Synthesis, photophysics, and reverse saturable absorption of 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl tethered [Ir(bpy)(ppy)2]PF6 and Ir(ppy)3 complexes (bpy = 2,2′-bipyridine, ppy = 2-phenylpyridine)

Abstract

We report the synthesis, photophysics, and reverse saturable absorption of five iridium(III) complexes 1–5 with 7-(benzothiazol-2-yl)-9,9-di(2-ethylhexyl)-9H-fluoren-2-yl (BTF) pendant attached to the 2-phenylpyridine ligand (1: [Ir(bpy)(BTF-ppy)₂]PF₆; 2: [Ir(bpy)(BTF––ppy)₂]PF₆; 3: Ir(ppy)₂(BTF-ppy); 4: Ir(ppy)(BTF-ppy)₂; 5: (BTF-ppy)₃, where bpy = 2,2′-bipyridine and ppy = 2-phenylpyridine). The effects of the extended π-conjugation of the ppy ligand and the increased number of BTF-ppy ligand, as well as the effects of neutral complex vs. cationic complex were evaluated. All complexes exhibit predominantly ligand-localized ¹π,π* transitions below 430 nm and charge-transfer transitions between 430 and 550 nm. They all emit at room temperature and at 77 K, mainly from the metal-to-ligand charge transfer (³MLCT)/ligand-to-ligand charge transfer (³LLCT) states for 1 and 2, and from the BTF-ppy ligand-centered ³π,π* excited states with significant contributions from the ³MLCT states for 3–5. The triplet excited states of 1–5 also manifest broad transient absorption (TA) in the visible to the near-IR spectral region. The electronic absorption, emission, and ns transient absorption are all red-shifted by extending the π-conjugation of the ppy ligand or increasing the number of BTF-ppy ligand. The energies of the lowest singlet and triplet excited states of the neutral complex 4 are lowered compared to those of its cationic counterpart 1; while the transient absorbing triplet excited state of 4 is much longer lived than that of 1. These complexes all exhibit strong reverse saturable absorption (RSA) for ns laser pulses at 532 nm, with a trend of 5 < 4 < 1 ≈ 3 < 2. This trend is primarily determined by the ratio of the excited-state absorption cross section to that of the ground state (σ_ex/σ₀) at 532 nm with the triplet quantum yield also playing a role for complexes 3–5. It appears that the increased number of BTF-ppy ligand reduces the RSA of the neutral complexes while the increased π-conjugation of the ppy ligand in the cationic complexes improves the RSA. However, the neutral complex 4 exhibits a weaker RSA at 532 nm than its cationic counterpart 1.