Hetero-binuclear near-infrared (NIR) luminescent Zn-Nd complexes self-assembled from the benzimidazole-based ligands

Article abstract of DOI:10.1016/j.saa.2012.08.048

With the compound [Zn(HL¹)₂(Py)] (H₂L ¹ = 2-(1H-benzo[d]imidazol-2-yl)-6-methoxyophenol), Py = pyridine) or [Zn(HL²)₂(Py)] (H₂L² = 2-(1H-benzo[d]imidazol-2-yl)-4-bromo

Full text of DOI:10.1016/j.saa.2012.08.048

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Hetero-binuclear near-infrared (NIR) luminescent Zn–Nd complexes
self-assembled from the benzimidazole-based ligands
Dan Zou a,b, Weixu Feng a, , Guoxiang Shi a,b, Xingqiang Lü a,b,c, , Zhao Zhang a,b, Yao Zhang a,b, Han Liu a,b
,
⇑
⇑
Daidi Fan a,b, Wai-Kwok Wong d, Richard A. Jones e
a Shaanxi Key Laboratory of Degradable Medical Material, Northwest University, Xi’an 710069, Shaanxi, China
b Shaanxi Key Laboratory of Physico-inorganic Chemistry, Northwest University, Xi’an 710069, Shaanxi, China
c Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou 350002, Fujian, China
d Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, China
e Department of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station A5300, Austin, TX 78712-0165, United States
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
Hetero-binuclear complexes [ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd, 1; Ln = Gd, 2) and [ZnLn(HL2)2(Py)(NO3)3]
(Ln = Nd, 3; Ln = Gd, 4) were obtained from two benzimidazole-based ligands H2L1 and H2L2, respectively.
The sensitization and energy transfer for the NIR luminescence of the Nd3+ ions in the two Zn–Nd com-
plexes (1 and 3) were discussed.
"
Hetero-binuclear Zn–Ln (Ln = Nd or
Gd) arrayed complexes assembled
from the benzimidazole-based
ligands.
"
"
Heavy-atom effect.
Enhancement of near-infrared
luminescent properties.
NH2
R = H, H2L1
;
HN
N
NH2
EtOH
1:1
R
R = Br, H2L2
OH
O
CHO
R
R = H; Br
OH
Py
Zn(OAc)2
O
HN
NH
HN
NH
N
Ln(NO3
)
3
N
N
N
N
N
R
Zn
R
R
Zn
O
R
O
O
O
Ln
O
O
O
O
O
O
R = H, [Zn(HL1 2(Py)];
R = Br, [Zn(HL2 2(Py)]
N
O
)
R = H, Ln = Nd, 1; Ln = Gd, 2;
R = Br, Ln = Nd, 3; Ln = Gd, 4
3
)
a r t i c l e i n f o
a b s t r a c t
With the compound [Zn(HL1)2(Py)] (H2L1 = 2-(1H-benzo[d]imidazol-2-yl)-6-methoxyophenol), Py = pyr-
idine) or [Zn(HL2)2(Py)] (H2L2 = 2-(1H-benzo[d]imidazol-2-yl)-4-bromo-6-methoxyphenol) as the pre-
cursor, complexes [ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd, 1; Ln = Gd, 2) or [ZnLn(HL2)2(Py)(NO3)3] (Ln = Nd,
3; Ln = Gd, 4) were obtained by the further reaction with Ln(NO3)3Á6H2O (Ln = Nd or Gd). The result of
their photophysical properties shows that the strong and characteristic near-infrared (NIR) luminescence
of Nd3+ ions for complexes 1 and 3 with emissive lifetimes in microsecond range, has been sensitized
from the excited state (1LC and 3LC) of the benzimidazole-based ligands, and the involvement of heavy
atoms (Br) at the ligand endows the enhanced NIR luminescent property.
Article history:
Received 24 May 2012
Received in revised form 30 July 2012
Accepted 21 August 2012
Available online 28 August 2012
Keywords:
Zn–Ln arrayed benzimidazole-based
complexes
Ó 2012 Elsevier B.V. All rights reserved.
UV–vis absorption
Visible and NIR luminescence
Sensitization and energy transfer
⇑
Corresponding authors. Tel./fax: +86 29 88302312.
E-mail addresses: fwxdk@foxmail.com (W. Feng), lvxq@nwu.edu.cn (X. Lü).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.08.048

360
D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
tra in the UV/vis region were recorded with a Cary 300 UV
Introduction
spectrophotometer, and steady-state visible fluorescence, PL exci-
tation spectra on a Photon Technology International (PTI) Alpha-
scan spectrofluorometer and visible decay spectra on a pico-N2
laser system (PTI Time Master). The quantum yield of the visible
luminescence for each sample was determined by the relative
comparison procedure, using a reference of a known quantum
yield (quinine sulfate in dilute H2SO4 solution, Uem = 0.546). NIR
emission and excitation in solution were recorded by PTI QM4
spectrofluorometer with a PTI QM4 Near-Infrared InGaAs detector.
The XRD patterns were recorded on a D/Max-IIIA diffractometer
There is considerable interest in the near infrared (NIR) lumi-
nescent Nd3+ complexes due to their potential applications in
bio-analysis [1] and materials science [2]. However, the f–f transi-
tions for the Nd3+ ion are partially forbidden, the absorption coef-
ficients and the emissive rates are very low [3]. In order to obtain
efficient emissions, many organic (cyclic or acyclic) ligands [4] and
d-block metal complexes [5] have been used as antennae or chro-
mophores for the effective sensitization of NIR luminescence of the
Nd3+ ion. Meanwhile, the role of suitable chromophores is to shield
the Nd3+ ion against OH-, CH- or NH-oscillators in order to avoid or
decrease luminescent quenching effect [6], and on the other hand,
to allow the realization of the energy level’s match of the excited
state of the chromophore to the Nd3+ ion’s exciting state [7].
