Synthesis, characterization, photophysical properties of lanthanide complexes with flexible tripodal carboxylate ligands

Article abstract of DOI:10.1016/j.poly.2012.07.020

Three series of lanthanide complexes [LnL₁, LnL₂ and LnL₃ Ln = Eu, Gd and Tb] have been synthesized with tripodal amide ligands. These ligands have similar benzoate antenna with different substituent groups at para-positio

Full text of DOI:10.1016/j.poly.2012.07.020

Polyhedron 52 (2013) 939–944
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, photophysical properties of lanthanide complexes
with flexible tripodal carboxylate ligands
a,
Chi-Fai Chan a, Hongguang Li a, Chunyu Seto a, Hoi-Lam Tam b, Ga-Lai Law c, , Ka-Leung Wong
⇑
⇑
a Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong
b Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong
c Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hum Hong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 20 July 2012
Three series of lanthanide complexes [LnL1, LnL2 and LnL3 Ln = Eu, Gd and Tb] have been synthesized with
tripodal amide ligands. These ligands have similar benzoate antenna with different substituent groups at
para-position (L1 = –OME, L2 = –H; L3 = –CF3). The antenna effect is known to play a prominent role in the
luminescence of the lanthanide ions emitting in the coordination complexes. The effect of the ligands
triplet and antenna effect have been studied here. The room and low temperature photophysical proper-
ties of the nine complexes have been monitored in solution with excitation at 330 nm. The emission
antenna effect (quantum yield and emission intensity) varied with the triplet state of ligand that comes
from the substituent on the benzoate antenna.
Dedicated to Alfred Werner on the 100th
Anniversary of his Nobel prize in Chemistry
in 1913.
Keywords:
Lanthanide
Luminescence
Terbium
Ó 2012 Elsevier Ltd. All rights reserved.
Europium
1. Introduction
the ultraviolet (UV) absorption properties absence of high fre-
quency vibrational modes [7], and location of donor excited states
The syntheses of lanthanide(III) complexes have been under
much spotlight due to their photoluminescent properties, with
europium and terbium complexes playing a prominent role [1].
Luminescent materials made with lanthanide(III) complexes have
been extensively used in optoelectronics [2] as well as in biological
applications such as the detection of various bioactive molecules
and in vitro imaging [3]. Emissive lanthanide(III) complexes are
taking over most organic-based imaging probes due to their unique
properties, e.g. long emission lifetimes, large Stokes shifts and fin-
gerprint emission patterns. In combination with time-resolved
microscopy, luminescence from lanthanide(III)-based imaging
probes becomes differentiable from biological autofluorescence
which leads to better imaging qualities.
of the ligand [7] identity are important criteria to be considered
[8]. Because the high phototoxicity of UV excitation is undesirable
when undergoing bio-imaging [9]. The tailoring of excited state
energies by substituents associated with coordinating benzoate li-
gands has yielded increased emission quantum yields [10]. How-
ever, there is a lack of systematic studies for the emission
quantum yield via linear excitation regarding the change of the
substituent groups in the same benzoate tripodal amide backbone.
Herein, we have synthesized nine lanthanide complexes with
three tripodal amide ligands and three lanthanide salts respec-
tively (Ln = Eu, Gd and Tb). Three tripodal amide ligands are conju-
gated with a benzoate chromophore, with the only variations at
the para-substituent groups (L1 = OCH3, L2 = H, L3 = CF3). The nine
lanthanide complexes are designated as Ln-L1, Ln-L2, Ln-L3 to indi-
cate complexion with different chromophores, where Ln = Eu, Gd
and Tb (Fig. 1). The mass spectra of the complexes are summarized
in the experimental section and supporting information. The envi-
ronmental-hypertensive emission spectra of the three europium
complexes have been examined in solution to investigate the coor-
dination environment/numbers of chelates on the europium(III)
ion. All the spectral measurements were conducted in DMSO, since
aqueous environment will quenching the luminescence instensity
significantly [11]. The photophsyical properties of these lanthanide
complexes have been compared via linear excitation. The triplet
states of these complexes have been evaluated with low tempera-
ture emission spectra with three Gd complexes (See Table 1).
Since lanthanide 4fN–4fN transitions are Laporte forbidden, the
direct excitation of electrons in tripositive lanthanide ions is inef-
ficient. In order to overcome such problem and achieve enhanced
luminescence, a sensitizing chromophore, so-called an antenna, is
used as a ligand to chelate to the emissive lanthanide center [4].
