A novel triazine?bearing calix[4]arene: Design, synthesis and gas sensing affinity for volatile organic compounds

Article abstract of DOI:10.1016/j.tet.2019.03.027

A novel triazine-calix[4]arene conjugate was designed and synthesized with the aim to study gas sensing against volatile organic compounds (VOCs) such as dichloromethane, chloroform and carbon tetrachloride. This novel compound was fully characterized by

Full text of DOI:10.1016/j.tet.2019.03.027

Accepted Manuscript
A novel triazine‒bearing calix[4]arene: Design, synthesis and gas sensing affinity for
volatile organic compounds
Erkan Halay, Yaser Acikbas, Rifat Capan, Selahattin Bozkurt, Matem Erdogan,
Ridvan Unal
PII:
S0040-4020(19)30312-6
https://doi.org/10.1016/j.tet.2019.03.027
TET 30213
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 21 November 2018
Revised Date: 4 March 2019
Accepted Date: 15 March 2019
Please cite this article as: Halay E, Acikbas Y, Capan R, Bozkurt S, Erdogan M, Unal R, A novel
triazine‒bearing calix[4]arene: Design, synthesis and gas sensing affinity for volatile organic
compounds, Tetrahedron (2019), doi: https://doi.org/10.1016/j.tet.2019.03.027.
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ACCEPTED MANUSCRIPT
A novel triazine‒bearing calix[4]arene: Design, synthesis and gas sensing affinity for
volatile organic compounds
a,b,
c
d
d
Erkan Halay
, Yaser Acikbas , Rifat Capan , Selahattin Bozkurt a,e, Matem Erdogan ,
Ridvan Unal e
a
Department of Chemistry, Scientific Analysis Technological Application and Research
Center, Usak University, Usak, 64200, Turkey
b Department of Chemistry and Chemical Processing Technologies, Banaz Vocational School,
Usak University, Usak, 64500, Turkey
c
Department of Materials Science and Nanotechnology, Faculty of Engineering, Usak
University, Usak, 64200, Turkey
d Department of Physics, Faculty of Science, University of Balıkesir, Balıkesir, 10145, Turkey
e Department of Medical Laboratory Techniques, Vocational School of Health Services, Usak
University, Usak, 64200, Turkey
Corresponding author. Department of Chemistry, Scientific Analysis Technological
Application and Research Center, Usak University, 1 Eylul Campus, Usak, 64200, Turkey.
E-mail address: erkan.halay@usak.edu.tr (E. Halay). Phone: +90 276 221 21 21 / 4852

ACCEPTED MANUSCRIPT
A novel triazine‒bearing calix[4]arene: Design, synthesis and gas sensing affinity for
volatile organic compounds
GRAPHICAL ABSTRACT
1

ACCEPTED MANUSCRIPT
A novel triazine‒bearing calix[4]arene: Design, synthesis and gas sensing affinity for
volatile organic compounds
ABSTRACT
A novel triazine-calix[4]arene conjugate was designed and synthesized with the aim to study
gas sensing against volatile organic compounds (VOCs) such as dichloromethane, chloroform
and carbon tetrachloride. This novel compound was fully characterized by spectroscopic
techniques such as FTIR, 1H and 13C NMR along with HRMS and BET analysis. The triazine
based calix[4]arene organic materials were fabricated onto quartz glasses and quartz crystal
substrates to form a thin film chemical sensor element by using Langmuir-Blodgett (LB)
technique. Quartz Crystal Microbalance, UV-Visible Spectroscopy, Atomic Force
Microscopy and Scanning Electron Microscopy techniques were employed to characterize all
these LB thin film layers. Fick’s Equations were used for analyzing the swelling process of
LB thin film sensor and diffusion coefficient values of organic vapours for swelling were
obtained. The initial experiments have revealed that new triazine appended calix[4]arene
derivative exhibited an effective chemical gas sensor characteristic with a large response to
dichloromethane vapour.
Keywords: Calix[4]arene, Triazine, Chemical sensors, Volatile organic compounds, Quartz
crystal microbalance.
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1. Introduction
In recent years, supramolecular network designing and synthesis has served a very
important purpose as it has been involved in various chemical and biological processes such
as pollution of environment, enzyme-substrate binding, cell migration and organic solute
transport [1]. In this sense, especially for the removal of harmful pollutants from the
environment, the effective design and synthesis of novel macrocyclic systems with the ability
of selective molecular recognition has become an important goal in supramolecular host-guest
chemistry [2−4]. Non-covalent interactions (NCI) occupying a prominent place in host-guest
chemistry such as hydrogen bonding, multipole-multipole interactions,
and anion- interactions are well-established essentials of chemical sensing in the
construction and description of these supramolecular architectures [5].
