Synthesis and characterization of Schiff base, Co(II) and Cu(II) metal complexes and poly(phenoxy-imine)s containing pyridine unit

Article abstract of DOI:10.1016/j.ica.2020.120040

In the first stage, two different Schiff base ligands were prepared by the condensation of 2-amino-5-bromopyridine and 2-amino-5-chloropyridine with salicylaldehyde in methanol at the molar ratio of 1:1. Ligands: C₁₂H₉ON₂B

Full text of DOI:10.1016/j.ica.2020.120040

Inorganica Chimica Acta 515 (2021) 120040
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Research paper
Synthesis and characterization of Schiff base, Co(II) and Cu(II) metal
complexes and poly(phenoxy-imine)s containing pyridine unit
⁎
İsmet Kaya , Sevil Daban, Dilek Şenol
Çanakkale Onsekiz Mart University, Department of Chemistry, Polymer Synthesis and Analysis Lab. 17020, Çanakkale, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
In the first stage, two different Schiff base ligands were prepared by the condensation of 2-amino-5-bromo-
pyridine and 2-amino-5-chloropyridine with salicylaldehyde in methanol at the molar ratio of 1:1. Ligands:
Schiff base
Schiff base-metal complex
Poly(phenoxy-imine)
Thermal analysis
Conductivity
C
12
H
9
ON
Schiff bases with Co(OAc)
Co (L1) (H O)(OCH )]H
OCH )] H O. In the third stage, ortho-vanillin (VAN) was polymerized via oxidative polycondensation in
2
Br (L1), C12
H
9
ON
·4H
O, [Cu
2
Cl (L2). In the second stage, four different metal complexes were obtained from
2
2
2
O and Cu(OAc)
2
·2H
2
O Molecular formulas of the metal complexes obtained were
[
2
2
2
3
2
(L1) (H O)(OCH
2
2
3
)]H O, [Co (L2) (H O)(OCH )]H and [Cu (L2) (H O)
2
2
2
2
3
2
O
2
2
2
(
3
2
aqueous alkaline medium with air as oxidant. Then, poly(phenoxy-imine)s were synthesized from condensation
reactions of poly (ortho-vanillin) (PVAN) with 2-amino-5-bromopyridine and 2-amino-5-chloropyridine in THF
medium at 60 °C. All synthesized compounds were characterized by spectral and electrochemical techniques.
Thermal properties of the compounds were determined by TGA-DTA and DSC analyses. The molecular weights of
the polymers were calculated from SEC analyses. Poly[2-((5-bromopyridin-2-yl-imino)methyl)-6-methox-
yphenol] (PVANBr) and poly[2-((5-chloropyridin-2-yl-imino)methyl)-6-methoxyphenol] (PVANCl) were de-
−3
−1
’
termined to have electrical conductivity of 10
S cm . Electrochemical band gaps (E ) of PVANBr and
g
PVANCl were calculated as 2.10 and 1.95 eV, respectively. Initial decomposition temperatures of
[
Co
2
(L1)
2
(H
2
O)(OCH
3
)]H
2
O, [Cu
2
(L1)
2
(H
2
O)(OCH
3
)]H
2
O, [Co
2
(L2)
2
(H
2
3
O)(OCH )]H
2
O
and [Cu
2
(L2)
2
(H O)
2
(
OCH
3
)]H
2
O were found to be 309, 271, 305 and 259 °C, respectively.
1
. Introduction
Conjugated Schiff base polymers and their derivatives have become
for different solvents and molarities [13]. Transition metal complexes
containing a Schiff base ligand are important, mostly due to their sig-
nificant applications in various areas such as catalysis [14–16], chemo-
sensors [17,18], luminescent materials [19,20], energy materials
[21,22], and biological fields [23]. In particular, complexes of metal
ions with octahedral (e.g., Cu(II), Co(II), Fe(II) and Zn(II)) coordination
preferences exhibit important physicochemical properties as well as
biological activity [24]. These types of compounds are used for metal
analyses, for device applications regarding telecommunications, optical
computing, storage, and information processing [25]. Lan-Qin Chai
et al. studied nickel(II) and cobalt(III) complexes constructed from
quinazoline type ligand in terms of electrochemical properties, calcu-
lation of HOMO and LUMO energies and antimicrobial activities [26].
