Synthesis, structure and spectroscopic properties of rare earth complexes with a new aryl amide 2,2′-bipydine derivative

Article abstract of DOI:10.1016/j.saa.2007.03.007

Solid complexes of rare earth nitrates and picrates with a new aryl amide ligand 3.3′-bis(benzylamido)-2,2′-bipyridine (L) were synthesized and characterized by elemental analysis, IR and molar conductivity measurements. The molecular structures of the complex [TbL₂(NO₃)₃H₂O]·2H₂O have been determined by single-crystal X-ray diffraction. The fluorescent properties of the Eu(III) and Tb(III) nitrates and picrates complexes in solid state were also investigated in detail. Under the excitation, these complexes exhibited characteristic emissions of europium and terbium ions. It is worth noting that the nature of the anion has a great effect upon the composition of the complexes as well as emission properties of them.

Full text of DOI:10.1016/j.saa.2007.03.007

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 49–55
Synthesis, structure and spectroscopic properties of rare earth
complexes with a new aryl amide 2,2ꢀ-bipydine derivative
Xue-Qin Songa,b, Jiang-Rong Zhenga, Wei-Sheng Liua,∗, Zheng-Hua Jua
a College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry,
Lanzhou university, Lanzhou 730000, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
Received 4 January 2007; received in revised form 14 February 2007; accepted 9 March 2007
Abstract
Solid complexes of rare earth nitrates and picrates with a new aryl amide ligand 3.3ꢀ-bis(benzylamido)-2,2ꢀ-bipyridine (L) were synthesized and
characterized by elemental analysis, IR and molar conductivity measurements. The molecular structures of the complex [TbL2(NO3)3H2O]·2H2O
have been determined by single-crystal X-ray diffraction. The fluorescent properties of the Eu(III) and Tb(III) nitrates and picrates complexes
in solid state were also investigated in detail. Under the excitation, these complexes exhibited characteristic emissions of europium and terbium
ions. It is worth noting that the nature of the anion has a great effect upon the composition of the complexes as well as emission properties of
them.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Bipyridine; Amide ligand; Rare earth complex; Fluorescence properties
1. Introduction
nate to rare earth metals should be able to absorb energy and
transfer it efficiently to the central metal [8].
The specific spectroscopic and magnetic properties of rare
earth ions have made them one of the essential components in
the preparation of new materials and ideal as probes in studies of
biological systems [1]. Among these studies, much attention has
been paid to the fluorescent rare earth complexes that are found
and used as labels and sensors for natural and medical science
[2,3], mainly due to their large Stokes shifts, narrow emission
profiles [4]. More and more chemists are attracted to design the
organized molecular architectures containing trivalent rare earth
ions (Eu3+, Tb3+) working as efficient light conversion devices
[5,6].
The emission originates from 4f–4f intra-atomic electronic
and magnetic dipole transitions that are parity-forbidden. As
such, the absorption cross sections for directly exciting trivalent
rare earth ions are small and the use of ligands to excite trivalent
rare earth ions through an energy-transfer process is, in general,
therefore much more effective [7]. In order to obtain strongly
fluorescent complexes, the chromophoric ligands which coordi-
Our group is interested in the supramolecular coordination
chemistry of rare earth ions with amide type ligands which
have strong coordination capability to the rare earth ions and
enhanced luminescence of rare earth complexes by provid-
ing some of proper conjugate absorption groups suitable for
energy transfer [9–11]. The chelate ligands 2,2-bipyridine (bpy),
1,10-phenanthroline (phen), and their substituted derivatives
have played an important role in the development of coordi-
nation chemistry [12,13]. Studies have also shown that rare
earth luminescence enhancement can be facilitated in com-
plexes possessing an aromatic ring that has a nitrogen atom,
suchas1,10-phenanthrolineorbipyridine[14–16]. Inthepresent
work, we designed and synthesized a new amide type bridged
ligand with a 3,3ꢀ-disubstituted-2,2ꢀpyridine antenna effect,
3,3ꢀ-bis(benzylamido)-2,2ꢀ-bipyridine (L) (Scheme 1) with two
relativelylong“arms”, andstudiedthecrystalstructureandlumi-
nescent properties of rare earth complexes with this new ligand.
