Red to near-infrared organometallic phosphorescent dyes for OLED applications

Journal of Organometallic Chemistry 751 (2014) 261e285

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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem

Review

Red to near-infrared organometallic phosphorescent dyes for OLED applications Cheuk-Lam Ho a, **, Hua Li b, Wai-Yeung Wong a, * a Institute of Molecular Functional Materials, Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, PR China b College of Life and Environmental Sciences, Minzu University of China, Beijing 100081, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 August 2013 Received in revised form 8 September 2013 Accepted 12 September 2013

In modern research on organic light-emitting diodes (OLEDs), cyclometalated iridium(III) complexes represent one of the most studied class of compounds. The high emission efficiency caused by the strong spin-orbit coupling in the presence of heavy metals leads to the mixing of singlet and triplet manifolds so that both the singlet and triplet excitons can be harvested. For OLEDs to be useful in displays application, true red, green, and blue emissions of sufficient luminous efficiencies and proper chromaticity are required. In recent years, the development of materials for phosphorescent red OLEDs has indeed gone through several important evolutional stages. However, the luminescent quantum yields of red-emitting iridium(III) phosphors tend to be intrinsically low which are governed by the energy gap law for triplet states in which the luminescence quantum yields tend to decrease with an increase in the emission wavelength. Many red organic dyes currently in use do not show a good compromise between device efficiency and color purity. In general, a dilemma facing red OLEDs was realized in which efficient and bright dopants are not red enough, and red-enough dopants are not efficient and bright. In this review article, we highlight the recent progress and current challenges of efficient OLEDs based on cyclometalated iridium(III) dyes which exhibit saturated red and near-infrared electroluminescence. Optimization of the phosphorescent red OLED efficiency/color purity trade-off and extension of the work to other organometallic phosphors are also presented and discussed. Ó 2013 Elsevier B.V. All rights reserved.

Dedicated to the 50th anniversary of Journal of Organometallic Chemistry Keywords: Iridium Near-infrared emission Organic light-emitting diodes Platinum Phosphorescence

Contents 1. 2.

3. 4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .261 Iridium-based red phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 2.1. Cyclometalating ligands derived from 1-phenylisoquinoline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 2.2. Cyclometalating ligands derived from 2-phenylquinoline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 2.3. Cyclometalating ligands derived from 2,4-diphenylquinoline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266 2.4. Cyclometalating ligands derived from pyridine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270 2.5. Other Ir(III)-based red phosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Red phosphors containing other transition metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .278 Near-infrared (NIR) emitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

1. Introduction * Corresponding author. Tel.: þ852 3411 7074; fax: þ852 3411 7348. ** Corresponding author. Tel.: þ852 3411 2450. E-mail addresses: [email protected] (C.-L. Ho), [email protected] (W.-Y. Wong). 0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2013.09.035

Since the first commercialization of organic light-emitting diodes (OLEDs) in 1997 by the pioneering company in Japan, OLEDs

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are considered to be the next generation of flat panel displays [1]. Some advantages of OLEDs which make them perfect candidates to replace the widely used liquid crystal displays and plasma display panels include high-efficiency, low-voltage, full color and ease of fabrication as large area flat panel displays in electronic devices [2]. OLEDs based on phosphorescent transition-metal complexes are attracting significant attention since they can greatly improve electroluminescence (EL) performance as compared with the conventional fluorescent OLEDs [1]. According to spin statistics, EL from small molecular fluorophors cannot exceed a maximum quantum yield of 25%, but in phosphorescent complexes, their EL can theoretically achieve quantum yields up to 100% since both triplet and singlet excitons can be harvested for the emission [3]. Among all the phosphors, cyclometalated iridium(III) complexes are acquiring the mainstream position in the field of organic displays because of their highly efficient emission properties, relatively short excited state lifetime and excellent color tunability over the entire visible spectrum [4]. Besides, other metal ions, such as Pt(II), Os(III) and Zn(II) can also be introduced into the emitter molecules to give efficient luminescence. Their developments are supported by the broad diversity of the possible structures surrounding around the metal ions. In particular, extensive research efforts have been made recently for gaining more achievements in phosphorescent white OLEDs as they can be used for the next generation solid-state lighting. White emission can be achieved by mixing three primary colors (red, green and blue) or two colors from an orange emitter complemented with a blue emitter [5]. To achieve highly efficient white OLEDs, there is a great demand for efficient and bright true red color phosphors. As compared to other colors, the design and synthesis of efficient red emitters is intrinsically more difficult, which is in accordance with the energy gap law [6,7]. Many red emitters suffer from poor compromise between device efficiency and color purity. The lower luminosity of a red device is due to its characteristic red emission in a spectral region where the eye has poor sensitivity. Moreover, the wide bandgap host used in OLED device and narrow bandgap red-emitting guest have a significant difference in the HOMO and/or LUMO levels between the guest and host materials. Thus, the guest molecules are thought to act as deep traps for electrons and holes in the emitting layer, causing an increase in the driving voltage of the device. Furthermore, self-quenching or tripletetriplet annihilation for red dopant molecules is an inevitable problem in such host-guest systems especially at high doping concentrations. Therefore, from a practical standpoint, a solution to the above issues based on materials design or/and device optimization is highly desirable. This review will comprehensively survey new red organometallic materials that have been used in phosphorescent OLEDs (PHOLEDs) in the past few years. Besides, since development of compounds with emission in the near-infrared (NIR) wavelength window (>700 nm) is rapidly emerging as an important area in a variety of biological and biomedical [8e10], telecommunications [11] and defense applications [12], a general overview of some molecular design strategies towards the various types of NIRemitting metal complexes is also given. Their structureeactivity relationship, photophysical and electroluminescence properties will also be discussed. 2. Iridium-based red phosphors 2.1. Cyclometalating ligands derived from 1-phenylisoquinoline Although a number of red phosphors have been synthesized, isoquinoline-type Ir(III) complexes, particularly those with 1phenylisoquinoline derivatives, are still the most studied one.

Quinoline/isoquinoline-based compounds have received much attention due to their high electron affinities [13,14]. The molecular design of red phosphorescent complexes with 1-phenylisoquinoline (piq) ligand is based on the fact that the highest occupied molecular orbital (HOMO) is principally composed of a mixture of iridium d and phenyl p orbitals, while the lowest unoccupied molecular orbital (LUMO) is predominantly localized on the p-orbitals of the piq chromophore. Greater p-electronic conjugation in the isoquinoline ring would significantly lower the LUMO level and notably reduce the HOMO-LUMO energy gap. Research in the field of red PHOLEDs began mostly with the well-known tris(1phenylisoquinoline)iridium(III) Ir(piq)3 (Ir-1) and Ir(piq)2(acac) (Ir-2). The piq unit as the ligand part of these Ir(III) complexes can partially suppress the tripletetriplet annihilation (TTA) and show short phosphorescent lifetime [15,16]. Both complexes exhibit a photoluminescence (PL) peak at around 620 nm, depending on the type of the host used in the device structure [17]. Since the initial works on Ir-1 and Ir-2 in red PHOLEDs, considerable focus was paid on Ir-1 and Ir-2 by developing suitable host materials in better matching the HOMO and LUMO energy levels to achieve a good exciton confinement within the emissive layer as well as studying the effects of charge trapping, concentration dependence of dopant and device structure for a better control of the red OLED efficiency. We will also describe some attractive red PHOLEDs based on Ir-1 or Ir-2 fabricated with different device structure and host. Kwon and co-workers reported the application of Ir-1 in different device configurations, including multiple quantum well structure, bilayer or single layer structure. They showed that by doping Ir-1 in the bis(10-hydroxybenzo[h]quinolinato)beryllium (Bebq2) matrix, a better match of the HOMO and LUMO energy levels between guest and host can be achieved as Bebq2 is a narrow bandgap material and works as a charge control layer with high electron mobility and very good electron-transporting (ET) characteristics. In the quantum confinement approach, multiple quantum well structures having various triplet quantum well devices from single to five quantum wells were employed (ITO/ TCTA:WO3/TCTA/multiple layer (Bepq2:Ir-1)x/Bepp2/Al) (ITO: indium tin oxide; TCTA: 4,40 ,400 -tri(9-carbazolyl)triphenylamine; Bepp2: bis(phenylpyridine)beryllium, x ¼ 1 to 5 quantum wells) [18]. Triplet energies in such devices are confined within the emitting layers. Among the five devices, a maximum external quantum efficiency (hext) of 14.8% was obtained with a twoquantum well device structure, with maximum current efficiency (hL) of 12.4 cd/A, low turn-on voltage (Von) of 2.5 V and excellent color stability with Commission Internationale de L’Eclairage (CIE) coordinates of (0.66, 0.33). A further increase of the number of quantum wells enhances the driving voltage with a slight drop in the efficiency. The same research group successfully demonstrated negligible barrier for electron transport in the red PHOLED by using the concept of multiple quantum well device structure and this should be very useful to the future development of OLED display technology. Simple bi-layered device with Ir-1 doped in the Bebq2 host would be a promising way to achieve efficient and low-cost red light production [19]. This device was fabricated in the configuration of ITO/NPB/Bebq2:Ir-1/LiF/Al. Thanks to the very small exchange energy value of 0.2 eV between singlet and triplet states by using the Bebq2 host, this device showed almost no barrier for the injection of charge carriers. Moderately high hL and hP values of 9.66 cd/A and 6.90 lm/W, respectively, were obtained. Later on, single layer device using thermal evaporation technique was also achieved. The key to this simplification is the direct injection of holes and electrons into the mixed host material through the opposite electrodes [20]. Mixed host system with NPB:Bebq2 (NPB: N,N0 -di-[(1-naphthyl)-N,N0 -diphenyl]-1,10 -biphenyl-4,40 -diamine)

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263

with reduced efficiency roll off which has been attributed to the good hole mobility and low hole injection (HI) barrier of BBTC host in the emission layer. Because of the attractive results of Ir-2, Wei et al. announced an obvious superiority of white OLEDs (WOLEDs) using complex Ir-2 doped in 4,40 ,400 -tri(N-carbazolyl)triphenylamine (TCTA) [27]. The WOLED with an optimal red dye doping concentration of 5 wt-% exhibited a high color rendering index (CRI) of 89 and a hP of 31.2 and 27.5 lm/W at the initial luminance and 100 cd/ m2, respectively. The device showed little variation of the CIE coordinates in a wide range of luminance. Also, there are numerous reports on the structural modification of the host material for Ir-1 and Ir-2 to improve the optical and electrical properties of related

at 1 wt-% doping concentration of Ir-1 showed the best performance. Such lightly doped device displayed low Von of 2.4 V, maximum hL and hP of 9.44 cd/A and 10.62 lm/W, respectively, with good CIE coordinates of (0.66, 0.33). This feature of red PHOLEDs paves the way to simplify the device structure and reduces the cost of device manufacture. Other researchers have improved the efficiencies of red PHOLEDs doped with Ir-1 by using different host materials in the past few years. For example, deep red Ir-1-based PHOLEDs doped in host materials 1,3,5-tris(3-(carbazol-9-yl)phenyl)benzene, 2,4,6-tris(3(carbazol-9-yl)phenyl)pyridine or 2,4,6-tris(3-(carbazol-9-yl) phenyl)pyrimidine showed superior efficiency and suppressed ef-

