Syntheses, structures and photophysical properties of new heterodinuclear Cd-Ln coordination complexes (Ln = Sm, Eu, Tb, Nd, Ho, Er)

Article abstract of DOI:10.1002/ejic.200800106

Six novel heterodinuclear d-f coordination complexes [LnCd(C ₈H₇O₃)₅(phen)(H₂O)] (Ln = Sm 1, Eu 2, Tb 3, Nd 4, Ho 5, Er 6; C₈H₇O₃ = 4-methoxybenzoato; phen = 1,10-phenanthroline) were synthesized by the hydrothermal method, and their structures were studied by single-crystal X-ray diffraction. The IR and UV/Vis/NIR absorption spectra and the luminescence spectra of the six complexes were determined at room temperature. They have the same molecular structure, and two neighboring molecules are connected by hydrogen bonds or by a Ln...O weak interaction to form dimer crystals. In the visible region, the emission spectra of complexes 1-3 show the intense characteristic bands of the corresponding Ln^III ions, which are mainly attributed to the efficient energy transfer from the d block to the f block, that is, to the good sensitization of the d block. Moreover, in the Cd-Ln coordination complexes, the Cd...Ln separation is very small, and the 4d orbitals of the Cd^II ion may influence the 4f orbitals of the Ln ^III ion, which probably causes some of the f levels of the Ln ^III ion to be adjusted, and this effect can be observed in the shifts or splittings of the absorption bands in the UV/Vis/NIR spectra of the complexes. Especially in complexes 4-6, some energy levels of the Ln ^III ions have shifts or splittings, hence the corresponding emission bands exhibit shifts or splittings in the NIR region. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800106

FULL PAPER
DOI: 10.1002/ejic.200800106
Syntheses, Structures and Photophysical Properties of New Heterodinuclear
Cd–Ln Coordination Complexes (Ln = Sm, Eu, Tb, Nd, Ho, Er)
Yu-Xian Chi,[a] Shu-Yun Niu,*[a] Zhao-Long Wang,[a] and Jing Jin[a]
Keywords: Heterodinuclear coordination complexes / Lanthanides / Cadmium / Luminescence
Six novel heterodinuclear d–f coordination complexes
[LnCd(C8H7O3)5(phen)(H2O)] (Ln = Sm 1, Eu 2, Tb 3, Nd 4,
Ho 5, Er 6; C8H7O3 = 4-methoxybenzoato; phen = 1,10-phen-
anthroline) were synthesized by the hydrothermal method,
and their structures were studied by single-crystal X-ray dif-
fraction. The IR and UV/Vis/NIR absorption spectra and the
luminescence spectra of the six complexes were determined
at room temperature. They have the same molecular struc-
ture, and two neighboring molecules are connected by hy-
drogen bonds or by a Ln···O weak interaction to form dimer
crystals. In the visible region, the emission spectra of com-
plexes 1–3 show the intense characteristic bands of the corre-
ficient energy transfer from the d block to the f block, that is,
to the good sensitization of the d block. Moreover, in the Cd–
Ln coordination complexes, the Cd···Ln separation is very
small, and the 4d orbitals of the CdII ion may influence the
4f orbitals of the LnIII ion, which probably causes some of the
f levels of the LnIII ion to be adjusted, and this effect can be
observed in the shifts or splittings of the absorption bands
in the UV/Vis/NIR spectra of the complexes. Especially in
complexes 4–6, some energy levels of the LnIII ions have
shifts or splittings, hence the corresponding emission bands
exhibit shifts or splittings in the NIR region.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
sponding LnIII ions, which are mainly attributed to the ef- Germany, 2008)
named the “antenna effect”. Numerous ligands can serve as
Introduction
the “antenna”,[11] for example β-diketones, flexible carbox-
ylic acids, pyridine derivatives, aromatic carboxylic acids
with highly conjugated and rigid structures, and so on.[12–16]
Currently, not only is the variety of ligands important, but
also introducing transition-metal ions into the system to
synthesize d–f heterometallic coordination complexes for
enhancing the luminescence of the f block by sensitization
of the d block. The absorption bands of the πǞπ* transi-
tions of the ligands are usually located in the ultraviolet
region, while the absorption bands (LǞL* or LǞM) of the
d block probably lie in the blue region or the longer wave-
length region, which will facilitate the energy transfer to the
LnIII ions (f block) and reduce the energy wasted. Thus the
