Tuning the self-assembly and luminescence properties of lanthanide coordination polymers by ligand design

Article abstract of DOI:10.1039/b800217g

Two new structure-related tripodal ligands featuring salicylamide pendant arms, 1,3,5-tris{[(2′-furfurylaminoformyl)phenoxyl]methyl}-2,4,6- trimethylbenzene (L^I) and 1,1,1-tris{[(2′-furfurylaminoformyl) phenoxyl]methyl}ethane (L^II) have been designed and synthesized with the ultimate aim of self-assembling lanthanide polymers with interesting luminescent properties. Among two series of Ln^III nitrate complexes (Ln = Pr, Nd, Sm, Eu, Gd, Tb or Dy) which have been characterized by elemental analyses, XRD, TGA and IR spectra, three new coordination polymers have been determined by X-ray diffraction analysis. The coordination polymer type {[Ln(NO₃)₃(L^I)]?nH₂O} _n possesses an unusual ladderlike double chain which can be further connected through π-π stacking interactions constructing a three-dimensional supramolecular structure. In contrast, the coordination polymer type {[Ln(NO₃)₃(L^II)]?nCH ₃OH}_n displays a (3,3)-connected puckered two-dimensional net with 4?8² topological notation. The photophysical properties of the Sm, Eu, Tb and Dy complexes at room temperature are investigated. The present work substantiates the claim that the supramolecular structure as well as the luminescent properties of the coordination polymer can be tuned by varying either the backbone group or the terminal group of the organic ligand.

Full text of DOI:10.1039/b800217g

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PAPER
www.rsc.org/dalton | Dalton Transactions
Tuning the self-assembly and luminescence properties of lanthanide
coordination polymers by ligand design†
Xue-Qin Song,a,b Wei-Sheng Liu,*a,c Wei Dou,a Jiang-Rong Zheng,a Xiao-Liang Tang,a Hong-Rui Zhanga and
Da-Qi Wangd
Received 21st January 2008, Accepted 9th April 2008
First published as an Advance Article on the web 9th May 2008
DOI: 10.1039/b800217g
Two new structure-related tripodal ligands featuring salicylamide pendant arms,
1,3,5-tris{[(2ꢀ-furfurylaminoformyl)phenoxyl]methyl}-2,4,6-trimethylbenzene (LI) and
1,1,1-tris{[(2ꢀ-furfurylaminoformyl)phenoxyl]methyl}ethane (LII) have been designed and synthesized
with the ultimate aim of self-assembling lanthanide polymers with interesting luminescent properties.
Among two series of LnIII nitrate complexes (Ln = Pr, Nd, Sm, Eu, Gd, Tb or Dy) which have been
characterized by elemental analyses, XRD, TGA and IR spectra, three new coordination polymers have
been determined by X-ray diffraction analysis. The coordination polymer type {[Ln(NO3)3(LI)]·nH2O}n
possesses an unusual ladderlike double chain which can be further connected through p–p stacking
interactions constructing a three-dimensional supramolecular structure. In contrast, the coordination
polymer type {[Ln(NO3)3(LII)]·nCH3OH}n displays a (3,3)-connected puckered two-dimensional net
with 4·82 topological notation. The photophysical properties of the Sm, Eu, Tb and Dy complexes at
room temperature are investigated. The present work substantiates the claim that the supramolecular
structure as well as the luminescent properties of the coordination polymer can be tuned by varying
either the backbone group or the terminal group of the organic ligand.
ligands such as sulfoxides,4 carboxylates,5 pyridones,6 lactames,7
Introduction
4,4ꢀ-bipyridine-N,Nꢀ-dioxide8 and nitrophenolate.9 Generally, the
The construction of coordination polymers has been a field of
polymer topology generated from the self-assembly of inorganic
rapid growth in supramolecular and materials chemistry due
(metal) species and organic ligands can be modified by the
to the formation of fascinating structures and their potential
chemical structure of the ligands chosen. As we know, the organic
applications as new materials.1 So far, most of the work reported
ligands serve to tether the metal centers and to propagate the
in the literature has been focused on the coordination polymers
structural information expressed in metal coordination prefer-
of d-block transition metal elements.2 In contrast with transition-
ences throughout the extended structure, as a result, properties of
metal complexes, covalence plays a minor role in metal–ligand
the organic ligands such as coordination ability, length, geometry,
bonds of lanthanide complexes, and the nature of the coordination
relative orientation of the donor groups and flexibility play a
sphere is controlled by a tenuous balance between coulombic
very important role in dictating polymer framework topology. In
interactions and ligand steric hindrance.3 The high coordination
the long run, this line of research can lead to predictions of the
number and flexible coordination geometry of lanthanide metal
topology and/or the periodicity of crystalline lattices generated
ions make it difficult to control the preparation of lanthanide
from the molecular structures of the participating small building
complexes. Therefore, the use of lanthanide ions as nodes for
blocks, one can anticipate that the relationship between polymeric
the construction of coordination polymers is more difficult than
structures and physical properties will eventually be elucidated as
their d-block metal analogues.4a Recently a few examples have
well.
appeared on lanthanide-based frameworks employing polydentate
Judicious choice of ligand is also required to ensure that the
chosen lanthanide metal is well shielded from the surrounding
aCollege of Chemistry and Chemical Engineering and State Key Laboratory
environment to prevent quenching of the excited state and
of Applied Organic Chemistry, Lanzhou University, Lanzhou, 730000, China.
enhance its emission. By deliberate incorporation of appropriate
E-mail: liuws@lzu.edu.cn; Fax: +86-931-8912582; Tel: +86-931-8912552
bSchool of Chemical and Biological Engineering, Lanzhou Jiaotong Univer-
sity, Lanzhou, 730070, China
multiple absorption groups suitable for energy transfer the tripodal
ligands could be used to develop strongly luminescent lanthanide
complexes.10 Owing to the structural features of salicylamide
derivatives, their lanthanide complexes have some advantages for
luminescence research. First, the ligand featuring salicylamide
pendant arms can strongly minimize the probability of solvent
molecule coordination (that is especially important for water
molecules); second, the high extinction coefficients of the ligand
featuring salicylamide pendant arms in the near UV-vis range
provide effective absorption of excitation energy which promotes
cState Key Laboratory of Coordination Chemistry, Nanjing University,
Nanjing, 210093, China
dDepartment of Chemistry, Liaocheng University, Liaocheng, 252000, China
† Electronic supplementary information (ESI) available: TGA diagrams,
XRD patterns, fluorescence spectra of the Sm and Dy complexes at room
temperature, phosphorescence spectra of the Gd complex of ligands LI and
◦
LII at 77 K and selected bond distances (A) and angles ( ) for polymers
1, 2, 3 (Fig. S1–S8 and Tables S1–S3). CCDC reference numbers 672449–
672452. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b800217g
˚
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more effective energy transfer to the lanthanide ion; third, the
presence of O and N atoms and aromatic rings may allow po-
tentially strong hydrogen bonds and p–p stacking interactions to
form supramolecular structures. We are interested in how different
types of rigid and flexible backbone as well as different terminal
coordination sites impact the coordination and supramolecular
motifs of a variety of lanthanide complexes based on ligands
with salicylamide arms. So far, several attractive supramolecular
complexes based on salicylamide derivatives, such as benzylsali-
cylamide derivatives and also related types of ligand have been
used to construct supramolecular complexes.11–13 However, the
coordination chemistry based on five-membered heterocyclic ring
containing tridentate ligands, such as LI and LII reported herein,
which are analogues of the extensive series of ligands studied
above, is still unprecedented.
