4-Phenyl-1,2,3-triazoles as Versatile Ligands for Cationic Cyclometalated Iridium(III) Complexes.

Five cationic iridium(III) complexes ( 1 - 5 ) were synthesized exploiting two triazole-based cyclometalating ligands, namely, 1-methyl-4-phenyl-1 H -1,2,3-triazole ( A ) and the corresponding mesoionic carbene 1,3-dimethyl-4-phenyl-1 H -1,2,3-triazol-5-ylidene ( B ). From the combination of these two ligands and the ancillary one, i.e., 4,4'-di- tert -butyl-2,2'-bipyridine (for 1 - 3 ) or tert -butyl isocyanide (for 4 and 5 ), not only the typical bis-heteroleptic complexes but also the much less explored tris-heteroleptic analogues ( 2 and 5 ) could be synthesized. The redox and emission properties of all of the complexes are effectively fine-tuned by the different ligands: (i) cyclometalating ligand A induces a stronger highest occupied molecular orbital (HOMO) stabilization compared to B and leads to complexes with progressively narrower HOMO-lowest unoccupied molecular orbital (LUMO) and redox gaps, and lower emission energy; (ii) complexes 1 - 3 , equipped with the bipyridine ancillary ligand, display fully reversible redox processes and emit from predominantly metal-to-ligand charge transfer (MLCT) states with high emission quantum yields, up to 60% in polymeric matrix; (iii) complexes 4 and 5 , equipped with high-field isocyanide ligands, display irreversible redox processes and high-energy emission from strongly ligand-centered triplets with long emission lifetimes but relatively low quantum yields (below 6%, both in room-temperature solution and in solid state). This work demonstrates the versatility of phenyl-triazole derivatives as cyclometalating ligands with different chelation modes (i.e., C ∧ N and C ∧ C:) for the synthesis of photoactive iridium(III) complexes with highly tunable properties.