Synthesis, characterization, and photophysical properties of first heterobinuclear Zn-Ln (Ln = La, Nd, Eu, Gd, Tb, Er, and Yb) Complexes Based on Asymmetric Schiff-Base Ligand

Article abstract of DOI:10.1080/00387010.2012.692427

With a novel asymmetric Schiff-base zinc complex ZnL(H 2 L = N-(3-methoxysalicylidene)-N-(5-(4-cyanophenyl)-3-methoxysalicylidene) phenylene-1, 2-diamine), obtained from phenylene-1, 2-diamine, 3-methoxysalicylaldehyde, and 5-(4-cyanophenyl)-3-methoxysali

Full text of DOI:10.1080/00387010.2012.692427

This article was downloaded by: [Linkopings universitetsbibliotek]
On: 01 March 2013, At: 04:42
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Spectroscopy Letters: An International Journal for
Rapid Communication
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lstl20
Synthesis, Characterization, and Photophysical
Properties of First Heterobinuclear Zn-Ln (Ln = La,
Nd, Eu, Gd, Tb, Er, and Yb) Complexes Based on
Asymmetric Schiff-Base Ligand
Dr. Shunsheng Zhao a , Xiangrong Liu b , Xingqiang Lü c & Wai-kwok Wong c
a College of Chemistry and Chemical Engineering, Xi'an University of Science and Technology,
Xi'an, P. R. ChinaMinistry of Education Key Laboratory of Synthetic and Natural Functional
Molecular Chemistry, Northwest University, Xiapos;an, P. R. China
b College of Chemistry and Chemical Engineering, Xi'an University of Science and Technology,
Xi'an, P. R. China
c Department of Chemistry and Centre for Advanced Luminescent Materials, Hong Kong
Baptist University, Waterloo Road, Kowloon Tong, Hong Kong, P. R. China
Accepted author version posted online: 10 Jul 2012.Version of record first published: 23 Jan
2013.
To cite this article: Dr. Shunsheng Zhao , Xiangrong Liu , Xingqiang Lü & Wai-kwok Wong (2013): Synthesis, Characterization,
and Photophysical Properties of First Heterobinuclear Zn-Ln (Ln = La, Nd, Eu, Gd, Tb, Er, and Yb) Complexes Based on
Asymmetric Schiff-Base Ligand, Spectroscopy Letters: An International Journal for Rapid Communication, 46:2, 109-116
To link to this article: http://dx.doi.org/10.1080/00387010.2012.692427
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to
anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should
be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,
proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in
connection with or arising out of the use of this material.

Spectroscopy Letters, 46:109–116, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0038-7010 print=1532-2289 online
DOI: 10.1080/00387010.2012.692427
Synthesis, Characterization, and
Photophysical Properties of First
Heterobinuclear Zn-Ln (Ln ¼ La, Nd, Eu,
Gd, Tb, Er, and Yb) Complexes Based on
Asymmetric Schiff-Base Ligand
Shunsheng Zhao1,
Xiangrong Liu2,
ABSTRACT With a novel asymmetric Schiff-base zinc complex ZnL(H2L
Xingqiang Lu¨ 3,
and Wai-kwok Wong3
1College of Chemistry and
¼ N-(3-methoxysalicylidene)-N’-(5-(4-cyanophenyl)-3-methoxysalicylidene)
phenylene-1, 2-diamine), obtained from phenylene-1, 2-diamine, 3-methox-
ysalicylaldehyde, and 5-(4-cyanophenyl)-3-methoxysalicylaldehyde, as the
Chemical Engineering, Xi’an
precursor, a series of heterobinuclear Zn-Ln complexes [ZnLnL(NO3)3
University of Science and
(CH3CN)] (Ln ¼ La, 1; Ln ¼ Nd, 2; Ln ¼ Eu, 3; Ln ¼ Gd, 4; Ln ¼ Tb, 5;
Technology, Xi’an, P. R. China;
Ln ¼ Er, 6; Ln ¼ Yb, 7) were obtained by further reaction with
and Ministry of Education Key
Ln(NO3)3 ꢀ 6H2O. Photophysical studies of these complexes show that
Laboratory of Synthetic and
Natural Functional Molecular
Chemistry, Northwest University,
Xi’an, P. R. China
2College of Chemistry and
Chemical Engineering, Xi’an
University of Science and
Technology, Xi’an, P. R. China
3Department of Chemistry and
Centre for Advanced
the strong and characteristic NIR luminescence with emissive lifetimes in
the microsecond range has been sensitized from the excited state of the
asymmetric Schiff-base ligand due to effective intramolecular energy transfer.
