Journal of Luminescence 143 (2013) 623–634
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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin
Structural characterization of some Schiff base compounds:
Investigation of their electrochemical, photoluminescence,
thermal and anticancer activity properties
Gökhan Ceyhan a, Muhammet Köse a, Mehmet Tümer a,n, İbrahim Demirtaş b,
Ayse Şahin Yağlioğlu b, Vickie McKee c
a
b
c
Chemistry Department, Kahramanmaraş Sütcü Imam University, 46100 Kahramanmaraş, Turkey
Chemistry Department, Çankırı Karatekin University, 18100 Çankırı, Turkey
Chemistry Department, Loughborough University, LE11 3TU Leics, UK
art ic l e i nf o
a b s t r a c t
Article history:
Received 11 January 2013
Received in revised form
27 March 2013
Accepted 5 June 2013
Available online 14 June 2013
Three Schiff base compounds, N,N′-bis(2,4-dimethoxy benzaldiimine)-1,4-diamino cyclohexane (IGA1),
N,N′-bis(2,3,4-trimethoxy benzaldiimine)-1,4-diamino cyclohexane (IGA2) and N,N′-bis(3,4,5-trimethoxy
benzaldiimine)-1,4-diamino cyclohexane (IGA3) were synthesized and characterized by the spectroscopic
and analytical methods. The electrochemical and photoluminescence properties of the compounds IGA1–
IGA3 have been investigated in the different conditions. All the synthesized Schiff base compounds IGA1,
IGA2 and IGA3 were screened for their cytotoxicity (HeLa and Vera cells). The structural characterization
of the Schiff base compounds was determined by single crystal X-ray diffraction studies. The molecules
IGA1 and IGA3 both lie on centers of symmetry but in IGA2 the molecule has no crystallographically
imposed symmetry. In the compound IGA1, Schiff base molecules are linked by π stacking interactions.
There is no evidence of π stacking in both IGA2 and IGA3, however there are some C–H⋯π and C–H…O
interactions in these compounds. The thermal stabilities of the compounds were investigated in the
nitrogen atmosphere.
& 2013 Elsevier B.V. All rights reserved.
Keywords:
Schiff base
Electrochemical
X-ray
Luminescence
Anticancer
1. Introduction
Schiff base condensation reactions of primary amines with
aldehydes and ketones result in the formation of imines which
contain a characteristic C¼ N double bond. Hugo Schiff described the
condensation between an aldehyde and an amine leading to a Schiff
base in 1864 [1]. Schiff bases decompose or polymerize rapidly
unless there is at least one aryl group bonded to the nitrogen or the
carbon atom of the C¼N double bond [2]. Schiff bases have played an
important role in the development of coordination chemistry, as they
readily form stable complexes with most of the transition metals
exhibiting different coordination modes and functionalities [3,4].
Schiff bases, and nowadays active and well-designed Schiff base
ligands are considered “privileged ligands” [5], because they are easily
prepared by a simple one-pot condensation of aldehydes and
primary amines in an alcohol solvent. Schiff bases have a wide
variety of applications in many fields, e.g., biological, inorganic and
analytical chemistry [6,7]. Application of many new analytical devices
requires the presence of organic reagents as essential compounds of
n
Corresponding author.: Tel.: +90 344 280 1444; fax: +90 344 280 1352.
E-mail addresses: [email protected], [email protected] (M. Tümer).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jlumin.2013.06.002
the measuring system. They are used in optical and electrochemical
sensors, as well as in various chromatographic methods, to enable
detection of enhance selectivity and sensitivity [8]. In addition, Schiff
bases are able to stabilize many different metals in various oxidation
states, controlling the performance of metals in a large variety of
useful catalytic transformations. Metal complexes of these bases have
numerous applications including antibacterial, antifungal [9,10] and
antiviral activities [11] as well as other biological applications [12].
Several applications have been related for these complexes in
chemical analysis [13], absorption and transport of oxygen [14], in
pesticides [15] and heterogeneous and homogeneous catalysis for
oxidation and polymerization of organic compounds [16,17].
Recently, we have synthesized and characterized a series of
Schiff base ligands and their early and transition metal complexes
[18–25]. They show excellent luminescence and electrochemical
properties by providing proper conjugate absorption groups suitable for energy transfer, which that could be used as a luminescent device. They also exhibit high antimicrobial and catalytic
activity [23–25].
In the present work, three Schiff base compounds N,N′-bis
(2,4-dimethoxy benzaldiimine)-1,4-diamino cyclohexane (IGA1),
N,N′-bis(2,3,4-trimethoxy benzaldiimine)-1,4-diamino cyclohexane
(IGA2) and N,N′-bis(3,4,5-trimethoxy benzaldiimine)-1,4-diamino
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G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
cyclohexane (IGA3) were prepared and characterized by analytical,
spectroscopic methods. The compounds were structurally characterized by X-ray diffraction studies. Additionally, electrochemical, thermal, luminescence and anticancer properties of the compounds were
investigated.
2. Experimental
2.1. Materials and measurements
All reagents and solvents were of reagent-grade quality and
obtained from commercial suppliers (Aldrich or Merck). Elemental
analyses (C,H,N) were performed using a LECO CHNS 932. Infrared
spectra were obtained using KBr disc (4000–400 cm−1) on a Perkin
Elmer Spectrum 400 FT-IR. The electronic spectra in the 200–
900 nm range were obtained on a Perkin Elmer Lambda 45
spectrophotometer. Mass spectra of the ligands were recorded
on a LC/MS APCI AGILENT 1100 MSD spectrophotometer. 1H and
13
C NMR spectra were recorded on a Bruker 400 MHz instrument.
