Preparation, crystal structure and luminescent properties of lanthanide picrate complexes with N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-acetamide (L)

Article abstract of DOI:10.1016/j.ica.2007.04.005

A new amide-based ligand derived from biphenyl, N-benzyl-2-{2′-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-aceamide (L) was synthesized. Solid complexes of lanthanide picrates with this new ligand were prepared and characterized by elemental analysis, conductivity measurements, IR and electronic spectroscopies. The molecular structure of [Eu(pic)₃L] shows that the Eu(III) ion is nine-coordinated by four oxygen atoms from the L and five from two bidentate and one unidentate picrates. All the coordinate picrates and their adjacent equivalent picrates form intermolecular π-π stacking. Furthermore, the [Eu(pic)₃L] complex units are linked by the π-π stacking to form a two-dimensional (2-D) netlike supramolecule. Under excitation, the europium complex exhibited characteristic emissions. The lifetime of the ⁵D₀ level of the Eu(III) ion in the complex is 0.22 ms. The quantum yield Φ of the europium complex was found to be 1.01 × 10^-3 with quinine sulfate as reference. The lowest triplet state energy level of the ligand indicates that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.

Full text of DOI:10.1016/j.ica.2007.04.005

Inorganica Chimica Acta 360 (2007) 3361–3368
www.elsevier.com/locate/ica
Preparation, crystal structure and luminescent properties of
lanthanide picrate complexes with N-benzyl-
2-{20-[(benzyl-methyl-carbamoyl)-methoxy]-
biphenyl-2-yloxy}-N-methyl-acetamide (L)
a
a
a
a
Yan-Ling Guo , Ya-Wen Wang , Wei Dou , Jiang-Rong Zheng ,
a,b,
c
Wei-Sheng Liu
*, Cheng-Yong Su
a
Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
b
State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, PR China
School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, PR China
c
Received 10 February 2007; accepted 3 April 2007
Available online 14 April 2007
Abstract
A new amide-based ligand derived from biphenyl, N-benzyl-2-{20-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-methyl-
aceamide (L) was synthesized. Solid complexes of lanthanide picrates with this new ligand were prepared and characterized by elemental
analysis, conductivity measurements, IR and electronic spectroscopies. The molecular structure of [Eu(pic)3L] shows that the Eu(III) ion
is nine-coordinated by four oxygen atoms from the L and five from two bidentate and one unidentate picrates. All the coordinate picrates
and their adjacent equivalent picrates form intermolecular p–p stacking. Furthermore, the [Eu(pic)3L] complex units are linked by the p–
p stacking to form a two-dimensional (2-D) netlike supramolecule. Under excitation, the europium complex exhibited characteristic
5
emissions. The lifetime of the D0 level of the Eu(III) ion in the complex is 0.22 ms. The quantum yield U of the europium complex
was found to be 1.01 · 10ꢀ3 with quinine sulfate as reference. The lowest triplet state energy level of the ligand indicates that the triplet
state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Lanthanide picrate complexes; Luminescent properties; p–p Stacking; Crystal structure
1. Introduction
success as luminescent probes [2,3]. Due to their long
(ms) luminescence lifetimes, sharp emission spectra with
large Stokes shifts (>200 nm), and unpolarized lumines-
cence, lanthanide ions demonstrate features not common
in the typical organic fluorophores.
