The coordination ability of ligands functionalized by azobenzene was manipulated, and two novel chelating ligands, (E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (CHNO, PBPM) and (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (CHNO, dmPBPM), were synthesized. The ligands can offer four coordinating atoms (one oxygen and three nitrogens) to act as tetradendate ligands, together with the two β-diketonates (4,4,4-trifluoro-1-phenylbutane-1,3-dionate, tfd), and the trifluoroacetate anion presented as a ligand and a counterion to form the quaternary units with lanthanide(III) ions (La, Eu, and Gd), [Ln(tfd)(PBPM)(CFCO)] (LnCHFNO) and [Ln(tfd)(dmPBPM)(CFCO)] (LnCHFNO), where the lanthanide(III) ions are nine coordinated with NO donor sets. All six complexes were structurally characterized, and four crystals were obtained and further analyzed by means of single-crystal X-ray diffraction. They all crystallized in the monoclinic P2/c space group with very similar lattice parameters, forming a monocapped twisted square antiprism. We successfully observed the photoluminescent properties of Eu(III) complexes at a wavelength of 614 nm in both solution and the solid state, as well as the trans-to-cis photoisomerization with the quantum yield (Φ = 10) of [Eu(tfd)(PBPM)(CFCO)] complex that was comparable to that of PBPM. Moreover, the trans-to-cis photoisomerization rates of complexes [Ln(tfd)(PBPM)(CFCO)] (La, Eu, Gd) (10-10 s ) were also at the same level as that of PBPM and much higher than azobenzene itself (10-10 s). With the aid of TD-DFT calculations, the luminescence of Eu(III) complexes was found to originate from the attenuation effect of β-diketonates. These features provide the foundation for the development of azobenzene-derived β-diketonates lanthanide(III) complexes with photoisomerization and photoluminescence dual functions.

Novel azobenzene-derived β-diketonates (4,4,5,5,6,6,6-heptafluoro-1-azobenzene-1,3-hexanedione (LA), 4,4,5,5,6,6,6-heptafluoro-1-(4-dimethylamino)azobenzene-1,3-hexanedione (LB)) were designed and their complexes with lanthanide cations (La(3+), Eu(3+), Gd(3+), Yb(3+)) were prepared and characterized by (1)H NMR, FT-IR, and elemental analysis. Three of the complexes were crystallized successfully and identified by X-ray diffraction. It was significant to find that LA showed remarkably reversible trans-to-cis isomerization properties, however, LB, bearing an electron donor compared with LA, slowed down the isomerization to an extent. The presence of Ln(iii) enhanced the reversible trans-to-cis isomerization properties of both LA and LB a little upon photoirradiation in organic solvents, and amazingly increased the fatigue resistance. In addition, the complexes doped in polymethyl methacrylate (PMMA) films produced a similar phenomenon as well as when in solution. Theoretical calculations based on time dependent density functional theory (TD-DFT) were performed for geometry optimization and to determine the excitation energies of LA and LB to gain further insight into the electronic structure of the complexes, and the data were consistent with the experimental results. The excellent reversible photoisomerization properties of the newly designed Ln(iii) complexes can offer important advantages that will help with the further study of these materials to reach their full potential in applications such as molecular switching devices.

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 (H 2 DL)] to form a neutral ternary complex ([Yb(tfd) 2 L], [Yb(tfd) 2 (NL)], or [Yb 2 (tfd) 4 (DL)], respectively), where the ytterbium(III) ion is eight-coordinated to N 3 O 5 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 t→c = 10 –2 ) at the same level as their pure ligands. The trans-to-cis photoisomerization rates of the complexes (10 –4 s –1 ) are slightly higher than those of the pure ligands and similar to azobenzene (10 –5 to 10 –4 s –1 ). 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 3+ (2F 5/2 , 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.

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