KEYWORDS asymmetric Schiff base, near-infrared, sensitization and energy
transfer
Luminescent Materials, Hong
Kong Baptist University,
INTRODUCTION
Waterloo Road, Kowloon Tong,
Hong Kong, P. R. China
The design and synthesis of luminescent lanthanide (Ln) complexes with
long-lived and characteristic line-like emission bands (near UV to visible and
NIR regions)[1] has attracted much recent interest, due to their potential
applications in bioassays, medicine, and materials science.[2–4] For lantha-
nide ions, because the f–f transitions are parity forbidden, the absorption
coefficients are normally very low, and the emissive rates are low, one
way to overcome this drawback is by using organic ligand, with strong
absorbing chromophores, to transfer energy to lanthanide (antenna
effect).[5] Thus, investigations in recent decades have focused on organic
dyes[6] and d-block metal complexes[7] (i.e., Cr(III), Ru(II), Pt(II), Re(I), or
Os(II)), which absorb light in the near-UV or visible ranges and have been
demonstrated to effectively transfer energy to the lanthanide ions indirectly.
Received 21 March 2012;
accepted 7 May 2012.
Address correspondence to
Dr. Shunsheng Zhao, College of
Chemistry and Chemical Engineering,
Xi’an University of Science and
Technology, NO. 58 of Yanta Road,
Xi’an 7100054, P. R. China. E-mail:
shshzhao@xust.edu.cn
109

SCHEME 1 Controlled synthesis route of Zn-Ln heterobimetallic Schiff-base complexes. (color figure available online.)
Our past research[8–12] has focused on the use of
compartmental salen-type Schiff-base ligands to bind
both 3d Zn(II) and 4f Nd(III), Er(III), or Yb(III) ions.
These Schiff-base zinc complexes, as suitable
chromophores, can act as antennae or sensitizers for
NIR of visible lanthanide luminescence, and the
p-conjugation of those salen-type Schiff-base ligands
helps to enhance luminescent properties. Due to the
energy transfer between antennae or sensitizers and
lanthanides mostly through the triplet excited state
of antennae to lanthanide ions, introduction of differ-
ent substituents with different electronic properties
(electron donor=electron acceptor) on the flanking
phenyl rings of the Schiff-base ligands, which could
increase the efficiency of intersystem crossing to
improve the quantum yield of the triplet excited state
of antennae, may consequently improve the NIR=
visible luminescent quantum efficiency of lanthanides.
Cyan group, a strongly electron-withdrawing group,
could potentially play an important role in changing
the electronic properties of Schiff-base ligands and
thus has been utilized in our study.
= N-(3-methoxysalicylidene)-N’-(5-(4-cyanophenyl)-
3-methoxysalicylidene) phenylene-1, 2-diamine).
For the improvement of lanthanide ions’ lumi-
nescence, we have therefore investigated the design,
synthesis, and use of an asymmetric Schiff-base
ligand to construct the Zn-Ln heterobinuclear com-
plexes. Herein, a novel zinc complex of asymmetric
Schiff-base ZnL (H2L = N-(3-methoxysalicylidene)-
N’-(5-(4-cyanophenyl)-3-methoxysalicylidene)
phenylene-1, 2-diamine), which has not been
reported before, has been prepared under control
of reaction time and ratio of starting reagents. With
the ZnL as the precursor, a series of heterobinuclear
Zn-Ln complexes [ZnLnL(NO3)3(CH3CN)] (Ln ¼ La,
1; Nd, 2; Eu, 3; Gd, 4; Tb, 5; Er, 6; Yb, 7) are
obtained respectively, as shown in Scheme 1. The
photophysical properties of these complexes are
reported, and the effective sensitization for the NIR
luminescence of the Ln3þ ions is also discussed.
EXPERIMENTAL
To the best of our knowledge, there was no litera-
ture available on the synthesis and luminescent
properties of Zn-Ln heterobinuclear complexes
based on asymmetric Schiff-base ligands, not even
zinc complex of asymmetric Schiff-base ZnL (H2L
Reagents and General Techniques
All chemicals for preparation were reagent grade
purchased commercially and were used without
further purification unless otherwise stated.
S. Zhao et al.
110

Acetonitrile for photophysical measurement was
dried with calcium hydride, distilled, and degassed
by N2 before use. 5-(4-Cyanophenyl)-3-methoxysali-
cylaldehyde was prepared from Suzuki coupling
reaction of 4-cyanophenylboronic acid and 5-
bromo-3-methoxysalicylaldehyde under catalysis of
Pd(PPh3)4. Elemental analyses were performed on
a Perkin-Elmer 240C elemental analyzer. Infrared
spectra were recorded on a Nicolet Nagna-IR 550
spectrophotometer in the region 4000–400 cmꢁ1
dropwise to a solution of o-phenylenediamine
(0.54 g, 5 mmol) in ethanol (100 mL) under room
temperature. The mixture was heated to reflux and
kept for 4 hr under protection of nitrogen, then
cooled down to room temperature and kept on
standby for 1 week. Product was obtained and col-
lected by filtration also as an orange-red crystal,
washed with absolute ethanol, and dried under vac-
uum. Yield 1.12 g (68.3%). 1H-NMR (400 MHz,
=
CDCl3):d=ppm 13.85 (1H, -OH), 8.72 (1H, -CH N),
1
using KBr pellets. H-NMR spectra were recorded
7.74 (2H, -Ph), 7.68 (2H, Ph), 7.29 (1H, -Ph), 7.19
(1H, Ph), 7.16–7.09 (2H, Ph), 6.84–6.80 (2H, Ph),
4.13 (2H, -NH2), 4.03 (3H, -OCH3). MS (FAB,Mþ).