TMS was used as internal standard and CDCl3 as solvent. The
thermal analysis studies of the compounds were performed on a
Perkin Elmer STA 6000 simultaneous Thermal Analyzer under
nitrogen atmosphere at a heating rate of 10 1C/min.
The single-photon fluorescence spectra of the Schiff base
compounds IGA1–IGA3 were collected on a Perkin Elmer LS55
luminescence spectrometer. All samples were prepared in spectrophotometric grade solvents and analyzed in a 1 cm optical
path quartz cuvette. The solution of ligands (4.0 10−5 mol L−1)
were prepared in ethanol, methanol, DMSO, n-butanol and DMF
solvents.
A stock solution of a concentration of 1 10–4 M of Schiff base
compounds was prepared in DMF for electrochemical studies.
Cyclic voltammograms were recorded on a Iviumstat Electrochemical workstation equipped with a low current module (BAS PA-1)
recorder. The electrochemical cell was equipped with a BAS glassy
carbon working electrode (area 4.6 mm2), a platinum coil auxiliary
electrode and a Ag+/AgCl reference electrode filled with tetrabutylammonium tetrafloroborate (0.1 M) in DMF and CH3CN solution and adjusted to 0.00 V vs. SCE. Cyclic voltammetric
measurements were made at room temperature in an undivided
cell (BAS model C-3 cell stand) with a platinum counter electrode
and an Ag+/AgCl reference electrode (BAS). All potentials are
reported with respect to Ag+/AgCl. The solutions were deoxygenated by passing dry nitrogen through the solution for 30 min
prior to the experiments, and during the experiments the flow was
maintained over the solution. Digital simulations were performed
using DigiSim 3.0 for windows (BAS, Inc.). Experimental cyclic
voltammograms used for the fitting process had the background
subtracted and were corrected electronically for ohmic drop.
Mettler Toledo MP 220 pH meters was used for the pH measurements using a combined electrode (glass electrode reference
electrode) with an accuracy of 70.05 pH.
Data collection for X-ray crystallography was completed using a
Bruker APEX2 CCD diffractometer and data reduction was performed using Bruker SAINT [26]. SHELXTL was used to solve and
refine the structures [27].
2.2. Synthesis of the Schiff base compounds
The benzaldehyde derivatives (2 mmol; 332 mg 2,4-dimethoxy
benzaldehyde for IGA1, 392 mg 2,3,4-trimethoxy benzaldehyde
for IGA2 and 3,4,5-trimethoxy benzaldehyde for IGA3) in ethanol (20 mL, anhydrous) and ( 7)trans-1,4-cyclohexanediamine
(1 mmol, 114 mg) in ethanol (20 mL, anhydrous) were mixed and
refluxed for about 4 h at 85 1C. The color of the solution changed to
light yellow. After cooling the solution, the resulting precipitate
was filtered and washed with cold ethanol. Single crystals of the
Schiff base compounds (IGA1, IGA2 and IGA3) suitable for X-ray
diffraction study were obtained by slow evaporation of the
compounds in ethanol. Physical properties and other spectroscopic
data are given in the experimental section.
IGA1: (C24H30N2O4). Yield: 84%, color: dirty yellow, m.p.: 198 1C.
Elemental analyses, found (calcd. %): C, 70.19 (70.22); H, 4.32
(7.37); N, 6.85 (6.82). 1H NMR (CDCl3, δ (ppm)): 8.61 (s, CH ¼
N, 2 H), 7.77–6.55 (m, Ar–H, 6 H), 3.84, 3.81 (s, OCH3, 12 H), 2.51–
1.06 (m, CH/CH2, 10 H). 13C NMR(CDCl3, δ (ppm)): 164.81 (CH¼ N),
152.38–112.92 (Ar–C), 58.50, 57.20 (OCH3), 54.77–25.62 (CH/CH2).
Mass spectrum (LC/MS APCI): m/z 411 [M]+ (100%), m/z 412
[M+2]+ (25%), m/z 413 [M+2]2+ (40%). FT-IR: (KBr, cm−1):
2962–2923 ν(CH2), 1625 ν(CH¼N).
IGA2: (C26H34N2O6). Yield: 82%, color: bright white, m.p.:
204 1C. Elemental analyses, found (calcd. %): C, 66.09 (66.05);
H, 7.15 (7.11); N, 5.95 (5.89). 1H NMR (CDCl3, δ (ppm)): 8.96
(s, CH¼N, 2 H), 7.60–7.12 (m, Ar–H, 4 H), 3.91–3.77 (s, OCH3, 18 H),
2.69–1.07 (m, CH/CH2, 10 H). 13C NMR(CDCl3, δ (ppm)): 159.36
(CH¼N), 155.86–112.09 (Ar–C), 59.70–56.05 (OCH3), 54.10–22.20
(CH/CH2). Mass spectrum (LC/MS APCI): m/z 471 [M]+ (100%), m/z
472 [M+2]+ (40%), m/z 473 [M+2]2+ (47%). FT-IR: (KBr, cm−1):
2960–2915 ν(CH2), 1630 ν(CH¼N).
IGA3: (C26H34N2O6). Yield: 88%, color: bright white, m.p.:
209 1C. Elemental analyses, found (calcd. %): C, 70.19 (66.05);
H, 4.32 (7.11); N, 6.85 (6.82). 1H NMR (CDCl3, δ (ppm)): 8.33
(s, CH ¼N, 2 H), 7.89–6.55 (m, Ar–H, 4 H), 3.94–3.90 (s, OCH3, 18 H),
2.79–1.02 (m, CH/CH2, 10 H). 13C NMR(CDCl3, δ (ppm)): 160.18
(CH¼N), 152.95–109.61 (Ar–C), 59.44–57.90 (OCH3), 53.65–23.55
(CH/CH2). Mass spectrum (LC/MS APCI): m/z 471 [M]+ (100%), m/z
472 [M+ 2]+ (37%), m/z 473 [M+2]2+ (45%). FT-IR: (KBr, cm−1):
2965–2917 ν(CH2), 1630 ν(CH ¼N).