Hammila had reported long ago that the antenna effect could also
occur by two-photon excitation of an appropriate antenna [5] and
therefore it is important to design antennae that could produce
efficient antenna effect. The binding strength of the ligands [6],
⇑
Corresponding authors. Tel.: +852 3411 2370.
E-mail addresses: ga-lai.law@polyu.edu.hk (G.-L. Law), klwong@hkbu.edu.hk
(K.-L. Wong).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.07.020

940
C.-F. Chan et al. / Polyhedron 52 (2013) 939–944
Additional hydrogen bonding is depicted in Fig. 1c. The amide
N
N3–H3 group forms a centrosymmetrically related pair of hydro-
gen bonds between H3 and O20. The amide N2–H2 group hydrogen
bonds to O100 in a molecule that is translated along the a-axis.
It is interesting to note that the tripodal molecule is quite rigid
and not flexible; this is due to the extensive intramolecular hydro-
gen bonding. There is also an intramolecular hydrogen bond be-
tween a carboxamido N–H group of one arm of the tripodal
ligand and a carbonyl O atom of a second carboxamide group [12].
O3N
O
N
N
Ln
O
N
Ln = Eu, Gd, Tb
O
L1 n = 1 R1 = -OCH3
Rn
L2 n = 2 R2 = -H
Rn
L3 n = 3 R3 = -CF3
2.2. Photophysical properties of lanthanide complexes
Rn
Fig. 1. The molecular structure of lanthanide complexes Ln-L1, Ln-L2 and Ln-L3.
(Ln = Eu, Gd and Tb).
The solution state electronic absorption, emission, and excita-
tion spectra were recorded for the lanthanide (Ln = Eu3+, Gd3+
and Tb3+) complexes at room temperature. The UV-absorption
bands of the complexes were red-shifted ꢀ8 nm after complexa-
tion. The extinction coefficients of all nine lanthanide complexes
are listed in Table 2. The complexes with benzyl chromophore
(L2) have shown the highest extinction coefficient at 280 nm
(ꢀ2.1 mmolꢁ1cmꢁ1) in their own series (i.e. europium complexes
in EuL1, EuL2, EuL3, Table 2). The excitation spectra (Fig. 3b) of
the complexes presented similar bands to their absorption spectra,
located at ꢀ280 nm and 330 nm, attributed to intraligand excita-
tions [13].
2. Results and discussion
2.1. Synthesis and characterization
A series of tripodal amide ligands are based on N-[2-(bis{2-
[(benzoyl)amino]ethyl}amino)-ethyl]benzamide, with various sub-
stituent groups in the para position (Fig. 2) were obtained from
tris(2-aminoethyl)amine with different species of methyl benzoate
with catalysis of thionyl chloride under room temperature. L3 gave
crystals for structural analysis and are shown in Fig. 1b. 1H NMR
and 13C NMR suggest structural information of these ligands (Sup-
plementary Figs. S1–S3). The product yields were found to be
approximately over ꢀ70%.
The room temperature 4fN emission spectra of europium and
terbium complexes under excitation at 330 nm (similar absorption
extinction of all nine complexes) are readily assigned (Fig. 4). For
Tb complexes, four structured narrow green emission bands at
480, 545, 580, 620 nm of Tb3+ (Fig. 4a) are assigned to an electronic
transitions 5D4 to 7FJ (J = 6, 5, 4, 3), whereas bands at 594, 620 and
700 nm of Eu3+ complexes correspond to 5D0 to 7FJ (J = 1., 2, 3, 4)
(Fig. 4b).
Rather than mass spectroscopy, analysis of the europium emis-
sion spectral profiles of three europium complexes allowed certain
information of their coordination environment to be obtained. In
literature, there are numbers of studies to show the sensitivity of
Ln3+ to its coordination can be inferred from their emission spectra,
even for Tb3+ (4f8, electrons are inner, shield electrons), the envi-
ronment can also be inferred by monitoring their low temperature
electronic spectra [14]. In europium, room temperature emission
spectra is available to give more instructive coordination informa-
tion due to the higher energy transitions from the non-degenerate
5D0 level which is fairly simple.
Single crystals of the ligand L3, which are suitable for X-ray
analysis, were obtained from slow evaporation of dichloromethane
solution at room temperature for a few days. Selected bond-dis-
tances and bond angles are listed in supporting information
Table S2.
