Synthesis, structures and luminescent properties of new heterobimetallic Zn-4f Schiff base complexes

Article abstract of DOI:10.1016/j.ica.2004.06.041

New stable heterobimetallic Zn-Ln (Ln = La, Gd, Nd, Yb, Er) Schiff base complexes have been synthesized and characterized. The photophysical properties of these Schiff base complexes can be tuned by changing the electronic properties of the substituents. By extending the conjugation, both the absorption and emission bands of the complexes show bathochromic shifts. Energy transfer is observed from the triplet state of the Zn-Ln complex to the excited state of lanthanide metal (Ln = Nd, Er, Yb) which then emits in the NIR region. A series of new 3d-4f heterobimetallic Schiff base complexes of the general formula [Zn(μ-L²)Ln(NO₃)₃(H ₂O)_n] (Ln = La 1, Nd 2, Gd 3, Er 4 and Yb 5; n = 1 or 2; H₂L² = N,N′-bis(3-methoxy-5-p-tolylsalicylidene) ethylene-1,2-diamine) are synthesized and characterized. Complexes 1, 2, 4 and 5 are structurally characterized by X-ray crystallography. The photophysical properties of these complexes are also investigated. At room temperature, complexes 1-5 exhibit similar solution absorption and emission spectra in the UV-Vis region. Furthermore, compounds 2, 4 and 5 exhibit solution emission corresponding to the lanthanide(III) ion in the near-infrared region at room temperature. The triplet state emission of the 3d-4f bimetallic complexes without energy transfer is also determined through the photophysical study of complex 3.

Full text of DOI:10.1016/j.ica.2004.06.041

Inorganica Chimica Acta 357 (2004) 4510–4521
www.elsevier.com/locate/ica
Synthesis, structures and luminescent properties of new
heterobimetallic Zn-4f Schiff base complexes
a
a,
a
a
Wing-Kit Lo , Wai-Kwok Wong *, Jianping Guo , Wai-Yeung Wong ,
b
King-Fai Li , Kok-Wai Cheah
b
a
Department of Chemistry and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,
Hong Kong, PR China
b
Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Waterloo Road, Kowloon Tong,
Hong Kong, PR China
Received 11 May 2004; accepted 26 June 2004
Available online 29 July 2004
Abstract
A series of new 3d–4f heterobimetallic Schiff base complexes of the general formula [Zn(l-L2)Ln(NO3)3(H2O)n] (Ln=La 1, Nd 2,
Gd 3, Er 4 and Yb 5; n=1 or 2; H2L2 =N,N0-bis(3-methoxy-5-p-tolylsalicylidene)ethylene-1,2-diamine) are synthesized and charac-
terized. Complexes 1, 2, 4 and 5 are structurally characterized by X-ray crystallography. The photophysical properties of these com-
plexes are also investigated. At room temperature, complexes 1–5 exhibit similar solution absorption and emission spectra in the
UV–Vis region. Furthermore, compounds 2, 4 and 5 exhibit solution emission corresponding to the lanthanide(III) ion in the
near-infrared region at room temperature. The triplet state emission of the 3d–4f bimetallic complexes without energy transfer is
also determined through the photophysical study of complex 3.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Heterobimetallic complexes; Lanthanide; Luminescence; Schiff bases; Zinc
1. Introduction
(Tb3+) and europium (Eu3+) complexes, which emit in
the visible region (green for Tb3+ and red for Eu3+
)
Lanthanide coordination compounds [1] become an
important research topic due to their application as con-
trast agents for NMR imaging [2,3], catalysts in RNA
hydrolysis [4], as active agents in cancer radiotherapy
[5], or as luminescent stains for protein labeling and sen-
sitive homogeneous immunoassays [6]. The lanthanide
ions are known for their unique optical properties such
as line-like emission spectra and long luminescence life-
times. Although lanthanide ions have intrinsically low
absorption cross-sections, their excitation can be done
via an antenna chromophore [7–9]. Most of the research
attention is placed on the luminescence of terbium
and can be used as pure green or red EL devices [10–
13]. Recently, there has been a growing interest in the
study of neodymium (Nd3+), ytterbium (Yb3+) and
erbium (Er3+) complexes [7,14–18], which emit in the
near-infrared region (NIR). The study of NIR lumines-
cence is of interest because the emission around 900–
1600 nm, which is highly transparent to biological
systems and fiber media, is valuable for fluoro-immuno-
assay and optical telecommunication.
There has been growing interest on dinuclear 3d–4f
and 4f–4f0 complexes, which exhibit interesting lumines-
cent and magnetic properties [19,20]. Recently, dinu-
clear 3d–4f Schiff base complexes have been employed
as building blocks to form extended structure [21],
2p–3d–4f [22] and 3d–3d0–4f systems [23]. The use of
*
Corresponding author. Tel.: +852-3411-7011; fax: +852-3411-
5862.
E-mail address: wkwong@hkbu.edu.hk (W.-K. Wong).