F N

O N Ir

Ir

O

Ir

O

O 3

2

2

2

Ir-1

N

O

Ir

N

O

Ir-2

Ir-3

Ir-4

F N

O

N

O

Ir Ph3P

O

O

2

2 Ir-5

N

Cl

Ir

Ir

Ir-6

ficiency roll-off [21]. All the devices showed high hext of over 18% and hP of w 19 lm/W at low current density due to the bipolar nature of the host materials. 2,7-Bis(phenylsulfonyl)-9-[4-(N,Ndiphenylamino)phenyl]-9-phenylfluorene (SAF) based red PHOLEDs exhibited a very low Von (2.4 V) and high EL efficiencies of 15.8% and 22 lm/W, superior to those of the corresponding device based on the conventional host material, 4,40 -N,N0 -dicarbazolylbiphenyl (CBP; 3.2 V, 8.5% and 8.4 lm/W) [22]. The efficiencies of SAFbased red PHOLEDs remained high at a practical brightness of 1000 cd/m2 (13.1%, 14.4 lm/W). Very long operational lifetime at high initial luminance for deep red PHOLEDs based on Ir-1 using double guest/host system was described by Gigli and co-workers [23]. Device with 15 wt-% of Ir-1 was doped in NPB and bis(2methyl-8-quinolinolato-N1,O8)-(1,10 -biphenyl-4-olato)aluminum (BAlq) which gave a maximum hL of 9.5 cd/A and a remarkably long device lifetime of more than 2700 h at a starting luminance of 6000 cd/m2. Host materials therefore play an important role in governing the energy barrier, emission color quality and efficiency of a device. For Ir-2, comparable PHOLED performance as Ir-1 has been reported. A deep red Ir-2-based PHOLED hosted by a bipolar triphenylamine/oxadiazole hybrid gave an hext up to 21.6%, hL of 15.9 cd/A and hP of 16.1 lm/W [24]. Compared with the device using the host 3,5-di(9-carbazol-9-yl)-N,N-diphenylaniline, the efficiencies of red PHOLEDs were slightly decreased but were still better than those using the common host N,N0 -dicarbazolyl-3,5-benzene (mCP) [25]. The maximum hext reached 19.2%. Comparable performance with hext of 19.3%, hL of 16.4 cd/A and hP of 13.0 lm/W has been reported in 2012 with the device structure of ITO/MoO3/NPD/BBTC:Ir-2/BCP/Bebq2/ LiF/Al (BBTC: 3,6-bis-biphenyl-4-yl-9-[1,10,40 ,100 ]terphenyl-4-yl-9carbazole; BCP: 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) [26]. This OLED showed nearly 100% internal quantum efficiency

2 Ir-7

devices. Nevertheless, all these results suggest that Ir(III) dopants Ir1 and Ir-2 are suitable candidates for highly efficient red and white OLEDs. In addition to Ir-1 and Ir-2, a number of isoquinoline based Ir(III) derivatives have been synthesized and studied. The introduction of various simple substituents (e.g. t-Bu, F, Me, OMe) on the ligands in the resulting Ir(III) complexes can not only alter the HOMOeLUMO level and emission wavelength but also minimize the molecular packing and concentration quenching of luminescence. Complex Ir3 with 7-methyl-1-p-tolylisoquinoline showed a blue shift in its PL spectrum as compared to Ir-2 which emitted a red light at 606 nm in neat film [28]. A highly efficient deep red PHOLED was fabricated by doping Ir-3 into a hole-blocking material 4-biphenyloxalato aluminum(III)bis(2-methyl-8-quinolinato)-4-phenylphenolate. This device showed CIE coordinates of (0.66, 0.34) when the doping concentration is above 2 wt-%, which is very close to the National Television System Committee (NTSC) standard red point (0.66, 0.33). A maximum luminance (Lmax) of 31317 cd/m2, a maximum hL of 21.6 cd/A and a half-lifetime of 13 h were achieved. The high efficiency can be obtained by the effective energy transfer from the host to the guest Ir-3 and the direct recombination of electrone hole pairs on the dopant. Liu and Wong introduced bulky and halogen substituent such as F, t-Bu and Me groups in complex Ir-2 to reduce the self-quenching effect which is believed to cause low device efficiency [29]. Complexes Ir-4 and Ir-5 showed red PL peak in dichloromethane at 618 and 628 nm, respectively. Red PHOLED devices consisting of indolo [3,2-b]carbazole/benzimidazole hybrid bipolar host material doped with Ir-4 and Ir-5 revealed maximum hext of 15.6 and 15.5%, respectively. By replacing the ancillary ligand from acetylacetone in Ir-2 to 2-acetyl-cyclohexane, Ir-6-doped devices with different host materials gave red emission peaks in the range of 618e636 nm [30].

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The best device showed Lmax of 576 cd/cm2. Complex Ir-7 with free phosphine unit showed an emission signal at 600 nm, which is about 20 nm shorter in wavelength than those of Ir-1 and Ir-2. Solution-processable OLED was fabricated with the structure of ITO/PEDOT:PSS/Ir-7/Ba/Al and red-shifted emission at lEL ¼ 615 nm was detected with CIE coordinates of (0.64, 0.34). Although the efficiencies of the devices based on Ir-7 were not superior to that doped with Ir-2, it opened up a significant progress in which luminescent Ir(III) units can be incorporated into organic polymer simply by phosphine coordination. This approach makes spincoating and inkjet printing processes possible during the device fabrication and it is conceived that we can further develop high

relative to Ir-12. It is interesting that a substantial increase of the PL intensity and elevation of the HOMO level with increasing dendritic generation from Ir-1 to Ir-12 and Ir-13 (HOMO: e4.99 eV for Ir-12; e 4.96 eV for Ir-13; e5.11 for Ir-1). This implies that the dendritic triphenylamine structure will effectively block the self-quenching of the triplet emission core and improve the HI and HT properties. Solutionprocessable complexes Ir-12 and Ir-13 were applied for the fabrication of deep red PHOLEDs. They emitted pure red light with lPL at around 640 nm with excellent CIE color coordinates of (0.70, 0.30). The PHOLEDs made from Ir-12 (or Ir-13) achieved the Lmax value of 7452 (6143) cd/m2 at 17 (16) V, peak hL of 5.82 (3.72) cd/A, hext of 11.65 (7.36)% and hP of 3.65 (2.29) lm/W. This class of materials with HT

N

O

N

Ir

Ir

O

N

N

2

3

X

X X=H X = OMe

Ir-8 Ir-9

X=H X = OMe

N

Ir-10 Ir-11

N

Ir

Ir 3

3

N

Ir-12

N

N

performance metallopolymers as electroluminescent materials or even generate white light by spontaneous emissions from both the polymer main chain and complex Ir-7. To solve the trade-off problem between device efficiency and color purity of red emitters, Wong et al. first reported the synthesis and characterization of a series of novel red-emitting Ir(III) complexes incorporating hole-transporting (HT) 9-arylcarbazole (Ir-8 to Ir-11) [31] and triphenylamine (Ir-12 and Ir-13) [17]. These functionalized red emitters improve the charge injection (HI) and HT in Ir-1 and Ir-2, help reduce the barrier height for HI by raising the HOMO levels and decrease the TTA by taking advantages of the bulky carbazole and triphenylamine groups. Impressive saturated red CIE coordinates of (0.68, 0.32) were reported by using both vacuum-deposited and spincoated techniques for the devices made from dopants Ir-8 to Ir-11. A hext as high as 12% was achieved by Ir-10 with hL of 10.15 cd/A and hP of 5.25 lm/W. The enhanced HT properties of the complexes have been demonstrated without the need of an additional HT layer in the device. While complex Ir-12 with triphenylamine unit emitted at 636 nm in the solution state and the emission wavelength slightly red-shifted with increasing the dendron size in Ir-13 (lPL ¼ 641 nm)

Ir-13

N

functionalities could provide a promising route for the rational design of heavy metal electrophosphors with superior device efficiency and color purity trade-off necessary for pure red-light generation. For small molecular materials, crystallization of their thin films may lead to the formation of excimers and exciplexes, which will decrease the device efficiency and impair the device stability. Moreover, at high doping concentration, the intermolecular interaction in thin film will lead to the self-quenching of luminescence. To address these issues, groups from Wong and Chen have adopted a 3-D bulky structure and sterically hindered configuration in the cyclometalating ligand to suppress close packing among the molecules in the solid state. They synthesized Ir(III)-based triplet emitters bearing fluorene derivatives with two sterically bulky triphenylamine units in Ir-14 [32] and spiro-annulate moiety in Ir15 [33]. The tortured geometries of these complexes not only render their highly amorphous properties, but also alleviate TTA and concentration quenching in order to enhance the PHOLED efficiency. By taking this advantage, only slight roll-off of device efficiencies was observed with increasing current density. The pure red-emitting Ir-14-based device (lEL ¼ 648 nm; CIE ¼ (0.70,

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

0.30)) attained Lmax of 6471 cd/m2 at 21 V, peak hext of 4.23%, hL of 1.69 cd/A and hP of 0.27 lm/W. The PHOLED device with Ir-15 showed an encouraging performance with peak hext of 4.6% and hL of 7.44 cd/A, however, poor CIE coordinates of (0.64, 0.36) was obtained which is far from the NTSC recommended red as compared to that for Ir-14. These works provide a viable platform to

265

demonstrated by Suh and Kwon [35]. The PL emission from the metal-to-ligand charge transfer (MLCT) state was blue-shifted for the 2-phenylquinoline derivative (Ir-19) as compared to the 1phenylisoquinoline one (Ir-2). The former one emitted orange-red light at 581 nm with a noteworthy increase in quantum efficiency (VPL ¼ 0.63 in film). Methylation of the phenyl moiety of the phq

N Ir N

N

O Ir O

N

2 3

Ir-15

Ir-14 BuO BuO N

O

Ir

Ir

Ir

O

O

O BuO

N

O

N

O

O

O

O

BuO

2 Ir-16

2

2

Ir-17

achieve anti TTA phenomenon and relieve the bottle-neck problem between red color purity and efficiency in small molecule PHOLEDs. In general, the development of red phosphorescent Ir(III) complexes with high quantum yields is still challenging because these compounds are intrinsically less emissive than blue and green emitters according to the energy gap law [6,7]. Yagi et al. reported a series of rigid and extended p-conjugation 0 framework of 1-(dibenzo[b,d]furan-4-yl)isoquinolinato-N,C3 Ir(III) complexes (Ir-16 to Ir-18) which yielded pure and saturated red PL emission with high quantum efficiency [34]. The PL quantum yield (VPL) of Ir-16 to Ir-18 is 0.61, 0.55 and 0.49, respectively, in toluene and much more emissive than Ir-1 (0.26 in toluene). Although a change of the O O ligand did not significantly affect the lPL, replacing dipivaloylmethanate (dmp) and acetylacetonate (acac) by 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate (bdbp) can increase the phosphorescence quantum efficiency and solubility of the complex. Pure red EL with CIE chromaticity coordinates of (0.68, 0.31) can be obtained for Ir-16 to Ir-18 based devices. The device with 0.51 mol-% Ir-16 led to a maximum performance among the three complexes with Lmax of 7270 cd/m2, hL of 3.9 cd/A, hext of 6.4% and hP of 1.4 lm/W. This work represents a good approach for getting highly emissive red phosphors with remarkable CIE values.

^

2.2. Cyclometalating ligands derived from 2-phenylquinoline

^

The nature of C N chelate around the Ir(III) center was found to be critical for determining the photophysical properties of the resulting phosphors. The impact of isomerization of C N chelate from 1-phenylisoquinoline to 2-phenylquinoline (phq) was

^

Ir-18

ligand increases the VPL to 0.70 and 0.76 for Ir-20 and Ir-21, respectively. A dramatic increase in VPL to 0.83 and 0.87 for Ir-22 and Ir-23 was observed if there is a further methylation of the quinoline ring of the phq chelate. The addition of methyl group on the C N ligand increases the intermolecular steric interaction that can result in reduced self-quenching effect and lead to an increment in VPL. Moreover, after such chemical structural modification, very clear red EL spectra with much narrower full width at half maximum (FWHM) were obtained for Ir-20 to Ir-23 in the wavelength range of 609e620 nm. Impressive device efficiency data were achieved for these five red dopants. The best performance was obtained for Ir-23 with peak hL of 30.1 cd/A, hext of 24.6% and hP of 32.0 lm/W; while the unsubstituted complex Ir-19 afforded maximum hL of 19.1 cd/A, hext of 15.7% and hP of 17.6 lm/W. Changing the ancillary ligand from an acac moiety to a 2,2,6,6tetramethylheptane-3,5-dionate exerts a significant influence on the device performance. This piece of work indicates that the orientation and substituent effects have a great impact on the intermolecular packing and device efficiency. In 2012, significant enhancement in device performance based on Ir-19 was carried out by Chang et al. by wisely managing the excitons in PHOLED [36]. Complex Ir-19 was employed together with a green emitter [Ir(ppy)2(acac)] which served as an exciton formation assistant and excitons can be delivered to the red emitter Ir-19. Introduction of a green emitter causes a minimal effect on the emission spectra and keeps the lEL at 600 nm but improves the emission intensity by a factor of w1.3. A high hext of 20.2% at 100 cd/m2, corresponding to hL of 33.3 cd/A and hP of 28.0 lm/W, has been achieved without the need of additional out-coupling enhancement. Detailed investigation on the nature of energy transfer should be carried out to gain more insight on the complex quantum mechanical processes

