Electrosynthesis and characterization of a new semi-conducting oligomer deriving from a disubstituted chalcone: 4-dimethylamino -4′-methoxychalcone

Article abstract of DOI:10.1016/j.molstruc.2020.129810

An oligo phenylene vinylene was electrosynthesized by the anodic oxidation of the 4-dimethylamino-4′-methoxychalcone (DMAMC) at a constant potential in nitromethane on a platinum electrode. ¹H and ¹³C NMR, FTIR and UV spectroscopy, c

Full text of DOI:10.1016/j.molstruc.2020.129810

Journal of Molecular Structure 1231 (2021) 129810
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Electrosynthesis and characterization of a new semi-conducting
oligomer deriving from a disubstituted chalcone: 4-dimethylamino
ꢀ
-4 -methoxychalcone
Ilhem Messaoudia, Imen Aribia, Zouhour Zaaboubb, Sahbi Ayachi c, Mohamed Othmand,
Ayoub Haj Saida,e,∗
a Université de Monastir, Laboratoire Interfaces et Matériaux Avancé (LIMA), Faculté des Sciences de Monastir, Boulevard de l’environnement, 5000 Monastir,
Tunisia
b Université de Monastir, Laboratoire de Micro-opto-électronique et Nanostructures (LMON), Faculté des Sciences de Monastir, Boulevard de l’environnement,
5000 Monastir, Tunisia
c Université de Monastir, Laboratoire de Physico-chimie des matériaux (LR01ES19), Faculté des Sciences de Monastir, Boulevard de l’environnement, 5000
Monastir, Tunisia
d Normandie Université´France, UNILEHAVRE, URCOM, EA 3221, FR 3038 CNRS, F-76600 Le Havre, France
e Centre de Recherche en Microélectronique et Nanotechnologie, Technopôle de Sousse, BP 334, Sahloul, 4054 Sousse, Tunisia
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
An oligo phenylene vinylene was electrosynthesized by the anodic oxidation of the 4-dimethylamino-4 -
methoxychalcone (DMAMC) at a constant potential in nitromethane on a platinum electrode. 1H and 13 C
NMR, FTIR and UV spectroscopy, confirmed the chemical structure of the obtained oligomer. In addition,
the latter was thermally stable up to 190 °C and exhibited two emission peaks in the yellow-orange
zone. The values of the corresponding optical and electrochemical band gaps were found to be 2.04 and
1.68 eV, respectively. Finally, a mechanism for the electro-oligomerization of DMAMC was proposed based
on the experimental study and a theoretical modelisation using DFT calculation.
Received 28 September 2020
Revised 12 December 2020
Accepted 21 December 2020
Available online 24 December 2020
Keywords:
Chalcone
Voltammetry
© 2020 Elsevier B.V. All rights reserved.
Electrosynthesis
Semi-conducting oligomer
Photoluminescence
1. Introduction
and the 3,4,5-trimethoxylated (TMC) chalcones [1–3]. We demon-
strated that, under these conditions, the electrochemical oxidation
Chalcone derivatives have recently attracted significant atten-
tion as part of the research of new functional materials for op-
toelectronics [1–4], non-linear optics [5–11], electrochemical and
optical chemosensing [12–18] thanks to their ease of synthesis and
the accessibility of their starting materials, their simple structural
modification, and their interesting optical property.
led to the formation of phenylenevinylene oligomers. In addition,
we showed that the presence of the methoxy group(s) on the B
ring of chalcones (scheme 1) has a crucial role in decreasing the
oxidation potential and in directing the reaction scheme by con-
ferring a distonic character to the radical cations. Indeed, the rate-
limiting step in the oligomerization process was the coupling of
two distonic isomers of radical cations obtained from the first an-
odic electron removal [1,2].
Although the electrochemical reduction of chalcones has been
well-studied [19–22], few studies have been devoted to their an-
odic oxidation [23–26]. Particularly, flavones and flavanonols have
been described as the resulting compounds of the anodic oxidation
In the present work, we are interested in the anodic oxidation
of a disubstituted chalcone, bearing the dimethylaminophenyl moi-
ety. The aim of this study is to assess the influence of introducing
the N,N dimethyl amino group donor on the electrochemical be-
havior of the chalcone at the voltammetric and preparative elec-
trolysis timescales. In fact, N-alkyl/aryl amine, acting as a donor
moiety, will affect the frontier molecular orbital energies and con-
sequently the electrochemical and the photophysical properties.
