Synthesis of new cyclotriphosphazene derivatives bearing Schiff bases and their thermal and absorbance properties

Article abstract of DOI:10.3906/kim-1905-60

In this study, a series of cyclotriphosphazene derivatives containing a Schiff base (3a–3d) were synthesized by the reactions of hexachlorocyclotriphosphazene (1) with bis-aryl Schiff bases (2a–2d) having different terminal groups (H, F, Cl, and Br). The products (3a–3d) were characterized by elemental and mass analyses, FT-IR, and ¹ H, ¹³ C, and ³¹ P NMR spectroscopies. Furthermore, the structure of compound 3a was also determined by X-ray crystallography. The thermal behaviors and the spectral properties of the new cyclotriphosphazene compounds (3a–3d) were investigated and the results were compared in the series.

Full text of DOI:10.3906/kim-1905-60

Turkish Journal of Chemistry
Turk J Chem
2020) 44: 31 – 47
TÜBİTAK
doi:10.3906/kim-1905-60
(
©
http://journals.tubitak.gov.tr/chem/
Research Article
Synthesis of new cyclotriphosphazene derivatives bearing Schiff bases and their
thermal and absorbance properties∗
Semih DOĞAN, Süreyya Oğuz TÜMAY, Ceylan MUTLU BALCI, Serkan YEŞİLOT,
∗
∗

Serap BEŞLİ
Department of Chemistry, Faculty of Science, Gebze Technical University, Gebze, Kocaeli, Turkey
Received: 24.05.2019 Accepted/Published Online: 03.09.2019 Final Version: 11.02.2020
•
•
Abstract: In this study, a series of cyclotriphosphazene derivatives containing a Schiff base (3a–3d) were synthesized by
the reactions of hexachlorocyclotriphosphazene (1) with bis-aryl Schiff bases (2a–2d) having different terminal groups
1
13
(
3
H, F, Cl, and Br). The products (3a–3d) were characterized by elemental and mass analyses, FT-IR, and H, C, and
1
P NMR spectroscopies. Furthermore, the structure of compound 3a was also determined by X-ray crystallography.
The thermal behaviors and the spectral properties of the new cyclotriphosphazene compounds (3a–3d) were investigated
and the results were compared in the series.
Key words: Cyclotriphosphazene, Schiff base, thermal stability, UV absorption, substituent effect
1. Introduction
Heterocyclic systems containing mainly phosphorus, nitrogen, sulfur, and oxygen atoms compose a large class
of inorganic compounds and they have applications in various areas [1–5]. The cyclophosphazenes, (NPCl2)3,4 ,
are among the most important members of these systems [3–7]. Cyclotriphosphazene and its derivatives have
particularly great stability thanks to their six-membered ring structures. In addition, their physical and chemical
properties can be adjusted via appropriate groups substituted on the phosphorus atoms [6–15].
Schiff bases (imines) are among the most popular classes of organic compounds due to their excellent
kinetic and thermodynamic stabilities and potential photoelectric properties [16–21]. Aryl Schiff bases, which
contain a classical π-conjugated system, have potential optoelectronic properties [16,17,22,23]. There are
important influences on the molecular properties due to the substituents on the aromatic rings of an aryl
Schiff base [16,24,25].
Although some cyclophosphazene derivatives containing Schiff bases were randomly synthesized in recent
years [8,26–32], their thermal stabilities and absorbance properties have not been investigated systematically. In
this study, it was planned to synthesize a thermally stable new type of heterocyclic structures by combining the
thermal properties of hexachlorocyclotriphosphazene and Schiff bases. Schiff bases (2a–2d) including H, F, Cl,
and Br atoms at the para position on the terminal phenyl ring were prepared and then cyclotriphosphazene (1)
was sequentially reacted with the bases. The thermal stabilities of Schiff base-substituted cyclotriphosphazenes
(
3a–3d) were determined by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)
∗
∗
This work is dedicated to our supervisor, Prof. Dr. Adem Kılıç, on his retirement.
Correspondence: besli@gtu.edu.tr
∗
3
1
This work is licensed under a Creative Commons Attribution 4.0 International License.

