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

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 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.