[27–32].
ꢀ
of 2 -hydroxychalcones [23–25]. More recently, we were interested
in the anodic oxidation of chalcones, substituted on ring B, in ace-
tonitrile on a platinum electrode, namely the 4-metoxylated (MC)
∗
Corresponding author.
E-mail addresses: ayoub.hajsaid@fsm.rnu.tn, ahajsaid@gmail.com (A.H. Said).
https://doi.org/10.1016/j.molstruc.2020.129810
0022-2860/© 2020 Elsevier B.V. All rights reserved.

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
TEAF and to minimize the volume of nitromethane. The organic
phase was dried with anhydrous Na2SO4, concentrated and then
precipitated in diethyl ether.
2.3. Material characterization
The 1H NMR and 13C NMR spectral data were acquired on a
Bruker AV 300 spectrometer. FTIR analysis was performed with a
Perkin Elmer Spectrum Two ATR-FTIR spectrometer, over the wave
Scheme 1. The chemical structure of the studied chalcone (DMAMC).
number range between 500 and 3500 cm−1
.
The investigation of the electrochemical behavior of the 4-
The UV–vis absorption spectra were seized on a Cary 5000
spectrophotometer. The PL spectra were carried out using laser
diode emitting as an excitation source (λex = 375 nm). The ther-
mogravimetric analysis (TGA) was acquired on TA Instruments Q50
(TA Instruments, USA) at a heating rate of 10 °C min–1, under ni-
trogen.
ꢀ
dimethylamino-4 -methoxychalcone (DMAMC) was carried out in
nitromethane, on a platinum electrode. The choice of nitromethane
was to avoid the nucleophilic attack of the solvent on the radi-
cal cations formed during the electrolysis. The obtained electrolysis
material was characterized by different physico-chemical methods
and its optical properties were explored. Finally, a Density func-
tional theory (DFT) study was performed on the geometrical struc-
ture and the electron density distribution of the DMAMC radical
cation. The theoretical data have been correlated to the experimen-
tal results and were exploit to propose a mechanistic scheme for
the main anodic process.
For the gel permeation chromatography analysis, a μ styragel
500 A-15 mm column was used (with a length of 300 mm and a
diameter of 7.8 mm). The temperature was 30 °C. The solvent was
tetrahydrofuran with a flow rate of 0.85 mL min−1. Polystyrene
was used as a standard. .
ꢀ
2.4. Synthesis of 4-dimethylamino-4 -methoxychalcone (DMAMC)
2. Experimental section
The general synthetic strategy employed to prepare the DMAMC
(see Scheme 1) was based on Claisen–Schmidt condensation. The
p-dimethylaminobenzaldehyde was treated with equimolar quan-
tity of the p-methoxyacetophenone in a basic solution of sodium
hydroxide (1.5 eq) in ethanol, at room temperature. The filtered
product was purified by recrystallization from a water- ethanol
mixture.
2.1. Chemicals
The p-dimethylaminobenzaldehyde (Aldrich, 98%) and the p-
methoxyacetophenone (AcrosOrganic, 98%) were commercially pur-
chased and used as received. The Nitromethane (Aldrich) was used
in the electrochemical study. The tetraethylammonium tetrafluo-
roborate (TEAF) used in the preparative scale electrolysis, while,
the tetrabutylammonium tetrafluoroborate (TBAF) used in the
voltammetric study. The diethyl ether and the Ethanol were from
Novachim. The Chloroform was from Scharlau.
DMAMC: Yield 70%, yellow powder.1H NMR (ppm): 3.04
(s, 6H), 3.88 (s, 3H), 6.71 (d, 2H,
2H, 8.7 Hz), 7.31 (d, 1H,
J
=
8.7 Hz), 6.94 (d,
J
=
J
=
15.6 Hz), 7.53 (d, 2H,
J = 8.7 Hz), 7.74 (d, 1H, J = 15.6 Hz), 7.99 (d, 2H, J = 9 Hz).