The crystal structure of the complex [TbL2(NO3)3H2O]·2H2O
shows that only one arm of each ligand bind one Tb atom. The
fluorescent properties of the solid complexes of europium(III)
and terbium(III) ions with this new ligand were also studied
in detail. And the results indicated that counter anion notably
∗
Corresponding author. Tel.: +86 931 89115151; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.03.007

50
X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
Scheme 1. The synthesis route of the ligand L.
affected the fluorescence characteristics of europium and ter-
bium ions.
Distilled water (10 ml) and methanol (5 ml) were added, after
4 days a pale flocculent solid was obtained. Recrystallization
in ethyl acetate afforded a white flocculent solid. Yield: 1.52 g
(78%) L: 1H NMR (CDCl3): 8.39 (dd, 2H), 7.85 (dd, 2H), 7.67
(t, 2H), 7.26–6.98 (m, 10H, ArH), 4.34 (dd, 4H).
2. Experimental
2.1. Materials
2.4. Synthesis of the complexes
The rare earth nitrates [17] and picrates [18] were prepared
according to literature method. Pyridine was dried according to
literature procedures. Other commercially available chemicals
were of reagent grade and were used without further purification.
A solution of 0.1 mmol rare earth nitrate in 5 ml of ethyl
acetate was added dropwise to the solution of 0.2 mmol L in
10 ml of ethyl acetate. The mixture was stirred at room tempera-
ture for 8 h. The precipitated solid complex was filtered, washed
with ethyl acetate three times and dried in vacuo over P4O10 for
48 h. All the nitrate complexes were obtained as white powders
which are stable in air, yield: 40%. Single crystals of nitrate
complex were grown from ethyl acetate and acetone (V:V = 3:1)
mixed solution with slow evaporation at room temperature. After
ca. 3 weeks, transparent colorless crystals were formed from the
solution which is suitable for X-ray diffraction analysis.
2.2. Methods
The metal ions were determined by EDTA titration using
xylenol orange as indicator. Carbon, Nitrogen and Hydro-
gen analyses were performed using an EL elemental analyzer.
Infrared spectra (4000–400 cm−1) were obtained with KBr discs
1
on a Therrno Mattson FTIR spectrometer. H NMR spectra
were recorded on a Brucker DRX 200 spectrometer in D2O or
CDCl3 solution with TMS as internal standard. Electronic spec-
tra were recorded with a Varian Cary 100 spectrophotometer in
ethyl acetate solution. Fluorescence measurements were made
on a Hitachi F-4500 spectrophotometer and a shimadzu RF-540
spectro-fluorophotometer equipped with quartz curettes of 1 cm
path length. The excitation and emission slit widths were 5 and
2.5 nm, respectively. The molar conductance values measure-
ments were carried out on a model DDS-307 type conductivity
bridge using 1.0 × 10−3 mol dm−3 solutions in acetone at 25 ◦C.
An ethyl acetate solution (5 cm3) of rare earth picrate
(0.1 mmol) was added dropwise to a solution of 0.1 mmol ligand
L in the (5 cm3) ethyl acetate. The mixture was stirred at room
temperature for 8 h. And then the precipitated solid complex was
filtered, washed with ethyl acetate, dried in vacuo over P4O10
for 48 h and submitted for elemental analysis, yield: 70%.
3. Results and discussion
3.1. Properties of the discussion
2.3. Synthesis of the ligand
Analytical data for the newly synthesized complexes, listed
in Table 1, conform to a 1:2 metal-to-L stoichiometry for nitrate
complex and 1:1 for picrate complex. All complexes are soluble
in DMF, DMSO, acetone, ethanol and methanol, but slightly sol-
uble in ethyl acetate and hexane. The molar conductance of the
complexes in acetone (see Table 1) indicates that all complexes
act as non-electrolytes.
2,2ꢀ-Binicotinic acid was prepared according to the literature
[19]. 1H NMR (D2O) 8.67 (dd, 2H), 8.57 (dd, 2H), 7.75 (t, 2H).
2.3.1. Preparation of
3,3ꢀ-bis(chlorocarbonyl)-2,2ꢀ-bipyridine and
3,3ꢀ-bis-(benzylamido)-2,2ꢀ-bipyridine (L)
2,2ꢀ-Binicotinic acid (1.07 g, 4.4 mmol) was directly dis-
solved in 20 ml of SOCl2 and refluxed until the white powder
disappeared completely. And the residue SOCl2 was removed
away completely under reduced pressure. The resulting 3,3ꢀ-
bis(chlorocarbonyl)-2,2ꢀ-bipyridine was dissolved in 20 ml of
dry pyridine immediately. Then the pyridine solution of ben-
zylamine (1.07 g, 10 mmol) was dropwise added into the
3,3ꢀ-bis(chlorocarbonyl)-2,2ꢀ-bipyridine solution under stirring
in the ice bath. Continuous stirring at room temperature for 10 h.
