Near-Infrared Photoluminescence and Reversible Trans-to-CisPhotoisomerization of Mononuclear and Binuclear Ytterbium(III) ComplexesFunctionalized by Azobenzene Groups

摘要

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 [(E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (HL), (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or (E)-4,4′-N′,N′-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (H2DL)] to form a neutral ternary complex ([Yb(tfd)2L], [Yb(tfd)2(NL)], or [Yb2(tfd)4(DL)], respectively), where the ytterbium(III) ion is eight-coordinated to N3O5 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-cisphotoisomerization was determined with the quantum yields (Φt→c = 10^–2) at the same level as theirpure ligands. The trans-to-cis photoisomerization rates of the complexes(10^–4 s^–1) are slightly higherthan those of the pure ligands and similar to azobenzene (10^–5 to 10^–4 s^–1). From time-dependentdensity functional theory calculations of the energy levels of thefirst excited triplet states of the ligands, the energies of the lowestexcited triplet states of all of the ligands are higher than the resonancelevel of Yb³⁺ (2F5/2, 1.2722 eV). We suggestthat these azo-containing ligands may participate in energy transferto the ytterbium ion, in addition to the main “antenna effect”ligand tfd. This is the first report of azobenzene group-functionalizedytterbium complexes with dual NIR luminescence and photoisomerizationproperties, indicating that azobenzene-containing lanthanide(III)complexes have potential applications as dual function materials inbiological systems.