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.