Pyrimidine-Based Mononuclear and Dinuclear Iridium(III) Complexes for High Performance Organic Light-Emitting Diodes

Abstract

Containing two nitrogen atoms, the electron-deficient pyrimidine ring has excellent coordinating capability with transition metal ions. However, compared with the widely used pyridine ring, applications of the pyrimidine ring in phosphorescent Ir(III) complexes are rare. In this research, two highly emissive pyrimidine-based mononuclear Ir(III) complexes and their corresponding dinuclear Ir(III) complexes were prepared with a simple one-pot reaction. The incorporation of the second Ir(III) center can lead to dramatic differences of both photophysical and electrochemical properties between the mono- and dinuclear complexes. Besides, these properties can also be fine-tuned with different substituents. Theoretical calculations have also been performed to understand their photophysical behaviors. The electroluminescent investigations demonstrate that the pyrimidine-based mono- and dinuclear Ir(III) complexes could show impressive device performance. The vacuum-deposited organic light-emitting diode (OLED) based on the mononuclear Ir(III) complex exhibited an external quantum efficiency (EQE) of 16.1% with almost no efficiency roll-off even at 10 000 cd m. More encouragingly, the solution-processed OLED based on the dinuclear Ir(III) complex achieved the outstanding EQE, current efficiency (CE), and power efficiency (PE) of 17.9%, 52.5 cd A, and 51.2 lm W, respectively, representing the highest efficiencies ever achieved by OLEDs based on dinuclear Ir(III) complexes.

Supramolecular iridium(III) assemblies

“…[47,67] Covalently linked dinuclear iridium complexes have been scarcely investigated as emitters in solid-state lighting, with only one example available for use in LEECs [68] and few for OLEDs. [69][70][71][72][73][74][75] However, iridium-based soft salts have demonstrated reasonable performance as emitter materials in OLEDs. The main advantage of using soft salts in lighting devices relies on the capacity to introduce two phosphorescent centers in one complex ion, while controlling the intermolecular separation of the two metal centers through non-covalent interactions.…”

Novel Emission Color‐Tuning Strategies in Heteroleptic Phosphorescent Ir(III) and Pt(II) Complexes

“…[42] As a result, the OLEDs using the dinuclear Ir(III) complex 83 achieves the outstanding EQE of 17.9 %, CE of 52.5 cd A À1 and PE of 51.2 lm W À1 , representing top-ranking EL results ever archived by the reported dinuclear Ir(III) complexes. [42]…”

“…[39,40] Introduction of N-chelating blocks easy to be reduced will induce a bathochromic emission in these complexes (13 l = 517 nm vs. 14 l = 526 nm, Figure 3), while the N-chelating blocks reluctant to be reduced will cause a hypsochromic emission (13 l = 517 nm vs. 15 l = 485 nm, Figure 3). [23,42,43] Extending p-conjugation of the C^N cyclometalating ppy-type ligands will generally lead to a bathochromic emission ( Figure 4). [23,24,39,44,45]…”

“…Recently, we found a novel and effective color-tuning strategy by dinuclearization of the ppy-type Ir(III) complexes. [42,63] Through employing pyrimidine-based ligands, novel dinuclear ppy-type Ir(III) complexes had been obtained ( Figure 27). Compared with the mononuclear complex 77 (l = 526 nm), the corresponding dinuclear ppy-type Ir(III) complexes 78 (l = 606 nm) and 79 (l = 607 nm) can exhibit much longer emission wavelength.…”

“…In addition, the emission color of these dinuclear ppy-type Ir(III) complexes can be tuned further by attaching different groups to the pyrimidine-based ligands (complexes 81-84, Figure 27). [42] R D 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 In addition, considering the unique MO energy levels ( Figures 28, 29 and 30), these dinuclear ppy-type Ir(III) complexes should possess ambipolar features which will benefit their EL performances. [42] As a result, the OLEDs using the dinuclear Ir(III) complex 83 achieves the outstanding EQE of 17.9 %, CE of 52.5 cd A À1 and PE of 51.2 lm W À1 , representing top-ranking EL results ever archived by the reported dinuclear Ir(III) complexes.…”