Synthesis, fluorescence study and biological evaluation of three Zn(II) complexes with Paeonol Schiff base

Article abstract of DOI:10.1016/j.saa.2009.06.037

The synthesis of three Paeonol Schiff base ligand and their Zn(II) complexes are reported. The complexes were fully characterized by IR, ¹H NMR, elemental analysis and molar conductivity. The experiment results show the three Zn(II) complexes can emit bright fluorescence at room temperature in DMF solution and solid state. The fluorescence quantum yields (Φ) of three Schiff base ligands and their Zn(II) complexes were calculated using quinine sulfate as the reference with a known Φ_R of 0.546 in 1.0N sulfuric acid. Furthermore, in order to develop these Zn(II) complexes' biological value, the antioxidant activities against hydroxyl radicals (OH{radical dot}) were evaluated. The results show the three complexes possess excellent ability to scavenge hydroxyl radicals.

Full text of DOI:10.1016/j.saa.2009.06.037

Spectrochimica Acta Part A 74 (2009) 415–420
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, fluorescence study and biological evaluation of
three Zn(II) complexes with Paeonol Schiff base
Dong-dong Qin, Zheng-yin Yang∗, Gao-fei Qi
College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 April 2008
Received in revised form 30 April 2009
Accepted 12 June 2009
The synthesis of three Paeonol Schiff base ligand and their Zn(II) complexes are reported. The complexes
were fully characterized by IR, 1H NMR, elemental analysis and molar conductivity. The experiment results
show the three Zn(II) complexes can emit bright fluorescence at room temperature in DMF solution and
solid state. The fluorescence quantum yields (˚) of three Schiff base ligands and their Zn(II) complexes
were calculated using quinine sulfate as the reference with a known ˚R of 0.546 in 1.0N sulfuric acid.
Furthermore, in order to develop these Zn(II) complexes’ biological value, the antioxidant activities against
hydroxyl radicals (OH•) were evaluated. The results show the three complexes possess excellent ability
to scavenge hydroxyl radicals.
Keywords:
Fluorescence
Quantum yields
Antioxidant activities
Schiff base
© 2009 Elsevier B.V. All rights reserved.
Zn(II) complexes
1. Introduction
In order to explore these zinc(II) complexes’ biological value, their
antioxidant activities against OH• were studied. The results show
Atpresent, luminescentcomplexesareattractingmoreandmore
interest from researchers due to their important applications in the
field of light-emitting diode devices [1], fluorescent sensor [2] and
biological probes [3,4]. Up to now, many metal complexes have been
studied for these uses, such as lanthanide(III) complexes [5], Pt(II)
and Ru(II) complexes [6]. It is known that some zinc(II) complexes
with nitrogen-containing ligands can exhibit intense emission at
room temperature and zinc also has a significant role in bioinor-
ganic chemistry [7]. Compared to other luminescent complexes,
zinc has its own advantages of cheap materials, no damage to
human body, and easy forming of complexes [8]. These excellent
properties inspire our interest to synthesize a new series lumi-
nescent zinc(II) complexes as a candidate for biological use or
fluorescence material. At the same time, Schiff base, a kind of very
important N, O donor ligand and effective antibacterial, antiviral
and antifungal agent becomes our first choice [9].
the complexes possess excellent activities.
2. Experimental section
2.1. Instrumentation
Elemental analyses were conducted on a Vario EL analyzer.
Infrared spectra (4000–400 cm−1) were determined with KBr disks
on a Therrno Mattson FTIR spectrometer. The ultraviolet spectra
were recorded on a Lambda 35 UV–vis spectrometer. 1H NMR spec-
tra were measured on a Bruker Avance Drx 200-MHz spectrometer.
The fluorescence spectra were recorded on a RF–5301PC spectroflu-
orophotometer produced by SHIMADZU. The antioxidant activities
were tested on a 721E spectrophotometer.
2.2. Materials and methods
The previous literatures [8,10] have reported some Schiff base
zinc(II) complexes, its Photoluminescent properties have been
studied; however, its biological activities against hydroxyl radical
are rarely evaluated. In this paper, we synthesized three paeonol
(2-hydroxy-4-methoxy-acetophenone, Fig. 2(a)) Schiff base ligands
and its zinc(II) complexes. It is found that all the zinc(II) complexes
canemitstrongfluorescenceinDMFsolutionandsolidstateatroom
temperature, moreover, their quantum yields (˚) were calculated.
2-hydroxy-4-methoxy-acetophenone, Diethylenetriamine, 1,3-
Diaminopropane, Zn(Ac)2·2H2O, quinine, Safranin, Mannitol, EDTA,
FeSO4·7H2O, H2O2, were produced in China. All materials and sol-
vents employed in this study were of analytical grade. EDTA–Fe(II)
and Na2HPO4–KH2PO4 buffers were prepared with twice distilled
water.
2.3. Calculation of quantum yields
∗
The fluorescence quantum yields of the ligands 1–3, and com-
plexes 1–3 were determined using quinine sulfate as the reference
Corresponding author. Tel.: +86 931 8913515; fax: +86 931 8912582.
E-mail address: yangzy@lzu.edu.cn (Z.-y. Yang).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.06.037

