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(111) Complexes

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-2-yl 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 (MLCT)-M-1,3 (metal-to-ligand charge transfer)/(LLCT)-L-1,3 (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1-5 arise from the (1)pi,pi* and intraligand ((ILCT)-I-1) transitions within the N<^>N or C<^>N ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-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 CAN ligands. In contrast, the (MLCT)-M-1,3/(LLCT)-L-1,3 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)pi,pi* state with some admixture of (MLCT)-M-3/(ILCT)-I-3/(LLCT)-L-3 characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the (MLCT)-M-3/(LLCT)-L-3 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 (MLCT)-M-3/(LLCT)-L-3 states with a small portion of N<^>N ligand-localized (3)pi,pi* 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 approximate to 2 > 4 approximate to 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 pi-conjugation in the N<^>N ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with pi-conjugated substituents only on the C<^>N ligands (3 and 4).