In view of this, we report here the syntheses of two new
members of a family of tripodal ligands, namely, 1,3,5-tris{[(2ꢀ-
furfurylaminoformyl)phenoxyl]methyl}-2,4,6-trimethylbenzene
(LI) and 1,1,1-tris{[(2ꢀ-furfurylaminoformyl)phenoxyl]methyl}-
ethane (LII) and describe the syntheses and the luminescence
properties of the resulting complexes formed with ions used
in fluoroimmunoassays (Ln = Sm, Eu, Tb, Dy). Three poly-
meric coordination complexes, namely, [Pr(NO3)3(LI)·H2O]n (1),
[Nd(NO3)3(LII)·0.5CH3OH]n (2), and [Sm(NO3)3(LII)]n (3), based
on the two ligands LI and LII, were structurally characterized by
single-crystal X-ray diffraction.
Structural analysis of ligand LI. The 1,3,5-trimethylbenzene
spacer exhibits symmetry and plays a crucial role in controlling
the steric configuration of the functional groups introduced
into its 2,4,6-positions. Namely, the functional groups attached
to the 2,4,6-positions are forced to position themselves at the
same side of the benzene ring of the spacer, since the ababab
configuration (a denotes ‘above’ and b denotes ‘below’) is the
most thermodynamically stable configuration due to the steric
repulsion between the neighboring substituents. The structure of
LI has been fully characterized by infrared spectroscopy, elemental
analysis and 1H NMR. To further confirm the structure of the new
ligand, the solid molecular structure of LI was determined using
single-crystal X-ray diffraction. The crystal structure of LI reveals
that the unsymmetric unit contains four crystallographically and
conformationally equivalent molecules. As shown in Fig. 1, LI
crystallizes in space group P21/n. The bridging trimethylbenzene
moiety and three furfurylsalicylamide arms are linked together
at the meta position by a methylene group, which gives rise to
ligand LI being asymmetric with an angular backbone driven
by the sp3 configuration of the bridging carbon atom. Three
arms of the ligand are severely twisted with a ca. 70◦ dihedral
angle between phenyl and the terminal furan moiety attached
to the phenyl ring. The dihedral angle between the phenyl rings
is 45.19, 68.33 and 78.39◦. Such an asymmetric bent organic
spacer with divergent properties is potentially useful in the con-
struction of polymeric complexes with photophysical properties.
The amide hydrogen atoms (H(11), H(14), H(36)) are hydrogen
bonded to the ether oxygen atom (O(3), O(3) · · · H(14) 1.94(8)
◦
˚
˚
Result and discussion
A, N(1)–H(14) · · · O(3) 134(2) , and O(3) · · · N(1) 2.620(17) A;
◦
˚
O(7), O(7) · · · H(11) 2.02(5) A, N(3)–H(11) · · · O(7) 135(17) , and
Synthesis and structural analysis of ligands LI and LII
˚
˚
O(7) · · · N(3) 2.68(6) A; O(4), O(4) · · · H(36) 2.00(3) A, N(2)–
◦
˚
H(36) · · · O(4) 134(70) , and O(4) · · · N(2) 2.705(17) A) to provide
three hydrogen bonded six-membered rings. It is no doubt
noteworthy that the robust intramolecular hydrogen bonds have a
template effect and participate in the stabilization of the complete
architecture.
The interesting coordination chemistry exhibited by salicylamide
derivatives encouraged us to undertake further studies on the
design and synthesis of new types of salicylamide ligand. Follow-
ing this approach, we expand benzene containing salicylamide to
furan-containing salicylamide. As shown in Scheme 1, LI and LII,
were successfully obtained which are pale white crystalline solids
and are soluble in common polar organic solvents such as ethyl
acetate, CHCl3, CH3OH, and DMF. This potentially facilitates
the solution reaction between the ligand and inorganic metal salts.
The IR spectra of these ligands showed an –NH absorption band
around 3400 cm−1. The strong absorption bands around 1646 cm−1
are consistent with carbonyl absorption, which is comparable to
those corresponding bands in known compounds.
Fig.
ellipsoids).
1
Molecular structure of LI (30% probability displacement
Scheme 1 Schematic illustration of ligands LI and LII.
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Table 1 Elemental analytical and IR spectral data for the complexes
Elemental analysesa (%)
IR (kmax/cm−1
)
=
C
H
N
Ln
m(C O)
PrLI(NO3)3·H2O
NdLI(NO3)3
50.08 (50.01)
50.49 (50.65)
50.27(50.38)
50.19 (50.31)
50.14 (50.08)
50.09 (50.01)
49.77 (49.85)
47.66 (47.50)
47.43 (47.35)
47.21 (47.23)
47.09 (47.16)
47.07 (46.92)
46.69 (46.85)
46.58 (46.70)
4.09 (4.11)
4.00 (3.99)
3.97 (3.96)
3.95 (3.96)
3.94 (3.94)
3.92 (3.93)
3.93 (3.92)
4.01 (4.03)
4.03 (4.02)
3.88 (3.87)
3.87 (3.86)
3.83 (3.84)
3.84 (3.84)
3.82 (3.83)
7.26 (7.29)
7.36 (7.38)
7.33 (7.34)
7.35 (7.33)
7.32 (7.30)
7.30 (7.29)
7.25 (7.27)
7.80 (7.82)
7.79 (7.80)
7.85 (7.87)
7.86 (7.86)
7.80 (7.82)
7.83 (7.81)
7.76 (7.78)
12.27 (12.22)
12.63 (12.67)
13.19 (13.14)
13. 22 (13.26)
13.60 (13.66)
13.84 (13.79)
13.99 (14.05)
13.08 (13.11)
13.42 (13.38)
14.04 (14.08)
14.16 (14.21)
14.59(14.63)
14.78 (14.76)
15.00 (15.04)
1610
1612
1610
1611
1612
1613
1610
1606
1607
1606
1606
1607
1608
1607
SmLI(NO3)3
EuLI(NO3)3
GdLI(NO3)3
TbLI(NO3)3
DyLI(NO3)3
PrLII(NO3)3·0.5CH3OH
NdLII(NO3)3·0.5CH3OH
SmLII(NO3)3
EuLII(NO3)3
GdLII(NO3)3
TbLII(NO3)3
DyLII(NO3)3
a Data in parentheses are calculated values.