Amide-based crown ethers offer many advantages in
extraction and analysis of the rare earth ions because of
their ring-like coordination structure and terminal group
effects [4,5]. The open-chain crown ethers containing amide
groups possess suitable molecular structure: a chain with
inflexible terminal groups. Therefore, they are excellent
reagents for activating ion-selective electrodes and extracts
of rare earth ions [6]. However, fluorescence properties on
open-chain crown ethers with lanthanide complexes have
Fluorescence spectroscopy has become a powerful tool
for monitoring biological events and elucidating the struc-
ture and function of biomolecules [1]. The sensitivity and
time-resolved nature of fluorescence, in addition to the
ready availability of necessary equipment, has made fluo-
rescence spectroscopy a popular choice for studying bio-
logical processes. Recently, lanthanides have found wide
*
Corresponding author. Address: Department of Chemistry and State
Key Laboratory of Applied Organic Chemistry, Lanzhou University,
Lanzhou 730000, China. Tel.: +86 931 8915151; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
0020-1693/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2007.04.005

3362
Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
been rarely reported [7]. Our group is interested in the
supramolecular coordination chemistry of lanthanide ions
with amide-based crown ethers that have strong coordina-
tion capability to the lanthanide ions and terminal group
effects [8]. As a part of our systematic studies, here we
introduced biphenyl groups as the basic molecular frame
and obtained a new open-chain crown ether ligand, N-
benzyl-2-{20-[(benzyl-methyl-carbamoyl)-methoxy]-biphenyl-
2-yloxy}-N-methyl-acetamide (L), and reported the
synthesis, crystal structure and luminescent properties of
the lanthanide picrate complexes with the new ligand.
The crystal structure of the complex Eu(pic)3L shows that
the Eu(III) ion could be effectively encapsulated and pro-
tected by one coordinated ligand and three coordinated
picrates. The p–p stacking formed by the equivalent
picrates, are very important for the assembly of the com-
plex molecule units into a two-dimensional (2-D) netlike
supermolecule and thus enhance the fluorescence intensity
of the complex in solid state. Under excitation, the euro-
pium complex exhibited characteristic emissions. The life-
using 10ꢀ3 mol dmꢀ3 solution at 25 ꢁC. IR spectra were
recorded on Nicolet FT-170SX instrument using KBr discs
in the 400–4000 cmꢀ1 region. 1H NMR spectrum was mea-
sured on a Varian Mercury plus 300 M spectrometer in
CDCl3 solution with TMS as internal standard. Fluores-
cence and phosphorescence measurements were made on
a Hitachi F-4500 spectrophotometer.
2.3. Synthesis of the ligand
The synthetic route for the ligand is shown in Scheme 1.
Anhydrous K2CO3 (5.6 g, 41 mmol) was added into the
20 mL DMF solution of 2,20-dihydroxybiphenyl (1.86 g,
10 mmol) at 100 ꢁC. After 1 h, a solution of N-methyl-N-
benzylchloroacetamide (5.92 g, 30 mmol) in 10 mL DMF
was added dropwise to the mixture and maintained at
100 ꢁC for 7 h. When cool, distilled water (60 ml) was
added and the turbid solution was extracted with chloro-
form (3 · 40 mL). The combined organic phases were
washed with water and dried with anhydrous Na2SO4.
The solvent was removed and the residue was chromato-
graphed to afford the ligand L; Yield: 80%.
5
time of the D0 level of the Eu(III) ion in the complex is
0.22 ms. The quantum yield U of the europium complex
was found to be 1.01 · 10ꢀ3 with quinine sulfate as refer-
ence. And the lowest triplet state energy level of the ligand
which was calculated from the phosphorescence spectrum
of the Gd complex at 77 K indicates that the triplet state
energy level of the ligand matches better to the resonance
level of Eu(III) than Tb(III) ion.
2.4. Synthesis of the lanthanide picrate complexes
To a solution of 0.2 mmol lanthanide picrate in 5 ml of
ethanol was added dropwise the solution of 0.2 mmol L in
10 ml of ethanol. The mixture was stirred at room temper-
ature for 6 h. The precipitated solid complex was filtered,
washed with ethanol and dried in vacuum over P2O5 for
48 h. All the complexes were obtained as yellow powders
in a yield of 85–90%. Slowly evaporating a solution of
the europium complex in CHCl3/CH3CH2OH at room
temperature, resulted in the formation of block crystals
after 2 months.
2. Experimental
2.1. Materials
Lanthanide picrate [9] and N-methyl-N-benzylchloroac-
etamide [10] were prepared according to literature meth-
ods. All commercially available chemicals were of A.R.
grade and were used without further purification.
