Linear and nonlinear optical properties of Ln-Zn heteronuclear complexes from a Schiff base ligand containing 8-hydroxyquinoline moiety

Article abstract of DOI:10.1016/j.inoche.2014.07.009

Starting from a Schiff base ligand containing 8-hydroxyquinoline moiety, namely, 3,3′-(1E,1′E)-(propane-1,3-diylbis(azan-1-yl-1-ylidene)) bis(methan-1-yl-1-ylidene) diquinolin-8-ol (H₂PBIQ), five heteronuclear Ln(III)-Zn(II) complexes (([Eu₂Zn(PBIQ) ₂(NO₃)₄]·CH₂Cl₂, 1), ([Tb₂Zn(PBIQ)₂(NO₃)₄] ·CH₂Cl₂, 2), ([Gd₂Zn(PBIQ) ₂(NO₃)₄]·CH₂Cl₂, 3), ([Nd₂Zn(PBIQ)₂(NO₃)₄] ·CH₂Cl₂, 4)), and ([Yb₂Zn(PBIQ) ₂(NO₃)₄]·CH₂Cl₂, 5) were obtained. Due to the low energy level resided in the excited state, the Schiff base ligand can sensitize near infrared emitting Ln(III) ions (Nd and Yb), while visible light emitting Eu and Tb ions cannot be excited. Instead, nonlinear optical properties were observed in Eu/Tb-Zn heteronuclear complexes.