13C NMR (ppm): 39–54.86–111.44–113.18–116.59–122.64–129.64–
129.97–131.55–144.29–151.49–162.50–188.39. M.p = 112 °C.
The ferrocenylchalcone Fe-cha (see Scheme 2), chosen as a ref-
erence in cyclic voltammetry, was prepared directly from acetyle-
ferrocene and p-(dimethylamino)benzaldehyde according to the
previously described procedure [33,34].
2.2. Electrochemical techniques
The voltammetric study was performed with a Voltalab10 ap-
paratus from Radiometer driven by the Volta Master software. All
cyclic voltammetric measurements were conducted at room tem-
perature in 25 mL of CH3NO2 solution containing TBAF (0.1 M)
as a supporting electrolyte. The three-electrode system contained
Fe-cha: Yield 56%, red powder. 1H NMR (ppm): 3.21 (s, 6H),
4.23 (s, 5H), 4.57 (s, 2H), 4.94 (s, 2H), 6.75 (d, 2H), 7.02 (d, 1H,
J = 9 Hz), 7.72 (d, 2H), 7.81 (d, 2H, J = 9 Hz), 7.72 (d, 2H), 7.81 (d,
1H, J = 9 Hz). 13C NMR (ppm): 40–69–70–72–81–112–117–122.5–
130–142–152–193. M.p = 140 °C.
a
platinum disk (2 mm diameter) as the working electrode,
Ag/AgCl/KCl (saturated) as the reference electrode, and a platinum
wire as the auxiliary electrode. The measurements were carried
out with seven different potential scan rates (20–300 mVs−1). The
ohmic drop compensation was activated during all experiments.
The measurements were performed at room temperature and the
cell was briefly deoxygenated with azote before each scan.
The number of exchanged electrons (ne) at the first oxidation
peak of DMAMC was obtained from the comparison of the peak
current to the monoelectronic oxidation peak of the ferrocenylchal-
cone (Fe-cha) recorded in the same conditions.
The electrochemical properties of the Fe-cha were measured at
a platinum working electrode in nitromethane 10−1 M TBAF so-
lution. The cyclic voltammetry analyses showed a reversible one-
electron oxidation at Epa = 0.673 V versus Ag/ AgCl when v = 0.1
Vs−1 (Fig. 1). The variation of the oxidation peak of Fe-ch with the
scan rate was studied. The peak potential separation (࢞Ep) was
close to 0.06 V and the anodic to cathodic current ratio was equal
to unity, irrespective of changing sweep rate.
The preparative electrolysis was carried out in
a two-
compartment cell under a nitrogen atmosphere, at a constant po-
tential of 1 V/ Ag/AgCl/KCl reference electrode. The separation be-
tween cell compartments was realized by a number 4 glass frit. A
Voltalab10 apparatus from Radiometer guided by the Volta Master
software was used. The anodic compartment contained one gram
(1 g) of the DMAMC dissolved in 50 mL (≈ 710−2 M) of a ni-
tromethane solution containing tetraethylammonium tetrafluorob-
orate TEAF (0.1 M) as supporting electrolyte. The working electrode
and the counter electrode were 2 cm x 2 cm platinum gauze. Ho-
mogenization of the solution was assured by a magnetic stirring.
The electrolysis solutions were washed with water to eliminate
Scheme 2. The chemical structure of the ferrocenylchalcone (Fe-cha).
2

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Fig. 1. Cyclic voltammogram corresponding to the oxidation of Fe-cha, 10−3 M in
CH3NO2, 10−1 M TBAF; recorded at a platinum electrode disk (d= 2 mm), scan rate
v: 100 mVs−1, Counter electrode: Pt wire.
Fig. 2. Cyclic voltammogram for oxidation of DMAMC, 2.10−3 M in CH3NO2; Scan
rate v: 100 mVs−1
.
