Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R′-phenyl)pyridine) 2 (1, R = fluoren-2-yl, R′ = H; 2, R = 7-benzothiazolylfluoren-2-yl, R′ = H; 3, R = H, R′ = fluoren-2-yl; 4, R = H, R′ = 7-benzothiazolylfluoren-2-yl; 5, R = R′ = 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2yl substituent on the 2-phenylpyridine (C ∧ N) and/or phenanthroline (N ∧ N) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV−vis absorption, emission, and transient difference absorption spectroscopy. Time-dependent density functional theory (TDDFT) calculations were performed to complement the experimental data and aid in our understanding of the characters of optical transitions. In addition, reverse saturable absorption was demonstrated at 532 nm for all complexes using nanosecond laser pulses. All complexes possess a weak low-energy tail that is attributed to 1,3 MLCT (metal-to-ligand charge transfer)/ 1,3 LLCT (ligand-toligand charge transfer) transitions. The major absorption bands below 475 nm for 1−5 arise from the 1 π,π* and intraligand ( 1 ILCT) transitions within the N ∧ N or C ∧ N ligands. Attachment of fluoren-2-yl or 7benzothiazolylfluoren-2-yl substituents on N ∧ N ligand results in stronger red-shifts of the main absorption band to 400 and 408 nm for 1 and 2, respectively, as compared to 321 and 361 nm in complexes 3 and 4 with the same substituents on the C ∧ N ligands. In contrast, the 1,3 MLCT/ 1,3 LLCT transitions in 1 and 2 are just slightly red-shifted as compared to those in 3 and 4. The emission of complexes 1 and 2 is attributed to the N ∧ N ligand-centered 3 π,π* state with some admixture of 3 MLCT/ 3 ILCT/ 3 LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the 3 MLCT/ 3 LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the C ∧ N and the N ∧ N ligands, the emission predominantly arises from the 3 MLCT/ 3 LLCT states with a small portion of N ∧ N ligand-localized 3 π,π* character. All complexes exhibit broadband triplet excited-state absorption in the visible to the near-IR region, with the major absorption bands bathochromically shifted in 1 and 2 as compared to those in 3 and 4. The stronger excited-state absorption leads to dramatic reverse saturable absorption (RSA) at 532 nm for nanosecond laser pulses. The RSA strength decreases as 5 ≈ 2 > 4 ≈ 1 > 3, which is primarily determined by the ratio of the triplet excited-state absorption cross section relative to that of the ground state. Extended π-conjugation in the N ∧ N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with πconjugated substituents only on the C ∧ N ligands (3 and 4).

A platinum 2,2 0 -bipyridine complex (1) bearing 2-(benzothiazol-2 0 -yl)-9,9-diethyl-7-ethynylfluorene ligands was synthesized and characterized. Its photophysical properties and nonlinear absorption characteristics were systematically investigated by UV-vis absorption, emission, and transient difference absorption spectroscopy, as well as Z-scan and nonlinear transmission techniques. Complex 1 exhibits a strong structureless 1 π,π* absorption band at 374 nm and a broad, weak metalto-ligand charge transfer ( 1 MLCT) transition in the visible region in CH 2 Cl 2 solution. It emits at approximately 565 nm with vibronic structures at room temperature in polar solvents, attributed to the acetylide ligand 3 π,π* excited state. In low-polarity solvents such as hexane and toluene, the emission band becomes structureless and red-shifted, which is assigned to the 3 MLCT state. The emission spectrum becomes more structured and slightly blue-shifted at 77 K in butyronitrile glassy matrix. In femtosecond and nanosecond transient absorption measurements, 1 exhibits both singlet and triplet excited-state absorption from 450 to 800 nm, which are tentatively attributed to the 1 π,π*/ 1 MLCT and 3 π,π*/ 3 MLCT, respectively. Z scan experiments were carried out using nanosecond and picosecond pulses at 532 nm, and picosecond pulses at a variety of other wavelengths in the visible and near-IR, and the experimental data were fitted by a five-level model using the excited-state lifetimes and estimated cross-section values from the photophysical study. In this way, values were obtained for the first and second singlet excited-state absorption cross sections and the triplet excitedstate absorption cross section throughout the visible and near-IR and for the two-photon absorption (TPA) cross section in the near-IR region. Our results demonstrate that 1 possesses extremely large ratios of the excited-state absorption cross sections to the ground-state absorption in the visible spectral region and, compared to the other two-photon absorbing platinum complexes, the largest two-photon absorption cross sections in the near-IR region. This makes complex 1 a very promising candidate for photonic devices that require large and broadband nonlinear absorption. Reverse saturable absorption of 1 in CH 2 Cl 2 solution at 532 nm for a nanosecond laser pulse was demonstrated. A remarkable transmission decrease was observed when the incident fluence increased.

