Synthesis and physicochemical properties of poly[2-(cyclohex-2-en-1-yl)aniline] as a new polyaniline derivative

Article abstract of DOI:10.1039/d1nj00596k

In this study, a new polyaniline (PANI) derivative, poly[2-(cyclohex-2-en-1-yl)aniline], was synthesized for the first time. It was shown by varying the synthetic parameters that the highest polymer yield was obtained if HNO₃and (NH₄)₂S₂O₈were used. It was found that variation in the conditions for synthesizing poly[2-(cyclohex-2-en-1-yl)aniline] resulted in changes in the optical properties, for example, the PANI derivative obtained in HClO₄medium and in the presence of (NH₄)₂S₂O₈demonstrated the highest luminescence quantum yield (0.05). Using thermogravimetric analysis of poly[2-(cyclohex-2-en-1-yl)aniline], the main three stages of the polymer thermal degradation were identified. Processing of thermogravimetric curves allowed us to calculate the kinetic and thermodynamic parameters of the thermal degradation of the polymer at various heating rates. The use of poly[2-(cyclohex-2-en-1-yl)aniline] as a sensitive material in resistive sensors showed a high response of electrical conductivity to the changes in humidity (% RH) and ammonia concentration in the environment. The presented results demonstrate an efficient method for PANI modification that favors an improvement in solubility, appearance of photoluminescent properties, and broadening the practical application.

Full text of DOI:10.1039/d1nj00596k

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Synthesis and physicochemical properties of
poly[2-(cyclohex-2-en-1-yl)aniline] as a new
polyaniline derivative†
Cite this: New J. Chem., 2021,
45, 6356
Anastasia N. Andriianova, *a Diana E. Gribko,b Ivan S. Petrov,b Ilnur Mullagaliev,b
Alina F. Sattarova,a Renat B. Salikhov,b Ildus B. Abdrakhmanova and
Akhat G. Mustafinab
In this study, a new polyaniline (PANI) derivative, poly[2-(cyclohex-2-en-1-yl)aniline], was synthesized for
the first time. It was shown by varying the synthetic parameters that the highest polymer yield was
obtained if HNO3 and (NH4)2S2O8 were used. It was found that variation in the conditions for synthe-
sizing poly[2-(cyclohex-2-en-1-yl)aniline] resulted in changes in the optical properties, for example, the
PANI derivative obtained in HClO4 medium and in the presence of (NH4)2S2O8 demonstrated the highest
luminescence quantum yield (0.05). Using thermogravimetric analysis of poly[2-(cyclohex-2-en-1-
yl)aniline], the main three stages of the polymer thermal degradation were identified. Processing
of thermogravimetric curves allowed us to calculate the kinetic and thermodynamic parameters of
the thermal degradation of the polymer at various heating rates. The use of poly[2-(cyclohex-2-en-
1-yl)aniline] as
a sensitive material in resistive sensors showed a high response of electrical
Received 4th February 2021,
Accepted 11th March 2021
conductivity to the changes in humidity (% RH) and ammonia concentration in the environment.
The presented results demonstrate an efficient method for PANI modification that favors an
improvement in solubility, appearance of photoluminescent properties, and broadening the practical
application.
DOI: 10.1039/d1nj00596k
rsc.li/njc
the most promising approaches to overcome this problem is to
introduce various substituents into the polymer chain.3 PANI
Introduction
As the organic electronics industry grew, the interest in the derivatives with alkyl, alkoxy, sulfo, and other functional groups
development of new conjugated polymers for application in have already been reported.3,11,12 These groups improve the
various electronic devices increased.1 PANI and its derivatives processing properties of the polymer. Of the existing methods
are considered to be the most promising and most widely for synthesizing aniline derivatives, the Claisen amino rearrange-
studied polymers in this field.2,3 This high interest is due to ment seems to be the simplest and most accessible method.12,13
the unique set of physical and physicochemical properties of The use of this method is relevant for incorporating a cycloalkenyl
this compound, such as ionic and electronic conductivity, substituent to an ortho position of the aniline aromatic ring since
redox activity, thermal and chemical resistance, low cost, and this reaction occurs selectively and in a high yield.14 It is also
easy conversion of a monomer to a polymer.3,4 The many known that the physicochemical properties of PANI and its
advantages of PANI favor its wider practical application in solar derivatives can be controlled by changing the synthesis conditions
cells,5 anti-corrosion coatings,6 membranes,7 bio/chemical (pH of the medium, the nature of the solvent and oxidant, reaction
sensors,8 organic light-emitting diodes,9 energy storage devices,10 time, and temperature).12,13 This approach allows one to control
and much more. The poor solubility of PANI in typical organic the properties of the final product to be used in sensors for
solvents is a known limitation of its practical application. One of detecting the humidity or ammonia levels in the environment.8
Excellent results have already been obtained with PANI and some
a Ufa Institute of Chemistry of the Russian Academy of Sciences,
Republic of Bashkortostan, Pr. Oktyabrya 71, Ufa 450054, Russia.