Full text of DOI:10.1016/j.inoche.2014.07.009

Inorganic Chemistry Communications 47 (2014) 13–16
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Linear and nonlinear optical properties of Ln–Zn heteronuclear complexes
from a Schiff base ligand containing 8-hydroxyquinoline moiety
Ling Chen a, Cheng Yan a, Bin-Bin Du a, Kai Wu a, Lu-Yin Zhang a, Shao-Yun Yin a, Mei Pan a,b,
⁎
a
MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, China
b
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Starting from a Schiff base ligand containing 8-hydroxyquinoline moiety, namely, 3,3′-(1E,1′E)-(propane-1,3-
diylbis(azan-1-yl-1-ylidene))bis(methan-1-yl-1-ylidene) diquinolin-8-ol (H2PBIQ), five heteronuclear Ln(III)–
Zn(II) complexes (([Eu2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 1), ([Tb2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 2), ([Gd2Zn(PBIQ)2(NO3)4]⋅
CH2Cl2, 3), ([Nd2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 4)), and ([Yb2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 5) were obtained. Due to the
low energy level resided in the excited state, the Schiff base ligand can sensitize near infrared emitting Ln(III) ions
(Nd and Yb), while visible light emitting Eu and Tb ions cannot be excited. Instead, nonlinear optical properties
were observed in Eu/Tb–Zn heteronuclear complexes.
Received 27 June 2014
Received in revised form 3 July 2014
Accepted 6 July 2014
Available online 8 July 2014
Keywords:
Schiff base ligand
NIR
© 2014 Published by Elsevier B.V.
Nonlinear optical properties
Schiff base type ligands are most widely applied in coordination
chemistry, due to their variety of chemical structures and versatile phys-
icochemical properties [1–4]. Coordination complexes obtained from
Schiff base ligands have been the focus of recent studies because of their
potential applications in various chemical and biological areas such as or-
ganic synthesis, catalyst, antimicrobial and antifungal agents, as well as
linear and nonlinear optical properties [5–7]. On the other hand, due to
its specific spatial and electronic structures, 8-hydroxyquinoline (Q) has
been extensively used in the preparation of coordination complexes, es-
pecially for potential applications in light emitting materials and OLED de-
vices [8–10]. Therefore, the introduction of 8-hydroxyquinoline moiety
into Schiff base might result in new type of ligands to assemble coordina-
tion complexes, for better understanding the energy transfer process and
tuning the optical properties.
Herein, a new Schiff base ligand containing 8-hydroxyquinoline
moiety, namely, 3,3′-(1E,1′E)-(propane-1,3-diylbis(azan-1-yl-1-
ylidene))bis(methan-1-yl-1-ylidene)diquinolin-8-ol (H2PBIQ) was
designed (Scheme 1). The co-assembly of H2PBIQ with Zn(OAc)2
and Ln(III) nitrates (Ln = Eu, Tb, Gd, Nd, Yb) afforded a series of iso-
morphous hetero-trinuclear complexes: ([Eu2Zn(PBIQ)2(NO3)4]⋅
CH2Cl2, 1), ([Tb2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 2), ([Gd2Zn(PBIQ)2
(NO3)4]⋅CH2Cl2, 3), ([Nd2Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 4), and ([Yb2-
Zn(PBIQ)2(NO3)4]⋅CH2Cl2, 5) [11–13]. The complexes are crystal-
lized in C2/c space group, in which the coordination unit contains two
PBIQ2− ligands, two Ln(III) metal centers and one Zn(II) metal center,
therefore forming a hetero-trinuclear structure. The coordination sphere
of Ln(III) ions in these complexes is ten-coordinated, in which the
hexadentate PBIQ2− ligand affords four N and two O atoms, and simulta-
neously, four O atoms from two nitrate anions help to satisfy the {N4O6}
bi-capped square antiprism coordination atmosphere, which has alto-
gether 10 vertices and 16 triangular faces. Meanwhile, the Zn(II) center
is encapsulated in a 4-coordinating tetrahedron geometry with four
bridging O atoms from 8-hydroxyquinoline groups in two different
PBIQ2− ligands (Scheme 1, Fig. 1a). The coordination units are further
stacked together by abundant hydrogen bonds and only minor cavities
can be observed along c direction (Fig. 1b).
The UV–vis absorption spectra of the complexes in DMSO solution
are similar, and we only take the spectrum of complex 1 as an example
(Fig. S1). As we can see, the major absorption peaks at 270 and 298 nm
can be attributed to singlet–singlet π–π* transitions of the heterocyclic
groups on the ligand. While the longest absorption band of the complex
extends into 500 nm, in accordance with its dark-red color, which is also
characteristic of a Schiff-base complex. Furthermore, the solid state ab-
sorption spectra of heteronuclear complexes 1–3 are shown in Fig. S2.
We can see that the complexes in the solid state have long absorption
bands extending above 500 nm, which is also due to the specific elec-
tronic structure in the Schiff base ligand. The triplet energy state of the
PBIQ2− ligand is estimated to be around 16,500 cm−1 according to the
phosphorescence spectra of complex 3 measured at 77 K as shown in
Fig. S3. It can be noted that the triplet energy level of the Schiff base li-
gand lies well below the energies of the main emitting levels of 5D0
for Eu3+ (17,500 cm−1) and 5D4 for Tb3+ (20,400 cm−1), therefore it
cannot sensitize these visible light emitting Ln3+ ions. Instead, due to
the coordination effect, the ligand based S1 → S0 luminescence is
⁎
Corresponding author at: School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, China.
E-mail address: panm@mail.sysu.edu.cn (M. Pan).
http://dx.doi.org/10.1016/j.inoche.2014.07.009
1387-7003/© 2014 Published by Elsevier B.V.

14
L. Chen et al. / Inorganic Chemistry Communications 47 (2014) 13–16
H2N
NH2
H2O, SeO2
CHO
dioxide
N
EtOH
N
OH
OH
8-hydroxyl-2-methylquinoline
2-Formyl-8-hydroxyquinoline
N
N
N
N
O
N
N
Zn(OAc)2 , Ln (III) nitrates
N
Zn
Ln
Ln
O
N
N
N
O
O
N
N
OH
HO
H2PBIQ
Ln =Eu, Tb, Gd, Nd, Yb
Scheme 1. Structure and syntheses of the ligand H2PBIQ and complexes 1–5 (coordinated nitrate anions are omitted for clarity).
intensified. Meanwhile, the emission contours of complexes 1–3 show
obvious difference. As we can see, Eu–Zn complex 1 has the widest
emission wavelength covering from 450 to 700 nm, Tb–Zn complex
2 from 475 to 670 nm, while Gd–Zn complex 3 emits within 470 to
650 nm (Fig. 2a). Therefore, the calculated CIE coordinates of the three
complexes fall into (0.375, 0.454), (0.459, 0.470), and (0.468, 0.479), re-
spectively, extending from near-green to pure-yellow region (Fig. 2b).
The luminescent lifetimes of the three complexes detected at 550,
580, and 590 nm were 0.179, 0.131, and 0.326 ns for complexes 1–3, re-
spectively. The obvious difference in the ligand-based luminescent life-
time might suggest distinct energy transfer rate or progress in these
three complexes.
On the other hand, the low energy state resided in the ligand is suit-
able for energy transfer to the emitting levels of 4F3/2 of Nd3+ and 2F5/2
of Yb3+ energy levels around 10,000 cm−1. Therefore, NIR emissions at
~900, 1050, 1340 nm (4F3/2
→ , , )
4I9/2 4I11/2 4I13/2 transitions for Nd3+
Fig. 1. The coordination structure of complexes 1–5 showing the coordination environment of Ln(III) (encapsulated in purple polyhedron)and Zn(II) (in cyan) metal centers (a, H atoms
omitted for clarity), and packing states along c direction (b).
Fig. 2. Solid state luminescence of heteronuclear complexes 1–3 at room temperature: emission spectra (a) and calculated CIE coordinates (b).

