Cyclometalation Geometry of the Bridging Ligand as a Tuning Tool for Photophysics of Dinuclear Ir(III) Complexes

Bridging ligands play a crucial role in design of luminescent dinuclear metal complexes. Bis-cyclometalating ligands gave rise to a large family of highly efficient emitters. Herein, we investigate the effect of switching the cyclometalating function of the bridging (chromophoric) ligand on photophysical properties of dinuclear Ir(III) complexes. The new dinuclear Ir(III) complex (Ir-1), comprising a bridging chromophoric ligand with two terminal cyclometalating phenyl derivatives, conjugated to the central twice nitrogen-coordinating thiazolo[5,4-d]thiazole derivative, displays red phosphorescence of decent efficiency in CH2Cl2 solution at room temperature (ΦPL = 12%, τ = 1.5 μs, and λ = 635 nm). This is several times more efficient compared to the properties of the earlier reported dinuclear Ir(III) complex IrIr, with a bridging ligand comprising terminal nitrogen-coordinating pyridine derivatives and a central cyclometalating thieno[3,2-b]thiophene derivative, under the same conditions (ΦPL = 3.5%, τ = 2.9 μs, and λ = 714 nm). This “C/N swap” within the bridging ligand caused blue-shifted and improved efficiency of phosphorescence of Ir-1. The origin of this effect is the significantly reduced exchange interaction in state T1 and, consequently, smaller ΔE(S1 – T1) energy gap. According to the density functional theory calculations, this comes from the more even (wider) distribution of the highest occupied molecular orbital within the bridging ligand and increased participation of the metal centers and halide atoms in the formation of states S1 and T1. Modulation of the substituent pattern on the bridging ligand in complex Ir-2, analogous to Ir-1, afforded selective tuning of the phosphorescence rate, whereas other properties of phosphorescence remained similar under the same conditions (ΦPL = 15%, τ = 3.1 μs, and λ = 632 nm).