2.5. Method of calculations
All calculations for the oxidized TMC have been performed us-
ing the most popular Becke’s three-parameter hybrid functional, B3
[35], with non-local correlation of Lee-Yange-Parr, LYP, abbreviated
as B3LYP, method [36]. This method, based on Density Functional
Theory (DFT) for a uniform electron gas (local spin density approx-
imation), is used with the 6–31 g (d,p) basis set. An open-shell
spin-unrestricted formalism was used for oxidized structure with
unpaired electrons (UB3LYP). All calculations reported in this work
were carried out with Gaussian 98 program [37].
3. Results and discussion
3.1. Voltammetric study
Fig. 3. Variation of the first peak potential Ep as a function of the logarithm of the
scan rate v, for DMAMC in CH3NO2, 10−1 M TBAF; C1 = 0.5 mM, (2) C2 = 1 mM,
(3) C3 = 2 mM, (4) C4 = 5 mM.
The cyclic voltammetry characterization of DMAMC in ni-
tromethane (0.1 M NBu4BF4) was carried out on a platinum disk
for different concentrations and at different scan rates ranging
from 20 to 300 mV−1. At 100 m Vs−1 and for a substrate concen-
tration C = 2.10−3 M, the cyclic voltammogram exhibited two an-
odic peaks, The first is chemically irreversible at a potential close
to 0.760 V, the second one is reversible and appears at 1.300 V /
ECS, as shown in Fig. 2.
Moreover, taking no account of the difference between the dif-
fusion coefficients DDMAMC and DFe-cha, the number of electrons
exchanged (ne) at the first oxidation peak of DMAMC is close
to one electron. This was obtained from the comparison of the
peak current of DMAMC to the one of the monoelectronic fer-
rocene/ferrocenium redox system of the Fe-cha recorded in the
same conditions [1,2]. It is worthy to note that the second peak
current is approximately one half of the first. This result corrobo-
rates that the second peak corresponds to the reversible oxidation
of a dimeric species.
To fully understand the mechanism of the electrochemical pro-
cess, the variation of the peak potential of DMAMC with the
scan rate or substrate concentration was studied. In anhydrous ni-
tromethane, the first peak potential variation is a linear function
of both the logarithms of the potential scan rate v and DMAMC
concentration (Ci). The corresponding slopes are close to 20 mV
(Fig. 3) and −20 mV per decade of v and (Ci/v), respectively
(Fig. 4).
Fig. 4. Variation of the first peak potential Ep as a function of log (Ci/v) (Ci con-
centration in M, v scan rate in mVs−1, for DMAMC in CH3NO2, 10−1 M TBAF.
and greatly facilitates the anodic oxidation. In fact, the first peak
potential of the studied chalcone was lowered by 0.83 V and
0.59 V compared to chalcones 4-metoxylated (MC) and 3,4,5-
trimethoxylated (TMC) on ring B, respectively [1,2]. However, at
the voltammetry timescale, the mechanism at the electrode, in a
similar way to previous studies, remains governed by a rate deter-
From this voltammetry study we conclude that the introduc-
tion of dimethyamine donor group raises the HOMO energy level
3

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Table 1
Positions and assignments of the IR vibration bands for DMAMC and O-DMAMC [27,38,39].
Mode
Wavenumber (cm−1
)
assignment
DMAMC
ODMAMC
C-H stretching
3005
3008
–
–
CH3 O- / CH3 N- protons stretching
2895/2803
1600
2898/2808
1600
=
C
C
C
C
C
C
C
C
O stretching
C stretching
=
1521
1508
–
–
–
N
O
C stretching
C stretching
1305–1244
1244–1160
986
1360–1254
1254–1167
Absent
946
–
=
=
=
=
C–H ethylenic out of plane bending (trans di substituted)
–
C
C
H ethylenic out of plane bending (trans tri substituted)
H aromatic out of plane bending
Absent
813
–
815–900
Absent
C ethylenic out of plane bending
561
The most important bands and their attribution are depicted in
Table 1.
We notice that the bands located at 561 and 986 cm−1 disap-
peared in the oligomer spectrum. These bands are attributed of the
=
C
C double bond of the ethylenic group in the enone linkage and
–
=
the out of plane bending of H C band (HC CH, E configuration),
respectively. However, new bands appeared at 900 and 946 cm−1
.