DOĞAN et al./Turk J Chem
techniques, while their absorbance spectral properties were investigated in different solvent systems by UV-Vis
spectrometry. Thermal analysis results demonstrated that the thermal stability of the cyclotriphosphazenes
containing Schiff bases was significantly higher than that of corresponding Schiff bases. Their UV absorption
spectra, which were measured in different solvent systems, showed that λmax of compounds from 3a–3d shifts
to the red region.
2
2
. Experimental
.1. Materials and physical measurements
Hexachlorocyclotriphosphazene (Aldrich) was purified by fractional crystallization from n-hexane. Dichloromethane
DCM; Merck), n-hexane (Merck), n-heptane (Merck), absolute ethanol (Merck), 4-aminophenol (Aldrich),
benzaldehyde (Aldrich), 4-fluorobenzaldehyde (Aldrich), 4-chlorobenzaldehyde (Aldrich), and 4-bromobenzaldehyde
Aldrich) were used as received. Tetrahydrofuran (THF; Merck) was distilled over a sodium/potassium alloy
(
(
under an atmosphere of dry argon. For sodium hydride with 60% dispersion in mineral oil (Merck, prior to
use the oil was removed by washing with dry n-hexane followed by decantation. All reactions were performed
under a dry argon atmosphere. CDCl3 and dimethyl sulfoxide (DMSO-d6) for NMR spectroscopy were ob-
tained from Merck. Analytical thin-layer chromatography (TLC) was performed on Merck silica gel plates
(Merck, Kieselgel 60, 0.25 mm thickness) with F254 indicator. Elemental analyses were obtained using an
ELEMENTAR Vario MICRO Cube. Mass analyses were recorded on a Bruker MALDI-TOF (matrix-assisted
laser desorption/ionization-time-of-flight) spectrometer using 1,8,9-trihydroxyanthracene as a matrix for com-
pounds 3a–3d. Infrared spectra were recorded on a PerkinElmer Spectrum 100 Two spectrometer with ATR in
−
1
1
13
31
the region of 4000–650 cm
.
H, C, and P NMR spectra were recorded for all compounds in CDCl3 for
3
a and 3b and DMSO-d6 for 3c and 3d on a Varian INOVA 500 MHz spectrometer using TMS as an internal
1
13
31
reference for H and C and 85% H3 PO4 as an external reference for P NMR measurements. Electronic
absorption spectra in solution were recorded using a Shimadzu 2101 PC UV spectrophotometer. The thermal
analyses were performed on Mettler Toledo TGA/SDTA 851 and Mettler Toledo DSC 822 devices. The TGA
were measured between 25 C and 700 C with 10 C/min and under 50 mL/min N2 flow. The DSC data were
recorded between room temperature (RT) and 300 C. The heating rate was also 10 C/min.
◦
◦
◦
◦
◦
2.2. X-ray crystallography
Intensity data were recorded on a Bruker APEX II QUAZAR diffractometer using monochromated Mo Kα
X-radiation (λ = 0.71073 Å). Absorption correction was performed by the multiscan method implemented
in SADABS [33] and space groups were determined using XPREP implemented in APEX2 [34]. Structures
were determined using the direct methods procedure in SHELXS-97 and refined by full-matrix least squares
2
on F using SHELXL-97 [35]. All nonhydrogen atoms were refined with anisotropic displacement factors
and C-H hydrogen atoms were placed in calculating positions and allowed to ride on the parent atom. The
final geometrical calculations were carried out with the PLATON [36] and MERCURY [37] programs and the
molecular drawings were done with the DIAMOND [38] program. Structure determination was deposited with
the Cambridge Crystallographic Data Centre with reference CCDC 1865974 for compound 3a. Although the
synthesis of compound 3a exists in the literature [26], the molecular structure of compound 3a has been
determined by X-ray crystallography for the first time in this study.
3
2

