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) ligand are reported in this paper. The two complexes possess intense and featureless diimine ligand localized 1ILCT (intraligand charge transfer) / 1π,π* absorption bands at ca. 330 and 430 nm, and weak 1,3MLCT (metal-to-ligand charge transfer) / 1,3LLCT (ligand-to-ligand charge transfer) absorption band at >500 nm. Both complexes exhibit weak dual phosphorescence at ca. 590 nm and 710 nm, which are attributed to the 3ILCT/3π,π* and 3MLCT/3LLCT states, respectively. The low-energy 3MLCT/3LLCT state also gives rise to 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 1ILCT/1π,π* absorption bands in its UV-vis absorption spectrum and the 3MLCT/3LLCT absorption band in the transient absorption spectrum and slightly enhances the RSA at 532 nm compared to those of Ir-2. This work represents the first report on dinuclear Ir(III) complexes that exhibit RSA at 532 nm.