A detailed investigation of light-harvesting efficiency of blue color emitting divergent iridium dendrimers with peripheral phenylcarbazole units

Abstract

The increase in phosphorescence efficiency was estimated by the energy transfer mechanism for divergent iridium dendrimers with peripheral phenylcarbazole units. A series of Ir-core/phenylcarbazole-end dendrons of the type, Ir(dfppy)₂(pic-Cz_n) (Gn, n = 0, 1, 2, and 3), was synthesized, where dfppy, pic, and Cz_n (n = 2, 4, and 8) are 2-(4,6-difluorophenyl)pyridine, picolinate substituted with Cz_n at the 3-position, and 4-(9-carbazolyl)phenyldendrons connected with 3,5-di(methyleneoxy)benzyloxy branches, respectively. Selective excitation of the Cz_n units of G1–G3 resulted in >90% quenching of the Cz fluorescence accompanied by the growth of phosphorescence from the Ir(dfppy)₂(pic) core as a consequence of energy transfer from the excited-singlet Cz_n chromophore to the core. The rate constants of energy transfer were determined by steady-state and transient spectroscopic measurements to be 4.32 × 10⁹ s⁻¹ (G1), 2.37 × 10⁹ s⁻¹ (G2), and 1.46 × 10⁹ s⁻¹ (G3), which were in good agreement with those calculated using the Förster model. The phosphorescence enhancements were 157 (G1), 213 (G2), and 264% (G3) when compared to the phosphorescence of the core Ir(dfppy)₂(pic-Ph₂) (G0), in which pic-Ph₂ is 3-(3,5-dibenzyloxybenzyl)picolinate.