The synthesis and characterization of azocalix[4]arene based chemosensors and investigation of their properties

Article abstract of DOI:10.1016/j.saa.2015.01.087

In the present study, azocalix[4]arenes were prepared by linking 4-methoxy, 4-methyl, 4-ethyl, 4-chloro, 4-bromo and 4-nitroaniline to calix[4]arene through a diazo-coupling reaction. A new family of azocalix[4]arene tetraester derivatives, (4a-f), have been prepared with the incorporation of ethyl ester units to azocalix[4]arene. Characterization of the synthesized azocalix[4]arenes was carried using elemental analyses, UV-vis, FT-IR and ¹H NMR spectroscopic techniques. The effect of varying pH levels and solvent types on the absorption ability of azocalix[n]arenes substituted with electron-donating and electron-withdrawing groups was examined. Thermal decomposition of azocalix[4]arene derivatives (4a-f) was investigated by means of thermogravimetry (TG), differential thermogravimetry (DTG) and differential thermal analysis (DTA) analyses. In conclusion of the examination of the extraction we found a selectivity characteristic of these compounds toward Ag⁺, Hg⁺ and Hg²⁺ cations.

Full text of DOI:10.1016/j.saa.2015.01.087

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
The synthesis and characterization of azocalix[4]arene based
chemosensors and investigation of their properties
b
c
a
Serkan Elçin a, , Gülbanu Koyundereli Çılgı , Alpaslan Bayrakdar , Hasalettin Deligöz
⇑
a Department of Chemistry Engineering, Faculty of Engineering, Pamukkale University, 20017 Denizli, Turkey
b Materials Science and Engineering Department Technology Faculty, Pamukkale University, 20017 Denizli, Turkey
c Department of Physics, Faculty of Science, Pamukkale University, 20017 Denizli, Turkey
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
In this study, azocalix[4]arene
tetraester derivatives (4a–f) were
synthesized.
Their thermal decomposition analysis
was performed by means of TG, DTA
and DTG.
Azocalix[4]arene derivatives have
showed a good selectivity toward
Hg2+ and Hg+ ions.
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study, azocalix[4]arenes were prepared by linking 4-methoxy, 4-methyl, 4-ethyl, 4-chloro,
4-bromo and 4-nitroaniline to calix[4]arene through a diazo-coupling reaction. A new family of azoca-
lix[4]arene tetraester derivatives, (4a–f), have been prepared with the incorporation of ethyl ester units
to azocalix[4]arene. Characterization of the synthesized azocalix[4]arenes was carried using elemental
analyses, UV–vis, FT-IR and 1H NMR spectroscopic techniques. The effect of varying pH levels and solvent
types on the absorption ability of azocalix[n]arenes substituted with electron-donating and electron-
withdrawing groups was examined. Thermal decomposition of azocalix[4]arene derivatives (4a–f) was
investigated by means of thermogravimetry (TG), differential thermogravimetry (DTG) and differential
thermal analysis (DTA) analyses. In conclusion of the examination of the extraction we found a selectivity
characteristic of these compounds toward Ag+, Hg+ and Hg2+ cations.
Received 4 November 2014
Received in revised form 14 January 2015
Accepted 29 January 2015
Available online 7 February 2015
Keywords:
Calix[n]arene
Solvent extraction
Tetraester
TG/DTA
Ó 2015 Elsevier B.V. All rights reserved.
Optimized structure
Introduction
Literature surveys show that the majority of the existing studies
have not only focused on the thermal behavior but also concen-
Calix[4]arenes play an important role in supramolecular
chemistry as molecular scaffolds for elaborating sophisticated
hosts [1–4]. Functional groups are introduced into the existing
trated on functionalization [6].
A variety of functional groups have been regio- and stereospe-
cifically introduced on the hydroxy groups of calix[4]arenes as rec-
ognition sites by linking moieties through etherification. However,
the transformation causes the location of the functional groups to
be apart from the calix skeleton [7,8]. This situation makes it diffi-
cult for guests to be affected by the steric effects of the calix skel-
eton and/or to interact with substituents which are present on it
(e.g., with residual hydroxy groups or linking hetero atoms). This
calix[n]arene framework by
a functionalized method either
through the ‘‘lower rim’’ (the oxygen-position of the phenolic moi-
eties) or the ‘‘upper rim’’ (the p-position of the aromatic nuclei) [5].
⇑
Corresponding author. Tel.: +90 258 296 3087; fax: +90 258 296 3262.
E-mail address: selcin@pau.edu.tr (S. Elçin).
http://dx.doi.org/10.1016/j.saa.2015.01.087
1386-1425/Ó 2015 Elsevier B.V. All rights reserved.

S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
179
issue may be addressed by cleaving the aryl–oxygen bonds,
R
thereby replacing the hydroxy groups with different functional
groups. However, such a transformation is quite difficult for
calix[4]arenes because of their steric and electronic environment
in proximity to the lower rim [9,10], in addition to the poor nuclei
fugacity of the phenolic hydroxy group.
N
a)
b)
-OCH3
-CH3
N
4
Calix[n]arenes are extensively used in selective extraction of
metals, nuclear waste treatment, catalysis, complexation of fuller-
enes and neutral molecules. Their synthesis procedure, in addition
to their physical characterization techniques (in terms of their
physical properties such as melting point, solubility and acid disso-
ciation constant, pKa) are complicated [1,2]. The high melting
points of these compounds necessitate a careful investigation of
their thermal behavior. Thermal analysis methods have been
extensively applied in the field of polymers, solid/liquid interface,
carbohydrate chemistry, minerals, energetic materials, pharma-
ceutical and biochemistry [11–13].
c)
e)
d)
f)
R=
-C2H5
-Br
-Cl
-NO2
O
C
O
O
Scheme 1. Diazo-coupled azocalix[4]arene tetraester derivatives (4a–f).
