Synthesis and luminescent properties of lanthanide complexes with structurally related novel multipodal ligands

Article abstract of DOI:10.1007/s10895-009-0571-y

To explore the relationship between the structure of the ligands and the luminescent properties of the lanthanide complexes, a series of lanthanide nitrate complexes with two novel structurally related multipodal ligands, 1,3-bis{[(2'-(2-picolylaminoformy

Full text of DOI:10.1007/s10895-009-0571-y

J Fluoresc (2010) 20:493–498
DOI 10.1007/s10895-009-0571-y
ORIGINAL PAPER
Synthesis and Luminescent Properties of Lanthanide Complexes
with Structurally Related Novel Multipodal Ligands
Qing Wang & Xu-Huan Yan & Wei-Sheng Liu &
Min-Yu Tan & Yu Tang
Received: 23 September 2009 /Accepted: 11 November 2009 /Published online: 21 November 2009
#
Springer Science + Business Media, LLC 2009
Abstract To explore the relationship between the structure of
the ligands and the luminescent properties of the lanthanide
complexes, a series of lanthanide nitrate complexes with two
novel structurally related multipodal ligands, 1,3-bis{[(2’-(2-
picolylaminoformyl))phenoxyl]methyl}benzene (LI) and 1,2-
bis{[(2’-(2-picolylaminoformyl))phenoxyl]methyl}benzene
(LII), have been synthesized and characterized by elemental
analysis, infrared spectra and molar conductivity measure-
ments. At the same time, the luminescent properties of the
Eu(III) and Tb(III) nitrate complexes in solid state and the Tb
(III) nitrate complexes in solvents were investigated at room
temperature. Under the excitation of UV light, these
complexes exhibited characteristic emissions of central metal
ions. The lowest triplet state energy levels T1 of these ligands
both match better to the lowest resonance energy level of Tb
(III) than to Eu(III) ion. The influence of the structure of the
ligands on the luminescent intensity of the complexes was
also discussed.
unique photophysical properties (microsecond to milli-
second lifetimes, characteristic and narrow emission bands,
and large Stokes shifts) [1–7]. Trivalent lanthanide ions are
fascinating in luminescence sources for their high color
purity and relatively long lifetimes of the excited states
arising from transitions within the partially filled 4f shell of
the Ln(III) ions. Among the luminescent lanthanide com-
plexes, Eu(III) and Tb(III) compounds show strong lumi-
nescence in the visible region, making them to be good
candidates for fluoroimmunoassays and structural probes
[1, 2, 8, 9].
The Laporte-forbidden f-f transitions have weak dipole
strengths, and the excited states may be easily quenched by
high-energy vibrations, particularly O-H oscillators from
water molecules both in the inner and outer coordination
spheres [10–12]. To achieve strong luminescence, con-
current control of solvation and hydration around the
lanthanide ion, and its high coordination number, should
be incorporated into the ligand design. Taking into account
the strong coordination capability of multipodal ligands and
the large coordination numbers (commonly 8–10) required
for Ln(III) ions, it is therefore not surprising that this type
ligands are preferred “antenna” for lanthanides [13–15].
This “antenna effect” involves energy transfer from excited
π molecular orbital of conjugated ligands to Ln(III) ions.
Furthermore, the complexation with a ligand provides
lanthanides with a certain degree of protection from the
surrounding water, hence increasing their luminescence
quantum yields. Our group has been interested in employ-
ing self-assembly strategies for luminescent lanthanide
complexes, and has recently used multipodal ligands as
sensitizers for lanthanide luminescence. As a part of our
systematic studies, two structurally related multipodal
ligands, 1,3-bis{[(2’-(2-picolylaminoformyl))phenoxyl]
methyl}benzene (LI) and 1,2-bis{[(2’-(2-picolylamino-
.
.
Keywords Multipodal ligands Lanthanide complexes
.