0020-1693/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2004.06.041

W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
4511
transition metal complexes as antenna chromophore for
2. Experimental
lanthanide ion sensitization has been reported [24]. Zin-
c(II) Schiff base complexes have been shown to be effec-
tive emitters [25]. We are interested in exploring the
possibility of using transition metal complexes with
Schiff base ligands (H2L1, H2L2 and H2L3) as antenna
chromophores for lanthanide ion sensitization. We have
recently communicated the preparation, structure and
luminescent properties of a series of bimetallic Zn-4f
Schiff base complexes, [Zn(l-L1)Ln(NO3)3(H2O)]
[H2L1 =N,N0-bis(3-methoxysalicylidene)ethylene-1,2-
diamine; Ln= Nd3+, Er3+ or Yb3+], and shown that the
zinc Schiff base could sensitize the emission of lantha-
nide ions in the NIR region [26]. We are interested in
the electronic effect of the substituents of the Schiff base
on the photophysical properties of the resulting Zn-4f
Schiff base complexes. Herein, we report the synthesis
of a new Schiff base ligand, N,N0-bis(3-methoxy-5-p-tol-
ylsalicylidene)ethylene-1,2-diamine (H2L2), and its Zn-4f
heterobimetallic complexes (Scheme 1). The Schiff base
and its Zn-4f complexes were fully characterized by ele-
2.1. Chemicals and measurements
Solvents and starting materials were purchased com-
mercially and used without further purification unless
otherwise stated. Elemental analyses (C, H and N) were
performed by the Shanghai Institute of Organic Chem-
istry, Chinese Academy of Sciences, China. Electronic
absorption spectra in the UV–Vis region were recorded
on a Hewlett–Packard 8453 UV–Visible spectrophotom-
eter, steady-state visible fluorescence and PL excitation
spectra on a Photon Technology International (PTI) Al-
phascan spectrofluorimeter and visible decay spectra on
a pico-N2 laser system (PTI Time Master) with kexc =337
nm. Quantum yields of visible emissions were computed
according to the literature method [27] using quinine sul-
fate in 0.1 N H2SO4 as the reference standard (U=0.55
in air-equilibrated water) [28]. NIR emission was detect-
ed by a liquid nitrogen cooled InSb IR detector (EG&G)
with a preamplifier and recorded by a lock-in amplifier
system. The third harmonics, 355-nm line of a Nd:YAG
laser (Quantel Brilliant B), was used as the excitation
light source. The emission spectra have been corrected
for the spectral response of the instrument. Infrared
spectra (KBr pellets) were recorded on a Nicolet Na-
gna-IR 550 spectrometer and NMR spectra on a JEOL
EX270 spectrometer. Chemical shifts of 1H and 13C
NMR spectra were referenced to internal deuteriated
solvents and then recalculated to SiMe4 (d 0.00). Low-
resolution mass spectra (LRMS) were obtained on a
Finnigan MAT SSQ-710 spectrometer in the positive
FAB mode. Electrospray ionization high-resolution
mass spectra (ESI-HRMS) were recorded on a QSTAR
mass spectrometer. Conductivity measurements were
carried out with a DDS-11 conductivity bridge for
10ꢀ4 moldmꢀ1 solutions either in CH3OH or CH3CN.
1
mental analyses, MS and H NMR and their solid-state
structures were ascertained by X-ray crystallography.
The photophysical properties of these Zn-4f Schiff base
complexes were also reported.
N
N
N
N
R
OH HO
R
OH HO
O
CH3
O
H3C
O
O
CH3
H3C
H2L1; R = H
H2L2; R = p-C6H4CH3
H2L3
O
O
(a)
(b)
H3C
OH
Br
OH
OCH3
OCH3
A
N
O
N
O
6
(c)
(d)
Zn
Ln
H3C
CH3
4
89
O
3
O
CH3
H3C
O
O
N
O
Ln= La, Nd, Gd, Er, Yb
Scheme 1. (a) 4-Methylphenylboronic acid, Pd(OAc)2, K2CO3, D.I. water, 2 days. (b) Ethylenediamine, absolute ethanol, reflux. (c) Zn(OAc)2,
absolute ethanol, reflux. (d) Ln(NO3)3 ÆxH2O, CH3CN, reflux.

4512
W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
2.2. Preparations of ligands and zinc(II) Schiff bases
2.2.3. Synthesis of ZnL2
To a stirred suspension of H2L2 (0.508 g and 1.00
mmol) in absolute ethanol (30 ml), Zn(OAc)2 Æ2H2O
(0.242 g and 1.10 mmol) was added and the reaction
mixture was heated under reflux overnight. The insolu-
ble yellow precipitate was filtered, washed with ethanol
and CHCl3 and dried under vacuum. ZnL2 was isolated
as a yellow solid. Yield: 0.442 g (77%). M.p. >300 ꢁC. 1H
NMR (DMSO-D6): d=8.54 (s, 2H, HC‚N), 7.51 (d,
J=8.0 Hz, 4H, CH-8), 7.20 (d, J=8.0 Hz, 4H, CH-9),
7.12 (d, J=2.0 Hz, 2H, CH-6), 7.09 (d, J=2.0 Hz, 2H,
2.2.1. Synthesis of 5-(40-methylphenyl)-3-methoxysali-
cylaldehyde (A)
When a suspension of 5-bromo-3-methoxysalicylalde-
hyde (3.061 g and 13.25 mmol), 1.3 equiv. of 4-methyl-
phenylboronic acid (2.341 g and 17.23 mmol), 20 mol%
palladium(II) acetate (0.595 g and 2.65 mmol) and 10
equiv. of potassium carbonate (18.313 g and 0.13 mol)
was stirred in D.I. water (75 ml) for 2 h, the color of
the mixture changed from yellow to brown. The reaction
mixture was stirred for another 2 days. HCl (6 N) was
then added until pH 7. The reaction mixture was then ex-
tracted with CH2Cl2 (3·50 ml). The organic layer was
evaporated to dryness and the crude product was puri-
fied by silica-gel column chromatography using a solvent
mixture of petroleum ether and CH2Cl2 as the eluent to
give a yellow solid of A. Yield: 2.56 g (80%). M.p. 93–
CH-4), 3.83 (s, 6H, OCH3), 3.32 (s, 4H,
–
NCH2CH2N–) and 2.31 (s, 6H, p-C6H4CH3). UV–Vis
(DMSO, 20 ꢁC): kmax/nm [log(e/dm3 molꢀ1 cmꢀ1)] 383
(3.84), 310 (4.52), 279 (4.66). FAB MS (+ve mode): m/
z=593 [M+Na]+. IR (KBr): m=3415m, 2917m,
1636vs, 1455s, 1395m, 1322m, 1275m, 1203m, 1120m,
975m, 820m cmꢀ1
.
1
95 ꢁC. H NMR (CDCl3): d=11.08 (s, 1H, OH), 9.99
(s, 1H, CHO), 7.46 (d, J=8.4 Hz, 2H, CH-6), 7.37 (d,
J=2.0 Hz, 1H, CH-8), 7.32 (d, J=2.0 Hz, 1H, CH-9),
7.27 (d, J=8.4 Hz, 2H, CH-4), 3.99 (s, 3H, OCH3) and
2.41 (s, 3H, p-C6H4CH3). 13C NMR (CDCl3): d=196.4,
150.6, 148.4, 137.2, 136.7, 133.1, 129.5, 126.4, 122.4,
120.7, 116.8, 56.6 and 21.2. UV–Vis (CH2Cl2, 20 ꢁC):
2.3. Synthesis of [Zn(l-L2)Ln(NO3)3(H2O)] complexes
Heterobimetallic complexes of the general formula
[Zn(l-L2)Ln(NO3)3(H2O)n] (Ln=La 1, Nd 2, Gd 3, Er
4 or Yb 5; n=1 or 2) were prepared by the same method.
