Luminescence of Pyrazinyl Pyrazolate Pt(II) Complexes Fine-Tuned by the Solid-State Stacking Interaction

Abstract

Three functional pyrazinyl pyrazolate Pt(II) complexes [Pt(fprpz)(2)] (1), [Pt(2fprpz)(2)] (2), and [Pt(5fprpz)(2)] (3), each with CF3, CF2H, and C2F5 substituents on pyrazolate, were synthesized from treatment of Pt(DMSO)(2)Cl-2 and respective pyrazinyl pyrazole chelates (fprpz)H, (2fprpz)H, and (5fprpz)H in refluxing tetrahydrofuran solution. Variations of these fluorinated substituents provided a profound effect on both the photo- and electroluminescence properties of as-prepared Pt(II) metal complexes in solution and solid states, respectively. More specifically, there exists a dominant ligand-centered (3)pi pi r* state contribution in both the solution state and doped thin films at a low concentration, which are strongly dependent upon the nature of the pyrazolate entity and tendency of self-aggregation. A systematic study demonstrates that the T-1 state properties can be fine-tuned by altering their functional substituents. Because Pt(II) complex 2 bears the least electron-deficient CF2H substituent, its thin film has shown the longest emissive wavelength in comparison to other derivatives. Upon formation of a vacuum-deposited thin film, the transition of the titled Pt(II) complexes is dominated by metal-metal-to-ligand charge transfer transition that can be tuned by the well-aligned stacking of the Pt(II) complexes, being more delocalized hence decreasing the energy upon increasing the stacking density. Moreover, we fabricated a series of organic light-emitting diodes (OLEDs) in an attempt to probe the concentration dependence of the doped emitter versus device performances. The electroluminescence of Pt(II) complex 1 shifted from sky blue to near infrared as the doping ratio gradually increased from 1 to 100 wt %. Broad-band white emission can also be realized by adjusting the concentration for optimal monomeric and aggregate emissions. With this remarkable feature, a highly efficient white OLED with external quantum efficiency up to 21.4% and spectral coverage from 450 to 800 nm was obtained at the doping level of 10 wt %, representing ideal candidates in developing solid-state lighting luminaries.