Wei Cao et al. synthesized new complexes involving a tridentate hy-
drazone ligand with Zn(II), Cd(II), Cu(II) and Co(III) ions. The com-
plexes were investigated for the location of the HOMO and LUMO
frontier orbitals of the molecular structures and antimicrobial activities
[27].
more attractive for researchers due to their thermal, optical, electronic,
mechanical, optoelectronic and fiber forming properties [1–5]. Het-
erocyclic compounds such as pyridine are good ligands due to the
presence of ring nitrogen atoms with a localized pair of electrons [6–9].
The presence of a nitrogen atom in the structure of pyridine provides
coordination sites to link directly with the transition metal ions for
Schiff bases [10,11]. Ayoub et al. synthesized new Schiff base ligands
containing pyridine. The octahedral geometry of all metal(II) com-
plexes was investigated with electronic and magnetic data. The kinetic
analysis of the thermogravimetric data was performed by using the
Coats-Redfern equation. The fluorescence properties of theses com-
H
plexes in DMSO, DMF, and CH
3
CN were studied. The effect of p in-
fluencing the fluorescence intensity is discussed. The catalytic activities
of metal(II) complexes were studied using H
2
2
O solution [12]. Turan
synthesized Schiff base ligands containing pyridine. The optical features
of Schiff base ligand and its metal complex compounds were compared
In this study, we synthesized poly (ortho-vanillin) (PVAN) via
⁎
Corresponding author.
E-mail address: ikaya@comu.edu.tr (İ. Kaya).
https://doi.org/10.1016/j.ica.2020.120040
Received 19 June 2020; Received in revised form 22 September 2020; Accepted 22 September 2020
Available online 28 September 2020
0020-1693/ © 2020 Elsevier B.V. All rights reserved.

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
oxidative polycondensation in an aqueous alkaline medium with air as
oxidant. Then, the poly (phenoxy-imine)s were obtained from con-
densation reactions of 2-amino-5-bromopyridine and 2-amino-5-chlor-
opyridine with PVAN in THF solution. Also, metal complexes were
7.01 (t, 2H, Ar-He, Ar-Hg).
2.4. Syntheses of metal complexes of L1 and L2 Schiff base ligands
formed using metal salts of Co(OAc)
·4H
2 2
O and Cu(OAc)
2
·2H
2
O with 2-
The solutions of Co(OAc) ·4H O (4 mmol) and Cu(OAc) ·2H O
2
2
2
2
[
(5-chloropyridine-2-yl-imino)methyl]phenol and 2-[(5-bromopyr-
(4 mmol) metal salts prepared in separate methanol (MeOH) (25 mL)
solutions were added to separate reaction flasks containing Schiff base
(4 mmol) dissolved in 25 mL THF. The mixture was stirred for 3 h at
room temperature in a magnetic stirrer heater. The precipitated com-
plex was filtered and washed with cold MeOH/THF (1:1). It was dried
in a vacuum oven at 50 °C for 24 h. The yields of L1-Co, L1-Cu, L2-Co
and L2-Cu were found to be 75, 78, 70 and 75%, respectively.
idine-2-yl-imino)methyl]phenol. The structures and characterizations
1
13
of polymers were completed with FT-IR, H NMR, C NMR, UV–Vis,
Fluorescence measurements, as well as CV and SEC analyses. The
characterizations of Schiff base-metal complexes were made with FT-IR,
UV–Vis and TGA measurements. Thermal stabilities of all compounds
were investigated with TG-DTA measurements. The T values of poly-
g
−
1
mers were determined from DSC measurements. Conductivity values of
the polymers were measured with four-point probe technique using a
Keithley 2400 Electrometer.
L1-Co: [Co (L1) (H O)(OCH )]H O (736.86 g mol ). L1-Cu:
−1
2
2
2
3
2
[Cu (L1) (H O)(OCH )]H O (746.08 g mol ). L2-Co: [Co (L2) (H O)
2
2
2
3
2
2
2
3
2
2
−
1
(OCH )]H O (647.86 g mol ). L2-Cu:[Cu (L2) (H O)(OCH )]H O
3
2
2
2
2
−
−1)
1
(
657.08 g mol ).
