Cheng-Huei Lin

Iridium(III) Complexes of a Dicyclometalated Phosphite Tripod Ligand: Strategy to Achieve Blue Phosphorescence Without Fluorine Substituents and Fabrication of OLEDs†

Organic light-emitting diodes (OLEDs) based on heavy transition-metal complexes are playing a pivotal role in next generation of, for example, flat panel displays and solid-state lighting. The readily available, Os-, Pt-, and in particular Ir-based phosphorescence complexes grant superior advantage over fluorescent materials. This is mainly due to heavyatom-induced spin–orbit coupling, giving effective harvesting of both singlet and triplet excitons. However, tuning of phosphorescence over the entire visible spectrum still remains a challenge. Particularly, designing new materials to show higher energy, such as deep-blue emission—with an ideal CIEx,y coordinate (CIE = Commission Internationale de L Eclairage) of (0.14, 0.09)—encounters more obstacle than the progress made for obtaining green and red colors. Representative blue phosphors are a class of Ir complexes possessing at least one cyclometalated 4,6-difluorophenyl pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and others. The majority of blue phosphors showed inferior color chromaticity with a sum of CIEx+y values being much greater than 0.3 or with single CIEy coordinate higher than 0.25. Such inferior chromaticity, in part, has been improved upon adoption of carbene-, triazolyl-, and fluorine-substituted bipyridine (dfpypy) based chelates. The above urgency prompted us to search for better and new blue phosphors. We produced a class of 2-pyridylazolate chelates possessing very large ligand-centered p–p* energy gap, as evidenced by the blue-emitting Os complexes. Subsequently, room-temperature blue phosphorescence was also visualized for the respective heteroleptic Ir complexes, particularly for those dubbed “nonconjugated” ancillary chelate(s). The nonconjugated ligands so far comprise a benzyl substituted pyrazole, an N-heterocyclic carbene, phosphines, and other ingenious molecular designs. Herein, we report the preparation of a novel class of heteroleptic Ir complexes by incorporation of tripodal, facially coordinated phosphite (or phosphonite), denoted as the P^C2 chelate, for serving as the ancillary, together with the employment of 2-pyridyltriazolate acting as blue chromophore. The reaction intermediate, which possesses an acetate chelate, was isolated and characterized to establish the synthetic pathway. The tridentate P^C2 ancillary chelate offers several advantages: 1) Good stabilization of complex and necessary long-term stability in application of for example, emitting devices. 2) The strong bonding of phosphorous donors is expected to destabilize the ligand field d–d excited state, thus minimizing its interference to the radiative process from the lower lying excited state. 3) P^C2 inherits profound and versatile functionality (see below) capable of fine-tuning the electronic character. As a result, highly efficient blue phosphorescence is attained with good OLED performance. Treatment of a mixture of [IrCl3(tht)3] (tht = tetrahydrothiophene) with an equimolar amount of triphenylphosphine (PPh3), triphenylphosphite {P(OPh)3}, and an excess of sodium acetate resulted in a high yield conversion (> 80%) into [Ir(P^C2)(PPh3)(OAc)] (1a); P^C2 = tripodal dicyclometalated phosphite (Scheme 1). Subsequent replacement of acetate in 1a with chelating 3-tert-butyl-5-(2-pyridyl)triazo-

Blue-emitting Ir(III) phosphors with 2-pyridyl triazolate chromophores and fabrication of sky blue- and white- emitting OLEDs

