Novel Design of Iridium Phosphors with Pyridinylphosphinate Ligands for High-Efficiency Blue Organic Light-emitting Diodes

Due to the high quantum efficiency and wide scope of emission colors, iridium (Ir) (III) complexes have been widely applied as guest materials for OLEDs (organic light-emitting diodes). Contrary to well-developed Ir(III)-based red and green phosphorescent complexes, the efficient blue emitters are rare reported. Like the development of the LED, the absence of efficient and stable blue materials hinders the widely practical application of the OLEDs. Inspired by this, we designed two novel ancillary ligands of phenyl(pyridin-2-yl)phosphinate (ppp) and dipyridinylphosphinate (dpp) for efficient blue phosphorescent iridium complexes (dfppy)2Ir(ppp) and (dfppy)2Ir(dpp) (dfppy = 2-(2,4-difluorophenyl)pyridine) with good electron transport property. The devices using the new iridium phosphors display excellent electroluminescence (EL) performances with a peak current efficiency of 58.78 cd/A, a maximum external quantum efficiency of 28.3%, a peak power efficiency of 52.74 lm/W and negligible efficiency roll-off ratios. The results demonstrated that iridium complexes with pyridinylphosphinate ligands are potential blue phosphorescent materials for OLEDs.

In recent years, considerable attention has been attached to organic light emitting diodes (OLEDs) due to their successful applications in solid-state lighting and full-color flat-panel display. To the present, cyclometalated iridium (Ir) (III) complexes are still the most promising phosphorescent guest materials for highly efficient OLEDs because of their short excited lifetime (microsecond time-scale), color tuning flexibility, high quantum yields and thermal stability [1][2][3][4][5][6][7][8][9][10][11][12][13][14] . Like light-emitting diode (LED), blue light is indispensable among the trichromatic emissions, which plays a pivotal role in saving energy and achieving pure white emission with high color rendering index (CRI). In comparison with extensively studied iridium-based red and green phosphorescent complexes, the efficient blue emitters are limited and the performances of blue OLEDs are still not satisfactory. Therefore, achieving stable and highly efficient blue phosphorescent iridium complexes and their corresponding devices still remains a significant challenge.
It is well known that functionalized 2-phenylpyridine derivatives were most often used for blue Ir(III) emitters as the main ligands. Generally, the modification of pyridine unit with electron-donating group would heighten the LUMO level, while the decoration of phenyl unit with electron-drawing group can lower the HOMO level. In addition, it is also possible to enlarge the HOMO-LUMO energy gap by designing and employing other N-heterocyclic moieties to replace the pyridyl or phenyl moiety of phenylpyridine ligand [15][16][17][18][19][20][21][22][23] . Furthermore, another way is using ancillary ligands with strong coordination fields, which also can simplify the synthetic conditions. Up-to-date, most efficient blue Ir(III) emitters were designed by combining these strategies, such as FIrpic (Ir(III)bis(4,6-(difluorophenyl)pyridinato-N,C')picolinate) 24,25 , and FIr6 (Ir(III)bis(4,6-(difluorophenyl) pyridinato-N,C 2' )-tetrakis(1-pyrazolyl)borate) 26,27 , etc. [28][29][30][31][32][33][34] . Among them, FIrpic is the most widely used blue emitter due to its good device performances, simple molecular structure and ease of synthesis. Although tremendous efforts have been made to develop appropriate host materials and optimize device structures, the efficiency and stability of OLEDs based on FIrpic are still suffering from acid-induced decomposition and poor electron mobility 35 . It is generally known that charge balance is vital for high efficiency and low efficiency roll-off of OLEDs. In most OLEDs, the electron mobility of the electron transport material is always lower than the hole mobility of the hole transport material. Therefore, bipolar host materials and phosphorescent emitters with high electron mobility were considered to be helpful to balance the distribution of carries [36][37][38][39][40][41][42][43] . Several groups have attempted to design highly efficient blue Ir(III) complexes with high electron mobility containing appropriate ligands [44][45][46][47][48][49][50][51] . For example, the Chi group used pyrazole and triazole because these nitrogen heterocycles owning more negative framework of ligands will increase the electron affinity and may be beneficial in improving the electron mobility 18,19 .