2.5. Crystal structure determination
2.2. Methods
X-ray diffraction data of Eu(pic)3L was collected on a
Bruker SMART CCD diffractometer, using graphite-
monochromatized Mo Ka (k = 0.71073 A) at 293(2) K.
The structures were solved by direct method and refined
by full matrix least-squares techniques on F2 with all non-
hydrogen atoms treated anisotropically. All calculations
˚
The metal ion was determined by EDTA titration using
xylenal orange as indicator. C, H and N were determined
using an Elementar Vario EL. Conductivity measurement
was carried out with a DDS-307 type conductivity bridge
Cl
N
O
N
O
O
N
DMF, K2CO3, 100ºC, 7h
OH
OH
O
O
L
Scheme 1. The synthetic route for the ligand L.

Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
3363
Table 1
Analytical and molar conductance data for the complexes (calculated values in parentheses)
Complex
Analysis (%)
C
Km (cm2 Xꢀ1 molꢀ1
)
H
N
Ln
La(pic)3L
Nd(pic)3L
Eu(pic)3L
Gd(pic)3L
Tb(pic)3L
Y(pic)3L
44.67(45.08)
44.72(44.91)
44.24(44.64)
44.12(44.48)
44.08(44.41)
46.49(46.84)
3.04(2.86)
3.02(2.84)
2.68(2.83)
2.97(2.82)
2.68(2.81)
2.74(2.97)
11.37(11.57)
11.36(11.53)
11.26(11.46)
11.21(11.42)
11.15(11.40)
12.11(12.02)
10.23(10.44)
10.51(10.78)
11.16(11.31)
11.24(11.64)
11.37(11.77)
6.72(6.95)
41.6
35.2
37.4
40.5
39.8
43.7
were performed with the program package SHELXTL. The
hydrogen atoms were assigned with common isotropic dis-
placement factors and included in the final refinement by
using geometrical restrains.
3.2. Crystal structure of Eu(Pic)3L
A summary of crystallographic data and details of the
structure refinements are listed in Table 2. The selected
bond lengths and bond angles are given in Table 3.
3. Results and discussion
The single-crystal X-ray analysis of the complex
Eu(Pic)3L reveals that the Eu(III) ion is coordinated with
9 oxygen donor atoms, five of which belong to two biden-
tate and one unidentate picrates and the remaining four to
the tetradentate ligand L (Fig. 1). The coordination polyhe-
dron around Eu(III) ion is a distorted tricapped trigonal
prism (Fig. 2). The ligand wraps around the metal ion with
its oxygen atoms and forms a ring-like coordination struc-
ture together with the Eu atom. The two phenyl rings
about the central bond in the molecule have a drastic twist-
ing, with the dihedral angle between them being 55.70ꢁ.
3.1. Properties of the discussion
Analytical data for the complexes listed in Table 1, indi-
cate that all the complexes conform to a 1:3:1 metal-to-pic-
rate-to-L stoichiometry Ln(pic)3L. The complexes are
soluble in DMF, DMSO, acetonitrile, acetone, ethyl ace-
tate and CHCl3, but slightly soluble in ethanol and metha-
nol. The molar conductances of the complexes in acetone
(see Table 1) indicates that all the complexes act as nonelec-
trolyte [11], implying that all the picrate groups are in the
coordination sphere.
˚
The La–O (C@O) distances (mean 2.367 A) are signifi-
cantly shorter than the La–O (C–O–C) distances (mean
˚
2.526 A). This suggests that the La–O (C@O) bond is
stronger than the La–O (C–O–C) bond.
Table 2
In the complex, the requirement of high coordination
number of the lanthanide ion is satisfied by the tetradentate
ligand, two bidentate and one unidentate picrates, thus,
no coordinated solvent molecules exist in the complex,
which can efficiently quench lanthanide luminescence, this
is important in the design of supramolecular photonic
devices [12].