Our past studies showed that in the formation of the series of
Zn–Ln heterometallic complexes from the compartmental Salen-
type Schiff-base ligands with the outer O2O2 moieties from MeO
groups [8] or without [9], and the asymmetric Salen-type Schiff-
base ligands with OH group [10], the Zn2+ complexes based on
the Salen-type Schiff-base ligands, as the suitable chromophores,
could effectively sensitize the NIR luminescence of the centered
Ln3+ ions. Meanwhile, from the viewpoint of enhanced NIR lumi-
nescent properties, the use of the flexible linker while not the rigid
one of the Salen-type Schiff-base ligands, could improve the NIR
luminescent quantum efficiency due to the effective intramolecu-
lar energy transfer from the ligand-centered 3LC and 1LC not just
1LC excited state to the Ln3+ ions. Moreover, the occupation of Py
(pyridine) at the axial position of the Zn2+ ion, was helpful for
the strong NIR luminescence, which should be resulted from the
effective decrease of the luminescent quenching effect arising from
OH-, CH- or NH-oscillators around the Ln3+ ion. Furthermore, the
involvement of heavy atoms (Br) on the Salen-type Schiff-base li-
gand could help to increase the quantum yield for the triplet state
formation and enhanced the NIR sensitization of the Ln3+ ions. As a
matter of fact, the use of the Salen-type Schiff-base ligands should
not be necessary to bind both Zn2+ and Ln3+ ions. Especially, the re-
cent report of one magnetic [Cu(HL1)2Tb(NO3)3] from the benz-
imidazole-based ligand H2L1 (H2L1 = 2-(1H-benzo[d]imidazol-2-
yl)-6-methoxyohenol) [11] stimulated us the further exploration
on the possibility of its Zn2+ and Ln3+ luminescent complexes, de-
spite much research on the luminescent Ln3+ complexes from the
benzimidazole-based ligands [12]. Herein, with the compound
[Zn(HL1)2(Py)] (Py = pyridine) or [Zn(HL2)2(Py)] (H2L2 = 2-(1H-
benzo[d]imidazol-2-yl)-4-bromo-6-methoxyphenol) as the pre-
cursor, complexes [ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd, 1; Ln = Gd, 2)
or [ZnLn(HL2)2(Py)(NO3)3] (Ln = Nd, 3; Ln = Gd, 4) were obtained
by the further reaction with Ln(NO3)3Á6H2O (Ln = Nd or Gd). The
sensitization and energy transfer for the NIR luminescence of the
Nd3+ ions in the two Zn–Nd complexes (1 and 3) were discussed.
with graphite-monochromatized Cu K
a radiation (k = 1.5418 Å).
Thermogravimetric analyses were carried out on a NETZSCH TG
209 instrument under flowing nitrogen by heating the samples
from 25 to 1000 °C.
Synthesis
2-(1H-benzo[d]imidazol-2-yl)-6-methoxyphenol (H2L1)
The ligand H2L1 was prepared according to the improved proce-
dure from the literature [11]. A solution of o-vanillin (3.04 g,
20 mmol) in absolute EtOH (10 ml) was added slowly to the solu-
tion of o-phenylenediamine (2.16 g, 20 mmol) in absolute EtOH
(10 ml) under a nitrogen atmosphere, and the resulting mixture
was stirred overnight at room temperature. The insoluble orange
precipitate was filtered off, and was added to absolute EtOH
(20 ml) and then the mixture was refluxed for 12 h. After cooling
to room temperature, the resulting off-white microcrystalline pre-
cipitate was filtered off, and washed with absolute EtOH and
diethyl ether. Yield: 1.46 g, 61%. Calc. for C14H12N2O2: C, 69.99;
H, 5.03; N, 11.66%; found: C, 69.91; H, 5.07; N, 11.65%. IR (KBr,
cmÀ1): 3337 (b), 1899 (w), 1836 (w), 1768 (w), 1625 (w), 1593
(w), 1532 (m), 1495 (s), 1475 (s), 1463 (s), 1450 (s), 1424 (s),
1391 (s), 1333 (m), 1304 (w), 1281 (m), 1262 (s), 1196 (w), 1181
(w), 1155 (w), 1088 (m), 1060 (s), 1009 (w), 981 (m), 954 (w),
930 (w), 909 (m), 863 (m), 831 (s), 788 (s), 745 (s), 717 (m), 651
(m), 633 (m), 603 (s), 571 (w), 555 (w), 520 (w), 501 (m), 440
(m), 419 (w). 1H NMR (400 MHz, CD3CN): d (ppm) 13.42 (s, 1H, –
NH), 13.27 (s, 1H, –OH), 7.67 (d, 1H, –Ph), 7.45 (m, 2H, –Ph), 7.32
(m, 2H, –Ph), 7.10 (d, 1H, –Ph), 6.93 (t, 1H, –Ph), 3.83 (s, 3H, –OMe).
The precursor of [Zn(HL1)2(Py)]
To a stirred solution of H2L1 (240 mg, 1 mmol) in absolute EtOH
(10 ml), solid Zn(OAc)2 2H2O (110 mg, 0.5 mmol) and absolute pyr-
idine (Py, 2 ml) were added separately, and the final mixture was
heated under reflux for 5 h. After cooling to room temperature,
the yellow insoluble precipitate was filtered out, washed with
absolute EtOH and CHCl3, and dried under vacuum. For [Zn(HL1)2(-
Py)]: yield: 230 mg, 74%. Calc. for C33H27N5O4Zn: C, 63.62; H, 4.37;
N, 11.24%; found: C, 63.51; H, 4.43; N, 11.25%. IR (KBr, cmÀ1): 3420
(w), 3058 (w), 2828 (w), 1625 (m), 1606 (m), 1536 (m), 1456 (vs),
1384 (w), 1349 (w), 1316 (m), 1292 (w), 1238 (s), 1203 (s), 1181
(m), 1164 (m), 1090 (m), 1070 (s), 1015 (w), 1000 (w), 933 (w),
912 (w), 852 (m), 781 (m), 733 (s), 665 (m), 632 (w), 562 (w),
507 (m), 434 (w). 1H NMR (400 MHz, CD3CN): d (ppm) 13.33 (s,
2H, –NH), 8.58 (d, 2H, –Ph), 7.79 (t, 2H, –Ph), 7.60 (t, 4H, –Ph),
7.39 (m, 2H, –Ph), 7.24 (t, 2H, –Py), 7.04 (m, 3H, –Py), 6.91 (d,
2H, –Ph), 6.62 (t, 2H, –Ph), 3.68 (s, 6H, –OMe).