In this context, we designed two novel ancillary ligands of phenyl(pyridin-2-yl)phosphinate (ppp) and dipyridinylphosphinate (dpp) for blue bis-cyclometalated Ir(III) complexes (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) (dfppy = 2-(2,4-difluorophenyl)pyridine, Fig. 1). The introduction of phosphoryl (P = O)moiety will improve the coordination fields of the ancillary ligand to make a hypsochromic shift. More importantly, P = O is a strong electron-withdrawing group capable of polarizing the molecule, which has been widely used in bipolar host materials and electron transport materials based on its excellent electron transport property [52][53][54][55][56][57][58][59][60][61] . In our former publications, we also reported some efficient OLEDs with Ir(III) complexes containing the electron transport P = O moiety 38,62,63 . The devices based on (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) displayed prominent electroluminescence (EL) performances with a peak current efficiency of 58.78 cd/A, a maximum external quantum efficiency (EQE max ) near 30% and low efficiency roll-off due to the introduction of P = O moiety and nitrogen heterocycle which can effectively balance the injection and transportation of charges and confine excitons in the emissive layer. Figure 1 showed the synthesis procedure for (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) complexes. The reaction of 2-bromopyridine with n-butyllithium and dichlorophenylphosphine (or phosphorus trichloride) gave the dipyridinyl phenyl phosphine (or tripyridinyl phosphine), which were further reacted with hydrogen peroxide and sodium hydroxide through a sequential oxidation and alkaline hydrolysis process to afford the corresponding Na(ppp) and Na(dpp) salts. Then, the two pyridinylphosphinates were employed with [(dfppy) 2 Ir(μ-Cl)] 2 dimer to form the relevant (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) complexes, which were characterized in detail by 1 H NMR, 31 P NMR, MS, and HRMS; single-crystal structure of (dfppy) 2 Ir(ppp) was also proved by X-ray crystallography ( Fig. 2(a)). The Ir metal is coordinated by two C^N main ligands and one phenyl(pyridin-2-yl) phosphinate ancillary ligand. The coordination ligands around the iridium center are in an octahedral geometry with the cis-N,O, cis-C,C, and trans-N,N chelating atoms. The Ir-C, Ir-N and Ir-O bond lengths are in the ranges of 1.965(8)-2.019(8) Å, 2.007(8)-2.176(7) Å and 2.169(6) Å, respectively, within the normal values for cyclometalated Ir(III) complexes (Table S1, Supporting Information). Furthermore, both complexes show good thermal stability, evaluated by thermogravimetric analysis (TGA) under a nitrogen atmosphere. The decomposed temperatures (T d ) of 5% weight loss are 389 °C for (dfppy) 2 Ir(ppp) and 396 °C for (dfppy) 2 Ir(dpp), respectively (Fig. S1), which are much higher than that of FIrpic (T d = 344 °C) 64 . These results suggest that both Ir(III) complexes are suitable for the application in OLEDs to guarantee no any decomposition during a long operation time.
The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra of (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) (CH 2 Cl 2 solution, 10 −5 M) at RT are depicted in Fig. 2(b), and the relevant data are collected in Table 1. In the absorption spectra the two complexes show intense bands before 350 nm due to the spin-allowed ligand-to-ligand charge transfer ( 1 LLCT) transitions of cyclometalated and ancillary ligands. While the metal-to-ligand charge transfer ( 1 MLCT and 3 MLCT) absorptions are distinguished at approximately 380 nm with lower extinction coefficients, which were confirmed by the theoretical calculation ( Fig. S2 and Table S2). Both complexes show intense blue phosphorescence with maximum emission peaks at 471 nm for (dfppy) 2 Ir(ppp) with a shoulder emission at 497 nm and 470 nm for (dfppy) 2 Ir(dpp) with a weaker emission at 496 nm, respectively. To ensure the reliability of quantum efficiency, the integrating-sphere system was used for measuring the absolute photoluminescence quantum yields in deaerated CH 2 Cl 2 , and the results were 0.35 for (dfppy) 2 Ir(ppp) and 0.25 for (dfppy) 2 Ir(dpp), respectively. In addition, the lifetimes are in the range of microseconds for the two The HOMO/LUMO energy levels of the iridium complexes are closely related to the choice of charge transport and host materials as well as design of OLED structure. Therefore, cyclic voltammetry experiments were carried out to calculate the HOMO and LUMO levels of the complexes (Fig. S4). The HOMO levels of (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) were found to be − 5.95 and − 5.96 eV, respectively, while the LUMO levels of (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) were estimated as − 3.00 and − 2.95 eV from the HOMO levels and UV-vis absorption spectra (Table S3).