π-stacking, cation-π
π
In this field, calix[n]arenes representing a versatile and powerful class of macrocycle
family have been extensively in use in the formation of supramolecular structures thanks to
their availability, bowl shaped unique constitution and easy functionalization at lower and
upper rims [6−8]. Among all kinds of calixarenes, calix[4]arene have been the most
extensively studied one as host molecules because of being the most easily modified and
arrangeable skeleton with the various reactions such as sulphonation, nitration, etherification,
esterification, formylation and also showing the best complexation properties owing to its
macrocyclic cavity and electronic features of substitutions [3, 9−12]. Thus, of course, besides
their primary use as a sensor in gas adsorption, ionic and molecular recognition,
calix[4]arenes have found numerous applications in diverse areas such as drug carriers,
transmembrane transport, fabrication of metal-organic frameworks, liquid crystals, stationary
phases and organic catalysts [13−22].
On the other hand, 1,3,5-triazine, also known as s-triazine, is another remarkable
molecule for supramolecular chemistry with six membered ring containing three nitrogens
2
owing to its high tendency to give reactions particularly substitution (SN ) reactions [23, 24].
Many attractive structures can be given as an example consisting of acceptor s-triazine core
containing 1,2,3-triazole, ferrocene, thiophene, bisphenylaminobenzene, tetrathiafulvalene,
2-pyridyl and styrylbenzene subunits as donors [25, 26]. In this sense, within the appropriate
choice of donor subunits and use of melamine derivatized triazine main frame, this molecule
has been a challenging subject for materials chemistry in terms of its use as functional
materials (e.g. luminescent active compounds, gas sensors, phosphorescent OLEDs,
absorption of surfactants and nanoporous membranes for desalination) [25].
However, to the best of our knowledge and literature survey, calix[4]arene molecules
along with s-triazine have been rarely studied on binding solvent molecules although they
have exhibited excellent complexation abilities for small organic molecules together. So we
have considered that triazine based calix[4]arene derivative can be effective for sensing
organic vapours such as chloroform, dichloromethane and carbon tetrachloride.
From the perspective of such information, in this study, we would like to report the
design and the synthesis of a new calix[4]arene derivative based on aniline bearing s-triazine
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conjugate system which was benefitted from its interactions (halogen-heteroatom, halogen-
halogen, halogen-π) with solvent molecules used in the experiments. After synthesis and
preparation of thin films utilizing LB thin film method, the related films were exposed to
some saturated VOC vapours in order to investigate the swelling period in chemical sensor
experiments [27]. These volatile organic compounds were easily detected by means of the
appropriate cavity and host–guest inclusion interaction [28]. With the help of Quartz Crystal
Microbalance (QCM) measurement system, variations in the resonance frequency were
monitored concurrently with swelling when organic vapour was introduced into a gas cell
[29]. The QCM results were fitted by using Early-time Fick’s law of diffusion equations. The
results revealed that our calix[4]arene LB film material has been an excellent sensor for
dichloromethane with high order sensitivity and selectivity, and deserve the chance for further
applications in the development of hazardous organic vapour sensing devices.
2. Results and discussion
2.1. Design, synthesis and characterization of V
Mono-substituted triazine derivative III was prepared through the condensation of
cyanuric chloride I and aniline II at 0°C under N2 atmosphere (Scheme 1). For the synthesis
of a triazine-calix[4]arene conjugate bearing two aniline units at the calixarene lower rim,
calixarene derivative IV [30], blocked in a cone conformation by the introduction of tert-butyl
groups at the upper rim, was selected as the starting main skeleton. Subsequently, another
substitution reaction between III and IV in acetone at reflux gave the target compound V with
80% yield (Scheme 1).
Scheme 1. Synthesis of the target triazine-calix sensor candidate V. Reagents and conditions:
(i) acetone, 0°C; (ii) K2CO3, acetone, III, reflux.
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The target novel conjugate was characterized by IR, 1H and 13C NMR spectroscopy and
HRMS analysis that provided compatible data for the expected molecular structure.
