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

Abstract

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 (C 25 H 22 N 6 O, PBPM) and ( E )-4-((4-(dimethylamino)phenyl)diazenyl)- N, N -bis(pyridin-2-ylmethyl) benzohydrazide (C 27 H 27 N 7 O, 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)(CF 3 CO 2 )] (LnC 47 H 34 F 9 N 6 O 7 ) and [Ln(tfd) 2 (dmPBPM)(CF 3 CO 2 )] (LnC 49 H 39 F 9 N 7 O 7 ), where the lanthanide(III) ions are nine coordinated with N 3 O 6 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 P 2 1 / 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 t→c = 10 –2 ) of [Eu(tfd) 2 (PBPM)(CF 3 CO 2 )] complex that was comparable to that of PBPM. Moreover, the trans-to-cis photoisomerization rates of complexes [Ln(tfd) 2 (PBPM)(CF 3 CO 2 )] (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.