Influence of different diimine (N^{^}N) ligands on the photophysics and reverse saturable absorption of heteroleptic cationic iridium(III) complexes bearing cyclometalating 2-{3-[7-(benzothiazol-2-yl)fluoren-2-yl]phenyl}pyridine (C^{^}N) ligands

Four heteroleptic cationic iridium(III) complexes containing cyclometalating 2-{3-[7-(benzothiazol-2-yl)fluoren-2-yl]phenyl}pyridine ligand and different diimine (N^{^}N) ligands (N^{^}N = 2-(pyridin-2-yl)quinoline (1), 1,10-phenanthroline (2), 2,2′-biquinoline (3), and 1,1′-biisoquinoline (4)) and a reference complex bearing 2-(pyridin-2-yl)quinoline and 2-phenylpyridine ligands (5) were synthesized and characterized. The influence of the diimine (N^{^}N) ligand on the photophysics of these complexes has been systematically investigated via spectroscopic methods and by time-dependent density functional theory (TDDFT). All complexes exhibit N^{^}N or C^{^}N ligand localized ¹π,π∗ transitions below 400 nm, and broad and structureless metal-to-ligand and ligand-to-ligand charge transfer (¹MLCT/¹LLCT) absorption bands between 400 and 450 nm, and weak ³MLCT/³LLCT absorption above 450 nm. Increasing the π-conjugation of the N^{^}N ligand causes enhanced molar extinction coefficients of the absorption bands and a bathochromic shift of the ³MLCT/³LLCT band. All complexes show orange to red phosphorescence at room temperature, with the emitting state being predominantly assigned to ³MLCT/³LLCT states for 1-5, but with some ³π,π∗ contributions for 3 and 5. Extending the π-conjugation of the N^{^}N ligand induces a pronounced red-shift of the emission band and decreases the emission lifetime and quantum yield. Complexes 1-5 exhibit relatively strong singlet and triplet transient absorption from 450 to 800 nm, where the reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm using nanosecond laser pulses demonstrate that complexes 1-5 are strong reverse saturable absorbers at 532 nm.