Heteroleptic Ir(III) complexes with 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazolate chromophore (bptz) and cyclometalating benzyldiphenylphosphine (bdp) or phenyl diphenylphosphinite (pdpit) ancillary (i.e. [Ir(bptz)2(bdp)] (1) and [Ir(bptz)2(pdpit)] (2)) are synthesized upon treatment of [IrCl3(tht)3] (tht = tetrahydrothiophene) with the relevant phosphine, followed by the addition of 2 equiv. of bptz chelate at elevated temperature. Their photophysical properties in solution were measured, along with the characteristics detected as dopants in thin solid films. For application, organic light emitting diodes (OLEDs) were also fabricated using 1 and 2 as dopants, achieving respective maximum efficiencies of 17.8% (44.8 cd A−1 and 46.3 lm W−1) and 9.1% (22.8 cd A−1 and 23.6 lm W−1). In addition, sky blue iridium complex 1 was used with red osmium complex [Os(bpftz)2(PPhMe2)2] (3) to fabricate phosphorescent OLEDs with a sophisticated red/blue/red emitting layer architecture, attaining a stable warm white color with CIE coordinates of (0.397, 0.411). This white OLED attained an electroluminescence efficiency of up to 18.1%, 39.6 cd A−1, and 35.7 lm W−1 for the forward direction.

Phosphorescent OLEDs assembled using Os(II) phosphors and a bipolar host material consisting of both carbazole and dibenzophosphole oxide

We report on the synthesis of a new series of Os(II) complexes (1–3) functionalized with 2-pyridyl (or 2-isoquinolyl) pyrazole chelates, together with a new diphosphine, 1,2-bis(phospholano)benzene chelate (pp2b). The resulting Os(II) complexes are fully characterized and their structural versus spectroscopic properties have been comprehended by absorption/emission together with computational approaches. The inherent electron richness, restricted rotational barrier and good steric hindrance of pp2b lead to the production of both orange and red phosphorescence with high quantum efficiency. For exploring these Os(II) based OLEDs, we also synthesized a bipolar material 5-[4-(carbazo-9-yl)phenyl] dibenzophosphole-5-oxide (CzPhO), possessing both carbazole donor and dibenzophosphole oxide acceptor. Successful fabrication of OLEDs using complexes 1 and 3 as the dopant and either 4,4′-N,N′-dicarbazolebiphenyl (CBP) or CzPhO as host is reported. For comparison, the CBP and CzPhO devices with 1 as the emitter showed peak efficiencies EQE of 10.9%, ηL of 21.7 cd A−1, and ηp of 11.9 lm W−1, and EQE of 14.3%, ηL of 34.8 cd A−1, and ηp of 45.2 lm W−1, respectively.

Stepwise Formation of Iridium(III) Complexes with Monocyclometalating and Dicyclometalating Phosphorus Chelates

With the motivation of assembling cyclometalated complexes without nitrogen-containing heterocycle, we report here the design and systematic synthesis of a class of Ir(III) metal complexes functionalized with facially coordinated phosphite (or phosphonite) dicyclometalate tripod, together with a variety of phosphine, chelating diphosphine, or even monocyclometalate phosphite ancillaries. Thus, treatment of [IrCl(3)(tht)(3)] with stoichiometric amount of triphenylphosphite (or diphenyl phenylphosphonite), two equiv of PPh(3), and in presence of NaOAc as cyclometalation promoter, gives formation of respective tripodal dicyclometalating complexes [Ir(tpit)(PPh(3))(2)Cl] (2a), [Ir(dppit)(PPh(3))(2)Cl] (2b), and [Ir(dppit)(PMe(2)Ph)(2)Cl] (2c) in high yields, where tpitH(2) = triphenylphosphite and dppitH(2) = diphenyl phenylphosphonite. The reaction sequence that afforded these complexes is established. Of particular interest is isolation of an intermediate [Ir(tpitH)(PPh(3))(2)Cl(2)] (1a) with monocyclometalated phosphite, together with the formation of [Ir(tpit)(tpitH)(PPh(3))] (3a) with all tripodal, bidentate, and monodentate phosphorus donors coexisting on the coordination sphere, upon treatment of 2a with a second equiv of triphenylphosphite. Spectroscopic studies were performed to explore the photophysical properties. For all titled Ir(III) complexes, virtually no emission can be observed in either solution at room temperature or 77 K CH(2)Cl(2) matrix. Time-dependent DFT calculation indicates that the lowest energy triplet manifold involves substantial amount of metal centered (3)MC dd contribution. Due to its repulsive potential energy surface (PES) that touches the PES of ground state, the (3)MC dd state executes predominant nonradiative deactivation process.