A typical procedure is given for [Zn(l-L2)La(NO3)3
(H2O)2].
k
max/nm [log(e/dm3 molꢀ1 cmꢀ1)] 366 (3.34), 257 (4.32).
FAB MS (+ve mode): m/z=243 [M+1]+. IR (KBr):
m=3445m, 2865m, 1664vs, 1488s, 1400s, 1270s, 1234s,
945s, 820s, 742s, 503m cmꢀ1. Anal. Calc. for C15H14O3
(M=242.27): C, 74.36; H, 5.82. Found: C, 74.67; H,
5.43%.
2.3.1. Synthesis of [Zn(l-L2)La(NO3)3(H2O)2] (1)
When La(NO3)3 Æ6H2O (0.055 g and 0.13 mmol) was
added to a suspension of ZnL2 (0.070 g and 0.12 mmol)
in CH3CN (30 ml) at refluxing temperature, a clear pale
yellow color was observed. The solution was refluxed for
12 h, cooled to room temperature and filtered. The fil-
trate, when allowed to evaporate slowly at room temper-
ature, gave yellow crystals of 1 in about a week. Yield:
0.067 g (62%). M.p. >300 ꢁC. 1H NMR (CD3CN):
d=8.59 (s, 2H, HC‚N), 7.56 (d, J=8.0 Hz, 4H, CH-
8), 7.42 (d, J=2.0 Hz, 2H, CH-6), 7.34 (d, J=2.0 Hz,
2H, CH-4), 7.28 (d, J=8.0 Hz, 4H, CH-9), 4.13 (s,
6H, OCH3), 3.96 (s, 4H, –NCH2CH2N–) and 2.37 (s,
6H, p-C6H4CH3). UV–Vis (CH3CN, 20 ꢁC): kmax/nm
[log(e/dm3 molꢀ1 cmꢀ1)] 362 (3.90), 270 (4.87). Fluores-
cence (CH3CN, 20 ꢁC): kexc/nm 365, 300; kem/nm 480.
FAB MS (+ve mode): m/z=832 [Zn(l-L)La(NO3)2]+
for 64Zn and 138La. IR (KBr): m=3424m, 1643s, 1459s,
1384vs, 1264m, 1208m, 1083m, 964m, 820m cmꢀ1. Anal.
Calc. for C32H30N5O13ZnLaÆ2H2OÆCH3CN (M=
973.99): C, 41.92; H, 3.83; N, 8.63. Found: C, 42.32;
H, 3.68; N, 8.41%.
2.2.2. Synthesis of H2L2
To a stirred solution of 1,2-diaminoethane (0.118 g
and 1.96 mmol) in absolute ethanol (30 ml), A (0.955
g and 3.94 mmol) was added. The resulting mixture
was refluxed overnight. After cooling to room tempera-
ture, the yellow crystalline product precipitated out, and
the product was washed with cold ethanol and petroleum
ether. The crude product was redissolved in CHCl3 and
evaporated to dryness to furnish H2L2 as a yellow solid.
Yield: 0.913 g (92%). M.p. 149–151 ꢁC. 1H NMR
(CDCl3): d=13.74 (s, 2H, OH), 8.37 (s, 2H, HC‚N),
7.41 (d, J=8.0 Hz, 4H, CH-8), 7.21 (d, J=8.0 Hz, 4H,
CH-9), 7.11 (d, J=2.0 Hz, 2H, CH-6), 7.04 (d, J=2.0
Hz, 2H, CH-4), 3.98 (s, 4H, –NCH2CH2N–), 3.94 (s,
6H, OCH3) and 2.38 (s, 6H, p-C6H4CH3). 13C NMR
(CDCl3): d=166.7, 148.4, 137.6, 136.5, 131.5, 129.6,
129.4, 126.5, 121.2, 118.3, 113.1, 59.4, 56.4 and 21.1.
UV–Vis
(CH2Cl2,
20
ꢁC):
k
max/nm
[log(e/
dm3 molꢀ1 cmꢀ1)] 353 (3.58), 262 (4.61). FAB MS (+ve
mode): m/z=509 [M+1]+, 531 [M+Na]+. IR (KBr):
m=3430m, 2917m, 1628vs, 1477s, 1389m, 1281s,
1213m, 1104m, 975m, 819s cmꢀ1. Anal. Calc. for
C32H32N2O4 (M=508.61): C, 75.57; H, 6.34; N, 5.51.
Found: C, 75.78, H, 6.54; N, 5.56%.
2.3.2. Synthesis of [Zn(l-L2)Nd(NO3)3(H2O)2] (2)
Nd(NO3)3 Æ6H2O (0.053 g and 0.12 mmol) and ZnL2
(0.062 g and 0.11 mmol) were used. Yellow crystals of 2
were obtained. Yield 0.087 g (88%). M.p. >300 ꢁC. UV–
Vis (CH3CN, 20 ꢁC): kmax/nm [log(e/dm3 molꢀ1 cmꢀ1)]
362 (3.83), 271 (4.81). Fluorescence (CH3CN, 20 ꢁC):