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D.-d. Qin et al. / Spectrochimica Acta Part A 74 (2009) 415–420
with a known ˚R of 0.546 in 1.0N sulfuric acid [11]. The area of the
emission spectrum was integrated using the software available in
the instrument and the quantum yield was calculated according to
the following equation: [8]
was collected and then recrystallized from methanol to obtain pure
ligand 3. Yield, 65%. Mp: 210–212 ◦C. 1H NMR (DMSO-d6, ppm): ı
13.63 (1H, s, –NH), 11.20 (1H, s, –OH), 7.91 ((1H, d, J = 6.6 Hz, 6-H),
7.52–7.61 (5H, m, Ph–H), 6.5 (1H, d, J = 6.6 Hz, 5-H) 6.45 (1H, s, 3-
H), 3.77 (3H, s, –OCH3), 2.43 (3H, s, N C–CH3). IR ꢁmax (cm−1):
ꢁ(C O): 1650, ꢁ(C N):1615.
ꢂ
ꢃ
ꢂ
ꢃ
ꢀ
ꢁ
ꢀS2
ꢀR2
˚
˚
AS
AR
OD
)
R
(
S
=
×
×
OD
(
)
R
S
2.8. Synthesis of (complex 1)
here ˚S and ˚R are the fluorescence quantum yields of the sam-
ple and reference, respectively, AS and AR are the areas under the
fluorescence spectra of the sample and the reference, respectively.
(OD)S and (OD)R are the respective optical densities of the sample
and the reference solution at the wavelength of excitation, and ꢀS
and ꢀR are the values of refractive index for the respective solvents
used for the sample and reference.
Zn(Ac)2·2H2O (0.110 g, 5 mmol) in 2 ml DMF was added to 20 ml
DMF solution containing ligand 1 (0.185 g, 5 mmol). After stirring at
room temperature for 2 h, the light yellow precipitate was collected
and washed with DMF three times, then with ethanol three times.
The complex was dried at vacuo 24 h. Yield, 80%, C21N2O4H24Zn
requires (%) C, 58.20; N, 6.47; H, 5.54. Found: C, 58.50; N, 6.24, H,
5.10. 1H NMR (DMSO-d6, ppm): ı 7.47 (2H, s, 6-H), 6.17–6.22 (4H,
2.4. Hydroxyl radical scavenging assay
3,5-H), 3.71 (6H, s, –OCH3), 3.62–3.71 (4H, C
N CH2), 2.35 (6H, s,
N
C
CH3), 2.05 (1H, m, –CH2–). ꢂM (S cm2 mol−1) 10−3 M DMF
The hydroxyl radicals (OH•) in aqueous media were generated
through the Fenton system. The solution of the tested compound
was prepared with DMF. The 5 ml assay mixture contained follow-
ing reagents: safranin (10 ␮M), EDTA-Fe(II) (80 ␮M), H2O2 (0.6%),
the tested compound (4–20 ␮M) and a phosphate buffer (pH 7.4).
The assay mixtures were incubated at 40 ◦C for 50 min in a water-
bath. After which, the absorbance was measured at 520 nm. All the
tests were run in triplicate and expressed as the mean standard
deviation (SD). The suppression ratio for OH• was calculated from
the following expression:
solution 25 ◦C = 16.2. IR ꢁmax (cm−1): ꢁ(C N):1599.
2.9. Synthesis of (complex 2)
Zn(Ac)2·2H2O (0.110 g, 5 mmol) in 5 ml methanol was added to
30 ml methanol solution containing ligand 2 (0.2 g, 5 mmol). After
stirring at room temperature for 2 h, the light yellow precipitate was
collected and washed with methanol three times, and then dried at
vacuo overnight. Yield, 85%. C22N3O4H29Zn requires (%) C, 55.00; N,
8.75; H, 6.04. Found: C, 54.61; N, 8.26, H, 5.90. 1H NMR (DMSO-d6,
ppm): ı 7.41 (2H, d, J = 9.6, 6-H), 6.1–6.14 (4H, 3,5-H), 3.64 (6H, s,
Ai − Ao
Scavenging effect(%) =
× 100
–OCH3), 3.33 (4H, s, C
N CH2), 2.9 (4H, s, HN–CH2), 2.32 (6H, s,
Ac − Ao
N
C
CH3). ꢂM (S cm2 mol−1) 10−3 M DMF solution 25 ◦C = 30.8. IR
where Ai = the absorbance in the presence of the tested compound;
A0 = the absorbance in the absence of the tested compound; AC = the
absorbance in the absence of the tested compound, EDTA-Fe(II) and
H2O2.
ꢁmax (cm−1): ꢁ(C N):1608.
2.10. Synthesis of (complex 3)
Zn(Ac)2·2H2O (0.055 g, 2.5 mmol) in 5 ml methanol was added
to 40 ml methanol solution containing ligand 3 (0.142 g, 5 mmol).
After stirring at room temperature for 2 h, the yellow precipitate
was collected and washed with methanol three times, then dried
at vacuo overnight. Yield, 70%. C32N4O6H30Zn requires (%) C, 60.86;
N, 8.87; H, 4.75. Found: C, 60.55; N, 8.53, H, 4.50. 1H NMR (DMSO-
d6, ppm): ı 8.09, 7.41, 6.2 (8H, PH–H), 3.7 (3H, s, –OCH3) 2.66 (3H,
2.5. Synthesis of Bi(2ꢀ-hydroxy-4ꢀ-methoxy-˛-methyl-
benzylidene)-1,3-trimethylenediimine.(ligand 1)
Paeonol (1.66 g, 10 mmol) and 1,3-Diaminopropane {0.37 g,
5 mmol} in 30 ml ethanol was refluxed for 10 h, a yellow pre-
cipitate was collected and recrystallized from DMF to give the
pure ligand 1. Yield, 50%. Mp: 175–177 ◦C. 1H NMR (DMSO-d6,
ppm): ı 7.50 (2H, d, J = 9.0 Hz, 6-H), 6.23 (2H, d, J = 9.0 Hz, 5-H),
6.19 (2H, s, 3-H), 3.71(6H, s, –OCH3), 3.62–3.71 (4H, t, J = 7.1 Hz,
C = N–CH2), 2.35(6H, s, N = C–CH3), 2.02(2H, m, J = 7.1 Hz, –CH2–).
IR ꢁmax (cm−1): ꢁ(C N):1605.
s, N
C
CH3). ꢂM (S cm2 mol−1) 10−3 M DMF solution 25 ◦C = 45.0.
IR ꢁmax (cm−1): ꢁ(HO–C N): 1605, ꢁ(H3C–C N): 1594.
3. Results and discussion
3.1. Structure of the complexes
2.6. synthesis of Bi(2ꢀ-hydroxy-4ꢀ-methoxy-˛-methyl-
benzylidene)-N,N’-diethyleneamine-1,5-diimine (ligand 2)
Fig. 1(a)–(c) shows the structure of the complexes 1–3, respec-
tively. These structures are in accord with the results of elemental
analysis, molar conductivity, IR and 1H NMR. It is notable that the
ligand 3 can exist as two forms as shown in Fig. 2(b). When ligand
3 was prepared, it was white; but after being exposed to the light
at room temperature for several months it partly became yellow,
complex 3 is also yellow. In the 1H NMR spectra, the–OH (ı 11.2)
and –NH (ı 13.63) can be observed in ligand 3; however, these two
peaks disappeared when the complex 3 was formed. In IR spec-
tra, ꢁ(C O) of ligand 3 appears at 1650 cm−1, this peak shifts to
1605 cm−1 for complex 3, so many shifts seem unusual. We think
the carbonyl group takes part in coordination as enolic form, that
Paeonol (1.66 g, 10 mmol) and Diethylenetriamine {0.515 g,
5 mmol} in 30 ml ethanol were refluxed for 10 h, solvent were
removed and diethyl ether was added, yellow solid was collected
and washed with diethyl ether several times. Yield, 85%. Mp:
121–123 ◦C. 1H NMR (DMSO-d6, ppm): ı 7.45 (2H, d, J = 8.7 Hz, 6-H),
6.17 (2H, dd, J = 8.7 Hz, J = 2.8 Hz, 5-H), 6.14 (2H, d, J = 2.8 Hz, 3-H),
3.71 (6H, s, –OCH3), 3.62 (4H, t, J = 6.6 Hz, C
N
CH2), 2.87 (4H, t,
J = 6.6 Hz, NH–CH2), 2.34 (6H, s, N
C CH3), 1.96 (1H, s, –NH–). IR
ꢁmax (cm−1): ꢁ(C N):1604.
2.7. Synthesis of 2-hydroxy-4-methoxy-˛-methylbenzylidene
is to say the O C NH group has changed to HO–C N. Another evi-
(benzoyl) hydrazone (ligand 3)
dence can be found in UV spectra in Fig. 3(c), the absorption band at
327 nm (ε = 22,790) for ligand 3 shifts to 383 nm (ε = 23649). The TG
curve of complex 2 shows weight loss between 180.6 and 271.7 ◦C,
corresponding to its DTA curve, there is one evident endothermic
Paeonol (1.66 g, 10 mmol) and benzoyl hydrazine (1.36 g,
10 mmol) in 20 ml ethanol were refluxed for 10 h, white precipitate