= –
These bands were assigned to the C C H aromatic out of plane
= –
bending of a 1,2,4 tri substituted benzene ring and the C C H
ethylenic out of plane bending of a trans tri substituted alkene, re-
spectively [39]. These results corroborate the involvement of both
the vinyl group and the phenyl ring (B) of chalcone in the coupling
reaction.
3.3.2. 1H NMR and 13 C NMR spectroscopy
The 1H NMR spectrum of DMAMC (Fig. 6a) showed two sig-
nals at 3.04 and 3.88 ppm attributed to the dimethylamine and
methoxy protons, respectively. The multiplet resonance signals at
chemical shift 6.71–8.04 ppm were assigned to the aromatic pro-
tons. In this region, doublets of the ethylenic protons appeared at
7.74 ppm (J = 15.6 Hz) and 7.31 ppm (J = 15.6 Hz), showing that
the ethylene moiety is in the trans-configuration E. The spectrum
of the obtained powder (Fig. 6b) exhibited three broad resonance
peaks, at the same shift as the DMAMC, attributed to the dimethy-
lamine, methoxy and aromatic protons, respectively.
Fig. 5. IR spectra of DMAMC (a) and O-DMAMC (b).
mining second-order homogeneous reaction succeeding a fast first
electron transfer. Most probably this reaction is a radical–radical
coupling reaction.
3.2. Preparative scale electrolysis
Finally, we noted that the integral ratio of the two signals
(methyl amine protons/methoxy protons) is 1.9. This value is un-
der the expected stoichiometric value 2.
In the light of the cyclic voltammetry results, preparative elec-
trolysis of DMAMC were carried out in a separated cell at a con-
stant potential as described in the experimental section. The elec-
trolysis was stopped after consumption of 2 F/mole of the starting
material. The solution obtained at the end of the electrolysis was
handled as described in the experimental section. After precipita-
tion in diethyl ether, a brown powder, corresponding to the ex-
pected oligomer, was collected by filtration. The rough yield, cal-
culated from the ratio of the weight of collected powder by the
weight of the starting material, was around 32%. The obtained
powder is soluble in most organic solvents such as chloroform,
dichloromethane and acetone.
The 13C NMR spectra of DMAMC and the oligomer O-DMAMC
are shown in Figs. 7 and 8, respectively. Carbons of DMAMC are
assigned as depicted by Fig. 6. In addition to signals appearing at
almost the same chemical shifts as for the starting monomer, the
oligomer spectrum showed the presence of new signals around
120 and 150 ppm. The 13C NMR spectrum recorded with DEPT
using a 135° pulse (Fig. 7b) showed that the new signals corre-
sponds to quaternary carbons. Moreover, this spectrum showed
that the ethylenic carbon peaks of the chain ends are strongly at-
tenuated (116–144 ppm). Finally, the signals relating to the carbons
of the carbonyl group appear in the region ranging from 190 to
200 ppm indicating that these carbons have different surround-
ing chemical environments. These results confirmed that the cou-
pling reaction occurred via an unsymmetrical linkage between the
vinyl group and the phenyl ring (B) of the chalcone, leading to a
phenylenevinylene like oligomer [1,2].
3.3. Structure characterization of the electrolysis product
3.3.1. IR spectroscopy
The recorded FT-IR spectra for DMAMC and the collected pow-
der are shown in Fig. 5. Both compounds present almost the same
infrared spectral pattern. However, we note that the IR spectrum
of the starting chalcone possesses needle-sharp bands whereas the
powder’s spectrum exhibits bands of broader width. These large
bands are characteristic of oligomeric materials. In fact, the pres-
ence of a complex mixture of rotamers and chains with differ-
ent lengths, in the oligomer powder, results in the band broad-
ening. The GPC analysis confirmed the formation of a short-chain
oligomer deriving from DMAMC (O-DMAMC) with an average de-
gree of polymerization (DP) close to 3 and a dispersity of 1.2.
3.4. Optical study
The UV–visible absorption spectra of the DMAMC and the
oligomer O-DMAMC in dilute chloroform solution are shown in
Fig. 9. The UV–visible spectrum of the DMAMC revealed two bands
with maxima located at 288 nm and 410 nm corresponding to n–
π∗ (B II) and π – π∗ (B I) transitions, respectively.