The EL spectra, current density -voltage -luminance (J -V -L), current efficiency -luminance (η c -L) and power efficiency -luminance (η p -L) curves of the devices (PPP-1 and DPP-1) are shown in Fig. 4 (the curves  for pic-1 are listed in Fig. S6), and the device performance data are listed in Table 2. Both devices showed typical emission maxima at 475 nm with shoulders at 500 nm, in accordance with the PL of the Ir(III) complexes in CH 2 Cl 2 solutions. The CIE (Commission Internationale de L'Eclairage) color coordinates are (x = 0.13, y = 0.37)   Table 2). Their outstanding EL performances may be attributed to the following facts: the LUMO/HOMO levels of the iridium complexes situated within those of 26DCzPPy, so the carriers can be trapped almost simultaneously by the phosphorescent dopants when they are injected into emission layer, which will benefit for the improvement of exciton recombination efficiency. The electron transport group P=O and nitrogen-containing heterocycle in the iridium complexes as well as lower LUMO level will be helpful for the electron transport to improve hole -electron balance. A better balanced charge transport will promote the recombination of electrons and holes and broaden the recombination zone as well as lead to the suppressed current leakage in the devices. As a result, a slow decay of the device efficiencies with increasing driving voltage demonstrates their relatively high device stability 67,68 .
It is observed that the performances of (dfppy) 2 Ir(ppp)-based device (PPP-2) are not markedly improved with a η c,max of 45.71 cd/A and a η p,max of 37.78 lm/W, respectively. However, the device based on (dfppy) 2 Ir(dpp) (DPP-2) achieved better EL performances with a lower turn-on voltage of 3.2 V, a higher η c,max of 58.78 cd/A, a EQE max of 28.3% and a η p,max of 52.74 lm/W, respectively. Most importantly, with the current density increase, the efficiency roll-off values are very small. For example, even at the luminance of 1000 cd/m 2 , current efficiency as high as 48.79 cd/A can be retained by the device DPP-2 with EQE near 25%. To the best of our knowledge, such performances are amongst the best results achieved from blue phosphorescent biscyclometalated Ir(III) emitters 35,69 . This ascendant result is attributed to more balanced electrons and holes transportation and distribution within emissive layer. For PPP-2, because the addition of TcTa will facilitate the hole injection into emitting layer, and more holes accumulation would lower the device efficiency consequently. But for device DPP-2, the pyridinyl and P=O groups in (dfppy) 2 Ir(dpp) are helpful for electron transport, which causes the higher electron density within the emitting layer of DPP-2; therefore, the insertion of TcTa layer assists to balance the carriers and improve the EL efficiency. The electron mobility test results of the Ir(III) complexes (Fig. 6) would confirm this hypothesis.
In conclusion, two novel bis-cyclometalated Ir(III) complexes containing phenyl(pyridin-2-yl)phosphinate (ppp) or dipyridinylphosphinate (dpp) as the ancillary ligands were synthesized and applied for blue phosphorescent OLEDs. The related photophysical, electrochemical and electroluminescent properties were thoroughly investigated. Device PPP-1 based on (dfppy) 2 Ir(ppp) with the structure of ITO/MoO 3 (3 nm)/TAPC (50 nm)/ (dfppy) 2 Ir(ppp) (10 wt%):26DCzPPy (15 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) displayed good EL  Table 2. Key EL data of the OLEDs. a V turn-on : turn-on voltage recorded at a brightness of 1 cd/m 2 ; b L max : maximum luminance; c η c,max : maximum current efficiency; d current efficiencies measured at brightness of 100 cd/m 2 and 1000 cd/m 2 ; e η p,max : maximum power efficiency; f power efficiencies measured at brightness of 100 cd/m 2 and 1000 cd/m 2 ; g EQE max : maximum external quantum efficiency.   Methods Materials and measurements. All reagents and chemicals were purchased from commercial sources and used without further purification. 