W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
4513
k
exc/nm 372, 305; kem/nm 875, 480. FAB MS (+ve
1467s, 1384vs, 1301s, 1265s, 1203m, 1187m, 1094m,
959m, 814m, 773m cmꢀ1. Anal. Calc. for C32H30N5O13Z-
nErÆH2OÆCH3CN (M=984.32): C, 41.48; H, 3.59; N,
8.54. Found: C, 41.88; H, 3.70; N, 8.25%.
mode): m/z=838 [Zn(l-L)Nd(NO3)2]+ for 64Zn and
144Nd. IR (KBr): m=3410m, 2928m, 1680vs, 1461s,
1384vs, 1260s, 1193m, 1104m, 1032m, 959m, 814s,
778m, 509m cmꢀ1. Anal. Calc. for C32H30N5O13ZnN-
dÆ2H2OÆCH3CN (M=979.32): C, 41.69; H, 3.81; N,
8.58. Found: C, 41.83; H, 3.81; N, 8.39%.
2.3.5. Synthesis of [Zn(l-L2)Yb(NO3)3 (H2O)] (5)
Yb(NO3)3 Æ5H2O (0.063 g and 0.12 mmol) and ZnL2
(0.072 g and 0.13 mmol) were used to afford yellow crys-
tals of 5. Yield: 0.070 g (58%). M.p. >300 ꢁC. UV–Vis
(CH3CN, 20 ꢁC): kmax/nm [log(e/dm3 molꢀ1 cmꢀ1)] 366
2.3.3. Synthesis of [Zn(l-L2)Gd(NO3)3(H2O)] (3)
Gd(NO3)3 Æ6H2O (0.055 g and 0.12 mmol) and ZnL2
(0.061 g and 0.11 mmol) were used. Yellow crystals of 3
were obtained. Yield: 0.060 g (60%). M.p. >300 ꢁC. UV–
Vis (CH3CN, 20 ꢁC): kmax/nm [log(e/dm3 molꢀ1 cmꢀ1)]:
362 (3.99), 269 (4.91). Fluorescence (CH3CN, 20 ꢁC):
(3.75), 267 (4.73). Fluorescence (CH3CN, 20 ꢁC): kexc
/
nm 370, 300; kem/nm 976, 480. FAB MS (+ve mode):
m/z=867 [Zn(l-L)Yb(NO3)2]+ for 64Zn and 173Yb. IR
(KBr): m=3430m, 1632s, 1472s, 1384vs, 1306s, 1270s,
1203m, 1198m, 1099m, 1037m, 959m, 814m, 778m
cmꢀ1. Anal. Calc. for C32H30N5O13ZnYbÆH2OÆCH3CN
(M=990.10): C, 41.23; H, 3.57; N, 8.49. Found: C,
41.22; H, 3.69; N, 8.33%.
k
exc/nm 365, 295; kem/nm 480. FAB MS (+ve mode):
m/z=851 [Zn(l-L)Gd(NO3)2]+ for 64Zn and 157Gd. IR
(KBr): m=3425m, 1363m, 1638s, 1462s, 1384vs, 1265s,
1203m, 1088m, 969m, 809m cmꢀ1. Anal. Calc. for
C32H30N5O13ZnGdÆH2OÆCH3CN (M=974.31): C,
41.19; H, 3.63; N, 8.62. Found: C, 41.52; H, 3.69; N,
8.53%.
2.4. X-ray crystallography
Pertinent crystallographic data and other experimen-
tal details are summarized in Table 1. Crystals of A
and H2L2 suitable for X-ray diffraction studies were
grown by slow diffusion of diethylether into a solution
of the respective compound in chloroform; whereas crys-
tals of 1, 2, 4 and 5 were grown by slow evaporation of a
solution of the respective compound in acetonitrile. In-
tensity data were collected at 293 K on a Bruker Axs
SMART 1000 CCD area-detector diffractometer using
graphite monochromated Mo Ka radiation (k=
2.3.4. Synthesis of [Zn(l-L2)Er(NO3)3(H2O)] (4)
Er(NO3)3 Æ5H2O (0.054 g and 0.12 mmol) and ZnL2
(0.062 g and 0.11 mmol) were used. Yellow crystals of 4
were obtained. Yield: 0.060 g (59%). M.p. >300 ꢁC.
UV–Vis
(CH3CN,
20
ꢁC):
k
max/nm
[log(e/
dm3 molꢀ1 cmꢀ1)] 360 (3.77), 267 (4.76). Fluorescence
(CH3CN, 20 ꢁC): kexc/nm 365, 300; kem/nm 1515, 480.
FAB MS (+ve mode): m/z=861 [Zn(l-L)Er(NO3)2]+ for
64Zn and 167Er. IR (KBr): m=3410m, 2948m, 1633vs,
Table 1
Crystal data and structure refinement
A
H2L2
1
2
4
5
Empirical formula
Molecular mass
Crystal system
Space group
C
15H14O3
C32H32N2O4
508.60
orthorhombic monoclinic
C34H39N6O16ZnLa C34H37N6O15ZnNd C32H32N5O14ZnEr C32H32N5O14ZnYb
242.26
991.99
monoclinic
P21/c
979.31
monoclinic
P21/n
943.26
tetragonal
I41cd
949.04
tetragonal
I41cd
Pbca
P/n
˚
a (A)
12.8138(14)
7.3384(8)
26.965(3)
90
17.4620(12)
7.1051(5)
21.5908(14)
90
8.0586(7)
17.7617(15)
28.170(2)
90
14.4326(9)
17.2692(12)
16.1154(11)
90
32.649(2)
32.649(2)
14.697(13)
90
32.6840(11)
32.6840(11)
14.6534(10)
90
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
90
90
95.6590(10)
90
91.764(2)
90
99.1480(10)
90
90
90
90
90
3
˚
U (A )
2535.6(5)
1024
2665.7(3)
1080
4030.2(6)
2000
3965.5(5)
1972
15666(2)
7504
16
15653.4(13)
7504
16
F(000)
Z
Dcalc (gcmꢀ3
l(Mo Ka) (mmꢀ1
h Range (ꢁ)
Reflections collected
Unique reflections
Rint
8
1.269
0.088
4
1.267
0.084
4
1.635
1.718
4
1.640
1.975
)
1.600
2.809
1.607
3.056
)
1.51–27.51
13846
2891
1.43–25.01
12775
7550
1.36–28.28
23857
9400
1.74–28.33
23662
9156
1.25–25.01
30971
6268
1. 25–28.31
45996
8253
0.0370
1616
0.0214
5486
0.0615
5611
0.0666
4898
0.1163
3445
0.0794
5130
Observed reflections
R1, wR2 (I>2r(I))a
R1, wR2 (all data)a
Goodness-of-fit on F2 0.971
0.0573, 0.1389 0.0476, 0.1341 0.0662, 0.1587
0.1045, 0.1675 0.0712, 0.1557 0.1240, 0.1870
0.0482, 0.1017
0.1207, 0.1254
0.988
0.0479, 0.0896
0.1027, 0.0996
0.920
0.0503, 0.1142
0.1078, 0.1396
1.033
1.033
P
1.059
P
P
P
2
2
1=2
a
R1 ¼ kF oj ꢀ jF ck= jF oj. wR2 ¼ f ½wðF 2o ꢀ F c2Þ ꢁ= ½wðF o2Þ ꢁg
.