E-mail: an.chem17@gmail.com
of its derivatives as sensitive material in resistive sensors.4,8,13
Thus, the purpose of this study was to synthesize poly(2-cyclohex-
2-en-1-yl)aniline, a new PANI derivative, and analyze the effect of the
nature of doping and oxidizing agents on its physicochemical
properties. The functionalization of PANI involved a preliminary
modification of the monomer. The process is based on the
b Bashkir State University, Republic of Bashkortostan, Z. Validi St 32, Ufa, 450076,
Russia
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1nj00596k
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synthesis of an N-alkenylaniline followed by its Claisen rearrange-
Polymer (C2). A solution of (NH4)2S2O8 (2.85 g, 12.5 mmol)
ment to an ortho-alkenylaniline. Poly-2-(cyclohex-2-en-1-yl)aniline as the oxidizing agent in 50 ml of 0.2 M HCl was slowly added
obtained using various oxidizing and doping agents was charac- with continuous stirring to a solution of 2-(cyclohex-2-en-1-yl)aniline
terized by IR, UV-visible, fluorescence spectroscopy, and scanning (1.72 g, 10.0 mmol) in 50 ml of 0.2 M HNO3. The reaction time was
electron microscopy. Moreover, the electrochemical characteris- 24 h at room temperature with continuous stirring. The precipitate
tics, kinetic and thermodynamic parameters of thermal destruc- that formed was filtered off and washed with 0.2 M HCl solution.
tion were calculated for the compounds studied. New PANI The resulting filtrate was dried in vacuo for 3 h at 40 1C. The
derivatives were successfully used as a sensitive material in polymer yield was 83%.
resistive-type sensors for determining the moisture level and
ammonia concentration.
Polymer (C3). A solution of FeCl3 as the oxidizing agent (2.03 g,
12.5 mmol) in 50 ml of 0.2 M HCl was slowly added with
continuous stirring to a solution of 2-(cyclohex-2-en-1-yl)aniline
(1.72 g, 10.0 mmol) in 50 ml of 0.2 M HCl. The reaction time was
24 h at room temperature with continuous stirring. The pre-
cipitate that formed was filtered off and washed with 0.2 M HCl
solution. The resulting filtrate was dried in vacuo for 3 h at 40 1C.
The polymer yield was 18%.
Materials and methods
Materials
Aniline (Merck) was distilled under reduced pressure (10 mmHg)
prior to use to remove impurities (n2D0 = 1.583). Dimethylsulfoxide
(DMSO), N-methylpyrrolidone (NMP), HCl, HClO4, HNO3, NaOH,
(NH4)2S2O8, FeCl3, and K2Cr2O7 were purchased from Merck and
were used as received.
Polymer (C4). A solution of (NH4)2S2O8 (2.85 g, 12.5 mmol)
as the oxidizing agent in 50 ml of 0.2 M HCl was slowly added
with continuous stirring to a solution of 2-(cyclohex-2-en-1-
yl)aniline (1.72 g, 10.0 mmol) in 50 ml of 0.2 M HCl. The
reaction time was 24 h at room temperature with continuous
stirring. The precipitate that formed was filtered off and
washed with 0.2 M HCl aq. solution. The resulting filtrate was
dried in vacuo for 3 h at 40 1C. The polymer yield was 61%.
Polymer (C5). A solution of (NH4)2S2O8 (2.85 g, 12.5 mmol)
as the oxidizing agent in 50 ml of 0.2 M HCl was slowly added
with continuous stirring to a solution of 2-(cyclohex-2-en-1-
yl)aniline (1.72 g, 10.0 mmol) in 50 ml of 0.2 M H2SO4. The
reaction time was 24 h at room temperature with continuous
stirring. The precipitate that formed was filtered off and
washed with 0.2 M HCl solution. The resulting filtrate was
dried in vacuo for 3 h at 40 1C. The polymer yield was 48%.
Polymer (C6). A solution of FeCl3 as the oxidizing agent (2.03 g,
12.5 mmol) in 50 ml of 0.2 M HCl was slowly added with
continuous stirring to a solution of 2-(cyclohex-2-en-1-yl)aniline
(1.72 g, 10.0 mmol) in 50 ml of 0.2 M HClO4. The reaction time was
24 h at room temperature with continuous stirring. The precipitate
that formed was filtered off and washed with 0.2 M HCl solution.
The resulting filtrate was dried in vacuo for 3 h at 40 1C. The
polymer yield was 21%.
Synthesis of the monomer
The 2-(cyclohex-2-en-1-yl)aniline monomer (C) was synthesized
by the following procedure (Scheme 1): 64 g (0.44 mol) of
3-bromocyclohexene obtained by the known procedure15 was
added dropwise to aniline (A) (148 g, 1.59 mol) in 1 : 4 ratio to
give an N-substituted aniline (B). After that, the reaction mixture
was kept at 140 1C for 3–4 h until compound B was completely
rearranged into 2-(cyclohex-2-en-1-yl)aniline (C) (Scheme 1). The
Claisen rearrangement of this monomer occurs selectively in
80% yield. Monomer C is a light yellow viscous liquid with a
boiling point of 120.0 1C (260 Pa), n2D0 = 1.5547. CHN (%): C
1
83.19, H 8.73, N 8.08, C11H15N. H NMR (CDCl3, d): 1.57–2.09
(6H, CH2), 3.15 (1H, CH), 3.34 (2H, NH2), 5.62 (2H, CH), 6.74–
7.03 (5H, Ph). 13C NMR (CDCl3, d): 21.12, 25.18, 29.15, 31.67,
114.77, 118.52, 125.71, 126.60, 127.48, 128.17, 129.13, 144.57
(Fig. S1, ESI†).
