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.