^

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N

O

N

O

Ir O

O 2 Ir-20

O

O

O 2

Ir-19

Ir

Ir

Ir

N

O

N

O

N

O

Ir

2

2

2

Ir-21

Ir-22

Ir-23

N

O Ir O

S 2 Ir-24

2.3. Cyclometalating ligands derived from 2,4-diphenylquinoline Another big family of red phosphors is based on the use of 2,4diphenylquinoline in Ir(III) complexes. From the standpoint of chemical structure, addition of a phenyl ring at the 4-position of 2phenylquinoline can contribute to the shift of emission wavelength as the effective conjugation length increases. Therefore, 2,4diphenylquinoline framework is able to shift the emission of Ir(III) complexes to the saturated red as the energy gap between the HOMO and LUMO levels is reduced. Complex Ir-25 was first reported by Shu et al. [37]. With respect to Ir-19, the 4-phenyl substituent in the 2,4-diphenylquinoline ligand leads to more reddish emission peak, which can be rationalized qualitatively by considering the decrease in the LUMO energy level that results from an increase in the p-conjugation length of the quinoline moiety induced by the 4-phenyl group. The phenyl group introduction on the quinoline ring shifts the reduction potentials of Ir-19 (2.41 and 2.64 V) to the less negative values (2.26 and 2.42 V) in Ir25 because the more electron-accepting heterocyclic portion governs the reductions of this Ir(III) complex, leaving the HOMO levels unaffected. For the sake of comparison, Kim et al. systematically designed and synthesized a series of 50 -substituted derivatives of Ir-25, i.e. with methoxy (Ir-26), methyl (Ir-27) and fluoride (Ir-28) units [38]. However, the emission wavelength of Ir(III) complex

with methoxy or methyl electron-donating group was not redshifted as expected. The emission wavelength of Ir-27 was the same as Ir-25, while the lEL was even blue-shifted in Ir-26 (lEL ¼ 605 nm) as compared to the non-substituted one. The electron-withdrawing fluoro substituent causes the lEL to be blueshifted to 610 nm. The author made a calculation on the HOMO and LUMO levels on Ir-25 to Ir-28 with supportive information for the emission wavelengths of these complexes (Fig. 1). Although the methoxy or methyl group increased the HOMO energy level, it strongly increased the LUMO more than the HOMO and led to less MLCT mixing within their Ir(III) complex. While the fluoro substituent lowered the HOMO level of Ir-28 intensively that caused the strongest MLCT between the Ir atom and the ligand, this resulted in the best device performance of Ir-28. The best performance of Ir-28 was achieved with hL of 15.8 cd/A and hP of 12.4 lm/W. Since the oxidation processes largely involve the Ir-aryl center whereas the reduction is generally considered to occur mainly on the heterocyclic portion of cyclometalating C N ligand, extending the conjugated aromatic motif of the ligand framework by replacing the phenyl ring in diphenylquinoline with fluorene in Ir29 and Ir-30 and naphthalene in Ir-31 effectively elevated the HOMO level as compared to Ir-25 with little change in the LUMO level. Likewise, a bathochromic shift in the emission wavelength in Ir-29 to Ir-31 relative to Ir-25 can be explained based on the same reason. With reference to Ir-25, the complex Ir-32 with the more electron-donating ability of methylthiophene than phenyl ring, an

^

-1

Energy Level (eV)

involved for future employment of this simple technique in obtaining higher performance PHOLEDs for displays as well as solid-state lighting applications. The impact of changing the phenyl group to the thiophene ring on 2-phenylquinoline in Ir-19 is not as remarkable as expected in comparison to the case of [Ir(ppy)2(acac)] and only a significant bathochromic shift in the PL maximum was noted for the latter [35]. Complex Ir-24 with 4-methyl-2-(thiophen-2-yl)quinoline showed a very sharp emission band with FWHM of only 48 nm and the emission maximum at a relatively short wavelength of 611 nm in the solution state as compared with the well-known deep red complex Ir-2 (lPL ¼ 620 nm; FWHM ¼ 58 nm). The narrow emission band width of an emitter is essential for reaching high luminous efficiency. A further decrease in FWHM to 39 nm for Ir-24 doped in a bipolar host and a VPL of 0.55 can benefit the dopant to get an attractive device performance. The best performance of Ir-24-based device showed a Lmax of 58688 cd/m2 and the maximum hext, hL and hP of 25.9%, 37.3 cd/A and 32.9 lm/W, respectively. Preliminary results showed that the operational lifetime of this device is estimated to be more than 2000 h at an initial luminance of 500 cd/m2.

LUMO HOMO

-2

-3

-4

-5

Fig. 1. Calculated HOMO and LUMO energy levels of Ir-25 to Ir-28.

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

267

N

O Ir O

2

X X=H = OCH3 = CH3 =F

Ir-25 Ir-26 Ir-27 Ir-28

N

O Ir O

C8H17

2

C8H17 Ir-29 Br

N

O

Br

Br

N

O

Ir

N

O Ir

Ir

O

O

O C6H13 Ir-30

2

S

2

2 Ir-32

Ir-31

C6H13

of 100 mA/cm2, the device could maintain a high efficiency of 8.16%. However, poorer efficiencies were reported for Ir-30 to Ir-32 as compared to Ir-29. Phenothiazine is a well-known heterocyclic compound with electron-rich sulfur and nitrogen heteroatoms. The phenothiazine group shows strong luminescence, high photoconductivity and reversible oxidation processes [39]. Moreover, the non-planar phenothiazine ring structure can restrict p-stacking aggregation, which prevents the detrimental pure singlet excitation recombination process [40]. By making use of the synergistic effects between phenothiazine and phenylquinoline, Jin et al. synthesized a series of Ir(III) complexes Ir-33 to Ir-35 with different ancillary ligands of acetylacetone, picolinic acid and picolinic acid N-oxide, respectively [41]. The change of ancillary ligands moves the

additional 0.12 V increase in the HOMO level was revealed. However, the HOMO level elevation in Ir-32 is not as much to the extent as in Ir-29eIr-31, which implies that it is more effective to move the emission color of the Ir(III) complexes to more reddish one by extending the p-conjugation length rather than replacing the phenyl ring with thiophene. Given the advantage from the presence of the long alkyl chains in fluorenyl groups of Ir-29 and Ir-30, both of them are amorphous solids which are in contrast to that of a crystalline solid in Ir-25. Bright red OLED was fabricated by using Ir-29 doped into a blend of PVK and 30 wt-% of PBD. The EL emission is a characteristic of Ir-29, with a maximum at 627 nm and CIE color coordinates of (0.68, 0.32), which satisfy the demand by NTSC. At a current density of 10.7 mA/cm2, the hext and hL were 10.27% and 11.0 cd/A, respectively. Even at a higher current density

O O

N

O

O

N

O

Ir

Ir

N O

N

O N Ir-33

S

C6H13

N Ir

2

N Ir-34

S

C6H13

2

N Ir-35

S

C6H13

2

268

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

thermal stability of the carbazole moiety, all these Ir(III) complexes offer excellent thermal stabilities and good electro-optical properties. Similar to the case for the phenothiazine-based one, the use of different ancillary ligand effectively modifies the photophysical and electrochemical properties of the resulting complexes. In comparison to their physical properties, the complexes with acac ancillary ligand tend to possess a relatively lower decomposition temperature. Poorer thermal stabilities were also detected for those bearing 2-(2-methoxyethoxy)ethyl chain. In terms of PL properties, complexes with acac show more red shifts in the emission wavelength (lPL ¼ 619 nm for Ir-36; 616 nm for Ir-37) than those with picolinic acid and picolinic acid N-oxide (lPL ¼ 595 nm for Ir-38 to Ir-41). The

emission of these complexes from red to the near infrared. They emit from the predominantly ligand-centered 3pep* excited state in the wavelength range of 626e712 nm. The HOMO level of Ir-33 is elevated relative to Ir-34 and Ir-35 from 4.82 eV to 4.95 eV and 4.94 eV, respectively, while their LUMO levels are almost the same (2.80 to 2.82 eV). Their suitable energy levels with the mixed host materials (TPD/TCTA/TPBI) (TPD: N,N0 -diphenyl-N,N0 (bis(3-methylphenyl)-[1,1-biphenyl]-4,40 -diamine; TCTA: 4,40 ,400 tris(carbazol-9-yl)triphenylamine, TPBI: 5-tris[2-N-phenyl-benzimidazolyl]benzene) are essential for effectively trapping both holes and electrons in the emitting layer within the OLEDs. However, poor performances of PHOLED devices doped with Ir-33 to Ir-35

N

O

N

O

O

Ir

Ir

O

N

O N

N

N

N Ir

N

2

2

N

Ir-37

Ir-36

O

Ir

O

O

N 2

2 Ir-39

Ir-38

O

O

O

O

O

O O

O

N

N Ir

Ir N O

N O

N

N

2

2 Ir-41

Ir-40

O

O

O

O

O

N

O

Ir

N Ir

N

N

N

2 N N

Ir-42

2 N

Ir-43

O

O

O

O

were recorded. The maximum hext of only 0.51% with CIE coordinates of (0.68, 0.30) in the deep red region was obtained for Ir35. This may be due to the low VPL for phosphorescence. Besides, a series of carbazole-based diphenylquinoline Ir(III) complexes (Ir-36 to Ir-41) were also reported by Jin and co-workers [42e45]. Systematic discussions on their structureeproperty relationship and efficiencies of PHOLEDs have been made by altering the ancillary ligands. Because of the inherent electron-donating nature, intense luminescence, good HT properties and high

O O

N Ir

N O N 2 Ir-44

device architecture employed was ITO/PEDOT/Ir-36-Ir-41/BAlq/ Alq3/Liq/Al. Both lEL of Ir-36 and Ir-37 were located at 623 nm with the CIE coordinates at (0.65, 0.34) but the lEL of Ir-38 to Ir-41 shifted to 604 nm and the emission maximum and the CIE values are relatively insensitive to the identity of alkyl or alkoxy chain. The performance of the Ir(III) complexes containing picolinic acid Noxide in PHOLEDs was remarkably higher than the other. It was believed that the electron-withdrawing nature and stronger negative inductive effect of picolinic acid N-oxide ligand could

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269

This work represents a great achievement on the research of solution-processed red emissive PHOLEDs. The use of 2pyrazinecarboxylic acid as the ancillary chelate in Ir-42 slightly red-shifted the lPL to 613 nm, however, if 5-methyl-2pyrazinecarboxylic acid was used instead in Ir-43, a blue shift in lPL was obtained (lPL ¼ 600 nm) [44]. The methyl unit mainly alters the LUMO levels from 2.70 to 2.85 eV of the Ir(III) complexes without affecting their HOMO levels. CIE coordinates of (0.60, 0.39) were obtained for both devices but the more efficient electron injection (EI) and ET in Ir-43 than Ir-42 make the device performance of the former better. This is determined by the better matching of the LUMO gap of Ir-43 with adjacent layers in the device. This device exhibited maximum hext, hL and hP of 3.38%, 6.69 cd/A and 1.80 lm/W, respectively. To investigate the structureeproperty relationship on the ligation position of Ir(III) complexes (i.e. C-2 versus C-3 positions of the carbazole moieties), complex Ir-44 was also prepared by the same research group which afforded molecules of different bandgaps as compared to Ir-40 [43]. The PL spectrum of Ir-44 showed an emission peak at 668 nm. Thus, by slightly modifying the molecular structure of the carbazoleequinoline chelate, the emission peak of the Ir(III) complex was red-shifted by 73 nm relative to Ir-40. Solution-processed PHOLEDs doped with Ir-40 or Ir-44 have respective peak hext of 8.74% with CIE coordinates of (0.62, 0.37) and 4.32% with deep-red CIE coordinates of (0.68, 0.28) using PVK as the

induce higher electron mobilities and result in an improved device performance. The highest Lmax of 18500 cd/m2 was obtained for Ir41 at 215 mA/cm2 with hext of 5.53% and hL of 8.89 cd/A. A slightly poorer performance was obtained for Ir-40 with hext and hL of 4.9% and 8.36 cd/A, respectively. To boost up the efficiencies of the red PHOLEDs by using complexes Ir-38 and Ir-39, different device configuration of ITO/PEDOT:PSS/PVK:OXD-7:TPD:Ir-38 or Ir-39/ cathode was adopted [45]. (OXD: 1,3-bis[5-(4-tert-butylphenyl)1,3,4-oxadiazole-2-yl]benzene) The use of TPD with PVK as a codopant would cause a reduction in the barrier for HI and can also serve as a HT layer which allows holes to penetrate more deeply into the emissive layer. The efficiencies of the red PHOLEDs based on Ir-38 and Ir-39 were greatly enhanced and were TPD concentration dependent. All the devices displayed emission peaks at 608 nm with excellent color purity at the CIE coordinates of (0.62, 0.38). The device efficiencies increased as the TPD concentration increased and reached the maximum at 16 wt-% due to the reduction of injection barrier from 0.6 to 0.3 eV in the presence of TPD. At this doping concentration, the Lmax of the OLEDs using Ir-38 and Ir-39 are 8768 and 6285 cd/m2, respectively. The OLEDs fabricated without the TPD have respective hext, hL and hP of 6.21%, 9.15 cd/A and 1.93 lm/W, respectively for Ir-38, and can be significantly increased to 10.56%, 6.42 lm/W and 17.5 cd/A at 16 wt-% of TPD. Similar observation was detected for Ir-39 and its maximum hext, hL and hP were 9.65%, 15.92 cd/A and 4.34 lm/W, respectively.