Synthesis of polymers
Polymer (C1). A solution of FeCl3 as the oxidizing agent (2.03 g,
Polymer (C7). A solution of FeCl3 as the oxidizing agent (2.03 g,
12.5 mmol) in 50 ml of 0.2 M HCl aq. solution was slowly added 12.5 mmol) in 50 ml of 0.2 M HCl was slowly added with
with continuous stirring to a solution of 2-(cyclohex-2-en-1- continuous stirring to a solution of 2-(cyclohex-2-en-1-yl)aniline
yl)aniline (1.72 g, 10.0 mmol) in 50 ml of 0.2 M H2SO4. The (1.72 g, 10.0 mmol) in 50 ml of 0.2 M HNO3. The reaction time
reaction time was 24 h at room temperature with continuous was 24 h at room temperature with continuous stirring. The
stirring. The precipitate that formed was filtered off and washed precipitate that formed was filtered off and washed with 0.2 M
with 0.2 M HCl solution. The resulting filtrate was dried in vacuo HCl solution. The resulting filtrate was dried in vacuo for 3 h at
for 3 h at 40 1C. The polymer yield was 39%.
40 1C. The polymer yield was 48%.
Polymer (C8). A solution of (NH4)2S2O8 (2.85 g, 12.5 mmol)
as the oxidizing agent in 50 ml of 0.2 M HCl was slowly added
with continuous stirring to a solution of 2-(cyclohex-2-en-1-yl)aniline
(1.72 g, 10.0 mmol) in 50 ml of 0.2 M HClO4. The reaction time
was 24 h at room temperature with continuous stirring. The
precipitate that formed was filtered off and washed with 0.2 M
HCl solution. The resulting filtrate was dried in vacuo for 3 h at
40 1C. The polymer yield was 54%.
Scheme 1 Synthesis of 2-(cyclohex-2-en-1-yl)aniline.14
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Apparatus and characterization
1H NMR and 13C NMR spectra were recorded on a BRUKER AM-300
spectrometer at 300 MHz (1H) and a BRUKER AVANCE-500
spectrometer at 500 MHz (1H) in CDCl3 or DMSO solution, using
TMS as the internal standard. IR spectra were obtained in KBr
pellets on a SHIMADZU IR Prestige-21 spectrometer. Elemental
analysis was performed on a Euro-2000 CHNS(O) analyzer. UV
spectra were recorded on a Shimadzu 2600 spectrophotometer in
DMSO, N-methylpyrrolidone, and CHCl3 solutions at 298 K,
wavelength range from 190 to 900 nm (gap width 2.0 nm, high
speed), quartz cells 1 cm thick. The study of the microstructure
of nanoparticles was carried out using a TESCAN MIRA 3 LMH
high-resolution scanning electron microscope (SEM) equipped
with a detector of scattered electrons and backscattered elec-
trons. The recording was carried out at an accelerating voltage of
10 kV in a vacuum of ca. 10ꢀ3 Pa. Thermogravimetric analysis
was performed on a Netzsch STA 449 F3 Jupiter instrument in a
nitrogen atmosphere (250 ml minꢀ1). The instrument sensitivity
was 0.001 mg. A sample weighing 10 mg was heated in the range
from 25 1C to 900 1C at various heating rates of 5, 10, 15, and
20 1C minꢀ1. Thermogravimetric curves were analyzed as func-
tions of mass loss versus temperature. The kinetic parameters of
polymer thermal destruction were determined using the Coats–
Redfern method.16 Fluorescence spectral analysis was performed
on an RF-5301 PC Shimadzu spectrofluorophotometer in DMSO
solution at room temperature. The quantum yield was calculated
using the formula (1):
Fig. 1 Calibration of humidity and ammonia sensors: (1) power supply
unit, (2) container with water or ammonia, (3) sealed cabinet, (4) polymer
film humidity sensor, (5) electronic hygrometer, (6) ammonia sensor,
(7) Arduino UNO, (8) display.
Results and discussion
Synthesis of the polymers
Table 1 shows the data on the yields of poly[2-(cyclohex-2-en-1-
yl)aniline] obtained in the presence of various doping acids and
oxidants (Scheme 2). As one can see, the highest yield is
obtained upon oxidative polymerization of the monomer in
an aqueous solution of HNO3 in the presence of (NH4)2S2O8.
The lowest yield was obtained in the presence of FeCl3. This is
natural since it provides mild oxidation conditions that require
a longer reaction time than the other oxidants. It is important
to note that K2Cr2O7 was also used as an oxidant. As a result,
the substituent degraded, numerous side products formed, and
the target polymer was not formed.
It is known that the main drawback of PANI lies in its low
solubility in the common organic solvents.3,12 The solubility of
polymers C1–C8 was estimated in various solvents. The results
are presented in Table 2. The polymers that we synthesized
showed higher solubility in organic solvents than PANI. It can be
Aref
Af
Df
jf ¼ jref
ꢁ
ꢁ
(1)
Dref
where jref is the relative quantum yield of anthracene in explained by the existence of alkyl substituents in the polymer
acetonitrile solution (0.2717); Aref and Af are the optical densities chain3 that cause higher chain flexibility than that of the rigid
of anthracene and the sample under study; Dref and Df are the PANI structure. Thus, the alkenyl substituent in the polymer
measured integral fluorescence intensities.
structure beneficially affects the solubility. It should be noted
Sitall was used as the substrate in resistive sensors. Aluminum that the nature of the doping agent and oxidant did not signifi-
electrodes were deposited on top of a Sitall substrate by thermal cantly affect the solubility of the samples that we synthesized.