Synthesis Luminescent properties
Introduction
Over the past decades, there has been much interesting in
design and synthesis of lanthanide complexes due to their
:
:
:
:
Q. Wang X.-H. Yan W.-S. Liu M.-Y. Tan Y. Tang (*)
Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province,
State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering,
Lanzhou University,
Lanzhou 730000, People’s Republic of China
e-mail: tangyu@lzu.edu.cn

494
J Fluoresc (2010) 20:493–498
Table 1 Analytical and
molar conductance data for the
complexes 1–6 (calculated
values in parentheses)
Complexes
Analysis(%)
C
Λm(cm2·Ω−1·mol−1
)
H
N
1
2
3
4
5
6
44.11(43.81)
43.87(43.56)
44.13(43.48)
44.21(43.81)
44.01(43.56)
43.94(43.48)
3.96(3.68)
3.79(3.66)
3.52(3.65)
3.56(3.68)
3.54(3.66)
3.51(3.65)
10.96(10.52)
10.82(10.46)
10.95(10.44)
10.64(10.52)
10.58(10.46)
10.54(10.44)
86.1
86.8
92.1
85.3
90.6
86.9
formyl))phenoxyl]methyl}benzene (LII), have been
designed and prepared successfully. A series of lanthanide
coordination compounds have been obtained, generally
formulated as [LnLI(NO3)2](NO3)·2H2O (Ln = Eu(1), Gd
(2), Tb(3)) and [LnLII(NO3)2](NO3)·2H2O (Ln = Eu(4), Gd
(5), Tb(6)). Under the excitation of UV light, Eu and Tb
complexes exhibit characteristic emission of corresponding
lanthanide ions. The lowest triplet state energy levels of the
ligands were calculated from the phosphorescence spectra
of the Gd complexes at 77 K. The results indicate that the
triplet state energy levels of the ligands both match better to
the resonance level of Tb(III) than Eu(III), and the structure
of the ligands affects the luminescence characteristics of the
complexes.
out with a DDSJ-308 type conductivity bridge using
10−3 mol·L−l solutions in methanol at 25 °C. 1H NMR
spectra were measured on a Varian Mercury-300 M
spectrometer in d-chloroform solutions, with TMS as
internal standard. Luminescence spectra and phosphores-
cence spectra were obtained on a Hitachi F-4500 fluores-
cence spectrophotometer. The quantum yield of the terbium
samples were determined by an absolute method using an
integrating sphere (150 mm diameter, BaSO4 coating) from
Edinburgh Instruments FLS920. The lifetime measurements
were measured on an Edinburgh Instruments FLS920
Fluorescence Spectrometer with Nd pumped OPOlette laser
as the excitation source.
Synthesis of the ligands LI, LII
Experimental section
The synthetic route for the ligands (LI, LII) is shown in
Scheme 1. Picolylsalicylamide was prepared according to
the literature methods [16]. Picolylsalicylamide (2 mmol)
and potassium carbonate (2 mmol) were refluxed in
acetone (25 cm3) for 30 min, then the o-Xylylene
dibromide or m-xylylene dibromide (1 mmol) was added
to the solution. The reaction mixture was refluxed
for 12 h. After cooling down, the mixture was filtered
and evaporated under reduce pressure to give the
crude product, which was purified by column chromatog-
raphy (petroleum ether / ethyl acetate, 2:1) on silica gel.
LI, yield 89%, m. p. 112–113 °C. Anal. Calcd for
Materials and methods
o-Xylylene dibromide and m-xylylene dibromide were
obtained from Alfa Aesar Co. The other commercially
available chemicals were of A.R. grade and were used
without further purification. Carbon, nitrogen and hydrogen
were determined on an Elementar Vario EL (see Table 1).