3.2. IR spectra
Tables 4 and 5 show the characteristic bands of the ligand
and its complexes. The IR spectrum of the free ligand shows
bands at 1628 and 1553 cm−1, which may be attributed to stretch
vibration of the carbonyl group of amide (C O) and(C N) of
bipyridine, respectively. In the nitrate complexes, the bands for
one of (C O) is shifted by about 24 (from 1628 to 1604) indicate
that one of the carbonyl of the ligand take part in coordination.

X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
51
Table 1
Analytical and molar conductance data for the complexes (calculated values in parentheses)
Complex
Analysis (%)
C
ꢀm (ꢁ−1 cm2 mol−1
)
H
N
Ln
[La(NO3)3L2·H2O]·2H2O
[Eu(NO3)3L2·H2O]·2H2O
[Gd(NO3)3L2·H2O]·2H2O
[Tb(NO3)3L2·H2O]·2H2O
[Er(NO3)3L2·H2O]·2H2O
La(Pic)3L
Eu(Pic)3L
Gd(Pic)3L
Tb(Pic)3L
Er(Pic)3L
51.36(51.03)
50.84(50.49)
50.54(50.28)
50.38(50.21)
50.13(49.87)
42.68(42.43)
42.13(41.99)
42.01(41.81)
41.90(41.75)
41.50(41.28)
4.18(4.12)
4.20(4.07)
4.18(4.06)
4.15(4.05)
4.10(4.02)
2.14(2.27)
2.13(2.24)
2.15(2.23)
2.09(2.23)
2.21(2.22)
12.69(12.59)
12.58(12.46)
12.28(12.40)
12.49(12.39)
12.23(12.30)
14.72(14.62)
14.68(14.47)
14.63(14.41)
14.45(14.39)
14.38(14.29)
11.08(11.35)
12.09(12.28)
12.41(12.66)
12.46(12.78)
13.20(13.66)
10.95(11.15)
11.88(12.07)
12.25(12.44)
12.31(12.56)
12.97(13.13)
53.6
54.3
55.2
54.9
47.8
50.3
64.5
58.6
60.8
61.5
arrangement with the dihedral angle of 70.09◦ which is in agree-
ment with IR spectra.
Notably, the vibration ν (C N) of pyridine ring at 1553 cm−1
is shifted towards higher frequency in the complexes as com-
pared to counterpart for the free ligand. This may be due to
coordination of the oxygen atoms of L to trivalent rare earth
ion which causes the fixing of L conformation. The fact that the
amide terminal group (benzylamine) has large sterically hin-
dered effect, which prevents both of carbonyl oxygen atoms of
one ligand from coordinating to lanthanide ion could account
for these changes.
The absorption bands assigned to the coordinated nitrates
were observed as two group bands at about 1492 (ν1) and
1290 (ν5) cm−1for the complexes. The differences between
the strongest absorption band ν1 and ν5 of nitrate group lie in
194–206 cm−1, indicating that coordinated nitrate groups in the
complexes are bidentate ligands [20], the ν3 of free nitrate group
disappears in the spectra of the complexes in agreement with
the results of the conductivity experiment. Additionally, strong
and broad bands at 3340 cm−1 indicate that H2O is existent in
the complexes, the N–H stretch vibration were also observed at
3422, 3217 and 3064 cm−1, confirming the elemental analysis.
In the picrate complexes, after forming the complex,
the characteristic frequency of the free ligand ν(C O) at
1628 cm−1shiftsca. 13 cm−1 towardslowerwavenumbers, indi-
cating that the oxygen atom of C O take part in coordination to
the metal ion. The OH out-of-plane bending vibration of the free
HPic at 1151 cm−1 disappears in the spectra of the complexes,
indicating that the H-atom of the OH group is replaced by rare
earth ions. The free HPic has νas(–NO2) and νs(–NO2) at 1555
and 1342 cm−1, respectively, which split into two bands at ca.
1577, 1543 cm−1and ca. 1364, 1333 cm−1. This indicates that
some of the nitryl O atoms take part in coordination [21].