Found: 344.3. Calculated for C21H17N3O2: 343.38.
on a JEOL EX-400 spectrometer with SiMe4 as
internal standard in CDCl3 at room temperature. Elec-
tronic absorption spectra in the UV=Vis region were
recorded with a Cary 300 UV spectrophotometer,
steady-state visible fluorescence and PL excitation
spectra on a Photon Technology International (PTI)
Alphascan 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 compari-
son procedure, using a reference of a known quan-
tum yield (quinine sulfate in 0.1 N H2SO4 solution,
Aem ¼ 0.546). NIR emission and excitation in solution
were recorded by a PTI QM4 spectrofluorometer with
a PTI QM4 Near-Infrared InGaAs detector.
Synthesis of ZnL
The asymmetric Schiff-base zinc complexes (ZnL)
could be synthesized through two different
procedures (Sch. 1).
Method A: HLa (486 mg, 2.0 mmol) was added par-
tially to a methanolic solution of Zn(OAc)2 ꢀ 2H2O
(440 mg, 2.0 mmol) under room temperature. The
clear solution obtained was stirred for 30 min. A sol-
ution of 5-(cyanophenyl)-3-methoxysalicylaldehyde
(460 mg, 2.0 mmol) in methanol was added dropwise
and the mixture was stirred for a further 24 hr under
room temperature. The precipitate was collected by
filtration; washed with methanol and diethyl ether,
sequentially; and dried under vacuum. Yield
Synthesis of HLa and HLb
N-(3-methoxysalicylidene)-phenylene-1, 2-
diamine HLa. An ethanolic solution of 2-hydroxy-3-
methoxybenzaldehyde (1.52 g, 10 mmol) was added
dropwise to a solution of phenylene-1,2-diamine
(1.08 g, 10 mmol) in ethanol (100 mL) under room
temperature. The mixture was heated to reflux and
kept refluxing for another 4 hr under protection of
nitrogen, then cooled down to room temperature
and kept on standby for 1 week. An orange-red crystal
was obtained and collected by filtration, washed with
absolute ethanol, and dried under vacuum, yield
1
923 mg (85.5%). H-NMR (400 MHz, DMSO-d6): d=
=
=
ppm 9.17 (1H, -CH N), 9.02 (s, 1H, -CH N),
7.91–7.85 (m, 6H, -Ph), 7.62 (d, 1H, -Ph), 7.40 (t,
2H, -Ph), 7.28 (s, 1H, -Ph), 7.03 (d, 1H, -Ph), 6.87
(d, 1H, -Ph), 6.45 (t, 1H, -Ph), 3.88 (s, 3H, -OCH3),
3.76 (3H, -OCH3), MS (FAB, þve). Found: 540.9.
Calculated for C29H21N3O4Zn: 540.90.
Method B: HLb (344 mg, 1.0 mmol) was added
partially to a methanolic solution of Zn(OAc)2 ꢀ 2H2O
(244 mg, 1.2 mmol) under room temperature. The
clear solution obtained was stirred for 30 min. A
solution of 3-methoxysalicylaldehyde (231 mg,
1.0 mmol) in methanol was added dropwise and
the mixture was stirred for a further 24 hr under room
temperature. The precipitate was collected by fil-
tration; washed with methanol and diethyl ether,
sequentially; and dried under vacuum. Yield
462 mg (85.5%). 1H-NMR and mass spectra are ident-
ical with that of the product from method A.
1
1.81 g (74.8%). H-NMR: (400 MHz, CDCl3) d=ppm
=
13.54 (1H, -OH), 8.63 (1H, N CH), 7.13–6.99 (4H,
-Ph), 6.90 (1H, Ph), 6.80 (2H, -Ph), 4.02 (2H, -NH2),
3.94 (3H, -OCH3), MS (FAB, Mþ). Found: 243.2.
Calculated for C14H14N2O2: 243.1.
N-(5-(4-cyanophenyl)-3-methoxysalicylidene)-
phenylene-1, 2-diamine HLb. An ethanolic
solution
methoxybenzaldehyde (1.26 g, 5 mmol) was added
of
5-(4-cyanophenyl)-2-hydroxy-3-
111
Heterobinuclear Zn-Ln Complexes Based on Asymmetric Schiff-Base

[ZnTbL(NO3)3(CH3CN)] 5: ZnL and Tb(NO3)3 ꢀ
6H2O were used, yield 65.2%. Element analysis: cal-
culated for C29H21N6O13TbZn: C 39.32, H 2.39, N
9.49; found: C 40.17, H 2.61, N 10.58; MS (FAB,
þve). Found: 823.4. Calculated for [M-(CH3-
CN)-(NO3)]þ: 823.84. FT-IR (KBr, cmꢁ1): 2221,
1617, 1584, 1549, 1465, 1384, 1328, 1271, 1230,
1193, 1099, 1076, 1041, 960, 833, 760, 783, 743,
696, 630, 551, 507, 435.