2
. Experimental materials
FT-IR (cm ; [Co (L1) (H O)(OCH )]H O: 3144 ν(–OH), 3070
2
2
2
3
2
ν(Ar-CH), 3040–2993 ν(Aliph.-CeH), 1597 ν(-C]N), 1566, 1520, 1426
2.1. Materials
ν(C]C), 1292 ν(CeO), 588 ν(M−N), 609 ν(M−O). [Cu (L1) (H O)
2
2
2
(
OCH
3
)]H O: 3104 ν(–OH), 3065 ν(Ar-CH), 3018–2998 ν(Aliph.-CeH),
2
All the organic solvents and reactive chemicals were purchased from
1608 ν(-C]N), 1577, 1536, 1439 ν(C]C), 1289 ν(CeO), 595 ν(M−N),
Merck Chemical Co. (Germany). All chemicals were used without pur-
ification as received.
576 ν(M−O). [Co (L2) (H O)(OCH )]H O: 3145 ν(–OH), 3075 ν(Ar-
2
2
2
3
2
CH), 3041–2995 ν(Aliph.-CeH), 1603 ν(-C]N), 1569, 1520, 1430
ν(C]C), 1290 ν(CeO), 590 ν(M−N), 574 ν(M−O). [Cu
(OCH )]H O: 3107 ν(–OH), 3064 ν(Ar-CH), 3017–2996 ν(Aliph.-CeH),
1609 ν(-C]N), 1580, 1536, 1439 ν(C]C), 1289 ν(CeO), 592 ν(M−N),
82 ν(M−O).
2
(L2)
2
(H
2
O)
2
.2. Synthesis of 2-[(5-bromopyridine-2-yl-imino)methyl) phenol
3
2
[
C
12
H
9
ON Br] (L1)
2
5
As a result of the condensation reaction, the synthesis of L1 [19] was
carried out by stirring salicylaldehyde (1.04 mL, 0.01 mol) with 2-
amino-5-bromopyridine (1.73 g, 0.01 mol) (Scheme 1) in separate
2.5. Syntheses of PVAN, PVANBr and PVANCl
1
00 mL reaction flasks within 50 mL methanol solution at 70 °C for 3 h
The PVANBr and PVANCl were carried out by condensation reaction
in two stages. During the first stage, the poly (ortho-vanillin) (PVAN)
was synthesized with air oxidant according to the method described in
the literature [28]. VAN (3 g, 0.025 mol) was dissolved in an aqueous
solution of KOH (10%, 0.025 mol). The reaction mixture was stirred at
in a magnetic stirrer heater under a condenser. The yield of L1 was
calculated to be 90%.
−
1
FT-IR (cm ): 3215 ν(–OH), 3045 ν(Ar-CH), 1609 ν(C]N), 1564,
1
1
492, 1450 ν(C]C), 1276 ν(CeO), 832 ν(C-Br). H NMR (DMSO‑d
6
): δ
−
1
ppm, 12.70 (s, 1H, –OH), 8.68 (s, 1H, –CH]N), 9.47 (s,1H, Ar-Ha),
90 °C for 25 h (Scheme 2). The air was passed at a rate of 8.5 L h
8
7
.16 (d, 1H, Ar-Hb), 7.48 (t, 2H, Ar-Hc, Ar-Hf), 7.80 (d, 1H, Ar-Hd),
.01 (t,2H, Ar-He, Ar-Hg).
during the course of the reaction. To prevent water loss in the reaction
mixture and unneutralize CO in air with KOH, the air was passed
2
through 200 mL of an aqueous solution of KOH (20%) before sending it
2
.3. Synthesis of 2-[(5-chloropyridine-2-yl-imino)methyl]phenol
into the reaction mixture. The mixture was neutralized with 3 mL HCl
(37%) at room temperature. Then, unreacted ortho-vanillin was re-
moved by steam distillation. The crude product was washed with water
(50 mL × 3), filtered and dried in a vacuum oven at 50 °C. During the
[
C
12
H
9
ON Cl] (L2)
2
As a result of the condensation reaction, the synthesis of L2 was
−
1
carried out by stirring salicylaldehyde (1.04 mL, 0.01 mol) with 2-
amino-5-chloropyridine (1.29 g 0.01 mol) (Scheme 1) in separate
second phase, PVAN (0.01 mol unit , 1.5 g) was dissolved in THF and
then the polymerization process was carried out by adding it to 2-
amino-5-bromopyridine (0.01 mol, 1.73 g) and 2-amino-5-chloroyr-
idine (0.01 mol, 1.29 g) and maintaining them in the magnetic stirrer
heater for 5 h at 60 °C (Scheme 2). The unreacted PVAN was removed
from the mixture through filtration. The precipitation of the product
was completed by removing the solvent from the remaining filtrate. The
yields of PVAN, PVANBr and PVANCl were found to be 85, 80 and 75%,
respectively.