3.4. Electronic spectra
The spectral profiles of the nitrate rare earth com-
plexes are very similar. Absorption spectra of L and
[TbL2(NO3)3H2O]·2H2O are presented in Fig. 2. The electronic
spectrum of the free ligand L in ethyl acetate displays only
one strong absorption band with maximum at 258 nm which
*
attributed to ␲−␲ transitions centered on the 2,2ꢀ-bipyridine
group is different from wavelengths compared to those of the
free 2,2ꢀ-bipyridine (232 and 279 nm), which exists in a trans
Table 2
Crystal data and structure refinement for the nitrate complex
CCDC
636943
C52H50N11O16Tb
1261.67
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
294(2) K
˚
0.71073 A
Monoclinic
P2(1)/m
˚
a = 10.0575(5) A
Unit cell dimensions
˚
b = 20.7418(11) A, β = 96.9640(10)
˚
c = 14.0835(7) A
3
˚
Volume
2916.3(3) A
Z
2
Density (calculated)
Absorption coefficient
F(0 0 0)
1.417 Mg/m3
1.287 mm−1
1264
Crystal size
Theta range for data collection
Index ranges
0.31 mm × 0.17 mm × 0.06 mm
1.76–23.93◦
−11 < = h < = 11, −19 < = k < = 23,
−15 < = l < = 16
13430
3.3. Crystal structure of [TbL2(NO3)3H2O]·2H2O
Reflections collected
A summary of crystallographic data and details of the struc-
ture refinements are listed in Table 2. The selected bond lengths
and bond angles are given in Table 3. The single-crystal X-
ray analysis of the complex [TbL2(NO3)3H2O]·2H2O exhibits
a distorted monocapped square antiprism. L functions as mon-
odentate ligand as shown in Fig. 1. The ninth coordination site
is occupied by the O atom (O8) of water molecular. Due to the
space obstacle, the two pyridine rings provide a non-coplanarity
Independent reflections
Completeness to theta = 23.93
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
4629 [R(int) = 0.0729]
98.7%
Full-matrix least-squares on F2
4629/190/472
1.017
R1 = 0.0639, wR2 = 0.1578
R1 = 0.0788, wR2 = 0.1646
1.023 and −0.987 e−3
Largest diff. peak and hole

52
X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
Table 3
◦
˚
Selected bond lengths (A) and angles ( ) for the complex [TbL2(NO3)3H2O]·2H2O
Tb(1)–O(9) 2.310(6)
Tb(1)–O(2)#1 2.434(9)
Tb(1)–O(6) 2.459(10)
Tb(1)–O(9)#1 2.310(6)
Tb(1)–O(5) 2.441(10)
Tb(1)–O(8) 2.430(8)
Tb(1)–O(4) 2.447(7)
Tb(1)–O(2) 2.434(9)
Tb(1)–O(4)#1 2.447(7)
O(9)–Tb(1)–O(9)#1 148.9(3)
O(9)–Tb(1)–O(2) 76.4(3)
O(9)–Tb(1)–O(2)#1 128.8(3)
O(2)–Tb(1)–O(2)#1 53.2(6)
O(8)–Tb(1)–O(5) 130.2(4)
O(9)–Tb(1)–O(4) 74.5(3)
O(2)–Tb(1)–O(4) 75.9(3)
O(9)–Tb(1)–O(4)#1 124.7(2)
O(2)–Tb(1)–O(4)#1 98.4(4)
O(4)–Tb(1)–O(4)#1 51.5(4)
O(8)–Tb(1)–O(6) 80.0(3)
O(5)–Tb(1)–O(6) 50.2(4)
O(9)–Tb(1)–O(8) 80.50(2)
O(9)#1–Tb(1)–O(8) 80.50(2)
O(9)#1–Tb(1)–O(2) 128.8(3)
O(9)#1–Tb(1)–O(2)#1 76.4(3)
O(9)–Tb(1)–O(5) 86.84(2)
O(2)–Tb(1)–O(5) 69.2(4)
O(9)#1–Tb(1)–O(4) 124.7(2)
O(2)#1–Tb(1)–O(4) 98.4(4)
O(9)#1–Tb(1)–O(4)#1 74.5(3)
O(2)#1–Tb(1)–O(4)#1 75.9(3)
O(9)–Tb(1)–O(6) 76.29(16)
O(2)–Tb(1)–O(6) 113.7(4)
O(4)–Tb(1)–O(6) 145.9(3)
O(8)–Tb(1)–O(2) 148.9(3)
O(8)–Tb(1)–O(2)#1 148.9(3)
O(9)#1–Tb(1)–O(5) 86.84(2)
O(2)#1–Tb(1)–O(5) 69.2(4)
O(8)–Tb(1)–O(4) 78.1(3)
O(5)–Tb(1)–O(4) 143.4(3)
O(8)–Tb(1)–O(4)#1 78.1(3)
O(5)–Tb(1)–O(4)#1 143.4(3)
O(9)#1–Tb#1–Tb(1)–O(6) 76.29(16)
O(2)#1–Tb(1)–O(6) 113.7(4)
O(4)#1–Tb(1)–O(6) 145.9(3)
Fig. 1. ORTEP diagram (30% probability ellipsoids) showing the coordination sphere of TbL2(NO3)3·2H2O.