sputtering in the vacuum vessel of a VUP-5 apparatus to a
Table 2 also shows the colors of the observed solutions of
thickness of about 400 nm, with a gap between the electrodes polymer samples. Colors of PANI solutions depends on quantity of
that was created using a shadow mask. A film of a polymer being quinoid and benzenoid units in the chain of polymer. This
studied was deposited in the 50 mm gap region by centrifugation phenomenon is clearly reflected in the comparison of the colora-
from a 0.1 g Lꢀ1 DMF solution. The resulting layer was thermally tion of PANI powders in its different forms of oxidation.3 According
annealed to remove the residual solvent by heating to 80 1C to the UV-visible absorption spectra in DMSO solution, for most
for 30 min. The following equipment was used in the experiments PANI derivatives, a yellow/brown color of the solution is observed,
for the measurements: MASTECH DC POWER SUPPLY HY3005D-
2 power supplies, a Tektronix DMM 4020 multimeter as the
ammeter, Arduino UNO with a display, DHT11 and MQ135
control sensors.
Table 1 Conditions for the synthesis of [2-(cyclohex-2-en-1-yl)aniline]
(C1–C8)
The relative humidity sensors were calibrated by means of a
DHT11 electronic hygrometer (5) in a specialized cabinet with
controlled humidity (Fig. 1). A vessel with water was placed into
the cabinet (2). The output signal was measured at specific
humidity values. The moisture growth rate was B0.1% per
minute. The ammonia sensors were calibrated in the same way
in a sealed cabinet where the ammonia vapor concentration
was monitored by a MQ135 sensor (6).18
Polymer
Acid (0.2 M)
Oxidant
Yield, %
C1
C2
C3
C4
C5
C6
C7
C8
H2SO4
HNO3
HCl
FeCl3
(NH4)2S2O8
FeCl3
(NH4)2S2O8
(NH4)2S2O8
FeCl3
39
83
18
61
48
21
48
54
HCl
H2SO4
HClO4
HNO3
HClO4
FeCl3
(NH4)2S2O8
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the C–H bonds of the aromatic ring. The signals at 2889 cmꢀ1
are characteristic of the C–H bond in the substituent. The main
peaks characteristic of the polymer structure of PANI derivatives
are found at 1514 and 1647 cmꢀ1
.
Scheme 2 Synthesis of poly-2-(cyclohex-2-en-1-yl)aniline.
They correspond to the vibrations of the benzoic and quinoid
moieties, respectively. The maximum at 1383 cmꢀ1 is characteristic
of the C–N bond. The presence of a peak at 1122 cmꢀ1 indicates
+
+
Table 2 The solubility of the polymers obtaineda
that the polymer is in a doped state (QN –/–N –).19 The maxima
around 748 cmꢀ1 correspond to the vibrations of the C–H bond in
the 1,2,4-substituted aromatic ring.
ꢂ
Solubility and color of the polymer solutionb
No.
C1
DMSO
N-Methylpyrrolidone
CHCl3
Elemental analysis (CHNS)
***
***
**
Light-brown
***
Brown
***
Brown
***
Yellow
***
Brown
***
Gray
***
Blue
***
Light-brown
***
Brown
***
Violet
***
Light-brown
***
Light-brown
***
Brown
***
Grey
***
Light-brown
**
Light-brown
*
Brown
*
Yellow
*
Light-brown
*
Grey
**
Gray-blue
**
The results of elemental analysis are in good agreement with
the calculated values (Table 3). According to the CHNS analysis,
it is evident that the structures of the polymers synthesized are
close to that of emeraldine, in agreement with the results of the
other analytical methods (Scheme 3).
C2
C3
C4
C5
C6
C7
C8
Optical properties
A study of the electronic spectra of the samples in various
solvents makes it possible to determine the effect of various
factors on the polymer structure more accurately.3,12 In view of
this, absorption spectra were obtained for this series of com-
pounds in solutions in DMSO, N-methylpyrrolidone, and CHCl3
with the same polymer concentration. The results of UV
spectroscopy of the samples of C1–C8 synthesized are shown
in Fig. 3. The spectra of the polymers studied contain an
absorption band in the region of 285–300 nm. The samples of
the PANI derivative considered in this study contain an alkenyl
substituent at the ortho position whose large volume increases
the torsion angle between the adjacent aromatic rings. This
effect is accompanied by a hypsochromic shift in the absorp-
tion spectra (Fig. 3) relative to the known spectrum of unsub-
stituted PANI.3,20 The shift is 20–30 nm for all the samples.
Moreover, a small maximum in the region of 470 nm in CHCl3
solution is observed in Fig. 3 for all the samples. The highest
intensity is observed for sample C3 synthesized in HCl using FeCl3
as the oxidizing agent. This maximum corresponds to the polaron–
p* electron transition in the polymer chain.21 The maximum
observed for all the samples in the region of 500–530 nm corre-
sponds to the n–p* electron transition in the quinoid fragment.
Variation of the synthesis conditions did not much affect the
positions of the absorption maxima. However, varying the synthesis
conditions changed the intensity of the absorption bands.
Brown
Brown
Brown
a
The solubility was determined using 0.05 g samples in 10 ml of a
solvent. *** soluble at room temperature; ** soluble on heating at
60 1C; * poorly soluble on heating at 60 1C.
b
which is due to the p–p* electronic transition. While in NMP
solution, for some samples, a violet or blue coloration is observed
due to the presence of a larger number of quinoid fragments in the
polymer structure.