Thermogravimetric analysis is
a valuable tool for the
determination of inclusion behavior of calix[n]arenes with guest
molecules such as toluene, xylene, chloroform, acetone, metha-
nol and alkyl ammonium [14,15]. Increasing temperature leads
to loss of small functional groups from the main calix[n]arene
body. As the temperature increases, the chain length of the
leaving functional groups increases. Calix[n]arenes are good
antioxidants of polyolefins such as polypropylene, and polyethyl-
ene [16–18]. It is reported that, more than 20,000 ton of pheno-
lic antioxidants were consumed during 1983 for the stabilization
of plastics in USA alone. This data clearly reveals the importance
of antioxidants in terms of the polymer industry [19]. The
present study is concerned with the synthesis of azocalix
[n]arenes and investigation of their thermal decomposition
kinetics.
Schatz et al. studied solid state inclusion of various organic sol-
vent molecules in p-tert-butylcalix[n]arene [15]. After their study,
synthesis and characterization of p-tert-butylcalix[6]arene ammo-
nium cation complexes have been reported. A similar study was
also reported by Radius et al., who prepared calix[4]arene-sup-
ported iron(III) complexes [20].
Experimental
All reagents and solvents were purchased from Merck, Sigma–
Aldrich and Carlo-Erba Company and used without further purifi-
cation.p-tert-Butylcalix[4]arene, calix[4]arene, p-substitute pheny-
lazocalix[4]arenes (3a–f) 25,26,27,28-tetrakis(ethoxycarbonylme
thoxy)-5,11,17,23-tetrakis[(4-methoxyphenyl)azo]calix[4]arene
(4a), 25,26,27,28-tetrakis (ethoxycarbonylmethoxy)-5,11,17,23-
tetrakis[(4-methylphenyl)azo]calix[4]arene (4b), 25,26,27,28-tet-
rakis(ethoxycarbonylmethoxy)-5,11,17,23-tetrakis[(4-ethylphe-
nyl)azo]calix[4]arene (4c), 25,26,27,28-tetrakis (ethoxycarbonylm
ethoxy)-5,11,17,23-tetrakis[(4-chlorophenyl)azo]calix[4]arene (4
d), 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-5,11,17,23-tet-
rakis[(4-bromophenyl)azo]calix[4]arene (4e) and 25,26,27,28-tet-
rakis(ethoxycarbonylmethoxy)-5,11,17,23-tetrakis[(4-nitrophenyl)
azo]calix[4]arene (4f) were synthesized as described in previously
reported methods [28–34].
Instrumental
As reported in [21–23], secondary and tertiary amine com-
plexes of calixresorcinarenes are formed in a solution with a com-
plementary fit with the calix cavity size. Calixresorcinarene forms
complexes with amines with a stoichiometry ratio of 1:2 in the
solid-state. There are several variations of decomposition struc-
tures of azocalixarenes/calixarenes, or azocalixarene metal com-
plexes as shown by X-ray crystallography. These structures
present variable positions of various metal cations, some of which
fill the cavity by interacting with the electron clouds and the others
by forming hydrogen bonds with phenolic oxygens.
Recently, synthesis and theoretical investigation of calix[4]ar-
ene derivatives and their complexes, polymeric calix[n]arene
derivatives and azocalix[n]arenes have been reported by our
research group [24]. We examined the selective extraction of
Fe3+ ion from aqueous phase into the organic phase as well as
the liquid–liquid extraction of transition metal ions using diazo-
coupled calix[n]arenes. However, there is still an important need
for systems those can explain color changes with ionic or molecu-
lar interactions [25–27].
In this study, we have mainly looked into the development of a
new class of chromogenic azocalix[4]arene chemosensors. This
work focuses on thermal behavior and decomposition of six
diazo-coupled azo substituted calix[4]arenes (4a–f) (Scheme 1).
There are an insufficient number of reports on the thermal decom-
position analysis of azocalix[4]arene-based compounds so far.
Hence, this study has been a research effort to fill this gap in the
academic literature.
Melting points of analyzed compounds were measured using an
Electrothermal IA9100 digital melting point apparatus in capillar-
ies sealed under nitrogen and they were used without correction.
1H NMR spectra were referenced to tetramethylsilane (TMS) at
0.00 ppm as the internal standard solution and were recorded on
a Bruker 400 MHz spectrometer at room temperature (25 1 °C).
IR spectra were recorded by a Mattson 1000 FTIR spectrometer
using KBr pellets. UV–vis spectra were obtained by a Shimadzu
1601 UV–Visible recording spectrophotometer. The elemental
analyses were performed in the Laboratories of TUBITAK (The
Scientific and Technological Research Council of Turkey).
Crystallization solvent was remained in some of the analytical
samples and affected the elemental analysis results. In such cases,
best fits between the analytical values and appropriate fractional
increments of solvents were used.
TG, DTG and DTA curves of analyzed compounds were obtained
simultaneously by using a Shimadzu DTG-60H thermal analyzer.
The measurements were realized in platinum crucible and flowing
air atmosphere (25 mL minꢁ1). Potential applications of the
calyx[n]arenes, such as selective extraction, azo dyes component
and antioxidant properties, are realized in air atmosphere. So we
chose air atmosphere. The analyzed temperature range was 25–
850 °C. The heating rate was set to 10 °C minꢁ1 and the sample
masses were between 3–5 mg. Highly sintered
a–Al2O3 was used
as the reference material. Before carrying out the experiments, it
is essential to calibrate the balance for buoyancy effects for the

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S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
quantitative estimation of mass changes. The material chosen for
the investigation of such effects was silver. The temperature was
calibrated measuring the melting points of indium and tin pro-
vided by Shimadzu.
12H, CH2ACH3), 3.50 (d, J = 13.83 Hz, 4H, ArCH2Ar), 4.25 (q,
J = 7.16 Hz, 8H, OACH2ACH3), 4.84 (s, 8H, OACH2AC@O), 5.03
(d, J = 13.77 Hz, 4H, ArCH2Ar), 7.26 (d, J = 8.83 Hz, 8H, ArH), 7.36
(s, 8H, ArH), 7.58 (d, J = 8.77 Hz, 8H, ArH).
Preparation of the ligands and the diazotization
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-bromophenyl)azo]calix[4]arene (4e)
Azocalix[4]arene (4e) was prepared as described above, using
ethyl bromoacetate with (3e) and K2CO3/NaI. A pale brown product
was acquired (yield, 0.73 g (56%) mp. 231–232 °C). [Found: C:
54.31; H: 4.09; N: 7.46]; C68H60Br4N8O12 requires C: 54.42; H:
p-tert-butylcalix[4]arene, calix[4]arene and azocalix[4]arenes
(3a–f) were synthesized as described previously [28–32].