D.-d. Qin et al. / Spectrochimica Acta Part A 74 (2009) 415–420
417
do not take part in formation of the complexes. In the complex 3,
the O–H and N–H proton signals of the free ligands disappear in
the 1H NMR, which indicate the phenoxo oxygen coordinates to the
Zn(II) ions and carbonyl has changed to enolic form. Moreover, in
1H NMR spectra of complexes 1 and 2, the resonance for the 6-H, 5-
H and 3-H (paeonol benzene ring) shifts by about 0.02–0.04 ppm,
towards high field compared to the corresponding signals of the
free ligands. However, the d and m peaks of benzene group in com-
plex 3 change to three broad peaks compared to the ligand 3. In
addition, it is interesting that the 1H signals in the group of –OCH3
and C N–CH2 also shift by about 0.07 and 0.29 ppm towards high
field when complex 2 forms.
3.3. Fluorescence property
Table 1 summarizes the absorption and emission data for lig-
ands (1–3) and complexes (1–3) in solution and the solid state. UV
spectra of these six compounds are shown in Fig. 3. As shown in
Table 1 and Fig. 3, the complexes 1–3 show three absorption bands,
which are similar to the ligands 1–3. For ligand 1 and complex 1, the
positions of their absorption bands are nearly no changes. But the
absorption band at 376 nm for ligand 2 shifts to 350 nm for complex
2, blue-shifted 26 nm; and absorption band at 327 nm for ligand 3
shifts to 383 nm for complex 3, which are maximum absorption
and red-shifted about 56 nm. The phenomenon of blue-shift and
red-shift may be due to the different structures of ligands 2 and
3, and the perturbation of the intraligand ␲–␲* transition by the
Zn(II) ions.
Fig. 4 shows the emission spectra of ligands 1–3 and com-
plexes 1–3 in DMF solution. It can be seen that ligands 1–3 emit
fluorescence at the same wavelength (414 nm). This suggests the
fluorescence of the three ligands is all produced by ␲–␲* transi-
tion of Paeonol group, other different groups cannot effect their
fluorescence. In comparison with the corresponding free ligands,
the maximum emission of complexes 1 and 2 in solution are the
same, only complex 3 exhibits slight red-shifts about 9 nm. On the
other hand, the complexes 1–3 show higher fluorescence intensity
than that of the free ligands, the changes of complexes 1 and 2
are higher than complex 3. These are supported by quantum yield
values, as shown in Table 1. Enhancement of fluorescence after
complexation may due to two reasons. First, when Zn(II) ions coor-
dinate to the ligand, the ligand’s photoinduced electron transfer
(PET) process, which can often quench the fluorescence, is pre-
vented. Second, the coordination of the ligands with the Zn atom
increases the rigidity of the ligands, which can diminish the loss
Fig. 1. Structures of complex 1(a), 2(b) and 3(c).
peak at 193.2 ◦C. This result confirms that water molecule takes part
in coordination.
3.2. IR and 1H NMR spectra
The ꢁ(C N) vibrations of ligands 1–3 are at 1605, 1604 and
1615 cm−1, respectively, in the complexes, these vibrations shift to
1599, 1608, and 1594 cm−1, respectively. The ꢁ(C N) vibration of
ligand 2 shifts to high wavenumbers about 4 cm−1 when complex
2 forms. This change is also an evidence of coordination of C
N
group [12]. In 1H NMR spectra, except for the signals of ligands,
there are not any other groups’ signals, such as CH3COO−, solvent
(methanol or DMF). These results indicate CH3COO− and solvent
Fig. 2. The structures of paeonol (a) and the structure of ligand 3 (b).

