Blue phosphorescent N-heterocyclic carbene chelated Pt(II) complexes with an α-duryl-β-diketonato ancillary ligand

C<^>C-star chelated Pt(II) compounds that contain an N-heterocyclic carbene (NHC) donor have recently attracted great interest as blue or white phosphorescent emitters for organic light-emitting devices (OLEDs). To overcome the tendency for excimer formation in Pt(II) compounds, an alpha-duryl-beta-diketonato ligand was selected as the ancillary ligand for blue phosphorescent C<^>C-star chelated Pt(II) compounds. Using this approach, a series of NHC-based C<^>C-star chelated Pt(II) compounds has been designed and synthesized. The chelate ligands used in the new C<^>C-star chelated Pt(II) compounds include 1-phenylimidazol-2-ylidene in Pt1, 1-phenyl-1,2,4-triazol-5-ylidene in Pt2, p-TMS-1-phenylimidazol-2-ylidene in Pt3, and p-TMS-1-phenyl-1,2,4-triazol-5-ylidene in Pt4. A single-crystal X-ray diffraction analysis revealed that the presence of the alpha-duryl-beta-diketonato ligand in the Pt(II) compounds effectively suppresses the dimer formation in the crystal lattice. Pt1, Pt2, Pt3, and Pt4 display blue phosphorescence at room temperature. The p-TMS substituted complex Pt3 was found to display the most efficient blue phosphorescence at lambda(em) = 468 nm with a Phi(PL) of 0.68. Spectroscopic and computational studies established that the blue phosphorescence in the phenyl-imidazolylidene chelated complexes originates mainly from a (MLCT)-M-3 state while that in the phenyl-triazolylidene chelated compounds arises from a (ILCT)-I-3 state. Electroluminescent devices using Pt1 and Pt3 as the dopant were fabricated which display both monomer and exciplex emission, leading to pure white light electroluminescence with CIE coordinates of (0.32, 0.31).