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–Cz1)2Ir(dpq)]+ (G1) and [(dfppy–Cz2)2Ir(dpq)]+ (G2), which have the dfppy ligand connected to carbazole-functionalized dendron Czn (n = 1, 2) [dfppy–Czn = 5-Czn-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 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 (τp = 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 Czn′–OH (τF = 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 μs that is nearly identical to that of G0 (6.8 μs), and the carbazole fluorescence lifetime is ∼11.5 ± 0.5 ns, which is again almost the same as that of Czn′–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 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.