Crystal data and structure refinements for europium picrate complex
Empirical formula
Temperature (K)
Crystal color
C101H77Cl3Eu2N22O50
293(2)
yellow
Crystal size (mm)
Formula weight
Crystal system
Space group
0.35 · 0.30 · 0.22
2809.12
triclinic
ꢀ
P1
˚
a (A)
12.4779(16)
15.014(3)
17.4846(18)
69.982(15)
81.329(11)
72.769(14)
2935.2(8)
1
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
Table 3
˚
Selected bond lengths (A) and bond angles (ꢁ) for Eu(pic)3L
Eu(1)–O(15)
Eu(1)–O(12)
Eu(1)–O(5)
Eu(1)–O(1)
Eu(1)–O(4)
2.288(3)
2.317(3)
2.323(3)
2.361(4)
2.374(3)
Eu(1)–O(2)
Eu(1)–O(6)
Eu(1)–O(3)
Eu(1)–O(22)
2.476(3)
2.499(3)
2.575(3)
2.612(4)
3
˚
V (A )
Z
Dcalc (Mg/m3)
F(000)
1.589
1414
˚
Radiation, graphite-monochromatized, k (A) 0.7107
Reflections collections
Independent reflection
h Range for data collection (ꢁ)
24405
11600
2.94–26.50
O(15)–Eu(1)–O(12)
O(12)–Eu(1)–O(5)
O(15)–Eu(1)–O(1)
O(5)–Eu(1)–O(1)
O(15)–Eu(1)–O(4)
O(12)–Eu(1)–O(4)
O(5)–Eu(1)–O(2)
O(1)–Eu(1)–O(2)
O(4)–Eu(1)–O(2)
O(15)–Eu(1)–O(6)
O(12)–Eu(1)–O(6)
85.60(13)
81.96(13)
94.06(14)
76.40(12)
82.50(13)
76.84(12)
75.12(11)
63.06(12)
89.96(11)
70.70(12)
71.35(12)
O(5)–Eu(1)–O(6)
O(1)–Eu(1)–O(6)
O(15)–Eu(1)–O(3)
O(1)–Eu(1)–O(3)
O(4)–Eu(1)–O(3)
O(2)–Eu(1)–O(3)
O(12)–Eu(1)–O(22)
O(5)–Eu(1)–O(22)
O(4)–Eu(1)–O(22)
O(2)–Eu(1)–O(22)
66.07(11)
78.21(12)
73.23(11)
74.47(11)
61.59(10)
66.73(10)
63.67(13)
68.10(12)
65.31(12)
69.81(12)
Index range
ꢀ15 6 h 6 15,
ꢀ18 6 k 6 18,
ꢀ21 6 l 6 20
Goodness-of-fit on F2
R [I > 2r(I)]
R (all data)
Largest difference in peak and hole (e A
1.029
R = 0.0443, wR = 0.1166
R = 0.0604, wR = 0.1213
1.205, ꢀ0.544
ꢀ3
˚
)

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Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
Fig. 1. ORTEP diagram (30% probability ellipsoids) show the coordination sphere of [Eu(pic)3L]. Hydrogen atoms are omitted for clarity.
Interestingly, all the three coordinated picrates in the
absorption bands at 1667 and 1110 cmꢀ1 which are
assigned to m(C@O) and m(C–O–C), respectively. In the
complexes, the bonds shift by ca. 51 and 28 cmꢀ1 towards
lower wave numbers, thus indicating that the C@O and
ether O-atoms take part in coordination to the metal ions.