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D.-d. Qin et al. / Spectrochimica Acta Part A 74 (2009) 415–420
Fig. 3. The UV–vis spectra of ligands 1–3 and complexes 1–3 in DMF solution (1 × 10−5 M).
of energy via vibrational motions and increase the emission effi-
ciency.
emission maxima of all the ligands and complexes in the solid state
are red-shifted compared to their corresponding emission max-
ima in solution. This phenomenon can be normally observed for
most fluorescent compounds in the solid state, probably due to ␲–␲
stacking of the aromatic rings in the molecules.
The emission spectra of ligands 1–3 and complexes 1–3 in the
solid state are shown in Fig. 5. Ligands 1–3 and complexes 1–3 emit
bright fluorescence in the solid state at room temperature. It can
be seen from Fig. 5 that the emission maxima of three complexes
are blue-shifted compared to those of the corresponding free lig-
and in the solid state, which is probably due to the disappearance
of the intramolecular hydrogen bonding in ligands 1–3 after coor-
dination with the zinc ions. In addition, it can be noted that the
3.4. Evaluation of antioxidant activity
As an important non-enzymatic antioxidant, Zn complexes are
attracting ample attention from researchers [13]. The scavenging
Table 1
Photoluminescent data of ligands 1–3 and complexes 1–3.
Compounds
Ligand 1
Absorption (nm) ε (dm3 mol−1cm−1
)
Excitation (ꢃmax, nm)
Emission (ꢃmax, nm)
Quantum yields (˚)a
Conditions
DMF, 298 K
275 (25753)
295 (18742)
377 (7938)
312
414
0.0063
389
313
501
413
Solid, 298 K
DMF, 298 K
Complex 1
Ligand 2
276 (10567)
324 (5286)
378 (3229)
0.0180
0.0086
385
313
474
414
Solid, 298 K
DMF, 298 K
275 (23135)
295 (17400)
376 (8312)
343
313
498
416
Solid, 298 K
DMF, 298 K
Complex 2
Ligand 3
276 (13204)
350 (10097)
0.0402
0.0062
389
313
446
414
Solid, 298 K
DMF, 298 K
287 (13567)
297 (16196)
327 (22790)
371
312
487
421
Solid, 298 K
DMF, 298 K
Complex 3
309 (12592)
383 (23649)
0.0094
391
474
Solid, 298 K
a
Determined using quinine sulfate in 1 M sulfuric acid as a standard.