N

N

N N

N

O

N

O

Ir

Ir

O

O 2

Ir-45

2 Ir-46

N N N

N

N N N

O Ir

N

O

Ir-47

2

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

host material. The combination of the high device performance and good color purity makes Ir-44 a good candidate for deep red phosphorescent dopant for solution-processed PHOLEDs. Dendrimers have proved to be good materials for solutionprocessable PHOLEDs, in which an Ir(III) complex is surrounded by a branched shell of cyclometalating ligand to prevent selfaggregation, phase segregation or concentration quenching of the emissive core in the solid state. By using this protection, Wang et al. reported carbazoleedendronized red dopants Ir-45 to Ir-47 by associating oligocarbazole units with Ir(III) complexes by covalent bonding to form single multifunctional dendrimers [46]. The oligocarbazole dendrons not only serve as red emitter but also work as the host material. All the dendrimers exhibited bright red emission between 615 and 622 nm, and their emission maxima show only a small red shift of 4e11 nm as compared to Ir-25. Effective tuning of the intermolecular interaction can be addressed by the dendron generation, and the reduced aggregation upon increasing the size of the dendron was confirmed by the observed increase of the emission lifetime. Good filmforming and charge-transporting properties of Ir-45 to Ir-47 enable them to be fabricated as PHOLEDs by using solutionprocessing technique with non-doped configuration: ITO/ PEDOT:PSS/dendrimer:TCCz-PBD/BCP/Alq/LiF/Al (TCCz: N-(4-(carbazol-9-yl)phenyl)-3,6-bis(carbazol-9-yl)carbazole). A Lmax of 1990 cd/m2 and a maximum hext of 6.3% (4.1 cd/A, 2.4 lm/W) with the CIE coordinates of (0.67, 0.33) have been demonstrated. It should be noted that the hext of Ir-47 is almost 4, 10 and 31 times higher than that of Ir-46, Ir-45 and Ir-25, which is attributed to the depressed self-quenching effect of the emissive cores upon increasing the size of the dendron. Doped devices were also studied by Wang and co-workers as a comparison. At the brightness of 100 cd/m2, the hext of non-doped device based on Ir47 at 5.0 wt-% is only about 16.7% lower than that of the corresponding doped device and is very close to that of device (5.4%) at the optimized doping level of 2 wt-% for Ir-25. This indicates that non-doped device structure can be used for this kind of complexes without significant loss in efficiency. Further improvement of the device performance was carried out by reducing the thickness of the BCP layer. Pure red light with the CIE coordinates of (0.65, 0.35) was obtained for Ir-47 and was found to be independent of the current density. An hext of 11.8%, a hL of 13.0 cd/A, and a hP of 7.2 lm/W at a brightness of 100 cd/m2 were obtained. This molecular design strategy of combining the host and dopant

N

O

N

O Ir

Ir O

O 2

2 Ir-48

Ir-49

to form a single multifunctional dendrimer is an efficient approach for the development of solution-processable non-doped red phosphorescent materials. Complex Ir-48, an isomer of Ir-25, with 2,3-diphenylquinoline as the cyclometalating ligand gave a bathochromic shift of 19 nm

1.0

EL Intensity (a.u.)

270

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) Fig. 2. EL spectrum for Ir-61-based red-emitting device.

in wavelength of its PL and EL as compared to Ir-25 [47]. The 3phenyl group increases the probability of pep conjugation of the cyclometalating ligand and lowers the triplet energy level, thus lowering the LUMO level to cause such a shift. A red PHOLED with 5 wt-% Ir-48 showed lEL ¼ 620 nm with a shoulder at about 668 nm. Its CIE coordinates changed slightly at different driving voltages (from (0.66, 0.34) at 9 V to (0.65, 0.34) at 18 V). A Von of 6 V, Lmax of 22040 cd/m2 at 18 V, hP of 2.4 lm/W, and hL of 11.4 cd/A at 15 V were achieved, demonstrating that Ir-48 is another dopant suitable for red PHOLEDs. Extending the p-electron delocalization at the quinoline ligand portion in Ir-49 resulted in a longer lPL at 604 nm with attractive VPL of 62% [48]. Quite short phosphorescent lifetime and good emission quantum yield of Ir-49 makes it to be a promising candidate for highly efficient red PHOLED. Accordingly, the device based on Ir-49 exhibited high performance with small rolloff in the corresponding device efficiency with increasing current density. It gave maximum hL of 17.4 cd/A and hext of 10.5%. Due to the blue shift of lEL (ca. 604 nm) of this device, it should possess great potential for high-efficiency two-element white light emission. 2.4. Cyclometalating ligands derived from pyridine derivatives Instead of using fused ring in quinoline, the conjugation length in pyridine-based cyclometalates of Ir(III) complexes should be extended either in the phenyl or pyridine ring to obtain red light. For Ir-50, the less electron-donating 2-position of carbazole lowers the energy of metal d orbital when it is ligated to the metal ion, giving rise to a lower energy level of the HOMO that is accompanied by a significant bathochromic shift in the emission wavelength to give the red color [49]. The carbazolyl ligand contribution to the excited states increases in the 2-ligated case as compared to the 3-ligated one. The good HI and HT properties of Ir-50 render it to be a dual functional triplet emitter in PHOLEDs. The Ir-50-based device exhibited a Lmax of 51000 cd/m2 at 15 V and respectable EL efficiencies (13.7%, 25.6 cd/A, 19 lm/W) with the CIE coordinates of (0.60, 0.39) obtained by the vacuum deposition technique. Replacing the phenyl ring at the 9-position of carbazole by a decyl chain in Ir-51 shows a similar lPL at 594 nm, however, both of the HOMO (4.74 eV) and LUMO (2.31 eV) levels are elevated in Ir-51 [50]. The color of Ir-51based solution-processed PHOLEDs with ITO/PEDOT:PSS/Ir51:CzOXD/BCP/Alq3/LiF/Al configuration showed a dopant concentration dependence. A Von of 11.3 V, a Lmax of 4894 cd/m2, and a

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

N

O

N

O

271

Ir

Ir

C6H13

O

O

C6H13

N C10H21

N

2

2

Ir-51

Ir-50

N Ir

CF3 CF3 N

C6H13

N

Ir

C6H13

Ir

3 Ir-54

N

C6H13

N

C6H13

3

3 Ir-53

Ir-52

maximum hL of 4.6 cd/A at 14.8 mA/cm2 were achieved at 5 wt-% with only a gentle decay in efficiencies with increasing current density. Similar to other Ir(III) phosphors, the difference in emission energies and electrochemical properties of carbazole-based Ir(III) complexes can be rationalized by substitution approach. Bryce and co-workers reported two red-emitting homoleptic Ir(III) complexes Ir-52 and Ir-53 with electron-withdrawing CF3 group attached at the 5- and 4-sites on the pyridine ring, respectively [51]. As compared to the unsubstituted one, the CF3 group shifts the oxidation potentials to more positive values, but the position of the CF3 group on pyridine ring shows little dependence on the HOMO level. A better performance of solution-processed PHOLEDs was observed in Ir-53 than in Ir-52 when they are blended with the high triplet energy poly(9-vinylcarbazole) (PVK) polymer host. The best performance was recorded at hL of 10.3 cd/A and hext of 5.6%. These studies provide valuable insights into chemically tailoring Ir

1.0

EL intensity (a.u.)

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm) Fig. 3. EL spectrum for Ir-62-based red-emitting device.

900

complexes of carbazolylpyridine ligands, leading to red PHOLEDs with good color tunability appropriate for full color display technology. Another approach to obtain red light emission for this class of Ir(III) complex is to extend the effective electronic conjugation of the pyridine ring. The earliest example for obtaining red emission in Ir(III) complex by using this approach was achieved by coupling two fluorene units at the ortho position on the pyridine ring in Ir54, and red emission located at 600 nm with a shoulder peak at 620 nm was observed [52]. By connecting the two fluorene units, the solid-state quenching can be minimized and the electronehole trapping on the Ir(III) complex was improved. A single active layer configuration with the PEDOT:PSS layer on ITO as the HI bi-layer electrode was employed and Ir-54 was doped in a PVK-PBD mixture. The emission quantum yield quickly increased as the doping concentration increased since the energy was efficiently transferred from the host to the guest until the triplet emission of Ir-54 became saturated at a very high doping concentration. The Lmax was over 2600 cd/m2 with maximum hext, hL and hP of 5%, 7.2 cd/A and 1.33 lm/W, respectively. Yang and Wu successfully encapsulated triphenylamine and dendritic triphenylamine cores on pyridine ring peripherally which also shift the phosphorescence peak to saturated red [53]. In solution, both Ir-55 and Ir-56 displayed a structureless emission centered at 608 nm. The triphenylamine chelates in Ir-55 and Ir-56 serve as good antenna for the charge transfer and/or energy transfer to the emissive center and maintain the high-lying HOMO level (ca. e5.2 eV) and sufficient triplet energy (ca. 2.9 eV). In addition, the arylamine group directly helps to get rid of the close molecular packing in the solid state. Their high phosphorescent quantum efficiencies (VPL ¼ 42 and 43% for Ir-55 and Ir-56, respectively) in poly(methyl methacrylate) (PMMA) doped films and short lifetimes benefit them to achieve high performance OLED devices. Devices based on Ir-55 at different doping concentrations were fabricated with a simple configuration (ITO/PEDOT:PSS/PVK:PBD:Ir-55/Ba/Al) and all of them exhibited bright red emission. Owing to the short phosphorescent lifetime of Ir-55 and hence the reduced TTA effect, the device parameters are maintained high as the luminance

272

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

N

N

N

N N

Ir

N N 3

Ir N

Ir-55

N

N 3 Ir-56

O

O

N

O

N

O

Ir

O

O

Ir

O

O

2 Ir-58

increases. The peak hext reached 13.8% at 10 wt-% level of Ir-55 and turns to brighter and more efficient value at hext of 15.3% with CIE coordinates of (0.61, 0.39) at 30 wt-% doping concentration. Such structural protection of the emissive core efficiently gave rise to such a high performance solution-processable red devices. Electronic effects with inductively electron-donating or -withdrawing groups have to be considered for their impact on orbital energies of the Ir(III) complexes. Adding an electron-withdrawing substituent on the pyridyl ring or phenyl ring can stabilize the LUMO or HOMO level, respectively, and result in narrowing of the HOMOeLUMO gaps that makes red light emission possible. Complexes Ir-57 to Ir-60 based on 5-benzoyl-2-phenylpyridine ligands with methyl moieties can tune the emission color bathochromically as compared to [Ir(ppy)2(acac)] [54]. Their peak emission wavelengths ranged from 618 to 648 nm in the red region of the visible spectrum. Compared to Ir-57 with a methyl group on the 4-position of the phenyl ring, the emission spectrum of Ir-58 with a methyl group at the 6-position was red-shifted by ca. 14 nm. The larger distorted angle between the phenyl and pyridine rings is caused by the 8-substituted methyl group in Ir-58 than in Ir-57, leading to a small energy gap in Ir-58 in comparison to Ir-57. While the PL spectra of Ir-59 and Ir-60 showed red-shifted emissions by ca. 15e 30 nm as compared with that of Ir-57 due to the increased electron density in the phenyl moiety caused by the introduction of two