Synthesis of [ZnLnL(NO3)3(CH3CN)]
(Ln ¼ La, 1; Nd, 2; Eu, 3; Gd, 4; Tb, 5;
Er, 6; Yb, 7)
General procedure: Asymmetric Schiff-base zinc
complex ZnL (54.1 mg, 0.1 mmol) was suspended in
CH3CN (20 mL) and heated to reflux. Lanthanide
nitrate (45 mg, 0.1 mmol) was added and the suspen-
sion became clear and was kept reflux for 30 min.
The clear solution was cooled down to room tem-
perature and filtrated. The filtrate was allowed to
evaporate the solvent slowly to give the target com-
plex. The product was collected by filtration, washed
with CH3CN, and dried under vacuum.
[ZnErL(NO3)3(CH3CN)] 6: ZnL and Er(NO3)3 ꢀ
6H2O were used, yield 65.2%. Element analysis: cal-
culated for C29H21ErN6O13Zn: C 38.95, H 2.37, N
9.40; found: C, 39.81; H 2.59, N 10.48; MS (FAB,
þve). Found: 832.1. Calculated for [M-(CH3-
CN)-(NO3)]þ: 832.17. FT-IR (KBr, cmꢁ1): 2223,
1616, 1585, 1549, 1466, 1384, 1328, 1272, 1232,
1193, 1099, 1076, 1041, 966, 833, 762, 785, 743,
693, 633, 552, 507, 437.
[ZnLaL(NO3)3(CH3CN)] 1: ZnL and La(NO3)3 ꢀ 6H2O
were used, yield 63.1%. Element analysis: calculated for
C29H21LaN6O13Zn: C 40.23, H 2.44, N 9.71; found: C
41.06, H 2.67, N 10.81; MS (FAB, þve). Found: 801.9.
Calculated for [M-(CH3CN)-(NO3)]þ: 801.96. FT-IR (KBr,
cmꢁ1): 2223, 1613, 1584, 1551, 1462, 1384, 1303, 1266,
1230, 1190, 1094, 1075, 1031, 965, 834, 811, 762, 741,
695, 659, 633, 577, 551, 508, 440.
[ZnYbL(NO3)3(CH3CN)] 7: ZnL and Yb(NO3)3 ꢀ
6H2O were used, yield 65.5%, Element analysis: cal-
culated for C29H21N6O13YbZn: C 38.70, H 2.35, N
9.34; found: C 39.57, H 2.57, N 10.42; MS (FAB,
þve). Found: 836.7. Calculated for [M-(CH3-
CN)-(NO3)]þ: 837.00. FT-IR (KBr, cmꢁ1): 2220,
1613, 1584, 1552, 1465, 1384, 1303, 1274, 1233,
1191, 1092, 1071, 1026, 964, 837, 812, 757, 747,
694, 662, 638, 587, 554, 507, 448.
[ZnNdL(NO3)3(CH3CN)] 2: ZnL and Nd(NO3)3 ꢀ
6H2O were used, yield 48.9%. Element analysis: cal-
culated for C29H21N6NdO13Zn: C 39.98, H 2.43, N
9.65; found: C 40.82, H 2.65, N 10.75; MS (FAB,
þve). Found: 809.1. Calculated for [M-CH3-
CN-(NO3)]þ: 809.15. FT-IR (KBr, cmꢁ1): 2240, 1613,
1584, 1552, 1463.2, 1384.4, 1303.4, 1270.7, 1234,
1193, 1097, 1077, 1033, 967, 835.8, 815, 758, 744,
694, 661, 635, 584, 558, 517, 443.
RESULTS AND DISCUSSION
Synthesis and Spectra Analysis
[ZnEuL(NO3)3(CH3CN)] 3: ZnL and Eu(NO3)3 ꢀ
6H2O were used, yield 55.81%. Element analysis: calcu-
lated for C29H21EuN6O13Zn: C 39.63, H 2.41, N 9.56;
found: C 40.48, H 2.63, N 10.66; MS(FAB, þve). Found:
816.8. Calculated for [M-(CH3CN)-(NO3)]þ: 816.88.
FT-IR (KBr, cmꢁ1): 2223, 1612, 1584, 1551, 1462,
1384, 1303, 1266, 1230, 1190, 1094, 1075, 1031, 965,
834, 811, 762, 741, 695, 659, 633, 577, 551, 508, 440.
[ZnGdL(NO3)3(CH3CN)] 4: ZnL and Gd(NO3)3 ꢀ
6H2O were used, yield 56.4%. Element analysis: cal-
culated for C29H21GdN6O13Zn: C 39.40, H 2.39; N
9.51; found: C 40.24, H 2.61, N 10.60; MS (FAB,
þve). Found: 822.1. Calculated for [M-(CH3-
CN)-(NO3)]þ: 822.16. FT-IR (KBr, cmꢁ1): 2225,
1618, 1589, 1550, 1467, 1384, 1328, 1273, 1235,
1194, 1098, 1075, 1041, 965, 834, 766, 783, 743,
694, 633, 552, 509, 439.
