— 1931 CIE chromaticity diagram of OLEDs of FIrpic (hollow square), FIrtaz (solid triangle), and FIrN4 (hollow trangle), respectively. 

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... hand, TBA[Ir(ppy) 2 (CN) 2 ] is juwst as “green” as FIrpic . 5 Furthermore, it is an anionic species and hence can not be used in the thermal vacuum-deposited OLEDs. Recently, FIr6 was reported to be a bluer phosphorophore [EL CIE x,y coordinates (0.16, 0.28)] than greenish-blue FIrpic with respectful performance in OLED brightness and efficiency. 6 T h e charge-neutral coordination arrangement for Ir(ppy) 2 P( n Bu) 3 CN can be considered as the modified version of ionic TBA[Ir(ppy) 2 (CN) 2 ] , although the thermal stability is in question and only doped polymer light-emitting diodes with low efficiency have been achieved. 7 FIrptaz and btfmptaz are two newcomers on the list with “almost blue” in color, but their OLED performances are not known. 8 In this report, two newly synthesized transition-metal- complex blue phosphorophores, FIrtaz and FIrN4 (Scheme 2), were synthesized: heteroleptic iridium complexes with 5-(2 ′ -pyridyl)-3-trifluoromethyl-1,2,4-triazole or 5-(2 ′ -pyridyl)tetrazole as the ancillary ligand. The triplet- state phosphorescent dopants, FIrtaz , FIrN4 , or FIrpic , appropriately dispersed in mCP (1,3- bis (9-carbazolyl)ben- zene) host material were characterized, and their EL performances were compared. Scheme 2 shows the synthetic procedure for the triplet-state blue phosphorophores. The dichloro-bridged iridium dim- mer complex FIr 2 Cl 2 was conveniently synthesized by the m e t h o d repo rted b y Non oyam a. 9 W h e r eas 5 -(2 ′ pyridyl)tetrazole is a known compound and can be prepared readily in one step from commercially available 2-cyano- pyridine and sodium azide in large scale, 10 the synthesis of 5-(2 ′ -pyridyl)-3-trifluoromethyl-1,2,4-triazole is less straightforward according to a modified procedure docu- mented in the literature. 11 The fabrication of OLEDs and their electroluminescence characterization have been described elsewhere. 12 ITO-coated glass with a sheet resistance of <50 Ω / ᮀ was used as the substrate. The device has a multilayer structure of ITO/NPB (30 nm)/mCP: FIrpic , FIrN4 , or FIrtaz (30 nm, 7%)/TPBI(30 nm)/LiF (0.5 nm)/Al (Scheme 3). Here, NPB is used as a hole-transporting layer, mCP as a host, TPBI as an electron-transporting and hole-blocking layer, LiF as an electron-injecting layer, and Al as the cathode. The current, voltage, and light intensity ( I - V - L ) measure- ments were made simultaneously using a Keithley 2400 pro- grammable source meter and a Newport 1835C Optical meter equipped with a Newport 818-ST silicon photodiode. The device was placed close to the photodiode such that all the forward light goes to the photodiode. The effective size of the emitting diode was 3.14 mm 2 , which is significantly smaller than the active area of the photodiode detector, a condition known as “under filling” satisfying the measure- ment protocol. 13 Only light emitting from the front face of the devices was collected and used in subsequent calcula- tions of external quantum efficiency according to the method described before. 13 The luminous flux (lm) has been previously defined, 14 and we adopted it for our characterization. Photoluminescence spectra (Fig. 1) of FIrtaz and FIrN4 have shown very similar spectra pattern with the highest energy emission peak maximum located at 459 or 460 nm, which is about 10 nm blue-shifted from the 470 nm of FIrpic . Similar blue-shifting (ca. 5 nm) was also observed for the long-wavelength emission side bands of FIrtaz and FIrN4 . As a result, both FIrtaz and FIrN4 showed notice- ably blue photoluminescence (PL) in comparison with the greenish-blue color of FIrpic in CH 2 Cl 2 solution at room temperature as pictured in quartz curvets excited with a UV lamp in Fig. 2. Three OLEDs were fabricated based on blue FIrN4 and FIrtaz or greenish-blue FIrpic dopant emitter co-evaporated with mCP as the host material (Scheme 3). Here, TPBI, instead of the commonly used BCP (2,9-dimethyl- 4,7- diphenyl-1,10-phenanthroline) or BAlq (aluminum (III) bis(2-methyl-8-quinolinato)4-phenylphenolate), 16 was adopted for the devices to confine excitons in the emissive zone. We found that blue-phosphorescence OLEDs using TPBI, instead of BCP, the hole-blocking layer has the advantage of higher current density that shifts the peak efficiency of the OLED to a more reasonable range of greater than 1 mA/cm 2 (see below). Furthermore, the new device structure (without using BCP hole-blocking layer) of the FIrpic OLED exhib- ited high external quantum efficiency over 12%, one of the highest of all FIrpic OLEDs. Similar to the PL spectra in solution, the extent