Pt(II) Phosphors Featuring Both Dicarbene and Functional Biazolate Chelates: Synthesis, Luminescent Properties, and Applications in Organic Light-Emitting Diodes

Pt(II) metal complexes [Pt(C^C)(X^X)] comprising three functional dianionic azolate chelates (X^XH₂: bipzH₂ = 5,5′-di(trifluoromethyl)-3,3′-bipyrazole, bitzH₂ = 5,5′-di(trifluoromethyl)-3,3′-bi-1,2,4-triazole, and phpzH₂ = 3-(trifluoromethyl)-5-(4-(trifluoromethyl)phenyl)-1H-pyrazole), together with three different charge-neutral dicarbene chelates (i.e., C^C = 1,1′-methylene bis(3-methyl-imidazol-2-ylidene), 1,1′-methylene bis(3-isopropyl-imidazol-2-ylidene), and 1,1′-(propane-1,3-diyl) bis(3-isopropyl-imidazol-2-ylidene), were synthesized and found to show bright solid-state emission depending on the associated X^X and C^C chelates. Pt(II) complexes 1a, 2, and 6 were examined by X-ray diffraction studies, confirming the square-planar skeleton. These Pt(II) metal complexes are found to be nonemissive in degassed solution at RT. The photophysical measurements as neat powder reveals emission maxima ranging from purple to sky blue emission and with high quantum yields for the majority of them. (Time-dependent) density functional theory (DFT/TD-DFT) calculations were executed to elucidate the emission process that was predominated by the combined3LLCT/³LMCT/³IL character, where LLCT and LMCT and IL stand for ligand-to-ligand charge transfer, ligand-to-metal charge transfer, and intraligand ππ∗ transition processes. Organic light-emitting devices comprising complex 5a achieved high efficiency (8.9%, 19.4 cd·A^-1, 22.5 lm·W^-1) with a sky blue emission showing CIE_x,y coordinates of (0.18, 0.32).