N

O

Ir

O 2 Ir-57

N

O

Ir

O

2

2 Ir-59

Ir-60

methyl units that destabilized the HOMO energy level of the Ir(III) complexes. Increased methyl group addition on the phenyl chromophore did not cause any significant difference in the absorption and PL spectra. Compared with those Ir(III) complexes with one methyl group, Ir-57 and Ir-58, the HOMO energy levels of the complexes with two methyl groups, Ir-59 and Ir-60, were approximately 0.11e0.18 eV higher than the complexes with one methyl group. The subtle structural and electronic changes in the ligands also significantly affect the EL properties of the complexes. The respective CIE coordinates of the devices were (0.62, 0.38), (0.65, 0.35), (0.66, 0.34) and (0.65, 0.35) for Ir-57 to Ir-60. Relative to the peak hext of the devices with Ir-57 and Ir-58, the value for Ir60 was increased by approximately 26% and 15%, respectively, at 20 mA/cm2, while that of device Ir-59 was decreased by 5.7% and 14%, respectively, at 20 mA/cm2. This study suggests that structural modification of the ligands of the Ir(III) phosphors could greatly improve the EL performance of PHOLEDs doped with them. In contrast to other typical Ir(III) complexes in the literature, complexes Ir-61 with boron functionality and Ir-62 with an electron-trapping fluorenone unit show an interesting color tuning principle [55,56]. They showed obvious red shift in their PL spectra when an electron-withdrawing chromophore was introduced to the phenyl moiety of phenylpyridine. They emitted at 605 and 615 nm in their solution state, respectively. The borylated group in

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

273

emission in the range of 652e656 nm under forward bias voltage. At a relatively low current density, hext from Ir-68 is higher than that from Ir-66 but the case is reversed as current density increases. This is attributed to the better HT properties in the largest dendrimer but fast decay of efficiency at high current density was observed. It can be conjectured that HT and ET become unbalanced caused by acceleration of the hole mobility rather than the electron mobility. Complex Ir-69 with mixed cyclometalating ligands showed peaks at 600 and 652 nm for the 20:80 wt-% Ir-69:CBP blend film [59]. It gave a pure red EL color with CIE coordinates of (0.64, 0.36). A preliminary PHOLED result based on Ir-69 was achieved with hL of 7 cd/A and hP of 4.0 lm/W at 100 cd/m2 in a bi-layer structure.

Ir-61 becomes one of the major LUMO contributors that results in a significant electron density transfer through the MLCT process to the B(Mes)2 group to stabilize the MLCT state strongly. Similarly, the electron-deficient fluorenone ring in Ir-62 acts as an electron sink in the MLCT process leading to its red emission and its EL spectrum lies in the red region of the CIE diagram (Figs. 2 and 3). Impressive device performance was obtained with Ir-61 doped in PVK host, which was found to have a low Von of 3.7 V, a Lmax of 16148 cd/m2 at 19.1 V, a hext of 9.4%, a hL of 10.3 cd/A and a hp of 5.0 lm/W. The efficiency roll-off at high current density is not severe in this device, implying that the TTA effect is not very significant at high current density, and this may be attributed to the suppression of selfquenching process by the bulky mesityl group. Reasonable

N

O Ir

N

O Ir

O

O B

2

O 2

Ir-61

PHOLED parameters were obtained with Ir-62 showing Von of 5.9 V, a Lmax of 4803 cd/m2, peak hext of 1.29%,hL of 1.48 cd/A and hp of 0.49 lm/W. These rare systems with promising EI and ET functionalities benefited from the electron-withdrawing moieties are vital for the optimization of future red-emitting devices. As pointed out above, a dendritic matrix around the phosphorescent dye at a molecular level can effectively control the selfquenching and thus optimize the optoelectronic properties. The site isolation effect provided by the bulky peripheral groups minimizes the undesired coreecore interaction. Highly branched dendrons Ir-63 to Ir-65 integrated with rigid polyphenylene chain attached to 2-benzo[b]thiophen-2-yl-pyridyl with a HT triphenylamine surface as end groups were reported [57]. The covalent linkage of inner chromophoric Ir(III) core and outer triphenylamine units by polyphenylene can enhance the hole capture and HI as well as improve the charge recombination for light emission. The polyphenylene could participate in the electrochemical process and charge transport of the dendrimers as well as the control of intermolecular interactions without changing the HOMO and LUMO levels by increasing the dendrimer generation. All of the three dendrimers possess similar PL emission bands at 621 nm with an additional shoulder at 670 nm and demonstrate similar EL peaks at 624 nm. Very close CIE coordinates to the NTSC standard for red sub-pixels were recorded for Ir-63 to Ir-65. Deep red light-emitting phosphorescent dendrimers Ir-66 to Ir-68 with carbazolyl moiety tethered to 2-benzo[b]thiophen-2-yl-pyridyl ligand through a nonconjugated spacer exhibited emission spectra with an identical shape and a characteristic phosphorescence emission at around 641e644 nm that are almost identical to that of the Ir(III) core [58]. High encapsulation of the Ir(III) unit in Ir-66 to Ir-68 by the multicarbazole dendrons ensures more efficient energy transfer within the complexes without spatial geometrical congestion, which results in increasing the PL intensity with dendrimer generation. High quality films obtained by spin-coating for host-free PHOLED systems were obtained based on Ir-66 to Ir-68. The dielectric nature with increasing size of the non-conjugated spacer showed a slightly higher Von for Ir-68-based device. They exhibited deep-red

Ir-62

2.5. Other Ir(III)-based red phosphors Besides the Ir(III)-based phosphors mentioned above, some other functionalized complexes also gave red EL bands. Complex Ir70 with 9,90 -dimethylfluorenyl-substituted benzothiazole cyclometalating ligands showed red phosphorescence at 591 nm with moderate VPL of 32.7% at 5 wt-% Ir-70 doped in CBP [60]. It showed alleviated self-quenching characteristics as VPL remained as high as 16% at the dopant concentration of 20 wt-%. Devices based on Ir-70 at different doping ratios were fabricated which emitted efficient red light at 600 nm (with shoulder at 653 nm) and CIE coordinates of (0.63, 0.36) accompanied by low efficiency roll-off at relatively high current density. The peak hL and hext of 28.5 cd/A and 15.6%, respectively, were obtained at 15 wt-% doping level. By replacing the methyl groups to octyl chains at the 9-position of fluorene in Ir-71, a slight red shift in PL (596 nm) was obtained [61]. A lower VPL of 16% was recorded for Ir-71 which resulted in a subsequent decrease in overall device efficiencies. The device containing dopant Ir-71 exhibited an almost saturated red emission at 598 nm with a maximum efficiency of 19.6 cd/A, 10.3 lm/W and 1.4% with CIE coordinates of (0.63, 0.37). Although Ir-72 with identical linear conjugation ligand between the benzothiazole moiety and biphenyl unit as compared to Ir-70 and Ir-71, it gave a significant bathochromic shift in its EL and PL emission. Because of the excellent HI and HT nature as well as electron-donating feature of carbazole unit in Ir-72, the HOMO level of Ir-72 was dramatically raised to a higher level (4.87 eV) than that of Ir-71 (5.18 eV), resulting in a smaller HOMO-LUMO bandgap and a further red shift of phosphorescence in Ir-72. Device based on Ir-72 emitted a saturated red light with a peak wavelength at 638 nm and a shoulder at 688 nm, which corresponded to the CIE coordinates of (0.67, 0.32). The performance of the solution-processed device doped with Ir-72 is not so remarkable, with a Lmax of 1706 cd/m2, and peak hL of 5.4 cd/A, hP of 4.4 lm/W and hext of 8.3%. It is generally accepted that the solution-processed devices exhibited an inferior performance than those from vacuum deposited ones.

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N N

N

N

N

N

N

N

N N N

N N

N

N

N Ir

N

Ir

Ir

S

S

S

3

3

3

Ir-64

Ir-63

Ir-65

N

N

O O

N

O

N O

N

O

N

O

O

O O

O

O

O

N O

O

O

O O

N

N

Ir

3

N

S

S

S

Ir-66

N

N

Ir

Ir

3

3

Ir-68

Ir-67

N

N Ir

RO

S

2

OR Ir-69

N

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

N

O

S

N

O

S

S

Ir O

O

O

N

O

Ir

Ir

275

C8H17

N

C8H17 2

2

2 Ir-70

Ir-71

Phthalazine compounds have a distorted heterocyclic biphenyl structure with two adjacent nitrogen atoms in one aromatic ring and have been widely used in red phosphorescent Ir(III) complexes with excellent luminescent properties. Compared with other common C^N]CH-based Ir(III) complexes, this C^N]N type of compounds showed high thermal stabilities. They were easily synthesized under mild synthetic conditions. Phthalazine-based ligands can bond to the Ir atom more strongly as compared to the phenylisoquinoline one as it would create a “proximity effect”. The strong bonding between metal and ligand would lead to efficient mixing of the singlet and triplet excited states and is beneficial for the observation of phosphorescence. The enlargement of the conjugated p-system in the phthalazine unit in Ir-73 to Ir-79 resulted in saturated red emission. Complex Ir-73 emitted red light peaking at 625 nm in CH2Cl2 solution, with a VPL of 20% [62]. The HOMO of Ir-73 is contributed both from the Ir(III) atom and ligands and the contribution in the LUMO comes mainly from the ligand, thus the emission of Ir-73 is assigned to a mixture of MLCT and ligand centered (pep*) transitions. The high thermal decomposition temperature (426  C) of Ir-73 demonstrates its potential in PHOLED application. The doped device at 5 wt-% Ir-73 presents the best performance with hext of 8.3% and CIE coordinates of (0.69, 0.30). However, these devices suffered from severe TTA at high excitation density. A tentative lifetime of Ir-73-based device was tested and a lifetime of 610 h at an initial luminance of 100 cd/m2 was measured. Wang et al. reported two functionalized phenylphthalazine Ir(III) derivatives Ir-74 and Ir-75 with carbazole and diphenylamine moieties for red PHOLEDs [63,64]. Similarly, the incorporation of carbazole and diphenylamine units are found to improve the charge balance in the EL process, extend the p-electron delocalization of the aromatic ligand chromophore and enhance the thermal stability of the compounds. Ir-74 and Ir-75 emitted at 615 nm and 614 nm, respectively, upon irradiation with 400 nm light. The HOMO level of Ir-75 was elevated to 5.16 eV by introducing the amine group as compared to Ir-74 (5.35 eV). The solution-processed devices using Ir-74 and Ir-75 as dopant were fabricated with the structure of ITO/PEDOT:PSS/Ir-74 or Ir75:PVK þ PBD/TPBI/Ba/Al. Both of the complexes emitted at around 620 nm with fine vibronic structure. Higher device efficiency was obtained for Ir-74 and it gave a Lmax of 2948 cd/m2 at the current density of 115.6 mA/cm2. A maximum hext of 20.2% corresponding to a hL of 11.3 cd/A was obtained at a current density of 0.18 mA/ cm2. The outstanding performance of this device can be explained by the high VPL of Ir-74 (VPL ¼ 0.46) and the reduction of nonradiative transition of the rigid ligand. The Ir-74-doped device at 4 wt-% doping level gave a maximum hext of 13.6%, corresponding to a hL of 7.4 cd/A at a current density of 0.73 mA/cm2. However, similar to Ir-73, both devices suffered from severe efficiency roll-off at high current density which is probably induced by the planar structure of the cyclometalating ligand, which tends to cause