d block can sensitize the luminescence of the f block more
efficiently. For example, in the Re–Ln and Pt–Ln complexes
based on polypyridine bridging ligands and in the cyanide-
bridged Ru–Ln coordination complexes, the d block sensi-
tizes the luminescence of the f block efficiently.[17] In ad-
dition, the selection of the central metal ions from the d
block is extensive, for example, OsII, CrIII, AuI, ZnII, CuI,
AgI, and so forth,[18–23] but the reports on heterometallic
Cd–Ln complexes are rare.[24,25] We selected the CdII ion
to synthesize Cd–Ln complexes. This choice has two main
reasons. Firstly, cadmium(II) coordination complexes have
excellent luminescence properties in the blue or the green
region, which may approach the emissive states of LnIII ions
and probably serve the energy transfer. Secondly, the coor-
dination number of the CdII ion is varied and is usually 6,
Luminescent lanthanide(III) coordination complexes
have attracted a great deal of attention for many years due
to their potential applications in areas such as bimetal
analysis and optical devices.[1–3] Lanthanide ions can show
linelike emission spectra with a narrow wavelength range
and high color purity. In addition, the emission properties
of this family of complexes are notable, and the emissions
cover an exceptionally wide spectral range: near-infrared
(NdIII, ErIII, YbIII), red (EuIII, PrIII, SmIII), green (ErIII
,
TbIII), and blue (TmIII, CeIII).[4] Recently, much attention
has been paid to the luminescence properties of lanthanide
complexes in the near-infrared region. Lanthanide com-
plexes that emit in the NIR region, a region where bio-
logical tissues and fluids are relatively transparent, have the
potential for use in chemosensor and fluoroimmunoassay
applications as well as optical amplification in lasers.[5–7]
However, the fǞf transition of lanthanide ions is parity-
forbidden. The absorption coefficients are very low, and the
intensity of the luminescence is weak.[8–10] To overcome this
drawback, organic ligands with intense and broad absorp-
tion bands are introduced as energy donors into the system
to sensitize the characteristic emissions of LnIII ions; this is
[a] School of Chemistry and Chemical Engineering, Liaoning Nor-
mal University,
Dalian 116029, P. R. China
E-mail: syniu@sohu.com
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
2336
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New Heterodinuclear Cd–Ln Coordination Complexes
7, or 8, which allows CdII ions to coordinate with organic hydrogen bonds coming from the coordinated water mole-
ligands to form novel structures.[26,27] The high coordina- cule and one carboxyl oxygen atom (O16A–H···O1B,
tion ability of CdII ions also prevents LnIII ions from enter- O16B–H···O1A, 2.720 Å) to form the dimer structure. In
ing cadmium(II) complexes, which makes it difficult to syn- the other complexes, the hydrogen bonds used to form the
thesize heterometallic Cd–Ln complexes. However, we are dimers are O16–H···O8 (2.717 Å) for 3, O16–H···O7
very much interested in the excellent luminescence proper- (2.719 Å) for 5, and O3–H···O14 (2.705 Å) for 6. However
ties of cadmium(II) coordination complexes, so we contin- in the crystal of [EuCd(C8H7O3)5(phen)(H2O)] (2) (Fig-
ued to try various reaction conditions and obtained a series ure 3), the molecular arrangement is a little different from
of Cd–Ln coordination complexes with luminescence prop- that of complex 1, that is, the dimers are connected by
erties.
Eu···O weak interactions (Eu1···O1, 4.246 Å) except for the
In this paper, the syntheses, structures, and photophysi- hydrogen bonds coming from the coordinated water and the
cal properties of six heterodinuclear Cd–Ln coordination carboxyl oxygen atom [O1–H···O15, 2.713(4) Å] (Figure 4).
complexes are reported. At the same time, in the six Cd–
Ln complexes, the sensitization and the influence of the d
block on the luminescence of the f block are discussed.
Results and Discussions
Crystal Structures of the Complexes
The formulas for the six complexes are identical with the
exception of the lanthanide ion: [LnCd(C8H7O3)5(phen)-
(H2O)] (Ln = Sm 1, Eu 2, Tb 3, Nd 4, Ho 5, Er 6). They
are isomorphous and crystallize in the triclinic space group
¯
P1. Therefore, only the structure of complex 1 is described
in detail.