Esterification
4.03; N: 7.47. IR (KBr)
t
: 1748 cmꢁ1 (AC@O), 1463 cmꢁ1 (N@N).
1H-NMR (CDCl3, 25 °C) dH: 1.31 (t, J = 7.14 Hz, 12H, CH2ACH3),
3.49 (d, J = 13.84 Hz, 4H, ArCH2Ar), 4.24 (q, J = 7.14 Hz, 8H, OACH2-
ACH3), 4.83 (s, 8H, OACH2AC@O), 5.03 (d, J = 13.78 Hz, 4H, ArCH2-
Ar), 7.35 (s, 8H, ArH), 7.42 (d, J = 8.77 Hz, 8H, ArH), 7.50 (d,
J = 8.73 Hz, 8H, ArH).
We have followed the procedure given in references [33,34] in
order to carry out the experiments.
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-methoxyphenyl)azo]calix[4]arene (4a)
The esterification reaction of calix[4]arene (3a) (1 g, 1.04 mmol)
and ethyl bromoacetate (0.93 mL, 8.32 mmol) has been performed
in a suspension of K2CO3 (2.15 g, 15.60 mmol) and NaI (1.25 g,
8.32 mmol) in dry acetonitrile (100 mL). The reaction mixture
was refluxed for 4 days and subsequently allowed to cool down
to room temperature. After evaporating the solvent by using a
rotary evaporator, the resulting residue was taken up in CHCl3
(100 mL). The obtained mixture was washed first with 0.5 N HCl
(250 mL) and then with water (300 mL). The organic layer was
dried with MgSO4. The addition of ethanol (50 mL) yielded the
product as a pale orange powder (yield, 1.01 g (74%), mp. 245–
246 °C). [Found: C: 66.19; H: 5.61; N: 8.67]; C72H72N8O16 requires
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-nitrophenyl)azo]calix[4]arene (4f)
Azocalix[4]arene (4f) was prepared as described above, using
ethyl bromoacetate with (3f) K2CO3/NaI. The color of the resultant
product was pale brown (yield, 0.69 g (52%), mp. 168–170 °C).
[Found: C: 59.91; H: 4.40; N: 12.47]; C68H60N12O20 requires C:
59.82; H: 4.43; N: 12.31. IR (KBr)
t:
1756 cmꢁ1 (AC@O),
1463 cmꢁ1 (AN@N), 1520 cmꢁ1 and 1342 cmꢁ1 (ArANO2). 1H-
NMR (CDCl3, 25 °C) dH: 1.32 (t, J = 7.14 Hz, 12H, CH2ACH3), 3.54
(d, J = 13.96 Hz, 4H, ArCH2Ar), 4.25 (q, J = 7.14 Hz, 8H, OACH2-
ACH3), 4.87 (s, 8H, OACH2AC@O), 5.10 (d, J = 13.93 Hz, 4H, ArCH2-
Ar), 7.44 (s, 8H, ArH), 7.74 (d, J = 9.04 Hz, 8H, ArH), 8.12 (d,
J = 9.03 Hz, 8H, ArH).
C: 66.25; H: 5.56; N: 8.58. IR (KBr)
t
: 1755 cmꢁ1 (AC@O),
1449 cmꢁ1 (AN@N). 1H-NMR (CDCl3, 25 °C) dH: 1.31 (t,
J = 7.14 Hz, 12H, CH2ACH3), 3.48 (d, J = 13.72 Hz, 4H, ArCH2Ar),
3.80 (s,12H, OACH3), 4.24 (q, J = 7,14 Hz, 8H, OACH2ACH3), 4.84
(s, 8H, OACH2AC@O), 5.02 (d, J = 13.76 Hz, 4H, ArCH2Ar), 6.78 (d,
J = 9.01 Hz, 8H, ArH), 7.33 (s, 8H, ArH), 7.64 (d, J = 8.98 Hz, 8H,
ArH).
Liquid–liquid extraction
A chloroform solution (10 mL) of ligand (10ꢁ3 M) and an aque-
ous picric acid solution (10 mL) of metal ions (2 ꢂ 10ꢁ5 M) were
shaken together at 25 °C for duration of an hour. An aliquot of
the aqueous solution was taken and its UV spectrum was recorded.
UV spectra of the aqueous picric acid solution of metal ions were
also recorded for comparison. The initial extraction procedure
was repeated twice. The extractability (Ex %) of the metal cations
was expressed by means of the following equation:
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-methylphenyl)azo]calix[4]arene (4b) [34]
Azocalix[4]arene (4b) was prepared as described above, using
ethyl bromoacetate with (3b) and K2CO3/NaI. A pale brown prod-
uct was obtained (yield, 1.12 g (81%) mp. 276–278 °C, Lit. mp.
283–285 °C).
Ex% ¼ ½ðA0 ꢁ AÞ=A0ꢃ ꢂ 100
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-ethylphenyl)azo]calix[4]arene (4c)
where A0 and A are the absorbance values in the absence and pres-
ence of ligands, respectively.
Azocalix[4]arene (4c) was prepared as described above, using
ethyl bromoacetate with (3c) and K2CO3/NaI. The product was dark
orange color (yield, 0.99 g (73%) mp. 215 °C). [Found: C: 74.26; H:
6.27; N: 8.72]; C76H80N8O12 requires C: 70.35; H: 6.21; N: 8.64. IR
Results and discussions
(KBr)
t
: 1750 cmꢁ1 (AC@O), 1465 cmꢁ1 (AN@N). 1H-NMR (CDCl3,
Syntheses and characterizations
25 °C) dH: 1.22 (t, J = 7.60 Hz, 12H, ArACH2ACH3), 1.31 (t,
J = 7.15 Hz, 12H, CH2ACH3), 2.64 (q, J = 7.60 Hz, 8H, ArACH2ACH3),
3.50 (d, J = 13.77 Hz, 4H, ArCH2Ar), 4.24 (q, J = 7.14 Hz, 8H, OACH2-
ACH3), 4.85 (s, 8H, OACH2AC@O), 5.03 (d, J = 13.72 Hz, 4H, ArCH2-
Ar), 7.11 (d, J = 8.58 Hz, 8H, ArH), 7.36 (s, 8H, ArH), 7.58 (d,
J = 8.51 Hz, 8H, ArH).