of blue-shifting of the emission wavelength remained the same for the EL spectra of FIrtaz and FIrN4 and devices ( λ ELmax , 464 and 462 nm, respectively) compared with that ( λ ELmax : 472 nm) of FIrpic devices (Fig. 3). The CIE x , y coordinates (0.14, 0.18) and (0.15, 0.24) of FIrtaz and FIrN4 devices are distinctively different from (0.16, 0.28) of greenish-blue FIrpic devices (Fig. 4). Particularly, the FIrtaz device is one of the bluest practical OLEDs based on phosphorescent transition-metal complexes. It is interesting to note that all EL spectra of blue OLEDs exhibit a long-wavelength emission side band or shoulder at 498, 488, and 490 nm for FIrpic , FIrtaz , and FIrN4 , respectively. Among these OLED devices, the FIrN4 device possesses the most pronounced longer-wavelength emission side band that detrimentally affects the color purity and causing the FIrN4 device to produce the greenish-blue color. On the other hand, the FIrtaz and FIrN4 devices have a very similar EL emission λ max , but the former has a significantly reduced emission side band at the lower energy side that allows the electroluminescence of FIrtaz to appear in the deep-blue region (CIE x , y : 0.14, 0.18). Thus, for the first time, a practical (bright enough and efficient enough) blue-phosphorescent OLED showing good blue color purity has been developed. A similar reason can be applied to FIrpic OLEDs. The EL wavelength of FIrpic is more than 10 nm longer than that of FIrN4 , but its CIE 1931 chromaticity (0.15, 0.28) is not much different from the of FIrN4 (0.15, 0.24). It is interesting to note that there is a subtle but seemly important difference between the PL and EL spectra of three phosphorophores. Unlike those that have quite similar intensities (77–82% to the main emission band) in solution PL spectra (Fig. 1), the intensity of the long-wavelength emission side band of the EL spectra (Fig. 3) varied largely, 63, 82, and 64%, relative to the main emission band of FIrpic, FIrN4, and FIrtaz , respectively (Table 1 and Fig. 3). The strong emission side band of FIrN4 also leads to the wide FWHM (full width at half-maximum) of the EL emission band (Table 1) that adversely affects the blue-color purity of the device. In addition to the wavelength of the main emission band, the intensity of the long-wavelength emission side band is vital to the blue-color purity of the OLED. FIrtaz and FIrN4 devices emit blue EL less effi- ciently than the greenish-blue FIrpic device. Whereas the FIrpic device reaches peak efficacies of 12.3% or 9.28 lm/W at 3.5 mA/cm 2 , only 6.1% (or 3.2 lm/W) and 5.8% (or 4.4 lm/W) can be achieved by FIrtaz and FIrN4 devices, respectively (Fig. 5). We have particularly noted that the drive current density in our devices is significantly higher than that reported previously, such as F I r6 doped devices. This is reflected in the EL efficiencies (either external quantum or power efficiencies) of the device, reaching a maximum in a much more reasonable current density range of 0.1–10 mA/cm 2 (Fig. 5). Most of the other known blue or near-blue phosphorescent OLEDs exhibit peak efficiencies at a very low drive current density of less than 0.1 mA/ cm 2 . 3b,3c,6a Nevertheless, both FIrtaz and FIrN4 OLEDs are significantly not as bright as the FIrpic OLED. The maximum blue electroluminance of both devices is around 10,000 cd/m 2 at 15 V and the highest current density is over 500 mA/cm 2 . Greenish-blue FIrpic OLED is more impressive because it can be as bright as 36,300 cd/m 2 (Fig. 6). At a low-current-density range of less than 1–2 mA/cm 2 (or less than 5–6 V), the bluest FIrtaz OLED exhibits much weaker EL intensity than the other two devices (Fig. 6). On the other hand, at a driving voltage of 5–6 V, the FIrtaz OLED has the highest allowed current density (Fig. 6). A low EL intensity and high current density put FIrtaz OLEDs in a bad situation in terms of EL efficiencies, particularly at the low-current-density range of less than 1–2 mA/cm 2 , even though it shows the best blue-color purity EL among the three devices. We have successfully developed two new blue-phosphorescence materials, FIrtaz and FIrN4 , containing bidentate pyridyl triazolate (or tetrazolate) ancillary ligand. OLEDs were fabricated with these two blue posphorophores and previously known FIrpic as well. We demonstrated that FIrN4 and FIrtaz generated blue electroluminescence with improved blue color purity relative to the greenish blue of FIrpic , although the FIrpic OLED was brighter and more efficient. Support from the Program Promoting University Academic Excellence from Ministry of Education, Taiwan, under the Contract No. 92-E-FA04-2-4, is acknowledged. We also thank the National Tsing Hua University and Academia Sinica for the financial ...