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

Abstract

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.[1] The readily available, Os II -, Pt II -, and in particular Ir III -based phosphorescence complexes grant superior advantage over fluorescent materials.[2] 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 CIE x,y coordinate (CIE = Commission Internationale de LEclairage) 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 III complexes possessing at least one cyclometalated 4,6-difluorophenyl pyridine {(dfppy)H} ligand, known as FIrpic, FIr6, FIrtaz, and others.[3] The majority of blue phosphors showed inferior color chromaticity with a sum of CIE x+y values being much greater than 0.3 or with single CIE y coordinate higher than 0.25.[4] Such inferior chromaticity, in part, has been improved upon adoption of carbene-, [5] triazolyl-, [6] and fluorine-substituted bipyridine (dfpypy) based chelates. [7] 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 II complexes.[8]Subsequently, room-temperature blue phosphorescence was also visualized for the respective heteroleptic Ir III complexes, [9] particularly for those dubbed "nonconjugated" ancillary chelate(s). The nonconjugated ligands so far comprise a benzyl substituted pyrazole, [10] an N-heterocyclic carbene, [11] phosphines, [12] and other ingenious molecular designs.[13]Herein, we report the preparation of a novel class of heteroleptic Ir III complexes by incorporation of tripodal, facially coordinated phosphite (or phosphonite), [14] denoted as the P^C 2 chelate, for serving as the ancillary, together with the employment of 2-pyridyltriazolate acting as blue chromophore.[15] The reaction intermediate, which possesses an acetate chelate, was isolated and characterized to establish the synthetic pathway. The tridentate P^C 2 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^C 2 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 [IrCl 3 (tht) 3 ] (tht = tetrahydrothioph...

Recent Progress in High‐Efficiency Blue‐Light‐Emitting Materials for Organic Light‐Emitting Diodes

“…Arylpyridine‐type blue triplet emitters have electron‐deficient aryl units as the HOMO determining unit and a pyridine‐derived unit as the LUMO determining unit to widen the HOMO‐LUMO gap of emitters for blue emission. Examples of electron‐deficient aryl units are F‐substituted phenyl, CN‐substituted phenyl, CF 3 ‐substituted phenyl and pyridinyl moieties . The electron‐deficient aryl units deepen the HOMO level of the triplet emitters and shift the emission spectrum to a blue emission wavelength.…”

Probing the Effects of the Number and Positions of −OCH3 and −CN Substituents on Color Tuning of Ir(III) Complex Derivatives through a Joint Computational and Experimental Study

“…Hence, a large 3 MLCT contribution will increase the quantum yield ( Φ PL ). We calculated MLCT% of all the derivatives under study following chou's work, as shown in Table . It can be seen that 3 MLCT% is affected by the different substituents and their grafted positions.…”

Recent Advances in Transition Metal Complexes and Light‐Management Engineering in Organic Optoelectronic Devices

“…Deep blue emissions were observed in the OLEDs fabricated from 24c showed a peak PE of 8.6 lm W −1 (CIE = 0.16, 0.11). In a later report, the efficiency was improved by the use of a phosphite tripod P ^ C ^ C type ligand in complexes 25a–c . It was suggested that the tripod ligand could enhance the long term stability of the complex in device operation.…”