Application of chiral cationic iridium(III) complexes for triplet-triplet annihilation up-conversion of photon energy in R-limonene

Converting photon energy in a green solvent is a challenge for developing environmentally friendly technologies. Triplet-triplet annihilation up-conversion (TTA-UC) in R-limonene is one such attempt. Since R-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 R-limonene to perform TTA-UC. Conversely, its racemic form was barely soluble due to its high crystallinity. When an R-limonene solution containing chiral [Ir(piq)₂(dmbpy)]PF₆ (ca. 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_nbpy)]⁺ (C_nbpy = 4,4'-nonyl (C₉)- or 4,4'-nonyldecyl (C₁₉)-2,2'-bipyridine), were investigated on the solubility and efficiency of TTA-UC.