Ir-72

aggregation of molecules at high current density and leads to excited-state intermolecular interactions. Therefore, further optimization of the devices based on Ir-74 and Ir-75 is needed for practical use. Two novel Ir(III) complexes Ir-76 and Ir-77 with carbazolebased phthalazine derivatives were reported recently [65]. They contain various functional units in order to achieve high triplet energy and good HT ability. For comparison, Ir-77 with triphenylamine unit (lPL ¼ 654 nm) instead of carbazole in Ir-76 (lPL ¼ 640 nm) showed a red shift of 14 nm in its PL profile which suggested that the carbazole moiety in this system increased the bandgap energy. The conjugation extension of the cyclometalating ligand and the pyridine ring of 2-picolinic acid also red-shifted the emission of Ir-77 as compared to Ir-74. Their HOMO levels are very similar (5.17 eV) which indicate that the introduction of carbazole group can regulate the energy level as the HOMO primarily resided on the Ir(III) center and 9-ethyl-9H-carbazole of the cyclometalating ligand. Their HOMO and LUMO levels were embedded between the HOMO of PVK and the LUMO of PBD which ensure holes and electrons are effectively trapped within the emissive layer in the device. The emission peak of Ir-76 appears at 656 nm, and that of Ir-77 at 662 nm and both correspond to saturated red emission ((0.678, 0.291) and (0.688, 0.287)). Aggregation emission peaks were observed at 707 nm for Ir-77-doped OLEDs. They exhibited a good light stability as their CIE coordinates remained almost unchanged at various doping concentrations. The overall luminance of Ir-77 was much lower than that of Ir-76 which indicate that more balanced charge recombination and favorable electrical excitation in the EL process was achieved in the latter case. The maximum hext values were 16.3% and 11.9% for Ir-76- and Ir-77-based devices, respectively. Integrating electron-rich thiophene ring with phthalazine in the homoleptic complexes Ir-78 and Ir-79 effectively red-shifted the emission bands as compared to the structurally similar compound Ir-74 [66]. Both complexes showed saturated red emission at 648 nm with FWHM of 35e 41 nm in the PL spectra. Change of the substituents in Ir-78 and Ir79 did not affect the effective conjugation length of 1-(thiophen-2yl)phthalazine derivatives that resulted in an almost identical EL emission at w656 nm. With the same device structure by spincoating technique, device using Ir-79 as the dopant has a higher efficiency and lower Von than device based on Ir-78 which contains 2,6-dimethylphenol group instead of carbazole. Ir-79-based device achieved a Lmax of 1589 cd/m2, an hL of 26 cd/m2 and an hext of 10.2%. The high thermal stability, outstanding EL properties and high PL efficiency of this class of Ir(III) materials with the C^N]N type structural ligand were believed to be promising for optoelectronic applications. Ir(III) complexes Ir-80 and Ir-81 with dibenzo-[f,h]quinoxaline derivatives were first reported by Cheng et al. as red emitters [67]. They emitted at around 610 nm with attractive VPL of 0.53

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N N N

N N

Ir

N

N N N

Ir

Ir

Ir O

3

3 Ir-73

Cl

Ir-74

Ir-75

N

Ir-76

N

N N

Ir O

2 N

O

N N

O

3

Cl

N

N N

N

N N

Ir

Ir

O

S

S

2

3

N

3

Ir-78

Ir-79

Ir-77

The PHOLED performances of three heteroleptic red phosphorescent Ir(III) complexes bearing two 2-(4-fluorophenyl)-3methyl-quinoxaline cyclometalated ligands together with one of the following ancillary ligands, triazolylpyridine (Ir-82), picolinate (Ir-83) and acetylacetonate (Ir-84), were examined by Johannes et al. [69]. They emitted from their triplet MLCT (3MLCT) states in the spectral range of 605e628 nm in solution state, suggesting that their emission colors are influenced by the ancillary ligand. The influence of 5d-electron density of the Ir(III) center on their HOMOs led to high quantum yields (VPL ¼ 0.39e 0.42) and short triplet lifetimes. Different ancillary ligands not only affect the emission wavelengths but also their electrochemical energy levels. The oxidation peaks are shifted to higher

and 0.48 for Ir-80 and Ir-81, respectively. The devices based on these metal complexes with exceedingly high brightness (65040 cd/m2 for Ir-80; 59560 cd/m2 for Ir-81) and respectable hext (11.9% for Ir-80; 10.4% for Ir-81), hL (23.3 cd/A for Ir-80; 21.7 cd/A for Ir-81) and hP (7.9 lm/W for Ir-80; 8.4 lm/W for Ir-81). Very recently, by applying the red phosphor Ir-81 with the device configuration of ITO/MoO3/NPB/TCTA/Be(PPI)2 or Zn(PPI)2:Ir-81/TPBI/LiF/Al, it further enhanced the device performance significantly (Be(PPI)2 and Zn(PPI)2: 2-(1-phenyl-1Hphenanthro[9,10-d]imidazol-2-yl)phenol-based beryllium and zinc complexes, respectively) [68]. Particularly, the Be(PPI)2based devices exhibited lower efficiency roll-off than Zn(PPI)2based one, which could be attributed to the more balanced

N

N N

O

N

O Ir

Ir O

O

2

2 Ir-80

carrier-transport characteristics of Be(PPI)2. Very low Von of 2.3 V was detected for both devices. The brightness of Be(PPI)2-based device reached as high as 38580 cd/m2 with hL of 15.9 cd/A, hP of 19.9 lm/W and hext of 15.1% with neglectable efficiency roll off. These results revealed that by a suitable choice of bipolar transport material for the dopant, a remarkable improvement in efficiency of PHOLED device can be obtained.

Ir-81

potentials and the first reduction peaks appear to be less negative with an increase in the p-accepting character of the ancillary ligand. The electron-withdrawing character of the fluorine substituent on these complexes leads to low-lying HOMO and LUMO energies as compared to other 2-phenylpyridyl-derived Ir(III) complexes. Limited efficiency roll-off over a wide range of current density in the devices were demonstrated by using Ir-82 to

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

N

N

N

N

N N

277

O

N

N

O

Ir

Ir

Ir

N

O O

N

N

Ir-82

2

2

2

F

F

F

Ir-83

Ir-84

N N

N Ir N N F3C

2 Ir-85

Ir-84. All the devices showed low Von and high brightness in the range of 2.4e2.9 V and 13252e28019 cd/m2, respectively. Device based on Ir-83 showed the highest device efficiencies with hext of 12% and hP of 14.6 lm/W. By using Ir-84, PHOLED with superior device stability with an extrapolated lifetime of 58000 h was realized. The device lifetimes are strongly correlated to the type of ancillary ligand used. Ir(III) complex Ir-85 bearing two substituted quinoxalines and an additional 5-(2-pyridyl)pyrazolate as the third coordinating ligand was found to be emissive in the saturated red region [70]. Such complex reveals a distorted octahedral geometry around iridium center due to the bulkiness and unsymmetrical environment of the ligands. It emitted at 642 nm in the solution state with VPL ¼ 0.4. A PLED was fabricated by using Ir-85 in a multilayer configuration. The best device performance was achieved at 1.8 wt% Ir-85 with Von of 9.4 V, Lmax of 7750 cd/m2 and CIE coordinates of (0.64, 0.31). The author ascribed that the EL of Ir-85 peaking at 640 nm which was dominated by the triplet excited states is mainly due to the supposition of both Forster energy transfer and direct charge-trapping/recombination at the iridium metal center. Ir(III) complex Ir-86 containing 4-phenyl quinazoline ligand anchored with HT diphenylamine unit displayed a strong red phosphorescent emission at 599 nm with a VPL of 11.6% in the solution state [71]. A much higher VPL was detected in doped film with PVK:PBD blend, and it increased from 31.2% to a maximum of 54.87% on going from 1 wt-% to 4 wt-% doping concentration. Solution-casted devices based on Ir-86 at different doping concentrations were fabricated, and complete energy transfer was achieved from the host to Ir-86. All the devices showed the main emission centered at 616 nm, and an additional weak emission peak at about 438 nm from the PVK-PBD host at 1% and 2% doping concentrations. The EL spectral stability was very good and the CIE plots were almost identical in which the coordinates were located near (0.62, 0.38). The device at 8% doping concentration achieved a Lmax of 7107 cd/m2 at a current density of 78.2 mA/cm2 with the Von of 13.5 V. The maximum hext and hL of the device at 8% doping concentration are 18.44% and 20.73 cd/A, respectively. The neglectable efficiency roll-off indicates the balance of electron and hole recombination in the host matrix, and complete energy and charge transfer from the host material to Ir(III) complex upon electrical excitation. By further optimization of the molecular and device structures with suitable energy transfer in PHOLEDs, promising devices could be achieved by using 4-phenylquinazoline

based Ir(III) complex. Employment of Ir(III) bis(carbene) complex Ir-87 as the phosphorescent emitter for the EL devices gave extremely high device efficiencies [72]. Basically, Ir(III) tris(carbene) complexes are known to have high triplet energy gaps and

N N

O

N

O

N

N

Ir

Ir

N

N

N

2

2

Ir-86

Ir-87

can be used as blue emitters. By using 1-(1H-pyrrol-2-yl)isoquinoline to form a heteroleptic Ir(III) complex Ir-87, it emitted at 599 nm. Device based on Ir-87 as the dopant emitter consisted of the layers: ITO/NPB/TCTA/CBP:Ir-87/BCP/Alq/LiF/Al which gave a red emission with the CIE coordinates of (0.60, 0.39) and revealed an extremely high hext of 24.9%, hL of 55.4 cd/A, hP of 43.6 lm/W and Lmax of 16572 cd/m2. Using a cationic Ir(III) complex Ir-88 as dopant, solutionprocessed red OLED was fabricated [73]. Here, TPBI layer was inserted to suppress the exciton quenching at the cathode interface and the peak hL of the device reached 4.2 cd/A and hext of 3.2%. The device emitted at 618 nm with CIE coordinates of (0.62, 0.38). +

N

N _ PF6

Ir N

N 2 Ir-88

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was probed in their neat film. Unlike most of the emissive materials, the solid-state FPL of Re-1 to Re-6 were found to increase relative to their solution values and were in the range of 7% for Re6e19% for Re-2. These imply that the intermolecular interactions of these dendrimers in neat films do not result in extra quenching with respect to their solution state. Single layer devices based on Re-1 to Re-6 with configuration comprising of ITO/neat dendrimer film/Ca/Al were fabricated. Of the six Re(I) complexes, Re-5 gave

WOLED was fabricated by co-doping blue-green emitter and redemitting complex Ir-88 in the device structure of ITO/PEDOT:PSS/ PVK:OXD-7:blue green emitter:Ir-88/TPBI/Cs2CO3/Al. The CIE coordinates of the WOLED changed from (0.43, 0.43) to (0.35, 0.44) when the biasing voltage increased from 5.0 to 8.0 V and the device emitted at 484 and 594 nm. The peak hL reached 16.7 cd/A, corresponding to an hext of 7.8% and an hP of 6.8 lm/W.

