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.