The larger shift for m(C@O) in the spectrum of the complex
suggests that the Ln–O (carbonyl) bond is stronger than
the Ln–O (ether) bond. The OH out-of-plane bending
vibration of the free Hpic at 1151 cmꢀ1 disappears in the
spectrum of the complex indicating that the H atom of
the OH group is replaced by Ln(III). The vibration m(C–
O) at 1265 cmꢀ1 is shifted towards higher frequency by
ca. 9 cmꢀ1 in the complex. This is due to the following
two effects. First, the hydrogen atom of the OH group is
replaced by Ln(III), increasing the p-bond character in
the C–O bond. Secondly, coordination of the oxygen atom
of L to Ln(III) causes the p-character to be weakened. The
free Hpic has mas (NO2) and ms (NO2) at 1555 and
1342 cmꢀ1, respectively, which splits into two bands at
1578, 1542 cmꢀ1 and 1359, 1331 cmꢀ1 in the complex. This
indicates that some of the O-atoms in the nitro group of
Picꢀ take part in coordination [14].
complex molecule stretch out like three hands and capture
their equivalent picrates of the adjacent molecules to form
p–p stacking [13] with the dihedral angles between them
being 0ꢁ, 0ꢁ and 0ꢁ, and the distances between their cent-
˚
˚
˚
roids being 3.779 A, 3.917 A and 3.647 A, respectively,
thus, producing a two-dimensional (2-D) netlike supramol-
ecule as shown in Fig. 3.
3.3. IR spectra
The main infrared bands of the ligand and its complexes
are presented in Table 4. The ‘‘free’’ ligand L exhibit two
1
3.4. H NMR spectra and mass spectra
1
L: H NMR (CDCl3): d 7.33–6.96 (m, 18H, ArH and
C12H8–), 4.62 (s, 2H, –O–CH2–CO–), 4.59 (d, J = 5.4 Hz,
2H, –N–CH2–), 4.49 (s, 2H, –O–CH2–CO–), 4.36 (d,
J = 24 Hz, 2H, –N–CH2–), 2.81–2.68 (m, 6H, –CH3).
Fig. 2. Coordination polyhedron of Eu(III) ion in Eu(pic)3L.

Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
3365
Fig. 3. The 2-D layers of [Eu(pic)3L] generated by intermolecular p–p stacking interactions. All the N-methylbenzylamines are omitted for clarity.
Table 4
The most important IR bands of the rare earth picrate complexes (cmꢀ1
The proton signal of the OH group in free HPic disap-
pears in the complexes, indicating that the H atom of the
OH group is replaced by Ln(III). The benzene ring protons
of the free HPic exhibit a singlet at 9.12 ppm. Upon coor-
dination, the signal moves to higher field. Only one singlet
is observed for the benzene ring protons of the three coor-
dination picrate groups, indicating fast exchange among
the groups in solution [15].
The mass spectrum of L shows a molecular ion at
m/z = 509 (M+H), confirming the molecular formula of
L as C32H32N2O4. Above the mass of C12H8 (m/z = 152)
are observed two major fragment ions at m/z = 330 (M –
[–O–CH2–CO–NC6H5(C2H5)]) and 181 (M–[–CO–
NC6H5(C2H5)]–[–O–CH2–CO–NC6H5(C2H5)]).
)
Complex
m(C@O) m(C–O–C) m(C–O) mas (–NO2) ms (–NO2)
Hpic
1265
1555
1342
L
1667
1615
1110
1081
1082
1082
1082
1082
1082
La(pic)3L
1273
1273
1274
1274
1274
1275
1576, 1542 1360, 1329
1577, 1541 1357, 1330
1577, 1542 1359, 1331
1578, 1542 1359, 1331
1578, 1542 1360, 1331
1578, 1542 1359, 1331
Nd(pic)3L 1615
Eu(pic)3L 1616
Gd(pic)3L 1616
Tb(pic)3L
Y(pic)3L
1616
1618
1
[La(pic)3 Æ L]: H NMR (CDCl3): d 8.89 (s, 6H, pic-),
7.36–7.04 (m, 18H, ArH and C12H8–), 4.68 (m, 4H, –N–
CH2–), 4.38 (s, 2H, –O–CH2–CO–), 4.22 (s, 2H, –O–
CH2–CO–), 2.80–2.73 (m, 6H, –CH3).