[SmCd(C8H7O3)5(phen)(H2O)] (1) is a heterodinuclear
d–f coordination complex, and each molecule of 1 contains
one SmIII ion, one CdII ion, five 4-methoxybenzoate groups,
one phen molecule, and one coordinated water molecule
(Figure 1).
Figure 2. The dimer structure of complex 1.
Figure 1. The molecular structure of complex 1.
Three 4-methoxybenzoato ligands bridge the SmIII and
CdII ions and the other two 4-methoxybenzoato ligands
chelate the SmIII ion with one coordinated water molecule,
which makes the SmIII ion eight-coordinate. The five-coor-
dinate CdII ion is coordinated to two N atoms from one
Figure 3. The molecular structure of complex 2.
Because the crystal quality of complex 4 is not good, its
phen molecule and three carboxyl oxygen atoms from three crystal structure can not be completely solved by the data
4-methoxybenzoato ligands. As shown in Figure 2, in the from the single-crystal X-ray diffraction analysis. However,
crystal two neighboring molecules are connected through in comparison with the other five complexes, complex 4 has
Eur. J. Inorg. Chem. 2008, 2336–2343
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Y.-X. Chi, S.-Y. Niu, Z.-L. Wang, J. Jin
FULL PAPER
Photophysical Properties of the Complexes
UV/Vis/NIR Spectra of the Complexes
The assignments for the UV/Vis/NIR spectra of the six
complexes and the two ligands are listed in Table 1. The
free ligands (4-methoxybenzoic acid and phen) exhibit the
πǞπ* transition bands, which are not shifted perceptibly in
the spectra of the complexes, indicating that the conjuga-
tion of the planar ligands changes little after coordinating
with the LnIII and the CdII ions.[28] Meanwhile, the spectra
of complexes 1, 4, 5, and 6 exhibit the characteristic absorp-
tion bands of the corresponding LnIII ions, while the spec-
tra of complexes 2 and 3 do not show those characteristics,
because their fǞf transitions are forbidden. The UV/Vis/
NIR spectra of all the complexes are given in the Support-
ing Information (Figures S1–S6). In the following dis-
cussion, the relationship between the absorption spectra
and the emission spectra will be mentioned.
Figure 4. The dimer structure of complex 2.
Luminescence Properties of the Complexes in the Visible
Region
We studied the room-temperature luminescence proper-
the same crystal system and space group, similar crystal cell ties of the six complexes in the solid state. The assignments
structure, and a similar IR spectrum; meanwhile, its elemen- of the emission bands are summarized in Table 2.
tal analysis results approximate its predicted molecular for-
Complexes 1–3 show emissions only in the visible region.
mula. We deduce that its molecular structure is similar to In Figure 5, with λEx. = 345 nm, complex 1 exhibits a broad
that of the other five complexes. However, we can not ex- and strong emission band at 360–500 nm, which should be
clude other structural modes.
assigned to a ligand-to-metal charge transfer (LMCT) of
Table 1. Assignments of absorption bands for the UV/Vis/NIR spectra of the complexes and the ligands.
Complex
λ [nm] (Assignment)
1
250, 336 (πǞπ*); 404 (6H5/2Ǟ4F7/2); 472 (6H5/2Ǟ4F5/2); 520 (6H5/2Ǟ4F3/2); 654 (6H5/2Ǟ4G5/2); 940 (6H5/2Ǟ6F11/2);
1080 (6H5/2Ǟ6F9/2); 1236 (6H5/2Ǟ6F7/2); 1382 (6H5/2Ǟ6F5/2); 1492 (6H5/2Ǟ6F3/2); 1662 (6H5/2Ǟ6F1/2
)
2
3
4
254, 336 (πǞπ*)
252, 374 (πǞπ*)
248, 342 (πǞπ*); 472 (4I9/2Ǟ4G11/2), 514 (4I9/2Ǟ4G9/2); 524 (4I9/2Ǟ4G7/2); 580 (4I9/2Ǟ4G5/2); 682 (4I9/2Ǟ4F9/2);
744 (4I9/2Ǟ4F7/2); 800 (4I9/2Ǟ4F5/2); 864 (4I9/2Ǟ4F3/2
)
5
6
248, 348 (πǞπ*); 420 (5I8Ǟ5G5); 452 (5I8Ǟ5F1); 480 (5I8Ǟ5F3); 540 (5I8Ǟ5F4); 646 (5I8Ǟ5F5); 884 (5I8Ǟ5I5);
1144, 1180 (5I8Ǟ5I6)
248, 350 (πǞπ*); 402 (4I15/2Ǟ4F9/2); 448 (4I15/2Ǟ4F5/2); 488 (4I15/2Ǟ4F7/2); 522 (4I15/2Ǟ4S3/2); 654 (4I15/2Ǟ4F9/2);
798 (4I15/2Ǟ4I9/2); 968 (4I15/2Ǟ4I11/2); 1514, 1488, 1416, 1376 (4I15/2Ǟ4I13/2
)
4-methoxybenzoic acid
phen
254, 294 (πǞπ*)
254, 340 (πǞπ*)
Table 2. Assignments of the emission bands in the visible region of the complexes and the ligands.