FT-IR spectra
The structure of the polymers obtained was confirmed using
spectral analysis methods. The FTIR spectra of C1–C8 are shown
in Fig. 2. All the samples show absorption at 3370–3425 cmꢀ1
that corresponds to the N–H bond vibration. The absorption
peak in the region of 2928–3000 cmꢀ1 is due to the vibrations of
Based on the UV spectra, the bandgap values were calculated
from the edge of the absorption band (Table 4). According to
the data presented in Table 4, the values of Eg are smaller than 4 eV,
which allows one to classify some of the samples synthesized as
semiconductor materials. It is important to note that samples C3,
C6, and C7 obtained using FeCl3 showed the smallest bandgap
values. This fact is probably due to the formation of the emeraldine
form of PANI due to oxidation of the monomer with FeCl3.
To get an idea about the theoretical bandgap, we performed a
quantum chemical calculation of the 2-(cyclohex-2-en-1-yl)aniline
hexamer using the B3LYP hybrid functional and the 6-31G(d) basis
set. This functional has an optimal ratio between the calculation
Fig. 2 IR spectra of samples C1–C8.
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Table 3 Elemental analysis results
No.
Formula
C (%)
N (%)
H (%)
S (%)
Cl (%)
Composition (found)
Composition (calculated)
C1
C2
C3
C4
C5
C6
C7
C8
(C12H12N)7ꢃClSO4
(C12H12N)7ꢃNO3SO4
(C12H12N)7ꢃ2Cl
66.97
67.39
75.42
66.62
65.43
69.31
74.71
66.27
6.47
6.75
7.15
6.76
6.29
6.41
8.16
6.43
7.52
6.29
7.11
6.19
6.03
5.93
6.15
6.22
5.05
3.54
—
3.31
4.50
—
1.62
—
5.11
2.57
—
5.94
2.62
3.52
C5.58H7.52N0.46S0.16Cl0.05
C5.62H6.29N0.48S0.11
C6.29H7.11N0.51Cl0.14
C5.55H6.19N0.48S0.10Cl0.07
C5.5H6.03N0.45S0.14
C5.78H5.93N0.46Cl0.17
C6.23H6.15N0.58Cl0.07
C5.52H6.22N0.46S0.09Cl0.10
C5.9H6.39N0.49S0.14Cl0.07
C5.8H6.27N0.55S0.14
C6.6H7.18N0.55Cl0.16
C6.32H7.18N0.55S0.09Cl0.08
C6.0H6.55N0.50S0.14
C6.31H6.83N0.53Cl0.15
C6.49H7.03N0.62Cl0.15
C6.03H7.54N0.50S0.07Cl0.08
(C12H12N)7ꢃClSO4
(C12H12N)7ꢃ2SO4
(C12H12N)7ꢃClClO4
(C12H12N)7ꢃNO3Cl
(C12H12N)7ꢃSO4ClO4
—
2.74
suitable spatial positions of atoms/groups by sequential addition
of monomers to each other followed by pre-optimization. The
spatial arrangement was fully optimized in the gas phase. The
advantage of hybrid functionals is that the estimated HOMO–
LUMO gas-phase gaps for a polymer that they provide are very
close to the Egap values obtained experimentally.22 To ensure the
potential energy minima, the frequencies of surface vibrations
were also analyzed at the same level of theory, and all the
frequencies were acknowledged to be real. All the calculations
were performed using the Gaussian 09 program, and the HOMO–
LUMO orbitals were built in Gauss-View.23 One can see in Fig. 9
that the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) form the external
Scheme 3 The structure obtained from CHNS analysis of the polymers.
accuracy and the time and resources it requires. Six molecules
of the corresponding monomer were arranged in the most
Fig. 3 UV spectra of samples C1–C8 in solutions in: (a) DMSO, (b) N-methylpyrrolidone, (c) CH3Cl.
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Table 4 Optical properties of polymers C1–C8
sample (Fig. 5b) showed a linear dependence, which confirms
that the luminescence is due to the polymer. Changing the
synthesis parameters showed that the use of (NH4)2S2O8 as the
oxidant resulted in a higher luminescence intensity and that
the maximum value was observed for the sample obtained in an
HClO4 solution. It is assumed that if bulky doping agents are
used, the protonation occurs much more slowly and the resulting
polymer has a low degree of doping, which leads to a higher
luminescence intensity, as it was found for PANI.25 It is important
to note that no shift of the emission maximum occurs in the
presented series of compounds, which allows us to assume that
the emission emitter is not affected by the synthesis conditions.
The luminescence quantum yield relative to anthracene was
calculated for the entire series of the samples studied.17 The
results are presented in Table 3. The highest quantum yield was
obtained for samples C2, C3, C6, and C8.
Based on the data in Table 4, the following important
conclusions can be drawn: (i) in most cases, the polymers
obtained in the presence of FeCl3 demonstrate the highest
luminescence quantum yields; (ii) a decrease in the bandgap
is accompanied by an increase in the luminescence intensity
due to a decrease in the energy of electron transition from the
valence band to the conduction band; (iii) varying the synthesis
conditions critically affects the photoluminescent properties of
the resulting samples.
Absorption
Polymer
l1, nm
lEx, nm
l
Em, nm
jf
Eg,a eV
C1
C2
C3
C4
C5
C6
C7
C8
289
288
301
293
287
290
288
289
330
420
440
350
420
440
370
380
481
486
485
439
487
494
486
489
0.04
0.04
0.04
0.03
0.02
0.04
0.03
0.05
2.07
2.11
1.82
1.99
1.86
1.80
1.83
1.97
a
Eg = 1240/ledge
.
boundaries of the electronic system. The difference between the
HOMO and LUMO (Egap) was calculated to be 1.95 eV. In general,
the theoretical bandgap is in good agreement with the optical
bandgap (Egap) obtained experimentally (Fig. 4).