1
00 mL reaction flasks within 50 mL methanol solution at 70 °C for 3 h
in a magnetic stirrer heater under condenser. The yield L2 was calcu-
lated to be 90%.
−
1
FT-IR (cm ): 3290 ν(–OH), 3050 ν(Ar-CH), 1605 ν(C]N), 1566,
1
1
495, 1450 ν(C]C), 1275 ν(CeO), 756 ν(C-Cl). H NMR (DMSO‑d
6
): δ
ppm, 12.71 (s, 1H, –OH), 8.58 (s, 1H,–CH]N), 9.46 (s, 1H, Ar-Ha),
8
.04 (d, 1H, Ar-Hb), 7.52 (t, 2H, Ar-Hc, Ar-Hf), 7.80 (d, 1H, Ar-Hd),
Scheme 1. Synthesis procedures of L1 and L2 Schiff base ligands.
2

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
Scheme 2. Syntheses procedures of PVAN, PVANBr and PVANCl.
−
1
FT-IR (cm ); VAN: 3250 ν(–OH), 3073 ν(Ar-CH), 2930 ν(Aliph-
CH), 1637 ν(C]O), 1588 ν(C]C). PVAN: 3325 ν(–OH), 3073 ν(Ar-CH),
done by size exclusion chromatography (SEC) with 100 Å and 7 nm
diameter loading material and 30 cm dual column (SCL-10A VP, Shi-
madzu Co., Japan). DMF and methanol mixture (v/v, 4/1, flow rate:
2
938 ν(Aliph-CH), 1650 ν(C]O), 1590, 1558, 1456 ν(C]C), 1252
−1
ν(CeO). PVANBr: 3287 ν(–OH), 3083 ν(Ar-CH), 2938 ν(Aliph-CH),
0.4 mL min ) was used as mobile phase. A refractive index detector
(RID) was used for measurement. Polystyrene was used as standard
[32]. Cyclic voltammetry (CV) measurements were carried out with a
1
620 ν(C]N), 1584, 1548, 1473 ν(C]C), 1260 ν(CeO), 832 ν(C-Br).
PVANCl: 3288 ν(–OH), 3053 ν(Ar-CH), 2937 ν(Aliph-CH), 1622 ν(C]
N), 1592, 1553, 1475 ν(C]C), 1257 ν(CeO), 782 ν(C-Cl).
CHI 660C Electrochemical Analyzer (CH Instruments, Texas, USA) at a
1
−1
H NMR (DMSO‑d
6
); PVAN: δ ppm, 10.27 (s, 1H, CH]O), 3.85(s,
potential scan rate of 20 mV s
[33]. A Shimadzu RF-5301PC spec-
3
1
H, –OCH3), 6.92–7.28 (Aromatic protons). PVANBr: δ ppm, 12.76 (s,
H, –OH), 8.66 (s, 1H, –CH]N), 6.94(s, 1H, Ar-Ha), 7.46 (s, 1H, Ar-
trofluorophotometer was used for fluorescence measurements of DMF
solutions. Measurements were made in a wide concentration range
−1
Hb), 7.51 (d, 1H, Ar-Hc), 8.16 (d, 1H, Ar- Hd), 9.46 (s, 1H, Ar-He).
PVANCl: δ ppm, 12.78 (s, 1H, –OH), 8.57 (s,1H, –CH]N), 6.92(s, 1H,
Ar-Ha), 7.38 (s, 1H, Ar-Hb), 7.52 (d, 1H, Ar-Hc), 8.02 (d, 1H, Ar- Hd),
between 3.125 and 25 mg mL to determine the optimal fluorescence
concentrations. Slit width in all measurements was 5 nm [34]. Con-
ductivities of the synthesized polymers were measured on a Keithley
2400 Electrometer. The pellets were pressed on a hydraulic press de-
9
.43 (s, 1H, Ar-He) 3.83 (–OCH3).
13
−2
C NMR (DMSO‑d
6
); PVANBr: δ ppm, 159.13 (–CH]N), 157.08
veloping up to 1687.2 kg cm . Iodine doping was carried out by ex-
(
(
C1-ipso), 141.94 (C2-ipso), 106.67 (C3-H), 121.80 (C4-ipso), 119.50
posure of the pellets to iodine vapor at atmospheric pressure and room
temperature in desiccator measurements were carried out over 24 h.
