A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates

Article abstract of DOI:10.1016/j.molliq.2014.10.012

Cobalt(II) and copper(II) complexes of a novel Schiff base ligand containing two -NN- chromophore groups, derived from the condensation of 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde with p-aminoazobenzene were synthesized and characterized by analytical

Full text of DOI:10.1016/j.molliq.2014.10.012

MOLLIQ-04485; No of Pages 10
Journal of Molecular Liquids xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II)
metal chelates
Gulbahar Kurtoglu a, Barıs Avar b, Huseyin Zengin c, Muhammet Kose a, Koray Sayin d, Mukerrem Kurtoglu a,
⁎
a
Department of Chemistry, Faculty of Science and Literature, Kahramanmaras Sutcu Imam University, 46100 Kahramanmaras, Turkey
Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bulent Ecevit University, 67100 Zonguldak, Turkey
Department of Chemistry, Faculty of Science and Literature, Gaziantep University, 27310 Gaziantep, Turkey
b
c
d
Department of Chemistry, Faculty of Science, Cumhuriyet University, 58140 Sivas, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 June 2014
Received in revised form 24 September 2014
Accepted 6 October 2014
Available online xxxx
Cobalt(II) and copper(II) complexes of a novel Schiff base ligand containing two –N_N– chromophore groups,
derived from the condensation of 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde with p-aminoazobenzene
were synthesized and characterized by analytical and spectroscopic methods. The X-ray powder diffraction anal-
ysis was used to determine the unit cell parameters of the synthesized ligand and its metal complexes. Self-isom-
erization via intramolecular proton transfer was investigated by UV–Vis and theoretical calculations. The effect of
solvent polarity on UV–Vis was examined. The electronic structures of compounds were also predicted with com-
putational chemistry methods. Theoretical vibrational and electronic spectra of the metal complexes were ob-
tained from these structures. Binding energies of the metal complexes were obtained by quantum chemical
calculations. Upon irradiation the ligands and their metal complexes gave blue light. The (1a) azo-aldehyde
and (2a) azo-azomethine ligands revealed quantum yields of 35 and 41% and their excited-state lifetimes
were 3.26 and 3.86 ns, respectively. Decreases in photoluminescence intensity and quantum yield upon complex-
ation with metal ions were noted. The thermal behavior to include activation energy (E), entropy (ΔS*), enthalpy
(ΔH*) and Gibbs free energy change (ΔG*), using Coats–Redfern (CR) and Horowitz–Metzger (HM) methods,
were examined.
Keywords:
Azo-azomethine
Theoretical studies
Solvent effect
Photoluminescence
X-ray powder diffraction
Thermal analyses
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
and using insignificant voltages, for flat panel displays prove to be impor-
tant [26,27]. Aromatic amines and polymeric arylamines have been used
Schiff bases derived from aromatic amines and aromatic aldehydes
and their metal complexes have a wide variety of applications in the
fields of biology [1], catalysis [2], materials [3] and inorganic [2] and
analytical chemistry [4]. These compounds have been found to pos-
sess a wide range of biological activities such as antibacterial [5],
antifungal [6], antimalarial [7], anticancer [8], antitubercular [9],
anti-inflammatory [10], and antiviral [11].
Azo dyes are key chromophores in the chemical industry as dyes and
pigments [12], food additives [13], indicators [14], radical reaction
initiators [15] and therapeutic agents [16]. The azo azomethine
compounds are commonly prepared by the condensation between
primary amine or hydrazine and aromatic aldehyde or ketone-linked –
N_N– chromophore group. Recent years have a great deal of interest in
the preparation and characterization of azo azomethines and their
metal complexes [17–22].
as hole transport layers for electroluminescent technology [28,29]. A fluo-
rescent dioxime-type ligand and its complexes have been reported in the
literature [30]; fluorescence emission intensity of the ligand decreased
with metal ion complex formation, where decreases in emission intensi-
ties were reported to be a result of the coordination complex of N-atom
of the ligand with the metal ions. Further, fluorescent studies using
metal complexes have been explored, for example, metal complexes in
excited states have been studied as components of luminescent sensors
and switches [31,32]. Developments in photochemistry and photophysics
using Ru(II)–polypyridine complexes have been reported by Balzani and
Juris, where these complexes helped to explain important electronic phe-
nomena and interactions [33]. Balzani et al. have also given a detailed dis-
cussion of photoactive dendrimers containing metal complexes [34]. A
multitude of research have been carried out on fluorescent sensors and
metal ions, for example, the transduction mechanisms for metal ion de-
tection by fluorescence were outlined by Formic et al. [35]. Small mole-
cule fluorescent sensors for the detection of several molecular
components of oxidative stress have also been reviewed by Hyman [36],
where design, function and application of probes to detect metal cations,
reactive oxygen species, and intracellular thiol-containing compounds
were addressed. Also the development of luminescent transition metal
Research and development for new organic materials as light emitting
devices continue [23–25]. Organic polymers having strong luminescence
⁎
Corresponding author.
E-mail addresses: mkurtoglu@ksu.edu.tr, kurtoglu01@gmail.com (M. Kurtoglu).
http://dx.doi.org/10.1016/j.molliq.2014.10.012
0167-7322/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

2
G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
complexes as chemosensors for cations and anions is presented in the
literature [37].
solution (60 °C) of 2-hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde
(1a) (0.226 g, 1 mmol) in the same solvent. The reaction mixture
was then refluxed for 2 h. A solid product was obtained and separat-
ed by filtration, then purified by crystallization from EtOH, washed
with Et2O, and then dried. Yield, 0.259 g (64%). m.p.: 188–189 °C.
