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Efficient Light Harvesting and Energy Transfer in a Red Phosphorescent Iridium Dendrimer

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journal contribution
posted on 2014-12-15, 00:00 authored by Yang-Jin Cho, Seong Ahn Hong, Ho-Jin Son, Won-Sik Han, Dae Won Cho, Sang Ook Kang
A series of red phosphorescent iridium dendrimers of the type [Ir­(btp)2(pic-PCn)] (Ir-Gn; n = 0, 1, 2, and 3) with two 2-(benzo­[b]­thiophen-2-yl)­pyridines (btp) and 3-hydroxypicolinate (pic) as the cyclometalating and ancillary ligands were prepared in good yields. Dendritic generation was grown at the 3 position of the pic ligand with 4-(9H-carbazolyl)­phenyl dendrons connected to 3,5-bis­(methyleneoxy)­benzyloxy branches (PCn; n = 0, 2, 4, and 8). The harvesting photons on the PCn dendrons followed by efficient energy transfer to the iridium center resulted in high red emissions at ∼600 nm by metal-to-ligand charge transfer. The intensity of the phosphorescence gradually increased with increasing dendrimer generation. Steady-state and time-resolved spectroscopy were used to investigate the energy-transfer mechanism. On the basis of the fluorescence quenching rate constants of the PCn dendrons, the energy-transfer efficiencies for Ir-G1, Ir-G2, and Ir-G3 were 99, 98, and 96%, respectively. The energy-transfer efficiency for higher-generation dendrimers decreased slightly because of the longer distance between the PC dendrons and the core iridium­(III) complex, indicating that energy transfer in Ir-Gn is a Förster-type energy transfer. Finally, the light-harvesting efficiencies for Ir-G1, Ir-G2, and Ir-G3 were determined to be 162, 223, and 334%, respectively.

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