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

Abstract

Three homoleptic Pt(II) metal complexes [Pt(im^Pz)₂] (<b>1</b>), [Pt(imⁱz)₂] (<b>2</b>), and [Pt(im^Mz)₂] (<b>3</b>) were synthesized from the treatment of Pt(DMSO)₂Cl₂ and functional imidazolyl pyrazole in refluxing tetrahydrofuran (THF). Alternatively, the heteroleptic Pt(II) complexes [Pt(im^Pz)(fppz)] (<b>4</b>), [Pt(imⁱz)(fppz)] (<b>5</b>), and [Pt(im^Mz)(fppz)] (<b>6</b>) were obtained from the treatment of a common intermediate [Pt(fppzH)Cl₂] with a corresponding imidazolyl chelate. Pt(II) complexes <b>1</b>, <b>2</b>, and <b>5</b> were studied by single-crystal X-ray diffraction to reveal the corresponding packing arrangement in their crystal lattices, among which both homoleptic complexes <b>1</b> and <b>2</b> formed monomeric species, while heteroleptic <b>5</b> aligned as a dimer with a nonbonding Pt···Pt contact of 3.574 Å. Subsequent photophysical examinations showed that the homoleptic <b>1</b>-<b>3</b> and heteroleptic <b>4</b>-<b>6</b> 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 <b>4</b>-<b>6</b> by grazing-incidence X-ray diffraction (GIXD) analyses. Finally, Pt(II) complex <b>6</b> 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 % <b>6</b> in the SimCP host exhibited a maximum external quantum efficiency (EQE) of 5.8% and CIE_{<i>x</i>,<i>y</i>} values of 0.20, 0.31. In contrast, OLEDs using a nondoped architecture (i.e., 100% of <b>6</b> 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_{<i>x</i>,<i>y</i>} values of 0.41, 0.55, manifesting their versatility in various degrees of stacking assemblies and hence facile color-tuning capability on OLEDs.