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, NO, or NPh). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized π,π* transitions mixed with ligand-to-ligand charge transfer (LLCT)/metal-to-ligand charge transfer (MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized π,π*/ILCT (intraligand charge transfer) transitions as well as the very weak LLCT/MLCT transitions at longer wavelengths. The electron-withdrawing NO substituent on the N^N ligand leads to a blue-shift of the π,π*/ILCT absorption band, while the electron-donating NPh substituent causes a pronounced red-shift of this band. The unsubstituted and NO-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 NPh-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 LLCT/MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.

Monocationic Iridium(III) Complexes with Far‐Red Charge‐Transfer Absorption and Near‐IR Emission: Synthesis, Photophysics, and Reverse Saturable Absorption

“…The chosen DFT‐based methodology represents one of the currently most popular density functionals and basis sets, which previously has shown good agreement with UV/Vis experimental data for various Pt(II), Ru(II) and Ir(III) complexes . The validity of various hybrid DFT functionals with different portions of Hartree–Fock (HF) exchange has been widely tested for description of geometries and optical properties of various transition metal complexes , .…”

“…In order to facilitate RSA of nanosecond laser pulses, an absorber should have a weak ground‐state absorption to populate the excited states, but long–lived triplet excited states, large triplet‐triplet excited‐state absorption coefficients, and a high quantum yield for triplet excited‐state formation. In recent years, the RSA of heavy transition‐metal complexes, such as octahedral Ir(III) complexes, have been extensively explored by our group, , , , and other groups, because these complexes displayed the aforementioned characteristics that well match the requirements for RSA. In addition, by structural modifications, both the ground‐ and excited‐state properties can be readily tuned in these complexes for optimization of the RSA.…”

“…Among which the lack of considerable ground‐state absorption in the red to the near‐infrared (NIR) regions remains to be one of the obstacles. In seeking solutions to red‐shift the ground‐state absorption but meanwhile keep a long–lived and strongly absorbing triplet excited state, immense effects have been put by our group on exploring N ^ N or C ^ N ligands suitable for reverse saturable absorbers , , , , . We discovered that incorporation of quinoxaline or benzo[ g ]quinoxaline unit into either the N ^ N or C ^ N ligand red–shifted the spin‐forbidden 3 π ,π* / 3 CT (charge transfer) absorption bands into longer wavelengths , , .…”

Pyrene‐functional star polymers as fluorescent probes for nitrophenolic compounds

“…When UV–vis spectra of the pyrene‐pendant star and linear copolymers as well as 1‐ethynylpyrene were taken in methylene chloride, they demonstrated very similar absorption profiles (Supporting Information Figure S22). These results were thought to indicate that the emergence of broad band in the UV–vis spectra of pyrene‐pendant star copolymers were resulted from π–π* electron transitions induced by close proximity of the pyrene pendant groups on the polymer chains …”

Synthesis and crystal structure of (1,8-naphthyridine-κ2 N,N′)[2-(1H-pyrazol-1-yl)phenyl-κ2 N 2,C 1]iridium(III) hexafluoridophosphate dichloromethane monosolvate

“…For example, d 6 iridium complexes with pseudo-octahedral coordination environments have been widely used in electroluminescent devices (sensors and light-emitting instruments) or photocatalysis because of their long excited-state lifetime, high quantum efficiency, luminescent colour adjustment and thermal stability (Lee et al, 2013;Fan et al, 2013). Among various iridium complexes, cyclometalated iridium(III) complexes are particularly attractive for the wide-range tunability of electronic structures via the rational molecular design of different components (Zhu et al, 2016). According to the set-up of cyclometalated iridium(III) cations with general formula [(N^N)Ir(C^N) 2 ] + in which N^N refers to a diimine ligand and C^N refers to a cyclometalated ligand, the combination and variation of N^N and C^N ligands provides the opportunity to modulate the properties of the target complexes (Goswami et al, 2014;Radwan et al, 2015).…”