As shown in Scheme 1, reaction of 2-hydroxy-3-
methoxybenzaldehyde and phenylene-1, 2-diamine,
in the molar ratio of 1:1 in ethanol under reflux for
4 hr, after several days’ standby and being cooled
down to room temperature, affords an orange
needle-like crystal. The 1H-NMR spectrum shows sig-
nals corresponding to phenyl groups (d 6.78–7.13,
7H), phenolic hydroxyl group (d 13.54, 1H), meth-
oxyl group (d 3.91, 3H), methylene (d 8.63, 1H),
and nonreacted amino group (d 4.02, 2H). The
FAB-MS spectrum exhibits one peak at m=z 243.2,
which corresponds to its molecular ion of the com-
pound HLa. Selected NMR and mass spectra data
are listed in Table 1.
By
reaction
of
5-(4-cyanophenyl)-3-
methoxysalicyaldehyde and phenylene-1,2-diamine,
with the similar procedure used above,
S. Zhao et al.
112

TABLE 1 Selected NMR, Mass, and FT-IR Spectra of HLa, HLb,
and ZnL
Addition of corresponding lanthanide nitrate to a
refluxing suspension of ZnL in fresh distilled dry
acetonitrile gives a clear solution, which gives the cor-
responding heterobimetalic Zn-Ln asymmetric
Schiff-base complexes after being cooled down to
room temperature and kept on standby for several
days. FT-IR (Table 2) spectra of these bimetallic com-
1H-NMR (ppm)
MS (FAB,
FT-IR (KBr),
cmꢁ1
Compound
CH¼N
OCH3
m=z)
HLa
HLb
ZnL
8.63
8.72
9.17,
9.02
3.94
4.03
3.88,
3.76
243.2
344.3
540
[MþH]þ
1616 C N
=
=
1617 C N
plexes 1–7, in which the characteristic absorptions of
ꢁ1
=
1627 C N
=
the n(C N) vibration are at 1612–1618 cm , similar to
that for the precursor ZnL, are slightly red-shifted by
9–15cmꢁ1 relative to those of ZnL (1627 cmꢁ1) upon
coordination of the lanthanide metal ions. Character-
istic absorptions of the n(N-O) vibration at 1462–
1467cmꢁ1 and 1384 cmꢁ1 should be attributed to the
coordinated nitrate anion. Characteristic absorptions
of the n(-CꢂN) vibration at 2220–2245 cmꢁ1 could
be found in these complexes. The FAB-MS spectra
of the seven complexes (1–7) exhibit one peak at
m=z 801.9 (1), 809.1 (2), 816.8 (3), 822.1 (4), 823.4
(5), 832.1 (6), or 836.7 (7), corresponding to the major
species [ZnLaL(NO3)2]þ for 1, [ZnNdL(NO3)2]þ for
2, [ZnEuL(NO3)2]þ for 3, [ZnGdL(NO3)2]þ for 4,
[ZnTbL(NO3)2]þ for 5, [ZnErL(NO3)2]þ for 6, or
[ZnYbL(NO3)2]þ for 7, further indicating the discrete
ZnLnL (Ln ¼ La, Nd, Eu, Gd Tb, Er, or Yb) molecule
structure of the complexes.
N-(5-(4-cyanophenyl)-3-methoxysalicylidene)-phen-
ylene-1, 2-diamine (HLb) could be obtained and has
1
been characterized by H-NMR and mass spectra.
1
The H-NMR spectrum of HLb shows signals corre-
sponding to phenyl groups (d 6.80–7.74, 10H), phe-
nolic hydroxyl group (d 13.85, 1H), methoxyl group
(d 4.03, 3H), methylene (d 8.72, 1H), and nonreacted
amino group (d 4.13, 2H). The FAB-MS spectrum
exhibits one peak at m=z 344.3, which corresponds
to its molecular ion of the compound HLb. Selected
NMR and mass spectra data are also listed in Table 1.
HLa and HLbhave good solubility in CH2Cl2 and
CHCl3 but slightly dissolve in methanol or ethanol.
However, HLa dissolves immediately when added
to a methanolic solution containing equivalent
Zn(OAc)2, indicating that the reaction between HLa
and Zn(OAc)2 takes place and forms a reaction inter-
mediate. The condensation between the amino
group and aldehyde group occurred upon the
addition of 5-bromo-2-hydroxy-3-methoxybenzalde-
hyde into the intermediate, which gave the asymmet-
ric Schiff-base zinc complex (ZnL). The zinc
complex has poor solubility in dichloromethane,
chloroform, methanol, or ethanol and, contrarily,
Photophysical Properties of the
Complexes
The photophysical properties of the complexes 1–
7 have been examined in dilute MeCN solution at
TABLE 2 Selected FT-IR and Mass Spectra of Heterobimetallic
Complexes
1
IR (cmꢁ1
)
MS (FAB-MS m=z)
good solubility in DMSO. The H-NMR spectrum of
ZnL in DMSO-d6 suggests the asymmetric Schiff-base
structure. Compared to the NMR spectrum of HLa
(Table 1), signals with the chemical shift at 13.85
(phenolic hydroxy) and 4.13 (amino) disappeared,
and the integral of the peak area suggests two differ-
=
1 ZnLaL
2 ZnNdL
3 ZnEuL
4 ZnGdL
5 ZnTbL
6 ZnErL
7 ZnYbL
2223 (ꢁCꢂN), 1614 (C N),
802
[M-CH3CN-NO3]þ
809
1462 (N-O), 1384 (N-O)
=
2240 (ꢁCꢂN), 1614 (C N),
1463 (N-O), 1384 (N-O)
[M-CH3CN-NO3]þ
817
=
2223 (ꢁCꢂN), 1613 (C N),
1462 (N-O), 1385 (N-O)
[M-CH3CN-NO3]þ
822
=
ent protons from the CH N group and 13 protons
=
2225 (ꢁCꢂN), 1617 (C N),
from the phenyl group. The FAB-MS spectrum of
ZnL shows only its molecular ion at m=z 519, which
also proves the asymmetric Schiff-base zinc complex
structure. ZnL could also be prepared according to
the route of method B, which allowed HLb to react
with Zn(OAc)2 first, followed by the addition of
2-hydroxy-3-methoxybenzaldehyde.