The FT-IR spectra were recorded on a Nicolet 360 FT-IR
instrument using KBr discs in the 4,000−400 cm−1 region
(see Table 2). Conductivity measurements were carried
Table 2 The most important IR
bands for the ligands (LI, LII)
−
Compounds ν(C = O) ν(Ar-O-C) ν(NO3
)
and the complexes 1–6 (cm−1
)
ν1
ν4
ν2
ν3
ν1–ν4 ν0
ν6
ν5
LI
1
1,645
1,611
1,612
1,613
1,642
1,609
1,609
1,609
1,116
1,112
1,112
1,112
1,103
1,109
1,109
1,109
1,489 1,315 1,029 815
1,491 1,315 1,029 815
1,491 1,315 1,029 815
174
176
176
1,384 843 757
1,384 844 757
1,384 840 757
2
3
LII
4
1,479 1,296 1,031 816
1,480 1,297 1,031 816
1,479 1,297 1,031 816
183
183
182
1,384 840 757
1,384 840 757
1,384 840 757
5
6

J Fluoresc (2010) 20:493–498
495
Scheme 1 The synthetic route for the ligands (LI=1,3-bis{[(2’-(2-picolylaminoformyl))phenoxyl]methyl}benzene, LII = 1,2-bis{[(2’-(2-
picolylaminoformyl))phenoxyl]methyl}benzene)
C34H30N4O4: C, 73.16; H, 5.42; N, 10.04; Found: C,
73.59; H, 5.18; N, 9.97%. H NMR (CDCl3, 300 MHz):
Result and discussion
1
4.73(d, 4H), 5.17(s, 4H), 6.97(d, 2H), 7.03–7.12(m, 4H),
7.23–7.58(m, 10H), 8.24(dd, 2H), 8.30(d, 2H), 8.85(s, 2H)
ppm.
Analytical data for the complexes, listed in Table 1, indicate
that the complexes all conform to a 1:1 metal-to-ligand
stoichiometry, [LnL(NO3)3]·2H2O (Ln = Eu, Gd, Tb; L =
LI, LII). All complexes are soluble in DMF, DMSO,
methanol, ethanol, acetonitrile and acetone, but slightly
soluble in THF and ethyl acetate. Conductivity measure-
ments for these complexes in methanol solution indicate
that all complexes act as a 1:1 type electrolyte [17],
implying that two nitrate groups are in coordination sphere.
So the formula of these complexes could be indicated as
[EuLI(NO3)2](NO3)·2H2O (1), [GdLI(NO3)2](NO3)·2H2O
(2), [TbLI(NO3)2](NO3)·2H2O (3) (LI = 1,3-bis{[(2’-
(2-picolylaminoformyl))phenoxyl]methyl}benzene), and
[EuLII(NO3)2](NO3)·2H2O (4), [GdLII(NO3)2](NO3)·2H2O
(5) and [TbLII(NO3)2](NO3)·2H2O (6) (LII = 1,2-bis{[(2’-
(2-picolylaMinoformyl))phenoxyl]methyl}benzene).
LII, yield 65%, m. p. 129–130 °C. Anal. Calcd for
C34H30N4O4: C, 73.16; H, 5.42; N, 10.04; Found: C, 73.23;
1
H, 5.33; N, 10.00%. H NMR (CDCl3, 300 MHz): 4.66(d,
4H), 5.32(s, 4H), 6.93(d, 2H), 6.99–7.09(m, 4H), 7.17(d, 2H),
7.29–7.35(m, 4H), 7.44–7.55(m, 4H), 8.16–8.21(m, 4H), 8.71
(s, 2H) ppm.
Preparation of the lanthanide complexes
0.1 mmol lanthanide nitrates was dissolved in 5 cm3 of
ethyl acetate, then the solution of 0.1 mmol ligand in 5 cm3
of ethyl acetate was added dropwise. The mixture was
stirred at room temperature for 6 h. And then the
precipitated solid complex was filtered, washed with ethyl
acetate, dried in vacuum over P4O10 for 48 h and submitted
for elemental analysis, yield 70–85%.
The main infrared bands of the ligands and their
complexes are presented in Table 2. The IR spectra of the
ligands LI and LII are shown in Fig. 1. The complexes
Fig. 1 The IR spectra of the ligands LI (blue) and LII (red)

496
J Fluoresc (2010) 20:493–498
possessing the same ligand have the similar IR spectra, of
which the characteristic bands have similar shifts, suggest-
ing that they have a similar coordination structure.
1–6, which indicate that free nitrate groups (D3h) are
present, in agreement with the results of the conductivity
experiments. Furthermore, both elemental analysis and IR
data are measured in solid state. In IR spectra, broad bands
at about 3,370 cm−1 are ascribed to the vibration of the
water molecules [19], thus we may deduced that the
complexes contain water molecules, which is in agreement
with the results of the elemental analysis.