4514
W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
˚
0.71073 A). The collected frames were processed with the
software SAINT [29] and an absorption correction was
applied (SADABS) [30] to the collected reflections. The
structures of all compounds were solved by direct meth-
ods (SHELXTL) [31] and refined against F2 by full matrix
least-squares analysis. All non-hydrogen atoms were re-
fined anisotropically for these structures. Hydrogen at-
oms were generated in their idealized positions and
allowed to ride on their respective parent carbon atoms.
0 ꢁC and room temperature, respectively. The resonance
disappears completely at 40 ꢁC. This suggests that both
resonance forms (Eq. (1)) are significant at room tem-
perature. The crystal structure of H2L2 depicted in
Fig. 2 revealed the presence of intramolecular hydrogen
bonding between the O–H group and the imino N of the
Schiff base with the Nꢂ ꢂ ꢂO distances of 2.5947(16) and
˚
2.5997(16) A and are similar to those reported for anal-
ogous Schiff bases [36]. The intramolecular H-bonds in
salicyl imines (Eq. (2)) have been thoroughly character-
ized as RAHB protons on the basis of crystal, NMR, IR
and computational data [34–36]. For the RAHB pro-
tons, the H-bond is strengthened by p-delocalization.
3. Results and discussion
3.1. Synthesis and structural characterization
The
Nꢂ ꢂ ꢂO
distances
in
these
compounds
˚
(2.65>d(Nꢂ ꢂ ꢂO)>2.48 A) are significantly shorter than
those in normal H-bonds. In addition, the 1H NMR sig-
nals of the strongly H-bonded protons are shifted highly
downfield (13<dNH <18 ppm).
5-(40-Methylphenyl)-3-methoxysalicylaldehyde
(A)
was prepared in good yield (80%) via the modified Su-
zukiꢀs coupling reaction [32] between 5-bromo-3-meth-
oxysalicylaldehyde and 4-methylphenylboronic acid.
The phenyl analog has been synthesized recently via a
O
H
1
similar procedure [33]. The H NMR spectrum of A in
R
N
R
N
H
CDCl3 exhibited a sharp singlet at d=11.08 for the O–
H proton. The chemical shift is typical of the reso-
nance-assisted hydrogen bonded (RAHB) proton of
the type O–Hꢂ ꢂ ꢂO‚C [34]. This is supported by the sol-
id-state structure of A (Fig. 1), which shows the presence
of intramolecular hydrogen bonding between the O–H
O
R = p-tolyl
O
˚
H
N
group and oxygen of the formyl group [2.661(2) A].
The Schiff base H2L2 was synthesized in excellent yield
(92%) by the condensation reaction between ethylene di-
amine and A in a 1:2 mole ratio. The 1H NMR spectrum
of H2L2 in CDCl3 showed a temperature-dependent res-
onance for the O–H protons. At ꢀ50 ꢁC, the O–H pro-
tons appear as a sharp singlet at d=14.00, which is
typical for RAHB proton of O–Hꢂ ꢂ ꢂN‚C [34,35]. The
resonance starts to broaden and shift upfield as the tem-
perature increases. It appears as a broad singlet
at d=13.74 (m1/2 =21.6 Hz) and 13.58 (m1/2 =43.2 Hz) at
R
R
N
H
O
ð1Þ
H
H
O
N
N
O
ð2Þ
The zinc Schiff base complex, ZnL2, was easily pre-
pared in 77% yield by reacting H2L2 with Zn(OAc)2 in
a 1:1 mole ratio in refluxing absolute ethanol for 12 h.
ZnL2 showed very poor solubility in refluxing CH3CN,
but when Ln(NO3)3 was added, it dissolved completely
to give a clear solution, which upon work-up, gave
heterobimetallic complexes of the general formula
[Zn(l-L2)Ln(NO3)3(H2O)n] (Ln=La 1, Nd 2, Gd 3, Er
4 or Yb 5; n=1 or 2) in 58–88% yield (Scheme 1). The
complexes were easily crystallized out by slow evapora-
tion of their solutions in acetonitrile. Elemental analyses
and FAB MS data also supported the formulation.
The structures of 1, 2, 4 and 5 were ascertained by
X-ray crystallography. Selected bond distances of the
[Zn(l-L2)Ln(NO3)3(H2O)n] complexes are summarized
in Table 2. In all cases, the crystal structures of neutral
[Zn(l-L2)Ln(NO3)3(H2O)n] complexes (Ln=La, Nd, Er
Fig. 1. Perspective drawing of A with 25% probability thermal
ellipsoids.

W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
4515
Fig. 2. Perspective drawing of H2L2 with 25% probability thermal ellipsoids.
Table 2
Selected bond distances (A) of Zn–Ln complexes
˚
1 (Ln=La)
2 (Ln=Nd)
4 (Ln=Er)
5 (Ln=Yb)
Zn–N(1)
Zn–N(2)
2.021(6)
2.009(6)
2.015
2.020(5)
2.016(4)
2.018
2.023(10)
2.058(10)
2.041
2.033(8)
1.987(8)
2.010
Average Zn–N
Zn–O(2)
Zn–O(4)
1.993(4)
2.018(5)
2.001
2.010(3)
1.989(4)
2.000
2.055(8)
2.016(8)
2.036
2.014(7)
2.029(7)
2.022
Average ZnAO
Ln–O(1)
Ln–O(2)
2.726(5)
2.514(5)
2.715(5)
2.471(4)
2.607
2.711(4)
2.382(3)
2.721(4)
2.378(3)
2.548
2.559(7)
2.284(8)
2.545(7)
2.234(8)
2.406
2.499(6)
2.220(7)
2.519(6)
2.249(7)
2.372
Ln–O(3)
Ln–O(4)
Average Ln–O
Znꢂ ꢂ ꢂLn
3.583
3.543
3.453
3.429
and Yb) revealed that the relatively soft transition
metal ion, Zn(II), is located in the inner N2O2 cavity
and the hard lanthanide(III) ion in the outer O4 cavity
of the Schiff base ligand. The zinc(II) ion is five-coordi-
nate and adopts a distorted square pyramidal geome-
try, with the imino nitrogen atoms N(1) and N(2),
and the phenolic oxygen atoms O(2) and O(3) forming
the square base, while the aqua oxygen (Ln=La and
Nd) or the monodentate nitrato oxygen (Ln=Er and
Yb) occupies the axial position. Depending on the size
of the Ln(III) ions, their coordination number varies
from 9 (Ln=Er, Yb), 10 (Ln=Nd) to 11 (Ln=La).