4

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Fig. 6. 1H NMR spectra of the DMAMC (a) and the O- DMAMC (b).
Fig. 7. 13 C NMR spectrum of DMAMC.
The first was attributed to the substituted benzoyl chromophore
(ArCO–). The second band was attributed to the substituted cin-
A slight difference was observed for the emission spectrum of O-
DMAMC in the thin film state indicating negligible interaction be-
tween solid states conjugated systems. Indeed, the presence of rel-
atively large groups considerably limits the stacking of oligomers
chains.
=
namoyl chromophore (ArCH CH–CO–) implicating the whole con-
jugated system corresponding to the intramolecular charge trans-
fer from the electron-donor amine substituent to the carbonyl
group acting as an electron acceptor. The oligomer spectrum ex-
hibited one absorption band located at 280 nm. Moreover, this
spectrum presents two shoulders at around 410 and 524 nm,
The optical study of the obtained material confirmed its semi-
conducting character. The broad emission spectrum could be con-
nected to the presence of oligomers with various conjugation
lengths [40]. The optical gap, deduced from the absorption onset,
was calculated to be 2.04 eV for the oligomer in solution.
The cyclic voltammetry (CV) studies of the O-DMAMC were in-
vestigated to determine the redox behavior and then to assess the
HOMO and LUMO energy levels according to a reported empirical
method [41–43]. The calculated EHOMO, ELUMO and Eg-el values were
found to be −4.99 eV, −3.31 eV and 1.68 eV, respectively.
=
the first one of which is attributed to the ArCH CH–CO– con-
jugated system absorption in a similar way than the chalcone
DMAMC.
The photoluminescent properties of the O-DMAMC were in-
vestigated in solution and as thin solid film (Fig. 10) under the
UV light excitation (λex = 375 nm). The recorded spectrum re-
vealed two emission maxima at about 560 and 610 nm (Fig. 10).
5

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Fig. 8. 13 C NMR spectrum of the oligomer O-DMAMC.
Table 2
Electrochemical and optical gap and λemission of oligomers
deriving from methoxylated chalcone (O-MC) trimethoxy-
lated Chalcone (O-TMC) and O-DMAMC.
Eg-el (eV)
Eg-op (eV)
λemission(nm)
O-MC
2.86
2.99
1.68
3.15
3.09
2.04
390
O-TMC
445
O-DMAMC
560–610
To compere the optoelectronic properties of O-DMAMC to
those of oligomers deriving from methoxylated chalcone (O-
MC) trimethoxylated Chalcone (O-TMC), the corresponding electro-
chemical and optical gap, and the λemission were summarized in
Table 2.
The obtained values confirmed the strong dependence of these
properties to the action of the dimethyl amine chromophore as a
stronger donating group. This effect was traduced by a significant
band-gap energy decrease and a light emission redshift. Notes that
the observed difference between the bandgaps obtained from the
optical method and from electrochemical analysis has been previ-
ously reported and explained for other conjugated polymers [44].
In fact, the optical electron transition leads to the formation of ex-
cited states, whereas the electrochemical reduction/oxidation gen-
erate species in ground states. Indeed, during the electrochemical
reactions other thermodynamic (salvation…) and kinetic effects are
involved and could affect the onset potential of the electrochemical
reaction.
Fig. 9. UV–visible absorption spectra of DMAMC (black line) and the oligomer O-
DMAMC (gray line), in chloroform solution.
3.5. Thermal analysis
The thermal behavior of the oligomer was investigated by ther-
mogravimetric analysis (TGA). In fact, the thermal behavior of the
organic semi-conducting materials is an important property for
both processing and applications [45]. The obtained thermogram is
shown in Fig. 11. The continuous curve represents the loss of mass
and the dotted line gives is its derivative with respect to the tem-
perature. TGA measurements revealed that the synthesized mate-
rial has a thermal stability until 190 °C. Beyond this temperature,
the obtained oligomer underwent a continuous multistep degra-
dation most probably corresponding to the decomposition of the
Fig. 10. Photoluminescence spectrum of O-DMAMC (λex = 375 nm).