It has been generally accepted that calix[n]arene based chemo-
sensors a stronger ion selectivity compared to diazo-coupling
based chemosensors. In some cases, this characteristic may be
enhanced with the use of appropriate functionalities. Our former
azocalix[n]arene based studies were mostly focused on derivatives
with functionalities appended to the lower rim [35,36].
Preparation of 25,26,27,28-tetrakis(ethoxycarbonylmethoxy)-
5,11,17,23-tetrakis[(4-chlorophenyl)azo]calix[4]arene (4d)
Each azocalix[n]arene compound carries a single p-substituted
phenylazo-chromogen group. Each of these chromogen groups
contains four free phenolic groups. Azocalix[4]arene compounds
are synthesized through diazo coupling reaction of corresponding
diazotized amines with calix[4]arenes. Azocalix[4]arene deriva-
tives are carried out through esterification of them.
Azocalix[4]arene (4d) was prepared as described above, using
ethyl bromoacetate with (3d) and K2CO3/NaI. The resultant prod-
uct was in orange color (yield, 0.82 g (61%) mp. 214–216 °C).
[Found: C: 61.84; H: 4.61; N: 8.36]; C68H60Cl4N8O12 requires C:
61.73; H: 4.57; N: 8.47. IR (KBr)
t:
1749 cmꢁ1 (AC@O),
In the present study, five new p-substituted (AOCH3, AC2H5,
ACl, ABr and ANO2) aniline derivatives were prepared. The effect
1464 cmꢁ1 (ANAN). 1H-NMR (CDCl3, 25 °C) dH: 1.31 (t, J = 7.15 Hz,

S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
181
of substituents on the color of azocalix[4]arene ester derivatives
(4a–f) was discussed. Their thermal behaviors, solvent extraction
and absorption properties were assessed. Until now there have
been no publications about the synthesis and wavelength shift
behavior of such compounds. Hence, this study has been a research
effort to fill this gap in the academic literature.
This study mainly focuses on five novel (4c–f and 4a) and one
existent (4b) azocalix[4]arene ester derivatives (4a–f). At first, p-
tert-butylcalix[4]arene was prepared by reaction of p-tert-butyl-
phenol with formaldehyde according to the Gutsche method
[28]. Treatment of p-tert-butylcalix[4]arene with aluminum chlo-
ride gave calix[4]arene according to the method described by Gut-
sche et al. [29]. In accordance with our former experience [30–32],
azocalix[4]arene derivatives have been synthesized from calix[4]-
arene by diazo coupling with the corresponding aromatic amines.
Then we synthesized the diazonium salt derivatives according to
the method reported by Morita et al. [30].
The synthesis of azocalix[4]arene tetraester derivatives (4a–f)
have been summarized in four steps starting from simple p-tert-
butylcalix[4]arene (Scheme 2). All synthetic compounds were
checked for their purity levels with thin layer chromatography
(TLC). They were characterized by FT-IR and NMR analyses as given
in the Experimental Section. Since the terminals of both pendants
of these conjugates possess ACH2COOC2H5 moieties, it is of inter-
est to study their recognition abilities toward electrophilic cou-
pling and solubilization properties of important molecules, such
as aromatic amines and ethyl esters. To elicit the selectivity of
these conjugates (AOCH3, ACH3, AC2H5, ACl, ABr and ANO2) as
well as the role of the four aromatic azo arms, appropriate con-
trolled molecular systems were generated by converting phenolic
groups into ACOOC2H5 moieties. The diazo-coupling reaction of
these, azocalix[4]arenes, namely, AN@NAPheAR, and their ethyl
ester version, namely, CH2COOC2H5, were synthesized and charac-
terized (Scheme 2) and used in the extraction studies.
The diazonium salts are excellent electrophiles and they attack
any available position on the calix[n]arene molecules to yield com-
plex mixtures. Purification of five novel azocalix[4]arene ester
derivatives resulted in corresponding residues in 52–81% yields.
The above reaction products were characterized by elemental
analysis, FT-IR and 1H-NMR spectral techniques. FT-IR Vmax values
of all products were recorded between 1748–1756 cmꢁ1 (AC@O),
1449–1465 cmꢁ1 (AN@N. The 1H-NMR spectrum of 4a revealed a
triplet peak for methyl protons at 1.31 (a), a quartet peak for meth-
ylene protons at 4.24 (b), two peaks for bridge methylene protons
(AB spin system) at d 3.35–3.54 and 5.02–5.10, a singlet peak for
methoxy protons at 3.80 (g), a singlet peak for carbonyl neighbor-
ing methylene protons at 4.84 (c).
Conformational studies have been realized by using 1H-NMR
spectra method at room temperature. The corresponding details
of the structure determination and refinement are given in Fig. 1.
The crystal structure of azocalix[4]arene ester derivatives exhib-
ited cone conformation and is in conformity with the results
obtained by NMR analysis. The structure exhibits rather exposed
azo and ester moieties those are amenable for interaction with
the incoming species. Similar structural features were observed
also with azocalix[4]arene where the terminal ACH2CH3 ester is
poised suitable for interaction.
Solvent effect
UV–vis absorption spectra were measured using a Shimadzu
160A spectrophotometer in the wavelength range 300–700 nm.
Absorption spectra of azocalix[4]arenes were measured with vari-
ous solvents and at different pH levels. The choice of solvent was
O
C
O
AlCl3
+
+
NaOH
Toluene
H
Reflux
H
HO
OH
OH
OH
OH
HO
OH
OH
OH
(2)
(1)
NH2
R
NaNO2 / HCl
DMF / THF
R
R
R
R
R
R
R
R
N
N
N
N
N
N
N
N
O
NaI / K2CO3
CH3CN
N
N
Br
N
N
N
+
N
N
O
N
O
O
O
O
H2C
H2C
CH2
CH2
C O
O
O
C
O C
O
C O
O
HO
OH
(3)
OH
OH
O
H2C
H3C
H2C
CH2
CH2
H3C
CH3
CH3
(4)
a)
f) -NO2
c)
e)
-Br
R=
-OCH3
b) -CH3
d)
-Cl
-C2H5
Scheme 2. Structure of azocalix[4]arene tetraester derivatives (4a–f).

