Functionalized Lanthanide(III) Complexes Constructed from Azobenzene Derivative and β-Diketone Ligands: Luminescent, Magnetic, and Reversible Trans-to-Cis Photoisomerization Properties.

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 (C25H22N6O, PBPM) and (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (C27H27N7O, 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)2(PBPM)(CF3CO2)] (LnC47H34F9N6O7) and [Ln(tfd)2(dmPBPM)(CF3CO2)] (LnC49H39F9N7O7), where the lanthanide(III) ions are nine coordinated with N3O6 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 P21/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 (Φt→c = 10-2) of [Eu(tfd)2(PBPM)(CF3CO2)] complex that was comparable to that of PBPM. Moreover, the trans-to-cis photoisomerization rates of complexes [Ln(tfd)2(PBPM)(CF3CO2)] (La, Eu, Gd) (10-3-10-2 s -1) were also at the same level as that of PBPM and much higher than azobenzene itself (10-5-10-4 s-1). 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.