1466 (N-O), 1384 (N-O)
[M-CH3CN-NO3]þ
823
=
2221 (ꢁCꢂN), 1613 (C N),
1460 (N-O), 1384 (N-O)
[M-CH3CN-NO3]þ
832
=
2223 (ꢁCꢂN), 1617 (C N),
1461 (N-O), 1384 (N-O)
[M-CH3CN-NO3]þ
837
=
2220 (ꢁCꢂN), 1614 (C N),
1451 (N-O), 1384 (N-O)
[M-CH3CN-NO3]þ
113
Heterobinuclear Zn-Ln Complexes Based on Asymmetric Schiff-Base

room temperature and are summarized in Table 3
and Figs. 1–3. As shown in Figure 1, the similar
ligand-centered solution absorption spectra (range
200–485 nm, peaks at about 305–310 nm) of com-
plexes 1–7 in the UV-visible region are observed,
which could be attributed to p-p transaction of the
asymmetric Schiff-base ligand. For complexes 1–7,
the similar residual visible emission bands (ca.
510 nm and s < 1 ns, almost undetectable) and low
quantum yields (Aem < 10ꢁ5) in dilute absolute MeCN
solution at room temperature are observed (Fig. 2).
Photoexcitation of the antennae in the range of
200–485 nm (kex ¼ 394 nm for 2, 388 nm for 6, or
390 nm for 7), as shown in Fig. 3, gives rise to the
FIGURE 1 Absorption spectra of complex 1–7 in acetonitrile at
room temperature (c = 2 ꢄ 10ꢁ5 M).
characteristic emissions of the Nd3þ ion (4 F3=2
!
2
4IJ=2, J ¼ 9, 11, 13), the Yb3þ ion (2F5=2! F7=2), and
the Er3þ ion (4I13=2! I15=2) in the NIR region. For com-
4
level of the excited state is lower than the energy
lever of Tb3þ so that it cannot transfer energy to
plex 2, the emissions at 872, 1066, and 1333 nm can be
4
the Tb3þ [14]
It is special for 3. The energy level is
.
4
4
4
4
assigned to F3=2 ! I9=2, F3=2 ! I11=2, and F3=2
!
higher than that of Eu3þ; however, no characteristic
emission of Eu3þ was detected. One explanation is
that there may be a deactivation pathway that
involves a charge-transfer from the ZnL excited state
to the Eu3þ ion.
4I13=2 transitions of the Nd3þ ion, respectively. For com-
plexes 6 and 7, the emissions at 1535 and 976 cmꢁ1 can
be attributed to the 4I13=2 ! I15=2 transition of the Er3þ
4
2
2
ion and the F5=2 ! F7=2 transition of the Yb3þ ion,
respectively. The complexes 1, 3, 4, and 5 don’t exhibit
NIR luminescence under the same condition, but have
the relatively stronger (506 nm for 1, 509 nm for 4,
510 nm for 5) or weakened (511 nm for 3) lumi-
nescence of the Schiff-base ligand in the visible region,
as shown in Table 3. For 1, it is because the La3þ ion
has no f-electron so that there is no f–f transition in
complex 1 and no NIR emission observed. For 4, it is
because the Gd3þ ion has no energy levels below
32,000 cmꢁ1, and therefore cannot accept any energy
from the antennae excited state.[13] For 5, the energy
It is worth noting that for complex 2, 6, or 7, the
similar excitation spectrum (kex ¼ 387 nm for 2,
379 nm for 6, or 385 nm for 7) was monitored at
the respective NIR emission peak (1066 nm for 2,
1535 nm for 6, or 976 nm for 7) or the residual visible
emission peak (510 nm) of complex 2 or 7, clearly
showing that both the visible and NIR emissions
are originated from the same p-pꢃ transitions of the
H2L Schiff-base ligand for complexes 2, 6, and 7,
TABLE 3 Selected Photophysical Data for Complexes 1–7
Absorption
Excitation
Emission
em=nm(UZn-L
104, s)
k
max=nm [log(e=
dm3molꢁ1cmꢁ1)]
k
ꢄ
kexc=nm
1 ZnLaL
2 ZnNdL
307 (4.51)
304 (4.59)
397
394
506 (1.47 ns)
511 (0.91 ns),
872 (1187 ns),
1066, 1333
3 ZnEuL
4 ZnGdL
5 ZnTbL
6 ZnErL
7 ZnYbL
304 (4.72)
307 (4.68)
308 (4.70)
305 (4.62)
307 (4.58)
399
395
396
388
390
511(0.93 ns), 1535
509 (1.30 ns)
510 (1.81 ns)
507 (1.10 ns), 1535
536 (1.01 ns),
FIGURE 2 UV-visible ranged excitation and emission spectra
of complex 1, 2, 4, 5, and 6 in acetonitrile at room temperature
(c = 2 ꢄ 10ꢁ5 M).
