Photophysics and reverse saturable absorption of cationic dinuclear iridium(III) complexes bearing fluorenyl-tethered 2-(quinolin-2-yl)quinoxaline ligands.

The synthesis, photophysics and reverse saturable absorption of two cationic dinuclear Ir(III) complexes bearing fluorenyl-tethered 2-(quinolin-2-yl)quinoxaline (quqo) ligands are reported in this paper. The two complexes possess intense and featureless diimine ligand localized ILCT (intraligand charge transfer)/π,π* absorption bands at 330 and 430 nm, and a weak MLCT (metal-to-ligand charge transfer)/LLCT (ligand-to-ligand charge transfer) absorption band at >500 nm. Both complexes exhibit weak dual phosphorescence at 590 nm and 710 nm, which are attributed to the ILCT/π,π* and MLCT/LLCT states, respectively. The low-energy MLCT/LLCT state also gives rise to a moderately strong triplet excited-state absorption at 490-800 nm. Because of the stronger triplet excited-state absorption than the ground-state absorption of these complexes at 532 nm, both complexes manifest a moderate reverse saturable absorption (RSA) at 532 nm for ns laser pulses. Expansion of the π-conjugation of the fluorenyl-tethered diimine ligand in Ir-1 causes a slight red-shift of the ILCT/π,π* absorption bands in its UV-vis absorption spectrum and the MLCT/LLCT absorption band in the transient absorption spectrum and slightly enhances the RSA at 532 nm compared to that in Ir-2. This work represents the first report on dinuclear Ir(III) complexes that exhibit RSA at 532 nm.