Structural analysis of lanthanide complexes based on tripodal
ligands LI and LII
of the structure of 1 it can be seen that two adjacent chains are
interlinked by two kinds of p–p staking interactions. The dihedral
angle between benzene rings (C(26), C(27), C(28), C(29), C(30),
C(31)) is 0◦ and the distance between the centroid of each ring is
Treatment of Ln(NO3)3·6H2O with the new ligands in ethyl
acetate–methanol solution in a 1 : 1 ratio yields a series of
complexes which, according to XRD, combustion analysis, and
IR exhibit the same structure and correspond to the formula
LnLI(NO3)3·nH2O and LnLII(NO3)3·nCH3OH (Ln = Pr, Nd, Sm,
Eu, Gd, Tb, Dy, n = 0, 0.5, 1) respectively. IR and elemental analy-
sis data for all complexes are summarized in Table 1. X-Ray quality
crystals of {[PrLI(NO3)3]·H2O}n, {[NdLII(NO3)3]·0.5CH3OH}n,
and [SmLII(NO3)3]n were obtained after several weeks of slow
evaporation of the ethyl acetate–methanol solution. Complexation
of the arms through the carbonyl functions is reflected in the
˚
˚
3.777 A and 3.736 A, while the aromatic p–p staking interactions
between two parallel furan rings are situated at a distance of
˚
3.786 A. In this way, the ladder-like polymer double chains are
connected together and construct a 3D supramolecular structure
(Fig. 2C). It is important to note that such an arrangement may
reduce steric hindrance and make the whole framework more
stable.
Structural analysis of [Nd(NO3)3(LII)]n (2). To confirm the
role of different backbone groups in the self-assembly process,
LII was used instead of LI to perform the reaction. Compounds
2 and 3 were obtained as crystals by combination of LII with
Ln(NO3)3·6H2O in an ethyl acetate–methanol system. Single-
crystal X-ray analysis revealed that they are isomorphous with
only very minor differences in their comparative geometries
reflecting the normal LnIII contraction effects, thus only 2 is
described here in detail. As shown in Fig. 3A-1, the NdIII center
lies in a monocapped square antiprism coordination environment
defined by nine O-donors from the carbonyl groups of three
tridentate ligands LII (O(4), O(6) and O(8)), and six O-donors
from three bidentate coordinated nitrate groups (O(10), O(11),
O(13), O(14), O(16) and O(17)). Each Nd3+ is coordinated to three
LII ligands; each LII ligand provides O(4) for one Nd3+, O(6)for the
second Nd3+ and O(8) for the third Nd3+, thus each LII ligand is
coordinated to three Nd3+ and each Nd3+ is coordinated to three
LII ligands to form a 1 : 1 complex. The Nd3+ cation acts as a three-
connecting node linking neighboring Nd3+ cation centers via the
ligands, forming a 2D coordination polymer. It is worth pointing
out that LII acts as a tridentate bridging spacer in 2 with all three O-
donors binding NdIII centers, which is markedly distinct from the
coordination behavior of the previous report.11 The only possible
explanation for this change might be the templating effect resulting
from the different terminal groups due to all the reactions being
carried out under exactly the same conditions including solvent
system, metal-to-ligand ratio and temperature. In addition, the
intramolecular N–H · · · O hydrogen bonds, formed among the
=
m(C O) vibration which is shifted towards lower energy by
ca. 36 cm−1. The complexes are soluble in DMF, DMSO and
methanol, slightly soluble in ethyl acetate, acetonitrile and acetone,
but insoluble in CHCl3 and diethyl ether.
Structural analysis of [Pr(NO3)3(LI)·H2O]n (1). As shown in
Fig. 2A-1, each PrIII center in 1 adopts a distorted monocapped
square antiprism coordination sphere, which is defined by three
oxygen donors from three LI ligands and three bidentate coordina-
tion nitrate oxygen groups. The Pr–O distances are from 2.398(5)
˚
to 2.584(7) A, which are comparable to the corresponding Pr–O
bond lengths found in related complexes. It is worth noting that
a surprisingly similar orientation of the three furfurylsalicylamide
arms is found between free ligand LI and its complex (Fig. 2A-2)
showing that the conformation of ligand LI does not change upon
coordination. Each praseodymium(III) center, shown as green
large spheres in Fig. 2, connects three ligands of LI. Similarly, each
ligand molecule is connected to three praseodymium atoms. The
combination of tripodal LI and nona-coordinated praseodymium
atoms in two adjacent chains are joined together by sharing the
O(8) carbonyl oxygen atom to build up an unusual ladderlike
double chain in which one of the ligand arms acts as the middle
rungl of the ladder (Fig. 2B) which is distinctly different from
previous report by Tang et al.12
It is interesting to note that evidence for intermolecular hydro-
gen bonding was not found. Upon further careful investigation
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Fig. 3 (A) ORTEP plot of complex 2 showing the local coordination
environment of NdIII with thermal ellipsoids at 30% probability (disordered
methanol molecules are omitted for clarity) and ligand LII in complex
2 showing a gathered-round fashion. (B) Schematic description of the
(3,3)-connected 2D net with 4·82 topological notation built on the
3-connected organic spacers (T1) and metal centers (T2): the connectors
of ligand and Nd atoms are colored in gray and purple, respectively.
which is quite different from LII itself. Each salicylamide arm had
a similar conformation and the arm is so long that their terminal
groups can “fold back”. Three oxygen atoms of the carbonyl
groups in the ligand, namely, O4, O6 and O8 are located in the
outer part of the whole ligand molecule. This conformation is
necessary for the ligand to bind three NdIII ions simultaneously.
Overall, the ligands in 2 and 3 were arranged in a virtually identical
fashion, with most of the differences due to slight changes in
folding at –CH2– groups or small rotations of C–C bonds which
were caused by lanthanide contraction effects.
To deeply understand the nature of the involuted frameworks, a
topological approach can be used, i.e., reducing multi-dimensional
structures to simple node-and-connection nets. Obviously, in this
framework, one NdIII ion is linked with three ligands through
carbonyl groups, which could be considered as an inorganic
three-connected node (T1; Fig. 3B). Likewise, each LII ligand is
connected to three NdIII ions through carbonyl groups, so it could
be regarded as an organic three-connected linker (T2; Fig. 3B).
