Effects of Extended π-Conjugation in Phenanthroline (N^∧N) and Phenylpyridine (C^∧N) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III) Complexes

Abstract

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).

Influence of a Naphthaldiimide Substituent at the Diimine Ligand on the Photophysics and Reverse Saturable Absorption of PtII Diimine Complexes and Cationic IrIII Complexes

“…In accordance with the trend reported for other Ir III complexes, incorporation of the BTF component to the C ∧ N ligand reduces the τ T value. [18,27] …”

“…[17,35] Our group also demonstrated that introducing the benzothiazolylfluorenyl (BTF) substituents to the N ∧ N ligand dramatically increased the T 1 state lifetimes for both Pt II diimine acetylide complexes [23] and heteroleptic Ir III complexes. [18,19,27] However, incorporation of the BTF motif into the C ∧ N ligands in the Ir III complex resulted in a reduced T 1 lifetime. [18,27] In addition to the aforementioned work, Weinstein's group reported a detailed study on the excited state dynamics of a Pt II diimine chloride complex bearing a strong electron-withdrawing naphthaldiimide (NDI) substituent on the diimine ligand.…”

“…[18,19,27] However, incorporation of the BTF motif into the C ∧ N ligands in the Ir III complex resulted in a reduced T 1 lifetime. [18,27] In addition to the aforementioned work, Weinstein's group reported a detailed study on the excited state dynamics of a Pt II diimine chloride complex bearing a strong electron-withdrawing naphthaldiimide (NDI) substituent on the diimine ligand. [39] It was found that there is little electronic interaction between the Pt(phen)Cl 2 [14,18,20,23,24,27] In this work, we synthesized a Pt II diimine complex and two cationic Ir III complexes bearing the NDI motif on the diimine (N ∧ N) ligand (complexes 2-4 in Figure 1).…”

“…[18,27] In addition to the aforementioned work, Weinstein's group reported a detailed study on the excited state dynamics of a Pt II diimine chloride complex bearing a strong electron-withdrawing naphthaldiimide (NDI) substituent on the diimine ligand. [39] It was found that there is little electronic interaction between the Pt(phen)Cl 2 [14,18,20,23,24,27] In this work, we synthesized a Pt II diimine complex and two cationic Ir III complexes bearing the NDI motif on the diimine (N ∧ N) ligand (complexes 2-4 in Figure 1). Pt II complex 1, which contains the 1,10-phenanthroline ligand, was synthesized to be a reference compound for the Pt II complex 2 in order to better understand the role of NDI in the presence of the BTF acetylide ligands.…”

“…In addition, the BTF-substituted phenylpyridine ligands are utilized in Ir III complex 4, because our previous studies revealed that the Ir III complexes bearing this ligand exhibited strong RSA at 532 nm due to the large ratio of the excited-state absorption cross section to the ground-state absorption cross section, although the introduction of the BTF substituent into the phenylpyridine ligands reduced the triplet excited-state lifetimes. [18,27,29] Even though this ligand has been utilized in our previous work, its influence on the photophysics and RSA of the Ir III complexes in the presence of an electronwithdrawing NDI substituent on the N ∧ N ligand is hard to predict, because the NDI substitution could completely change the nature of the lowest triplet excited state. Therefore, comparison between complexes 4 and 3 would allow us to further elucidate the effect of the substituent on the C ∧ N ligand when the NDI substituent is introduced into the N ∧ N ligand.…”