Effects of Extended 蟺-Conjugation in Phenanthroline (NN) and Phenylpyridine (CN) Ligands on the Photophysics and Reverse Saturable Absorption of Cationic Heteroleptic Iridium(III

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• 作者：Yuhao Li ; Naveen Dandu ; Rui Liu ; Zhongjing Li ; Svetlana Kilina ; Wenfang Sun
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：March 27, 2014
• 年：2014
• 卷：118
• 期：12
• 页码：6372-6384
• 全文大小：723K
• 年卷期：v.118,no.12(March 27, 2014)
• ISSN：1932-7455

文摘

Five new iridium(III) complexes (3,8-R-Phen)Ir(2-(3-R鈥?phenyl)pyridine)₂ (1, R = fluoren-2-yl, R鈥?= H; 2, R = 7-benzothiazolylfluoren-2-yl, R鈥?= H; 3, R = H, R鈥?= fluoren-2-yl; 4, R = H, R鈥?= 7-benzothiazolylfluoren-2-yl; 5, R = R鈥?= 7-benzothiazolylfluoren-2-yl) with fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituent on the 2-phenylpyridine (CN) and/or phenanthroline (NN) ligands were synthesized and characterized. Their photophysical properties were investigated systematically via UV鈥搗is 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 ^1,3MLCT (metal-to-ligand charge transfer)/^1,3LLCT (ligand-to-ligand charge transfer) transitions. The major absorption bands below 475 nm for 1鈥?b>5 arise from the ¹蟺,蟺* and intraligand (¹ILCT) transitions within the NN or CN ligands. Attachment of fluoren-2-yl or 7-benzothiazolylfluoren-2-yl substituents on NN 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 CN ligands. In contrast, the ^1,3MLCT/^1,3LLCT 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 NN ligand-centered ³蟺,蟺* state with some admixture of ³MLCT/³ILCT/³LLCT characters for 1. In contrast, the emission of 3 and 4 emanates exclusively from the ³MLCT/³LLCT states. For complex 5, which contains 7-benzothiazolylfluoren-2-yl substituents on both the CN and the NN ligands, the emission predominantly arises from the ³MLCT/³LLCT states with a small portion of NN ligand-localized ³蟺,蟺* 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 鈮?2 > 4 鈮?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 蟺-conjugation in the NN ligand obviously increases the RSA of complexes 1, 2, and 5 in comparison to those with 蟺-conjugated substituents only on the CN ligands (3 and 4).