Fig. 2 shows a perspective view of L3 with the packing diagrams
of L3 projected down the c-axis. The three tripodal arms of the li-
gand are extended, in spite of the intramolecular hydrogen bond-
ing that involves the amide N1-H1 group and both N4 and O3.
Table 1
Selected bond length (Å, left) and angle (right) of ligand L3.
L3
O(1)
O(2)
N(1)
N(1)
N(2)
N(3)
N(3)
N(4)
C(1)
C(4)
C(5)
C(6)
C(7)
C(3)
C(13)
C(2)
H(5)
C(13)
C(22)
H(23)
C(11)
C(2)
C(5)
C(6)
C(7)
C(8)
1.233(5)
1.230(6)
1.457(6)
0.84(5)
1.324(6)
1.449(7)
0.88(5)
1.466(5)
1.514(6)
1.381(6)
1.383(8)
1.370(9)
1.381(8)
1.381(9)
1.495(7)
1.383(6)
0.950(6)
0.950(8)
1.48(1)
In Fig. 4b, the relative intensity of the electric-dipole allowed
D
J = 2 (ꢀ618 nm) transitions compared to the
DJ = 1 magnetic di-
pole transitions was almost constant. (EuL1 = 1:9.2; EuL2 = 1:2.2;
EuL3 = 1:2.3) The relative intensity of the hypersensitive J = 2 tran-
sitions allow us to determine the symmetry and the polarisability
of the capping axial donor. The slight variation may be induced by
the steric hindrance of the three substituents groups in the ligand
[15]. We also compared the relative emission intensity ratios of
D
J = 1 to DJ = 2 with our previously reported polymeric europium
complexes (with known molecular structures) that are formed by
the same ligand L2 and information are shown in the Fig. S4. The
C(8)
C(9)
C(13)
C(14)
C(15)
C(16)
C(17)
C(21)
C(14)
C(19)
H(15)
H(16)
C(20)
C(22)
ratio of
DJ = 1 to DJ = 2 in the polymeric form is around 1:6 [16].
The comparison of luminescence quantum efficiency of lantha-
nide complexes were previously based upon the energy of the
respective ligand triplet state and the lanthanide emissive states
[17]. In the present study, the triplet state of three antenna are af-
fected by their substituent groups, and governed the variation of
emission quantum efficiency. We have chosen excitation at
330 nm since all three chromophores showed similar absorption
coefficients at that wavelength. The emission quantum yields of
terbium and europium analogues are summarized in the Table 2.
The emission quantum yields are obtained by a commercially
available integrated sphere [18].
1.518(8)
L3
C(6)
C(5)
C(4)
O(1)
C(2)
C(2)
C(3)
O(5)
O(4)
O(3)
O(3)
N(2)
N(2)
N(2)
C(4)
C(9)
O(1)
N(2)
C(3)
H(6)
H(6)
122.1(3)
117.4(2)
119.4(2)
120.5(2)
122.2(2)
120(1)
116(1)

C.-F. Chan et al. / Polyhedron 52 (2013) 939–944
941
Fig. 2. The ORTEP diagram and hydrogen bonding of L3.
chromophores coordinated. The lowest energy level of Gd3+ (6P7/
2) is much higher than the energy levels of the ligands; therefore,
it is reasonably assumed that no energy transitions take place be-
tween the ligand and metal [19]. The phosphorescence of these Gd
complexes was measured under cold conditions (77 K) in solution-
matrix state. Strong emission bands of the three Gd complexes are
located at 395 nm, 383 nm and 380 nm for GdL1–L3, which were
blue-shifted for ꢀ40 nm compared with their room temperature
emission spectra (Fig. 5). The luminescence lifetimes were re-
corded to verify their fluorescent or phosphorescent nature. The
emission lifetimes measured for ca. 405 nm were on a nanosecond
scale (S1 ? S0) whereas that at ca. 450 nm were on a microsecond
scale (T1 ? S0) for the gadolinium complexes GdL1, GdL2 and GdL3.