Of course, in the pioneering studies conducted in the past, structural determination of
calixarene derivatives had left a somewhat clouded picture behind because of the
regiochemical preferences [31]. After that, thanks to many synthetic studies trying to clarify
this, it was elucidated that a given system could be fixed in preferred conformations by using
appropriate reagent and reaction conditions [32−34]. In this regard, the solvent and the base
themselves have some direct influence on the formation of isomers in the synthesis of
calixarene derivatives [35−38]. This can be explained by the ion-pair tightness. When
considered from this point of view, due to the strong metal template effect of K+ keeping two
OH groups in their original position through the K+−phenoxide interaction, phenoxide anions
having K+ as a counteraction is not reactive. Thus, the conformation is apparently
immobilized in the di-O-substituted stage [35]. In other words, even in the presence of excess
III, the reaction stops in the di-O-substitution stage and the solvent (acetone) along with the
base (K2CO3) we used in our work lead to the formation of di-O-substituted isomer (distal-
cone calix derivative). Moreover, as the most probative, observing the sufficient number of 1H
NMR signals described in detailed below and coupling constants (for bridging methylene
groups, J =13.6 Hz) has proved the substitution pattern.
1
In the H NMR spectrum of compound V, singlets were observed for both tert-butyl
groups of calix ring at 1.32 and 1.06 ppm, while the methylene bridge protons of this cone
appeared as a pair of doublets located at 3.93 ppm and 3.41 ppm. Besides, the presence of
resonances relative to the remaining calix protons along with aromatic protons of aniline units
between 6.86-7.20 ppm evidenced the formation of the desired triazine-calix[4]arene
13
conjugate (see Supplementary data). In the C NMR spectrum, methyl carbons of tert-butyl
groups in calix ring were observed at 31.2 and 31.6 ppm, while triazine ring carbons appeared
at 171.3, 164.8 and 150.2 ppm (see Supplementary data). Moreover, the target compound V
was further characterized by the presence of etheric C–O–C stretching band at 1169 cm−1 in
the FT-IR spectrum (see Supplementary data).
2.2. Isotherm Graph and Transfer Ratio
Fig. 1 indicates the surface arrangement of triazine based calixarene monolayer (π-A
isotherm graph) at the air-water interface at a value of pH 6. This graph was used to analyze
the surface behavior and to calculate the area per molecule, am , for a floating monolayer at
the air-water interface given by [39]:
AMw
am =
(1)
cNAV
where A is the limited area of the molecules compressed by the moveable barriers (taken from
Fig. 1), Mw (1058.14 g mol-1) is the molecular weight of the triazine based calixarene, c (0.54
mg mL-1) is the solution concentration, V (400 µL) is the solution volume spread over the air-
water interface and NA (6.023x1023) is the Avogadro’s number. Using Fig. 1 and a method of
extrapolating the linear region of isotherm to zero pressure, am was estimated to be 1.06 nm2.
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A suitable surface pressure from Fig. 1 for the triazine based calixarene LB film deposition
process was found to be 22 mN m−1.
Fig. 1. Isotherm graph of the triazine based calixarene monolayer.
The deposition process of each LB film monolayer can be monitored to calculate the transfer
ratio value (TR) given by:
TR = A A2
(2)
1
where A is given by reduced area of the monolayer on the water surface during the
1
deposition process and A2 is the area of coated LB film on the solid substrate. Decreasing of
the monolayer area on the water surface for each bilayer should be equal to the transfer
process of monolayer onto the solid substrate. Using Equation 2 TR ratio is calculated to be
96% for the triazine based calixarene and this result confirms that a uniform and reproducible
LB film deposition was occurred during the LB film fabrication. It is also consistent with
previous studies where TR value was calculated over 94% for the calixarene LB thin films
[28, 40]. As a result, monolayers at air-water interface based on calixarene are suitable
coating materials for LB films and can easily transfer onto quartz glasses and quartz crystal
with a high transfer ratio.
2.3. SEM, AFM and BET results
Scanning Electron Microscopy (SEM) observations were made in a FEI Quanta 650
Field Emission model scanning electron microscope operated at 5 kV. SEM images were used
for a comparison of SEM image of the bare quartz glass substrate (Fig. 2a) with the triazine
based calixarene LB film (Fig. 2b) suggesting that a matrix of the triazine based calixarene
molecules have been formed.
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Fig. 2. SEM images of (a) the bare quartz glass substrate, (b) triazine based calixarene coated
quartz glass substrate.
The surface investigations were conducted by using a Quesant-350 Atomic Force
Microscope (AFM). The microscope was worked in contact mode and the typical force
constant of the cantilever was used as 0.15 Nm-1. The scan rate was applied as 1 Hz to
investigate the thin film surface morphology. The 3D and 2D AFM images of LB film of the
triazine based calixarene surfaces are presented in Figs. 3a–b, respectively. 2D AFM image
(500 nmx500 nm) and the cross section of an calixarene thin film image were obtained by
selecting the horizontal section as profile. Fig. 3b indicates the values of Z-axis (Red cursor:
82.764 nm and Blue cursor: 89.033 nm) on X and Y scale in nm. It can be said that the
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triazine based calixarene LB films on bare glass surface are uniform, close-packed, and
homogeneous. AFM image shows that the triazine based calixarene LB film has a grainy
form. This may be an advantage for gas sensing applications due to its high surface to volume
ratio [41].