Complex
λEx. [nm] (Bandwidth Ex., Bandwidth Em.)
λEm. [nm] (Assignment)
1
345 (3 nm, 3 nm)
360–500 (LMCT), 562 (4G5/2Ǟ6H5/2), 598 (4G5/2Ǟ6H7/2),
644 (4G5/2Ǟ6H9/2
)
2
358 (3 nm, 1 nm)
580 (5D0Ǟ7F0), 592 (5D0Ǟ7F1), 617 (5D0Ǟ7F2), 655 (5D0Ǟ7F3),
703 (5D0Ǟ7F4)
3
4
5
6
347 (1 nm, 1 nm)
392 (3 nm, 3 nm)
356 (3 nm, 3 nm)
354 (3 nm, 3 nm)
297 (3 nm, 1 nm)
344 (3 nm, 3 nm)
490 (5D4Ǟ7F6), 547 (5D4Ǟ7F5), 585 (5D4Ǟ7F4), 623 (5D4Ǟ7F3)
400–500 (LMCT)
370–450 (LMCT)
370–500 (LMCT)
328 (LLCT)
4-methoxybenzoic acid
phen
363, 381, 407 (LLCT)
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New Heterodinuclear Cd–Ln Coordination Complexes
the d block (Cd–L section),[29,30] because the emission band 5D4Ǟ7F4 (585 nm), and 5D4Ǟ7F3 (624 nm).[33,34] The lumi-
is different from that of the free ligands (4-methoxybenzoic nescence of the d block is quenched in the two complexes,
acid and phen) in shape and position; moreover, the emis- and the characteristic emissions of the LnIII ions are en-
sions of the ligands have been quenched. In the emission hanced by the better sensitization of the d block.
spectrum of complex 1 (Figure 5), the other three peaks at
562, 598 and 644 nm are weaker and correspond to
4
4
4G5/2Ǟ6H5/2, G5/2Ǟ6H7/2, and G5/2Ǟ6H9/2 transitions of
the SmIII ion,[14,31] which indicates that the energy transfer
from the d block to the f block is incomplete and most of
the energy is used for luminescence from the d block itself.
Figure 7. The emission spectrum of complex 3 (λEx. = 347 nm,
Bandwidth Ex.: 1 nm, Bandwidth Em.: 1 nm).
Comparing the emission spectra of complexes 1–3, we
can find that the characteristic emission intensity of the
SmIII ion in complex 1 is much weaker than that of the
EuIII ion in 2 and the TbIII ion in 3, because in complex 1,
the luminescence of the d block is not quenched completely,
that is, the energy transfer from the d block (Cd–L section)
to the f block (Sm–L section) is incomplete. We think that,
the efficiency with which the luminescence of the LnIII ion
can be sensitized depends on the matching between the en-
ergy levels of the LnIII ions and the ligand (4-methoxyben-
zoic acid) or the d block. According to “a theory of sensi-
tized luminescence in solids” by Dexter[35] and the state-
ment that the region in which organic ligands can be sensi-
tized most effectively is ∆E = 2100–3200 cm–1 by Sato et al.
and others,[36] the 4-methoxybenzoic acid ligand, having its
πǞπ* transition band at 294 nm (34014 cm–1), can transfer
the energy to the excited states of the LnIII ion in the
30814–31914 cm–1 range (Figure 8). However, the energy
gap between the states in this range and the emissive states
Figure 5. The emission spectrum of complex 1 (λEx. = 345 nm,
Bandwidth Ex.: 3 nm, Bandwidth Em.: 3 nm).
