Luminescent Diiridium Complexes with Bridging Pyrazolates: Characterization and Fabrication of OLEDs Using Vacuum Thermal Deposition

Abstract

efficient and durable emitters that are capable to show RGB colors constitute one of the key components for the successful advancement of OLED technology. [1] However, development of metal based phosphorescent emitters, particularly the blue-emitting phosphors, is still far from success in term of emission efficiency and chemical stability. [2] This is attributed to the higher energy demanded for the excited states of blue-emitting phosphors versus those of the lower-energy green and red emitters, so that the blue emitters are more subject to the upper lying metal-centered dd excited state via thermal population, causing quenching and faster decomposition at the long-lived triplet states. [3] Both scenarios are expected in giving emitters with inadequate efficiency and stability, which account for the grand challenges in the design of metal based phosphors.The majority of metal based OLED phosphors were mononuclear Ir(III) metal complexes bearing three bidentate chelates in either homoleptic and heteroleptic modes, among which the representative examples are [Ir(ppy) 3 ] [4] and MS-2 [5] (Scheme 1, top); both possess the class of phenylpyridinato cyclometalates. According to the chemical thermodynamics, however, these tris-bidentate complexes are expected to be less stable compared to the emitters bearing A series of novel diiridium complexes (1-4) bearing both functional 2-pyrazolyl-6-phenyl pyridine chelate and bidentate phenyl imidazolylidene chelate are synthesized, for which the pyrazolate fragment of the tridentate 2-pyrazolyl-6-phenyl pyridine also behaves as the bridge to hold two iridium atoms in close vicinity. Their structure is unambiguously confirmed using X-ray structure determination on the corresponding derivative 2a bearing 1,3-bis(4-fluorophenyl)-1H-Imidazolyl cyclometalate. Their photophysical and electrochemical properties are studied and further affirmed by the computational approaches. All these Ir(III) metal complexes 1-4 are very stable in both solution and solid film with near unity emission quantum efficiency. As opposed to most of diiridium complexes documented in literature, 1-4 are volatile and suitable for fabrication of organic light emitting diodes (OLEDs) under vacuum evaporation. The corresponding electroluminescent devices exhibit superior performance, among which external quantum efficiency of 27.6% using 2 as dopant stands for the record high of OLEDs using dinuclear Ir(III) complexes. They also offer a low roll-off at high luminance, demonstrating their potential en route to high performance OLEDs.

Unusually Fast Phosphorescence from Ir(III) Complexes via Dinuclear Molecular Design

“…Both complexes show intense T 1 →S 0 phosphorescence with spectral maxima at λ max =558 nm and λ max = 605 nm for 5 and at λ max =575 nm and λ max = 618 nm for 6, as measured in degassed toluene (c ≈ 10 5 M) at room temperature ( Figure 2). The resulting radiative rate k r = 0.78•10 6 s -1 of 5, calculated as k r = Φ PL /τ, is a high value for phosphorescence although still comparable to other mono-nuclear Ir(III) complexes and even some polynuclear systems reported earlier 27,46 . However, the almost three times larger value k r =2.27•10 6 s -1 of the di-nuclear 6 is remarkable.…”

“…As a result, complex 6 is an emitter with an emission rate a few times higher than other Ir(III) phosphors used in OLEDs. 10,14,17,19,21,23,[25][26][27][29][30][31][32] Moreover, by the emission rate, 6 also outcompetes the materials exhibiting thermally activated delayed fluorescence (TADF) [62][63][64][65][66][67][68] which are posed as alternative OLED emitters. 62,[69][70][71][72] The proposed di-nuclear design affords outstanding phosphorescence efficiency and also leaves room for photophysical tuning through modifications of the bridging C^NN^C ligand and N^C^N ligand and also through modifications at the position of the halogen anion.…”

“…6,8,9 Indeed, cyclometallated Pt(II) and Ir(III) complexes are often employed as emitters in phosphorescent OLEDs (PhOLEDs). 7,[10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] Apart from a high emission quantum yield, it is desirable that an OLED emitter shows a high radiative rate (short radiative decay time) in order to utilize more excitons per unit of time.…”

Cyclometalated Ir(iii) complexes towards blue-emissive dopant for organic light-emitting diodes: fundamentals of photophysics and designing strategies

“…16). 173 Among them, 149 exhibits blue-shifted emission peaks at 468, 500, and 536 nm due to the combined inductive and mesomeric effects. Importantly, very high PLQYs of >0.90 were observed for all diiridium complexes in both solution and film states at RT.…”

Are substitutes to Cd-based quantum dots in displays more sustainable, effective, and cost competitive? An alternatives assessment approach

“…OEs are a rapidly evolving field, but recent developments can be approximately categorized according to their method of exciton recombination: phosphorescence (Ph) and thermally activated delayed fluorescence (TaDF). 32 These categories are thoroughly explored in several reviews [33][34][35] , but we chose to focus on the best-performing materials identified by Bräse et al 32 For OEs, a green, red, and blue chemical substance of each generation were chosen for packages 2-OLED [36][37][38] (Ph) and 3-OLED [39][40][41] (TaDF). It is unclear which alternative substance is used by industry, but we decided to investigate these because of their impressive performance in academic papers.…”