To examine the ability of the ligands to be antenna
groups for sensitized luminescence from lanthanides, we
measured the luminescent properties of the Eu3+, Tb3+
complexes with these two ligands. The luminescence
spectra of the two ligands and their complexes in solid
state (the excitation and emission slit widths were 1.0 nm)
and [TbL(NO3)2](NO3)·2H2O (L = LI, LII) in acetonitrile,
acetone, methanol, ethanol and DMF solutions (concentra-
tion: 1.0×10−3 mol L−1, the excitation and emission slit
widths were 2.5 nm) were recorded at room temperature.
Excited by the absorption band at 327 nm and 325 nm,
the “free” ligands LI and LII exhibit broad emission bands
(λmax=445 nm, λmax = 447 nm) in solid state respectively.
It can be seen from Fig. 2 that these four complexes 1, 3, 4,
The characteristic bands of the ν(C = O), ν(C-O-C) of
free ligand LI and LII are shown at 1,645 cm−1
,
1,116 cm−1, and 1,642 cm−1, 1,103 cm−1 respectively. In
the IR spectra of the complexes, the low-energy bands
remain unchanged, but the high-energy bands of complexes
1–3 red shift to about 1,612 cm−1 (∆ν=33 cm−1) and
complexes 4–6 red shift to about 1,609 cm−1 (∆ν=33 cm−1)
as compared to the free ligand LI and LII respectively, thus
indicating that only the oxygen atoms of C = O take part in
coordination to the lanthanide ions.
The absorption bands of complexes 1–3 assigned to the
coordinated nitrate groups (C2v) are observed at about
1,490 cm−1 (ν1), 1,315 cm−1 (ν4), 1,029 cm−1 (ν2) and
815 cm−1 (ν3) [18]. The separation of two strongest
frequency bands |ν1–ν4| lies about 175 cm−1, clearly
−
establishing that the NO3 groups in the solid complexes
coordinate to the lanthanide ions as bidentate ligands [19].
In addition, three absorptions at 1,384 cm−1, 840 cm−1 and
757 cm−1 can be seen clearly in the spectra of complexes
Fig. 2 Emission spectra of the complex 1 a, 3 b, 4 c, 6 d in solid state

J Fluoresc (2010) 20:493–498
497
Fig. 3 Emission spectra of the complex 3 a, 6 b in different solutions at room temperature
and 6 show the characteristic emissions of Eu3+ and Tb3+.
This indicates that the two ligands are good organic
chelators to absorb and transfer energy to the lanthanide
ions. Notably, in the spectra of Eu3+ complexes, the 5D0 →
The fluorescence quantum yield Φ of the complex 6 in
solid state was found to be 41.1 0.1% using an integrating
sphere, which is two times more than the complex 3 (15.8
0.1%). The luminescence decay of the complexes 3 and 6 are
best described by single exponential processes with signifi-
cantly longer lifetimes of τ=1.215 0.001 ms and τ=1.372
0.001 ms, indicating the presence of one distinct emitting
species, respectively. We deduced that the electrostatic factors
in the ligand-metal bonding, which may be affected by the
different position of the substituents in the ligands, influenced
the triplet state energy level of the ligands and made the
fluorescence quantum yield of the LII complex higher.
We can also see from Fig. 3 that in the five solvents, the
two complexes 3 and 6 have the strongest luminescence in
acetonitrile solutions, and the luminescence intensities
become weaker from ethanol, acetone, methanol and
DMF solution similarly. This is due to the coordinating
effects of solvents, namely solvate effect [23]. Together
with the raising coordination abilities of acetonitrile,
ethanol, acetone, methanol, and DMF for the lanthanide
ions, the energy transfer from the triplet excited states of the
ligands LI and LII to the emitting level of the Tb3+ could
not be carried out perfectly because the oscillatory motions
of the entering molecules may consume more energy.
Otherwise, we deduced that the acetonitrile molecules
coordinated to the lanthanide ions can also absorb and
transfer the energy to the central ions.