Fig. 2. Absorption spectra in ethyl acetate of L (left) and Tb complex (right).

X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
53
Table 4
The most important IR bands for the rare earth nitrate complexes(cm−1
)
−
−
−
−
V3 (NO3 )
Compound
ν (C O)
ν (C N)
V1 (NO3
)
V4 (NO3
)
V2 (NO3
)
|V1–V4|
L
1628
1553
1578
1578
1579
1578
1578
–
–
–
–
–
[La(NO3)3L2·H2O]·2H2O
[Eu(NO3)3L2·H2O]·2H2O
[Gd(NO3)3L2·H2O]·2H2O
[Tb(NO3)3L2·H2O]·2H2O
[Er(NO3)3L2·H2O]·2H2O
1624, 1604
1623, 1606
1626, 1606
1624, 1605
1625, 1604
1492
1494
1494
1491
1493
1298
1307
1289
1308
1287
1031
1027
1026
1027
1030
818
818
817
817
817
194
187
205
183
206
conformation [22]. It is interesting to note that complexation
with rare earth ions resulted in almost no shift of the maximum.
Theligandincludesabipyridinechromophoreandabenzylchro-
mophore, however, it is the bipyridine that sensitizes the metal
centred emission.
not for the Tb picrate complex. Under identical experimental
conditions, the fluorescence characteristic emissions in solid
state are listed in Table 6. The emission spectra of the ligand
and complexes are shown in Fig. 3.
Excited at 318 nm, the “free” ligand exhibits broad emis-
sion bands (λmax = 417 nm) in solid state. The efficient energy
transfer from ligand to centre ions (antenna effect) is one of
key factors to achieve rare earth characteristic fluorescence. It
is shown in Fig. 3 that the complexes show the characteristic
emissions of Eu3+ or Tb3+. This indicates that the ligand L is
a good organic chromophore to absorb and transfer energy to
metal ions.
3.5. Fluorescence studies
The fluorescence characteristics of the ligand, europium and
terbium complexes in solid state were measured at room tem-
perature. Among these four complexes, fluorescence of the
complexes of Eu, Tb nitrates and Eu picrate were observed but
Fig. 3. (a) Emission spectrum of L in solid state, (b) emission spectrum of [Eu(NO3)3L2·H2O]·2H2O in solid state, (c) emission spectrum of [Tb(NO3)3L2·H2O]·2H2O
in solid state and (d) emission spectrum of Eu(Pic)3L in solid state.

54
X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
Table 5
The most important IR bands of the rare earth picrate complexes (cm−1
)
Compound
L
ν (C O)
1628
ν (C N)
1553
V (C–O)
–
Vas (–NO2)
–
Vs (–NO2)
–
HPic
–
–
1265
1274
1273
1273
1274
1274
1555
1342
La(Pic)3L
Eu(Pic)3L
Gd(Pic)3L
Tb(Pic)3L
Er(Pic)3L
1616
1616
1615
1616
1616
1578
1578
1578
1578
1578
1578, 1543
1578, 1542
1578, 1543
1578, 1543
1578, 1543
1363, 1332
1364, 1333
1364, 1333
1364, 1333
1364, 1333
Table 6
Fluorescence data for the ligand and Eu and Tb complexes
Slit(nm)
λex (nm)
318
λem (nm)
RFI
Assignment
L
5
5
417
390.9
7
[Eu(NO3)3L2·H2O]·2H2O
397
593
616
581
38.57
104.9
29.18
5D0 → F1
7
5D0 → F2
7
5D0 → F0
7
Eu(Pic)3L
2.5
5
449
318
592
617
492
546
177.6
1502
909.1
2134
5D0 → F1
7
5D0 → F2
7
5D4 → F6
7
[Tb(NO3)3L2·H2O]·2H2O
5D4 → F5
7
586
132.4
5D4 → F4
It is noted that the intensity ratio of the 5D0–7F2 transition to
the 5D0–7F1 transition is widely used as a measure of the coor-
dination state and the site symmetry of the europium ion, since
the 5D0–7F1 emission is independent of the ligand environment,
and primarily magnetic dipole in character, while the 5D0–7F2
emission is essentially purely electric dipole in character, and
its intensity is very sensitive to the crystal field symmetry. In the
two spectrums of Eu(III) complexes, the transition 5D0–7F2 is
the strongest in the transitions with about three or eight times
the strength of the transition of 5D0–7F1. The results show that
Eu(III) has the lower symmetric coordination environment [23].
