Specific molecular design of tetradentate Platinum(II) complexes for enhancing the electroluminescent performances of blue-phosphorescent organic light-emitting diodes

One of the greatest challenges in the organic light-emitting diode (OLED) industry is to embody the extremely efficient and electrochemically stable blue phosphors. This study presents two tetradentate-ligand-based Pt(II) complexes, namely Pt1N and Pt2N, which are functionalized with 1H-imidazopyridine moieties and bulky groups to enhance the electronic charge-transfer (CT) characteristics and prevent intermolecular interactions, respectively. Both complexes were found to exhibit blue phosphorescence in the solution and rigid states. Interestingly, the enhanced CT characteristics accelerated the radiative decay processes, as confirmed by the resulting short phosphorescence decay lifetimes (∼1.6 μs). When incorporated into phosphorescent OLEDs, Pt1N and Pt2N delivered superior performances, demonstrating low operating voltages (∼4.0 V), high efficiencies (maximum CE/EQE: 36.6 cd/A/19.9 % for Pt1N and 38.4 cd/A/20.6 % for Pt2N; CE, current efficiency; EQE, external quantum efficiency), and significantly long operation lifetimes (time required to reach 15 % degradation of the initial luminescence of 1000 cd/m⁻²(−|-) = 20–119 h) within the sky-blue electroluminescence region (CIE_y = 0.29–0.33). In particular, the devices that were able to fully harvest the triplet excitons demonstrated maximized EQE values of > 24 % with a CIE_y of 0.22. These findings underscore the effectiveness of enhancing steric hindrance and promoting radiative decay via specific molecular design to achieve efficient and stable blue phosphors.