Fig. 3. a) 3D AFM image of the triazine based calixarene coated glass b) 2D AFM image (500x500
nm2) and the cross section of the triazine based calixarene coated glass.
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The raw data provided by the BET analyzer instrument is in the form of a table indicating the
volume of gas (nitrogen) introduced per mass of sample (cc/g), along with the relative
ꢀ
pressure ( ꢂ ) during analysis. Also, the values of the adsorbed monolayer volume (ꢃ ) and
ꢄ
ꢀꢁ
the specific surface area (S) can be calculated by using Equation 4 and Equation 5. Table 1
shows some data for the triazine based calixarene sample.
Table 1. Pressure and volume data from BET analysis of the triazine based calixarene.
Volume
Adsorbed
Monolayer
Vm (cc/g)
Relative
pressure
(p/p0)
Volume
Adsorbed
Va (cc/g )
BET Surface
Area (S)
(m2/g)
1
Organic
material
ꢀꢁ
ꢃ ꢊ ꢂ − 1)
ꢅ
ꢀ
0.0453
0.0903
0.1361
0.1821
0.2288
1.611
1.914
2.152
2.366
2.508
0.0294
0.0518
0.0731
0.0941
0.1183
2.0651
2.0269
2.0515
2.0763
2.0339
326.5984
320.5612
324.4382
328.3669
322.6164
The triazine
based
calixarene V
The generalized BET equation for gas adsorption can be described as follows [42]:
ꢆ ꢈ ꢉ
ꢇ
ꢃ =
ꢅ
(3)
ꢐ
ꢊ
)
ꢊ
)
ꢉ ꢌꢉ [ꢍꢎ ꢈꢌꢍ ꢏ ꢑ]
ꢋ
ꢐ
ꢋ
Here ꢃ is the adsorbed volume of gas, ꢃ is the adsorbed monolayer volume, ꢀ is the
ꢅ
ꢄ
equilibrium gas pressure, ꢀꢁ is the saturation pressure and C is the BET constant (61.47). This
ꢀ
equation can be rearranged as a linear function of ꢂ as follows:
ꢀꢁ
ꢐ
ꢐ
ꢐ
ꢋ
ꢊ
ꢐ
ꢋ
)
ꢌꢍꢑ[ꢏ
ꢈꢌꢍ ꢎꢍꢑ]
ꢃꢒ = ꢆ ꢏ
(4)
(5)
ꢓ
ꢈ
The specific surface area (S, surface area per unit mass) can be found by the equation:
ꢆ
ꢕ ꢖ
ꢇ
ꢔ = ꢗꢗꢘꢁꢁ ꢄ
The constant N is Avogadro’s number, A is the cross-sectional surface area of a single
adsorbed gas molecule (0.162 nm2 for nitrogen), m is the mass of nanomaterials used in the
measurement (0.2753 g) and 22400 represents the Standard Temperature and Pressure (STP)
volume of one mole of gas. This surface area (S) is given in units of area/mass (m2/g).
Fig. 4 and the inset in Fig. 4 show plotted data from Table 1. The data in Table 1 can be used
to calculate the specific surface area of the triazine based calixarene measured according to
Equations 4 and 5. For the data shown in Table 1 and Fig. 4, the triazine based calixarene
exhibits a BET surface area (the specific surface area) of about 324.51 ± 3.09 m2/g. In the
literature, similar analyses were carried out by Alahmadi et al. for the systematic and
comparative study of the MCM-41 mesoporous silica functionalized p-sulfonatocalix[4]arene
(C4S) and p-tert-butyl-calix[4]arene (PC4). In the relevant study, the BET surface areas of
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MC4S and MPC4 were obtained as 452 m2/g and 339 m2/g, respectively [43]. In this context,
it could be said that the BET value of our calix derivative V has been compatible for a porous
structure.
Fig. 4. BET plot of the triazine based calixarene. Inset: The adsorbed volume of nitrogen plotted as a
function of relative pressure as measured during BET surface analysis of the triazine based calixarene.
2.4. UV-Visible results
Fig. 5 shows the UV-Vis absorption spectrum of calix[4]arene solution in chloroform
along with UV-Vis absorption spectrum of various layers of calix[4]arene LB films fabricated
onto a quartz glass substrate. The one of calix[4]arene LB film spectra exhibited an absorption
band at 250 nm which coherence with the spectrum of chloroform solution (at 250 nm).
