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

We prepared dendrimers of heteroleptic iridium(III) complexes, [(dfppy–Cz<sub>1</sub>)<sub>2</sub>Ir(dpq)]<sup>+</sup> (<b>G1</b>) and [(dfppy–Cz<sub>2</sub>)<sub>2</sub>Ir(dpq)]<sup>+</sup> (<b>G2</b>), which have the dfppy ligand connected to carbazole-functionalized dendron Cz<sub><i>n</i></sub> (<i>n</i> = 1, 2) [dfppy–Cz<i><sub>n</sub></i> = 5-Cz<i><sub>n</sub></i>-2-(4,6-difluorophenyl)pyridine, dpq = 2,3-bis-(2-pyridyl)-qinoxaline, Cz<sub>1</sub> = 4-(9-carbazolyl)benzyloxymethyl, and Cz<sub>2</sub> = 4-[1,3-bis(9-carbazolyl)benzyloxy]benzyloxymethyl]. While parent complex [(dfppy)<sub>2</sub>Ir(dpq)]<sup>+</sup> (<b>G0</b>) shows an intense emission at ∼635 nm with a lifetime of 1 μs 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 (τ<sub>p</sub> = 44 ns for <b>G1</b> and 115 ns for <b>G2</b>) at room temperature. Upon exciting the carbazole chromophore of <b>G1</b> and <b>G2</b> 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 <b>G1</b> and 62 ps for <b>G2</b>, shorter by 2-orders of magnitude than that of free carbazole dendron Cz<sub><i>n</i></sub>′–OH (τ<sub>F</sub> = 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 <b>G1</b> and <b>G2</b> is ∼7 μs that is nearly identical to that of <b>G0</b> (6.8 μs), and the carbazole fluorescence lifetime is ∼11.5 ± 0.5 ns, which is again almost the same as that of Cz<sub><i>n</i></sub>′–OH (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 <b>G1</b> and <b>G2</b> 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 <b>G1</b> and <b>G2</b> by laser pulses.