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

A series of red phosphorescent iridium dendrimers of the type [Ir­(btp)₂(pic-PC_n)] (Ir-G_n; 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 (PC_n; n = 0, 2, 4, and 8). The harvesting photons on the PC_n 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 PC_n 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-G_n 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.