To further study the optical properties of the polymers synthe-
sized, we obtained the photoluminescence spectra of C1–C8 in
DMSO solution (Fig. 5). It is known that polymers comprising a
p-conjugated aromatic system exhibit photoluminescence.24
It was previously found that unsubstituted PANI exhibits photo-
luminescent properties and that incorporation of an alkenyl sub-
stituent at the aromatic ring and varying the synthesis conditions
allow the intensity of the polymer luminescence to be controlled.13
A distinctive feature of ortho-substituted PANI derivatives is that
photoluminescence increases with an increase in the substituent
volume and a decrease in the degree of doping.13
Thermal properties
The thermal stability was studied for sample C4 at heating rates
of 5, 10, 15, and 20 1C minꢀ1 (Fig. 6). It was found that the
thermal destruction of the polymer occurred in a few stages: at
the first stage, 80–200 1C (I), the loss of moisture, solvent, and
doping agent occurs; at the second stage, 200–300 1C (II), the
C–C bond between the main polymer chain and a substituent
breaks; and at the third stage, 300–600 1C (III), degradation of
the main polymer chain begins.12
Studies on the luminescent properties of poly[2-(cyclohex-2-
en-1-yl)aniline] synthesized under various conditions revealed
that the luminescence intensity changed depending on the
synthesis conditions (Fig. 5). Varying the concentration of C4
The thermogravimetric analysis data were used to determine
the kinetic parameters of the thermal degradation of C4 polymer
by the Coats–Redfern method.26 Based on the assumption that
the degradation of C4 is a first-order reaction, the activation
energies and pre-exponential factors were calculated using the
equation:27
ꢀ
ꢁ
ꢀꢀ
ꢁꢀ
ꢁꢁ
ꢀ
ꢁ
lnð1 ꢀ aÞ
AR
2RT
Ea
ln ꢀ
¼ ln
1 ꢀ
ꢀ
T2
bEa
E
RT
where A is the pre-exponential factor (minꢀ1), Ea is the activation
energy (kJ molꢀ1), T is the temperature in K, t represents time in
min, R is the gas constant, and a represents the mass loss
fraction. The conversion ‘‘a’’ that represents the mass loss
fraction with respect to temperature can be calculated as:
m0 ꢀ mt
a ¼
m0 ꢀ mf
Fig. 4 The electron distribution in the 2-(cyclohex-2-en-1-yl)aniline hexamer’s
LUMO and HOMO energy levels obtained at the B3LYP/6-31G(d) level of
theory in the gas phase. The blue, gray, white, and green balls represent N,
C, and H atoms, respectively.
where m0 and mf represent the initial and final masses of the
samples in g, and mt represents the mass of a sample at any
time ‘‘t’’ during thermal degradation.
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Fig. 5 (a) Luminescence spectra of samples C1–C8 in DMSO solution (10ꢀ2 g L); (b) dependence of the luminescence intensity on the concentration of
sample C4 (1.25 ꢁ 10ꢀ4–10ꢀ2 g L); (c) photoluminescence excitation–emission map of C3.
A plot of ln{ꢀln(1 ꢀ a)/T2} versus 1/T was built to determine substituent at an ortho position of the aromatic ring decreased the
the activation energy and the pre-exponential factor. This depen- thermal stability. This fact was previously established for the
dence allows the best calculation of kinetic parameters based on majority of existing ring-substituted PANI derivatives.3
the values of the slope and intersection with axis y (Fig. 7).
Based on thermogravimetric analysis, thermodynamic para-
The high correlation coefficient (R2 = 0.99) of the linear approxi- meters of the process were calculated, such as the enthalpy
mation of the experimental data by the Coats–Redfern method (DH, kJ molꢀ1), Gibbs free energy (DG, kJ molꢀ1), and entropy
allows us to conclude that the thermal degradation of C4 actually (DS, kJ molꢀ1 Kꢀ1 26 (Table 6). These values were obtained from
)
follows a first-order reaction equation. The kinetic parameters the following equations:
calculated from the linear correlation are presented in Table 5.
One can see from the data in Table 5 that the pre-exponential factor
DH = Ea ꢀ RT
ꢀ
ꢁ
increases with an increase in the heating rate. However, its value
decreases with each subsequent stage, I - II - III.
No correlation was found between the activation energy and
the heating rate. At the same time, Ea increases with an increase
in the reaction temperature, i.e., in the series of degradation
stages I - II - III.
kBT
hA
DG ¼ Ea þ RTmax ln
DH ꢀ DG
DS ¼
Tmax
where kB = 1.381 ꢁ 10ꢀ23 m2 kg sꢀ2 Kꢀ1 is the Boltzmann
constant, Tmax is the temperature where the maximum degra-
dation occurs, h = 6.626 ꢁ 10ꢀ34 m2 kg sꢀ1 is the Planck
constant, and R = 0.008314 kJ molꢀ1 Kꢀ1 is the universal gas
constant.
The activation energies of C4 thermal degradation were com-
pared with the literature data obtained for PANI at a heating rate of
20 1C minꢀ1 and calculated in a similar way by the Coats–Redfern
method (I – 160 kJ molꢀ1; II – 230 kJ molꢀ1; III – 322 kJ molꢀ1).