Intramolecular energy transfer from the triplet state of the lig-
and to the resonance level of the rare earth ion is one of the most
importantprocessesinfluencingthefluorescencequantumyields
of rare earth complexes [24]. The energy difference between the
triplet state energy level of the ligand and the lowest excited state
level of rare earth cannot be too large or too small. In order to
acquire the triplet excited state T1 of the ligand L, the phospho-
rescence spectra of the Gd(III) nitrate complexes were measured
at 77 K in a methanol solution. The triplet state energy levels
T1 of the ligand L in the nitrate and picrate complexes, which
were calculated from the shortest wavelength phosphorescence
bands [25] of the corresponding Gd(III) complexes, are 23,585
and 21,245 cm−1, respectively. We deduced that the electrostatic
factors in the ligand–metal bonding, which may be affected by
the different counter anion in these two series complexes, influ-
enced the triplet state energy level of the ligand L [26] and made
this T1 energy be lowered in the picrate complexes. These energy
levels are both above the lowest excited resonance level 5D0 of
Eu(III) (17,300 cm−1) and 5D4 (20,500 cm−1) of Tb(III). Thus,
the absorbed energy could be transferred from ligand to the Eu
or Tb ions. And we may deduce that the triplet state energy
level T1 of this ligand L in the nitrate complexes matches bet-
ter to the lowest resonance level of Tb(III) (ν = 3085 cm−1) than
Eu(III) ( ν = 6285 cm−1) ion, because such big ν (T1–5D0) could
result in the non-radioactive deactivation of the europium emit-
ting state and quench the fluorescence of the Eu nitrate complex
[26]. Comparing the emission spectra of these two nitrate com-
plexes, we could find that the fluorescence intensity of terbium
nitrate complex is much stronger than that of Eu(III) complexes.
And the triplet state energy level T1 of this ligand L in the picrate
complexesmatchesbettertothelowestresonancelevelofEu(III)
(ν = 3945 cm−1) than Tb(III) (ν = 745 cm−1) ion, because such
small ν(T1–5D4) could result in the non-radioactive deactivation
of the terbium emitting state via a back-energy transfer process
(T1 ← Tb(5D4)) and quench the luminescence of the Tb picrate
complex [26]. Actually we do not observe the fluorescence of
the Tb picrate complex at room temperature.
4. Conclusions
According to the data and discussion above, the new aryl
amide ligand 3,3ꢀ-dibenzylamido)-2,2ꢀ-bipydine can form sta-
ble solid complexes with rare earth nitrates and picrates. When
the ligand formed the rare earth complexes, obvious changes
in IR spectra were observed. In the complexes, rare earth ions
were coordinated to the C O oxygen atoms of the ligand L.
The fluorescent properties of the Eu and Tb complexes in solid
state were investigated. Under the excitation, the complexes
exhibited characteristic fluorescence of europium and terbium
ions. And the lowest triplet state energy levels of the ligand
in the nitrate and picrate complexes indicate that the counter

X.-Q. Song et al. / Spectrochimica Acta Part A 69 (2008) 49–55
55
anion of the complexes is very essential in determining both the
composition and the fluorescent properties of the rare earth com-
plexesbyinfluencingtheelectrostaticfactorsintheligand–metal
bonding.
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5. Supplementary material
Crystallographic data for the structure analysis have been
deposited with the Cambridge Crystallographic Data Center,
CCDC No.636943. Copies of this information may be obtained
free of charge from the Director, CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK (e-mails: linstead@ccdc.cam.ac.uk;
deposit@ccdc.cam.ac.uk, directline:+441223762910;Tel:+44
1223 336408; fax: +44 1223 336033).
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This work was supported by the National Natural Sci-
ence Foundation of China (Grants 20431010, 20021001 and
J0630962).
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at .
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