellow phosphorescent dopant in the fabrication of white organic light‐emitting devices (WOLEDs). WOLEDs based on conventional structures with yellow emission from Y‐Pt in combination with blue emission from bis(4,6‐difluorophenyl‐pyridinato‐N,C²′) (picolinate) iridium(III) (FIrpic) show a total η_power of up to 31 lm W^?1. A two‐fold increase in η_power by utilizing a modified WOLED structure comprising of a composite blue host is realized. With this modified device structure, the total η_power and driving voltage at a luminance of 1000 cd m^?2 can be improved to 61 lm W^?1 and 7.5 V (i.e., 10 V for control devices). The performance improvement is attributed to an effectively broaden exciton formation‐recombination zone and alleviation of localized exciton accumulation within the FIrpic‐doped composite host for reduced triplet‐triplet annihilation, yielding blue light‐emission with enhanced intensity. The modified device structure can also adopt a higher concentration of Y‐Pt towards its optimal value, leading to WOLEDs with high efficiency.     

Facile Synthesis of Highly Efficient Lepidine‐Based Phosphorescent Iridium(III) Complexes for Yellow and White Organic Light‐Emitting Diodes

Highly efficient lepidine‐based phosphorescent iridium(III) complexes with pentane‐2,4‐dione or triazolpyridine as ancillary ligands have been designed and prepared by a newly developed facile synthetic route. Fluorine atoms and trifluoromethyl groups have been introduced into the different positions of ligand, and their influence on the photophysical properties of complexes has been investigated in detail. All the triazolpyridine‐based complexes display the blueshifted dual‐peak emission compared to the pentane‐2,4‐dione‐based ones with a broad single‐peak emission. The complexes show emission with broad full width at half maximum (FWHM) over 100 nm, and the emissions are ranges from greenish–yellow to orange region with the absolute quantum efficiency (Φ_PL) of 0.21–0.92 in solution, i.e., Φ_PL = 0.92 ( 18 ), which is the highest value among the reported neutral yellow iridium(III) complexes. Furthermore, high‐performance yellow and complementary‐color‐based white organic light‐emitting diodes (OLEDs) have been fabricated. The FWHMs of the yellow, greenish–yellow OLEDs are in the range of 94–102 nm, which are among the highest values of the reported yellow or greenish–yellow‐emitting devices without excimer emission. The maximum external quantum efficiency of monochrome OLEDs can reach 24.1%, which is also the highest value among the reported yellow or greenish–yellow devices. The color rendering indexes of blue and complementary yellow‐based white OLED is as high as 78.     

White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

Amorphous Diphenylaminofluorene‐Functionalized Iridium Complexes for High‐Efficiency Electrophosphorescent Light‐Emitting Diodes

Two new phosphorescent iridium(III ) cyclometalated complexes, [Ir(DPA‐Flpy)₃] ( 1 ) and [Ir(DPA‐Flpy)₂(acac)] ( 2 ) ((DPA‐Flpy)H = (9,9‐diethyl‐7‐pyridinylfluoren‐2‐yl)diphenylamine, Hacac = acetylacetone), have been synthesized and characterized. The incorporation of electron‐donating diphenylamino groups to the fluorene skeleton is found to increase the highest occupied molecular orbital (HOMO) levels and add hole‐transporting ability to the phosphorescent center. Both complexes are highly amorphous and morphologically stable solids and undergo glass transitions at 160 and 153 °C, respectively. These iridium phosphors emit bright yellow to orange light at room temperature with relatively short lifetimes (< 1 μs) in both solution and the solid state. Organic light‐emitting diodes (OLEDs) fabricated using 1 and 2 as phosphorescent dopant emitters constructed with a multilayer configuration show very high efficiencies. The homoleptic iridium complex 1 is shown to be a more efficient electrophosphor than the heteroleptic congener 2 . Efficient electrophosphorescence with a maximum external quantum efficiency close to 10 % ph/el (photons per electron), corresponding to a luminance efficiency of ～ 30 cd A^–1 and a power efficiency of ～ 21 lm W^–1, is obtained by using 5 wt.‐% 1 as the guest dopant.     