G1

G1 G1

G1 G1

OC OC

N

N

N Re

Cl CO

Re-1

OC

Re

OC

OC

Cl

Re

N Cl

Cl

OC

CO

N N G1OC Re Cl G1

CO

OC

Re-5

Re-4

CO

Re-6

R=

OR

RO

Re

OC

Re-3

Re-2

N N G1OC Re Cl G1

N

OC

CO

OC

CO

G1

N

N

N

G1 =

2+

N

N

N N F3C

R

CF3 N

N

N N

X2

L

L

PPh2Me N N Os

N

Os

N

PPh2Me

Os-2 Os-3 Os-4 Os-5

Os-1

L P As P As

R CH=CH CH2-CH2 CH=CH CH=CH

X _ CF3CF3CF3CO2 CF3CF3CF3CO2 _ CH3C6H4SO3_ CF3CF3CF3CO2_

3. Red phosphors containing other transition metals

the best device performance with an hext of 0.4%, hL of 0.8 cd/A and hP of 0.2 lm/W at 100 cd/m2. The lack of optimum performance

The research on red phosphors based on Ir(III) complexes has drawn increasing attention due to their high device efficiencies, however, metal complexes derived from other transition elements, such as Re(I), Os(II), Pt(II) and Zn(II), can also be utilized as red emitters. Burn and Samuel synthesized a series of (1,10phenanthroline)rhenium(I)tricarbonyl complexes Re-1 to Re-6 with different dendritic generation which comprised of biphenyl units and 2-ethylhexyloxy surface groups [74]. All the complexes exhibited broad PL spectra and emitted in the range of 629e645 nm in the solution state. Attachment of conjugated phenyl group as dendron in Re-2 to Re-6 effectively shifted their PL maxima to the red as compared to the parent complex Re-1. Similar observation

from the device may be due to the imbalance of charge injection with electrons being injected more readily than holes. In spite of the modest solid-state VPL, it is believed that the performance of the devices based on Re(I) complexes could be improved by the use of an insoluble HT layer or by blending the dendrimer with a suitable host material. In the OLED application, the long device lifetime is speculated to be an intrinsic property, where radiative phosphors significantly shorten the lifetime of potentially reactive triplet states in the conductive host material [75]. Due to the strong back bonding from Os(II) ion to the ligands, the emission from the lowest triplet state of Os(II) complexes is MLCT in character with a relatively short lifetime

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

(<1.8 ms). Unlike Pt(II) and Ir(III) complexes, the application of Os(II) complexes in PHOLED has rarely been explored. Applying Os-1 into a simple three-layer device configuration of ITO/PEDOT:PSS/Os-1/ TPBI/LiF/Al gave a strong red emission with the CIE coordinates of (0.61, 0.39) [76]. Complete energy and charge transfer from the host to Os-1 was observed as its EL spectrum consists only of the phosphor emission without any residual emission from the host or other adjacent layers even at the high driving current. The Os(II) complexes Os-2 to Os-5 reported by Jen et al. were also found to be useful in EL applications [77]. They all emitted red EL colors in the range of 620e650 nm when they were fabricated in PHOLED together with a blend of PVK and PBD. The corresponding CIE coordinates for Os-2 to Os-5 were (0.60, 0.39), (0.61, 0.39), (0.65, 0.33) and (0.65, 0.34). The change of counter ion from heptafluorobutyrate in Os-2 to tosylate in Os-4 increased the brightness of the device but decreased the overall efficiency. Therefore, counter ions also play important role in optimizing the device performance. The overall efficiencies of these devices are not high; the best performance was found for Os-3, which has efficiency and brightness of 0.82% and 590 cd/m2, respectively. Very recently, structural modifications on Os-1 have been attempted by Chi and Liao which led to an enhancement in PHOLED efficiencies using these Os(II) based materials. Phosphine coordinated species Os-6 and Os-7 with tetradentate bis(pyridylpyrazolate) are believed not only to give longer p-conjugation length as compared to common bidentate ligand, but also to enhance the metal chelate stabilization energy [78]. Both of the complexes are highly emissive in solution and solid state with the peak wavelengths appearing at 628 and 634 nm, respectively. The slight blue shift in emission wavelength in Os-6 as compared to Os-7 is mainly attributed to the stronger electron-withdrawing properties of the ancillary PPh2Me in the former. The PHOLED devices made of Os-6 and Os-7 showed satisfactory deep red emission with the CIE coordinates located at (0.63, 0.36) and (0.65, 0.35), respectively. Relative to the Ir-1 based device fabricated under identical condition, Os-6 and Os-7 showed structureless emission peaks centered CF3 N

CF3

N

N

N Os

N

CF3

PPh2Me

N

PPh2Me Os-6

O

N

N Os

N CF3

86]. Therefore, Zhu and co-workers have sought for a new approach to reduce phosphorescent lifetime and prevent aggregation of the compounds, thereby enhancing the carrier-transporting properties by inserting non-planar triarylamine in Pt-2 to Pt-4, while the nonfunctionalized Pt-1 was also synthesized for comparison [87]. They presented intense dual emission peaks at room temperature which were assigned as the ligand-centered fluorescence and phosphorescence. The low-lying energy bands are in the red region (635e 638 nm). The addition of diarylamino group also increases the VPL of Pt-2 to Pt-4 (1.7e2.3%), which are almost 20 times higher than that of Pt-1 (0.09%). Similar to Ir(III) complex, incorporation of the electron-rich diarylamino moiety into the Pt(II) complex can raise the HOMO energy level significantly (5.49 to 5.52 eV for Pt-2 to Pt-4; e5.71 eV for Pt-1) and therefore improve the HT property of the complex. Pt-1 to Pt-4 based double layer PLEDs were fabricated using a blend of PFO and PBD as the host matrix. Unlike their PL, only one EL peak at 639e700 nm was observed for each device at different doping concentrations. A maximum hext of 8.6% and brightness of 1501 cd/m2 were obtained using Pt-2 at 4% doping concentration. This luminescent efficiency is two times higher than that of Pt-1. All the results suggest that the incorporation of diarylamino unit to the Pt(II) complexes can improve the EL properties and reduce the p-p stacking and PtPt interactions efficiently. By fusing an electron-deficient fluorenone ring system to pyridine, a substantial decrease in the triplet emission energy was obtained in Pt-5. This red electrophosphor with enhanced EI/ET features was prepared by using an electron trapping fluorenone chromophore in the ligand [56]. The dominant emission of Pt-5 is centered at 603 nm together with a subordinate weak emission peak in the green spectral region at 520 nm. The electron-deficient ring plays a crucial role as an electron acceptor in the charge transfer of electron density within the complex. Unlike other cyclometalated Pt(II) complexes [88], the LUMO of Pt-5 is delocalized over a larger area, resulting in destabilizing the LUMO level and red-shifted phosphorescence emission. However, Pt-5 exhibited a relatively low VPL (1%) and the performance parameters for the Pt-5-based red

PPhMe2

N N

279

N

O

O Pt

Pt

N

R

O

N

N

N

O Pt

N

N

O

PPhMe2 Os-7

O

R

Pt-1 R=H Pt-2 Pt-3 CH3 C(CH3)3 Pt-4

at 616 and 620 nm, respectively, which are in contrast to that of Ir-1 that gave an intense peak at 620 nm with a shoulder at 674 nm. Moreover, both devices with dopants Os-6 and Os-7 exhibited EL properties superior to that of the reference device with Ir-1. The device with Os-6 showed the best performance characteristics with the maximum hL of 14.0 cd/A, hext of 9.8% and hP of 9.2 lm/W. It is believed that the efficiencies of Os-6 and Os-7 can be further improved if a lower driving voltage can be achieved. Cyclometalated Pt(II) complexes have also attracted considerable attention in the context of their PHOLED application, as they have strong spin-orbital coupling at room temperature which result in highly phosphorescent emission [79e83]. However, as compared to Ir(III) complexes, cyclometalated Pt(II) complexes display relatively lower emission efficiency and longer phosphorescence lifetime which suffer from pronounced TTA and exhibit rapidly decreased emission efficiency at high current density [84e

Pt-5

N

N Pt

Pt

N

N

N

N

Pt-6

Pt-7

PHOLED were 0.81%, 0.80 cd/A and 0.22 lm/W with Von of 9.7 V. One of the features of this device is the absence of severe efficiency rolloff at high current density, implying that the TTA effect is not very significant even at high current density. This is presumably attributable to the relatively short triplet lifetime of Pt-5. This approach has great potential to excel in the future advance of other color-

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tunable emitting systems induced by electron-withdrawing functionalities. Later on, Fukagawa reported two Pt(II) phenyliminofunctionalized dopants Pt-6 and Pt-7 which showed high VPL that are comparable to those efficient red Ir(III) complexes [89]. These two complexes were designed to have a more rigid structure than common cyclometalated Pt(II) complexes and they are expected to suppress the vibration and rotation around the Pt atom, resulting in higher phosphorescence efficiency from the MLCT state. The PL spectrum of a 6 wt-% doped Pt-6/Bebq2 shows an emission peak maximum at 621 nm with a VPL of 58%. Such high VPL is very rare for Pt(II) complexes. By applying Pt-6 or Pt-7, low power consumption and high stability devices were demonstrated with good color saturation and CIE coordinates of (0.66, 0.34). The maximum hext and hP were 19.5% and 25.5 lm/W for Pt-6 and 19.3% and 30.3 lm/W for Pt-7, respectively. These values are comparable to the devices reported for red PHOLEDs using Ir(III) complexes. The estimated half-life of the device was about 10000 h at an initial luminance of 1000 cd/m2. This finding could expand the range of design possibilities of novel phosphorescent dopant materials for efficient and stable PHOLEDs. 4. Near-infrared (NIR) emitters

and stable dimesityboryleoligothiophene groups on the cyclometalating ligands which can extend the emission wavelength to the NIR region and enhance the charge transfer properties of the complexes by connection of the accepting boryl group with thiophene donor [106]. The bulky mesityl group promotes the formation of amorphous films with high glass transition temperatures which is crucial for long term stability of the devices. Upon photoexcitation, NIR-3 and NIR-4 exhibited dual emission in their PL spectra. The relative ratios of fluorescence to phosphorescence emission intensities of NIR-3 and NIR-4 are different as the emission of the heteroleptic congener NIR-4 is mainly caused by fluorescence (470 nm) with very weak phosphorescence only (>750 nm) but the intensity of both emissions is almost the same in homoleptic NIR-3. The EL color of these complexes also displayed dual emission bands consisting of both visible and NIR peaks and their relative ratio of the EL peak height varied with the doping

N

O Ir N

O

O

Ir

In addition to red emitters, there is a growing interest in the NIR emission from metal complexes through photoluminescence [90e 95] and electroluminescence [96e100]. NIR OLEDs are of interest due to their applications in a number of areas, including infrared signaling and displays, telecommunications and wound healing [8,11]. The most common approach that has been taken to develop NIR emitting devices is to employ lanthanide complexes. However, due to the low intrinsic VPL of the metal centered F states in lanthanide complexes (0.1e0.5%) which suffer from low overall quantum efficiency of their OLED devices [96e98,100e102]. Compared with other color phosphorescent materials, the molecular design and synthesis of efficient NIR-emitting phosphors is intrinsically more difficult since their VPL tend to decrease as the emission wavelength increases in accordance with the energy gap law [7,103,104]. Therefore, researchers are focused on modifying the ligands around the metal center rather than those with the lanthanide ion to induce a red shift in emission wavelength and to improve the overall VPL of other metal complexes. Qiao and Qiu reported the strategy of synthesizing a Ir(III) complex NIR-1 by modifying the quinoline portion of the ligand [48]. Through the use of phenyl-benzoquinoline derivatives with fused aromatic rings, the effective conjugation length will significantly increase and NIR1 exhibited an unexpectedly large red shift in wavelength to the NIR region with a peak maximum at 708 nm and a shoulder at around 780 nm. The infrared portion constitutes about 70% of the total emission output. Its emission VPL is as high as 2.5%. Its NIR device was fabricated in the configuration of ITO/NPB/Alq3:10% NIR-1/ Alq3/Mg:Ag and exhibited voltage-independent EL at 720 nm which coincides with that in the solution PL spectrum. A quite short lifetime of about 0.57 ms for NIR-1 and a relatively faster radiative rate at 4.4  104 s1, should be partially responsible for the slow reduction of efficiency as determined by TTA for its device at high current density. The forward light output is up to 4.6 mW/cm2 and the peak hext is nearly 1.1%. NIR-1 in such a simple device structure is very interesting and worthy of further study. The use of 1pyrenyl-isoquinoline as the cyclometalating ligand in NIR-2 extends the EL emission to the NIR spectral regime with a peak EL wavelength centered at 720 nm [105]. The peak hext was above 0.25% when NIR-2 was doped into a PVK:PBD matrix together with the layers of hole blocking and EI. Recently, Wong and co-workers designed two Ir(III) complexes NIR-3 and NIR-4 with very robust

O 2

2 NIR-2

NIR-1

N

O

N

Ir

Ir O

S

S 3

2 S

S B

NIR-3

B

NIR-4

concentration. The relative intensities for their NIR emissions in the EL spectra are dramatically higher than those in their PL spectra as the doping concentration increases. Although the overall OLED performances for NIR-3 and NIR-4 are poor (hext: 0.19% for NIR-3; 0.13% for NIR-4), it is believed that through further material development and device optimization, this kind of NIR emissive phosphors with electron-withdrawing borylated substituent would show better potential in night-vision displays or sensors operating in the NIR regime. In this field, terdentate cyclometalating ligands based on 1,3di(2-pyridyl)benzene Pt(II) complexes NIR-5 to NIR-7 with square-planar geometry favor face-to-face intermolecular interactions, including the formation of excimers or aggregates, which lead to their broad coverage of the PL spectrum by the combination of monomer and excimer emission [107]. They showed intense luminescence at room temperature, displaying vibrationally structured emission spectra with maxima in the green region at around 500 nm followed by weak excimer bands in the range of 700e750 nm. The trend in emission energy follows the order of NIR-7 > NIR-6 > NIR-5 in dilute solution, and this

