Heteroleptic cationic iridium(iii) complexes bearing naphthalimidyl substituents: synthesis, photophysics and reverse saturable absorption

Abstract

Three heteroleptic cationic iridium(iii) complexes containing a cyclometalating 2-[3-(7-naphthalimidylfluoren-2'-yl)phenyl]pyridine ligand and different diimine (N^N) ligands (N^N = 2,2'-bipyridine (bpy, ), 1,10-phenanthroline (phen, ), and 5,5'-bis[7-(benzothiazol-2'-yl)fluoren-2'-yl]-2,2'-bipyridine (BTF-bpy, )) were synthesized and characterized. The photophysics of these complexes was systematically investigated via spectroscopic methods and by time-dependent density functional theory (TDDFT). All complexes possess a very weak charge-transfer tail at ca. 450-570 nm; and two intense absorption bands in the region of 290-350 nm and 350-450 nm, respectively. The emission of in CH2Cl2 emanates predominantly from the C^N ligand-localized (3)π,π* state. These emitting excited states also give rise to broadband triplet excited-state absorption in the visible to the near-IR region (i.e. 420-800 nm for and , and 460-800 nm for ). The kinetics of fs transient absorption (TA) reveals that the lowest singlet excited-state lifetimes of these complexes vary from 1.43 ps to 142 ps. The stronger excited-state absorption of compared to their respective ground-state absorption in the visible spectral range leads to strong reverse saturable absorption (RSA) at 532 nm for ns laser pulses. The trend of transmission signal decrease follows > > . Extending the π-conjugation of the N^N ligand increases the strength of RSA. In addition, the naphthalimidyl (NI) substitution at the cyclometalating ligand dramatically increases the triplet excited-state lifetimes and broadens the triplet excited-state absorption to the NIR region compared to the respective Ir(iii) complexes with a benzothiazolyl substituent on the cyclometalating ligand.

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 , , .…”

Toward Broadband Reverse Saturable Absorption: Investigating the Impact of Cyclometalating Ligand π-Conjugation on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium Complexes

“…In the past few years, our group and other groups have demonstrated the RSA of Ir­(III) complexes with varied degrees of π-conjugation or substituents on the diimine (N ∧ N) and/or cyclometalating (C ∧ N) ligands. ,,− These studies showed that extending the π-conjugation of the N ∧ N ligands via attaching π-conjugated substituents on the bipyridine or phenanthroline ligand dramatically increased the lowest triplet excited-state (T 1 ) lifetime by incorporating more N ∧ N ligand-associated 3 π,π*/ 3 ILCT (intraligand charge transfer) characters into the T 1 state. , Meanwhile, the predominant 3 π,π*/ 3 ILCT nature of the T 1 state greatly extended the triplet excited-state absorption to the near-IR region. In contrast, the similar modification on the C ∧ N ligands reduced the T 1 lifetime and weakened the T 1 absorption by introducing more 3 MLCT/ 3 LLCT characters into the T 1 state. , Interestingly, fusing aromatic rings to the C ∧ N ligand increased the T 1 lifetime by switching the T 1 state from the 3 MLCT/ 3 LLCT nature to the C ∧ N ligand localized 3 π,π*/ 3 ILCT nature .…”

Benzothiazole-decorated iridium-based nanophotosensitizers for photodynamic therapy of cancer cells

“…1,10-Phenanthroline was chosen as the core of the N^N ligand, resulting in a higher triplet excited-state lifetime and triplet excited-state quantum yield compared with N^N ligands with a 2,2′-bipyridine core. 43,44 The C^N ligand could increase the d* orbital energy of Ir( iii ), further reducing the non-radiative d,d* transition and resulting in a higher triplet excited-state quantum yield. 45 Phenylpyridine was chosen as a core part of the C^N ligand and fluorene was attached to phenanthroline and phenylpyridine to enhance the visible light-absorption ability.…”