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S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
Fig. 1. 1H-NMR spectra of azocalix[4]arene tetraester derivatives (4a).
selected based on their polarities. The analysis has been mainly
focused on the shift effect of solubility and pH. Effects of substitu-
ents have also been considered. More detailed analysis of the
effects of various solvents and pH change has been performed only
for azocalix[4]arene (4a–f) solutions.
Typical absorption spectra of azocalix[4]arenes (4a–f) in differ-
ent solvents are shown in Fig. 2. It can be seen that the absorption
maxima measured in DMF and DMSO solutions are slightly shifted
to higher wavelengths (red shift) as compared with the corre-
sponding values measured in other solvents. This shift can simply
be explained by the conjugation of substituted azo derivatives in
the calixarene structure.
examined. The kmax values of no compound changed with varying
compound concentration.
These compounds existed in an equilibrium state and its evi-
dence is provided by isosbestic points in the visible spectra of
the compound 4d in different solvents (Fig. 3). These compounds
(4a–f) showed one absorption peak in DMF and DMSO. The equi-
librium of compounds (4a–f) in DMF and DMSO may exist between
tautomeric form and anionic form.
Solvent extraction
Effect of various pH levels and solution concentrations on the
absorption intensities have been studied in a more detailed way.
The absorption spectra of azocalix[4]arenes (4a–f) were recorded
in various solvents at a concentration of ꢄ10ꢁ6 to ꢄ10ꢁ8 M; the
recorded results are summarized in Table 1. The spectra of azoca-
lix[4]arenes were found to exhibit strong solvent dependency,
which did not show regular variation with the polarity of solvents.
It was observed that the absorption spectra of the compounds
in chloroform solution generally differ from the absorption spectra
in all other solvents. The kmax of the compounds shifted consider-
ably in DMF and DMSO (e.g. for compound 4a kmax is 345 nm in
CHCl3, 349 nm in DMF and 350 nm in DMSO; for compound 4c is
332 nm in CHCl3, 336 nm in DMF and 338 nm in DMSO; for com-
pound 4e is 338 nm in CHCl3, 340 nm in DMF and 343 nm in
DMSO).
The UV spectra of azocalix[4]arenes (4a–f) in chloroform solu-
tion showed two bands in the region 300–700 nm. The relatively
small difference in the kmax may have been caused by the polarity
change of the absorbing system due to solvent interactions. Table 1
also shows that the presence of electron-donating or electron-
withdrawing groups has not resulted in any marked increase or
decrease in the kmax in the visible 300–500 nm.
Designation of azocalix[4]arene tetraacetates as receptors is
based on their well-recognized complexation abilities. If these
compounds are immobilized in the cone conformation, they exhibit
high complexation affinity toward selected metal cations. To visu-
alize the complexation phenomenon, we appended tetraacetate
groups in the opposite direction of the upper rim chromophore
groups. This functionalization allows the monitoring of complexa-
tion phenomenon using UV–vis spectroscopy.
In recent years, the various sensors have been reported for
selective and sensitive determination of metals, biomolecules and
organic compounds [37–39]. The present study is focused on inves-
tigating the ionophoric properties of the upper rim functionalized
azocalix[4]arene derivatives (4a–f) toward selected metal cations
(Na+, K+, Sr2+, Ag+, Hg+, Hg2+, Co2+, Ni2+, Cu2+, Zn2+, Cr3+, Al3+
,
La3+). The extraction properties of these compounds (4a–f) have
been evaluated by solvent extraction processes of selected metal
picrates (Table 2 and Fig. 4).
An investigation of the ionic recognition properties of synthe-
sized chemosensors in terms of alkaline (Na+, K+), alkaline-earth
(Sr2+) and transition metal ions (Ag+, Hg+, Hg2+, Co2+, Ni2+, Cu2+
,
Pb2+, Zn2+, Cr3+, Al3+ and La3+) was also conducted. Azo group
(AN@NA) containing molecular receptors have been expected to
exhibit pronounced changes in the course of interaction with tran-
sition metal ions. However, the azocalix[4]arenes (4a–f) showed
The effects of concentration, acidity and alkalinity of these azoc-
alix[4]arene derivatives (4a–f) on their absorption maxima were

S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
183
1,5
1
1,5
Acetonitrile
Acetonitrile
Acetic acid
Chlorof orm
DMF
Acetic acid
Chlorof orm
1
DMF
DMSO
DMSO
Methanol
Methanol
0,5
0
0,5
0
300
350
400
nm
450
500
300
350
400
nm
450
500
a
b
1,5
1
Acetonitrile
Acetic acid
Chloroform
DMF
1.5
Acetonitrile
Acetic acid
Chloroform
DMF
1
0.5
0
DMSO
DMSO
Methanol
Methanol
0,5
0
300
350
400
nm
450
500
300
350
400
nm
450
500
c
d
1,5
1
1,5
1
Acetonitrile
Acetic acid
Chlorof orm
DMF
Acetonitrile
Acetic acid
Chloroform
DMF
DMSO
DMSO
Methanol
Methanol
0,5
0,5
0
0
300
350
400
450
500
300
350
400
450
500
nm
nm
e
f
Fig. 2. Absorption spectra of azocalix[4]arene (4a–f).
Table 1
Influence of solvent on kmax (nm) of azocalix[4]arenes (4a–f).