2.3. Anticancer activity studies of the Schiff base compounds
2.3.1. Preparation of samples
Stock solutions of the samples were prepared in DMSO and
diluted with Dulbecco's modified eagle medium (DMEM). DMSO
final concentration is below 1% in all tests.
2.3.2. Cell lines and cell culture
HeLa, Vero and C6 cancer cell lines were grown in Dulbecco's
modified eagle medium (DMEM) supplemented with 10% fetal
bovine serum (FBS), 2% penicilin streptomycin. The medium was
changed twice a week.
2.3.3. Cell proliferation assay
Antiproliferative effects of the plants were investigated on Vero
cells (African green monkey kidney), C6 cells (Rat Brain tumor cells)
and HeLa cells (human uterus carcinoma) using proliferation BrdU
ELISA assay [28,29]. Cultured cells were grown in 96-well plates
(COSTAR, Corning, USA) at a density of 3 104 cells/well. In each
experimental set, cells were plated in triplicates and replicated twice.
The cell lines were exposed to two concentrations of methanolic
extracts of different organs (flower, steam and root) of CC, for 24 h at
37 1C in a humidified atmosphere of 5% CO2. 5-Fluorouracil, cisplatin
were used as standart compounds. Cells were than incubated for
overnight before applying the BrdU Cell Proliferation ELISA assay
reagent (Roche, Germany) according to manufacturer's procedure. The
amount of cell proliferation was assessed by determining the A450 nm
of the culture media after addition of the substrate solution by using a
microplate reader (Ryto, China). Results were reported as percentage
of the inhibition of cell proliferation, where the optical density
measured from vehicle-treated cells was considered to be 100% of
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Table 1
Crystallographic data of the Schiff base compounds.
625
Table 2
Selected bond lengths [Å] and angles [deg] for Schiff base compounds.
Identification code
IGA1
IGA2
IGA3
Empirical formula
Formula weight
Crystal system
Space group
Unit cell
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
Volume (Å3)
Z
Abs. coeff. (mm−1)
Refl. collected
Ind. Refl. [Rint]
R1, wR2 [I 42s(I)]
R1, wR2 (all data)
CCDC number
C24H30N2O4
410.50
Monoclinic
P2(1)/n
C26H34N2O6
470.55
Triclinic
P−1
C26H34N2O6
470.55
Monoclinic
C2/c
11.1446(10)
8.0684(7)
12.0014(10)
90
97.149(1)
90
1070.77(16)
2
0.087
10,592
2666 [0.0257]
0.0383, 0.0974
0.0476, 0.1031
918,291
9.5532(13)
10.4875(15)
12.9490(18)
105.158(2)
102.273(2)
91.963(2)
1218.0(3)
2
0.091
12,402
6019 [0.0274]
0.0463, 0.1165
0.0730, 0.1301
918,292
37.378(3)
5.2182(5)
13.4972(12)
90
109.994(1)
90
2473.9(4)
4
0.090
11,904
3057 [0.0284]
0.0412, 0.1224
0.0537, 0.1418
918,293
proliferation. All assays were repeated at least twice using HeLa and C6
cells. Percentage of inhibition of cell proliferation was calculated as
follows:
½1−ðAsamples =Acontrol Þ 100:
2.4. Statistical analysis
IGA1
C(9)–N(1)
C(10)–N(1)
C(1)–O(1)
C(2)–O(1)
C(4)–O(2)
C(5)–O(2)
1.2696(14)
1.4623(14)
1.4350(13)
1.3655(13)
1.3679(13)
1.4232(15)
C(9)–N(1)–C(10)
C(1)–O(1)–C(2)
C(4)–O(2)–C(5)
117.17(10)
117.00(9)
117.52(9)
IGA2
C(10)–N(1)
C(11)–N(1)
C(14)–N(2)
C(17)–N(2)
C(3)–O(2)
C(4)–O(2)
C(5)–O(3)
C(6)–O(3)
C(21)–O(4)
C(22)–O(4)
C(23)–O(5)
C(24)–O(5)
C(25)–O(6)
C(26)–O(6)
1.2708(18)
1.4649(18)
1.4658(19)
1.2713(19)
1.3790(17)
1.4321(18)
1.3684(17)
1.4271(19)
1.3609(17)
1.4253(19)
1.3820(17)
1.426(2)
1.3727(18)
1.430(2)
C(10)–N(1)–C(11)
C(14)–N(2)–C(17)
C(1)–O(1)–C(2)
C(3)–O(2)–C(4)
C(5)–O(3)–C(6)
C(21)–O(4)–C(22)
C(23)–O(5)–C(24)
C(25)–O(6)–C(26)
116.06(13)
116.15(13)
114.82(12)
112.76(11)
117.86(12)
117.30(12)
113.21(12)
116.12(13)
IGA3
C(10)–N(1)
N(1)–C(11)
C(2)–O(1)
C(3)–O(1)
C(4)–O(2)
C(5)–O(2)
C(6)–O(3)
C(7)–O(3)
1.2661(17)
1.4657(15)
1.3585(15)
1.4326(15)
1.3737(14)
1.4373(16)
1.3631(15)
1.4263(16)
C(2)–O(1)–C(3)
C(4)–O(2)–C(5)
C(6)–O(3)–C(7)
C(10)–N(1)–C(11)
117.28(10)
114.60(10)
117.31(10)
117.61(11)
The results of investigation in vitro are means 7SD of nine
measurement. Differences between groups were tested with
ANOVA. p values of o0.01 were considered as significant.
