Cyclometalated Platinum(II) β-Diketonate Complexes as Single Dopants for High-Efficiency White OLEDs: The Relationship between Intermolecular Interactions in the Solid State and Electroluminescent Efficiency

Abstract

Three cyclometalated platinum(II) β-diketonate complexes were developed to investigate the relationship between intermolecular interactions in the solid state according to the position of a bulky trimethylsilyl (TMS) substituent and organic light-emitting diode performance. A variety of intermolecular interactions was observed (i.e., Pt(II)···π, Pt···Pt, and π–π interactions) that were dependent on the presence or absence of TMS and its position on the 2′,6′-difluoro-2,3′-bipyridine ligand. Hirshfeld analysis was performed for the quantitative analysis of intermolecular interactions, indicating that the incorporation of TMS at the 4- or 5-position of the ligand results in strong intermolecular interactions between the Pt(II) complexes. All three complexes show blue to sky blue emissions with high photoluminescent quantum efficiency of ∼0.6–0.8. A relatively remarkable decrease in the highest occupied molecular orbital energy was observed compared to the increase in the lowest unoccupied molecular orbital energy, which is attributed to the stronger electronegativity of difluorinated-bipyridine than phenylpyridine. All complexes effectively formed excimers under low doping concentration, which made it possible to prepare single-doped white organic light-emitting diodes (WOLEDs). WOLEDs were successfully fabricated using the three different Pt(II) complexes as single dopant materials, resulting in over 10% external quantum efficiency. Moreover, the Pt(II) complex substituted with TMS at the 5-position was used as a single dopant at 3 wt % in a nonoptimized WOLED, exhibiting an external quantum efficiency of 12.3%, white emission with Commission Internationale de L’Eclairage (1931) coordinates (x, y) of (0.40, 0.42), and good color rendering index (80), providing one of the highest performance results among single-doped WOLEDs.