DOĞAN et al./Turk J Chem
2
2
.3. Synthesis
.3.1. Synthesis of Schiff bases (2a–2d)
The preparations of Schiff bases (2a–2d) were as reported in the literature [16,39]. Schiff bases were prepared
by the condensation of 0.1 mol of the corresponding benzaldehyde and 0.1 mol of 4-aminophenol in ethanol for
3
h under reflux. The products crystallized while cooling and were recrystallized from ethanol several times for
purification.
2.3.2. General procedure used for synthesis of compounds 3a–3d
Compound 2a (2.13 g; 10.8 mmol) was dissolved in 10 mL of dry THF in a 100 mL three-necked round-bottomed
flask and NaH (60% oil suspension, 0.45 g; 10.8 mmol) in 5 mL of dry THF was quickly added dropwise to the
stirred solution under an argon atmosphere. Then a solution of hexachlorocyclotriphosphazene (1) (0.40 g; 1.2
mmol) in 5 mL of THF was prepared and added by dropping funnel dropwise to the stirred solution of 2a. The
reaction was stirred under reflux for 48 h. The reaction was followed by TLC on silica gel plates using n-hexane-
THF (3:2) as the mobile phase. The reaction mixture was filtered to remove the sodium chloride and any other
insoluble material. The excess THF was removed under reduced pressure and then the reaction mixture was
extracted with DCM/distilled water phase for separation from excess Schiff base. n-Hexane was slowly added
to the solution and the solids precipitated. The resulting solids were filtered, washed with n-hexane, and then
dried at room temperature. Compound 3a was crystallized from the n-heptane-THF (2:1). A summary of the
preparation of all compounds is given in Table 1.
a, b
Table 1. Preparation of compounds 3a–3d.
◦
Compound 1 Schiff base
Sodium hydride Yield % Mp ( C) Isolated Cmp
g
0
0
0
mmol
.40 1.20
.40 1.20
.40 1.20
.40 1.20
g
mmol
g
mmol
2a 2.13 10.80 0.45 10.80
2b 2.32 10.80 0.46 10.80
2c 2.50 10.80 0.47 10.80
2d 2.98 10.80 0.48 10.80
66
61
65
62
172
211
239
262
3a
3b
3c
3d
0
a
All reactions carried out for 48 h under reflux.
b
n-Hexane/THF (3:2) solvent system was used as mobile phase for TLC analysis.
Anal. calc. for 3a; C78 H60 N9 O6 P3 : C, 71.39; H, 4.61; N, 9.61%, M, 1312.3. Found: C, 71.59; H, 4.68;
+
−1
N, 9.18%, [M+H] , 1313.2. FT-IR (cm ) 3059 (Ar-C-H), 1626 (HC=N), 1497 (C=C), 1168, 1202 (P=N),
1
9
51 (P-O-C-aryl). H NMR, CDCl3 , 298 K, δ (ppm) 8.50 (s, H; H C=N), 7.91 (d, 2H, aryl), 7.49 (t, 3H,
aryl), 7.21 (d, 2H, aryl), 6.88 (d, 2H, aryl). C NMR, CDCl3 , 298 K, δ (ppm) 167.28, 160.20, 148.79, 134.45,
29.00, 128.83, 128.68, 121.93, 121.68. P NMR, CDCl3 , 298 K, δ (ppm) 9.46.
1
3
3
1
1
Anal. calc. for 3b; C78 H54 F6 N9 O6 P3 : C, 65.96; H, 3.83; N, 8.88 %, M, 1420.3. Found: C, 66.18;
+
−1
H, 3.91; N, 8.27%, [M+H] , 1421.5. FT-IR (cm ) 3023 (Ar-C-H), 1627 (HC=N), 1506 (C=C), 1164, 1174
1
(
7
P=N), 953 (P-O-C-aryl). H NMR, CDCl3 , 298 K, δ (ppm) 8.32 (m, H; H C=N), 7.78 (m, 2H, aryl), 7.07–
1
3
.03 (m, 4H, aryl), 6.87 (m, 2H, aryl).
44.70, 132.67, 130.58, 122.34, 116.00, 115.94, 155.82.
C NMR, CDCl3 , 298 K, δ (ppm) 165.57, 163.54, 157.07, 154.37,
3
1
1
P NMR, CDCl3 , 298 K, δ (ppm) 9.38.
3
3

