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 combinedLLCT/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 CIEx,y coordinates of (0.18, 0.32).