R3
R4
2.5. X-ray structure solution and refinement for the compound
X-ray diffraction data for all three compounds were collected at
150(2) K on a Bruker Apex II CCD diffractometer using Mo-Kα
radiation (λ¼ 0.71073 Å). The structures were solved by direct
methods and refined on F2 using all the reflections [28]. All the
non-hydrogen atoms were refined using anisotropic atomic displacement parameters and hydrogen atoms bonded to carbon were
inserted at calculated positions using a riding model. The crystal
data and details of the structure solution and refinement are given in
Table 1, selected bond lengths and angles are given in Table 2.
R2
R1
N
N
R1
R4
R2
R3
3. Results and discussion
The title compounds IGA1–IGA3 were obtained from the reactions of the (7)trans-1,4-cyclohexanediamine and methoxy carbonyl compounds in ethanol solution at high yields. Proposed
structures of the synthesized Schiff base compounds are given in
Fig. 1. Analytical and spectroscopic data for the compounds are
given in the experimental section and agree well with the
expected values. Because the synthesized compounds have polar
groups, such as –OCH3 and –CH ¼N, they are soluble in polar
organic solvents, such as EtOH, MeOH, CHCl3 etc. In addition, the
compounds are stable for a long time at room temperature
without decomposition to oxidized products.
The 1H(13C) NMR spectral studies of the Schiff base compounds
were done using CDCl3 as solvent and obtained data are given in
the experimental section. In the 1H NMR spectra of the compounds IGA1–IGA3, the singlets in the 8.96–8.33 ppm range can be
attributed to the proton of the azomethine (CH ¼ N) group. The
multiplets in the 7.89–6.55 ppm range can be assigned to the
protons of the aromatic ring protons. The singlet in the 3.94–
R2, R4: H; R1, R3 : -OCH3 (IGA1); R4: H; R1, R2, R3 : -OCH3 (IGA2); R1: H; R2, R3, R4: -OCH3
(IGA3)
Fig. 1. Proposed structures of the synthesized Schiff base compounds. R2, R4: H; R1,
R3: –OCH3 (IGA1); R4: H; R1, R2, R3: –OCH3 (IGA2); R1: H; R2, R3, R4: –OCH3 (IGA3).
3.77 ppm range can be attributed to the methoxy protons.
The protons of the cyclohexane ring are shown in the 2.79–
1.02 ppm range as the multiplets.
Three Schiff base compounds IGA1–IGA3 have the symmetric
structure. In order to well understand the structures of the ligands,
the 13C NMR spectra were recorded. In the 13C NMR spectra of
the compounds IGA1–IGA3, the carbon atoms of the azomethine
groups were shown in the 159.36–164.81 ppm range. The signals
of the aromatic ring C atoms of the ligands were shown in the
109.61–155.86 ppm range. The carbon atoms of methoxy groups of
the compounds have different enviroments on the benzene rings.
The carbon atom at the ortho position of the compounds IGA1 and
IGA2 is shown at 57.20 and 56.05 ppm, respectively. The methoxy
carbon atoms on the para position of the benzeneoid rings were
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G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
shown in the 58.50–57.90 ppm range. The carbon atoms of the
cyclohexane ring were shown in the 54.77–22.20 ppm range
The infrared spectral data of the compounds are given in the
experimental section. In the FT-IR spectra of the compounds,
the sharp bands in the 2965–2915 cm−1 range can be attributed
to the aliphatic ν(CH2) vibration of the methoxy groups. The
azomethine ν(CH¼N) vibration of the compounds was shown in
the 1630–1625 cm−1 range.
The mass spectral data of the Schiff base compounds are given
in the experimental section. The mass spectra for the compounds
were characterized by a peak corresponding to Schiff base fragmentation. The mass spectra of the compounds IGA1–IGA3 show
the molecular ion peaks ([M]+, 100%) at m/z 411 and 471. Moreover,
the fragmentation peaks at m/z 412 and 472 (25%, 40% and 37%)
with 413 and 473 (40%, 47% and 45%) can be attributed to the
[M+2]+ and [M+1]+ ions, respectively.
3.1. Crystal structures of the Schiff base compounds IGA1–IGA3
General views of all three compounds are shown in Fig. 2, all
the bond lengths are within the normal ranges. The molecular
structures are closely similar, differing principally in the position
of the methoxy groups in the compounds of IGA2 and IGA3 and the
number of methoxy groups and positions in the compound IGA1.
The molecules IGA1 and IGA3 both lie on centers of symmetry but
in the compound IGA2 the molecule has no crystallographically
imposed symmetry. For this reason, the two phenyl rings are
necessarily coplanar in the compound IGA1 and IGA3, but are tilted
at 57.60(4)1 to each other in the compound IGA2.
In all compounds, there are many interactions between the
molecules in the crystal lattice and methoxy groups play a central
role in the molecular arrangement. In the compound IGA1, the
molecules are linked by π…π stacking interactions. There are two
sets of interactions in the compound IGA1 and the same contacts
are extended between the other symmetry-related molecules.
First, C3–C9 edge of the one of the molecule is stacked with the
same section of the adjacent molecule; C3 and C9n are separated
by 3.497 Å (symmetry operation n2−x, −y, 1−z), second, C4–C7 edge
of the one of the molecule is stacked with the same section of the
neighboring molecule, C6 and C6nn are seperated by 3.409 Å
(symmetry operation nn2−x, 1−y, 1−z) (Figs. 3 and 4). There is no
evidence of π⋯π stacking in both IGA2 and IGA3, however there are
some C–H⋯π and C–H⋯O interactions in these compounds.