976 (10,826 ns)
S. Zhao et al.
114

C-H oscillators of the Schiff-base ligand H2L, besides
the fewer opportunities of nonradiative migration
2
from the single emitting transition (2F5=2 ! F7=2
)
of the Yb3þ ion in complex 7.[18]
CONCLUSION
In conclusion, with the phenylene-1,2-diamine,
3-methoxysalicylaldhyde, and 5-(4-cyanophenyl)-
3-methoxysalicylaldehyde as starting materials, novel
asymmetric Schiff-base zinc complex ZnL and Zn-Ln
heterobinuclear complexes [ZnLnL(NO3)3(CH3CN)]
(1–7) are obtained. The results of their photophysi-
cal studies show that the strong and characteristic
NIR luminescence with emissive lifetimes in the
microsecond range has been sensitized from the
excited state of the ligand due to the effective intra-
molecular energy transfer. Although the asymmetric
Schiff-base ligand used has not significantly
enhanced the NIR luminescent efficiency of lantha-
nide ions compared to that of symmetric ligands, this
work affords a method for specific design and prep-
aration of heterometallic complexes from the asym-
metric Schiff-base ligands appended with different
functional groups for fine tuning of NIR=visible lumi-
nescent properties of lanthanide complexes.
FIGURE 3 Near-infrared excitation spectra (monitored at
1066 nm for 2, 1535 nm for 6, and 976 nm for 7) and NIR emission
spectra of complex 2, 6, and 7 in acetonitrile at room temperature
(c = 2 ꢄ 10ꢁ5 M).
which suggests that the energy transfer from the
antenna to the Ln3þ ions takes place efficiently.[13]
Moreover, for complexes 2 and 7, the respective
luminescent decay curves obtained from time-
resolved luminescent experiments can be fitted
mono-exponentially with a time constant of microse-
conds (1.19 ms for 2 at 872 nm and 10.83 ms for 7 at
976 nm), and the intrinsic quantum yield ALn
(0.476% for 2 or 0.542% for 7) of the Ln3þ emission
may be estimated by ALn ¼ sobs=s0, where sobs is the
observed emission lifetime and s0 is the ‘‘neutral life-
time,’’ viz 0.25 ms and 2.0 ms for the Nd3þ and Yb3þ
ions, respectively,[15] which indicates the presence of
a single emitting center for both 2 and 7 in dilute
MeCN solution.[16] Due to the limitation of our instru-
ment, we were unable to determine the sobs value for
the Er3þ ion and thus could not estimate the ALn
value for the Er3þ ion, though the NIR emission of
the Er3þ ion is observed for complex 6. The quantum
efficiencies of both 2 and 7 are a little higher than
those of binuclear ZnLn from the symmetric
salen-type Schiff-base ligands,[17] which should result
from increasing efficiency of intersystem crossing of
the asymmetric ligand, thus enhancing the sensitiza-
tion process. From the viewpoint of the energy level
match, though the energy gap (4F3=2, 11,062 cmꢁ1) of
ACKNOWLEDGMENT
This work is funded by the National Natural
Science Foundation of China (21103135, 21073139),
Open Foundation of Ministry of Education Key
Laboratory of Synthetic and Natural Functional
Molecular Chemistry at Northwest University
(2010025), Natural Science Basic Research Plan
in Shaanxi Province of China (Program No.
2011JQ2011), Cultivation Foundation of Xi’an
University of Science and Technology (Program No.
2010QDJ030), Scientific Research Program Funded
by Shaanxi Provincial Education Commission (Pro-
gram No. 12JK0622), and Hong Kong Research Grants
Council (HKBU 202407 and FRG=06-07=II-16) in P. R.
of China.
the Nd3þ ion in complex 2 is smaller than that (2F5=2
,
REFERENCES
9931 cmꢁ1) of the Yb3þ ion in complex 7, a higher
quantum efficiency of 7 is shown than that of 2,
possibly because the excited state of the Nd3þ ion
is more sensitive to being quenched by the distant
1. Comby, S.; Bu¨ nzli, J.-C. G. Lathanide near-infrared luminescence
in molecular probe and device. In Handbook on the Physics and
Chemistry of Rare Earths; K. A. Gschneidner Jr, J.-C. G. Bu¨ nzli, V. K.
Pecharsky, Eds.; Elsevier Science B. V.: Amsterdam, 2007; vol. 37,
221–224.
115
Heterobinuclear Zn-Ln Complexes Based on Asymmetric Schiff-Base

2. Kido, J.; Okamoto, Y. Organo lanthanide metal complexes for
electroluminescent materials. Chem. Rev. 2002, 102, 2357–2368.
