Boranil Chelates as Antennae for Ytterbium(III) Luminescence Sensitization

Five original ligands based on bipyridine and phenanthroline chelating units connected to boranil moieties are synthesized and characterized, including single-crystal X-ray diffraction. For classical boranil moieties, large absorption coefficients are observed, together with intense fluorescence. For sulfur-containing boranils, absorption coefficients and fluorescence quantum yields are smaller, but larger Stokes shifts are observed, together with singlet oxygen generation. These features are in line with the time-dependent density functional theory study, which also reproduces well the behavior of the yttrium(III) complexes; more specifically, the positioning of the ligand triplet states, which is in the range of 15,500-20,000 cm^-1, as well as the larger spin-orbit coupling allowing better intersystem crossing for sulfur-containing boranils. In addition to ligand-based fluorescence, all five ytterbium complexes show near-infrared emission with quantum yields (Φ_Yb^L) in the range of 0.0027-0.0040, lifetimes in the range of 8.8-11.4 μs, and brightness (B = ε·Φ_Ln^L) in the range of 121-163 M^-1·cm^-1 (with 350 nm excitation).