Ligands
MeCN
344
AcOH
344
CHCl3
345
DMF
DMSO
MeOH
348
MeOH + HCl
MeOH + KOH
4a
349
438
339
435
336
435
338
437
340
334
364
350
435
340
434
338
433
340
433
343
433
358
353
482
335
458
335
441
335
434
338
436
357
349
334
335
4b
4c
4d
4e
4f
334
333
335
338
358
333
332
335
338
354
334
332
336
338
359
335
333
433
334
433
336
434
364
337
673
343
363

184
S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
0,5
0,25
0
0,5
MeOH
MeOH
MeOH / HCl
MeOH / KOH
MeOH / HCl
MeOH / KOH
0,25
0
300
350
400
450
500
300
350
400
450
500
nm
nm
a
b
1,25
1
MeOH
MeOH
1
0,75
0,5
MeOH / HCl
MeOH / HCl
MeOH / KOH
0,75
0,5
0,25
0
MeOH / KOH
0,25
0
300
350
400
nm
450
500
300
350
400
nm
450
500
c
d
1
0,75
0,5
1,25
MeOH
MeOH
1
0,75
0,5
MeOH / HCl
MeOH / KOH
MeOH / HCl
MeOH / KOH
0,25
0
0,25
0
300
350
400
450
500
300
350
400
nm
450
500
nm
e
f
Fig. 3. Effect of acid and base absorption spectra of azocalix[4]arene (4a–f).
Table 2
Extraction of metal picrates with azocalix[4]arene derivatives 4a–f.a
Ligands
Picrate salt extracted (%)
Na+
K+
Sr2+
Ag+
Hg+
Hg2+
Co2+
Ni2+
Cu2+
Pb2+
Zn2+
Cr3+
Al3+
La3+
4a
4b
4c
4d
4e
4f
63.0
63.0
63.0
4.3
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
18.4
13.2
13.6
4.0
4.2
5.0
15.7
16.5
17.0
19.6
18.5
14.4
23.8
26.7
23.4
28.7
28.0
17.7
63.0
63.0
5.0
3.6
3.5
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
63.0
3.2
63.0
63.0
63.0
63.0
63.0
63.0
7.9
6.0
6.0
6.9
6.8
6.5
4.1
3.9
63.0
7.1
6.9
3.4
63.0
63.0
63.0
3.1
63.0
4.0
63.0
63.0
63.0
63.0
4.4
3.7
a
H2O/CHCl3 = 10/10 mL (v/v): [picric acid] = 2 ꢂ 10ꢁ5 M, [ligand] = 1 ꢂ 10ꢁ3 M, [metal nitrate] = 1 ꢂ 10ꢁ2 M; 298 K, 1 h contact time.
a
6
2%.

S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
185
35.0
30.0
25.0
20.0
15.0
10.0
5.0
4a
4b
4c
4d
4e
4f
0.0
Na+
K+ Sr2+ Ag+ Hg+ Hg2+ Co2+ Ni2+ Cu2+ Pb2+ Zn2+ Cr3+ Al3+ La3+
METALS
Fig. 4. Extraction percentages of azocalix[4]arene derivatives (4a–f).
technique provides additional information about structure analysis
and molecular interactions [40–42].
Table 3
The thermo-analytical results of all compounds (4a–f).
In the current study, thermal behavior and thermal stability of
novel azocalix[4]arene derivatives (4a–f) are investigated. The
relationship between thermal stability and substituted groups
was determined. The thermo-analytical results are summarized
in Table 3. The DTA curves show that all compounds melt before
decomposition. Melting points of 4a–f compounds were identified
as 249, 279.20, 220.05, 218.40, 238.73, 166.71 °C, respectively.
These values are consistent with melting points which were mea-
sured using an Electrothermal IA9100 digital melting point
apparatus.
The compounds 4a–c, f decompose in two exothermic stages.
The first stage corresponds to the decomposition of ester groups
and occurred in 260–352 °C temperature range averagely. Reaction
peak temperatures are very close to each other and their average
value is 315.65 °C. The second stage is related to the decomposition
of complete structure and ends with (67.99–73.91%) experimental
mass loss. This value is compatible with average theoretical mass
loss (61.54–73.64%). This reaction starts immediately after the
end of the first decomposition reaction and completes at 658 °C
averagely. The TG and DTA curves of these compounds are given
in Fig. 5.
Compound Ti–Tf (°C) Tpeak (°C) Mass loss (%(exp.)) Mass loss (%(theo.)) Tmelting
4a
4b
4c
4d
260–340 317.93
340–657 572.19
260–346 316.97
346–665 597.75
260–345 314.49
345–653 602.87
260–344 321.38
344–610 539.06
610–666 642.65
260–327 322.25
327–612 559.83
612–802 640.24
260–377 313.22
377–655 526.97
26.58
73.91
26.36
73.61
26.07
73.13
27.55
64.50
8.20
23.34
66.48
9.95
37.02
67.99
26.36
73.64
27.72
72.28
26.52
73.48
26.07
65.43
8.50
22.98
66.56
10.47
38.46
61.54
249.00
279.20
220.05
218.40
4e
4f
238.73
166.71
substantially stronger affinities toward Hg2+ compared to the other
cations.
Thermal behavior
The decomposition of (4d and e) compounds occur in three
stages. The first decomposition stage takes place in (260–344 to
260–327 °C), respectively. Experimental mass loss values are
27.55% (theo: 26.07%) and 23.34% (theo: 22.98%), respectively. In
Thermal analysis plays an important role in studying the struc-
ture and stability of calix[n]arenes. The determination of thermal
stability of calix[n]arenes is a very important in terms of finding
potential application areas. The thermogravimetric analysis
Fig. 5. The TG and DTA curves of (4a–f) compounds.

186
S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
Fig. 6. The TG and DTA curves of (4d and e) compounds.
Conclusions
In summary, the synthesis and characterization of six novel
azocalix[4]arene tetraester derivatives (4a–f) were studied by
means of FT-IR, 1H NMR spectroscopic techniques as well as ele-
mental analysis. These compounds were examined the effect of
varying pH levels and solvent types on the absorption ability of
azocalix[4]arenes substituted with electron-donating and elec-
tron-withdrawing groups. Thermal decomposition of azoca-
lix[4]arene derivatives (4a–f) was investigated by means of
thermogravimetry (TG), differential thermogravimetry (DTG) and
differential thermal analysis (DTA) analyses. In conclusion of the
examination of the extraction we found a selectivity characteristic
of these compounds toward Ag+, Hg+ and Hg2+ cations.