6

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Scheme 3. Electro-oligomerization mechanism.
pendant chains and the conjugated backbone, successively. Finally,
the oligomer exhibited acceptable thermal stability for optoelec-
tronic applications.
The polymerization mechanism can be considered as sequences
of coupling of radical cations and deprotonation reactions as pre-
viously proposed for the electro-oligomerization of methoxylated
chalcones [1,2]. In fact, the voltammetric study indicated that the
rate-limiting step is the coupling of two radical cations issued from
the first electron transfer. This reaction involves two distonic iso-
mers leading to a non-symmetric radical cations coupling. In fact,
two mesomeric forms involving the donating effect of the amino
3.6. Electro-oligomerization mechanism
The obtained results showed that the oxidation of chalcone
leads to the formation of a semi-conducting conjugated material.
7

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
Table 3
The bond lengths for DMAMC and its radical cation (RC).
–
Bond (A°)
C1 C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C1
C5-C7
C7-C8
C8-C9
C9-O10
C2-N17
DMAMC
RC
1.415
1.433
1.418
1.435
1.384
1.369
1.409
1.427
1.407
1.425
1.387
1.371
1.457
1.434
1.352
1.366
1.478
1.498
1.232
1.232
1.381
1.351
Table 4
The charge and the spin density distribution over DMAMC and its radical cation (RC).
C1
C2
C3
C4
C5
C6
C7
C8
C9
N17
O10
DMAMCa
RCa
SDb
−0.134
−0.099
0.125
0.354
0.366
0.025
−0.135
−0.104
0.106
−0.128
−0.104
−0.048
0.128
0.146
0.246
−0.147
−0.123
−0.051
−0.107
−0.100
−0.085
−0.150
−0.108
0.304
0.361
−0.508
−0.493
−0.433
0.127
0.359
−0.465
−0.043
0.286
a
The Mulliken atomic charge (with hydrogen summed into heavy atom) for selected atoms.
The atomic spin density.
b
sible stereostructures related to the trans–cis configuration. The
presence of possible rotamers (s-cis / s-trans) relative to the con-
formation of the carbonyl groups further complicates this struc-
tural problem. Moreover, the calculated value of integral ratio of
the methyl amine protons signal / methoxy protons signal, in
1HNMR study, may indicate that the oligomer suffer from a bond
breaking side reaction resulting in the dimethylamine phenyl moi-
ety loss. In fact, the carbon-carbon double bond cleavage of chal-
cones was previously described during their chemical oxidation.
This oxidative cleavage mainly leads to the formation of the initial
benzaldehyde and ketone. [50–54].
4. Conclusions
In this paper we have studied the anodic oxidation
ꢀ
platinum
of
a
substitued chalcone, namely the 4-dimethylamino-4 -
Fig. 11. TGA and DTG thermograms of the oligomer O-DMAMC.
methoxychalcone (DMAMC), in nitromethane at
a
working electrode. This work deals with the assesment of the
dimethylamine group effect compeering with methoxylated chal-
cone previuosly described. Regarding this objective we have
showed that i) The presence of the dimethylamine group fun-
cionnal group keep unchanged the behavior of the studied
chalcone at the voltammetry timescale and the mechanism at the
electrode remains governed by a rate determining radical–radical
coupling reaction. ii) The electro-oligomerization of chacones
remains the major reaction pathways at the prepartive scale.
group (I and II in Scheme 3) could describe the structure of the
DMAMC radical cation.
This intuitive assumption was consolidated by the electronic
structure of the DMAMC radical cation given by the DFT cal-
culation. In Table 3 we present the bond lengths for DMAMC
and its radical cation. The spin density distribution was given in
–
Table 4.We note that the C C bond lengths in the cinnamoyl moe-
ity changed considerably in the radical cation structure. Specially,
the C3-C4 and the C6-C1 bonds are close to localized double bond;
however, the other bonds are longer than those of an aromatic
ring. Moreover, the C2-N17 is shortened in the radical cation and
a positive charge appeared on the nitrogen atom. Furthermore,
Table 3 indicates that the highest SD was located on carbon C5
(0.246) and C8 (0.304).