In the emission spectra of complexes 2 and 3 (Figures 6
and 7), only the characteristic emission bands of EuIII and
TbIII appear, while the luminescence of the d block is
quenched completely. The emission spectrum of complex 2,
excited at 358 nm, exhibits the five characteristic emissions
of the EuIII ion. Two intense peaks at 592 and 617 nm are
5
5
assigned to D0Ǟ7F1 and D0Ǟ7F2 transitions, and three
weak peaks at 580, 655, and 703 nm correspond to
5
5
5D0Ǟ7F0, D0Ǟ7F3, and D0Ǟ7F4, respectively.[28,32] The
intensity of the 5D0Ǟ7F2 transition (electric dipole) is
stronger than that of the D0Ǟ7F1 transition (magnetic di-
5
pole), and this indicates that the coordination environment
of the EuIII ion is asymmetric, which is confirmed by the
crystallographic analysis.[3] In the spectrum of complex 3,
excited at 347 nm, the intense emission bands of the TbIII
5
5
(4G5/2, SmIII 1; D0, EuIII 2; D4, TbIII 3) of the LnIII ions
5
5
ion are observed: D4Ǟ7F6 (490 nm), D4Ǟ7F5 (547 nm),
Figure 6. The emission spectrum of complex 2 (λEx. = 358 nm,
Bandwidth Ex.: 3 nm, Bandwidth Em.: 1 nm).
Figure 8. Diagram of the energy levels of complexes 1 (SmIII), 2
(EuIII), 3 (TbIII), 4-methoxybenzoic acid, and the d block.
Eur. J. Inorg. Chem. 2008, 2336–2343
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Y.-X. Chi, S.-Y. Niu, Z.-L. Wang, J. Jin
FULL PAPER
Table 3. The elemental analysis results and the IR spectral band assignments for the complexes.
Elemental Analysis: calcd. for C52H45N2CdLnO16 (found)
1
2
3
4
5
6
C
H
N
Cd
Ln
51.03 (51.33)
3.75 (3.73)
2.33 (2.30)
9.39 (9.24)
12.76 (12.36)
51.06 (51.26)
3.75 (3.72)
2.34 (2.30)
9.40 (9.23)
12.81 (12.48)
50.66 (50.97)
3.73 (3.70)
2.34 (2.29)
9.28 (9.17)
13.16 (12.97)
51.32 (51.59)
3.78 (3.75)
2.36 (2.32)
9.36 (9.28)
12.31 (11.91)
50.44 (50.72)
3.71 (3.68)
2.32 (2.28)
9.24 (9.13)
13.48 (13.39)
50.21 (50.63)
3.72 (3.68)
2.29 (2.27)
9.21 (9.11)
13.88 (13.56)
IR (KBr) [cm–1]
1
2
3
4
5
6
ν(OH)
3428
3434
3426
3433
3443
3433
ν(Ar–H)
ν(CH3)
ν(C=C)
ν(asCOO–)
ν(sCOO–)
ν(Ar–O)
ν(C–C)
3073, 3003
2935, 2836
1532, 1513
1605
1416
1256
3073, 3004
2937, 2837
3073, 3004
2935, 2836
3075, 3004
2935, 2837
3006
3071, 3006
2932, 2837
1535, 1510
1606
1422
1257
2933, 2838
1533, 1514
1606
1423
1258
1552, 1529, 1514 1559, 1530, 1512 1529, 1514
1605
1418
1259
1605
1418
1258
1173
1606
1418
1258
1172
1172
1173
1174
1173
ν(C–N)
1103
1103
1103
1103
1104
1104
ν(C–O)
1030
1029
1029
1030
1030
1030
δ(C–H, Ar)
δ(Ar–H)
ν(Cd–N)
ν(Cd–O)
ν(Ln–O)
1311
862, 786, 728
545
511
420, 379
1312
861, 786, 727
539
508
419, 378
1312
863, 786, 727
540
511
420, 379
1311
860, 786, 727
540
511
420, 377
1312
858, 785, 727
541
511
419, 379
1312
857, 786, 728
543
513
419, 382
is large, so most of the energy from the ligand will be the TbIII ion is the smallest, and the wasted energy is less
wasted in the nonradiative intersystem crossing, and it is in the intersystem crossing, so the characteristic emission
too hard to sensitize the luminescence of the LnIII ion. bands of TbIII in complex 3 are stronger than those of SmIII
However, Figure 8 shows that the energy gap between the in 1 and EuIII in 2. Meanwhile, Figure 8 shows that there
6
emissive state (T1, 23500 cm–1) of the d block and the emis- are six FJ (J = 1/2–11/2) states between the emissive state
sive state of the LnIII ions presents more feasibility for en- (4G5/2) and the ground states (6HJ) of the SmIII ion. When
4
ergy transfer and sensitization. So the d block dominates these excited electrons in the emissive G5/2 state return to
6
the role of sensitizing the luminescence of the LnIII ion in the ground states, they must pass the FJ states. This pro-
the complexes.