Experimental section
Materials and methods
All chemicals were commercial products of reagent grade and
were used without further purification. Elemental analyses were
performed on a Perkin–Elmer 240C elemental analyzer. Infrared
spectra were recorded on a Nicolet Nagna-IR 550 spectrophotom-
eter in the region 4000–400 cmÀ1 using KBr pellets. 1H NMR spec-
tra were recorded on a JEOL EX270 spectrometer with SiMe4 as
internal standard in CD3CN at room temperature. ESI-MS was per-
formed on a Finnigan LCQDECA XP HPLC–MSn mass spectrometer
with a mass to charge (m/z) range of 4000 using a standard electro-
spray ion source and CH3CN as solvent. Electronic absorption spec-
[ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd, 1; Ln = Gd, 2)
To a stirred solution of [Zn(HL1)2(Py)] (0.063 g, 0.10 mmol) in
absolute MeOH (5 ml), the solution of Ln(NO3)3Á6H2O (0.10 mmol,
Ln = Nd, 0.043 g; Ln = Gd, 0.044 g) in absolute acetone (4 ml) were
added, sequentially, and the mixture was reflux for about 3 h. The
respective yellow clear solution was then cooled to room temper-
ature and filtered. Diethyl ether was allowed to diffuse slowly into

D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
361
the respective solution at room temperature and pale yellow
microcrystallines 1–2 were obtained in about three weeks, respec-
tively. For 1: Yield: 0.065 g, 68%. Calc. for C33H27N8O13ZnNd: C,
41.58; H, 2.85; N, 11.75%; found: C, 41.69; H, 2.91; N, 11.76%. IR
(KBr, cmÀ1): 3270 (w), 2336 (w), 2103 (w), 1895 (w), 1769 (w),
1652 (m), 1625 (m), 1609 (m), 1571 (m), 1540 (m), 1453 (vs),
1385 (s), 1318 (s), 1303 (s), 1201 (m), 1180 (m), 1162 (m), 1120
(w), 1092 (m), 1055 (s), 991 (m), 931 (w), 909 (w), 863 (m), 846
(m), 817 (w), 782 (m), 744 (s), 703 (m), 658 (m), 630 (w), 561
(w), 539 (w), 508 (m), 442 (m), 420 (m). 1H NMR (400 MHz, CD3-
CN): d (ppm) 14.59 (s, 2H), 9.40 (d, 2H), 8.61 (m, 2H), 7.72 (m,
4H), 7.61 (m, 2H), 7.48 (m, 2H), 6.92 (m, 3H), 6.46 (m, 2H), 5.72
(m, 2H), À2.65 (s, 6H). ESI-MS (CH3CN) m/z: 950.01 ([MÀH]+,
100%). For 2: Yield: 0.060 g, 62%. Calc. for C33H27N8O13ZnGd: C,
41.02; H, 2.82; N, 11.60%; found: C, 40.98; H, 2.86; N, 11.58%. IR
(KBr, cmÀ1): 3427 (w), 2318 (w), 2041 (w), 1967 (w), 1765 (w),
1637 (m), 1616 (m), 1585 (m), 1571 (m), 1551 (m), 1522 (m),
1457 (vs), 1387 (s), 1330 (s), 1303 (s), 1286 (m), 1263 (m), 1234
(m), 1195 (m), 1167 (m), 1120 (w), 1096 (m), 1074 (m), 1023
(m), 966 (m), 850 (m), 815 (w), 784 (m), 740 (s), 666 (m), 648
(w), 632 (w), 582 (w), 556(w), 540 (w), 512 (m), 442 (m), 415
(m). ESI-MS (CH3CN) m/z: 966.03 ([MÀH]+, 100%).
1179 (m), 1059 (m), 986 (s), 847 (m), 798 (s), 766 (vs), 703 (s),
634 (m), 566 (m), 507 (s), 454 (m). 1H NMR (400 MHz, CD3CN): d
14.71 (s, 2H, –NH), 9.42 (s, 1H, –Ph), 8.62 (s, 2H, –Ph), 7.75 (d,
2H, –Ph), 7.60 (t, 2H, –Ph), 7.49 (t, 2H, –Ph), 7.10 (t, 2H, –Ph),
6.42 (m, 4H, –Ph), 5.67 (s, 2H, –Ph), -3.14 (s, 6H, –MeO). ESI-MS
(CH3CN) m/z: 1105.84 ([MÀH]+, 100%). For 4: Yield: 0.055 g, 62%.
Calc. for C33H25Br2N8O13ZnGd: C, 35.26; H, 2.24; N, 9.97%; found:
C, 35.25; H, 2.31; N, 9.91%. IR (KBr, cmÀ1): 3316 (w), 3090 (w),
2932 (w), 2110 (w), 1936 (w), 1594 (m), 1480 (w), 1451 (s),
1385 (m), 1317 (m), 1248 (s), 1179 (m), 1060 (m), 986 (s), 847
(m), 799 (vs), 746 (vs), 703 (vs), 634 (m), 545 (m), 508 (s), 452
(m). ESI-MS (CH3CN) m/z: 1121.85 ([MÀH]+, 100%).
X-ray crystallography
Single crystals of 1Á1.5MeOHÁ1.5H2OÁ0.5Py and 3ÁPy of suitable
dimensions were mounted onto thin glass fibers. All the intensity
data were collected on a Bruker SMART CCD diffractometer
(Cu-K
a
radiation and k = 1.54184 Å for 1Á1.5MeOHÁ1.5H2OÁ0.5Py;
Mo-K
a
radiation and k = 0.71073 Å for 3ÁPy) in and scan
U
x
modes. Structures were solved by direct methods followed by
difference Fourier syntheses, and then refined by full-matrix
least-squares techniques against F2 using SHELXTL [13]. All other
non-hydrogen atoms were refined with anisotropic thermal
parameters. Absorption corrections were applied using SADABS
[14]. All hydrogen atoms were placed in calculated positions and
refined isotropically using a riding model. Crystallographic data
and refinement parameters for the complexes are presented in Ta-
ble 1. Relevant atomic distances and bond angles are collected in
Table 2.