As part of our continuing interest in azo-azomethine chemistry, we
synthesized numbers of such compounds [38–40]. We now report on
the successful synthesis of a novel azomethine ligand containing –
N_N– moiety and its Co(II) and Cu(II) metal chelates and their
photoluminescence properties were investigated. The structures of the
prepared compounds were confirmed by spectroscopic techniques to
include UV–Vis, FT-IR, 1H and 13C NMR data and elemental analysis.
The theoretical study of the synthesized compound and its metal com-
plexes was carried out by using different programs and methods. The
crystallinity of the prepared compound and its metal complexes was
studied by X-ray powder diffraction. The thermal stability of the pre-
pared ligands and its metal complexes was also investigated using a
thermogravimetric analysis (TGA), along with the kinetics and thermo-
dynamic data evaluation.
⋀
m: 2.70 Ω−1 cm2 mol−1. Elemental analyses for C25H19N5O
(405.45 g/mol): Found: C, 74.15; H, 5.096; N, 17.06%. Calcd.: C, 74.06; H,
4.72; N, 17.27%. IR (KBr, cm−1): 3450 υ(O–H), 3049 υ(aromatic C–H),
1616 υ(C_N), 1480 υ(N_N). 1H NMR (ppm, in CDCl3): 13.672 (s, 1H,
OH), 8.846 (s, 1H, CH_N), 8.121 (s, 1H, Ar–H), 7.204–7.189 (m, 1H,
Ar–H). 13C NMR (ppm, in CDCl3): 164.045, 162.78, 152.67, 152.59,
151.51, 149.92, 145.70, 131.19, 130.68, 129.16, 129.13, 128.15, 127.92,
124.32, 122.94, 122.66, 122.04, 118.86, 118.28.
2.4. Preparation of copper(II) complex (2b)
Cu(CH3COO)2·H2O (0.10 g, 0.5 mmol) in 10 mL of hot MeOH–CHCl3
(1:1) was added to 20 mL of the azo-azomethine ligand (2a) (0.405 g,
1.0 mmol) in CHCl3. The mixture was refluxed for 3 h on water bath.
The volume of the solution was reduced to one-third of its original vol-
ume and left overnight. The dark green precipitate was filtered off,
washed with cold MeOH and Et2O and dried. Yield, 0.62 g (79%). m.p.:
275 °C. ⋀m: 3.10 Ω−1 cm2 mol−1. μeff = 1.78 B.M. Elemental analyses
for C50H36CuN10O2 (872.43 g/mol): Found: C, 68.21; H, 4.42; N,
15.58%. Calcd.: C, 68.83; H, 4.16; N, 16.05%. IR (KBr, cm−1): 3049
υ(aromatic C–H), 1611 υ(C_N), 1472 υ(–N_N–), ~556 υ(M–O),
513 υ(M–N).
2. Experimental
2.1. Reagents
Salicylaldehyde was purchased from Merck and used as received.
Dimethylformamide (DMF), chloroform (CHCl3), dichloromethane
(CH2Cl2), carbon tetrachloride (CCl4), hexane, toluene, methanol,
ethanol, 9,10-diphenylanthracene, acetone and diethyl ether were pur-
chased from Aldrich Chem. Co. and Merck and used as received.
Reagents used were purchased from Aldrich, Fluka and Merck and
used without further purification. The azo-aldehyde compound, 2-
hydroxy-5-[(E)-phenyldiazenyl]benzaldehyde (1a) was prepared ac-
cording to the published paper [38].
2.5. Preparation of cobalt(II) complex (2c)
To a solution of (0.202 g, 0.5 mmol) 4-[(E)-phenyldiazenyl]-2-[(E)-
({4-[(E)-phenyldiazenyl]phenyl}imino)methyl]phenol (2a) in 20 mL
CHCl3, 10 mL of CoCl2·6H2O (0.0059 g, 0.25 mol) was added. The solu-
tion was refluxed at 40–50 °C with stirring for 2 h, and the resulting
mixture was kept in the refrigerator for one day. The reddish brown
complex was filtered and washed with cold MeOH and dried under vac-
2.2. Techniques
The melting points of the azo-azomethine compounds were deter-
mined with an Electrothermal LDT 9200 apparatus. Infrared spectra
were recorded on a PerkinElmer FT-IR spectrophotometer using KBr
discs. 1H and 13C NMR spectra were collected using Bruker AC spectrom-
eter 600 MHz, with samples dissolved in CDCl3, and chemical shifts
expressed in δ with TMS as an internal standard. Elemental analysis
for C, H and N was performed on a PerkinElmer 240 elemental analyzer
in Inonu University, Malatya, Turkey. The synthesized azo-azomethine
ligand and its Co(II) and Cu(II) complexes were examined in spectro-
photometric grade different solvents. The UV–Vis spectra were record-
ed from 190 to 1100 nm using a PG Instruments Ltd T80+ UV/VIS
scanning spectrophotometer on 2.0 × 10−5 mol/L sample concentra-
tions, prepared in spectrophotometric grade different solvents, and
contained in 1 cm optical path quartz cuvette. The X-ray powder diffrac-
tion patterns were recorded on a vertical type Rigaku D-max/B diffrac-
tometer with CuKα radiation generated at 30 kV and 30 mA. Samples
were measured from 20° to 50° (2θ) with a step size of 0.02 Ί and a
count time of 1 s per step. Ligand and complex fluorescence spectra
were obtained using a PerkinElmer LS55 spectrometer. Samples were
prepared in spectrophotometric grade DMF, where solution concentra-
tions were 1.0 × 10−5 mol/L and excitation was achieved at 338 nm.