Acknowledgements
The authors are grateful to the Scientific Research Projects
Council of Pamukkale University (PAU BAP, 2012KRM003).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2015.01.087.
Fig. 7. The optimized structure of (4b) molecule.
References
this stage, fragmentation occurred in ester groups. In the second
stage, the decomposition of compound proceeds until it reaches
its halogen-benzene structure residue. Peak temperatures of sec-
ond stage are 539.06 and 559.83 °C, respectively. Final halogen-
benzene structure of the compounds decomposes in the last stage.
The third exothermic peak of (4d) compound is greater than that of
(4e) compound due to the stronger bond between chlorine and
benzene groups. The TG and DTA curves of these compounds are
shown in Fig. 6.
[1] C.D. Gutsche, Calixarenes revisited, in: J.F. Stoddart (Ed.), Monographs in
Supramolecular Chemistry, The Royal Society of Chemistry, Cambridge, 1998.
[2] A. Casnati, R. Ungaro, Z. Asfari, J. Vicens, in: Z. Asfari, J. Harrowfield, J. Vicens
(Eds.), Calixarenes 2001, Kluwer Academic Publishers, Dordrecht, 2001, pp.
365–384.
[3] N. Iki, S. Miyano, Can thiacalixarene surpass calixarene, J. Incl. Phenom.
Macrocycl. Chem. 41 (2001) 99–105.
[4] N. Morohashi, F. Narumi, N. Iki, T. Hattori, S. Miyano, Thiacalixarenes, Chem.
Rev. 106 (2006) 5291–5316.
[5] B. Dhawan, S.-I. Chen, C.D. Gutsche, Calixarenes 19. Studies of the formation of
calixarenes via condensation of p-alkylphenols and formaldehyde, Macromol.
Chem. 188 (1987) 921–950.
[6] Y. Masuda, Y. Zhang, C. Yan, B. Li, Synthesis and characterization of
scandium(III) complexes with p-tertbutylcalix[n]arenes (n = 4,6,8), J. Alloys
Compd. 275 (1998) 872–876.
Optimization studies
[7] W. Aeungmaitrepirom, A. Hagege, Z. Asfari, J. Vicens, M. Leroy, Solvent
extraction of selenate and chromate using a diaminocalix[4]arene, J. Incl.
Phenom. Macrocycl. Chem. 40 (2001) 225–229.
[8] P. Jurecka, P. Vojtisek, K. Novotny, J. Rohovec, I. Lukes, Synthesis,
characterisation and extraction behaviour of calix[4]arene-based phosphonic
acids, J. Chem. Soc., Perkin Trans. 2 (7) (2002) 1370–1377.
[9] C.G. Gibbs, P.K. Sujeeth, J.S. Rogers, G.G. Stanley, M. Krawiec, W.H. Watson, C.D.
Gutsche, Syntheses and conformations of the p-tert-butylcalix[4]arenethiols, J.
Org. Chem. 60 (1995) 8394–8402.
Probable Three-dimensional structure of the (4b) molecule, was
drawn with GaussView 5.0 molecular imaging program [43] and
space settlements of atoms were determined. All theoretical calcu-
lations were performed by Gaussian 09W package program [44].
Showed in Fig. 7 the crystal structure of azocalix[4]arene ester
derivative (4b) exhibited as cone conformation.

S. Elçin et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 142 (2015) 178–187
187
[10] H. Al-Saraierh, D.O. Miller, P.E. Georghiou, Narrow-rim functionalization of
calix[4]arenes via Sonogashira coupling reactions, J. Org. Chem. 70 (2005)
8273–8280.
[11] M.N. Radhakrishnan Nair, G.V. Thomas, M.R. Gopinathan Nair,
Thermogravimetric analysis of PVC/ELNR blends, Polym. Degrad. Stab. 92
(2007) 189–196.
[12] F. Yao, Q. Wu, Y. Lei, W. Guo, Y. Xu, Thermal decomposition kinetics of natural
fibers: activation energy with dynamic thermogravimetric analysis, Polym.
Degrad. Stab. 93 (2008) 90–98.
[28] C.D. Gutsche, M. Iqbal, Para-tert-butylcalix[4]arene, Org. Synth. 68 (1990)
234–237.
[29] C.D. Gutsche, M. Iqbal, D. Stewart, Calixarenes. 18. Synthesis procedures for
para-tert-butylcalix[4]arene, J. Org. Chem. 51 (1986) 742–745.
[30] Y. Morita, T. Agawa, E. Nomura, H.J. Taniguchi, Syntheses and NMR behavior of
calix[4]quinone and calix[4]hydroquinone, Org. Chem. 57 (1992) 3658–3662.
[31] Ö.Ö. Karakußs, H. Deligöz, Azocalixarenes. 7: synthesis and study of the
absorption properties of novel mono-azo substituted chromogenic
calix[4]arenes, Turk. J. Chem. 35 (2011) 87–98.
[13] S. Vyazovkin, Thermal analysis, Anal. Chem. 80 (2008) 4301–4316.
[14] M. Lazzarotto, F.F. Nachtigall, E. Schnitzler, E.E. Castellano, Thermo gravimetric
analysis of supramolecular complexes of p-tert-butylcalix[6]arene and
ammonium cations: crystal structure of diethylammonium complex,
Thermochim. Acta 429 (2005) 111–117.
[15] J. Shatz, F. Schildbach, A. Lentz, S. Rastatter, Thermal gravimetry, mass
spectrometry and solid-state 13C NMR spectroscopy-simple and efficient
methods to characterize the inclusion behaviour of p-tert-butylcalix[n]arenes,
J. Chem. Soc., Perkin Trans. 2 (1) (1998) 75–78.
[32] Ö.Ö. Karakusß, H. Deligöz, Azocalixarenes. 8: synthesis and investigation of the
absorption spectra of di-substituted azocalix[4]arenes containing
chromogenic groups, J. Incl. Phenom. Macrocycl. Chem. 61 (2008) 289–296.
[33] F. Arnaud-Neu, E.M. Collins, M. Deasy, G. Ferguson, S.J. Haris, B. Kaitner, A.J.
