Tuning the photophysics and reverse saturable absorption of heteroleptic cationic iridium(III) complexes via substituents on the 6,6′-bis(fluoren-2-yl)-2,2′-biquinoline ligand

To understand the effects of the terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes and to obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state, we synthesized three heteroleptic cationic iridium(III) complexes bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted 6,6′-bis(7-R-fluoren-2-yl)-2,2′-biquinoline (N^N) ligand (R = H, NO₂, or NPh₂). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized ¹π,π∗ transitions mixed with ligand-to-ligand charge transfer (¹LLCT)/metal-to-ligand charge transfer (¹MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized ¹π,π∗/¹ILCT (intraligand charge transfer) transitions as well as the very weak ^1,3LLCT/^1,3MLCT transitions at longer wavelengths. The electron-withdrawing NO₂ substituent on the N^N ligand leads to a blue-shift of the ¹π,π∗/¹ILCT absorption band, while the electron-donating NPh₂ substituent causes a pronounced red-shift of this band. The unsubstituted and NO₂-substituted complexes (complexes 1 and 2, respectively) are moderately emissive at room temperature (RT) in solution as well as at 77 K in the glassy matrix, while the NPh₂-substituted complex (3) is weakly emissive at RT, but the emission becomes much brighter at 77 K. Complexes 1 and 2 show very broad and strong triplet excited-state absorption from 460 to 800 nm with moderately long lifetimes, while complex 3 exhibits weak but broad absorption bands from 384 to 800 nm with a longer lifetime than those of 1 and 2. The nonlinear transmission experiment manifests that complexes 1 and 2 are strong reverse saturable absorbers (RSA) at 532 nm, while 3 shows weaker RSA at this wavelength. These results clearly demonstrate that it is feasible to tune the ground-state and excited-state properties of the Ir(III) complexes via the terminal substituents at the diimine ligand. By introducing the fluoren-2-yl groups to the 2,2′-biquinoline ligand to extend the diimine ligand π-conjugation, we can obtain Ir(III) complexes with reasonably long-lived and strongly absorbing triplet excited state while red-shifting their ^1,3LLCT/^1,3MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.