The standard 9,10-diphenylanthracene was used for quantum effi-
ciency calculations [41,42]. Thermogravimetric analysis (TG and
DTG) and differential thermal analysis (DTA) experiments were per-
formed using a SII Exstar TG/DTA 6200 in the temperature range
of 25–1000 °C with a heating rate of 20 °C min−1 under flowing
dry air atmosphere (30 mL/min).
uum. Yield, 0.309 g (70%). m.p.: 254 °C. ⋀m: 51 Ω−1 cm2 mol−1. μeff
=
3.33 B.M. Elemental analyses for C51H46ClCoN10O6 (989.36 g/mol):
Found: C, 61.86; H, 4.960; N, 14.08%. Calcd.: C, 61.91; H, 4.696; N,
14.16%. IR (Kr, cm−1): 3320 υ(O–H-hydrated and methanol), 3058
υ(aromatic C–H), 2930 (Me–H), 1612 υ(C_N), 1480 υ(–N_N–), 826
(coordinated water), 549 υ(M–O), 510 υ(M–N).
2.6. Computational method
All calculations were made by Gaussian 09 package programs [43–45].
Hartree–Fock (HF) and density functional theory (DFT/B3LYP) methods
with 3-21G and LANL2DZ basis sets were used for obtaining the opti-
mized structure of tautomers and copper(II) and cobalt(II) complexes.
The calculated vibrational frequencies were scaled by 0.9, 0.9393, 0.964
[46] and 0.9978 [47] for HF/3-21G, HF/LANL2DZ, B3LYP/3-21G and
B3LYP/LANL2DZ, respectively. The best level was found as HF/3-21G
level from the correlation of experimental and calculated vibrational fre-
quencies. Tautomer interacting with metal cation was determined by cal-
culating the orbital character of HOMO and HOMO-1. Time dependent-HF
(TD-HF) method was used for UV–Vis spectra calculations. The UV–Vis
spectra were calculated in vacuum, water and chloroform. Solute–solvent
interactions were taken into account by the conductor-like polarizable
continuum model (CPCM) [48].
3. Results and discussion
2.3. Preparation of 4-[(E)-phenyldiazenyl]-2-[(E)-({4-[(E)-phenyldiazenyl]
phenyl}imino) methyl]phenol (2a)
3.1. Synthesis and solubility
A hot solution (60 °C) of p-aminoazobenzene (0.197 g, 1 mmol) in
The synthesized ligand and its metal complexes were characterized
10 mL of MeOH + 10 mL of CHCl3 mixture was mixed with a hot
by IR, UV–Vis, NMR spectra, magnetic susceptibility measurements and
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
3
molar conductance. The analytical data of the ligand, Co(II) and Cu(II)
3.2. 1H and 13C NMR spectra
metal chelation along with their physical properties are given in the
Experimental section. It was noted that the metal(II) complexes are
air stable in the solid state and completely soluble in CHCl3, DMF, and
DMSO and partially soluble in MEOH and EtOH. Elemental analysis
data are given in the Experimental section and are in well agree-
ment with theoretical values. The synthesized complexes showed
1:2 metal–ligand stoichiometry. The ligand acts as a bidentate
ligand, with the metals coordinating via phenolic oxygen and
azomethine nitrogen donor atoms. Single crystals of the new azo-
azomethine ligand and its transition metal chelates could not be
isolated from organic solvents, and so no definite structures are
available. However, structures of the ligand and Co(II) and Cu(II)
compounds were proposed by the analytical and spectroscopic
data as shown in Scheme 1, and Fig. 1, respectively.
The 1H and 13C NMR spectra of azo-azomethine ligand (2a) were re-
corded in CDCl3. The 1H NMR spectrum of the ligand presents the signal
at 13.67 ppm, assigned to the phenolic OH proton. The data of the ligand
(2a) showed a sharp singlet at 8.85 ppm assigned to the azomethine
group proton (CH_N) [18]. Furthermore, the signals in the range of δ
8.06–7.28 ppm were attributed to the aromatic protons [40] (Fig. S1).
In 13C NMR spectrum of the ligand, the signals at δ 164 and
162 ppm were assigned to the carbon shifts of C_N and C–OH
groups, respectively [22] (Fig. S2). All the other aromatic carbon
shifts were observed in the range of δ 118–152 ppm. Both 1H and
13C NMR spectra of the ligand showed that there was no significant
organic impurity in the sample.
The molar conductance of 10−3 M solutions of the ligand and its
metal complexes in dimethylsulfoxide (DMSO) solvent were measured
on a Mettler Toledo Seven Multi conductivity meter. The reported
values are averages of triplicate measurements. All the measurements
were taken at room temperature for freshly prepared solutions. The
molar conductance values of Cu(II) and Co(II) complexes were found
to be 3.10 and 51 Ω−1 cm2 mol−1, respectively. The low molar con-
ductivity value of Cu(II) complex indicates that it is non-electrolyte in
DMSO, however Co(II) complex behaves as 1:1 electrolyte. The molar
conductance value suggests that in the Co(II) complex the phenolic
group of one of the ligand remained protonated and the complex
contains a chloride counter ion. Suggested formulation of the Co(II)
complex is in good agreement with spectral and analytical data.