Lough, M.A. McKervey, E. Marques, B.L. Ruhl, M.J. Schwing-Weill, E.M. Sewardt,
Synthesis, X-ray crystal structures, and cation-binding properties of alkyl
calixaryl esters and ketones, a new family of macrocyclic molecular receptors,
J. Am. Chem. Soc. 111 (1989) 8681–8691.
[34] K. Lang, P. Proskova, J. Kroupa, J. Moravek, I. Stibor, M. Pojarova, P. Lhotak, The
synthesis and complexation of novel azosubstituted calix[4]arenes and
thiacalix[4]arenes, Dyes Pigm. 77 (2008) 646–652.
[16] a) W. Feng, L.H. Yuan, S.Y. Zheng, G.L. Huang, J.L. Qiao, Y. Zhou, The effect of p-
tert-butylcalix[n]arene on
c-radiation degradation of polypropylene, Radiat.
Phys. Chem. 57 (2000) 425–429.
[35] H. Deligöz, M. Yılmaz, Selective complexation of Na+ by polymeric
calix[4]arene tetraesters, J. Polym. Sci. A Polym. Chem. 33 (1995) 2851–2853.
[36] H. Deligöz, Ö.Ö. Karakusß, G.K. Çılgı, Structural analysis of calix[n]arene-iron(III)
complexes (n = 4, 6, 8) and thermal decomposition of the parent
calix[n]arenes, J. Coord. Chem. 60 (2007) 73–83.
[17] T. Zaharescu, S. Jipa, R. Setnescu, C. Santos, B. Gigante, I. Mihalcea, C. Podina,
L.M. Gorghiu, Thermal stability of additivated isotactic polypropylene, Polym.
Bull. 49 (2002) 289–296.
[18] S. Jipa, T. Zaharescu, R. Setnescu, T. Setnescu, M. Dumitru, L.M. Gorghiu, I.
Mihalcea, M. Bumbac, Effect of calixarenes on thermal stability of
polyethylenes, Polym. Degrad. Stab. 80 (2003) 203–208.
[37] H. Karimi-Maleh, A.L. Sanati, V.K. Gupta, M. Yoosefian, M. Asif, A. Bahari, A
voltammetric biosensor based on ionic liquid/NiO nanoparticle modified
carbon paste electrode for the determination of nicotinamide adenine
dinucleotide (NADH), Sens. Actuators, B 204 (2014) 647–654.
[19] J. Pospisil, Mechanistic action of phenolic antioxidants in polymers-a review,
Polym. Degrad. Stab. 20 (1988) 181–202.
[20] J. Zeller, S. Koenig, U. Radius, Synthesis and structural analysis of
calix[4]arene-supported iron(III) complexes, Inorg. Chim. Acta 357 (2004)
1813–1821.
[21] E. Utzig, O. Pietraszkiewicz, M. Pietraszkiewicz, Thermal analysis of calix[4]
resorcinarene complexes with secondary and tertiary amines, J. Therm. Anal.
Calorim. 78 (2004) 973–980.
[38] H. Karimi-Maleh, F. Tahernejad-Javazmi, A.A. Ensafi, R. Moradi, S. Mallakpour,
H. Beitollahi, A high sensitive biosensor based on FePt/CNTs nanocomposite/N-
(4-hydroxyphenyl)-3,5-dinitrobenzamide modified carbon paste electrode for
simultaneous determination of glutathione and piroxicam, Biosens.
Bioelectron. 60 (2014) 1–7.
[39] M. Najafi, M.A. Khalilzadeh, H. Karimi-Maleh,
A new strategy for
[22] M. Ashram, N. Al-Ghezawi, I.A. Awaad, G. Asman, Synthesis and physical
determination of bisphenol A in the presence of Sudan I using a ZnO/CNTs/
ionic liquid paste electrode in food samples, Food Chem. 158 (2014) 125–131.
studies of polycarbonate of p-tert-butylcalix[4]arene:
a highly selective
receptor for Na+, Turk. J. Chem. 33 (2009) 647–656.
[40] H. Deligöz, Ö.Ö. Karakusß, G.K. Çılgı, H. Cetisßli, A Study on the Thermal
[23] C. Liu, F. Luo, Y. Bi, W. Liao, H. Zhang,
A
zinc/p-sulfonatocalix[4]arene/
Behaviours of Parent Calix[4]arenes and Some Azocalix[4]arene Derivatives,
Thermochim. Acta 426 (2005) 33–38.
phenanthroline supramolecular compound with novel (H2O)10 water clusters,
J. Mol. Struct. 888 (2008) 313–317.
[24] H. Deligöz, Azocalixarenes: synthesis, characterization, complexation,
extraction, absorption properties and thermal behaviours, J. Incl. Phenom.
Macrocycl. Chem. 55 (2006) 197–218.
[25] H. Deligöz, N. Ercan, The synthesis of some new derivatives of calix[4]arene
containing azo groups, Tedrahedron 58 (2002) 2881–2884.
[26] I. Sßener, F. Karcı, E. Kılıç, H. Deligöz, Azocalixarenes. 3: synthesis and
investigation of the absorption spectra of hetarylazo disperse dyes derived
from calix[4]arene, Dyes Pigm. 62 (2004) 141–148.
[41] Ö.Ö. Karakußs, G.K. Çılgı, H. Deligöz, Thermal analysis of two series mono- and
di-azocalix[4]arene derivatives, J. Therm. Anal. Calorim. 105 (2011) 341–347.
[42] S. Elçin, H. Deligoz, Di-substituted azocalix[4]arenes containing chromogenic
groups: Synthesis, characterization, extraction, and thermal behavior,
Tetrahedron 69 (2013) 6832–6838.
[43] R. Dennington, T. Keith, J. Millam, GaussView, Version 5, Semichem Inc.,
Shawnee Mission KS, USA, 2009.
[44] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman,
et al., Gaussian 09, revision A.1., Gaussian Inc., Wallingford CT, 2009.
[27] M.S. Ak, H. Deligöz, Azocalixarenes. 6: synthesis, complexation, extraction and
thermal behaviour of four new azocalix[4]arenes, J. Incl. Phenom. Macrocycl.
Chem. 59 (2007) 115–123.