cess must cause part of the energy to be wasted, and the
In the following, we will further discuss the sensitization energy transfer from the d block to the f block will be affec-
by the d block with a look at the relationship between the ted, so the luminescence of the SmIII ion is weaker and the
UV/Vis/NIR absorption spectra and the emission spectra luminescence of the d block still exists in complex 1.
of the complexes. In the absorption spectrum of complex 1
Complexes 4–6 mainly show luminescence in the NIR
(Table 3, Figure S1), the 6H5/2Ǟ4G5/2 transition band of the region, while in the visible region they only show weak
SmIII ion is the weakest, while in the absorption spectra of emissions (LMCT) of the d block (Figures S7–S9 in the
complexes 2 and 3 (Table 3, Figures S2 and S3), the Supporting Information).
7F0Ǟ5D0 transition of the EuIII ion is forbidden, as is the
Luminescence Properties in the NIR Region
7F6Ǟ5D4 transition of the TbIII ion. However, after the het-
erodinuclear Cd–Ln complexes are formed, the d block can
At room temperature, the NIR emission spectra of com-
sensitize the luminescence of the LnIII ions, that is, the en- plexes 4–6 in the solid state were measured by an FLS 920
ergy can be transferred from the emissive state (T1) of the combined time-resolved and steady-state fluorescence spec-
5
d block to the emissive states (4G5/2, SmIII 1; D0, EuIII 2; trometer with a Ge detector. The emission spectrum of
5D4, TbIII 3) of the LnIII ions. When these excited electrons complex 4 presents two emission bands at 913 nm and
return to the ground states from the emissive states of the 992 nm (λEx. = 657 nm) (Figure 9), which can be assigned
4
4
LnIII ions, the energy being released by radiation, com- to F3/2Ǟ4I9/2 and F3/2Ǟ4I11/2 transitions of the NdIII ion,
plexes 1–3 will show the corresponding characteristic emis- respectively. The two emission bands of complex 4 show
sions of the LnIII ions. If the d block does not sensitize shifts relative to the bands of the reported Nd com-
the f block, the emissions of the LnIII ions, but only those plexes[37,38] and the theoretical values.[39] The absorption
depending on electronic transitions of the LnIII ions them- bands of the NdIII ion in complex 4 (Figure S4 in the Sup-
selves, will not appear in the spectra of complexes 1–3. So, porting Information) also present shifts relative to those
the characteristic emissions of the f block mainly profit of the isolated NdIII ion, for which the shift of the
from the sensitization of the d block in complexes 1–3. In 4I9/2Ǟ4F3/2 transition corresponding to 864 nm is known
addition, as it is shown in Figure 8, the energy gap between (the theoretical value is 899 nm) to have a broad band (840–
5
4
the emissive state (T1) of the d block and the D4 state of 920 nm). This indicates that the F3/2 state of the NdIII ion
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New Heterodinuclear Cd–Ln Coordination Complexes
generates the shift and splittings, so the emission bands (992 ing the UV/Vis/NIR absorption spectrum of complex 6
and 913 nm) also present shifts in complex 4. This is caused (Figure S6), we can find that the absorption band corre-
by the formation of the heterometallic d–f coordination sponding to the 4I15/2Ǟ4I13/2 transition is broad and in-
complex.
cludes two split peaks: the strong peak (split into 1488 and
1514 nm) and the weak shoulder peak (split into 1376 and
1416 nm). So when the excited electrons return to the
ground state (4I15/2) from the split state (4I13/2), complex 6
shows two emission bands both of which are split (Fig-
ure 11). In complex 6, the observed emission spectrum is
consistent with the absorption spectrum, which further
proves the reliability of the assignments for the emission
spectrum of complex 6.
Figure 9. The NIR emission spectrum of complex 4 (λEx.
657 nm).
=
For complex 5 with λEx. = 985 nm, the emission at
5
1001 nm can be assigned to the F5Ǟ5I7 transition of the
HoIII ion, while both emission bands at 1378 and 1513 nm
5
can be assigned to the F5Ǟ5I6 transition,[40] which is at-
5
tributed to the splittings of the I6 state. This can be illus-
trated by the absorption spectrum of complex 5 (Figure S5):
5
the absorption band corresponding to the I8Ǟ5I6 transi-
Figure 11. The NIR emission spectrum of complex 6 (λEx.