7F2 transition is much more intense than the D0 → F1
transition, showing that the Eu(Ш) ion is not in a centro-
symmetric coordination site [20]. Intramolecular energy
transfer from the triplet state of the ligand to the resonance
level of the Ln(Ш) ion is one of the most important
processes influencing the luminescence quantum yields of
Ln(Ш) complexes [21]. The energy difference between the
triplet state energy level of the ligand and the lowest excited
state level of Ln(Ш) cannot be too large or too small. So
considering the emission spectra of these four complexes 1,
3, 4, and 6, it can be concluded that the triplet energy of
ligand LІ and LП are in an appropriate level to Tb3+ in the
complexes, which make the energy transition from the
ligands to Ln(Ш) more easily in these two Tb3+ complexes.
In order to acquire the triplet excited states T1 of the ligands,
the phosphorescence spectra of the Gd(III) complexes were
measured at 77 K in a methanol-ethanol mixture (V:V=1:1).
The triplet state energy levels T1 of the ligands LI and LII
in the complexes, which were calculated from the shortest
wavelength phosphorescence band [22] of the Gd com-
plexes, are 24,667 and 24,814 cm−1, respectively. These
energy levels are above the lowest excited resonance level
5D0 of Eu (III) (17,286 cm−1) and 5D4 (20,545 cm−1) of Tb
(III). Thus the absorbed energy could be transferred from
ligands to the Eu or Tb ions. And this result also indicates
that the triplet state energy levels T1 of the ligands LI and
LII match better to the lowest resonance level of Tb(III)
(Δν=4,122, 4,269 cm−1) than to Eu(III) (Δν=7,381,
7,528 cm−1) ion. The larger energy gap between the ligands
triplet and the europium ion excited states led to observa-
tion of stronger sensitization of terbium complexes than the
europium complexes.
5
7
Conclusions
In summary, a series of lanthanide complexes carrying two
novel structurally related multipodal ligands, 1,3-bis{[(2’-
(2-picolylaminoformyl))phenoxyl]methyl}benzene (LI) and
1,2-bis{[(2’-(2-picolylaminoformyl))phenoxyl]methyl}-
benzene (LII) as antenna have been synthesized and

498
J Fluoresc (2010) 20:493–498
characterized. When the ligands (LI and LII) formed the
lanthanide complexes, obvious changes in IR spectra were
observed. In the complexes, lanthanide ions were coordi-
nated to the C = O oxygen atoms of the ligands. Comparing
the luminescence spectra of the europium(III) and terbium
(III) complexes, we found that these novel multipodal
ligands in Tb complexes are more effective in energy-
transfer than in Eu complexes. The lowest triplet state
energy levels of the ligands LI and LII indicate that the
triplet state energy levels of the ligands match better to the
resonance level of Tb3+ than Eu3+. Under the same
condition, the complex 6 give longer lifetimes and higher
quantum efficiencies than complex 3. So we may deduce
that the position of the substituents of the ligands is very
essential in determining the luminescent properties of the
lanthanide complexes by influencing the electrostatic
factors in the ligand-metal bonding.
8. Montalti M, Prodi L, Zaccheroni N, Charbonnière L, Douce L,
Ziessel R (2001) A luminescent anion sensor based on a europium
hybrid complex. J Am Chem Soc 123:12694–12695
9. Bünzli JCG, Piguet C (2002) Lanthanide-containing molecular
and supramolecular polymetallic functional assemblies. Chem
Rev 102:1897–1928
10. Beeby A, Clarkson IM, Dickins RS, Faulkner S, Parker D, Royle
L, de Sousa AS, Williams JAG, Woods MJ (1999) Non-radiative
deactivation of the excited states of europium, terbium and
ytterbium complexes by proximate energy-matched OH, NH and
CH oscillators: an improved luminescence method for establishing
solution hydration states. J Chem Soc Perkin Trans 23:493–503
11. Maas H, Currao A, Calzaferri G (2002) Encapsulated lanthanides
as luminescent materials. Angew Chem Int Ed 41:2495–2497
12. Binnemans K (2009) Lanthanide-based luminescent hybrid
materials. Chem Rev. In press: doi: 10.1021/cr8003983.