DOĞAN et al./Turk J Chem
Anal. calc. for 3c; C78 H54 Cl6 N9 O6 P3 : C, 61.68; H, 3.58; N, 8.30%, M, 1519.0. Found: C, 61.75;
+
−1
H, 3.67; N, 7.87%, [M+H] , 1518.6. FT-IR (cm ) 3040 (Ar-C-H), 1625 (HC=N), 1494 (C=C), 1168, 1180
1
(
(
P=N), 949 (P-O-C-aryl). H NMR, DMSO-d6 , 298 K, δ (ppm) 8.45 (s, H; H C=N), 7.80 (d, 2H, aryl), 7.42
1
3
d, 2H, aryl), 7.14 (d, 2H, aryl), 7.06 (d, 2H, aryl).
37.50, 130.90, 129.86, 129.46, 128.99, 121.89, 121.67, 115.66. P NMR, DMSO-d6 , 298 K, δ (ppm) 9.44.
C NMR, DMSO-d6 , 298 K, δ (ppm) 158.53, 154.66,
3
1
1
Anal. calc. for 3d; C78 H54 Br6 N9 O6 P3 : C, 52.46; H, 3.05; N, 7.06%, M, 1785.7. Found: C, 52.87;
+
−1
H, 3.14; N, 6.88%, [M+H] , 1785.6. FT-IR (cm ) 3033 (Ar-C-H), 1624 (HC=N), 1496 (C=C), 1165, 1200
1
(
(
1
P=N), 951 (P-O-C-aryl). H NMR, DMSO-d6 , 298 K, δ (ppm) 8.44 (s, H; H C=N), 7.72 (d, 2H, aryl), 7.57
1
3
d, 2H, aryl), 7.14 (d, 2H, aryl), 7.05 (d, 2H, aryl).
33.92, 131.73, 130.05, 125.36, 121.90, 121.47, 113.39.
C NMR, DMSO-d6 , 298 K, δ (ppm) 158.39, 135.51,
P NMR, DMSO-d6 , 298 K, δ (ppm) 9.43.
3
1
3
3
. Results and discussion
.1. Synthesis and characterization of compounds
Schiff base ligands (2a–2d) bearing four different side groups at the para position on the terminal phenyl ring
were prepared from the condensation reaction of 4-aminophenol with benzaldehyde, p-fluoro-, chloro-, and
bromobenzaldehyde in ethanol [16,39]. Full substituted cyclotriphosphazene products (3a–3d) were synthe-
sized from the nucleophilic substitution reactions of chlorocyclotriphosphazene (1) with Schiff bases including
hydroxyl groups in high yields. The general synthesis route and the structures of compounds 3a–3d are shown
in Scheme 1 and Scheme 2. All compounds (3a–3d) were characterized by FT-IR, elemental analysis, MALDI-
1
13
31
TOF mass spectrometry, and H, C, and P NMR spectroscopies. The molecular structure of compound
a was also determined by X-ray crystallography. The analysis of the data for each new compound is provided
as part of the analytical data in Section 2.3.
3
Scheme 1. The synthesis pathway of Schiff bases 2aÐ-2d.
The FT-IR spectra of compounds 3a–3d showed characteristic stretching bands at 3023–3059 cm
−
1
(aromatic
C–H groups), 1624–1627 cm (C=N stretching vibrations), 1494–1506 cm (alkenyl C=C stretch), 1150–1202
−
1
−1
−
1
−1
cm (P=N), and 949–953 cm (P–O–C), as expected. The FT-IR spectra of compounds 2b and 3b are given
as an example in Figure 1. When the IR spectrum of compound 2b was compared with that of compound 3b,
−
1
it was seen that the OH vibration in the 3103–3126 cm
region disappeared in the spectrum of compound 3b
−
1
and a new band appeared in the 949–953 cm region because of P–OAr absorption. The characteristic P=N
stretching vibrations of cyclophosphazenes were observed between 1164 and 1174 cm
−
1
as sharp bands.
3
1
The proton decoupled P NMR spectra of compounds 3a–3d exhibit an A3 type spin system due to
same phosphorus environments within the molecule, as expected, as shown in Supplementary Figures S1–S3.
3
1
The P NMR spectrum of compound 3b, shown as an example in Figure 2, exhibited a unique sharp singlet
at 9.38 ppm.
3
4

