Versatile Pt(II) Pyrazolate Complexes: Emission Tuning via Interplay of Chelate Designs and Stacking Assemblies

Three homoleptic Pt(II) metal complexes [Pt(im(P)z)(2)] (1), [Pt(im(i)z)(2)] (2), and [Pt(im(M)z)(2)] (3) were synthesized from the treatment of Pt(DMSO)(2)Cl-2 and functional imidazolyl pyrazole in refluxing tetrahydrofuran (THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(im(P)z)(fppz)] (4), [Pt(imiz)(fppz)] (5), and [Pt(im(M)z)(fppz)] (6) were obtained from the treatment of a common intermediate [Pt(fppzH)Cl-2] with a corresponding imidazolyl chelate. Pt(II) complexes 1, 2, and 5 were studied by single-crystal X-ray diffraction to reveal the corresponding packing arrangement in their crystal lattices, among which both homoleptic complexes 1 and 2 formed monomeric species, while heteroleptic 5 aligned as a dimer with a nonbonding Pt center dot center dot center dot Pt contact of 3.574 angstrom. Subsequent photophysical examinations showed that the homoleptic 13 and heteroleptic 4-6 exhibited the structured sky-blue pi pi* emission and structureless light-green-emitting metal-metal-to-ligand charge transfer (MMLCT) emission in the solid state, respectively. A shortened Pt...Pt interaction of approximately 0.34-0.35 nm was confirmed in thin films of all heteroleptic Pt(II) complexes 4-6 by grazing-incidence X-ray diffraction (GIXD) analyses. Finally, Pt(II) complex 6 was employed as a dopant in the fabrication of organic light-emitting diode (OLED) devices with varied doping ratios, among which OLEDs with only 1 wt % 6 in the SimCP host exhibited a maximum external quantum efficiency (EQE) of 5.8% and CIEx,y values of 0.20, 0.31. In contrast, OLEDs using a nondoped architecture (i.e., 100% of 6 in the emitting layer (EML)) achieved a maximum EQE of 26.8%, current efficiency (CE) of 91.7 cd A(-1), and power efficiency (PE) of 80.1 lm.W-1 and CIEx,y values of 0.41, 0.55, manifesting their versatility in various degrees of stacking assemblies and hence facile color-tuning capability on OLEDs.