Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

Abstract

A series of 2-aryloxazolo[4,5-f][1,10]phenanthroline ligands (N^N ligands) and their cationic iridium(III) complexes (1-11, aryl = 4-NO-phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh-phenyl (4), 4-NH-phenyl (5), pyridin-4-yl (6), naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9), anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of π-conjugation (6-11) on oxazolo[4,5-f][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f][1,10]phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized π,π*/ILCT (intraligand charge transfer) transitions, mixed with LLCT (ligand-to-ligand charge transfer)/MLCT (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong ILCT/LLCT/MLCT transitions at 350-450 nm, and broad but very weak LLCT/MLCT absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO or NPh substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.

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 , , .…”

Two-way vapochromism of a luminescent platinum(ii) complex with phosphonic-acid-functionalized bipyridine ligand

“…[15][16][17][18][19][20][21][22] These complexes can detect small lattice distortions induced by the adsorption of vapour through highly sensitive electronic transitions involving intermolecular metallophilic interactions. Other promising approaches recently developed involve the use of host-guest interactions between the adsorbing material and vapour molecules [23][24][25][26][27][28] or the use of ligand substitution reactions at metal centres. [29][30][31][32][33][34][35][36][37] Most of the vapochromic behaviour reported so far is essentially based on the colour differences between vapouradsorbed and vapour-released phases.…”

Osmaindenes: Synthesis and Reversible Mechanochromism Characteristics

“…Based on an extensive literature search, [28][29][30] we speculate that mechanical stimulation may lead to changes in the molecular morphology of these complexes, which may also be responsible for its reversible mechanochromism behavior. Therefore, in order to further study the mechanochromism behavior, three different solid states of these complexes were tested by X-ray diffraction (XRD) measurements.…”