A logical conclusion was made that incorporation of a bulky alkyl
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The results of scanning electron microscopy are shown in
Fig. 8. The samples C1–C8 studied formed inhomogeneous
supramolecular structures during the synthesis. The morphology of
the resulting samples significantly differs from the supramolecular
structure of PANI.3,12,29 PANI derivatives with alkyl substituents
tend to form a globular morphology where the spheres are
distinctly separated from each other.12,30 The observed difference
from the PANI’s fibrillar morphology31 is probably due to the
presence of a substituent at the aromatic ring of the polymer chain
and due to different synthesis conditions.
The supramolecular polymer structure changes significantly
upon variation of the acids and oxidants used in the synthesis.
As one can see from Fig. 8, polymers C1, C2, and C8 demon-
strate a structure consisting of elongated non-uniform cylinders/
wires. The structures of samples C1 and C2 contain a small
number of spherical moieties, while sample C8 entirely consists
of elongated particles. This difference is probably due to the
interaction of the monomer with the reaction medium, whose
mechanism is determined by the hydrophobicity of monomer C
and its ability to form a salt with an acid residue. On treatment
with an (NH4)2S2O8 in solutions of hydrochloric and sulfuric acids,
the oxidative polymerization occurs at a higher rate than the
processes involving FeCl3. In view of this, more polymerization
centers are formed in reactions with (NH4)2S2O8, and the morphol-
ogy of the polymer becomes more finely dispersed. This conclu-
sion is based on the morphology of samples C4, C5 and C3, C7. As
one can see, samples C4 and C5 have finely dispersed structures
that are slightly finer than that of sample C7 and a few times finer
than that of C3. Sample C6 features a more homogeneous
morphology with few globules, probably due to the nature of the
doping acid that promotes the formation of such a structure.
Fig. 6 Mass loss (a) and (b) mass loss rates versus time for the thermal
degradation of C4 at 5, 10, 15, and 20 1C minꢀ1 under an inert atmosphere
(nitrogen).
The data presented in the table show that the enthalpy, like
the activation energy, does not change with an increase in the
heating rate. Positive DH values allow us to conclude that an
external impact is required for the degradation of the polymer
chain. The high DG values indicate a fairly low susceptibility to
this reaction.28 It is important to note that as the heating rate
increases, the Gibbs free energy decreases, which indicates that
the reaction occurs more readily. The DS values in Table 6 are
negative. These values show that the decomposition products have
more ordered structures than the initial polymer C4. Unlike DG,
the entropy value increases with an increase in the heating rate.
Sensing properties
The industry of automated systems has been developing in
recent decades, which increases the demand for various
sensors.32–34 Electronic sensors of humidity and ammonia
based on films of conjugated organic polymers whose electro-
physical parameters depend on the environmental conditions
are considered as a promising approach in this field.18 The
PANI derivatives C1–C8 that we obtained were used as sensitive
elements to create moisture and ammonia sensors in the form
of thin-film resistors. The polymer films were obtained by
centrifugation from solutions.18 The dependence of the elec-
trical conductivity of PANI derivatives on the air relative
Scanning electron microscopy (SEM)
The study of the morphology of various PANI derivatives makes humidity was studied (Fig. 9a).
it possible to assess the contribution of the substituent to the
One can see in Fig. 9a that as the environment humidity
formation of the supramolecular structure. In addition, mor- increases, the electrical conductivity also increases. The most
phology has a significant impact on the applied characteristics significant increase in conductivity is observed for sample C4
of the polymer. For example, depending on the type of device, synthesized using (NH4)2S2O8 as the oxidizing agent in an
the organic semiconductor layer must interface with different aqueous solution of hydrochloric acid. The smallest change
materials and have a different morphology. Thus, the detecting in conductivity in each series of compounds was found for
layer of sensors and catalyst carrier must have a developed polymers C2, C3, and C8. It is important to note that these
highly permeable structure with a large specific surface area. compounds were obtained in different acids but with the same
In the other hand, for use in electrochromic devices or LEDs, oxidizing agent, i.e., FeCl3. This fact is probably due to the redox
dense and homogeneous layers of strictly regulated thickness state formed due to monomer polymerization in the presence of
are required.1,3,5–11
FeCl3. This oxidant favors the formation of a smaller amount of
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Fig. 7 Plots according to the CR method for the (I) first, (II) second and (III) third step of C4 thermal degradation at 5, 10, 15, and 20 1C minꢀ1
.
Table 5 Kinetic parameters of C4 thermal degradation
Table 6 Thermodynamic parameters of C4 thermal degradation
Degradation stage b (g minꢀ1) Ea (kJ molꢀ1) A ꢁ 10ꢀ2 (minꢀ1) R2
Degradation Tmax
,
DH
DG
DS
stage
K
b, g minꢀ1 (kJ molꢀ1
)
(kJ molꢀ1
)
(J molꢀ1 Kꢀ1
)
I
5
10
15
20
5
22.20
29.08
25.35
22.23
21.75
29.17
30.22
28.63
28.92
30.85
31.22
30.30
4.50
7.19
13.76
16.18
0.49
4.42
7.83
8.17
0.80
1.88
2.78
3.46
0.996
0.994
0.995
0.996
0.998
I
366
5
10
15
20
5
10
15
20
5
19.16
26.04
22.30
21.18
17.33
24.76
25.81
24.21
22.84
24.77
25.14
24.22
121.90
127.35
121.64
118.03
176.17
173.89
172.42
170.64
238.57
235.29
233.28
231.03
ꢀ280.71
ꢀ276.82
ꢀ271.42
ꢀ270.07
ꢀ299.13
ꢀ280.86
ꢀ276.11
ꢀ275.76
ꢀ295.13
ꢀ287.98
ꢀ284.73
ꢀ282.90
II
III
10
15
20
5
10
15
20
0.996 II
0.991
0.995
0.991
0.991 III
0.993
531
731
10
15
20
0.991
oxidized quinoid moieties of the polymer chain, which affects
the electrical conductivity of the polymer. The results obtained absorbs moisture, which results in swelling of the polymer
for sample C7 showed the best linearization. A number of followed by unfolding of the compact spiral-shaped polymer
experiments were carried out for this polymer to determine the chain. The formation of a more aligned structure of a PANI
reproducibility of the results by running ten successive tests for derivative favors the charge transfer along the polymer
sensitivity to the ambient humidity (Fig. 9b).
