Nonlinear Absorbing Cationic Bipyridyl Iridium(III) Complexes Bearing Cyclometalating Ligands with Different Degrees of π-Conjugation: Synthesis, Photophysics, and Reverse Saturable Absorption

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 1 π,π* transitions as the major absorption bands and lower-energy 1 MLCT (metal-to-ligand charge transfer)/ 1 LLCT (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 3 π,π* (C^N ligand localized) / 3 MLCT for 1, 3 π,π*/ 3 MLCT/ 3 LMCT (ligand-to-metal charge transfer) for 2 – 4, pure 3 π,π* transitions for 5 and 6, and 3 π,π*/ 3 MLCT/ 3 LMCT/ 3 LLCT for 7 . Complex 5 possesses the lowest emission energy because the larger conjugation and the most delocalized character of the 3 π,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 3 π,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 .