DOĞAN et al./Turk J Chem
Scheme 2. The synthesis pathway of compounds 3aÐ-3d.
1
The H NMR spectra of compounds 2a–2d and 3a–3d have similar chemical shifts, except for some
obvious differences. For example, while OH protons were observed at 9.53–9.57 ppm for compounds 2a–2d,
1
the signals disappeared in the H NMR spectra of compounds 3a–3d. The azomethine protons for compounds
3
a–3d were observed between 8.32 and 8.50 ppm as singlets in the spectra. The aromatic protons were observed
1
between 6.87 and 7.91 ppm. The H NMR spectra of compounds 2d and 3d are given as examples in Figure 3.
The MALDI-TOF mass spectra of 3a–3d provided definitive characterization and the results are given
in Section 2. The mass spectrum of compound 3d is given as an example in Figure 4.
3.2. Characterization of compound 3a by X-ray crystallography
The molecular structure of compound 3a was characterized by X-ray crystallography and its crystal structure is
shown in Figure 5. The crystallographic data and refinement parameters are summarized in Table 2. Selected
bond lengths and angles are given in Table S1. The molecular structure of compound 3b (each terminal phenyl
ring contains fluorine atoms at the para position) was also confirmed by X-ray analysis, as shown in Figure
S4, but the crystal structure could not be fully elucidated due to crystallographic problems. Although different
crystallization systems and methods were tried, quality crystals could not be obtained.
Compound 3a is crystallized in the monoclinic space group C 2/c, with a = 46.790 (4) Å; b = 7.9041 (6)
3
5

DOĞAN et al./Turk J Chem
Figure 1. FT-IR spectra of a) compound 2b and b) compound 3b.
3
1
Figure 2. Proton decoupled P NMR spectrum of compound 3b.
◦
◦
Å; c = 39.079 (3) Å; α, γ = 90 ; and β = 114.969 (4) (shown in Table 2). The phosphazene consists of a
six-membered ring, (PN)3 , which is substituted with six bis-aryl Schiff base groups on the three P atoms. Three
of the six arms are located on one side of the ring, while the others are on the other side. The six-membered
N3 P3 ring is a very slightly twisted conformation in the compound with the maximum deviation from the plane
of the cyclotriphosphazene ring being only 0.066 Å (on P2 atom) with a total puckering amplitude QT of 0.154
Å, as shown in Figure S5 and Table S1. Normally, P-N bond lengths in the hexachlorocyclotriphosphazene ring
3
6

DOĞAN et al./Turk J Chem
1
Figure 3. H NMR spectra of a) compound 2d and b) compound 3d in d -DMSO.
6
Figure 4. Mass spectrum of compound 3d.
3
7

DOĞAN et al./Turk J Chem
Figure 5. A view of the molecular structure for 3a with the atom-numbering scheme. Displacement ellipsoids are drawn
at the 50% probability level. The hydrogen atoms have been omitted and only one orientation of the disordered C51
and C52 carbon atoms of compound 3a has been presented for clarity.
◦
are equal to each other and the P-N bond distance is 1.581 Å. N-P-N and P-N-P bond angles are 118.40 and
◦
1
21.40 , respectively. In compound 3a, the average values of endocyclic bond parameters of the phosphazene
◦
ring are 1.578 Å for the P-N bond distance;nd 117.19 for the N-P-N angle. The P-N-P bond angles are a little
larger than the N-P-N bond angles and the average value of P-N-P angles is 121.55 (Table S1). Thus, the P-N
◦
bond lengths are in the normal ranges and all of these bond parameters were compatible with those reported
for fully substituted cyclotriphosphazene derivatives [29,40,41].
The investigation of the crystal structure of compound 3a shows that there is no classical hydrogen bond
but there are many short intermolecular contacts where separation between donor and acceptor atoms is less
than 3.5 Å. These contacts are observed predominantly between the hydrogen atoms and heteroatoms (C , N ,
O). The orientations of the phenyl rings allow intermolecular π-π interactions, which may be effective in the
stabilization of this self-organization. The intramolecular π-stacking interaction is 4.409 Å in the solid state,
as shown in Figure S6.
3.3. Thermal properties
Cyclotriphosphazene, consisting of alternating phosphorous and nitrogen atoms, exhibits unusual thermal
properties such as flame retardancy and self-extinguishing ability [42–44]. Additionally, it can be decorated
with thermally and chemical stable groups via nucleophilic substitution reactions in order to further increase its
thermal stability. Here, the cyclotriphosphazene ring was appended with Schiff bases, which are also attractive
compounds owing to cheapness, ease of synthesis, and their chemical and thermal stability.
3
8