There is one four-membered ring, and two eight-membered rings
around the three-connected metal node, thus forming a three-
connected linker. Because the three-connected nodes and three-
connected linkers are arranged in a ratio of Nd/L = 1 : 1, the
short Schl
Fig. 2 (A) ORTEP plot of complex 1 showing the local coordination
environment of PrIII with thermal ellipsoids at 30% probability (disordered
water molecules are omitted for clarity) and ligand LI in complex 1 showing
a divergent fashion. (B) View of the 1D ladder-like coordination polymer
of the PrIII complex with the LI ligand along the a-axis and c-axis (hydrogen
atoms as well as crystalline water molecules are omitted for clarity). (C) The
3D supramolecular structure of 1 constructed by p–p stacking interactions
which are indicated with dashed orange lines (viewd along the a-axis,
hydrogen atoms as well as crystalline water molecules are omitted for
clarity).
a¨fli symbol of the topology can be expressed as 4·82
(Fig. 3B). Ultimately, this special assembly may be related to
the flexible coordination of the lanthanide centers and flexible
conformations of the ligand. Thus, polymer 2 affords the (3,3)-
connected puckered 2D net with 4·82 topological notation.
carbonyl oxygens, amide nitrogen atoms and nitrate ligands, play
a vital role in stabilizing the 2D coordination structure.
It is interesting to note that the ligand LII was arranged with all
three salicylamide arms in a gathered-round fashion (Fig. 3A-2)
TGA, XRD and MS studies. To examine the thermal stability
of the resulting polymers, thermal gravimetric analysis (TGA) was
carried out (Fig. S1 and S2, ESI†) on crystalline samples of the
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compounds in the range 25–625 ◦C. The two kinds of complex
exhibited very similar mass losses over the entire operating range
respectively. The◦mass decrease of the coordination polymers of LI
began at ∼188.2 C, while the coordination polymers of LII showed
higher◦ thermal stability with mass remaining constant until
∼234 C, at which point decomposition commenced. The X-ray
powder diffraction pattern of the decomposition product obtained
at 625 ◦C gave diffraction peaks corresponding to lanthanide oxide
and some other minor peaks belonging to unknown compounds. It
is obvious that the coordination polymers of LII are thermally more
stable than that of LI, which could be attributed to the fact that
Ln–O bonds are normally much stronger in complexes of LII than
in LI. To explore the other complex structures, with the exception
of 1, 2 and 3, powder X-ray diffraction (XRD) of those complexes
was investigated. The XRD patterns (Fig. S3 and S4, ESI†) of
the other complexes showed the main reflections remaining nearly
identical with the 1, 2 and 3 samples, respectively, which supported
the notion that the two kinds of complexes are isostructural.
To get structural information in solution, we have successfully
obtained the positive-ion ESI mass spectra of the coordination
polymers of ligands LI and LII in ethyl acetate–methanol solution
respectively, although the measurements of their matrix-assisted-
laser-desorption ionization (MALDI) mass spectra were unsuc-
cessful. In the ESI mass spectrum of the GdIIIcomplex of ligand
LII, 1 : 1, 1 : 2, 1 : 3, 2 : 3, 3 : 2 species were detected (Fig. 4). The
first two species were observed as [GdLII(NO3)2]+ (m/z 1013.4)
and [GdLII2(NO3)2]+ (m/z 1744.8), in addition, the 3 : 1 species
was observed as a dicationic species [GdLII3(NO3)]2+ (m/z 1207.6),
2 : 3 and 3 : 2 species were observed as penta-cationic species
[Gd2LII3(NO3)]5+ (m/z 516.0) [Gd3LII2(NO3)4]5+ (m/z 437.0). This
observation strongly indicates that the polymeric complexes of the
ligands observed in the solid state are maintained in solution too.
The idea behind the use of the two ligands is to explore the
coordination chemistry and self assembly principles of tripodal
type ligands. There is no doubt that tripodal bridging ligands
with three coordination sites are expected to result in more
unprecedented coordination polymers with novel topologies. As
discussed above, LI herein exhibits a divergent spacer to link
metal ions into 1D ladder-like polymeric complexes. In contrast
to LI, LII act as a convergent ligand and binds metal ions into the
(3,3)-connected puckered 2D net with 4·82 topological notation.
These results substantially indicate that the different rigidity of the
backbone groups as well as the different aromatic terminal groups
play a central role in determining the supermolecular structure of
the coordination polymers.
UV-vis spectroscopic studies
As shown in Fig. 5, the absorption spectra of the ligand LI in
acetonitrile features one main band located at around 294 nm
(e/dm3 mol−1 cm−1, 11 800) which can be assigned to characteristic
p–p* transitions centered on the salicylamide units. It is interesting
to note that the UV-vis absorption spectrum of LII in acetonitrile is
almost the same as that observed for LI (293 nm, e/dm3 mol−1 cm−1,
9920) which shows that the backbone groups do not substantially
affect the electronic properties of the salicylamide moiety. The
Fig. 4 Positive-ion ESI mass spectra measured in ethyl acetate and methanol mixed solution, the inset shows an enlargement around the peaks m/z
1207 and 1747.
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Sensitization of lanthanide-centered emission by ligands
Upon UV irradiation, the free ligands emit a strong blue lumi-
nescence (apparent kmax at ca. 463 nm for LI and ca. 454 nm for
LII that can be easily detected with the naked eye. However, upon
complexation this emission is replaced by the typical luminescence
color arising from the corresponding lanthanide cation. These
colors can also be easily visualized using a standard laboratory
UV lamp (kex ≈ 365 nm), and no loss in signal intensity over a
period of several weeks was observed, indicating good stability of
the complexes.
For the SmIII complex of LII, the emission intensity is the
weakest, but two characteristic bands can still be observed. They
Fig. 5 Electronic absorption spectra of LI (dotted line) and LII (solid line)
in acetonitrile (5 × 10−5 M).
6
are attributed to 4G5/2 → HJ (J = 5/2, 7/2) transitions (Fig. S5,
ESI†). The solid state excitation and emission spectrum of the
europium complex of LII at room temperature is shown in Fig. 7.
The excitation spectrum of the europium complex of LII exhibits
a series of sharp lines characteristic of the EuIII energy-level
structure, and can be assigned to transitions between the 7F0, 1 and
5L6, 5D2, 1 levels16 which indicates that the EuIII luminescence is not
efficiently sensitized by the ligand. There are five characteristic
peaks of EuIII shown in the emission spectra which are attributed
large molar absorption coefficient for the ligands indicates that the
two tripodal ligands have a strong ability to absorb light. When
Ln3+ was added to an acetonitrile solution of LI or LII, respectively,
the absorption spectrum showed little change which proves that the
lanthanide ions do not substantially affect the electronic properties
of the salicylamide moiety either. But it must be noted that the
complexes of LI tended to precipitate more easily because of their
poor solubility in acetonitrile than those of LII. Weak absorption
bands of the Nd complex of LII were also observed at k = 490–
550, 550–640, 650–700, 715–775, 775–845 and 845–925 nm, as
shown in Fig. 6. The electronic spectra in the visible region of
the NdIII complex exhibits alterations in intensity and shifts in
position of the absorption bands relative to the corresponding
LnIII aquo ions which have been attributed by Jørgensen to the
effect of crystal fields upon interelectronic repulsion between the
4f electrons, and is related to covalence in the metal–ligand bond,
assessed by Sinha’s parameter (d), and the nephelauxetic ratio
(b).14,15 The b value (1.0028) for the NdIII complex is more than
unity and the d value (−0.2134) is negative, suggesting that the
Nd3+–O bond of the complex has weaker covalency than that of
the Nd3+ aquo ion.