Accordingly, a new o-phenylenevinylene oligomer was formed.
The obtained oligomer was characterized by various spectroscopic
techniques: 1H and 13C NMR, FTIR and UV. The thermal study
showed that the resulting material was stable up to 190 °C.The
global electro-oligomerization mechanism reaction is similar to
that proposed for metoxylated chalcone. The performed DFT
calculation supported this mechanism and demonstrated that
the dimethylamine group played a crucial role in conferring the
distonic character to the radical cation, issued from the first
electronic transfer, and consequently in directing the reaction
scheme iii) The dimethylamine group notabely influenced the
absorption/emission properties of the obtained oligomer. In fact,
the donating effect of the dimethylamine group resulted in a
significant band-gap energy decrease and a light emission redshift.
Finally, in this contribution, we have brought out useful infor-
mation for establishing correlation between the structural modifi-
cations of chalcones, their electrochemical reactivity and the ab-
sorption/emission properties of the resulting oligomer. In addi-
tion, we showed that the electro-oligomerization of chalcones is
a promising facile synthetic route with a broad scope, a tolerance
of functional groups and enable the access, with acceptable yields,
to new organic semiconductors with modulated properties.
All these results are in agreement with the isomeric forms pre-
viously described.Thus, it is most probably that the construction of
–
a C C linkage has been developed through the non-symmetric C5-
C8 cross-coupling. This non-symmetric coupling seems presenting
a good compromise between electronic and steric effects. The re-
sulting hydrodimer (III) leads, after a proton loss, to the intermedi-
ate (IV). This latter cationic intermediate should be able to undergo
an intramolecular rearrangement, under the aromatization driving
force, leading to dimer with loss of a proton. This rearrangement
occurs in a similar way than the acid catalysed rearrangement of
ꢀ
4,4 disubstituted cyclohexadienone [46–49]. The oxidation of the
obtained dimer, leads to supplementary couplings with other oxi-
dized monomers to form oligomers.
Unfortunately, it is not possible to describe rigorously the ob-
tained oligomers structure because of the complexity of the pos-
8

I. Messaoudi, I. Aribi, Z. Zaaboub et al.
Journal of Molecular Structure 1231 (2021) 129810
In the future, the optical and electrical properties of the ob-
tained oligomer will be investigated for optoelectronic applications.
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S. Easwaramoorthi, R. Nandhakumar,
A simple chalcone based ratio-
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Declaration of Competing Interest
[14] Y. Shan, Q. Wu, N. Sun, Y. Sun, D. Cao, Z. Liu, R. Guan, Y. Xu, X. Yu, Two indole
chalcone derivatives as chemosensor for cyanide anions, Mater. Chem. Phys.
186 (2017) 295–300, doi:10.1016/j.matchemphys.2016.10.056.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
[15] Y. Wei, G. Qin, W. Wang, W. Bian, S. Shuang, C. Dong, Development of flu-
orescent FeIII sensor based on chalcone, J. Lumin. 131 (2011) 1672–1676,
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jorganchem.2007.03.044.
CRediT authorship contribution statement
Ilhem Messaoudi: Investigation, Writing - original draft. Imen
Aribi: Investigation. Zouhour Zaaboub: Investigation. Sahbi Ay-
achi: Investigation. Mohamed Othman: Investigation. Ayoub Haj
Said: Conceptualization, Writing - original draft, Supervision.
[17] J. Maynadie, B. Delavaux-Nicot, D. Lavabre, S. Fery-Forgues, Monosubstituted
ferrocenyl chalcones: effect of structural changes upon the ability to detect
calcium by absorption spectroscopy, J. Organomet. Chem. 691 (2006) (2006)
1101–1109, doi:10.1016/j.jorganchem.2005.11.021.
[18] Z. Xu, W. Yang, C. Dong, Determination of human serum albumin using
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an intramolecular charge transfer fluorescence probe: 4 -dimethylamino-2,5-
Acknowledgements
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This research was supported by the Ministry of Higher Educa-
tion and Scientific Research, Tunisia.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2020.129810.
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