In all three terbium and europium complexes, the energy gap
between the triplet states of the chromophore/antenna to Tb
Table 2
The photophysical properties of LnL1, LnL2, LnL3 (Ln = Eu, Gd and Tb).
e
(DMSO,
s
s
/
mmolꢁ1 cmꢁ1
)
(DMSO,
(DMSO,
(DMSO,%)a
ns)a
77 K l
s)a
Eu-L1
Eu-L2
Eu-L3
Gd-L1
Gd-L2
Gd-L3
Tb-L1
Tb-L2
Tb-L3
1.01
1.54
2.1
1.03
1.50
2.05
1.06
1.48
2.1
–
–
–
6.7
5.5
5.8
–
–
–
–
–
–
21
18
16
–
–
–
1.2
3
1.5
–
–
–
1.5
1.5
3.3
a
With 330 nm excitation.
(ꢀ20500 cmꢁ1) and Eu (17500 cmꢁ1) are greater than 2000 cmꢁ1
,
hence no back energy transfer occurred. However, the energy level
of the 5D0 state for Eu3+ is ca. 17500 cmꢁ1, which is 3000 cmꢁ1 low-
er than the Tb3+ (5D4) energy level so the energy gap between T1
and 5D0 in EuL3 (–CF3) becomes ꢀ6000 cmꢁ1 and compared to that
between T1 and 5D4 in TbL3 (–CF3) which is ꢀ2500 cmꢁ1, it is rea-
sonable that the energy transfer from the ligand is better converted
via the ligand triplet [20] and thus Tb complex TbL3 exhibit better
luminescence properties.
Terbium (TbL3) complexes with a trifloromethyl group at the
chromophore (–CF3, L3) gave the highest quantum yields among
their series with excitation at 330 nm, but in europium complexes,
EuL2 is the brightest one. In this study, the triplets of these euro-
pium or terbium complexes have been determined through the
low temperature emission spectra from their Gd analogues
(GdL1, GdL2 and GdL3) in order to examine in details the geometry
and electronic structure of the ligand and the number of
(a)
(b)
Eu-L
1
em = 618 nm
Eu-L
1
Eu-L
2
Eu-L
3
0.2
Eu-L
2
Eu-L
3
0.0
280
300
320
340
300
350
400
260
Wavelength/nm
Wavelength/nm
Fig. 3. (a) The UV absorption spectra and (b) excitation spectra of complexes EuL1, EuL2, EuL3 in a solution of DMSO (0.1 mM).

942
C.-F. Chan et al. / Polyhedron 52 (2013) 939–944
(a)
(b)
Fig. 4. The emission spectra of (a) terbium complexes TbL1, TbL2, TbL3 and (b) europium complexes EuL1, EuL2, EuL3 in a solution of DMSO with excitation at 330 nm.
(0.5 mM, abs330nm = 0.1).
ethyl)ethane-1,2-diamine (100 mg, 0.68 mmol) and triethylamine
(1 ml, 6.8 mmol) in dry DCM (10 ml) at 0 °C under nitrogen gas.
The resulting solution was stirred more 5 h at rt. and washed with
0.5 M NaOH aq. and 0.5 M HCl. The organic layer was dried over
anhydrous sodium sulfate, filtered and concentrated. Silica gel
flash column chromatography (DCM: MeOH = 20:1 of the residue
gave a pale yellow solid (284 mg, 0.62 mmol, 91%) as the title prod-
uct. 1H NMR (CDCl3, 400 MHz): d 7.65 (dd, J = 4, 8 Hz, 6H), 7.30 (m,
3H), 7.19 (s, 3H), 7.08 (m, 6H), 3.57 (q, J = 6 Hz, 6H), 2.74 (t, J = 8 Hz,
6H); 13C NMR (CDCl3, 400 MHz): d 168.2, 133.7, 131.1, 128.2, 127.0,
53.7, 37.9. ESI-MS = 458.2.
3. Conclusion
In conclusion, nine lanthanide complexes with tripodal amide
ligands have been synthesized and the crystal structure of one tri-
podal amide ligand L3 was obtained. The nine complexes differ in
their substituents groups on the benzoate antenna which is shown
to influence the electronic density of the ring and hence the
luminescent properties.
4. Experimental
4.1. Synthesis of Tris[2-(benzoylamino)ethyl]amine (L1)
4.2. Synthesis of Tris[2-(4-meoxyl-benzoylamino)ethyl]amine (L2)
Benzoyl chloride (0.32 ml, 2.74 mmol) was dissolved in dry
DCM (5 ml), then dropped into the solution of N1,N1-bis(2-amino-
4-methoxybenzoyl chloride (0.32 ml, 2.74 mmol) was dis-
solved in dry DCM (5 ml), then dropped into the solution of

C.-F. Chan et al. / Polyhedron 52 (2013) 939–944
943
λex = 330nm
λex = 330nm
GdL1
20000000
15000000
10000000
5000000
0
298K
77K
10000000
GdL2
GdL3
298K
77K
5000000
0
10000000
λex = 330nm
298K
77K
500
5000000
0
400
600
700
800
Wavelength/nm
Fig. 5. The emission spectra of gadolinium complexes (GdL1–GdL3) in a solution of Me-THF with excitation at 330 nm at 298 K and 77 K. (0.5 mM).