L. Chen et al. / Inorganic Chemistry Communications 47 (2014) 13–16
15
and 980 nm (2F5/2 → 2F7/2 transition for Yb3+) were observed for com-
plexes 4 and 5 (Fig. 3). Meanwhile, based on the results of time-resolved
luminescent experiments, luminescent decay curves can be fitted
mono-exponentially with time constants of microseconds for the two
complexes (1.1 μs for 4 and 2.5 μs for 5). Therefore, the intrinsic quan-
tum yields ФLn can be estimated by ФLn = τobs / τ0, where τobs is the ob-
served emission lifetime and τ0 is the “neutral lifetime” of Nd3+
(0.25 ms) and Yb3+ ions (2.0 ms), respectively (see Table 1) [14]. For
the two NIR emitting complexes, ФLn values of 0.6 and 0.1% are obtained,
respectively.
The NLO properties of heteronuclear complexes 1 and 2 were mea-
sured by Z-scan technique using a tunable nanosecond laser system
(532 nm, 21 ps, 0.39 µJ). Firstly, the NLO absorption components were
evaluated by Z-scan experiment under an open aperture (OA) configu-
ration, as shown in Fig. 4. The curves demonstrate a negative nonlinear
absorption for both two complexes. It is obvious that the theoretical
curves (red line) qualitatively reproduce well the general pattern of ob-
served experimental data (black and blue dots). This fact suggests an ef-
fective third order characteristic for experimentally detected NLO
effects. The two-photon absorption coefficients β of complexes 1 and
2 were calculated to be 6 × 10−11 and 3.5 × 10−11 m/W, respectively.
Furthermore, the NLO refractive effects of the complexes were assessed
by dividing the normalized Z-scan data obtained under closed aperture
(CA) configuration by the normalized Z-scan data obtained under the
open aperture (OA) configuration. From the experimental results, com-
plex 1 does not show obvious NLO refractive effect, while complex 2 has
a second-order hyperpolarizabilities γ calculated to be −9.5 × 10−18 m2/W,
which means that the compound has a self-defocusing property.
In summary, the coordination of Ln(III) ions with a Schiff base ligand
results in a series of isomorphous d–f heteronuclear complexes, with
Fig. 3. Solid state luminescence of heteronuclear complexes 4–5 at room temperature.
Table 1
Photophysical properties of complexes 1–5.
Sample
Emission center/nm
τ/ns or μs
Complex 1 (Zn + Eu)
Complex 2 (Zn + Tb)
Complex 3 (Zn + Gd)
Complex 4 (Zn + Nd)
550
580
590
0.179 ns
0.131 ns
0.326 ns
1.1 μs
870, 906, 1054*(detected
for emission lifetime), 1341
980
Complex 5 (Zn + Yb)
2.5 μs
1.010
1.005
1.000
0.995
0.990
1.01
1.00
0.99
0.98
0.97
0.96
NLO absorption
NLO absorption
0.985
Complex 1
Complex 2
0.980
-30
-20
-10
0
10
20
30
-30
-20
-10
0
10
20
30
A
A
1.06
1.04
1.02
1.00
0.98
0.96
NLO refractive
Complex 2
-30
-20
-10
0
10
20
30
A
Fig. 4. Nonlinear absorption and refractive Z-scan curves of heteronuclear complexes 1 and 2.