2-(1H-benzo[d]imidazol-2-yl)-4-bromo-6-methoxyphenol (H2L2)
The benzimidazole-based ligand H2L2 was prepared in the same
way as that of H2L1 except o-phenylenediamine (2.16 g, 20 mmol)
and
5-bromo-2-hydroxy-3-methoxybenzaldehyde
(4.62 g,
20 mmol) were used. The off-white microcrystalline precipitate
was obtained by washing with absolute EtOH and diethyl ether.
Yield: 1.62 g, 70%. Calc. for C14H11BrN2O2: C, 52.69; H, 3.47; N,
8.78%; found: C, 52.66; H, 3.49; N, 8.77%. IR (KBr, cmÀ1): IR (KBr,
cmÀ1): 3934 (w), 3362 (w), 2473 (w), 2344 (w), 1922 (w), 1628
(m), 1464 (m), 1444 (m), 1385 (m), 1327 (s), 1249 (vs), 1114
(m), 983 (s), 911 (m), 825 (vs), 735 (vs), 708 (vs), 601 (v), 548
(w). 1H NMR (400 MHz, CD3CN): d 13.65 (s, H, –NH), 13.40 (s, 1H,
–OH), 7.63 (d, 1H, –Ph), 7.49 (d, 1H, –Ph), 7.37 (m, 1H, –Ph), 7.31
(m, 1H, –Ph), 7.12 (m, 2H, –Ph), 3.93 (s, 3H, –MeO).
Results and discussion
Synthesis and characterization
As shown in Scheme 1, treatment of o-phenylenediamine with
o-vanillin or 5-bromo-2-hydroxy-3-methoxybenzaldehyde in 1:1
molar ratio in absolute EtOH at ambient temperature and the sub-
The precursor of [Zn(HL2)2(Py)]
The method was similar to that used for [Zn(HL1)2(Py)] (above).
H2L2 (319 mg, 1 mmol), Zn(OAc)2Á2H2O (110 mg, 0.5 mmol) and Py
(2 ml) were employed. Finally, the yellow precipitate was filtered
out, washed with absolute EtOH and CHCl3, and dried under vac-
uum. For [Zn(HL2)2(Py)]: yield: 245 mg, 70%. Calc. for C33H25Br2N5-
O4Zn: C, 50.76; H, 3.23; N, 8.97%; found: C, 50.70; H, 3.31; N, 8.98%.
IR (KBr, cmÀ1): 3436 (w), 3184 (w), 2930 (w), 2343 (w), 2119 (w),
1626 (m), 1530 (s), 1471 (m), 1382 (w), 1313 (s), 1232 (vs), 1179
(m), 1069 (s), 1000 (w), 887 (m), 841 (m), 799 (vs), 740 (vs), 634
(m), 567 (m), 508 (s), 419 (s). 1H NMR (400 MHz, CD3CN): d
13.49 (s, 2H, –NH), 8.57 (s, 1H, –Ph), 7.82 (s, 2H, –Ph), 7.58 (d,
2H, –Ph), 7.39 (t, 2H, –Ph), 7.26 (t, 2H, –Ph), 7.14 (t, 2H, –Ph),
7.09 (m, 4H, –Ph), 6.98 (s, 2H, –Ph), 3.70 (s, 6H, –MeO).
Table 1
Crystallographic data and refinement parameters for complexes 1Á1.5MeOHÁ1.5H2-
OÁ0.5Py and 3ÁPy.
Compound
1Á1.5MeOHÁ1.5H2OÁ0.5Py 3ÁPy
Empirical formula
Formula weight
Crystal system
Space group
k/Å
a/Å
b/Å
c/Å
C
37H38.5N8.5O16ZnNd
C38H30N9O13Br2ZnNd
1090.14
1067.87
Triclinic
Triclinic
P-1
P-1
1.54184
0.71073
11.4753(6)
15.6752(9)
26.4003(16)
93.742(5)
90.597(5)
106.176(5)
4549.1(4)
4
11.407(4)
14.335(5)
15.257(6)
106.740(7)
103.261(7)
91.570(7)
2313.7(15)
2
aꢀ/
bꢀ/
[ZnLn(HL2)2(Py)(NO3)3] (Ln = Nd, 3; Ln = Gd, 4)
c
ꢀ/
V/Å3
To a stirred solution of [Zn(HL2)2(Py)] (0.078 g, 0.10 mmol) in
absolute MeCN (5 ml), the solution of Ln(NO3)3 6H2O (0.10 mmol,
Ln = Nd, 0.043 g or Ln = Gd, 0.044 g) in absolute acetone (4 ml)
was added, and the mixture was refluxed about 3 h. The respective
yellow clear solution was then cooled to room temperature and fil-
tered. Diethyl ether was allowed to diffuse slowly into the respec-
tive solution at room temperature and pale yellow
microcrystallines 3 or 4 was obtained after three weeks, respec-
tively. For 3: Yield: 0.080 g, 71%. Calc. for C33H25Br2N8O13ZnNd:
C, 35.67; H, 2.27; N, 10.09%; found: C, 35.72; H, 2.33; N, 10.08%.
IR (KBr, cmÀ1): 3310 (w), 3090 (w), 2935 (w), 2114 (w), 1935
(w), 1595 (m), 1474 (w), 1457 (s), 1385 (m), 1314 (s), 1248 (s),
Z
q
/g cmÀ3
1.559
1.708
Crystal size/mm
0.31 Â 0.27 Â 0.23
0.28 Â 0.26 Â 0.22
l
/mmÀ1
9.916
3.424
Data/restraints/
parameters
Quality-of-fit indicator
No. Unique reflections
No. Observed reflections
8748/0/1144
8139/0/548
1.020
0.970
8748
8139
10321
11489
Final R indices [I > 2
r
(I)]
R1 = 0.0863
WR2 = 0.2289
R1 = 0.1015
WR2 = 0.2484
R1 = 0.0975
WR2 = 0.2227
R1 = 0.2289
WR2 = 0.3097
R indices (all data)

362
D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
Table 2
gle crystal of 1Á1.5MeOHÁ1.5H2OÁ0.5Py or 3ÁPy suitable for X-ray
diffraction studies was obtained by allowing Et2O to diffuse into
the respective mixture (MeOH–Py) at room temperature.