Similar result was obtained for the UV-Vis spectra of 5,27-(Dipropylmorpholinoacetamido)-
26,28-dihydroxycalix[4]arene LB films exhibited at 253 nm [28]. The inset in Fig. 5 also
shows a plot of the absorption intensity at 250 nm relative to the number of layers of LB films
and reveals almost a linear increase in the absorption intensity with the number layers. The
linear relationship between absorbance and number of layers suggests that a regular
deposition of the triazine based calixarene monolayer takes place, resulting in fairly uniform
LB films. This means that similar amount of the triazine based calixarene is transferred during
each LB deposition.
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Fig. 5. UV-Vis spectra of the triazine based calixarene in a chloroform solution and the triazine based
calixarene LB film on the quartz glass substrate. Inset: linear increase of absorbance
as a function of layer numbers.
2.5. QCM results
QCM measurement technique allows us to study the mass change of quartz crystal
surface deposited LB thin film as a function of resonance frequency. In here, a mass change
per unit area ( ∆m ) versus the resonance frequency change ( ∆f ) on the triazine based
calixarene LB film layer/multilayer quartz crystal is calculated from the Sauerbrey equation
[44]. Fig. 6 shows the deposition of triazine based calixarene LB films onto a quartz crystal
substrate for ten layers. It is clear that number of LB film layers is increased the resonance
frequency shift is also increased. This linearity between ∆f and N shows that a regular
deposition of the triazine based calixarene onto the QCM substrate occurred adequately and
equal mass of the triazine based calixarene is deposited per unit area. The frequency shifts of
44.91 Hz/per layer for the triazine based calixarene LB films are determined from the slope of
Fig. 5. The amount of deposited mass on the quartz crystal substrate per bilayer is estimated
as 692.76 ng (2.61 ng mm-2) for the triazine based calixarene LB film using Fig. 6 and the
Sauerbrey equation.
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Fig. 6. The frequency shifts of the triazine based calixarene LB film against the number of layers.
2.6. Gas sensing properties of the triazine based calixarene
To investigate the chemical sensor properties of the triazine based calixarene LB film
for hazardous organic vapour, the kinetic response of calixarene thin film sensor to
dichloromethane, chloroform and carbon tetrachloride vapours is recorded by measuring the
frequency changes as a function of time. The triazine based calixarene LB film is periodically
exposed to the organic vapours for 2 minutes, followed by injection of dry air for a further 2
minutes period. Fig. 7 shows the kinetic response of triazine based calixarene LB film to the
saturated vapours.
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Fig. 7. The frequency change of the triazine based calixarene LB thin film against
various chlorinated aliphatic hydrocarbons.
The QCM kinetic study was carried out by injection of dry air and organic vapour for
2 minutes, periodically. To ensure the response stability of triazine based calixarene LB film
sensor, at the beginning film was exposed to dry air for 120 seconds and there was certainly
no change at all seen in Fig. 7. When the triazine based calixarene film was exposed to
saturated organic vapours, surface adsorption effect took place and a sudden increase can be
seen. After the vapour molecules start diffusing into the triazine based calixarene film,
frequency begins to change downward exponentially. For the recovery of film, dry air was
introduced into the cell at the time of 240 s. Desorption of vapour molecules start between
240 and 244 s, and frequency values instantaneously return to initial values. These surface
adsorption, diffusion and desorption processes refer to reproducibility, reversibility and
sensitivity of the triazine based calixarene LB thin films. The triazine based calixarene LB
thin film sensor is remarkably more selective and sensitive to dichloromethane vapour than
other organic vapours. QCM kinetic measurements were repeated three times to ensure
reproducibility of the sensor response. Table S1 shows the frequency change ( ∆f ), response
and recovery times of the triazine based calixarene LB film against organic vapours
mentioned above. Calculations have been made for each organic vapour and are shown in
Table S1. Also, the reproducibility of triazine based calixarene LB thin film sensor for
dichloromethane vapour with different concentration values is shown in Fig. S1. When the
concentration of vapour percentage increases, the frequency shifts increases proportionately.
The limit of detection (LOD) of the calixarene LB film sensor was calculated by the
measured sensor sensitivity (Hz / ppm). LOD was defined by [28]:
LOD=3σ/S
(6)
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where, σ is the noise level of the fabricated QCM sensor, and S is the sensitivity to a
specific analyte of the sensor. The sensitivity of LB film sensor was obtained from the
frequency shift curve when exposed to dichloromethane in Fig. S4. The approximate value of
the curve was obtained from this figure. In this study, the resonance frequency was recorded
in air for use as the absolute frequency of the QCM system, and the frequency response was
stable within ± 1 Hz over a period of 25-40 min. Therefore, the frequency noise was estimated
at 1 Hz. The sensitivity of the calixarene QCM sensor to dichloromethane vapor was obtained
as 5.99 Hz ppm-1. Also, the detection limit performance of the calixarene QCM sensor to
dichloromethane vapor was calculated as 0.50 ppm, at room temperature.