The Ln(III) ions are surrounded by four O atoms from
the Schiff base, two from the bridging phenolic groups
and two from the methoxy groups, with the remaining
coordination sites by O atoms from aqua and nitrato
ligands. For 1, the remaining coordination sites are oc-
cupied by seven O atoms, six from three bidentate nit-
rato ligands and one from the aqua oxygen (Fig. 3); for
2, by six O atoms, four from two bidentate nitrato lig-
ands, one from monodentate nitrato ligand and one
from the aqua oxygen (Fig. 4); and for 4 (Fig. 5) and
5 (Fig. 6), by five O atoms, four from two bidenate nit-
rato ligands and one from the aqua oxygen. The results
illustrate that as the ionic size decreases, the lanthanide
ion lowers its coordination number to relieve the con-
gestion by converting one of its bidentate nitrato ligand
to a less steric demanding monodentate nitrato ligand.
Eventually, the coordination environment of the lan-
thanide is so congested that it has to transfer the nitra-
to ligand to the Zn(II) ion. The distance between Zn
˚
and Ln ions is 3.429–3.583 A and is too long for any
significant interaction. The average Zn–N and Zn–O
bond lengths are in the range of 2.010–2.041 and
˚
2.000–2.036 A, respectively, which are comparable to
those of other similar Zn(II) Schiff base complexes
[37,38]. The average Ln–O bond length lies within
˚
2.372–2.607 A, which is comparable to those of other
reported heterobimetallic Zn-4f Schiff base complexes
[38]. The decrease in Ln–O distance in the series
La3+ >Nd3+ >Er3+ >Yb3+ is in agreement with the lan-
thanide contraction and reflects a decrease in ionic ra-
˚
dii: 1.17>1.12>1.03>1.01 A [39].
3.2. Solution behavior
Conductivity measurements showed that complexes 1–
5 behaved as electrolytes in methanol and non-
electrolytes in acetonitrile. This indicates that the

4516
W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
Fig. 3. Perspective drawing of [Zn(l-L2)La(NO3)3(H2O)2] (1) with 25% probability thermal ellipsoids.
Fig. 4. Perspective drawing of [Zn(l-L2)Nd(NO3)3(H2O)2] (2) with 25% probability thermal ellipsoids.
Fig. 5. Perspective drawing of [Zn(l-L2)Er(NO3)3(H2O)] (4) with 25% probability thermal ellipsoids.
complexes dissociated one or more of their coordinated
nitrates to form cations in methanol but remained neutral
and undissociated in acetonitrile. Similar observation has
been reported for [LnL0(NO3)3] complexes, where L0 is a
neutral tripodal ligand bearing benzimidazole and pyri-
zation (ESI) mass spectral data. The ESI high-
resolution mass spectra (positive mode) of complexes 1–
5 in methanol displayed similar patterns and exhibited
mass peaks at m/z corresponding to [(ZnL2)2Ln(NO3)2]+,
[(ZnL2)Ln(NO3)2]+, [(ZnL2)2Ln(NO3)]2+, [(ZnL2)2Ln]3+
and [(ZnL2)+H]+. For instance, complex 1 displayed
mass peaks at m/z 1407.1763 [(ZnL2)2La(NO3)2]+,
833.0401 [(ZnL2)La(NO3)2]+, 672.5905 [(ZnL2)2La-
1
dine groups [40]. H NMR study showed that the com-
plexes exist as a mixture of species in methanol solution.
For instance, the Zn–La complex 1 exhibited more than
one set of proton signals for the Schiff base ligand whose
relative intensities vary with temperature. This suggests
that there are two or more species present in methanol so-
lution. This is further supported by the electrospray ioni-
(NO3)]2+
,
427.7329 [(ZnL2)2La]3+ and 571.1547
[(ZnL2)+H]+, which deviate less than 10 ppm of the the-
oretical peak values of 1407.1789 (C64H60N6O14Zn2La),
833.0316 (C32H30N4O10ZnLa), 672.5949 (one-half of

W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
4517
Fig. 6. Perspective drawing of [Zn(l-L2)Yb(NO3)3(H2O)] (5) with 25% probability thermal ellipsoids.
C64H60N5O11Zn2La), 427.7338 (one-third of C64H60-
N4O8Zn2La) and 571.1569 (C32H31N2O4Zn), respectively.
The observed isotopic distribution patterns of the
[(ZnL2)2La(NO3)2]+, [(ZnL2)La(NO3)2]+, [(ZnL2)2La
(NO3)]2+, [(ZnL2)2La]3+ and [(ZnL2)+H]+ peaks match
well with the expected theoretical signals. Furthermore,
MS–MS studies on the above ions showed that
[(ZnL2)2La(NO3)]2+, [(ZnL2)2La]3+ and [(ZnL2)+H]+
exist independently in solution and are not fragmented
ions of [(ZnL2)2La(NO3)2]+. This suggests that when dis-
solved in methanol, [Zn(l-L2)Ln(NO3)3(H2O)n] gave a
complex mixture containing cations of [(ZnL2)2
Ln(NO3)2]+, [(ZnL2)Ln(NO3)2]+, [(ZnL2)2Ln(NO3)]2+
and [(ZnL2)2Ln]3+ (Chart 1) and neutral species of
[Zn(l-L2)Ln(NO3)3(H2O)n] and ZnL2. The existence of
the [(ZnL2)2Ln(NO3)2]+ species was further supported
by the characterization and X-ray crystal structure of
the phenylene bridged Schiff base complex [(ZnL3)2
Nd(NO3)2(H2O)2][NO3], isolated from a methanol solu-
tion of [Zn(l-L3)Nd(NO3)3(EtOH)] [41]. For the related
[Zn(l-L1)Pr(NO3)3(H2O)] complex, it dissociated into a
mixture containing at least [Zn(l-L1)Pr(NO3)3(H2O)]
and ZnL1 when dissolved in DMSO and remained undis-
sociated in acetone [20f].