Selected interatomic distances/Å and bond angles/° with esds for complexes
1Á1.5MeOHÁ1.5H2OÁ0.5Py and 3ÁPy.
1Á1.5MeOHÁ1.5H2OÁ0.5Py
3ÁPy
The IR spectra of complexes 1–4 show two strong absorptions at
1451–1457 and 1314–1320 cmÀ1, which are typical for the biden-
tate NO3À groups, and the bands appearing at 766 or 758 cmÀ1 as-
Zn(1)–N(1) 2.053(14)
Zn(1)–N(3) 2.026(14)
Zn(1)–N(5) 2.083(12)
Zn(1)–O(2) 2.042(8)
Zn(1)–O(3) 2.117(11)
Nd(1)–O(1) 2.549(9)
Nd(1)–O(2) 2.421(9)
Nd(1)–O(3) 2.354(9)
Nd(1)–O(4) 2.667(9)
Nd(1)–O(5) 2.561(11)
Nd(1)–O(7) 2.603(10)
Nd(1)–O(8) 2.504(10)
Nd(1)–O(10)
2.609(10)
Nd(1)–O(11)
2.512(11)
Nd(1)–O(13)
Zn(1)–N(1) 2.006(16)
Zn(1)–N(3) 2.004(15)
Zn(1)–N(5) 2.124(16)
Zn(1)–O(2) 2.061(11)
Zn(1)–O(3) 2.100(13)
signed as the m(C–Br) vibration in complex 3 or 4, respectively. The
1H NMR spectrum identification of H2L1 or H2L2 in CD3CN with a
sharp singlet at d = 13.27 ppm or 13.40 ppm for the –OH proton,
exhibits the typical resonance-assisted hydrogen bonded (RAHB)
proton of the type O–HÁ Á ÁN [15]. As to the room temperature 1H
NMR spectrum of complex 1 or 3 in CD3CN, one set of the photon
resonances of the (HL1)À or (HL2)À ligand while with large shifts (d
from 14.59 to À2.65 ppm for 1 and d from 14.71 to À3.14 ppm for
3) are observed, due to the Nd3+-induced shift, significantly spread
in relative to those of the free ligand (d from 13.42 to 3.83 ppm for
H2L1 or d from 13.65 to 3.93 ppm for H2L2) and the precursor (d
from 13.33 to 3.68 ppm for [Zn(HL1)2(Py)] or d from 13.49 to
3.70 ppm) for [Zn(HL2)2(Py)]. The ESI-MS spectra of the four Zn–
Ln complexes (1–4) exhibit the strongest peak at m/z 950.00 (1),
966.03 (2), 1105.84 (3) or 1121.85 (4), respectively, corresponding
to the major species [ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd, 1; Ln = Gd,
2) or [ZnLn(HL2)2(Py)(NO3)3] (Ln = Nd, 3; Ln = Gd, 4), further indi-
cating that the discrete Zn–Ln (Ln = Nd or Gd) molecule exists in
the respective dilute MeCN solution. Thermogravimetric analysis
(TGA) of the four polycrystalline complexes shows the similar
weight loss patterns, as shown in Figs. 1s and 2s, respectively.
The gradual weight loss (ca. 10% for 1Á1.5MeOHÁ1.5H2OÁ0.5Py or
2Á1.5MeOHÁ1.5H2OÁ0.5Py and ca. 7% for 3ÁPy or 4ÁPy) occurred be-
tween 25 and 210 or 180 °C, indicative of the loss of the respective
solvated molecules, and the frameworks decomposed at ca. 308 or
315 °C with an observed abrupt weight loss.
Zn(2)–N(9) 2.060(10)
Zn(2)–N(11) 2.019(12)
Nd(1)–O(1) 2.641(12)
Nd(1)–O(2) 2.488(11)
Zn(2)–N(13) 2.110(12)
Zn(2)–O(15) 2.041(9)
Zn(2)–O(16) 2.101(8)
Nd(1)–O(3) 2.380(12)
Nd(1)–O(4) 2.780(13)
2.468(11)
Nd(1)–O(5) 2.561(15)
Nd(1)–O(7) 2.528(12)
Nd(1)–O(8) 2.617(14)
Nd(1)–O(10) 2.575(16)
Nd(1)–O(11) 2.571(15)
Nd(1)–O(13) 2.540(14)
Nd(2)–O(14) 2.563(9)
N(1)–Zn(1)–N(3) 101.1(5) Nd(1)–O(15) 2.434(8)
N(1)–Zn(1)–N(5) 93.2(5)
N(1)–Zn(1)–O(2) 88.4(4)
N(1)–Zn(1)–O(3) 162.6(4) Nd(1)–O(18)
2.571(11)
Nd(1)–O(16) 2.374(9)
Nd(1)–O(17) 2.692(9)
Nd(1)–O(20) 2.626(9)
Nd(1)–O(21) 2.517(9)
N(9)–Zn(2)–N(11)
105.9(4)
N(9)–Zn(2)–N(13) 95.9(5) Nd(1)–O(23) 2.604(9)
N(1)–Zn(1)–N(3)
107.3(7)
N(1)–Zn(1)–N(5)
94.7(6)
N(1)–Zn(1)–O(2)
87.9(6)
N(1)–Zn(1)–O(3)
164.3(6)
N(9)–Zn(2)–O(15) 87.1(4) Nd(1)–O(24)
2.512(10)
N(9)–Zn(2)–O(16)
162.4(4)
Nd(1)–O(26)
2.465(10)
NH2
R = H, H2L1;
HN
N
Structures of Complexes 1 and 3
NH2
EtOH
R = Br, H2L2
OH
1:1
R
CHO
X-ray quality crystals of 1Á1.5MeOHÁ1.5H2OÁ0.5Py as the repre-
sentative of the two Zn–Ln complexes based on the H2L1 ligand
were obtained, and the tables of selected crystal properties are gi-
ven in Tables 1 and 2. Complex 1Á1.5MeOHÁ1.5H2OÁ0.5Py crystal-
lizes with two independent hetero-binuclear molecules and
solvates of MeOH, H2O and Py in the asymmetrical unit shown in
Fig. 3s. As shown in Fig. 1, in both Zn–Nd molecules, two deproto-
nated (HL1)À ligands sandwich one Zn2+ ion through their N,O sites
and one Nd3+ ion through their O2 site, endowing a relative
arrangement close to head-to-head. Each Zn2+ (Zn1 or Zn2) ion
has a five-coordinate environment and adopts a distorted square
pyramidal geometry, composed of the inner N2O2 core from two
deprotonated benzimidazole-based (HL1)À ligands as the base
plane, and one N atom from the coordinated Py at the apical posi-
tion. The Nd3+ ion in both Zn–Nd molecules is bridged the respec-