1 H and 31 P NMR spectra were measured on a Bruker AM 400 spectrometer. Mass spectra (MS) were obtained with ESI-MS (LCQ Fleet, Thermo Fisher Scientific). High resolution mass spectra (HRMS) were measured with a LTQ-Orbitrap XL (Thermofisher, USA). Absorption and photoluminescence spectra were measured on a UV-3100 spectrophotometer and a Hitachi F-4600 photoluminescence spectrophotometer, respectively. The decay lifetimes and absolute photoluminescent quantum yields were measured with an Edinburgh Instruments FLS-920 fluorescence spectrometer equipped with an integrating sphere in degassed CH 2 Cl 2 solution at room temperature. Cyclic voltammetry measurements were conducted on a MPI-A multifunctional electrochemical and chemiluminescent system (Xi'an Remex Analytical Instrument Ltd. Co., China) at room temperature, with a polished Pt plate as the working electrode, platinum thread as the counter electrode and Ag-AgNO 3 (0.1 M) in CH 3 CN as the reference electrode, tetra-n-butylammonium perchlorate (0.1 M) was used as the supporting electrolyte, using Fc + /Fc as the internal standard, the scan rate was 0.1 V/s. X-ray crystallography. The single crystals of complexes and ligand were carried out on a Bruker SMART CCD diffractometer using monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Cell parameters were retrieved using SMART software and refined using SAINT on all observed reflections. Data were collected using a narrow-frame method with scan widths of 0.30° in ω and an exposure time of 10 s/frame. The highly redundant data sets were reduced using SAINT and corrected for Lorentz and polarization effects. Absorption corrections were applied using SADABS supplied by Bruker. The structures were solved by direct methods and refined by full-matrix least-squares on F2 using the program SHELXS-97. The positions of metal atoms and their first coordination spheres were located from direct-methods E-maps; other non-hydrogen atoms were found in alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in calculated position and refined as riding atoms with a uniform value of Uiso.

Fabrication and measurements of OLEDs.
Indium-tin-oxide (ITO) coated glass with a sheet resistance of 10 Ω/sq was used as the anode substrate. Prior to film deposition, patterned ITO substrates were cleaned with detergent, rinsed in de-ionized water, dried in an oven, and finally treated with oxygen plasma for 5 minutes at a pressure of 10 Pa to enhance the surface work function of ITO anode (from 4.7 to 5.1 eV). All the organic layers were deposited with the rate of 0.1 nm/s under high vacuum (≤ 2 × 10 −5 Pa). The doped layers were prepared by co-evaporating dopant and host material from two individual sources, and the doping concentrations were modulated by controlling the evaporation rate of dopant. MoO 3 , LiF and Al were deposited in another vacuum chamber (≤ 8.0 × 10 −5 Pa) with the rates of 0.01, 0.01 and 1 nm s −1 , respectively, without being exposed to the atmosphere. The thicknesses of these deposited layers and the evaporation rate of individual materials were monitored in vacuum with quartz crystal monitors. A shadow mask was used to define the cathode and to make ten emitting dots (with the active area of 10 mm 2 ) on each substrate. Device performances were measured by using a programmable Keithley source measurement unit (Keithley 2400 and Keithley 2000) with a silicon photodiode. The EL spectra were measured with a calibrated Hitachi F-7000 fluorescence spectrophotometer. Based on the uncorrected EL fluorescence spectra, the Commission Internationale de l'Eclairage (CIE) coordinates were calculated using the test program of Spectrascan PR650 spectrophotometer. The EQE of EL devices were calculated based on the photo energy measured by the photodiode, the EL spectrum, and the current pass through the device.
Measurement and calculation of electron mobility. The electron mobility measurement of (dfppy) 2 Ir(ppp) and (dfppy) 2 Ir(dpp) was used the transient EL technique on the devices of ITO/TAPC (50 nm)/Ir(III) complex (60 nm)/LiF (1 nm)/Al (100 nm). A rectangular voltage pulse (amplitude 8~10 V, pulse duration 50 μs) was applied to an OLED by using a pulse generator (Rigol Model DG1022). The time-dependent EL was detected by placing a miniature photomultiplier tube (PMT, Hammamatsu Model H6780) directly on the top of the OLED. The output photocurrent from the PMT was sourced into a sensing resistor in which the transient EL signal was displayed on a storage oscilloscope (Tektronix Model TDS2000B). The electron mobility can be roughly calculated with the equation of μ e = d 2 /(t d · V), where d is the thickness of the emitting layer, V is the driving voltage, and t d is the delay time.