7
7
7
to 5D0 → F0 (580 nm), 5D0 → F1 (592 nm), 5D0 → F2 (616 nm),
5D0 → F3 (648 nm) and D0 → F4 (693 nm). The sharpness of
7
5
7
the non-degenerate transition (5D0 → F0) indicates that all the Eu
7
sites in the sample have essentially identical ligand coordination
and structural environments. The intensity of the 5D0
→
7F2
transition (electric dipole) is stronger than that of the 5D0 → F1
transition (magnetic dipole), which indicates that the coordination
environmentofthe EuIII ion is asymmetric.17 For TbIII, the emission
intensity is stronger than that of the other three in the same
conditions. There are three characteristic peaks shown in Fig. 8
7
5
7
which are assigned to the D4 → FJ (J = 4, 5, 6) transitions.
For DyIII, three characteristic bands can be seen in the emission
spectra, which are attributed to transitions of 457 nm (4I15/2
→
6H15/2), 481 nm (4D9/2 → H15/2) and 577 nm (4D9/2 → H13/2
)
6
6
(Fig. S6, ESI†). Compared with the emission spectra of the four
complexes (Table 2), the transition intensity changes in the order
TbIII > DyIII > EuIII > SmIII which means that the energy transfer
Fig. 7 Room-temperature excitation and emission spectra for europium
complex of ligand LII (kex = 397 nm, excitation and emission passes =
2.5 nm) in the solid state with emission monitored at approximately 616 nm.
Fig. 6 Absorption spectra of the NdIII complex (0.02 M acetonitrile
solution) showing f–f transitions.
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Table 2 Photophysical data of the complex
kmaxa/nm
s/ms
Ub (%)
LI
Sm
Eu
Tb
Dy
Sm
Eu
Tb
Dy
564 (50.56), 598 (82.57)
593 (138), 619 (498)
492 (2803), 546 (7434), 580 (389.2)
482 (2622), 573 (1866)
562 (96.5), 97 (117)
580 (54.57), 593 (283.6) 618 (536.1), 648 (14.62), 693 (16.17)
492 (>10 000), 546 (>>10 000), 583 (2099), 590 (1843)
457 (76.53), 482 (2622), 573 (1866)
—
—
0.558
1.021
—
0.050
1.613
2.014
0.069
0.82
5.2
—
0.3
2.6
44
LII
1.6
a Excitation and emission passes = 2.5 nm. Data in parentheses for the relative fluorescence intensity are given in a.u. b Luminescence lifetimes and
quantum yield values are reported here with an error of 15%.
Energy transfer between the ligand and LnIII
To demonstrate the energy transfer process, the phosphorescence
spectrum of the Gd complex of LI and LII were measured for triplet
energy-level data. From the phosphorescence spectra (Fig. S7 and
S8, ESI†), the triplet energy level (3pp*) of the Gd complex, which
corresponds to its lower wavelength emission edge, is 22 727 cm−1
(440 nm) and 24 449 cm−1 (409 nm) respectively. Because the lowest
6
excited state, P7/2 (E(6P7/2) = 32 000 cm−1) of GdIII is too high
to accept energy from the ligand, the data obtained from the
phosphorescence spectra actually reveal the triplet energy level
of ligands in lanthanide complexes. The singlet state energy (1pp*)
level of the ligand is estimated by referencing its absorbance edge,
which is 32 467 and 32 258 cm−1 (308 nm and 310 nm) respectively.
In general, the sensitization pathway in luminescent europium
complexes consists of excitation of the ligands into their excited
singlet states, subsequent intersystem crossing of the ligands to
Fig. 8 Room-temperature emission for ligand LII
its terbium complex (---, kex = 318 nm) in the solid state (excitation and
emission passes = 1 nm), The inset shows the lifetime decay curve of the
Tb complex of ligand LII.
(
, kex = 326 nm) and
their triplet states, and energy transfer from the triplet state to the
5DJ manifold of the EuIII ions, followed by internal conversion to
the emitting 5D0 state. Finally, the EuIII ion emits when a transition
to the ground state occurs.18 Moreover, electron transition from the
higher excited states, such as 5D3 (24 800 cm−1), 5D2 (21 200 cm−1),
5
5
and D1 (19 000 cm−1), to D0 (17 500 cm−1) becomes feasible by
internal conversion, and most of the photophysical processes take
place in this orbital. Consequently, most europium complexes give
rise to typical emission bands at ∼ 581, 593, 614, 654, and 702 nm,
from the organic ligands to TbIII and DyIII is more effective than
that to EuIII and SmIII. Furthermore, the intensity of the four
complexes based on the LI ligand is relatively weak, possibly due
to the steric effects of the backbone group, which prevents the
salicylamide chromophore of LI from being accessible to the LnIII
ion reducing the luminescent intensity of the lanthanide ions.
In addition to steady state measurements, the luminescence
lifetimes of the Sm(4G5/2), Eu(5D0), Tb(5D4) and Dy(4F9/2) excited
states and quantum yield determinations upon ligand excitation
for each of the differing complexes were determined with the
results also summarized in Table 2. We note that both the lumines-
cence lifetimes and quantum yield values were strongly influenced
by the backbone groups. The relatively long luminescence lifetimes
and larger quantum yield values are an indication that the tripodal
ligand provides a significant level of protection from non-radiative
deactivation of lanthanide cations by the solvent molecules which
is consistent with the single crystal analysis. In view of the fact
that both U and s have diminished by about the same factor, it is
evident that the main problem lies in a decrease of sensitization
efficiency and not in an increase of non-radiative deactivation of
the lanthanide excited state. This is most likely due to insufficient
inter-system crossing (ISC) of LI as further supported by the
energy transfer discussed below.
5
corresponding to the deactivation of the D0 excited state to the
7FJ ground states (J = 0–4). In a similar way, the 4f electrons
of the TbIII ion are excited to the 5DJ ion manifold from the
ground state. Finally, the TbIII ion emits when the 4f electrons
undergo a transition from the excited-state of 5D4 to the 7FJ
3
1
ground states (J = 6–4). The gap between the pp* and pp*
1
3
states (DE = pp* − pp* = 9740 cm−1 or 7809 cm−1) of ligands is
favorable for a relatively efficient intersystem crossing process.19
Latva’s empirical rule20 states that an optimal ligand-to-metal
3
5
energy transfer process for LnIII needs (DE = pp* − DJ ) 2500–
4000 cm−1 for EuIII and 2500–4500 cm−1 for TbIII. The triplet energy
level of the two salicylamide ligands (22 727 cm−1 and 24 449 cm−1)
is higher than the 5D0 level of EuIII (17 500 cm−1) and the 5D4 level
of TbIII (20 400 cm−1). This therefore supports the observation of
stronger sensitization of the terbium complexes than the europium
complexes because of the smaller overlap between the ligand triplet
and europium ion excited states. It is important to note that
the stronger sensitization of the LII complexes than that of LI
3
complexes is mainly due to the higher pp* energy of LII which
depends on the backbone groups.