Magnetic susceptibility measurements were carried out on a
Sherwood Scientific magnetic balance according to the Gouy method
3.3. IR spectra
The infrared spectral data of the bidentate ligand and its metal com-
plexes are presented in the Experimental section. The spectra of the
Co(II) and Cu(II) complexes of the azo-azomethine ligand are shown
in Fig. S3. The azo-azomethine ligand (2a) exhibits a medium intense
band at 3450 cm−1 due to the intramolecular hydrogen bonded (O–
H), where its absence in the spectra of the Co(II) and Cu(II) complexes
indicates deprotonation of the phenolic group and coordination of the
oxygen atom to the metal ion. Infrared spectrum of the ligand shows
υ(C_N) peak at 1616 cm−1 and absence of C_O peak at around
1656 cm−1 in the azo aldehyde indicates Schiff base formation [20]. A
downward shift (υ = 4–5 cm−1) in υ(C_N) (azomethine) is observed
with coordination, indicating that the azomethine group nitrogen is in-
volved in coordination. It is expected that coordination of nitrogen to
the metal atom would reduce the electron density in the azomethine
absorption [39]. In the FT-IR spectra, the ligand and its complexes
exhibit bands at 3049–3058 cm−1 that are assignable to vibrations of
aromatic C–H stretching. The synthesized compounds show a band
in the region 1480 cm−1 to 1472 cm−1 which may be assigned to
υ(–N_N–) chromophore groups in the structure [40]. New bands,
and Hg[Co(SCN)4] was used as a calibrant. The effective magnetic mo-
pffiffiffiffiffiffiffiffiffiffi
ments were calculated using the relation μeff ¼ 2:828
χ
mT, where T
is the absolute temperature and χm is the molar susceptibility corrected
using Pascal's constants for diamagnetism of all atoms in the com-
pounds. The magnetic measurement studies suggest that the Cu(II)
and Co(II) complexes exhibit paramagnetic behaviors with μeff values
of 1.78 and 3.33, respectively. The magnetic moment of the Cu(II) com-
plex (1.78 μB) indicates the presence of one unpaired electron. The
Co(II) complex shows a value of 3.33 μB indicating octahedral geometry.
which are not present in the ligand appeared around 556–549 cm−1
,
corresponding to M–O and 510–513 cm−1 to M–N vibrations support
the involvement of N and O atoms in coordination with metal center
Scheme 1. Preparation of 4-[(E)-phenyldiazenyl]-2-[(E)-({4-[(E)-phenyldiazenyl]phenyl}imino)methyl]phenol and its possible tautomers (i):NaNO2-HCl,
hydroxybenzaldehyde, (iii): MeOH-CHCl3mixture(1:1), reflux.
0 ºC, (ii): 2-
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
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4
G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Fig. 1. The proposed structure of metal(II) complexes; (2b) and (2c).
[18]. A less intense band at ~1476 cm−1 in the spectrum of the ligand
may be assigned to υ(–N_N–).
the range of 325–340 nm depending on the polarity of the solvents.
These absorptions were assigned to π → π* transitions due to chromo-
phore groups in the structure. Absorption intensity in hexane which is
an apolar and aprotic solvent has the lowest value. However, in DMF
(polarity index of 6.1), absorption intensity and wavelength have the
highest value. Solvents used in UV–Vis measurement have polarity
index decreasing with order of DMF, EtOH, MeOH, CHCl3, CH2Cl2,
C6H5CH3, CCl4 and n-C6H12. Once the polarity of the solvent increases,
there is a slight increase in λmax and ε (L mol−1 cm−1) values. In DMF,
an additional absorption maximum was observed at 455 nm. This absorp-
tion was attributed to the existence of tautomeric form in highly polar sol-
vent. In DMF and EtOH, there is an absorption shoulder at around 355 nm
attributed to the n → π* transitions.
Maximum absorbance wavelengths (λmax) of the azo-azomethine
ligand (2a) were observed in the 345–400 nm range in various solvents
(Fig. 2). Absorption spectra of (2a) in EtOH, MeOH, CHCl3, CH2Cl2,
toluene, CCl4 and hexane were almost identical. However, in DMF,
both absorption intensities and maximum absorption wavelengths in-
creased. The maximum absorption band at 455 nm observed for azo-
salicylaldehyde (1a) has completely disappeared in the case of the
azo-azomethine compound (2a).
3.4. Electronic spectra
Electronic spectra of the synthesized compounds were studied in
spectrophotometric grade solvents of different polarities. The data of
the azo compounds 1a, 2a, 2b and 2c are summarized in Table 1. The
molar absorptivity coefficient values of the studied compounds in-
creased as the polarity of the solvent increases. This increase in molar
absorptivity coefficient values of the compounds indicated that the ab-
sorption capacities of these are getting much better as the polarity of
the solvent increases.
UV–Vis spectra of the azo salicylaldehyde compound (1a) in various
solvents are shown in Fig. S4. Maximum absorptions were observed in
Table 1
UV–Vis data of the synthesized dye and its metal chelates.
Compound
Solvent
λ
max (nm)
Absorbance
ε (L mol−1 cm−1) × 104
(1a)
DMF
EtOH
340
339
338
335
334
333
330
325
400
362
361
360
359
351
350
345
385
375
370
365
360
355
350
345
390
370
365
360
355
352
350
345
2.804
2.532
2.233
2.046
1.903
1.733
1.587
1.425
2.862
2.751
2.553
2.234
2.001
1.786
1.638
1.431
2.885
2.652
2.419
2.114
1.897
1.693
1.501
1.332
2.919
2.779
2.393
2.081
1.903
1.739
1.595
1.449
14.02
12.66
11.17
10.23
9.52
8.67
7.94
7.13
14.31
13.76
12.77
11.17
10.01
8.93
UV–Vis spectra of the metal complexes (2b) and (2c) in various
solvents are shown in Figs. 3 and 4, respectively. Absorption spectra
of the complexes in various solvents are similar. The maximum ab-
sorbance values are observed at 385, 375, 370, 365, 360, 355, 350
and 345 nm. In DMF, maximum absorption wavelengths shifted to
longer values and the highest absorption values were observed. No
d–d transition due to the presence of metal ion was observed in
the concentration studied.