13. Renaud F, Piguet C, Bernardinelli G, Bünzli J-CG, Hopfgartner G
(1999) Nine-coordinate lanthanide podates with predetermined
structural and electronic properties: facial organization of unsym-
metrical tridentate binding units by a protonated covalent tripod. J
Am Chem Soc 121:9326–9342
14. Gade LH (2002) Tripodal amido complexes: molecular “claws” in
main group and transition metal chemistry. Acc Chem Res
35:575–582
Acknowledgements This work was supported by the National
Natural Science Foundation of China (Project No. 20401008) and
the program for New Century Excellent Talents in University (NCET-
06-0902).
15. Hamann CS, Zelewsky AV, Barbieri A, Barigelletti F, Muller G,
Riehl JP, Neels A (2004) Diastereoselective formation of chiral
tris-cyclometalated iridium (III) complexes: characterization and
photophysical properties. J Am Chem Soc 126:9339–9348
16. Michio K (1976) The dual fluorescence nature of some n-
monosubstituted salicylamides. Bull Chem Soc Jpn 49:2679–2682
17. Geary WJ (1971) The use of conductivity measurements in
organic solvents for the characterization of coordination com-
pounds. Coord Chem Rev 7:81–122
18. Carnall W, Siegel S, Ferrano J, Tani B, Gebert E (1973) New
series of anhydrous double nitrate salts of the lanthanides.
Structural and spectral characterization. Inorg Chem 12:560–564
19. Nakamoto K (1978) Infrared and raman spectra of inorganic and
coordination compounds, 4th edn. Wiley, New York
20. Liu WS, Tan MY, Wang X, Zhang SY (1990) Study on the ternary
coordination of compounds of lanthanide elements with 1, 10-
phenanthroline-N-oxide and acetylacetore. Acta Chim Sin 48:
1090–1095
21. Latva M, Takalo H, Mukkala VM, Marachescu C, Rodriguez-Ubis
JC, Kankare J (1997) Correlation between the lowest triplet state
energy level of the ligand and lanthanide(III) luminescence
quantum yield. J Lumin 75:149–169
References
1. Yam VWW, Lo KKW (1998) Recent advances in utilization of
transition metal complexes and lanthanides as diagnostic tools.
Coord Chem Rev 184:157–240
2. Parker D, Dickins RS, Puschmann H, Crossland C, Howard JAK
(2002) Being excited by lanthanide coordination complexes: aqua
species, chirality, excited-state chemistry, and exchange dynamics.
Chem Rev 102:1977–2010
3. Faulkner S, Pope SJA (2003) Lanthanide-sensitized lanthanide
luminescence: terbium-sensitized ytterbium luminescence in a
trinuclear complex. J Am Chem Soc 125:10526–10527
4. Xu G, Wang ZM, He Z, Lü Z, Liao CS, Yan CH (2002) Synthesis
and structural characterization of nonanuclear lanthanide complexes.
Inorg Chem 41:6802–6807
5. Rajapakse HE, Reddy DR, Mohandessi S, Butlin NG, Miller LW
(2009) Luminescent terbium protein labels for time-resolved
microscopy and screening. Angew Chem Int Ed 48:4990–4992
6. Montgomery CP, Murray BS, New EJ, Pal R, Parker D (2009)
Cell-penetrating metal complex optical probes: targeted and
responsive systems based on lanthanide luminescence. Acc Chem
Res 42:925–937
22. Gutierrez F, Tedeschi C, Maron L, Daudey JP, Poteau R, Azema J,
Tisnès P, Picard C (2004) Quantum chemistry-based interpreta-
tions on the lowest triplet state of luminescent lanthaides
complexes. Part 1. Relation between the triplet state energy of
hydroxamate complexes and their luminescence properties. J
Chem Soc Dalton Trans 9:1334–1347
7. Mooer EG, Samuel APS, Raymond KN (2009) From antenna to
assay: lessons learned in lanthanide luminescence. Acc Chem Res
42:542–552
23. Liu HQ, Cheung TC, Che CM (1996) Cyclometallated platinum
(II) complexes as luminescent switches for calf-thymus DNA.
Chem Commun 9:1039–1040