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[LnLII (NO3)3] (Ln = Pr, Nd, Sm, Eu, Gd, Tb or Dy). To a clear
solution of LII (0.073 g, 0.1 mmol) in ethyl acetate (5 ml) was added
Ln(NO3)3·5H2O (0.1 mmol) in 5 ml of ethyl acetate. The resulting
solution was left stirring overnight at room temperature to afford
a pale white solid which was filtered off, washed three times with
ethyl acetate, and dried over CaCl2 for 1 d to give the product in
50–60% yield. One millimole of ligand and 1 equiv. (1 mmol) of
the metal nitrates were dissolved in a hot methanol + ethyl acetate
(v/v = 1 : 10) solution to make a concentrated solution. Then
the flask was cooled, and the mixture was filtered into a sealed
25–40 ml glass vial for crystallization at room temperature. After
about three weeks crystals suitable for analysis were obtained.
Experimental
Synthesis of the ligands
LI. The ligand 1,3,5-tris{[(2ꢀ-furfurylaminoformyl)phenoxyl]-
methyl}-2,4,6-trimethylbenzene (LI) was prepared by the follow-
ing synthetic route. To a solution of 1,3,5-tris(bromomethyl)-2,4,6-
trimethylbenzene21 (1.31 g, 3.3 mmol) in dry acetone was added
K2CO3 (0.69 g, 5 mmol) and the mixture stirred and heated for
10 min, furfurylsalicylamide22 (2.39 g, 11 mmol) in 50 ml of acetone
was added dropwise over 30 min and the resulting solution stirred
and heated to reflux for 12 h. After cooling, the inorganic salts
were then separated by filtration and the solvent removed from the
filtrate under reduced pressure. The crude product was purified
by chromatography on silica, gradient elution from petroleum
ether (boiling range: 60–90 ◦C) to 1 : 5 petroleum ether–ethyl
acetate. 2.32 g, Yield 87%; mp: 158.7 ◦C. Analytical data, calc.
Materials and instrumentation
The commercially available chemicals were used without further
purification. All of the solvents used were of analytical reagent
grade.
for C48H45N3O9: C, 71.36; H, 5.61; N, 5.20; Found: C, 71.59, H,
5.59, N, 5.18%; IR (KBr, m, cm ): 3379 (s, NH), 1646 (s, C O),
−1
=
The metal ions were determined by EDTA titration using
xylenol orange as indicator. C, N and H were determined using
an Elementar Vario EL. Melting points were determined on
a Kofler apparatus. Thermogravimetric analyses (TGA) were
performed with a WCT-2A thermoanalyzer under air atmosphere
1599 (m), 1518 (s), 1469 (m), 1292 (m), 1223 (s), 978 (m), 755 (m).
1H NMR (CDCl3, 300 MHz): d: 2.36 (s, 9H), 4.39 (d, J = 5.2, 6H,
NHCH2), 5.18 (s, 6H, OCH2), 6.89 (d, J = 3.0, 3H, Ar), 6.09 (t,
3H, Ar), 7.02–7.21 (m, 9H, Ar), 7.50–7.58 (m, 3H, Ar), 8.02 (s,
3H, NH), 8.27–8.31 (dd, 3H, Ar).
◦
(25–625 ◦C) at a heating rate of 10 C min−1. X-Ray powder
diffraction (XRD) patterns were obtained on a Rigaku D/Max-
II X-ray diffractometer with graphite-monochromatized Cu-Ka
radiation. IR spectra were recorded on a Nicolet FT-170SX
instrument using KBr discs in the 400–4000 cm−1 region. 1H NMR
spectra were measured on a Bruker DRX 300 spectrometer in
CDCl3 solution with TMS as internal standard. Electronic spectra
were recorded with a Varian Cary 100 spectrophotometer in
acetonitrile solution. Mass spectra (electrospray ionization time-
of-flight, ESI-TOF, positive mode) were recorded on Mariner
ABI Mass spectrometer. Fluorescence measurements were made
on a Hitachi F-4500 spectrophotometer and a shimadzu RF-
540 spectrofluorophotometer equipped with quartz curettes of
1 cm path length with a xenon lamp as the excitation source.
An excitation slit of 2.5 or 1 nm and an emission slit of 2.5
or 1 nm were used for the measurements in the solid state.
The 77 K solution-state phosphorescence spectra were recorded
with solution samples loaded in a quartz tube inside a quartz-
walled optical Dewar flask filled with liquid nitrogen in the
phosphorescence mode.
LII. The
ligand
1,1,1-tris{[(2ꢀ-furfurylaminoformyl)-
phenoxyl]methyl}ethane (LII) was prepared by
a similar
synthetic route. To a solution of 1,1,1-tris(p-tosyloxymethyl)-
propane23 (2.98 g, 5 mmol) in dry DMF was added K2CO3
(1.00 g, 7 mmol) and the mixture stirred and heated for 10 min,
furfurylsalicylamide (3.08 g, 16.5 mmol) in 50 ml of DMF was
added dropwise over 30 min and the resulting solution stirred and
heated to reflux for 48 h. After cooling, the inorganic salts were
then separated by filtration and the solvent removed from the
filtrate under reduced pressure. The crude product was purified
by chromatography on silica, gradient elution from petroleum
ether to 1 : 8 petroleum ether–ethyl acetate. 1.75 g, Yield 48%; mp:
126.1 ◦C. Analytical data, calc. for C42H41N3O9: C, 68.93; H, 5.65;
N, 5.74; Found: C, 68.74, H, 5.67, N, 5.75%; IR (KBr, m, cm−1):
=
3406 (s, NH), 1647 (s, C O), 1601 (m), 1530 (m), 1486 (m), 1295
(m), 1224 (s), 1012 (m), 753 (m). 1H NMR (CDCl3, 300 MHz): d:
0.63 (t, 3H CH3), 1.35 (m, 2H, CH2), 4.18 (s, 6H, OCH2), 4.49 (d,
J = 3.6, 6H, NHCH2), 6.09–6.12 (m, 6H, Ar), 6.81 (d, J = 5.4,
3H, Ar), 7.11 (s, 3H, Ar), 7.11–7.15 (t, 3H, Ar), 7.18–7.22 (t, 3H,
NH), 7.99–8.02 (dd, 3H, Ar).