1
The H NMR spectra of the ‘‘free’’ ligand L, the dia-
3.5. Electronic spectra
magnetic Ln(III) picrate complex La(Pic)3L were measured
in CDCl3. The spectrum of L exhibits two multiplets at ca.
7.26 and 2.75 ppm, two doublets at 4.59 ppm (J = 5.4 Hz)
and 4.36 ppm (J = 24 Hz), and two singlets at 4.62 and
4.49 ppm assigned to C12H8– and –C6H5 protons, –CH3
protons, two groups of –N–CH2– protons, two groups of
–O–CH2–CO– protons, respectively.
Upon coordination, the proton signal of –O–CH2–CO–
in the lanthanum complex is shifted to higher field by
ca.0.25 ppm, and the signal of –N–CH2– moves to lower
field. This is probably due to the inductive effect of Ln–
O(L) bonds and a change in the conformation of the ligand
in the complexes.
The electronic spectra in the visible region of the Ln(III)
complexes exhibit alternations in intensity and shifts in
position of the absorption bands relative to the corre-
sponding Ln(III) aquo ions. The shift has been attributed
by Jorgensen to the effect on the crystal field of interelec-
tronic repulsion between the 4f electrons, and is related
to the covalent character of the metal–ligand bond,
assessed by Sinha’s parameter (d), the nephelauxetic ratio
(b) and the bonding parameter (b1/2) [16]. Absorption spec-
tra of the Eu(III) complex was registered in chloroform
solution at room temperature and the covalent parameters
were calculated (Table 5). The values of b, which are less

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Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
Table 5
4000
Covalent parameters for the neodymium picrate complex
wb55
3500
3000
2500
2000
1500
1000
500
Frequency (cmꢀ1
)
Assignment
Covalent parameters
ExpDec1 fit of row_1 wb55
Complex
4I9/2–4F3/2
b = 0.9984
d = 0.1603
Ex = 421nm
Nd(pic)3L
11261
12469
13405
14771
17212
17414
19048
4F5/2
4F7/2
b
1/2 = 0.0283
4F9/2
4G5/2
2G7/2
4G7/2
5D0 7F2
0
500
450
400
350
300
250
200
150
100
-500
0
2
4
6
8
Time (ms)
Fig. 5. The time-resolved fluorescence spectroscopy of the europium
complex at 77 K.
500
5D0 7F1
CHCl3
ethyl acetate
400
acetone
acetonitrile
300
540
560
580
600
620
640
660
680
700
720
Wavelength (nm)
200
100
0
Fig. 4. The emission spectrum of the europium picrate complex in solid
state at room temperature.
than unity, and positive values of d and b1/2 support the
existence of partial covalent bonding between metal and
ligand [17].
540
560
580
600
620
640
660
3.6. Luminescence study in solid state and in solutions at
room temperature
Wavelength (nm)
Fig. 6. The emission spectrum of the europium picrate complex in
different solutions at room temperature.
The emission spectra of [Eu(pic)3L] at room temperature
in the solid state (the excitation and emission slit widths
were 1.0 nm, Fig. 4) and solution (the excitation and emis-
sion slit widths were 10.0 nm, Fig. 6) were recorded. The
emission spectra are similar to each other and show charac-
teristic emission bands of an Eu(III) ion at about 592 and
Table 6
Fluorescence data for the europium picrate complex
Complex
Eu(pic)3L
Solvent
kex (nm)
kem (nm)
RFI
Assignment
7
solid state
421
593
614
178
491
5D0 ! F1
7
5D0 ! F2
614 nm, corresponding to 5D0 ! F1 and 5D0 ! F2 transi-
tions. The fluorescence intensities at 614 nm are stronger
than those at 592 nm, so that the peak height at 614 nm
was used to measure the fluorescence intensities of an
Eu(III) complex. The fluorescence characteristics of the
europium complex in solid state and in CHCl3, ethyl ace-
tate, acetone, acetonitrile, and DMF solutions (concentra-
tion: 1.0 · 10ꢀ3 mol Lꢀ1) are listed in Table 6. It can be
seen that the Eu complex shows strong emission when
excited with 421 nm in the solid state. The intensity ratio
7
7
7
CHCl3
461
461
461
461
592
613
592
613
593
613
592
614
67
433
32
155
24
117
16
5D0 ! F1
7
5D0 ! F2
7
AcOEt
5D0 ! F1
7
5D0 ! F2
7
acetone
acetonitrile
DMF
5D0 ! F1
7
5D0 ! F2
7
5D0 ! F1
7
52
5D0 ! F2
7
7
value g (5D0 ! F2/5D0 ! F1) is 2.8, indicating a low sym-
metry for the electrostatic field surrounding Eu(III) [18].