3. Bu¨ nzli, J.-C. G.; Eliseeva, S. V. Lanthanide NIR luminescence for
telecommunications, bioanalyses and solar energy conversion. J. Rare
Earths 2010, 28, 824–842.
10. Zhao, S. S.; Lu¨ , X. Q.; Hou, A. X.; Wong, W.-Y.; Wong, W.-K.; Yang,
X. P.; Jones, R. A. Heteronuclear trimetallic and 1D polymeric 3d-4f
Schiff-base complexes with OCNꢁ and SCNꢁ ligands. Dalton Trans.
2009, 9595–9602.
11. Yang, X. P.; Jones, R. A.; Wong, W.-K.; Oye, M. M.; Holmes, A. L.
Design and synthesis of a near infra-red luminescent hexanuclear
Zn-Nd prism. Chem. Commun. 2006, 1836–1838.
4. Bu¨ nzli, J.-C. G. Lanthanide luminescence for biomedical analyses and
imaging. Chem. Rev. 2010, 110, 2729–2755.
5. Sabbatini, N.; Guardigli, M.; Lehn, J. M. Luminescent lanthanide
complexes as photochemical supramolecular devices. Coord. Chem.
Rev. 1993, 123, 201–228.
6. Bu¨ nzli, J.-C. G.; Piguet, C. Taking advantage of luminescent
lanthanide ions. Chem. Soc. Rev. 2005, 34, 1048–1077.
12. Hui, Y. N.; Feng, W. X.; Wei, T.; Lu¨ , X. Q.; Song, J. R.; Zhao, S. S.;
Wong, W.-W.; Jones, R. A. Adjustment of coordination environment
of Ln3þ ions to modulate near-infrared luminescent properties of
Ln3þ complexes. Inorg. Chem. Commun. 2011, 14, 200–204.
13. Albrecht, M.; Osetska, O.; Klankermayer, J.; Fro¨ hlich, R.; Gumy, F.;
Bu¨ nzli, J.-C.-G. Enhancement of near-IR emissions by bromine
substitution in lanthanide complexes with 2-carboxamide-8-hydroxy-
quinoline. Chem. Commun. 2007, 1834–1836.
7. (a) Ward, M. D. Transition-metal sensitized near-infrared
luminescence from lanthanides in d-f heteronuclear arrays. Coord.
Chem. Rev. 2007, 251, 1663–1677; (b) Chen, F. F.; Chen, Z. Q.; Bian,
Z. Q.; Huang, C. H. Sensitized luminescence from lanthanides in d-f
bimetallic complexes. Coord. Chem. Rev. 2010, 254, 991–1010;
(c) Ward, M. D. Mechanisms of sensitization of lanthanide(III)-
based luminescence in transition metal=lanthanide and anthracene=
lanthanide dyads. Coord. Chem. Rev. 2010, 254, 2634–2642.
8. Lo, W. K.; Wong, W.-K.; Wong, W.-Y.; Guo, J. P.; Yeung, K.-T.;
Cheng, Y.-K.; Yang, X. P.; Jones, R. A. Heterobimetallic Zn(II)-Ln(III)
phenylene-bridged Schiff base complexes, computational studies,
and evidence for singlet energy transfer as the main pathway in
the sensitization of near-infrared Nd3þ luminescence. Inorg. Chem.
2006, 45, 9315–9325.
14. Weber, M. J. Radiative and multiphonon relaxation of rare-earth ions
in Y2O3. Phys. Rev. 1968, 171, 283–291.
15. Bu¨ nzli, J.-C. G. In Lanthanide Probe in life, Chemical and Earth
Sciences, Theory and Practice; J.-C. G. Bu¨ nzli, G. R. Choppin, Eds.
Elsevier Science B. V.: Amsterdam, 1989, ch. 7, pp. 219–293.
16. Carnall, W. T.; Fields, P. R.; Rajnak, K. Electronic energy levels of trivalent
lanthanide aquo ions. II. Gd3þ. J. Chem. Phys. 1968, 49, 4443–4446.
17. Dexter, D. L. A theory of sensitized luminescence in solid. J. Chem.
Phys. 1953, 21, 836–850.
18. Beeby, A.; Clarkson, I. M.; Dockins, R. S.; Faulkner, S.; Parker, D.;
Royle, L.; de Souda, A. S.; Williams, J. A. G.; Woods, M. Non-radiative
deactivation of the excited states of europium, terbium and ytterbium
complexes by proximate energy-matched OH, NH and CH oscillators:
an improved luminescence method for establishing solution hydration
states. J. Chem. Soc., Perkin Trans. 2 1999, 493–504.
9. Bi, W. Y.; Lu¨ , X. Q.; Chai, W. L.; Song, J. R.; Wong, W.-K.; Yang, X.
P.; Jones, R. A. Effect of heavy-atom (Br) at the phenyl rings of
Schiff-base ligands on the NIR luminescence of their bimetallic Zn-Nd
complexes. Z. Anorg. Allg. Chem. 2008, 634, 1795–1800.
S. Zhao et al.
116