tive Zn2+ ion by two phenoxo oxygen atoms of two (HL1)À ligands
with the similar ZnÁ Á ÁNd separation of 3.636(3) Å. Each Nd3+ ion is
ten-coordinate. In addition to the four oxygen atoms from two
deprotonated benzimidazole-based (HL1)À ligands, they complete
their coordinatiÀon environments with six oxygen atoms from three
bidentate NO3 anions. The lengths (2.026(14)–2.053(14) or
2.019(12)–2.060(10) Å) of Zn–N (the benzimidazole (HL1)À ligands,
N1 and N3 or N11 and N13) bonds are smaller than that (2.083(12)
or 2.110(12) Å) of Zn–N (Py, N5 or N15) bond while between those
(2.042(8)–2.117(11) or 2.041(9)–2.101(8) Å) of Zn–O (phenoxo, O2
and O3 or O15 and O16) bonds. The Nd–O bond lengths depend on
the nature of the oxygen atoms: they vary from 2.354(9)–
O
R
R = H; Br
OH
Py
Zn(OAc)2
O
HN
NH
O
HN
O
NH
Ln(NO3)3
N
N
N
N
N
N
R
Zn
R
R
Zn
O
R
O
O
O
Ln
O
O
O
O
N
O
R = H, [Zn(HL1)2(Py)];
R = Br, [Zn(HL2)2(Py)]
R = H, Ln = Nd, 1; Ln = Gd, 2;
R = Br, Ln = Nd, 3; Ln = Gd, 4
3
Scheme 1. Syntheses of the ligands, the precursors and the series of hetero-
binuclear Zn–Ln complexes 1–4.
sequent refluxing in absolute EtOH, produced the off-white benz-
imidazole-based ligand H2L1 or H2L2 in the good yield, respec-
tively. Reaction of the benzimidazole-based ligand H2L1 or H2L2
and Zn(OAc)2Á2H2O in 2:1 molar ratio in the presence of excess
absolute pyridine (Py) afforded the respective precursor
[Zn(HL1)2(Py)] or [Zn(HL2)2(Py)] in good yield of ca. 70%. Further
reaction the precursor [Zn(HL1)2(Py)] or [Zn(HL2)2(Py)] with the
Ln(NO3)3Á6H2O (Ln = Nd or Gd) resulted in the formation of two
series of the hetero-binuclear Zn–Ln complexes [Zn(HL1)2(-
Py)Ln(NO3)3] (Ln = Nd, 1 or Ln = Gd, 2), or [Zn(HL2)2(Py)Ln(NO3)3]
(Ln = Nd, 3 or Ln = Gd, 4), respectively. For complexes 1 and 3, Sin-
À
2.667(9) Å, and the bond lengths from oxygen atoms of NO3 an-
ions are longer than those from phenoxo oxygen atoms, while
slightly shorter than those from –OMe groups. Moreover, the large
dihedral angles (14.8(3)° and 23.9(3)° or 20.7(3)° and 23.2(3)°) in

D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
363
between the solvate Py and the Zn–Nd molecule, as shown in
Fig. 4s, is also observed. It is worth noting that in the formation
of two Zn–Nd arrayed complexes 1Á1.5MeOHÁ1.5H2OÁ0.5Py and
3ÁPy, incomparable of that of magnetic [Cu(HL1)2Tb(NO3)3] [11]
where the Cu2+ ion is four-coordinate and has a square-planar
geometry, the occupation of Py at the axial position of Zn2+ ions
is considered to completely avoid the further coordination of sol-
vents around the Nd3+ ions of the two Zn–Nd complexes. As to
the bulk purity of the polycrystalline complexes 1Á1.5MeOHÁ1.5H2-
OÁ0.5Py and 3ÁPy, it is convincingly established by X-ray powder
diffraction measurements. As shown in Figs. 5s and 6s, respec-
tively, for each of the two complexes, the peak positions of the
measured pattern closely match those of the respective simulated
1Á1.5MeOHÁ1.5H2OÁ0.5Py or 3ÁPy, confirming that a single phase is
formed for each complex.
Photophysical properties
The photophysical properties of the precursor [Zn(HL1)2(Py)] or
[Zn(HL2)2(Py)] and complexes 1–4 have been examined in dilute
MeCN solution at room temperature or 77 K, and summarized in
Table 3 and Figs. 3–7. As shown in Fig. 3, the similar ligand-cen-
tered solution absorption spectra (225–230, 298–301 and 339–
340 nm) of complexes 1–2 to that (226, 301 and 358 nm) of the
precursor [Zn(HL1)2(Py)] in the UV–vis region are observed, while
the lowest energy absorptions of complexes 1–2 are blue-shifted
by 18–19 nm upon the further coordination of Ln3+ ions. For com-
Fig. 1. Perspective drawing of one of the hetero-binuclear part in complex
1Á1.5MeOHÁ1.5H2OÁ0.5Py, H atoms, the other hetero-binuclear part and solvates
are omitted for clarity.
both Zn–Nd molecules between the benzimidazole moieties and
the substituted aryl rings, should be due to the coordination of me-
tal ions. Besides the inter-molecular N10-H10Á Á ÁN17 hydrogen
bonding with the NÁ Á ÁN distance of 2.850(2) Å between the solvate
Py and one of the Zn–Nd molecules, as shown in Fig. 3s, the other
solvate molecules (MeOH or H2O) are not bound to the framework
and they exhibit no observable interactions with the host
structure.