Intermolecular C–H…π and C–H…O interactions for both IGA2
and IGA3 are shown in Figs. 5 and 6, respectively.
3.2. The effect of different solvents on the absorption spectra
of the Schiff base compounds
The absorption spectra of the compounds IGA1–IGA3 were
investigated in different solvents such as, CH2Cl2, CH3CN, CHCl3,
EtOH and MeOH. Typical absorption spectra of the Schiff base
Fig. 2. Crystal structures of the Schiff base compounds with atom labeling (thermal ellipsoid, 50% probability). Hydrogen atoms are shown as arbitrary spheres IGA1
(a), IGA2 (b) and IGA3 (c).
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Fig. 3. π–π interactions in IGA1. Hydrogen atoms are omitted for clarity. Symmetry codes: n2−x, −y, 1−z,
627
nn
2−x, 1−y, 1−z.
Fig. 4. Packing diagram of IGA1 showing ππ interactions.
Fig. 5. Packing diagram of the compound IGA2 viewing down the c axis, π⋯HC and O⋯HC interactions are shown as dashed lines.
Fig. 6. Packing diagram of the compound IGA3 viewing down the b axis, O⋯HC interactions are shown as dashed lines.
compound IGA3 in different solvents are shown in Fig. 7. The
corresponding absorption wavelength maxima and molar extinction coefficients (ε) values are given in Table 3. In the spectra of the
Schiff base compound IGA3, while the highest absorption maxima
are seen in CH2Cl2 solution, the lowest absortion bands are seen in
the EtOH solution. The other compounds IGA1 and IGA2 also show
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G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Fig. 7. Absorption spectra of the Schiff base compound IGA3 in different solvents.
similar properties as the compound IGA3. The absorption wavelength maxima and molar extinction coefficient values of the
Schiff base compounds in different solvents varied from 382 to
204 nm and from 2.04 to 3.82 (1 10−4 M−1 cm−1), respectively,
with increasing the polarity of solvents. In the spectra of the Schiff
base compounds, there is a broad bands in the region 280–382 nm
involving mainly π-π∗ transitions. According to modern molecular orbital theory [30], any factors that can influence the electronic
density of conjugated system must result in the bathochromic or
hypsochromic shift of absorption bands. The band at around
382 nm exhibits a solvatochromic shift, characteristic of a large
dipole moment, and frequently suggestive of a large hyperpolarizability. Moreover, a negative solvatochromism, i.e. a hypsochromic
shift is observed, thus indicating a reduction of the dipole moment
upon electronic excitation.
The absorption spectra of the compound IGA1 show three
bands in the 330–204 nm range. The bands in the CH2Cl2 and
EtOH solutions were shifted to the longer wavelengths. The
compound IGA2 has also four bands in the 382–205 nm range.
In addition to, the compound IGA3 has two absorption bands in the
382–263 nm range. The bands in the 382–310 nm range can be
attributed to the n–πn transitions. The strong absorption bands in
the 279–204 nm range can be attributed to the π–πn transitions.
The effect of different concentrations on the absorption spectra of
the Schiff base compounds
In order to investigate the effect of concentration on the
absorption properties, the spectra of the compounds were investigated in different concentrations (1 10−3 M−1 10−7 M) and
obtained data are given in Table 4. The absorption spectra of the
compounds IGA2 are given in Fig. 8 and the spectra of the
compounds IGA1 and IGA3 are given in the Supplemantary
material section. In the 1 10−3 M concentration, the compound
IGA1 has two absorption bands at 284 and 310 nm wavelengths.
In the 1 10−7 M concentration, these bands shift to the lower
regions and their intensities decrease from 1 10−3 M to
1 10−7 M concentration. In the 1 10−3 M concentration, the
compound IGA2 has three absorption bands at 278, 328 and
382 nm wavelengths. While the band at 278 has the highest
absorption maxima, the band at 328 nm has the lowest absorption. Against to the 1 10−7 M concentration, the bands at 278 and
382 nm shift to the lower regions and their intensities decrease.
In the other hand, the compound IGA3 has one maximum absorption
band at 380 nm in the 1 10−3 M, and this band shifts to the lower
regions and its intensity decreases towards to the 1 10−7 M.
3.3. The effect of different solvent on the photoluminescence
properties of the Schiff base compounds
The photoluminescence properties of the Schiff base compounds IGA1–IGA3 were studied in the ethanol, methanol, acetonitrile, CHCl3 and CH2Cl2 solvents using 4.8 10−5 M solutions.
At room temperature, the Schiff base compounds exhibit similar
emission spectra in the UV–vis region (Table 3). The emission and
excitation spectra of the Schiff base compound IGA3 in various
solvents are shown in Fig. 9 and the spectra of the compounds
IGA1 and IGA2 are given in the Supplemantary material section.
The spectra of the compounds IGA1–IGA3 show one emission
band in the 365–320 nm range shorter wavelength (SW) region in
the ethanol, methanol, acetonitrile, CHCl3 and CH2Cl2 solutions.
The photoluminescence emission peaks of the Schiff bases apparently produce red shift with the introduction of the electron
donating groups. The introduction of the electron donating groups
by mesomeric and inductive effects causes the fluorescence
characteristic emission peaks of the Schiff bases to red shift in
the range of 25–30 nm. The reason is that the electron density of
the phenyl ring is increased with the δ-π hyper conjugation
effect. The Schiff bases with methoxy substituents possess p-π
conjugation that can increase their photoluminescence emission
intensity. As the Schiff bases have p-methoxy substitute groups,
they show a good conjugation and rigid planar structure. All Schiff
base compounds have both highest emission and excitation bands
in the methanol solution. On the other hand, they have lowest
emission and excitation bands in the CH2Cl2 solution.