+
+
N
O
N
Zn
N
O
N
O
R
O
R
R
Zn
Ln
O
R
R
O
H3C
O2NO Ln
H3C
CH3
ONO2
O
O
CH3
CH3
H3C
O
O
O
O
N
O
R
O
N
Zn
O
2
N
+
[ (ZnL2)2Ln(NO3)2]+
2
[ (ZnL ) Ln(NO3)2]
2+
3+
N
O
N
N
O
N
Zn
Ln
Zn
Zn
Ln
Zn
R
O
R
R
R
O
R
O
O
O
O
H3C
O2NO
H3C
O
O
H3C
H3C
CH3
CH3
CH3
CH3
O
O
O
O
R
O
N
R
O
N
R
N
N
2+
2
[ (ZnL2)2Ln ]3+
[ (ZnL )2 Ln(NO3)]
Chart 1.

4518
W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
However, complexes 1–5 only displayed one set of re-
physical properties are summarized in Table 4. The ab-
1
sonances attributed to the Schiff base ligand in their H
NMR spectra when dissolved in CD3CN. For instance,
complex 1 in CD3CN displayed a temperature-inde-
pendent 1H NMR spectrum, which only exhibited a
set of proton resonances attributed to the Schiff base lig-
and at d=8.58 (s, 2H, HC‚N), 7.56 (d, J=8.0 Hz, 4H,
CH-4), 7.42 (d, J=2.0 Hz, 2H, CH-6), 7.34 (d, J=2.0
Hz, 2H, CH-4), 7.28 (d, J=8.0 Hz, 4H, CH-9), 4.13 (s,
6H, OCH3), 3.97 (s, 4H, –NCH2CH2N–) and 2.37 (s,
6H, p-C6H4CH3). The relative intensity and chemical
shifts of the resonances did not change and no other
new signal was observed over the temperature range of
ꢀ40 to 60 ꢁC. This indicates that there is only one spe-
cies present in acetonitrile solution and the solution
structure of the complex in acetonitrile is similar to its
solid-state structure. The 1H NMR data of H2L2,
ZnL2, 1, 2, 4 and 5 are given in Table 3. The assignment
of the chemical shifts is based on the relative intensity
and coupling constants of the signals as well as by com-
parison with the related [Zn(l-L2)Ln(NO3)3(H2O)]
(Ln=La and Pr) complex [20b]. Attempts to obtain a
1H NMR spectrum for 3 were unsuccessful.
sorptions between 267 and 366 nm can be assigned to
pfip* transitions of the Schiff base ligand. The visible
emission of compounds 1–5, with lifetimes (s) ranging
from 1.9 to 4.6 ns and quantum yields (Uem) of 0.38–
2.17·10ꢀ3 for 1 and 3, and <10ꢀ5 for 2, 4 and 5, can
be assigned to the intra-ligand emission. The absorption,
emission and excitation (monitored at 480 nm) spectra
of 1 (Fig. 7) are slightly red shifted as compared to the
corresponding
unsubstituted
[Zn(l-L1)Ln(NO3)3-
(H2O)n] complexes [26]. This shows that we can tune
the photophysical properties of the Schiff base complexes
by changing the electronic properties of the substituents.
By extending the conjugation, both the absorption and
emission of the complexes are red shifted. The photolu-
minescence excitation (PLE) spectrum reflects where the
maximum contribution of the photoluminescence (PL)
comes from. Under steady-state conditions, excited elec-
trons relax from the highest excited states to the lowest
excited states before they recombine radiatively or non-
radiatively. In this case, the PLE spectrum showed ex-
actly this situation; the maximum PL contribution came
from the lowest excited bands with decreasing contribu-
tion from the higher energy band. Thus, although the
absorption bands coincide with the excitation bands,
as they should, the relative intensities of the energy
bands by the two techniques can reveal the origin of
the physical process measured. The emission spectra
and decay time measurements for Gd3+ complexes
3.3. Luminescent properties
At room temperature, complexes [Zn(l-L2)Ln
(NO3)3(H2O)n] exhibited similar solution absorption
and emission spectra in the UV–Vis region. Their photo-
Table 3
1H NMR data (chemical shifts in ppm versus SiMe4) in CD3CN with intensity ratios given in parenthesesa
CH2
HC‚N
CH-4b
CH-6b
CH-8b
CH-9b
OCH3
CH3
OH
H2L2
3.98 (4)
3.32 (4)
3.96 (4)
7.53 (4)
59.94 (4)
47.91 (4)
8.37 (2)
8.54 (2)
8.59 (2)
11.21 (2)
55.32 (2)
45.26 (2)
7.04 (2)
7.09 (2)
7.34 (2)
0.38 (2)
6.20 (2)
2.80 (2)
7.11 (2)
7.12 (4)
7.42 (2)
7.99 (2)
13.29 (2)
13.15 (2)
7.41 (4)
7.51 (4)
7.56 (4)
6.46 (4)
19.79 (4)
17.30 (4)
7.21 (4)
7.20 (4)
7.28 (4)
5.93 (4)
13.23 (4)
12.01 (4)
3.94 (6)
3.83 (6)
4.13 (6)
2.38 (6)
2.31 (6)
2.37 (6)
1.84 (6)
5.44 (6)
5.43 (6)
13.74 (2)
ZnL2c
1
2
4
5
a
ꢀ5.28 (6)
ꢀ10.23 (6)
ꢀ7.95 (6)
We were unable to obtain a 1H NMR spectrum for 3.
Please refer to Scheme 1 for the numbering of the phenyl protons.
The spectrum was recorded in DMSO-D6.
b
c
Table 4
Photophysical data of compounds 1–5 in CH3CN
Absorption, kmax/nm [log(e/dm3 molꢀ1 cmꢀ1)]
Excitation, kexc/nm
Emission, kem/nm (s/ns, Uem ·103)
1
2
3
4
5
362 (3.90), 270 (4.87)
362 (3.83), 271 (4.81)
362 (3.99), 269 (4.91)
360 (3.77), 267 (4.76)
366 (3.75), 267 (4.73)
365, 300
372, 305
365, 295
365, 300
370, 300
480 (1.9, 2.17)
480 (4.6)a, 875 (1305)b
480 (1.9, 0.38)c
480 (4.3)a, 1515b,d
480 (4.3)a, 976 (14590)b
a
b
c
U
em <10ꢀ5
.
