Photo- and Electroluminescence from 2-(Dibenzo[b,d]furan-4-yl)pyridine-Based Heteroleptic Cyclometalated Platinum(II) Complexes: Excimer Formation Drastically Facilitated by an Aromatic Diketonate Ancillary Ligand

To clarify the ancillary ligand effect on excimer formation of heteroleptic cyclometalated platinum(II) complexes, we investigated photo- and electroluminescence behavior for green-phosphorescent [(dibenzo[ b, d ]furan-4-yl)pyridinato- N, C 3 ′ ]platinum(II) 1,3-diketonates Pt-1 and Pt-2 (1,3-diketonate ancillary ligands; 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dionate ( bdbp ) and dipivaloylmethanate ( dpm ) for Pt-1 and Pt-2, respectively). The X-ray crystallographic study reveals that both Pt-1 and Pt-2 form dimeric pairs in the solid states, indicating that they are likely to form the excimers. In PMMA films doped with Pt-1, red-shifted photoluminescence at >600 nm, assignable to the excimer emission, increasingly emerges along with the original monomer emission at 516 and 552 nm as the doping level of Pt-1 is increased to 44 wt %. Pt-2 -doped films give quite modest excimer-based photoluminescence even when heavily doped to the same extent as Pt-1 -doped films. This clearly indicates that the aromatic ancillary ligand ( bdbp ) effectively facilitates the excimer formation rather than the aliphatic ( dpm ). Similar efficient excimer formation is also observed for electroluminescent devices such as poly(9-vinylcarbazole)-based polymer light-emitting diodes (PLEDs) doped with Pt-1 . The Pt-1 -doped PLEDs afford electroluminescence from green to orange when the doping level of Pt-1 is varied from 7.1 to 24 wt %. Thus, referring to the Commission Internationale de L’Eclairage (CIE) chromaticity coordinate, the color tuning is available from CIE = (0.40, 0.57) to (0.56, 0.43) (@maximum luminance). We also found that the excimer-based luminescence of Pt-1 is enhanced in the PLEDs more than in polymer thin films upon photoexcitation. This result shows the difference in the mechanism of triplet excimer formation between photo- and electroluminescence and indicates that the efficient triplet excimer formation is caused by direct charge recombination of anion and cation radicals of Pt-1 .