A series of platinum(II) diimine complexes with different substituents on fluorenyl acetylide ligands (1a-1e) were synthesized and characterized. The influence of the auxiliary substituent on the photophysics of these complexes has been systematically investigated spectroscopically and theoretically (using density functional theory (DFT) methods). All complexes exhibit ligand-centered (1)π,π* transitions in the UV and blue spectral region, and broad, structureless (1)MLCT/(1)LLCT (1a, 1b, 1d and 1e) or (1)MLCT/(1)LLCT/(1)π,π* (1c) absorption bands in the visible region. All complexes are emissive in solution at room temperature, with the emitting state is tentatively assigned to mixed (3)MLCT/(3)π,π* states. The degree of (3)π,π* and (3)MLCT mixing varies with different substituents and solvent polarities. Complexes 1a-1e exhibit relatively strong singlet and triplet transient absorption from 450 to 800 nm, at which point reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm by using nanosecond laser pulses demonstrate that 1a-1e are strong reverse saturable absorbers and could potentially be used as broadband nonlinear absorbing materials.

We report the synthesis, photophysics, and reverse saturable absorption together with time-dependent density functional theory modeling of seven cationic iridium(III) complexes bearing one 2,2′-bipyridine ligand and two cyclometalating ligands (C^N ligand) with varied degrees of π-conjugation (HC^N = benzo[H]quinoline in 1, 1-phenylisoquinoline in 2, 1-(2-pyridyl)naphthalene in 3, 2-(2-pyridyl)naphthalene in 4, 1-(2-pyridyl)pyrene in 5, 1,2-diphenyl-pyreno[4,5-d]imidazole in 6, and 3-(2-pyridyl)perylene in 7). All complexes possess ligand-localized 1π,π* transitions as the major absorption bands and lower-energy 1MLCT (metal-to-ligand charge transfer)/1LLCT (ligand-to-ligand charge transfer) transitions in their ultraviolet–visible absorption spectra. The extended π-conjugation in the cyclometalating ligands of complexes 5–7 causes a significant red-shift of the major absorption bands with increased molar extinction coefficients with respect to those of complexes 1–4 that contain less conjugated C^N ligands. All complexes are emissive in solutions at room temperature and in glassy matrix at 77 K. The emitting states are assigned to 3π,π* (C^N ligand localized) /3MLCT for 1, 3π,π*/3MLCT/3LMCT (ligand-to-metal charge transfer) for 2–4, pure 3π,π* transitions for 5 and 6, and 3π,π*/3MLCT/3LMCT/3LLCT for 7. Complex 5 possesses the lowest emission energy because the larger conjugation and the most delocalized character of the 3π,π* transition within the C^N ligand in this complex. Complexes 1, 4, and 7 possess larger contribution of charge transfer characters in their lowest triplet excited states. Therefore, the transient absorption of these three complexes is broad but short-lived (90–300 ns). In contrast, complexes 2, 3, 5, and 6 all give long-lived (2.0–19.5 μs) triplet transient absorption in the visible spectral region of ca. 450–700 nm, which can be regarded as emanating predominantly from the C^N ligand-centered 3π,π* state. The reverse saturable absorption (RSA) of these complexes was evaluated at 532 nm for nanosecond laser pulses. The results demonstrate that these complexes, except for 7, all exhibit strong RSA for nanosecond laser pulses at 532 nm, with a trend of 7 < 1 < 4 < 6 < 5 ≈ 2 ≈ 3.

Four heteroleptic cationic iridium(III) complexes containing cyclometalating 2-{3-[7-(benzothiazol-2-yl)fluoren-2-yl]phenyl}pyridine ligand and different diimine (N ∧ N) ligands (N ∧ N = 2-(pyridin-2-yl)quinoline (1), 1,10-phenanthroline (2), 2,2′-biquinoline (3), and 1,1′-biisoquinoline (4)) and a reference complex bearing 2-(pyridin-2-yl)quinoline and 2-phenylpyridine ligands (5) were synthesized and characterized. The influence of the diimine (N ∧ N) ligand on the photophysics of these complexes has been systematically investigated via spectroscopic methods and by time-dependent density functional theory (TDDFT). All complexes exhibit N ∧ N or C ∧ N ligand localized 1 π,π* transitions below 400 nm, and broad and structureless metal-toligand and ligand-to-ligand charge transfer ( 1 MLCT/ 1 LLCT) absorption bands between 400 and 450 nm, and weak 3 MLCT/ 3 LLCT absorption above 450 nm. Increasing the π-conjugation of the N ∧ N ligand causes enhanced molar extinction coefficients of the absorption bands and a bathochromic shift of the 3 MLCT/ 3 LLCT band. All complexes show orange to red phosphorescence at room temperature, with the emitting state being predominantly assigned to 3 MLCT/ 3 LLCT states for 1−5, but with some 3 π,π* contributions for 3 and 5. Extending the πconjugation of the N ∧ N ligand induces a pronounced red-shift of the emission band and decreases the emission lifetime and quantum yield. Complexes 1−5 exhibit relatively strong singlet and triplet transient absorption from 450 to 800 nm, where the reverse saturable absorption (RSA) could occur. Nonlinear transmission experiments at 532 nm using nanosecond laser pulses demonstrate that complexes 1−5 are strong reverse saturable absorbers at 532 nm.