This conclusion agrees with the crystal structure of the
complex.
The result of luminescence lifetime study by time-
resolved laser-induced fluorescence spectroscopy of the
europium complex obtained at 77 K is shown in Fig. 5.

Y.-L. Guo et al. / Inorganica Chimica Acta 360 (2007) 3361–3368
3367
5
The lifetime of the D0 level of the Eu(III) ion in the com-
from two bidentate and one unidentate picrates. The coor-
dinate picrates can form intermolecular p–p stacking,
which is very important for the assembly of the complex
molecule units into a two-dimensional (2-D) netlike super-
molecule, and thus enhance the fluorescence intensity of the
europium complex in solid. The lifetime of the 5D0 level of
the Eu(III) ion in the complex is 0.22 ms. The quantum
yield U of the europium complex was found to be
1.01 · 10ꢀ3 with quinine sulfate as reference. The lowest
triplet state energy level of the ligand indicates that the trip-
let state energy level of the ligand matches better to the res-
onance level of Eu(III) than Tb(III) ion. To sum up, we
designed a new open-chain crown ether ligand which can
form stable complexes with lanthanide picrates and sensi-
tize Eu(III) emission. Further studies of the ligand are
under way.
plex obtained from the delayed curve is 0.22 ms.
From Table 6, it also can be seen that in DMF solution,
the fluorescence of europium was quenched which is con-
tributed to the decomposition of the complex in this sol-
vent, and in the other four solvents, the complex has the
similar excitation and emission wavelengths. In CHCl3
solution the europium picrate complex has the strongest
luminescence, followed by ethyl acetate and then acetone
and acetonitrile. This is due to the coordinating effects of
the solvents, namely solvate effect [19] where vibrational
quenching of the complex excited state may occur through
high energy oscillators on the solvent molecule.
The fluorescence intensity of the europium complex in
solution is much lower than the one in solid state. We con-
sider this is on one hand due to the breakage of the p–p
stacking formed by coordinated equivalent picrates which
favor to the energy transfer for europium [20], and on
the other hand due to the coordinating effects of solvents.
The fluorescence quantum yield U of the europium pic-
5. Supplementary material
CCDC 635705 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge via http://www.ccdc.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-
336-033; or e-mail: deposit@ccdc.cam.ac.uk.
rate complex in CHCl3 (concentration: 1.0 · 10ꢀ5 mol Lꢀ1
)
was found to be 1.01 · 10ꢀ3 with quinine sulfate as Ref.
[21].
Compared with the europium complex, the characteris-
tic fluorescence of the terbium complex was not deter-
mined, either in solid state or in solutions. The reason is
probably that the energy gap between the triplet state levels
of the ligand and the lowest resonance level of the euro-
pium favor to the energy transfer process for europium.
In order to acquire the triplet excited state T1 of the ligand
L, the phosphorescence spectra of the Gd(III) picrate com-
plex was measured at 77 K in a chloroform–methanol–eth-
anol mixture (v:v:v, 1:5:5). The triplet state energy levels T1
of the ligand L, which was calculated from the shortest
wavelength phosphorescence band [22] of the correspond-
ing Gd(III) complexes, is 21930 cmꢀ1. This energy level is
above the lowest excited resonance level 5D0 of Eu(III)
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