Besides, the experimental data of Fig. 7 were analysed (see Supplementary data) using
the Early-time Fick’s law of diffusion by following Fick’s second equation which was
reviewed by Crank [45]:
t1
(7)






∆ft
D
d2
2
= 4
∆f∞
π
where
d
is the thickness of the thin film,
D
is the diffusion coefficient, ∆ft and ∆f∞ are the
normalized frequency shift at any time, t and saturation point in ∆f , respectively. Fig. S2
displays the normalized frequency shift against swelling time. The normalized ∆f values
[∆ft ∆f∞ ] are plotted in Fig. S3 for the square root of swelling time according to Equation 7.
The slopes of the linear relations in Fig. S3 found the diffusion coefficients,
swelling of the triazine based calix[4]arene film.
D
for the
As shown in Fig. 7, the resonance frequency change ( ∆f ) of triazine based
calix[4]arene thin film sensor for the organic vapours are observed as dichloromethane >
chloroform > carbon tetrachloride. The values of resonance frequency change are found to be
115 Hz, 57 Hz and 14 Hz for dichloromethane, chloroform and carbon tetrachloride,
respectively. Also, the values of diffusion coefficient are found to be 12.51x10-15 cm2s-1,
6.61x10-15 cm2s-1 and 1.53x10-15 cm2s-1 for dichloromethane, chloroform and carbon
tetrachloride, respectively. Similar studies were carried out for the kinetic vapour sensing
measurements of calix[4]resorcinarene LB films using ellipsometry, QCM, and SPR
techniques. The results were explained by capillary condensation of VOC vapours in the
nanoporous matrix of the calixarene LB films which result from swelling of the LB films due
to surface adsorption effect [46]. The calix[4]resorcinarene thin film was exposed to several
organic vapours and displayed a fast and reversible response. These results were also
explained that the host-guest interaction between the cavity molecules and the organic vapour
molecules [47, 48]. Also, the kinetic responses of LB films of calix[4]arenes containing
p-tert-butyl groups and without p-tert-butyl groups for chloroform, benzene, toluene, and
ethanol vapours were investigated as a function of time by using QCM technique. The results
showed that after attaching tert-butyl groups onto the calix[4]arene structure, the response of
LB film to chloroform vapour increased [13]. In addition to other gases, similarly, in our
present work we investigated the sensitivity of our triazine based compounds against
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chloroform vapour by using QCM technique. We observed that the response of our LB thin
film chemical sensor against dichloromethane was almost two times greater than for
chloroform. This result could be explained with the substituted OH groups of calix[4]arene at
1,3-position by aniline bearing triazine units. Thanks to introducing of these units to the
calix[4]arene skeleton, while the cavity has been expanding, the penetration of organic
vapours has been greater.
The interaction between calix[4]arene LB thin film and relevant harmful organic
vapours is considered to be a physical absorption via hydrogen bonding or dipole/dipole
interaction [49]. The high values of resonance frequency change and diffusion coefficient
obtained for dichloromethane vapour are compared with chloroform and carbon tetrachloride
vapours. The inference which may be illuminated by the organic vapoursʹ high and low dipole
moment values was previously presented in the literature [27, 50]. The order between dipole
moment values and diffusion coefficient of organic vapours is similar (Table S1). This can be
also explained with the fact that the molar volume of dichloromethane (64.10 cm3 mol-1) is
lower than chloroform (80.70 cm3 mol-1) and carbon tetrachloride (97.10 cm3 mol-1). In this
way, the penetration of chloroform and carbon tetrachloride molecules into the calix[4]arene
LB thin films is slower and difficult while dichloromethane molecules can easily diffuse into
the related films.
