Photodynamic Behavior of Heteroleptic Ir(III) Complexes with Carbazole-Functionalized Dendrons Associated with Efficient Electron Transfer Processes

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• 作者：Ah-Reum Hwang ; Won-Sik Han ; Kyung-Ryang Wee ; Hyun Young Kim ; Dae Won Cho ; Byoung Koun Min ; Suk Woo Nam ; Chyongjin Pac ; Sang Ook Kang
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：January 19, 2012
• 年：2012
• 卷：116
• 期：2
• 页码：1973-1986
• 全文大小：770K
• 年卷期：v.116,no.2(January 19, 2012)
• ISSN：1932-7455

文摘

We prepared dendrimers of heteroleptic iridium(III) complexes, [(dfppy鈥揅z₁)₂Ir(dpq)]⁺ (G1) and [(dfppy鈥揅z₂)₂Ir(dpq)]⁺ (G2), which have the dfppy ligand connected to carbazole-functionalized dendron Cz_n (n = 1, 2) [dfppy鈥揅z_n = 5-Cz_n-2-(4,6-difluorophenyl)pyridine, dpq = 2,3-bis-(2-pyridyl)-qinoxaline, Cz₁ = 4-(9-carbazolyl)benzyloxymethyl, and Cz₂ = 4-[1,3-bis(9-carbazolyl)benzyloxy]benzyloxymethyl]. While parent complex [(dfppy)₂Ir(dpq)]⁺ (G0) shows an intense emission at 635 nm with a lifetime of 1 渭s assigned to dpq-based metal-to-ligand charge-transfer (MLCT) phosphorescence, excitation of the dendrimers at either carbazole (309 nm) or MLCT band (355 nm) resulted in markedly weaker and much shorter-lived MLCT emission (蟿_p = 44 ns for G1 and 115 ns for G2) at room temperature. Upon exciting the carbazole chromophore of G1 and G2 at 309 nm, furthermore, both the carbazole fluorescence and the MLCT emission were very weak at room temperature. It was found that the lifetime of carbazole fluorescence is 20 ps for G1 and 62 ps for G2, shorter by 2-orders of magnitude than that of free carbazole dendron Cz_n鈥测€揙H (蟿_F = 6.1 ns). These observations demonstrate that both the excited-singlet state of carbazole and the triplet MLCT state of the Ir(dpq) core are efficiently quenched in the dendrimers. At 77 K, however, the MLCT emission lifetime for both G1 and G2 is 7 渭s that is nearly identical to that of G0 (6.8 渭s), and the carbazole fluorescence lifetime is 11.5 卤 0.5 ns, which is again almost the same as that of Cz_n鈥测€揙H (11.5 ns). Since the apparent quenching of either carbazole fluorescence or MLCT emission observed at room temperature does not occur at 77 K, the temperature-dependent emission behavior of G1 and G2 for both the carbazole fluorescence and the MLCT phosphorescence was attributed to the participation of activated processes, that is, electron transfer from excited-singlet carbazole to the Ir(dpq) core as well as from the ground-state carbazole unit to the triplet MLCT Ir(dpq) core. This mechanism was supported by transient-absorption spectroscopic experiments that demonstrate the generation of the carbazole radical cation after exciting G1 and G2 by laser pulses.