Near-Infrared Photoluminescence and Reversible Trans-to-Cis Photoisomerization of Mononuclear and Binuclear Ytterbium(III) Complexes Functionalized by Azobenzene Groups.

Two mononuclear and one binuclear ytterbium complexes with dual near-infrared (NIR) photoluminescence and reversible trans-to-cis photoisomerization functions were synthesized and characterized. The central ytterbium(III) ion coordinates with two β-diketonate (4,4,4-trifluoro-1-phenylbutane-1,3-dionate (tfd)) ligands and one deprotonated azobenzene-containing tetradentate ligand [()-4-(phenyldiazenyl)-,-bis(pyridin-2-ylmethyl) benzohydrazide (HL), ()-4-((4-(dimethylamino)phenyl)diazenyl)-,-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or ()-4,4'-','-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (HDL)] to form a neutral ternary complex ([Yb(tfd)L], [Yb(tfd)(NL)], or [Yb(tfd)(DL)], respectively), where the ytterbium(III) ion is eight-coordinated to NO donor sets. X-ray crystallographic analysis shows that all three complexes form a trigonal dodecahedron geometry with similar -N=N- distances that are slightly longer than those of the pure azobenzene-containing ligands. The NIR luminescence properties of the Yb(III) complexes were determined at a wavelength of about 980 nm with quantum yields in the range of 0.4-0.6% in ethanol and acetonitrile solutions at room temperature, and trans-to-cis photoisomerization was determined with the quantum yields (Φ = 10) at the same level as their pure ligands. The trans-to-cis photoisomerization rates of the complexes (10 s) are slightly higher than those of the pure ligands and similar to azobenzene (10 to 10 s). From time-dependent density functional theory calculations of the energy levels of the first excited triplet states of the ligands, the energies of the lowest excited triplet states of all of the ligands are higher than the resonance level of Yb (2F, 1.2722 eV). We suggest that these azo-containing ligands may participate in energy transfer to the ytterbium ion, in addition to the main "antenna effect" ligand tfd. This is the first report of azobenzene group-functionalized ytterbium complexes with dual NIR luminescence and photoisomerization properties, indicating that azobenzene-containing lanthanide(III) complexes have potential applications as dual function materials in biological systems.