C5-H), 124.74 (C6-ipso), 166.40 (C7-ipso), 118.85 (C8-H), 110.60 (C9-
H), 151.81 (C10-ipso), 139.68 (C11-H), 67.62 (–OCH3). PVANCl: δ
ppm, 156.70 (–CH]N), 151.64 (C1-ipso), 139.01 (C2-ipso), 109.80
3. Results and discussion
(
C3-H), 124.78 (C4-ipso), 119.37 (C5-H), 129.96 (C6-ipso), 165.38 (C7-
ipso), 121.26 (C8-H), 117.92 (C9-H), 147.89 (C10-ipso), 137.17 (C11-
H), 56.38 (–OCH3).
3.1. Spectral analysis of the synthesized compounds
The structures of the synthesized monomer and polymer were illu-
1
13
2
.6. Characterization techniques
minated by the FT-IR, UV–Vis, H NMR and C NMR analyses.
Accordingly, the imine (–C]N–) band belonging to the formation of L1
−
1
−1
The infrared spectra were measured by PerkinElmer FT-IR Spectrum
and L2 Schiff base ligand was observed at 1609 cm and 1605 cm
,
−
1
one (ATR sampling accessory 4000–550 cm ) [29]. Ultraviolet–visible
respectively. While the –OH band belonging to VAN was observed at
−1 −1
(
UV–Vis) spectra were measured by Analytikjena Specord 210 Plus. The
3250 cm , the fact that this band is lower at 3325 cm
in PVAN
absorption spectra of monomers and polymers were recorded by using
indicates that the polymer formed. The stretching vibration bands of
−5
DMSO at 25 °C. The concentrations of solutions were 1.2 × 10 M for
all UV–Vis spectra measurements. The optical band gaps of compounds
were calculated from their absorption edges as follows; (Eg = 1242/
C]O group of VAN and PVAN were observed at 1637 and 1650, re-
−1
spectively. In PVAN, it was also observed at 1650 cm
due to con-
−1
jugation because this band was shifted 13 cm . The formation of a
1
13
−1
λonset) [30]. H and C NMR spectra (Bruker AC FT-NMR 400 MHz
CeO band at 1252 cm is indicative of PVAN. The imine bands seen at
−1
and 100.6 MHz) were obtained using DMSO‑d
6
as a solvent and tetra-
1620 and 1622 cm
belonged to the formation of PVANBr and
methylsilane as an internal standard at room temperature [31].
Thermal data were obtained by using a Perkin Elmer Diamond Ther-
mogravimetric and Differential Thermal Analyzer (TG/DTA). Glass
transition temperatures of polymers were examined using Perkin Elmer
Pyris Sapphire Differential Scanning Calorimetry (DSC) [29]. The cal-
culation of the weight average molecular weight (Mw), number average
molecular weight (Mn), and polydispersity index (PDI) values were
PVANCl synthesized from condensation reactions of 2-amino-5-bromo-
pyridine and 2-amino-5-chloro-pyridine with PVAN. The C-Br and C-Cl
−
1
−1
specific bands of the structures were seen at 832 cm and 756 cm
,
respectively. The bands belonging to the formation of the complex
metal-nitrogen (M−N) and metal–oxygen (M−O) were given in the
synthesis method [35]. FT-IR spectra of L1-Co, L1-Cu, L2-Co and L2-Cu
are added to Supporting information in Figs. S1 and S2, respectively.
3

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
The phenolic CeO bending vibrations for L1-Co, L1-Cu, L2-Co and L2-
Table 2
−
1
Cu were observed at 1292, 1289, 1290 and 1289 cm , respectively,
which further confirms the participation of phenolic oxygen in co-
ordination with the metal ions.
Optical and electrochemical values of the synthesized compounds.
a
b
c
d
Compounds
HOMO (eV)
LUMO (eV)
E'g (eV)
Eg (eV)
λonset (nm)
1
L1
−5.67
−5.52
−5.59
−5.35
−5.34
−3.17
−3.21
−3.46
−3.25
−3.39
2.50
2.31
2.13
2.10
1.95
3.11
3.12
2.30
2.82
2.83
400
398
539
440
439
The H NMR spectra and data of L1 and PVANBr are given in
L2
Supporting information, Figs. S3 and S4, respectively. The chemical
shift values of all the protons belonging to the synthesized materials are
PVAN
PVANBr
PVANCl
1
given in the experimental section. In the H NMR spectrum of the Schiff
base, the proton signals and split of seven different proton, imine
a
b
c
Highest occupied molecular orbital.