N1,N1-bis(2-aminoethyl)ethane-1,2-diamine (100 mg, 0.68 mmol)
and triethylamine (1 ml, 6.8 mmol) in dry DCM (10 ml) at 0 °C
under nitrogen gas. The resulting solution was stirred more 5 h
at rt. and washed with 0.5 M NaOH aq. and 0.5 M HCl. The organic
layer was dried over anhydrous sodium sulfate, filtered and con-
centrated. Silica gel flash column chromatography.
L2 gave a white solid (335 mg, 0.61 mmol, 90%) as the title prod-
uct. 1H NMR (CDCl3, 400 MHz): d 7.57 (d, J = 4, Hz, 6H), 7.04 (t,
J = 6 Hz, 3H), 6.51 (d, J = 4 Hz, 6H), 3.72 (s, 9H), 3.57 (q, J = 6 Hz,
6H), 2.73 (t, J = 6 Hz, 6H); 13C NMR (CDCl3, 400 MHz): d 167.7,
161.7, 128.9, 126.0, 113.2, 55.1, 53.2, 37.5. ESI-MS = 548.3.
vacuum, three white solid as the title complexes. Eu-L2 = 737; Tb-
L2 = 743; Gd-L3 = 828 [MꢁNO3ꢁ1].
4.6. Synthesis of complexes Eu-L3, Tb-L3 and Gd-L3
L3 (20 mg, 0.030 mmol) and Ln(NO3)3 (mmol) were stirred in
MeOH (10 ml) at 50 °C for 24 h. Solvent was removed under vac-
uum, three white solid as the title complexes. Eu-L3 = 837; Tb-
L3 = 843; Gd-L3 = 841 [MꢁNO3ꢁ1].
4.7. Photophysical measurements
4.3. Synthesis of Tris[2-(4-trifluoromethyl-benzoylamino)ethyl]amine
(L3)
UV–Vis absorption spectra in the spectral range 200–1100 nm
were recorded by a HP UV-8453 spectrophotometer. Single-photon
luminescence and lifetime spectra were recorded using a Edin-
burgh Instruments FLS920 Combined Fluorescence Lifetime and
Steady State Spectrophotometer (185–850 nm) equipped with a
single photon counting photomultiplier in a Peltier Cooled Hous-
ing. The spectra were corrected for detector response and stray
background light phosphorescence. The solution state quantum
yields of the lanthanide complexes were measured by a demount-
able 142 mm (inner) diameter barium sulfide coated integrating
sphere supplied with two access ports.
Low-temperature (77 K) phosphorescence spectra were ob-
tained by exciting samples with a xenon lamp. The transparent
glassy materials were formed by mixing samples with 2-meth-
yltetrahydrofuran. The samples were placed in a tailor-made
quartz tube housed in an Oxford Instruments liquid nitrogen
cryostat.
4-(Trifluoromethyl)benzoyl chloride (0.32 ml, 2.74 mmol) was
dissolved in dry DCM (5 ml), then dropped into the solution of
N1,N1-bis(2-aminoethyl)ethane-1,2-diamine (100 mg, 0.68 mmol)
and triethylamine (1 ml,6.8 mmol) in dry DCM (10 ml) at 0 °C un-
der nitrogen gas. The resulting solution was stirred more 5 h at rt.
and washed with 0.5 M NaOH aq. and 0.5 M HCl. The organic layer
was dried over anhydrous sodium sulfate, filtered and concen-
trated. Silica gel flash column chromatography.
L3 gave a white solid (368 mg, 0.59 mmol, 87%) as the title prod-
uct. 1H NMR (CDCl3, 400 MHz): d 8.56 (t, J = 4 Hz, 3H), 7.87 (d,
J = 4 Hz, 6H), 7.60 (d, J = 4 Hz, 6H), 3.38 (q, J = 8 Hz, 6H), 2.71
(t, J = 8 Hz, 6H); 13C NMR (CDCl3, 400 MHz): d 165.1, 138.0, 130.7,
129.4, 127.9, 125.0, 52.7, 37.5. The crystal data of L3 is listed in
Table S1. ESI-MS = 662.2.
