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 (1)pi,pi* transitions below 400 nm, and broad and structureless metal-to-ligand and ligand-to-ligand charge transfer ((MLCT)-M-1/(LLCT)-L-1) absorption bands between 400 and 450 nm, and weak (MLCT)-M-3/(LLCT)-L-3 absorption above 450 nm. Increasing the pi-conjugation of the N<^>N ligand causes enhanced molar extinction coefficients of the absorption bands and a bathochromic shift of the (MLCT)-M-3/(LLCT)-L-3 band. All complexes show orange to red phosphorescence at room temperature, with the emitting state being predominantly assigned to (MLCT)-M-3/(LLCT)-L-3 states for 15, but with some (3)pi,pi* contributions for 3 and 5. Extending the pi-conjugation of the N<^>N ligand induces a pronounced red-shift of the emission band and decreases the emission lifetime and quantum yield. Complexes 15 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 15 are strong reverse saturable absorbers at 532 nm.