Cover Picture: White Electroluminescence from a Single‐Polymer System with Simultaneous Two‐Color Emission: Polyfluorene as the Blue Host and a 2,1,3‐Benzothiadiazole Derivative as the Orange Dopant (Adv. Funct. Mater. 7/2006)

New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized by Wang and co‐workers on p. 957. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue and orange emission from the corresponding emitting species. A single‐layer device has been fabricated that has performance characteristics roughly comparable to those of organic white‐light‐emitting diodes with multilayer device structures. New single‐polymer electroluminescent systems containing two individual emission species—polyfluorenes as a blue host and 2,1,3‐benzothiadiazole derivative units as an orange dopant on the main chain—have been designed and synthesized. The resulting single polymers are found to have highly efficient white electroluminescence with simultaneous blue (λ_max = 421 nm/445 nm) and orange emission (λ_max = 564 nm) from the corresponding emitting species. The influence of the photoluminescence (PL) efficiencies of both the blue and orange species on the electroluminescence (EL) efficiencies of white polymer light‐emitting diodes (PLEDs) based on the single‐polymer systems has been investigated. The introduction of the highly efficient 4,7‐bis(4‐(N‐phenyl‐N‐(4‐methylphenyl)amino)phenyl)‐2,1,3‐benzothiadiazole unit to the main chain of polyfluorene provides significant improvement in EL efficiency. For a single‐layer device fabricated in air (indium tin oxide/poly(3,4‐ethylenedioxythiophene): poly(styrene sulfonic acid/polymer/Ca/Al), pure‐white electroluminescence with Commission Internationale de l'Eclairage (CIE) coordinates of (0.35,0.32), maximum brightness of 12 300 cd m^–2, luminance efficiency of 7.30 cd A^–1, and power efficiency of 3.34 lm W^–1 can be obtained. This device is approximately two times more efficient than that utilizing a single polyfluorene containing 1,8‐naphthalimide moieties, and shows remarkable improvement over the corresponding blend systems in terms of efficiency and color stability. Thermal treatment of the single‐layer device before cathode deposition leads to the further improvement of the device performance, with CIE coordinates of (0.35,0.34), turn‐on voltage of 3.5 V, luminance efficiency of 8.99 cd A^–1, power efficiency of 5.75 lm W^–1, external quantum efficiency of 3.8 %, and maximum brightness of 12 680 cd m^–2. This performance is roughly comparable to that of white organic light‐emitting diodes (WOLEDs) with multilayer device structures and complicated fabrication processes.     

Synthesis of red-emitting cationic Ir (III) complex and its application in white light-emitting electrochemical cells

Construction of intramolecular π-π packing between cyclometalated and ancillary ligand have been proved to be a feasible way to design cationic Ir(III) phosphors for stable light-emitting electrochemical cells (LECs). Employing blue-green (C1) and yellow-emitting (C2) Ir(III) complexes as emitters, in which such π-π interactions between ligands occur, the stable and efficient LECs with peak current efficiency of 21.9 cd/A and 20.3 cd/A, respectively, were realized. To achieve the white electroluminescence, herein, a new red-emitting cationic Ir (III) complex (C3) using 2-(5-phenyl-2-phenyl-2H-1,2,4-triazol-3-yl)pyridine as ancillary ligand was designed and synthesized. A warm white light emitting, by doping C3 into the blue-green emitting C1, exhibits the current efficiency of 10.1 cd/A. In addition, the obtained LECs are more stable than those of previous reports due to presence of intrinsic super-cage structures in the systems. The results indicate that the efficient white LECs hold promising applications in the practical solid state lighting.     

Tetraphenylimidazole‐Based Excited‐State Intramolecular Proton‐Transfer Molecules for Highly Efficient Blue Electroluminescence

Aiming for highly efficient blue electroluminescence, we have designed and synthesized a novel class of tetraphenylimidazole‐ based excited‐state intramolecular proton‐transfer (ESIPT) molecules with covalently linked charge‐transporting functional groups (carbazole‐ and oxadiazole‐functionalized hydroxyl‐substituted tetraphenylimidazole (HPI), i.e., HPI‐Cbz and HPI‐Oxd, respectively). High T_g (ca. 130 °C) amorphous films of HPI‐Cbz and HPI‐Oxd showed intense and ideal blue‐light emission (λ_max = 462 and 468 nm, Φ_PL = 0.44 and 0.38) with a large Stokes shift of over 160 nm and a narrow full width at half‐maximum of less than 65 nm. Organic light‐emitting devices using HPI‐Cbz and HPI‐Oxd as the emitting layer generated an efficient blue electroluminescence (EL) emission peaking at around 460 nm with excellent CIE coordinates of (x, y) = (0.15, 0.11). A maximum external quantum efficiency of 2.94%, and a maximum brightness of 1 229 cd m⁻² at 100 mA cm⁻², as well as a low turn‐on voltage of 4.8 V were achieved in this work.     

