Designing Highly Efficient Phosphorescent Neutral Tetrahedral Manganese(II) Complexes for Organic Light-Emitting Diodes

Phosphorescent transition-metal complexes have played the vital role in the rapid development of organic light-emitting diodes (OLEDs) as the most promising candidates for next-generation flat-panel display and solid-state lighting techniques. In this work, novel and low-cost phosphorescent neutral tetrahedral manganese(II) complexes (DBFDPO-MnX₂, X = Br, or Cl) based on dibenzofuran-based phosphine oxide derivative as ligand are designed and synthesized. The manganese(II) complexes exhibit intense green phosphorescence with high photoluminescence quantum yields (PLQYs) of as high as 81.4% (DBFDPO-MnBr₂). Using complex DBFDPO-MnBr₂ as dopant, a green OLED with current efficiency (CE_max) of 35.47 cd A⁻¹, power efficiency (PE_max) of 34.35 lm W⁻¹, and external quantum efficiency (EQE_max) of 10.49% is fabricated. Interestingly, red exciplex emission is also observed in electroluminescence, arising from the interaction between the host materials (bis(2-(2-hydroxyphenyl)-pyridine)beryllium (Bepp₂) or 1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBi)) and the dopant (DBFDPO-MnBr₂). The exciplex-based red OLED in this study exhibits the maximum CE and PE reaching 18.64 cd A⁻¹ and 17.92 lm W⁻¹, respectively, which are among the up-to-date highest values for exciplex-based red OLEDs. Beneficial from the exciplex, it has the great potential to broaden the electroluminescent spectra with manganese(II) complex.