MeOH
CHCl3
CH2Cl2
Toluene
CCl4
Hexane
DMF
EtOH
MeOH
CHCl3
CH2Cl2
Toluene
CCl4
Hexane
DMF
EtOH
MeOH
CHCl3
CH2Cl2
Toluene
CCl4
Hexane
DMF
EtOH
MeOH
CHCl3
CH2Cl2
Toluene
CCl4
(2a)
(2b)
(2c)
8.19
7.16
14.43
13.26
12.10
10.57
9.49
8.47
7.51
6.66
14.60
13.90
11.97
10.41
9.52
8.70
7.98
7.25
Hexane
λ
max: maximum absorption wavelength; Abs: maximum absorption intensity; ε: molar
absorptivity coefficient; sample concentration was 2.0 × 10−5 mol L−1
.
Fig. 2. UV–Vis spectra of (2a) azo azomethine dye in various solvents.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
5
Table 2
Correlation coefficients between experimental and calculated stretching frequencies of
the tautomer (1) and its Co(II) and Cu(II) complexes.
Level
Tautomer (1)
(2b)
(2c)
HF/3-21G
0.9974
0.9907
0.9925
0.9975
0.9981
0.9970
0.9981
0.9968
0.9974
0.9936
0.9876
0.9970
HF/LANL2DZ
B3LYP/3-21G
B3LYP/LANL2DZ
where n is the atomic orbital coefficient and ∑n2 is the sum of the
squares of all atomic orbital coefficients in a specific molecular orbital.
The atomic orbital coefficients (greater than 0.1) were used in calculat-
ing the orbital characters. The contour diagrams and oxygen and nitro-
gen character of HOMO and HOMO-1 of all tautomers are given in Fig. 6.
According to Fig. 6, total donor atom characters of HOMO and
HOMO-1 for tautomer (1) are higher than tautomers (2) and (3). There-
fore, tautomer (1) interacts with metal cation. Optimized structures of
copper(II) and cobalt(II) complexes with tautomer (1) and atomic labels
are presented in Figs. 7 and S6, respectively. Some geometric parameters
are listed in Table 4.
Fig. 3. UV–Vis spectra of the Co(II) complex (2c) in various solvents.
3.5. Computational studies
Molecular geometries of (2b) and (2c) complexes were found as
distorted tetrahedral and octahedral, respectively. The bond length be-
tween oxygen atoms (numbered with 22 and 70) and central metal is
shorter than between nitrogen atoms (numbered with 24 and 72) and
metal. The angle (24N–97(Cu/Co)–72N) of the copper(II) complex is
wider than cobalt complexes. The most important difference between
(2b) and (2c) structures is that two water molecules coordinated to
cobalt(II) ion in (2c) complex.
3.5.1. Stretching frequencies and optimized structures
Tautomers were optimized at HF/3-21G, HF/LANL2DZ, B3LYP/3-21G
and B3LYP/LANL2DZ levels. Infrared spectra of the tautomer (1) and its
metal complexes with copper(II) and cobalt(II) ions were calculated.
The optimal level was determined by correlation with the experimental
and theoretical stretching frequencies. The correlation coefficients are
given in Table 2.
As can be seen in Table 2, the best level for vibrational frequencies
was found as HF/3-21G. IR spectra of tautomer (1) and its complexes
with Cu(II) and Co(II) ions at this level are represented in Fig. S5.
The calculated and experimental frequencies for tautomer (1) and
Cu(II) and Co(II) complexes are given in Table 3.
Optimized structures of the tautomers (1–3) are presented in Fig. 5.
The determination of tautomer interacting with a metal cation is impor-
tant for optimized structures of the synthesized metal complexes. Tauto-
mer gives four electrons from HOMO and HOMO-1 when it coordinates
with metal cations. Therefore, orbital character of HOMO and HOMO-1
is important to determine the coordinating tautomer.
3.5.2. UV–Vis spectra of the Co(II) and Cu(II) complexes
The UV–Vis spectra of the Co(II) and Cu(II) complexes were calculat-
ed at TD-HF/3-21G level. The solvent effect was investigated for elec-
tronic absorption spectrum. All calculations were made in vacuum,
water and chloroform with CPCM method. Wavelengths of bands and
shoulder in UV–Vis spectra are given in Table S1. The calculated
UV–Vis spectra of the synthesized metal complexes in vacuum and
two solvents are presented in Fig. S7.
Two bands and a shoulder were calculated at UV–Vis spectrum of
the (2b) complex in vacuum, whereas two bands were calculated in
water and three bands were obtained in chloroform. One absorption
band was calculated for (2c) complex in vacuum, water and chloroform.
Characters of oxygen and nitrogen atoms at HOMO and HOMO-1
were calculated with Eq. (1) [49].
3.6. X-ray powder diffraction
n2
OC% ¼ X ꢀ 100
ð1Þ
n2
In the absence of single crystal, X-ray powder diffraction data are use-
ful for structure determination, especially for deducing accurate cell pa-
rameters [50–52]. Structure determination by X-ray powder diffraction
data has gone through a recent surge since it has become important to
get to the structural information of materials, which do not yield good
quality single crystals [22]. The X-ray powder diffraction measurements
of the two ligands (1a and 2a) and their complexes (2b) and (2c)
were recorded using Cu-Kα source in the range of 20°–50° 2θ and the
Table 3
Calculated and experimental stretching frequencies of tautomer (1) and its complexes.