3. Conclusions
A new calix[4]arene derivative with triazine π-conjugate system has been engineered to
perform chemical sensing. For this purpose, two mono-substituted triazine rings bearing
aniline group have been tethered to the calix[4]arene scaffold. The target triazine-
calix[4]arene conjugate has been obtained in good yield and the exact structure of this new
1
13
compound was established by FT-IR, H-, C-NMR and HRMS analysis. The triazine based
calix[4]arene LB film transferred onto the substrates with a reasonable high transfer ratio
value of ~96%. A linear relationship between absorbance/resonance frequency and the
triazine based calix[4]arene LB film layers indicates that the triazine based calix[4]arene
molecule was orderly deposited as equal mass per unit area onto quartz glass and quartz
crystal substrates for UV-Vis and QCM results, respectively. The frequency shift per layer
and the deposited mass onto a quartz crystal were found to be 44.91 Hz per layer and 692.76
ng per layer (2.61 ng mm−2), respectively. The triazine based calix[4]arene LB films were
tested for their hazardous vapour sensing properties using QCM technique. According to the
obtained results from sensing studies, it can be concluded that the triazine based calix[4]arene
LB film sensor exhibited an effective chemical gas sensor characteristic with a large response
to various chlorinated aliphatic hydrocarbons vapours (dichloromethane, chloroform and
carbon tetrachloride). In conclusion, it was anticipated that good reorganization of units
around calixarene skeleton along with H-bonding ability,
π-π stacking and so host-guest
interactions have played an important role in vapour sensing process.
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4. Experimental
4.1. General
All commercially available (Merck and Sigma Aldrich) reagents and solvents were used
as purchased. All experiments were performed under an atmosphere of dry nitrogen. While
the reactions were monitored by thin layer chromatography (TLC) applied on silica gel 60
F254 pre-coated aluminum plates visualizing the spots under UV light at 254 and 366 nm, flash
column chromatography was carried out using silica gel 60 (230 × 400 mesh) for purification
of crude products. Melting points were measured on Gallenkamp electrothermal WA11373
melting point instrument in a sealed capillary glass tube without correction. FT-IR spectra
were obtained on a Perkin‒Elmer Spectrum Two spectrophotometer equipped with ATR
sampling accessory. The 1H (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a
Varian VNMRJ 400 spectrometer and reported in units of part per million (ppm) using
tetramethylsilane (TMS) as internal standard (δ = 0 ppm). High resolution mass spectrum
(HRMS) was recorded under electrospray ionization (ESI) conditions in positive ion mode
using an Agilent 6230 TOF-MS instrument. The specific surface area was determined by the
Brunauer-Emmett-Teller (BET) method using a Costech Sorptometer KELVIN-1042
instrument. Sample was degassed at 100 °C during 2 hours in helium flow (carrier gas) before
nitrogen adsorption.
4.2. Synthesis of the compounds III and V
4.2.1. 2-Phenylamino-4,6-dichloro-s-triazine (III)
Intermediate III was prepared in accordance with the method described in the
literature [51]. To compound I (1.0 g, 5.42 mmol) dissolved in acetone (30 mL) at 0°C,
aniline (1 eq) in 20 mL acetone was added dropwise and allowed stirring at 0°C for 2 hours
along with TLC monitoring. After full conversion, the reaction was neutralized with 10 mL
10% Na2CO3 solution and then poured into the crashed ice. The precipated desired product
was filtered, washed with cold water, and dried to obtain the title compound (1020 mg, 78%
yield) as a white solid. M.p 197-198°C. FTIR (ATR) ν cm–1: 3350 (NH), 1621 (C=N), 1608
(C=C), 1215 (C-N), 789 and 749 (C-Cl). 1H NMR (CDCl3) δ ppm: 7.67 (bs, 1H, NH), 7.51 (d,
J =7.6 Hz, 2H, ArH), 7.39 (t, J = 7.6 Hz, 2H, ArH), 7.23-7.18 (m, 1H, ArH). 13C NMR
(CDCl3) δ ppm: 171.34, 170.15, 164.03, 135.61, 129.29, 125.87, 121.33.
4.2.2. 25,27-(4-chloro-2-phenylamino-1,3,5-triazin)-26,28-dihydroxycalix[4]arene (V)
Firstly, a mixture of p-tert-butylcalix[4]arene IV [30] (570 mg, 0.88 mmol) and K2CO3
(2.4 eq) was stirred in 20 mL of acetone at room temperature for 24 hours. Then compound
III (3 eq) in 20 mL of acetone was added and the reaction mixture was refluxed for 14 hours
along with TLC monitoring. After the completion of the reaction, the solvent was evaporated
under reduced pressure and flash column chromatography with silica gel was performed on
the crude material using Hexane:EtOAc (10:1 6:1) to afford V (745 mg, 80% yield) as a
white solid. M.p 309-311°C. FTIR (ATR) ν cm–1: 3526, 2956, 2927, 1603 (C=C), 1552, 1169
(C-O-C), 1121 (C-O), 752 (C-Cl). 1H NMR (CDCl3) δ ppm: 7.20 (m, 4H, ArH), 7.12 (m, 4H,
ArH), 7.03 (m, 2H, ArH), 6.94 (m, 4H, ArH), 6.86 (m, 4H, ArH), 3.93 (d, J = 13.6 Hz, 4H,
16

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ArCH2Ar), 3.41 (d, J =13.6 Hz, 4H, ArCH2Ar), 1.32 (s, 18H, C(CH3)3), 1.06 (s, 18H,
13
C(CH3)3). C NMR (CDCl3) δ ppm: 171.3, 164.8, 150.2, 150.1, 148.6, 148.4, 142.3, 136.9,
131.9, 128.8, 127.3, 125.7, 125.4, 124.1, 119.5, 34.1, 33.9, 31.6, 31.2. HRMS (ESI-TOF)
calcd for C62H67Cl2N8O4 [M+H]+ 1057.4862, found 1057.4875; for C62H66Cl2N8O4Na
[M+Na]+ 1079.4721, found 1079.4726; for C62H66Cl2N8O4K [M+K]+ 1095.4421, found
1095.4459.
