Dual emissive ytterbium(III) complexes with π-conjugated BODIPY-bipyridine ligands

Four BODIPY-functionalized bipyridine ligands (B1, B2, B3 and B4) were synthesized for sensitizing the near-infrared emission of Yb(III) ions. In these ligands, a BODIPY moiety was conjugated to 2,2'-bipyridine through an acetylene spacer at its C2 position, whereas its C6 position was substituted by H (B1), iodine (B2), 4-carboxylphenylacetylene (B3) or 4-thiocyanophenylacetylene (B4). The ligands exhibit strong absorption in the visible region and readily form stable complexes with ytterbium(III) trishexafluoroacetylacetonate (hfac^-) hydrate in dichloromethane. Single-crystal X-ray diffraction analysis for Yb-B2 showed that the BODIPY unit almost falls into the bipyridine plane with Yb(III) being coordinated by six O from hfac^- and two N atoms from bipyridine. All complexes exhibit strong absorption between 530 and 570 nm and can sensitize the ytterbium(III) for its emission at 980 nm under the UV-VIS light illumination. Interestingly, the visible emission from the complex increased when excited at 375 nm, making the complexes dual emissive, which is due to the increased absorption of the complex at 375 nm and inefficient energy transfer from the BODIPY moiety to the Yb(III) ion.