The excitation spectra of the compounds IGA1–IGA3 were investigated in ethanol, methanol, acetonitrile, CHCl3 and CH2Cl2 solutions
and obtained data are given in Table 3. The spectra of the compound
IGA3 are given in Fig. 9. The excitation spectra of the compounds IGA1–
IGA3 resemble one other. These spectra consist of the strong π-πn
band with the long-wavelength edge at 262 nm and a weak
intraligand charge transfer (ILTC) band with the edge at 305 nm.
In the excitation spectra of the compounds in acetonitrile, EtOH and
MeOH solutions, the band around ∼277 nm shifted to longer wavelength region around ∼300 nm in the CHCl3 and CH2Cl2 solutions. It
may be that the extended π-conjugation would induce an excited state
resonance contribution of the methoxy groups to the benzene rings in
the increased polarity.
3.4. The effect of different concentration on the photoluminescence
properties of the Schiff base compounds
The effect of different concentrations on the photoluminescence properties of the Schiff base compounds was investigated in
the 1.0 10−3–1.0 10−7 M range in the DMF solution. At room
temperature, the Schiff base compounds exhibit similar emission
spectra in the UV–vis region (Table 4). The emission and excitation
spectra of the Schiff base compound IGA3 in the DMF are shown in
Fig. 10 and the spectra of the compounds IGA1 and IGA2 are given
in the Supplemantary material section. In the 1.0 10−3 M concentration, the compounds have highest emission peaks in the
548–482 nm range. As the concentration of the compounds
decreases, the emission peaks of the compounds shifted to the
lower regions. For example, in the 1.0 10−7 M concentration, the
Table 3
UV–vis absorption, emission and excitation spectral data of the Schiff base compounds IGA1–IGA3 in the different solvents.
Solvent
λmax (nm)
IGA1
IGA2
IGA3
Ems.
Exc.
Abs. (εmax, M−1 cm−1)
Ems.
Exc.
Abs. (εmax, M−1 cm−1)
Ems.
Exc.
Abs. (εmax, M−1 cm−1)
Methanol
365
277
311(3.11 10−4)
331
262
215(2.15 10–4), 260(2.60 10–4)
340
268
220(2.20 10−4), 279(2.79 10−4)
Acetonitrile
355
281
224(2.24 10−4), 267(2.67 10−4),
308(3.08 10−4), 340(3.40 10−4)
328
265
212(2.12 10−4), 260(2.60 10−4), 320(3.20 10−4), 367(3.67 10−4)
335
272
221(2.21 10−4), 275(2.75 10−4),
343(3.43 10−4)
Ethanol
345
292
227(2.27 10−4), 268(2.68 10−4), 309(3.09 10−4)
325
270
258(2.58 10−4), 352(3.52 10−4)
332
277
220(2.00 10−4), 279(2.79 10−4)
−4
−4
340
300
220(2.22 10 )
322
275
225(2.25 10 )
328
280
220(2.20 10−4), 270(2.70 10−4)
Dichloromethane
336
305
268(2.68 10−4), 307(3.07 10−4)
320
277
259(2.59 10−4), 370(3.20 10−4)
326
288
277(2.77 10−4)
Ems.
Exc.
Abs. (εmax, M−1 cm−1)
Table 4
UV–vis absorption, emission and excitation spectral data of Schiff base compounds IGA1–IGA3 in the different concentrations (solvent, DMF).
Concentration (M)
λmax (nm)
IGA1
1 10−3
IGA2
Ems.
Exc.
Abs. (εmax, M−1 cm−1)
504
407
230(2,30 10−4), 284(2,84 10−4), 330(3.30 10−4)
205(2.05 10−4), 278(2.78 10−4), 328(3.28 10−4), 382(3.82 10−4)
536
430
279(2.79 10−4), 382(3.82 10−4)
−4
−4
−4
−4
534
427
274(2.74 10−4), 381(3.81 10−4)
−4
−4
−4
−4
404
210(2,10 10 ), 278(2.78 10 ), 322(3.22 10 )
530
440
222(2.22 10 ), 257(2.57 10 ), 325(3.25 10 ), 360(3.60 10 )
530
425
270(2.70 10−4), 380(3.80 10−4)
1 10−6
486
402
208(2,08 10−4), 275(2.75 10−4), 318(3.18 10−4)
520
437
220(2.20 10−4), 255(2.55 10−4), 329(3.29 10−4), 370(3.70 10−4)
516
423
265(2.65 10−4), 379(3.79 10−4)
513
421
263(2.63 10−4), 378(3.78 10−4)
400
−4
442
490
482
−4
548
1 10
1 10
−4
Abs. (εmax, M−1 cm−1)
−5
−7
−4
Exc.
1 10
405
−4
Ems.
−4
500
−4
IGA3
222(2,22 10 ), 281(2.81 10 ), 327(3.27 10 )
−4
−4
−4
204(2,04 10 ), 270(2.70 10 ), 310(3.10 10 )
540
510
441
435
223(2.23 10 ), 263(2.63 10 ), 320(3.20 10 ), 380(3.80 10 )
−4
−4
−4
−4
210(2.10 10 ), 245(2.45 10 ), 328(3.28 10 ), 375(3.75 10 )
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Chloroform
629
630
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Fig. 8. Absorption spectra of the Schiff base compound IGA2 in the different concentrations.