chain.36,37 It is also assumed that the saturation of the polymer
Presumably, an increase in the electrical conductivity of a with moisture is accompanied by the formation of a complex
polymer film with an increase in the ambient humidity is due due to the transfer of an unshared electron pair from the
to an increase in the mobility of the doping ion that is weakly nitrogen atom in the polymer to water molecules.36,37
bound by van der Waals forces to the polymer chain.35 With
Moreover, samples C1–C8 were studied as sensitive materials
an increase in the environment humidity, the polymer film in an ammonia sensor (Fig. 10). It was found that ammonia
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Fig. 8 SEM images of C1–C8.
Fig. 9 Dependence of electrical conductivity on the relative humidity of air for resistive sensors based on samples: (a) C1–C8 (at U = 5 V) and (b)
reproducibility of C7.
vapors affected the conductivity of C2, C3, C7, and C8 samples
(Fig. 10). The time to restoration of the previous conductivity
value was about 1–2 s. However, samples C1, C4, C5, and C6 did
not show a noticeable change in conductivity.
The results obtained are explained by the protonation/
deprotonation of the polymer chain. An increase in the concen-
tration of ammonia in the environment leads to a decrease in
the degree of doping. It is assumed that when the polymer
interacts with NH3, ammonia molecules absorb protons from
PANI, forming energetically favorable ammonium (NH4+), and
as a result, PANI is dedoped and conductivity decreases.
Conclusions
Fig. 10 Dependence of the electrical conductivity of films in resistive
In this work, a hitherto unknown PANI derivative, poly[2-(cyclohex-2-
en-1-yl)aniline], was synthesized and its physicochemical properties
sensors based on samples C2, C3, C7, and C8 on the concentration of
ammonia in the environment.
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12 A. Andriianova, A. Shigapova, Y. Biglova, R. Salikhov,
I. Abdrakhmanov and A. Mustafin, Chinese J. Polym. Sci.,
2019, 37(8), 774.
13 L. R. Latypova, A. N. Andriianova, S. M. Salikhov,
I. N. Mullagaliev, R. B. Salikhov, I. B. Abdrakhmanov and
A. G. Mustafin, Polym. Int., 2020, 69, 804.
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17 S. Pashaei, S. Hosseinzadeh and H. Hosseinzadeh, Polym.
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20 M. Aymen, S. Sami, S. Ahmed, G. Fethi and B. M. Abdellatif,
J. Phys. D: Appl. Phys., 2013, 46(33), 335103.
were studied. It was shown that the highest polymer yield was
observed if HNO3 and (NH4)2S2O8 were used. Moreover, varying
the conditions for the synthesis of poly[2-(cyclohex-2-en-1-yl)aniline]
allowed us to achieve the highest fluorescence intensity and
quantum yield (0.05) for the sample obtained in HClO4 medium
in the presence of (NH4)2S2O8. Thermogravimetric analysis of
poly[2-(cyclohex-2-en-1-yl)aniline] allowed us to obtain the
kinetic and thermodynamic parameters of the polymer thermal
degradation. The use of poly[2-(cyclohex-2-en-1-yl)aniline] as a
sensitive material in moisture and NH3 sensors showed a high
sensitivity of the electrical conductivity to the changes in humid-
ity and NH3 concentration in the environment. The advantages
of absorption sensors based on films of the polymers in question
include their small hysteresis. Thus, the reported results demon-
strate an efficient method for PANI modification that favors an
improvement in the solubility, appearance of photoluminescent
properties, and a wider practical application.
21 A. N. Andriianova, A. G. Mustafin and I. B. Abdrakhmanov,
ChemistrySelect, 2019, 4(38), 11307.
Conflicts of interest
¨
22 S. Stafstrom, J. L. Bredas, A. J. Epstein, H. S. Woo,
D. B. Tanner, W. S. Huang and A. G. MacDiarmid, Phys.
Rev. Lett., 1987, 59(13), 1464.
There are no conflicts to declare.
23 U. Salzner, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2014,
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24 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman and H. Nakatsuji, Wallingford,
CT, 2014, p. 201.
25 C. A. Parker, Photoluminescence of Solutions with Applications
to Photochemistry and Analytical Chemistry, Elsevier, New
York, 1968.
Acknowledgements
The reported study was funded by RFBR, project number 19-33-
90147\19. This work was supported by the Ministry of Science
and Higher Education of the Russian Federation as part of the
state task no. AAAA-A19-119020890014-7.
26 J. Y. Shimano and A. G. MacDiarmid, Synth. Met., 2001,
123(2), 251.
27 S. R. Naqvi, R. Tariq, Z. Hameed, I. Ali, M. Naqvi,
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