Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

[Image: see text] The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C(∧)N) are reported. The complexes (OC-6-54)-[PtCl(2)(C(∧)N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from (3)LC(C(∧)N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C(2)-symmetrical species (OC-6-33)-[PtCl(2)(C(∧)N)(2)], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating (3)LMCT states. In contrast, the isomers (OC-6-42)-[PtCl(2)(C(∧)N)(trz)] are not luminescent because they present a (3)LMCT state as the lowest triplet.