Effects of Varying the Benzannulation Site and π Conjugation of the Cyclometalating Ligand on the Photophysics and Reverse Saturable Absorption of Monocationic Iridium(III) Complexes

Abstract

A series of monocationic iridium(III) complexes, [Ir(C^N) 2 (pqu)] + PF 6 − [pqu = 2-(pyridin-2-yl)quinoline, C^N = 2-phenylquinoline (1), 3-phenylisoquinoline (2), 1-phenylisoquinoline (3), benzo[h]quinoline (4), 2-(pyridin-2-yl)naphthalene (5), 1-(pyridin-2-yl)naphthalene ( 6), 2-(phenanthren-9-yl)pyridine ( 7), 2-phenylbenzo[g]quinoline (8), 2-(napht ha l e n -2 -y l )q u i n o l i n e ( 9 ) , a n d 2 -( n a p h t h a l e n -2yl)benzo[g]quinoline (10)], were synthesized in this work. These complexes bear C^N ligands with varied degrees of π conjugation and sites of benzannulation, allowing for elucidation of the effects of the benzannulation site at the C^N ligand on the photophysics of the complexes. Ultraviolet−visible (UV−vis) absorption and emission of the complexes were systematically investigated via spectroscopic techniques and time-dependent density functional theory calculations. Their triplet excited-state absorption and reverse saturable absorption (RSA) were studied by nanosecond transient absorption (TA) spectroscopy and nonlinear transmission techniques. The fusion of phenyl ring(s) to the phenyl ring or the 4 and 5 positions of the pyridyl ring of the C^N ligand resulted in red-shifted UV−vis absorption and emission spectra in complexes 2, 5−7, 9, and 10 compared to those of the parent complex 0, while their triplet lifetimes and emission quantum yields were significantly reduced. In contrast, the fusion of one phenyl ring to the other sites of the pyridyl group of the C^N ligand showed an insignificant impact on the energies of the lowest singlet (S 1 ) and triplet (T 1 ) excited states in complexes 1, 3, and 4 but noticeably affected their TA spectral features. The fusion of the naphthyl group to the 5 and 6 and positions at the pyridyl ring did not influence the S 1 energy of complex 8 but altered the nature of the T 1 states in 8 and 10 by switching them to the benzo[g]quinoline-localized 3 π,π* state, which resulted in completely different emission and TA spectra in these two complexes. The site-dependent variations of the ground-and excited-state absorption induced strong but varied RSA from these complexes for 4.1-ns laser pulses at 532 nm, with the RSA strength decreasing in the trend of 3 > 7 ≈ 4 ≈ 9 ≈ 6 > 8 ≈ 1 ≈ 2 ≈ 5 > 10.

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

“…As the NTOs in Table and Table S1 show, the low‐energy absorption band(s) at >420 nm exclusively emanate from the 1 LLCT/ 1 MLCT transitions. In view of the dark or extremely small oscillator strength of the S 1 and S 2 transitions in these complexes, the very weak absorption bands beyond 500 nm for Ir1 and Ir2 , and beyond 575 nm for Ir3 could have contributions from the spin‐forbidden 3 π , π* / 3 CT transitions that have been reported in many other Ir(III) complexes , , , , . The intense absorption bands at 320–420 nm (Table S2) are dominated by the dppi ligand–localized 1 π , π* transitions, and mixed with some 1 MLCT/ 1 LLCT transitions.…”

“…The triplet excited‐state quantum yields were then obtained using the relative actinometry method, with the benzene solution of SiNc (ε 590 = 70,000 M –1 cm –1 , Φ T = 0.20) being used as the reference. The setup and details of the nonlinear transmission measurement were the same as those reported earlier by our group , …”

“…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 , , .…”