Jui-Yi Hung

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 to True-Blue Phosphorescent IrIII Complexes Bearing a Nonconjugated Ancillary Phosphine Chelate: Strategic Synthesis, Photophysics, and Device Integration

We report the design and synthesis of Ir(III) complexes functionalized with substituted pyridyl cyclometalate or azolate chromophores, plus one newly designed nonconjugated phosphine chelate, which not only greatly restricts its participation in the lowest-lying electronic transition but also enhances the coordination strength. These two key factors lead to fine-tuning of the phosphorescence chromaticity toward authentic blue and simultaneously suppress, in part, the nonradiative deactivation. This conceptual design presents a novel strategy in achieving heretofore uncommon, high-efficiency blue and true-blue phosphorescence. The fabrication of the organic light-emitting devices (OLEDs) employing phosphorescent dopants [Ir(dfpbpy)(2)(P(wedge)N)] (1b) and [Ir(fppz)(2)(P(wedge)N)] (3) was successfully made, for which the abbreviations (dfpbpy)H, (fppz)H, and (P(wedge)N)H represent 2-(4,6-difluorophenyl)-4-tert-butylpyridine, 3-(trifluoromethyl)-5-(2-pyridyl)pyrazole, and 5-(diphenylphosphinomethyl)-3-(trifluoromethyl)pyrazole, respectively. Of particular interest is the 3-doped OLEDs, which exhibit remarkable maximum efficiencies of 6.9%, 8.1 cd A(-1), and 4.9 lm W(-1), together with a true-blue chromaticity CIE(x,y) = 0.163, with 0.145 recorded at 100 cd m(-2).

Phosphorescent Ir(III) complexes bearing double benzyldiphenylphosphine cyclometalates; strategic synthesis, fundamental and integration for white OLED fabrication

A new Ir(III) complex [Ir(bdp)2(OAc)] (1) was prepared by the treatment of IrCl3(tht)3 with approx. two equivalent of benzyldiphenylphosphine in refluxing decalin solution, bdpH = benzyldiphenylphosphine and tht = tetrahydrothiophene. Complex 1 proves to be a versatile precursor, which could further react with various triazolate chelates such as 5-pyridyl-3-trifluoromethyl-1,2,4-triazole (fptzH), 3-tert-butyl-5-(2-pyridyl)-1,2,4-triazole (bptzH), 5-(1-isoquinolyl)-3-tert-butyl-1,2,4-triazole (iqbtzH) and 5-(1-phenanthridinyl)-3-tert-butyl-1,2,4-triazole (pbtzH) to afford the emissive complexes [Ir(bdp)2(fptz)] (2), [Ir(bdp)2(bptz)] (3), [Ir(bdp)2(iqbtz)] (4), and [Ir(bdp)2(phbtz)] (5), respectively. Single crystal X-ray diffraction studies of 1 and 5 revealed a distorted octahedral Ir(III) metal core, both possess two mutually orthogonal bdp cyclometalates, and the respective PPh2 donors reside at the cis-orientation. Formation of complexes 2–5 can be envisioned as simple replacement of acetate with the incoming N-heterocyclic triazolate chelates. As for photophysical properties, the structural variation leads to salient difference in emission features among complexes 2–5. Combining theoretical approaches, the results are rationalized by the contribution from the degree of ligand π-conjugation, together with the occurrence of ligand-to-ligand charge transfer (LLCT) and intra-ligand ππ* transition in the lowest lying excited state. The orange-red and white light-emitting OLEDs were then fabricated using 5 as dopant, for which the respective devices gave peak efficiencies of 13.6% photons/electron, 33.3 cd A−1, 29.8 lm/W and with CIEx,y = 0.530, 0.467 at 100 cd m−2, and peak efficiencies of 13.0% photons/electron, 28.0 cd A−1, 22.8 lm/W, and with CIEx,y = 0.356, 0.348 at 1000 cd m−2.

Blue-emitting Ir(III) phosphors with ancillary 4,6-difluorobenzyl diphenylphosphine based cyclometalate

Treatment of difluorobenzyldiphenylphosphine with the Ir(III) dimer [(dfppy)2Ir(mu-Cl)]2 gives (N,N)-trans-[Ir(dfppy)2(dfbdpH)Cl], followed by skeletal isomerization to form its (N,N)-cis analogue, and then the fully cyclometalated complex [Ir(dfppy)2(dfbdp)]; the last complex and its derivative are suitable for fabrication of true-blue phosphorescent OLEDs.

Authentic-Blue Phosphorescent Iridium(III) Complexes Bearing Both Hydride and Benzyl Diphenylphosphine; Control of the Emission Efficiency by Ligand Coordination Geometry

Sequential treatment of IrCl(3) x nH(2)O with 2 equiv of benzyl diphenylphosphine (bdpH) and then 1 equiv of 3-trifluoromethyl-5-(2-pyridyl) pyrazole (fppzH) in 2-methoxyethanol gave formation to three isomeric complexes with formula [Ir(bdp)(fppz)(bdpH)H] (1-3). Their molecular structures were established by single crystal X-ray diffraction studies, showing existence of one monodentate phosphine bdpH, one terminal hydride, a cyclometalated bdp chelate, and a fppz chelate. Variation of the metal-ligand bond distances showed good agreement with those predicted by the trans effect. Raman spectroscopic analyses and the corresponding photophysical data are also recorded and compared. Among all isomers complex 1 showed the worst emission efficiency, while complexes 2 and 3 exhibited the greatest luminescent efficiency in solid state and in degassed CH(2)Cl(2) solution at room temperature, respectively. This structural relationship could be due to the simultaneously weakened hydride and the monodentate bdpH bonding that are destabilized by the trans-pyrazolate anion and cyclometalated benzyl group, 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.