Application of chiral cationic iridium( <scp>iii</scp> ) complexes for triplet–triplet annihilation up-conversion of photon energy in <i>R</i> -limonene

Converting photon energy in a green solvent is a challenge for developing environmentally friendly technologies. Triplet-triplet annihilation up-conversion (TTA-UC) in <i>R</i>-limonene is one such attempt. Since <i>R</i>-limonene is a nonpolar medium, conventional organic compounds or neutral metal complexes are used as a donor. In this study, cationic chiral iridium(III) complexes are used for the first time. The studies were based on the finding that a cyclometalated cationic iridium(III) complex in a chiral form, Δ- or Λ-[Ir(piq)₂(dmbpy)]⁺ (piqH = 1-phenyisoquinoline, dmbpy = 4,4'-dimethyl-2,2'-bipyridine), was soluble enough in <i>R</i>-limonene to perform TTA-UC. Conversely, its racemic form was barely soluble due to its high crystallinity. When an <i>R</i>-limonene solution containing chiral [Ir(piq)₂(dmbpy)]PF₆ (<i>ca.</i> 2 × 10^-5 M) and DPA (0.3-2.6 mM) was irradiated with laser light at 445 nm, the up-conversion of photon energy to 430 nm was achieved with a quantum yield as high as 4.5% under air. Despite the slight difference in solubility between the enantiomers, no stereoselectivity was observed in TTA-UC. The effects of long alkyl chains attached to a donor molecule, or Δ- or Λ- [Ir(piq)₂(C_<i>n</i>bpy)]⁺ (C_<i>n</i>bpy = 4,4'-nonyl (C₉)- or 4,4'-nonyldecyl (C₁₉)-2,2'-bipyridine), were investigated on the solubility and efficiency of TTA-UC.