Lowest unoccupied molecular orbital.
Electrochemical band gap.
(
–CH]N-) and hydroxyl (–OH) bands were observed as expected. The
signals for –OH and –CH]N- protons of L1 and L2 were observed at
2.70 and 8.68 ppm; and 12.71 and 8.58 ppm, respectively. The signals
for –OH and –CH]N- protons of PVANBr and PVANCl were observed at
1
d
Optical band gap.
1
3
1
2.76 and 8.66 ppm; and 12.78 and 8.57 ppm, respectively. C NMR
This causes the energy difference between the HOMO−LUMO energy
levels in the molecule to decrease. The optical band gap values of the
compounds were calculated as mentioned in the literature [30] and the
values are given in Table 2. The π → π* and n → π* electronic tran-
sitions of L1, L2, L1-Co, L1-Cu, L2-Co and L2-Cu compounds were ob-
served at 313 and 352 nm; 311 and 351 nm; 310 and 403 nm; 287 and
spectrum and data of PVANBr are given in Supporting information, Fig.
S5. The observation of proton signals of the peaks belonging to five
different protons, imine (–CH]N-), methoxy (–OCH3) and aldehyde
groups (–CHO) are indicators that the synthesis of PVANBr was suc-
1
3
cessfully accomplished [36,37]. According to C NMR spectra of the
polymers, eleven different carbons were seen. The transformation of the
sixth and seventh carbons into ipso carbons as a result of imine bond
formation is understood from the decrease in band intensity. The sig-
nals for –CH]N-and C1-OH carbon atoms in PVANBr and PVANCl were
observed at 159.13 and 157.08 ppm; and 156.70 and 151.64 ppm, re-
spectively.
3
92 nm; 306 and 386; 280 and 370 nm, respectively. The d → d*
electronic transitions of metal complexes were observed between 420
and 500 nm.
The fact that polymers have a lower optical band gap value than
Schiff bases is an indicator of polyconjugation. Additionally, in the
spectra of metal-complex compounds of L1 and L2, the new bands
observed in the range of 480–380 nm can be attributed to the charge
transfer from the ligand to the metal or from the metal to the ligand
center [38]. The bands in the 580–485 nm range in Co(II) complexes
come from the d → d* transitions of Co(II) ion. For the other complexes,
the d → d* transition was observed in the range of 690–580 nm.
The cyclic voltammetry measurements of the synthesized materials
were carried out by using a triple electrode system. The electrochemical
characteristics of the synthesized Schiff bases and their polymers were
investigated by using glassy carbon electrode (GCE) as working elec-
trode, Ag wire as reference electrode, Pt wire as auxiliary electrode and
the acetonitrile solution of TBAPF as solvent. Potential current vol-
tammograms for Schiff base ligand (L1, L2) and polymers (PVAN,
PVANBr, PVANCl), for reduction values (Ered) of the protonated imine
The mean molar mass of the imine structured polymers synthesized
by the condensation reaction determined by size exclusion chromato-
graphy (SEC) and results are given in Table 1. According to SEC chro-
matograms, PVANBr and PVANCl were in three fractions. The % radius
of the first, second and third fractions of PVANBr and PVANCl were
found to be 24%, 32% and 44% and 25%, 35% and 40%, respectively.
The weight average molecular weight of the first, second and third
−1
fractions of PVANBr and PVANCl was calculated as 45500 g mol ,
−1
−
1
−1
−1
2
0100 g mol and 1460 g mol ; and 50200 g mol , 22000 g mol
−1
and 1450 g mol , respectively. The total average molecular weight
−1
values of PVANCl and PVANBr were found to be 20800 g mol
and
−
1
1
7950 g mol , respectively.
For UV–Vis spectrophotometer measurements of monomers and
polymers, methanol and DMSO, respectively, were used as solvent and
−
5
(–HC]N) nitrogen in the negative screening by oxidation values (Eox)
the concentration of solutions were 1 × 10 M. The π → π* and n →
π* transitions of VAN and PVAN were observed at 264 and 342 nm; 265
and 346 nm, respectively. The π → π* and n → π* electronic transitions
of PVANCl and PVANBr were observed at 285 and 317 nm; 285 and
of phenolic –OH groups in the positive scan are given in Fig. 1. The
decrease in the oxidation peak of the polymers to 1050 mV occurred,
while it was 1340 mV in the Schiff bases as a result of CeOeC coupling
during the oxidative polycondensation of the phenol group [39]. As
seen in Table 2, the values for the electrochemical band gaps (E'g) were
calculated according to the oxidation–reduction peak potential and the
values of the optical band gap (Eg) were calculated to be compatible
with each other [40]. The calculations were made by using the fol-
lowing equations [41].