4.3. LB film preparation
The investigation of NIMA 622 LB trough (a fully computer-controlled device) of an
area of 1200 cm2 considered the transfer of triazine based calixarene monolayer from the air-
water interface to on suitable substrates. Solutions of the triazine based calixarene in
chloroform with a concentration of 0.49 mg ml-1 were prepared to take π-A isotherm graph.
A duration of 15 minutes was given for the solvent to evaporate before the isotherm graph
was taken and the temperature control unit was used to control the temperature of water
subphase. All experimental data were collected at room temperature. The isotherm graph’s
results were performed again three times more and found to be reproducible. After examining
the π-A isotherm graphs, the floating monolayer at the water surface (at the surface pressure
of 22.5 mN m-1) was found to be stable for a period of 15 minutes. The value of surface
pressure was selected for the LB film which is deposited on quartz glass and on quartz crystal
substrates for both UV-Visible and QCM measurements. The LB film’s deposition mode was
of the Z-type, and the vertical dipping process was performed with a speed of 25 mm min-1 at
the selected surface pressure for both the down and up strokes. After each up-stroke LB film
samples were allowed to dry. Under the similar surface pressure, various numbers of
monolayer were placed on these substrates.
4.4. QCM technique
For QCM studies, the triazine based calix[4]arene material was fabricated onto quartz
crystal substrates. After each deposition period, the LB film sample was left drying and the
change in mass was checked utilizing the QCM measurement system controlled with a home-
made computer. Dedicated software permits online recording of the variations in quartz
resonance frequency. All the measurements were performed on an in-house designed
oscillating circuit at room temperature with standard quartz crystals having a nominal
resonance frequency of 3.5 MHz. The frequency of oscillation was checked as an element of
time utilizing a PC after the quartz crystal was embedded into the electronic control unit. The
frequency variation values which specify the amount of reaction, were measured with a
precision of 1 Hz while providing organic vapour. Also, a gas cell was built to investigate the
triazine based calixarene LB film reaction while exposing to organic vapours by measuring
the frequency change. The calixarene thin film sensor was periodically exposed to organic
vapours for 2 minutes and afterwards was allowed to recuperate after injection of dry air. And
during exposure to organic vapours the variation in resonance frequency were recorded in real
time. In order to observe the reproducibility of triazine based calix[4]arene thin film sensor
this procedure was performed over several times.
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Acknowledgements
The financial support from the Research Foundation of Usak University (Project No:
2018/MF002) is gratefully acknowledged. Additionally, we thank UBATAM (Usak
University, Scientific Analysis Technological Application and Research Center) for analyses.
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Scheme and Figure Captions:
Scheme 1. Synthesis of the target triazine-calix sensor candidate V. Reagents and conditions:
(i) acetone, 0°C; (ii) K2CO3, acetone, III, reflux.
Figure 1. Isotherm graph of the triazine based calixarene monolayer.
Figure 2. SEM images of (a) the bare quartz glass substrate, (b) triazine based calixarene coated
quartz glass substrate.
Figure 3. a) 3D AFM image of the triazine based calixarene coated glass b) 2D AFM image (500x500
nm2) and the cross section of the triazine based calixarene coated glass.
Figure 4. BET plot of the triazine based calixarene. Inset: The adsorbed volume of nitrogen plotted as
a function of relative pressure as measured during BET surface analysis of the triazine based
calixarene.
Figure 5. UV-Vis spectra of the triazine based calixarene in a chloroform solution and the triazine
based calixarene LB film on the quartz glass substrate. Inset: linear increase of absorbance
as a function of layer numbers.
Figure 6. The frequency shifts of the triazine based calixarene LB film against the number of layers.
Figure 7. The frequency change of the triazine based calixarene LB thin film against
various chlorinated aliphatic hydrocarbons.
23