DOĞAN et al./Turk J Chem
Table 2. Crystal data and structure refinement details for compound 3a.
Compound
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
3a
C78H60N9O6P3
1312.26 g/mol
120 (2)
Monoclinic
C 1 2/c 1
46.790 (4)
7.9041 (6)
39.079 (3)
b (Å)
c (Å)
αα )
◦
◦
90
◦
◦
ββ ( )
γγ )
114.969 (4)
◦
◦
90
−
3
Volume/Å
Z
13101.9 (18)
8
1.331
0.155
−
3
Density calc./g cm
−
1
µ/mm
F(000)
Crystal size/mm
Radiation
5472
3
0.18 × 0.24 × 0.30
Mo Kα (λ = 0.71073 Å)
◦
◦
2
θ range for data collection/
2.94 to 25.00
Completeness
0.992
Index ranges
–55 ≤ h ≤ 53, –9 ≤ k ≤ 9, –46 ≤ l ≤ 46
Reflections collected
Independent reflections
Data/restraints/parameters
74825
11463 [R (int) = 0.0546]
11463 / 66 / 872
1.044
2
Goodness-of-fit on F
FinalR indexes [I ≥ 2σ(I)]
Final Rindexes [all data]
Largest diff. peak/hole/e Å
R1 = 0.0654, wR2 = 0.1560
R1 = 0.0971, wR2 = 0.1690
0.758 and –0.562
−
3
The melting points (Tm) of all compounds (2a–2d and 3a–3d) were determined by DSC technique and
are shown in Figure 6 and Figures S7–S9. The endothermic sharp peak belonging to the melting point in the
DSC thermograms of compounds 3a–3d showed that the compounds were obtained in high purity, as shown
in Figure 6. DSC analysis results indicated that the melting points of compounds 3b, 3c, and 3d were higher
than those of corresponding Schiff base compounds 2b, 2c, and 2d, excluding 3a, as shown in Table 3. When
the DSC curves given in Figure 7 are examined, it can be said that the melting point increased in the order of
terminal groups, which is hydrogen, fluorine, chlorine, and bromine.
◦
TGA thermograms of the compounds (2a–2d and 3a–3d) were recorded between RT and 700 C under 50
mL/min N2 flow. TGA was utilized to evaluate the thermal stability of the Schiff bases (2a–2d) and compounds
◦
(
3a–3d). The onset decomposition temperatures (Ton) were recorded at a heating rate of 10 C/min. The
3
9