3
15 nm, respectively (Supporting information, Fig. S6). The location of
these bands depends on the groups attached to the aromatic ring and
the polarity of the solvent. On this basis, many aromatic substances
contain a series of sharp bands with long wavelength. The bands which
occurred through the transition of n → π* were rather broad and their
wavelengths were observed as red-shifted. Absorption bands for the
electronic transition of Schiff base ligands, metal complexes and poly-
mers are given in Supporting information, Fig. S6. PVANBr and PVANCl
of polymers had a red shift observed due to the polyconjugated bond
structure compared to their monomers. The new absorption band was
observed between 350 and 400 nm at the UV–Vis spectra of polymers
due to polyconjugation. The polyconjugation of the polymers increased
the HOMO energy levels while it decreased the LUMO energy level.
EHOMO = (4.39 + Eox)
(1)
ELUMO = (4.39 + Ered)
(2)
E = ELUMO
EHOMO
g
(3)
In the metal complex formations of the Schiff bases, the CV mea-
surement was not taken because the oxygen atom of the hydroxyl group
Table 1
The number average molecular weight (Mn), weight average molecular weight (Mw), polydispersity index (PDI), and % values of polymers.
Total
I. Fraction
II. Fraction
III. Fraction
Polymer
Mw
Mn
PDI
Mw
Mn
PDI
%
Mw
Mn
PDI
%
Mw
Mn
PDI
%
PVAN
13,450
17,950
20,800
11,100
15,400
16,400
1.21
1.17
1.27
40,000
45,500
50,200
34,200
40,100
42,100
1.17
1.13
1.19
%20
%24
%25
15,900
20,100
22,000
12,100
16,400
15,300
1.31
1.23
1.44
%30
%32
%35
1340
1460
1450
1210
1300
1290
1.11
1.12
1.12
%50
%44
%40
PVANBr
PVANCl
4

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
Table 3
Fluorescence spectra data of compounds at optimum concentrations in 100 mL
DMF.
Compounds
Conct.(mg mL−1)
aλEx
bλmax(Em)
cIEm
dΔλST
VAN
6.25
12.50
25.00
12.50
287
285
292
296
304
307
356
368
539
603
219
216
23
29
63
61
PVAN
PVANBr
PVANCl
a
b
c
Excitation wavelength for emission.
Maximum emission wavelength.
Maximum emission intensity.
Stoke's shift values.
d
The fluorescence spectra of the synthesized substances were taken
using DMSO at room temperature. The excitation and emission slit were
set as 5 nm for all the analyses. The wavelength of emission bands for
the Schiff base ligand and polymers synthesized by condensation
method are given in Fig. 2. While the Schiff base ligand and its metal
complexes did not show any fluorescence properties, it was observed
that the polymers showed the fluorescence property due to their poly-
conjugated structure. It was also seen that the emission intensity in-
creased when the concentration decreased, and after a certain point
there was no observable change in the intensity even when con-
centration decreased, as seen in Fig. 2. Similar results were observed in
the literature [42]. According to the values for optimum concentration
in Table 3, the fluorescence intensities of PVANBr and PVANCl were
found to be the same, but it was observed that the wavelength of
PVANCl had more red shift. The Stoke's shift (ΔλST) value in Table 3 is
significant for the production of fluorescence sensors. High Stoke's shift
Fig. 1. Cyclic voltammograms of L1, L2, PVAN, PVANBr and PVANCl (Scan
−
1
speed: 200 Mv.s , support electrolyte: TBAPF6).
and the nitrogen atom of the imine group formed a complex with Cu
(
OAc)
2
·2H
2
O and Co(OAc)
2
·4H O, since it is not an oxidizing and re-
2
ducing group.
Fig. 2. Fluorescence spectra of various concentrated solutions of VAN (a) PVAN (b) PVANBr (c) PVANCl (d) in DMF. Slit width: 5 nm for λEx and λEm; concentration
of the compounds: 0.01 mg mL
−
1
.
5

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
Fig. 3. TGA curves of all compounds.
Table 4
Thermal degradation values of the synthesized compounds.
