Cyclometalated Platinum(II) β-Diketonate Complexes with Extremely High External Quantum Efficiency for White Organic Light-Emitting Diodes

Three cyclometalated Pt(II) β-diketonate compounds with sky-blue to green phosphorescence are prepared by modifying the substituents of the 2′,6′-dimethoxy-2,3′-bipyridine (OMe₂pypy, C^N) chelate ligand. These molecules have the general formula Pt(C^N)(O^O), where O^O = acetylacetonate; one is unsubstituted (1), one possesses a trimethylsilyl (TMS) substituent (2), and the other is substituted with dimesitylboron (BMes₂) (3). The intermolecular interactions of 2 are greater than those of 1 and 3, based on color mapping and surface area results from Hirshfeld analyses. Time-dependent density functional theory calculations reveal that the electronic transitions of monomeric 1 and 3 are predominantly intraligand or ligand-to-ligand charge transfer mixed with metal-to-ligand charge transfer transitions, while for the dimeric forms, dimeric 2, an additional strong metal–metal-to-ligand charge transfer transition is observed. White organic light-emitting diode (WOLED) devices using an 1,3-bis(N-carbazolyl)benzene (mCP) or mCP:TSPO1(diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide) mixed host and 1–3 as dopants are fabricated. With a dopant concentration of 20 wt% in the emissive layer, compound 2 exhibited bright yellow–green emission with a maximum external quantum efficiency (EQE) of 24.9%. This result corresponds to one of the highest EQEs among the reported C^N chelated Pt(II) β-diketonate-based WOLEDs.