B
Y. Jia et al.
HO
N
Cd(OAc)2·2H2O Ln(tta)3·2H2O
CH2Cl2/MeOH
N
{[Ln(H2L)(tta)2(OAc)]·0.5H2O]n
OH
Scheme 1. Schematic representation of complexes 1–4.
4
intramolecular hydrogen bonding with the deprotonated phenol
oxygen atoms, thus indicating that a proton migration occurs in
the complexation reactions.
Simulation 4
3
Luminescence Properties
The UV-vis absorption data of complexes 1–4, Eu(tta)ꢀ2H2O,
and H2L are presented in Fig. S2 (Supplementary Material). In
MeOH, H2L consists of three main absorptions at ,215, 255,
315 nm which are assigned to the p–p* transitions of Ar-OH
and the imine group. As for 1, there are three sets of absorption
bands at ,237, 270, and 346 nm, which are due to the intra-
ligand transitions of the ligand. The emission spectra of complex
1 in the solid state tested at room-temperature are presented in
Fig. 3. Based on a maximum excitation at 368 nm in the solid
state, the emission spectrum of EuIII is observed at 614 nm with a
strong and unique emission peak. Four typical emission bands of
the EuIII ion are observed, 590 (5D0-7F0), 614 (5D0-7F2), 650
(5D0-7F3), and 696 nm (5D0-7F4). The free H2L ligand does
not exhibit luminescence under similar conditions. The H2L
provides efficient energy transfer for the sensitisation of the
Ln3þ ion. The lifetime of 1 is 417.19 ms in the solid state.
Simulation
3
2
1
1
Simulation
40
10
20
30
50
2q [deg.]
Fig. 1. PXRD patterns for 1–4.
IR Spectra
The IR absorption spectra of the ligand and complexes 1–4 are
shown in Fig. S1 (Supplementary Material). Taking 1 as an
example, the broad but weak O–H stretching vibration of the
free H2L ligand at ,2855 cmꢁ1 is replaced by a band at
,3695 cmꢁ1 in complex 1 due to the N–H vibration in C=Nþ–
H. This band indicates that the hydrogen atoms are still involved
in intramolecular H-bonding with the phenolic oxygen. The
C=N stretching vibration in complex 1 shifts to a higher
wavenumber (by 20–30 cmꢁ1 to ,1657 cmꢁ1).[21]
X-Ray crystallographic analysis has revealed that complexes
1–4 are isomorphic, as revealed by comparison of their powder
X-ray diffraction (PXRD) patterns (Fig. 1). The PXRD patterns
of complexes 1–4 are in agreement with the simulated ones from
the respective single-crystal X-ray data. Taking 1 as an example,
the unit cell of complex 1 crystallises in the monoclinic space
group P21/n. The perspective view of the molecular structure of
1 was shown as a dodecahedron[22] in Fig. 2b. Crystallographic
details are provided in Table 1 and selected bond lengths of
the coordination environment of the metal centres are listed in
Table S1 (Supplementary Material). The organic ligand H2L is
involved in linking the one dimensional chain structure. As a
result, each H2L acts a bridge between two EuIII atoms. Cd2þ
ions are not observed in the structure. The unique EuIII ions are
eight-coordinated by two different H2L anionic oxygen atoms,
two oxygen atoms from an acetate anion, and four oxygen atoms
from two tta ligands. The Eu–O (tta) distances range from 2.395
NIR Luminescence
The NIR emission spectra of complex 4 in the solid state tested at
room temperature are presented in Fig. 4. Excited at 380 nm
withmaximumexcitation, thetypical NIRemissionband of YbIII
centred at 980 nm was observed, which is assigned to the
2F5/2-2F7/2 transition. Two other broad bands centred at 993
and 1028 nm are also observed. The free H2L ligand does not
exhibit NIR luminescence under the same conditions. The H2L
provides efficient energy transfer for the sensitisation of the
Yb3þ ion. The lifetime of 4 was t1 ¼ 9681.93 ns in the solid state.
In general, the widely accepted energy transfer mechanism
of the luminescent lanthanide complexes is proposed by
Crosby.[23] In order to make the energy transfer effective, the
energy-level match between the lowest triplet energy level (T1)
of the ligand and the lowest excited state level of the LnIII
ion becomes one of the most important factors dominating
the luminescence properties of the complexes. On account
of the difficulty in observing the phosphorescence spectrum
of the ligand, the emission spectrum of the complex {[Gd(H2L)
(tta)2(OAc)]ꢀ0.5H2O}n (2) at 77 K is used to estimate the triplet
state energy level of the ligand. The singlet state energy level of
H2L is estimated by referring its absorbance edge, which is
24814 cmꢁ1 (403 nm). The triplet (T1) energy levels are calcu-
lated by referring to the lower wavelength emission peaks of the
corresponding phosphorescence spectrum of the GdIII complex,
which are 20325 cmꢁ1 (492 nm) (Fig. S3, Supplementary Mate-
rial). It is known that the gap DE(T1–LnIII) should be intermedi-
ate for maximum energy transfer, otherwise too large or too
small a gap would decrease the efficiency of energy transfer.
According to Latva et al.,[24] the ligand-to-metal transfer pro-
cess could occur effectively when the energy gap is in the region
of 2500–3200 cmꢁ1. Therefore, the energy gaps between the
triplet state of H2L and the resonance energy level of EuIII was
˚
(3) to 2.409(3) A. Notably, the Eu–O bond distances for H2L
atoms O4 and O8 of the ligands (2.293(2) and 2.310(2)) are
shorter than those oxygen atoms of acetate (2.461(3) and 2.484
(2)). The oxygen atoms (O6, O7) of the acetate group chelate the
europium ion. One Schiff base ligand displays a curved config-
uration in which the backbone oxygen atoms (O4, O15) link Eu1
and Eu2 ions. The O4–Eu–O8 angle is 158.698. The hydrogen
atoms located on the two nitrogen atoms are involved in