4.4. Synthesis of complexes Eu-L1, Tb-L1 and Gd-L1
Acknowledgments
L1 (20 mg, 0.044 mmol) and Ln(NO3)3 (0.044 mmol) were
stirred in MeOH (2 ml) at r.t. for 24 h. Solvent was removed under
vacuum, three white solid as the title complexes. Eu-L1 = 825;
Tb-L1 = 831; Gd-L1 = 829 [MꢁNO3ꢁ1].
The work described in this paper was partially supported by
grants from the Hong Kong Baptist University (FRG1/11-12/034).
Appendix A. Supplementary data
4.5. Synthesis of complexes Eu-L2, Tb-L2 and Gd-L2
CCDC 890169 contains the supplementary crystallographic data
for L3. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
L2 (20 mg, 0.036 mmol) and Ln(NO3)3 (0.036 mmol) were
stirred in MeOH (5 ml) at rt. for 24 h. Solvent was removed under

944
C.-F. Chan et al. / Polyhedron 52 (2013) 939–944
[9] J.-C.G. Bünzli, C. Piguet, Chem. Rev. 102 (2002) 1897.
[10] G.-L. Law, C. Man, D. Parker, J.W. Walton, Chem. Commun. 46 (2010) 2391.
[11] D. Parker, R.S. Dickins, H. Puschmann, C. Crossland, J.A.K. Howard, Chem. Rev.
102 (2002) 1977.
[12] J. Xu, T.M. Corneillie, E.G. Moore, G.-L. Law, N.G. Butlin, K.N. Raymond, J. Am.
Chem. Soc. 133 (2011) 19900.
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2012.07.020.
[13] H.-B. Xu, J. Li, L.-Y. Zhang, X. Huang, B. Li, Z.-N. Chen, Cryst. Growth Des. 10
(2010) 4101.
References
[14] G.-L. Law, K.-L. Wong, W.-M. Kwok, P.A. Tanner, Wing-Tak Wong, J. Phys.
Chem. B 111 (2007) 10858.
[15] J.-H. Jia, G.-L. Law, K.-L. Wong, P.A. Tanner, W.-T. Wong, Inorg. Chem. 47 (2008)
9431.
[1] D. Parker, Aust. J. Chem. 64 (2011) 239.
[2] G. Mancino, A.J. Ferguson, A. Beeby, N.J. Long, T.S. Jones, J. Am. Chem. Soc. 127
(2005) 524.
[3] C.P. Montgomery, B.S. Murray, E.J. New, R. Pal, D. Parker, Acc. Chem. Res. 42
(2009) 92511.
[16] G.-L. Law, K.-L. Wong, Y.-Y. Yang, Q.-Y. Yi, W.-T. Wong, P.A. Tanner, Inorg.
Chem. 46 (2007) 9754.
[4] P. Primus, M.U. Kumke, J. Phys. Chem. A 116 (2012) 1176.
[5] D.G. Smith, G.-L. Law, R. Pal, D. Parker, K.L. Wong, Chem. Commun. 47 (2011)
7347.
[6] G.-L. Law, K.-L. Wong, C.W.-Y. Man, W.-T. Wong, S.-W. Tsao, M.H.-W. Lam,
P.K.-S. Lam, J. Am. Chem. Soc. 130 (2008) 3714.
[7] L.M.P. Lima, A. Lecointre, J.-F. Morfin, A. de Blas, D. Visvikis, L.J. Charbonnière,
C. Platas-Iglesias, R. Tripier, Inorg. Chem. (2011) 12508.
[8] H.L. Handl, R.J. Gillies, Life Sci. 77 (2005) 361.
[17] D. Parker, J.W. Walton, L. Lamarque, J.M. Zwier, Eur. J. Inorg. Chem. 25 (2010)
3961.
[18] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.C. Rodríguez-Ubis, J.
Kankare, J. Lumin. 75 (1997) 149.
[19] G.F. de Sá, S. Alves Jr., B.J.P. da Silva, E.F. da Silva Jr., Opt. Mater. 11 (1998) 23.
[20] K.-L. Wong, G.-L. Law, K.-K. Lau, H.-L. Tam, K.-W. Cheah, W.-T. Wong, Eur. J.
Inorg. Chem. (2007) 5419.