title: 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.
abstract: 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-localized1π,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-localized1π,π*/1ILCT (intraligand charge transfer) transitions as well as the very weak1,3LLCT/1,3MLCT transitions at longer wavelengths. The electron-withdrawing NO2substituent on the N^N ligand leads to a blue-shift of the1π,π*/1ILCT absorption band, while the electron-donating NPh2substituent 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 their1,3LLCT/1,3MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58108-6050, United States.
authors.forename: Xiaolin
authors.initials: X
authors.lastname: Zhu
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58108-6050, United States.
authors.forename: Levi
authors.initials: L
authors.lastname: Lystrom
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58108-6050, United States.
authors.forename: Svetlana
authors.initials: S
authors.lastname: Kilina
authors.affiliation: Department of Chemistry and Biochemistry, North Dakota State University , Fargo, North Dakota 58108-6050, United States.
authors.forename: Wenfang
authors.initials: W
authors.lastname: Sun