Frequency
Tautomer (1)
(2b)
(2c)
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
νO–H
3514
3043
2884
1683
1578
1461
–
3450
3049
2930
1616
1567
1480
–
–
–
3422
3053
2968
1527
1501
1377
552
3320
3058
2930
1612
1557
1480
549
νC–H(aromatic)
νC–H(aliphatic)
νC_O/C_N
νC_C
νN_N
νM–O
3038
2975
1585
1509
1364
545
3049
2925
1611
1575
1472
556
νM–N
–
–
492
513
508
510
Fig. 4. UV–Vis spectra of the Cu(II) complex (2b) in various solvents.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

6
G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Fig. 5. Optimized structures of the tautomers (1–3) of (2a) dye at HF/3-21G level.
diffraction patterns are shown in Fig. 8. The diffractograms and associated
data depict the 2θ value for each peak, relative intensity and interplanar
spacing (d-values). The X-ray diffraction patterns of compounds with re-
spect to major peaks of relative intensity greater than 10% were indexed
using DICVOL91 program included in the X'PertHigh-Score Plus software;
the indexing method also gives Miller indices (hkl), unit cell parameters
and unit cell volume. The corresponding parameters are summarized in
Table S2. The results indicated that the ligands and complexes have
monoclinic crystal system. Moreover, with help of the diffraction data,
the crystallite sizes were estimated according to the highest value of in-
tensity compared with the other peaks. The crystallite size calculations
were performed using Debye–Scherrer equation [53] and the obtained
values indicated that the particles were nano-sized. Similar results were
also reported in the literature [39,54,55].
having a quantum yield of 35% and an excited-state lifetime of 3.26 ns.
The wavelength of 479 nm was the maximum luminescent intensity
and 129 nm the full width at half maximum for (2a), having a quantum
yield of 41% and an excited-state lifetime of 3.86 ns.
It was noted that photoluminescence intensity and quantum yield of
(2a) increased compared to its precursor (1a) as a result of the coupling
reaction. A possible explanation may be the extensive π-electron delo-
calization being able to form a large conjugated system (2a).
Fig. 9 depicts a comparison of excitation and emission spectra of
ligands and complexes. Metal complex photoluminescence intensity
and quantum yield decreased compared to that of their ligand (2a).
Ligand (2a) had a 348 nm excitation maximum wavelength that shifted
to 346 nm upon transition metal ion binding, though decreases in exci-
tation intensity for (2a) were dependent on the nature of transition
metal ion. Upon excitation at 338 nm, the ligand had a 479 nm emission
maximum which shifted to 429 nm upon transition metal ion binding. A
possible reason for emission intensity reductions may be coordination
complex formation of the O and N-atom on (2a) with the metal ions.
Thus the energy transfer from the excited state of the ligand (2a) to
metal ions may be possible, hence non-radiated transition of the ligand
excited state increases and therefore decreases the fluorescence emis-
sion. The extent of fluorescence quenching increased with metal ion
3.7. Photoluminescence studies
Absorption and photoluminescence spectra were collected by exci-
tation at 338 nm. The ligands having (1a) and (2a) azo chromophore
group showed a strong blue light. Ligand photoluminescence spectra
are given in Fig. 9. The wavelength of 446 nm was the maximum lumi-
nescent intensity and 119 nm the full width at half maximum for (1a),
(1)
(2)
(3)
Tautomer
Tautomer
Tautomer
Fig. 6. Contour diagrams and orbital characters of HOMO and HOMO-1.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
7
Fig. 7. Optimized structure of (2b) at HF/3-21G level and atomic labels.
complex formation as a result of having a lower d-orbital electron
number. Emission maxima was in the order of Cu(II) b Co(II) for the
complexes. Thus this allows for an interaction between the metal and
the ligand (2a). Fluorescence enhancement through complexation allows
for the advancement of photochemical technologies. Fig. 9 shows a de-
crease in photoluminescence intensity and quantum yield upon metal
complexation of ligand (2a). Further, maximum photoluminescence
peak for ligand (2a), shifted from 479 to 429 nm, while the other peaks
become more intense and distinct. Table 5 gives photoluminescence
data for the ligand and its complexes.
3.8. Thermal properties
Thermal study (TG/DTG/DTA) of the synthesized compounds
was investigated in the temperature range of 25–1000 °C with a heating
rate of 20 °C min−1 under dry air using α-Al2O3 as reference. The TG,
DTG and DTA curves of the compounds are shown in Figs. S8 and 10.
Table 4
Selected bond lengths (Å) and angles (deg.) of Cu(II) and Co(II) complexes at HF/3-21G
level in vacuum.
(2b)
(2c)
Bond length (Å)
12N–13N
1.352
1.353
22O–97(Cu/Co)
23C–24N
24N–97(Cu/Co)
34N–35N
60N–61N
70O–97(Cu/Co)
71C–72N
1.866
1.866
2.005
1.395
1.354
1.854
1.312
2.036
1.356
1.842
1.452
2.025
1.352
1.354
1.955
1.387
1.981
1.355
72N–97(Cu/Co)
82N–83N
Bond angle (°)
22O–97(Cu/Co)–24N
22O–97(Cu/Co)–70O
22O–97(Cu/Co)–72N
24N–97(Cu/Co)–70O
24N–97(Cu/Co)–72N
70O–97(Cu/Co)–72N
92.7
88.3
170.4
97.1
88.5
173.2
86.9
132.3
108.3
106.5
128.1
93.5
Fig. 8. X-ray powder diffraction patterns of 1a, 2a, 2b, and 2c.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

8
G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Fig. 9. Photoluminescence spectra of the dye and its metal chelates in DMF; samples were
excited at 338 nm.
The mass loss against temperature steps for the ligand and its metal
complexes is summarized in Table 6. The experimental mass losses
were in good agreement with the calculated mass loss values.