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

the porphyrin ring, which is the longest emission wavelength demonstrated to date for a phosphor in an EL device. The solution VPL revealed a decline as the electronic conjugation is extended and therefore the VPL is only 0.08 for NIR-15. It was clearly demonstrated that the large difference observed for the solution VPL did not directly correlate with the performance of the chromophores in the devices; on the other hand, the solid-state lifetime was demonstrated to be an accurate prediction parameter of the relative device performance both in PLEDs and OLEDs. It was also found that the addition of 3,5-ditert-butylphenyl groups in place of phenyl groups on the benzoporphyrin ring periphery resulted in increased device efficiency. Although the efficiency improvement obtained with the disubsti-

suggested that the substituents in the pyridyl rings are important in determining the excimer energy. The induction of electronwithdrawing substituents at the 4-position of the pyridyl rings serves to stabilize the excimer formation and leads to a red shift in emission wavelength which favors the excimer-based NIR OLED application. The emissive layer consisting of NIR-5 to NIR-7 at 5 wt% in TCTA and neat film in OLEDs were examined. The emission colors of the devices at 5 wt-% level are green-yellow and are mainly characterized by monomolecular electronic transitions while those from the neat film fall in the deep red/NIR region resulting from the unique excimer-like emission. The corresponding CIE coordinates of the latter red/NIR-emitting devices were F F3C

F

CF3 F3C N

Pt

N

281

F

F

CF3 N

Pt

N

F3C

N

Pt

Cl

Cl

Cl

NIR-5

NIR-6

NIR-7

(0.65, 0.33), (0.65, 0.35) and (0.67, 0.32) for NIR-5 to NIR-7, respectively. The EL quantum efficiency of NIR-5 is lower (<0.35%) as compared to the others. An enhancement of hext of 0.55% was obtained for NIR-6 by introducing fluoro substituents into the central ring, whilst the OLEDs with neat NIR-7 film gave hext of 1.7%. Therefore, the OLED quantum efficiency and NIR emission emanating from this kind of complexes was shown to depend on the substituents and their positions in the central and lateral rings of the terdentate ligand. A class of phosphorescent platinoporphyrins based on p-extended tetraphenyltetrabenzoporphyrin molecular framework also showed good performance in the NIR-emitting OLEDs. Solution-processed device based on NIR-8 with the architecture of ITO/PEDOT:PSS/ PVCz:OXD-7:NIR-8/CsF/Al/Ag exhibited the EL peak at approximately 770 nm that is related to the triplet energy level of 1.6 eV [108]. The Von of NIR-8 based devices increased with the doping concentration. Replacing the tetraphenyltetrabenzoporphyrin by tetraphenyltetranaphtho [2,3] porphyrin in NIR-9 around the Pt(II) ion significantly red-shifted the phosphorescence emission and the lEL is located at 883 nm with a weak vibronic shoulder at around 1000 nm [109]. Its VPL is 0.22 with a triplet state lifetime of 0.85 ms. Both VPL and lifetime of NIR-9 (0.7 and 53 ms, respectively) are less than that of NIR-8 [110], indicating that the rate of nonradiative decay increases with decreasing triplet excited state energy. PLEDs with the structure of glass/ITO/PEDOT:PSS/NIR-9:PVK:PBD/LiF/Ca/Al were fabricated by spin-coating. The PLEDs turned on at an applied voltage of 6 V and the EL peak was centered at 896 nm. The device features hext of 0.4% and hP of 0.22 lm/W. The relatively low efficiency and high voltage operation may be due to the thick emissive layer and the high EI and HI barriers at the electrodes. OLEDs based on NIR-9 were also fabricated by the same research group via vacuum thermal evaporation. Light emissions from doped or undoped OLEDs operated at lower voltage (2 V) as compared to the PLEDs. For the undoped device, a maximum hext of 3.8% and hP of 19 lm/W were achieved at low current densities. Lower efficiencies with a maximum of hext ¼ 3.3% and hP ¼ 17 lm/W were detected for n-doped device based on BPhen. In 2011, a family of Pt(II) porphyrin complexes with different substituents on tetraphenyltetrabenzoporphyrin (NIR-10 to NIR-15) has been synthesized [111]. They have demonstrated that substituents play a major role in the determination of porphyrinate photophysical properties by various expanded conjugation. The PL and EL emission of NIR-15 was shifted to 1022 and 1005 nm, respectively, by inserting fluorenes on

N

CF3

tuted Pt-benzoporphyrins was not as high as that predicted by their solution VPL values, record high hext were obtained for PLEDs and OLEDs emitting in the NIR region with hext of 3.0 and 9.2%, respectively, for NIR-12 (PLED) and NIR-15 (OLED). Zinc(porphyrin)-based oligomers NIR-16 and NIR-17 represent an alternative approach as NIR emitters [112]. The linear NIR-16 and cyclic porphyrin NIR-17 hexamers have a red-shifted emission at 873 and 920 nm, respectively, in their thin film state. The film with NIR-16 showed a relatively low VPL which is lower than 1%. However, the addition of 4-benzylpyridine in NIR-17 narrowed both the absorption and emission spectra which suggested a suppression of aggregates and led to a significant increase in VPL to 7.7%. Taking this advantage, the OLED made of NIR-17 gave an order of magnitude higher in quantum efficiency as compared to NIR-16. This work successfully showed that Zn(II) porphyrin oligomers can serve as a good candidate for NIR emitters as well as color tuner through control of the conjugation length. Osmium(II) complexes NIR-18 and NIR-19 with isoquinolyl triazolate provide another avenue for the design of charge neutral and NIR emitting materials [113]. The trans-geometry of the phosphine donors in NIR-18 offers the most effective extended p-delocalization and hence red-shifted emission peak and lower excited state energy gap as compared to the cis-configured complex NIR-19. Both NIR-18 and NIR-19 exhibited phosphorescence with the peak wavelengths located at 805 and 731 nm, respectively, in the solution state. Vacuumdeposited NIR-emitting EL devices employing 6 wt% of NIR-18 or NIR19 in Alq3 host materials were fabricated which gave an intense emission at 814 and 718 nm, respectively. Efficiency roll-offs at high current densities are not significant in both devices and these may be associated to the rather short excited state lifetimes (0.04 ms for NIR18; 0.16 ms for NIR-19). These also imply that these devices can be stably driven to high currents and high optical output. Forward radiant emittance reached 65.02 and 93.26 mW/cm2, while the maximum hext is 1.5 and 2.7% for NIR-18 and NIR-19, respectively. The non-ionic nature, intrinsic lower lying excited state and relatively short lifetime of Os(II) phosphors are believed to be the key factors that make this kind of complexes to be useful for NIR-emitting devices. 5. Conclusions In this review, we summarize the recent advances in different classes of molecular phosphorescent dyes suitable for use as red or

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N N

N

N Pt

N

N

N

N

N

N

NIR-10

NIR-9

N N

N

N

Pt

Pt N

N Pt

NIR-8

N

N Pt

N

N

NIR-12

N

N

NIR-13

C6H13

C6H13 N Pt N C6H13

N

C6H13

C6H13 C6H13

NIR-15

N

NIR-14

C6H13

N

N Pt

Pt N

N

NIR-11

N

N

C6H13

C.-L. Ho et al. / Journal of Organometallic Chemistry 751 (2014) 261e285

N

Ar

Ar

N

N

N Zn

(C6H13)3Si

Si(C6H13)3

Ar

6

Ar

Si(C6H13)3 N

N

N

N

N Zn

(C6H13)3Si

283

6

NIR-17

NIR-16

N

N N Me2PhP

N Os

N N N

NIR-18

NIR triplet emitters in vacuum-deposited or spin-coated PHOLEDs. Functionalization of metal complexes in terms of the metal center and/or ligand structure can effectively tune the HOMO and LUMO levels of the dopant which would produce red or NIR dopant with desirable photophysical, thermal, redox and EL properties. All of these factors would have dramatic effects on the efficiency of the resulting red PHOLEDs, since the injection and transport of charge carriers in the devices is much influenced by the energy differences of the LUMO and HOMO levels between the host and the dopant. Given the ease of synthesis and advantages of electrophosphorescence and solution processing that some of these electrophosphors can offer, this can be seen as a big step forward in the advance of simple high-performance red PHOLEDs. On the other hand, it is challenging for scientists to design and invent stable red-emitting phosphors for white light generation from all-phosphorescent WOLEDs. One common way to obtain WOLEDs is based on an array of red, green and blue PHOLEDs (i.e. a three-color system) Therefore, the development of suitable redemitting phosphor as the red light source should continue to receive considerable research attention. Although red phosphor has been commercialized since 2003, there is still room for developing new materials with highly efficient saturated red (with CIE coordinates corresponding to the NTSC standard red color) or NIR emission and short triplet state lifetime. To realize appropriate performance levels for practical applications, optimization of the trade-off problem between efficiency and color purity is required. It is nice to see that some of the materials developed to date can provide a good avenue for the rational design of thirderow transition metal electrophosphors which reveal a superior device efficiency/color purity trade-off necessary for pure red light generation. Other phenomena such as saturation of triplet emissive states and tripletetriplet annihilation in red phosphors should also be avoided in order to give efficient and stable red or white light sources for lighting and display applications. More work should still

Me2 P

Os

PPhMe2 N

N N

N N

P Me2

N N N

N

NIR-19

be focused in these areas to realize useful devices and we look forward to these exciting developments in the upcoming future.

Acknowledgments W.-Y. Wong acknowledges the financial support from the National Basic Research Program of China (973 Program) (2013CB834702), Hong Kong Baptist University (FRG2/11-12/156), Hong Kong Research Grants Council (HKBU203011 and HKUST2/ CRF/10) and Areas of Excellence Scheme, University Grants Committee of HKSAR, China (Project No. [AoE/P-03/08]). We also thank the 985 Program and 111 Project from Minzu University of China (CUN985-07-08 and 111 Project B08044) for financial support. References [1] L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong, J. Kido, Adv. Mater. 23 (2011) 926e952. [2] M.B. Khalifa, M. Mazzeo, V. Maiorano, F. Mariano, S. Carallo, A. Melcarne, R. Cingolani, G. Gigli, J. Phys. D 41 (2008) 155111-1e155111-3. [3] Y. Kawamura, K. Goushi, J. Brooks, J.J. Brown, H. Sasabe, C. Adachi, Appl. Phys. Lett. 86 (2005) 071104-1e071104-3. [4] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750e753. [5] G.-J. Zhou, W.-Y. Wong, S. Suo, J. Photochem. Photobio. C Photochem. Rev. 11 (2010) 133e250. [6] C.-T. Chen, Chem. Mater. 16 (2004) 4389e4400. [7] S.D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 118 (1996) 1949e1960. [8] M.R. Robinson, R.P. Eaton, D.M. Haaland, G.W. Koepp, E.V. Thomas, B.R. Stallard, P.L. Robinson, Clin. Chem. 38 (1992) 1618e1622. [9] A.E. Boyer, M. Lipowska, J.M. Zen, G. Patonay, V.C.W. Tsang, Anal. Lett. 25 (1992) 415e428. [10] M.I. Daneshvar, J.M. Peralta, G.A. Casay, N. Narayanan, L. Evans, G. Patonay, L. Strekowski, J. Immunol. Methods 226 (1999) 119e128. [11] J.-C.G. Bünzli, S.V. Eliseeva, J. Rare Earths 28 (2010) 824e842. [12] A. Rogalski, K. Chrzanowski, Opto-Electron. Rev. 10 (2002) 111e136. [13] C. Adachi, M.A. Baldo, S.R. Forrest, S. Lamansky, M.E. Thompson, R.C. Kwong, Appl. Phys. Lett. 78 (2001) 1622e1624. [14] P. Coppo, E.A. Plummer, L.D. Cola, Chem. Commun. (2004) 1774e1775.

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