TG
DSC
Compounds aTon bWmax.T
20%
50%
% Char
at
c
Tg
d
weight
weight
(°C)/ ΔCp (J
−1
−1
Loss (°C) Loss (°C) 1000 °C
g
K
)
L1
195
210
309
271
305
233
200
130
408
279
339
229
187
720
534
706
–
–
–
–
–
–
L2
235
–
L1-Co
L1-Cu
L2-Co
322, 627
280, 605
320, 632,
25.80
15.85
17.39
7
50
L2-Cu
259
162
155
155
272
268
222
165
194
484
487
272
–
26.30
38.00
25.00
53.00
–
PVAN
185, 369
165, 358
158, 390
120/0.077
125/0.281
126/0.369
PVANBr
PVANCl
a
b
c
The onset temperature.
Maximum weight temperature.
Glass transition temperature.
d
Change of specific heat during glass transition.
Scheme 3. The proposed conductivity mechanism of poly(phenoxy-imine) with
iodine molecules.
value provides marginal basic line signal and helps the matter available
for the production of fluorescence sensors [43].
3.2. Thermal studies
TG curves for Schiff bases, metal complexes and polymers are shown
in Fig. 3. The values of T20, T50, initial decomposition (Ton), glassy
transition temperatures (Tg), and the specific heat exchanges (ΔCp)
during glass transition are given in Table 4. When the decomposition
temperatures of Schiff base ligand, their metal complexes and polymers
are compared, the highest initial decomposition temperature is ob-
served for the metal complexes. Ton values of L1, L2, L1-Co, L1-Cu, L2-
Co, L2-Cu, PVAN, PVANBr and PVANCl were calculated as 195, 210,
3
09, 271, 305, 259, 162, 155 and 155 °C, respectively. The % char
amounts of PVAN, PVANBr, and PVANCl were 38, 25 and 53%, re-
spectively, at 1000 °C. While L1 and L2 Schiff base ligands were de-
graded in one step, their metal complex compounds were degraded in a
few steps. In seen that Table 4, the thermal stabilities of L1-Co, L1-Cu,
L2-Co and L2-Cu complexes are higher than Schiff base ligand and
Fig. 4. Electrical conductivity changes of the I2-doped and undoped polymers
vs. doping time at 25 °C.
6

İ. Kaya, et al.
Inorganica Chimica Acta 515 (2021) 120040
polymers. The final decomposition steps for L1-Co, L1-Cu, L2-Co and
L2-Cu metal complexes are attributed to complete decomposition of the
complexes leaving M residues (M = Cu and Co). The thermal de-
gradations of L1 and L2 compounds were completed at 650 and 850 °C,
respectively. The % char amounts of L1-Co, L1-Cu, L2-Co and L2-Cu
were found to be 25.80, 15.85, 17.39 and 26.30%, respectively, at
complexes were more stable than those of the polymers. This high
thermal strength can be utilized in producing refractory materials for
astronomical studies. According to the SEC analysis of the synthesized
polymers, the total molar mass was calculated to be in the range of
17950–20800 g mol−1
.
1
000 °C. These results confirmed the formation of metal complexes.
CRediT authorship contribution statement
Theoretical metal amounts in L1-Co, L1-Cu, L2-Co and L2-Cu were
found to be 14.39, 15.35, 16.15 and 17.19%, respectively. It was also
observed that the initial decomposition temperatures of L1 and L2 were
higher than those of polymers. As was previously mentioned, the reason
can be the CeOeC coupling seen in Fig. 3 as well as CeC coupling
during polymer synthesis. The weak etheric bond occurring from
CeOeC coupling causes the degradation of polymers at lower tem-
peratures, resulting in formation of non-heat resistant polymers. When
the initial degradation temperatures (Ton) and the glass transition
temperatures (Tg) of synthesized polymers are compared based on
polymer types, the calculated glass transition temperatures are lower
İsmet Kaya: Writing - review & editing, Supervision, Formal ana-
lysis, Writing - original draft. Sevil Daban: Investigation, Formal ana-
lysis, Methodology, Resources, Writing - original draft. Dilek Şenol:
Software, Conceptualization, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
than the initial degradation temperatures, as it is expected. The T
g
values of PVAN, PVANBr and PVANCl were 120, 125 and 125 °C, re-
Appendix A. Supplementary data
spectively.
Supplementary data to this article can be found online at https://
doi.org/10.1016/j.ica.2020.120040.
3.3. Electrical conductivities
The electrical conductivity values of polymers which were obtained
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