DOĞAN et al./Turk J Chem
Figure 6. The DSC curves of compounds 2c and 3c
Figure 7. The DSC curves of compounds 3a–3d heated
◦
◦
◦
heated under nitrogen to 300 C at a heating rate of 10
under nitrogen to 300 C at a heating rate of 10 C/min.
◦
C/min.
Table 3. Thermal analysis results of compounds 1, 2a–2d, and 3a–3d.
o
◦
◦
DSC ( C) (Tm) First mass loss in TGA ( C)
Remaining amount at 700 C (%)
Compounds
Ton
Tdm
Cyclotriphosphazene
1
114
159
167 1.4
Schiff bases
2
2
2
2
a
b
c
187
153
184
207
206 275
198 290
234 292
234 280
14.5
19.6
38.5
38.6
d
Cyclotriphosphazene derivatives
3
3
3
3
a
b
c
175
211
239
262
398 458
366 427
379 431
396 436
62.5
56.5
56.6
62.9
d
T
T
T
m
: Melting temperature.
on: Onset temperature, starting point of the decomposition processes.
dm: Maximum point of decomposition temperature.
curves of the TGA measurements are given in Figures S10–S12. The onset decomposition temperatures
◦
(
(
Ton), maximum point of decomposition temperatures (Tdm), and remaining substance amounts at 700
C
%) are given in Table 3. All compounds decomposed in one step. The thermal stabilities of the new
cyclotriphosphazene derivatives (3a–3d) are obviously higher than those of the Schiff bases (2a–2d) and
◦
cyclotriphosphazene (1). For example, while the decomposition temperature (Tdm) of 2a was 275 C, the
◦
Tdm value was 458 C for compound 3a, as shown in Figure 8.
The Ton and Tdm values of products decreased in the order of hydrogen (3a), bromine (3d), chlorine
(
3c), and fluorine (3b) derivatives of the Schiff bases, respectively (Table 3). The thermal stability of the
4
0

DOĞAN et al./Turk J Chem
◦
Figure 8. TGA curves of compounds 2a and 3a from 25
Figure 9. TGA curves of compounds 3a–3d from 25
C
◦
C to 700 C at a heating rate of 10 ◦ C/min under N
◦
◦
◦
2
to 700 C at a heating rate of 10 C/min under N
of 50 mL/min.
2
flow
flow of 50 mL/min.
products slightly reduced in the order of 3a, 3d, 3c, and 3b, as shown in Figure 9. The main factor affecting
this trend is the size of the halogen group (Ph-X; X = F, Cl, Br) substituted on the Schiff base [45,46]. It is seen
that as the size of the halogen atoms increases, the cross-linking process on decomposition decreases. Another
factor explaining this trend is the increase of bond dissociation energy in the order of C-Br < C-Cl < C-F.
The char yields of new cyclotriphosphazene derivatives 3a–3d (62.5% for 3a; 56.5% for 3b; 56.6% for 3c;
6
2.9% for 3d) are quite high because of the P-N skeleton and cross-linking process on decomposition (Table 3).
3.4. UV-Vis absorption properties of 3a–3d
The optical properties of new cyclotriphosphazene derivatives (3a–3d) containing Schiff bases and precursor
Schiff bases (2a–2d) were evaluated with UV-Vis spectroscopy. All of the spectral measurements were carried
◦
out in a spectroscopic quartz cuvette by using a micropipette at 25 C. UV-Vis electronic absorption spectra of
compounds 2a–2d and 3a–3d were plotted in CH2 Cl2 and the epsilon values of all compounds in CH2 Cl2 were
−
5
calculated and are given Table 4. As shown in Figure 10, 1 × 10
M of compounds 2a–2d and 3a–3d
demonstrated the absorption maximum between 266–339 nm, attributed to π-π* transitions for benzene rings
and azomethine moieties [47–50]. Importantly, the UV-Vis electronic absorption wavelengths of compounds
3a–3d were similar to those of their precursor Schiff bases (2a–2d), which showed that there was no effective
ground state interaction with the appended Schiff base on the cyclotriphosphazene scaffold. On the other
hand, the absorbances of compounds 3a–3d were much higher than those of the corresponding Schiff bases
(
2a–2d) at the same concentration, which was most probably due to the increasing number of absorbing Schiff
base groups on the cyclotriphosphazene scaffold [51]. The molar absorption coefficients of 3a–3d were as
4
5
−1
−1
high as 10 to 10 L mol
cm , which were determinative for π-π* state transition characteristics of
cyclotriphosphazene derivatives (3a–3d) [52]. These obtained results are quite reasonable because it is well
known that cyclophosphazenes are optically inert in the UV-Vis region and their photophysical properties can
be adjusted according to the appended moiety [10,53–55].
The normalized UV-Vis electronic absorption spectra in CH2 Cl2 of compounds 3a–3d are given in
Figure 11. As can be seen, the maximum wavelengths of electronic absorption of compounds 3a–3d changed
4
1