Fig. 9. The fluorescence emission and excitation spectra of the Schiff base compound IGA3 in the different solvents.
peak of the compound IGA1 has been shifted to 482 nm wavelenght. The emission peaks of the other compounds also shift to
the lower regions (510, 513 nm). The quantity of the compounds
has the effect on the emission peaks. In the excitation spectra of
the compounds, while the excitation values of the compounds are
in the 442–407 nm range in the 1 10−3 M concentration, the
values have been shifted to the 435–400 nm range in the
1 10−7 M concentration. As a result, different concentrations of
the compounds have effect on their photophysical properties.
3.5. The electrochemical properties of the Schiff base compounds
Electrochemical properties of the Schiff base compounds
(IGA1–IGA3) were studied in DMF—0.1 M NBu4BF4 as supporting
electrolyte at 293 K. In order to study the effects of the solution
concentration and the scan rates, we used the solutions in two
different concentrations (1 10−3 and 1 10−4 M) and the scan
rates (100, 150, 200, 250, 500, 750, 1000 mV/s) and against an
internal ferrocence-ferrocenium standard. The obtained data are
given in Table 5. The electrochemical curves of the compound IGA3
are shown in Fig. 11 and the curves of the compounds IGA1 and
IGA2 are given in the Supplemantary material section. The compounds IGA1–IGA3 in the 100–1000 mv/s range show the three
anodic peak potentials in the −1.46 to 1.35 V range.
In the cv curves of the compound IGA1, there are seven
reversible redox processes in the 1 10−3 and 1 10−4 M concentrations. These reversible redox processes are in the −0.26 to 1.16 V
(Epa) and −0.28 to 1.12 V (Epc). On the other hand, the Schiff base
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
631
Fig. 10. The fluorescence emission and excitation spectra of the Schiff base compound IGA1 in the different concentrations (1.0 10−3–1.0 10−7 M, DMF solutions).
Table 5
The electrochemical data of the Schiff base compounds IGA1, IGA2 and IGA3.
Compound
Concentration
Scan rate (mV/s)
Epa (V)
Epc (V)
E1/2 (V)
Ipa/Ipc
ΔEp (V)
IGA1
1 10−3
100
250
500
750
1000
100
250
500
750
1000
−0.65, 0.93
−0.60, 1.02
−0.46, 0.10
−0.51, 0.09, 0.88
−0.49, 0.05, 0.92
−0.56, −0.04, 0.92
−0.44, 0.04, 1.06
−0.33, 0.14, 1.16
−0.31, 0.16, 1.20
−0.26, 0.22, 1.28
0.89, −0.91
0.95, 0.02, −0.96
0.95, −0.04, −1.04
0.94, −0.09, −1.07
0.92, −0.15, −1.13
1.05, −0.99
1.05, 0.05, −1.23
1.08, −0.13, −1.34
0.99, −0.27, −1.44
1.01, −0.28, −1.46
0.91
0.96
–
0.91
0.92
–
1.05
1.12
–
−0.27
1.04
1.07
0.44
0.93
1.00
0,56
1.00
1.07
0.55
0.92
0.26
0.36
0.14
0.51
0.64
0.43
0.01
0.08
0.21
0.02
100
250
500
750
1000
100
250
500
750
1000
−0.08,
−0.55,
−0.48,
−0.43,
−0.40,
−0.56,
−0.49,
−0.42,
−0.36,
−0.33,
0.54
−0.01,0.57
0.05, 1.05
0.08, 1.08
0.16, 1.20
−0.03, 1.18
0.02, 1.09
0.11, 1.15
0.16, 1.21
0.20, 1.25
0.67, 0.14, −0.94
0.70, 0.07, −1.07
0.69, −0.04, −1.24
0.69, −0.04, −1.31
0.79, −0.04, −1.36
0.92, 0.09, −0.88
1.04, 0.13, −0.13
1.00, 0.09, −1.24
0.99, 0.06, −1.32
0.96, −1.37
–
–
–
–
–
–
1.07
–
–
–
0.80
0.81
1.52
1.56
0.88
0.33
1.04
1.09
1.22
0.34
0.86
0.50
0.36
0.39
0.20
0.32
0.05
0.15
0.10
0.29
100
250
500
750
1000
100
250
500
750
1000
−0.65, −0.16
−0.62, −0.04
−0.51, −0.02, 0.98
−0.61, 0.46, 0.95
−0.42, 0.18, 1.05
−0.55, 0.02, 1.06
−0.43, 0.09, 1.14
−0.32, 1.26
−0.31, 0.17, 1.35
−0.30, 0.19, 1.31
1.01, −1.02, 0.08
0.98, 0.07, −1.03
0.93, −1.12
0.72, 0.08, −0.87
0.89, −0.15, −1.26
1.08, −0.92
1.00, −1.24
0.97, −1.39
0.97, −1.48
0.93, −0.22, −1.49
–
–
–
–
–
1.07
–
–
–
–
0.63
0.55
0.87
0.70
1.17
0.98
1.14
1.30
1.40
1.36
0.67
0.41
0.05
0.26
0.16
0.02
0.14
0.29
0.38
0.38
1 10−4
IGA2
1 10−3
1 10−4
IGA3
1 10−3
1 10−4
Supporting electrolyte: [NBu4](BF4) (0.1 M). All the potentials are referenced to Ag+/AgCl; where Epa and Epc are anodic and cathodic potentials, respectively. E1/2 ¼ 0.5 (Epa+Epc), ΔEp ¼Epa−Epc.
compounds IGA2 and IGA3 have only one reversible redox processes at 1.06 and 1.08 V (Epa) and 1.04 and 1.08 V (Epc). Reversible
processes of the compounds are shown in Fig. 12. In this process,
the oxygen atoms of the methoxy groups of the organic compounds give the electrons to the benzenoid rings and then to the
nitrogen atoms by the resonance. This process occurs as the
reversible. While the compound IGA1 has two methoxy groups
(ortho and para positions on the benzene rings), the other
compounds IGA2 and IGA3 have three methoxy groups (ortho,
meta, para and meta, para and meta on the benzene rings,
respectively). Although the methoxy groups decrease electron
density of the benzenoid rings by the inductive effect, the electron
density increase by the mesomeric effect. At the 100 mV/s scan
rate, the cathodic peak potentials of the compound IGA1 shifted to
the higher positive regions than the other compounds IGA2 and
IGA3. Electron donating groups to the benzene rings shift the
potentials from the positive to negative regions. This situation
were seen in these compounds.