It was observed that the (1a) azo aldehyde decomposed in one
stage, while the (2a) ligand decomposed in two stages. For (1a)
compound, the first stage appeared at 85–260 °C with a mass loss
of 81.6% (C12H9N2), accompanied by two weak endothermic peaks
with Tmax = 128 and 233 °C on the DTA curve, which may be attrib-
uted to decomposition of azo-benzene group except the aldehyde
and hydroxyl groups.
In the (2a) ligand, the first decomposition stage was observed in the
range of 190–424 °C with weight loss of 51.9% corresponding to
Fig. 10. TG, DTG and DTA curves of (i) Cu(II) and (ii) Co(II) metal complexes.
C
13H10N3. The endothermic peak at 187 °C, and the exothermic peak
at 371 °C in DTA curve correspond to the weight loss. The rest of the
organic moiety decomposed in the second decomposition step in the
range of 424–724 °C with Tmax = 519 °C and 560 °C in DTA curve.
The TG curve of Cu(II) shows two decomposition stages. The molec-
ular backbone of the complex is thermally stable up to about 325 °C. The
first decomposition stage appeared at 250–374 °C for Cu(II) complex,
with a mass loss of 21.5%, accompanied by an exothermic peak with
of methanol and water solvates. An exothermic peak in this region
with Tmax = 68 °C on the DTA curve. The second decomposition stage
occurs within the temperature range of 91–205 °C with a weight loss
of about 3.5% (Cl + H2O), corresponding to the loss of coordinated
water molecule and chloride ion. The third stage of decomposition pro-
ceeds within the temperature range of 205–401 °C with an estimated
mass loss of 16.5% (C12H9N2); an exothermic peak may be attributed
to the loss of one of the azo-benzene groups. Also, in this region the
DTA curve shows an endothermic peak at Tmax = 267 °C. The final
step of the thermal decomposition was observed in the temperature
range of 401–832 °C with a mass loss of 51.8% accompanied by an exo-
thermic peak at Tmax = 569 °C in DTA. In this step all organic residues
were removed from the structure leaving metal oxide as the final product
(CoO). When the metal complexes were heated to higher temperatures,
they decomposed to give oxides of the MO type.
Tmax = 331 °C on the DTA curve, which may be attributed to the remov-
al of one of the azo-benzene groups (C12H9N2). In the second decompo-
sition stage the rest of the other organic moiety of the complex
decomposed within the temperature range of 374–647 °C with an exo-
thermic peak in this region (Tmax = 461 °C), leaving CuO as the final
residue.
The TG curve of the Co(II) complex shows four decomposition
stages. The first step shows a sudden weight loss in the temperature
range of 26 to 91 °C with a mass loss of 21.2% indicating the removal
Table 5
Photoluminescence data for the prepared compounds.
Entity
λ
maxEx (nm)
InEx
582
λ
maxEm (nm)
InEm
572
ϕf (%)
τf (ns)
(1a)
346
446
35
3.26
(312;324;364)
348
(325;369;392)
347
(326;367;386)
346
(393;414;471;501)
479
(408;438;521)
433
(407;468;506)
429
(2a)
(2b)
(2c)
718
477
385
704
469
378
41
30
25
3.86
2.76
2.29
(325;367;385)
(412;468;508)
λmaxEx: maximum excitation wavelength; InEx: maximum excitation intensity; λmaxEm: maximum emission wavelength; InEm: maximum emission intensity;ϕf: quantum yield; τf:
excited-state life time.
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
9
Table 6
Thermal data of the compounds.
Compound
Decomp. stage
DTA peak temp. (°C) (endo. ↓, exo. ↑)
128 ↓, 233 ↓
TG range (°C)
DTGmax (°C)
217
Mass loss obs. %
Decomposed assignment
(1a)
I
85–260
81.6
18.4
51.9
48.1
–
C12H9N2
–
Residue
I
II
Residue
I
II
Residue
I
II
III
IV
Residue
(2a)
(2b)
(2c)
187 ↓, 371 ↑
519 ↑, 560 ↑
190–424
424–724
357
560
C13H10N3
C12H9N2O
–
331 ↑
461 ↑
250–374
374–647
327
463
21.5
75.7
2.8
21.2
3.5
16.5
51.8
7
C12H9N2
Rest of the organic moiety
CuO
H2O + MeOH
3H2O
C12H9N2
Rest of the organic moiety
CoO
68 ↑
130 ↑, 195 ↑
267 ↓
26–91
55
91–205
205–401
401–832
130
274
572
569 ↑
3.9. Kinetic studies
where, h is the Planck's constant 6.626 × 10−34 J s and k is the Boltzmann
constant 1.38 × 10−23 J K−1
.
The kinetics and thermodynamic parameters for non-isothermal
decomposition of the ligands and their metal complexes were deter-
mined by the Coats–Redfern (CR) [56] and Horowitz–Metzger (HM)
[57] methods.