=
985 nm).
tion is broad (1100–1200 nm), and the strongest peak lies
at 1144 nm with a shoulder at 1180 nm, indicating that the
5I6 state of the HoIII in complex 5 generates splittings. So
both of the two observed emissions at 1378 and 1513 nm
In comparison with the results in some relevant papers,
the NIR emission spectra of complexes 4–6 present split-
tings or shifts,[42,43] which are mainly attributed to the for-
mation of the heterodinuclear Cd–Ln coordination com-
plexes. From the crystal structures of the complexes, the
Cd···Ln separation of the molecule is very small (in the
3.66–3.72 Å region), thus the 4d orbitals of the CdII ion
may influence the 4f orbitals of the LnIII ion, which proba-
bly causes part of the f levels of the LnIII ion to be adjusted.
5
are assigned to the F5Ǟ5I6 transition (Figure 10).
Figure 10. The NIR emission spectrum of complex 5 (λEx.
985 nm).
=
In some papers, ErIII complexes usually only exhibit one
emission band at about 1535 nm corresponding to the
4I13/2Ǟ4I15/2 transition,[1,41] whereas complex 6 exhibits two
emission bands at 1343 and 1515 nm (λEx. = 985 nm) (Fig-
ure 11). We think that both of the emission bands should
be assigned to the 4I13/2Ǟ4I15/2 transition, because the
Figure 12. Diagram of the NIR luminescence processes of com-
4I13/2 state of the ErIII ion in complex 6 is split. By observ- plexes 4, 5, and 6.
Eur. J. Inorg. Chem. 2008, 2336–2343
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2341

Y.-X. Chi, S.-Y. Niu, Z.-L. Wang, J. Jin
FULL PAPER
As a result, some of the levels of the LnIII ions in complexes
4–6 present splittings and shifts that are clearly visible in
the UV/Vis/NIR spectra. So the corresponding emission
bands of complexes 4–6 exhibit shifts or splittings in the
NIR region. Figure 12 gives an illustration of the possible
emission processes of complexes 4–6.
Experimental Section
Materials and Methods: Ln(NO3)3·nH2O were prepared by dissolv-
ing the corresponding lanthanide oxide (99.99%) in an excess of
nitric acid. Cd(NO3)2·4H2O, phen, and 4-methoxybenzoic acid
were AR and used as purchased. The elemental analyses were car-
ried out with a PE-240C elemental analyzer and PLASMA-II ICP,
and the structures of the complexes were determined with a Bruker
SMART APEX II X-diffractometer. The IR spectra were recorded
with a JASCO FT/IR-480 spectrometer with pressed KBr and the
UV/Vis/NIR spectra (diffuse reflectance) were determined with a
JASCO V-570 spectrometer. The excitation and emission spectra
were measured with a JASCO FP-6500 fluorescence spectrometer,
and the NIR spectra were determined with a FLS 920 combined
time-resolved and steady-state fluorescence spectrometer with a Xe
lamp as the light source and a Ge detector.
Conclusions
Six new heterodinuclear coordination complexes (Cd–Ln
where Ln = Sm, Eu, Tb, Nd, Ho, Er) were synthesized by
the hydrothermal method, and their structures were deter-
mined by single-crystal X-ray diffraction (except for com-
plex 4). They have the same molecular structure and two
neighboring molecules are connected by hydrogen bonds or
weak interactions to form dimer crystals. The study of the
photophysical properties indicates that the d block influ-
ences the f block in two aspects: (i) In these complexes,
Syntheses of the Complexes
Synthesis of SmCd(C8H7O3)5(phen)(H2O) (1): A solution of 4-
methoxybenzoic acid (0.14 g, 1 mmol) in MeOH (8 mL) and H2O
(5 mL) was adjusted to pH = 6 with NaOH (1 molL–1) and HAc.
the d block can sensitize the luminescence of the f block. A solution of Cd(NO3)2·4H2O (0.08 g, 0.25 mmol) and Sm-
(NO3)3·nH2O (0.12 g, 0.25 mmol) in H2O (8 mL) was added to the
above solution while stirring and heating, but there was a white
precipitate, which was dissolved by adding HAc. After the solution
of phen (0.05 g, 0.25 mmol) in EtOH (5 mL) was added to the
above mixed solution, the final solution was sealed in a Teflon-
lined autoclave and heated at 90 °C for 140 h. After cooling to am-
bient temperature, the reaction solution was filtered, and yellow
crystals were obtained, washed out with the mother liquor and air-
dried.