The involvement of heavy atoms (Br) did not lead to a signifi-
cant change of the hetero-binuclear Zn–Ln structures from the
replacement of the benzimidazole-based ligand H2L2. X-ray quality
crystals of 3ÁPy as the representative of the two Zn–Ln complexes
based on the H2L2 ligand were obtained, and the tables of selected
crystal properties are also given in Tables 1 and 2. Complex 3ÁPy
crystallizes with one independent hetero-binuclear molecule and
one solvate of Py in the asymmetrical unit, as shown in Fig. 2.
The slightly shorter Zn–N (benzimidazole) bond lengths
(2.004(15)–2.006(16) Å) and the slightly longer ZnÁ Á ÁNd separation
(3.688(3) Å) in complex 3ÁPy than those of complex 1Á1.5
MeOHÁ1.5H2OÁ0.5Py, respectively, should be resulted from the
with-drawing electronic effect of Br atoms with the use of the
benzimidazole-based ligand H2L2. The inter-molecular N4–
H4Á Á ÁN9 hydrogen bonding with NÁ Á ÁN distance of 2.769(3) Å
plex 1, the residual visible emission band (ca. 503 nm and
s < 1 ns,
almost undetectable) and the low quantum yield (Uem < 10À5) in
dilute absolute MeCN solution at room temperature are observed,
but photo excitation of the antenna at the range of 275–500 nm
(kex = 377 nm), as shown in Fig. 4, gives rise to the characteristic
emissions of the Nd3+ ion (4F3/2 ? 4IJ/2, J = 9, 11, 13) in the NIR re-
gion: The emissions at 907, 1085 and 1354 nm can be assigned to
4F3/2 ? 4I9/2 4F3/2 ? 4I11/2 and 4F3/2 ? 4I13/2 transitions of the Nd3+
,
ion, respectively. The precursor [Zn(HL1)2(Py)] or the complex 2
does not exhibit the NIR luminescence under the same condition,
just has the typically strong luminescence (kem = 528 nm,
s
= 1.37 ns and / = 14.3 Â 10À3 for the precursor [Zn(HL1)2(Py)])
or the weakened (kem = 503 nm,
s
= 0.52 ns and / = 0.11 Â 10À3
for 2) of the benzimidazole-based ligand H2L1 in the visible region,
as shown in Fig. 5. It is worth noting that for complex 1, the similar
excitation spectrum monitored at the NIR emission peak
(kem = 1085 nm) to that of complex 2 at the weakened visible emis-
sion peak (kem = 503 nm), clearly showing both the visible and NIR
emissions are originated from the same
p–
p⁄ transitions of the
benzimidazole-based ligand H2L1, suggests that the energy transfer
from the antenna to the Ln3+ ions takes place efficiently [16]. More-
over, for complex 1, the luminescent decay curves obtained from
time-resolved luminescent experiments can be fitted mono-expo-
nentially with time constant of microseconds (1.26
and 1.28 s at 1085 nm), and the intrinsic quantum yield UNd (ca.
0.51%) of the Nd3+ emission may be estimated by UNd
where obs is the observed emission lifetime and 0 is the ‘‘natural
ls at 907 nm
l
=
sobs/s0,
s
s
lifetime’’, viz 0.25 ms for the Nd3+ ion [17], which indicates the
presence of single emitting center for complex 1 in dilute MeCN
solutions [18]. Due to the limitation of our instrument, we were
unable to determine the sobs value at 1354 nm for Nd3+ ion.
As a reference compound, complex 2 allows the further study of
the antenna luminescence in the absence of energy transfer, be-
cause the Gd3+ ion has no energy levels below 32000 cmÀ1, and
therefore cannot accept any energy from the antennae excited
state [19]. In dilute MeCN solution at 77 K, complex 2 displays
the stronger antennae fluorescence than that (kem = 503 nm,
s
= 0.52 ns and / = 0.11 Â 10À3) at room temperature on the same
Fig. 2. Perspective drawing of complex 3ÁPy, H atoms and solvates are omitted for
clarity.
condition, which shows the higher luminescent intensity

364
D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
Table 3
The photophysical properties of the precursors [Zn(HL1)2(Py)], [Zn(HL2)2(Py)] and complexes 1–4 at 2 Â 10À5 M in absolute MeCN solution at room temperature or 77 K.
Compound
Absorption
kab/nm [log(
Excitation
kex/nm
Emission
e
/dm3 molÀ1 cmÀ1)]
kem/nm (
s
,
U
 103)
[Zn(HL1)2(Py)]
1
2
226(0.38), 301(0.22), 358(0.11)
225(0.47), 298(0.28), 340(0.24)
230(0.30), 301(0.32), 339(0.24)
422
377
385
528(s, 1.37 ns, 14.3)
503(w), 907(m, 1.26 ls), 1085(s, 1.28 l
503(m, 0.52 ns, 0.11)
s), 1354a
486(s, 1.13 ns, 77 K), 535(s, 2.24 ms, 77 K)
541(s, 1.81 ns, 19.4)
511(w), 902(m, 1.54
512(m, 0.85 ns, 0.20)
[Zn(HL2)2(Py)]
3
4
232(0.53), 303(0.0.39)
243(0.54), 300(0.48), 341(0.42)
242(0.54), 301(0.48), 343(0.41)
331, 451
411
414
ls), 1082(s, 1.57 l
s), 1352a
495(s, 1.44 ns, 77 K), 544(s, 3.41 ms, 77 K)
a
Due to the limitations of the instrument, we were unable to measure the lifetime of the NIR luminescence at 1300 nm above.
0.8
Ex for
[
Zn
(
HL1
) (Py)]
2
2.5x106
HL1
)
]
Em for
Ex for
Em for
[
Zn
(
HL1
) (Py)
]
[
(
Py)
Zn
(
2
2
2
1
2
2.0x106
1.5x106
1.0x106
0.6
0.4
0.2
0.0
2
5.0x105
0.0
300
400
500
600
700
800
200
250
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 5. Visible emission and excitation spectra of precursor [Zn(HL1)2(Py)] and
complex 2 in MeCN solution at 2 Â 10À5 M at room temperature.
