Phosphorescent Chiral Cationic Binuclear Iridium(III) Complexes: Boosting the Circularly Polarized Luminescence Brightness

We report the synthesis and characterization of two chiral binuclear iridium(III) complexes (ΛΛ and ΔΔ) prepared from enantiopure building blocks [μ-Cl2(Δ-Ir(C^N)2)2] and [μ-Cl2(Λ-Ir(C^N)2)2]. These building blocks have been obtained by chiral preparative high-performance liquid chromatography of the neutral iridium(III) complex Irpiv (piv = 2,2,6,6-tetramethylheptane-3,5-dionate) followed by selective degradation of the ancillary ligand. For comparison purposes, we also synthesized a monomer (IrL1) and a dimer (Ir2L2, mixture). All the complexes exhibit similar emission properties, emitting in the orange-red region of the spectra with a good photoluminescence quantum yield (λmax = 610-625 nm, Φ ∼ 25%, τ ∼ 800-900 ns). However, the ΛΛ and ΔΔ complexes are optically active, indicating that no isomerization occurred during the different synthetic steps, as evidenced by both the circular dichroism spectra and their circularly polarized luminescence (CPL). The capital gain of the dimers (Ir2L2, ΛΛ, and ΔΔ) is a 4-fold brightness (B380 = ε380 nm × Φ) compared to the monomer (IrL1) and the CPL brightness (BCPL = B380 × glum/2) of the binuclear complexes being among the highest reported to date for chiral iridium(III) complexes.