16
L. Chen et al. / Inorganic Chemistry Communications 47 (2014) 13–16
[10] N.M. Shavaleev, H. Adams, J. Best, R. Edge, S. Navaratnam, J.A. Weinstein, Inorg.
Chem. 45 (2006) 9410.
either intensified red emission from the ligand or sensitized NIR emis-
sion from Ln(III) ions. Furthermore, nonlinear optical properties were
also observed in these d–f heteronuclear complexes. These properties
provide potential applications in such areas as optical amplification,
medical diagnostic probe, and OLED devices.
[11] Preparation of the ligand: 3,3′-(1E,1′E)-(propane-1,3-diylbis(azan-1-yl-1-ylidene))
bis (methan-1-yl-1-ylidene)diquinolin-8-ol (H2PBIQ). A mixture of 8-hydroxyl-2-
methylquinoline (4 g, 25 mmol), selenium dioxide (3.5 g, 31 mmol), H2O (3 mL),
1,4-diethylene dioxide (300 mL) was stirred at 80 °C for 24 h under the protection
of nitrogen. After cooling, the mixture was filtered and the filtrate evaporated. The
solid was purified by silica-gel column chromatography (developing solvent—
petroleum ether:ethyl acetate, 95:5, v/v) to yield pure yellow needle crystal
of 2-Formyl-8-hydroxyquinoline. Yield = 90%; 1H NMR (400 MHz, CDCl3):
δ 10.25 (s, 1H), 8.35 (d, J = 8.5 Hz, 1H), 8.18 (s, 1H), 8.09 (d, J = 8.5 Hz, 1H),
7.66 (t, J = 8.0 Hz, 1H), 7.46 (d, J = 8.2 Hz, 1H), 7.31 (d, J = 7.7 Hz, 1H).
2-Formyl-8-hydroxyquinoline (0.173 g, 1 mmol), 1,3-diaminopropane (42 μL,
0.5 mmol) were added to 20 mL ethanol and stirred at room temperature.
After two hours, the reaction mixture was filtered and then got the final ligand
H2PBIQ. Selected IR data (KBr pellet, cm−1): 3401.79 (OH), 1645.26 (C_N). The
ligand was not stable by itself, so no further characterization was performed.
[12] Preparations of the complexes: A solution of the ligand H2PBIQ (0.05 mmol, 19 mg)
in dichloromethane (2 mL) was added to the bottom of a tube. And then, in turn,
10 mL mixture solution (dichloromethane: ethanol, v/v = 50:50,) and 2 mL solu-
tion of Eu(NO3)3·6H2O (0.05 mmol, 23 mg) in ethanol were carefully added into
the tube. After laying at room temperature for a month, red needle-like crystals of
1 suitable for single crystal diffraction were obtained from the middle liquid layer.
Anal. Calcd. (%) for C46H36N12O16Eu2Zn (1·0.5CH2Cl2): C, 39.51; H, 2.627; N, 11.95.
Found (%): C, 39.72; H, 2.829; N, 11.30. Complexes 2–5 were obtained by the
same synthetic method as that for 1. Complex 2: Anal. Calcd. (%) for C46H36N12O16
Tb2Zn (2·0.5CH2Cl2): C, 38.82; H, 2.599; N, 11.69. Found (%): C, 38.82; H, 2.851;
N, 10.97. Complex 4 Anal. Calcd. (%) for C46H36N12O16Nd2Zn: C, 40.42; H, 2.65; N,
12.30. Found (%): C, 40.33; H, 2.778; N, 11.62. Phase purity of the complexes was
confirmed by PXRD as shown in Fig. S4.
Acknowledgments
The authors are grateful to the 973 Program of China
(2012CB821701), the NSFC Projects (91222201, 21373276, 21121061,
21173272), the NSF of Guangdong Province (S2013030013474), the FRF
for the Central Universities, and the RFDP of Higher Education of China
for funding.
Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2014.07.009. These data include MOL files
and InChiKeys of the most important compounds described in this
article.
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