Quantum yields were determined by the optically dilute
method24 using the following equation:
Syntheses of the complexes
[LnLI(NO3)3] (Ln = Pr, Nd, Sm, Eu, Gd, Tb or Dy). To a clear
solution of LI (0.081 g, 0.1 mmol) in ethyl acetate (10 ml) was added
Ln(NO3)3·5H2O (0.10 mmol) in 5 ml of ethyl acetate. The resulting
solution was left stirring overnight at room temperature to afford
a pale white solid which was filtered off, washed three times with
ethyl acetate, and dried over CaCl2 for 1 d to give the product
in 70–80% yield. A layer of a solution (10 mL) of ethyl acetate
was carefully layered over an 4 mL ethyl acetate solution of LI
(0.01 mmol, 8.1 mg). Then, an methanol solution (2 mL) of
lanthanide nitrate salts was carefully layered over the buffer layer.
Pale green crystals appeared after 4–5 weeks and were collected
and dried in air after being washed with ethyl acetate.
Here A is the absorbance at the excitation wavelength (k), I is
the intensity of the excitation light at the same wavelength, n is the
refractive index, and D is the integrated luminescence intensity.
The subscripts “x” and “r” refer to the sample and reference,
respectively. In this case, quinine sulfate in 1.0 N sulfuric acid was
used as the reference (Ur = 0.546).25
The luminescence decays were recorded using a pumped dye
laser (Lambda Physics model FL2002) as the excitation source.
The nominal pulse width and the linewidth of the dye-laser output
were 10 ns and 0.18 cm−1, respectively. The emission of a sample
was collected by two lenses into a monochromator (WDG30),
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Table 3 Crystal data and structure refinement parameters for ligand LI and polymers 1, 2 and 3
Compound
LI
1
2
3
Empirical formula
Temperature/K
M
C48H45N3O9
293
807.84
C48H47N6O19Pr
293
1152.83
Monoclinic
P2(1)/c
14.395(2)
17.708(3)
24.171(4)
106.782(2)
5898.9(17)
4
C42.5H43N6NdO18.5
298
1078.07
Monoclinic
P2(1)/c
16.657(7)
16.373(7)
18.370(9)
98.696(7)
4952(4)
C42H41N6O18Sm
294
1068.16
Monoclinic
P2(1)/c
16.6382(6)
16.2654(5)
18.6824(6)
98.911(2)
4994.9(3)
4
Crystal system
Space group
Monoclinic
P2(1)/n
8.9278(2)
15.1760(4)
31.0232(8)
95.2180(10)
4185.86(18)
4
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
4
Dc/kg m−3
l/mm−1
1.282
0.089
1.298
0.896
1.446
1.125
1.420
1.250
F(000)
1704
0.45 × 0.32 × 0.28
2352
2192
2164
Crystal size/mm
0.36 × 0.32 × 0.29
1.76–25.00
−16 to 17, −21 to 19, −28
to 28
0.30 × 0.27 × 0.25
1.67–25.00
−19 to 18, −19 to 18, −15
to 21
0.15 × 0.13 × 0.10
2.21–25.05
−19 to 15, −19 to 19, −21
to 22
h Range for data collection/◦ 1.88–25.50
Index ranges, hkl
−10 to 10, −18 to 17, −36
to 37
Reflections collected/unique
Rint
Data/restraints/params
Goodness-of-fit on F2
Final R indices [I > 2r(I)]
R indices (all data)
22130/7759
0.0414
7759/0/702
1.015
30035/10300
0.0660
10300/446/712
1.053
25588/8712
0.0901
8712/616/619
1.022
22725/8209
0.1010
8209/0/578
0.964
R1 = 0.0529, wR2 = 0.1263 R1 = 0.0660, wR2 = 0.1431 R1 = 0.0702, wR2 = 0.1880 R1 = 0.0666, wR2 = 0.1198
R1 = 0.1135, wR2 = 0.1578 R1 = 0.1672, wR2 = 0.2285 R1 = 0.1548, wR2 = 0.2545 R1 = 0.1788, wR2 = 0.1413
detected by a photomultiplier and processed by a Boxcar Average
(EGG model 162) in line with a microcomputer.
ligand-to-metal energy transfer. It is worth noting that the change
of backbone as well as terminal groups is a decisive factor in
determining the coordination environments of the metal centers,
the topologies of the polymeric products as well as luminescence
properties.
X-Ray crystallography
The X-ray diffraction data were collected on a CCD area detector
with a graphite-monochromated Mo-Ka radiation source (k =
˚
0.71073 A). All of the structures were solved by direct methods and
Acknowledgements
refined by full-matrix least-squares cycles on F2. The coordinates
of the non-hydrogen atoms were refined anisotropically, while
hydrogen atoms were included in the calculation isotropically but
not refined. The furan ring and crystalline water in complex 1, crys-
talline methanol in complex 2 exhibit disorder over two positions
and were refined isotropically with half site occupancy. Details of
crystallographic parameters, data collection and refinements are
listed in Table 3. Selected bond distances and angles for polymers
1, 2 and 3 are listed in Tables S1, S2 and S3† respectively. CCDC
6724449 for LI, CCDC 672450 for 1, CCDC 672451 for 2 and
CCDC 672452 for 3.
The authors acknowledge support from the National Natural Sci-
ence Foundation of China Research (Grants 20771048, 20431010,
20621091 and J0630962).
Notes and references
1 (a) B. Moulton and M. J. Zaworotko, Chem. Rev., 2001, 101, 1629;
(b) M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M.
O’Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319; (c) S.
Kitagawa and S. Noro, Comprehensive Coordination Chemistry II,
Elsevier Ltd., Oxford, U. K., 2004, vol. 3; C. Janiak, Dalton Trans.,
2003, 2781.
Conclusions
2 (a) S. R. Batten and R. Robson, Angew. Chem., 1998, 110, 1558; (b) A. J.
Blake, N. R. Champness, P. Hubberstey, W.-S. Li, M. A. Withersby and
M. Schro¨der, Coord. Chem. Rev., 1999, 183, 117.
This study demonstrates that the two tripodal ligands with
furfurylsalicylamide arms 1,3,5-tris{[(2ꢀ-furfurylaminoformyl)-
phenoxyl]methyl}-2,4,6-trimethylbenzene (LI) and 1,1,1-tris{[(2ꢀ-
furfurylaminoformyl)phenoxyl]methyl}ethane (LII) are capable of
coordinating lanthanide ions with carbonyl oxygen atoms and
generate novel lanthanide coordination polymers. The resulting
polymers provide the lanthanide ions with a nona-coordination
environment and prevents solvent and water molecules from en-
tering the coordination sphere, effectively resulting in an enhanced
sensitization of the lanthanide due to a decrease in the vibronic
quenching effect. Luminescence studies also demonstrated that
the salicylamide ligands exhibit a good antennae effect with
respect to the TbIII ion due to efficient intersystem crossing and
3 D. G. Karraker, J. Chem. Educ., 1970, 47(6), 424.
4 (a) X.-H. Bu, W. Weng, W. Chen and R.-H. Zhang, Inorg. Chem., 2002,
41, 413; (b) X.-H. Bu, W. Weng, M. Du, W. Chen, J.-R. Li, R.-H. Zhang
and L.-J Zhao, Inorg. Chem., 2002, 41, 1007; (c) J. R. Li, X. H. Bu, R. H.
