Cyclometalated platinum(II) complexes serve as core guest materials for high-performance phosphorescent organic light-emitting diodes (OLEDs) due to their excellent exciton utilization efficiency and structural modifiability. In this work, rigid 5H-benzo[d]benzo[4,5]imidazo[1,2-a]imidazole (BZI) was adopted as the fundamental building block. Two tetradentate cyclometalated platinum(II) complexes, namely Pt-py and Pt-bt, were rationally designed and synthesized by introducing pyridine and benzothiazole moieties, respectively. Both complexes exhibit outstanding thermodynamic and electrochemical stabilities. Theoretical simulation, photophysical measurements and electroluminescent device results reveal that Pt-py possesses a highly rigid molecular skeleton and weak excited-state geometric distortion, which effectively suppresses non-radiative decay. It exhibits high photoluminescence quantum yield and radiative decay rate, as well as excellent carrier trapping capability, enabling efficient charge recombination and sufficient host-guest energy transfer in the emitting layer. The corresponding doped OLED exhibit outstanding electroluminescent properties and extremely low efficiency roll-off, with a maximum luminance of 65782 cd•m-2, current efficiency of 83.3 cd•A-1, power efficiency of 86.5 lm•W-1 and external quantum efficiency (EQE) of 23.4%. Its efficiency roll-off is only 4.7% at a practical luminance of 1000 cd•m-2. In contrast, Pt-bt with an extended conjugated plane achieves obvious emission redshift and realizes orange-red phosphorescence. However, the enlarged conjugated structure aggravates excited-state structural distortion and weakens carrier trapping ability, resulting in incomplete host-to-guest energy transfer and severe exciton quenching. Restricted by the energy gap law, the resultant device shows mediocre electroluminescence performance, with a low maximum EQE of 10.4% and serious efficiency roll-off. These results indicate that the rigid BZI moiety provides a structural basis for the efficient phosphorescence emission of Pt(II) complexes. Rational selection of coordinated fragments enables the regulation of emission color, efficiency and device stability. This work offers insights for the molecular design of novel tetradentate Pt(II)-based phosphorescent materials and their applications in OLEDs.
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