Time-resolved spectroscopic analysis of the light-energy harvesting mechanism in carbazole-dendrimers with a blue-phosphorescent Ir-complex core†
In order to investigate the light-energy harvesting mechanism, a series of dendrimers with a heteroleptic iridium(III) complex core, [Ir(dmb)2(pic-Czn)] (Gn: n = 1, 2, and 3), with 2,6-difluoro-3-(4-methylpyridin-2-yl)benzonitrile (dmb) as the cyclometallating ligand and 3-hydroxypicolinate (pic) as the ancillary ligand, connected to carbazole-based dendrons (Czn: n = 2, 4, and 8) was synthesized. The Ir centred complex [Ir(dmb)2(pic-OCH3), G0] shows a blue emission at <500 nm, which is assigned to metal-to-ligand charge-transfer (3MLCT) phosphorescence. This phosphorescence was enhanced with increasing generations due to the increase in the total absorbance of the Cz-dendron. The light-harvesting efficiencies determined by various methods were approximately 160 (G1), 220 (G2), and 330% (G3). The energy transfer efficiencies for G1–G3 from the peripheral Cz-dendron to the Ir-core complex were above 97%, determined using the reduction in the lifetime of the excited 1Cz*-dendrons. G1–G3 showed a transient absorption (TA) band at 600 nm, which was attributed to the Sn ← S1 transition of the Cz-dendrons. The fast decay of these TA bands was consistent with the fast emission decay times. The time-resolved TA band correlated with the core Ir-complex was observed at 500 nm, though it overlapped and interfered with the intense TA band of the Cz-dendrons. Therefore, we attempted a global analysis by singular value decomposition (SVD). The determination of the absorption spectra of the individual species participating in the energy transfer process by SVD analysis can distinguish between different mechanistic models. The analysed rate constants were consistent with the results determined by the emission decays.