We report the utilization of both pyrid-2-yl-imidazolylidene and dianionic bipz chelates as constituents in syntheses of a new series of charge-neutral Pt(II) complexes 1-4, among which complex 4 revealed remarkable triboluminescence, i.e., generation of photoemission upon grinding or cracking of the solid sample. The triboluminescence is found to be sensitive to the subtle changes of the associated substituents of pyrid-2-yl-imidazolylidene chelate, as verified by the disappearance of the triboluminescence for complexes 1-3. Alternatively, the well-ordered solid packing of 3, as indicated by the grazing incidence X-ray scattering experiment, serves as an ideal emitter for the fabrication of highly efficient OLEDs, rendering high external quantum efficienciy (25.9%) and luminesce efficiency (90 cd A) at the practical brightness of 100 cd m. The rather low roll-off in efficiency (24.4%, 85 cd A at high brightness of 1000 cd m) is attributed to the short excited-state lifetime of 3 (∼800 ns) in the solid state, which in turn is associated with the MMLCT transition character.
Three
homoleptic Pt(II) metal complexes [Pt(imPz)2] (1), [Pt(imiz)2] (2), and [Pt(imMz)2] (3)
were synthesized from the treatment of Pt(DMSO)2Cl2 and functional imidazolyl pyrazole in refluxing tetrahydrofuran
(THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(imPz)(fppz)] (4), [Pt(imiz)(fppz)] (5), and [Pt(imMz)(fppz)] (6) were
obtained from the treatment of a common intermediate [Pt(fppzH)Cl2] 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···Pt
contact of 3.574 Å. Subsequent photophysical examinations showed
that the homoleptic 1–3 and heteroleptic 4–6 exhibited the structured sky-blue
ππ* 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 CIE
x,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 CIE
x,y
values of 0.41, 0.55, manifesting their versatility in various
degrees of stacking assemblies and hence facile color-tuning capability
on OLEDs.
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