Pyrazolate-Bridged NHC Cyclometalated [Pt₂] Complexes and [Pt₂Ag(PPh₃)]⁺ Clusters in Electroluminescent Devices

The ionic transition metal complexes (iTMCs) [{Pt(C^∧C*)(μ-Rpz)}₂Ag(PPh₃)]X (HC^∧C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazole-2-ylidene, X = ClO₄/PF_6; Rpz = pz 1a/2a, 4-Mepz 1b/2b, and 3,5-dppz 1c/2c) were prepared from the neutral [{Pt(C^∧C*)(μ-Rpz)}₂] (Rpz = pz A, 4-Mepz B, and 3,5-dppz C) and fully characterized. The "Ag(PPh₃)" fragment is in between the two square-planar platinum units in an "open book" disposition and bonded through two Pt-Ag donor-acceptor bonds, as shown by X-ray diffraction (d_Pt-Ag ∼ 2.78 Å, 1a-1c). ¹⁹⁵Pt{¹H} and ³¹P{¹H} NMR confirmed that these solid-state structures remain in solution. Photoluminescence studies and theoretical calculations on 1a, were performed. The diphenylpyrazolate derivatives show the highest photoluminescence quantum yield (PLQY) in the solid state. Therefore, 2c and its neutral precursor C were selected as active materials on light-emitting devices. OLEDs fabricated with C showed a turn-on voltage of 3.2 V, a luminance peak of 21,357 cd m^-2 at 13 V, and a peak current efficiency of 28.8 cd A^-1 (9.5% EQE). They showed a lifetime t₅₀ of 15.7 h. OLEDs using 2c showed a maximum luminance of 114 cd m^-2, while LECs exhibited a maximum luminance of 20 cd m^-2 and a current efficiency of around 0.2 cd A^-1, with a t₅₀ value of 50 min.