Manipulating Charge‐Transfer Character with Electron‐Withdrawing Main‐Group Moieties for the Color Tuning of Iridium Electrophosphors

A new and synthetically versatile strategy has been developed for the phosphorescence color tuning of cyclometalated iridium phosphors by simple tailoring of the phenyl ring of ppy (Hppy = 2‐phenylpyridine) with various main‐group moieties in [Ir(ppy‐X)₂(acac)] (X = B(Mes)₂, SiPh₃, GePh₃, NPh₂, POPh₂, OPh, SPh, SO₂Ph). This can be achieved by shifting the charge‐transfer character from the pyridyl groups in some traditional iridium ppy‐type complexes to the electron‐withdrawing main‐group moieties and these assignments were supported by theoretical calculations. This new color tuning strategy in Ir^III‐based triplet emitters using electron‐withdrawing main‐group moieties provides access to Ir^III phosphors with improved electron injection/electron transporting features essential for highly efficient, color‐switchable organic light‐emitting diodes (OLEDs). The present work furnished OLED colors spanning from bluish‐green to red (505–609 nm) with high electroluminescence efficiencies which have great potential for application in multicolor displays. The maximum external quantum efficiency of 9.4%, luminance efficiency of 10.3 cd A⁻¹ and power efficiency of 5.0 lm W⁻¹ for the red OLED (X = B(Mes)₂), 11.1%, 35.0 cd A⁻¹, and 26.8 lm W⁻¹ for the bluish‐green device (X = OPh), 10.3%, 36.9 cd A⁻¹, and 28.6 lm W⁻¹ for the bright green device (X = NPh₂) as well as 10.7%, 35.1 cd A⁻¹, and 23.1 lm W⁻¹ for the yellow‐emitting device (X = SO₂Ph) can be obtained.     

Blue‐Emitting Iridium(III) Complexes for Light‐Emitting Electrochemical Cells: Advances,Challenges, and Future Prospects

Light‐emitting electrochemical cells (LECs) are one of the most promising technologies for solid‐sate lighting. Among them, LECs based on phosphorescent iridium(III) complexes have attracted significant research interest in the past 15 years, because of their high efficiency and tunable emission color across the entire visible spectrum. To fabricate white LECs for lighting, high‐performance blue LECs are the first prerequisite. Huge efforts have been devoted to improving the performances of blue LECs based on iridium(III) complexes either by developing new blue‐emitting complexes or by engineering the devices. Nevertheless, blue LECs have still shown much lower performances (brightness, efficiency, stability, etc.) compared to the red, orange‐red, yellow, and green counterpart devices. In particular, a single blue LEC with satisfactory blue‐color purity, high efficiency, and high stability is still missing. Here, the advances in blue‐emitting iridium(III) complexes for LECs and the device engineering on LECs using the complexes are reported. The challenges ahead are discussed, and future prospects are outlined.     

Green Ir(III) complexes with multifunctional ancillary ligands for highly efficient solution-processed phosphorescence organic light-emitting diodes with high current efficiency

Two new highly efficient green emitting heteroleptic Ir(III) complexes, namely, bis[5-(2-ethylhexyl)-8-(trifluoromethyl)benzo[c][1,5]naphthyridin-6(5H)-one]iridium-4-((3,5-di(9H-carbazol-9-yl)benzyl)oxy)picolinate (Ir-HT) and bis[5-(2-ethylhexyl)-8-(trifluoromethyl) benzo[c][1,5]naphthayridin-6(5H)-one]iridium-4-((4-(5-phenyl-1,3,4-oxadiazol-2-yl)benzyl) oxy)picolinate (Ir-ET) were designed and synthesized for solution-processed phosphorescence organic light-emitting diodes (PHOLEDs). These new Ir(III) complexes are based on amide-bridged trifluoromethyl (-CF₃) substituted phenylpyridine unit as main ligand and 1,3-bis(N-carbazolyl)benzene (mCP) unit and 1,3,4-oxadiazole (OXD) unit functionalized picolinate (pic) as an ancillary ligand. These multifunctional groups were attached into the 4-position of pic ancillary ligands via ether linkage. Interestingly, the solution-processed PHOLED device using Ir-HT as a dopant exhibited a maximum external quantum efficiency (EQE_max) of 20.92% and a maximum current efficiency (CE_max) of 64.04 cd A⁻¹. Whereas PHOLED device using Ir-ET displayed a EQE_max of 20.68% and a CE_max of 65.02 cd A⁻¹. This is one of best CE with high EQE for green Ir(III) complexes via solution-processed PHOLEDs using multifunctional ancillary ligands so far.