Due to the limitations of our instrument, we were unable to determine the quantum yield of the NIR luminescence of 2, 4 and 5.
The lifetime of 3 in CH3CN at 77 K was 2.23 ns (monitored at 480 nm), 2.90 ns (fluorescence; monitored at 515 nm) and 6.10 ms (phospho-
rescence; monitored at 515 nm).
d
Due to the limitations of our instrument, we were unable to measure the lifetime of the NIR luminescence of 4.

W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
4519
room temperature, which is minimized at 77 K. The lig-
and-centered singlet excited state (1LC) is estimated to
be about 20830 cmꢀ1 and triplet state (3LC) to be
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
19420 cmꢀ1
.
Other than the visible emission, compounds 2, 4 and
5 also exhibited emission corresponding to the lantha-
nide(III) ion in the NIR region (Fig. 9). These NIR
emissions are very similar to those reported for Nd3+
,
Er3+ and Yb3+ [14,44]. For 2, the emissions at 875 and
4
905 nm can be assigned to F3/2 fi4I9/2, 1068 nm to
4
4F3/2 fi4I11/2 and 1356 nm to F3/2 fi4I13/2 transitions
of Nd3+. For 4, the emission at 1515 nm can be assigned
4
to I13/2 fi4I15/2 transition of Er3+ and for 5, the emis-
sion at 976 nm can be assigned to the 2F5/2 fi2F7/2 tran-
sitions of Yb3+. From Fig. 9, we can observe that the
intensity of the Ln3+ emission follows the order: Yb>N-
d>Er, typical of other related system [44]. Due to the
limitations of our instrument, we were unable to meas-
ure the excitation spectrum (monitored at 1515 nm)
and the NIR luminescence lifetime of 4. Compounds 2
and 5 show very similar excitation (monitored at the
NIR emission peak) and absorption spectra. The excita-
tion spectrum of 2 (Fig. 10) monitored at 875 nm is sim-
ilar to that monitored at 480 nm. This clearly shows that
both visible and NIR emissions are originated from the
pfip* transitions of the Schiff base ligand. The sensiti-
zation process leading to the observable NIR lumines-
cence probably proceeds via excitation of the Schiff
base to its singlet state (S1) and the energy is then trans-
ferred to its triplet state (T1) by intersystem crossing,
which is, in part, followed by energy transfer to the ex-
cited state of the lanthanide ion (Ln3+)* to produce the
final luminescence. The overall NIR luminescence quan-
tum yield (Utot) is the product of triplet yield (UISC), en-
ergy transfer yield (UET) and intrinsic luminescence
quantum yield of the lanthanide ion (ULn). Due to
the limitations of our instrument, we were unable to
250
300
350
400
450
500
550
600
650
700
wavelength /n.m.
Fig. 7. Absorption (——), excitation (ꢂ ꢂ ꢂ, monitored at 480 nm) and
emission (–––) spectra of 1 in CH3CN at room temperature.
allowed the identification of the lowest ligand triplet
state in the complexes [42–45]. This is possible because
the metal centered (MC) electronic levels of Gd3+ are
known to be located at 31000 cmꢀ1, typically well above
the ligand-centered electronic levels of aromatic ligands
[46]. Therefore, ligand-to-metal energy transfer and the
consequent MC luminescence cannot be observed. Fig.
8 shows the variable temperature emission spectra of 3
in acetonitrile. Time-resolved spectra of 3 showed that
the lifetimes at 480 and 515 nm were both 1.9 ns at room
temperature, indicating that the emissions came from
the same state and corresponded to a ligand-centered
(LC) short-lived fluorescence, and at 77 K, on top of
the weak LC fluorescence (s=2.3 ns at 480 nm and 2.9
ns at 515 nm), an intense LC long-lived phosphores-
cence was also observed at 515 nm (s=6.10 ms). This in-
dicates that both LC singlet and triplet states exist at 77
K. The results show that the 3LC state is efficiently
quenched by thermally activated vibrational modes at
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
1
0.0002
x10
0.0000
-0.0002
800
900 1000 1100 1200 1300 1400 1500 1600 1700
wavelength /nm
0
300
400
500
600
700
wavelength /nm
Fig. 9. Room temperature near-infrared luminescence spectra of 2
(——), 4 (ꢂ ꢂ ꢂ) and 5 (–––) in CH3CN upon excitation at 355 nm. The
Fig. 8. Absorption (–Æ–Æ–Æ–Æ), excitation (–––) and emission spectra at
room temperature (——) and at 77 K (ꢂ ꢂ ꢂ) of 3 in CH3CN.
concentration of each solution is 1.1·10ꢀ4 moldmꢀ3
.

4520
W.-K. Lo et al. / Inorganica Chimica Acta 357 (2004) 4510–4521
deposited with the Cambridge Crystallographic Data
4
Centre (Deposition No. 234508–234513). Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (Fax: + 44-1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
2
Acknowledgement
Financial support from the Hong Kong Baptist Uni-
versity and the Hong Kong Research Grants Council
(HKBU 2038/02P) is acknowledged.
0
250
300
350
400
450
500
wavelength /nm
Fig. 10. Absorption (——) and excitation (ꢂ ꢂ ꢂ; monitored at 870 nm)
spectra of 2 in CH3CN at room temperature.
References
[1] J.-C.G. Bunzli, Rare Earths, in: R. Saez-Puche, P. Caro (Ed.),
¨
Editorial Complutense, Madrid, Spain, 1998, p. 223ff.
[2] J.A. Peters, J. Huskens, D.J. Raber, Prog. Magn. Reson.
Spectrosc. 28 (1996) 283.
measure the overall quantum yields of the NIR lumines-
cence of the complexes directly. However, the intrinsic
luminescence quantum yields of lanthanide ions can be
estimated from the equation ULn =sobs/s0 [47], in which
sobs is the observed emission lifetime and s0 is the radi-
ative or ꢁnaturalꢀ lifetime of Ln3+, which is 0.25 and 2
ms for Nd3+ and Yb3+, respectively [14]. Thus, the max-
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