DOĞAN et al./Turk J Chem
−
5
Figure 10. UV-Vis absorption spectra of 1 × 10
c, 3c; d) 2d, 3d in dichloromethane.
M 3a–3d with their precursors 2a–2d: a) 2a, 3a; b) 2b, 3b; c)
2
Table 4. Absorption properties of compounds 2a–2d and 3a–3d.
−
1
−1
)
Compounds λabs* (nm) ε* (L mol cm
2
2
2
2
3
3
3
3
a
b
c
d
a
b
c
266, 333
266, 328
272, 338
274, 339
265, 318
266, 321
271, 324
278, 330
38800
30200
31700
31300
92700
57500
139900
154000
d
*
Values are given for dichloromethane.
for not only the first absorption bands (266–278 nm) but also the second absorption bands (318–330 nm) in
the following order: 3d > 3c > 3b > 3a (Br > Cl > F > H). When the substitution at the Schiff base on
the cyclotriphosphazene changed from H to Br, red shifts of the first and second electronic absorption bands
were determined to be 13 nm and 22 nm, respectively. These alterations of the electronic absorption maxima
of 3a–3d can be attributed to the increase of polarizability of heavy atoms, which was consistent with previous
reports in the literature [10,24,25].
The solvent system and working concentration can significantly affect the UV-Vis properties of molecules
[24,56]. Therefore, solvent effect on absorption of compounds 3a–3d were investigated with different solvents
4
2

DOĞAN et al./Turk J Chem
Figure 11. Normalized UV-Vis absorption spectra of 3a–3d in dichloromethane.
such as cyclohexane, 1,4-dioxane, THF, dichloromethane, DMSO, and water, as shown in Figure S13. Although
water and cyclohexane were used for dissolution, the UV-Vis spectra of compounds 3a–3d were not obtained
due to the solubility problem of the compounds. As can be seen from Figure S13, compounds 3a–3d have good
solubility in organic solvents (1,4-dioxane, THF, dichloromethane) and moderate solubility in a polar solvent
(DMSO). In addition, it can be understood that the UV-Vis electronic absorption of compounds 3a–3d was
not affected by the change of solvents and π-π* transitions between 266 and 330 nm were consistent without
any shift. The effects of concentration on the UV-Vis electronic absorption spectra of compounds 3a–3d
−
5
−6
were examined at various concentrations between 10 and 10
M, as shown in Figures S14–S17. The UV-
Vis absorbance of compounds 3a–3d gradually decreased without any red or blue shift when concentrations
−
5
−6
of solutions were diluted from 1 × 10
to 1 × 10
M, which indicated that no inter- or intramolecular
interaction existed between attached Schiff base groups on the cyclotriphosphazene scaffold.
After evaluation of the UV-Vis electronic absorption properties of cyclotriphosphazene derivatives (3a–
3
d) containing Schiff bases, the fluorescence properties of compounds 3a–3d were investigated at various
−
5
−6
concentrations between 10 and 10
M in 1,4-dioxane, THF, dichloromethane, and DMSO, which were
excited between 250 and 350 nm. However, compounds 3a–3d showed no appreciable fluorescence properties in
these conditions. These results obtained for the fluorescence properties of Schiff base derivatives are expected
and consistent with the literature because it is well known that C=N isomerization is the predominant decay
process of the excited states for Schiff base derivatives with an unbridged C=N structure such that those
compounds are often nonfluorescent [56–60].
4. Conclusions
To summarize, we report on the synthesis and characterization of a series of cyclotriphosphazene derivatives
containing Schiff base ligands and their thermal and absorbance properties. It was found that the newly
synthesized compounds (3a–3d) have good thermal properties, making them potentially suitable for some
industrial applications, such as flame-retardant additives to polymers. It was also found that the electronic
absorption maxima of 3a–3d are slightly shifted to the red region when the substitution at the Schiff base on
cyclotriphosphazene changes from H to Br.
4
3

DOĞAN et al./Turk J Chem
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