D.-d. Qin et al. / Spectrochimica Acta Part A 74 (2009) 415–420
419
Fig. 4. Emission spectra of ligands 1–3 and complexes 1–3 in DMF (1 × 10−5 M, slit = 3 nm).
ability of the ligands1–3 and complexes 1–3 against hydroxyl rad-
ical is shown in Fig. 6. It can be seen that the inhibitory effects of
all compounds on hydroxyl radical are related to concentration. At
a concentration from 3 to 15 ␮M, the percentage scavenging effect
values are about 0–50.9%, 1.5–25.4% and 3–20.1% for ligands 1–3,
respectively;andvaluesareabout5.3–57.9%, 8–72.45and6.9–72.4%
for complexes 1–3, respectively. It is obvious that scavenging activ-
ities of the complexes are stronger than that of ligands. The results
indicate Zn takes an important role in the scavenging ability of
complexes [13].
Fig. 5. Emission spectra of ligands 1–3 and complexes 1–3 in the solid state (slit = 1.5 nm, low sensitivity)

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D.-d. Qin et al. / Spectrochimica Acta Part A 74 (2009) 415–420
Fig. 6. Scavenging effect of tested compounds on hydroxyl radical. Values are means SD (n = 3).
4. Conclusion
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A series of Schiff base ligands and their Zn complexes has been
synthesized and fully characterized. The research results indicate
the three Zn complexes can emit bright fluorescence in solution
and solid state. It is suggested that the three complexes are a new
class of fluorescence material. Furthermore, the high scavenging
ability of three Zn complexes against hydroxyl radical makes them
ideal candidates for biological use.
Acknowledgements
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This work is supported by the National Natural Science Founda-
tion of China (20475023) and Gansu NSF (0710RJZA012).
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