Zhang, C. Y. Duan, K. M. C. Wong and V. W. W. Yam, New J. Chem.,
2004, 28, 261; (d) J. R. Li, X. H. Bu and R. H. Zhang, Inorg. Chem.,
2004, 43, 237; (e) S. J. Dalgarno, M. J. Hardie, J. L. Atwood and C. L.
Raston, Inorg. Chem, 2004, 43, 6351.
5 (a) L. Pan, X. Huang, J. Li, Y. Wu and N. Zheng, Angew. Chem.,
2000, 112, 537; (b) T. M. Reineke, M. Eddaoudi, M. O’Keeffe and
O. M. Yaghi, Angew. Chem., Int. Ed, 1999, 38, 2590; (c) L. Pan, M. K.
Adams, H. E. Herandez, X. T. Wang, C. Zheng, Y. Hattori and K.
Kaneko, J. Am. Chem. Soc., 2003, 125, 3062; (d) T. M. Reineke, M.
Eddaoudi, D. Moler, M. O’Keeffe and O. M. Yaghi, J. Am. Chem. Soc,
2000, 122, 4843; (e) X. D. Guo, G. S. Zhu, Q. R. Fang, M. Xue, G. Tian,
3590 | Dalton Trans., 2008, 3582–3591
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
J. Y. Sun, X. T. Li and S. L. Qiu, Inorg. Chem., 2006, 44, 3850; (f) C.
Daiguebonne, N. Kerbellec, K. Bernot, Y. Ge´rault, A. Deluzet and O.
Guillou, Inorg. Chem., 2006, 45, 5399; (g) C. Qin, X.-L. Wang, E.-B.
Wang and Z.-M. Su, Inorg. Chem., 2005, 44, 7122; (h) Z.-H. Zhang, T.
Okamura, Y. Hasegawa, H. Kawaguchi, L.-Y. Kong, W.-Y. Sun and N.
Ueyama, Inorg. Chem., 2005, 44, 6219; (i) C. Marchal, Y. Filinchuk, D.
Imbert, J.-C. G. Bu¨nzli and M. Mazzanti, Inorg. Chem., 2007, 46, 6242.
6 (a) D. M. L. Goodgame, S. Menzer, A. M. Smith and D. J. Williams,
Chem. Commun., 1997, 339; (b) D. M. L. Goodgame, S. P. W. Hill and
D. J. Williams, Inorg. Chim. Acta, 1998, 272, 131.
7 (a) G. A. Doyle, D. M. L. Goodgame, S. P. W. Hill and J. Williams,
Chem. Commun., 1993, 207; (b) L. H. Carrad, D. M. L. Goodgame,
S. P. W. Hill and D. J. Williams, J. Chem. Soc., Dalton Trans., 1993, 1003;
(c) D. M. L. Goodgame, S. Menzer, A. T. Ross and D. J. Williams, Inorg.
Chim. Acta, 1996, 251, 141.
12 Y. Tang, J. Zhang, W.-S. Liu, M.-Y. Tan and K.-B. Yu, Polyhedron,
2005, 24, 1160.
13 X.-Q. Song, W. Dou, W.-S. Liu, Y.-L. Guo and X.-L. Tang, Inorg. Chem.
Commun., 2007, 10, 1058.
14 C. K. Jorgensen and B. R. Judd, Mol. Phys., 1964, 8, 281.
15 S. P. Sinha, Spectrochim. Acta, 1966, 22, 57.
16 M. Albin, R. R. Wright, Jr. and W. D. Horrochs, Inorg. Chem., 1985,
24, 2491.
17 A. F. Kirby, D. Foster and F. S. Richardson, Chem. Phys. Lett., 1983,
95, 507.
18 (a) R. Pavithran, N. S. Saleesh Kumar, S. Biju, M. L. P. Reddy, S. Jr.
Alves and R. O. Freire, Inorg. Chem., 2006, 45, 2184; (b) R. Pavithran,
M. L. P. Reddy, S. Alves, Jr., R. O. Freire, G. B. Rocha and P. P. Lima,
Eur. J. Inorg. Chem., 2005, 20, 4129; (c) M. Shi, F. Li, T. Yi, D. Zhang,
H. Hu and C. Huang, Inorg. Chem., 2005, 44, 8929.
8 (a) D.-L. Long, A. J. Blake, N. R. Champness, C. Wilson and M.
Schro¨der, J. Am. Chem. Soc., 2001, 123, 3401–3402; (b) D.-L. Long,
A. J. Blake, N. R. Champness and M. Schro¨der, Chem. Commun., 2000,
136.
9 M. R. Bu¨rgstein, M. T. Gamer and P. W. Roesky, J. Am. Chem. Soc.,
2004, 126, 5213.
19 (a) F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. Vander Tol and
J. W. J. Verhoeven, J. Am. Chem. Soc, 1995, 117, 9408; (b) J.-C. G. Bu¨nzli
and C. Piguet, in Encylopedia of Materials: Science and Technology,
ed. K. H. J. Buschow, R. W. Cahn, M. C. B. Flemings, B. Ilschner,
E. J. Kramer and S. Mahajan, Elsevier Science Ltd, Oxford, 2001,
chapter 1.10.4, p. 4465.
10 (a) X.-P. Yang, C.-Y. Su, B.-S. Kang, X.-L. Feng, W.-L. Xiao and H.-Q.
Liu, J. Chem. Soc., Dalton Trans., 2000, 3253; (b) X.-P. Yang, B.-S.
Kang, W.-K. Wong, C.-Y. Su and H.-Q. Liu, Inorg. Chem., 2003, 42,
169; (c) R. Sun, S. N. Wang, H. Xing, J. F. Bai, Y. Z. Li, Y. Pan and
X. Z. You, Inorg. Chem., 2007, 46, 8451.
11 J. Zhang, Y. Tang, N. Tang, M.-Y. Tan, W.-S. Liu and K.-B. Yu, J. Chem.
Soc., Dalton Trans., 2002, 832.
20 M. Latva, H. Takalo, V. M. Mukkala, C. Matachescu, J. C. Rodriguez-
Ubis and J. Kanakare, J. Lumin., 1997, 75, 149.
21 K. Michio, Bull. Chem. Soc. Jpn., 1976, 49, 2679.
22 Z. Jiri, P. Magdalena, H. Petr and T. Milos, Synthesis, 1994, 110, 1132.
23 S. Farber and R. T. Conley, Synth . Commun., 1974, 4(4), 243.
24 G. A. Crosby and J. N. Demas, J. Phys. Chem., 1971, 75, 991.
25 S. R. Meech and D. J. Philips, J. Photochem., 1983, 23, 193.
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