Especially, complexes 1–3 have intense characteristic emis-
sions in the visible region, owing to efficient sensitization
of the f block by the d block. (ii) In these heterodinuclear
Cd–Ln complexes, the Cd···Ln separation is smaller, so the
4d orbitals of the CdII ion may influence the 4f orbitals of
the LnIII ion, which probably causes the intramolecular
levels to be adjusted. So some of the f levels of the LnIII
ion are adjusted, and this is reflected in the UV/Vis/NIR
spectra of the complexes. Especially in complexes 4–6, some
absorption bands show shifts or splittings relative to those
of the isolated LnIII ion. So the corresponding emission
bands of complexes 4–6 present shifts or splittings relative
to those of some reported Ln complexes.
The preparation methods of complexes 2–6 are similar to that of
complex 1, except that the corresponding Ln(NO3)3·nH2O were
used instead of Sm(NO3)3·nH2O, and the time of reaction and
crystallization was different. The results of elemental analyses and
the assignments for the IR spectra of all six complexes are listed in
Table 4. Crystal data and structure refinement for the complexes.
Complex
1
2
3
4
5
6
Empirical formula
Formula mass
Crystal system
Space group
T [K]
C52H45N2O16CdSm C52H45N2O16CdEu
C52H45N2O16CdTb C52H45N2O16CdNd C52H45N2O16CdHo C52H45N2O16CdEr
1216.65
triclinic
P1
1218.26
triclinic
P1
1225.22
triclinic
P1
1210.5
1231.23
triclinic
P1
1233.6
triclinic
triclinic
¯
¯
¯
¯
¯
¯
P1
P1
293(2)
293(2)
293(2)
293(2)
293(2)
293(2)
λ [Å]
a [Å]
b [Å]
c [Å]
α [°]
β [°]
0.71073
12.380(2)
13.899(2)
16.804(4)
108.446(3)
104.769(3)
104.394(2)
2475.4(9)
2
0.71073
12.3489(16)
13.9651(18)
16.776(2)
108.7080(10)
104.4330(10)
104.313(2)
2479.2(6)
2
0.71073
12.3443(19)
13.945(2)
16.752(4)
108.548(2)
104.561(2)
104.370(2)
2471.0(8)
2
0.71073
12.323(3)
13.950(3)
16.030(3)
84.41(3)
67.96(3)
75.47(3)
2472.5(9)
0.71073
12.3358(12)
13.9507(13)
16.696(2)
108.5700(10)
104.499(2)
104.3450(10)
2463.3(5)
2
0.71073
12.334(2)
13.941(2)
16.652(4)
108.530(3)
104.539(3)
104.338(2)
2455.0(9)
2
γ [°]
V [Å3]
Z
Dcalcd. [gcm–3]
µ [mm–1]
F (000)
1.632
1.678
1218
1.632
1.756
1220
1.647
1.924
1224
1.660
2.100
1228
1.666
2.205
1226
θ range [°]
Reflns. collected
1.97 to 29.17
15555
1.82 to 29.05
15569
1.83 to 29.09
15452
1.83 to 29.04
15459
1.83 to 29.05
15348
Ind. Reflns. (Rint
)
11438 (0.0161)
11418 (0.0192)
11418/2/761
1.015
11372 (0.0213)
11372/0/693
1.030
11362 (0.0146)
11362/2/657
1.018
11276 (0.0162)
11276/0/721
1.030
Data/restraints/parameters 11438/2/657
GOF on F2
1.018
R1 [I Ͼ 2σ (I)]
wR2 [IϾ 2σ (I)]
R1 [all data]
0.0329
0.0722
0.0446
0.0790
0.0356
0.0753
0.0500
0.0824
0.0349
0.0835
0.0452
0.0901
0.0286
0.0639
0.0363
0.0682
0.0306
0.0688
0.0397
0.0736
wR2 [all data]
2342
www.eurjic.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2008, 2336–2343

New Heterodinuclear Cd–Ln Coordination Complexes
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Published Online: April 2, 2008
Eur. J. Inorg. Chem. 2008, 2336–2343
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2343