632
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Fig. 11. Cyclic voltammograms of the Schiff base compound IGA3 in the presence of 0.1 M NBu4BF4-DMF solution at different scan rates and in the 1.0 10−4 M concentration.
OCH3
R4
+OCH
R2
R4
3
R2
R1
R1
.
N
N
-4e+4e-
N
N.
R1
R4
R2
OCH3
R1
R4
R2
+OCH3
Fig. 12. The reversible redox process of the Schiff base compounds.
compounds prepared here, yet show much less antiproliferative
activity against HeLa cell line than 5-FU and cis-platin. The
molecular structures of the compounds IGA2 and IGA3 are very
similar, differing only in the position of the methoxy groups (both
have six methoxy groups). This shows that the position of the
methoxy groups have an effect on the antiproliferative activities.
Additionally, the compound IGA1 has shown higher antiproliferative activities against HeLa cell line than compound IGA2 although
the compound IGA1 has less number of methoxy groups.
In order to compare thermal stabilities of the Schiff base
compounds IGA1–IGA3, thermal studies were done. The thermogravimetric analyses for the compounds were carried out within
the temperature range from ambient temperature up to 1000 1C.
The thermal curves of the compounds IGA1–IGA3 are given in
Fig. 14. As seen also from the thermal curves, the compounds are
not stable at the high temperature. Thermal decomposition of the
compound IGA1 starts at the lower temperature than the other
compounds IGA2 and IGA3. Mass loss of the compound IGA1 starts
at 180 1C temperature. Thermal stabilities of the compounds IGA2
and IGA3 are more stable than the IGA1. The compounds IGA2 and
IGA3 are stable up to 350 1C temperature. The compounds are fully
decomposed to the CO2 and H2O gases at 410 1C temperature.
The compounds IGA1–IGA3 show the quasi-reversible and
irreversible processes at the other potentials.
4. Conclusion
3.6. Anticancer activity studies of the Schiff base compounds
The antiproliferative activities of the Schiff base compounds
IGA1–IGA3 were investigated against HeLa, C6 and Vero cell lines.
According to the results; all of the compounds have shown cell
selective activity against HeLa and C6 cell lines. However, the
activities of the compounds have increased to depending increase
of doses against all of the cell lines. The antiproliferative activities
of the Schiff base compounds IGA1–IGA3 against C6 cell line (a),
HeLa cell line (b) and Vero cell line (c) are given in Fig. 13a–c.
The antiproliferative activities of the compounds IGA1–IGA3 and
standard compounds showed the following order at 500 mM
against HeLa cell line: 5-FU 4cis-platin 4IGA3 4IGA1 4IGA2. The
Schiff base compound IGA3 has been found to show the highest
antiproliferative activity against HeLa cell line amongst the
In this study, we synthesized three Schiff base compounds
IGA1–IGA3 and characterized by the analytical and spectroscopic
methods. Single crystals for the structure determination of the
compounds have been obtained from the ethanol solution. Electrochemical, thermal, anticancer and photophysical properties of
the compounds were investigated. In order to determine the
effects of the concentration and scan rate, the electrochemical
properties of the compounds were investigated in the different
concentration and at the scan rates. From the obtained results,
both concentration and scan rate have effect on the cathodic and
anodic peak potentials. The luminescence properties of the compounds were investigated in the different solvents and concentrations. In the anticancer activity studies, the effect of the mehoxy
groups on the benzene ring has been shown. The compounds IGA1
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
633
Fig.13. (a–c) The antiproliferative activities of compounds IGA1, IGA2 and IGA3 against C6 cell line (a), HeLa cell line (b) and Vero cell line (c). The values represent the
mean 7SEM (n¼3). nP o 0.01 when compared to control groups (one-way ANOVA following the Duncan's multiple comparison test).
Fig. 14. The thermal curves of the IGA1–IGA3 compounds.
and IGA3 both lie on centers of symmetry, but the molecule IGA2
has no crystallographically imposed symmetry. In the compound IGA1, Schiff base molecules are linked by π…π stacking
interactions. There is no evidence of π⋯π stacking in both IGA2
and IGA3, however there are some C–H⋯π and C–H⋯O interactions in these compounds.
634
G. Ceyhan et al. / Journal of Luminescence 143 (2013) 623–634
Acknowledgments
We are grateful to The Scientific & Technological Research
Council of Turkey (TUBITAK) (Project number: 109T071) for the
support of this research.
Appendix A. Supporting information
Full crystallographic data for the Schiff base ligands IGA1–IGA3
were deposited with the Cambridge Crystallographic Data Center,
CCDC numbers: 918291 (IGA1), 9182932 (IGA2) and 918293 (IGA3).
Copies of this information may be obtained by writing your request
to: The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44-1223-336-033; e-mail: [email protected] or www: http://
www.ccdc.cam.ac.uk).
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.jlumin.2013.06.002.
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