The calculated results are given in Table 7. The kinetics and thermody-
namic data obtained by the two methods are comparable. The correlation
coefficients (r) of the Arrhenius plots of the thermal decomposition steps
were found to lie in the range of 0.98203–0.99975, showing a good fit
with linear function. The kinetics data obtained from the TG curves,
showed that the activation energies of decomposition were in the range
of 48.97–434.80 kJ mol−1. High activation energies reveal thermal stabil-
ity of the complexes. The entropy of the activation (ΔS*) gave negative
values in all compounds, except the Cu(II) complex. The negative
ΔS* value indicates that complexes are formed spontaneously, and
a more ordered activated state as a result of decomposition of
other products. Further, positive ΔS* values indicate dissociation
character of degradation [58]. The enthalpy (ΔH*) and Gibbs free
energy change (ΔG*) values have positive values in the ranges of
45.62–429.81 kJ mol−1 and 57.66–250.44 kJ mol−1, respectively,
representing endothermic pathway at all thermal steps. The positive
value of ΔG*, shows that the final residue free energy is greater than
that of the precursor compound, thus all the decomposition stages
proceed as non-spontaneous processes [59].
i. Coats–Redfern method
"
#
ꢂ
ꢀ
ꢁꢃ
1−n
1−ð1−αÞ
AR
βE
2RT
E
E
RT
ln
¼ ln
1−
−
forðn≠1Þ
ð1Þ
ð2Þ
ð1−nÞT2
ꢂ
ꢃ
ꢂ
ꢀ
ꢁꢃ
ln ð1−αÞ
AR
βE
2RT
E
E
ln
−
¼ ln
1−
−
forðn ¼ 1Þ
RT
T2
where, α is the fraction of material decomposed, n is the order of the
decomposition reaction, R is the gas constant (8.314 J mol−1 K−1),
β is the heating rate and T is the temperature corresponding to
weight loss.
ii. Horowitz–Metzger (HM) method
4. Conclusions
"
#
1−n
1−ð1−αÞ
1−n
ART2s
βE
E
RTs
Eθ
ln
¼ ln
−
þ
forðn≠1Þ
ð3Þ
ð4Þ
RT2s
In this study, a novel azo-azomethine ligand of 2-hydroxy-5-[(E)-
phenyldiazenyl]benzaldehyde with p-aminoazobenzene and its two
metal chelates were synthesized and characterized by analytical and
spectral techniques. Compound (2a) acts as a bidentate ligand and
formed stable complexes with Co(II), and Cu(II) ions. Examination of
the infrared spectra shows that the azo-azomethine ligand is bidentate
and coordinates through the phenolic oxygen and azomethine nitrogen
atoms. The azo-salicylaldehyde and azo-azomethine compounds gave
photoluminescence at 446 and 479 nm, where the maximum lumines-
cence intensities and quantum efficiencies of metal complexes were
greatly affected. The luminescence peak shifted from 479 to 429 nm
upon complexation.
ART2s
βE
E
Eθ
RT2s
ln ½−ln ð1−αÞꢁ ¼ ln
−
þ
forðn ¼ 1Þ
RTs
where, θ = T − Ts, Ts is the temperature at the DTG peak.
The plot of the left-hand side of Eqs. (1) and (2) against 1 / T and
against θ for Eqs. (3) and (4) gives a slope from which activation energy,
E was calculated and pre-exponential factor, A was determined from the
intercept.
The thermodynamic parameters of entropy (ΔS*), enthalpy (ΔH*) and
Gibbs free energy changes (ΔG*) were calculated from the following
equations:
An X-ray powder diffraction study suggested the monoclinic
crystal system for the ligands and metal complexes. Also, the thermal
study revealed thermal stability of the synthesized metal complexes
and indicated that all the decomposition steps were non-spontaneous
processes.
ꢀ
ꢁ
Ah
kTs
ΔSꢂ ¼ R ln
ð5Þ
Acknowledgments
ΔHꢂ ¼ E−RTs
ð6Þ
ð7Þ
The authors thank Dr. Mehmet Gülcan, Department of Chemistry,
Van Yüzüncüyıl University, Van, Turkey, for magnetic susceptibility
and conductivity measurements.
ΔGꢂ ¼ ΔHꢂ−TsΔSꢂ
Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012

10
G. Kurtoglu et al. / Journal of Molecular Liquids xxx (2014) xxx–xxx
Table 7
Kinetic and thermodynamic parameters of thermal decomposition of the synthesized dye and its metal chelates.
Compound
Step
I
n
Method
E kJ mol−1
A (s−1
)
ΔS*(J K−1 mol−1
)
ΔH*(kJ mol−1
)
ΔG*(kJ mol−1
)
r
(1a)
0
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
HM
CR
68.25
85.57
80
114.77
120
1.1 × 105
1.9 × 107
−152.53
−109.74
−170
−106.02
−153.72
−118.88
393
64.18
81.50
74.76
109.53
113
138.27
398.34
429.81
183.28
183.71
47.51
56.87
48.3
139
0.99974
0.99944
0.99607
0.99964
0.99975
0.99802
0.9915
0.99273
0.99911
0.99616
0.99762
0.99368
0.99029
0.99291
0.98203
0.98577
0.99453
0.99379
135.27
181.86
176.32
241
(2a)
(2b)
(2c)
I
0.25
0.75
2
1.75 × 104
3.8 × 107
II
I
1.62 × 105
1.07 × 107
4.19 × 1033
3.46 × 1036
3.4 × 1011
4.17 × 1011
1.55 × 106
6.89 × 107
4.3 × 104
145.2
403.33
434.80
189.40
189.83
50.24
59.60
51.63
48.97
115.22
113.56
95.21
104.24
237.3
162.54
160.55
206.59
205.77
89.23
448.77
II
I
1.75
0.75
2
−31.67
−29.97
−127.2
−95.65
−158.71
−162.61
−82.02
−77.18
−192
57.66
II
III
IV
112.26
111.15
197.48
151.23
250.44
243.40
2.69 × 104
5.92 × 108
1.06 × 109
1.65 × 103
1.63 × 104
45.62
110.67
109.01
88.20
97.21
2
HM
CR
HM
1.25
−173
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Appendix A. Supplementary data
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Please cite this article as: G. Kurtoglu, et al., A novel azo-azomethine based fluorescent dye and its Co(II) and Cu(II) metal chelates, J. Mol. Liq.
(2014), http://dx.doi.org/10.1016/j.molliq.2014.10.012