Pt(II) metal complexes are known to exhibit strong solid-state aggregation and are promising for realization of efficient emission in fabrication of organic light emitting diodes (OLED) with nondoped emitter layer. Four pyrimidine-pyrazolate based chelates, together with four isomeric Pt(II) metal complexes, namely: [Pt(pm2z) 2 ], [Pt(tpm2z) 2 ], [Pt(pm4z) 2 ], and [Pt(tpm4z) 2 ], are isolated and systematically investigated for their structure-property relationships for practical OLED applications. Detailed single molecular and aggregated structures are revealed by photophysical and mechanochromic measurements, grazing-incidence X-ray diffraction, and theoretical approaches. These results suggest that these Pt(II) emitters pack like a deck of playing cards under vacuum deposition, and their emission energy is not only affected by the single molecular designs, but notably influenced by their intermolecular packing interaction, i.e., Pt···Pt separations that are arranged in the order: [Pt(tpm4z) 2 ] > [Pt(pm4z) 2 ] > [Pt(tpm2z) 2 ] > [Pt(pm2z) 2 ]. Nondoped OLED with emission ranging from green to red are prepared, to which the best performances are recorded for [Pt(tpm2z) 2 ], giving maximum external quantum efficiency (EQE) of 27.5% at 10 3 cd m −2 , maximum luminance of 2.5 × 10 5 cd m −2 at 17 V, and with stable CIE x,y of (0.56, 0.44).computers, notepads, and cell phones. Their efficiencies have been improved during the past two decades using suitable third-row transition-metal phosphors as emitters. Among these emitters, Pt(II) metal complexes have been attracting intensive studies owing to their higher stability and intense luminescence at room temperature (RT). [1] However, in contrast to the Ir(III) emitters with d 6 -electronic configuration and octahedral coordination structure, [2] the Pt(II) emitters possesses distinctive d 8 -configuration and square planar geometry, which induced a greater tendency in forming π-π-stacking interaction between adjacent molecules in solid state. As for application, this aggregation is capable of tuning emission to lower energy region of the visible spectra and even further into the near-infrared (NIR), which allows versatile prospective in applications for this class of emitters.Many aggregated Pt(II) complexes have been synthesized, for which their structures can be differentiated into two classes. One is best represented by those bearing a linear Pt n architecture and with Pt···Pt distance of less than 3.4 Å; the latter is the sum of van del wall radii of

Dependence of external quantum efficiency (EQE) and radiative decay lifetime of blue-emitting OLED is established with photostable, bis-tridentate Ir(iii) emitters, giving max. EQE and luminescence of 12.2% and 18 980 cd m−2, and with CIEx,y coordinates between (0.16, 0.22) and (0.17, 0.27).