Fig. 3. UV–vis spectra of precursor [Zn(HL1)2(Py)] and complexes 1–2 in MeCN
solution at 2 Â 10À5 M at room temperature.
3.0
Ex for 1
NIR Em for
Ex for
NIR Em for
1
3
2
(HL
[
Zn
)
Py
]
3
2
2.5
2.0
1.5
1.0
0.5
0.0
3
4
200
300
400
500
800
1000
1200
1400
1600
Wavelength (nm)
Wavelength (nm)
Fig. 4. NIR emission and excitation spectra of complexes 1 and 3 in MeCN solution
at 2 Â 10À5 M at room temperature.
Fig. 6. UV–vis spectra of precursor [Zn(HL2)2(Py)] and complexes 3–4 in MeCN
solution at 2 Â 10À5 M at room temperature.
(kem = 486 and 535 nm) and the distinctively longer luminescence
lifetimes (1.13 ns and 2.24 ms). This result shows that the sensiti-
zation of the NIR luminescence for complex 1 should arise from
both the 1LC (20576 cmÀ1) and the 3LC (18692 cmÀ1) excited state
of the benzimidazole-based ligand H2L1 at low temperature. If the
antennae luminescence lifetime of complex 2 is to represent the
excited-state lifetime in the absence of the energy transfer, the en-
ergy transfer rate (kET) in the complex 1 can thus be calculated
from kET = 1/sq À 1/su [20], where sq is the residual lifetime of
the Zn2+-based emission undergoing quenching by the respective
Ln3+ ion, and su is the weakened while unquenched lifetime in
the reference complex 2, so the energy transfer rates for the Nd3+
ion in complex 1 may all be estimated to be above 5 Â 108 sÀ1
,
which could well imply the reason to the effective energy transfer
for complex 1.
By extension of the conjugation through the involvement of
heavy atoms (Br) from H2L1 to H2L2, as shown in Table 3 and
Fig. 6, the absorption spectra of the precursor [Zn(HL2)2(Py)] and

D. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 359–366
365
[ZnLn(HL1)2(Py)(NO3)3] (Ln = Nd,
1
or Ln = Gd, 2) and
4.0x106
3.5x106
3.0x106
2.5x106
2.0x106
2
Em for
Ex for
Ex for
Em for
[
Zn
(
HL
)
(Py)
]
[ZnLn(HL2)2(Py)(NO3)3] (Ln = Nd, 3 or Ln = Gd, 4) with two energy
donors around the Ln3+ ion are obtained, respectively. The results
of their photophysical studies show that the strong and character-
istic NIR luminescence of Nd3+ ions with emissive lifetimes in the
microsecond range, has been sensitized from the excited state
(both 1LC and 3LC) of the benzimidazole-based ligand due to the
effective intramolecular energy transfer, and the involvement of
heavy atoms (Br) at the ligand endows the enhanced NIR lumines-
cent property. While in facilitating the NIR sensitization, the spe-
cific design of hetero-metallic polynuclear complexes from the
benzimidazole-based ligands is now under way.
2
2
L
) (Py)]
2
[Zn
(
H
4
4
1.5x106
1.0x106
5.0x105
0.0
Acknowledgements
250 300 350 400 450 500 550 600 650 700 750 800
This work is funded by the National Natural Science Foundation
(21173165, 20871098), the Program for New Century Excellent
Talents in University from the Ministry of Education of China
(NCET-10-0936), the research fund for the Doctoral Program
(20116101110003) of Higher Education of China, the State Key
Laboratory of Structure Chemistry (20100014), the Provincial Nat-
ural Foundation (2011JQ2011) of Shaanxi, the Education Commit-
tee Foundation of Shaanxi Province (11JK0588), Hong Kong
Research Grants Council (HKBU 202407 and FRG/06-07/II-16) in
P. R. of China, the Robert A. Welch Foundation (Grant F-816), the
Texas Higher Education Coordinating Board (ARP 003658-0010-
2006) and the Petroleum Research Fund, administered by the
American Chemical Society (47014-AC5).
Wavelength (nm)
Fig. 7. Visible emission and excitation spectra of precursor [Zn(HL2)2(Py)] and
complex 4 in MeCN solution at 2 Â 10À5 M at room temperature.
complexes 3–4 are all red-shifted relative to those of the precursor
[Zn(HL1)2(Py)] and complexes 1–2, respectively. Moreover, for the
precursor [Zn(HL2)2(Py)], the red-shifted emission maximum
(kem = 541 nm, shown in Fig. 7) by about 13 nm, and the slightly
higher quantum yield (/ = 19.4 Â 10À3) than those of the corre-
sponding precursor [Zn(HL1)2(Py)] are observed, respectively. Sim-
ilarly, for complex 3 in dilute MeCN solution at room temperature,
the almost quenched visible emission (kem = 511 nm) and the
strong characteristic NIR emissions of the Nd3+ ion (4F3/2 ? 4IJ/2
,
Appendix A. Supplementary data
J = 9, 11, 13), as shown in Fig. 4, also suggest that the effective sen-
sitization from the antenna of (HL2)À ligands to the Nd3+ ion takes
place [16]. Through the further investigation on the emission of the
reference complex 4, especially at 77 K, the strong antennae fluo-
rescence (kem = 495 and 544 nm) and two luminescence lifetimes
(1.44 ns and 3.41 ms) show that the sensitization of the NIR lumi-
nescence for complex 3 should also arise from both the 1LC
(20202 cmÀ1) and the 3LC (18382 cmÀ1) excited state of the benz-
imidazole-based ligand H2L2 at low temperature.
We were naturally interested in the effect of heavy atom (Br) on
the NIR luminescent property of the two Zn–Nd complexes 1 and 3.
Although there was almost no position shift of the NIR emissions of
Nd3+ ions in solution at room temperature, each of the emission
intensities of complex 3 was distinctively higher than the respec-
tive one of complex 1, which is probably the result of the promo-
tion of 1LC ? 3LC intersystem crossing by the involvement of
heavy atoms [21]. On the other hand, for complex 3, the longer life-
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.08.048.
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the (HL2)À ligand and the emitting level (4I11/2, 9424 cmÀ1) for
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